Optics: measuring and testing – By light interference – For dimensional measurement
Reexamination Certificate
2001-07-27
2003-12-23
Kim, Robert H. (Department: 2882)
Optics: measuring and testing
By light interference
For dimensional measurement
C356S516000
Reexamination Certificate
active
06667809
ABSTRACT:
BACKGROUND
Scanning microscopy techniques, including near-field and confocal scanning microscopy, conventionally employ a single spatially localized detection or excitation element, sometimes known as the scanning probe [“Near-field Optics: Theory, Instrumentation, and Applications,” M. A. Paesler and P. J. Moyer, (Wiley-New York) (1996); “Confocal Laser Scanning Microscopy,” C. Sheppard,
BIOS
(Scientific-Oxford and Springer-New York) (1997).] The near-field scanning probe is typically a sub-wavelength aperture positioned in close proximity to a sample; in this way, sub-wavelength spatial resolution in the object-plane is obtained. An aperture smaller than a free space optical wavelength of an optical beam used in a near-field microscopy application is hereinafter referred to as a sub-wavelength aperture. The confocal scanning probe employs diffraction-limited optics to achieve resolution of the order of the optical wavelength. Spatially extended images are acquired by driving the scanning probe in a raster pattern.
Effects of background beams in certain near-field microscopy systems generally are a significant source of systematic and statistical errors.
SUMMARY OF THE INVENTION
The invention features systems and methods for near-field, interferometric microscopy in which one or more phase retardation plates are positioned in the measurement and/or reference arms to reduce the contribution to the interference signal of background sources including, e.g., a beam component scattered from a near-field aperture used to couple a probe beam to a sample. The systems may operate in either reflective or transmissive modes. Furthermore, the microscopy systems using the aperture may be designed to investigate the profile of a sample, to read optical date from a sample, and/or write optical date to a sample.
In general, in one aspect, the invention features an interferometric optical microscopy system for imaging an object. The system includes: (i) a beam splitter positioned to separate an input beam into a measurement beam and a reference beam; (ii) a measurement beam source array positioned to receive the measurement beam, the measurement beam source array including a mask having an array of measurement apertures each configured to radiate a portion of the measurement beam to the object, the object interacting with the radiated measurement beam portions to direct signal radiation back through the apertures to define a measurement return beam; (iii) a reference beam source array positioned to receive the reference beam, the reference beam source array including an array of elements each configured to radiate a portion of the reference beam, the radiated reference beam portions defining a reference return beam; (iv) a multi-element photo-detector; (v) imaging optics positioned to direct the measurement and reference return beams to the photo-detector and configured to produce overlapping conjugate images of the array of reference elements and the array of measurement apertures on the photo-detector, wherein the conjugate image for each measurement aperture overlaps with the conjugate image of a corresponding reference element to produce an optical interference signal indicative of a particular region of the object; and (vi) at least one phase mask positioned to contact the return measurement beam and the return reference beam, wherein the at least one phase mask causes the conjugate image for each reference element and each measurement aperture to have an asymmetric profile along at least a first dimension.
Embodiments of the microscopy system may include any of the following features.
The at least one phase mask may include a first phase mask and a second phase mask, wherein the first phase mask is positioned to contact the return measurement beam and not the return reference beam, and wherein the second phase mask is positioned to contact the return reference beam and not the return measurement beam. For example, the first phase mask may be positioned in a pupil plane of the imaging optics for the return measurement beam and the second phase mask may be positioned in a pupil plane of the imaging optics for the return reference beam.
Alternatively, the at least one phase mask may include a first phase mask positioned to contact both of the return measurement beam and the return reference beam. For example, that first phase mask is positioned in a pupil plane of the imaging optics for both the return measurement beam and the return reference beam.
The at least one phase mask may divide the transverse profile of the return measurement beam and the return reference beam into multiple sections along the first dimension and imparts a relative phase shift equal to &pgr;+2&pgr;n, where is n is an integer, to half of the multiple sections.
The phase mask may further cause the conjugate image for each reference element and each measurement aperture to have an asymmetric profile along a second dimension orthogonal to the first dimension. For example, the at least one phase mask may divide the transverse profile of the return measurement beam and the return reference beam along the first dimension into multiple sections and impart a relative phase shift equal to &pgr;+2&pgr;n
1
, where is n
1
is an integer, to half of the multiple sections corresponding to the first dimension, and the at least one phase mask may further divide the transverse profile of the return measurement beam and the return reference beam along a second dimension orthogonal to the first dimension into multiple sections and impart a relative phase shift equal to &pgr;+2&pgr;n
2
, where is n
2
is an integer, to half of the multiple sections corresponding to the second dimension.
The system may further include a second, at least one phase mask positioned to contact the return measurement beam and the return reference beams, wherein the second, at least one phase mask causes the conjugate image for each reference element and each measurement aperture to have an asymmetric profile along a second dimension orthogonal to the first dimension.
The system may further include a pinhole array positioned adjacent the photodetector, wherein each pinhole is aligned with a separate set of one or more detector elements, and wherein the imaging system causes the conjugate image for each measurement aperture to align with a corresponding pinhole of the pinhole array.
The mask in the measurement beam source array may further include an array of measurement scattering elements, wherein each measurement scattering element is adjacent a corresponding one of the measurement apertures and has transverse dimensions comparable to the corresponding measurement aperture. Each measurement scattering element scatters a portion of the measurement beam, and the measurement return beam further includes the portions of the measurement beam scattered by the measurement scattering elements. In such cases, the imaging optics are further configured to produce a conjugate image of the array of measurement scattering elements that overlaps with the conjugate image of the array of reference elements, wherein the conjugate image for each measurement scattering element overlaps with the conjugate image of a corresponding reference element to produce an optical interference signal that provides an estimate of scattering from the adjacent measurement aperture.
For embodiments including the scattering elements, the system may further include a pinhole array positioned adjacent the photo-detector, wherein each pinhole is aligned with a separate set of one or more detector elements, and wherein the imaging system causes the conjugate image for each measurement aperture and each measurement scattering element to align with a corresponding pinhole of the pinhole array.
Each reference element may include a reflective element.
Each reference element may include a transmissive aperture.
In general, in another aspect, the invention features an interferometric optical microscopy system for imaging an object, the system including: (i) a beam splitter po
Artman Thomas R
Fish & Richardson P.C.
Kim Robert H.
Zetetic Institute
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