|Publication number||US20050110986 A1|
|Application number||US 11/020,603|
|Publication date||May 26, 2005|
|Filing date||Dec 21, 2004|
|Priority date||Dec 8, 1994|
|Also published as||US7084967, US20040057044, US20050036137|
|Publication number||020603, 11020603, US 2005/0110986 A1, US 2005/110986 A1, US 20050110986 A1, US 20050110986A1, US 2005110986 A1, US 2005110986A1, US-A1-20050110986, US-A1-2005110986, US2005/0110986A1, US2005/110986A1, US20050110986 A1, US20050110986A1, US2005110986 A1, US2005110986A1|
|Inventors||Mehrdad Nikoonahad, Stanley Stokowski, Keith Wells, Brian Leslie|
|Original Assignee||Mehrdad Nikoonahad, Stokowski Stanley E., Wells Keith B., Leslie Brian C.|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (93), Referenced by (14), Classifications (10)|
|External Links: USPTO, USPTO Assignment, Espacenet|
This application is a continuation of U.S. patent application Ser. No. 09/954,287, filed Sep. 11, 2001, which is a continuation of U.S. patent application Ser. No. 09/760,558, filed Jan. 16, 2001, now U.S. Pat. No. 6,636,302, which is a continuation of U.S. patent application Ser. No. 09/213,022, filed Dec. 16, 1998, now U.S. Pat. No. 6,215,551, which is a continuation of U.S. patent application Ser. No. 08/499,995, filed Jul. 10, 1995, now U.S. Pat. No. 5,883,710, which is a continuation-in-part application of parent application entitled “Optical Scanning System for Surface Inspection,” by Mehrdad Nikoonahad, Keith B D. Wells and Brian C. Leslie, Ser. No. 08/351,664, filed Dec. 8, 1994, now abandoned. This application is also related to the patent application entitled “Optical Wafer Positioning System,” by Mehrdad Nikoonahad, Philip R. Rigg, Keith B. Wells and David S. Calhoun, Ser. No. 08/361,131, filed Dec. 21, 1994 (“Related Application”), which has since issued as U.S. Pat. No. 5,530,550. Both prior applications are incorporated by reference herein in their entirety.
This invention relates in general to surface inspection systems, and in particular, to a high speed scanner system for inspecting anamolies on surfaces such as semiconductor wafers, photomasks, reticles, ceramic tiles, and other surfaces.
The size of semiconductor devices fabricated on silicon wafers has been continually reduced. At the time this application is filed, for example, semiconductor devices can be fabricated at a resolution of a half micron or less and sixty-four (64) megabyte DRAMs are being fabricated with 0.35 micron design rule. The shrinking of semiconductor devices to smaller and smaller sizes has imposed a much more stringent requirement for sensitivity of wafer inspection instruments which are called upon to detect contaminant particles and pattern defects that are small compared to the size of the semiconductor devices. On the other hand, it is desirable for wafer inspection systems to provide an adequate throughput so that these systems can be used in production runs to detect defective wafers.
In U.S. Pat. No. 4,898,471 to Stonestrom et al. assigned to the present assignee to the present application, the area illuminated on a wafer surface by a scanning beam is an ellipse which moves in the scan direction. In one example given by Stonestrom et al., the ellipse has a width of 20 microns and a length of 115 microns. Light scattered by anomalies or patterns in such illuminated area is detected by photodetectors placed at azimuthal angles in the range of 80 to 100° The signals detected by the photodetectors are used to construct templates. When the elliptical spot is moved in the scan direction to a neighboring position, scattered light from structures within the spot is again detected and the photodetector signal is then compared to the template to ascertain the presence of contaminant particles or pattern defects as opposed to regular pattern. In Stonestrom et al., the scanning beam scans across the entire wafer to illuminate and inspect a long narrow strip of the wafer extending across the entire dimension of the wafer in the scanning direction. The wafer is then moved by a mechanical stage in a direction perpendicular to the scanning direction for scanning a neighboring elongated strip. This operation is then repeated until the entire wafer is covered.
While the system of Stonestrom et al. performs well for inspecting wafers having semiconductor devices that are fabricated with coarser resolution, with the continual shrinking of the size of the devices fabricated, it is now desirable to provide an improved inspection tool that can be used to detect very small size anomalies that may be difficult to detect using Stonestrom et al.'s system.
This invention is based on the recognition that very small anamolies can be detected by reducing the size of the area that is illuminated by the scanning light beam. Light scattered from structures in the spot will include background, such as light scattered by pattern on the surface, as well as light that is scattered by anomalies such as contaminant particles, pattern defects or imperfections of the surface. Such background can have a significant amplitude. For this reason, if the anamoly is of a size which is small compared to the size of the illuminated area, the scattered light from such anamoly may be overwhelmed by and become undetectable from the background. By reducing the size of the illuminated area or spot size, the ratio of the light intensity scattered by an anomaly to that of the background will be increased, thereby increasing detection sensitivity. However, with a smaller spot size, it will be more difficult to maintain the uniformity of the spot along a long straight scan line across the entire wafer. By breaking up the scan path into short segments, it is possible to employ a smaller spot size while at the same time maintaining uniformity of the spot along the path. From the system point of view, by reducing the length of the scan, the size of the collection optics for detecting forward scattered light becomes more manageable.
Thus one aspect of the invention is directed towards a method for detecting anamolies on a surface, comprising the steps of directing a beam of light at a grazing angle towards the surface, causing relative motion between the beam and the surface so that the beam scans a scan path covering substantially the entire surface; and collecting light scattered along said path for detecting anamolies. The scan path includes a plurality of arrays of scan path segments, wherein each of at least some of such scan path segments has a span shorter than the dimensions of the surface.
As used in this application, “minimum width” of the illuminated area or spot on the surface to be inspected is defined as the minimum dimension of a boundary around the area or spot along any direction on the surface, where the boundary is defined as the locations on the surface where the illumination light intensity is a predetermined fraction or percentage of the maximum intensity of illumination in the area or spot. In the description of the preferred embodiment, for example, the boundary is where the light illumination intensity is 1/e2 of the maximum intensity of illumination in the area or spot, e being the natural number. The minimum dimension is the minimum distance between two parallel lines that enclose between them the boundary of the area or spot. The term “minimum width” is explained in more detail below.
Another consideration of the invention is to provide an adequate throughput while data is collected at a moderate rate for defect detection so that the data collection and processing system employed need not be overly complex and expensive.
Thus another aspect of the invention is directed towards a method for detecting anamolies on the surface of a semiconductor wafer, comprising directing a beam of light towards a surface to illuminate an area of the surface defining a spot having a spot size whose minimum width is in the range of about 5 to 15 microns, causing relative motion between the beam and the wafer so that the beam scans a path covering the entire surface; and collecting light scattered along said path for detecting anamolies. The spot size and the directing and causing steps are such that the beam scanning substantially inspects the entire surface of the wafer at a throughput in excess of about 40 wafers per hour for 150 millimeter diameter wafers, at a throughput in excess of about 20 wafers per hour for 200 millimeter diameter wafers, and at a throughput in excess of about 10 wafers per hour for 300 millimeter diameter wafers.
Yet another aspect of the invention is directed towards a method for detecting anamolies on a surface, comprising the steps of directing a beam of light towards the surface to illuminate an area of the surface defining a spot having a spot size whose minimum width is in the range of about 5 to 15 microns, causing relative motion between the beam and the surface so that the beam scans a path covering substantially the entire surface; and collecting light scattered along said path for detecting anamolies. The spot size and said directing and causing steps are such that the surface is inspected at a speed not less than about 1.5 cm2/s.
Still another aspect of the invention is directed towards a method for detecting anamolies on a surface, comprising the steps of directing a beam of light towards said surface to illuminate an area of the surface defining a spot having a spot size whose minimum width is in the range of about 5 to 15 microns, causing relative motion between the beam and the surface so that the beam scans a path covering substantially the entire surface; and collecting light scattered along said path for detecting anamolies. The surface has dimensions of not less than 200 millimeters in any direction along the surface. The directing and causing steps are such that the beam scans substantially the entire surface in about 50 to 90 seconds.
Another aspect of the invention is directed towards a system for detecting anamolies on a surface, comprising means for directing a beam of light at a grazing angle toward said surface; means for causing relative motion between the beam and the surface so that the beam scans a scan path covering substantially the entire surface; and means for collecting light scattered along said path for detecting anamolies. The scan path includes a plurality of arrays of scan path segments, wherein each of at least some of such scan path segments has a span shorter than the dimensions of the surface.
One more aspect of the invention is directed towards a system for detecting anamolies on a surface of a semiconductor wafer, comprising means for directing a beam of light towards said surface to illuminate an area of the surface defining a spot having a spot size whose minimum width is in the range of about 5 to 15 microns; means for causing relative motion between the beam and the wafer so that the beam scans a path covering substantially the entire surface; and means for collecting light scattered along said path for detecting anamolies. The spot size and said directing and causing means are such that the beam scanning substantially inspects the entire surface of the wafer at a throughput in excess of about 40 wafers per hour for 150 millimeter diameter wafers, at a throughput in excess of about 20 wafers per hour for 200 millimeter diameter wafers, and at a throughput in excess of about 10 wafers per hour for 300 millimeter diameter wafers.
Yet another aspect of the invention is directed towards a system for detecting anamolies on a surface, comprising means for directing a beam of light toward said surface to illuminate an area of the surface defining a spot having a spot size whose minimum width is in the range of about 5 to 15 microns; means for causing relative motion between the beam and the surface so that the beam scans a path covering substantially the entire surface; and means for collecting light scattered along said path for detecting anamolies. The spot size and said directing and causing means are such that the surface is inspected at a speed not less than about 1.5 cm2/s.
Still one more aspect of the invention is directed towards a system for detecting anamolies on a surface, comprising means for directing a beam of light toward said surface to illuminate an area of the surface defining a spot having a spot size whose minimum width is in the range of about 5 to 15 microns; means for causing relative motion between the beam and the surface so that the beam scans a path covering substantially the entire surface; and means for collecting light scattered along said path for detecting anamolies. The surface has dimensions of not less than 200 millimeters in any direction along the surface. The directing and causing means are such that the beam scans substantially the entire surface in about 50 to 90 seconds.
It is a further object of the present invention to classify detected anomalies and determine their size while increasing the confidence and accuracy of the detection system by reducing false counts.
These objects have been achieved with an apparatus and method for detecting anomalies of sub-micron size, including pattern defects and particulate contaminants, on both patterned and unpatterned wafer surfaces. For the purposes of this application, a particulate contaminant is defined as foreign material resting on a surface, generally protruding out of the plane of the surface. A pattern defect is in the plane of the surface and is usually induced by contaminants during a photolithographic processing step. The device employs a plurality of collector channels symmetrically disposed, in the azimuth, on opposite sides of the center of a scan line. In addition to the collector channels, other detector channels are employed to enhance the detection of anomalies. The collector and detector channels are collectively referred to as inspection channels. Also, an interchannel communication apparatus is employed to compare and adjust data received from each of the inspection channels which facilitate detecting and characterizing anomalies. A laser beam illuminates a localized spot on a wafer surface with the beam having a grazing angle of incidence, and the spot is scanned over a short scan line. The wafer is orientated so that the streets of the patterns on the die are not oblique with respect to the scan line, i.e., the streets are either perpendicular or parallel to the scan line. The surface is moved in a serpentine fashion, along adjacent striped regions, as the spot is scanned over its entire area. The position of the inspection channels, as well as the polarization of the beam, allows distinguishing, inter alia, pattern defects from particulate contaminants. The detector channels include an imaging channel which combines the advantages of a scanning system and an imaging system while improving signal/background ratio of the present system. The inspection channels collect light and feed it to a light detector for producing an electrical signal corresponding to the collected light intensity. The interchannel communication apparatus is a processor which stores, in memory, the information carried by the signals from the inspection channels, with the memory addresses corresponding to spatial positions on the surface. The processor constructs maps from the stored information, representing the anomalies detected on the surface. The maps from the inspection channels are compared by performing various algorithms and logical operations, e.g., OR, AND and XOR, to characterize the detected anomalies.
In operation, each wafer is scanned with a beam incident thereon at a grazing angle and the light scattered and specularly reflected from the wafer's surface are simultaneously collected with the above mentioned inspection channels. Previously, the wafer has been aligned so that the streets on the die are not oblique with respect to the scan line. Light collected is converted into electrical signals which are further processed by dedicated electronics. A processor analyzes the information carried by the signals and produces various maps representing the light intensity detected at various beam positions. The maps are compared either in the analog domain or digitally to identify and characterize anomalies. If compared digitally, the maps are binarized which allows performing various algorithmic and logical, e.g. OR, AND and XOR, operations on the data they represent, thereby allowing a user to choose a desired level of confidence in the detected anomalies. The binarization can take place against either a constant or a variable threshold, further reducing the occurrence of false counts. The variable threshold is dependent upon the local reflectivity and can be derived from a reflectivity channel which determines local reflectivity of the surface based upon detecting specularly reflected light.
The invention has advantages over the previous scanning techniques in that it provides a small spot that scans at speeds far in excess of those of the prior art, while providing the added feature of classifying anomalies. Further, controlling the polarization of the incident beam and the light detected results in an excellent ratio of particle to pattern signal.
For simplicity, identical components in the different figures of this invention are labeled by the same numerals.
In many lasers, the laser beam produced has a Gaussian intensity distribution, such as that shown in
As a broader definition, “minimum width” of the elliptical spot 10 a may be defined as the minimum distance between two parallel lines that enclose between them the boundary of the area or spot. In reference to spot 10 in
In the description above, it is indicated that for a spot which is relatively small compared to the size of the surface to be inspected, it will be difficult to maintain uniformity of the spot across a scan line which spans the entire length or width of the wafer. In reference to
In view of the above problems, Applicants have invented a surface inspection system where the size of the area illuminated by the scanning light beam can be reduced while maintaining uniform detection sensitivity by causing the scanning light beam to scan short scan path segments having a spatial span less than the dimension of the surface it is scanning, as illustrated in the preferred embodiment in
The surface inspection system of this invention will now be described in reference to
In order to move the illuminated area that is focused onto surface 40 for scanning the entire surface, the AOD 30 causes the deflected beam 32 to change in direction, thereby causing the illuminated spot 10 on surface 40 to be scanned along a scan line 50. As shown in
The deflection of beam 32 by AOD 30 is controlled by chirp generator 80 which generates a chirp signal. The chirp signal is amplified by amplifier 82 and applied to the transducer portion of AOD 30 for generating sound waves to cause deflection of beam 32 in a manner known to those skilled in the art. For a detailed description of the operation of the AOD, see “Acoustooptic Scanners and Modulators,” by Milton Gottlieb in Optical Scanning, ed. by Gerald F. Marshall, Dekker 1991, pp. 615-685. Briefly, the sound waves generated by the transducer portion of AOD 30 modulates the optical refractive index of an acoustooptic crystal in a periodic fashion thereby leading to deflection of beam 32. Chirp generator 80 generates appropriate signals so that after being focused by lens 36, the deflection of beam 32 causes the focused beam to scan along a scan line such as line 50 in the manner described.
Chirp generator 80 is controlled by timing electronic circuit 84 which in the preferred embodiment includes a microprocessor. The microprocessor supplies the beginning and end frequencies f1, f2 to the chirp generator 80 for generating appropriate chirp signals to cause the deflection of beam 32 within a predetermined range of deflection angles determined by the frequencies f1, f2. The auto-position sensor (APS) optics 90 and APS electronics 92 are used to detect the level or height of surface 40 and form a part of the Related Application. Detectors such as detector 111 b collects light scattered by anamolies as well as the surface and other structures thereon along scan line 50 and provides output signals to a processor in order to detect and analyze the characteristics of the anamolies.
Surface 40 may be smooth (118) or patterned (119). The incident focus beam 38 is preferably in the range of about 10-85° to the normal direction 150 to the surface 40 and more preferably within the range of 50-80° from the normal; in
Improved Sensitivity of Detection
From the point of view of sensitivity of detection, it is desirable to design the illumination optics portion of system 20 so that the minimum width w of the illuminated spot 10 is minimized. The minimum width w is proportional to the focal length of lens 36 and inversely proportional to the beam diameter of beam 28 and 32. Therefore, the minimum width w can be reduced by reducing the focal length of lens 36 or increasing diameter of beam 28, or both. If the focal length of lens 36 is increased, however, this will increase the length of scan line 50 which may be undesirable. If the diameter of beam 28 becomes comparable to the clear aperture of the crystal in AOD 30, this will produce higher level sidelobes which is undesirable. As noted above, increased level of sidelobes will increase background signal level. Applicants discovered that it is preferable for the ratio k between the clear aperture of the crystal in the AOD 30 to diameter of beam 28 and 32 to exceed 1.2.
It is possible to increase the beam diameter of beam 28 and 32 by employing a long AOD crystal, while maintaining k to be above 1.2. However, in addition to cost considerations, a larger AOD crystal will cause larger losses, thereby degrading the diffraction efficiency of the AOD device. For this reason, it is desirable to employ AOD crystals that are as small as possible, while at the same time meeting the sensitivity and throughput requirements. Assuming that the beam 28 that is entering the AOD 30 has a Gaussian intensity profile, the clear aperture of the AOD, D, satisfies”
where π is the ratio of the circumference of a circle to its diameter.
Where l is the scan line of scan path segment 50, v is the acoustic velocity in the AOD crystal 30, w is the length of the short axis of the elliptical spot (or the minimum width of the spot if not elliptical) on surface 40, Δf or (f2-f1) is the bandwidth of the AOD 30. The constant k is preferably in the range 1.2-5. In one embodiment, k is 1.7 and l is in the range of about 2-10 millimeters.
For a semiconductor wafer inspection instrument to be used for wafer inspection in actual production for inspecting the entire surface of the wafer, throughput considerations are paramount. Therefore, in addition to sensitivity capability described above, it is also desirable for the wafer inspection system of this invention to have a high throughput. The time required for inspecting semiconductor wafers first includes the time required for the illuminating light beam to scan the entire surface of the wafer. To perform the above-described short scan path segment scans, the time required to scan the entire surface depends on a number of factors. One factor obviously is the angle of illumination of the illuminating beam, or the value of 0, that is the angle between the illuminating beam and normal 150 to surface 40 to be inspected shown in
In most laser beams, the beam intensity has a Gaussian distribution not only in the Y direction but also in the X direction. For this reason, after the illuminating beam completes the scanning operation for scanning a short scan path segment such as segment 50 as shown in
As described above, the minimum width (that is, length of short axis) of the spots 10, 10′, 10″ is w. If the angle between the illuminating light beam and normal 150 to the surface 40 to be inspected is θ as shown in
In the scanning process described above in reference to
As known to those skilled in the art, when AOD 30 is used to cause beam 38 to scan along each short scan path segment such as 50, time will be required at the beginning of the scan for the sound waves generated by the transducer portion of the AOD to reach the far end of the AOD crystal so as to begin deflecting the beam. This is accounted for by a quantity called the duty factor η given by equation 2 below, and therefore, the total ts it takes system 20 to scan the entire surface of a wafer with radius R is given by equation 3 below:
From equation 3 above, it is evident that the shorter the time T to scan along a scan path segment such as 50, the shorter will be the time required to scan the entire wafer and therefore the higher the throughput. The time T is referred to as the chirp duration which also determines the data rate. The speed of the electronic circuit for processing the data ultimately sets a lower limit for the chirp duration.
From equation 1 above, for a given spot size, length of the scan path segment and the value of k, it is evident that the larger the bandwidth Δf or f2-f1, the smaller will be the clear aperture required of the AOD. To get maximum bandwidth from the AOD, the AOD should be operated at the highest possible frequency and one then expects to get one octave bandwidth around the center frequency of the transducer. However, the acoustic losses in the AOD crystal increase with the center frequency of operation. Large acoustic losses can cause two major problems: reduction in diffraction efficiency and thermal errors induced in the crystal. A reduction in the diffraction efficiency reduces the sensitivity of the system to small particles. When the AOD transducer is operated at high frequencies, more of the acoustic energy will be converted into heat which sets up thermal gradients in the AOD crystal. Such thermal gradients would cause errors by degrading the focal spot which in turn leads to a reduction in sensitivity for detecting anamolies. It is therefore advantageous to minimize the acoustic losses by selecting as low a center frequency of the transducer as possible. A compromise should then be found to yield acceptable detection sensitivity as well as acceptable throughput. Applicants found that a center frequency in the range of 50-300 megahertz and a bandwidth preferably within the range of 50-250 megahertz would be acceptable. The AOD 30 is preferably driven by a linear frequency modulated (FM) chirp signal from generator 80 in
From equation 3 above, it is seen that the larger the angle θ, the higher will be the throughput, since the illuminated spot will cover a larger area of the surface. But as noted above, the larger the spot size, the lower will be the sensitivity of detection. In the preferred embodiment, θ is in the range of 10-85° and more preferably in the range of 50-80°.
Also from equation 3 above, it is evident that the larger the number of samples taken across the illuminated spot diameter, the more time it would take to scan the wafer. In the preferred embodiment, the number of samples taken across the illuminated spot diameter along both orthogonal axes (X, Y) is in the range of 2-10. Where four samples are taken along at least the X axis, N is 4 in equation 3.
For sensitivity considerations, it is preferable for the minimum width w of the illuminated area to be in the range of 5-15 microns. If θ is in the range of 50-80°, then the illuminating beam will illuminate the scan path segments such as 50 at such speed that the surface is inspected at a speed not less than about 2.5 cm2/s, and more preferably in a range of about 2.5-3.8 cm2/s.
From equation 3 above, if the time required for moving the wafer or the illumination beam so that the illuminated spot is transferred between adjacent strips such as strips 54, 56 is taken into account, then the average speed for scanning the entire surface 40 will be reduced compared to that for scanning a short scan path segment such as segment 50. Furthermore, the speed for inspecting the entire wafer is further reduced because each pixel on the wafer is scanned multiple times as described above in reference to
In the preferred embodiment, generator 80 supplies a linear FM chirp signal to drive the AOD so that the chirp duration is preferably in the range of 20-200 microseconds, and more preferably in the range of about 80-120 microseconds. The beam 28 before deflection by the AOD 30 has at least one cross-sectional dimension (e.g. the longer dimension) in the range of about 4-12 millimeters. Preferably, the scan lens 36 is placed substantially at one focal length away from AOD 30 so that beam 38 scans the surface 40 telecentrically.
From the above, it will be evident that the objective of the invention of the high sensitivity and high throughput surface inspection system has been achieved while moderate data rate (e.g. 22 Mhz) at modest cost for the data sampling and processing electronics can still be achieved. This system is capable of inspecting patterned wafers with 0.35 micron design rule, such as patterned wafers for 64 and 256 megabit DRAM technology. The system is capable of detecting contaminant particles and pattern defects on memory and logic devices. With the present state-of-the-art robotic implementation for removing and replacing wafer 40 on stage 124 ready for system 20 to inspect and the inherent delay (about 25 seconds/wafer) involved therein, system 20 described above is capable of inspecting in excess of about 40 wafers per hour for 150 millimeter diameter wafers (6-inch wafers), in excess of about 20 wafers per hour for 200 millimeter diameter wafers (8-inch wafers) and in excess of about 10 wafers per hour for 300 millimeter diameter wafers (12-inch wafers).
While in the invention described above, the scan path segments are described and illustrated as straight lines, it will be understood that it is also possible for curved scan lines to be employed, such as where the wafer is rotated about an axis instead of translated along straight lines in the X and Y directions as described above. While in the preferred embodiment described above, the short scan path segments form arrays, each array covering a substantially rectangular strip of the wafer, it will be understood that other different arrangements of the scan path segments are possible for covering the entire or substantially the entire surface 40; such and other variations are within the scope of the invention. As the spot 10 approaches the edge of surface 40, the length of the scan path segment may be reduced so that the spot does not fall outside surface 40. All the advantages described are obtained even though the segments are of different lengths if each of at least some of the segments has a span shorter than the dimensions of the surface. Also, the AOD 30 may be replaced by a polygonal scanner or galvanometer. While the invention has been described by reference to preferred embodiments, it will be understood that modifications and changes can be made without departing from the scope of the invention which is to be defined only by the appended claims.
The present invention, as shown in
As shown in
The beam 1014 has a wavelength of 488 nm and is produced by an Argon ion laser. The optical axis 1048 of the beam 1014 is directed onto the wafer surface 1016 at an angle, Θ. This angle, Θ, is in the range of 55-85° with respect to the normal to the wafer surface 1012, depending on the application. The scanning means includes the deflector 1016 and the translation stage 1024 upon which the wafer rests. The position of the wafer on the stage 1024 is maintained in any convenient manner, e.g., vacuum suction. The stage 1024 moves to partition the surface 1012 into striped regions, shown as 1025, 1026 and 1027 with the deflector 1016 moving the beam across the width of the striped regions.
Providing the groups of collector channels, at differing azimuthal angles, facilitates classifying detected anomalies, by taking advantage of a discovery that laterally scattered light is more sensitive to detecting pattern defects, and forwardly scattered light is more sensitive to detecting particulate contaminants. To that end, channels 1010 a and 1010 b are positioned to collect laterally scattered light, representing pattern defects, and channels 1011 a and 1011 b are provided collect forwardly scattered light, representing particulate contamination.
To detect particulate contaminants on a pattern surface, the variable polarization filter 1118 would attenuate scattered light that is not in a P state of polarization, if the beam were S-polarized. Were beam 1014 in a P state of polarization, the collector channels would collect scattered light that was S-polarized, whereby the variable polarization filter 1118 would attenuate all other scattered light impinging on the channel. For detecting particulates on a bare surface, beam 1014 would be in a P state of polarization and the collector channels would collect all light scattered therefrom to maximize the capture rate.
Referring again to
Although the above-described example discussed comparing maps from signals generated by a pair of collector channels, this is not the only manner in which the system may operate. It is to be understood that maps formed from signals generated by the detector channels may also be compared to identify and classify anomalies, by performing algorithms and logical operations on the data, as described above. Comparing signals to a variable threshold level provides an instructive example, because the threshold level is derived from the bright field reflectivity/autoposition channel 1020, shown in
The variable threshold level is dependent upon the local reflectivity. To that end, the bright field reflectivity/autoposition channel 1020, is positioned in front of the beam 1014 to collect specularly reflected light. The bright field signal derived from this channel carries information concerning the pattern, local variations in reflectivity and height. This channel is sensitive to detecting various defects on a surface. For example, the bright field signal is sensitive to representing film thickness variations, discoloration, stains and local changes in dielectric constant. Taking advantage of bright field signal sensitivity, the bright field signal is used to produce the variable threshold level 1040, shown in
P s /P b =σ/A b h
where Ps is the optical power scattered by a particle, Pb is the background optical power, Ab is the area of the beam on the surface and σ and h are constants. This shows that the ratio of the scattering cross section to the area of the beam determines the signal to background ratio.
With an imaging-based channel, all the scattered power from an anomaly is imaged onto one array element. The power distributed in background, however, is imaged over a range of elements, depending upon the magnification of the system. Assuming a linear magnification M, at the image plane the background power over an area is as follows:
providing an effective background power per array element as
P b =P i hA c /M 2 A b
where Ac is the area of an array element. Therefore, the signal to background ratio is given by the following:
P s /P b =M 2 σ/A c h
This shows that the signal to background ratio is independent of the spot diameter, providing an improved signal to background ratio given by:
i=M 2 A b /A c
If imaging is not desired, another PMT-based collector channel similar to the one shown in
Referring again to
In operation, the beam 1014 is scanned over the surface 1012, producing both scattered and specularly reflected light, which are simultaneously detected. The light scattered laterally, forwardly and upwardly is simultaneously detected by the collector channels and the imaging system. The specularly reflected light from the wafer's surface 1012 is detected by the bright field reflectivity/autoposition channel 1020. Light detected by the inspection channels is converted into electrical signals which are further processed by dedicated electronics, including a processor 1500. The processor 1500 constructs maps from the signals produced by the inspection channels. When a plurality of identical dies are present on the wafer surface 1012, a detection method may be employed whereby periodic feature comparisons are made between adjacent die. The processor compares the maps from the inspection channels either in the analog domain or digitally, by performing logical operations on the data, e.g., AND, OR and XOR, to detect anomalies. The processor forms composite maps, each representing the detected anomalies by a single group of symmetrically disposed collector channels. The composite maps are then compared so that the processor may classify the anomalies as either a pattern defect or particulate contamination. Typically, the wafer surface 1012 has been aligned so that the streets on the die are not oblique with respect to the scan line, using the information carried by the electrical signal produced by the alignment/registration channel. Proper alignment is a critical feature of this invention, because periodic feature comparison is performed to locate anomalies.
While the above described apparatus and method for detecting anomalies has been described with reference to a wafer surface, it can easily be seen that anomaly detection is also possible for photomasks and other surfaces, as well as producing reflectivity maps of these surfaces. The invention is capable of detecting anomalies of submicron size and affords the added advantage of classifying the type of anomaly and identifying its size and position on the surface. This information is highly useful to wafer manufacturers as it will permit locating the step in the wafer manufacturing process at which point an anomaly occurs.
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|International Classification||G01N21/94, G01N21/95, G01N21/956|
|Cooperative Classification||G01N21/956, G01N21/94, G01N21/9501|
|European Classification||G01N21/94, G01N21/956, G01N21/95A|