Optics: measuring and testing – Inspection of flaws or impurities – Transparent or translucent material
Utility Patent
1999-06-22
2001-01-02
Font, Frank G. (Department: 2877)
Optics: measuring and testing
Inspection of flaws or impurities
Transparent or translucent material
C356S369000, C356S237100
Utility Patent
active
06169601
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to methods and apparatus for performing optical inspection of smooth manufactured surfaces of a substrate such as silicon wafers, magnetic media, glass plates, or the like, so as to detect defects including particles on the surface and subsurface defects such as pits and scratches in the surface and voids below the surface. More particularly, the invention relates to methods and apparatus for discriminating between particles and subsurface defects.
BACKGROUND OF THE INVENTION
Optical inspection techniques are increasingly being used for inspecting smooth surfaces of articles such as silicon wafers, computer disks, glass plates, and the like, for detecting very small defects. In many applications, it is desirable to be able to detect particles on the surface, pits in the surface, voids beneath the surface, microscopic scratches, and other types of defects.
Optical inspection methods based on the scattering of light from a defect have been developed and have been used for several years as a means of detecting and mapping defects and contamination on smooth surfaces. Most such methods have not been capable of discriminating between particles and subsurface defects, but merely detect the presence of a defect and its size.
In some applications, however, it can be important to be able to distinguish particles from subsurface defects. For example, in the semiconductor industry, silicon wafers that are found to have particles on the surface after polishing can be subjected to further cleaning operations in order to cure the defects. However, if the defects are pits in the surface or voids beneath the surface, further cleaning will not be effective for curing the defects. If the wafer inspection system is not capable of discriminating between the two types of defects, then inevitably time and resources will be futilely expended attempting to remedy some defective wafers having pits and/or voids. Furthermore, if the manufacturer is unable to classify defects as pits or voids, it is more difficult to take appropriate steps to reduce the incidence of pits and voids, which typically are caused during the bulk manufacturing of silicon from which wafers are made.
Similarly, patterned wafers are typically inspected following a chemical-mechanical polish (CMP) operation in order to detect surface defects in the polished surface of the patterned wafer. In the course of the CMP operation, microscopic scratches (e.g., on the order of 0.25 &mgr;m wide by 0.25 &mgr;m deep by 5 &mgr;m long) are sometimes formed in the oxide film layer of the wafer. This can be caused, for example, by contamination of a polishing pad by foreign matter. It is important to be able to distinguish between such scratches and particles on the surface of the wafer. If the defect can be identified as a scratch, then adjustments can be made to the CMP process in order to prevent or reduce the scratching.
Accordingly, recent efforts in the optical inspection field have focused on discriminating between particles and subsurface defects such as pits, scratches, and voids. For example, commonly owned U.S. Pat. application Ser. No. 08/958,230 filed Oct. 27, 1997, hereby incorporated herein by reference, discloses methods and apparatus for distinguishing between pits and particles by detecting variations in the intensity of scattered light with angular position relative to the defect.
SUMMARY OF THE INVENTION
The present invention enables discrimination of particles and subsurface defects by detecting changes in the total intensity of light scattered over a wide angular range as a function of the incident angle of a polarized light beam, or as a function of the polarization of the incident beam.
In accordance with a preferred embodiment of the invention, a method of differentiating particles and subsurface defects comprises directing a first beam of P-polarized light to impinge at a first incident angle on the surface being inspected, collecting light scattered from the surface over a predetermined area, and integrating the intensities of the scattered light over this area to form a first integrated response. A second beam of the P-polarized light is directed to impinge on the surface at a second incident angle which is greater than the first incident angle, and light scattered from the surface is collected and the intensities of the scattered light are integrated over the same area to form a second integrated response. The first and second integrated responses are compared to determine whether a defect is a particle or a subsurface defect. Particles are characterized by the second integrated response being greater than the first integrated response, while subsurface defects are characterized by the second integrated response being less than the first integrated response.
In a preferred embodiment of the invention, the first beam is directed at an incident angle of about 5-20 degrees (measured from the surface normal) and the second beam is directed at an incident angle of about 50-75 degrees. The scattered light advantageously is collected over a range of angles from negative to positive as measured from a plane normal to the surface.
Various devices may be used for collecting the scattered light and integrating the intensities to produce the integrated responses. In one preferred embodiment, a wide angle collector is used to collect the scattered light. In another preferred embodiment, a plurality of collectors are distributed over the angular range for collecting the scattered light.
In accordance with another preferred embodiment of the invention, particles are distinguished from subsurface defects on the basis of the ratio of the integrated response produced by a P-polarized incident beam to the integrated response produced by an S-polarized incident beam at the same incident angle. More specifically, the defect is a particle if the ratio of P-to-S integrated response is greater than a predetermined value; conversely, if the P-to-S ratio of integrated response is less than the predetermined value, then the defect is a subsurface defect such as a pit, scratch, or void. Defects can be classified as particles based on these ratios regardless of the composition of the particles or their shapes.
In still another embodiment of the invention, an incident beam containing both P-polarized and S-polarized light can be used, such as circularly or elliptically polarized light. Two collectors are disposed symmetrically on either side of the incident plane. One of the collectors detects P-polarized light components of the scattered light, and the other collector detects S-polarized light components of the scattered light. The P-to-S ratio is then used to classify defects as pits or particles.
REFERENCES:
patent: 3904293 (1975-09-01), Gee
patent: 4469442 (1984-09-01), Reich
patent: 4740708 (1988-04-01), Batchelder
patent: 4794264 (1988-12-01), Quackenbos et al.
patent: 4794265 (1988-12-01), Quackenbos et al.
patent: 4889998 (1989-12-01), Hayano et al.
patent: 4893932 (1990-01-01), Knollenberg
patent: 4933567 (1990-06-01), Silva et al.
patent: 4966457 (1990-10-01), Hayano et al.
patent: 5355212 (1994-10-01), Wells et al.
patent: 5389794 (1995-02-01), Allen et al.
patent: 5424536 (1995-06-01), Moriya
patent: 5465145 (1995-11-01), Nashashige et al.
patent: 5486919 (1996-01-01), Tsuji et al.
patent: 5625193 (1997-04-01), Broude et al.
patent: 5712701 (1998-01-01), Clementi et al.
patent: 5936726 (1999-08-01), Takeda et al.
patent: 6034776 (2000-03-01), Germer et al.
patent: 5-142156 (1993-06-01), None
patent: 06229939 (1994-08-01), None
Eremin Yuri A.
Scheer Craig A.
Stover John C.
ADE Optical Systems
Alston & Bird LLP
Font Frank G.
Lauchman Layla
LandOfFree
Method and apparatus for distinguishing particles from... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for distinguishing particles from..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for distinguishing particles from... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2448882