Ashutosh Chakraborty

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.

Performance analysis of 3-D monolithic integrated circuits

3-D monolithic integration (3DMI), also termed as sequential integration, is a potential technology for future gigascale circuits. Since the device layers are processed in sequential order, the size of the vertical contacts is similar to traditional contacts unlike in the case of parallel 3-D integration with through silicon vias (FSVs). Given the advantage of such small contacts, 3DMI supports stacking active layers such that fine-grain integration of 3-D circuits can be implemented. This paper extends the idea of constructing the standard cells across two active layers, forming 3-D cells, to reduce the overall area and interconnect wirelength of a circuit. To demonstrate the effect of the 3DMI technology on these important parameters of circuit design, two important communication blocks are evaluated. Specifically, a low-density-parity-check (LDPC) decoder as a sample of interconnect-dominated circuit and a data-encryption-standard (DES) block, which is good instance of a gate dominated circuit, are investigated. By employing 3-D cells in the conventional design flow chain, there is more than 10% decrease in wirelength for both circuits (in wirelength driven placement mode). However, when subjected to timing driven placement a slight reduction in delay (1.6%) is observed for an LDPC decoder, whereas for the DES block considerable delay reduction (14.22%) is achieved.

Dynamic Thermal Clock Skew Compensation Using Tunable Delay Buffers

The thermal gradients existing in high-performance circuits may significantly affect their timing behavior, in particular, by increasing the skew of the clock net and/or altering hold/setup constraints, possibly causing the circuit to operate incorrectly. The knowledge of the spatial distribution of temperature can be used to properly design a clock network that is able to compensate such thermal non-uniformities. However, redesign of the clock network is effective only if temperature distribution is stationary, i.e., does not change over time. In this paper, we specifically address the problem of dynamically modifying the clock tree in such a way that it can compensate for temporal variations of temperature. This is achieved by exploiting the buffers that are inserted during the clock network generation, by transforming them into tunable delay elements. Temperature-induced delay variations are then compensated by applying the proper tuning to the tunable buffers, which is computed offline and stored in a tuning table inserted in the design. We propose an algorithm to minimize the number of inserted tunable buffers, as well as their tunable range (which directly relates to complexity). Results show that clock skew is kept within original bounds with worst-case power and area penalty of 3.5% and 5.5% respectively.

CELONCEL: effective design technique for 3-D monolithic integration targeting high performance integrated circuits

3-D monolithic integration (3DMI), also termed as sequential integration, is a potential technology for future gigascale circuits. Since the device layers are processed in sequential order, the size of the vertical contacts is similar to traditional contacts unlike in the case of parallel 3-D integration with through silicon vias (TSVs). Given the advantage of such small contacts, 3DMI enables manufacturing multiple active layers very close to each other. In this work we propose two different strategies of stacking standard cells in 3-D without breaking the regularity of the conventional design flow: a) Vertical stacking of diffusion areas (Intra-Cell stacking) that supports complete reuse of 2-D physical design tools and b) vertical stacking of cells over others (Cell-on-Cell stacking). A placement tool (CELONCEL-placer) targeting the Cell-on-Cell placement problem is proposed to allow high quality 3-D layout generation. Our experiments demonstrate the effectiveness of CELONCEL technique, fetching us an area gain of 37.5%, 15.51% reduction in wirelength, and 13.49% improvement in overall delay, compared with a 2-D case when benchmarked across an interconnect dominated low-density-parity-check (LDPC) decoder at 45nm technology node.

Dynamic thermal clock skew compensation using tunable delay buffers

The thermal gradients existing in high-performance circuits may significantly affect their timing behavior, in particular by increasing the skew of the clock net and/or altering hold/setup constraints, possibly causing the circuit to operate incorrectly. The knowledge of the spatial distribution of temperature can be used to properly design a clock network that is able to compensate such thermal non-uniformities. However, re-design of the clock network is effective only if temperature distribution is stationary, i.e., does not change over time. In this work, we specifically address the problem of dynamically modifying the clock tree in such a way that it can compensate for temporal variations of temperature. This is achieved by exploiting the buffers that are inserted during the clock network generation, by transforming them into tunable delay elements. Temperature-induced delay variations are then compensated by applying the proper tuning to the tunable buffers, which is computed off-line and, stored in a tuning table inserted in the design. We propose an algorithm to minimize the number of inserted tunable buffers, as well as their tunable range (which directly relates to complexity). Results show that clock skew is kept within original bounds with minimum area and power penalty. The maximum increase in power is 23.2% with most benchmarks exhibiting less than 5% increase in power

Analysis and optimization of NBTI induced clock skew in gated clock trees

NBTI (Negative Bias Temperature Instability) has emerged as the dominant PMOS device failure mechanism for sub-100 nm VLSI designs. There is little research to quantify its impact on skew of clock trees. This paper demonstrates a mathematical framework to compute the impact of NBTI on gating-enabled clock tree considering their workload dependent temperature variation. Circuit design techniques are proposed to deal with NBTI induced clock skew by achieving balance in NBTI degradation of clock devices. Our technique achieves up-to 70% reduction in clock skew degradation with miniscule (<0.1%) power and area penalty.

Skew management of NBTI impacted gated clock trees

Negative bias temperature instability (NBTI) has emerged as the dominant failure mechanism for PMOS devices in nanometer integrated circuit (IC) designs, thus limiting their lifetime. There are several existing research works that mitigate impact of NBTI on gate delay and reliability. However, its impact on one of the most important components of modern IC design-the clock tree-has not been researched enough. Clock gating impacts the extent of NBTI-induced VTH degradation of clock buffers leading to nonuniform NBTI degradation and, thus, increased clock skew. In this paper, we propose a practical design-time technique of modifying the clock gating implementation by selecting NAND or NOR gate as output stage of integrated clock gating cells with the objective of minimizing NBTI-induced clock skew. This selection intelligently modulates the signal probability and delay equations of clock signal paths with no extra hardware penalty. We formulate the skew minimization problem as an integer linear program which determines the optimal NAND or NOR assignment of clock gating buffer. Experimental results demonstrate the effectiveness of our method as the NBTI-induced clock skew is reduced by more than 74% compared to the traditional method. The impact of voltage and temperature variation on the proposed technique was analyzed and we observed reduced but still significant reduction in clock skew under variation as compared to the traditional clock gating technique.

Thermal resilient bounded-skew clock tree optimization methodology

The existence of non-uniform thermal gradients on the substrate in high performance ICs can significantly impact the performance of global on-chip interconnects. This issue is further exacerbated by the aggressive scaling and other factors such as dynamic power management schemes and non-uniform gate level switching activity. In high-performance systems, one of the most important problems is clock skew minimization since it has a direct impact on the maximum operating frequency of the system. Since clocks are routed across the entire chip, the presence of thermal gradients can significantly alter their characteristics because wire resistance increases linearly as the temperature increases. This often results in failure to meet original timing constraints thereby rendering the original topology unusable. Therefore it is necessary to perform a temperature aware re-embedding of the original topology to meet timing under these temperature effects. This work primarily explores these issues by proposing two algorithms that re-structure an existing clock tree topology to compensate for such temperature effects and as a result also meet timing constraints

Layout level timing optimization by leveraging active area dependent mobility of strained-silicon devices

Advanced MOSFETs such as strained silicon (SS) devices have emerged as critical enablers to keep Moores law on track for sub-100 nm technologies. Use of strained silicon devices provides performance improvement equivalent to use of next generation devices, without actually requiring scaling. Traditionally, the research in the field of SS has been focussed on device modeling and process characterization. Recently, the dependence of mobility of a SS MOSFET device on its poly-to-poly distance has been reported. In this work, we propose a new methodology to exploit this dependence to achieve cycle time reduction of a design at the layout level. To the best of our knowledge, this is the first research work to tackle timing closure by layout modifications using active area dependent mobility of SS devices. Our methodology shows consistent improvement for benchmark designs mapped onto various 90 nm commercial standard cell libraries. This work enables reduction of cycle time by as much as 6.31% (and on an average 5.25%) very late in the design closure cycle without requiring any optimization iterations.

An integrated nonlinear placement framework with congestion and porosity aware buffer planning

Due to skewed scaling of interconnect versus cell delay in deep submicron CMOS, modern VLSI timing closure requires extensive buffer insertion. Inserting a large number of buffers may cause not only dramatic cell migration but also routing hotspots. If buffering is not controlled well, it may fail to close a design. Placement with buffer porosity (i.e., cell density) awareness can allocate space for inserting these buffers, and buffering with congestion awareness can improve the routability. Therefore, there is essential need for a placement framework with explicit porosity and congestion control. In this paper, we propose the first integrated nonlinear placement framework with porosity and congestion aware buffer planning. We demonstrate the integration of increasingly refined cell porosity and routing congestion aware buffer planning and insertion methodology in a high quality nonlinear placer. Our experiments show the improvement of average routing overflow by 69%, average wirelength by 28% and average buffer count by 40%, compared with the traditional placement framework without buffer planning.