Substrate inspection system having two scattered light detecting bundles orthogonally positioned to each other

Surface defects on a substrate (11) are detected by raster scanning the surface (10) with a laser beam (12). Light scattered by the substrate at a large angle with respect to a normal to the surface is collected by at least two bundles of light guide fibers (21,22) located nearly in the plane of the substrate surface, one fiber bundle parallel to the laser scan with the other positioned perpendicularly thereto. The light collected by each bundle is separately processed by a computer (20) to determine the size, in three dimensions, and orientation of defects on the substrate surface. In a preferred embodiment, four light guide fiber bundles (31,32,33 and 34) are used around the substrate surface with the outputs being separately processed.

TECHNICAL FIELD 
This invention relates to methods for inspecting flat surfaces, and more 
particularly, to methods for detecting defects on a flat surface by 
scanning the surface with a laser beam and detecting light scattered at a 
large angle with respect to a normal to the surface. 
BACKGROUND OF THE INVENTION 
U.S. Pat. No. of Jablonowski, 4,286,293 issued Aug. 25, 1981 (assigned to a 
subsidiary of the present assignee company) is directed to a method for 
inspecting gold conductors on ceramic substrates by scanning the substrate 
surface with a laser beam and detecting light reflected and scattered from 
the surface for display on a video monitor. It was noted that light from 
the gold conductors was reflected at a relatively small angle with respect 
to a normal to the surface while light scattered from the ceramic surface 
was predominantly scattered at a high angle with respect to the surface 
normal. The patent teaches that greater contrast on the video display 
between the gold conductors and the ceramic substrates can therefore be 
obtained by subtracting the light scattered at a high angle from the light 
received at a relatively low angle to the normal. Thus, the patent teaches 
the use of two optical fiber waveguides for directing light to two 
detectors and combining subtractively the detector outputs. 
The U.S. Pat. No. of Heebner et al. 4,441,124, issued Apr. 3, 1984 (issued 
to a subsidiary of the present assignee company) teaches how the 
principles of the Jablonowski patent can be used for silicon wafer 
inspection, particularly for detecting the presence of spurious 
particulates. For this purpose, the light which is displayed on a monitor 
is that which is scattered at a high angle with respect to the wafer 
surface normal. To efficiently gather this light, a ring defined by the 
ends of light guides or optical fibers is arranged around the periphery of 
the wafer to be inspected. The presence of a particulate is then 
manifested by an increase of scattered light detected by these optical 
fibers arranged essentially coplanar with the surface of the wafer (i.e., 
at a high angle of nearly 90 degrees with respect to the wafer surface 
normal). In order to provide greater contrast for the display, light 
scattered and reflected at a relatively low angle with respect to the 
surface normal is also detected and may be subtracted from the signal 
generated by the light scattered at a high angle with respect to the 
normal. Thus, to emphasize light scattered at a high angle with respect to 
the surface normal, Heebner teaches subtracting light scattered at a low 
angle; whereas, in Jablonowski, to emphasize light scattered at a low 
angle, light scattered at a high angle is subtracted. 
In adapting the Heebner technique to the inspection of alumina substrates, 
we have determined that enhancing contrast through a subtraction of light 
reflected at a low angle is not necessary. However, we have also 
determined that there is a need to define more accurately the nature of 
any defects on the surface of alumina substrates being inspected. 
Specifically, it would be useful to be able to define with some accuracy 
the dimensions of any defect detected and its topography. 
SUMMARY OF THE INVENTION 
In accordance with the invention, these objectives are attained, in a 
method for inspecting surfaces using a scanning laser beam, by collecting 
light scattered from the surface by two arrays of optical fibers located 
nearly in the plane of the surface, one array being parallel to the 
direction of the laser scan and the other positioned perpendicularly 
thereto. Each of the two arrays is connected to a separate light detector 
for generating information that is processed separately from the 
information generated by the other array of optical fibers. 
As with the Heebner method, the signals generated are compared with 
information stored in the computer describing the scattered light that 
would be expected from a defect-free surface. By arranging the fibers 
along two separate dimensions, however, one can obtain more accurate 
information in two dimensions of any defect encountered by the scanning 
beam. For example, if the defect has a length dimension parallel to the 
direction of the beam scan, that length will be accurately detected by the 
optical fibers arranged perpendicularly with respect to the direction of 
the beam scan, while the width dimension of the defect will be accurately 
recorded by the fiber array arranged parallel to with the direction of the 
beam scan. 
Measurement in the third dimension, and even more accuracy in the other two 
dimensions, can be obtained by using four separate arrays of fibers 
surrounding the periphery of the surface to be inspected, each array being 
connected to a separate detector which is separately processed by the 
computer. Thus, for inspecting a typical substrate having four sides, one 
array is arranged along each of the four sides. This permits accurate 
detection of light scattered in any of four directions from a defect, thus 
permitting more accurate mapping by the computer of the geometry of the 
defect. Each of the four channels are of course separately processed by 
the computer with information received being compared with stored 
information, the comparison being used to generate information describing 
the nature of the defect. 
Separate processing of the information from the arrays should not be 
confused with separate processing of light scattered at a small angle and 
light scattered at a large angle with respect to the normal as in the 
Jablonowski or Heebner patents for the purpose of obtaining greater 
contrast. All of the light gathered by the fiber optic arrays of our 
invention is light scattered at a relatively large angle with respect to a 
normal of the surface. If it were desired to collect light scattered at a 
small angle with respect to the normal for the purpose of providing 
greater contrast in the manner described by Heebner and Jablonowski, that 
could also be done; but, as mentioned before, we have found that for 
alumina substrate inspection, it is not always necessary. 
Another advantage of the invention as described is that the computer can be 
easily programmed to distinguish pit defects (i.e., depressions in the 
surface) from burr defects (i.e., protuberances from the surface). As will 
become more clear from the Detailed Description, a pit that is initially 
encountered by the scanning laser beam tends first to scatter light in the 
direction of the scan, while a burr tends initially to scatter light in a 
direction opposite the direction of the scan. With appropriate stored 
information, the video display can then display the defect as a pit or 
burr in addition to displaying its dimensions. Additionally, in the 
inspection of alumina substrates, pits can be discriminated from burrs by 
comparing the Fourier frequency components of the scattered light pulses 
with stored information; as will be explained, a relatively low frequency 
content indicates pits, and a relatively high frequency content indicates 
burrs. 
These and other objects, features and advantages of the invention will be 
better understood from the following Detailed Description taken in 
conjunction with the accompanying drawing description.

Detailed Description 
Referring now to FIG. 1, there is illustrated a method for inspecting the 
flat upper surface 10 of an article 11 to determine the presence of 
defects, particulates and other departures from surface flatness. a laser 
beam 12 from a laser 13 raster scans the surface 10. Raster scanning may 
be accomplished in a known manner by using a motor 14 to drive a rotating 
mirror 15 and another motor 16 to drive a stage supporting the article 11. 
The rotating mirror from which the laser beam 12 is reflected causes the 
beam to scan the surface in an x direction as shown by arrow 17 while the 
motor 16 slowly moves or steps the article 11 in a y direction as shown by 
arrow 18. For each x direction scan by the laser beam, the article should 
be moved in the y direction a distance approximately equal to (1/e) times 
the diameter of the laser beam. The laser 13 and motors 14 and 16 are 
controlled and synchronized by a computer 20. 
As is known, defects on flat surfaces such as silicon wafers and ceramic 
substrates are best determined by selectively gathering light scattered at 
a high angle with respect to the normal to the surface. For this purpose, 
arrays of optical fibers 21 and 22 are arranged along two sides of the 
article 11 nearly coplanar with the surface 10. For most purposes, the 
angle between the optical fibers and a normal to the surface 10 should be 
greater than 80 degrees but less than 90 degrees; that is, it should be 
located just slightly above the surface 10 and preferably at nearly 90 
degrees to the surface normal. Each time the scanning laser beam 12 
encounters a defect on surface 10, light scattered at a high angle with 
respect to the surface normal is received by optical fibers 21 and 22 and 
is converted to an electrical signal by light detectors 23 and 24. These 
signals are directed to the computer 20 where they are combined with 
information giving the location of the laser beam 12 on the surface 10. 
This information is used to give an indication of the location of the 
defect which may be displayed on a video monitor if desired. 
In accordance with one embodiment of the invention, the outputs of 
detectors 23 and 24 are separately processed by computer 20 to give a more 
dependable description in two dimensions of the defect. That is, the 
outputs of the detectors 23 and 24 are separately compared with stored 
information in the computer that discriminates between light scattered 
from a defect and that scattered from a surface that is flat within 
predetermined limits, and because the optical fibers are arranged in a 
specific direction, they can give better directionoriented information. 
This can be appreciated from a consideration of FIG. 2, which shows 
illustratively a burr defect 25 having a length in the x direction and 
shorter width in the y direction. It can be appreciated that when the 
laser beam scans the surface in the x direction at the position 26, it 
scatters light predominantly toward optical fibers 21 that are connected 
to detector 23 of FIG. 1. Thus, it is quite convenient to compare the 
output of detector 23 with stored information in the computer concerning 
the scattered light that would indicate a defect and use this comparison 
information to indicate the length in the x direction of the defect 25. 
Likewise, when the scanning laser beam is at position 27, light is 
scattered predominantly toward optical fibers 22, which are connected to 
detector 24 of FIG. 1. The computer is therefore programmed to compare the 
output of detector 24 with stored information so as to give information 
concerning the dimension in the y direction of any defect encountered. 
Experiment has shown that much more accurate information can be gathered 
and displayed in this manner than having all of the optical fibers 
connected to a single detector as is true in the Heebner patent. 
While the simplest embodiment of the invention makes use of two arrays of 
optical fibers, more accurate determinations and mapping of 
threedimensional characteristics of defects can be made by using four 
separate arrays surrounding the flat surface to be inspected. FIG. 3 is a 
simplified schematic drawing of such a system that has been built and 
demonstrated by us particularly for the purpose of inspecting the flat 
upper surfaces of alumina ceramic substrates. A ceramic substrate 30 to be 
inspected is surrounded by four separate arrays of optical fibers 31, 32, 
33 and 34. While, for purposes of clarity, only one optical fiber of each 
of the arrays 32 and 34 is shown, it is to be understood that each array 
is coextensive with one side of the substrate 30. The surface of the 
ceramic substrate is scanned by a laser beam generated by a 16 milliwatt 
helium-neon laser 36. The laser beam is directed through a beam expander 
37 to a rotating polygon mirror 38, and then through a telecentric 
scanning lens 39 to the substrate surface. The rotating polygon mirror 38 
gives raster scanning in the x direction while a motorized stage 41 
supporting the ceramic substrate gives continuous movement in the y 
direction. The (1/e) diameter of the impinging beam is 25 microns, and the 
stage advances 25 microns during each raster scan. Arranged on a rack as 
shown are a computer 42, power supplies 43, electronic circuitry 44 and 
stage controller 45. The computer is connected to a keyboard and monitor 
47 on which a visual display of defect locations and geometries can be 
made. Each of the optical fiber arrays is connected by an optical fiber 
bundle 48 to a photomultiplier tube 49. Each of the photomultiplier tubes 
independently generates a signal dependent on light gathered by one of the 
fiber optic arrays and transmits the signal to the computer 42 
(interconnection not shown). 
One can appreciate from FIG. 2 the advantage of having four rather than 
only two arrays of optical fibers; light may be scatteredby scattered by 
burr 25 in the y direction away from fibers 21, and it may be scattered in 
the x direction opposite fibers 22. Fibers for collecting such light will 
contribute to output accuracy. 
Another advantage of having four arrays of optical fibers can be 
appreciated from FIGS. 4 and 5. FIG. 4 illustrates that when a scanning 
laser beam first encounters a pit, light is initially scattered in the 
direction of scan. Conversely, FIG. 5 illustrates that when a scanning 
laser beam first encounters a burr, it scatters light predominantly in a 
direction opposite that of the laser beam scan. Thus, the computer can be 
programmed with appropriate statistical information so that a distinction 
can be made between pits and burrs. The appropriate comparisons are of 
course possible because each of the four arrays of optical fibers are 
connected to a separate detector, the output of which is separately 
processed by the computer. 
We have found that a further distinction between pits and burrs on alumina 
substrates can be made by assessing the Fourier components of the light 
pulse created by the defect. A burr defect is characteristically sharper 
in geometry than a pit; that is, it characteristically has a steeper slope 
with respect to the surface. As a consequence, the light pulse created by 
a burr has steeper slopes and therefore a higher proportion of high 
frequency components than a pit. Thus, frequency analysis can easily be 
employed by the apparatus to indicate the nature of the defect, with a 
high proportion of high frequency components being designated as a burr 
and high proportion of low frequency components being designated as a pit. 
An electronic block diagram of the apparatus of FIG. 3 is illustrated in 
FIG. 6. Each of the photomultiplier tubes 49 of FIG. 3 corresponds to a 
separate photomultiplier tube (PMT) 50 with a filter. The filter is tuned 
to the frequency of the helium-neon laser and filters out other ambient 
light. Each of the PMTs 50 is connected to a signal conditioning circuit 
51, the purpose of which is to normalize and amplify the electrical 
signals. The outputs of the signal conditioning circuits are directed to 
threshold detectors 52. Each of the defects to be detected results in a 
burst or pulse of scattered light detected by the appropriately placed 
optical fibers. The detectors 52 define a threshold by information stored 
in the computer and record a response only to those pulses that have a 
height and a width that exceeds the stored threshold. The defect pulse is 
then converted to a digital representation of its height and width, and 
thus can be processed by the computer apparatus. 
A mirror-driver power supply encoder 53 is an encoder mounted on a rotating 
shaft of the mirror 38, and it generates a signal indicative of the 
position of the scanning laser beam on the substrate surface. Likewise, a 
stage controller linear-encoder 54 generates a signal describing the 
position in the y direction of the stage supporting the substrate. These 
signals are combined in a phase-locked loop 56 which in turn transmits 
information to control circuitry 57. Control circuitry 57 may be 
programmed to screen out all signals originating from certain locations on 
the substrate surface, which is useful if only certain "window" portions 
of the substrate surface are to be inspected. This screening is done by 
the control circuitry 57 based on information stored in a dual-port memory 
58. The control circuitry 54 and the analog to digital conversion circuits 
52 are connected to the dual-port memory 58 which accommodates a very high 
speed of input and interface with a computer 59. The dual-port memory 58 
provides information to computer 59 at an appropriate speed for its 
operation. 
A laser detector 61, together with control circuitry 62, monitors the power 
output of laser 36. A sample sensor 63 and control circuitry 64 detect the 
presence of the substrate 30 in the support carriage. Prior to inspection, 
an air jet (not shown) is used to clear the surface of the substrate of 
dust particles. An air pressure sensor 65 and control circuitry 66 
monitors the air pressure constituting the air jet. A door sensor 67 and 
control circuitry 68 are used for safety purposes to prevent operation if 
an operator's hand is near the system. 
Interface circuits 70 couple these sensor circuits to the computer 59. The 
computer 59 is connected to a color monitor 72 for giving a visual 
indication of imperfections and to a keyboard 73 for providing input 
information. 
It is often desirable to use the invention for inspecting ceramic 
substrates upon which circuit patterns have already been formed. High 
density circuit patterns typically used in modern hybrid integrated 
circuits have conductors that are sufficiently close together to diffract 
the light of the scanning laser beam. We have found that this diffracted 
light is best separated from the desired scattered light by arranging the 
substrate such that the laser beam scans the regular pattern at about a 45 
degree angle. We have found that under that condition the angle with 
respect to the normal of diffracted light is a minimum, and therefore 
inherently separated from the desired scattered light which is always at a 
high angle with respect to the surface normal. 
This is illustrated in FIG. 7 in which conductors 75 located on the surface 
of a ceramic substrate 76 are arranged to be at a 45 degree angle with 
respect to the direction of scan of the laser beam 77. Under this 
condition, the light 79 diffracted by the conductive pattern will be in a 
direction perpendicular to the axis of the conductor pattern; whereas, 
light 80, which is received in accordance with the invention, is scattered 
at a other angles with respect to the direction of the diffracting 
surface. 
While the invention has been described as a method for inspecting the 
surfaces of ceramic substrates, it is apparent that the principles of the 
invention may be applied to the inspection of any flat surface, including 
metal surfaces. Particularly, it may be useful for inspecting flat 
surfaces of photolithographic masks, which must normally be free of minor 
surface imperfections. The details of the specific embodiment described 
for inspecting alumina substrates should not be construed as being 
essential and should not limit the invention. Various other embodiments 
and modifications of the invention may be devised without departing from 
the spirit and scope of the invention.