Optical Scanning System for Surface Inspection
In an optical scanning system for detecting particles and pattern defects on a sample surface, a light beam is focused to an illuminated spot on the surface and the spot is scanned across the surface along a scan line. A detector is positioned adjacent to the surface to collect scattered light from the spot where the detector includes a one- or two-dimensional array of sensors. Light scattered from the illuminated spot at each of a plurality of positions along the scan line is focused onto a corresponding sensor in the array. A plurality of detectors symmetrically placed with respect to the illuminating beam detect laterally and forward scattered light from the spot. The spot is scanned over arrays of scan line segments shorter than the dimensions of the surface. A bright field channel enables the adjustment of the height of the sample surface to correct for errors caused by height variations of the surface. Different defect maps provided by the output of the detectors can be compared to identify and classify the defects. The imaging function of the array of sensors combines the advantages of a scanning system and an imaging system while improving signal/background ratio of the system.
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This is a continuation of application Ser. No. 10/412,458, filed Apr. 10, 2003; which application is a continuation of application Ser. No. 09/571,303, filed May 8, 2000, now abandoned; which application is a continuation of application Ser. No. 08/868,292, filed Jun. 3, 1997, now U.S. Pat. No. 6,081,325; which application claims the benefit of the filing date of Application No. 60/018,973, filed Jun. 4, 1996. These applications are hereby incorporated by reference as if fully set forth herein.BACKGROUND OF THE INVENTION
The present invention pertains to the field of optical surface inspection. Specifically, the present invention pertains to illumination and light collection optics for inspecting semiconductor wafers and the like.
Monitoring anomalies, such as pattern defects and particulate contamination, during the manufacture of semiconductor wafers is an important factor in increasing production yields. Numerous types of defects and contamination, especially particles, can occur on a wafer's surface. Determining the presence, location and type of an anomaly on the wafer surface can aid in both locating process steps at which the anomaly occurred and determining whether a wafer should be discarded.
Originally, anomalies were monitored manually by visual inspection of wafer surfaces for the presence of particulate matter. These anomalies, usually dust or microscopic silicon particles, caused many of the wafer pattern defects. However, manual inspection proved time-consuming and unreliable due to operator errors or an operator's inability to observe certain defects. The ever increasing size of the wafer surface, along with the decreasing dimensions of the components thereon, resulted in a sharp increase in the number of components on the wafer's surface. The need for automation became manifest.
To decrease the time required to inspect wafer surfaces, many automatic inspection systems were introduced. A substantial majority of these automatic inspection systems detect anomalies based on the scattering of light. For example, see U.S. Pat. No. 4,601,576 to L. Galbraith, assigned to the assignee of the present invention. These systems include two major components: illumination optics and collection-detection optics. Illumination optics generally consists of scanning a wafer surface with a coherent source of light, e.g., a laser. Anomalies present on the wafer's surface scatter incident light. The collection optics detect the scattered light with reference to the known beam position. The scattered light is then converted to electrical signals which can be measured, counted and displayed as bright spots on an oscilloscope or other monitor.
The illumination optics plays a major role in establishing the detection sensitivity of the inspection system. The sensitivity is dependent upon the size of the spot scanned on the wafer and the illumination angle. The smaller the spot size, the more sensitive the system is to detecting anomalies. However, decreasing the spot size increases the time required to scan the wafer surface and therefore reduces throughput.
The sensitivity of both the illumination and collection-detection optics is dependent upon the texture of the surface of the wafer illuminated. If the surface illuminated is patterned, this reduces the sensitivity of the system because such areas produce scatter which makes it difficult to determine the presence of an anomaly. To abrogate scatter due to patterned features, the angle of incidence of the spot on the surface is increased, with respect to the normal to the surface. However, too great of an angle, i.e., a grazing angle with respect to the surface, will also reduce the sensitivity of the system. Moreover, increasing the angle of incidence, increases the effective size of the spot, thereby reducing the sensitivity of the system. Thus, a trade-off exists between sensitivity and inspection rate of the system. The sensitivity of the collection-detection optics is generally a factor of the detector's azimuthal position with respect to the scanning beam and elevation.
Accordingly, many illumination and collection-detection techniques have been proposed that take advantage of the aforementioned concepts. In addition, efforts have been made to provide for constant scanning of the wafer's surface to further increase the speed of the inspection. In U.S. Pat. No. 5,317,380, Allemand discloses a beam of laser light brought to focus as an arcuate scan line on a surface, at a grazing angle of incidence. A pair of light detectors are provided to collect light which is scattered away from the beam in a forward direction so that the angle of collection is constant over the entire scan line.
U.S. Pat. No. 4,912,487 to Porter et al. discloses a laser pattern writing and inspection system that illuminates a target surface with an argon ion laser beam. An acousto-optical deflector is driven with a chirp signal and placed in the path of the beam to cause it to sweep out raster scan lines. The target is placed on a stage capable of bi-directional movement. The beam has an angle of incidence normal to the target and the stage moves so that it is scanned along adjacent contiguous strips of equal width.
U.S. Pat. No. 4,889,998 to Hayano et al., discloses an apparatus and method for detecting foreign particles on a pellicle using a beam of light that is scanned across the pellicle with light detected by a plurality of detectors grouped in pairs. Two pairs of detectors are positioned to collect rearwardly scattered light. The difference in intensity of scattered light detected by each detector is monitored, whereby the position of the particle on the pellicle is determined by analyzing the intensity variations.
In U.S. Pat. No. 4,898,471 to Stonestrom et al., an apparatus and method for detecting particles on a patterned surface are disclosed wherein a single light beam is scanned, at a grazing angle of incidence, across the surface. The surface contains a plurality of identical dies with streets between them. With the beam scanning parallel to the streets, a single channel collection system detects scattered light from an azimuthal angle that maximizes particle signals while reducing pattern signals. A processor constructs templates from the detected light which corresponds to individual dies and then compares the templates to identify particles on the dies.
In U.S. Pat. No. 4,617,427, Koizumi et al., a wafer is mounted on a feed stage connected to a rotary drive which provides a constant speed helical scan of a wafer surface. An S-polarized laser beam is scanned thereon at varying angles of incidence. The angle of incidence is dependent upon whether the wafer is smooth or patterned. A single detector is positioned perpendicular to the wafer's surface for collecting scattered light and includes a variable polarization filter that attenuates scattered light in an S polarization state, when the surface is patterned, and does not attenuate light if the wafer is smooth.
In U.S. Pat. No. 4,441,124 to Heebner, a laser is scanned over the surface of a wafer at an angle normal thereto. The laser beam is scanned by deflecting it with a galvanometer and an acousto-optic deflector in synchronization with the scanning beam rate of a video monitor. A photodetector employing a ring-type collection lens monitors the intensity of light scattered substantially along the wafer surface. This arrangement was employed to take advantage of the finding that a patterned wafer having no particulate matter thereon will scatter substantially no light along the wafer surface, while a wafer having particulate matter on it will scatter a portion of the light impinging thereon along the surface.
Another particle detection apparatus and method is disclosed in U.S. Pat. No. 4,391,524, to Steigmeier et al., wherein a laser beam is scanned at an angle normal to the wafer's surface. In addition to rotating, the wafer stage is provided with movement along one axis that results in the wafer being scanned in a spiral fashion. A single detector is positioned perpendicular to the surface to collect scattered light. Threshold circuitry is employed to discriminate between the defects monitored.
It is an object of the present invention to provide a high-speed apparatus which is capable of scanning a laser beam across the surface of either a patterned or unpatterned wafer to detect anomalies thereon with sizes on the order of a fraction of a micron.
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.SUMMARY OF INVENTION
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 may be in or below the plane of the surface and is usually induced by contaminants during a photolithographic processing step or caused by crystal defects in the surface.
One aspect of the invention is directed towards an optical scanning system for detection of anomalies, such as particles and pattern defects on a surface, comprising means for directing a focused beam of light onto a sample surface to produce an illuminated spot thereon and means for scanning the spot across the surface along a first scan line. The system further comprises a first detector positioned adjacent to said surface to collect scattered light from the spot wherein the detector includes a one-dimensional or two-dimensional array of sensors and means for focusing scattered light from the illuminated spot at each of a plurality of positions along the scan line onto a corresponding sensor in the array.
Another aspect of the invention is directed towards an optical scanning method for detection of anomalies, such as particles and patterns on a surface, comprising the steps of directing a focused beam of light onto a sample surface to produce an illuminated spot thereon; scanning a spot across the surface along a first scan line; positioning a first detector adjacent to said surface to collect scattered light from the spot, wherein the detector includes a one-dimensional or two-dimensional array of sensors; and focusing scattered light from the illuminated spot at each position along the scan line onto a corresponding sensor in the array.BRIEF DESCRIPTION OF THE DRAWINGS
For simplicity in description, identical components are identified by the same numerals in this application.DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention, as shown in
As shown in
The beam 14 has a wavelength of 488 nm and is produced by an Argon ion laser. The optical axis 48 of the beam 14 is directed onto the wafer surface 12 at an angle, θ. This angle, θ, is in the range of 55-85° with respect to the normal to the wafer surface 12, depending on the application. The scanning means includes the deflector 16 and the translation stage 24 upon which the wafer rests. The position of the wafer on the stage 24 is maintained in any convenient manner, e.g., vacuum suction. The stage 24 moves to partition the surface 12 into striped regions, shown as 25, 26 and 27 with the deflector 16 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 10a and 10b are positioned to collect laterally scattered light, representing pattern defects, and channels 11a and 11b are provided to collect forwardly scattered light, representing particulate contamination.
To detect particulate contaminants on a pattern surface, the variable polarization filter 118 would attenuate scattered light that is not in a P state of polarization, if the beam were S polarized. Were beam 14 in a P state of polarization, the collector channels would collect scattered light that was S polarized, whereby the variable polarization filter 118 would attenuate all other scattered light impinging on the channel. For detecting particulates on a bare surface, beam 14 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 20, shown in
The variable threshold level is dependent upon the local reflectivity. To that end, the bright field reflectivity/autoposition channel 20, is positioned in front of the beam 14 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 40, shown in
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
where Ac is the area of an array element. Therefore, the signal to background ratio is given by the following:
This shows that the signal to background ratio is independent of the spot diameter, providing an improved signal to background ratio given by:
If imaging is not desired, another PMT-based collector channel similar to the one shown in
In many lasers, the laser beam produced has a Gaussian intensity distribution, such as that shown in
To maintain uniform detection sensitivity, the scanning light beam is preferably caused to scan short sweeps 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 application will now be described with reference to
In order to move the illuminated area that is focused onto surface 240 for scanning the entire surface, the AOD 230 causes the deflected beam 232 to change in direction, thereby causing the illuminated spot 210 on surface 240 to be scanned along a sweep 250. As shown in
The deflection of beam 232 by AOD 230 is controlled by chirp generator 280 which generates a chirp signal. The chirp signal is amplified by amplifier 282 and applied to the transducer portion of AOD 230 for generating sound waves to cause deflection of beam 232 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 230 modulate the optical refractive index of an acoustooptic crystal in a periodic fashion thereby leading to deflection of beam 232. Chirp generator 280 generates appropriate signals so that after being focused by lens 236, the deflection of beam 232 causes the focused beam to scan along a sweep such as sweep 250 in the manner described.
Chirp generator 280 is controlled by timing electronics circuit 284 which in the preferred embodiment includes a microprocessor. The microprocessor supplies the beginning and end frequencies f1, f2 to the chirp generator 280 for generating appropriate chirp signals to cause the deflection of beam 232 within a predetermined range of deflection angles determined by the frequencies f1, f2. The illumination sensor optics 20 and adaptive illumination control 292 are used to detect and control the level of illumination of spot 210. The optics 20 and adaptive illumination control 292 are explained in detail in U.S. Pat. No. 5,530,550.
Detectors such as detectors 10a, 10b, 11a, 11b of
In most laser beams, the beam intensity has a Gaussian intensity 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 sweep such as sweep 250 as shown in
Detector 120 includes a one-dimensional or two-dimensional array of sensors. To enable time delayed integration as described below, detector 120 employs a two-dimensional array of sensors as illustrated in
In reference to
For the purpose of illustration, it is assumed that when spot 210 is scanned along the sweep immediately prior to sweep 250, light scattered from the pixel P is focused by lens assembly 119 onto sensor 121(1)(3) of detector 120 in
To enable time delayed integration, processor 500 causes the signal in sensor 121(1)(3) obtained by detecting light scattered by pixel P during the previously described scan to be transferred to sensor 121(2)(3), so that the signal obtained during the prior scan will be added to that obtained by sensor 121(2)(3) from detecting the light scattered by pixel P during sweep 250. Similarly, processor 500 causes the thus accumulated signal in sensor 121(2)(3) to be transferred to sensor 121(3)(3) prior to the sweep 250′, so that the signal thus accumulated can be added to that obtained by sensor 121(3)(3) by detecting the light scattered from pixel P during sweep 250′. In this manner, time delayed integration is performed by accumulating the signals obtained from light scattering from pixel P during four sequential sweeps and is read out as the output signal for such pixel. The same can be done for other pixels on the surface of the wafer. While in the preferred embodiment, the amount of overlap and the sampling rate are controlled so that each illuminating spot is divided into 16 pixels, it will be understood that the spot may be divided into a smaller or greater number of pixels by altering the amount of overlap between sequential sweeps and by altering the sampling rate; such and other variations are within the scope of the invention.
The above-described process may be performed for all of the pixels in the illuminated spot where processor 500 simply causes all of the signals in each linear array or line of sensors, such as line 121(1) to be transferred to corresponding sensors in the next line 121(2), and this process is carried out for all of the lines, from line 121(1) to the next to the last line 121(N-1), so that time delay integration is performed for all of the pixels. The number of sensors in each line is preferably large enough to cover all the pixels in each sweep. In order to avoid edge effects, it may be desirable to include enough lines of sensors to cover all of the possible positions of the pixels in the illuminated spot along the X direction of the wafer.
As described above, signals obtained by light scattering from the pixel are accumulated over four sequential sweeps. The final accumulated signal is then read out by processor 500 as the output of detector 120 for such pixel. Processor 500 then constructs a defect map from a two-dimensional array of such accumulated signals from the outputs of detector 120. Such map may be compared to the defect maps obtained by processor 500 from detectors 10a, 10b, 11a and 11b to obtain an AND, a union and an XOR map for the purpose of identifying anomalies. Thus, an AND map would comprise only of anomalies present in a map from one detector and in a map or maps from one or more of the remaining detectors. A union map comprises anomalies present in at least one of the maps of two or more detectors. An XOR map comprises anomalies present in the map of only one detector but not in the map or maps of the remaining detectors. As noted above, the above maps are useful for classifying defects. Thus, if an anomaly is present in the map of one detector but not in the map of the other symmetrically placed detector, then the anomaly is probably not symmetrical. Or if an anomaly is present in the map of detectors 10a, 10b for detecting laterally scattered light but not in the maps of detectors 11a, 11b for detecting forward scattered light, then the anomaly may be more likely to be a pattern defect than a particle.
The XY stage 24 is controlled by a controller (not shown) in communication with processor 500. As this controller causes the stage 24 to move the wafer by a quarter of the X dimension of the spot 210, this is communicated to processor 500, which sends control signals to detector 120 to cause a transfer of signals between adjacent lines of sensors and sends control signals to timing electronics 284 as shown in
In the preferred embodiment, the illuminated spot has a spot size whose minimum dimension is in the range of about 2 to 25 microns.
Referring again to
In operation, the beam 14 is scanned over the surface 12, 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 12 is detected by the bright field reflectivity/autoposition channel 20. Light detected by the inspection channels is converted into electrical signals which are further processed by dedicated electronics, including a processor 500. The processor 500 constructs maps from the signals produced by the inspection channels. When a plurality of identical dies are present on the wafer surface 12, 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, in the manner described above, 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 12 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 have 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.
1. An optical scanning system for detection of anomalies on a surface comprising:
- optics directing a focused beam of radiation onto a sample surface to produce an illuminated spot thereon;
- one or more sensor(s);
- one or more optical element(s), each element collecting radiation scattered from the illuminated spot on the surface along a channel and directing the collected and scattered radiation to one of the sensor(s), causing each of the sensor(s) to provide output signals in response thereto, each of the sensor(s) sensing radiation scattered from the surface in directions away from a specular reflection direction and different from those of radiation sensed by the other sensor(s);
- a bright field detector detecting a specular reflection of the radiation in the beam from the illuminated spot on the surface to provide an output signal; and
- a device causing relative motion between the beam and the surface so that the beam is caused to illuminate different parts of the surface and so that the sensor(s) and the detector provide output signals in response to radiation from different parts of the surface illuminated by the beam.
International Classification: G01N 21/88 (20060101);