Center for High Resolution Electron Microscopy CENTER FOR SOLID STATE SCIENCES, ARIZONA STATE UNIVERSITY OFFICE OF NAVAL RESEARCH Two-dimensional electrical characterization of ultra shallow source/drain extensions for nanoscale MOSFETs Uttam Singisetti presented by Science and Engineering of Materials Program Arizona State University Advisor: Professor Stephen Goodnick Electrical Engineering Department

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

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 SourceDrain Oxide Junction Depth Gate Overlap

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. Heavily B doped Si Silicon Nitride Lithography Spin SOG RTD Al Etch Mask Nitride Mask P doped SOG

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 Al Etch Mask Si Substrate RIE with CF 4 gas only Oxide

Center for High Resolution Electron Microscopy CENTER FOR SOLID STATE SCIENCES, ARIZONA STATE UNIVERSITY OFFICE OF NAVAL RESEARCH Al Etch Mask Nitride Silicon Substrate RIE with optimized values of power and pressure and CF 4 and O 2 gas flow

Center for High Resolution Electron Microscopy CENTER FOR SOLID STATE SCIENCES, ARIZONA STATE UNIVERSITY OFFICE OF NAVAL RESEARCH

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) Sputtered Ions (P, B) Cs + Ion Gun Quadrupole Mass Analyzer Back Scattered Ions 13 kV -1 kV

Center for High Resolution Electron Microscopy CENTER FOR SOLID STATE SCIENCES, ARIZONA STATE UNIVERSITY OFFICE OF NAVAL RESEARCH MJD SIMS Analysis carried out using 14 keV Cs + primary ion source in the CAMECA IMS 3F equipment at ASU The Metallurgical Junction Depth (MJD) as determined from SIMS is 30 nm and 60 nm respectively for the two junctions

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

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 SIMS profile of a delta doped P sample measures in CAMEC IMF 3F The “knock-on” effect seen is quite significant Delta Layer

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 This System uses 3 keV Cs + primary ion and has post sputter acceleration system The SIMS profile shows a higher surface concentration and drops rapidly, which is typical of P junctions

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 Philips CM200 FEG TEM, ASU Lorentz lens Hologram Electrostatic Biprism Field Emission Gun CCD camera Object Wave Reference wave through vacuum USJ Sample Digital Hologram

Center for High Resolution Electron Microscopy CENTER FOR SOLID STATE SCIENCES, ARIZONA STATE UNIVERSITY OFFICE OF NAVAL RESEARCH Digital Hologram Reconstruction Digital Hologram Fourier Transform Inverse Fourier Transform Complex Image Phase Image Thickness Image

Center for High Resolution Electron Microscopy CENTER FOR SOLID STATE SCIENCES, ARIZONA STATE UNIVERSITY OFFICE OF NAVAL RESEARCH Reconstructed phase image of 30 nm MJD sample Bright region indicates presence of a junction n+n+ Nitride Vacuum 100 nm p Phase Images are converted to potential image by Where C E is the interaction constant Which depends on the acceleration voltage of the electrons, V 0 mean inner potential of Si 1-D Scan Cr from Sample Preparation

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 for 100% activation Conversion of 1D Potential Profiles to 1D Electric Field and Total Charge Distribution * Ref: The potential profile is simulated from the SIMS profile using a self- consistent Poisson Solver *

Center for High Resolution Electron Microscopy CENTER FOR SOLID STATE SCIENCES, ARIZONA STATE UNIVERSITY OFFICE OF NAVAL RESEARCH 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 Derived From Electron Holography

Center for High Resolution Electron Microscopy CENTER FOR SOLID STATE SCIENCES, ARIZONA STATE UNIVERSITY OFFICE OF NAVAL RESEARCH EJD ~ 27 nm Simulation of the Electric Field and Total Charge concentration from the SIMS profile using a Poisson Solver Simulated from SIMS data

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 200 nm 1D Scan Nitride

Center for High Resolution Electron Microscopy CENTER FOR SOLID STATE SCIENCES, ARIZONA STATE UNIVERSITY OFFICE OF NAVAL RESEARCH 1-D Potential profile for the 65 nm USJ from the from EH and Simulation of SIMS profile EJD 1-D Electric field and total charge from EH and Simulation gave an EJD value of ~60 nm

Center for High Resolution Electron Microscopy CENTER FOR SOLID STATE SCIENCES, ARIZONA STATE UNIVERSITY OFFICE OF NAVAL RESEARCH Two-Dimensional Analysis of the USJs Nitride Mask 100 nm n+n+ p Si ~ 30 nm ~ 5nm Vacuum Cr from TEM Sample Preparation The dark contour line is the halfway point of the total variation of the potential in the Space charge region 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 200 nm n+n+ p Si Nitride mask ~ 65 nm Vacuum ~ 5nm 2-D Potential Image from EH for the 65 nm MJD Sample 200 nm ~ 65 nm Nitride Si 2-D charge image (arbitrary units) 2-D Poisson Equation

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 100 nm Oxide p n+n+ ~ 50 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

Center for High Resolution Electron Microscopy CENTER FOR SOLID STATE SCIENCES, ARIZONA STATE UNIVERSITY OFFICE OF NAVAL RESEARCH Nitride Si Stress Simulation near the nitride mask edge in ATHENA Process Simulator Presence of high stress near the edge 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 Nitride Si Nitride Mask 100 nm n+n+ p Si Vacuum Cr from TEM Sample Preparation Oxide Si 100 nm Oxide p n+n+ ~ 50 nm

Center for High Resolution Electron Microscopy CENTER FOR SOLID STATE SCIENCES, ARIZONA STATE UNIVERSITY OFFICE OF NAVAL RESEARCH The LPCVD nitride is under high stress, it can relieve stress by generating Frenkel Pairs at the Si/Si 3 N 4 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 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

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 Resistance Microscopy can substantiate the observed anisotropy in diffusion

Center for High Resolution Electron Microscopy CENTER FOR SOLID STATE SCIENCES, ARIZONA STATE UNIVERSITY OFFICE OF NAVAL RESEARCH Silicon Al Oxide Questions or Comments ?