Calibration device and method for an optical defect scanner

A calibration device for an optical scanner for optically detecting microscopic defects on object surfaces is formed on a substrate having a characteristic pattern of a plurality of different arrays of artificially created defects. Each array is arranged by size and spacing of the artificial defects to represent an actual defect size. Each artificially created defect of a given array is of the same size. Each defect is provided with a surface which in response to an incident beam of light scatters the light. The response of the system to the scattered light forms a characteristic pattern which corresponds to actual defects.

This invention relates to a calibration device and a method for calibrating 
an optical scanner for detecting microscopic defects by scattered light 
from the surface of an object. 
BACKGROUND OF THE INVENTION 
Silicon wafers useful in the manufacture of semiconductor devices require 
close scrutiny to detect defects as soon as possible in the manufacturing 
process. Several apparatus are known in the art for detecting microscopic 
defects on the surface or near the surface of such devices. One such 
apparatus utilizes a laser beam that is scanned over the surface of a 
wafer and includes means for detecting scattered radiation from the wafer 
surface. The specular reflection is blocked from the detection device by 
suitable arrangement of the lenses and spatial filters. If the surface of 
the wafer has an imperfection such as dirt, hills, scratches and the like, 
the laser beam will be scattered from the imperfection. There are also 
scattering processes such as Raman scattering, etc., which occur, but the 
intensity due of the light to such scattering effects is usually 
negligible. The scattered light from the wafer is collected from about the 
main axis of the lens and is focused on a detector. The scattered light is 
converted to electrical impulses which can be counted or in the 
alternative can be displayed as a bright spot on an oscilloscope. See 
copending U.S. application Ser. No. 000,813, filed by E. F. Steigmeier et 
al. on Jan. 4, 1979, now U.S. Pat. No. 4,314,763, issued Feb. 9, 1982, 
entitled "DEFECT DETECTION SYSTEM" for a detailed description of such a 
scanning apparatus. 
It is difficult to calibrate such a scanning apparatus to predictable 
dimensions of the observed microscopic defects. The usual method for 
calibrating such an apparatus is to observe the displayed scan by electron 
or optical microscopic techniques. Such a procedure of calibration is 
difficult because it is a cumbersome procedure requiring calibration at 
locations other than where the scanner is located and is more time 
consuming. There is a need in the art to provide means for calibrating 
such apparatus quickly and preferably at the site of the apparatus without 
the use of optical or electron microscopy. 
SUMMARY OF THE INVENTION 
According to the present invention, a calibration device serving as an 
object with simulated microscopic defects comprises a semiconductor wafer 
having a characteristic pattern of a plurality of different arrays of 
artificial defects. Each array of such defects is formed of a plurality of 
evenly spaced defects, each defect of a particular array having the same 
dimension. The pattern of the calibration device when suitably positioned 
in the machine serves to simulate an object having microscopic defects of 
unknown dimension when exposed to an incident beam at a preselected 
intensity value. The calibration device will provide an illuminated 
pattern that simulates a preselected size of a microscopic defect on the 
surface of a given object.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
Before proceeding to a detailed description of the calibration device of 
the invention, reference is made to FIG. 1 illustrating a suitable optical 
scanner for which the calibration device is used. The apparatus shown in 
FIG. 1, described in detail in the above-identified application, comprises 
an optical system 10 which includes a light source 16 providing a beam of 
light 12' passing through a series of prisms 18 and 20 and then through a 
focusing means, such as lens 22, providing beam 12. The light source 16 
provides a light of any selected wavelength and includes light in the 
infrared (IR), visible or ultraviolet (UV) light spectrum. Light source 16 
may be a low power laser, for example, a HeNe laser producing light at 
6323 angstroms in wavelength which is focused by lens 22 into a spot 250 
.mu.m in diameter. Beam 12 of the laser light is projected onto the 
surface 14 of an object such as the unit under test (UUT). The unit under 
test may be a wafer of silicon as used in the manufacture of integrated 
circuits (IC) and other semiconductor devices. The position of the light 
source 16 is not critical, but the position of the beam 12 between the 
prism 20 and object surface is important. The axis of the beam is 
preferably substantially perpendicular to the surface 12. Light generated 
by laser 16 is scanned over the surface 14 of the UUT and is reflected 
back through the lens 22 via beam pattern 24 and collected on a 
photodetector 26 which is positioned along the axis of the beam 12. Lens 
22 serves a first of two spatial filters to specular reflected light along 
the axis of beam 12. The defects that appear on the surface 14 of UUT may 
be as small as 1 .mu.m in area. A defect may extend beyond the diameter of 
the laser beam, namely beyond the 250 .mu.m, in which case its shape, as 
distinguished from its mere size, will be detected by the scanning 
process. Surface defects scatter a sufficient amount of light beyond prism 
20 so as to be detected by detector 26. 
A defect for the purposes of this description shall mean any imperfection 
to an ideally optically flat surface or in the underlying structure below 
the surface of the object under test. An imperfection to the optically 
flat surface includes particulates (dust, etc.) nicks, hills, scratches, 
depressions, etc., which are detected when there is little or no 
penetration of the incident light. An imperfection in the underlying 
structure includes inclusions (foreign particles), bubbles in the form of 
voids, microcrystalline grain boundaries, etc., which are detected when 
there is substantial penetration of the incident light. The depth of 
penetration is a function of the wavelength of light and the material. In 
general, since an optically flat surface, will not scatter light, the 
defect will have surface portions that are not optically flat with respect 
to the incident light. 
An aperture mask 25 acts as the second of two spatial filters in optical 
system 10 and prevents ambient light from being projected onto the 
detector 26. The output of detector 26 is applied to an amplifier circuit 
28 which provides an output signal to either or both a counter display 30 
or a cathode ray tube (CRT) display 32. Counter 30 counts the number of 
defects that are detected during a scan of beam 12. The CRT display 32 
provides a visual display of the relative spatial distributions of the 
locations of the defects on the UUT. Amplifier 28 is analogue in nature 
and produces an amplified output of the detector output proportional to 
the input signal it receives from detector 26 at terminal 28a. This 
results in gray scale in the display 32, the intensity of the indications 
of defects on the CRT screen being indicative of the defects. A more 
detailed schematic of amplifier 28 is shown in FIG. 2 to be described. 
In the form of the scanner shown in FIG. 1, the beam 12 scans the UUT in 
spiral fashion and the electron beam of the display 32 is also scanned in 
spiral fashion. The UUT may be a circular surface and for such purposes a 
spiral pattern is useful. If desired, the pattern may be converted into a 
X-Y display which is achieved by the coordinate transformation system 60 
which transforms polar coordinates of the beam striking at surface 14 into 
suitable rectangular coordinates which are applied as X-Y coordinate 
inputs for the display 32. A detailed description of the polar coordinate 
system is not given here, but for a more detailed description see the 
above-identified copending application Ser. No. 000,813, described above 
and hereby incorporated by reference. In brief, the coordinate system 
includes a polarizer P.sub.R, spaced, stationary analyzers P.sub.y and 
P.sub.x, detectors 74 and 76 excited by the photocells L.sub.x and L.sub.y 
which are energized by power supply 73. The system 60 includes a shaft 62 
rotating in direction 64 over table support 42 slideable by motor 40. A 
gear 66 connected to shaft 62 is meshed with gear 48 so that the polarizer 
P.sub.R rotates at a predetermined angular speed, typically one half the 
angular speed of the UUT on table 44 rotated by shaft 46 in direction 38. 
The light from the diodes L.sub.x and L.sub.y are passed through the 
polarizer P.sub.R and detected by detector 74 and 76 and applied to the 
processing circuit 54. A wiper arm 50 is connected (dashed line 56) to the 
table 42 and moves with the table 42 as the table translates in the 
direction x'. The wiper arm 50 is part of a potentiometer 52 which is 
connected to processing circuit 54 for position control purposes. The 
processing circuit 54 provides the X and Y signals for application to the 
CRT 32 in the manner described in the above-identified application. 
In operation, when the incident beam 12 is positioned at the center of UUT, 
the output of amplifier 28 is zero. As the beam 12 is moved from the 
center, signals are detected by detector 26 and applied to amplifier 
circuit 28 and applied to display 32. The display is scanned in an X-Y 
direction, providing a visual display corresponding to the scattered 
reflections from the beam 12. The display appears as bright spots and 
positions of the spots on the display screen correspond to the spatial 
distribution of the locations of the defects on or close to surface of the 
UUT. 
Reference is now made to FIG. 2, which shows particularly the amplifier 
circuit 28 in relation to other portions of the system. Laser 16 provides 
a fixed intensity beam 12 which is scattered as beam 24 which in turn is 
detected by detector 26. The output of detector 26 is coupled via terminal 
28a to a preamplifier 100 which in turn is coupled to a threshold current 
amplifier 102 and thence to single pole, double throw switch 105 and 
eventually switch 106. The threshold signal from amplifier 102 is either 
applied directly to an inverter 104 or, in the alternative to an output 
amplifier 108. Single pole, double throw switch 107 inserts either one of 
diodes 110 or 112 in the circuit depending upon the insertion or removal 
of inverter 104 by switches 105 and 106. Inverter 104 is used, if desired, 
to invert the output signal of amplifier 102 whereby the display of a 
detected defect will be inverted. Output resistor 114 connected to ground 
provides the output signal which is applied to the cathode of the CRT 32, 
as at terminal 116. 
A threshold control network 120 provides a means for controlling to a 
predetermined or preselected value the intensity I of the beam of the CRT 
display 32. The network 120 comprises a reference potentiometer 122 formed 
of serial resistors 124, 126 and 128 connected between +15 volts and -15 
volts. An adjustment tap 130 is connected to one terminal of a single 
pole, double throw switch 132, the common terminal of which is connected 
to another switch 134 and thence to the input 102a of threshold amplifier 
102. This provides in a test mode an adjustable voltage to the threshold I 
amp 102 to provide a test signal for display on the CRT display 32. 
Theshold amplifier 102 is a suitable operational amplifier having a first 
input 102b and a second input 102a. The network 120 with the switches 132 
and 134 in the position as shown provides a control voltage to terminal 
102a of amplifier 102 as the test mode of operation during which the laser 
is scanning the UUT or calibration wafer. For normal operation to preset 
the predetermined threshold at which the CRT beam provides a predetermined 
intensity I, a threshold intensity adjustment potentiometer 140 is 
connected by ganged switch 147 between either one of a pair of selectable 
resistors 142 and 144 connected in common to +15 volts, the other 
terminals being connected to switch 147 through a pair of resistors 146 
and 148 to -15 volts. The resistors can be selected to provide different 
voltage ranges to thereby change the intensity of the CRT beam over a wide 
range of values as desired. In operation, with switches 134 and 132 
positioned to the "normal" position opposite to that shown in FIG. 2, the 
intensity threshold (I) control 140 will be in the circuit. By adjusting 
potentiometer 140, the intensity (I) of the CRT beam may be adjusted to a 
predetermined value. Suitable calibration indicia on the potentiometer are 
provided (not shown) as a repeatable reference of the selected position of 
the potentiometer 140. 
In addition to the threshold (I) intensity control network 120, a second 
threshold (D) control 150 is provided to modify the intensity signal 
(I.sub.UUT) applied to the terminal 116 of CRT 32. The D threshold 
adjustment 150 provides a reference adjustment of a threshold amplifier 
152 whose input is coupled to the output of a gate 154 which in turn is 
triggered by flip-flop 156 responding to one shot 158. Gate 154 is coupled 
to transistor 160 which in turn is coupled to counter 30a of display 30 
shown in FIG. 1. With momentary contact switch 162 normally in the 
position shown, triggered events will register in the display 30 and with 
the switch 162 momentarily in the other position the display 30 is cleared 
to "0000". Switch 164 is a switch for controlling the CRT beam 12 to be 
"on" or "off" in accordance with the position as shown. The coordinate 
transformation system 60, described above for FIG. 1, is shown in block 
form coupled to the terminals X and Y of the CRT scope display 32 to 
provide the X-Y display pattern described above. 
Thus, the threshold intensity (I) potentiometer 140 and threshold (D) 
potentiometer 150 provide an adjustable detection sensitivity control of 
the scattered light. The sensitivity of detection can be further increased 
by changing the gain of the amplifier 108. The intensity of the CRT 
display beam is adjusted by the threshold I potentiometer 140 to increase 
the detector signal sufficiently to the level at which the CRT 30 displays 
the detected signal. 
In operation with a UUT in position on the table 44, the scanner provides a 
beam 12 which in turn provides a scattered beam 24 which, after detection, 
will provide a display on CRT 32. Defects that may appear will be counted 
and displayed in counter display 30. However, there is difficulty in 
calibrating the apparatus so that the dimensions of the defect are known. 
Opitcal or electronic microscopic techniques have heretofore been used to 
determine the dimensions of such defects. 
A calibration device 170 of the invention is illustrated in FIG. 3A. The 
calibration device 170 is suitably formed on a wafer in which a plurality 
of patterns 172 are provided in the form of a plurality of artificial 
defects such as islands of dots 174. In the alternative and preferred 
form, dots 174 are etched into the surface of the wafer as will be 
described further. In brief for the present purposes, it should be 
understood that the response of the scanner will be different for 
different types of surface defects. For example, a dust particle with a 
very rough and irregular surface, would probably scatter more light than a 
smooth surface such as a surface exhibited by a latex sphere of the type 
used in the art to calibrate an electron microscope. Conversely, a pit 
developed by etching might scatter very effectively due to off-axis 
specular reflection from the etched surfaces. It is for this reason that 
the preferred embodiment of the invention provides for the dots in the 
form of etched pits rather than islands of deposited materials. Dots in 
the form of islands tend to be quite smooth particularly with vertical 
walls and flat surfaces that do not scatter well. Since the calibration 
device 170 of the invention serves to simulate an actual microscope defect 
on the surface of UUT, it is important that the scattered light from the 
artificial defects of the calibration device provide an intensity of the 
reflected beam that approximates if not equals the scattered light 
intensity from actual defects. It should be noted that specular 
reflections are very strong as compared to scattered light. Moreover, an 
off-axis specular reflection is much stronger than off-axis scattered 
light. Nevertheless, it should be appreciated that, in this environment, 
there are many types of defects seen which all scatter with different 
effectiveness. 
The patter 172 of dots 174 shown in FIG. 3C is suitably developed on the 
wafer 170 by a mask and known photolithographic techniques. A suitable 
mask making procedure such as a manufacturing electron-beam system (MEBES) 
is used. Each of dots 174 serve as the fundamental artificial microscopic 
defect from which the scattering of the light emanates. Each dot 174 is 
essentially a square on the wafer surface 170 and can be defined in any 
appropriate material such as silicon dioxide (SiO.sub.2) or silicon (Si). 
The dots 174 are arranged into an array 176 of a group, for example, of 13 
by 13 dots 174. The number of dots in an array must be large enough to 
form a visible pattern on the CRT display 32. If the array 176 was formed 
of too few a number of dots 174, for example, four dots, the observed 
pattern would not be distinctive enough to be unambiguous as compared to 
other four-dot patterns representing a different calibration size. Some of 
the dots can be omitted from the array 176 as indicated by the omitted 
portions 178, 180, etc., in FIG. 3C. The missing dots are useful in 
providing a means for identifying an array within the pattern or 
orientation of the array or pattern in the display. Furthermore, a pattern 
of missing dots can be arranged to depict the numbers 5, 10, 20, etc., 
depicting thereby the calibration size of the particular array within the 
pattern 172. The dots 174 which make up or form an array 176 may be of 
various side dimensions ranging from 50 microns to 1 micron. 
In general, there are two types of arrays 176. In one form of the array, 
dots 174 are spaced from each other on 300 micron spacing centers. In the 
other form of the two arrays dots 174 are spaced on 100 micron centers. 
Arrays on 300 micron centers are formed of dots having a side dimension of 
50, 20, 10, 50, 3 or 1 micron. The arrays formed of 100 micron-spaced dots 
174 are provided with either 5, 3, or 1 .mu.m dots. For purposes of this 
description an array of 50 micron dots on 300 micron centers may be called 
a 50/300 array as indicated by array 182 of the pattern 172 shown in FIG. 
3B. Similarly, 5 micron dots 174 on 100 micron centers may be called a 
5/100 array represented by array 190 of FIG. 3B. The arrays 182, 184, etc. 
are arranged into a pattern 172 as shown in FIG. 3B and shown within the 
circle 172a of FIG. 3A. The patterns 172 are repeated across the entire 
surface of the wafer 170. Masks are suitably provided to provide a 4" mask 
set on a 3" wafer for example. The calibration device 170 was developed on 
bulk silicon wafers which have grown on them a layer of silicon dioxide 
about 2000 angstroms thick. The plurality of patterns 172 was defined in 
the oxide film with standard wet chemical techniques. Patterns may also be 
generated directly on a surface of bulk silicon wafers by plasma etching. 
In the preferred form of pattern development, etched pits are formed into 
the surface of the wafer 170. As indicated above, the pits of each of the 
dots 174 scatter more light than the islands of silicon dioxide forming 
dots 174 and simulate better than normal type of surface contamination. 
Wafers have been made in the form shown in FIG. 3B but without a 1/300 
array 186, namely, without a pattern of dots 1 .mu.m in diameter on 300 
.mu.m centers. However, a calibration device including array 186 can be 
made within the state of the art of etching and mask manufacture. Such a 
calibration device will accordingly provide resolution down to 1 micron 
defects. Nevertheless, devices made with arrays with 3 to 50 micron dots 
are still very effective in detecting defects of concern to the 
semiconductor industry. It will be noted, as shown in FIG. 3C, that 
dimension A represents the dimension of each individual dot 174 whereas 
dimension B represents the spacing between the dots, as described 
hereinabove. 
In operation, the scanner is aligned as necessary as described in the 
aforementioned co-pending application, now U.S. Pat. No. 4,314,763, and 
the calibration wafer 170 is placed on the table 44. If it is desired to 
provide an output for display by counter 30 only, the threshold D 
adjustment 150 (FIG. 2) is set to a predetermined value, nominally half of 
the full scale of the potentiometer. The potentiometer 140 for the 
intensity threshold I with the switches 132 and 134 in the left position 
from that shown in FIG. 2 is adjusted to give a maximum count on the 
display 30. A distinct maximum will be observed representing a counting of 
all of the light scattering dots 174 on the wafer 170. 
If a CRT output display is desired, the CRT 32 may now be calibrated for 
optimum response by increasing the CRT intensity control (not shown) until 
a similar display to that shown in FIG. 4G (to be described further 
hereinafter) is obtained. The response of the CRT display 32 and the 
counter 30 should now be equivalent. That is to say, any count of defects 
(as defined above) that were scanned and detected to provide a count will 
also appear as a visual display of a sequence of spatially displaced spots 
on the CRT display 32. To set the response of the scanner to be sensitive 
to a particular size of microscopic defect, the threshold I potentiometer 
140 should be set to a minimum or zero value and the wafer 170 scanned. 
Some defects may be observed even at this low setting. The potentiometer 
140 (threshold I) is increased until a portion of the array 176 of squares 
is seen. FIG. 4A illustrates such a display. The photographs of FIGS. 
4A-4G are, it is to be noted, full size photographs as seen by an operator 
viewing a typical CRT display. One advantage of the calibrator of this 
invention is the ability of an operator to visually observe a pattern and 
determine from the observed pattern the size of the defect. The portion of 
the arrays 176 as seen in FIG. 4A are the portion of pattern 172 
represented by block 182 only of FIG. 3B. As the threshold (I) 
potentiometer 140 is adjusted to increase the sensitivity of the detection 
system, the next display from the calibrator device 170 will be the 20/300 
array represented in FIG. 3B as array 182 plus array 186. Thus, array 182 
is still displayed and, in addition, the array 186 appears alongside the 
array 182. It should be understood that the individual dots 174 in each of 
the arrays 182 and 186 are not actually seen in the display although there 
may appear to be a regular omitted-dot pattern in the array such as 
portions 178 and 180 of array 176, FIG. 3C. The area of the missing dots 
represented by portions 178 and 180 of FIG. 3C can at times be seen with 
appropriate adjustment of the CRT. Increasing the threshold (I) 
potentiometer 140 still further will result in the observation of either 
the 10/300 array 188 or the 5/100 array 190, as illustrated by FIGS. 4C 
and 4D, respectively. Note that arrays 190, 194, 196 are repeated 10 times 
each within the pattern 172 as seen in FIG. 3B. 
The diameter of laser beam 12 is nominally 250 .mu.m as explained above and 
the response to the 5/100 array 190 is greater than the response to the 
10/300 array 188, simply because more scattering centers manifested by the 
dots 174 are within the beam 12 at a single instant in the closely-spaced 
5/100 arrays 190. 
The next array of the pattern to appear with further increase of the 
threshold intensity (I) potentiometer 140 is the 3/100 array 194, which 
appears as a doubling of the width of the 5/100 arrays 190. This display 
is shown in FIG. 4E. The calibration procedure is continued until the 
5/300 array 184 (FIG. 4F) and the 3/300 array 192 (FIG. 4G) become visible 
on the display 32. The 5/100 bar pattern 190 is seen in FIG. 4D. The 
double bar pattern of 190 and 194 is seen in FIG. 4G. A device can be 
made, but not shown in the photographs, with arrays of 1 .mu.m dots on 100 
.mu.m centers represented in dotted lines by bar pattern 196 shown in FIG. 
3B. 
By noting the various settings of the threshold I potentiometer 140 at 
which the various arrays illustrated by FIGS. 4A through 4G appear in the 
display, the sensitivity of the apparatus may be adjusted to respond to 
any desired dimensional range of defects. It is possible that all of the 
arrays 176 would not be seen before the signal is washed out by noise and 
surface scattering from the silicon surface even in places where there are 
no microscopic defects or other features. Since the mask forming the 
calibration device 170 incorporates some blank areas such as portions 178 
and 180 (FIG. 3C), this washing out effect can be observed from the CRT 
display 32. However, these portions are not easy to observe. The areas of 
arrays 187 (1/300) and 196 (1/100), can be used to observe the washing-out 
effect. Even if such arrays are not used, the area to the left of the 
array 182 (50/300) and array 187 (1/300) could be used for such purposes. 
As mentioned hereinabove, it is preferred that the dots forming the 
artificially created defects, causing the fundamental source of scattered 
light, be in the form of a recess in the general form of a square and an 
etched surface within the recess. In systems in which the dots 174 were in 
the form of islands, the correlation of the response characteristics of 
the artificially created defects of a specific size to the response 
characteristics of actual defects of the same or similar size was less 
than fully satisfactory. Microscopic evaluation of the structural features 
of the island form of dots shows vertical sidewalls and a smooth top 
surface. Any scattering comes from only the edges of the silicon dioxide 
island and, as such, the island reponds as if it were a much smaller 
defect thereby disqualifying the desired simulation of an actual 
microscopic defect or particle of the same dimension. The material used 
for the island may also have an effect. The preferred choice of silicon 
dioxide is made on the basis of durability and ease of patterning, but the 
wafers 170 could be fabricated with any suitable semiconductor material. 
It is possible to choose a surface which could be matched in scattering 
response to typical ambient contamination. For example, the user will 
empirically determine what kinds of contamination are likely to be 
encountered, and attempt to match the scattering efficiency of like-sized 
artificial defects to the scattering efficiency of that contamination. 
Examples of such materials might be photoresists, polycrystalline silicon, 
or metals. In the preferred form, a selective etch to form pits in &lt;100&gt; 
silicon would be a reasonable choice resulting in a faceted pit which 
might be expected to scatter very effectively. 
It will now be appreciated that the calibration device described above 
provides a means to calibrate optical scanners useful in detecting defects 
in semiconductor materials without the use of any other calibration 
equipment. The pattern of the device is constructed such that the response 
of the scanner to micron sized particles can be visualized on a 
macroscopic scale in the order of 1 cm as seen in FIG. 4A, for example. 
The pattern is constructed of arrays of surface features with typical 
sizes ranging from 1 to 50 microns in such a way that the scanner will 
respond to individual features of the pattern. When this occurs, the 
individual features combine to form a macroscopic pattern of artificial 
microscopic defects on the display. Adjustments of the sensitivity of the 
scanner by potentiometer 140 to display the various features of the 
pattern is used as a means to calibrate the scanner. In this way the 
scanner is unambiguously calibrated to a particular sensitivity. Thus, the 
scanner when calibrated allows the operator to observe a defect and 
determine the size thereof to a reasonable degree of accuracy by noting 
the adjusted value of potentiometer 140 at which the defect was displayed. 
This potentiometer value is compared to the calibration values as 
described above. 
While the artificially created defects simulate defects that occur in or on 
the semiconductor substrate, it should be understood that the accuracy of 
the wafer is dependent on the degree of scattering that is effected by the 
individual and combined defects. What is essential is that the 
artificially created defect scatters light rather than manifests only 
simple on-axis specular reflection. 
The calibration device described may be arranged in any desired form other 
than that illustrated above. In practice, the calibration device 170 is 
used to quickly adjust potentiometer 140 of the laser scanner to, for 
example, an acceptable threshold value for grading production wafers. If 
the acceptable quality of wafers is one, for example, having defects not 
greater than 20 .mu.m, the scanner is adjusted to provide an output 
corresponding to FIG. 4B. Wafers are screened accordingly.