David Turgis

A 460MHz at 397mV, 2.6GHz at 1.3V, 32b VLIW DSP, embedding F MAX tracking

Wide-voltage-range-operation DSPs bring more versatility to achieve high energy efficiency in mobile applications to increase signal processing complexity and handle a large range of performance specifications. This paper describes a 32b DSP fabricated in 28nm UTBB FDSOI technology [1]. Body-bias-voltage (VBB) scaling from 0V up to ±2V (Pwell/Nwell) decreases the DSP core VDDMIN to 397mV and increases clock frequency by +400% at 500mV and +114% at 1.3V. In addition to technology gains, dedicated design features are included to increase frequency over the full VDD range, considering parameter variations. As depicted in Fig. 27.1.1, the 32b datapath VLIW DSP is organized around a MAC dedicated to complex arithmetic and two dedicated operators: a cordic/divider and a compare/select. Data enters the circuit through a serial interface and code is run from a 64×32b register file. It has been shown in [1] that a given operating frequency can be achieved at a lower VDD in UTBB FDSOI compared to bulk by applying a forward-body bias. An additional design step is achieved in this work by (1) increasing the frequency at low VDD thanks to a specific selection and design of standard cells with respect to power vs. performance and (2) dynamically tracking the maximum frequency to cope with variations.

Proposal of a new ultra low leakage 10T sub threshold SRAM bitcell

The tendency for low energy consumption in systems-on-chip results in a need for memories operating in the near- and sub-threshold regions. This paper gives a comparative study of Static Random Access Memory (SRAM) bitcells working under Ultra-Low Voltage in 32nm CMOS. A new 10T SRAM bitcell is then proposed and features low leakage current. It is capable of operation under ULV (~300mV) and allows bit-interleaving technique that is critical to cope with multiple bit soft-errors for reducing dynamic and static power consumption compared to state-of-the-art bitcells.

Efficient yield estimation through generalized importance sampling with application to NBL-assisted SRAM bitcells

We consider the general problem of the efficient and accurate determination of the yield of an integrated circuit, through electrical circuit level simulation, under variability constraints due to the manufacturing process. We demonstrate the performance of our general-purpose Importance Sampling based algorithm for the case of an industrial SRAM application. Section 1 reviews the notions of yield and how this translates to very low probability of failure estimation. We explain in detail the working principle of statically and dynamically write-assisted SRAM bitcells and the limitations of the classical static approximations for yield estimation of these structures. Section 2 explores naïve statistical approaches for the determination of low probabilities and the notion of variance reduction. Section 3 explains the detailed mathematical background of our new algorithm. We show how the original extremely challenging problem is translated to a simpler convex optimization problem, and how the algorithm produces a standard confidence interval associated to a probability or quantile. Section 4 presents the quantitative results and speedup factors obtained on an industrial SRAM bitcell for the analysis of a write failure mechanism. Results obtained on static negative bit line assisted SRAM compares favorably to results obtained with extrapolation of static models and prove the accuracy of the method, while results obtained on dynamic negative bit line assisted SRAM allow to quantitatively optimize the assist circuitry and help to reduce its over-design margins.

Optimization of a voltage sense amplifier operating in ultra wide voltage range with back bias design techniques in 28nm UTBB FD-SOI technology

Advanced SoC designs regularly use Dynamic Voltage and Frequency Scaling (DVFS) to achieve high performance and low power targets of portable systems. In this paper, we focus on optimization of a Voltage Sense Amplifier (VSA) in 28nm Ultra-Thin Body and BOX Fully Depleted SOI (UTBB FD-SOI) technology to achieve high performance operations over the Ultra Wide Voltage Range (UWVR) from 1.3V to 0.4V. We use Flip-Well design methodology along with forward body bias modulation to extend operation range of the VSA and also reduce sense amplifier read time by 28%, while saving power consumption by up to 59% compared to Bulk technology.

Low Standby Power Capacitively Coupled Sense Amplifier for wide voltage range operation of dual rail SRAMs

Dual Rail SRAMs are widely used to enable Dynamic Voltage and Frequency Scaling (DVFS) in SRAMs where array voltage cannot be scaled down. DVFS operating points are limited by maximum differential supported between two supplies of the SRAM. To extend gains of DVFS, we propose a Low Standby Power - Capacitively Coupled Sense Amplifier (LSTP-C2SA) that enables further lowering of periphery supply in Dual Rail SRAMs without leading to SRAM cell instability. We present a design method to optimally size the coupling capacitance in LSTP-C2SAs. Designs with LSTP-C2SA are shown to consume 43% lesser read power in DVFS operation at 0.4V in 28nm UTBB FD-SOI when compared to an implementation with standard latch sense amplifier. Silicon measurements confirm LSTP-C2SA functionality at 0.35V.

UTBB FD-SOI front- and back-gate coupling aware random telegraph signal impact analysis on a 6T SRAM

This work investigates the impact of Random Telegraph Signal (RTS) noise on a 6-Transistor Single-P-Well Static Random Access Memory (6T-SRAM) in 28nm Ultra-Thin Body and Buried Oxide Fully-Depleted Silicon-On-Insulator (UTBB FD-SOI) technology. RTS noise impact is observed through Write-Ability measurements on a 143Kb SRAM macro. A SPICE-level bias- and time-dependent RTS model peculiar to UTBB FD-SOI, introducing the back-gate as a second RTS noise source and considering the front- and back-gate coupling, is used for simulations to confirm silicon observations. It is shown that the body-biasing feature of UTBB FD-SOI does not introduce critical RTS noise compared to the one originated from the device front gate.

Impact of Random Telegraph Signals on 6T high-density SRAM in 28nm UTBB FD-SOI

This work investigates the impact of Random Telegraph Signal (RTS) noise on a 6 Transistors single P-well Static Random Access Memory (6T-SRAM) manufactured in 28nm Ultra-Thin Body and Buried Oxide Fully-Depleted Silicon-On-Insulator (UTBB FD-SOI) technology. A SPICE-level bias-and time-dependent RTS model peculiar to UTBB FD-SOI, which considers both front- and back-gate of the device as RTS sources, is presented. The Bit-Error-Rate is evaluated on silicon dies through the write-ability (WA) failure criterion and with a dedicated back-biasing strategy. Simulations evidence the role of RTS-induced dynamic variability with respect to process variability and show a good agreement with measurements.

High-Density 4T SRAM Bitcell in 14-nm 3-D CoolCube Technology Exploiting Assist Techniques

In this paper, we present a high-density four-transistor (4T) static random access memory (SRAM) bitcell design for 3-D CoolCube technology platform based on 14-nm fully depleted - silicon on insulator MOS transistors to show the compatibility between the 4T SRAM and the 3-D design and the considerable density gain that they can achieve when combined. The 4T SRAM bitcell has been characterized to investigate the critical operations in terms of stability (retention and read) taking into account the postlayout parasitic elements. Thus, failure mechanisms are exposed and explained. Based on this paper, a data-dependent dynamic back-biasing scheme improving the bitcell stability is developed. A specific read-assist circuit is also proposed in order to enable a large number of bitcells per column in a memory array. Finally, the designed bitcell offers up to 30% area gain compared to a planar six-transistor SRAM bitcell in the same technology node.

A 32 kb 0.35–1.2 V, 50 MHz–2.5 GHz Bit-Interleaved SRAM With 8 T SRAM Cell and Data Dependent Write Assist in 28-nm UTBB-FDSOI CMOS

An optimized co-design of SRAM cell, assist schemes, and layout is proposed to achieve wide voltage range operation of SRAM from 0.35–1.2 V at all process corners. A differential read asymmetric 8 T memory cell and a data dependent differential supply and body modulation write assist scheme are proposed. We also propose a layout that reduces metal capacitance of wordlines by 54% and also enables bit-interleaving. The proposed assist scheme can be combined with conventional assist schemes to further lower minimum write operational voltage of the SRAM by 70–130 mV at iso-performance without causing reliability concerns. A 32 kb instance is fabricated in 28-nm UTBB-FDSOI technology and efficiency of the proposed scheme is demonstrated with lowest write voltage of 0.32 V. Multiple read assist schemes have been used to simultaneously lower read voltage to 0.35 V. 50 MHz operation is measured when integrated in a DSP processor at 0.358 V. Low voltage and wide voltage range figure of merits are also defined to benchmark the proposed solutions with other works.

28nm FDSOI technology sub-0.6V SRAM Vmin assessment for ultra low voltage applications

Vmin measurements in 28nm FDSOI technology on 128Mb SRAM bitcells from -40°C to 125°C are reported in this paper. Adding the silicon ageing behavior and the process variability, we have developed a complete model and demonstrated end-of-life SRAM Vmin of 0.6V and 0.5V on 20Mb with 0.120μm2 and 0.152μm2 bitcells, respectively. This is the first report of a such extensive SRAM Vmin assessment at the 28nm node. The construction of write limited bitcells, combined with write assist design technique, was found to be the most efficient way to achieve ultra low Vmin in 28nm FDSOI technology. In addition, Vmin retention below 0.4V is demonstrated in 0.120μm2 bitcells, leading to the enablement of ultra-low leakage bitcells with 2pA/cell in retention mode.