Compute-in-memory (CIM) is a new computing paradigm that addresses the memory-wall problem in hardware accelerator design for deep learning. The input vector and weight matrix multiplication, i.e., the multiply-and-accumulate (MAC) operation, could be performed in the analog domain within memory sub-array, leading to significant improvements in throughput and energy efficiency. Static random access memory (SRAM) and emerging non-volatile memories such as resistive random access memory (RRAM) are promising candidates to store the weights of deep neural network (DNN) models. In this review, firstly we survey the recent progresses in SRAM and RRAM based CIM macros that have been demonstrated in silicon. Then we discuss general design challenges of the CIM chips including analog-to-digital conversion (ADC) bottleneck, variations in analog compute, and device non-idealities. Next we introduce the DNN+NeuroSim benchmark framework that is capable of evaluating versatile device technologies for CIM inference and training performance from software/hardware co-design's perspective.

Compute-in-Memory Chips for Deep Learning: Recent Trends and Prospects

When the international technology roadmap of semiconductors (ITRS) started almost five decades ago, the metal oxide effect transistor (MOSFET) as units in integrated circuits (IC) continuously miniaturized. The transistor structure has radically changed from its original planar 2D architecture to today’s 3D Fin field-effect transistors (FinFETs) along with new designs for gate and source/drain regions and applying strain engineering. This article presents how the MOSFET structure and process have been changed (or modified) to follow the More Moore strategy. A focus has been on methodologies, challenges, and difficulties when ITRS approaches the end. The discussions extend to new channel materials beyond the Moore era.

Miniaturization of CMOS.

The performance of silicon complementary metal–oxide–semiconductor integrated circuits can be enhanced through the monolithic three-dimensional integration of additional device layers. For example, silicon integrated circuits operate at low voltages (around 1 V) and high-voltage handling capabilities could be provided by monolithically integrating thin-film transistors. Here we show that high-voltage amorphous oxide semiconductor thin-film transistors can be integrated on top of a silicon integrated circuit containing 100-nm-node fin field-effect transistors using an in-air solution process. To solve the problem of voltage mismatch between these two device layers, we use a top Schottky, bottom ohmic contact structure to reduce the amorphous oxide semiconductor circuit switching voltage. These contacts are used to form Schottky-gated thin-film transistors and vertical thin-film diodes with excellent switching performance. As a result, we can create high-voltage amorphous oxide semiconductor circuits with switching voltages less than 1.2 V that can be directly integrated with silicon integrated circuits. High-voltage amorphous oxide semiconductor thin-film transistors can be integrated on top of a silicon integrated circuit containing 100-nm-node fin field-effect transistors using an in-air solution process.

Monolithic integration of high-voltage thin-film electronics on low-voltage integrated circuits using a solution process

In this paper, we present an optimization flow for monolithic 3D ICs called Pin-3D Optimizer. Compared with the state-of-the-art RTL-to-GDS flows that rely on ad-hoc technology file tweaks and RC scaling, Pin-3D offers a streamlined method to run commercial 2D IC tools to obtain high-quality monolithic 3D IC designs. Specifically, Pin-3D supports effective legalization, routing, timing closure, and ECO optimization for monolithic 3D IC designs. We propose a novel optimization methodology where the cells in each tier of a 3D IC are optimized using cell data and constraints of the full 3D design. The optimizations in a tier also directly influence the timing, power in the other tiers, leading to better overall PPA of the 3D IC. With the help of two industry processors designed with a 28 nm technology node, we show that Pin-3D provides up-to 9.0% smaller wirelength and 88% smaller total negative slack than die-by-die M3D flows. We also observe up-to 8.7% lower power and 26% smaller wirelength than 2D ICs. In addition, Pin-3D is the first flow that supports routing and timing optimization in heterogeneous logic-on-logic monolithic 3D ICs. We demonstrate this capability by performing area-balanced tier partitioning, routing, and timing closure of a 3D design with different technologies on each die.

Pin-3D: a physical synthesis and post-layout optimization flow for heterogeneous monolithic 3D ICs

Resistive random access memory (RRAM)-based compute-in-memory architecture helps overcome the bottleneck caused by large memory transactions in the convolutional neural network (CNN) accelerators. However, their deployment using 2-D IC technology faces challenges, as today’s RRAM cells remain at legacy nodes above 20 nm due to high programming voltages. Besides, power-hungry analog-to-digital converter (ADC) units limit the throughput of RRAM accelerators. In this article, we present the first-ever heterogeneous (multiple technology nodes) mixed-signal monolithic 3-D IC designs of the RRAM CNN accelerator. Our RRAM remains at legacy 40-nm nodes in one tier, but CMOS periphery scales toward advanced 28/16 nm in another tier. Our 3-D designs overcome the bottleneck caused by ADCs and offer up to $4.9\times $ improvement in energy efficiency in TOPS/W and up to 50% reduction in footprint area over 40-nm 2-D IC designs. Compared with existing 2-D works, our 3-D architecture offers up to $28.6\times $ improvement in energy efficiency.

Heterogeneous Mixed-Signal Monolithic 3-D In-Memory Computing Using Resistive RAM

The 3D sequential integration, of active devices requires to limit the thermal budget of top tier processing to low temperature (LT) (i.e. $\mathrm{T}_{\text{TOP}}=500^{\circ}\mathrm{C})$ in order to ensure the stability of the bottom devices. Here we present breakthrough in six areas that were previously considered as potential showstoppers for 3D sequential integration from either a manufacturability, reliability, performance or cost point of view. Our experimental data demonstrate the ability to obtain 1) low-resistance poly-Si gate for the top FETs, 2) Full LT RSD epitaxy including surface preparation, 3) Stability of intermediate BEOL between tiers (iBEOL) with standard ULK/Cu technology, 4) Stable bonding above ULK, 5) Efficient contamination containment for wafers with Cu/ULK iBEOL enabling their re-introduction in FEOL for top FET processing 6) Smart Cut™ process above a CMOS wafer.

J. Aubin

Papers

Breakthroughs in 3D Sequential technology