Optics: measuring and testing – By light interference – For dimensional measurement
Reexamination Certificate
2002-04-17
2004-09-07
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
For dimensional measurement
C356S454000
Reexamination Certificate
active
06788422
ABSTRACT:
BACKGROUND
This invention relates to wavelength-tuning, phase-shifting interferometry.
Interferometric optical techniques are widely used to measure optical thickness, flatness, and other geometric and refractive index properties of precision optical components such as glass substrates used in lithographic photomasks.
For example, to measure the surface profile of a measurement surface, one can use an interferometer to combine a measurement wavefront reflected from the measurement surface with a reference wavefront reflected from a reference surface to form an optical interference pattern. Spatial variations in the intensity profile of the optical interference pattern correspond to phase differences between the combined measurement and reference wavefronts caused by variations in the profile of the measurement surface relative to the reference surface. Phase-shifting interferometry (PSI) can be used to accurately determine the phase differences and the corresponding profile of the measurement surface.
With PSI, the optical interference pattern is recorded for each of multiple phase-shifts between the reference and measurement wavefronts to produce a series of optical interference patterns that typically span a full cycle of optical interference (e.g., from constructive, to destructive, and back to constructive interference). The optical interference patterns define a series of intensity values for each spatial location of the pattern, wherein each series of intensity values has a sinusoidal dependence on the phase-shifts with a phase-offset equal to the phase difference between the combined measurement and reference wavefronts for that spatial location. Using numerical techniques known in the art, the phase-offset for each spatial location is extracted from the sinusoidal dependence of the intensity values to provide a profile of the measurement surface relative the reference surface. Such numerical techniques are generally referred to as phase-shifting algorithms.
The phase-shifts in PSI can be produced by changing the optical path length from the measurement surface to the interferometer relative to the optical path length from the reference surface to the interferometer. For example, the reference surface can be moved relative to the measurement surface. Alternatively, the phase-shifts can be introduced for a non-zero optical path difference by changing the wavelength of the measurement and reference wavefronts. The latter application is known as wavelength tuning PSI and is described, e.g., in U.S. Pat. No. 4,594,003 to G. E. Sommargren.
Typically, high-stability light sources are desirable in wavelength-tuning PSI applications as instabilities in the light source (e.g., the mode characteristics) can corrupt PSI data. Mode instabilities, or mode-hops as they are commonly known, cause an unknown and random jump in the phase and frequency of the light source. Accordingly, it is not usually possible to extract an accurate phase from corrupted data.
Because PSI measurements are often required to be extremely accurate and repeatable, highly-stable laser light sources are typically used to prevent mode-hops from occurring, or to stabilize the light before it enters the interferometer. Laser diodes are an inexpensive coherent light source whose wavelength can be tuned by e.g., varying the diode current. Unfortunately, laser diodes often exhibit unpredictable long-term operating mode characteristics. The short cavity of the laser diode, its sensitivity to vibration, optical feedback and temperature, and the unpredictability of aging effects can make the mode characteristics of the laser diode difficult to control. Externally stabilizing the output of a laser diode by coupling the laser diode with an external cavity, or carefully controlling the laser diode environment and fixing the operating mode at a well-defined position can sufficiently improve laser diode stability for use in e.g., PSI applications.
SUMMARY
Quasi-stable light sources, such as bare laser diodes, are often overlooked for high-stability applications, like phase-shifting interferometry (PSI). However, the inventors have devised approaches for using a bare laser diode, or other quasi-stable light source, as a source in PSI. Accordingly, the invention is directed to PSI apparatus and methods that utilize non-stabilized light sources, e.g., light sources having mode-instabilities. The inventors have recognized that certain wavelength tunable light sources can be used as a light source for wavelength-tuned interferometers, despite their modal stability. Notably, the inventors have devised PSI implementations wherein laser diodes can be used for interferometric measurements, without the use of additional mode-stabilizing apparatus. In particular, PSI data is acquired regardless of the modal stability of the light. An algorithm, implemented during or after data acquisition, identifies data corrupted by mode instabilities, and eliminates the data from further analysis. Accordingly, the algorithm outputs analyzed data that is free from corruption associated with mode-hops and the like. Moreover, identifying a phase shifting range over which corrupted data sets are collected allows future data to be acquired using a different phase shifting range, potentially avoiding mode instabilities altogether.
In implementations described below, the user accepts that mode hops will occur, rather than trying to fix the operating mode over the use lifetime of the light source. If a controller detects a mod-hop, the controller either changes the light source environment to move the mode-hop out of the wavelength tuning range, or the controller determines the position where the mode-hop occurred and avoids that position during phase processing. These implementations make two assumptions: (i) the range between adjacent mode-hops is larger than the tuning range required by the PSI algorithm for determining a phase; (ii) the light source environment can be changed in sufficiently small increments to move the mode-hop out of the light source tuning range, without moving another mode-hop into the tuning range.
In general, in one aspect, the invention features an interferometry method. The interferometry method includes positioning a measurement surface within an interferometer that derives measurement and reference wavefronts from a tunable coherent light source that exhibits mode-instabilities within a range of wavelengths. The interferometry method further includes measuring an interference signal at each of multiple positions of a series of optical interference patterns produced by the interferometer. Each pattern in the series corresponds to one of multiple wavelengths in the wavelength range of the source. The interferometry method also includes identifying whether a portion of the interference signals is corrupted by a mode-instability in the light source.
Implementations of the interferometry method can include one or more of the following features.
Identifying corrupted portions of the interference signals can include extracting multiple phase values corresponding to different portions of each of at least two of the interference signals. The method can include determining a surface profile corresponding to the different portions. Furthermore, the interferometry can include comparing surface profiles and identifying a corrupt portion based on the comparison. Comparing the surface profiles can include determining a parameter related to a difference between a pair of surface profiles, e.g., the average of the difference at the multiple positions. This parameter can be compared to a noise figure. Comparing the surface profiles can further include comparing the difference between the first mentioned pair of surface profiles to the difference between a second pair of surface profiles.
The interferometry method can include determining a final surface profile from non-corrupted portions of the interference signals. The final surface profile can be determined by averaging surface profiles extracted from the non-corrupted portions of the inter
Connolly Patrick
Turner Samuel A.
Zygo Corporation
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