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Patent US5032734 - Method and apparatus for nondestructively measuring micro defects in materials - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Sign in<nobr>Advanced Patent Search</nobr>PatentsA method and apparatus are disclosed for nondestructively measuring the density and orientation of crystalline and other micro defects on and directly below the surface of a properly prepared material such as a semiconductor wafer. The material surface is illuminated with a probe beam of electromagnetic...http://www.google.com/patents/US5032734?utm_source=gb-gplus-sharePatent US5032734 - Method and apparatus for nondestructively measuring micro defects in materialsAdvanced Patent SearchPublication numberUS5032734 APublication typeGrantApplication numberUS 07/597,857Publication dateJul 16, 1991Filing dateOct 15, 1990Priority dateOct 15, 1990Fee statusPaidPublication number07597857, 597857, US 5032734 A, US 5032734A, US-A-5032734, US5032734 A, US5032734AInventorsFred D. Orazio, Jr., Robert B. Sledge, Jr., Robert M. Silva, deceasedOriginal AssigneeVti, Inc.Export CitationBiBTeX, EndNote, RefManPatent Citations (17), Non-Patent Citations (5), Referenced by (43), Classifications (18), Legal Events (5) External Links: USPTO, USPTO Assignment, EspacenetMethod and apparatus for nondestructively measuring micro defects in materialsUS 5032734 AAbstract A method and apparatus are disclosed for nondestructively measuring the density and orientation of crystalline and other micro defects on and directly below the surface of a properly prepared material such as a semiconductor wafer. The material surface is illuminated with a probe beam of electromagnetic radiation which is limited to a nondestructive power level or levels. Polarization and wavelength or wavelengths of the electromagnetic radiation are selected according to certain characteristics of the material so that penetration depth is controlled. Specific orientation of the material with respect to the probe beam and the detector is required to detect that portion of the probe beam scattered from the defects of interest, surface or subsurface, without interference from other scatter sources and to identify the orientation of the defects. Maps of scatter intensity versus position are made according to the density of the defects encountered.
The invention having thus been described, the following is claimed: 1. A method of simultaneously measuring the distribution of surface and subsurface micro defects on a predetermined area of a material, comprising the steps of:(a) generating a beam of electromagnetic radiation with substantially equal S and P polarized components; (b) directing the beam towards the surface at a predetermined fixed angle of incidence and focusing the beam to expose a small portion of the material to the electromagnetic radiation; (c) directing the lines of sight of two detectors toward the surface of the material to the point where the beam intercepts the surface, one line of sight and detector for only P polarized electromagnetic radiation and the other line of sight and detector for only S polarized electromagnetic radiation; (d) limiting the extent of the scattered electromagnetic radiation entering each detector in order to detect the scatter coming from a small solid angle around the line of sight of each detector, for detecting a portion of the scattered electromagnetic radiation and converting it to an electrical signal proportional to the detected intensity from each detector; (e) producing relative rotation between the beam and the material about an axis substantially perpendicular to the surface and at the point where the beam intercepts the surface and determining the rotational positions of selected scatter signature for that point; (f) producing relative lateral movement between the beam and the material to expose an adjacent portion of the material to the electromagnetic radiation; (g) repeating above steps (e) and (f) for each portion of the material exposed until the predetermined area is covered in a contiguous manner; and (h) mapping the selected scatter intensity versus the coordinate position of each point of measurement for the predetermined area, the scatter intensity sensed at the P polarization detector being related to the subsurface defects and the scatter intensity sensed at the S polarization detector being related to the surface defects and the rotational positions being related to the defect orientations. 2. A method of simultaneously measuring the distribution of surface and subsurface micro defects on a predetermined area of a material, comprising the steps of:(a) generating a beam of electromagnetic radiation with elliptical polarization; (b) directing the beam towards the surface at a predetermined fixed angle of incidence and focusing the beam to expose a small portion of the material to the electromagnetic radiation; (c) directing the lines of sight of two detectors toward the surface of the material to the point where the beam intercepts the surface, one line of sight and detector for only P polarized electromagnetic radiation and the other line of sight and detector for only S polarized electromagnetic radiation; (d) limiting the extent of the scattered electromagnetic radiation entering each detector in order to detect the scatter coming from a small solid angle around the line of sight of each detector, for detecting a portion of the scattered electromagnetic radiation and converting it to an electrical signal proportional to the detected intensity from each detector; (e) producing relative rotation between the beam and the material about an axis substantially perpendicular to the surface and at the point where the beam intercepts the surface and determining the rotational positions of selected scatter signature for that point; (f) producing relative lateral movement between the beam and the material to expose an adjacent portion of the material to the electromagnetic radiation; (g) repeating above steps (e) and (f) for each portion of the material exposed until the predetermined area is covered in a contiguous manner; and (h) mapping the selected scatter intensity versus the coordinate position of each point of measurement for the predetermined area, the scatter intensity sensed at the P polarization detector being related to the subsurface defects and the scatter intensity sensed at the S polarization detector being related to the surface defects and the rotational positions being related to the defect orientations. 3. A method of measuring the distribution of subsurface micro defects caused by random processes such as ion implantation, on a predetermined area of a material, comprising the steps of:(a) generating a beam of electromagnetic radiation; (b) directing the beam towards the surface at a predetermined fixed angle of incidence and focusing the beam to expose a small portion of the material to the electromagnetic radiation; (c) directing the line of sight of the detector toward the surface of the material to the point where the beam intercepts the surface; (d) limiting the extent of the scattered electromagnetic radiation entering the detector in order to detect the scatter coming from a small solid angle around the line of sight, for detecting a portion of the scattered electromagnetic radiation and converting it to an electrical signal proportional to the detected intensity; (e) producing relative rotation between the beam and the material about an axis substantially perpendicular to the surface and at the point where the beam intercepts the surface and determining the rotational position of the selected minimum scatter for that point; (f) producing relative lateral movement between the beam and the material to expose an adjacent portion of the material to the electromagnetic radiation; (g) repeating above steps (e) and (f) for each portion of the material exposed until the predetermined area is covered in a contiguous manner; and (h) mapping the selected minimum scatter intensity versus the coordinate position of each point of measurement for the predetermined area. 4. Apparatus for simultaneously measuring the distribution of surface and subsurface micro defects on a predetermined area of a material, comprising the steps of:(a) means for generating a beam of electromagnetic radiation with substantially equal S and P polarized components; (b) means for directing the beam towards the surface at a predetermined fixed angle of incidence and focusing the beam to expose a small portion of the material to the electromagnetic radiation; (c) means for directing the lines of sight of two detectors toward the surface of the material to the point where the beam intercepts the surface, one line of sight and detector for only P polarized electromagnetic radiation and the other line of sight and detector for only S polarized electromagnetic radiation; (d) means for limiting the extent of the scattered electromagnetic radiation entering each detector in order to detect the scatter coming from a small solid angle around the line of sight of each detector, for detecting a portion of the scattered electromagnetic radiation and converting it to an electrical signal proportional to the detected intensity from each detector; (e) means for producing relative rotation between the beam and the material about an axis substantially perpendicular to the surface and at the point where the beam intercepts the surface and determining the rotational positions of selected scatter signature for that point; (f) means for producing relative lateral movement between the beam and the material to expose an adjacent portion of the material to the electromagnetic radiation; (g) means for repeatably engaging means (e) and (f) for each portion of the material exposed until the predetermined area is covered in a contiguous manner; and (h) mean for mapping the selected scatter intensity versus the coordinate position of each point of measurement for the predetermined area, the scatter intensity sensed at the P polarization detector being related to the subsurface defects and the scatter intensity sensed at the S polarization detector being related to the surface defects and the rotational positions being related to the defect orientations. 5. A method of measuring the distribution of micro defects on a predetermined area of a material, comprising the steps of:(a) generating a beam of electromagnetic radiation; (b) directing the beam towards the surface at a predetermined fixed angle of incidence; (c) directing a detector's line of sight toward the surface of the material to the point where the beam intercepts the surface; (d) detecting a portion of the scattered electromagnetic radiation and converting it to an electrical signal related to the detected intensity; (e) producing relative rotation between the beam and the material about an axis substantially perpendicular to the surface and at the point where the beam intercepts the surface and determining the rotational position of selected scatter for that point; (f) producing relative lateral movement between the beam and the material to expose an adjacent portion of the material to the electromagnetic radiation; (g) repeating above steps (e) and (f) for each portion of the material exposed until the predetermined area is covered in a contiguous manner; and (h) mapping the selected scatter intensity versus the coordinate position of each point of measurement for the predetermined area. 6. Apparatus for measuring the distribution of micro defects on a predetermined area of a material, comprising the steps of:(a) means for generating a beam of electromagnetic radiation; (b) means for directing the beam towards the surface at a predetermined fixed angle of incidence; (c) means for directing a detector's line of sight toward the surface of the material to the point where the beam intercepts the surface; (d) means for detecting a portion of the scattered electromagnetic radiation and converting it to an electrical signal related to the detected intensity; (e) means for producing relative rotation between the beam and the material about an axis substantially perpendicular to the surface and at the point where the beam intercepts the surface and determining the rotational position of selected scatter for that point; (f) means for producing relative lateral movement between the beam and the material to expose an adjacent portion of the material to the electromagnetic radiation; (g) means for repeatably engaging means (e) and (f) for each portion of the material exposed until the predetermined area is covered in a contiguous manner; and (h) means for mapping the selected scatter intensity versus the coordinate position of each point of measurement for the predetermined area. Description
This application is a continuation-in-part of application Ser. No. 437,109, filed Nov. 16, 1989, which is a continuation-in-part of application Ser. No. 301,721, filed Jan. 26, 1989, which is a continuation-in-part of application Ser. No. 218,542, filed July 13, 1988, abandoned, which is a continuation-in-part of application Ser. No. 918,518, filed Oct. 14, 1986, abandoned, which is a continuation-in-part of application Ser. No. 724,966, filed Apr. 19, 1985, abandoned.
One technique currently used to measure crystalline damage is described in U.S. Pat. Nos. 4,352,016 and 4,352,017. This approach measures the reflectance of ultraviolet light, at two wavelengths, from the surface of a semiconductor wafer. This technique is known to be insensitive to damage at any depth in the material primarily because of the use of ultraviolet light which is a shallow penetrator in semiconductor materials. A second factor significantly limiting sensitivity is the reflectance measurement itself. Such measurements are notoriously difficult to make and result in looking for small variations in large numbers, which is one of the reasons why this technique requires measurements at two wavelengths. The practical application of this reflectance technique shows up these deficiencies.
A second approach is described in U.S. Pat. No. 4,391,524. This approach can measure the light scattered from the surface and subsurface regions but because of the geometry of the measurement, important data is lost. There are three factors which bear on this assessment which are independent of the wavelength selected for the probe beam. First, the angle of incidence of the probe beam is 0�. This eliminates any possibility of determining the directional nature of the defects, or of using polarization to help discriminate between surface and subsurface defects. Secondly, the detector subtends a large solid angle thus integrating scatter from all directions, again making impossible the determination of directional defects, and at the same time diluting the signature of the defects it is designed to measure. And finally, the detector line of sight is also at 0�, or near 0�. This introduces significant amounts of surface scatter into the measured signal which is nearly impossible to separate from the subsurface scatter under these conditions. Subtle variations in surface scatter will mask the scatter from the subsurface that are the purpose of the measurement. The result is a measurement that is insensitive to oriented defects, which most subsurface defects are, and even insensitive to many very small surface defects which are also oriented.
The incident angle of the beam, the viewing angle of the detector, the polarization of the beam and the polarization of the detected radiation are used to enhance the scatter signature from the defects of interest, and at the same time minimize scatter from unwanted sources on or in the material. The relative rotation of the material with respect to the probe beam is used to determine the orientation of the defects and enhance the scatter signature from the oriented defects. This relative rotation can also be used to minimize scatter from a known defect orientation on or in the material, in order to enhance the scatter from defects that are not oriented or are oriented in a different direction. For instance, on diamond turned parts, selecting an orientation parallel to the scribe marks left by the diamond turning will eliminate the high scatter associated with these marks and allow detection of other, much smaller, defects on or below the surface. Another application of this approach could be used to examine semiconductor wafers before and after ion implant to detect the extent of the damage caused by the ion implant process. For this application, the wafer would be measured and the rotational position would be selected to minimize the scatter from the polishing induced subsurface defects which are highly oriented. The wafer would then be ion implanted which will introduce new subsurface defects. These defects are not oriented but are random in nature. The wafer would be measured again with the rotational position selected to minimize the scatter as before. A comparison between the first measurement and the second will yield a number related to the density of defects introduced by the ion implant. The purpose here is to minimize the effect of the polishing induced subsurface defects in order to detect the defects caused by the ion implant alone. This same approach could be used with any process that introduces randomly oriented surface or subsurface defects.
FIG. 9 illustrates an embodiment of the invention which is similar to FIG. 5 but which detects both surface and subsurface defects simultaneously.
It is possible to combine S and P polarization to get circular polarization. Circular polarization, in this case, refers to the state where the electric vector is uniform in length and rotates about the direction of propagation of the wave in either a clockwise or counterclockwise motion. A circularly polarized beam can be considered half P polarized and half S polarized. A uniformly unpolarized beam would have the same effect. Using such an arrangement for the probe beam would result in both P and S polarized scattered light being sent to the detector. By separating the scattered light into its P and S components and using a separate detector for each, surface and subsurface defects could be measured simultaneously. Another variation on this approach would be to use elliptically polarized light where the rotating electric vector is not uniform in length and the S and P components are not equal. This would be done to tailor the ratio of surface to subsurface scatter desired for a given input power.
The angle of incidence A can be varied between 0� and 90�. Typically, large angles of incidence provide the best results because the penetrating electromagnetic radiation interacts with the lineated subsurface defects which act like a grating and scatter the light back out through the surface. The blown-up view 12 of FIG. 1A is to give a better understanding of what the real surface and subsurface are like and to show the increasing density of defects with depth. The low defect zone 19 is a region between the surface and the heaviest zone of defects often described as the M-layer in semiconductors and the Beilby layer in optics. It is an area of recrystallized or amalgamated material which can shield the subsurface defects, that this invention measures, from ordinary detection.
For subsurface measurements the angle of incidence A of the beam 16 should be as close as possible to Brewster's angle (the angle of minimum reflectance at P polarization, sometimes called the polarizing angle) in order to maximize the amount of energy transmitted into the material, and at the same time minimize the energy scattered from the surface. As an example, Brewster's angle for silicon is about 75�. Typically, angles of incidence larger than 55� begin to cause problems because the impact point of the incident beam 16 begins to spread across the surface. The reflectance of P polarized light at a wavelength of 632.8 nanometers and an angle of incidence of 55�, is about 15%. This means that 85% of the light penetrates the material. In a practical sense, this percentage of light, with an energy density of 30 watts/cm2 in a 0.25 mm diameter beam, will be able to detect defects in silicon that are 4 to 5 microns below the surface, when using a suitable detector. It is possible to reshape the beam cross section from circular to elliptical, with the major axis of the ellipse perpendicular to the plane of incidence, so that a more circular cross section will be projected on the surface at large angles of incidence. Other angles A, less than 55�, can be used but with poorer results in terms of signal-to-noise ratio which is used herein as the ratio of detected subsurface scatter power to total detected power.
For the detection of surface defects using S polarization, the angle of incidence A of beam 16 should also be large to maximize the amount of surface scatter and minimize the subsurface scatter. This is necessary because for any given material and a given wavelength of the incident beam polarized S, the greater the angle of incidence, the greater the reflectance. Since there is no "Brewster's angle" for S polarization, the largest angle possible should be selected. As a matter of convenience in the design and construction of equipment, Brewster's angle could be used and the polarization switched from S to P depending on what kind of defects are being measured, surface or subsurface. Again, using circularly polarized, or uniformly unpolarized, light would allow surface and subsurface defects to be measured simultaneously as long as there is a separate detector for each polarization.
The angle D of the detector line of sight 26 should be positioned in the direction opposite the reflected beam 22. The angular difference between A and D should be small, less than 30�, for large values of A, and large, greater than 30�, for small values of A. This is true because the energy which is scattered from the subsurface must traverse some thickness of material (19 in FIG. 1A) and emerge from a high index to a low index. This causes a severe refraction of the scattered light toward the surface 15 of the material 10 being tested, especially for high index materials such as semiconductors. Other angles D and A may be used but with poorer results in terms of signal-to-noise ratio. Values of D which place it near the specularly reflected beam 22, increase the surface scatter component of the detected signal to the point where the subsurface measurements cannot be made with great accuracy.
The angle R of the plane of incidence 28 is very important in determining the orientation of the subsurface defects in a particular material. The nature of the subsurface defects in many materials is in the form of lineations which are generated by the processes of sawing, grinding, polishing and even cleaning. These lineations are in the form of zones of defects which are significantly longer than they are wide. In effect, these lineations form a fine grating. Hence the scatter from these features is highly oriented so that when the plane of incidence 28 is perpendicular to them, the scatter is very intense in the plane of incidence. At any other angle, the scatter is significantly less or nonexistent in the plane of incidence. This means that the angle R is directly related to the orientation of the subsurface defects. By selectively examining the scatter intensity versus angle R, the direction of maximum scatter, and hence the orientation of the defects, can be determined. This directionality can then be directly related to the process used to form the surface. Thus effects of process variations can be directly observed. Ideally, an angle R and the maximum scatter associated with it would be determined for each position on the material.
Other positions for the angle R may also be important from a crystallographic perspective. For instance, certain kinds of defects, like stacking faults, are oriented in certain crystallographic directions. Therefore it would be appropriate, if detection of this particular type of defect was important, to chose the R angle corresponding to this particular known direction so as to detect the stacking faults while minimizing the effects of other defects. There are other situations where there is a known direction that must be avoided because the signal from that direction will overwhelm the signal from the defects of interest. This is the case when measuring diamond turned parts. The scribe marks left in the surface and subsurface by the diamond turning are so large that the signal from them will not allow smaller defects to be detected. By avoiding the direction that is perpendicular to the scribe marks and concentrating on the direction parallel to the scribe marks, small surface and subsurface defects can be detected. A similar approach can be used when looking for subsurface defects caused by ion implants. In this case the minimum scatter direction is selected because the ion implant defects are randomly oriented. This has the effect of minimizing the scatter from the polishing induced subsurface damage. Other variations on the use of the R angle are possible such as when looking for a second maximum or other second order effects which would be overlooked by using only the maximum R angle position.
The following figures are major embodiments of the present invention. The first is shown schematically in FIG. 3. In this embodiment a source of electromagnetic radiation or laser 32 and the detector 34 are fixed, and the test part 10 is moved in the X and Y directions and rotated about the Z axis. The detector 34 may be a photomultiplier tube or a solid state detector. The various motions of the test part are accomplished with computer controlled, motor driven micropositioning stages 39 and 37 for motion along the X and Y axes respectively and stage 41 for rotation about the Z axis. The three stages are assembled so that the Z axis always intercepts the test surface at the intersection of the X-Y coordinates being measured. At each X and Y position the angle R is varied from 0� to 360�, a maximum reading is taken with the detector 34, fed to a computer, and a map of the maximum scatter intensity versus position is generated. This is only one of many possible sets of data. Maps from directions other than the maximum, or multiple directions, can also be mapped. These maps are the output of the measurement device of the invention. The beam 16 is conditioned prior to interacting with the test part 10. A spatial filter/beam expander 36 gives the beam a Gaussian cross section and focuses the beam to a given diameter at the test part surface, to increase the energy density to a level suitable for the measurement. Other beam cross sections are acceptable and may even perform better such as a tophat which has a constant intensity across the beam diameter, but the Gaussian cross section is acceptable and is the easiest to generate. A polarizer 38 insures the correct polarization, S or P, depending on whether surface or subsurface defects are to be measured. Combinations of S and P polarization are possible but in this case the detector must be able to select the polarization of interest or the result will be a combination of surface and subsurface effects. Beamsplitter 30 divides the beam so that a small portion of the energy is directed to detector 31 the output of which is fed to the computer. This information is used to correct the output of detector 34 for variations in the input power from laser 32. Cylindrical lens 33 reshapes the beam cross section to an elliptical shape so that the foot print of the beam on the test part surface is circular. The reflected beam 22 is stopped with an absorber 40 to eliminate spurious reflections.
FIG. 4 is the schematic of an embodiment where the detector 34 is fixed and the test part 10 moves only in rotation about the Z axis, which is also fixed with respect to the test part 10. The probe beam 16 is scanned in the X and Y directions using movable mirrors 42 and 44 which oscillate in planes disposed at 90� to each other. A lens 46 corrects for angular beam variations caused by the scanning. The beam stop 40 is large to accommodate the scanned beam and insures that the beam does not reflect or scatter back to the detector and cause erroneous readings. With this arrangement, the scanning can be done very quickly over large areas. The angle R must still be changed, using micropositioning stage 41, by rotating the material under examination, but the material is fixed in the X and Y directions.
FIG. 6 is an embodiment of the invention designed for high speed defect measurements on semiconductor wafers or other flat surfaced materials. In this case the test part 10 moves by means of micropositioning stages 39 and 37 in the X and Y directions and the beam is rotated about the Z axis by means of an air bearing supported optical element 65. The air bearing 66 allows the optical element to rotate at a very high speed and thus allows measurements to be made quickly. The probe beam 16 is generated by a laser 32, passes through optics 36 to filter and shape the beam as before. The optical element 38 converts the beam to circular polarization. The beam must be coincident with the Z axis which is the axis of rotation of the air bearing 66. The beam passes through the rotating optical element 65 which consists of two prisms 67 and 68 separated by an opaque material 69. The prism 67 reflects the beam at 90� to the Z axis and through the face of prism 67 which has a polarizing film 70 on the surface. This converts the circularly polarized beam 16 to a P polarized condition for subsurface measurements in this case. The beam is then reflected from annular mirror 71 to give it the proper angle of incidence on test part 10 and the reflected beam 22 travels back to mirror 71 and through the face of prism 68 which has a polarizing film 72 which only allows S polarized light through. The effect will be to reduce the intensity of the reflected beam 22 by a significant factor and convert the remaining light to S polarization. The beam is then reflected from the angled face of prism 68 and through the bottom face of prism 68 which has a polarizing film 73 which will only allow P polarized light through, again reducing the intensity of the beam. The remaining light impacts on the angled internal surface of rotating element 65 which has a black absorbing coating 74. The purpose of this part of the optical element 65, and the circuitous path for the reflected beam 22, is to eliminate any reflection of the probe beam back into the optical system where it might interfere with the measurement. For surface measurements, polarizing films 70, 72 and 73 would be set for S, P and S polarization respectively. The light scattered from the subsurface defects in the test part 10 is collected by annular mirror 75 and reflected back into rotating optical element 65 and back to annular mirror 50 along path 26. The light passes through focusing optic 76 and to detector 34. Vertical micropositioning stages 77 adjust the height of the rotating element 65 above the test part 10 to accommodate for any thickness variations between test parts. A computer C provides the data analysis and mapping function by converting the data to colors according to a preset scale and mapping the colors according to the X-Y coordinates on the part being tested.
The effect of using two, three or more wavelengths in the measurement apparatus described, is to allow separate wavelengths to penetrate to different depths and be detected separately. Each detector receives a different signal containing the scatter signature of the defects from just below the surface to whatever depth that wavelength can penetrate in the material being tested. The wavelength that penetrates the least will contain the least information while the next deepest penetrating wavelength will contain all the previous information plus new information about deeper defects. If the shallower information is subtracted from the deeper information, the result will be just information about the deeper defects. If these depths are well known, then a zone of defects at a known depth can be identified. The same process can be continued with the next wavelength to obtain a third depth zone, the first being the one obtained by the shallowest penetrating wavelength alone and the second depth zone the one obtained by substracting the first from the second deepest penetrating wavelength. More or fewer wavelengths can be used depending on the amount of information desired and the complexity of the instrument one is willing to accept. The subtraction of information must be done with due regard to the rotational orientation of the defects since defect orientation can change with depth. The best approach is to take the rotational data for each wavelength at a given point and subtract it getting both a magnitude and direction for that point in the desired depth zone.
FIG. 9 shows an embodiment of the invention which is in some ways similar to that shown in FIG. 5. The difference being that the circularly polarized probe beam 16 is not linearly polarized before impinging on the surface of the test part 10. The result is that the scattered light 26 contains both P and S polarized components. This scattered light is reflected by mirrors 58 and 56 up to polarizing beamsplitter 92. This beamsplitter will transmit only P polarized light and reflect only S polarized light. The P polarized scattered light 93 reflects from annular mirror 94 which has a large central opening. The light enters detector 95 which responds equally to P and S polarized light. As the optical bench 52 rotates on stage 41, the light entering detector 95 will change from P to S for every 90� of rotation of optical bench 52 even though the light exiting beamsplitter 92 is always P polarized with respect to the optical bench 52.
The light reflected from polarizing beamsplitter 92 is S polarized and is reflected again from polarizing beamsplitter 96. The S polarized scattered light follows path 97 and is reflected from annular mirror 98 which has only a small hole for the incident beam 16. The scattered light enters detector 99 which is identical to detector 95. The electrical output of detector 95 corresponds to the variations of the P polarized scattered light and the electrical output of detector 99 corresponds to the variations of the S polarized scattered light. This embodiment of the invention allows the simultaneous detection of S and P polarized light so that surface and subsurface defects can be detected from the same place on the test part.
Finally, when using P and S polarization together to examine the surface and subsurface simultaneously, the variations of scatter with R angle may be different for each polarization at a given spot. This may require a more complex analysis of the results to get an accurate representation of the location of the defects, surface or subsurface. Such an analysis may also effect the determination of the defect orientation. The result of this analysis would be a scatter signature which could be used to isolate the defects of interest for the particular material being examined.
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OF OHIO, OHIOFree format text: ASSIGNMENT OF ASSIGNORS INTEREST.;ASSIGNORS:ORAZIO, FRED D. JR.;SLEDGE, ROBERT B. JR.;SILVA, ROBERTM. (DECEASED);AND OTHERS;REEL/FRAME:005486/0843Effective date: 19901011RotateOriginal ImageGoogle Home - Sitemap - USPTO Bulk Downloads - Privacy Policy - Terms of Service - About Google Patents - Send FeedbackData provided by IFI CLAIMS Patent Services©2012 Google