Syed M. Alam

Power Delivery Design for 3-D ICs Using Different Through-Silicon Via (TSV) Technologies

3-D integrated circuits promise high bandwidth, low latency, low device power, and a small form factor. Increased device density and asymmetrical packaging, however, renders the design of 3-D power delivery a challenge. We investigate in this paper various methods to improve 3-D power delivery. We analyze the impact of through-silicon via (TSV) size and spacing, of controlled collapse chip connection (C4) spacing, and of dedicated power delivery TSVs. In addition to considering typical cylindrical or square metal-filled TSVs (core TSVs), we also investigate using coaxial TSVs for power delivery resulting in reduced routing blockages and added coupling capacitance. Our 3-D evaluation system is composed of a quad-core chip multiprocessor, a memory die, and an accelerator engine, and it is evaluated using representative SPEC benchmark traces. This is the first detailed architectural-level analysis for 3-D power delivery. Our findings provide clear guidelines for 3-D power delivery design. More importantly, we show that it is possible to achieve 2-D-like, or even better, power quality by increasing C4 granularity and by selecting suitable TSV size and spacing.

A comprehensive layout methodology and layout-specific circuit analyses for three-dimensional integrated circuits

In this paper, we describe a comprehensive layout methodology for bonded three-dimensional integrated circuits (3D ICs). In bonded 3D integration technology, parts of a circuit are fabricated on different wafers, and then, the wafers are bonded with a glue layer of Cu or polymer based adhesive. Using our layout methodology, designers can layout such 3D circuits with necessary information on inter-wafer via/contact and orientation of each wafer embedded in the layout. We have implemented the layout methodology in 3DMagic. Availability of 3DMagic has led to interesting research with a wide range of layout-specific circuit analyses, from performance comparison of 2D and 3D circuits to layout-specific reliability analyses in 3D circuits. Using 3DMagic, researchers have designed and simulated an 8-bit encryption processor mapped into 2D and 3D FPGA layouts. Moreover, the layout methodology is an essential element of our ongoing research for the framework of a novel reliability computer aided design tool, ERNI-3D.

Layout-Specific Circuit Evaluation in 3-D Integrated Circuits

In this paper, we describe a comprehensive layout methodology for bonded three-dimensional integrated circuits (3D ICs). In bonded 3D integration technology, parts of a circuit are fabricated on different wafers, and then, the wafers are bonded with a glue layer of Cu or polymer based adhesive. Using our layout methodology, designers can layout such 3D circuits with necessary information on inter-wafer via/contact and orientation of each wafer embedded in the layout. We have implemented the layout methodology in 3DMagic. Availability of 3DMagic has led to interesting research with a wide range of layout-specific circuit evaluation, from performance comparison of 2D and 3D circuits to layout-specific reliability analyses in 3D circuits. Using 3DMagic, researchers have designed and simulated an 8-bit encryption processor mapped into 2D and 3D FPGA layouts. Moreover, the layout methodology is an essential element of our ongoing research for the framework of a novel Reliability Computer Aided Design tool, ERNI-3D.

Electrical modeling and characterization of through-silicon vias (TSVs) for 3-D integrated circuits

The integration of chips in the third dimension has been explored to address various physical and system level limitations currently undermining chip performance. In this paper, we present a comprehensive analysis of the electrical properties of through silicon vias and microconnects with an emphasis on single via characteristics as well as inter-TSV capacitive and inductive coupling in the presence of either a neighboring ground tap or a grounded substrate back plane. We also analyze the impact of technology scaling on TSV electrical parasitics, and investigate the power and delay trend in 3-D interstratum IO drivers with those of global wire in 2-D circuits over various technology nodes. We estimate the global wire length necessary to produce an equivalent 3-D IO delay, a metric useful in early stage design tools for 3D floorplanning that considers the electrical characteristics of 3D connections with TSVs and microconnects.

Through-Silicon Via (TSV)-induced noise characterization and noise mitigation using coaxial TSVs

Through-Silicon Via (TSV) is a critical interconnect element in 3D integration technology. TSVs introduce many new design challenges. In addition to competing with devices for real estate, TSVs can act as a major noise source throughout the substrate. We present in this paper a comprehensive study of TSV-induced noise as a function of several critical design and process parameters including substrate type, signal slew rate, TSV height, ILD thickness, and TSV-to-device and TSV-to-TSV spacing. We create a SPICE model for simulating TSV-to-device and TSV-to-TSV noise couplings in two different types of substrates: a lightly doped bulk substrate, and a lightly doped thin epitaxial layer on top of a heavily doped bulk. Our SPICE model provides small error when compared with a detailed Finite Element Analysis Method. Our findings show the importance of using a grounded backplane in reducing noise and how coaxial TSVs further mitigate TSV-induced noise.

System-level comparison of power delivery design for 2D and 3D ICs

Three-dimensional integrated circuits (IC) promise high bandwidth, low latency, low device power, and a small form factor. Increased device density and asymmetrical packaging, however, render 3D power delivery design a challenge. In this paper, we provide a system-level comparison of power delivery for 2D and 3D ICs. We investigate various techniques that can impact the quality of power delivery in 3D ICs. These include through-silicon via (TSV) size and spacing, controlled collapse chip connection (C4) spacing, and a combination of dedicated and shared power delivery. Our evaluation system is composed of quad-core chip multiprocessor, memory, and accelerator engine. Each of these modules is running representative SPEC benchmark traces. Our findings are practical and provide clear guidelines for 3D power delivery optimization. More importantly, we show that it is possible to achieve 2D-like or even better power quality by increasing C4 granularity and selecting suitable TSV size and spacing.

Inter-Strata Connection Characteristics and Signal Transmission in Three-Dimensional (3D) Integration Technology

In a general case of 3D integrated circuit (IC) technology, it is desirable to design a die for 3D integration with flexibility to facilitate integration with a number of other circuit dies. We present a generic circuit technique which minimizes power consumption and circuit area while allowing reliable signal transfer between 3D dies as well as enabling the design of a bonded interface circuitry without a complete knowledge of inter-strata connection configurations. We also present parasitic RC characteristics of inter-strata connection elements, such as micro-bumps and through-substrate vias, and discuss the technology scaling trends. An inter-strata signal transmission, according to our method, has receive and transmit circuitry with programmable power supply which can be independently controlled for achieving optimum power and signal drive. In addition, the receive circuitry includes hysteresis to allow superior signal integrity in the presence of inter-strata parasitic variations

Circuit level reliability analysis of Cu interconnects

Copper (Cu) based interconnect technology is expected to meet some of the challenges of technology scaling in the pursuit of higher performance. However, Cu interconnects are still susceptible to electromigration-induced failure over time. We describe a new hierarchical approach for predicting the reliability of Cu-based interconnects in circuit layouts, and present an RCAD tool, SysRel, for such an analysis. We propose a (jL) product filtering algorithm with a classification of separate via-above and via-below treatments in Cu interconnect trees. After the filtering of immortal trees, a default model is applied to the remaining trees to compute reliability figures for individual units. SysRel utilizes joint stochastic reliability metrics based on the desired lifetime of a chip and combines reliability figures from individual fundamental reliability units. Simulation results with a 32-bit comparator circuit layout demonstrate the significance of our methodology in selectively identifying critical nets and their impacts on overall reliability.

Circuit-level reliability requirements for Cu metallization

Under similar test conditions, the electromigration reliability of Al and Cu interconnect trees demonstrate significant differences because of differences in interconnect architectural schemes. The low critical stress for void nucleation at the Cu and interlevel diffusion-barrier interface leads to varying failure characteristics depending on the via position and configuration in a line. Unlike Al technology, a (jL) product-filtering algorithm with a classification of separate via-above and via-below treatments is required for Cu interconnect trees. A methodology and tool for circuit-level interconnect-reliability analyses has been developed. Using data from the literature, the layout-specific circuit-level reliability for Al and dual-damascene Cu metallizations have been compared for various circuits and circuit elements. Significantly improved test-level reliability in Cu is required to achieve equivalent circuit-level reliability. Moreover, the required improvement will increase as low-k/low-modulus dielectrics are introduced, and as liner thicknesses are reduced.

Thermal-electrical co-optimisation of floorplanning of three-dimensional integrated circuits under manufacturing and physical design constraints

Although the stacking of multiple strata to produce three-dimensional (3D) integrated circuits (ICs) improves interconnect length and hence reduces power and latency, it also results in the exacerbation of the thermal management challenge owing to the increased power density. There is a need for design tools to understand and optimise the trade-off between electrical and thermal design at the device and block levels. This study presents results from thermal-electrical co-optimisation for block-level floorplanning in a multi-die 3D IC under various manufacturing and physical design constraints. A method for temperature computation based on linearity of the governing energy equation is presented. This method is combined with previously reported electrical delay models for 3D ICs to simultaneously optimise both the maximum temperature and the interconnect length. It is shown that co-optimisation of thermal and electrical objectives results in a floorplan that is attractive from both perspectives. Physical design constraints because of cost-effective 3D manufacturing such as using fully or partly identical dies using reciprocal design symmetry (RDS), differentiated technology in each die and thinned die/wafer are discussed and their impact on the thermal-electrical co-optimisation is investigated. In some cases, the cheapest manufacturing choice, such as using identical die, for each layer may not result in optimal thermal and electrical design. Results presented in this work highlight the need for thermal and electrical co-design in multi-strata microelectronics, and for reconciling manufacturing and design considerations in order to develop practical design tools for 3D ICs.