Received 13 June 2016; revised 24 February 2017; accepted 8 March 2017. Date of publication 20 March 2017; date of current version 7 June 2017. Digital Object Identifier 10.1109/TETC.2017.2684781 NP-Dynamic Skybridge: A Fine-Grained 3D IC Technology with NP-Dynamic Logic JIAJUN SHI, MINGYU LI, MOSTAFIZUR RAHMAN, SANTOSH KHASANVIS, AND CSABA ANDRAS MORITZ J. Shi, M. Li, and C.A. Moritz are with the Department of Electrical and Computer Engineering, University of Massachusetts, Amherst, MA 01003 M. Rahman is with the School of Computing and Engineering, University of Missouri, Kansas City, MO 65211 S. Khasanvis is with BlueRISC Inc., Amherst, MA 01002 CORRESPONDING AUTHOR: J. SHI ([email protected]) ABSTRACT A new 3D IC fabric named NP-Dynamic Skybridge is proposed that provides fine-grained vertical 3D integration for future technology scaling. Relying on a template of vertical nanowires, it expands our prior work to incorporate and utilize both n- and p-type transistors in a novel NP-Dynamic circuit-style compatible with true 3D integration. This enables a wide range of elementary logics leading to more compact circuits, simple clocking schemes for cascading logic stages and low buffer requirement. We detail new design concepts for larger-scale circuits, and evaluate our approach using a 4-bit nanoprocessor implemented in 16 nm technology node. A new pipelining scheme specifically designed for our 3D NP-Dynamic circuits is employed in the nanoprocessor. We compare our approach with 2D CMOS as well as state-of-the-art tran- sistor-level monolithic 3D IC (T-MI) approach. Benchmarking results for the 4-bit nanoprocessor show bene- fits of up to 56.7x density, 3.8x power and 1.7x throughput over 2D CMOS. Compared with T-MI, our new 3D fabric showed 31x density, 3x power and 1.4x throughput improvement. Additional evaluation of 4-bit and 8-bit CLA designs shows that significantly improved gains can be achieved for our 3D approach over 2D CMOS with increasing circuit bit-width, indicating potential for future scalability. INDEX TERMS 3D IC fabric, NP-Dynamic circuits, emerging technologies, vertical nanowires, nanoscale computing fabrics, nanoprocessor I. INTRODUCTION In the Skybridge fabric, uniform n-type transistors are used in 2D CMOS integrated circuit (IC) technology scaling faces a dynamic circuit style [6], and n-type transistor based NAND severe challenges that result from device scaling limitations and AND-of-NAND compound gates are the elementary logic [1], [2], interconnect bottleneck that dominates power and functions. Multiple clock signals are used to control the cas- performance [3], etc. 3D ICs with die-die and layer-layer cading of elementary logic stages, and buffers are used in- stacking using Through Silicon Vias (TSVs) [4] and Mono- between stages for signal propagation and restoration in large- lithic Inter-layer Vias (MIVs) [5] have been explored in recent scale designs, which introduce limitations. years to generate circuits with considerable interconnect sav- Here, we propose NP-Dynamic Skybridge fabric [12] that ing for continuing technology scaling. However, these 3D IC incorporates both n- and p-type transistors to build a new technologies still rely on conventional 2D CMOS’s device, class of fine-grained 3D integrated circuits. It follows a fab- circuit and interconnect mindset showing only incremental ric-centric mindset to integrate both n- and p-type circuit benefits [8] while adding new challenges such as thermal components in the physical layer for 3D compatibility. The management, manufacturing and reliability issues [4], [5]. use of both n- and p-type transistors allows a wide range of In [6], [7], a vertical nanowire based 3D integrated circuit elementary logic functions to be supported including NAND, fabric called Skybridge was proposed showing a pathway for AND-of-NAND, NOR and OR-of-NOR, which provide high truly fine-grained 3D integration. In this 3D fabric, core IC flexibility for implementation of logic functions. This results fabric aspects from device to circuit-style, connectivity [9], in compact circuit designs with short interconnects and low thermal management [10] and manufacturing pathway [11] buffer requirement. The use of NP-Dynamic circuit style also are co-architected keeping 3D compatibility in mind, and uses allows a simple clocking scheme (since alternately cascading the vertical dimension instead of a multi-layered 2D mindset. n- and p-type logic stages maps intrinsically to the signal 2168-6750 ß 2017 IEEE. Translations and content mining are permitted for academic research only. Personal use is also permitted, but republication/redistribution requires IEEE permission. 286 See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. VOLUME 5, NO. 2, APRIL-JUNE 2017 Shi et al.: NP-Dynamic Skybridge: A Fine-Grained 3D IC Technology with NP-Dynamic Logic monotonicity requirement of dynamic circuits). The achieved towards a functional circuit is an ongoing long-term effort. design with unique 3D components has good connectivity [9] Figure 1A shows the envisioned NP-Dynamic Skybridge; and routability [13] to address high-density interconnects in Using a similar process flow described in [11], vertical nano- larger-scale circuits, which leads to compact circuit design wires, are constructed primarily through masking and high with significant power efficiency and high performance. aspect ratio etching on heavily doped silicon bulk (other meth- In this paper, the NP-Dynamic Skybridge approach is ods are also possible). Architected fabric components are con- extensively evaluated at the circuit-level and system-level to structed on these nanowires by using material deposition investigate its benefits in circuit designs with complex and techniques [11]. In this section, we present the core compo- high-density interconnects. For circuit-level evaluation, its nents that enable fine-grained integration of both n- and p-type core fabric components including fabric structures facilitat- nanowires in Skybridge fabric. Detailed explanation of mate- ing both n- and p-type integration and elementary logic gates rial selection and working mechanism are presented to illus- combined with TCAD and HSPICE simulations are pre- trate how these components are used in unison to achieve sented in detail. For system-level evaluation, a 4-bit Wire desired functionality and 3D compatibility with circuits imple- Stream Processor (WISP-4) [14] design is used as bench- mented across both horizontal and vertical dimensions. mark. A new pipelining scheme specifically designed for NP-Dynamic 3D circuit style is used in the WISP-4 design to A. VERTICAL NANOWIRES reduce the operation time of each stage in pipeline for Vertical nanowires are the fundamental building blocks that improved throughput. Device-to-circuit fabric evaluation is enable vertical stacking of designed core Skybridge compo- presented for WISP-4 accounting for characteristics of nents. The nanowires serve multiple functions – they can act selected materials, circuit style, placement and routing, and as (i) logic nanowires that have stacked transistors to imple- 3D layouts, which shows up to 56.7x density benefit, 3.8x ment required logic gates, (ii) routing nanowires to carry power efficiency and 1.7x benefit in throughput against 2D electrical signals along the vertical dimension, and (iii) heat CMOS with the equivalent technology node. We also com- dissipating nanowires to extract and sink heat generated dur- pare our approach with the state-of-the-art approach of con- ing circuit operation to the bulk substrate [10]. ventional 3D IC direction, the transistor-level monolithic 3D The nanowire formation step precedes all manufacturing IC (T-MI) [15] using equivalent technology node. Our 3D steps, and is done after wafer preparation. Wafer preparation approach showed up to 31x density benefit, 3x power effi- involves stacking heavily doped n-type and p-type silicon ciency and 1.4x benefit in throughput against T-MI 3D IC. layers to create a dual-doped silicon wafer (Figure 1B). This Additionally, we used 4-bit and 8-bit CLAs as benchmark to can be achieved by bonding heavily doped n-type and p-type evaluate NP-Dynamic Skybridge’s gain over 2D CMOS as substrates using techniques that are similar to the ones the bit-width of design grows; NP-Dynamic’s 8-bit CLA described in literature [16], [17] and currently used for con- design has 24 percent increased latency and 16 percent ventional 3D ICs. A silicon dioxide layer is used between the increased power compared with 4-bit CLA design while 2D n-type and p-type doped silicon layers for isolation. Vertical CMOS’s 8-bit CLA design has 2x latency and 25 percent nanowire patterning can be achieved through inductively increased power compared with 4-bit CLA design. coupled plasma etching [11], [18] and has been experimen- The rest of the paper is organized as follows: Section II tally demonstrated as shown in Figure 2A. presents the proposed 3D fabric’s core components. In Section III, we show the elementary gates and their fan-in sensitivity B. VERTICAL GATE-ALL-AROUND (VGAA) analysis. Section IV shows a device-to-circuit evaluation TRANSISTOR methodology for this new fabric. Then, in Section V, we show VGAA junctionless transistors are used as active devices, and the benchmarking of a 4-bit nanoprocessor (WISP-4) with an are formed on nanowires through consecutive material depo- optimized pipelining scheme for improved throughput versus sition steps [11]. These junctionless transistors use uniform 2D CMOS and T-MI 3D. Section VI presents a preliminary doping with no abrupt variation in Drain/Source/Channel study of gain versus bit-width utilizing carry look-ahead adder regions (Figure 1D-E), which simplifies manufacturing designs. Section VII concludes the paper. requirements and is especially suitable for this fabric. Their channel conduction is modulated by the workfunction differ- II. ENABLING 3D INTEGRATION: CORE COMPONENTS ence between the heavily doped channel and the gate [19].