Jae-Seok Yang

TSV stress aware timing analysis with applications to 3D-IC layout optimization

As the geometry shrinking faces severe limitations, 3D wafer stacking with through silicon via (TSV) has gained interest for future SOC integration. Since TSV fill material and silicon have different coefficients of thermal expansion (CTE), TSV causes silicon deformation due to different temperatures at chip manufacturing and operating. The widely used TSV fill material is copper which causes tensile stress on silicon near TSV. In this paper, we propose systematic TSV stress aware timing analysis and show how to optimize layout for better performance. First, we generate a stress contour map with an analytical radial stress model. Then, the tensile stress is converted to hole and electron mobility variations depending on geometric relation between TSVs and transistors. Mobility variation aware cell library and netlist are generated and incorporated in an industrial timing engine for 3D-IC timing analysis. It is interesting to observe that rise and fall time react differently to stress and relative locations with respect to TSVs. Overall, TSV stress induced timing variations can be as much as ± 10% for an individual cell. Thus as an application for layout optimization, we can exploit the stress-induced mobility enhancement to improve timing on critical cells. We show that stress-aware perturbation could reduce cell delay by up to 14.0% and critical path delay by 6.5% in our test case.

Double patterning layout decomposition for simultaneous conflict and stitch minimization

Double patterning lithography (DPL) is considered as a most likely solution for 32 nm/22 nm technology. In DPL, the layout patterns are decomposed into two masks (colors), and manufactured through two exposures and etch steps. If the spacing between two features (polygons) is less than certain minimum coloring distance, they have to be assigned opposite colors. However, a proper coloring is not always feasible because two neighboring patterns within the minimum distance may be in the same mask due to complex pattern configurations. In that case, a feature may need to be split into two parts to resolve the conflict, resulting in stitch insertion which causes yield loss due to overlay and line-end effect. While previous layout decomposition approaches perform coloring and splitting separately, in this paper, we propose a simultaneous conflict and stitch minimization algorithm with an integer linear programming (ILP) formulation. Since ILP is in class NP-hard, the algorithm includes three speed-up techniques: (1) grid merging; (2) independent component computation; and (3) layout partition. In addition, our algorithm can be extended to handle design rules such as overlap margin and minimum width for practical use as well as off-grid layout. Our approach can reduce 33% of stitches and remove conflicts by 87.6% compared with two phase greedy decomposition.

A new graph-theoretic, multi-objective layout decomposition framework for double patterning lithography

As Double Patterning Lithography(DPL) becomes the leading candidate for sub-30nm lithography process, we need a fast and lithography friendly decomposition framework. In this paper, we propose a multi-objective min-cut based decomposition framework for stitch minimization, balanced density, and overlay compensation, simultaneously. The key challenge of DPL is to accomplish high quality decomposition for large-scale layouts under reasonable runtime with the following objectives: a) the number of stitches is minimized, b) the balance between two decomposed layers is maximized for further enhanced patterning, c) the impact of overlay on coupling capacitance is reduced for less timing variation. We use a graph theoretic algorithm for minimum stitch insertion and balanced density. An additional decomposition constraints for self-overlay compensation are obtained by integer linear programming(ILP). With the constraints, global decomposition is executed by our modified FM graph partitioning algorithm. Experimental results show that the proposed framework is highly scalable and fast: we can decompose all 15 benchmark circuits in five minutes in a density balanced fashion, while an ILP-based approach can finish only the smallest five circuits. In addition, we can remove more than 95% of the timing variation induced by overlay for tested structures.

Stress-driven 3D-IC placement with TSV keep-out zone and regularity study

Through-silicon via (TSV) fabrication causes tensile stress around TSVs which results in significant carrier mobility variation in the devices in their neighborhood. Keep-out zone (KOZ) is a conservative way to prevent any devices/cells from being impacted by the TSV-induced stress. However, owing to already large TSV size, large KOZ can significantly reduce the placement area available for cells, thus requiring larger dies which negate improvement in wirelength and timing due to 3D integration. In this paper, we study the impact of KOZ dimension on stress, carrier mobility variation, area, wirelength, and performance of 3D ICs. We demonstrate that, instead of requiring large KOZ, 3D-IC placers must exploit TSV stress-induced carrier mobility variation to improve the timing and area objectives during placement. We propose a new TSV stress-driven force-directed 3D placement that consistently provides placement result with, on average, 21.6% better worst negative slack (WNS) and 28.0% better total negative slack (TNS) than wirelength-driven placement.

Double Patterning Layout Decomposition for Simultaneous Conflict and Stitch Minimization

Double patterning lithography (DPL) is considered as a most likely solution for 32 nm/22 nm technology. In DPL, the layout patterns are decomposed into two masks (colors), and manufactured through two exposures and etch steps. If the spacing between two features (polygons) is less than certain minimum coloring distance, they have to be assigned opposite colors. However, a proper coloring is not always feasible because two neighboring patterns within the minimum distance may be in the same mask due to complex pattern configurations. In that case, a feature may need to be split into two parts to resolve the conflict, resulting in stitch insertion which causes yield loss due to overlay and line-end effect. While previous layout decomposition approaches perform coloring and splitting separately, in this paper, we propose a simultaneous conflict and stitch minimization algorithm with an integer linear programming (ILP) formulation. Since ILP is in class NP-hard, the algorithm includes three speed-up techniques: (1) grid merging; (2) independent component computation; and (3) layout partition. In addition, our algorithm can be extended to handle design rules such as overlap margin and minimum width for practical use as well as off-grid layout. Our approach can reduce 33% of stitches and remove conflicts by 87.6% compared with two phase greedy decomposition.

Overlay aware interconnect and timing variation modeling for double patterning technology

As double patterning technology (DPT) becomes the only solution for 32-nm lithography process, we need to investigate how DPT affects the performance of a chip. In this paper, we present an efficient modeling of timing variation with overlay which is inevitable for DPT. Our work makes it possible to analyze timing with overlay variables. Since the variation of metal space caused by overlay results in coupling capacitance variation, we first model metal spacing variation with individual overlay sources. Then, all overlay sources are considered to determine the worst timing with coupling capacitance variation. Non-parallel pattern caused by overlay is converted to parallel one with equivalent spacing having the same delay to be applicable of a traditional RC extraction flow. To verify our work, we use identical interconnects having different positions and different layout decompositions. Experimental result shows that the delay has a variation from 7.8% to 9.1% depending on their locations. The well decomposed structure shows only 2.7% delay variation.

Robust clock tree synthesis with timing yield optimization for 3D-ICs

3D integration has new manufacturing and design challenges such as timing corner mismatch between tiers and device variation due to Through Silicon Via (TSV) induced stress. Timing corner mismatch between tiers is caused because each tier is manufactured in independent process. Therefore, inter-die variation should be considered to analyze and optimize for paths spreading over several tiers. TSV induced stress is another challenge in 3D Clock Tree Synthesis (CTS). Mobility variation of a clock buffer due to stress from TSV can cause unexpected skew which degrades overall chip performance. In this paper, we propose clock tree design methodology with the following objectives: (a) to minimize clock period variation by assigning optimal z-location of clock buffers with an Integer Linear Program (ILP) formulation, (b) to prevent unwanted skew induced by the stress. In the results, we show that our clock buffer tier assignment reduces clock period variation up to 34.2%, and the most of stress-induced skew can be removed by our stress-aware CTS. Overall, we show that performance gain can be up to 5.7% with our robust 3D CTS.

Dealing with IC manufacturability in extreme scaling

As the CMOS feature enters the era of extreme scaling (14nm, 11nm and beyond), manufacturability challenges are exacerbated. The nanopatterning through the 193nm lithography is being pushed to its limit, through double/triple or more general multiple patterning, while non-conventional lithography technologies such as extreme ultra-violet (EUV), e-beam direct-write (EBDW), and so on, still have grand challenges to be solved for their adoption into IC volume production. This tutorial will provide an overview of key overarching issues in nanometer IC design for manufacturability (DFM) with these emerging lithography technologies, from modeling, mask synthesis, to physical design and beyond.

Chemical-mechanical polishing aware application-specific 3D NoC design

In this paper, we propose the first chemical-mechanical polishing (CMP) aware application-specific three-dimensional (3D) network-on-chip (NoC) design that minimizes through-silicon-via (TSV) height variation, thus reduces its bonding failure, and meanwhile optimizes conventional NoC design objectives. Our 3D NoC design assigns cores to proper silicon layers, determines the 3D NoC topology, allocates routing paths, and then floorplans cores, routers and TSV arrays by a CMP-aware manner. The key idea behind this 3D NoC design flow is to determine the CMP-aware 3D NoC topology where TSV arrays with low and uniform metal density are inserted between adjacent layers. Experimental results show that our CMP-aware 3D NoC design can achieves lower TSV height variation, higher performance and lower power consumption than the previous state-of-the-art 3D NoC designs.

Layout optimizations for double patterning lithography

Deep sub-wavelength lithography is one of the most fundamental challenges for future scaling beyond 32nm as the industry is currently stuck at the 193nm lithography. Many ingenious technologies/tricks are developed to push the limit of 193nm lithography, e.g., immersion lithography and computational lithography. But they may not be sufficient for 22nm patterning. Meanwhile, next-generation lithography, such as EUV (Extreme Ultra-Violet) lithography may not be available for mass production in the near future. As a practical solution, double patterning lithography (DPL) has become a leading candidate for 22nm (and likely 16nm) lithography process. DPL poses new challenges for overlay control, layout decomposition, and up-stream physical designs. In this paper, we will discuss some recent advancements and challenges in layout decompositions and DPL friendly layout optimizations.