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A method and apparatus used to reduce glitches in measurements made by an accumulation of phase changes over a range greater than 2π. Embodiments of the invention feature the use of a glitch filter that receives a phase input indicating a position value. The glitch filter reduces glitches by using a

A method and apparatus used to reduce glitches in measurements made by an accumulation of phase changes over a range greater than 2π. Embodiments of the invention feature the use of a glitch filter that receives a phase input indicating a position value. The glitch filter reduces glitches by using a feedback term that has been reduced modulo 2π.The method includes measuring a plurality of values of a measurement signal, determining a current position value based on each of the plurality of measured values, wherein determining includes determining a delta value based on a current measured value and a previous position value, wherein the delta value is reduced modulo 2π to a range of ±π, and determining the current position value based on the determined delta value, and outputting at least one of the determined current position value and a velocity value.

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1. A method comprising:measuring a plurality of values of a time-varying measurement signal from an interferometer, the measured values indicative of changes in position of an object monitored by the interferometer;determining a current position value for the object based on each of the plurality of

1. A method comprising:measuring a plurality of values of a time-varying measurement signal from an interferometer, the measured values indicative of changes in position of an object monitored by the interferometer;determining a current position value for the object based on each of the plurality of measured values; andoutputting at least one of the determined current position value and a velocity value,wherein the determining comprises:determining a delta value based on a current measured value and a previous position value, wherein the delta value is reduced modulo 2π to a common range spanning 2π, anddetermining the current position value based on the determined delta value.2. The method of claim 1, wherein determining the current position value based on the determined delta value comprises accumulating each of a plurality of current delta values.3. The method of claim 2, wherein each of the plurality of current delta values is scaled before accumulating.4. The method of claim 3, further comprising:scaling using an amplifier.5. The method of claim 1, further comprising:digitizing the measurement signal to produce a digitized representation of the plurality of measured values; anddetermining a phase value for one or more of the digitized representations.6. The method of claim 5, wherein determining a delta value comprises processing a portion of the digitized representation.7. The method of claim 1, further comprising:determining the phase values using a frequency-selective process.8. The method of claim 7, wherein the frequency-selective process comprises at least one of a discrete fourier transform (DFT) and a fast fourier transform (FFT) and a digital Phase Locked Loop (PLL).9. The method of claim 7, further comprising:determining a center frequency of the frequency-selective process based on the determined velocity value.10. The method of claim 1, further comprising:adjusting at least one of the measured values using a data age adjustment value to compensate for data age.11. The method of claim 10, further comprising:determining a dynamic data age adjustment value for the at least one of the measured values.12. The method of claim 10, wherein the data age adjustment value is constant for the at least one of the measured values.13. The method of claim 1, wherein the plurality of values represent time position values.14. The method of claim 13, further comprising:adjusting at least one of the measurement values using a data age adjustment value to compensate for data age.15. The method of claim 14, further comprising:determining a dynamic data age adjustment value for the at least one of the measurement values.16. The method of claim 14, wherein the data age adjustment value is constant for the at least one of the measurement values.17. The method of claim 1, wherein the object whose position is being monitored by the interferometer comprises a wafer supported on at least one moveable stage, the method further comprising:imaging spatially patterned radiation onto the wafer;adjusting the position of said at least one stage relative to the imaged radiation; andmeasuring the position of the wafer relative to the imaged radiation based on the determined current position value.18. The method of claim 1, wherein the object whose position is being monitored by the interferometer comprises a mask, the method further comprising:supporting a wafer on at least one moveable stage;directing a source of radiation through the mask and lens assembly to produce spatially patterned radiation;adjusting the position of said mask relative to radiation from said source, said lens assembly imaging said spatially patterned radiation onto the wafer; andmeasuring the position of said mask relative to said radiation from said source based on the determined current position value.19. The method of claim 1, wherein the object whose position is being monitored by the interferometer comprises at least one stage, the method further comprising:providing a pattern of radiation using a write beam source;supporting a substrate on the at least one stage;directing said write beam of radiation onto the substrate;positioning said at least one stage and said write beam of radiation relative to one another; andmeasuring the position of said at least one stage relative to said write beam based on the determined current position value.20. The method of claim 1, wherein the common range is ?π to +π.21. An apparatus comprising:an electronic processing unit configured to receive a time-varying measurement signal from an interferometer, determine a plurality of measurement values for the received signal, determine a current position value based on each of the measurement values, and output at least one of the current position value and a velocity value, wherein the measurement values are indicative of changes in position of an object monitored by the interferometer,the electronic processing unit including a filter component configured to receive a current measurement value and determine a delta value based on a current measurement value and a previous position value, the determined delta value reduced modulo 2π to a common range spanning 2π.22. The apparatus of claim 21, wherein the filter component comprises:a subtractor to subtract the current position value from the current measurement value, and output the delta value.23. The apparatus of claim 21, wherein the filter component also comprises:an accumulator to receive and accumulate a plurality of delta values.24. The apparatus of claim 23, wherein the filter component also comprises:a scaling component to receive and scale the delta value before accumulating.25. The apparatus of claim 21, also comprising a digitizer to convert the measurement signal into digital representations corresponding to each of the plurality of measurement values; anda phase meter to determine a phase value for one or more of the digitized representations.26. The apparatus of claim 25, wherein the filter component processes only a portion of each digital representation.27. The apparatus of claim 21, wherein the electronic processing unit includes a frequency-selective phase meter to determine a phase value for each of the received measurement signals.28. The apparatus of claim 27, wherein the phase meter determines the phase value by at least one of a discrete fourier transform (DFT), a fast fourier transform (FFT), and a digital Phase-Locked-Loop unit (PLL).29. The apparatus of claim 27, wherein a center frequency of the frequency-selective phase meter is based on the determined velocity value.30. The apparatus of claim 21, further comprising:a data age adjustment component to determine a data age adjustment value to compensate for data age for each of the measurement values.31. The apparatus of claim 30, wherein the data age adjustment value is determined dynamically.32. The apparatus of claim 31, wherein the data age adjustment value is constant for each of the values of the measurement signal.33. The apparatus of claim 21, wherein the electronic processing unit includes a phase meter component to determine phase values that represent time position values.34. The apparatus of claim 33, further comprising:a data age adjustment unit to determine a data age adjustment value to compensate for data age.35. The apparatus of claim 34, wherein the data age adjustment value is determined dynamically.36. The apparatus of claim 34, wherein the data age adjustment value is constant for each of the values of the measurement signal.37. The apparatus of claim 21, wherein the common range is ?π to +π.38. The apparatus of claim 21, further comprising the interferometer for providing the measurement signal.