WO1999032879A1 - Method and apparatus for reducing roughness scatter as a noise source in wafer scanning systems - Google Patents

Method and apparatus for reducing roughness scatter as a noise source in wafer scanning systems Download PDF

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WO1999032879A1
WO1999032879A1 PCT/US1998/027752 US9827752W WO9932879A1 WO 1999032879 A1 WO1999032879 A1 WO 1999032879A1 US 9827752 W US9827752 W US 9827752W WO 9932879 A1 WO9932879 A1 WO 9932879A1
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light
surface
workpiece
scattered light
topographic
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PCT/US1998/027752
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French (fr)
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Michael E. Fossey
John C. Stover
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Ade Optical Systems Corporation
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    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00Investigating or analysing materials by the use of optical means, i.e. using infra-red, visible or ultra-violet light
    • G01N21/84Systems specially adapted for particular applications
    • G01N21/88Investigating the presence of flaws or contamination
    • G01N21/94Investigating contamination, e.g. dust

Abstract

In the optical inspection of polished Si wafers, p-polarized light is incident on the wafer surface and p-polarized light scattered light is detected out of plane at an orientation (υs, ζs), where the scattering angles υs, ζs are selected to minimise the contribution due to surface microroughness.

Description

METHOD AND APPARATUS FOR REDUCING ROUGHNESS SCATTER AS A NOISE SOURCE IN WAFER SCANNING SYSTEMS Field and Background of the Invention This invention relates to surface inspection systems, and more particularly to the inspection of the surface of wafers of the type used as substrates for microelectronics devices.

Wafer particle scanners have been used for a number of years to detect and map surface defects on wafers. They operate by scanning a laser beam over the wafer surface and recording the location and magnitude of light scattered from the surface. When the beam encounters particles or pits of sufficient size, a large increase in scatter is detected above the normal background noise signals. The smallest particle that can be detected by the scanning system is determined by these background noise signals. A significant source of background noise arises from surface roughness (or topographic) scatter due to microroughness of the wafer surface. Summary of the Invention

The present invention provides a method and apparatus whereby the noise contributions from topographic scatter can be reduced. The signal-to-noise ratio is thus increased, increasing the sensitivity of the scanning system and enabling it to detect smaller defects. The present invention is based upon an understanding of the scattering behavior of light from rough surfaces, and in particular, a recognition of how the scattering behavior of a polarized incident light beam can be advantageously exploited when collecting and analyzing light scattered from a wafer surface so as to minimize the component of the collected scattered light resulting from topographic scatter and to maximize the component of scattered light resulting from scattering by defects present on the wafer surface.

The measured polarization state of a light signal can be characterized by calculating four quantities (SO, SI, S2, and S3) that comprise what is known as a Stokes Vector. Once measured (applying known techniques), these four values can be used to calculate four other quantities. Three of these define the polarization state of a completely polarized beam of monochromatic light. They are the two sinusoidal light electric field amplitudes in perpendicular directions and the propagation phase angle between them. These three quantities can be used to describe the polarization state of a completely polarized beam. The fourth quantity that can be found from the Stokes Vector is the fractional depolarization. If light is 100% polarized, then it can be removed (or filtered out) by using a filter composed of a quarter wave plate in front of a polarizer. The filter is adjusted by individually rotating the components about their surface normals. These filtering techniques, which are well established in the literature, are part of a field of study known as polarimetry.

If a 100% polarized source (such as a laser) is used, then sample characteristics determine whether any of the reflected light is depolarized. In general, samples that scatter the light many times before it leaves the reflector (such as a piece of paper) greatly depolarize the light. On the other hand, scatter signals that result from single scatter events (such as topographic scatter from very smooth surfaces) are changed in polarization state from the incident beam state, but are still 100% polarized. Silicon wafer, mirrors and other reflectors that are optically smooth scatter topographically in this manner. Some surface features, such as PSL spheres on a wafer surface, also do not depolarize the light, although they produce a different polarization sate than the topographic scatter. Other surface features, such as deep pits or particles that are not isotropic in their material characteristics, will depolarize the scatter signal. The changes in the polarization signal and the amount of depolarization are available to be exploited as means to discriminate one feature from another.

The fact that topographic scatter from smooth surfaces remains 100% polarized allows it to be eliminated from any location on the scattering hemisphere with the appropriate filter. From dielectric surfaces (non-metals), the filtering is easier and can be accomplished with only a polarizer at the appropriate angle; the wave plate is not required. For the case of metals (or materials that have a non-zero absorption coefficient), a wave plate is required for filtering out of the incident phase. Silicon is almost a dielectric surface since it has a very small absorption coefficient, and excellent filtering can be accomplished using just a polarizer. This invention is concerned with making use of a source configuration (e.g. high angle P polarized light) that naturally creates a lower topographic scatter region at a special location on the scattering hemisphere and then filtering that location with a polarizer.

Topographic scatter from optically smooth surfaces is a well-understood and documented phenomenon. The relationship for normalized scatter density is:

Ps I Ω \6π2 cos' 0. cos θ,QS(f) (1)

Pi λ4

where Ps is the power scattered into solid .angle Ω in a direction defined by the angles θs and φs (polar and azmuthal respectively) under the condition of π watts of wavelength λ incident at angle θi from the surface normal. The quantity Q is the polarization factor (also sometimes referred to as the unit Jones vector) and describes material properties of the substrate. S (j) is the surface power spectral density function and describes the surface topography.

There are actually four Q terms to cover the two orthogonal polarization states possible at the source and in the scattered field. Equations for the four terms are given below, where ε is the complex dielectric constant of the surface.

{ε -\) sφ

&• = (2)

(cos#, +yjε-sm2 6>,)(cos6>. + JJ--sin2 θ )

Figure imgf000005_0001
Notice in equation (1) that if Q could be forced to zero, then the topographic scatter would also reduce to zero no matter how rough the surface. Examination of the four Q's reveals that for ε >1 (which is true for all but free space) there are some special conditions where this can be forced to happen. Qsp and Qps are zero in the incident plane (φ = 0 and 180 degrees) and Qss is zero in the perpendicular plane (φ=90 and 270 degrees). These relationships have been remarked on previously in published literature and used as means to reduce surface scatter in favor of particle scatter. Qpp has a different form of numerator than the other three expressions. It can be forced to zero for the right choice of incident angle and scatter angles for any real ε (that is a dielectric material). It can be forced to a minimum for any complex value of ε (that is a metal). The present invention involves making use of this particular effect to reduce topographic scatter when scanning for particulate scatter on semiconductor wafers.

First consider the special case of θs=θi and a real value of ε. In this situation there is one θ value for which the Qpp numerator goes to a minimum. For φ=0, this corresponds to Brewsters Angle (the incident angle for which the p-polarized source reflectance becomes zero. However, notice in Equation (5) that if φ is slightly different than zero, then adjusting either θs alone or θi alone will bring the numerator back to zero. This means that there are other positions on the scattering hemisphere, away from the specular reflection and out of the plane of the incident light and specularly reflected light beam, for which Qpp is zero. In fact, if the numerator is set equal to zero and solved for φs, the result is an equation in θs (for fixed θi) that defines a line of zero p-polarized scatter on the hemisphere. Measurements made on this line through an analyzer rotated to pass only p-polarized light will contain a greatly reduced topographic scatter component. The facts that the measurement aperture is finite (extending on both sides of the line) and that the analyzer will not be perfect will allow some topographic signal. For the situation of a silicon wafer, ε is almost real (ε=4.4 + iθ.07), the Qpp component is not quite zero, but is small. There are reasons for not using an analyzer - involving changes in the polarization state of the scatter signal from particles of different materials. In this case, the total topographic scatter signal is proportional to the sum of Qps and Qpp. If this sum is evaluated for silicon, a minimum can be found as a function of the various angles. For analyzing a silicon wafer using a wafer scanner geometry of the particular type disclosed herein, the optimum position appears to be at about φs=45 degrees and θs=50 degrees.

From the foregoing discussion, it will be seen that the present invention provides a method of and apparatus for detecting defects on the surface of a workpiece, such as a wafer, which comprises the steps of: directing a polarized light beam onto the surface of the workpiece at a non-normal incident angle; collecting light scattered from the surface of the workpiece at a selected location in the scattering hemisphere, the scattered light including a component resulting from defects present on the workpiece surface and a component due to topographic roughness of the workpiece surface; filtering the collected scattered light through a filter oriented to minimize the topographic scatter component in the collected scattered light; and detecting the thus-filtered light.

In another aspect, the present invention provides a method of detecting defects on the surface of a workpiece, comprising the steps of directing a P-polarized light beam onto the surface of the workpiece at a non-normal incident angle; collecting light scattered from the surface of the workpiece, the scattered light including a component resulting from defects present on the workpiece surface and a component due to topographic roughness of the workpiece surface, and wherein the collecting step is performed at a location in the scattering hemisphere out of the plane defined by the incident light beam and selected so that the value of the unit Jones vector for P-polarized incident and P-polarized scattered fields is minimized; and filtering the collected scattered light through a polarizer oriented to minimize the topographic scatter component in the collected scattered light.

Brief Description of the Drawings

Some of the features and advantages of the invention having been described, others will become apparent from the detailed description which follows, and from the accompanying drawings, in which:

Figure 1 is a perspective view of a surface inspection system according to the present invention. Figure 2 illustrates a transporter of a surface inspection system according to the present invention arranged for rotatively and translationally transporting a workpiece, such as a wafer, along a material path.

Figure 3 is a schematic side elevational view of the surface inspection system.

Figure 3 A is a fragmentary view of a light channel detector of the surface inspection system.

Figure 4 is a schematic side elevational view of the optical scanning system. Figure 5 schematically illustrates rotational and translational travel of a wafer through an inspection area.

Figure 6 is a diagrammatic side view of the collector showing the locations of the segmented optics for collecting light scattered.

Figure 7 is a diagrammatic top view of the collector showing the locations of the segmented optics.

Figure 8 is a diagrammatic perspective view of the collector showing the locations of the segmented optics

Figures 9-12 are plots showing modeled polarization coefficient Qpp at different angles theta, for the substrates silicon, titanium, aluminum, and tungsten respectively.

Figure 13 is a plot showing actual BRDF data on a tungsten surface. Figure 14 is a plot of Differential Scattering Cross Section (DSC) from two different particle sizes plotted along with the surface DCS, and showing the results obtained using a previously known scanner geometry. Figure 15 is a plot similar to Figure 14, for a similar scanner geometry, but wherein the detector is located out-of-plane and polarized as taught by the present invention.

Description of Illustrative Embodiment The present invention will be described more fully hereinafter with reference to the accompanying drawings in which specific embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the illustrated embodiments set forth herein; rather, these illustrated embodiments are provided so that this disclosure will be thorough and complete and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout. Figure 1 is a perspective view of a surface inspection system 20 for detecting defects such as particles, pits and the like on a surface of a workpiece W or article, such as a silicon wafer. Portions of the system 20 are broken away for purposes of clarity and shown by phantom lines to illustrate various elements of the surface inspection system 20. The surface inspection system 20 is suitably used for inspecting the surface of unpattemed wafers W, both with and without deposited films. The system 20 preferably includes means for translationally transporting a workpiece W along a material path P, means associated with the translational transporting means for rotating the workpiece W as it travels along the material path P, means for scanning the surface S of the workpiece W during rotative and translational travel along the material path P, and means for collecting light reflected and scattered from the surface S of the workpiece W. As illustrated in Figure 1, the surface inspection system 20 is arranged as a workstation including a worktable 21. Positioned on the worktable 21 is a generally closed and substantially lightproof housing 22, a video display 23, a keyboard 25, and a mouse 26. A cabinet 27 is suspended from the worktable for carrying a system controller 50. Adjacent the cabinet 27 is a shelf unit 28 for carrying a printer 29 and associated printing paper 29a. The housing 22 has been partially broken away to better illustrate the inspecting arrangement of the present invention. The inspection of the wafer W preferably is conducted in an inspection zone Z on an inspection table 31. A robotic wafer handling device 32 is located adjacent the inspection station 20 to load and unload wafers W from a cassette 33 onto the table 31. The cassette 33 holds a number of wafers W and is loaded into the cabinet 27 through a door (not shown). The handling of the wafers W inside the housing 22 is done automatically without contact by human hands to avoid contamination or smudges.

As best illustrated in Figures 1-3, the surface inspection system 20 preferably includes means for translationally transporting a workpiece W along a material path P. The means for transporting a workpiece W is illustrated as a transporter 40 arranged to translationally transport a workpiece W along a material path P and preferably through an inspection zone or area Z. The translational transporter 40, as illustrated, preferably includes a gear 42, a motor 41 including a shaft 41a arranged for rotating the gear 42, and guides 36, 37 having teeth formed integral therewith. The motor 41 and gear 42 mounted on the motor shaft 41a form a chuck for the system 50. The motor 41 of the chuck is preferably mounted to a stage member 43 having a plurality of flanges 43a extending upwardly therefrom which receives the workpiece W, i.e., silicon wafer, thereon along edges of the workpiece W as illustrated. This mounting technique for the workpiece W reduces smudges or other surface problems which may be associated with positioning the lower surface of the workpiece so as to abuttingly contact an upper surface of the stage member 43. The stage member 43 preferably is translationally transported along stage guide members 38, 39 secured to an underside thereof. Other translational and/or rotating means such as a piston and cylinder configuration mounted to the stage member and a motor for rotating the stage member as understood by those skilled in the art may also be used according to the invention. Also, means for rotating a workpiece W, illustrated as a rotator 45, is associated with the transporter 40 and arranged to rotate a workpiece W during translational travel along the material path P. The rotator 45 as illustrated preferably includes a motor 46 mounted to an underside of the stage member for providing rotation of the wafer mounted thereon at a predetermined speed. The transporter 40 and the rotator 45 preferably are synchronized and arranged with a scanner 80 so as to form a spiral-shaped narrow angle scan (α) across the surfaces of the workpiece during rotational and translational travel along the material path P.

As illustrated in Figures 1 and 3-5, a scanner 80 is positioned and arranged to scan a surface of a workpiece W during rotational and translational travel along the material path P. It will also be understood, however, by those skilled in the art that the scanner 80 may be arranged for rotational and/or translational movement while the workpiece W is stationary, or translationally or rotatively moved. In addition, other material paths P may be used, e.g., neither the workpiece W nor the scanner 80 may be translationally moved and the workpiece tested in only a rotational path. Accordingly, the present invention includes a P-polarized light source 81 or a light source coupled with a P-polarized filter positionally aligned with the light source to generate a P-polarized light beam B therefrom, means for receiving the light source and scanning a surface S of a workpiece W, i.e., a mirror 82, lenses 84, 86, deflector 85, and means for imparting a rotational and translational scan of the workpiece W, i.e., the transporter 40 and the rotator 45.

The scanner 80 preferably includes a light source 81, i.e., laser, arranged to either generate a linear polarized or P-polarized light beam B therefrom or coupled with a P-polarized filter positionally aligned with the light source. The P-polarized light preferably has a spot size that includes a full width, half-maximum of less than 0.1 millimeters. The scanner also includes means positioned to receive the light beam B and arranged for scanning the light beam B along a relatively narrow scan path (α) across a surface S of the workpiece W as the workpiece W rotationally and translationally travels along the material path P. The light source 81 is preferably a visible-light laser with a relatively short wavelength, such as Argon-Ion or solid state, as understood by those skilled in the art. The laser 81 is also preferably the combination of a laser with external optics as understood by those skilled in the art. The laser 81 preferably has a beam diameter of about 0.6 millimeters ("mm").

The scanning means preferably includes a deflector 85, as illustrated, positioned to receive the light beam B and arranged to deflect the light beam B along a relatively narrow scan path (α). The deflector 85 is preferably an acousto-optical (AO) deflector as illustrated or a resonant scanner, and the relatively narrow scan path (α) is preferably no greater than 0.1 radians and, more particularly, in the range of 0.025-0.040 radians. The scan path α preferably directionally corresponds to the path P of translational travel and, as best illustrated in Figure 4, preferably is in a generally parallel direction therewith as illustrated by the arrows. The deflection is accomplished by exciting a crystal with high frequency sound waves, for example, which interact with the incident light wave in such a way to shift the light beam B and thereby change the angle of propagation. It will be understood that various frequencies of the crystal will responsively cause the light passing therethrough to be deflected at correspondingly various angles of propagation. If the frequency of the sound waves is swept in a sawtooth pattern, the laser beam B is scanned through an angle (α) proportional to the frequency. The AO deflector 85 preferably provides a constant scanning speed which, in turn, provides a consistent or a predetermined time response for particles or defects detected from an article surface. Although the present invention is described with reference to an AO deflector 85, other means for providing narrow angle scans as may also be used according to the present invention as understood by those skilled in the art, such as a galvanometer, a piezoelectric scanner, a resonant scanner, a rotating mirror, a scanning head, other electronic scanners, or the like. Also, a beam expander 82 is preferably positioned between the laser source 81 and the deflector 85 to expand the light beam B prior to entering the acousto-optical deflector 85. The beam expander 82 preferably provides means for more fully filling the active aperture of the deflector 85 to best utilize the scan angle of the deflector 85.

The scanner 80 also preferably includes means positionally aligned with the deflector 85 and arranged for directing the light beam from the narrow scan path (α) toward a surface S of a workpiece W at a relatively high angle of incidence (θi) (measured from normal to the workpiece) as the workpiece W rotatively and translationally travels along the material path P. Although a high angle of incidence (θi) is preferred, the angle of incidence may be any angle other than normal to the workpiece W to provide the advantages of the present invention. The angle of incidence is preferably greater than 45 degrees from normal to the article surface, i.e., less than 45 degrees from the surface of the workpiece W and, more particularly, is preferably in the range of 65-85 degrees from normal to the article surface.

The directing means is illustrated as a mirror 82 and a plurality of optical lenses 84, 86 arranged to direct the light beam B from the laser 81 toward the surface S of the workpiece W to be inspected. As the light beam B travels from the AO deflector 85, the beam B passes through a cylindrical lens 84 which preferably angularly orients the light beam B for a linear scan of the surface of the article during translational and rotational movement of the article through the inspection zone. A stop member 87 is positionally aligned with the cylindrical lens 84 positioned closely adjacent the AO deflector 85 to stop the relatively small portion of light which is not linearly oriented for the scan of the surface of the workpiece W. The optical lens 86 positioned after the cylindrical lens 84 is a focusing or f-theta lens, as understood by those skilled in the art, arranged for focusing the light beam on the surface of the workpiece W.

The scanner 80 according to the present invention preferably scans the beam of light B in a radial direction with rotating motion and linear, lateral, or translational motion (Y) to implement a spiral scan pattern as best illustrated in Figure 3. Nevertheless, any other material path P for the workpiece W may also be used to provide the advantages of the invention.

As best illustrated in Figures 1, 3, 3 A, and 6-7, means for collecting light from the surface of a workpiece is preferably a collector 100 having a light channel detector 110 arranged for detecting light specularly reflected from the surface S of a workpiece W and a dark channel detector 120 positioned adjacent the light channel detector 110 for detecting light scattered from the surface S of a workpiece W. The light channel detector 110 may be a PMT or a photodiode, but preferably, as understood by those skilled in the art, is a quadrant-cell device, i.e., detector, arranged for X-Y coordinate positioning detection so that deviation in the path of reflected light, i.e., during detection of a defect or particle, may be determined. Such quadrant-cell detectors are manufactured by Advanced Photonix, Inc., formerly Silicon Detector Corp., of Camarillo, California. Although a particular configuration is illustrated, it will be understood that various other rectangular or multiple cell, i.e., bi-cell, configurations may also be used according to the present invention.

The dark channel detector 120 preferably includes a plurality of collectors 121, 123, 125, 127 positioned closely adjacent each other and arranged for collecting components of the scattered light at different respective predetermined angles from the surface S of the workpiece W. The plurality of collectors 121, 123, 125, 127 of the dark channel detector 120 form segmented optics having at least two collectors positioned adjacent each other. The plurality of collectors 121, 123, 125, 127 as illustrated will be understood by those skilled in the art to be compound lenses, and other lens arrangements may also be used according to the present invention. One of the collectors 121 is a forward channel collector. It is positioned in the plane of incident light beam forwardly of where the beam scans the wafer W and is arranged to collect light components scattered forwardly from the surface S of the workpiece W at a relatively small angle θa. A center channel collector 123 is positioned closely adjacent the forward channel collector 121, also in the plane of the incident light beam, and is arranged to collect light components scattered substantially normal from the surface S of the workpiece W at a relatively medium angle θb. A back channel collector 125 is positioned adjacent the center channel collector 123 out of the plane of the incident light beam and is arranged to collect light components scattered backwardly from the surface S of the workpiece W at a relatively large angle θc. An additional off-axis collector 127 is positioned at a polar angle θs and at an azmuthal φs selected as described herein for minimizing the topographic scatter component of the scattered light. The dark channel detector 120 further includes a forward channel detector 122, a center channel detector 124, a back channel detector 126, and an off-axis channel detector 128, each respectively positioned in optical communication with a corresponding collector 121, 123, 125, 127. A polarizing filter 129 is positioned in front of the off-axis channel detector 128. Filter 129 is oriented to minimize the light scattered by the rough surfaces. In the illustrated embodiment, the collector 127 is located where the P component of the scattered light is zero, and the polarizing filter 129 is oriented eliminate the S polarized light scattered from the surface. Signals from the respective detectors 122, 124, 126 and 128 are directed to electronic signal discrimination circuitry 150.

As best illustrated in Figures 1, 3, and 6, the relative respective angles θa, θb, θc of the plurality of collectors 121, 123, 125 are preferably determined with respect to the angle of reflection θr of light from the surface S of the workpiece W and with respect to forward , backward , and substantially normal light component scattering which occurs relative to the angle of incidence θi of the scan. For example, if the angle of incidence θi is relatively high, e.g., -75° from normal (15° from horizontal), then the forward scattering or small angle θa is preferably about +22° to +67°, the substantially normal scattering or medium angle θb is about -25° to +20°, and the backward scattering or large angle θc is about -72° to -27.

The dark channel collector 120 and associated signal discrimination circuitry 150 according to the present invention preferably analyzes the characteristics of surface S and particle scattering such as for use on polished wafers and various deposited films. When certain conditions, i.e., most related to allowable levels of surface roughness, are met, the distribution of light scattered from a surface (BRDF) can be expressed as:

BRDF = [16π2 cos θi cos θs Q S(fx, fy)]/λ4 where θi is the incident angle, θs is the scattering angle, Q is the reflectance (or polarization coefficient) at the wavelength and polarization of the incident light, S is the power spectral density characteristic of the surface roughness, λ is the wavelength of incident light, fx, fy are the spatial frequencies which, in turn, are expressed in terms of incident and scattering angles as follows: fx = (sin θs cos φs - sin θj) / λ fy = sin θs sin φs / λ. In these equations, θ always refers to angles in the plane of incidence and φ represents azimuthal (out-of-plane) angles. The shape of the BRDF curve is defined by S(fx,fy) and the cosine terms in the above equation, which provide acceptable results given good power spectral density information. The magnitude of the curve is determined primarily by the reflectance Q.

The reflectance of the surface of the article and a particle or defect detected thereon is dependent on the dielectric constant of the material or particle inspected, i.e., silicon, aluminum. Also, the reflectance of a material or particle illuminated with P polarized light, preferable according to the present invention, will have different characteristics than a material or particle illuminated with S polarized light. The reflectance of a dielectric illuminated with P polarized light is zero at a certain angle, i.e., Brewster's angle, and is a function of the index of refraction (n) as follows: θb = tan'1 n. Metals and other absorbing materials, for example, exhibit curves with similar shapes; however, the reflectance for P polarization reaches a non-zero minimum. The angle where this non-zero minimum occurs may be referred to as a pseudo-Brewster's angle or alternately as the principal angle. The principal angle is dependent on the complex dielectric constant (n' = n-ik) and can be found by evaluating the following expression on an iterative basis:

(n2 + k2)1/2 = sin2θp / cos θp. For example, although aluminum and silicon are different, aluminum being a strong absorber and silicon exhibiting dielectric characteristics with a high index, the principal angle of both materials is close to the same, i.e., about 78° (aluminum 78.1 °, silicon 77.8°). Almost all other materials of interest will have indices no larger than aluminum or silicon and therefore will have principal angles equal to or less than about 78°, e.g., silicon dioxide has an index of refraction of about 1.65 which equates to a principal angle of about 58.8°. The characteristics of dielectric films are also dependent on the substrate and film thickness, and the reflectance curve may exhibit minima at several angles. Data representative of these various values from the BRDF curve, the indices of refraction, and dielectric constants may be used to determine particle or defect identification information for the surface inspection system 50. A relative signal to noise ratio for determining the scattering angle may be obtained by taking the particle response and dividing by the square root of the BRDF. This provides an equivalent signal to noise ratio in the limit where the dominant source of noise is the Poisson fluctuation of the "haze" signal as understood by those skilled in the art. This comparison of the resulting angles of collected light from the dark channel collector 120, i.e., scattered light, is compared to known characteristic data such as in a data table to assist with this determination and then classify the defect accordingly. These comparative steps are preferably performed according to predetermined command signals, i.e., a software program, resident in the structural hardware or on a stored disk or the like as understood by those skilled in the art.

It can be demonstrated through measurement and modeling that there are certain combinations of input polarization, detector polarization and optical geometry that cause the intensity of scattered light from surface roughness to almost disappear. In particular, this involves appropriate positioning of the off-axis collector 128 and orientation of its polarization filter 129. The ideal location for collector 128 can be determined by plotting Qpp, the polarization factor, from equation (5) above. Figures 7 and 8 show the geometry and position for the collectors 122, 124, 125 and 127 of the scanning apparatus illustrated and described herein. In Figures 9 to 12, Qpp is plotted for several important materials, namely silicon, titanium, aluminum and tungsten, for this particular exemplary geometry, through a plane separated from the incident plane by 45 degrees

To analyze detection performance on an actual tungsten surface, BRDF was measured on an industry standard. POLAR output, which is in the form of Differential Scattering Cross Section (DSC) from two different PSL sphere sizes (130 nm and 398 nm) is plotted along with the surface DSC (derived from the measured BRDF) in the two charts of Figures 14 and 15. The first chart (Figure 14) represents the configuration and detection optics of an earlier scanner design where there is no out-of- plane detection. The second chart (Figure 15) represents the same basic configuration, but with the collector located out-of-plane and including the effect of a polarizer in the detection optics.

From this data for the various materials of interest, it has been determined that locating the detector 128 as shown in Figures 7 and 8, centered -51 degrees from vertical and including a polarizer oriented to capture only the P polarized light, will provide substantial performance improvement on the materials of interest. Since the dips on aluminum and silicon are fairly narrow, it may be advantageous to add an aperture stop in front of the detector to narrow the field of view. The analysis using measured tungsten surface data shows two to three orders of magnitude improvement in particle signal strength relative to the surface scatter. Similar levels of improvement appear to be obtainable on aluminum and polysilicon substrates.

Claims

THAT WHICH IS CLAIMED IS:
1. A method of detecting defects on the surface of a workpiece, said method comprising: directing a polarized light beam onto the surface of the workpiece at a non-normal incident angle; collecting light scattered from the surface of the workpiece at a selected location in the scattering hemisphere, the scattered light including a component resulting from defects present on the workpiece surface and a component due to topographic roughness of the workpiece surface; filtering the collected scattered light through a filter oriented to minimize the topographic scatter component in the collected scattered light; and detecting the thus-filtered light
2. A method according to claim 1, wherein the step of collecting scattered light is performed at a location in the scattering hemisphere out of the plane defined by the incident light beam and selected so that the value of one of the polarization factors for the scattered field is minimized.
3. A method according to claim 2, wherein the polarized light beam which is directed onto the surface of the workpiece is P-polarized, and wherein said location for collecting scattered light is selected so that the value of the polarization factor for the P- polarized incident and P-polarized scattered fields is minimized.
4. A method according to claim 3, wherein said filtering step comprises filtering the collected scattered light through a polarizer oriented to minimize the topographic scatter component in the collected scattered light.
5. A method according to any one of claims 1 to 4, wherein said filtering step comprises filtering the collected scattered light through a polarizer and a wave plate, each independently oriented to minimize the topographic scatter component in the collected scattered light.
6. A method of detecting defects on the surface of a workpiece, said method comprising: directing a P-polarized light beam onto the surface of the workpiece at a non normal incident angle θi; collecting light scattered from the surface of the workpiece, the scattered light including a component resulting from defects present on the workpiece surface and a component due to topographic roughness of the workpiece surface, wherein the collecting is performed at a selected location in the scattering hemisphere out of the plane defined by the incident light beam and represented by the angles θs and φs, the selected angles θs and φs being such as to minimize the value of Qpp in the equation:
Figure imgf000019_0001
filtering the collected scattered light through a polarizer oriented to minimize the topographic scatter component in the collected scattered light.
7. Apparatus for detecting defects on the surface of a workpiece, comprising: a light source for oriented to direct a polarized light beam onto the surface of the workpiece at a non-normal incident angle; a light collector positioned for collecting light scattered from the surface of the workpiece at a selected location in the scattering hemisphere, the scattered light including a component resulting from defects present on the workpiece surface and a component due to topographic roughness of the workpiece surface; a filter positioned for receiving and filtering the collected scattered light, the filter being oriented to minimize the topographic scatter component in the collected scattered light; and a detector for detecting the thus-filtered light.
8. Apparatus according to claim 7, wherein the collector of scattered light is positioned at a location in the scattering hemisphere out of the plane defined by the incident light beam and selected so that the value of one of the polarization factors for the scattered field is minimized.
9. Apparatus according to claim 8, wherein the light source produces a P-polarized light beam, and wherein said selected location for collecting scattered light is such that the value of the polarization factor for the P-polarized incident and P-polarized scattered fields is minimized.
10. Apparatus according to claim 9, wherein said filter comprises a polarizer oriented to minimize the topographic scatter component in the collected scattered light.
11. Apparatus according to claim 7, wherein said filter comprises a polarizer and a wave plate, each independently oriented to minimize the topographic scatter component in the collected scattered light.
12. Apparatus according to claim 7, wherein said light source directs a P- polarized light onto the surface of the workpiece at a non normal incident angle θi; and wherein said light collector is positioned at a selected location in the scattering hemisphere out of the plane defined by the incident light beam and represented by the angles θs and φs, the selected angles θs and φs being such as to minimize the value of Qpp in the equation:
Figure imgf000020_0001
a polarizing filter for filtering the collected scattered light, said polarizing filter being oriented to minimize the topographic scatter component in the collected scattered light.
PCT/US1998/027752 1997-12-22 1998-12-22 Method and apparatus for reducing roughness scatter as a noise source in wafer scanning systems WO1999032879A1 (en)

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