llllllllllllll|Il|||||lllllIIIIIllllllllllIllllllllllllllllllllllllllllllll USOO5479252A United States Patent   Patent Number: Worster et al.   LASER IMAGING SYSTEM FOR INSPECTION AND ANALYSIS OF SUB-MICRON PARTICLES 5,479,252 Date of Patent: Dec. 26, 1995 OTHER PUBLICATIONS A. F. Slomba et al., “A Laser Flying Spot Scanner for Use in Automated Fluorescence Antibody Instrumentation,” Journal of The Association For The Advancement of Medi cal Instrumentation’ VOL 6, N0 3’ May_Jun_ 1972’ pp_ 23O_234_  Inventors: Bruce W. Worster, Saratoga; Dale E. Crane; Hans J. Hansen, both of Pleasamon; ChriSmPher R- Farley’ San Jose; Ken K‘ Lee’ Los Altos’ all of Calif H. M. Nier, “Automatic Moving Part Measuring Equip ment,” IBM Technical Disclosure Bulletin, vol. 22, No. 7, Dec. 1979, pp. 2856-2857. G. I. Brakenho?" et al., “Confocal Scanning Light Micros  Assignee; Um-apoime Corporation, San Jose, Calif copy with High Aperture Immersion Lenses,” Journal of Microscopy, vol. 117, pt. 2, Nov. 1979, pp. 2l9—232. (List continued on next page.)  Appl. No.1 80,014 Primary Examiner-Robert P. Limanek Assistant Examiner—David B. Hardy  Filed: Attorney, Agent, or Firm-Skjerven, Morrill, MacPherson, Jun. 17, 1993  Int. c1.6 ................................................... .. G01N 21/88 Frankhn 8‘ Fuel; Alan H‘ Macpherson  U.S. Cl. .................. .. 356/237; 356/369; 250559.42;   250/559'48 Field Of Search ................................... .. 356/237, 369, A laser irna in s stem is used to anal ze defects on g wafers g y that have been detected y by patterned semiconductor 356/317, 445; 250/563, 572 ABSTRACT wafer defect detecting systems (wafer scanners). The laser imaging system replaces optical microscope review stations  ' References Cited now utilized in the semiconductor fab environment to exam ine detected optical anomalies that may represent wafer U‘S‘ PATENT DOCUMENTS defects. In addition to analyzing defects, the laser imaging system can perform a variety of microscopic inspection Re. Carlsson et ... . .. . . . . . . .. functions including defect detection and metrology The 2,758,502 8/1956 Scott et a1. ................ _. 356/100 et a1‘ "" " laser imaging System uses confocal laser scanning mime copy. techniques,_and operates under class I cleanroom 3:049:04? 8/1962 Polanyi 665i.-'::........,............':: 88/14 condm‘ms and wlthout exposure "f the wafers to Operator contamination or air?ow. Unlike scanning electron micro scopes (SEMs) that have previously been used for defect (List continued on next page.) analysis, the laser imaging system will not damage samples or slow processing, costs signi?cantly less to implement than an SEM, can produce a three dimensional image which provides quantitative dimensional information, and allows FOREIGN PATENT DOCUMENTS 0052392 0112401 0155247 1185339 2132852 2152697 2184321 6/1932 7/1984 9/1985 European paL Off , European Pat. Off. . European Pat. O?”. . 3/ 1970 United Kingdom - 7/1984 8/1985 6/1987 United Kingdom . United Kingdom . United Kingdom . WO79/O1027 11/1979 sub-surface viewing of defects lying beneath dielectric lay ers. The laser imaging system is adaptable to cluster or in-situ applications, where examination of defects or struc tures during on-line processing can be performed. WIPO . 29 Claims, 6 Drawing Sheets are r 214 U* E 6 215 CPU Graphics Processor‘ 5,479,252 Page 2 US. PATENT DOCUMENTS 3,187,627 3,360,659 6/1965 Kapany ..................................... .. 88/39 12/1967 Young ..... . . . .. 250/236 & Co. (Publishers) Ltd, pp. 52-56. T. Wilson et al., “Dynamic Focusing in the Confocal Scan ning Microscope,” Journal of Microscopy, vol. 128, Pt. 2, Nov. 1982, pp. 139-143. 3,497,694 2/1970 Jura et a1. . 250/202 3,602,572 8/1971 . . . .. 350/7 I. J. Cox, “Electronic Image Processing of Scanning Optical 3,705,755 12/1972 . . . . .. 350/6 Microscope Images,” International Conference on Elec 3,719,776 Norris Baer .. . . . . ... ... ..... 3/1973 Fujiyasu et al. .. 178/67 R 3,764,512 10/1973 Greenwood et a1. .. 3,775,735 11/1973 3,782,823 Funk et a1. ........ .. D. K. 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Mirkin .. ... . . ... ..... . . . . . . .. 350/91 356/237 .... .. 250/306 250/461 2 250/458.1 4,366,380 12/1982 4,379,135 4/1983 Sasaki et a1. .. .... .. 436/536 4,379,231 4/1983 Shii et a1. . . . . .. ....... ... . . . . .. 250/306 250/311 4,405,237 9/1983 Manuccia et al. . 4,406,015 9/1983 Koga ........... .. 4,406,525 9/1983 Itoh et al. .... .. 4,407,008 9/1983 Schmidt et al. ........................ .. 358/93 4,455,485 256/301 378/50 .. 350/486 6/1984 Hosaka et al. . 4,485,409 ll/1984 4,549,204 10/1985 Bertero et al. . 4,631,581 Schumaeher .......................... .. 358/294 12/1986 Carlsson .................................. .. 358/93 4,636,069 l/1987 Balasubramanian .. 4,733,063 3/1988 Kimura et al. ...... .. 356/71 .. 250/201 4,827,125 5/1989 Goldstein et a1. 4,863,226 9/1989 Houpt et al. 5,034,613 5,035,476 5,046,847 5,091,652 7/1991 7/1991 9/1991 2/1992 Denk et al. Ellis et a1. Nakata et a1. Mathies et al. 5,117,466 5/1992 Buican et al. 5,122,653 5,127,726 6/1992 Ohki ............ .. 7/1992 Moran ......... .. 250/216 356/237 7/1992 Brelje et al. .. 356/318 5,127,730 5,153,428 5,162,641 .. 250/234 350/6.5 250/458.1 350/6.5 356/237 250/458.1 . . .. 625-626. Philip G. Stein, “Image-Analyzing Microscopes,” Analyti cal Chemistry, vol. 42, No. 13, Nov. 1970, pp. 103A-106A. G. J. Brakenhoff, “Imaging Modes In Confocal Scanning Light Microscopy (CSLM),” Journal of Microscopy, vol. 117, pt. 2, Nov. 1979, pp. 233-242. W. Jerry Alford et al., “Laser Scanning Microscopy,” Pro ceedings of the IEEE, vol. 70, No. 6, Jun. 1982, pp. 641-651. H. J. B. Marsman et al., “Mechanical Scan System for Microscopic Applications,” Review of Scienti?c Instru Sheppard ......... .. 4,223,354 tronic Image Processing, Jul. 26-28, 1982, pp. 101-104. 204/299 . . . . . .. 382/6 10/1992 Ellis ............ .. 250/234 11/1992 Fountain ............................... .. 250/201 OTHER PUBLICATIONS ments, vol. 54, Aug. 1983, pp. 1047-1052. Shura Agadshanyan et al., “Morphoquant-An Automatic Microimage Analyzer of the JENA Optical Works,” JENA Review, JR 6, 1977, pp. 270-276. C. J. R. Sheppard et al., “Optical Microscopy with Extended Depth of Field,” Proc. R. Soc. Lond. A, vol. 387, 1983, pp. 171-186. N. Aslund et al., “PHOIBOS, A Microscope Scanner Designed for Micro-Fluorometric Applications, Using Laser Induced Fluorescence,” Physics IV, Royal Institute of Technology, S-100 44 Stockholm 70, (1983 Publication), pp. 338-343. Eric A. Ash (edited by), Scanned Image Microscopy, Aca demic Press, 1980, pp. 183-225. P. Davidovits et al., “Scanning Laser Microscope for Bio logical lnvestigations,” Applied Optics, vol. 10, No. 7, Jul. 1971, pp. 1615-1619. I. J. Cox et al., “Scanning Optical Microscope Incorporating a Digital Framestore and Microcomputer,” 2219 Applied Optics, vol. 22, May 1983, No. 10, pp. 1474-1478. Paul Davidovits et al., “Scanning Laser Microscope,” Nature, vol. 223, Aug. 23, 1969, p. 831. A. Boyde et al., “Tandem Scanning Reflected Light Micros copy of Internal Features in Whole Bone and Tooth Samples,” Journal of Microscopy, vol. 132, Pt. 1, Oct. 1983, pp. l-7. Mojmir Petran et a1. “Tandem-Scanning Re?ected Light Microscope,” Journal of the Optical Society of America, vol. 58, No. 5, May 1968, pp. 661-664. D. K. Hamilton et al., “Three-Dimensional Surface Mea surement Using the Confocal Scanning Microscope,” Applied Physics B 27, 1982, pp. 211-213. David A. Agard, “Three-Dimensional Architecture of a Polytene Nucleus,” Nature, vol. 302, Apr. 21, 1983, pp. C. J. R. Sheppard et al., “Depth of Field in the Scanning 676-680. Microscope,” Optics Letters, vol. 3, No. 3, Sep. 1978, pp. Kenneth R. Castleman, Digital Image Processing, 1979 Prentice-Hall, Inc., 1979, pp. 351-359. 115-117. I. J. Cox et al., “Digital Image Processing of Confocal Images,” Image And Vision Computing, 1983 Butterworth IBM Tech. Disclosure Bulletin, vol. 18, No. 12, 1976, p. 1474. US. Patent Dec. 26, 1995 Sheet 1 of 6 106 FIG. 1 5,479,252 US. Patent Dec. 26, 1995 Sheet 2 of 6 5,479,252 .UIm 8329&20c6%891 mom wow EN wow 5 Q NE mom 8N\EN mmmZN mom omm 5NNON mmm I IE“) EI] US. Patent Dec. 26, 1995 Sheet 3 of 6 5,479,252 I- ________________________ — _I I I : I GALVANOMETER sCANNER REsONANT SCANNER I I 309 :/ l l I I : I LINE SCANNER _, PIXEL CLOCK DRIVER GENERATOR I I PAGE SCANNER I DRIVER I I I I I | : : ‘ : PAGE sCANNER CONTROLLER ; r """""""""" T". I i SCANNER NODE I : I SYNC l I I A I—————————————————————————— — —I BRIGHT/DARK FIELD sTEPPER I I 308\ LON , I ROBOT 306 ‘__ LAsER ATTEN. FILTER WHEEL ‘ ’ PREAUGNER l ‘_________I LINE SELECT <-> CONTROLLER x STAGE <______J' FILTER WHEEL Y STAGE 304 VIDEO CAMERA 0 308 \y FILTER WHEEL 2 STAGE CONTROLLER LON I LAS ER PS NODE II LASER POWER SUPPLY H G - 3A LAsER HEAD I ‘ __. I ' WHITE LIGHT I .' I I LAMP MODE HALOGEN POWER SUPPLY I <_______, CONTROLLER I WHITE LIGHT LAMP I ' : US. Patent Dec. 26, 1995 Sheet 4 of 6 5,479,252 | I l | FIG. 3 B ' I ' PIEzO STAGE POSITION SENSOR I ' PMT l l I I TuRRET I l l —’ : i FINE Z-AXIS PMT/ PREAMP CONTROLLER NODE 310 1 T | TuRRET NODE ; IL V l l I l l l I I I l | L________—__.> l l—_._._.> LASER VIDEO I AND SYNC I I l I SDP FRAME GRABBER l, 307 f 305 v v f I WHITE LIGHT OAMERA i, 302 f v l l LON : 'NTERFACE BS2320 SERIAL poms SDP INTERFACE BOARD _____________________________________ __ , OPTION SLOT ‘I OPTION SLOT 0 303 STARTER VIDEO SILICON ORAPI-IIOS IRIS INDIGO / I WORKSTATION GIO BUS : US. Patent Dec. 26, 1995 Sheet 5 0f 6 5,479,252 | ' FIG. 3C CONFOCAL I MIRROR I . ' ' SINGLE LINE LASER POWER LASER SHUWER MONITOR l I l I OPTICS SRUTIERS I POWER MON NODE CONTROL l ' , CASSETTE SWITCH NODE I I | | OOOR ‘_--> I I l <______. SOLENOID NODE l T ———l HOUSE CHUCK CHUCK WAFER VACUUM SENSOR VACUUM SOLINOIDS VACUUM SENSOR DOOR SOLENOID I l KEY TO FIG. 3 FIG. 3A FIG. 38 FIG. 3 FIG. 36 POSITION SENSOR 5,479,252 1 2 LASER IMAGING SYSTEM FOR INSPECTION AND ANALYSIS OF SUB-MICRON PARTICLES The video systems generally cost about three times as much as the laser scanning systems, i.e., the laser scanning systems typically cost approximately $350,000 while the video systems typically cost approximately $1,000,000. However, while the laser scanning systems are more effec tive in detecting bumps than in detecting pits, the video CROSS-REFERENCE TO RELATED systems work well in detecting either bumps or pits, and can APPLICATIONS also sense subsurface defects. As these wafer scanners were developed, the need to This application is related to the commonly owned, c0 identify positively the nature, e.g., type of material, type of pending U.S. patent application entitled “Surface Extraction defect (defects are classi?ed broadly as particulate or pro cess ?ow defects; there are many sub-types within each of from a Three~Dimensional Data Set,” by Ken K. Lee, application Ser. No. 08/079,193, ?led on the same date as the these classi?cations), and the precise location and size of the defects was not appreciated. This information is important present application and incorporated by reference herein. 15 BACKGROUND OF THE INVENTION for several reasons. Identi?cation of the nature of the defect can be used to determine the origin of the defect. The number, location and size of the defects can be used to calculate the density of defects in general, and, along with 1. Field of the Invention identi?cation of the nature of the defects, the density of This invention relates to lasers and, in particular, to a laser particular types of defects. This information can then be imaging system for use in analyzing defects on semicon 20 used to more closely monitor and/or to modify process steps ductor wafers. in the chip production process. 2. Related Art As the need for more precise defect analysis has become Semiconductor chip manufacturers have increasingly apparent, semiconductor manufacturers’ demand for the sought to improve yields in their production processes. Key ability to “revisit” defects (or a subset of them) found by the to this effort is the reduction of particulate contamination 25 above-described wafer scanners, for purposes of positive during wafer processing. As the line widths of features on identi?cation of the nature, location and size of the defects, the chip have shrunk from 10 microns several years ago to has led to the hasty design and production of review stations one micron and below today (with line widths approaching based on laboratory microscopes with precision wafer han 0.3 micron or less expected in the next few years), the ability dling stages that allow an operator to close in on and to detect and control smaller and smaller particles to achieve evaluate the previously detected defects. Revisiting of the higher degrees of cleanliness has become paramount. Addi defects by the review stations is done off~line from the defect tionally, production of acceptable chips requires accurate detection process so as not to limit the throughput of the performance of each of the process steps carried out on the wafer scanners. Little engineering was done in the design of wafer. The value of product on each wafer has also increased these review stations: in particular with respect to the optics dramatically, due to the increasing complexity of semicon and cleanliness (e.g., the review stations typically use off ductor devices (many more layers and process steps) and the the-shelf, visible light, research~style microscopes). development of larger wafers (up to 200 mm diameter), As noted above, the decreasing line widths of features on ?irther accentuating the need for defect detection and con— current and future semiconductor chips increase the impor trol. 40 tance of detection of contaminants and other defects having Instrument suppliers have addressed a portion of this a diameter, width, or other characteristic dimension on the problem by developing defect detecting systems which scan order of 0.1 to 0.3 microns. The visible light, off-the-shelf wafers (wafer scanners) during production for anomalous microscopes currently being used in defect review stations optical sites that are characteristic of particulate contamina lack sufficient resolution to resolve defects of such small size, or to resolve this size structure on larger defects to aid tion (but may represent other ?aws as well). Defects can be either a pit or a bump in the surface of the wafer. In one type of wafer scanner, in which a laser beam is focussed on and scanned over the surface of the chip (laser in identi?cation. Visible light scanning microscopes (both scanning system), anomalous optical sites are identi?ed by comparing the light scatter from locations on known good chips to the light scatter from the corresponding locations on the chips being tested. If the two light scatters are different, than an anomalous optical site has been detected. Wafer scanners of this type are made by Tencor Instruments of Mountain View, Calif. as Model 7500, and by Inspex of Billerica, Mass. as Model TPC 8500. In another type of wafer scanner, a video picture is taken 50 the use of conventional microscopes increases the risk of contamination of the semiconductor chips during the review process, since a (relatively dirty) human is in close proxim ity to the wafer surface and because the presence of the 55 Consequently, the semiconductor processing industry has good chip and compared to a corresponding video picture taken of a chip to be tested. Typically, these video systems 60 use white light imaging. The video pictures are analyzed by comparing them on a pixel by pixel basis, i.e., numerical data representing the video image at each pixel is compared and, if the difference falls outside of a pre-established acceptable difference, an anomalous optical site is identi?ed. 65 as, for example, Model 2131. microscope causes turbulent ?ow near the wafer which tends to pull in nearby contaminants to the wafer. with a conventional video camera of the surface of a known KLA of San Jose, Calif. makes a wafer scanner of this type white light and laser-based) that are built by modifying off-the-shelf microscopes can improve the resolution sig ni?cantly, but they are currently in limited use, mostly as part of complex and expensive research setups. Additionally, attempted to use scanning electron microscopes (SEMs) that will provide increased resolution and perform energy dis persive (EDX) analysis. In EDX analysis, X-rays are directed toward the surface of the semiconductor chip. By measuring the wavelength spectrum of the reflected light, information can be gleaned regarding the types of material present on the wafer surface. Unfortunately, EDX analysis requires high voltage (up to approximately 40,000 volts) SEMs; bombardment of the wafer surface with electrons from high voltage SEMs causes damage to the wafer, 5,479,252 3 4 rendering the wafers unusable for further processing. Recently, low voltage SEMs (100—1000 volts) have seen microscopy techniques, including multiline visible light limited use in wafer fabs for “critical dimension” measure ments of line widths, but low voltage SEMs are too slow to improving resolution even further due to the shorter wave use except on a sample basis, and, in addition, provide no lengths of the ultraviolet light. The laser imaging system has The laser imaging system utilizes confocal laser scanning lasers, and can be optionally ?tted with an ultraviolet laser, analytical (i.e., EDX) capability. Further, in both high and resolution on the order of 0.1 to 0.2 microns. The laser low voltage SEMs, the time to load samples into the SEM and pump down the load-lock chamber containing the SEM imaging system can also be used for metrology. Additional capabilities of the instrument include ?uores~ cence of contaminants (for assistance both in locating them is relatively long, undesirably slowing down processing of the wafers. As a result, defect revisiting with SEMs is usually done olT-line in a quality control or analysis labo 10 against the complex background of patterned wafers, and in identifying their origins), a variety of software to assist the ratory. In an attempt to overcome the limitations of SEMs, some major semiconductor producers have begun to use systems which include both low and high voltage SEMs. However, such systems are expensive, selling in the $1,000,000 to $1,500,000 range. 15 operator in evaluating and classifying the defect, commu nications and data storage capabilities for providing trend analysis on-line or off-line, and capacity for image storage. For future product line expansion, the laser imaging SUMMARY OF THE INVENTION system is adaptable to cluster or in~situ applications, where examination of defects or structures during on-line process According to the invention, a laser imaging system that allows hands-o?~ operation and operates under class 1 clean BRIEF DESCRIPTION OF THE DRAWINGS ing can be performed. room conditions, has several distinct advantages over con ventional systems for sub-micron particle structure evalua tion. In one embodiment, the laser imaging system “revisits” defects on production semiconductor wafers, where the defects are ?rst detected (but not analyzed or evaluated) by 25 FIG. 1 is a perspective view of a laser imaging system according to the invention. FIG. 2 is a schematic diagram of a laser imaging system according to the invention illustrating the operation of the conventional wafer scanners such as are available from laser imaging system. vendors as Inspex, KLA, or Tencor Instruments, among FIGS. 3A, 3B, and 3C combined are a schematic diagram of the electronics associated with the laser imaging system according to the invention. others. The laser imaging system replaces and outperforms conventional microscopes now used to analyze defects on production semiconductor wafers. Signi?cantly, the laser imaging system according to the FIG. 4 is a view of a display screen resulting from analysis of an area of the surface of a semiconductor chip by a laser invention is the ?rst defect review tool whose optics and imaging system according to the invention. functionality have been designed explicitly for ef?cient performance of the dedicated revisit task. Unlike scanning electron microscopes (SEMs) that have previously been used for defect analysis, the laser imaging system will not damage samples or slow processing, and costs signi?cantly 35 A laser imaging system according to the invention is used less to implement than an SEM. Further, while SEMs can produce images with resolution on the nanometer scale, they have certain limitations. For example, the SEM image has an extended depth of ?eld, like a photograph taken through a high f-stop aperture, but this image contains no quantitative depth information. Some methods of dealing with this de?ciency are sample tilting or coating to produce a “shad owing” effect or perspective change, but these methods require additional process steps and cost, may damage the wafer, and do not completely resolve the‘ problem. Unlike the SEM, the laser imaging system according to the invention operates in air with class 1 cleanroom com to analyze defects on semiconductor wafers that have been 40 45 laser imaging system can perform a variety of microscopic inspection functions including defect detection and metrol ogy. 50 FIG. 1 is a perspective view of laser imaging system 100 according to the invention. Laser imaging system 100 includes housing 102 made of stainless steel. Laser imaging system 100 occupies a footprint which ?ts inside a 48" standard clean hood. Laser imaging system 100 has con trolled intemal air?ow (clean air from the cleanroom is rendering techniques, which provides quantitative dimen 55 with correct perspective maintained, without necessity for sample tilting or coating. Additionally, the laser imaging drawn in through the top of laser imaging system 100 and exhausted from laser imaging system 100 outside of the cleanroom), maintaining class 1 conditions in the wafer area, which is isolated from the operator console. A cassette of wafers (not shown) of a given size, e.g., system has an ability the SEM cannot match: sub-surface viewing of defects lying beneath dielectric layers. Combined with three-dimensional analysis software, the user is able to examine cross sections of the defect and surrounding mate rial, and to assess the impact on circuit layers of the wafer. The laser imaging system presents a real time video image with resolution superior to a conventional microscope. An detected by patterned wafer defect detecting systems (wafer scanners). The laser imaging system replaces optical micro scope review stations now utilized in the semiconductor fab environment to examine detected optical anomalies that may represent wafer defects. In addition to analyzing defects, the patibility. Also unlike the SEM, the laser imaging system can produce a three dimensional image, using simple image sional information. The image can be stored and recalled for later viewing. The image can be rotated or tilted or shaded, DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION wafers ranging from 3 inches (75 mm) to 8 inches (200 mm) 60 in diameter, is positioned on cassette platform 101. One of a set of interchangeable mounting plates (not shown), there being a different mounting plate for each cassette size (i.e., wafers of dilTerent sizes are held by different cassettes), is attached to cassette platform 101. Typically, defects have operator can view the image on a conventional computer 65 previously been identi?ed on the wafers by a defect detect ing system, as described above. Wafers from the cassette are the wafers to operator contamination or air?ow. loaded through wafer door 103 formed in housing 102 into display, with comfortable ergonomics, and without exposing 5,479,252 5 6 a wafer processing area housed by optics housing section precise de-skew alignment, the system can accurately trans 107 of housing 102. The wafers are either loaded by the operator or by a robot 104 that is part of a standard machine late any speci?ed location on the wafer into the ?eld of view of the microscope with an accuracy of a few microns. The heart of laser imaging system 100 is the laser scan interface (SMIF) i.e., micro-environmentally controlled, interface. The SMIF interface, which is a “box” for trans ferring wafers in which clean room conditions are main ning microscopic optics module (“optics head”) which includes elements 201, 202, 203, 204, 205, 207, 208, 210, 211, 212, 219, 220, 221, 222, and 223 shown in FIG. 2 tained, is conventional and is available from Asyst Tech nologies in Milpitas, Calif. (various models are available below. The optics head includes a laser, confocal beam and can be used with the invention). After inspection, the wafers are unloaded by either the operator or the robot 104. An optional 3-cassette carousel (not shown) may be mounted on laser imaging system 100 allowing sorting of wafers after inspection. Robot 104 is a conventional precision, high reliability (less than one wafer drop per million transfers) robotic wafer handler, such as is available from MECS in Japan as part no. scanning optics, and ultraviolet and visible photo detection electronics, together with commercial microscope compo nents to achieve high quality real time confocal images. Laser imaging system 100 will produce a complete XY scanned laser image, in a single plane of focus, at video rates. The resulting image is displayed on a high resolution 15 UTX~1000. Robot 104 will reliably sense, load, and unload wafers from cassettes, interchangeably handling 75 mm to conventional microscope. 200 mm wafers. Robot 104 senses missing or skewed wafers in the cassette(s), as well as the presence or absence of a wafer on the robot arm or vacuum chuck 224 (see FIG. 2). Robot 104 (and other components of laser imaging system 100) is designed to eliminate any wafer contamination (laser imaging system 100 maintains Class 1 compatible cleanli ness while handling wafers). Robot 104 has su?icient utility backup (power, air, vacuum) to protect any wafer in transit 25 on robot 104 from damage. Upon restart after a power failure, all wafer locations (cassette slots, robot arms, vacuum chuck 224 (FIG. 2), plus missing cassettes, are sampled for the presence or absence of wafers and/or cassettes, and appropriate responses made. When loading wafers, robot 104 removes wafers from cassette platform 101 and performs a pre-alignment step, using pre-aligner 105 which senses a notch and/or ?at(s) on the wafer. Optionally, any bar code (identifying the particu— lar wafer) which may be present on the wafer may also be 30 semiconductors. An example of a laser that can be used with the invention is the Model 2204-25ML air-cooled argon ion 35 interference problems that can occur for a speci?c wave 40 45 (such as Helium-Neon or Helium Cadmium) could also be mounted on a conventional computer controlled ?lter wheel (not shown) within optics housing section 107. Notch ?lter 202 isolates a laser line or lines. A notch ?lter for use with the invention is available from Edmund Scienti?c of Bar rington, NJ. as part no. 43120. Other ?lters are available 55 from the same source for other wavelengths. The light having the selected wavelength(s) passes from notch ?lter 202 to polarizing beam splitter 203. Polarizing various aspects of the system. The X and Y axes de?ne a beam splitter 203 is attached to selectable notch ?lter 202 60 using conventional optical mounts. Polarizing beam splitter 203 preferentially re?ects light only of the proper polariza tion and directs the light to spatial ?lter 204. The polariza tion of the light emitted from laser 201 is oriented so that most of the light is re?ected by polarizing beam splitter 203 at 90 degrees into the focusing optics of spatial ?lter 204. A for the mis-orientationof the patterns on the wafer with respect to the wafer ?at and the robot 104 placement error), and automatically focuses using the laser as described scope ?eld of view with etched ?ducial marks or other pre-speci?ed structures on the wafer surface. After this wavelengths strongly, but will allow transmission of others to perform the imaging desired. Other wavelength lasers used to supply light at other wavelengths. Laser 201 produces polarized light at several discrete wavelengths. The light passes through a “notch” ?lter 202 bay 106 of housing 102. below. The operator accomplishes ?ne alignment of the wafer (dc-skew point) by lining up the visible light micro give good results for all materials, ?lm thicknesses, and surface properties. Additionally, in many cases, it is desir able to observe through one or more top layers of material (typically dielectric) which will re?ect or absorb some a link or network such as Ethernet, RS232, etc. A computer plane parallel to the patterned surface of the wafer and the Z-axis is perpendicular to the patterned surface of the wafer.) The system translates the wafer to the ?rst “de-skew” point (i.e., pre'determined orientation of the wafer that accounts laser produced by Uniphase Corporation, San Jose, Calif. It is important to perform imaging with a selection of wave lengths of laser light to overcome absorption, re?ection, and length for a given material. That is, one wavelength will not either by diskette or other media, or by communication via After pre-alignment, the wafer is loaded into the wafer processing area through wafer door 103 onto the optical unit’s XYZ-stage (translational motion) and is secured on a conventional vacuum chuck 224 (FIG. 2). (In this descrip tion, a Cartesian coordinate system is used in description of sample at which it is desired to obtain imaging data. Laser imaging system 100 includes an air cooled, multi line argon ion laser 201 which provides up to six different wavelengths of light for imaging surfaces and structures in While the wafer is being loaded, a ?le of data from the for use with laser imaging system 100 is available from Silicon Graphics in Mountain View, Calif. as part no. SGI XS24Z. A disk drive, available from Silicon Graphics in Mountain View, Calif. as part no. P3-F252, and tape drive, available from Hamilton/Avnet in Mountain View, Calif. as Maynard (Archive) 215018, are attached within disk drive FIG. 2 is a schematic diagram of laser imaging system 100 according to the invention illustrating the operation of laser imaging system 100. Laser imaging system 100 uses the basic principles of confocal microscopy, in which illu minating light passes through a pinhole, and the image of this pinhole is then cast by the system optics on the sample to be viewed. The light scattering from the sample returns through the system optics to the pinhole, but only light from the focal plane of the imaging (objective) lens returns through the pinhole, i.e., light from the plane through the read at this time. defect detecting system, specifying the wafer coordinates of the detected defects, is transferred to the laser imaging system computer within housing 102 (not visible in FIG. 1), monitor, also in real time. Thus, the operator can scan through different levels of focus in real time, as with a 65 small portion of the light passes through polarizing beam splitter 203 to a conventional power monitor diode (not shown) mounted behind polarizing beam splitter 203, where 5,479,252 7 8 the light is absorbed. A polarizing beam splitter for use with the invention is available from Melles Griot of Irvine, Calif. objective lenses 205 and autofocus (one lens is focused and focus o?’sets stored in the computer are used to automati cally focus the other lenses) of each objective lens 205. A as part no. 03PBB003. turret for use with the invention is available from Olympus of Japan as part no. BL0920. Turret 223 is designed to accommodate three to six objective lenses 205, and can Spatial ?lter 204 consists of optics which expand the beam and then focus it on a pinhole aperture. The diameter of the pinhole aperture is selected according to well-known techniques to re-image the light through the downstream optics and a selected one of a plurality of objective lenses 205 to produce a diffraction-limited spot on wafer 206. The diameter of the pinhole aperture is also selected to allow easy alignment of the beam of light and a signi?cant amount 10 of high power light to pass through the aperture. A spatial ?lter for use with the invention is available from Melles Griot of Irvine, Calif. as Compact Spatial Filter Newport/ 910. Spatial ?lter 204 is attached to polarizing beam splitter 203 by conventional optics mounts. Subsequent optics within spatial ?lter 204 and between the pinhole assembly and the scanner mirrors collimate the tube lens 211 as a standard component are mounted together with a ?ange and held by a locking screw. The turret] illuminator assembly bolts to the optics baseplate. According to the principles of confocal imaging, the light striking wafer 206 is scattered and a portion of the light light, and direct the light to mirrors mounted on X-Y beam scanner 207. An X-Y beam scanner for use with the inven 20 tion is available from General Scanning of Watertown, Mass. as part no. GOO-3011003. X-Y beam scanner 207 is attached to spatial ?lter 204 by conventional optics mounts. The mirrors in X-Y beam scanner 207 can oscillate their angle with respect to the beam of light passing through X-Y 25 operating at 8 kHz, the other a servo controlled unit, on the sample, very little light returns through the aperture. operating at 13 or 26 Hz (but capable of other speeds). The 30 aperture reaches the polarizing beam splitter 203, which, being oppositely polarized, passes through polarizing beam 35 tern and allowing more light to reach the tube lens 211, 45 below, are standard in?nity corrected optics. Quarter wave plate 210 is attached to scan lens 208 and is positioned to convert the linearly polarized laser light to circularly polarized laser light. A quarter wave plate for use with the invention is available from Melles Griot of Irvine, the raster scan, a map of light intensity in the focal plane of the objective lens 205 is constructed. This map can either be stored in the memory of system computer 214, or analyzed by surface data processor 213, which stores the readings, and makes a comparison of the intensity with previously stored maps from other scans, as described below. The light intensity map is also written directly into the video memory of the system computer 214 and may be displayed live on the computer display 215 in an appropriate window, as described below. To obtain a three dimensional image, the optics head works with the ?ne z-stage control 216 to develop an expanded depth-of-?eld image. The sample height is 50 stepped over a pre—selected vertical interval (typically 12 nm or some multiple thereof) using the ?ne z-stage control 216. After each complete raster scan at a particular sample height, Calif. as part no. 02WRM005. Beam splitter 209 is attached to quarter wave plate 210 by a conventional optical mount, and is explained in more detail below. Tube lens 211 is attached to beam splitter 209 by a conventional optical mount and works with objective lens 205 to de-magnify the raster scanned pinhole image and project it on the wafer 206. splitter 203 undeviated and is imaged on the photodetector 212. By measuring the light intensity at each XY location of providing a more uniform brightness across the raster pat described below, without distorting the image. The tube lens Consequently, signals in the confocal optics get darker, not merely blurred, as occurs with conventional optics, when the sample is out of focus. Light which passes through the scanner 207 traces out a raster pattern in space. A raster scan 211 and objective lenses 205, described in more detail the ?eld lens, scan lens 208, and mirrors of X-Y scanner 207 204. If the light spot was in focus on the sample, the image is imposed on the aperture. If the light spot was out of focus oscillating galvanometers, one a high speed resonant unit of 256 or 512 lines is produced at approximately 26 or 13 frames per second, and is imaged at the back focal plane of the tube lens 211. This raster pattern is imaged in space by the scan lens 208 in the plane of the ?eld lens (not shown, but between beam splitter 209 and quarter wave plate 210). A scan lens for use with the invention is available from Applied Optics of Pleasanton, Calif. as part no. 000424. Scan lens 208 is attached to X-Y beam scanner 207 by conventional optical mounts. The ?eld lens serves to collect high angle light, reflected back into objective lens 205, returning through the optical path described above. As the retuming light passes through quarter wave plate 210, the returning light is con verted to light linearly polarized and 90” out of phase with respect to the polarization of the light originally emitted by laser 201. The light continues back along the path through until the light reaches the pinhole aperture of spatial ?lter beam scanner 207. X-Y beam scanner 207 includes two servo steps in small increments, so that the X-Y beam handle low power (magni?cations of 5, l0 and 20 times actual size) as well as medium power (magni?cation of 50 times actual size) and high power, high N.A. (numerical aperture, a conventional designation for the light gathering property of an objective lens in which higher numbers indicate a broader cone of gathered light) objective lenses 205 (magni?cations of 100 and 150 times actual size and 0.95 NA). Turret 223 and a vertical illuminator containing the height of the sample is changed using ?ne z-stage control 216, and a new raster scan perfonned, as described above, 55 to obtain a map of light intensity in the focal plane of objective lens 205 (at the new sample height) by measuring the light intensity at each XY location of the raster scan. A tube lens for use with the invention is available from X-Y stage control 218 is used to position the defect or Olympus of Japan as part of their vertical illuminator model region of interest in the ?eld of view. The X-Y stage control SLM220. 60 is then held still while the ?ne z-stage control 216 is used as described above. The image of the light spot is focused and demagni?ed by the objective lens 205 in the focal plane of the objective lens A three-dimensional image can be obtained from the 205. Objective lenses 205 for use with the invention are multiple XY light intensity maps in one of two ways. First, available from Olympus of Japan by specifying l00xBF l-LM590. Many interchangeable lenses are available. Objective lenses 205 are mounted on a computer controlled motorized turret 223 that enables automatic changing of as noted above, the XY data from each raster scan can be 65 analyzed by surface data processor 213 by comparing the light intensity at each point of the XY scan with correspond ing points of a “master map.” This “master map” stores the 5,479,252 10 maximum light intensity values found at each XY point, these values resulting from previous comparisons of XY light intensity maps. The Z-axis location of the maximum light intensity at each XY location is also stored. After all of the XY light intensity maps have been obtained and com pared to the “master map,” the data representing the light intensity maximumat each XY location and the Z-axis location of each light intensity maximum are used to con struct the three-dimensional image of the wafer surface. With this method, it takes about 5 seconds to acquire all of the light intensity data and extract the surface. Alternatively, especially if the wafer is multilayered, i.e., producing multiple peaks at each XY location along the Z~axis (which might occur, for instance, where transparent layers are formed), each light intensity map can be stored in system computer 214, along with the Z-axis height of each lengths pass to the video camera, a conventional microscope image can be obtained, in addition to the laser image, by using a conventional microscope illuminator 220 and video camera 219, charge coupled device (CCD). The white light imaging is accomplished without the use of microscope eyepieces that would result in undesirable proximity of the operator to the wafer being analyzed that may result in contamination of the wafer. Rather, the microscope image is displayed on a computer display (simultaneously with the 10 computer display 215, using software described in more detail below, or on a separate video monitor display (not shown). 15 The white light microscope image is produced alone or simultaneously with the live laser image by video camera 219 available from COHV of Danville, Calif. as part no. 8215-1000 which views the sample in white light emitted by microscope illuminator 220, and inserted into the optical path by beam splitter 221. A microscope illuminator for use map. If it is desired to create a three-dimensional image of the surface of the wafer, the XY light intensity maps are successively compared to determine the maximum light intensity at each XY location. The Z-axis location of the maximum light intensity at each XY location is stored and, at the conclusion of the series of comparisons of the XY light intensity maps, is used with the maximum light intensity laser image, if desired), either in a separate window on 20 with the invention is available from Olympus of Japan as part no. 5LM220. A beam splitter for use with the invention is available from Melles Griot of Irvine, Calif. as part no. O3BSC007. Filter 222 blocks the laser line in use, but passes broad bands of light having other wavelengths, so that laser light from laser 201 is prevented from saturating the image data to construct the three-dimensional image of the wafer surface. With this method, it takes about 35 seconds to 25 at video camera 219 with re?ected light. A ?lter for use with the invention is available from Edmund Scienti?c of Bar acquire all of the light intensity data, then extract the surface rington, NJ. as part no. 22754. Video camera 219 and ?lter using a processor in system computer 214. wheel 222 are mounted on brackets which position video A process for constructing a three-dimensional image of camera 219 and ?lter 222 in line with beam splitter 221. a surface from a three~dimensional data set is described in Beam splitter 221 is mounted on the turret assembly with more detail in commonly owned, co-pending US. patent conventional optical mounts. application entitled “Surface Extraction from a Three-Di mensional Data Set,” by Ken K. Lee, application Ser. No. To get a white light image alone, laser imaging system 08/079,193, ?led on the same date as the present application, 100 can remove beam-splitter 221 and substitute a mirror the pertinent disclosure of which is hereby incorporated by (not shown) so that only the video camera light path is 35 active. Then, the blocking ?lter (mounted on a ?lter wheel) reference. can be removed and the full spectrum white light image The raster scan is repeated 13 times per second for a 512 viewed. by 512 pixel image, or faster for smaller (i.e., fewer pixels FIGS. 3A, 3B, and 3C combined are a schematic diagram such as 256 by 256) images. (Note that raster scan sizes of the electronics associated with laser imaging system 100. other than 512 by 512 or 256 by 256 can be used.) A complete three~dimensional volume data set will typically 40 total data array of size 512 by 512 by 64. For a 512 by 512 pixel image, the total time to accumulate the data to con struct the three—dimensional image of the surface (assuming FIGS. 3A, 3B and 3C show all analog and digital electron ics, plus power supplies, for complete operation of laser include 64 raster scans (other numbers can be used), for a imaging system 100. Laser imaging system 100 operates on 220 volts (200-240 volt nominal), 50/60 Hz single phase 45 electric power (or the European and Japanese equivalents). The SDP Frame Grabber 301 interfaces with photo detec tor 212 (FIG. 2) and is synchronized with the scanner electronics 309, and ?ne z-stage control 310, to digitize the The light intensity at each data point is stored in system photodetector data and produce a three-dimensional map of computer 214 as an 8-bit quantity. A simple map of a three-dimensional surface is created using the three-dimen 50 light intensity which can either be stored directly in the 64 raster scans, i.e., vertical height steps) is approximately ?ve seconds. computer memory, or processed to immediately extract a sional graphics (such as the Silicon Graphics Inc. Graphics Library, available as part of the XS24Z computer package) surface image. The SDP Frame Grabber 301 interfaces through the SDP interface 302 to the system computer 303 (also shown as 214 in FIG. 2). SDP Frame Grabber 301 is of system computer 214 by plotting the X, Y, and Z position of each maximum intensity point, and displaying the map as a continuous surface. The brightness of each point on the 55 surface is determined by the light intensity measured at that point. The map display may be done in gray scale, in false fast and enables surface data to be extracted from the volume data. The system computer 303 is a high speed RISC graphical workstation, such as a Silicon Graphics Iris Indigo XS24Z color converted from the gray scale, in a mode showing shape (position) only, or shape with height represented in manufactured by Silicon Graphics of Mountain View, Calif, gray scale. or equivalent, capable of handling concurrent tasks of robot functions, stage motion, operator interface, and optics con The capacity for white light imaging, in addition to the laser imaging described above, is another feature of laser trol, while also performing image processing functions. In imaging system 100. ‘As noted above, beam splitter 209 is attached between quarter wave plate 210 and tube lens 211. By imposing beam splitter 209 in the path of light from laser 201 just prior to tube lens 211, and using suitable ?ltering that blocks the re?ected laser light but lets other wave 65 addition, system computer 303 must work with a windowing user interface and high resolution color graphics. The X, Y, and coarse Z stage controllers 304 communicate with system computer 303 via an RS-232C interface 305, as do the robot and pre-aligner controllers 306. 5,479,252 11 12 The balance of the system electronic functions commu The user can utilize laser imaging system 100 as a white nicate through Local Operating Network (LON) interface light microscope. The microscope view is presented in a different window than the laser view. In white light mode, the user can select views (e.g., two-dimensional), translate (X-Y movement) and focus on details (Z-direction move 307 built on the same interface slot as RS-232C interface 305. The LON 308 itself is a pair of wires that plug into each node serially around the system. Each node contains a local processor and ?rmware for LON communications, self diag nosis, and local operation of certain functions. ment), change objective lens magni?cation. The overall system software is designed for operation of laser imaging system 100 in both operator and engineering All user interface is via an operator console that is part of system computer 303 and which includes computer display 215, a mouse/trackball, a joystick controller, and a keyboard. 10 mode, optionally using defect ?les supplied by various wafer scanners in a variety of formats. Both operator and The operator console may optionally be remotely mounted engineering mode are password protected separately, as (i.e., outside the cleanroom). Image processing and analysis explained in more detail below. In engineering mode, the user can use a recipe develop ment editor to develop recipes for routine inspection of functions may be controlled from the console. Through these controls and the windowing software, the operator can set up, program and operate any part of laser imaging system speci?ed types of wafers at speci?c process steps for that wafer, i.e., to pre-specify operating parameters for use by 100 including wafer selection and handling, defect editing and selection, automatic and/or manual wafer loading, defect classi?cation, etc. For example, the joystick control operators working on a speci?c process level and product. The recipes can specify which screen and windows are to be used, enable the laser wavelength and power to be used to be selected, speci?cation of the number of slices of data and ler allows the user to move the coarse Z-stage control 217 in small increments, to bring an object or region of interest on the wafer into view. Alternatively, for enhanced ease in making very small lateral movements, the operator can use their spacing (in nanometers), autofocus to be speci?ed, and the mouse to point and click to cause the X-Y stage control 218 to change position. 25 Engineering mode also allows access to system mainte The operator has three modes from which to select nance functions such as utilizing the LON access to run viewing: white-light conventional microscope optics (“white light mode”), real time laser scanning optics (“laser diagnostic checks on the electronics or recalibrate XY and Z stage motion In operator mode, the operator loads, inspects and clas si?es lots of wafers per the predetermined recipe associated with the particular lot number and wafer ID. The operator has limited options to alter the inspection sequence. mode”), or both laser and white light optics simultaneously (“combination mode”). In white light mode, the operator can select from one of several objective lenses, varying effective magni?cation of the image. (The laser image scales simul taneously with the white-light image.) Laser or white light imaging of a region of the wafer the offset in the z-direction (vertical direction) from the autofocus position to the ideal viewing position to be preset. 35 Utilities are available in pop-up menus to enable manual produces data regarding the wafer characteristics in the imaged region. The data is stored on system computer 214. control of the ?ne Z-stage control 216, coarse Z-stage control 217 and X-Y stage control 218, polling of stage After imaging of a region of the wafer, the operator exam ines each defect image as laser imaging system 100 presents it to him. If the defect is not in view (if, for instance, the ment of a wafer from the ?at ?nder or stage after a power variables, and robot manual control (e.g., to allow move 40 defect location data from the wafer scanner is slightly erroneous), or the operator wishes to examine a larger or different area, the joystick controller allows him to “cruise” the wafer. After the operator examines the defect, the opera tor classi?es the defect, optionally records the image, and proceeds to examine the next defect. Upon completion of review of all desired defects, or other inspection tasks, the display of LON nodes, direct control of system functions, 45 store wafer images and to bring them up in Library win dows. These images are usually stored as bit maps and can 50 of the screen of computer display 215, as explained in more detail below, the operator sees a real-time narrow depth of ?eld laser image, which may be zoomed to higher e?‘ective 55 re?ectivity of the wafer) may also be employed to automate most of the operator’s tasks in acquiring the three-dimen sional image. The three-dimensional image may be exam ined in pseudocolor, pro?led, shadowed, rotated, etc. for comparison between inspection systems, thus enabling the operator to compare the image produced by laser imag ing system 100 with familiar images. The bit maps can be displayed in special windows to help operators classify matic ranging (automatic selection of vertical distance to traverse in obtaining imaging data), automatic focus (auto matic focusing of objective lens at desired vertical location) and automatic gain control (automatic adjustment of the photodetector gain to compensate for differences in the be used to represent typical defects for comparison by the operator with the defect currently being classi?ed. Laser imaging system 100 can be con?gured to bring up such images automatically as di?°erent defect classi?cations are selected. Bit map images from other devices, such as white light microscopes or SEMs can also be stored and recalled, magni?cations. The operator can translate the sample in the Z-direction (vertical direction) to cover an entire vertical region of interest. The operator can also select a range of vertical motion, and have laser imaging system 100 con struct a 3-dimensional image of the region speci?ed. Auto e.g., open and close the laser shutter, can be accomplished. As an aid to workers reviewing defects (especially during production), laser imaging system 100 has the capability to wafer is returned to its cassette, and the next wafer loaded, repeating the process above. In the live laser image, presented in a particular window failure). Diagnostics can also be called up via a pop-up window displaying all LON nodes and system variables. The status of LON nodes can be checked and revised. From the 60 65 defects. The system software includes different levels of password protection. At one level, engineering personnel can access laser imaging system 100 to set up predetermined recipes for screen con?guration, laser scan parameters, and defect codes. At a different level, operators can access laser imag ing system 100 to call up the recipe for the particular wafer level and product being used in order to examine defects on wafers to be inspected. This feature, combined with auto 5,479,252 13 14 matic focus, ranging, and gain control, allows competent operation of laser imaging system 100 with a minimum of The Laser Window directly displays the live laser image produced by the scanning laser beam. Controls for changing operator training. the focus of the laser through its range are available. Laser imaging system 100 uses an Ethernet interface that supports standard ?le transfer and management (data and number of slices to utilize for a three-dimensional image, Additional controls include autofocus, laser intensity, zoom, recipe upload/download, etc.). Laser imaging system 100 and the step size of three-dimensional image slices, plus other imaging control features. includes software for generating output report ?les for use by data analysis (trending, statistical analysis, etc.) software The 2-D Window utilizes the acquired three-dimensional surface image, and presents a two-dimensional image of a slice through the three-dimensional data. In “XY” mode, the 2-D Window displays a top view in false color of the Wafer as well as printed reports. A number of confocal images may be stored in ?les for subsequent review on or off line from laser imaging system 100. As noted above, laser imaging system 100 includes com surface, i.e., a projection of an effectively in?nite depth of ?eld image of the wafer surface. In “X2” and “Y2” modes, the surface image is represented as surface pro?les of selected vertical slices. In the two-dimensional images, a rubber band metrology box (i.e., a variable size cursor box) can be used to determine the size (in the plane chosen) of the puter display 215. Computer display 215 displays pictorial and numerical results of the analysis of the defects on the semiconductor chips, and lists menu selections for control of laser imaging system 100. Laser imaging system 100 pro vides the capacity for displaying any number of different display screens on computer display 215. Each display defect. In “cut” mode, the “ Y” mode view includes cursor lines which can be controlled by, for instance, a mouse to 20 select “XZ” mode or “YZ” mode slices. If the 2-D Window of windows that can be displayed on a display screen is is used with a volume data set rather than a surface data set, screen is de?ned by the number, type, size and location of the windows included within the display screen. The number limited only by the size of the windows to be displayed. Generally, the types of windows that can be displayed on each screen are icon windows, picture windows and infor mation windows. FIG. 4 is a view of display screen 400 resulting from analysis of an area of the surface of a semiconductor chip by laser imaging system 100. Display screen 400 includes a the “XY” mode display shows a single slice scan of data, rather than a surface outline. The “XZ” mode and “Y2” 25 mode show vertical cuts through the data set. Special options allow pro?les of volume data sets to show multiple layer structures. This occurs by analyzing volume data from multilayer semi-transparent samples. The 3D Window projects a perspective view of the surface image. The 3D Window may be rotated, tilted, zoomed, shaded, etc. by the operator to obtain a desired plurality of windows of various types. In FIG. 4, windows 401a, 401b, 401e, 401d, 401e, 401f and 401g are icon windows; windows 4020, 402b, 4020 are picture windows; image for analysis. and window 403 is an information window. In FIG. 4, the icon windows list choices for pictorial The Wafer Map Window displays the defect map of the wafer under inspection, the defect map having been pro display in the picture windows. For instance, icon windows duced by a wafer scanner that is not part of laser imaging 401a and 401d command display of a two-dimensional 35 image in one of the pictorial windows, e.g., the surface pro?le seen in window 402b or the planar surface view seen in FIG. 402a. Icon window 401a commands display of a three-dimensional image in one of the pictorial windows, e.g., the three~dimensional surface image seen in window 402C. The pictorial windows 402a, 402b and 402C show two or three-dimensional images of the semiconductor chip being analyzed, as discussed above. The information win dow 403 gives tabular information regarding the size and location of particular defects on the chip and is describe in 45 more detail below. Laser imaging system 100 includes a number of pre de?ned screens (in one embodiment, on the order of 4-5 list. The Cassette Map Window diagrammatically represents a loaded cassette. The operator can select and load any wafer from the cassette by, for instance, using a mouse to “point and absorption. It is always a problem to focus microscopes 55 pictorial window is stored as an icon window when not in use. Windows in different regions of the screen may be , through and selected directly by highlighting items on the There is a major advantage to using a laser with multiple lines to image surfaces, to account for different re?ectivities to de?ne an unlimited number of screens, each screen having interchanged by “clicking and dragging” the window to the pop-up text window (the defect locations are given as coordinates of a Cartesian coordinate system), scrolled imaging system 100. laser imaging system 100 includes the capability for a user new location. defect. A rubber band metrology box may be used to display a portion of the defect map to higher precision in an enlarged view. Alternatively, a list of defects can be brought up in a and click” on the wafer. The operator can also unload the wafer to a cassette on the cassette platform 101 of laser screens), i.e., screens having windows of a pre-determined type and size located at pre-determined locations. However, any desired combination of windows according to window type, window size and window location. The screen (window and arrangement of windows) to be displayed is selected by the operator. For each screen, each system 100. Defects are shown as a color-to-size coded dot on the screen. The operator can select a defect to revisit by, for instance, using a mouse to “point and clic ” on the automatically. Confocal optics are a natural way to perform an autofocus due to the extremely narrow depth of ?eld. However, a single spot autofocus may not work well if a very dark spot on the sample is being imaged. By scanning the laser spot in real-time, and averaging over the return 60 signal, a much more reliable autofocus is obtained. If a particular material is strongly absorbing, different wave lengths may be selected. Alternatively, a laser ?lter may be used which transmits each wavelength of the laser inversely proportionally to the intensity of the wavelength and so 65 illuminates the sample with multiline laser light which is sure to select at least one wavelength which re?ects strongly. in white-light only mode for optimum viewing of the ' white-light image. In a simpler mode, the laser can be used with no ?lter, and The Microscope Window contains a live video presenta tion of the white-light image taken from the wafer surface through the microscope objective lens. This image may be viewed simultaneously with the laser image or can be view 5,479,252 15 16 the computer can adjust laser power and photodetector gain In its simplest mode, laser imaging system 100 works on opaque, diffusely scattering surfaces. to achieve a good autofocus. False color can be used to see small defects, either on the surface or under the surface. By converting images to false color from black and white, very small objects may be detected that would otherwise go unnoticed, because the human eye can discern thousands of separate hues, while small differences in brightness are hard for the eye to detect. This is one way in which laser imaging system 100 exceeds the “Rayleigh Criterion” (a common measure of resolution) in its ability to see structures on the order of 0.1 to 0.2 micron size. Other aids in small feature detection are the The laser imaging system according to the invention is unique in its capability for combining live video, live laser scan, three-dimensional imaging, wafer maps, and wafer surface pro?les all on one screen. By selecting screens with different size and composition of windows the operator may view all the relative data needed for completion of his task. The system according to the invention can be easily 10 displays, etc. The necessary adaptations to laser imaging improvement in resolution obtained with confocal optics, system 100 would include some software changes and some and the oversampling (overlapping images) performed by changes in the material handling system. Appendix A accompanying this speci?cation is a draft laser imaging system 100 that enables detection of features as small as 0.1 to 0.2 rrricrons. User’s Manual for laser imaging system 100 and is herein As an option, the laser optics can be used to obtain a incorporated by reference. ?uorescence image of the wafer, detecting visible light in preselected wavelength bands. Fluorescence using white light or laser light in the same system is possible. Illumi nating light of selected wavelengths will ?uoresce certain 20 materials, such as skin ?akes or photoresist, the material emitting light at a longer wavelength. Suitable ?lters in front We claim: length, but pass the longer wavelength, thus enabling iden 25 1. An imaging system comprising: means for inspecting a semiconductor die to determine the e?icacy of a process previously performed on the semiconductor die, the means for inspecting compris mg a multiline laser light source that emits laser light of a ?lter passing the longer wavelengths in front of the photo detector 212. Polarized light images of objects or structures on the sample may be taken by illuminating the objects or struc plurality of wavelengths, tures with polarized white light and viewing the sample through a cross polarized ?lter. Optically active materials such as quartz will appear bright against the dark polariza tion extinction background. Optional bright?eld/dark ?eld objective lenses and illumination may be used to help locate particles and other defects on the wafer surface. The same technique may be used with the confocal optics, Various embodiments of the invention have been described. The descriptions are intended to be illustrative, not limitative. Thus, it will be apparent to one skilled in the art that certain modi?cations may be made to the invention as described without departing from the scope of the claims set out below. of the video camera block the illuminating radiation wave ti?cation of the material. This may also be done with the laser, ?uorescing with a short wavelength, and placing a adapted for use in other materials science industries such as production of magnetic media, thin ?lm heads, ?at panel means for directing the laser light toward the semicon ductor die, and 35 means for measuring a ?rst intensity of laser light re?ected from the semiconductor die; and means for analyzing a defect on the semiconductor die to determine the nature and origin of the defect. 2. An imaging system as in claim 1, wherein the means for by using linearly polarized light. Inserting a second quarter analyzing further comprises means for determining at least wave plate in the beam adds another 90 degree rotation, so one material of which the defect is constituted. that re?ected light from the sample no longer passes to the 40 3. An imaging system as in claim 1, wherein the means for detector, but light further rotated by passing through the analyzing further comprises means for determining at least optically active medium is allowed to pass and shows as a bright spot on the live laser scan. one dimension of the defect. A second major use of conventional microscope stations, different from the review or “revisit” function, is the more 45 general defect detection function, where preselected sites on a wafer are inspected for e?icacy of a previous process step. Laser imaging system 100 according to the invention is directly usable for this application, which faces exactly the same problems as defect imaging: decreasing size of objects of interest, a lack of resolution, and the need for three dimensional imaging. The hardware required is exactly the tion of the defect on the semiconductor die. 5. An imaging system as in claim 1, wherein the laser emits one of said plurality of wavelengths at a second intensity, and wherein the means for inspecting further comprises a second ?lter that allows said one of said plurality of wavelengths to be transmitted to the semicon ductor die with a third intensity inversely proportional to the second intensity. same, as is the system control software. Only a variation in the application software is needed, so that predetermined inspection sites may be speci?ed instead of defect map sites. Overlay registration is another possible use of laser imag ing system 100. Laser imaging system 100 can view pho toresist layers on top of underlying structures to ascertain accuracy of placement of the photoresist layer with respect 4. An imaging system as in claim 1, wherein the means for analyzing further comprises means for determining a loca 55 6. An imaging system as in claim 1, further comprising: means for supporting the semiconductor die; means for isolating the semiconductor die within a region of the system, and wherein air within the region con forms to Class 1 cleanroom conditions. 7. An imaging system as in claim 6, further comprising means for remotely loading and unloading the semiconduc to features of the underlying structures. Laser imaging system 100 can also be used to view tor die onto and off of the means for supporting such that air within the region conforms to Class 1 cleanroom conditions. through transparent (e.g., dielectric or glass) layers to allow 8. An imaging system as in claim 1, wherein the imaging determination of the vertical site of defects, or to provide system can resolve features on the object as small as some specialized inspection. For example, stress voids in 65 approximately 0.1 microns. metal layers below dielectric layers can be viewed. The 9. An imaging system as in claim 8, wherein the laser light depth of metal plugs in glass insulators can also be seen. source is an ultraviolet laser. 5,479,252 18 17 20. An imaging system as in claim 11, wherein: 10. An imaging system as in claim 8, wherein images are rendered in false color. the white light is polarized; and the means for imaging an object using white light further 11. An imaging system comprising: a laser imaging means for imaging a surface of an object comprises means for allowing only light re?ected from using laser light, the laser imaging means comprising optically active material in or on the object to pass through to the means for measuring the intensity of a laser light source that emits a beam of laser light; a spatial ?lter for focusing the beam on a pinhole white light. aperture; 21. An imaging system for detection or analysis of defects a lens arranged between the pinhole aperture and the surface, the lens having a focal plane; means for scanning the beam and directing the scanned beam toward the object through the lens; and means for measuring the intensity of laser light re?ected from the object and back through the lens and the pinhole aperture, wherein the measured intensity of laser light re?ected from the object is at in or on an object, the system comprising: means for producing light of a ?rst polarity; means for directing the light of a ?rst polarity through a pinhole aperture and toward the object such that a portion of the light of a ?rst polarity is re?ected by a surface of the object; a lens mounted between the pinhole aperture and the surface, the lens for focusing the light of a ?rst polarity a maximum intensity when the surface lies in the to a focal point on or within the object; focal plane of the lens; and a white-light imaging means for imaging a surface of an object using white light, the white-light imaging means 20 comprising is at a maximum intensity when the surface and the a white light source that emits white light; means for directing the white light toward the object; means for measuring the intensity of white light re?ected from the object; and a ?lter that prevents laser light from impinging upon the means for measuring the intensity of white light 25 focal point of the lens are coincident; means for changing the polarization of the light re?ected by the object so that the polarization of the light re?ected by the object is of a second polarity different than the ?rst polarity; means for ?ltering the re?ected light so that the re?ected and allows white light to impinge upon the means for measuring the intensity of white light; wherein the laser light imaging is performed at the same time as the white light imaging. 12. An imaging system as in claim 11, wherein one of the laser light or the white light is emitted at a ?rst wavelength means for measuring the intensity of the light re?ected from the object and back through the pinhole aperture, wherein the intensity of light re?ected from the object 30 light passing through the means for ?ltering has an intensity directly proportional to the distance between the focal point of the lens and the surface; and means for analyzing the intensity of to create an image of the object. that causes ?uorescence of a particular material in or on the 22. A laser imaging system for imaging a surface of an ?lter, and prevents light of the ?rst wavelength from passing through the ?lter. pinhole aperture and output a scanned beam toward the 13. An imaging system as in claim 11, wherein: the means for imaging an object using laser light further a lens arranged between the beam scanner and the surface, comprises means for displaying the laser light image; a sensor generating an output signal proportional to the object such that the one of the laser light or white light is 35 object, the system comprising: a laser light source that emits a beam of laser light, the re?ected from the material at a second wavelength, and beam focussed on a pinhole aperture; wherein the imaging system further comprises a ?lter that allows light of the second wavelength to pass through the a beam scanner con?gured to receive the beam from the and the means for imaging an object using white light further comprises means for displaying the white light image. 14. An imaging system as in claim 13, wherein an image surface; the lens having a focal plane; and 45 of the object can be displayed as a two-dimensional view of a surface of the object. 50 15. An imaging system as in claim 14, wherein an image of the object can be displayed as a two-dimensional view of a slice through the object. 16. An imaging system as in claim15, wherein an image intensity of laser light re?ected from the object and back through the lens, the beam scanner, and the pinhole aperture to impinge on the sensor, wherein the output signal of the sensor indicates a maximum inten sity of re?ected laser light when the surface lies in the focal plane of the lens. 23. The imaging system of claim 22, wherein the laser is a multiline laser light source that emits laser light at a plurality of wavelengths. 24. An imaging system as in claim 23, wherein the laser of the object can be displayed as a three-dimensional view 55 emits one of said plurality of wavelengths at a ?rst intensity, of a surface of the object. and wherein the means for inspecting further comprises a 17. An imaging system as in claim 16, wherein at least ?lter that allows said one of said plurality to be transmitted two of the views can be displayed simultaneously. 18. An imaging system as in claim 11, wherein: to the semiconductor die with a second intensity inversely proportional to the ?rst intensity. 25. An imaging system as in claim 23, wherein the beam travels along a ?rst path between the laser and the surface, the system further comprising: a beam splitter arranged in the ?rst path of the beam the laser light is polarized; and the means for imaging an object using laser light further comprises means for allowing only light re?ected from optically active material in or on the object to pass through to the means for measuring the intensity of laser light. 19. The imaging system of claim 11, wherein the scanning means comprises a resonant mechanical scanner. between the laser and the lens, the beam splitter con 65 ?gured (l) to pass a ?rst portion of light travelling along the ?rst path and to re?ect a second portion of light travelling along the ?rst path so that the second 5,479,252 19 20 portion of light travels along a second path not parallel to the ?rst path, and (2) to pass a third portion of light travelling to the beam splitter along the second path and a white light source that emits white light; a re?ective body for directing the white light toward the object; and to re?ect a fourth portion of light travelling to the beam a camera for white-light imaging, the camera con?gured to receive a portion of the white light re?ected from the surface. splitter along the second path so that the fourth portion of light travels along the ?rst path; a white light source that emits white light along the second path to impinge the beam splitter and illuminate 27. The imaging system of claim 26, wherein the laser light imaging and the white light imaging are performed the surface; a camera arranged to receive light travelling along the second path; and a ?lter arranged between the beam splitter and the camera, the ?lter preventing laser light from impinging upon the camera and allowing white light re?ected from the surface and re?ected by the beam splitter to impinge upon the camera. 26. An imaging system as in claim 22, further comprising: 10 simultaneously. 28. An imaging system as in claim 22, wherein the object is enclosed within a region of the system and wherein air within the region conforms to Class 1 cleanroom conditions. 29. An imaging system as in claim 22, wherein the beam scanner is a resonant mechanical scanner.
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