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Lithography Defects: Reducing and Managing Yield Killers through Photo Cell Monitoring by Ingrid Peterson, Gay Thompson, Tony DiBiase and Scott Ashkenaz, KLA-Tencor Corporation Rebecca Howland Pinto, Ph.D., Consultant
Defectivity challenges in the lithography module
Recent results have shown that lithography defects previously thought to be cosmetic can affect yield by as much as 15 percent.1 Stains and minor color variations can be translated into blocked contacts, bridging, missing or extra pattern defects, and CD variations during subsequent steps in the process. As a result, managing defectivity during photolithography is as crucial to the contribution of the photo cell to yield as are proper design, control of critical dimension (CD) and overlay, film parameters and electrical parameters. Many leading semiconductor manufacturers have found that the most effective methodology for controlling defectivity in the lithography module is to supplement after-develop inspection (ADI) with macro and micro photo cell monitoring (PCM) using test wafers. The traditional approach has been to inspect product wafers in-line after the resist has been developed, using a high numerical-aperture, brightfield micro inspection system together with a brightfield/darkfield macro inspection system. While this approach is highly effective for capturing micro defects (such as developer spots, resist lifting and collapse, uncleared patterns, developer nozzle impact patterns, resist-developer residue and amine contamination) and macro defects (such as missing photoresist, focus spots, gross overlay errors and scratches) some compelling studies have shown that several of these defect types have a low capture rate on product wafers. The low capture rate is primarily due to interference from defects from previous layers, or underlying grain or color variation that makes detection of a defect, that is itself represented as a minor color change, challenging. The resulting delay in detecting a tool or process problem can have serious financial consequences, especially for back-end-of-the-line (BEOL) layers, where priorlevel “noise� is highest. Consequently, lithography process excursions may not be evident until electrical testing several weeks later.2 A photo cell monitor, sometimes called an excursion monitor,2 is a resist-onsilicon or resist-on-oxide-on-silicon patterned test wafer fully processed through the lithography cluster. It may use the same reticles as the product wafer, or use a reticle specifically designed for photo cell monitoring. Macro and micro defectivity are measured on the PCM wafer, the defects are classified using automatic defect classification (ADC) and the critical dimensions (CDs) Spring 2000
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PCM Defect
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Defect After Etch
Figure 1. PCM defects can accurately represent defects on product wafers, and find them earlier in the process. On the left are three examples of defects found on PCM wafers that may cause defects of the types shown on the right, detected on product wafers after etch.
of the resist are measured using scanning electron microscopy (SEM).3 Statistical process control (SPC) charts use the categorized defectivity and metrology data to monitor the performance of the lithography cluster and identify excursions and trends. The advantage to using a PCM methodology is that the wafer has
21XX micro wafer inspection system
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only one resist film, which dramatically reduces the noise introduced into the inspection by underlying layers, which inevitably have defects, grain, and film-thickness variations — especially in the backend layers. The topography of a PCM wafer is also much simpler than that of a product wafer. As a result the PCM wafer can provide a very sensitive monitor for yield-limiting excursions caused by either equipment- or process-induced defects. Furthermore, the experience of KLA-Tencor’s Yield Management Consulting (YMC) group has shown that roughly 90 percent of defect types seen on product wafers at ADI can be detected by PCM (Figure 1). Using KLA-Tencor’s 21XX micro and 2401 macro inspection systems, the PCM methodology provides the most efficient and effective inspection strategy for detecting process excursions in the lithography module (Figure 2). The complete equipment set would include reticle requalification using STARlight SL3UV, unpatterned wafer inspection to qualify and monitor the health of the tools using SP1TBI, automatic defect classification using IMPACT ADC on off-
2401 macro wafer inspection system
Figure 2. Poor spin quality can be obser ved visually and captured using automated micro inspection and/or automated macro inspection. In most instances the lowest cost of ownership system that can detect the defect will provide the most benefit. In this case an intelligent sampling strategy would allow the high-speed macro inspection system to capture this defect first – making the micro inspection unnecessar y.
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line optical and SEM review stations, and PMC-Net analysis for aid in tracing the source of the defects and managing the data flow. Finally, the Sample Planner™ program can help develop a custom sampling plan for the fab’s specific needs. The equipment set and methodology described in this paper have arisen from collective expertise and case studies from many fabs, with the aim of minimizing the contribution of the lithography module to yield loss. This is accomplished through efficient capture and classification of critical defect types which in turn enable quick response to defect excursions, and best return on investment of the defectivity equipment set, while improving the overall equipment effectiveness of the lithography clusters. Financial impact of a PCM program
Motorola estimated recently that replacing manual macro inspection with automated macro inspection would prevent more than $4M scrap per year, with a differential average net yearly equipment cost of ownership of $0.4M.4 This estimate was based on assuming a scrap rate from the lithography module of 0.01 percent and 5000 wafer starts/week. In a pilot study they found that the greatest benefit was obtained from 100 percent inspection of output from a new photo cell, which produced sporadic failures due to multiple coat and develop stations, and new software issues. The magnitude and quality of this financial benefit can be applied to macro PCM as well as to macro ADI. The use of PCM has been shown to be effective in managing lithography defectivity. It provides a high signalto-noise ratio and early detection of problems which might not otherwise be recognized, or might be recognized only at test, once yield has been impacted, and after many lots of wafers may have been affected. Yield impacts of 15 percent have been seen where defects were not otherwise under effective control.
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Comets Striations Spins Microbubbles Lifting No resist
Surface Prep
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• Missing • Wrong width • Miscentering
Resist Spin
Edge Bead Removal
Hard Bake
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Soft Bake
Align and Expose
Post Exposure Bake • • • • • •
Scumming Developer spots Residue Resist collapse Satellite spots No develop Deformed contacts
Missing Focus error Gross misalign Aperture blade error No exposure Hot spots
Figure 3: A wide variety of defect types can arise in the lithography cell.
The yield impact has significant cost, both through lost material, and through lost fab manufacturing capacity. In the case of multi-product ASIC and foundry fabs, the rise in defectivity may have significant impact on the fab’s ability to satisfy a customer with a small number of wafers of a particular product. PCM methodology for defect reduction
Defectivity control in the lithography module relies on three steps: 1) optimization for detection of photo problems; 2) establishment of the process performance baseline; and 3) improvement of the baseline. Introducing the PCM approach to complement ADI information addresses each step. Photo defects are difficult to detect on product wafers because the noise from the topography, grain and color of the underlying layers confounds the detection of current-layer defects, as mentioned previously. In particular, many lithography defects have low
topography, subtle color variation, and/or small physical extent. All of these characteristics mean that the signal for optical detection is small. Thus, reducing the denominator in the signal-to-noise ratio by removing the additional challenge of detection on a product wafer raises the capture probability of defects having these characteristics. Micro PCM Defect Types
Developer spots (Figures 3 and 4) are observed on developer equipment from every vendor, and are one category of lithography-related defects whose capture rate benefits from using a photo-cell monitor wafer. Often classified as missing pattern or extra pattern during in-line monitoring after-etch, and commonly detected on the perimeter of the wafer, these defects can be caused by splash-back after the develop cycle. Possible causes include poor exhaust in the developer cup, developer cup design, or the type of developer nozzle. This defect type is not seen by traditional develop Spring 2000
track particle monitors using bare silicon wafers since its mechanism is dependent upon the surface tension between resist and developer. The surface properties of the resist-coated PCM wafer, however, are favorable for detecting this defect type.5 Developer residue ( Figures 3 and 4) is a defect type found on PCM wafers that monitor via and contact pattern. Because the defects are very low contrast and have color variation similar to that of developer spots, they are hard to find on production wafers during ADI due to noise from underlying layers. Their spatial distribution is distinctive. Very dense, radial, and typically following the pattern on the wafer, the spatial distribution of developer residue provides an important clue to the source of this type of defect. Developer residues can cause blocked contacts or blocked via openings on the product wafer, which can lead to significant yield loss (Figures 6 and 7). Probable causes include developer CO2 or resist-
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Defect
Typical Cause
Comet
• Dried resist on dispense nozzle
Missing and extra pattern, CD error
• Contamination or perturbation on wafer surface
Example
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• Contamination in resist • Changes in surface tension conditions
Lifting Missing and extra pattern
• Inadequate soft bake
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• Missed PEB • Over/under priming • Resist dispense rate
Microbubble Broken leads for smaller design rules
• ARC/resist interfacial interaction between substrate, primer and coating
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• Air leak in the resist pump • Mis-adjusted suck back Developer Spot Missing or extra pattern after etch
Developer Residue Blocked contacts after etch
• Splash-back after the develop cycle • Poor exhaust in the developer cup, developer cup design, or developer nozzle type
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• Developer-CO2 or resist-developer interaction • Ph shock
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• Non-optimized develop/rinse conditions Closed or Deformed Contact
• Residue on the contact hole, unoptimized develop program
Satellite Spot
• Resist-developer interaction
Blocked contact or extra pattern after etch Developer Nozzle Residue Bridge between lines
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• By-product of develop and resist chemistries • Contamination of developer nozzle
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Figure 4. Detecting these common defect types before the wafers leave the lithography cell allows simpler and faster identification of the source of the defects, and results in fewer wafers scrapped or dispositioned for re-work.
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are typically near the center of the wafer or out near the edge, and are due to exhaust imbalance, EBR pressure control problems, or spurts. EBR width errors come from improper setup of the EBR nozzle or optical EBR exposure. These can result in a reduction of usable wafer area (if too large) or contamination from flaked resist (if too small). Excursions resolved using Klarity Defect IMPACT classified defects Klarity observed signatures Figure 5. In production, IMPACT ADC classified the Satellite and E 2 Nozzle defects with better than 95 percent accuracy and purity, while Klarity Defect observed their spatial signatures. The trend chart shows that both problems were fixed, and excursions in both defect types were monitored. 9
Microbubbles (Figures 3 and 4) are examples of defects whose size (<< 0.5 µm) makes them difficult to detect at ADI. This defect, which can cause broken leads for devices having critical dimensions less than 0.35 µm, is readily detectable using PCM wafers with high sensitivity inspection. Probable causes include resist dispense rate, ARC/resist interfacial interaction, and interaction between substrate, primer and coating. While the resist process is typically very clean, an example of a resistrelated defect that can be found through PCM is amine contamination of the deep ultra-violet (DUV) resist. The contamination can degrade the profile of the resist. Macro PCM Defect Types
While many lithography process defects are readily detected using micro inspection, some may also be detectable using macro inspection, at substantially higher throughput. Many errors in resist coating, exposure, develop, and rinse result in defects which cover a relatively large area. Resist coating errors can come from a number of causes, from poor surface
prep to problems with the resist dispense. These defects can cover large portions of a wafer, but may also be as small as 50 µm. Because they are relatively large, and tend to occur in clusters, they can have significant yield impact. Resist film defects such as comets may come from contamination in the resist. A well-defined head points toward the center of the wafer, and a wake of resist thickness variation flares out toward the wafer edge. The head will cause a hard defect, while the tail may result in significant CD variations. Edge Bead Removal (EBR) splashback comes from solvent which is aspirated and deposits back on the wafer surface during spin, resulting in spots of cleared resist. These spots Cup 3-2 Defect Density (Defects/cm2)
developer interaction; developer precipitates, rinse deficiencies, or resist and developer surfactant bonding.
Hot spots or focus spots are regions where the stepper does not focus properly. These may be due to backside particles which remain on the wafer or are transferred to the stepper chuck. With today’s high numerical-aperture lenses, the susceptibility to hot spots is increasing. Several of the developer errors listed as micro PCM defects may also manifest themselves at a size detectable by macro inspection. Inspection equipment set
Apart from the PCM wafers themselves, the inspection equipment set for defect reduction in the lithography module using a PCM approach is identical to that used for ADI. A brightfield inspection system having a high numerical aperture, such as KLA-Tencor’s 21XX, is optimized for capturing low-contrast micro defects like developer spots, residues and microbubbles. Defects having characteristic spatial distributions, such as comets, striations, spin defects, scratches, etc. require a macro inspection system like KLA-Tencor’s 2401. Contact PCM
Cup 3-3
Poly PCM
Develop Process Related Defects
Develop Process Related Defects
Time Figure 6. Using a PCM wafer based on the actual contact reticle instead of a lines-and-spaces PCM wafer provided much improved correlation to the observed defect density on the product.
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Defect Type: Isolated Closed Contacts
SEM Image of Closed Contact
Optical Image of Closed Contact Defect Description:
Low-Magnification SEM Image
High-Magnification SEM Image
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Killer Defect
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Can be eliminated by optimizing develop program, and/or eliminating bubbles in the developer puddle
Posible cause: micro-bubbles in the developer
Defect Type: Resist residue EDX Analysis Results
Optical Images
Defect Description:
SEM Images
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Defect can be caused by nozzle residue
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Can be a killer defect if it lands on top of contact
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Can be reduced by optimizing develop program (dispense speed and time, DI water rinse speed)
Defect Type: Area of Missing Pattern 0.00-0.20 0.20-0.50
Defect Description:
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0.50-1.00
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1.50-2.00 2.00-3.00
The way these two systems are used together is primarily based on defect type, but also on minimizing cost of ownership. The 2401 is more sensitive to low-level hot spots, and its higher throughput and lower price make it more cost-effective than the 21XX for detecting large-area errors such as bad spin, comets, striations, edge-bead removal errors, etc. (Figure 2.) The 21XX is essential for detecting small, localized defects. While the best strategy for implementing the tools depends upon individual circumstances including the complexity of the lithography process and operation of the fab, typically the 21XX and 2401 would be used to scan each PCM wafer every shift during development, and daily during production. The 2401 would typically be used first to filter macro issues. The wafers would then be inspected for micro defects by the 21XX. Both micro and macro PCM inspection are used for each path through the track and resist combination, since each may fail independently. After the defects are detected, automatic defect classification is used to separate the defects into types. Once defects are classified, SPC can be applied to track by defect type or by grouping killer versus non-killer defects. The kill ratio of individual defect types would be determined from short loop PCM experiments since the capture rate for many of the lithography-related yield-limiting defects is low for after-develop inspection on product wafers.
3.00-5.00 5.00-8.00 8.00-10.00 10.00-9999.00 0
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SEM Images of Area of Missing Pattern
Figure 7. Defect types in the lithography cell associated with missing contacts after etch were uncovered using the contact PCM technique. These defect types included: (a) isolated closed contacts; (b) resist residue; and (c) areas of missing pattern.
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Optical ADC should be performed in conjunction with SEM review since some defect types cannot be distinguished optically, but only through SEM review. After-develop inspection complements the lithography defect reduction program by capturing topographyrelated process-integration defects. Micro ADI might be employed for 2 or 3 wafers per lot, while macro ADI would be used on 50 percent to 100 percent of the wafers in each lot.
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Case study 1: AMD SDC
AMD SDC discovered the value of using a photo cell monitor for a critical lithography process having six levels of metal. They found by failure analysis that they had a problem with missing contacts and vias, but neither their in-line inspection results after etch or after develop detected an excursion.6 When they first implemented a PCM methodology they used a lines-and-spaces reticle that mimicked the poly step. They still found poor correlation between the contact failures and their inspection results.7 The breakthrough came when they created a new PCM using the contact reticle itself. This contact PCM increased their sensitivity to subtle, low-topography defects by reducing the noise contributed to the measurement by the underlying layers of the product wafers. Furthermore, some defect types depend upon the amount of unexposed photoresist on the wafer and its interaction with the developer. These defect types are usually not detected reliably using a lines and spaces reticle (Figure 6). Using the PMC based on the actual contact reticle enabled the mystery to be solved. Three defect types in the photo cell were found to be associated with the missing contacts after etch: isolated closed contacts, resist residue and areas of missing pattern (Figure 7). The low noise of the contact PCM also allowed AMD to detect a repeater on the contact reticle, which they were unable to detect with any inspections using product wafers.7 The missing contact and via problems were found to have four different sources: 1) randomly isolated missing contacts and vias due to a developer program optimization issue; 2) randomly isolated missing contacts caused by a residue falling on the open contact and via after develop, which blocked the etch;
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3) large areas of missing contacts and vias caused by large residues and bubbles in the develop process; 4) the repeater mentioned above. Although 1) and 2) were indistinguishable under optical review, subsequent SEM review clarified that the sources of these defect types were very different. Case Study 2: IBM
IBM began to experiment with PCM methodologies when they were dissatisfied with the correlation between product inspections and yield in the photolithography module. After six months of PCM operation, they found that all process excursions were detected, and response time to the excursions had improved significantly.2 The defect types that were identified successfully using PCM included starburst defects, hexamethyl-disilazane (HMDS) flaking, focus spots, printed defects, and dose and resist-thickness variations. These defect types are described and illustrated in Reference 2. Each of them causes yield-limiting defects down the line. Furthermore, the PCM approach allowed the engineers to identify equipment problems quickly, reducing the rework and scrap costs. In some cases, the source of the defects was found months before it may have been found using traditional methods. Case Study 3: NEC UK
NEC were interested in using automated macro inspection and a PCM wafer to monitor their stepper.8 They created a diffraction-grating reticle with 0.3 µm lines and spaces, and performed a focus-exposure matrix with focus offsets and leveling offsets. They inspected the wafers on the 2401 and used the results to estimate the process windows. The 2401 detected a slight defocus process excursion, and a slight field tilt process excursion. (See article on page 35.) Summary
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photoresist can be cost-effective and can provide defect capture in many cases superior to that of after-develop inspection. The superior defect capture using a PCM wafer instead of product arises from its single-layer design, which removes the noise contributed by the topography, grain and color variation of underlying layers. This is a particularly important strategy when the defects are low-contrast, which is typical for many of the lithography-related defect types. The complementary strategy of line monitoring using micro and macro ADI on production wafers makes it possible to (1) catch excursions of critical defects including topography related process integration defects not seen on the PCM wafer; (2) make quick go/no-go determinations of the health of the production lot and process tool module; and (3) disposition product wafers more effectively. As part of a complete defect reduction program in the lithography module, the use of a photo cell monitor for micro and macro process tool monitoring can provide significant improvement in defect detection, better process tool health plans and faster response time to process excursions. These benefits translate directly into significant savings in rework and scrap costs, and raise the contribution of the lithography module to device yield. Numerous fabs have demonstrated significant improvements in lithography defectivity by establishing defect management through PCM. References 1. Ingrid Peterson, “Defect Reduction Methodology in the Lithography Module,” Proceedings from the XIII Annual Meeting of the SPIE Microlithography Conference, March, 1999, pp 520-528. 2. Eric H. Bokelberg and Michael E. Pariseau, “Tracking the performance of photolithographic processes with excursion monitoring,” MICRO, January 1998.
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Focus Offset Results Wafer No. +ve Offset Detected -ve Offset Detected 1 0.05 N -0.05 N 0.1 N -0.1 N 2 0.15 N -0.15 N 0.2 N -0.2 N 3 0.25 N -0.25 N 0.3 N -0.3 Y 4 0.35 N -0.35 Y 0.4 Y -0.4 Y 5 0.45 Y -0.45 Y 0.5 Y -0.5 Y 6 0.55 Y -0.55 Y 0.6 Y -0.6 Y 7 0.65 Y -0.65 Y 0.7 Y -0.7 Y Figure 8. Defocus process excursions were identified by the 2401 by performing a focus-exposure matrix using a PCM wafer, after a problem with the stepper was suspected.
3. John Allgair, Gong Chen, Steve Marples, David Goodstein, John Miller and Frank Santos, “Feature Integrity Monitoring for Process Control Using a CD SEM,” presented at SPIE’s 25th Annual International Symposium on Microlithography, February, 2000. 4. Arnold Yanof, Vincent Plachecki, Frank Fischer, Marcelo Cusacovich, Chris Nelson and Mark Merrill, “Implementation of Automated Macro After Develop Inspection in a Production Lithography Process,” p r e sented at SPIE’s 25th Annual International Symposium on Microlithography, February, 2000. 5. Ingrid. B. Peterson, “Importance of Defect Reduction in the Lithography Module,” Yield Management Solutions, Autumn 1998. 6. Christina Cheung, Robert Chiu, Khoi Phan, Ingrid Peterson, Andy Phillips, Kevin Kan, “Contact Photo Cell Monitor (PCM) for an Advanced BEOL Lithography Process,” in Proceedings of the SEMICON/West Yield Management Solutions Seminar, July, 1999. 7. Private communication. 8. Iain Rutherford, Brian Haile, and Tony DiBiase, “Production QC and Tool Monitoring Using and Automated Post Develop Macro Inspection System,” to be published in Proceedings of the SEMICON/Europa Yield Management Solutions Seminar, April, 2000. 9. Frank Poag, Douglas Paradis, Mahesh Reddy and Jon Button, “Defect Yield Management using the KLATencor Intelligent Line Monitor,” in Proceedings of the SEMICON/ Southwest Yield Management Solutions Seminar, October, 1999. 24
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About the Authors Dr. Ingrid B. Peterson is currently Solutions Development Manager for Lithography and Parametric Module Solutions at KLA-Tencor. She joined KLA-Tencor in 1995 and has held positions as Applications Development Manager for Reticle Inspection Analysis products and as a consultant for KLA-Tencor’s Yield Management Consulting Group. Prior to joining KLA-Tencor, Ingrid worked as a staff process engineer in photolithography, was a staff research scientist at the Max Planck Institute for Solid State Physics in Stuttgart, Germany and was an adjunct assistant professor in the Physics Department at the University of California Los Angeles. Ingrid earned a Ph. D. in Physics from the University of California at Santa Barbara. Gay Thompson is currently Field Marketing Manager for Defect Module Solutions at KLA-Tencor. She joined KLA-Tencor in 1996 and has held positions in New Production Introduction and Product Marketing for KLA-Tencor’s wafer inspection products. Prior to joining KLA-Tencor, Gay worked as a program manger in the U.S. Air Force, managing the development, production and launch of satellite systems. Gay earned a B.S.E.E. from MIT and a M.S.E. from the University of Texas at Austin. Tony DiBiase is a director in the Yield Management Consulting division of
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KLA-Tencor. Since 1980, Tony has held various positions in maskmaking, process integration, metrology, lithography development, and maskmaking at National Semiconductor, Synertek, and Synergy Semiconductor. He has a B.S. in Chemistry from the University of Cincinnati. Scott Ashkenaz is KLA-Tencor's Vice President of Strategic Marketing for Patterning and Parametric Process Module Control Solutions (PMCS), which provides lithographic and parametric alignment among KLATencor's products to improve wafer fab productivity and capability. Prior to joining KLA in 1985, Scott was the Mask Lithography Manager for Austrian Microsystems, and responsible for AMI's advanced mask development program. He attended the Bachelor's and Master's programs in Photographic Science and Engineering at Rochester Institute of Technology. A former director of marketing with KLA-Tencor, Rebecca Howland Pinto is an independent consultant in technical marketing. A frequent contributor to Yield Management Solutions, Ms. Pinto has published numerous articles and lectured worldwide during her 10 years in the industry. She has a Ph.D. in applied physics from Stanford University, and an A.B. in physics from Dartmouth College. circle RS#046
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