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