Variations in topography and material properties of the surface layer of a body are observed in microscopic imaging using a scanning capacitance probe. The acronym SCaM identifying the process and apparatus is derived from the phrase scanning capacitance microscope. The material properties observable by SCaM are the surface-electric property representative of the complex dielectric constant of the surface material and the surface-mechanical property representative of the elastic constant of the surface material.
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1. A method for determining variations in the topography and material properties of the surface layer of a body of material comprising:
(a) scanning said surface layer with a capacitance probe to generate a first signal representing capacitance variations between the surface layer and said probe; (b) scanning a recording medium in synchronism with the scanning of the surface layer with said probe; (c) recording said first signal on the recording medium as a second signal; and (d) generating a visual display from said second signal from said recording medium presenting thereby capacitance variations as visually discernible variations of an image consisting of points which correlate on a one-to-one basis with points on the surface layer, the image variations manifesting features of the surface layer which cause variations in capacitance between the surface layer and the capacitance probe.
2. The method of
3. The method of
scanning said surface layer with said probe by moving said body layer in an x-y raster scan while holding said probe in a fixed position.
4. The method of
scanning said surface layer with said probe by rotating said body while moving said probe in a radial direction.
5. A method for determining variations in the topography and material properties of the surface layer of a disc upon which is impressed a spiral groove, comprising the steps of:
(a) rotating said disc about a vertical axis; (b) scanning said surface layer by a capacitance probe having an electrode riding in said groove, thereby generating a scan of the disc in radius and angle to generate a capacitance signal; (c) recording said signal as an image on photographic film by (c1) generating an x-y raster scan of a cathode ray tube in synchronism with the radius and angle scan of said surface layer of said disc; (c2) modulating the intensity of the cathode ray tube beam with said signal to display thereby capacitance variations as visually discernible variations of an image consisting of points which correlate on a one-to-one basis with points on the surface layer, the image variations manifesting features of the surface layer which cause variations in capacitance between the surface layer and the capacitance probe; and (c3) photographing the image displayed on the face of the cathode ray tube. 6. A method for determining variations in the topography and material properties of the surface layer of a disc upon which is impressed a spiral groove, comprising the steps of:
(a) rotating said disc about a vertical axis; (b) scanning said surface layer by a capacitance probe having an electrode riding in said groove, thereby generating a scan of the disc in radius and angle to generate a first signal; (c) filtering said first signal with an anti-aliasing filter to provide a second signal; (d) sampling said second signal with an analog to digital converter having a fixed sampling rate which is synchronized to said radius and angle scan of said surface layer to convert said second signal to digital form to provide digital samples of said second signal; and (e) storing said digital samples in a computer memory, said digital samples representing capacitance variations consisting of data points which correlate on a one-to-one basis with points on the surface layer, the capacitance variations representing features of the surface layer which cause variations in capacitance between the surface layer and the probe.
7. The method of
8. The method of
generating an image from said digital samples by scanning the computer memory in synchronism with a raster scan of a cathode ray tube and modulating the intensity of the cathode ray tube beam with a signal derived from the contents of the computer memory.
9. The method according to
before rotating said disc, coating the surface with a conformal metallic layer having a thickness great enough to prevent variations in the mechanical and electrical properties of said surface layer from affecting the capacitance probe whereby only topographical variations in said
surface are determined. 10. A method for determining variations in the electrical properties of the surface layer of a body of material comprising the steps of: (a) scanning said surface layer with a capacitance probe to generate a first signal representing capacitance variations between said surface layer and said probe; (b) scanning a recording medium in synchronism with the scanning of the surface layer with said probe; (c) recording said first signal on the recording medium as a second signal; (d) generating a first image from said second signal corresponding to a predetermined surface portion of said surface layer; (d1) storing said first image in memory; (e) coating said surface layer with a conformal metallic coating having a thickness in excess of the Thomas-Fermi shielding range for electrons in the material comprising the metal of said metallic coating, said coating being thin enough to prevent significant changes in the mechanical properties of said surface layer; (f) after step (e) repeating steps (a) to (c) to generate a third signal representing capacitance variations and recording of said third signal as a fourth signal for generating a second image of said predetermined surface portion; (g) generating said first image from memory; and (h) comparing said first image to said second image to determine variations in electrical properties of said surface as manifested by the presence of features in the first image which are absent in the second image. 11. A scanning capacitance microscope for determining variations in the opography and material properties of the surface layer of a body of material comprising: (a) means for scanning said surface layer with a capacitance probe guided directly over said surface layer to generate a first signal in analog form representing capacitance variations between the surface layer and said probe; (b) means for forcing said probe against said surface with sufficient force to deform said surface, (b) (c) means for scanning a recording medium in synchronism with the scanning of the surface layer with said probe; (c) (d) means for converting said first signal into a digital signal and means for recording said first signal in digital form on the recording medium as a second signal; and (d) (e) means for generating a visual display from said second signal from said recording medium presenting thereby capacitance variations as visually discernible variations of an image consisting of points which correlate on a one-to-one basis with points on the surface layer, the image variations manifesting features of the surface layer which cause variations in capacitance between the surface layer and the capacitance probe. 12. The microscope according to claim 11 further including means for rotating said body and means for moving said probe in a radial direction over said layer. 13. The microscrope according to claim 11, further including a scan generator means for controlling the scan of said probe and for controlling the scan of said recording medium so that the recording medium is scanned in synchronism with said surface layer. 14. The microscope according to claim 13, wherein said probe is supported in a fixed position over said surface layer and said surface layer is translated in an X and Y scan under said probe. 15. The microscope according to claim 13, wherein said surface layer is fixed and said probe is translated over said surface in an X and Y scan. 16. The microscope according to
17. A method of microscopy comprising continuously scanning the surface of a body with only one capacitance probe by providing relative movement between the surface and the probe to generate a continous signal representing the variation in capacitance between the body and the probe, and processing the signal to produce an image of the topography and material properties of the surface layer.
18. A method according to
19. A method according to
20. Microscopy apparatus comprising means for continuously scanning the surface of a body with only one capacitance probe by providing relative movement between the surface and the probe to generate a continuous signal representing the variation in capacitance between the body and the probe, and processing the signal to produce an image of the topography and material properties of the surface layer.
21. An apparatus according to
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This invention relates to microscopy and, more particularly, to microscopy utilizing capacitance signals generated from a capacitance probe scanned over the surface of a body of material.
Microscopes are essentially devices which create maps or displays of the variation of some property of an object under study. Different types of microscopes map variations of different properties of a material to provide contrast in a generated image of the material.
Optical microscopy techniques can be used to generate maps of the variations of some properties of certain materials. However, diffraction effects and depth of field limitations present formidable difficulties when attempting to discern variations in the properties of materials in which the variations are on a scale of the order of 5-10 angstroms. Optical microscopes use light with wavelengths of the order of a few thousand angstroms; the resolution of optical microscopes, it should be noted, is at best about 2500 angstroms.
Electron microscopy techniques have been used in the mapping of very fine variations in topography of certain materials. However, the electron microscope, even though overcoming the diffraction and depth of field difficulties experienced by optical microscopes, nevertheless, is limited by its field of view. Moreover, electron microscopes have another limitation in that the preparation of a specimen of a material to be evaluated typically requires cutting an area or portion of interest out of the material in order to provide a specimen small enough to fit inside the vacuum chamber of the electron microscope.
Acoustic microscopy techniques are used to determine absorption spectra and the Raman frequency modes of material. See U.S. Pat. Nos. 4,028,933 and 4,267,732 for a detailed description of these techniques. Acoustic microscopes can be used to discern topographic, mechanical and thermal properties of a material. Acoustic microscopes, however, cannot discern electrical properties of materials.
Furthermore, optical, electron and acoustical microscopes provide a common difficulty in that they can present a great number of extraneous features which are not relevant to certain kinds of material evaluation.
In the art of video disc records, and the manufacture of the discs useful in the art, it is important to be able to determine certain properties of the disc.
It is known that the video disc that has been recorded with information comprising both video and audio signals still contains in the playback mode extraneous signals which are termed noise. These noise signals contribute deleteriously to the quality of the video and audio signals that are eventually displayed in a TV-monitor. Variations in the (1) geometry of the groove or what may be termed variations from the desired topography, (2) the mechanical stiffness of the groove, or more particularly, the mechanical stiffness of the surface layer of the material and (3) the complex dielectric of the material of the disc all contribute to the noise signals. While it is desirable that these properties be identified, no known process heretofore has been able to provide such information. The optical, acoustic, and electron microscopes can, in principle, discern variations in the geometry. However, in practice, optical and acoustic microscopy of the variation of the groove geometry of concern in the video disc art is impossible because the variations of interest are more than 1,000 times smaller than the groove itself and because the dimensions of the variation of interest are at or beyond the limits of optical microscopes.
Electron microscopes can discern the variations of geometry which are of interest, but only over such a small field of view as to make the interpretation of a display from an electron microscope very difficult. Electron microscopes also probe fairly deeply into the surface of the material of a disc, thereby further complicating interpretation of the displays, which are usually in microphotograph (or, simply, "micrograph") form. There is a need, therefore, for a system that functions as a microscope to provide detailed and enlarged mapping displays manifesting or representing the variations in the properties of the surface of materials.
The present invention comprises a method for determining variations in the topography and material properties of the surface layer of a body of material by scanning the surface of the body with a capacitance probe to generate a signal representing capacitance variations between the surface and the probe.
An image of the surface of the body is formed by scanning a recording medium in synchronism with the scanning probe and recording on that medium the capacitance signal from the probe in such a manner that it may be viewed by an observer as variations in brightness or color of a two-dimensional map of the surface of the body. Topography and the material properties of mechanical stiffness and complex dielectric constant can be discerned from the display.
FIG. 1 is a block schematic of a system embodying the present invention;
FIG. 2 is a schematic of a capacitance probe suitable for practicing one form of the invention embodied as a stylus riding over a grooved disc;
FIG. 3 is a block schematic of a scan generator useful in practicing the invention;
FIG. 4 is a block schematic of a preferred system for practicing the invention utilizing video discs;
FIGS. 5a and 5b are micrographs illustrating the capacitance microscope display of a surface portion of a video disc containing no recorded signals;
FIGS. 6a and 6b are micrographs of the surface of a disc before and after, respectively, the disc has been coated with a metal;
FIG. 7 is a schematic of a modified form of a capacitance probe for developing a variable contact force;
FIG. 8 is a schematic of a system for scanning a probe in x-y coordinates;
FIG. 9a is a micrograph of the pricipleconvention an 206 and 208 208 and 206 of this embodiment. In the alternative, one may use a piezoelectric transducer of the type disclosed in U.S. Patent 4,162,511, issued to M. Toda, et al. on July 24, 1979 for the translator mechanism.
The scanning movement is under control of a triangle wave generator 212. Generator 212 provides a triangular wave at a frequency of 20 Hz applied simultaneously to the input of translator 208, the input to the Y axis scan 214 and to the X input of an oscilloscope 216. The generator 212 is suitably an Interstate model F77. Translator 208, responding to the triangular wave from generator 212, causes the probe 204 to be moved in a linear motion over the sample 210 for a distance of about 300 micrometers for each cycle of the triangular wave.
The Y axis scan and synchronization circuit 214 in response to the triangular wave from generator 212 provides a control signal for the Y axis translator 206 as well as the input signal to the Y axis input of oscilloscope 216.
The circuit of 214 may be implemented using the circuit described above as shown in FIG. 3. In the alternative, the circuit described in the above-mentioned patent application of Matey and Corson, Ser. No. 143,028, now U.S. Pat. No. 4,307,419 may be used for circuit 214. If the circuit of FIG. 3 is used for the scan and synchronization circuit 214, the output of the triangular wave generator 212 is coupled to the trigger input 52, in lieu of the optical position encoder 53 shown in FIG. 3. In addition, the analog output on path 20 to terminal 78 in is then coupled to drive the Y axis translator 206. Since the analog output of FIG. 3 at terminal 78 thereof will increase by one step of the digital-to-analog (D/A) converter 62 for each cycle of the triangular wave from generator 212, the probe 204 will be moved across the sample 210 in a rectangular raster.
Since the same voltages which are applied to the X translator 208 and the Y translator 206 are also applied to the X and Y inputs of the oscilloscope 216, the beam of the oscilloscope will generate a geometrically similar raster on the screen of the oscilloscope.
The output capacitance voltage signal from probe 204 is coupled to a capacitance-to-voltage converter 202 of the type similar to converter 18 illustrated in FIG. 1. The converter output of converter 202 is applied to the Z-axis of the oscilloscope 216. The input to the Z-axis thereby generates variations in the intensity of the raster causing an image of the sample 210 as in the manner described hereinabove. Moreover, a camera 218 may be used to record the image displayed by the oscilloscope 216.
Reference is made to FIGS. 9a and 9b. The micrograph of FIG. 9a is that developed by an optical microscope of a silicon wafer having metallic gratings suitably formed on the surface thereof. The grating spacing is 28 μm and the height is 200 angstroms. The width of the grating lines is about 5 μm. One grating line is seen in the drawing FIG. 9a as 230a.
The micrograph of FIG. 9b was generated by the scanning capacitance microscope illustrated in FIG. 8. FIG. 9b shows another portion of the same silicon wafer shown in FIG. 9a. The grating is clearly visible in FIG. 9b. One of the grating lines similar to line 230a of FIG. 9a is seen in FIG. 9b as line 230b.
The grating line 230b is noticably curved in FIG. 9b. The curved lines in FIG. 9b are obviously distortions of the apparent straight grating lines in FIG. 9a. These distortions are the result of non-linearity in the x and y translators used in the embodiment of FIG. 8. The translators used in this embodiment are voice coil translators originally designed and used to provide velocity correction in a video disc player as described hereinabove. The use of high linearity voice coil translators would alleviate this distortion.
The invention can be practiced with a capacitance probe which does not contact the surface at all, thereby exerting no force on the surface. Reference is made to FIGS. 10a and 10b showing, respectively, in schematic form the elevation and plan view of a suitable probe 136 arranged to maintain capacitive coupling between the surface of the sample 138 and the electrode portion of the probe 136 without contacting the surface. The probe 136 is formed by a wire 140 of about one micron thickness extending through the center of a tube 142 of about 1000 microns in diameter filled with epoxy. The electrode 140 extends upwardly through the tube 142 and is connected to a stripline 144 arranged on the upper surface of the probe terminating at a terminal 146 for connection to the utilization circuit. The probe is carried on an arm 148 rigidly mounted at terminal 150 above the surface of the sample.
In operation, the sample 138 is moved with an appropriate raster scan relative to the probe 136 such that the electrode 140 provides a capacitive input to the circuit at terminal 146. Sample 138 is provided with a metallic portion 152 to establish the capacitive circuit to the reference ground as shown.
In the embodiment of FIG. 8 the sample 210 is fixed with the probe 204 scanning thereover. If desired, the invention may be practiced using a variation of this in which the probe 204 is fixed and the sample 210 is scanned under the fixed probe. In such an embodiment the probe 204 would be structured as disclosed in FIGS. 10a and 10b described above and the sample 210 would be translated in the X and Y directions by suitably coupling the Y and X translators 208 and 206, respectively, to effect such scanning. In another embodiment, the sample 138 (FIG. 10a) may be scanned by rotating the sample 138 while moving the probe 140 in a radial direction.
The magnification achieved by this microscope is determined in the same manner as the magnification for any scanning microscope i.e., by the relative sizes of the raster scanned on the sample and the raster scanned on the recording medium. As an example, assume we are scanning a sample and that the scanning probe will move in a square raster of width W over the sample. Then, if the recording medium (for example, a photographic film) is scanned in a square raster of width W', the magnification of the resulting micrograph is M=W'/W. In general, it should be understood the magnification need not be isotropic. Tus Thus, in FIG. 9b, a nearly isotropic magnification of about 125X was used to illustrate the use of SCaM on semiconductor materials. In FIG. 6a and 6b, an anisotropic magnification of about 200X in the horizontal direction and about 50X in the vertical direction was used to illustrate the use of SCaM on video disc materials.
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