Transcript Document

Fysische en Analytische Chemie
Prof. S. De Gendt and Prof. C. Vinckier
How Trace-Analytical Techniques Contribute to the Research and Development
of Si, Ge and III/V Semiconductor Devices – some examples
S Monolayer passivation on Ge
• Approach of chemisorption of (sub)monolayers as
preparation for high-k deposition to realize smooth interface
• Combination of Total Reflection X-Ray Fluorescence
(TXRF) and Reflection High Energy Diffraction (RHEED):
determination of bonding geometry of S on Ge
• Quantitative TXRF: accuracy within 10% if relative
sensitivity factors are determined
• Application for gas phase deposition H2S on Ge
Introduction
CMOS for sub-32 nm technology: high-k
dielectrics on high-mobility substrates
All CMOS Si processes need to be reengineered: cleaning, dopant activation,
passivation, dielectric deposition, etc…
Our work:
how trace analytical
techniques
total reflection
X-ray Fluorescence (TXRF)
• Conventional technique for coverage analysis of layers =
Rutherford Backscattering Spectroscopy
• Grazing incidence – XRF technique with high potential also
for light layers on heavy substrates (problem for RBS)
- combination of experimental recording angle dependent XRF curves and
theoretical modeling [4]: rather complex method
- demonstrated for ALD HfO2 on Si
• TXRF
6.E+14
Atomic Absorption
5.E+14
4.E+14
Spectrometry (AAS)
3.E+14
2.E+14
can assist
1.E+14
- labor intensive; reference method
• Application for Si layers on Ge
and
1 monolayer S on Ge
Wet etch + GFAAS
- fast; simple calibration
-Deviation TXRF from
GFAAS and GIXRF:
origin in inaccurate
calibration factor
GI-XRF
GF-AAS
TXRF
Epi growth rate on Si
3.5
Si thickness (nm)
S surface concentration
(/cm2)
7.E+14
Si passivation and HfO2
dielectric layers on Ge
3.0
2.5
2.0
- All techniques:
saturation curve: real
effect in growth
1.5
1.0
0.5
0.0
0.E+00
1
2
3
4
5
6
0
7
50
100
150
200
Deposition time (sec)
Process
Fig. 1: Optimization of S coverage on Ge wafers in a deposition
process from gaseous H2S, using Direct-TXRF analysis.
- Growth at start faster
than on Si: formation of
super islands?
Fig. 5: Coverage analysis of Si on Ge using different techniques: Direct
analysis using grazing incidence – XRF and TXRF, or via wet-chemical
etching in combination with GF-AAS. The growth rate of the epi process
on Si wafers is presented as a reference.
Contamination control
on bulk Ge and GaAs
As dopants in Ge
• Dopant concentration to be tuned to the application
• Conventional application of trace analytical techniques
• Determination As concentration is challenging application
- minor development: sensitivity same than for Si -> no additional calibration standards
- higher detection limits than on Si due to more background scattering
• Vapor Phase Decomposition – Droplet Collection (VPD-DC) pre concentration
- major development for each substrate: different wetting properties and matrix removal
- Applications in cleaning of Ge and GaAs substrates
Techniques
Analytical
- TXRF: Atomika 8300W,
WL excitation, 70% crit angle
- GFAAS: Perkin Elmer 4110ZL
- VPD-DC: WSPS, GeMeTec
Materials
1000
no clean
HF 0.5%
HF 2%
HF 5%
DL
Ge wafers
100
10
1
10000
Metal conc (E10 at/cm2)
2
Surface concentration (1e10 at/cm )
4
10
GaAs wafers
Before clean (0,0)
Before clean (12.5,0)
1000
Before clean (-12.5, 0)
Before clean (0,-12.5)
100
After clean (0,0)
After clean (12.5,0)
After clean (-12.5,0)
10
After clean (0,-12.5)
Detection limit
0.1
1
0.01
K
Ca
Ti
Cr
Fe
Ni
- Ge: Umicore, <100>, 100mm
- GaAs: in house deposited MOCVD
or CMK, <100>
Ca
Ti
Cr
Fe
Ni
Cu
Zn
Zn
Fig 2: Application of Direct- and VPD-DC-TXRF in cleaning experiments: (a) Cleaning of
controlled contaminated Ge wafers using dilute HF chemistries; measurement before clean: DirectTXRF, after clean: VPD-DC-TXRF; and (b) Cleaning of ‘real process’ GaAs wafers using
controlled oxidation using H2O2 based solutions and etching in HCl based solutions. (no bar
indicates a result < detection limit).
Processing
- Cleaning: immersion in statical chemical
bath, HPW overflow rinsing, N2 dry
- S-passivation: H2 bake, H2S deposition,
temp 80-330 deg C, ambient N2 or H2
- Epi Si deposition: ASM Epsilon 2000
reactor, pressure 5.3x103 Pa (40 torr), using
N2 as a carrier gas
- HfO2 deposition: ASM ALCVD™ Pulsar
2000 reactor, from HfCl4 and H2O precursors
at 300°C and 1 Torr
- Mass spectrometry (SIMS, ICPMS): interference 70Ge (21%) and 75As (100%)
- X-ray methods (ED-XRF): Interference GeK (10.98 keV) and AsK at (10.53 keV)
• Selected method: wet chemical etch + graphite furnace – AAS
- selective excitation atomic absorption lines, matrix removal in the ashing steps
- etching chemistry: ammonia/peroxide mixture
- optimization of etch volume: detection limits down to 31016 As/cm3 Ge
• Application on As dopants in GeOI substrates
Dopant concentration
(As/cm3)
• Direct-TXRF for Ge substrates
Sample 1
1.4E+18
Sample 2
1.2E+18
1.0E+18
8.0E+17
6.0E+17
4.0E+17
2.0E+17
0.0E+00
1
10
Etch volume (mL)
Fig. 4: Method optimization for As dopant determinations
in GeOI wafers: validation study for the use of low etch
volumes to improve detection limits with one order of
magnitude; comparison of results obtained in 1 mL versus
10 mL solution volume.
Conclusions
• Trace analytical techniques maintain role as work horse in contamination monitoring but more applications beyond this demonstrated
•Advantages TXRF
• Quantitative character with accuracies > 90%: showed beneficial for S passivation study
• Here demonstrated for thin layer analysis Si on Ge
• Other methods GI-XRF and GFAAS
•Demonstrated useful for validation purposes (see Si on Ge); GFAAS as technique in case strong analyte selectivity is required (see: As analysis in Ge)
• Recently also Inductively Coupled Plasma Mass Spectrometry available for applications in Micro- and Trace Analytical Chemistry