CA1329997C

CA1329997C - Method and apparatus for nondestructively measuring subsurface defects in materials

Info

Publication number: CA1329997C
Application number: CA000605305A
Authority: CA
Inventors: Fred D. Orazio, Jr.; Robert M. Silva; Robert B. Sledge, Jr.
Original assignee: VTI Inc
Current assignee: VTI Inc
Priority date: 1988-07-13
Filing date: 1989-07-11
Publication date: 1994-06-07
Anticipated expiration: 2011-06-07
Also published as: EP0439881A1; EP0439881B1; US4933567A

Abstract

METHOD AND APPARATUS FOR NONDESTRUCTIVELY
MEASURING SUBSURFACE DEFECTS IN MATERIALS
Abstract of the Disclosure A method and apparatus are disclosed for nondestructively measuring the density and orientation of crystalline and other micro defects 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 and with a wavelength, or wavelengths, 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 subsurface region without interference from the surface scatter and to identify the orientation of the defects. Maps of scatter intensity versus position are made according to the density of the defects in the subsurface.

Description

Docket 5403 METHOD ANV APPARATUS FOR NONDESTRUCTIVELY
MEAS _ING SUBSURFACE DEFECTS IN MATERIALS

Background of the Invention Many of the materials used in the manufacture of semiconductors, optics and a variety of other applications require the highest quality materiaL available to meet the performance 25 requirements expected in the future. This i5 particularly important when it relates to the quality of the surface, the crystalline structure and the impuritie~ in the material. In the case of semiconductors like silicon and gallium arsenide for instance, a single crystalline defect or impurity near the surface of the 30 material can significantly degrade the performance of an inte~rated circuit, or keep it from operatin~ at all. Material defects in optics made from silicon! sapphire and special glasses can have catastrophic results when used with hi~h powered lasers or when second order optical effect are being used. These situations have 35 been recognized for ~ome time, and a variety of equipment has been disclosed or developed to measure the surface character of these special materials. For examp:Le U. S. Pat. No. 4,314,763, entitled "D~FECT VET~CTION SYSTEM", discloses one of several techniques used to measure surface defects and contam.ination on semlconductors.

~ ~ . ', ' ' .
' Docket 5403 1 3 2 q ~ 9 7 The measurement of the crystalline and other micro defects directly below the surface, however, has been much more difficult. For example U. S. Pat. No. 4,391,524, entitled "METHOD FOR
5 DETERMINING THE QUALITY OF LIG~IT SC~TTERING MATERIAL", similar to the one previously mentioned, discloses one technique developed for that purpose. A second approach is described in U.S. Pat. No.
4,352,016, entitled "METHOD AND hPPARATUS FOR DETÆRMINING THE
QUALITY OF A SEMICONDUCTOR SURFACE" and U.S. Pat. No. 4,314,017, 10 entitled "APPARATUS FQR D~TERMINING TH~ QUAI.ITY OF A SEMICONDUCTOR
SURFACE". All of these measurement techniques have signiPicant limitations when measuring the subsurface crystalline damage or other micro defects which are most important to the improvement and use of these materials.
The term defects, as used herein, refers to any of a var:iety o~
structural crystalline defects, either grown-in or processing induced, like slips, dislocations, stacking faults and even buried scratch traces as well as defects which are formed when foreign 20 material is incorporated into the crystal structure such as inclusions, precipitates and impurity clusters and other impurity related defects. Basically, there are three ways of generating defects in the materials of interest. First, defects can be grown into the material when it is manufactured in its bulk form. For instance, when single crystals of silicon or gallium arsenide are grown, dislocations can form in the boule due to thermal stresses induced during the growing process, or impurities in the starting material are incorporated into the crystal. Secondly, after the material is manufactured, it must be cut into usable pieces and the 30 surfaces ground and polished in preparation for further processing. These steps of cutting, yrinding and polishing also introduce slips and dislocations into the crystal structure, but generally just below the prepared surface. Impurities can also be introduced into the material during these op~rations by diffusion 35 and other mechanisms. This second class of defects is generally 1,000 to 1,000,000 times greater in number than the defects grown into the original boule of material. Not only are the numbers larger, but as stated, these defects are near the surface while the 1 32q~7 Docket 5403 grown-in defects are distributed throughout the volume of the material. Thirdly, defects such as stacking faults, precipitates, dislocation lines and ion implantation induced defects can be generated by various fabrication processes typica~y used in the 5 processing of semiconductor wafers. The same i5 true of optical materials not only for crystalline defects but also for buried defects which can be generated in amorphous and polycrystalline materials by the preparation processes. These subsur~ace defects will e~fect the way light is transmitted through or reflected from an 10 optical material. Another effect, which is just beginning to be understood, is the connection between subsurface defects of all types and coating defects. Since optics and electronics both make extensive use of coatings, such effects are of great importance. For instance, epitaxial layers grown on semiconductor wafers can have 15 stacking faults grown-in ~uring the manufacturing process ancl these can be related to the defects already existing in the subs-trate wafer.

One technique currently used to measure crystalline damage is 20 described in U. S. Patents No. 4,352,016 and No. 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 25 shallow penetrator in semiconductor materials. ~ second factor significantly limiting sPnsitivity is the reflectance measurement itself. Such measurements are notor.iously difficult to make and result in looking for small variations in large numbers, which is one of the reasons why thls techni~ue requires measurements at two 30 wavelengths. The practical application of this reflectance technique shows up these deficiencies.

A second approach is de3cribed in U. S. Patent No.
4,391,524. This approach can measure the light scattered from the 35 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 ., ;

' 1 32q~7 Do~ket 5403 probe beam is O degrees. 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 5 integrating sca-tter from all directions, again making impossible the determination of directional defects, and at the same time diluting the si~nature of the defects it is designed to measure. And finally, the detector line of sight is also at O degrees, or near O
degrees. This introduces significant amounts of surface scatter into 10 the measured signal which is nearly impossible to separate from the subsurface scatter. Subtle variatlons 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 defect direction and very sensitlve to the surface character of the 15 test part.

Other technigues which purport to measure subsurface defects using a scatter measurement technique all measure the total integrated scatter from the surface of the test part. This technique 20 known as TIS integrates the scatter from the surface and subsurface as well as from all directions. The result is a measurement of mostly surface roughness for which this technique was originally designed. The surface scatter component of the total scatter from a material is very large, and will overwhelm the subsurface component 25 if the scattered light is collected anywhere near the specularly reflected beam.

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Docket 5403 1 3 2 9 q ~ 7 Summary of the Invention The object of the present invention is to improve significantly the capability of measuring the density of s;ubsurface crystalline and other micro defects in sem.iGonductors, optics and other special materials and coatings. The present invention is directecl to a nondestructive method of measuring the density and orientation of 10 crystalline or other micro defects directly below the surface of a material which allows penetration of electromagnetic radiation by measuring the radiation scattered from the ~ubsurface defect sites.
In accordance with the invention, the material to be measured must have a low sur~ace roughness and must be clean so that the surface 15 scatter does not contribute si~nificantly to the total scatter signal.
The material must be illuminated with a beam of electromagnetic radiation at a properly selected wavelength so that the penetration depth of the radiation is controlled. The intensity of the beam must be sufficient to provide a scatter intensity large enough to measure 20 and for best effect it should be coherent as is gener~ted by a laser. However the power density of the beam should not be so high that damage i5 done to the material being measured.

It is also possible to use multiple selected wavelengths to 25 simultaneously detect defects at different depths. By separat:lng the scatter signals for each wavelength and subtracting the signal of the shallow penetrating wavelengths from that oE the deeper penetrating wavelengths, it is possible to do d0pth gradient defect detection. In effect, detecting a zone of defects at some depth while 30 eliminating the effect~ of the defects nearer the surface. This can also be done by changing the laser intensity since a higher intensity will put more power at a greater depth thus allowing the detection of defects at a greater depth.

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 subsurface and at the same time minimize scatter ~rom the ', . , 1 3299q7 Docket 5403 surface of 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. The measurement technique of the invention is particularly 5 useful for measuring the processing induced defects in single crystal semiconductor materials such as silicon, gallium arsenide, indium phosphide and mercury cadm.ium telluride.

In order to use the data generated by such a system constructed in accordance with the .invention, it is desirable to plot and display the data in the form of a map of the subsurface defects.
This is done by substituting color variations for scatter intensity variations and plotting these color variations on the map. By adjusting the color distribution on a map, certain features can be 15 brought out or enhance~. For instance, isolatecl impurity clusters ~ust below the surface will show up as individual high scatter spots which in many cases are higher than the background scatter level. These spots can be isolated as a single color with a high contrast to show their distribution. Many such displays are 20 possible as specific defects are associated with a specific scatter signature. Black and white maps can also be made using shades of gray as an indicator of scatter intensity.

Other features and advantages of the invention will become 26 apparent from the following description, the accompanying drawings and the appended claims.

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Docket 5403 1 3 2 9 q q 7 Brief Descri~tion_of the Drawin~s FIG. 1 is a greatly enlarged cross section of a material with subsurface defects bein~ illuminated by a beam of electromagnetic radiation in accordance with the invention;

FIG. la is an enlarged portion at the intersection of the beam 10 and the material;

FIG. 2 is a perspective view o~ the material surface and schematically illustrates the orientation of the beam and the detector line o~ si~ht relative to the material surface;
FIG. 3 illustrates an embodiment of the invention wherein the material moves under a fixed beam and a fixed detector;

FIG. 4 illustrates an embodiment wherein the beam is scanned 20 over the material laterally in the X and Y directions while the material rotates about the Z axis and the detector is fixed;

FIG. 5 illustrates an embodiment wherein the material moves laterally in the X and Y directions and the beam of electromagnetic radiation rotates about the Z axis;

FIG. 6 illustrates an embodiment of the invention designed for very hi~h speed mapping of semiconductor wafers or other flat surfaced materials; and FIG. ~ illustrates an embodiment of the invention which is similar to FIG. 6 but which uses three wavelen~ths for the probe beam simultaneously.

3~ FIG. 8 illustrates an embodiment of the invention which is ~imilar to FIG. 6 but which varies the power in the probe beam.

. . . ..
' i ' . ' ' ' . :'' .'~ " -Docket 5403 1 3 2 ~ 9 q 7 Description of the Preferred Embodiments FIG. 1 and FIG. la shows a material or test part 10, such as an optic or a semiconductor wafer, having a surface 15 illuminated by a beam 16 oF electromagnetic radiation direct-ed at an angle of incidence A. The material or part surface 15 to be measured should be smooth with only a microroughness present as shown in the 10 enlargement 12 in FIG. la. For example, at a wavelength L of 6328 angstroms, a root mean square (rms) microroughness on the order of 0.005 times L, or about 30 angstroms or less, typically insures that surface scatter 17 does not overpower the near subsurface scatter 18. A150, the surface 1~ must be clean in order to prevent the 15 scatter due to surface contamination from overpowering the scatter 18 from the subsurPace defects.

The inco~ing beam 16 has a wavelength, polarization intensity and angle of incidence designed to optimize the trans~i~sion into the material 10 to a chosen depth. This increases the scatter 18 from the subsurface while decreasing the scatter from the sur~ace 17. The wavelength is selected by examining a material property, the extinction coe~ficient k, so that the penetration depth of the radiation L/(2 pi k), where pi is the well known constant (3.1~159), is 25 greater than the depth of the defects i~ the subsurface de~ect zone 20, but shallow enough so that the radiation does not penetrate to the back side of a thin part. As an example, the processing defects in a silicon wafer typically extends to no more than 2 or 3 micrometers and certainly no more than a few tens of micrometers, 30 while the wafer thickness i5 on the order of 350 to 400 micrometers.
At the same time, a beam with a wavelength of 6328 angstroms will have a penetration depth in silicon of about 3.7 micrometers.

The penetration depth gives a relative indication of how far 35 below the surface the probe beam will penetrate before being completely extinguished. The actual detection depth, the depth at whlch significant defects can be seen by their scatter slgnatures, is roughly proportional to the penetration depth. The detPction depth .

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Docket 5403 1 3 2 9 9 ~ 7 g is strongly related to the energy density ~power/axea) of the probe beam at the surface, the sensitivity of the scattered light detector system and the other physical parameters of the system such as polarization, anyle of incidence, detected solid angle and the system 5 siynal-to-noise ratio which, in this instanc~e, is defined as the detected scatter signal divided by the total system noise.

The polarization i9 selected ~or minimum intensity in the reflected beam 22 and maximum intensity in the transmitted beam 24.
10 For all cases, the maximum transmitted intensity is yielded by P
polarized light. The P in this case refers to the state where the electric vector 25 of the incident radiation 16 is para~el to the plane of incidence. This insllres that the intensity of the transmitted beam 24 is maxi~ized and thus the scatter 18 ~rom the 15 subsurEace defects is maximized.

Other polarizations can be used to achieve a variety of resul-ts. For instance, if S polarization is used (S in this case refers to the state where the electric vector is perpendicular to 20 the plane of incidence), the reflected intensity is maximized and the transmitted intensity is minimized thus enhancing the surface scatter 1~ over the subsurface scatter 18.

The intensity of the incident beam 16 must be sufficient to 25 provide a subsurface scatter intensity 18 larye enough to measure at the depth of interest. Since the intensity drops off rapidly with depth, it is possible to control the detection depth by adjusting the intensity. The energy density of the beam at the surface (power/area) must not approach the damage threshold of the material 30 being examined. Even low power beams, when focused to a tight spot, can have energy densities which can cause a large charge buildup on the surface sufficient to damage the cry~tal structure thus creating the very defeots being measured.

The angle of incidence A can be varied between O and 90 degrees. 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 .

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Dvcket 5403 ~ 3 2 9 9 9 7 scatter the light back out thr~ugh the surface. The blown-up view 12 of FIG. lA 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 5 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.
FIG. 2 shows a schematic o~ the surface 15 of the part 10 under test. The X, Y and Z axes are shown with the Z axis perpendicular to the surface 15. The incoming beam 16 has an angle of incidence A.
The line of sight 26 of a detector (not shown) is determined by the 15 angle D. The incident beam 16, the reflected beam 22, the detector line of si~ht 26 and the Z axis are all in the same plane 28. The angle made by the plane 28 with the X axis is the an~le R. The Z
axis, the incident beam 16 and the detector line of sight 26 all intercept at the same point on the surface of the test part, the 20 point being measured.

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 25 maximize the amount of energy transmitted into the material, and at the same time minimize the energy scattered from the surface. As an exa~ple, Brewster's angle for silicon is about ~5 degrees. Typically, angles of incidence larger than 55 degrees begin to cause problems because the impact point of the inciderlt beam 16 beglns to spread 30 across the surface. It is possible to reshape the beam cross section from circular to elliptical, with the major axis of the ellipse perpendiGular to the plane of incidence 50 that a more circular cross section will be proj@cted on the surface at large angles of incidence. Qther angles A can be used but with poorer results in 35 terms of signal-to-noise ratio which is used herein as the ratio of detected subsurface scatter power to total detected power.

Docket 5403 1 3 2 9 9 q 7 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 degrees, for large values of A, and large, greater than 30 degrees, 5 for small values of A. This is true because the ener~y which is scattered from the subsurface inust travers~e some thickness of material (19 in FIG. lA) 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 matPrial 10 being tested, especially for high inclex 10 materials su~h as semiconductors. Other angles D and A may be used but with poorer results in terms o~ signal-to-noise ratio. Values of D which place it near the 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 line of sight 26 of the detector is of great importance to the invention because the solid angle 27, that i9 intercepted by the detector, is small in comparison with other measurement techniques.
In most scatter measurement approaches the scatter is gathered from 20 a large solid angle. This has the effect of integrating or averaging the scatter signal over the large solid angle. In effect, this averaging approach dilutes the particular kind of information that is important in the measurement of sub~urface defects, and that is the angular position information. Another, equally important effect, is the enhancement of the scatter signature because the defect sites act like a grating and force the scatter in one particular direction while virtually eliminating the scatter from all other directions. The invention described herein will operate best when the solid angle intercepted by the detector is less than 0.1 ancl preferably between 30 0.001 and 0.01 steradians.

The angle R of the plane of incidence Z8 is very important in determining the orientation of the subsurface defects in a particular material. The nature of the subsurface defects in many 35 materials is in the form of lineations which are generated by the processes of sawlng, grinding, polishing and even cleanin~. These lineations are in the form of zones of clefects which are significantly longer than they are wide. In effect, these lineations ~ , Docket 5403 1 3 2 9 q 9 7 form a fine grating. Hence the scatter ~rom these lineations is highly oriented so that when the plane of incidence 28 is perpendicular to them, the scatter is very inten~e in the plane of incidence. At any other angle, the scatter is slgnificantly less or 5 nonexistent in the plane of incidence. This means that the angle R
ls directly related to the orientation of the ~ubsurface defects. By selectively examining the scatter ~ntensity versus angle Ri the direction of maximum scatter, and hence the direction of the defects, can be determined. This directionality can then be directly related 10 to the process used to form the surface. Thus effect~ 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 stackin~ faults, are oriented in certain crystallographic directions. Therefore it would be appropriate, if dqtection of this particular type of defect was important, to chose the R angle 20 corresponding to this particular known direction so as to detect the stacking faults while minimizing the effects of other defects. 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 seatter detector can be a solid state device or a photomultiplier type of device. In any case it i8 possible to chop or pulse the beam to enhance detection of the scatter signal, a technique well known in the art. This may be necessary when 30 examining very high quali-ty single crystal material to achieve the highest possible signal-to-noise ratio.

Individually the aspects of the invention just discussed, the angle of incidence A, the detector line of sight angle D, the Z axis 35 angle R and the solid angle intercepted by the detector 27, are only moderately useful in improvlng the scatter signature from subsurface defects. Taken together, these four aspects o~ the invention greatly improve the sensitivity of the measurement Docket 5403 1 3 2 9 9 q 7 apparatus ~o the most subtle variations of subsurface defect5. Each embodiment of the invention described in the ~ollowing paragraphs, is chosen to take special care to enhance the ability of the apparatus to make the best possible use of these elements of the 5 present invention.

The following fiyures are major embodiments of the present invention. The first is shown schematically in FIG. 3. In this embodîment a source of electromagretic radiation or laser 32 and the 10 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 micropositioniny stages 3~ and 37 for motion along the X and 15 Y axes r~spectively 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 be:lng measured. At each X and Y position the an~le R is varied from 0 to 360 degrees, a maximum reading is taken with the detector 34, fed to 20 a computer, and a map o~ the maximum scatter inter~sity 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 bearn 16 is conditioned 25 prior to interacting with the test part 10. A spatial filter/beam expander 36 gives the beam a Gaussian cross section of a given diameter, and a polarizer 38 insures P polarization. Beamsplitter 30 divides the beam so that a small portion of the energy is directed to detector 31 the output of which i~ also fed to the computer. This 30 information i5 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 35 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

' ' ' Docket 5403 1 3 2 q 9 q 7 axis, which is also fixed with respect to the test par~ 10. The probe beam 16 i5 scanned in the X and Y directions using movable mirrors 42 and 44 which oscillate in planes disposed at 90 degrees to each other. A lens 46 corrects for angular beam variations caused by the 5 scanning. The beam stop 40 is large to accommodate the scanned beam and insures that ~he beam does not re~lect or scatter back to the detector a~d cause erroneous readings. With this arrangement, the scanning can be done very quickly over large areas. The angle R
must still be cha~ged, using micropositioning stage 41, by rotating 10 the material under examination, but the material is fixed in the X
and Y directions.

With the embodiment just described, it is not possible to locate an X-Y coordinate and then rotate about a Z axis throu~h that 15 particular point on the test part. The R angle must be ~ixed and the entire surface o~ the test part scanned in X and Y to get one complete map. This would requ:ire considerab:ly more processing to obtain a final composite map or would be used when the orientation of the defects are known from previous measurements.
FIG. 5 shows an embodiment wherein 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 with stage 41. This separation of axes simplifies the design in many respects 25 particularly with respect to the lack of electrical connections to the rotating portion of the apparatus. The beam i5 generated by a laser 32 and conditioned as before except that element 38 converts the incident radiation to circular polarization instead of P
polarization. Then it i5 directed by steering mirror 48 through the 30 center of annular mirror 50 and through a hole in the center of rotation stage 41. At this point the beam 16 is coincident with the Z
axis. The beam is redirected by mlrror 56 through polarizing filter 62, which now converts the incident radiation to P polarization, and cylindrical focusing lens 64. Finally the beam 16 impinges on mirror 35 60 which gives it the proper angle of incidence on the test part 10.
The reflected beam 22 is intercepted by beam absorber 40. The scattered electromagnetlc radiation is gathered by mirror 58 and reflected by mirror 56 through the hole in stage 41 to mirror 50 Docket 5403 1 3 2 ~ q ~ 7 along path 26 to the detector 34. All the optical co~nponents on optical bench 52 rotate about the Z axis on the movable part of stage 41.

FIG. 6 is an embodiment of the invention desi~ned for high speed subsurface de~ect measurements on semiconductor wa~ers 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
directlons and the beam i5 rotated about the Z axis by means of an 10 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 15 polarization. The beam mu~t be coincident with the Z ax:is wh.ich is the axis o~ rotation o~ the air bearing 66. The beam passes through the rotating opt:Lcal element 65 which consists of two pr:lsms 67 and 68 separated by an opaque material 69. The prism 6~ reflects the beam at 90 de~rees to the Z axis and through the face of prism 67 20 which has a polarizing film ~0 on the surface. This converts the circularly polarized beam 16 to a P polarized condition. The beam is then reflected from annular mirror ~1 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 25 film ~2 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 ~3 which will 30 only allow P polarized light through, again reduciny the intensity of the beam. The remaining light impacts on the angled internal surface of rotating element 65 which has a black absorbing coating ~4. The purpose of this part of the optical element 65, and the circuitous path for the reflected beam 22, is to eliminate any 35 reflection of the probe beam back into the opt:ical system where it might interfere with the measurement. The light scattered from the subsurface de~ects in the test part 10 is collected by annular mirror ~5 and reflected back into rotating optical element 65 and . -::

': '~' :

Docket 5403 1 3 2 q 9 ~ 7 back to annular mirror 50 along path 26. The light passes through focusing optic 76 and to detector 34. Vertical micropositioning stages 7~ adjust the height of the rotating element 65 above the test part 10 to accommodate for any thickness variations between 5 test parts. A computer C prov.ides 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.

FIG. 7 i5 an embodiment of the invention that is similar to that shown in FIG. 6 except that there are three lasers at different wavelengths and three detectors for those wavelengths. Each Or the three lasers 78, 79 and 80 is operatln0 at a different discrete wavelength. Each of the wavelengths is chosen to obtain noticeably 15 different penetrat:ion depths in the materlal belng measurecl. Each laser has a spatial filter/beam expander 36 to focus and filter the output, and a part of ~he beam, in each case, is split off by mirror 30 and captured by detector 31 to monitor the beam power. Mirrors 81, 82 and 83 are ~esigned to be highly reflective at the wavelength 20 of its corresponding laser while transparent at the other wavelengths. The result is a single beam 16 containiny three discrete wavelengths which is circularly polarized by optical element 38. This single composite beam is reflected by rotating element 65 to annular ~irror 71 and directed on to test part 10. The 25 light at the three wavelengths i5 scattered back to mirror ~5, through rotating element 65 and along path 26. The light is focused by lens ~6 and collimated by optlcal element 90 and reflected to detectors B4, 85 and 86 by traverse mirrors 8~, 88 and 89 whi.ch are again designed to reflect strongly at the wavelength selected but be 30 transparent at the other wavelengths. Therefore, each mirror will reflect only one wavelength to its associated detector which is tuned and filtered for that wavelength. For example, mirror 89 will reflect a single wavelength to detector 86 while passin~ the other two wavelengths to the following two mirrors.
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 separa-tely. Each Docket 5403 1 3 2 9 9, 7 detector receives a different signal containing the scatter signature of the defects from just below the surface to whate~er depth that wavelength can penetrate in the material being tested. The wavelength that penetrates the :least will contain the 5 least information while the next deepest penetratiny wavelength will contain all the previous information plus new information about deeper defects. If the shallower information :is subtracted from the deeper in~ormation, the result will be just information about the deeper defects. If the~e depths are well known, then a zone of 10 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 subtracting the first from the second deepest penetrating 15 wavelength. More wavelengths can be used depending on the amount of information desired and the complexit~ of the in~trumerlt one i8 willing to accep-t. The subtraction o~ information must be done with due regard to the rotational orientation of the de~ects since defect orientation can change with depth. The bes-t approach is to take the 20 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.

The benefit of this embodiment of the invention is to allow the 25 determination of the depth of the defects accurately and nondestructively. This depth information is not available with the previous embodiments of the invention.

Another embodiment of the invention, shown in FIG. 8, allows 30 defect depths to be determined in a different way. This e~bodiment is also very similar to that shown in FIG. 6. The difference being that la~er 32 now has a power modulator 91 that is controlled by the computer C. This allows the power output of the laser to be changed and controlled so that different power levels can be available in the incident beam 16. Since the wavelen~th and power of the probe beam are selected to achleve a predetermined detection depth and the probe beam power decreases exponentially once it has entered the test material, lowering the power will reduce the detection Docket 5403 1 3 2 9 q ~ 7 depth. This i5 true because there is an absolute power level below which the power sca-ttered from the defects that emerges from the surface is so small that it is below the noise level of the system. Hence changing the power level changes the detection depth.

This detection depth change with power can be used just the same a~ wavelen~th changes, as described for FIG. 7, are used to determine the depth of defect zones. In this case, however, the application must be slightly dif~erent. One possible way to apply 10 this technique would be to actually make several passes over the test part, each at a different power level and then subtract the appropriate data to get the depth information. Another way would be to modulate the power level very rapidly so that at each rotational position two or more power levels are available. Each power level 15 would be stored as a separate piece of data for that location on the test part. The data would then be appropriately subtracted to extract the depth lnformation.

This approach has some advantages over the multiple 20 wavelength approach described for FIG. 7. The resulting equip~ent i5 less oomplicated and therefore less expensive. It is possible to have computer control over the detection depth that is desired and the control is much finer in that slight variations in the detection depth are possible. The main difficulty i5 that increasiny the 25 detection depth by increasing the power can rapidly lead to power levels that will damage the test part, particularly semiconductor parts, because of the nonlinear nature of the absorption of light under the test conditions.

Another approach to be considered is the combinatiQn of the embodiments described for FIG. 7 and FIG. 8 which would involve addlng modulator 91 to the lasers ~8, ~9 and 8Q in FIG. ~. This would allow both multiple wavelengths and power levels to be combined to optimize the operation of the equipment for the material bein~
35 tested and the defect depth desired.

All of the embodiments of the present invention must be isolated from the environment in two important ways. The apparatus , Docket 5403 1 3 2 ~ ~ q 7 must be vibration isolated to remove the effects of subtle movement between the prohe beam and the test part. Vibrations will cause gross inaccuracies since adjacent portions of the test part may have vastly different scatter signatures. Also the apparatus must be 5 isolated in a clea~ environ~ent to remove the problem of contamination of the surface being measurecl. This is necessary because the serlsitivity of the apparatus to contamination of any kind, and in particular to particulate colltamination, is very great.

While the measurement methods and apparatus herein described con~stitute preferred embodiments of the invention, it is to be understood that the invention i5 not limited to the precise methods and apparatus described, and that changes may be made therein without departlng from the scope and spirit of the inventlon as 15 defined in the appended claims.

The invention having thus been described, the Pollowing is claimed:

Claims

1. A method of measuring the distribution of crystalline or other micro defects directly below the surface of a predetermined area of a material which allows penetration of electromagnetic radiation, comprising the steps of:
(a) generating a beam of electromagnetic radiation having a predetermined wavelength capable of penetrating the material to some depth;
(b) directing the beam towards the surface at a predetermined fixed angle of incidence generally close to Brewster's angle and focusing the beam to expose a small portion of the material to the electromagnetic radiation;
(c) directing a detector's line of sight toward the surface of the material to the point where the beam intercepts the surface and at an acute angle relative to the incident beam and in the same direction from normal as the incident beam;
(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 perpendicular to the surface and at the point where the beam intercepts the surface and determining the rotational position of selected maximum 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; and (h) mapping the selected maximum scatter intensity versus the coordinate position of each point of measurement for the predetermined area, the scatter intensity being proportional to the crystalline quality of the material and the rotational position of the maximum scatter being related to the directional orientation of the defects directly below the surface.

2. A method as defined in claim 1 wherein the wavelength of the electromagnetic radiation is selected according to the optical characteristics of the material so that penetration to only a depth less than the material thickness will occur.

3. A method as defined in claim 1 and including the step of spatially filtering the beam to achieve a predetermined and uniform power distribution at the surface of the material.

4. A method as defined in claim 1 and including the step of chopping or pulsing the beam to enhance detection.

5. A method as defined in claim 1 wherein the beam is focused to one millimeter diameter or less.

6. A method as defined in claim 1 wherein the radiation energy deposited on the material is limited to prevent damage to the material such as heating or electrical breakdown.

7. A method as defined in claim 1 and including the step of continuously monitoring the beam power to compensate for errors due to power fluctuations.

8. A method as defined in claim 1 and including the step of displaying the scatter data as a map with different colors used to indicate scatter intensities.

9. A method of measuring the distribution of crystalline or other micro defects directly below the surface of a predetermined area of a material which allows penetration of electromagnetic radiation, comprising the steps of:
(a) generating a beam of electromagnetic radiation polarized P and having a predetermined wavelength capable of penetrating the material to some depth;

(b) directing the beam towards the surface at a fixed angle of incidence generally close to Brewster's angle and focusing the beam to expose a small portion of the material to the electromagnetic radiation;
(c) directing a detector's line of sight toward the surface of the material to the point where the beam intercepts the surface and at an acute angle relative to the incident beam and in the same direction from normal as the incident beam;
(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 and limiting the radiation entering the detector to polarized P radiation, 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 perpendicular to the surface and at the point where the beam intercepts the surface and determining the rotational position of selected maximum 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; and (h) mapping the selected maximum scatter intensity versus the coordinate position of each point of measurement for the predetermined area, the scatter intensity being proportional to the crystalline quality of the material and the rotational position being related to the directional orientation of the defects directly below the surface.

10. A method as defined in claim 9 wherein the extent of the scattered electromagnetic radiation entering the detector is limited to between 0.001 steradian and 0.01 steradian.

11. A method of measuring the distribution of crystalline or other micro defects directly below the surface of a predetermined area of material which allows penetration of electromagnetic radiation, comprising the steps of:
(a) generating a beam of electromagnetic radiation having a predetermined wavelength capable of penetrating the material to some depth;
(b) directing the beam towards the surface at a predetermined fixed angle of incidence generally close to Brewster's angle and focusing the beam to expose a small portion of the material to the electromagnetic radiation;
(c) directing a detector's line of sight toward the surface of the material to the point where the beam intercepts the surface and at an acute angle relative to the incident beam and in the same direction from normal as the incident beam;
(d) limiting the extent of the scattered electromagnetic radiation entering the detector to less than 0.1 steradian 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 perpendicular to the surface and at the point where the beam intercepts the surface and determining the rotational position of selected maximum 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; and (h) mapping the selected maximum scatter intensity versus coordinate position of each point of measurement for the predetermined area, the scatter intensity being proportional to the crystalline quality of the material and the rotational position being related to the directional orientation of the defects directly below the surface.

12. Apparatus for measuring the distribution of crystalline or other micro defects directly below the surface of a predetermined area of a material which allows penetration of electromagnetic radiation, comprising:
(a) means for generating a beam of electromagnetic radiation having a predetermined wavelength capable of penetrating the material to some depth;
(b) means for directing the beam towards the surface at a fixed predetermined angle of incidence generally close to Brewster's angle and focusing the beam to expose a small portion of the material to the electromagnetic radiation;
(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 and at an acute angle relative to the incident beam and in the same direction from normal as the incident beam;
(d) means for 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) means for producing relative rotation between the beam and the material about an axis perpendicular to the surface and at the point where the beam intercepts the surface and determining the rotational position of the selected maximum 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; and (h) means for mapping the selected maximum scatter intensity versus the coordinate position of each point of measurement for the predetermined area, the scatter intensity being proportional to the crystalline quality of the material and the rotational position of the maximum scatter being related to the directional orientation of the defects directly below the surface.

13. A method of measuring the distribution of crystalline or other micro defects directly below the surface of a predetermined area of a material which allows penetration of electromagnetic radiation, comprising the steps of:
(a) generating a beam of electromagnetic radiation polarized P and having a predetermined wavelength capable of penetrating the material to some depth;
(b) directing the beam towards the surface at a fixed angle of incidence generally close to Brewster's angle and focusing the beam to expose a small portion of the material to the electromagnetic radiation;
(c) directing a detector's line of sight toward the surface of the material to the point where the beam intercepts the surface and at an acute angle relative to the incident beam and in the same direction from normal as the incident beam;
(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 and limiting the radiation entering the detector to polarized P radiation, 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 perpendicular to the surface and at the point where the beam intercepts the surface and selecting a rotational position 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; and (h) mapping the scatter intensity versus the coordinate position of each point of measurement for the predetermined area, the scatter intensity being proportional to the crystalline quality of the material in the selected rotational position.

14. A method of measuring the distribution of crystalline or other micro defects directly below the surface of a predetermined area of a material which allows penetration of electromagnetic radiation, comprising the steps of:
(a) generating multiple beams of electromagnetic radiation polarized P having discrete predetermined wavelengths capable of penetrating the material to different depths;
(b) making the multiple beams coincident and directing the beams toward the surface at a fixed angle of incidence generally close to Brewster's angle and focusing the beams to expose a small portion of the material to the electromagnetic radiation;
(c) making the line of sight of the multiple detectors coincident and directing that line of sight toward the surface of the material to the point where the multiple beams intercept the surface and at an acute angle relative to the incident beams and in the same direction from normal as the incident beams;
(d) limiting the extent of the scattered electromagnetic radiation entering the detectors in order to detect the scatter coming from a small solid angle around the line of sight and limiting the radiation entering the detectors to polarized P radiation, for detecting a portion of the scattered electromagnetic radiation and converting it to an electrical signal proportional to the detected intensity for each of the multiple wavelengths;
(e) producing relative rotation between the beams and the material about an axis perpendicular to the surface and at the point where the beams intercept the surface and determining the rotational position of the selected scatter in a depth zone for that point by subtracting the scatter versus rotational position data for a shallow penetration wavelength from the corresponding data for a deeper penetrating wavelength and where the number of depth zones possible is related to the number of wavelengths used;
(f) producing relative lateral movement between the beams and the material to expose an adjacent portion of the material to the multiple wavelengths of electromagnetic radiation;
(g) repeating above steps (e) and (f) for each portion of the material exposed until the predetermined area is covered; and (h) mapping the selected scatter intensity versus the coordinate position of each point of measurement for the predetermined area and depth zone, the scatter intensity being proportional to the crystalline quality of the material in that zone and the rotational position being related to the directional orientation of the defects in that zone.

15. A method of measuring the distribution of crystalline or other micro defects directly below the surface of a predetermined area of a material which allows penetration of electromagnetic radiation, comprising the steps of:
(a) generating a beam of electromagnetic radiation polarized P and having a predetermined wavelength capable of penetrating the material to some depth;

(b) directing the beam towards the surface at a fixed angle of incidence generally close to Brewster's angle and focusing the beam to expose a small portion of the material to the electromagnetic radiation;
(c) directing a detector's line of sight toward the surface of the material to the point where the beam intercepts the surface and at an acute angle relative to the incident beam and in the same direction from normal as the incident beam;
(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 and limiting the radiation entering the detector to polarized P radiation, 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 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; and (h) mapping the selected scatter intensity versus the coordinate position of each point of measurement for the predetermined area in such a way so as to enhance known features by making them all one color in contrast to any background colors.

16. Apparatus for measuring the distribution of crystalline or other micro defects directly below the surface of a predetermined area of a material which allows penetration of electromagnetic radiation, comprising:

(a) means for generating multiple beams of electromagnetic radiation polarized P having discrete predetermined wavelengths capable of penetrating the material to different depths;
(b) means for making the multiple beams coincident and directing the beams toward the surface at a fixed angle of incidence generally close to Brewster's angle and focusing the beams to expose a small portion of the material to the electromagnetic radiation;
(c) means for making the line of sight of multiple detectors coincident and directing that line of sight toward the surface of the material to the point where the multiple beams intercept the surface and at an acute angle relative to the incident beams and in the same direction from normal as the incident beams;
(d) means for limiting the extent of the scattered electromagnetic radiation entering the detectors in order to detect the scatter coming from a small solid angle around the line of sight and limiting the radiation entering the detectors to polarized P radiation, for detecting a portion of the scattered electromagnetic radiation and converting it to an electrical signal proportional to the detected intensity for each of the multiple wavelengths;
(e) means for producing relative rotation between the beams and the material about an axis perpendicular to the surface and at the point where the beams intercept the surface and determining the rotational position of the selected scatter in a depth zone for that point by subtracting the scatter versus rotational position data for a shallow penetration wavelength from the corresponding data for a deeper penetrating wavelength and where the number of depth zones possible is related to the number of wavelengths used;
(f) means for producing relative lateral movement between the beams and the material to expose an adjacent portion of the material to the multiple wavelengths of electromagnetic radiation;

(g) means for repeatably engaging means (e) and (f) for each portion of the material exposed until the predetermined area is covered; and (h) means for mapping the selected scatter intensity versus the coordinate position of each point of measurement for the predetermined area and depth zone, the scatter intensity being proportional to the crystalline quality of the material in that zone and the rotational position being related to the directional orientation of the defects in that zone.

17. Apparatus for measuring the distribution of crystalline or other micro defects directly below the surface of a predetermined area of a material which allows penetration of electromagnetic radiation, comprising:
(a) at least one laser for generating a beam of electromagnetic radiation having a predetermined wavelength capable of penetrating the material to some depth;
(b) means including a rotatably supported reflector and a stationary annular reflector for directing the beam towards the surface at a fixed predetermined angle of incidence to expose a small portion of the material to the electromagnetic radiation and for rotating the beam about an axis perpendicular to the surface at the point where the beam intercepts the surface;
(c) at least one detector;
(d) means including a rotatably supported reflector and a stationary annular reflector for directing the line of sight of the detector toward the surface of the material to the point where the beam intercepts the surface and at an acute angle relative to the incident beam;
(e) means for 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 selected portion of the scattered electromagnetic radiation and for determining the rotational position of the selected portion;
(f) means for moving the material in a lateral direction for successively exposing adjacent portions of the material to the electromagnetic radiation until the predetermined area is covered; and (g) means for mapping the selected scatter intensity versus the coordinate position of each point of measurement for the predetermined area, the scatter intensity being proportional to the crystalline quality of the material and the rotational position of the selected scatter being related to the directional orientation of the defects directly below the surface.

18. A method of measuring the distribution of crystalline or other micro defects directly below the surface of a predetermined area of a material which allows penetration of electromagnetic radiation, comprising the steps of:
(a) generating a beam of electromagnetic radiation polarized P having a predetermined wavelength capable of penetrating the material;
(b) controlling the power level of the electromagnetic radiation for selecting different detection depths;
(c) directing the beam towards the surface at a fixed angle of incidence generally close to Brewster's angle and focusing the beam to expose a small portion of the material to the electromagnetic radiation;
(d) directing the detector's line of sight toward the surface of the material to the point where the beam intercepts the surface and at an acute angle relative to the incident beam and in the same direction from normal as the incident beam;
(e) 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 and limiting the radiation entering the detector to polarized P radiation, for detecting a portion of the scattered electromagnetic radiation and converting it to an electrical signal proportional to the detected intensity;
(f) producing relative rotation between the beam and the material about an axis perpendicular to the surface and at the point where the beam intercepts the surface and determining the rotational position of the selected scatter in a depth zone for that point by subtracting the scatter versus rotational position data for a shallow penetrating power from the corresponding data for a deeper penetrating power whereby the number of possible depth zones is related to the number of power levels used;
(g) producing relative lateral movement between the beam and the material to expose an adjacent portion of the material to the electromagnetic radiation;
(h) repeating above steps (e) and (f) for each portion of the material exposed until the predetermined area is covered; and (i) mapping the selected scatter intensity versus the coordinate position of each point of measurement for the predetermined area and depth zone, the scatter intensity being proportional to the crystalline quality of the material in that zone and the rotational position being related to the directional orientation of the defects in that zone.

19. A method of measuring the distribution of crystalline or other micro defects directly below the surface of a predetermined area of a material which allows penetration of electromagnetic radiation, comprising the steps of:
(a) generating multiple beams of electromagnetic radiation polarized P having discrete predetermined wavelengths capable of penetrating the material to different depths and having controllable and changeable power levels for each beam;
(b) controlling the power level of the multiple wavelengths of electromagnetic radiation for selecting different detection depths other than those provided by the multiple wavelengths;
(c) making the multiple beams coincident and directing the beams toward the surface at a fixed angle of incidence generally close to Brewster's angle and focusing the beams to expose a small portion of the material to the electromagnetic radiation;
(d) making the line of sight of the multiple detectors coincident and directing that line of sight toward the surface of the material to the point where the multiple beams intercept the surface and at an acute angle relative to the incident beams and in the same direction from normal as the incident beams;
(e) limiting the extent of the scattered electromagnetic radiation entering the detectors in order to detect the scatter coming from a small solid angle around the line of sight and limiting the radiation entering the detectors to polarized P radiation, for detecting a portion of the scattered electromagnetic radiation and converting it to an electrical signal proportional to the detected intensity for each of the multiple wavelengths;
(f) producing relative rotation between the beams and the material about an axis perpendicular to the surface and at the point where the beams intercept the surface and determining the rotational position of the selected scatter in a depth zone for that point by subtracting the scatter versus rotational position data for a shallow penetrating wavelength and power from the corresponding data for a deeper penetrating wavelength and power whereby the number of possible depth zones is related to the number of wavelengths used and the power levels chosen;
(g) producing relative lateral movement between the beams and the material to expose an adjacent portion of the material to the multiple wavelengths and power levels of electromagnetic radiation;

(h) repeating above steps (e) and (f) for each portion of the material exposed until the predetermined area is covered; and (i) mapping the selected scatter intensity versus the coordinate position of each point of measurement for the predetermined area and depth zone, the scatter intensity being proportional to the crystalline quality of the material in that zone and the rotational position being related to the directional orientation of the defects in that zone.

20. A method of measuring the distribution of crystalline or other micro defects of 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 a detector's line of sight toward the surface of the material to the point where the beam intercepts the surface and at an acute angle relative to the incident beam and in the same direction from normal as the incident beam;
(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 perpendicular to the surface and at the point where the beam intercepts the surface and determining the rotational position of selected maximum 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; and (h) mapping the selected maximum scatter intensity versus the coordinate position of each point of measurement for the predetermined area.

21. Apparatus for measuring the distribution of crystalline or other micro defects of a predetermined area of a material, comprising:
(a) means for generating a beam of electromagnetic radiation;
(b) means for directing the beam towards the surface at a fixed predetermined angle of incidence and focusing the beams to expose a small portion of the material to the electromagnetic radiation;
(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 and at an acute angle relative to the incident beam and in the same direction from normal as the incident beam;
(d) means for 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) means for producing relative rotation between the beam and the material about an axis perpendicular to the surface and at the point where the beam intercepts the surface and determining the rotational position of the selected maximum 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; and (h) means for mapping the selected maximum scatter intensity versus the coordinate position of each point of measurement for the predetermined area.