Waferscale S-MCM for High Performance Computing
Rabindra Nath DasVladimir BolkhovskyAlex WynnRavi RastogiScott ZarrDmitri ShapiroManuel doCantoL. M. JohnsonEric A. Dauler
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This paper describes a novel strategy to combine laser direct write (LDW) and optical lithography (I-line) to fabricate 200 mm waferscale superconducting multi-chip modules (S-MCM) for interconnecting multiple active superconducting electronics chips based on single flux quantum (SFQ) logic for next generation cryogenic processing systems. The packaging roadmap includes the development of S-MCM (48 mm x 48 mm) using a nearly full I-line reticles, followed by reticle stitching to fabricate the largest possible stitched S-MCM (96 mm x 96 mm) using a four mask/layer process. The stitching process starts with sequential exposure of multiple I-line photomasks, with small overlap (stitched area), to realize larger combined circuit areas for design- critical S-MCM layers with minimum linewidths of 0.8-1 gm. The packaging roadmap further extends the S-MCM size utilizing laser direct write (LDW) lithography to make wider (> 1 gm) features such as fan-out circuits, extending the stitched circuit area to include the entire 200 mm wafer as a single S-MCM. Process control monitors (PCM) include snake/comb test structures, critical dimension (CD) cells, transmission lines, daisy chains etc. Niobium-indium-based microbump technology was developed to demonstrate full-size (20 x 20 mm 2 ) SFQ flip-chips on an S-MCM with low 4 K interconnect resistance (50-100 μΩ) at the SFQ chip to S-MCM interface. Confocal micrographs show a uniform niobium-indium microbump-based interconnect network and X-ray images show the desired deformation of niobiumindium microbumps after flip-chip bonding. Full-size (20x20mm 2 ) flip-chip daisy chains with 10k and 100k bumps maintained high Nb critical current post-bonding (> 50 mA), demonstrating a viable platform for building larger superconducting computing systems.Keywords:
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The Critical Dimension Uniformity (CDU) specification on photomasks continues to decrease with each successive node. The ITRS roadmap for optical masks indicates that the CDU (3 sigma) for dense lines on binary or attenuated phase shift mask is 3.4nm for the 45nm half-pitch (45HP) node and will decrease to 2.4nm for the 32HP node. The current capability of leading-edge mask shop patterning processes results in CDU variation across the photomask of a similar magnitude. Hence, we are entering a phase where the mask CDU specification is approaching the limit of the capability of the current Process of Record (POR). Mask shops have started exploring more active mechanisms to improve the CDU capability of the mask process. A typical application is feeding back the CDU data to adjust the mask writer dose to compensate for non-uniformity in the CDs, resulting in improved quality of subsequent masks. Mask makers are currently using the CD-SEM tool for this application. While the resolution of SEM data ensures its position as the industry standard and continued requirement to establish the photomask CD Mean to Target value, a dense measurement of CDs across the reticle with minimal cycle time impact would have value. In this paper, we describe the basic theory and application of a new, reticle inspection intensity-based CDU approach that has the advantage of dense sampling over larger areas on the mask. The TeraScanHR high NA reticle inspection system is used in this study; it can scan the entire reticle at relatively high throughput, and is ideally suited for collecting dense CDU data. We describe results obtained on advanced memory masks and discuss applications of CDU maps for optimizing the mask manufacturing process. A reticle inspection map of CDU is complementary to CD-SEM data. The dense data set has value for various applications, including feedback to mask writer and engineering analysis within the mask shop.
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A next reticle format is considered from a view point of photomask fabrication process. In order to determine optimal reticle thickness corresponding to 230 mm size, processes feasibility for the thickness are examined. Cr dry etching is studied in terms of plasma condition and patterning accuracy. It is found that Cr dry etching with accurate critical dimension (CD) control is possible with substrate as thick as 12.7 mm. Post exposure bake (PEB) process for chemically amplified (CA) resists is also examined in terms of CD uniformity on substrates. Relationship of substrate thickness and CD uniformity is estimated with PEB controllability and CA resists characteristics. In addition to this, it is demonstrated that actual temperature uniformity for substrate with a thickness of 9 mm is equivalent to that for a present thickness of 6.35 mm. From these results in overall, it is proposed that substrate thickness of 9 mm is feasible for actual photomask production.
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This chapter is concerned with masks for optical lithography. Photomasks used in present-day semiconductor manufacturing are reduction reticles, where the pattern is formed in a chromium layer over a fused silica substrate. The pattern represents one level of an integrated circuit (IC) design. An optical stepper forms a four to five demagnified image of the reticle on the wafer. The reticle usually contains one or more identical circuit patterns plus wafer process test patterns and marks for aligning the reticle in the optical stepper. Typically, 20 to 25 different mask levels are required for a complete IC device.
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With growing implementation of low k1 lithography on DUV scanners for wafer production, detecting and analyzing photomask critical dimension (CD) errors and semitransparent defects is vital for qualifying photomasks to enable high IC wafer yields for 130nm and 100nm nodes. Using the TeraStar pattern inspection system's image computer platform, a new die-to-database algorithm, TeraFlux, has been implemented and tested for the inspection of small "closed" features. The algorithm is run in die-to-database mode comparing the energy flux difference between reticle and the database reference for small closed features, such as, contacts, trenches, and cells on chrome and half-tone reticles. The algorithm is applicable to both clear and dark field reticles. Tests show the new algorithm provides CD error detection to 6% energy flux variation with low false defect counts. We have characterized the sensitivity and false defect performance of the die-to-database energy flux algorithm with production masks and programmed defect test masks. A sampling of inspection results will be presented. Wafer printability results using the programmed defects on a programmed defect test reticle will be presented and compared to the inspection defect sensitivity results.
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A lithographic technique which can significantly reduce the effect of photomask defects is investigated. It is based on exposures of multiple reticle fields containing identical patterns, and is especially suitable for 1 × wafer steppers. The principle, requirements, and initial experimental results of this method are presented.
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Non-uniformity in reticle CDs can cause yield loss and/or performance degradation during chip manufacturing. As a result, CD Uniformity (CDU) across a reticle is a very important specification for photomask manufacturing. In addition the photomask CDU data can be used in a feedback loop to improve and optimize the mask manufacturing process. A typical application is utilizing CDU data to adjust the mask writer dose and compensate for non-uniformity in the CDs, resulting in improved quality of subsequent masks. Mask makers are currently using the CD-SEM for data collection. While the resolution of SEM data ensures its position as the industry standard, an output map of CDU using the reticle inspection tool has the advantage of denser sampling over larger areas on the mask. High NA reticle inspection systems scan the entire reticle at high throughput, and are ideally suited for collecting CDU data on a dense grid. In this paper, we describe the basic theory of a prototype reticle inspection-based CDU tool, and results on advanced memory masks. We discuss possible applications of CDU maps for optimizing the mask manufacturing process or in adjusting scanner dose to improve wafer CD uniformity.
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Critical dimensions (CD) measured in resist are key to understanding the CD distribution on photomasks. Vital to this understanding is the separation of spatially random and systematic contributions to the CD distribution. Random contributions will not appear in post etch CD measurements (final) whereas systematic contributions will strongly impact final CDs. Resist CD signatures and their variations drive final CD distributions, thus an understanding of the mechanisms influencing the resist CD signature and its variation play a pivotal role in CD distribution improvements. Current technological demands require strict control of reticle critical dimension uniformity (CDU) and the Advanced Mask Technology Center (AMTC) has found significant reductions in reticle CDU are enabled through the statistical analysis of large data sets. To this end, we employ Principle Component Analysis (PCA) - a methodology well established at the AMTC1- to show how different portions of the lithographic process contribute to CD variations. These portions include photomask blank preparation as well as a correction parameter in the front end process. CD variations were markedly changed by modulating these two lithographic portions, leading to improved final CDU on test reticles in two different chemically amplified resist (CAR) processes.
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