Optics: measuring and testing – By light interference – For dimensional measurement
Reexamination Certificate
2002-05-13
2004-04-06
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
For dimensional measurement
Reexamination Certificate
active
06717680
ABSTRACT:
TECHNICAL FIELD
This invention relates to interferometry, and more particularly to multiple phase shifting interferometry.
BACKGROUND
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 span a full cycle of optical interference (e.g., from constructive, to destructive, and back to constructive interference). In PSI, typically, a single phase shifting device is employed to shift the phase between the reference and measurement wavefronts to produce intensity modulation at a particular frequency.
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, such as a Fourier decomposition of the intensity variation, 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.
Unfortunately, PSI measurements can be complicated by spurious reflections from other surfaces of the measurement object because they too contribute to the optical interference. In particular, light from all locations in the interferometer, including scattering from small surface defects such as scratches, pits or dust (or volume defects such as bubbles) can influence the interferogram. These defects act as light scattering centers, producing characteristic ring patterns called Newton rings or “Bulls eye” patterns that can imprint onto the measured phase map, thereby affecting the extracted surface topography. In such cases, the net optical interference image is a superposition of multiple interference patterns produced by pairs of wavefronts reflected from the multiple surfaces or defects of the measurement object and the reference surface.
SUMMARY
In general, the multi-phase shifting interferometric system extends and improves phase-shifting interferometry by minimizing measurement errors resulting from additional, unwanted reflections of non-measurement surfaces and surface defects that contaminate the optical interference pattern. The multi-phase shifting interferometric system minimizes these measurement errors by exploiting the sensitivity of the PSI extracting algorithms to different modulation frequencies. In particular, each PSI extracting algorithms exhibits a specific frequency dependence whereby certain frequency components, determined by the exact algorithm, are weighted more heavily than other frequency components such that the algorithm suppresses certain frequency components relative to others. The multi-phase shifting interferometric system uses the frequency modulating sensitivity of the PSI extracting algorithms to filter and suppress the contribution of unwanted optical interference in the measured phase by interferometrically modulating the optical interference pattern resulting from additional, unwanted reflections of non-measurement surfaces and surface defects at frequencies that are different from the desired optical interference pattern and fall in frequency regions at which the algorithms exhibit low sensitivity.
The multi-phase shifting interferometric system includes at least two independent phase shifting components that each independently shift the phase in the interferometric cavity. Examples of phase shifting components include, but are not limited to, translatable measurement and reference surfaces, tunable light sources, polarizing optics, and phase shifting components in tandem interferometric systems. Together, the multiple-phase shifting components operate cooperatively to produce an interference modulation for the desired cavity interference at one frequency and an interference modulation due to the interference produced from other undesired sources at a different frequency. The multi-phase shifting interferometric system chooses phase shifting rates so that the undesired interference modulation occurs in frequency regions where the phase extraction algorithm exhibits reduced sensitivity and the desired interference modulation occurs in frequency regions of high algorithm sensitivity.
In general, in a first aspect the invention features a method for performing phase-shifting interferometry, which includes differentially modulating an interference signal derived from an interferometer to cause a first interference component of the interference signal to modulate at a first frequency and a second interference component of the interference signal to modulate at a second frequency, wherein the first interference component of the interference signal originates from an interferometric cavity of,interest in the interferometer and the second interference component of the interference signal originates from a defect in the interferometer.
Embodiments of the invention can further include any of the following features. The method can include differentially modulating the interference signal comprises independently shifting a phase in the interferometric cavity using at least two independent phase shifting components. The interferometric cavity can include a measurement object. The independent phase shifting can include using a first of the phase shifting components to modulate a position of a first surface (e.g., a reference surface or a measurement surface of a measurement object) that defines part of the interference cavity. The independent phase shifting can include using a second of the phase shifting components to modulate the position of a second surface that defines another part of the interference cavity. Moreover, the first surface can be a measurement surface and the second surface can be a reference surface. The desired interference intensity can modulate at a frequency related to:
v
1
−v
2
wherein v
1
and v
2
are the modulation rates of the first surface and the second surface, respectively.
Furthermore, in some embodiments the independent phase shifting can include using a first of the phase shifting components to modulate a wavelength of an input beam to the interferometer. In such cases, the interference signal phase variation due to the modulation of the wavelength can be related to
2
n
∂
k
∂
t
(
x
1
-
x
2
)
wherein n is a refractive index,
k
=
2
π
λ
wherein &lgr; is the wavelength the input beam in the interferometer,
∂
k
∂
t
is a wavelength scan rate, and x
1
and x
2
are positions of surfaces that define the interferometric cavity. The independent phase shifting further can include using a second of the phase shifting components to modulate x
2
. The interference signal phase variation due to the wavelength modulation and the position modulation can be related to:
2
n
∂
k
∂
t
(
Deck Leslie L.
Kuchel Michael
Connolly Patrick
Turner Samuel A.
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