uttam singisetti

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



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



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



International Technology Roadmap for Semiconductors (ITRS) identifies the features for future generations

2003 ITRS Requirements for Ultra Shallow Junctions for source/drain extensions



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



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



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



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



Vertical Diffusion mask is critical for accurate 2-D profiling of USJs

Al Etch Mask

Si Substrate

RIE with CF4 gas only

Oxide



Al Etch Mask

Nitride

Silicon Substrate

RIE with optimized values of power and pressure and CF4 and O2 gas flow



Al Etch mask

Nitride edge



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



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



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



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



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



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



Digital Hologram Reconstruction

Digital Hologram Fourier Transform

Inverse FourierTransform

Complex Image

Phase ImageThickness Image

),(

),(ln2),(

yxA

yxAyxt

ref

holoin

)Re

Imarctan(),( yx



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



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*



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



EJD ~ 27 nm

Simulation of the Electric Field and Total Charge concentration from the SIMS profile using a Poisson Solver

Simulated from SIMS data



Similar 1-D analysis was carried out for the 65 nm USJ and USJ with oxide mask

p

n+ 200 nm

1D Scan

Nitride



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



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



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



2-D Analysis of the USJ with oxide diffusion mask

100 nm

Oxide

p

n+

~ 50 nm



• 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



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



Stress simulation for Si substrate under oxide mask shows an order of magnitude less stress than with a nitride mask

Oxide

Si



Nitride

SiNitride Mask

100 nm

n+

p

Si

Vacuum

Cr from TEM Sample

Preparation

Oxide

Si

100 nm

Oxide

p

n+

~ 50 nm



• 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



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



Silicon

Al

Oxide

Questions or Comments ?

uttam singisetti

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