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Two-dimensional electrical characterization of ultra shallow source/drain extensions for nanoscale MOSFETs presented by Uttam Singisetti Advisor: Professor Stephen Goodnick Electrical Engineering Department Science and Engineering of Materials Program Arizona State University Center for High Resolution Electron Microscopy CENTER FOR SOLID STATE SCIENCES, ARIZONA STATE UNIVERSITY OFFICE OF NAVAL RESEARCH Outline of the Talk • Background and Motivation for the work • Fabrication of ultra shallow junctions (USJ) • One-dimensional (1-D) Secondary Ion Mass Spectroscopy analysis of USJs • Electron holography (EH) technique and 1-D analysis using EH • 2-D Electron Holography Results of the USJs • Interpretation of results and conclusion Center for High Resolution Electron Microscopy CENTER FOR SOLID STATE SCIENCES, ARIZONA STATE UNIVERSITY OFFICE OF NAVAL RESEARCH MOSFET Scaling and ITRS Requirements Moore’s Law has been driving force for the continued scaling of transistors http://www.intel.com/research/silicon/mooreslaw.htm Center for High Resolution Electron Microscopy CENTER FOR SOLID STATE SCIENCES, ARIZONA STATE UNIVERSITY OFFICE OF NAVAL RESEARCH International Technology Roadmap for Semiconductors (ITRS) identifies the features for future generations 2003 ITRS Requirements for Ultra Shallow Junctions for source/drain extensions Center for High Resolution Electron Microscopy CENTER FOR SOLID STATE SCIENCES, ARIZONA STATE UNIVERSITY OFFICE OF NAVAL RESEARCH Major challenges are • Ultra shallow junction depths to reduce short channel effects • Low sheet resistance • High lateral abruptness • 2-D control of the doping profile (Gate Overlap or lateral diffusion) Poly gate Oxide Junction Depth Gate Overlap Source Drain Center for High Resolution Electron Microscopy CENTER FOR SOLID STATE SCIENCES, ARIZONA STATE UNIVERSITY OFFICE OF NAVAL RESEARCH ASU Nano-CMOS Process Aim: To fabricate sub-50 nm gate length NMOSFET and integrate with Si Single Electron Transistor (SET) Key Fabrication Steps are Source/Drain Fabrication by Rapid Thermal Diffusion (RTD) from heavily doped Spin-on-Glass (SOG) Self-aligned Gate Sidewall Spacers by RPECVD oxide/nitride and Reactive Ion Etching (RIE) Gate length definition by Electron Beam Lithography Status 300 nm and 90 nm n channel MOSFETS fabricated successfully Failure of 70 nm gate length MOSFET due to Source-Drain overlap Center for High Resolution Electron Microscopy CENTER FOR SOLID STATE SCIENCES, ARIZONA STATE UNIVERSITY OFFICE OF NAVAL RESEARCH Motivation • Fabricate ultra shallow junctions below 40 nm using Rapid Thermal Diffusion • One-dimensional chemical characterization of the USJs using SIMS • One-dimensional electrical characterization by Electron Holography • Two-dimensional characterization of the USJs and estimation of the lateral diffusion in USJs Center for High Resolution Electron Microscopy CENTER FOR SOLID STATE SCIENCES, ARIZONA STATE UNIVERSITY OFFICE OF NAVAL RESEARCH Fabrication of Ultra Shallow Junctions • Deposit 200 nm of LPCVD silicon nitride on heavily B doped p-type substrate • Nitride film is patterned by optical lithography and reactive ion etching to open diffusion windows • P doped Spin-on-Glass is spun and baked to drive away solvents • Rapid thermal diffusion carried out in a TAMRAK RTA equipment • SOG removed by etching in HF and 100 nm Cr metal deposited for TEM sample preparation for electron holography. Silicon Nitride Al Etch Mask P doped SOG Lithography Spin SOG RTD Heavily B doped Si Nitride Mask Center for High Resolution Electron Microscopy CENTER FOR SOLID STATE SCIENCES, ARIZONA STATE UNIVERSITY OFFICE OF NAVAL RESEARCH Vertical Diffusion mask is critical for accurate 2-D profiling of USJs RIE with CF4 gas only Al Etch Mask Oxide Si Substrate Center for High Resolution Electron Microscopy CENTER FOR SOLID STATE SCIENCES, ARIZONA STATE UNIVERSITY OFFICE OF NAVAL RESEARCH RIE with optimized values of power and pressure and CF4 and O2 gas flow Al Etch Mask Nitride Silicon Substrate Center for High Resolution Electron Microscopy CENTER FOR SOLID STATE SCIENCES, ARIZONA STATE UNIVERSITY OFFICE OF NAVAL RESEARCH Al Etch mask Nitride edge Center for High Resolution Electron Microscopy CENTER FOR SOLID STATE SCIENCES, ARIZONA STATE UNIVERSITY OFFICE OF NAVAL RESEARCH Two USJs with nitride mask and one USJ with oxide mask were fabricated following the procedure discussed 1-D chemical analysis was carried out by Secondary Ion Mass Spectroscopy (SIMS) 13 kV Back Scattered Ions -1 kV Cs+ Ion Gun Quadrupole Mass Analyzer Sputtered Ions (P, B) Center for High Resolution Electron Microscopy CENTER FOR SOLID STATE SCIENCES, ARIZONA STATE UNIVERSITY OFFICE OF NAVAL RESEARCH SIMS Analysis carried out using 14 keV Cs+ primary ion source in the CAMECA IMS 3F equipment at ASU Dopant Concentration (cm -3) 1022 Diffused Phosphorous Diffused Phosphorous Substrate Boron 1021 1020 The Metallurgical Junction Depth (MJD) as determined from SIMS is 30 nm and 60 nm respectively for the two junctions 1019 MJD 1018 1017 0 20 40 60 80 100 Depth from Si surface (nm) Center for High Resolution Electron Microscopy CENTER FOR SOLID STATE SCIENCES, ARIZONA STATE UNIVERSITY OFFICE OF NAVAL RESEARCH MJD of 50 nm as determined from for USJ with oxide diffusion mask 1021 Dopant Concentration (cm -3) Diffused Phosphorus Substrate Boron 1020 1019 1018 1017 0 20 40 60 80 100 Depth from Si surface (nm) Center for High Resolution Electron Microscopy CENTER FOR SOLID STATE SCIENCES, ARIZONA STATE UNIVERSITY OFFICE OF NAVAL RESEARCH Recoil Implantation or “knock-on” effect in SIMS 1020 Phosphorus profile of a delta doped layer Concentration (cm-3) Delta Layer 1019 SIMS profile of a delta doped P sample measures in CAMEC IMF 3F The “knock-on” effect seen is quite significant 1018 1017 0 10 20 30 40 50 Depth from Si Surface (nm) 60 70 Center for High Resolution Electron Microscopy CENTER FOR SOLID STATE SCIENCES, ARIZONA STATE UNIVERSITY OFFICE OF NAVAL RESEARCH CAMECA IMS 6F at North Carolina State University has been optimized for minimal “knock-on” effects for P measurement 1022 This System uses 3 keV Cs+ primary ion and has post sputter acceleration system Diffused P Substrate B Concentration (cm-3) 1021 1020 The SIMS profile shows a higher surface concentration and drops rapidly, which is typical of P junctions 1019 1018 1017 0 10 20 30 40 50 Depth from the Si surface (nm) 60 70 Center for High Resolution Electron Microscopy CENTER FOR SOLID STATE SCIENCES, ARIZONA STATE UNIVERSITY OFFICE OF NAVAL RESEARCH Electron Holography a Transmission Electron Microscopy Technique Field Emission Gun Digital Hologram Object Wave USJ Sample Electrostatic Biprism Reference wave through vacuum Lorentz lens CCD camera Hologram Philips CM200 FEG TEM, ASU Center for High Resolution Electron Microscopy CENTER FOR SOLID STATE SCIENCES, ARIZONA STATE UNIVERSITY OFFICE OF NAVAL RESEARCH Digital Hologram Reconstruction Inverse Fourier Transform Digital Hologram Complex Image Fourier Transform Aholo ( x, y ) t ( x, y ) 2in ln A ( x, y ) ref Im ( x, y ) arctan( ) Re Thickness Image Phase Image Center for High Resolution Electron Microscopy CENTER FOR SOLID STATE SCIENCES, ARIZONA STATE UNIVERSITY OFFICE OF NAVAL RESEARCH Vacuum Cr from Sample Preparation Bright region indicates presence of a junction Phase Images are converted to potential image by n+ Nitride ( x, y ) V ( x, y ) V0 C E t ( x, y ) 1-D Scan p Where CE is the interaction constant Which depends on the acceleration voltage of the electrons, V0 mean inner potential of Si 100 nm Reconstructed phase image of 30 nm MJD sample Center for High Resolution Electron Microscopy CENTER FOR SOLID STATE SCIENCES, ARIZONA STATE UNIVERSITY OFFICE OF NAVAL RESEARCH 1-D Measured and Simulated Potential Profiles Simulation EH data 1.6 Potential (V) The potential profile is simulated from the SIMS profile using a selfconsistent Poisson Solver* 1.2 Conversion of 1D Potential Profiles to 1D Electric Field and Total Charge Distribution 0.8 dV ( x) E ( x) dx 0.4 0 0 10 20 30 40 50 Distance from Si Surface(nm) Simulation for 100% activation * Ref:http://www.nd.edu/~gsnider 60 d 2V ( x) ( x) 2 dx Si ( x) q( N D ( x) N A ( x) p( x) n( x)) Center for High Resolution Electron Microscopy CENTER FOR SOLID STATE SCIENCES, ARIZONA STATE UNIVERSITY OFFICE OF NAVAL RESEARCH Derived From Electron Holography Electrical Junction Depth (EJD) is the point where the total charge goes to zero. This is the point of inflection on the 1D potential profile The EJD from Electron Holography is ~ 25 nm Center for High Resolution Electron Microscopy CENTER FOR SOLID STATE SCIENCES, ARIZONA STATE UNIVERSITY OFFICE OF NAVAL RESEARCH Simulated from SIMS data EJD ~ 27 nm Simulation of the Electric Field and Total Charge concentration from the SIMS profile using a Poisson Solver Center for High Resolution Electron Microscopy CENTER FOR SOLID STATE SCIENCES, ARIZONA STATE UNIVERSITY OFFICE OF NAVAL RESEARCH Similar 1-D analysis was carried out for the 65 nm USJ and USJ with oxide mask Nitride 1D Scan p n+ 200 nm Center for High Resolution Electron Microscopy CENTER FOR SOLID STATE SCIENCES, ARIZONA STATE UNIVERSITY OFFICE OF NAVAL RESEARCH 1.5 Potential (V) Simulation EH Data 1 1-D Potential profile for the 65 nm USJ from the from EH and Simulation of SIMS profile 0.5 1-D Electric field and total charge from EH and Simulation gave an EJD value of ~60 nm EJD 0 0 20 40 60 80 100 Depth from Si Surface (nm) 120 140 Center for High Resolution Electron Microscopy CENTER FOR SOLID STATE SCIENCES, ARIZONA STATE UNIVERSITY OFFICE OF NAVAL RESEARCH Two-Dimensional Analysis of the USJs Cr from TEM Sample Preparation Vacuum The dark contour line is the halfway point of the total variation of the potential in the Space charge region n+ p ~ 5nm ~ 30 nm Nitride Mask Si 100 nm Rescaled 2-D Potential Image from EH Center for High Resolution Electron Microscopy CENTER FOR SOLID STATE SCIENCES, ARIZONA STATE UNIVERSITY OFFICE OF NAVAL RESEARCH 2-D Potential Image from EH for the 65 nm MJD Sample 2-D charge image (arbitrary units) Nitride mask Vacuum Nitride 2-D Poisson Equation ~ 5nm Si ~ 65 nm ~ 65 nm Si 200 nm 200 nm n+ p Center for High Resolution Electron Microscopy CENTER FOR SOLID STATE SCIENCES, ARIZONA STATE UNIVERSITY OFFICE OF NAVAL RESEARCH 2-D Analysis of the USJ with oxide diffusion mask n+ Oxide ~ 50 nm p 100 nm Center for High Resolution Electron Microscopy CENTER FOR SOLID STATE SCIENCES, ARIZONA STATE UNIVERSITY OFFICE OF NAVAL RESEARCH • The lateral diffusion USJs with nitride mask is retarded compared to the lateral diffusion in USJs with oxide mask • The stress induced in Si substrate due to nitride film could be the factor for observed lateral diffusion • The diffusion constant (D) and equilibrium concentration of interstitials are dependent on stress in Si substrate D AX ( P ) f H AX ( P) D AX ( P 0) exp kT * C AX (P) f H AX ( P ) * C AX ( P 0) exp kT Center for High Resolution Electron Microscopy CENTER FOR SOLID STATE SCIENCES, ARIZONA STATE UNIVERSITY OFFICE OF NAVAL RESEARCH Stress Simulation near the nitride mask edge in ATHENA Process Simulator Nitride Presence of high stress near the edge Si This can be correlated to the observed diffusion profile in EH Center for High Resolution Electron Microscopy CENTER FOR SOLID STATE SCIENCES, ARIZONA STATE UNIVERSITY OFFICE OF NAVAL RESEARCH Stress simulation for Si substrate under oxide mask shows an order of magnitude less stress than with a nitride mask Oxide Si Center for High Resolution Electron Microscopy CENTER FOR SOLID STATE SCIENCES, ARIZONA STATE UNIVERSITY OFFICE OF NAVAL RESEARCH Cr from TEM Sample Preparation Vacuum Nitride n+ Si p Nitride Mask Si 100 nm n+ Oxide Oxide ~ 50 nm Si 100 nm p Center for High Resolution Electron Microscopy CENTER FOR SOLID STATE SCIENCES, ARIZONA STATE UNIVERSITY OFFICE OF NAVAL RESEARCH • There is a supersaturation of vacancies and undersaturation of interstitials in the Si substrate underneath nitride film, this is due to the dynamic state of the nitride film • The LPCVD nitride is under high stress, it can relieve stress by generating Frenkel Pairs at the Si/Si3N4 interface. The Si interstitials go into the film and relieve the stress. The vacancies are injected into the substrate which cause an undersaturation of interstitials via recombination reaction • This could suppress the diffusion of phosphorus under the nitride film as phosphorus predominantly diffuses via an interstitial mechanism • The observed anisotropy could be due to any of the above discussed factors or a combination of these factors Center for High Resolution Electron Microscopy CENTER FOR SOLID STATE SCIENCES, ARIZONA STATE UNIVERSITY OFFICE OF NAVAL RESEARCH Conclusion and Future Work Two-dimensional electrical junction depth (EJD) delineation was carried out on ultra shallow junctions Reduced lateral diffusion was observed for junctions with a nitride mask than with an oxide mask Stress in the Si substrate under nitride mask was simulated as a possible factor for the observed phenomenon Diffusion mask dependent lateral diffusion can be used to engineer source/drain extensions in nano-scale MOSFETS via “Defect Engineering” Complimentary measurements using Scanning Spreading Microscopy can substantiate the observed anisotropy in diffusion Resistance Center for High Resolution Electron Microscopy CENTER FOR SOLID STATE SCIENCES, ARIZONA STATE UNIVERSITY OFFICE OF NAVAL RESEARCH Questions or Comments ? Al Oxide Silicon Center for High Resolution Electron Microscopy CENTER FOR SOLID STATE SCIENCES, ARIZONA STATE UNIVERSITY OFFICE OF NAVAL RESEARCH