Single Crystal Silicon Growth in Silicon-Indium
Solute under Lateral Diffusion Epitaxy (LDE)
Luke H. L. Yu
Department of Materials
Science and Engineering
Supervisor: Dr. Kitai
701* Seminar
October 30, 2012
Outline
• Introduction
• Objectives
• Lateral Diffusion Epitaxy
• Challenges
• Results
• Future work
• Summary
2
G. Dhanaraj, et al. Crystal Growth Technologies for
Silicon Photovoltaics, in Handbook of Crystal Growth,.
a) Czochralski Process b) Float zone growth
Current Technologies
3
1) High temperature process
2) Insufficient usage of raw material
3) Costly cutting and polishing processes
4) Kerf loss of material
W. Koch et al., Bulk Crystal Growth and Wafering for PV, 2003
Issues
4
• Invent a new method to manufacturing single crystalline silicon
• Reduce the overall process cost by reducing dicing process
• Photovoltaic substrate application
W. Koch et al., Bulk Crystal Growth and Wafering for PV, 2003
Objectives
5
Typical process flow for
manufacturing silicon wafers
Combine the crystal growth, silicon, and
flattening processes into one single step
+
Low Temperature Growth
Self-Supported Substrate
(Self separating)
Uniform Thickness
(Thick film growth)
+
Lateral Bulk Film Growth
by Liquid Phase Epitaxy
Approaches
6
Liquid Phase Epitaxy (LPE)
• A method to grow semiconductor crystal layers
from the melt on solid substrates
• The method is mainly used for the growth of
compound semiconductors
• Uniform and high quality layers can be produced
• Inexpensive process
Conventional LPE Disadvantages:
• Not suitable for film thickness considerably
smaller than µm level
• Limited lateral overgrowth potential
Innovent Technologieentwicklung, Jena et al.
Film Growth - LPE
7
Lateral Diffusion Epitaxy (LDE)
8
SiO2 Plate
Graphite Frame
Substrate
• A compound graphite slider boat with an oxidized silicon plate is
placed a set distance (gap) above the seed line on the substrate
• Indium is used for the growth solution and is capable of growing
p-type doped single-crystalline silicon
• The plate restricts vertical platelet growth and allows lateral
growth of the platelet
Purge gas Inlet
Exhaust Outlet Vacuum System Three zone tube furnace
Pushing rod Growth solution boat
Substrate slider boat
Equipments
From LPE to LDE
9
Substrate Preparation
Oxidation Patterning Seed line etching
SiO2/Si Si Mask/SiO2/Si
Final Substrate
Seed line
Variables
• Solvent selection
• Substrate selection
• Temperature region
• Cooling rate
• Seed line width
• Lateral growth gap
• Vacuum condition
• Purge gas
• Seed line orientation
• Wetting enhancement
10
Variables – Solvent selection
• Low melting point
• Large silicon solubility
• Stable with graphite crucible
• Strong wetting on seed line
• Easy to remove from substrate
• Low cost and low toxic
Bo Li, 2012, Lateral diffusion LPE growth of Sc-Si R.W. Olesinski et al., 1985, Alloy Phase Diagram 11
Variables – Temperature
Bo Li, 2012, Lateral diffusion LPE growth of Sc-Si
High growth Temp. Low growth Temp.
Pro • high growth rate
• low impurity level
• low vapour pressure
• steady doping level
Con
• increase container degrade rate
• post-growth strain
• non-steady doping level
• poor nucleation
• high impurity level
1. Increase temperature to 950oC to dissolve
silicon atoms
2. Hold temperature to reach super saturation
3. Decrease temperature from 950 to 850oC
with 0.25oC / min rate
4. Natural cool to room temperature
12
Variables – Substrate
• Back contact & mechanical support
• Lattice constant match
• Similar thermal expansion coefficient
• lower grade of sc-Si
• (111) orientation
• n-type
• Plate materials: SiO2
• Growth inhibitor
• non stick to grown film
13
Grown silicon
Grown silicon
Seed line Silicon Substrate
Seed line
Silicon Substrate
Bo Li,2012, Lateral diffusion LPE growth of Sc-Si
Preliminary Results
14
15
• Limited epitaxy lateral overgrowth (ELO)
Challenges
• Ledges, kinks formation
•Wetting problem & strong etching back effect
Improvements
16
Methods Effects
Placement of Silicon oxide plate
between substrate and source •Prevent vertical overgrowth
• Improve surface smoothness
Reduction of the gap between
substrate and plate (0.5mm to 0.25mm)
• Improve the aspect ration
(width/height)
Reposition the seed line
(Downward-oriented seed line)
• Reduce gravity effect (indium is
denser than silicon)
•Uniform deposition
Thin silicon film deposition on
substrate /plate
• Enhance surface wetting
• Improve diffusion mechanism
Contribution from density effect
17
Facts
at room temperature…
• ρIndium = 7.31 g.cm–3
• ρsilicon = 2.33 g.cm–3
Findings
• Under isothermal condition, dissolution of silicon
is higher on the lower substrate, while the growth
is larger on the upper substrate
• Contributed by the density difference, and the
transport phenomena of solute by diffusion and
buoyant-driven convection
Seed line oriented downward • Reposition the seed line on the
upper plate
• Utilizing the lower substrate for
enhance lateral growth
• The lateral growth gap is 0.5 mm
SiO2 Plate
Graphite Frame
Substrate
Seed line oriented upward Seed line oriented downward 18
Results: plain view
19
Aspect Ratio…
20
• To identify the width over thickness relationship
• Higher the aspect ratio, the stronger lateral
overgrowth
• Aspect Ratio: Half Width/Thickness
• Thickness correction factor: 1.943
0
20
40
60
0 200 400 600 800 1000
Hei
gh
t (μ
m)
Width (μm)
Cross Section Profile
Grown silicon
Seed line
Silicon Substrate
Aspect Ratio Continue…
21
0
20
40
60
80
100
0 200 400 600 800
He
igh
t (μ
m)
Width (μm)
2D Cross Section Profile
3D cross section profile
Average aspect ratio: 3.144
Average thickness: 80 μm
Average width: 503 μm
Aspect Ratio Continue…
22
Maximum aspect ratio: 32.051
0
20
40
60
80
100
0 500 1000
He
igh
t(μ
m)
Width (μm)
Cross Section Profile
Growth Mode
• Island growth
• Step flow growth due to off cut
silicon substrate ( 3 o)
• Ideally, layer by layer growth
could provide premium quality 23
Diffraction pattern
24 Bo Li et al. 2012, Single Crystalline Si Substrate Growth by Lateral Diffusion Epitaxy, unpublished manuscript
• 2θ scan from 25o to 70o
• 2θ of Si(111) = 28.45o
• Rocking curve for the Si strip with (111)
orientation
• Peak at 12.4o
• Full width at half max (FWHM) of 0.03o
Summary
• Lateral Diffusion Epitaxial growth of single crystalline Si at temperature
ranging from 950 to 850 °C are conducted on (111) sc-Si substrate
• Aspect ratio (width/height) is used to analyze the epitaxial layer
overgrowth (ELO)
• The average aspect ratio is 3.144, with an average width of 500 μm
• Maximum aspect ratio of 32.051 was obtained by downward oriented
seed line setup, and the width is over 1000 μm
Future work
• Electric property test on carrier mobility, resistivity, doping level
• Peel off test and SOP for peel off test
• SEM and XRD on peel off sample
25
Questions
Publications
PCT patent application: Semiconductor formation by lateral diffusion liquid phase
epitaxy, Application No. PCT/CA2012/050327, filed: May 17, 2012, A. H. Kitai,
B. Li, H. L. Yu
Luke H. L. Yu, Bo Li, Huaxiang Shen, Adrian H. Kitai. “Lateral Diffusion Epitaxy (LDE) of
Single Crystal Silicon with Downward Facing Substrate” Photonic North 12: proceedings of
Photonic North 2012, 8412-53, Montreal: SPIE, June 2012
B. Li, H. L. Yu, H. Shen and A. H. Kitai, Single Crystalline Si Substrate Growth
by Lateral Diffusion Epitaxy, Journal of Crystal Growth (submitted, June 2012)
Acknowledgements
This work is supported by the NSERC. I would like to thank Dr. Kitai for
giving me the opportunity to work with this project, and also Dr. Li and
Huaxiang Shen’s valuable assistance and suggestions to carry out the
project.
26
Extra slides
• Variables – Solvent, why not Al?
• Variables – Substrate, why (111) Si?
• Variables – Growth impeding gap
• Variables – Wetting enhancement
• LDE growth roadmap
• Lift off Process
• Deposition rate
• Dissolution of Si in In – Density effect
27
Variables – Solvent, why not Al?
• High doping concentration
• Band gap narrowing
• Hole-hole interaction
• High melting point (660° C)
28
Variables – Substrate, why (111) Si?
• Lattice match
• Heterogeneous nucleation on the seed line
• Non-silicon surface present a more difficult barrier to nucleation
• Constraint growth sites on the substrate
• Avoid deposition on the crucibles or free surface in the melt
• The dominant growth mechanism can be deduced from the
dependence of the growth rate (υ) on the super cooling (δT)
υ = B0 * δTm * exp(-B1/ δT) * [1-exp(-B2 * δT)]
where exponent m and values of Bi are characteristics of the surface-
related crystal growth mechanism (Brice, 1973)
Peter Capper and Michael Mauk, Liquid Phase Epitaxy of Electronics, Optical and
Optoelectronc Materials, 2007 29
Variables – Growth impeding gap
Methods Effects
Reduction of the gap between
substrate and plate (0.5mm to 0.25mm)
• Improve the aspect ration
(width/height)
LDE strip growth with 0.5mm gap LDE strip growth with 0.25mm gap
Problems
• Diffusion limited and indium segregation on substrate
• Heavy etching back to seed line (trench forming)
30
Variables – Wetting enhancement
Methods Effects
Thin silicon film deposition on
substrate /plate
• Enhance surface wetting
• Improve diffusion mechanism
Plate (Downward-oriented seed line)
n-type (111) silicon substrate
SiO2 Mask SiO2 Mask
Polycrystalline Silicon Thin Film
Silicon Oxide Mask
Silicon substrate
• A thin layer between the lateral
growth gap was deposited by electron
beam evaporation technique
• The polycrystalline silicon film is in
range from 25 to 50nm
Problems
• Multiple nucleation cites available
• Heavy etching back effect (surface
indium atoms undercuts the seed line)
• Impurity level increase
31
LDE growth roadmap
Growth Type
Quarter (mm) Plate
Half (mm) Plate
Pre-wetting
Down
TF
HP-PW-D-TF
NTF
HP-PW-D-NTF
Up
TF
HP-PW-U-TF
NTF
HP-PW-U-NTF
No Pre-wetting
Down
TF
HP-NPW-D-TF
NTF
HP-NPW-D-NTF
Up
TF
HP-NPW-U-TF
NTF
HP-NPW-U-NTF
Lateral Diffusion Plate
Wetting Process
Seed line Position
Surface Wetting Enhancement
with Thin Film of Silicon
32
Lift off Process
Mylar film
Cover glass
Mylar film
33
Deposition rate
Dependence of growth rate with cooling rate • Cooling rate: 0.25oC/min
• Growth rate estimate: 0.3 µm/min
• True average growth rate:
~ 0.2µm/min
• Lateral average growth rate :
~ 0.42µm/min
William C. O’Mara et al. Handbook of semiconductor silicon technology, page 271 34
Dissolution of Si in In – Density effect
A U¨mit Cos kun et al. Simulation of dissolution of silicon in
indium melt solution by spectral methods, 2002 35
Selective region growth
36
• Growth front different Increase of number of nucleation sites per unit length • Edge effect on the platelets • silicon atom distribution was not uniform
Growth on larger substrate Growth on smaller substrate