Method and apparatus for detecting particles on a surface of a semiconductor wafer having repetitive patterns

An apparatus for detecting particles on the front surface of a patterned semiconductor wafer having repetitive patterns includes a laser for illuminating an area on the front surface at grazing angle of incidence with a beam of polarized light. A lens collects light scattered from the area and forms a Fourier diffraction pattern of the area illuminated. A Fourier mask blocks out light collected by the lens at locations in the Fourier diffraction pattern where the intensity is above a predetermined level indicative of background information and leaves in light at locations where the intensity is below the threshold level indicative of possible particle information. The Fourier mask includes an optically addressable spatial light modulator and a crossed polarizer with the Fourier diffraction pattern being used as both a read beam and a write beam for the spatial light modulator. A camera detects scattered light collected from the area by the lens and not blocked out by the Fourier mask.

BACKGROUND OF THE INVENTION 
The present invention relates generally to a method and apparatus for 
detecting the presence of particles on the surface of an object and more 
particularly to a method and apparatus for detecting contaminant particles 
on the surface of a patterned semiconductor wafer having repetitive 
patterns using the principle of light scatttering. An example of a 
patterned semiconductor wafer having repetitive patterns is a memory 
wafer. 
There are a variety of existing ways for detecting and measuring the number 
and sizes of particles on the surface of a semiconductor wafer for the 
purpose of rejecting those wafers which have on their surface one or more 
particles above certain sizes or an excessive number of particles. One of 
the more simple methods involves having a human operator inspect the wafer 
using a light field/dark field microscope. Using the eye, the operator 
actually counts the number of particles and also identifies the size of 
the particles, such as those between 1 and 20 microns, and then rejects 
those wafers which have particles of or above a certain size or which have 
an excessive number of particles. This method, however, is highly 
inaccurate and very expensive both in terms of wages for the human 
operator and in terms of the number of rejects both after the inspection 
and after production of the chips (when an erroneously passed wafer is 
found to have an electrical defect, e.g., show circuits, because of the 
presence of contaminant particles). 
In U.S. Pat. No. 5,317,380, issued May 31, 1994, and assigned to Inspex, 
Inc. there is disclosed a method and apparatus for detecting particles on 
a surface of an object, such as a virgin or patterned semiconductor wafer, 
ceramic tile, or the like. In one embodiment, an apparatus is provided in 
which a scanning beam of laser light is brought to focus as an arcuate 
scan line on a surface of the object at a grazing angle of incidence using 
an off-axis hypertelecentric mirror. A pair of light detectors are 
positioned at a meridional angle of about 30 degrees and at an azimuthal 
angle of about 4 degrees to measure forward scattered light from the 
surface. The object is then moved translationally so that the beam can 
scan another line of the surface. A light trap is provided to trap light 
that is reflected by the surface, and a series of masks are provided to 
mask light which is scattered by the hypertelecentric mirror and in the 
case of pattered objects, light which is diffracted by the pattern 
imprinted on the object. 
In U.S. Pat. No. 4,898,471, issued Feb. 6, 1990, and assigned to Tencor 
Instruments, a system for detecting particles and other defects on a 
patterned semiconductor wafer, photomask, or the like is disclosed. The 
system includes a light source for emitting a beam of light. A polarizing 
filter is used to polarize the beam of light in a direction substantially 
parallel to the surface of the patterned semiconductor wafer to be 
examined. The beam is enlarged in cross-sectional diameter by a beam 
expander placed along the path of the beam after the polarizing filter. 
The beam is then caused to scan by a deflection mirror. A telecentric lens 
brings the scanning beam to focus on the patterned wafer at a shallow 
angle of incidence, the beam striking the wafer surface substantially 
parallel to the pattern streets formed on the wafer. A light collection 
system for detecting side scattered light is positioned in the plane of 
the scan line. The light collection system, which includes a lens for 
focusing the side scattered light, a polarizing filter oriented in a 
direction substantially parallel to the surface of the patterned wafer, 
and a photomultiplier tube for detecting light incident thereon and 
transmitting electrical signals in response thereto, receives light 
scattered in a direction less than 15 degrees above the surface and at 
angel relative to the beam direction in a range from about 80 degrees to 
100 degrees. A processor constructs templates from the electrical signal 
corresponding to individual patterns and compares the templates to 
identify particles. 
In U.S. Pat. No. 4,806,744 issued Feb. 21, 1989 and assigned to Insystems, 
Inc., there is disclosed an inspection system which employs a Fourier 
transform lens and an inverse Fourier transform lens positioned along an 
optic axis to produce from an illuminated area of a patterned specimen 
wafer a spatial frequency spectrum whose frequency components can be 
selectively filtered to produce an image pattern of defects in the 
illuminated area of the wafer. Depending on the optical components 
configuration of the inspection system, the filtering can be accomplished 
by a spatial filter of either the transmissive or reflective type. The 
lenses collect light diffracted by a wafer die aligned with the optic axis 
and light diffracted by other wafer dies proximately located to such die. 
The inspection system is useful for inspecting only dies having many 
redundant circuit patterns. The filtered image strikes the surface of a 
two-dimensional photodetector array which detects the presence of light 
corresponding to defects in only the illuminated on-axis wafer die. 
Inspection of all possible defects in the portions of the wafer surface 
having many redundant circuit patterns is accomplished by mounting the 
wafer onto a two-dimensional translation stage and moving the stage so 
that the illuminated area continuously scans across the wafer surface from 
die to die until the desired portions of the wafer surface have been 
illuminated. The use of a time delay integration technique permits 
continuous stage movement and inspection of the wafer surface in a raster 
scan fashion. 
In U.S. Pat. No. 4,895,446 to M. C. Maldari et al., there is disclosed a 
method and apparatus for detecting the presence of particles on the 
surface of an object such as the front side of a patterned semiconductor 
wafer. A vertically expanded, horizontally scanning, beam of light is 
directed onto an area on the surface of the object at grazing angle of 
incidence. A video camera positioned above the surface detects light 
scattered from any particles which may be present on the surface, but not 
specularly reflected light. The surface is angularly repositioned 
(rotated) relative to the incident light beam so that the diffracted light 
form the surface and the pattern of lines on the surface is at a minimum. 
The object is then moved translationally to expose another area to the 
incident light beam so that the entire surface of the object or selected 
portions thereof can be examined, one area at a time. The patent also 
discloses the use of a mark containing a pattern corresponding to the 
Fourier transform of the patterned surface to mask off light scattered 
from the pattern on the surface but not any particles that may be present 
on the surface. 
In U.S. Pat. No. 4,377,340 to G. P. Green et al., there is disclosed a 
method and apparatus for detecting and measuring the number and sizes of 
impurities on the surface of a material, such as a semiconductor wafer, 
wherein a beam of high intensity collimated light from a xenon arc lamp is 
directed onto the surface at normal incidence in the absence of any 
extraneous light, through a collimating mirror and a pin hole device and 
where at the particles will scatter the light, and wherein the surface is 
viewed by a high light sensitive TV camera which is positioned off-axis to 
pick up scattered light but not specularly reflected light for display on 
a viewing screen. 
In U.S. Pat. No. 4,342,515 to M. Akiba et al., there is disclosed an 
inspection apparatus for detecting unfavorable foreign matters existent on 
the surface of an object such as a semiconductor wafer. The apparatus 
includes a collimated beam generator portion which projects a collimated 
beam towards the object to be inspected from a side thereof and a 
mechanism which senses light reflected from the surface of the object, 
through a polarizer plate. In accordance with the disclosed technique for 
using the apparatus, the signal-to-noise ratio between a detection signal 
generated by a pattern of the foreign matter to be detected and a signal 
generated by a normal pattern of the object surface and sensed as a noise 
component are said to be enhanced. 
In U.S. Pat. No. 3,782,836 to D. F. Fey et al., there is disclosed a 
surface irregularity analyzing system which includes structure for 
directing light toward a surface in a direction having a certain angular 
relationship to the surface. If the light strikes irregularities in the 
surface it is reflected in a direction having an angular relationship to 
the surface other than equal and opposite the incident direction. The 
amount of light reflected from irregularities in the surface is 
determined, either photographically or photoelectrically using a detector 
positioned over the surface, to provide an analysis of irregularities in 
the surface. 
In U.S. Pat. No. 2,947,212 to R. C. Woods, there is disclosed a method of 
detecting surface conditions on a strip of sheet metal having line 
markings in which light from a light source is directed towards the 
surface of the sheet metal in a direction generally perpendicular to the 
line markings. Non-specular reflection in a selected direction which is 
perpendicular to the lines, and which is preferably between the angle of 
incidence and the angle of specular reflection, is monitored by a 
photoelectric cell which is able to detect a surface flaw by variation in 
the intensity of the reflected light. The light in the incident beam may 
be polarized and the light in the selected non-specular reflected beam 
filtered to pass only such polarized light. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a new and improved 
method and apparatus for detecting the presence of contaminant particles 
on the surface of a patterned semiconductor wafer having repetitive 
patterns using the principle of scattered light. 
It is another object of the present invention to provide a method and 
apparatus as described above in which background scatter is filtered out 
in a new and novel manner. 
It is a further object of the present invention to provide a method and 
apparatus as described above which is designed for use in dark field and 
bright field illumination applications. 
Other objects, as well as features and advantages of the present invention, 
will be set forth in part in the description which follows, and in part 
will be obvious from the description or may be learned by practice of the 
invention. The objects, features, and advantages of the invention may be 
realized and attained by means of the instrumentalities and combinations 
particularly pointed out in the appended claims. 
Apparatus for detecting particles on the front surface of a patterned 
semiconductor wafer having repetitive patterns according to this invention 
comprises means for illuminating an area on said front surface with a beam 
of polarized light, optical means for collecting light scattered from the 
area illuminated, the optical means forming a Fourier diffraction pattern 
of area illuminated, self-programmable Fourier mask means for blocking out 
from the light collected by the optical means areas in the Fourier 
diffraction pattern whose intensity is above a predetermined level 
indicative of background information and letting pass through areas in the 
Fourier diffraction pattern whose intensity is below the threshold level 
indicative of possible particle information, the Fourier mask means 
including an optically addressable spatial light modulator and a crossed 
polarizer, and a camera for detecting light scattered from area collected 
by the optical means and not blocked out by the Fourier mask means. 
A method for detecting particles on the front surface of a patterned 
semiconductor wafer having repetitive pattern according to this invention 
comprises illuminating an area on the front surface with a beam of 
polarized light, collecting light scattered from the area illuminated and 
forming a Fourier diffraction pattern of the light scattered from the area 
illuminated, blocking out from the light collected areas in the Fourier 
diffraction pattern whose intensity is above a predetermined level 
indicative of background information and letting pass through areas in the 
Fourier diffraction pattern whose intensity is below the threshold level 
indicative of possible particle information, the blocking being achieved 
using an optically addressable spatial light modulator which rotates the 
polarization of the Fourier diffraction pattern at locations where the 
intensity is below a predetermined threshold level and a crossed 
polarizer, and detecting scattered light collected from the area 
illuminated and not blocked out.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The present invention is directed to a method and apparatus for detecting 
the presence of contaminant particles on the surface of a patterned 
semiconductor wafer having repetitive patterns using the principle of 
scattered light. 
In accordance with the invention an area on the surface to be examined is 
illuminated with a beam of polarized light. A lens collects light 
scattered from the area illuminated and forms a Fourier diffraction 
pattern of the scattered light. A Fourier mask blocks out light in areas 
in the Fourier diffraction pattern above a predetermined intensity level 
indicative of background information on the wafer and does not block out 
light in areas which is below the predetermined intensity level indicative 
of possible particles. The unblocked light is then detected by a camera. 
The procedure is repeated for other areas on the surface. 
The Fourier mask comprises an optically addressable spatial light modulator 
and a crossed polarizer. 
Referring now to the drawings there is illustrated in FIG. 1 an apparatus 
11 for use in detecting the presence of particles on the front surface 12 
surface of a patterned semiconductor wafer 13 having repetitive patterns. 
Apparatus 11 includes a holder 15 for holding wafer 13. Holder 13 is 
mounted on a stage 17 which is movable in two mutually perpendicular 
directions by a pair of motors 19 and 21, the particular details of the 
mechanical arrangement for moving stage 17 not being a part of this 
invention. 
Apparatus 11 also includes a light source 21, a self-programmable Fourier 
mask 23, a first light detector 25 and a second light detector 27. 
Light source 21 generates a high intensity, plane polarized, coherent, 
monochromatic beam of light and may be, for example, a ND:YAG laser or a 
helium-neon laser. 
Self-programmable Fourier mask 23 which is shown in detail in FIG. 2 
includes a liquid crystal spatial light modulator 29 and a crossed 
polarizer 31. Liquid crystal spatial light modulator 29 which is shown in 
detail in FIG. 2 includes a back layer of glass 33, a first transparent 
electrode 35 in front of back layer 33, a photoconductor 37, such as 
amorphous silicon, in front of first transparent electrode 35, a 
dielectric mirror 39 in front of photoconductor 37, a liquid crystal 41 in 
front of dielectric mirror 39, a second transparent electrode 43 in front 
of liquid crystal 41 and a front layer of glass 45 in front of second 
transparent electrode 43. Dielectric mirror 39 includes a front side 47 
and a rear side 49. Liquid crystal spatial light modulator 29 is 
preferably a ferromagnetic liquid crystal type spatial light modulator. 
The thicknesses of layers 33, 37, 39, 41 and 45 may be as follows: layers 
33 about 5 mm, layer 37 about 3 microns, layer 39 about 1 micron, layer 41 
about 1 micron and layer 45 about 5 mm. For simplicity, layers 37, 39 and 
41 on spatial light modulator 29 are shown in FIG. 1 as a single line 50. 
Electrodes 35 and 43 are on the order of about 1 micron thick. 
First light detector 25 is a high sensitivity video camera and second light 
detector 27 may be a standard video camera. Each camera may be, for 
example, CCD type cameras. 
Apparatus 11 also includes a first lens 51, a second lens 53, a third lens 
55, a fourth lens 57 and a fifth lens 59. 
In the operation of apparatus 11, light from source 21 is directed onto 
front surface 12 of semiconductor wafer 13. Light source 21 is arranged so 
as to strike surface 12 at a grazing angle of incidence i.e. at an angle 
of between around 0 and 5 degrees. Light scattered upward from the area on 
surface 12 which is illuminated by the beam of light from source 21 is 
imaged by first lens 51 in combination with second lens 53 at intermediate 
image plane 61. As can be appreciated, the image formed at intermediate 
image plane 61 includes light scattered from any particles which may be 
present on the area of the surface illuminated and, in addition, light 
scattered from the pattern lines of the pattern on the area of the surface 
illuminated. A Fourier diffraction pattern of the scattered light is 
formed at the back focal plane 63 of lens 51. 
The image of area of surface 12 illuminated by light source 21 which is 
formed at intermediate image plane 61 passes through a beamsplitter 64 and 
is reimaged by third lens 55 at first light detector 25 after being 
reflected off mirror 39 and being reflected off beamsplitter 64. In 
Fourier mask 23 light whose intensity is above a predetermined intensity 
level and corresponding to pattern lines on the surface of wafer 13 is 
blocked out. 
Second lens 53 in combination with third lens 55 forms an image of the 
Fourier diffraction pattern from plane 63 on liquid crystal 41 in spatial 
light modulator 29 after passing through beamsplitter 64 and from there is 
reflected off mirror 39. A portion of the light passed by lens 53 
corresponding to the Fourier diffraction pattern is reflected off of a 
beamsplitter 65 located between lens 53 and lens 55, then reflected off of 
a pair of mirrors 67 and 69 and then in combination with fourth lens 57 
enters spatial light modulator 29 from the rear and is brought to focus at 
photoconductor 37. Thus, lens 53 and lens 55 are used to image the Fourier 
diffraction formed at plane 63 on liquid crystal 41 while lens 53 and 57 
along with beamsplitter 65 and mirrors 67 and 69 are used to image the 
Fourier diffraction pattern formed at plane 63 onto photoconductor 37. The 
beam of light striking mirror 47 from liquid crystal 41 constitutes a 
"read" beam while the beam of light striking photoconductor 37 constitutes 
a "write" beam. The write beam and read beam are axially aligned on their 
corresponding sides of mirror 39 and are of the same size (magnitude). 
Thus there is point to point correspondence of the write and read beams at 
spatial light modulator 39. 
In those areas (locations) where the intensity of the write beam is below a 
predetermined threshold level indicative of possible particle information, 
the polarization of the corresponding areas on the read beam on reflection 
from mirror 39 will be rotated 90 degrees. On the other hand, in those 
areas where the write beam is above the preselected threshold level 
indicative of background information, the polarization of corresponding 
areas on the read beam on reflection from mirror 39 will remain the same; 
i.e. will not be rotated. 
Light reflected from mirror 39 is deflected off beamsplitter 64 and strikes 
crossed polarizer 31 which allows areas on the beam where the polarization 
has been rotated 90 degrees to pass through and blocks areas on the beam 
where the polarization has not been rotated. Light passed through 
polarizer 31 is then brought to focus on detector 25. 
Thus, detector 25 records an image of light scattered from surface 12 whose 
intensity is below the predetermined threshold level and caused by 
particles and not scattered light whose intensity is above the threshold 
level and caused by pattern lines. 
Apparatus 11 also includes a beamsplitting mirror 73 which is movable up 
and down as shown by arrows A. When mirror 73 is in the position shown, 
the Fourier diffraction pattern after it is processed by Fourier mask 23 
will be imaged by lens 59 onto detector 27 while the scattered light from 
surface 12 will be imaged onto detector 25 after it is processed by 
spatial light modulator 23. On the other hand, when mirror 73 is moved out 
the light path, as shown by the dotted lines only an image of the portion 
of wafer 11 illuminated, without the pattern lines, will be formed on 
camera 25 and no image will be formed on detector 27. 
Wafer 13 is then moved translationally so that other areas on surface 12 
may be examined, in a similar manner, one at a time. 
As can be appreciated, mask 23 is self-programmable in that it is not 
limited to use with a wafer having a particular pattern of lines but 
rather can be used with any wafer having patterns that are repetitive, 
even if the diffraction pattern changes from area to area on the wafer. 
A ray trace of the light path from wafer 13 to detector 25 is shown in FIG. 
3. Also shown in FIG. 3 is the spacing of some of the components. 
Referring now to FIG. 4 there is shown another embodiment of an apparatus 
constructed according to this invention, the apparatus being identified by 
reference numeral 81. 
Apparatus 81 differs from apparatus 11 in that a portion of the Fourier 
diffraction pattern formed in the Fourier plane is not used directly as 
the write beam, as in apparatus 11, but, rather, is converted into 
electrical signals which are processed and then converted back to a video 
image which is then used as the write beam. 
Apparatus 81 includes a light source 83, a first light detector 85, a 
second light detector 87, a first lens 89, a second lens 91, a third lens 
93, a fourth lens 95, a fifth lens 97, a first beamsplitter 99, a second 
beamsplitter 101 and a third beamsplitter 103 corresponding, respectively 
to light source 21, light detector 25, light detector 27, first lens 51, 
second lens 53, third lens 55, fourth lens 57, fifth lens 59 beamsplitters 
65 and 71 and movable mirror 73, respectively, in apparatus 11. 
Apparatus 81 also includes a self-programmable Fourier mask 23. 
However, instead of mirrors 67 and 69, apparatus 81 includes a sixth lens 
105, a third light detector 107, a processor 109, a CRT 111 and a seventh 
lens 113. Detector 107 is identical in construction to detector 87. 
Lens 105 in combination with lens 91 images the Fourier diffraction pattern 
formed in Fourier plane 115 of lens 89 into light detector 107 where the 
image is converted into a stream of digital electrical signals. The stream 
of digital electrical signals are processed in processor 109, as maybe 
desired. The processing may include raising the overall gain of the image 
or blocking out selected areas. The output of processor 109 is fed into 
CRT 111 which converts the digital electrical signals into a video image. 
The video image from CRT 111 is collected by lens 113 and then reimaged by 
lens 95 through glass 33 onto photoconductor 37 in spatial light modulator 
29. At the same time, lens 91 in combination with lens 93 images the 
Fourier diffraction pattern through glass layer 45, transparent electrode 
43 and liquid crystal 41 onto mirror 39. Lens 89 in combination with lens 
91 forms an image of the area illuminated by light source 83 at image 
plane 117. The image formed at image plane 117 is then collected by lens 
93 and passed through Fourier mask 85, using beamsplitter 101. The 
filtered image is then brought to focus at detector 85. The diffraction 
pattern at Fourier plane 115 is imaged at detector 87 after it passes 
through mask 23. 
A ray trace of light from wafer 13 to camera 91 is shown in FIG. 5. 
The embodiments of the present invention recited herein are intended to be 
merely exemplary and those skilled in the art will be able to make 
numerous variations and modifications to it without departing from the 
spirit of the present invention. For example, an array of photodiodes can 
be used in place of any or each one of the cameras. Also, cameras 25 and 
85 could be replaced by non-imaging detectors, such as a photomultiplier 
tube if an image of the area illuminated is not desired. All such 
variations and modifications are intended to be within the scope of the 
present invention as defined by the claims appended hereto.