Cost Modeling and Projection for Stacked Nanowire Fabric

Abstract— To continue scaling beyond 2-D CMOS with 3-D integration, any new 3-D IC technology has to be comparable or better than 2-D CMOS in terms of scalability, enhanced functionality, density, power, performance, cost, and reliability. Transistor-level 3-D integration carries the most potential in this regard. Recently, we proposed a stacked horizontal nanowire based transistor-level 3-D integration approach, called SN3D [1][2] that solves scaling challenges and achieves tremendous benefits with respect to 2-D CMOS while keeping manageable thermal profile. In this paper, we present the cost analysis of SN3D and show comparison with 2-D CMOS, conventional TSV based 3-D (T3-D) and Monolithic 3-D (M3-D) integrations. In our cost model, we capture the implications of manufacturing, circuit density, interconnects, bonding and heat in determining die cost and evaluate how cost scales as transistor count increases. Since, SN3D is a new 3-D IC fabric, based on our proposed manufacturing pathway [2] we assumed complexity of fabrication steps as proportionality constants in our cost estimation model. Our analysis revealed SN3D have 86% and 74% reduction in area; 55% and 43% reduction in interconnect distribution and total interconnect length required; reduced metal layer requirement; and 70% and 68% reduction in total cost in comparison to 2-D CMOS and Monolithic 3-D (M3-D) integrations respectively.

Index Terms—3-D IC, 3-D CMOS, Fine-Grained 3-D, SN3D, 3- D Manufacturing, 3-D cost, SN3D Cost
I. INTRODUCTION Transistor-level 3-D integration is considered the most promising direction for replacing 2-D CMOS due to its density and performance benefits. Our proposal for transistor-level 3- D integration called stacked horizontal nanowire based 3-D IC fabric (SN3D), is based on stacked horizontal nanowires and uses architected features for circuit functionality, interconnection, and thermal management. Previously we reported huge gains with SN3D design [1-3]. In this paper, we focus on cost aspects and show a detailed comparison with 2-D CMOS, TSV based 3-D CMOS (T3-D) and monolithic 3-D (M3-D) integrations.
Fig. 1 shows core components of the fabric and overview of circuit mapped SN3D fabric. Stacked suspended horizontal nanowires (Fig. 1A) are the building blocks, which are prefabricated, and predopped. Architected fabric components: Gate-All-Around junctionless nanowire FETs (Fig. 1B), Common Contact (CC- Fig. 1C.i), Common Gate (CG- Fig. 1C.ii), Horizontal Bridges (HB- Fig. 1C.iii), Horizontal Insulation (HI-Fig. 1D) and Fabric Vias (FV- Fig. 1E) are formed onto these nanowires through material deposition techniques[4][5]. Active devices here are junctionless nanowire transistors that do not require any doping variation for Drain/Source/Channel regions. Previously we demonstrated Cost Modeling and Projection for Stacked Nanowire Fabric
A) Stacked Nanowires C) Fabric Interconnect Features (i) Common Contact Contact (Ni) Gate (TiN) Gate (Ti) (ii) Common Gate (iii) Horizontal Bridge F) SN3D Fabric-View Nanowire (Si) B) Nanowire Transistor NW Channel Gate Oxide(HfO2) Contact Gate (Ti/TiN) Spacer (Si3N4) Contact (Al) Bridge E) Vias Nano-Vias (W) Horizontal Isolation (SU8/SiO2) D) Horizontal Insulation (HI) Substrate Metal Stack Via Insulation Nanowire Gate Bridge Common Contact Fig. 1: (A) Stacked Nanowires; (B) Junction-less Gate All Around Nanowire Transistor; (C) Fabric-Interconnects, (i) Common Contact (CC), (ii) Common Gate (CG), (iii) Horizontal Bridge (HB); (D) Horizontal Insulation (HI); (E) Fabric-Vias; (F) SN3D Fabric-Overview

junctionless device behavior experimentally [4][5]. The CG feature allows multiple junctionless devices to be gated with single input in vertical and/or horizontal directions. This enables optimal placement of common gated devices and minimizes inter-device connectivity requirements. The CCs provide common contact for stacked NWFETs, serve as an interconnection between adjacent transistors’ source and drain contacts. Functionally, CG and CC carry common signals; however, physically they are composed of different material stacks according to channel-gate work-function and Ohmic Contact requirements respectively. The HI (Fig. 1D) provides isolation between adjacent Gate, Source and Drain contacts in the vertical direction. The HBs [4][5] serve dual purpose: connectivity and heat extraction. These Bridges along with CG and CC allow routability in 3-D and is very different from 2-D CMOS and other through silicon via based 3-D CMOS approaches where routing mostly take place in the 2-D plane and through vias that connect two layers/dies. Finally, fabric vias (Fig.1E) are used for input/output signals, and to

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