トランジスタ・アレイ方式に基づくアナログレイアウトにおける密度最適化

In integrated circuit design of advanced technology nodes, layout density uniformity significantly influences the manufacturability due to the CMP variability. In analog design, especially, designers are suffering from passing the density checking since there are few useful tools. To tackle this issue, we focus on a transistor-array(TA)-style analog layout, and propose a density optimization algorithm consistent with complicated design rules. Based on TA-style, we introduce a density-aware layout format to explicitly control the layout pattern density, and provide the mathematical optimization approach. Hence, a design flow incorporating our density optimization can drastically reduce the design time with fewer iterations. In a design case of an OPAMP layout in a 65nm CMOS process, the result demonstrates that the proposed approach achieves more than 48× speed-up compared with conventional manual layout, meanwhile, it shows a good circuit performance in the post-layout simulation.北九州市立大

Electrical recommendations and formulas for metal fill in radio-frequency integrated circuits

With increasing transistor operating frequencies, interconnects and passive devices are becoming performance limiters in integrated circuit (IC) designs. To combat this, the interconnect layers above the active silicon are trending toward low-κ dielectrics and Cu metallization. The use of these new materials has popularized chemical mechanical polishing (CMP) to planarize the several interconnect layers. Unfortunately, the mechanical trade-offs of CMP require metal pattern density uniformity and additional dummy metal shapes fill in regions of low density. These metal fills act as parasitics that increase the capacitances in interconnects and passive devices – hindering their performance. This work analyzes and optimizes the parasitic capacitive impact of rectangular metal fills on key passive components. Our systematic analysis of fills below a metal-insulator-metal (MIM) capacitor reveals an optimal design: large, square fills with lengths roughly 40% of MIM capacitors plate length. We fabricated such a MIM capacitor in a 250nm process showing a reduction in the substrate capacitance by half (compared to default tiling). Fill’s impact on interconnects, such as transmission lines, is also investigated. A detailed study of schemes that use grounded fills as shielding between interconnects informs an optimal grounding strategy. The strategy provides maximal isolation while minimizing capacitive loading. In fact, compared to no fills the addition of our metal fill shield increases loading by 58% while providing 58dB more isolation between example interconnects fabricated in a 130nm process. The capacitive impact of adding metal fills is found to be more significant as process dimensions shrink. In a 65nm process the inter-level dielectric constant is 3.5, but the addition of 50% density fills causes the effective dielectric constant to be 5.5. A semi-empirical, closed-form formula is developed to calculate this effective dielectric constant. The formula is accurate to within <1% for a wide range of metal fill densities, sizes, aspect ratios, and process dimensions. This is a significant improvement over state-of-the-art formulas which are found to be accurate to within ~10%. Our high accuracy is maintained when applied to multiple layers with and without staggering. Moreover, we successfully apply the formula to calculating ground/substrate capacitances of MIM capacitors, microstrip transmission lines, and a spiral inductor. This may speed up the calculation by hundredfold or even thousandfold. Results are compared to fabricated MIM capacitors and microstrips. Calculations and measurements match to within <5% for the capacitors and <2% for the microstrips

Advanced analog layout design automation in compliance with density uniformity

To fabricate a reliable integrated circuit chip, foundries follow specific design rules and layout processing techniques. One of the parameters, which affect circuit performance and final electronic product quality, is the variation of thickness for each semiconductor layer within the fabricated chips. The thickness is closely dependent on the density of geometric features on that layer. Therefore, to ensure consistent thickness, foundries normally have to seriously control distribution of the feature density on each layer by using post-processing operations. In this research, the methods of controlling feature density distribution on different layers of an analog layout during the process of layout migration from an old technology to a new one or updated design specifications in the same technology have been investigated. We aim to achieve density-uniformity-aware layout retargeting for facilitating manufacturing process in the advanced technologies. This can offer an advantage right to the design stage for the designers to evaluate the effects of applying density uniformity to their drafted layouts, which are otherwise usually done by the foundries at the final manufacturing stage without considering circuit performance. Layout modification for density uniformity includes component position change and size modification, which may induce crosstalk noise caused by extra parasitic capacitance. To effectively control this effect, we have also investigated and proposed a simple yet accurate analytic method to model the parasitic capacitance on multi-layer VLSI chips. Supported by this capacitance modeling research, a unique methodology to deal with density-uniformity-aware analog layout retargeting with the capability of parasitic capacitance control has been presented. The proposed operations include layout geometry position rearrangement, interconnect size modification, and extra dummy fill insertion for enhancing layout density uniformity. All of these operations are holistically coordinated by a linear programming optimization scheme. The experimental results demonstrate the efficacy of the proposed methodology compared to the popular digital solutions in terms of minimum density variation and acute parasitic capacitance control

Layout regularity metric as a fast indicator of process variations

Integrated circuits design faces increasing challenge as we scale down due to the increase of the effect of sensitivity to process variations. Systematic variations induced by different steps in the lithography process affect both parametric and functional yields of the designs. These variations are known, themselves, to be affected by layout topologies. Design for Manufacturability (DFM) aims at defining techniques that mitigate variations and improve yield. Layout regularity is one of the trending techniques suggested by DFM to mitigate process variations effect. There are several solutions to create regular designs, like restricted design rules and regular fabrics. These regular solutions raised the need for a regularity metric. Metrics in literature are insufficient for different reasons; either because they are qualitative or computationally intensive. Furthermore, there is no study relating either lithography or electrical variations to layout regularity. In this work, layout regularity is studied in details and a new geometrical-based layout regularity metric is derived. This metric is verified against lithographic simulations and shows good correlation. Calculation of the metric takes only few minutes on 1mm x 1mm design, which is considered fast compared to the time taken by simulations. This makes it a good candidate for pre-processing the layout data and selecting certain areas of interest for lithographic simulations for faster throughput. The layout regularity metric is also compared against a model that measures electrical variations due to systematic lithographic variations. The validity of using the regularity metric to flag circuits that have high variability using the developed electrical variations model is shown. The regularity metric results compared to the electrical variability model results show matching percentage that can reach 80%, which means that this metric can be used as a fast indicator of designs more susceptible to lithography and hence electrical variations

EDA Solutions for Double Patterning Lithography

Expanding the optical lithography to 32-nm node and beyond is impossible using existing single exposure systems. As such, double patterning lithography (DPL) is the most promising option to generate the required lithography resolution, where the target layout is printed with two separate imaging processes. Among different DPL techniques litho-etch-litho-etch (LELE) and self-aligned double patterning (SADP) methods are the most popular ones, which apply two complete exposure lithography steps and an exposure lithography followed by a chemical imaging process, respectively. To realize double patterning lithography, patterns located within a sub-resolution distance should be assigned to either of the imaging sub-processes, so-called layout decomposition. To achieve the optimal design yield, layout decomposition problem should be solved with respect to characteristics and limitations of the applied DPL method. For example, although patterns can be split between the two sub-masks in the LELE method to generate conflict free masks, this pattern split is not favorable due to its sensitivity to lithography imperfections such as the overlay error. On the other hand, pattern split is forbidden in SADP method because it results in non-resolvable gap failures in the final image. In addition to the functional yield, layout decomposition affects parametric yield of the designs printed by double patterning. To deal with both functional and parametric challenges of DPL in dense and large layouts, EDA solutions for DPL are addressed in this thesis. To this end, we proposed a statistical method to determine the interconnect width and space for the LELE method under the effect of random overlay error. In addition to yield maximization and achieving near-optimal trade-off between different parametric requirements, the proposed method provides valuable insight about the trend of parametric and functional yields in future technology nodes. Next, we focused on self-aligned double patterning and proposed layout design and decomposition methods to provide SADP-compatible layouts and litho-friendly decomposed layouts. Precisely, a grid-based ILP formulation of SADP decomposition was proposed to avoid decomposition conflicts and improve overall printability of layout patterns. To overcome the limited applicability of this ILP-based method to fully-decomposable layouts, a partitioning-based method is also proposed which is faster than the grid-based ILP decomposition method too. Moreover, an A∗-based SADP-aware detailed routing method was proposed which performs detailed routing and layout decomposition simultaneously to avoid litho-limited layout configurations. The proposed router preserves the uniformity of pattern density between the two sub-masks of the SADP process. We finally extended our decomposition method for double patterning to triple patterning and formulated SATP decomposition by integer linear programming. In addition to conventional minimum width and spacing constraints, the proposed decomposition method minimizes the mandrel-trim co-defined edges and maximizes the layout features printed by structural spacers to achieve the minimum pattern distortion. This thesis is one of the very early researches that investigates the concept of litho-friendliness in SADP-aware layout design and decomposition. Provided by experimental results, the proposed methods advance prior state-of-the-art algorithms in various aspects. Precisely, the suggested SADP decomposition methods improve total length of sensitive trim edges, total EPE and overall printability of attempted designs. Additionally, our SADP-detailed routing method provides SADP-decomposable layouts in which trim patterns are highly robust to lithography imperfections. The experimental results for SATP decomposition show that total length of overlay-sensitive layout patterns, total EPE and overall printability of the attempted designs are also improved considerably by the proposed decomposition method. Additionally, the methods in this PhD thesis reveal several insights for the upcoming technology nodes which can be considered for improving the manufacturability of these nodes

Modeling of pattern dependencies in the fabrication of multilevel copper metallization

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2007.Includes bibliographical references (p. 295-303).Multilevel copper metallization for Ultra-Large-Scale-Integrated (ULSI) circuits is a critical technology needed to meet performance requirements for advanced interconnect technologies with sub-micron dimensions. It is well known that multilevel topography resulting from pattern dependencies in various processes, especially copper Electrochemical Deposition (ECD) and Chemical-Mechanical Planarization (CMP), is a major problem in interconnects. An integrated pattern dependent chip-scale model for multilevel copper metallization is contributed to help understand and meet dishing and erosion requirements, to optimize the combined plating and polishing process to achieve minimal environmental impact, higher yield and performance, and to enable optimization of layout and dummy fill designs. First, a physics-based chip-scale copper ECD model is developed. By considering copper ion depletion effects, and surface additive adsorption and desorption, the plating model is able to predict the initial topography for subsequent CMP modeling with sufficient accuracy and computational efficiency. Second, a compatible chip-scale CMP modeling is developed.(cont.) The CMP model integrates contact wear and density-step-height approaches, so that a consistent and coherent chip-scale model framework can be used for copper bulk polishing, copper over-polishing, and barrier layer polishing stages. A variant of this CMP model is developed which explicitly considers the pad topography properties. Finally, ECD and CMP parts are combined into an integrated model applicable to single level and multilevel metallization cases. The integrated multilevel copper metallization model is applied to the co-optimization of the plating and CMP processes. An alternative in-pattern (rather than between-pattern) dummy fill strategy is proposed. The integrated ECD/CMP model is applied to the optimization of the in-pattern fill, to achieve improved ECD uniformity and final post-CMP topography.by Hong Cai.Ph.D

Characterization and modeling of pattern dependencies in copper interconnects for integrated circuits

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2002.Includes bibliographical references (p. 173-176).Copper metallization has emerged as the leading interconnect technology for deep sub-micron features, where electroplating and chemical mechanical polish (CMP) processes have a vital role in the fabrication of integrated circuits. The processes both suffer from a similar problem: the copper electroplated profiles and the polished surface exhibit pattern dependent topography. In this thesis, a methodology for the characterization and modeling of pattern dependent problems in copper interconnect topography is developed. For the electroplating process, the methodology consists of test structure and mask design to examine feature scale copper step height and the height of copper array regions as a function of underlying layout parameters. Semi-empirical response surface models are then generated with model parameters extracted from conventional and superfill plating processes. Once the models are calibrated, layout parameters including pattern density, line width distributions, and line length are extracted for each cell in a 40 gm by 40 tm discretization of any random chip layout. Then, a chip-scale prediction is achieved by simulating generalized average heights for each grid cell across the entire chip. The prediction result shows root mean square errors of less than 1000 A for array height and around 500 A for step height. This methodology provides the first known chip-scale prediction of electroplated topography. For pattern dependencies in copper CMP, this thesis focuses on the development of test structures and masks (including multi-level structures) to identify key pattern effects in both single-level and multi-level polishing.(cont.) Especially for the multi-level studies, electrical test structures and measurements in addition to surface profile scans are seen to be important in accurately determining thickness variations. The developed test vehicle and characterization of copper dishing and oxide erosion serve as a basis for further pattern dependent model development. Finally, integration of electroplating and CMP chip-scale models is illustrated; the simulated step and array heights as well as topography pattern density are used as an input for the initial starting topography for CMP simulation of subsequent polishing profile evolution.by Tae Hong Park.Ph.D

Post assembly process development for Monolithic OptoPill integration on silicon CMOS

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2004.Includes bibliographical references (leaves 108-110).Monolithic OptoPill integration by means of recess mounting is a heterogeneous technique employed to integrate III-V photonic devices on silicon CMOS circuits. The goal is to create an effective fabrication process that enables the volume production of high performance optoelectronic integrated circuits (OEICs). This thesis focuses on the development of post-assembly processes and technologies, in which InGaAs/InP P-i-N photodiodes were integrated as long wavelength photodetectors with an optical clock receiver circuit. Fabrication procedures, challenges experienced, and results accomplished are presented for each process step including the formation of alloyed and non-alloyed ohmic contacts on n-type and p-type InGaAs contact layers, active area definition by dry-etching InGaAs/InP with ECR-enhanced RIE, BCB passivation and planarization, via opening by dry-etching BCB with RIE, and top contact metallization. In conjunction, an InP-based test heterostructure was fabricated into discrete photodiodes. Decoupling the fabrication and benchmarking of III-V photonic device from the Si-CMOS electronic circuit allowed for the independent electrical and optical characterization of the photodetectors. Measurements and analysis of the P-i-N photodiodes will assist the forthcoming analysis of the final OEIC. Preliminary results and discussions of the calibration sample are presented in this thesis.by Yi-Shu Vivian Lei.S.M