uttam singisetti
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
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
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
Source Drain
Oxide
Junction DepthGate Overlap
Center for High Resolution Electron Microscopy
CENTER FOR SOLID STATE SCIENCES, ARIZONA STATE UNIVERSITY OFFICE OF NAVAL RESEARCH
ASU Nano-CMOS ProcessAim: 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 CF4 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 CF4 and O2 gas flow
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)
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
1017
1018
1019
1020
1021
1022
0 20 40 60 80 100
Diffused PhosphorousDiffused PhosphorousSubstrate Boron
Dop
ant C
once
ntr
atio
n (c
m-3
)
Depth from Si surface (nm)
MJD
SIMS Analysis carried out using 14 keV Cs+ primary ion sourcein 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
1017
1018
1019
1020
1021
0 20 40 60 80 100
Diffused PhosphorusSubstrate Boron
Do
pa
nt
Co
nc
entr
ati
on
(cm
-3)
Depth from Si surface (nm)
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 3FThe “knock-on” effect seen is quite significant
1017
1018
1019
1020
0 10 20 30 40 50 60 70
Phosphorus profile of a delta doped layer
Co
nce
ntr
atio
n (
cm-3
)
Depth from Si Surface (nm)
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
1017
1018
1019
1020
1021
1022
0 10 20 30 40 50 60 70
Diffused PSubstrate B
Co
nce
ntr
atio
n (
cm-3
)
Depth from the Si surface (nm)
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
vacuumUSJ 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 FourierTransform
Complex Image
Phase ImageThickness Image
),(
),(ln2),(
yxA
yxAyxt
ref
holoin
)Re
Imarctan(),( yx
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+
Nitride
Vacuum
100 nm
p
Phase Images are converted to potential image by
0),(
),(),( V
yxtC
yxyxV
E
Where CE is the interaction constant Which depends on the acceleration voltage of the electrons, V0 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
0
0.4
0.8
1.2
1.6
0 10 20 30 40 50 60
SimulationEH data
Po
ten
tial
(V
)
Distance from Si Surface(nm)
1-D Measured and Simulated Potential Profiles
Simulation for 100% activation
Conversion of 1D Potential Profiles to 1D Electric Field and Total Charge Distribution
dx
xdVxE
)()(
Si
x
dx
xVd
)()(
2
2
))()()()(()( xnxpxNxNqx AD * Ref:http://www.nd.edu/~gsnider
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
p
n+ 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
0
0.5
1
1.5
0 20 40 60 80 100 120 140
SimulationEH Data
Po
ten
tia
l (V
)
Depth from Si Surface (nm)
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+
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 nmn+
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 PoissonEquation
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+
~ 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
kT
HCC
fPAX
PAXPAX)(*
)0(*
)( exp
kT
HDD
fPAX
PAXPAX)(
)0()( exp
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
SiNitride Mask
100 nm
n+
p
Si
Vacuum
Cr from TEM Sample
Preparation
Oxide
Si
100 nm
Oxide
p
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/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
• 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 ?