Optics: measuring and testing – By light interference – For dimensional measurement
Reexamination Certificate
2001-07-27
2003-04-22
Hannaher, Constantine (Department: 2878)
Optics: measuring and testing
By light interference
For dimensional measurement
C356S496000, C356S512000
Reexamination Certificate
active
06552805
ABSTRACT:
BACKGROUND
This invention is related to optical imaging and metrology of semiconductor, data-storage, and biological materials, structures, and devices.
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.
Positioning the near-field scanning probe in close proximity, preferably in a non-contact mode, is known in the field as “the approach problem.”
SUMMARY OF INVENTION
The invention features systems and methods for near-field, interferometric microscopy in which a mask having an array of sub-wavelength apertures is used to couple near-field probe beams to a sample. The periphery of the mask further includes one or more larger apertures to couple light to the sample that forms the basis of an interferometric signal indicative of the relative distance between the mask and the sample. The interferometric signal can be the basis of a control signal in a servo system that dynamically positions the mask relative to the sample. In some embodiments, light coupled to the sample though the large aperture is scattered by the sample and detected through one of the sub-wavelength aperture and mized with a reference beam component to produce interferometric control signal. In general, the systems may operate in either reflective or transmissive modes, and may be used 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: a beam splitter positioned to separate an input beam into a measurement beam and a reference beam; a measurement beam source array positioned to receive the measurement beam; a reference beam source array positioned to receive the reference beam; a multi-element photo-detector; and imaging optics.
The measurement beam source array includes a mask having an array of measurement apertures and a control aperture adjacent one of the measurement apertures, wherein the control aperture has transverse dimensions larger than the transverse dimensions of the adjacent measurement aperture. Each of the measurement apertures and the control aperture is configured to radiate a portion of the measurement beam to the object. The object interacts with the radiated measurement beam portions and in response directs signal radiation back through the measurement apertures to define a measurement return beam. The transverse dimensions of the control aperture are selected to cause the signal radiation directed back through the measurement aperture adjacent the control aperture to be dominated by radiation derived from the control aperture. The reference beam source array includes an array of elements each configured to radiate a portion of the reference beam, the radiated reference beam portions defining a reference return beam.
The imaging optics are 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. 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.
Embodiments of the microscopy system may further include any of the following features.
The system may further include a positioning system for supporting the object relative to the measurement beam source array, and an electronic controller coupled to the photo-detector and the positioning system. During operation the electronic controller causes the positioning system to adjust the separation between the measurement beam source array and the object in response to a control signal derived from the interference signal corresponding to the measurement aperture adjacent the control aperture.
The mask may include multiple control apertures each adjacent one of the measurement apertures, wherein each control aperture has transverse dimensions larger than the transverse dimensions of the corresponding adjacent measurement aperture. For example, the multiple control apertures may surround the periphery of the array of the measurement apertures.
The system may further include a source for providing the input beam. Furthermore, each of the measurement apertures may have a transverse dimension less than a vacuum wavelength of the input beam provided by the source. Moreover, the control aperture may have a transverse dimension greater than or equal to the vacuum wavelength of the input beam provided by the source.
The control aperture may be located at the periphery of the array of measurement apertures. Furthermore, an end of each measurement aperture adjacent the object, other than the measurement aperture adjacent the control aperture, may lie in a first common plane, and an end of each of the control aperture and the measurement aperture adjacent the control aperture may be displaced relative to the first common plane. For example, such ends of the control aperture and the measurement aperture adjacent the control aperture may lie in a second common plane parallel to the first common plane. The system may further include a source providing the input beam, wherein the first common plane is displaced from the second common plane by an amount equal to about twice the wavelength of the input beam.
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 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. In such cases, the measurement return beam further includes the portions of the measurement beam scattered by the measurement scattering elements. 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 indicative of scattering from the adjacent measurement aperture.
Furthermore, in embodiments involving the scattering elements, the control aperture may located at the periphery of the array of measurement apertures, and an end adjacent the object of each measurement aperture and each measurement scattering element other than the measurement aperture adjacent the control aperture and the measurement scattering element adjacent the measurement aperture adjacent the control aperture may lie in a first common plane, and wherein an end adjacent the object of each of the control aperture, the measurement aperture adjacent the control aperture, and the measurement scattering site adjacent the measurement aperture adjacent the control aperture may be displaced relative to the first common plane. Furthermore, the ends of the control aperture and the measurement aperture adjacent the control aperture may lie in a second common plane parallel to the first common plane. Also, the system may further include a source providing the input beam, and the first common plane may be displaced from the second common plane by an amount equa
Fish & Richardson P.C.
Hannaher Constantine
Lee Shun
Zetetic Institute
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