ECS 2008 Presentaion

NiSi Formation in Electrochemically-Etched Silicon Nanowires W. Xu* ,1 , Vadim Palshin 2 , J.C. Flake** ,1 1 Gordon and Mary Cain Department of Chemical Engineering 2 Center for Advanced Microstructure and Devices Louisiana State University, Baton Rouge LOUISIANA STATE UNIVERSITY

O utline
Overview
Electroless SiNW fabrication
SiNW Integration and NiSi formation
Electrical Measurement
XPS & XANES Characterization
Passivation & Surface Functionalization
Conclusion
Silicon nanowire sensor Fernando Patolsky, Gengfeng Zheng & Charles M Lieber, NATURE PROTOCOLS VOL.1 NO.4 2006 Silicon nanowire solar cell B. Tian, et al. Nano Lett. Vol.449, NO.18 2007 LOUISIANA STATE UNIVERSITY

Conventional Integration with VSL grown silicon nanowires Silicon nanowire transistor (SBFETs) W. Weber. et al. Nano Lett. Vol. 6, No. 12 2006
Grow Silicon nanowire via VLS method
Deposit fabricated SiNWs on dielectric substrate
Find the deposited SiNWs and make electrode by lithography involved metal deposition
Anneal to form ohmic contact
Grow silicon nanowire via VLS method
Randomly deposit fabricated SiNWs on patternted electrodes by lithography
Selective electroless nickel on top of the electrode
Anneal to form ohmic contact
Characteristic of this approach: Find and wire time consuming High vaccum, high temperature,high costs Doping requires p-or n- type growth sequences “ Find and Wire” SiNW deposition onto electrodes Characteristic of this approach: Advantage over “find and wire” Difficult to position nanowires, dop ing SBFET requires higher potentials VLS relatively high cost same as “find and wire” C. M Lieber et al SCIENCE VOL 291 851-853 2001 . LOUISIANA STATE UNIVERSITY

Integration of Electroless SiNWs: Fabrication of SiNWs via electroless etching SiNWs deposition on lithographically patterned electrodes Thermal annealing to form self-aligned NiSi contacts Electrical measurement Advantages over conventional approach: Low cost compared to VLS L arge scale fabrication R elative simple silicidation process Low process temperatu re Enables FET / nanowire sensor fabrication. Characterization Separation of SiNWs from parent substrate LOUISIANA STATE UNIVERSITY

Why NiSi?
Low resistivity
Low silicon consumption during silicidation
Low processing temperature ( 350-750 °C)
High temperature stability
The Compatibility with conventional silicon manufacturing
LOUISIANA STATE UNIVERSITY Summary of various metal silicides properties H. Iwai et al. Microelectronic Engineering 60 (2002) 157 –169

SiNW s fabrication via electroless etching
Self-assembled localized microscopic electrochemical cell model Silver atoms depositing on the silicon wafer can form nuclei that can behave as cathode, and the area surrounding these nuclei behaves as an anode and will be etched away and dissolved into HF solution. The half reactions are: Cathode: Ag+e - ->Ag Anode: Si + 6F - -> SiF 6 2- + 4e - The presence of the microelectrochemcal cell leads to the selective etching of silicon substrate, and finally forms silicon nanowires (K.Peng et al Adv. Mater. 14 (2002) 1164. ) . LOUISIANA STATE UNIVERSITY

Cross-section SEM imagines of fabricated SiNWs on substrate SEM imagines of sep a rated SiNWs deposited On substrate Large quantity of SiNWs deposited on substrate To p view SEM imagines of fabricated SiNWs on substrate S iNWs on substrates Se perated SiNWs LOUISIANA STATE UNIVERSITY

E lectrical measurement SEM images of Inter-digited electrodes （ IDE ） SEM image of one single SiNW across electrode The fabricated SiNWs were deposited on IDE electroplated with nickel, and annealed at 450°C in reducing atmosphere. Cross-section sketch of SiNWs across the electrodes LOUISIANA STATE UNIVERSITY

Electrical measurement Based on the estimation that there are 612 SiNWs across the IDE( by SEM analysis) 　 and the average diameter of nanowire is 150 nm. 　 The corresponding resistivity: Same order of magnitude of silicon for annealed nanowires suggests the lateral NiSi formation is occurred at the end of nanowires and the nanowires is not fully silicized. LOUISIANA STATE UNIVERSITY Bu lk resistance Bare IDE IDE after SiNW deposition IDE after anneal Resistance >1·10 9 Ω 　（ＯＣ） 4·10 6 Ω 2·10 3 Ω NiSi Silicon IDE after anneal Resistivity 10μΩ∙cm 1-5 Ω∙cm ～ 0.35 Ω∙cm

TEM images of SiNW with NiSi
Bright region inside the structure --- single crystal silicon
Bright region along the wall --- amorphous silicon dioxide
Dark region on top right --- NiSi phase
LOUISIANA STATE UNIVERSITY

The bulk sample shows 1:1 nickel to silicon after 30 min suptter
The SiNWs sample shows 1:1 nickel to silicon with relatively high oxygen concentration
The oxygen concentration is due to the difficulty to strip silicon oxide in dense nanowire arrays
XPS analysis Bulk silicon sample SiNWs sample LOUISIANA STATE UNIVERSITY

XANES provides detail examination of electronic environment
Comparison of SiNWs sample Ni K-edge XANES in total electron yield( TEY) mode with literature
The XANES spectra shows similar spactra with bulk NiSi
XANES analysis S. J. Naftel, I. Coulthard, T. K. Sham, S. R. Das and D. X. Xu, Physical Review B , 57 , 9179 (1998) Ni K-edge XANES in TEY mode on NiSi nanowire sample LOUISIANA STATE UNIVERSITY

Electroless fabrication of SiNWs
Separation and integration SiNWs with Patterned nickel electrodes
Silicidation by thermal annealing
Ohmic behavior after silicidation
Problems needs to be addressed
Passivation – surface functionalization
Doping

Directly Functionalize (Si-C, no intermediate oxide) of silicon nanowire surface as a possible solution to passivate the surface and avoid oxidation. For example anodic electrografting of SiNW surfaces with alkyl Grignards (or cathodic electrografting with alkynes) Passivation of Si surface Absorption infrared spectra. [a] ethyl hydrosilylated; [b] ethyl electrografted; [c] methyl hydrosilylated; [d] methyl electrografted. AFM images of passivated silicon surfaces. [A] ethyl anodic electrografting; [B] ethyl hydrosilylation; [C] methyl anodic electrografting; [D] methyl hydrosilylation. Anodic electrografting Thermal hydrosilylation Oxides (XPS) LOUISIANA STATE UNIVERSITY

LOUISIANA STATE UNIVERSITY Functionalization of Nanowires ATR FTIR spectra showing neat hexynoic acid top and surfacebound hexynoic acid electrografted to a silicon nanowire surface. FTIR spectra showing BSA top and immobilized BSA onto a silicon nanowire substrate. Z. R. Scheibal et al, Electrochemical and Solid-State Letters, 11 (8) 2008

Conclusion
Ele ctrole ss fabrication of SiNW
Separation and Integration with Lithographically patterned electrode
TEM, XPS, XANES Demonstrate the formation of NiSi (1:1) phase formation with pre- patternted electrodes
Control surface chemistry Si-R
Passivation
Functionalization
Current work focuses on building SiNW sensors (doping)

Acknowledgements: Sponsors: ATMI, Louisiana Board of Regents D. Cao Materials Characterization center, LSU S. S. Vegunta and J. N. Ngunjiri, Department of Chemical Engineering, LSU Thank you for your attention ! LOUISIANA STATE UNIVERSITY

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