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Si and Ge NW FETs, NiSi-Si-NiSI conductor hetero-structures and manufacturing steps Csaba Andras Moritz Associate Professor University of Massachusetts, Amherst [email protected] From Nanodevices to Nano Computing Carbon Nanotubes (CNT) Transistors or Diodes Nanoarray Nanocircuit Lauhon et al., Nature 420,57 Semiconductor Nanowires (NW) Nanocomputing a0 b0 a0 b0 clk s0 s0 a1 b1 a1 b1 Down clk s1 s1 a2 b2 a2 b2 Down clk s2 s2 a3 b3 a3 b3 Down Up s3 s3 Down Up Up c0 c0 clk s0 s0 Up s0 s0 c2 c2 c1 c1 s0 s0 Copyright - Csaba Andras Moritz , ECE, UMass Amherst s1 s1 s0 s0 s0 s0 c4 c4 c3 c3 s2 s2 s3 s3 2 Nanowires From Lieber, Nanoscience: Building a Big Future Copyright - Csaba Andras Moritz , ECE, UMass Amherst 3 Nanowires Copyright - Csaba Andras Moritz , ECE, UMass Amherst 4 Nanowire Materials From Lieber, Nanoscience: Building a Big Future Copyright - Csaba Andras Moritz , ECE, UMass Amherst 5 Comparison of NWs and CNTs Controlled doping of CNTs is not possible Specific growth of semiconducting and conducting tubes is not possible These properties depend sensitively on diameter and helicity in CNTs Semiconductor NWs overcome these limitations Vast knowledge in the semiconductor industry Remain semiconducting independent on diameter Controlled doping demonstrated, e.g., with Boron for p-type and Phosphorus for n-type for SiNWs Change the conductivity of SiNWs over many orders of magnitude Measured with Transmission Electron Microscopy (TEM) Copyright - Csaba Andras Moritz , ECE, UMass Amherst 6 P and N-type SiNW (FETs) Yi Cui et al, The Journal of Physical Chemistry, 2000 Copyright - Csaba Andras Moritz , ECE, UMass Amherst 7 Electron charging Yi Cui et al, 2000 Copyright - Csaba Andras Moritz , ECE, UMass Amherst 8 Specializing NWs Control of composition, structure, size, doping Diameter controlled during growth As small as 3nm Stable electronic characteristics Copyright - Csaba Andras Moritz , ECE, UMass Amherst 9 FETs PFETs and NFETs in SiNWs, GaNi NWs Both PFETs and NFETs in same material with Si and Ge NWs and CNTs Greytak et al, American Institute for Physics, 2004 IBM Nanoscience Group lead by Davouris demonstrated CNTs Copyright - Csaba Andras Moritz , ECE, UMass Amherst 10 Ge based complementary FETs Complementary doping demonstrated in Si, GaN, and now Ge Has been used to assemble inverters, bipolar transistors and light emitting diodes Achieving p-FET and n-FET in same material was challenging Ge has higher electron and hole mobility than Si and both P and N type devices have been demonstrated Copyright - Csaba Andras Moritz , ECE, UMass Amherst 11 Synthesis of p and n-type Ge NWs Core-shell method, doping with PH3 for N and B2H6 for P From Greytak et al, 2004 Copyright - Csaba Andras Moritz , ECE, UMass Amherst 12 P and N-type Ge FETs From Greytak et al, 2004 Ge NWs with Ti S-D contacts Vd – drainsource bias voltage, Id the current through the channel,Vggate voltage Curves characteristic of MOS FETs Yield 86% Copyright - Csaba Andras Moritz , ECE, UMass Amherst 13 Comparison with Si and GaN FETs Higher on currents than in those devices Higher mobilities and smaller Vth possible Deposition of Ge oxynitride or SiGe capping layer Optimization of the doping procedure Copyright - Csaba Andras Moritz , ECE, UMass Amherst 14 Nanoarrays Nanowires are aligned with LongmuirBlodgett fluidic alignment Can be packed into NW arrays Copyright - Csaba Andras Moritz , ECE, UMass Amherst 15 Nanoarrays Copyright - Csaba Andras Moritz , ECE, UMass Amherst 16 Metal/semiconductor nanowire heterostructures MW-NW contacts Lithographically defined metal contacts with electrodes Problem: size scale – much larger than nanoscale Cannot be used for interconnect between FETS on a grid Integrated interconnect and contact solution based on selective transformation of Si NWs into NiSi nanowires Yue Wu et al, Nature 2004. Copyright - Csaba Andras Moritz , ECE, UMass Amherst 17 Why NiSi? Has been shown to have low resistivity (10 uOhmcm) Compatibility with Si manufacturing FET with NiSi/p-Si/NiSi junction Si channel of 20-nm in a 10-nm diameter structure Ability to form ohmic contacts with p and n type silicon High maximum currents – 29-nm NiSi-NW would carry 1.84 mA Current density comparable to CNTs Copyright - Csaba Andras Moritz , ECE, UMass Amherst 18 NiSi/Si Nanowire Heterostructures 。 Deposit React Etch Si NWs toatremove Ni 550 (green) C to formexcess to NiSi NWNW (blue) Ni(brown) Form Selectively Lithography NiSi segments deposit mask Ni Form NWs NiSi as masks segments on Si NWs Wu et al., Nature Vol. 430, pp. 61, 2004 Copyright - Csaba Andras Moritz , ECE, UMass Amherst 19 Modulation doped NWs for decoders Copyright - Csaba Andras Moritz , ECE, UMass Amherst 20 Very large scale integration Nanowires assembled to form structures of 1,000 to 30,000 Assembled and interconnected > 80% yield Copyright - Csaba Andras Moritz , ECE, UMass Amherst 21 Our approach: Nano circuits based on nanoarrays and FETs Why not use 2-terminal devices? There are several approaches resembling PLA and cell-based FPGA like nanoFabrics, nanoPLA, CMOL We are interested in building processor datapaths Need for latching etc Much higher density can be achieved even in 2-D fabrics Even in 2-terminal arrays there is a need for signal restoration based on FETs (see nanoPLA) We want to know what the benefits would be and what the challenges are from an architects point-of-view Copyright - Csaba Andras Moritz , ECE, UMass Amherst 22 Manufacturing steps for large scale 2-D computing with NW FETs (NASICs) Combination of self-assembly and nano lithography Self-assembly Form NW array with correct doping of wires Initial metallization between crosspoints using one set of wires as the mask Create channel regions for FETs at cross-points Nanolitography and conventional lithography Additional specialization of crosspoints with NiSi metallization Sub 10-nm imprint lithography Micro-nano interfacing Stephen Chou et al, University of Minnesota, 1997 Not based on modification of chemical structure by radiation, its resolution is immune to many factors that limit the resolution of conventional lithography, such as wave diffraction, scattering and interference in resist, and the chemistry of the resist and developer selective chemical modification (Zhong et al Science 2003) Several other proposals (coded NWs radial doping, Distributed pin array, etc) CMOS support structures Copyright - Csaba Andras Moritz , ECE, UMass Amherst 23
