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Chapter 15 Accuracy and Repeatability

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1. C 0 5 temperature controlled environment Pressure 760 mm Hg 25 mm Hg possible storm fronts during measurement pressure not controlled Humidity 50 10 humidity controlled environment LASER SYSTEM CONFIGURATION ON CMM Agilent 10780C Agilent 10716A Receiver Agilent 10716A High resolution High Resolution Interferometer Interferometer oy Agilent 10707A S Beam Bender z q Agilent 10724A A Plane Mirror SO x Reflector a gt Agilent 10701A X lt 50 Beam Splitter b Agilent 10724A p Plane Mirror K Agilent 10700A Reflector lt 33 Beam Splitter Agilent 10780C Receiver lt Agilent 10707A Beam Bender it ae 10780C f Receiver Agilent 10717A Wavelength gt Tracker Agilent 10707A Beam Bender Agilent 10707A Beam Bender Agilent 10724A Plane Mirror Reflector Agilent 10716A Agilent 5517B Laser Head Agilent 10700A Agilent 10780C High Resolution 33 Beam Receiver Interferometer Splitter Figure 15 9 Laser system configuration for a precision Coordinate Measuring Machine CMM 15 28 User s Manual Chapter 15 Accuracy and Repeatability Examples Determining System Accuracy and Repeatability A list of parameters needed to calculate the system s measurement accuracy and repeatability for this application is provided the following subsections The laser head and optics component specifications are taken from this
2. The components of the wavelength tracker are aligned at the factory to minimize any cosine or Abb errors In many cases it may not be possible to completely eliminate these sources of error but every effort should be made to minimize them The paragraphs below discuss methods of installing and compensating for these errors Minimizing deadpath errors Deadpath error is an error introduced due to an uncompensated length of laser light between the interferometer and the retroreflector when the machine is at its Zero position Deadpath is the difference in optical path lengths between the reference and measurement components of the beam when the positioning stage or machine is at its zero position as defined by the machine s coordinate system Unequal beam components produce an optical path length difference that will not be properly compensated during changing environmental conditions resulting in a measurement error The optical path can differ due to unequal path lengths or different optics thickness or composition in the beam path Deadpath error can be minimized in most applications by a combination of the following e Minimizethe distance D Mount the interferometer as close to the retroreflector as possible when the machine is at its zero position as defined by its own coordinate system This minimizes the unequal path length cases e Minimize unequal path treatments as much as possible Minimize the number of o
3. provides a compensation accuracy of 1 4 ppm and a repeatability better than 1 4 ppm depending on the temperature range The second method of compensation uses a differential refractometer the Agilent 10717A Wavelength Tracker The wavelength tracker uses an optical technique to provide compensation repeatability as small as 0 14 ppm Since it is a differential refractometer only changes in the air s index of refraction are measured This means the initial compensation number must be determined from another source which also determines the compensation accuracy One popular method for accurately determining the initial compensation number is to measure a known standard or artifact with the laser system on the machine Alternatively high accuracy external sensors or the Agilent 10751C D Air Sensor can be used to obtain the initial compensation value The repeatability of the Agilent 10717A Wavelength Tracker s compensation number is given by the equation Repeatability 0 067ppm F ei xAT a AAP A This equation shows that the compensation number s repeatability is a function of ambient temperature and pressure This temperature and pressure dependency is based on the materials used to construct the Agilent 10717A Wavelength Tracker 15 10 User s Manual Chapter 15 Accuracy and Repeatability The Components of System Accuracy and Repeatability Additional information about wavelength of light compensation is provided in Ch
4. s Manual Chapter 15 Accuracy and Repeatability The Components of System Accuracy and Repeatability e The second approach is to choose an interferometer model which permits the minimum deadpath in the installation wherever possible While Agilent interferometers can usually be installed with essentially zero deadpath the application itself sometimes imposes constraints For example in some cases the Agilent 10715A may be the interferometer of choice because it has a remote reference mirror which minimizes deadpath when the interferometer itself cannot be located at the zero point During use of the interferometer system there are two alternative methods to minimize deadpath effects e The first method is to always move the moving optic typically the measurement reflector to the position where the deadpath distance is zero that is where measurement path length equals reference path length before resetting the laser system This aligns the machine s zero point to the zero deadpath position If you always do this no further compensation will be required e Thesecond method which you should use when it is not possible to align the machine s zero point to the zero deadpath position at reset is to provide deadpath compensation via software in the system controller Note that when using the Agilent 10719A in its angle measuring configuration the software correction is the only method possible since the measurem
5. 15 Accuracy and Repeatability The Components of System Accuracy and Repeatability Any changein air density which is a function of air temperature air pressure humidity and composition affects the index of refraction Thus a change in air density alters the required compensation of the laser measurement Without proper compensation system accuracy and repeatability will be degraded F or example assuming a standard and homogeneous air composition a one ppm error will result from any one of the following conditions e a1 C 1 8 F change in air temperature e a2 5mm 0 1 inch of mercury changein air pressure e an 80 changein relative humidity The wavelength compensation number WCN is the inverse of the index of refraction that is xr 2 WCN ne V Since the laser interferometer system counts the number of wavelengths of distance traveled actual displacement can be determined as follows Actual displacement wavelength counts x WCN xAy 3 This equation shows that uncertainty in the wavelength compensation number directly affects the interferometer measurement This error is a proportional term and is specified in parts per million The wavelength compensation number can be derived by a direct measurement of index of refraction using a refractometer or by using empirical data Without a refractometer it is best simply to measure the air pressure temperature and relative humidity and then relate this dat
6. 43 Chapter 15 Accuracy and Repeatability Achieving Optimum System Accuracy and Repeatability Achieving Optimum System Accuracy and Repeatability To achieve the best measurement accuracy and repeatability from a laser interferometer system in your application 1 Whenever possible make the measurements in a tightly controlled stable environment Also use the appropriate compensation methods to correct for atmospheric and material temperature effects 2 When designing a machine to use a laser interferometer system minimize both deadpath distances and Abb offsets If a deadpath exists on the machine correct for it during measurements 3 For each measurement axis be sure to properly align optical components during installation to minimize the amount of cosine error 4 Usethe proper components for the particular application If significant changes in environmental conditions are expected use automatic compensation and interferometers with minimal thermal drift Additional details are presented below Minimizing environmental effects The relative importance of typical atmospheric effects and material temperature errors is shown in Figure 15 19 Measurement errors due to material temperature errors are especially important in many applications Ideally all distance measurements with the laser system would be made in a temperature controlled room held at exactly 20 C 68 F the standard temperature Then the machine or
7. D need not be measured with precision The error in measuring D simply shows up as an uncompensated deadpath AD This value would be much smaller than the error dueto User s Manual 15 55 NOTE Chapter 15 Accuracy and Repeatability Non Uniform Environments The ability to correct for deadpath error in software does not eliminate the necessity of minimizing deadpath for proper location of the interferometer wherever possible If the deadpath D is large compared to the distance traveled L then the predominant error is a zero shift due to uncertainty in determining the change in air wavelength and this error cannot be eliminated in software Minimizing Abb error Abb offset errors occurs when the measuring point of interest is displaced from the actual measuring scale location and there are angular errors in the positioning system A very important advantage of laser systems is that the Abb error evident in almost all positioning systems is very easily reduced Abb offset error will make the indicated position either shorter or longer than the actual position depending on the angular offset The amount of measurement error resulting from Abb offset is Offset distance x tangent of offset angle Figure 15 7 illustrates Abb error and demonstrates the requirement for minimizing angular error and placement of the measurement path In Figure 15 7 A the measurement axis is coincident with the leadscrew centerline
8. System Accuracy and Repeatability Optics nonlinearity Optics nonlinearity occurs as a result of the optical leakage of one polarization component into the other The interferometer optical element in a laser interferometer system can contribute to measurement uncertainty because of its inability to perfectly separate the two laser beam components vertical and horizontal polarizations Optics nonlinearity error is periodic with a period of one wavelength of optical path change or a 360 phase shift between the reference and measurement frequencies Nonlinearity caused by optical leakage affects all interferometer systems whether they are single frequency or two frequency Leakage of one laser beam component into the other occurs for two reasons First the light leaving any laser source is not perfectly polarized linearly instead it is slightly elliptical Second the interferometer optical element is unable to perfectly separate the two laser beam components Figure 15 1 shows a computed error plot of nonlinearity versus optical path length change for worst case conditions when using a linear interferometer The peak to peak phase error is 5 4 corresponding to a worst case peak to peak error of 4 8 nanometers of distance Using a statistical model the RSS Root Sum of Squares value is 4 2 nanometers worst case peak to peak including the contribution from the laser head This nonlinearity error is a fixed term and is diffe
9. Worst Case Microns Figure 15 15 Worst case System Accuracy with and without Atmospheric Compensation for the Wafer Stepper example 15 40 User s Manual Chapter 15 Accuracy and Repeatability Examples Determining System Accuracy and Repeatability WORST CASE SYSTEM ACCURACY WITH ATMOSPHERIC COMPENSATION I C WAFER STEPPER D Laser Wave Mi 2 Compensation 3 Cosine 4 Deadpath Electronic 6 Non Linear Thermal Drift Hil With Atmospheric Compensation 0 00 0 02 0 04 0 06 0 08 Positional Error at 0 2m Worst Case Microns Figure 15 16 Worst case System Accuracy with Atmospheric Compensation for the Wafer Stepper example Another potential source of error that should be included in the total accuracy budget is the flatness of the measurement mirrors In X Y stage applications long mirrors are attached to two sides of the stage as shown in Figure 15 14 Because the mirrors are not perfectly flat a measurement change occurs in one axis as the other axis is moved Since a mirror flatness of 4 20 is recommended for correct operation of the laser system this would induce a maximum measurement error of 0 03 micron To compensate for this measurement error map the mirror flatness then make the correction via softwarein the controller User s Manual 15 41 Chapter 15 Accuracy and Repeatability Examples
10. and is measuring a displacement of the carriage at the leadscrew This figure illustrates the displacement error E which is generated at the measurement probe tip due to angular motion 8 of the carriage Figure 15 7 B shows the same carriage motion as Figure 15 7 A but with the measurement axis coincident with the probe path In this case the measurement system measures the actual displacement and there is no offset error A helpful rule of thumb for approximating the error attributable to angular motion is that for each arcsecond of angular motion the error introduced is approximately 0 1 micron per 20 mm of offset 5 microinches per inch of offset When considering a specific application make every effort to direct the measurement path as close as possible to the actual work area where the measurement process takes place In Figure 15 22 a machine slide is shown with the interferometer and retroreflector placed to minimize Abb error The measurement axis is placed at approximately the same level as the work table and is also measuring down the center of the machine slide 15 56 User s Manual Chapter 15 Accuracy and Repeatability Non Uniform Environments MINIMIZE ABBE ERROR Machine Slide lt gt Top View Interferometer Retroreflector Laser Beam Working Surface Machine Slide an Side View Figure 15 22 Positioning of measurement axis to minimize Abb error For X Y stage applications the lase
11. designing and installing the laser interferometer system A more detailed discussion of these error components follows User s Manual 15 3 Chapter 15 Accuracy and Repeatability The Components of System Accuracy and Repeatability Laser wavelength An interferometer system generates optical fringes when relative movement occurs between the measurement optics of the system Each fringe generated represents displacement by a fraction of the laser s wavelength However fringes are also generated if the laser wavelength changes causing an apparent distance change measurement even when there is no actual displacement of an optic This apparent movement is measurement error The laser source of any interferometer system has some type of frequency stabilization to maintain its wavelength accuracy and repeatability A laser interferometer system s accuracy is fundamentally based on the laser s wavelength accuracy The system s repeatability is based on the laser s wavelength stability Laser wavelength accuracy and stability are specified in parts per million ppm of the laser frequency They are proportional errors that is the measurement error is a function of the distance measured All laser sources for Agilent laser transducer systems have the same wavelength accuracy and stability specifications These values are specified in a vacuum Lifetime wavelength accuracy for the laser heads is 0 1 ppm standard and 0 02 ppm with
12. face at measurement zero position Agilent 10715A Zero deadpath condition cannot be achieved with this interferometer design because the reference and measurement mirrors cannot be coplanar Distance between front face of reference mirror and front face of measurement mirror Agilent 10716A Zero deadpath condition exists when the measurement mirror is flush with the interferometer s measurement face Distance between interferometer measurement face and measurement mirror at measurement zero position Agilent 10719A Zero deadpath condition exists when the measurement mirror is 19 05 mm 0 750 inch farther from the interferometer s measurement face than the reference mirror is M R 19 05 metric or M R 0 750 English where M Measurement Mirror distance from interferometer R Reference Mirror distance from interferometer at measurement zero position Agilent 10721A Zero deadpath condition exists when the measurement mirror is 19 05 mm 0 750 inch farther from the interferometer s measurement face than the reference mirror is 15 18 M R 19 05 metric or M R 0 750 English where M Measurement Mirror distance from interferometer Reference Mirror distance from interferometer at measurement zero position User s Manual Chapter 15 Accuracy and Repeatability The Components of System Accuracy an
13. optional calibration Wavelength stability of the laser heads is typically 0 02 ppm over their lifetime and 0 002 ppm over one hour Electronics error Electronics error stems from the method used to extend basic optical measurement resolution in an interferometer system The basic resolution of an interferometer system is 4 2 when using cube corner optics The resolution can be electronically or optically extended beyond 1 2 In an Agilent laser measurement system the electronics error equals the uncertainty of the least resolution count That is electronic error equals the measurement resolution It is the quantization error of the electronic counter in the system Other methods of electronic resolution extension can cause jitter and nonlinearity in measurement data thus adding other errors 15 4 User s Manual Chapter 15 Accuracy and Repeatability The Components of System Accuracy and Repeatability The electronics error term is a fixed error equal to the least resolution count on Agilent systems When using an Agilent laser measurement system there are three possible linear measurement resolutions depending on the interferometer chosen Table 15 2 lists the measurement resolutions for each interferometer available with this system when used with the Agilent 10885A PC Axis Board the Agilent 10895A VME Laser Axis Board the Agilent 10897B High Resolution VMEbus Laser Axis Board or the Agilent 10898A High Resolutio
14. part would be at its true size and the wavelength compensation number determined earlier could be used directly 15 44 User s Manual Chapter 15 Accuracy and Repeatability Achieving Optimum System Accuracy and Repeatability RELATIVE EFFECT OF ERRORS 550 Material Temperature Error Of 1 C Steel I N Error Micrometers Air Temperature Error Of 1 C Air Pressure Error Of 5 mm Hg 0 10 20 30 40 50 Measured Distance Meters Figure 15 19 Relative effect of errors in atmospheric and material temperature User s Manual 15 45 Chapter 15 Accuracy and Repeatability Achieving Optimum System Accuracy and Repeatability Laser measurement errors from environmental effects can be corrected by using a combined compensation term called the Total Compensation Number or TCN It contains a Wavelength of Light compensation term WCN and a Material Temperature compensation term MTC These terms were described individually earlier in this chapter The WCN is Equation 2 andthe MTC is Equation 5 The TCN is determined from the WCN and MTC as follows TCN WCN XMTC 13 Expanding the WCN and MTC terms we get WCN MTC air wavelength X 1 Linear Thermal Coefficient x Material Temperature Standard Temperature TCN 14 vacuum wavelength The Wavelength of Light term compensates for changes in the laser wavelength The material temperature term corrects the measurem
15. the carriage is positioned by a leadscrew and the measurement axis is at the leadscrew centerline This figure illustrates the displacement Abb error E which is generated at the measurement probe tip due to unwanted angular motion 9 of the carriage during the measurement Figure 15 7 B shows the same carriage motion as Figure 15 7 A but 15 22 User s Manual ABBE OFFSET Chapter 15 Accuracy and Repeatability The Components of System Accuracy and Repeatability with the measurement axis coincident with the probe path Here the measurement system measures the actual displacement thus no Abb error exists In general reducing the Abb offset will reduce sensitivity to unwanted angular motions A Measurement Axis at Leadscrew B Measurement Axis at Probe Path Measurement Axis 1 TEE l j l Abb Offset Actual __ Probe Distance Measurement Path Axis of Movement Measured __ Distance Measured le Error in lt Distance gt Measurement of Movement Figure 15 7 Abb error As a general rule Abb error is approximately 0 1 micron per 20 mm of offset for each arc second of angular motion Abb error can occur with any type of displacement transducer In high accuracy applications where it is not possible to completely eliminate the Abb effect you may measure the unwanted angular displacement directly and then correct for Abb errors via software A variety of interferom
16. value The absolute accuracy is dependent on this initial value Some methods of determining an initial compensation number are by e using an Agilent 10751C D Air Sensor e using look up tables such as those in Chapter 16 Wavelength of Light Compensation Numbers of this manual e measuring temperature pressure and humidity and then inputting these values into the automatic compensation board e measuring a known standard length User s Manual 15 47 NOTE Chapter 15 Accuracy and Repeatability Achieving Optimum System Accuracy and Repeatability To calculate the initial compensation number by measuring a known standard or artifact use the following formula Compensation Measured length from laser system on machine 14 umber Sar E PIER Ray ERE PEE EASE OTE TS RE Actual length from a Standards laboratory If relative compensation is satisfactory for your application the default values of initial compensation may be used See the laser electronics documentation for your system for details Sensor placement To correct for the effects of air conditions on the laser reading place the Agilent 10717A Wavelength Tracker or Agilent 10751C D Air Sensor where it can accurately monitor the conditions influencing the laser beam Mount the sensor as close as possible to the measurement path soit monitors the condition of these laser beams Agilent 10717A Wavelength Tracker When you use the wavelength tr
17. 0 12 15 24 User s Manual Chapter 15 Accuracy and Repeatability Determining System Accuracy and Repeatability Cosine error can be eliminated by taking care to orient the measurement laser beam parallel to the actual axis of travel Use the proper alignment procedures for each type of interferometer F or example e with interferometers using plane mirror reflectors Agilent 10706A B Agilent 10715A Agilent 10716A the resulting cosine error is less than 0 05 ppm e with interferometers that use cube comer reflectors Agilent 10702A Agilent 10705A the cosine error in parts per million is approximately equal to 31250 L2 where L is the measured distance in millimeters Determining System Accuracy and Repeatability The measurement accuracy and repeatability of a laser interferometer system are determined by summing all the error components previously discussed The error components used to determine the measurement repeatability are a subset of the accuracy components Table 15 4 shows the list of components for these error budgets and how the totals are determined As shown in Table 15 4 the only differences between the two error budgets are the laser wavelength terms and the cosine error not being part of the repeatability error budget Table 15 4 Laser interferometer system accuracy and repeatability error Laser Interferometer System Accuracy is the Sum of Repeatability is the Sum of Proportional Terms Laser Wavel
18. 0897B or Agilent 10898A electronics is Measurement Resolution 0 0025 micron Optics nonlinearity Nonlinearity when using the Agilent 10716A High Resolution Interferometer is 0 001 micron Optics thermal drift This error term should be included when determining long term repeatability The error depends on the degree of thermal cycling that the interferometer experiences With the Agilent 10716A in this application typical thermal drift will be 0 04 micron Optics Thermal Drift degree C Abb error Since this error term is independent of the type of measurement scale used but strongly dependent on how the machine is designed and built specific errors will not be included in this example However the errors should be included when calculating the error budget for an actual machine when the Abb offset is known and angular errors can be measured or estimated x 0 5 degree C x 0 02 micron Abb Error 0 micron assumed User s Manual 15 31 Chapter 15 Accuracy and Repeatability Examples Determining System Accuracy and Repeatability Cosine error If the proper alignment procedure for the Agilent 10716A is followed the worst case cosine error is Cosine Error 0 05 ppm Cosine Error in microns 0 05 ppm 1 0 m 40 05 micron CMM system accuracy calculation Now the appropriate components can be summed together to obtain system measurement accuracy and repeatability Worst case system accura
19. 15 Accuracy and Repeatability Chapter 15 Accuracy and Repeatability Introduction ntroduction This chapter introduces the basic concepts techniques and principles that determine the overall measurement performance of Agilent laser measurement systems Two examples of modeling a laser system s accuracy and repeatability are provided Understanding the error components in the laser interferometer system will help you use the modeling technique described in this chapter The measurement accuracy and repeatability is determined by summing the error components in the system s error budgets Before proceeding with the discussion of each component in the accuracy and repeatability error budgets review the definitions of accuracy and repeatability given below e Accuracy The maximum deviation of a measurement from a known standard or true value e Repeatability The maximum deviation between measurements under the same conditions and with the same measuring instrument This also refers to how stable the measurement will be over time The Components of System Accuracy and Repeatability The system measurement accuracy and repeatability error budgets share many of the same error components System measurement repeatability is divided into short term and long term repeatability Short term repeatability is the measurement stability over a period shorter than one hour long term repeatability is stability over a period longer than on
20. 15 Accuracy and Repeatability Examples Determining System Accuracy and Repeatability WORST CASE SYSTEM ACCURACY CMM EXAMPLE Laser Wave Hi 2 Compensation G Cosine 4 Deadpath Electronic 6 Non Linear T Thermal Drift i With Atmospheric Compensation Without Atmospheric Compensation J I fa 0 2 4 6 8 10 Positional Error at 1 0m Worst Case Microns Figure 15 10 Worst case System Accuracy with and without Atmospheric Compensation for the CMM example WORST CASE SYSTEM ACCURACY WITH ATMOSPHERIC COMPENSATION CMM EXAMPLE Laser Wave Mi 2 Compensation GB Cosine 4 Deadpath Electronic 6 Non Linear T Thermal Drift il 0 0 0 1 0 2 0 3 Positional Error at 1 0m Worst Case Microns Figure 15 11 Worst case System Accuracy with Atmospheric Compensation for the CMM example User s Manual 15 33 Chapter 15 Accuracy and Repeatability Examples Determining System Accuracy and Repeatability CMM system repeatability calculation Calculation of laser system long term repeatability in this example is the same as system accuracy except that the cosine error term 40 05 micron is not included Therefore system repeatability in this example will be With Atmospheric Without Atmospheric Compensat
21. 25 50 25 00 24 50 p Agilent 10715A 24 48 Figure 15 4 Comparison of optics thermal drift between 7 2 9 6 Interferometers 12 14 4 Time Agilent 10706B and Agilent 1071 24 00 23 50 23 00 24 3 sunyesodway 15 15 User s Manual Chapter 15 Accuracy and Repeatability The Components of System Accuracy and Repeatability Deadpath error Deadpath error is caused by an uncompensated length of the laser beam between the interferometer and the measurement reflector with the positioning stage or machine at its zero position the position at which the laser system is reset The deadpath distance is the difference in the optical path lengths of the reference and measurement components of the laser beam at the zero position If not properly compensated during changing environmental conditions these unequal beam components can produce a measurement error Figure 15 5 A shows the unequal path lengths for a conventional linear interferometer The deadpath length is designated as D In this diagram the reference component is fg and the measurement component is fa The fa optical path is longer than the fg path by D Assume that the measurement reflector a cube corner in this example moves the distance L see Figure 15 5 B to a new position and comes to rest Assume that while the cube corner is at rest the environmental conditions surroundi
22. Component Parameters Laser Wavelength Measurement Distance L Laser Specifications Atmospheric Compensation Measurement Distance L Environmental Conditions Compensation Performance Material Thermal Expansion Measurement Distance L Material Temperature Material Cosine Error Measurement Distance L Interferometer Type Misalignment Angle Deadpath Error Deadpath Distance Environmental Conditions Compensation Performance Electronics Error Resolution Interferometer Type Electronics Optics Non Linearity Interferometer Type Optics Thermal Drift Interferometer Type Temperature Changes Abb Error Abb Offset Angular Changes Precision Coordinate Measuring Machine CMM example The typical configuration for this application is shown in Figure 15 9 It uses Agilent 10716A High Resolution interferometers and the Agilent 10717A Wavelength Tracker This CMM has a working measurement volume of 1 0m x 1 0 m x1 0 m User s Manual 15 27 Chapter 15 Accuracy and Repeatability Examples Determining System Accuracy and Repeatability Dimensions see figure below Maximum distance measured L 1 0 m Deadpath distance D 0 1 m Cosine Error 0 05 ppm Agilent 10716A aligned according to procedure in this manual Nonlinearity 1 0 nm Agilent 10716A Abb error none assume zero offset Measurement resolution 2 5 nanometers Agilent 10716A ENVIRONMENT Temperature 20
23. D E F Material Temperature Sensor It has an accuracy of 0 1 C and a measurement repeatability better than 0 1 C Linear coefficients of expansion for various commonly used materials are presented in Chapter 17 Material Expansion Coefficients of this manual User s Manual 15 11 Chapter 15 Accuracy and Repeatability The Components of System Accuracy and Repeatability Optics thermal drift In a laser interferometer system changes in temperature of some optical components during the measurement can cause measurement uncertainty This effect occurs in the measurement optic the interferometer in the form of a change in optical path length with temperature This change in optical path length appears as an apparent distance change This optical path length change is caused by the two laser beam components horizontal and vertical polarizations passing through different amounts of glass as shown in Figure 15 2 With a conventional plane mirror interferometer such as the Agilent 10706A beam component f4 travels through more glass than does fg Beam component fa makes twice as many trips through the polarizing beam splitter as does fg Component fa also makes two round trips through the quarter wave plate whereas fg does not pass through the quarter wave plate at all UNEQUAL PATH LENGTHS E RaT NANS Y ate Z Y As o E From Laser X Wy fa iii fa f To Receiver aoh a gt A Y j A
24. Determining System Accuracy and Repeatability IC Stepper system repeatability calculations Long term repeatability Calculation of laser system long term repeatability in this exampleis the same as system accuracy except that the cosine error term 40 01 micron is not included Therefore laser system long term repeatability will be With Atmospheric Without Atmospheric Compensation Compensation Direct Sum Total 0 057 micron 2 715 microns Worst Case RSS sum Typical 0 043 micron 2 710 microns Figure 15 17 is a graph of the worst case long term repeatability Again the importance of atmospheric compensation is shown Figure 15 18 shows in more detail the worst case long term repeatability with atmospheric compensation WORST CASE SYSTEM LONG TERM REPEATABILITY I C WAFER STEPPER Laser Wave Mi 2 Compensation G Deadpath 4 Electronic 6 Non Linear 6 Thermal Drift Hi 3 6 With Atmospheric Compensation 4 2 B Without Atmospheric Compensation 4 f yyy YY 0 1 2 3 Positional Error at 0 2m Worst Case Microns Figure 15 17 Worst case System Long term Repeatability with and without Atmospheric Compensation for the Wafer Stepper example 15 42 User s Manual Chapter 15 Accuracy and Repeatability Examples Determining System Accuracy and Repeatability WORST CASE SYSTEM LONG TERM REPEATAB
25. ILITY WITH ATMOSPHERIC COMPENSATION I C WAFER STEPPER 4 Laser Wave Ei 2 Compensation G Deadpath 4 Electronic Non Linear 6 Thermal Drift il With Atmospheric Compensation 0 00 0 02 0 04 0 06 0 08 Positional Error at 0 2m Worst Case Microns Figure 15 18 Worst case System Long term Repeatability with Atmospheric Compensation for the Wafer Stepper example Short term repeatability In this example calculation of system short term repeatability is the same as long term repeatability except 1 long term laser wavelength error is replaced by short term error and 2 optics thermal drift is not included The atmospheric compensation error is assumed to be the same However under normal operating conditions atmospheric pressure changes would generally be substantially less than those used in this example for the short periods of interest in IC fabrication With Atmospheric Without Atmospheric Compensation Compensation Direct Sum Total 0 050 micron 2 708 microns Worst Case RSS sum Typical 0 042 micron 2 700 microns As seen from these values the difference between system long term and short term repeatability is only a few nanometers If the assumed short term environmental changes especially atmospheric pressure are much smaller then short term repeatability will be significantly smaller User s Manual 15
26. Material temperature compensation is accurate only when the part and the machine are at thermal equilibrium with their surroundings Changing temperature can change thermal gradients in both the machine and the part In this case the primary machine errors are due to complex bending effects which distort machine geometry in addition to simple thermal expansion These effects are extremely difficult if not impossible to describe mathematically Changing temperatures also affect the measurement optics resulting in optics thermal drift as described earlier in this chapter Therefore if a machine is operated in a poor environment its accuracy may be limited by its own geometry thermal expansion and optics thermal drift In this case the most practical solution is to improve the environment and use optics that are thermally stable Air turbulence Air Turbulence is an important factor to be considered during installation of a laser system It is usually caused by variations in air temperature The major effect of air turbulence is reduction of amount of signal at the receiver This reduction is due to either physical deflection of the laser beam or degradation of the beam s coherence Excessive air turbulence may cause complete loss of measurement signal This loss of signal will be detected by the Agilent electronics which will output an error signal One application where serious consideration must be given to air turbulence is atemperat
27. Stage Mirror LEGEND A fA lt gt fp lt gt fa and fg f r Rounded corners are used to help you trace paths Figure 15 2 Conventional plane mirror interferometer with unequal path lengths that result in optics thermal drift 15 12 User s Manual Chapter 15 Accuracy and Repeatability The Components of System Accuracy and Repeatability When a change in temperature occurs the physical size of the optical elements changes as does their index of refraction Both changes contribute to an apparent change in distance This type of interferometer has a typical thermal drift value of 0 5 micron per degree C This measurement error is a fixed value and is only a function of the change in interferometer temperature not the distance measured Optics thermal drift can be reduced by either controlling the temperature of the measurement environment or by using interferometers that are insensitive to temperature changes To reduce the temperature sensitivity of an interferometer the beam components need to travel through the same type and amount of glass Several interferometers available for Agilent laser measurement systems significantly reduce the optics thermal drift error e TheAgilent 10715A Differential Interferometer has a thermal drift on the order of fractions of a nanometer per C e TheAgilent 10706B High Stability Plane Mirror Interferometer has a thermal drift optics that of a conventional plane mirror inter
28. a to the refractive index using the formulas by Barrel amp Sears or Edlen The accuracy and repeatability of the compensation number derived by the empirical method depends on the accuracy of the formula used and the ability to measure the atmospheric conditions 2 Barrell H amp Sears J E 1939 Phil Trans Roy Society A258 1 64 3 Edlen B The Refractive Index of Air Metrologia 1966 2 71 80 Birch K P Downs MJ Metrologia 1993 30 155 162 Birch K P Downs MJ Metrologia 1994 31 315 316 Estler W Tyler Applied Optics 24 6 1985 808 815 User s Manual 15 9 Chapter 15 Accuracy and Repeatability The Components of System Accuracy and Repeatability The empirical method suffers from the following disadvantages compared to using a refractometer e itis an indirect measurement which is subject to sensor error e itis an approximation good to only 0 05 ppm e itis slow in response due to sensor time constants and calculation time e it requires periodic calibration of the sensors e it ignores air composition changes such as Carbon dioxide and Chemical vapors Agilent laser position transducer systems generally provide two methods of atmospheric compensation In the first method an air sensor is available that 1 measures air temperature and pressure 2 allows a selectable humidity setting and 3 calculates a compensation number for the system This product the Agilent 10751C D Air Sensor
29. acker mount the unit on a stable surface so that alignment is maintained Agilent 10751C D Air Sensor The air sensor should not be placed directly below the measurement beam path because the heat from the air sensor will affect the laser beam The Agilent 10751C D Air Sensor base contains a magnet to aid in securing it to magnetic materials For permanent mounting fasten the sensor using the 10 32 tapped hole on the bottom of the unit AGILENT 10751C D AIR SENSOR ORIENTATION R AT 2 6 Figure 15 20 Air sensor orientation 15 48 User s Manual Chapter 15 Accuracy and Repeatability Achieving Optimum System Accuracy and Repeatability NOTE The Air Sensor should be mounted with its arrow pointing up to maximize accuracy as shown in Figure 15 20 Agilent 10757D E F Material Temperature Sensor When monitoring material temperature to account for material expansion the Agilent 10757D E F Material Temperature Sensor should be placed on the part of the machine closest to the workpiece The material temperature sensor contains a magnet to aid in securing it to ferrous materials For permanent mounting a clamp can be used to secure it If two material temperature sensors are used they should be placed to determine the average temperature of the workpiece After attaching a probe to the workpiece allow at least 10 minutes for the probe temperature to stabilize at the workpiece temperature WOL compensation method compari
30. apter 16 Wavelength of L ight Compensation Numbers of this manual Material thermal expansion Since a part or machine s dimensions are a function of temperature a correction for material expansion or contraction may be required This correction relates the distance measurement back to a standard temperature of 20 C 68 F To achieve this correction the temperature of the part or machine during the time of the measurement and its coefficient of linear thermal expansion must be known The method of correction is to electronically change the effective laser wavelength e g through the controller software by an amount sufficient to correct for thermal expansion or contraction This correction or compensation term is known as Material Temperature Compensation and is defined as Material Temperature Compensation 1 a AT 5 where a coefficient of linear thermal expansion AT T 20 C Therefore the compensated distance measurement at standard temperature is L L Material Temperature Compensation 6 where L length at 20 C L length at temperature T Assuming a known coefficient of thermal expansion the magnitude of this error is a function of the object s temperature and the temperature sensor s measurement accuracy and repeatability This error termis also a proportional term specified in parts per million The material temperature sensor for Agilent laser systems is the Agilent 10757
31. cy and repeatability is determined by directly summing these components However a more realistic but still conservative system repeatability is the vector sum RSS Root Sum of Squares of the individual components System accuracy and repeatability will be calculated with and without atmospheric compensation to show the importance of compensating for changes in atmospheric conditions The results are presented in Table 15 6 Table 15 6 System accuracy with and without atmospheric compenstation Laser Wavelength Error Compensation Error System Accuracy Calculation With Atmospheric Compensation Without Atmospheric Compensation microns microns Material Thermal Expansion 0 0 0 0 Deadpath Error Electronics Error Optics Non Linearity Optics Thermal Drift Abb Error Cosine Error 0 015 0 90 0 0025 0 0025 0 001 0 001 0 02 0 02 0 0 0 0 0 05 0 05 Direct Sum Total 0 26 micron 9 99 microns RSS sum where s are not 0 22 micron 9 95 microns independent and is an offset The following equation is used to calculate the RSS sum RS sum sum of squares of independent terms sum of not independent terms 2 offset Figure 15 10 graphically presents this accuracy data and shows the importance of using atmospheric compensation Figure 15 11 shows in more detail the relative magnitude of each component when using atmospheric compensation 15 32 User s Manual Chapter
32. d Repeatability Table 15 3 Deadpath mirror positions and values for Agilent interferometers Continued Interferometer Agilent 10735A Mirror Position for Minimal Deadpath Zero deadpath condition cannot be achieved with this interferometer Because of interferometer design zero deadpath would require that measurement reflector be inside the interferometer 6 59 mm 0 259 inch behind the measurement face Deadpath Value Distance between interferometer measurement face and cube corner face at measurement zero position plus 6 59 mm 0 259 inch Agilent 10736A Zero deadpath condition cannot be achieved with this interferometer Because of interferometer design zero deadpath would require that measurement reflector be inside the interferometer 6 59 mm 0 259 inch behind the measurement face Distance between interferometer measurement face and cube corner face at measurement zero position plus 6 59 mm 0 259 inch Agilent 10736A 001 Zero deadpath condition cannot be achieved with this interferometer Because of interferometer design zero deadpath would require that measurement reflector be inside the interferometer For measurement axis 1 or measurement axis 3 zero deadpath would require that the measurement reflector be inside the interferometer 6 59 mm 0 259 inch behind the measurement face For the bent measurement axis measurement axis 2 zero deadpath would require that the m
33. d in measuring the artifact Consequently in this example accuracy and repeatability of atmospheric compensation information will be equal Using Equation 4 given earlier in this chapter and the specified environmental conditions accuracy and repeatability of compensation information from wavelength tracker can be determined User s Manual 15 29 Chapter 15 Accuracy and Repeatability Examples Determining System Accuracy and Repeatability Compensation accuracy and repeatability 0 06 ppm 0 002 ppm 0 067ppm degree C x 0 5 degreeC MMHG x25 mmH g 40 15 ppm At maximum distance the position uncertainty due to compensation will be Compensation Error 1 0 m 40 15 x 107 40 15 micron With no atmospheric compensation the error would be 9 0 ppm This translates to a position uncertainty at the maximum distance of 1 m of 9 0 microns Material thermal expansion On a CMM with a laser interferometer system used as the position scale material compensation should be done to the measured part not the machine Therefore the material temperature error term depends on the type of material being measured and the specifications of the temperature sensor This can be a significant error if the temperature of the part is not tightly controlled or compensation is not adequate For example with a 0 5 m part made of steel a 0 00 ppm C and using the Agilent 10757D E F Material Temperature Sensor the resulting
34. e hour Error components that make up the accuracy and repeatability error budgets are shown in Table 15 1 15 2 User s Manual Chapter 15 Accuracy and Repeatability The Components of System Accuracy and Repeatability Table 15 1 Error components for accuracy and long and short term repeatability error budgets System Error Budgets Long Term Short term Error Components by Category Accuracy Repeatability Repeatability Intrinsic Laser Wavelength Electronics Error Optics Nonlinearity Environmental Atmospheric Compensation Material Thermal Expansion Optics Thermal Drift Installation Deadpath Error Abb Error Cosine Error Both the accuracy and the repeatability error budgets have several components Some of these components are affected by the operating environment while others are affected by the system installation The error components can be categorized as either proportional or fixed terms Proportional error terms are generally specified in parts per million ppm The resulting measurement error is a function of the distance measured by the interferometer system Fixed error terms are noncumulative Fixed terms are given in units of length such as nanometers or microns The resulting measurement errors are not a function of the measured distance Environmental and installation error components are often the largest contributors to the error budgets Be sure to keep them in mind when
35. easurement reflector be inside the interferometer 34 42 mm 1 355 inches behind the measurement face For measurement axis 1 or measurement axis 3 distance between interferometer measurement face and measurement mirror at measurement zero position plus 6 59 mm 0 259 inch behind the measurement face For the bent measurement axis measurement axis 2 distance between interferometer s beam bender measurement face and measurement mirror at measurement zero position plus 34 42 mm 1 355 inches Agilent 10776A Zero deadpath condition exists when the measurement cube corner is flush with the interferometer s measurement face Distance between interferometer measurement face and cube corner face at measurement zero position Agilent 10770A Zero deadpath condition exists when the angular reflector face is parallel to the interferometer s measurement face Difference in beam path lengths between interferometer and angular reflector at measurement zero position Agilent 10774A When used with the straightness reflector the reference and measurement beam paths are the same length in air Deadpath does not exist Agilent 10775A When used with the straightness reflector the reference and measurement beam paths are the same length in air User s Manual Deadpath does not exist 15 19 NOTE Chapter 15 Accuracy and Repeatability The Components of Sys
36. ects of thermal changes of the interferometer should be included With the Agilent 10706B High Stability Plane Mirror Interferometer typical thermal drift will be 0 04 micron Optics Thermal Drift degree C x 0 1 degree C 0 004 micron 15 38 User s Manual Chapter 15 Accuracy and Repeatability Examples Determining System Accuracy and Repeatability Abb error In X Y stage applications it is usually easy to have the interferometer measurement axis in line with the wafer Therefore Abb offset will be zero and no Abb error will occur Abb Error 0 micron Cosine error If the proper alignment procedure for the Agilent 10706B High Stability Plane Mirror Interferometer is followed the worst case cosine error is Cosine Error 0 05 ppm Cosine Error in microns 40 05 ppm x 0 2 m 0 01 micron IC Stepper System accuracy calculation Now you can sum the appropriate components together to obtain system measurement accuracy and repeatability Worst case system accuracy and repeatability is determined by directly summing these components However a more realistic but still conservative system repeatability is the vector sum RSS Root Sum of Squares of the individual components System accuracy and repeatability will be calculated with and without atmospheric compensation to show the importance of compensating for changes in atmospheric conditions The results are presented in Table 15 7 Table 15 7 IC Step
37. ence Cube Corner E E o manc a m m m m m a m mal M gt u I Measurement I Cube Corner LEGEND me p fg X gt f4 and fg Figure 15 21 Equal path length correction User s Manual 15 53 Chapter 15 Accuracy and Repeatability Non Uniform Environments Compensation for deadpath errors Correction for deadpath error unequal path length is necessary if there is a change in the laser wavelength due to environmental conditions Compensation for deadpath error can be done by correcting for the deadpath distance D in software in the controller In this case the general relation True Position Wavelength counts due to motion x vacuum wavelength x TCN is expanded to be True Position Accumulated Counts Deadpath Counts x Wavelength Conversion Factor x TCN Deadpath in selected units Accumulated raw counts is the actual output from the electronics rather than the number of wavelengths For the Agilent 10716A interferometer a displacement count equals 1 256 where is the wavelength of the laser in air for Agilent laser electronics When using one the interferometers listed below an actual displacement count is equal to 1 128 where is the wavelength of the laser in air for Agilent laser electronics e Agilent 10706A B Plane Mirror Interferometer e Agilent 10715A Differential Interferometer e Agilent 10719A One Axis Differential Interferometer e Agilent 10721A Two Axis Di
38. ength Accuracy Laser Wavelength Stability Atmospheric Compensation Atmospheric Compensation Material Thermal Expansion Material Thermal Expansion Cosine Error not applicable Deadpath Error Deadpath Error Fixed Terms Electronics Error Resolution Electronics Error Resolution Optics Non Linearity Optics Non Linearity Optics Thermal Drift Optics Thermal Drift Abb Error Abb Error User s Manual 15 25 Chapter 15 Accuracy and Repeatability Examples Determining System Accuracy and Repeatability All these terms can be directly summed to determine the worst case system accuracy and repeatability However taking the vector sum of the individual components results in a more realistic or typical system performance Again these components are categorized into proportional terms or fixed terms The resulting measurement errors from proportional terms are a function of the distance measured Fixed terms are noncumulative and the resulting measurement errors are not a function of the distance measured Repeatability error components can also be divided into short term lt 1 hour and long term gt hour components F or short term repeatability only a subset of the total error components is included Generally the optics and material thermal effects are negligible over a short period of time and these components are deleted from the short term repeatability error budget Additionally short term laser wavelength stab
39. ent back to standard temperature Recall from the earlier section on atmospheric compensation that the laser position transducer counts the number of wavelengths of motion traveled This measurement can then be corrected for atmospheric effects by multiplying the distance by a correction factor the WCN The result was given in Equation 3 Actual Displacement true position Wavelength counts xWCN x vacuum Wavelength 3 We can now combine the compensation for both atmospheric and material temperature effects and calculate the true length of the object at standard 20 C temperature Using equations 3 and 13 we get Actual Length Wavelength counts x TCN x vacuum Wavelength 15 Laser compensation capability The laser system electronics can accept a manually entered Total Compensation Number TCN or automatically determine the TCN if a compensation board is installed 15 46 User s Manual Chapter 15 Accuracy and Repeatability Achieving Optimum System Accuracy and Repeatability Manual compensation For manual compensation the Total Compensation Number TCN is entered through the system controller to the Agilent laser electronics The TCN can be calculated via Equation 13 or 14 See Chapter 16 Wavelength of Light Compensation Numbers for Wavelength Compensation numbers and the method to calculate them manually See Chapter 17 Material Expansion Coefficients for information about Material Tempera
40. ent and reference path lengths are inherently unequal by 19 05 mm 0 750 inch By expanding Equation 3 the corrected actual displacement can be represented as Actual displacement Accumulated Counts Deadpath Counts x AY X WCN4 Deadpath distance 8 Accumulated counts is the displacement measured in units of LRCs Least Resolution Counts Deadpath counts is the deadpath distance in terms of compensated LRCs using the initial compensation number WCNO Ay R is equal to the LRC in units of length where R is the amount of resolution extension The compensation number at the time of measurement is WCN1 In most cases when you enter a deadpath distance into the software a positive value corresponds to the case in which the measurement path length is longer than the reference path length H owever for the Agilent 10719A and Agilent 10721A differential interferometers the User s Manual 15 21 Chapter 15 Accuracy and Repeatability The Components of System Accuracy and Repeatability deadpath distance sign depends on the measurement mirror position during reset For example if the measurement and reference mirrors are located coplanar during reset the deadpath distance is 19 mm 0 750 inch Even with this correction a small error still remains because of the repeatability of the compensation number determination This deadpath correction error is given as Deadpath Correction Error Deadpat
41. eters can serve this purpose particularly the Agilent 10719A when used as an angle measuring optic the Agilent 10735A or the Agilent 10736A for plane mirror of X Y stage applications User s Manual 15 23 Chapter 15 Accuracy and Repeatability The Components of System Accuracy and Repeatability Cosine error Misalignment of the measurement axis the laser beam to the mechanical axis of motion results in an error between the measured distance and the actual distance traveled This error is called cosine error because its magnitude is proportional to the cosine of the angle of misalignment Cosine error is common to all position transducers If the laser alignment is unchanged over time the cosine error will not change Therefore cosine error is part of the accuracy budget but not part of the repeatability budget Figure 15 8 illustrates cosine error using a ruler as a scale with an angle 0 between the measurement axis and the scale axis Measured length L is related to scale length Lz by L L cos 9 11 COSINE ERROR Scale Axi ae Scale Length Ls ae cale Axis Measured Length L 4 l Axis of Travel L L cos 0 Figure 15 8 Cosine error Cosine error is a proportional term that is the resulting measurement error is a function of the distance measured by the interferometer Therefore the cosine error can be represented in parts per million as Cosine error in ppm 1 cos 0 x 1
42. ferometer typically 0 04 micron C Other interferometers incorporating a similar high stability design include the Agilent 10716A Agilent 10719A Agilent 10721A Agilent 10735A and Agilent 10736A Figure 15 3 is an optical schematic of the Agilent 10706B High Stability Plane Mirror Interferometer In the Agilent 10706B the reference beam cube comer has been replaced by a quarter wave plate with a high reflectance coating on the back This optical design allows the measurement and reference beams to have the same optical path lengths in the glass essentially eliminating measurement errors caused by temperature changes of the optics 4 Baldwin D R amp Siddall G J A double pass attachment for the linear and plane mirror interferometer Proc SPIE Vol 480 p 78 83 1984 User s Manual 15 13 Chapter 15 Accuracy and Repeatability The Components of System Accuracy and Repeatability EQUAL PATH LENGTHS IN HIGH STABILITY INTERFEROMETER High Reflector eee Quarter wave Plates Yj ay Ate Yj From L peo oo Sg rom Laser gt E g ME Y fA fB 7 i fa To Receiver lt 44 aa gt Z A y LA LEGEND lt fA me n p E ig lt X gt f4 and fg f r Rounded corners are used to help you trace paths Figure 15 3 Agilent 10706B High Stability Plane Mirror Interferometer Beam Paths The optical path lengths for the two beams may differ slightly due to the normal dimensio
43. fferential Interferometer e Agilent 10735A Three Axis Differential nterferometer e Agilent 10736A Three Axis Differential Interferometer e Agilent 10736A 001 Three Axis Differential Interferometer with Beam Bender For the interferometers listed below a displacement count equals A 64 e Agilent 10702A Linear Interferometer e Agilent 10766A Linear Interferometer e Agilent 10705A Single Beam Interferometer Deadpath counts is the deadpath length D in terms of counts These counts have to be appropriate for the optics being used 15 54 User s Manual Chapter 15 Accuracy and Repeatability Non Uniform Environments You must input the terms Deadpath Counts and deadpath in selected units with the correct conversion factor These terms can be determined as follows For 4 256 Optics 5 4 0442888 x 10 Deadpath Counts Initial TCN D For A 128 Optics 5 2 0221444 x 10 Deadpath Counts Thitial TON gt For A 64 Optics 5 _ 1 0110722 x 10 Deadpath Counts Tnitial TCN D where D is the deadpath distance measured in millimeters The wavelength conversion factor is also dependent on which measurement optics are used For 4 256 optics 6 millimeters Wavelength Conversion Factor 2 4726175 x 10 count For 4 128 optics 6 millimeters count Wavelength Conversion Factor 4 9452351 x 10 For A 64 optics Wavelength Conversion Factor 9 8904902 x iG mii meteis count The deadpath distance
44. h Distance x Wavelength Compensation Number Repeatability 9 The error in measuring the deadpath distance can generally be ignored if its measurement tolerance is within 40 5 mm Deadpath error and deadpath correction error are both proportional values that are specified in parts per million However the measurement error is a function of deadpath distance rather than the distance measured by the interferometer Using the Agilent 10717A Wavelength Tracker and software correction the deadpath correction error will be less than 40 14 ppm x deadpath distance Abb error Abb error was first described by Dr Ernst Abb of Zeiss If errors of parallax are to be avoided the measuring system must be placed co axially in line with the line in which displacement giving length is to be measured on the work piece In simple terms Abb error occurs when the measuring point of interest is displaced from the actual measuring scale location and unwanted angular motion occurs in the positioning system Abb error makes the indicated position either shorter or longer than the actual position depending on the angular offset The Abb error is a fixed term and can be represented as Abb error offset distance x tangent of offset angle Ag tan 6 Figure 15 7 shows an example of Abb error and illustrates the requirements for minimizing angular error and minimizing offset of the scale from the measurement path In Figure 15 7 A
45. h can affect both the laser system and also the accuracy of the machine itself is thermal gradients created by localized heat sources e g motors electromagnetics lamps etc located on or near the machine You should shield the measurement path from these types of heat sources A key benefit of the Agilent 10780F Agilent E1708A and Agilent E1709A remote receivers is that they allow remote mounting of the receiver electronics eliminating its 2 watts of heat from the measurement area The remote fiber optic pickup is entirely passive and dissipates no heat A local heat source which can affect the laser system enough to cause measurement signal loss also tends to degrade the geometric accuracy of the machine through warping or bending Therefore you should consider thermally isolating the heat source from the machine as well as the measurement path Optics installation effects When planning the installation of the laser head and optics on a specific machine important points to remember are e Install the interferometer and retroreflector to minimize deadpath errors e Align the laser beam path parallel to the axis of motion to minimize cosine errors e Select the measurement paths to minimize Abb error e Usethermally stable optics User s Manual 15 51 Chapter 15 Accuracy and Repeatability Non Uniform Environments These effects are not a concern for the optical axis used for the Agilent 10717A Wavelength Tracker
46. humidity controlled environment LASER SYSTEM CONFIGURATION ON I C WAFER STEPPER Agilent 10717A Wavelength Tracker Agilent 10706B High Stability Plane Mirror Interferometer X Y STAGE X Y ee Agilent 10780C Agilent 10780C Receiver Receiver A 33 A Laser 100 67 Agilent 10706B gt gt High Stability Plane Mirror inl Agilent 10700A Agilent 10701A Interferometer 33 Beam 50 Beam Agilent 10780C Splitter Splitter g Receiver Figure 15 14 Laser System Configuration for an Integrated Circuit Wafer Stepper Each error component will be calculated individually and then summed to determine system repeatability 15 36 User s Manual Chapter 15 Accuracy and Repeatability Examples Determining System Accuracy and Repeatability Laser wavelength error The timerequired for an operation by IC fabrication equipment is often only a few minutes Thus accuracy long term stability and short term stability need to be calculated Laser Wavelength Stability 0 002 ppm short term This translates to a maximum distance error of Laser Wavelength Stability Error 0 2 m 40 002 x 10 6 short term 0 0004 micron Laser Wavelength Stability 0 02 ppm long term Laser Wavelength Stability Error 0 2 m 0 02 x10 long term 0 004 micron Laser Wavelength Accuracy 0 02 ppm with optional calibration Laser Wavelength Accuracy Error 0 2 m 0 02 x 10 40 004 micr
47. ility is used instead of long term wavelength stability and atmospheric changes especially pressure will also be smaller Examples Determining System Accuracy and Repeatability The examples below illustrate the calculation of measurement accuracy and repeatability of Agilent laser measurement systems for two typical applications In the first example the laser system is part of a precision coordinate measuring machine CMM and monitors the position of the touch probe on the machine In this example accuracy and long term repeatability will be determined In the second example the laser measurement system is built into an integrated circuit manufacturing system such as a wafer stepper or inspection machine and controls the position of the wafer stage F or this example accuracy long term repeatability and short term repeatability will be determined Short term repeatability is calculated for the wafer stepper application because process time for wafer exposures is typically very short lt 2 minutes Table 15 5 shows a list of parameters needed to calculate each error component 5 Steinmetz C R Displacement Measurement Repeatability in Tens of Manometers with Laser Interferometry Proc SPIE Vol 921 p 406 420 1988 15 26 User s Manual Chapter 15 Accuracy and Repeatability Examples Determining System Accuracy and Repeatability Table 15 5 Parameters needed to calculate each error component System Error
48. ion Compensation Direct Sum Total 0 21 micron 9 94 microns Worst Case RSS sum Typical 0 17 micron 9 90 microns Figure 15 12 is a graph of the worst case repeatability Again it shows the importance of atmospheric compensation Figure 15 13 shows in more detail the worst case repeatability with atmospheric compensation WORST CASE SYSTEM REPEATABILITY CMM EXAMPLE 4 Laser Wave Hi 2 Compensation Z 3 Deadpath 4 Electronic Non Linear 6 Thermal Drift Hil With Atmospheric Compensation Without Atmospheric Compensation 0 2 4 6 8 10 Positional Error at 1 0m Worst Case Microns Figure 15 12 Worst case System Repeatability with and without Atmospheric Compensation for the CMM example 15 34 User s Manual Chapter 15 Accuracy and Repeatability Examples Determining System Accuracy and Repeatability WORST CASE SYSTEM REPEATABILITY WITH ATMOSPHERIC COMPENSATION CMM EXAMPLE 1 Laser Wave fi 2 Compensation 3 Deadpath 4 Electronic 5 Non Linear 6 Thermal Drift Hi With Atmospheric Compensation f oa bj oh 0 0 0 1 0 2 0 3 Positional Error at 1 0m Worst Case Microns Figure 15 13 Worst case System Repeatability with Atmospheric Compensation for the CMM example IC Wafer Stepper example In th
49. is example the laser system is built into an Integrated Circuit Wafer Stepper and controls the position of the wafer stage A typical configuration for this application is shown in Figure 15 14 It uses Agilent 10706B High Stability Plane Mirror nterferometers and an Agilent 10717A Wavelength Tracker Following is a list of parameters needed to calculate the system accuracy and repeatability The laser head and optics component specifications are taken from this manual system resolution specifications for Agilent laser transducer electronics Agilent 10885A Agilent 10895A Agilent 10897B and Agilent 10898A are taken from the manual of the respective electronic board and the Agilent 10751C D Air Sensor and Agilent 10757D E F Material Temperature Sensor environmental specifications are provided in this chapter User s Manual Chapter 15 Accuracy and Repeatability Examples Determining System Accuracy and Repeatability Dimensions see figure below Maximum distance measured L 0 2 m Deadpath distance D 0 1 m Cosine Error 0 05 ppm Agilent 10706B aligned according to procedure in this manual Nonlinearity 2 2 nm Agilent 10706B Abb error none assume zero offset Measurement resolution 5 nanometers Agilent 10706B ENVIRONMENT Temperature 20 C 0 1 temperature controlled environment Pressure 760 mm Hg 25 mm Hg possible storm fronts during measurement pressure not controlled Humidity 50 10
50. manual system resolution specifications for Agilent laser transducer electronics Agilent 10885A Agilent 10895A Agilent 10897B and Agilent 10898A are taken from the manual of the respective electronic board and the Agilent 10751C D Air Sensor and Agilent 10757D E F Material Temperature Sensor environmental specifications are provided in this chapter Each error component is calculated individually and summed in the appropriate error budget to determine system accuracy and repeatability Laser wavelength error When using a CMM both accuracy and long term repeatability need to be calculated Laser Wavelength Stability 0 02 ppm long term This translates to a maximum distance uncertainty of Laser Wavelength Stability Error 1 0 m 40 02 x 10 long term 40 02 micron Laser Wavelength Accuracy 0 02 ppm with optional calibration Laser Wavelength Accuracy Error 1 0 m 40 02 x 10 0 02 micron Atmospheric compensation Since the wavelength tracker provides relative compensation information the initial compensation number from another source determines the compensation accuracy In this example the initial compensation number is derived from measuring a known artifact or standard with the laser system on the machine The accuracy of measuring the artifact or standard is the sum of the laser system measurement repeatability machine repeatability and touch probe accuracy It is assumed that no error is induce
51. measurement accuracy and repeatability will be Measurement Accuracy g x temperature sensor repeatability x part length 10 0 ppm 1 dere CS 0 1 degree C x 0 5m 40 5 micron The Agilent 10757D C E temperature sensor has a measurement repeatability equal to its accuracy Measurement Repeatability 0 5 micron Since this error is independent of the type of measurement scale but strongly dependent on the type of material and temperature sensor performance specific errors will not be included in this example However this error should be included when calculating the error budget for an actual machine Material Thermal Expansion 0 micron assumed 15 30 User s Manual Chapter 15 Accuracy and Repeatability Examples Determining System Accuracy and Repeatability Deadpath error Deadpath error is a function of deadpath distance method of compensation and environmental conditions With no compensation for deadpath Equation 7 determines the error Deadpath Error 0 1 m 49 x 10 40 9 micron With deadpath correction and using Wavelength Tracking Compensation Equation 9 determines the error Deadpath correction error 0 1 m 40 15 x 10 40 015 micron Electronics error With Agilent laser interferometer systems the electronics error equals measurement resolution When using the Agilent 10716A High Resolution Interferometer system measurement resolution for Agilent 10885A Agilent 10895A Agilent 1
52. n VMEbus Dual Laser Axis Board User s Manual 15 5 Chapter 15 Accuracy and Repeatability The Components of System Accuracy and Repeatability Table 15 2 System measurement resolution for each interferometer Interferometer Fundamental Optical Resolution System Resolution Note 1 System Resolution Note 2 Agilent 10702A 2 2 316 5 nm 12 5 pin 2 64 10 0 nm 0 4 pin 0 512 1 2 nm 0 047 pin Agilent 10705A Agilent 10706A Agilent 10706B Agilent 10715A Agilent 10716A Agilent 10719A Linear Angular Agilent 10721A Linear Angular Agilent 10735A Linear Yaw Pitch Agilent 10736A Agilent 10736A 001 Agilent 10766A Agilent 10770A Angular 2 316 5 nm 12 5 pin M4 158 2 nm 6 2 pin 4 158 2 nm 6 2 pin M4 158 2 nm 6 2 pin 8 79 1 nm 3 1 pin 4 158 2 nm 6 2 pin 1 71 arcsec 8 3 urad M4 158 2 nm 6 2 pin 2 56 arcsec 12 4 urad 1 64 10 0 nm 0 4 pin 4 128 5 0 nm 0 2 pin 1 128 5 0 nm 0 2 pin 4 128 5 0 nm 0 2 pin 1 256 2 5 nm 0 1 pin 4 128 5 0 nm 0 2 pin 0 05 arcsec 0 26 urad 1 128 5 0 nm 0 2 pin 0 08 arcsec 0 39 urad 4 512 1 2 nm 0 047 pin 1 1024 0 62 nm 0 024 yin 4 1024 0 62 nm 0 024 yin 1 1024 0 62 nm 0 024 yin 1 2048 0 31 nm 0 012 pin 1 1024 0 62 nm 0 024 yin 0 007 arcsec 0 03 prad 1 1024 0 62 nm 0 024 yin 0 01 arcsec 0 05 urad Three axes each the same as the Agilent 10706B See listing ab
53. nal tolerances in the thicknesses of the quarter wave plates and in the geometry of the beam splitter These small variations result in the small thermal drift of the Agilent 10706B Since either optical path length may be longer than the other depending on the actual optical elements used the thermal drift may be positive or negative Figure 15 4 is a plot of the thermal drift performance of the Agilent 10706B Agilent 10716A and Agilent 10715A interferometers as compared to a conventional plane mirror interferometer e The left vertical scale is thermal drift in microns e Theright vertical scale is the interferometer s temperature in C e The horizontal scale is time e Thethermal drift of the conventional plane mirror interferometer Agilent 10706A closely tracks the optics temperature changes at a rate of approximately 0 5 micron per C e The Agilent 10715A shows essentially zero drift 15 14 User s Manual Measurement Drift Microns 1 75 1 50 1 25 1 00 75 H 50 25 P 0 00 25 Chapter 15 Accuracy and Repeatability The Components of System Accuracy and Repeatability e TheAgilent 10706B and Agilent 10716A show much smaller drift than the conventional plane mirror interferometer typically 0 04 micron per degree C MEASUREMENT DRIFT AND TEMPERATURE VS TIME A Tees T emperature 27 00 26 50 26 00 lt Conventional Plane Mirror Interferometer
54. ng the laser beam change The laser beam wavelength changes over the entire path D L due to these environmental changes and so should be compensated Since a laser interferometer system measures only wavelengths of motion which involves only the distance L the system will not correct for the wavelength change over D This will result in an apparent shift in the zero position on the machine This Zero shift is deadpath error and occurs whenever environmental conditions change during a measurement 15 16 User s Manual Chapter 15 Accuracy and Repeatability The Components of System Accuracy and Repeatability DEADPATH A Reflector at Initial Position ay 5 l AI easuremen Ve A fB Reflector fa fg fa From Laser P gt _s s i fa fB I fa To Receiver lt lt lt _ lt B After Reflector Movement r IN Measurement Ve A fB Reflector fA fp fa From Laser X gt gt f A To Receiver lt A gt LEGEND f a gt fa m lt m gt fg lt f and fB Figure 15 5 Deadpath caused by unequal lengths from initial point Deadpath error can be represented as Deadpath Error Deadpath distance x AWCN 7 where AWCN Change in wavelength compensation number during the measurement time Deadpath effects should be considered when designing a laser interferometer into an application or when using it Table 15 3 li
55. on Atmospheric compensation Since the wavelength tracker provides relative compensation information the initial compensation number from another source determines the compensation accuracy In this example the initial compensation number is obtained by measuring a known artifact or standard with the laser system The accuracy of measuring the artifact is the sum of the laser system measurement repeatability machine repeatability and the accuracy of the alignment mark sensing system It is assumed that no error is induced in measuring the artifact on the machine Consequently in this example accuracy and repeatability of the atmospheric compensation information will be equal Using Equation 4 and the specified environmental conditions accuracy and repeatability of compensation information from wavelength tracker can be determined Compensation accuracy and repeatability 0 06 ppm 0 002 ppm 0 067ppm degreac x 0 1 degree C mm HG x 25mmH g 0 14 ppm At maximum distance the position error due to compensation will be Compensation Error 0 2 m x 40 14 x 10 0 028 micron With no atmospheric compensation the error would be 9 0 ppm This translates into a position error of 1 8 microns User s Manual 15 37 Chapter 15 Accuracy and Repeatability Examples Determining System Accuracy and Repeatability Material thermal expansion This error depends on the machine design and the position that is measured
56. or controlled On a wafer stepper the wafer is positioned relative to the optical column If the measurement axes are placed to allow measurements between the wafer and optical column for example using an Agilent 10719A or Agilent 10721A differential interferometer material temperature effects may be ignored This assumes the material expansion in the measurement path is equal to that in the reference path Material Thermal Expansion 0 micron assumed Deadpath error Deadpath error is a function of deadpath distance method of compensation and environmental conditions With no compensation for deadpath Equation 7 determines the error Deadpath Error 0 1 m x 40 9 x 10 40 9 micron With deadpath correction and the use of the wavelength tracker Equation 9 determines the error Deadpath correction error 0 1 m x 40 14 x 10 40 014 micron Electronics error With Agilent laser interferometer systems the electronics error equals the measurement resolution When using the Agilent 10706B High Stability Plane Mirror Interferometer system measurement resolution for the Agilent 10885A Agilent 10895A Agilent 10897B or Agilent 10898A electronics is Measurement Resolution 0 005 micron Optics nonlinearity Nonlinearity when using the Agilent 10706B High Stability Plane Mirror Interferometer is 40 0022 micron Optics thermal drift Because the measurement repeatability of this piece of equipment is important the eff
57. ove 2 128 on three axes 0 04 arcsec 0 2 urad 0 05 arcsec 0 24 urad 2 1024 on three axes 0 005 arcsec 0 025 urad 0 006 arcsec 0 03 urad Three axes each the same as the Agilent 10706B See listing above Three axes each the same as the Agilent 10706B See listing above 2 316 5 nm 12 5 pin 20 0 arcsec 97 0 urad 1 64 10 0 nm 0 4 pin 0 63 arcsec 3 0 prad 2 512 11 2 nm 0 047 pin 0 08 arcsec 0 38 urad Notes 1 The system resolution is based on using 32X electronic resolution extension This is available with the Agilent 10885A and Agilent 10895A 2 The system resolution is based on using 256X electronic resolution extension This is available with the Agilent 10897B and Agilent 10898A electronics 3 The Agilent 10719A interferometer makes a single measurement which may be either linear or angular optically subtracted depending on the installation The linear and angular measurements are mutually exclusive and therefore not simultaneous 4 The Agilent 10721A interferometer makes a two adjacent linear measurements which can be subtracted electronically to give an angular measurement with a linear measurement simultaneously 5 The Agilent 10735A Agilent 10736A and Agilent 10736A 001 interferometers make linear and angular measurements so they have both linear and angular resolution specifications 15 6 User s Manual Chapter 15 Accuracy and Repeatability The Components of
58. per System Accuracy with and without Atmospheric Compenstation System Accuracy Calculation With Atmospheric Compensation Without Atmospheric Compensation microns microns Laser Wavelength Error Compensation Error Material Thermal Expansion 0 0 0 0 Deadpath Error 0 014 0 90 Electronics Error 0 005 0 005 Optics Non Linearity 0 0022 0 0022 Optics Thermal Drift 0 004 0 004 Abb Error 0 0 0 0 Cosine Error 0 01 0 01 Direct Sum Total 0 067 micron 2 725 microns RSS sum where s are not 0 053 micron 2 710 microns independent and is an offset User s Manual 15 39 Chapter 15 Accuracy and Repeatability Examples Determining System Accuracy and Repeatability Use the following equation to calculate the RSS sum RS sum sum of squares of independent terms sum of not independent terms2 offset Figure 15 15 graphically presents this accuracy data and shows the importance of using atmospheric compensation Figure 15 16 shows in more detail the relative magnitude of each component when using atmospheric compensation WORST CASE SYSTEM ACCURACY I C WAFER STEPPER 1 Laser Wave Ii 2 Compensation G Cosine 4 Deadpath Electronic 6 Non Linear 7 Thermal Drift i 2 With Atmospheric Compensation G Without Atmospheric Compensation Positional Error at 0 2m
59. ptics such as windows used in the beam path e Usean Agilent 10715A Differential Interferometer or Agilent 10706B High Stability Plane Mirror Interferometer instead of the Agilent 10706A Plane Mirror Interferometer Some unequal path treatment cannot be avoided with the Agilent 10706A Plane Mirror Interferometer The other interferometers have negligible difference in their treatments F igure 15 2 shows that component fa travels through more glass than does fg It makes twice as many trips through the interferometer as does fa and also two round trips through the quarter wave plate This unequal treatment of fa and fg causes deadpath errors under changing conditions e Correct the residual distance D in software in the controller 15 52 User s Manual Chapter 15 Accuracy and Repeatability Non Uniform Environments e Equalize the path lengths of fg and fy by moving the reference cube corner a distance D from the interferometer See Figure 15 21 Assuming the atmospheric conditions are equivalent and the distances between the cube corners and the interferometer are equal this configuration would not have deadpath errors due to unequal path lengths Take care when using this method of reducing deadpath because any drift in the position of the reference cube corner will also show up as a measurement error This drift can result from non rigid mounting and thermal expansion for example EQUAL PATH LENGTH CORRECTION Refer
60. r system can minimize Abb errors Plane mirror interferometers used with plane mirrors mounted at 90 to each other on the top edges of an X Y stage create a very accurate positioning system which eliminates Abb error F igure 15 23 shows a typical installation for an X Y stage The principal advantage of this type of positioning system is that the measurement in both X and Y axes takes place at the work surface plane If there are angular errors in the cross slides of the stage any displacement of the work surface due to these errors is measured by the laser User s Manual 15 57 Chapter 15 Accuracy and Repeatability References X Y STAGE MEASUREMENT Work Surface Plane Laser Beam Plane Mirror Laser Beam Interferometer gt NAS A i WN Plane Mirror NAN 4 go Interferometer A Receiver Plane Mirrors Y Axis ye Receiver Figure 15 23 X Y Stage measurement with Agilent 10706A Plane Mirror Interferometer References 1 Quenelle R C Nonlinearity in Interferometer M easurenents Agilent Technologies J ournal p 1 0 April 1983 2 Barrell H amp Sears J E 1939 Phil Trans Roy Society A258 1 64 3 Edlen B TheRefractive ndex of Air Metrologia 1966 2 71 80 4 Birch K P Downs MJ Metrologia 1993 30 155 162 5 Birch K P Downs MJ Metrologia 1994 31 315 316 6 Estler W Tyler Applied Optics 24 6 1985 808 815 7 Baldwin D R amp Siddall G J A double pass a
61. rent for each interferometer Quenelle R C Nonlinearity in Interferometer Measurements Agilent Technologies Journal p 10 April 1983 User s Manual 15 7 Chapter 15 Accuracy and Repeatability The Components of System Accuracy and Repeatability NONLINEARITY ERROR VS OPTICAL PATH LENGTH CHANGE 1 1 N 1 w I Nonlinearity Error degrees of phase Optical Path Length Change degrees of phase Figure 15 1 Worst case error resulting from imperfect separation of two beam components Atmospheric compensation The atmospheric compensation error term is usually the single largest component in an error budget The magnitude of this error depends on the accuracy of the compensation method the atmospheric conditions in which the laser system is operating and how much the atmospheric conditions change during a measurement The laser wavelength is specified as the vacuum wavelength Av In vacuum the wavelength is constant to the degree specified by the stability specification but in an air atmosphere the wavelength depends on the index of refraction of the atmosphere Since most laser interferometer systems operate in air it is necessary to correct for the difference between Ay and the wavelength in air Ag This correction is referred to as atmospheric or wavelength of light WOL compensation The index of refraction n of air is related to Ay and A by Ay 1 15 8 User s Manual Chapter
62. son The method of atmospheric WOL compensation used is important in determining the overall laser system measurement accuracy Table 15 8 summarizes the laser system accuracy for various methods of atmospheric compensation as a function of different atmospheric conditions Table 15 8 Laser system measurement accuracy comparison Environment Pressure 760 mm Hg 25 mm Hg Relative Humidity 50 10 Temperature Control No Compensationt at 20 C 9 0 ppm 9 9 ppm 14 0 ppm Compensation using Agilent 10751C D Air 1 4 ppm 1 5 ppm 1 6 ppm Sensor at 20 C typical Wavelength Tracking Compensation 0 15 ppm 0 19 ppm 0 44 ppm Measurement in Vacuum 0 1 ppm 0 1 ppm 0 1 ppm These accuracy specifications include the laser head term but exclude electronics accuracy and interferometer nonlinearity terms t No compensation means that no correction in compensation number occurs during environmental changes System accuracy equals these values measurement repeatability plus accuracy of initial compensation value User s Manual 15 49 Chapter 15 Accuracy and Repeatability Non Uniform Environments Non U niform Environments Compensation for environmental effects is practical only when the material being measured is at a constant temperature and when the medium through which the measurement laser beam passes is not disturbed such as by air turbulence Changing temperature conditions
63. sts the minimum deadpath mirror position s and the deadpath values for Agilent interferometers User s Manual 15 17 Chapter 15 Accuracy and Repeatability The Components of System Accuracy and Repeatability Table 15 3 Deadpath mirror positions and values for Agilent interferometers Interferometer Agilent 10702A Mirror Position for Minimal Deadpath Zero deadpath condition exists when the measurement cube corner is flush with the interferometer s measurement face Deadpath Value Distance between interferometer measurement face and cube corner face at measurement zero position Agilent 10705A Zero deadpath condition exists when the measurement cube corner is flush with the interferometer s measurement face Distance between interferometer measurement face and cube corner face at measurement zero position Agilent 10706A Zero deadpath condition cannot be achieved with this interferometer Because of interferometer design zero deadpath would require that measurement reflector be inside the interferometer 7 62 mm 0 300 inch behind the measurement face Distance between interferometer measurement face and cube corner face at measurement zero position plus 7 62 mm 0 300 inch Agilent 10706B Zero deadpath condition exists when the measurement mirror is flush with the interferometer s measurement face Distance between interferometer measurement face and cube corner
64. tem Accuracy and Repeatability During system design there are two key approaches to minimizing deadpath effects e One approach is to locate the stationary optic typically the interferometer as close as possible to the zero point of the moving optic The zero point is established at the time the laser system is reset This will minimize or eliminate deadpath in most applications This is shown in Figure 15 6 which shows how to eliminate deadpath in a basic optical layout for an interferometer system OPTICAL CONFIGURATION WITH AND WITHOUT DEADPATH A With Deadpath Zero Position Deadpath Measurement Length B Without Deadpath Zero Position Figure 15 6 Optical configuration with and without deadpath It is important to understand that the zero deadpath condition occurs when the reference and measurement optical paths have equal length For some interferometers this may NOT correspond simply to bringing the interferometer and measurement mirror as close as possible F or example due to the differential design of the Agilent 10719A and Agilent 10721A interferometers the zero deadpath condition occurs when the mirror is 19 mm 0 750 inch FARTHER from the interferometer than the reference mirror is located This condition makes the reference and measurement path lengths equal because the reference beam travels an additional 19 mm 0 750 inch inside the interferometer 15 20 User
65. ttachment for the linear and plane mirror interferometer Proc SPIE Vol 480 p 78 83 1984 8 Steinmetz C R Displacement Measurement Repeatability in Tens of Manometers with Laser Interferometry Proc SPIE Vol 92 1 p 406 420 1988 15 58 User s Manual Product specifications and descriptions in this Laser and Optics User s Manual document subject to change without notice For complete manual order Copyright C 2002 Agilent Technologies Paper version p n 05517 90045 Printed in U S A 07 02 CD version p n 05517 90063 This is a chapter from the manual titled This chapter is p n 05517 90141
66. ture compensation numbers Manual compensation can also be done without deriving or looking up the factors by using the appropriate Agilent automatic compensation board for the Agilent laser electronics The compensation board computes compensation factors from the environmental data atmosphere and machine or part temperature entered manually through the controller to the Agilent electronics Automatic compensation With most Agilent laser electronics the necessary information for wavelength compensation can be obtained automatically by using the appropriate Agilent automatic compensator board and environmental sensors WOL compensation is provided by using either the Agilent 10751C D Air Sensor to measure air temperature pressure and humidity or the Agilent 10717A Wavelength Tracker to measure the laser wavelength change directly The Agilent 10757D E F Material Temperature Sensor provides the temperature data for the Material Temperature term The Agilent automatic compensation board automatically provides an updated total compensation number TCN The Agilent 10717A Wavelength Tracker and its accompanying Agilent 10780C Agilent 10780F Agilent E1708A or Agilent E1709A receiver provide the Agilent automatic compensation board with information indicating any changes in the laser wavelength Unlike the air sensor the wavelength tracker measures relative differential changes in the laser wavelength with respect to an initial
67. ure controlled environment Although it would appear that such an environment would be ideal temperature controlled areas often exhibit greater air turbulence than non controlled areas This turbulence is caused by incomplete mixing of new air from the temperature control unit with existing air creating thermal gradients or pockets Although such environments are good for a machine s thermal stability the short term fluctuations can cause measurement signal degradation in the laser system 15 50 User s Manual Chapter 15 Accuracy and Repeatability Non Uniform Environments Reducing air turbulence In an uncontrolled environment the effects of air turbulence can be minimized by protecting the laser beam with some type of cover Since this would normally be done for protection against beam interruption air turbulence effects will usually not bea significant installation factor in a typical environment Protection against air turbulence problems which occur in a controlled environment depends largely on the specific application F or systems such as integrated circuit lithography equipment in small closel y controlled enclosures it may be sufficient to provide constant air flow over the measurement paths In other cases such as large coordinate measuring machines protecting the laser beams with covers prevents air turbulence effects from interfering with the measurement Avoiding thermal gradients One source of air turbulence whic

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