Transcript Nitride mask

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
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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
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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
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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
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Vertical Diffusion mask is critical for accurate 2-D profiling of USJs
RIE with CF4 gas only
Al Etch Mask
Oxide
Si Substrate
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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
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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
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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
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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
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Digital Hologram Reconstruction
Inverse
Fourier
Transform
Digital Hologram
Complex Image
Fourier Transform
 Aholo ( x, y ) 

t ( x, y )  2in ln 
 A ( x, y ) 
 ref

Im
 ( x, y )  arctan( )
Re
Thickness Image
Phase Image
Center for High Resolution Electron Microscopy
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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
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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
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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
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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
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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
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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
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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
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Questions or Comments ?
Al
Oxide
Silicon
Center for High Resolution Electron Microscopy
CENTER FOR SOLID STATE SCIENCES, ARIZONA STATE UNIVERSITY
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