cavitation erosion thesis

7/29/2019 cavitation erosion thesis

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The Pitfalls of Pit Contacts:

Electroless Metallization for c-Si Solar Cells

by

Kate Fisher

B.E, Photovoltaics and Solar Energy, 2004


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Originality Statement

I hereby declare that this submission is my own work and to the best of my knowledge it

contains no materials previously published or written by another person, nor material which to

a substantial extent has been accepted for the award of any other degree or diploma at UNSW or

any other educational institution, except where due acknowledgement is made in the thesis. Any

contribution made to the research by others, with whom I have worked at UNSW or elsewhere,

is explicitly acknowledged in the thesis.

I also declare that the intellectual content of this thesis is the product of my own work,

except to the extent that assistance from others in the projects design and conception or in style,

presentation and linguistic expression is acknowledged.

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Abstract

This thesis focuses on improving the adhesion of electroless metal layers plated to pit

contacts in interdigitated, backside buried contact (IBBC) solar cells. In an electrolessly plated,

pit contact IBBC cell, the contact grooves are replaced with lines of pits which are intercon-

nected by the plated metal. It is shown, however, that electroless metal layers, plated by the

standard IBBC plating sequence, are not adherent on pit contact IBBC solar cells. The cause

of this adhesion problem is investigated by examining the adhesive properties of each of the

metal layers in the electroless metallization sequence on planar test structures. This investiga-

tion reveals that Pd activation of heavily P diffused Si impedes Ni silicide growth and that, in

the absence of a silicide at the Ni/Si interface, an electrolessly plated Cu layer will cause the un-

derlying Ni layer to peel away from the substrate. It is also found that the Ni silicidation process

itself intermittently causes the unreacted Ni to spontaneously peel away from the substrate. An

electroless metallization sequence that results in thick, adhesive Cu deposits on planar

surfaces is developed in this thesis. It is shown that this process leads to the formation of a Ni

silicide on both n- and p- type, heavily diffused surfaces. Fully plated, pit contact IBBC solar

cells were not able to be fabricated during the course of this work but it is reasonable to expect

that the modified plating sequence developed in this work will result in the metal layers being

adhesive on these cells.


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Dedication

...to a world with fewer problems...


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Acknowledgements

Firstly, I would like to thank my supervisor, Dr Jeffery Cotter for providing me with the

opportunity to carry out this research. I would not have been able to undertake it without the

financial support of employment in the same field (and physical space) as my research. My

understanding of solar cells and skills in the laboratory have benefited from this employment

opportunity and your guidance as a boss and a supervisor has helped very much in realising this

thesis.

Thanks also goes to Dr Tom Puzzer for training and enlightening discussions on electron

microscopy and willingness to divulge an astonishing wealth of information about analytical

measurement techniques. Particular thanks also to Jenny Norman and other staff at the electron

microscope unit for helping me wrestle with the Cambridge (it was a beast but Im still sad to

see it go).

Special thanks to Stuart Wenham for being so forthcoming with processing tips and tricks

for buried contact solar cells. Thanks also for the historical information.

I would also like to thank Malcom Abbott for his fine proof reading skills and ability to

utilise them to great effect at very short notice. Thanks to Florence Chen and Jeff also on this

front.

I would like to acknowledge the love, support and not inconsiderable cooking skills of

my partner Sami.

Parents, as usual, thanks.


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List of Abbreviations

AED Autocatalytic Electroless Deposition

BGD Boron Groove Diffusion

BPSG Boro-Phospho-Silicate Glass

BSF Back Surface Field

BSG Boro-Silicate Glass

CMOS Complementary Metal Oxide Semiconductor

c-Si Crystalline silicon

CZ Czochralski

DSBC Double-Sided Buried Contact

EB Electron Beam

EDS Energy Dispersive Spectroscopy

EDTA Ethyl-Diamine-Tetraacetic Acid

EWT Emitter Wrap Through

FZ Float zoned

IBBC Interdigitated Backside Buried Contact

IC Intergrated Circuit

PCD Photoconductance Decay

PERL Passivated Rear Locally Diffused

PGD Phosphorus Groove Diffusion

PSG Phospho-Silicate Glass


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RIE Reactive Ion Etching

RTP Rapid Thermal Processing

SEI Secondary Electron Image

SEM Scanning Electron Microscope

SPM Sulfuric Peroxide Mixture

SSBC Single-Sided Buried Contact

TCA Trichloroethane

UNSW University of New South Wales

Voc Open Circuit Voltage


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Contents

Chapter

1 Introduction 1

1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 Thesis Statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.3 Thesis Contributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.4 Thesis Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2 Literature Review: Electroless Metallization 7

2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.2 Electroless Plating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.2.1 Palladium Immersion Activation . . . . . . . . . . . . . . . . . . . . . 9

2.2.2 Autocatalytic Electroless Depositions . . . . . . . . . . . . . . . . . . 11

2.3 Ni Silicidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2.3.1 Silicidation of Pure Ni Deposits . . . . . . . . . . . . . . . . . . . . . 15

2.3.2 Silicidation of AEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

2.4 Electroless Metallization of Silicon Solar Cells . . . . . . . . . . . . . . . . . 20

2.4.1 Planar AED Contacts . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

2.4.2 Buried AED Contacts . . . . . . . . . . . . . . . . . . . . . . . . . . 24

2.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27


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3 Research Methodology 28

3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

3.2 Background Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

3.2.1 Buried Contact Solar Cell Structures . . . . . . . . . . . . . . . . . . . 28

3.2.2 Plating Buried Contact Solar Cells . . . . . . . . . . . . . . . . . . . . 31

3.3 Experimental Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

3.3.1 Electroless Plating . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

3.3.2 Ni Sintering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

3.3.3 Ni Cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

3.3.4 Pd Cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

3.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

4 Experimental Results and Discussion 43

4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

4.2 Preliminary Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

4.2.1 Pits and Grooves: a Structural Comparison . . . . . . . . . . . . . . . 44

4.2.2 Pits and Grooves: a Photoconductance Comparison . . . . . . . . . . . 45

4.2.3 Metal Adhesion Problems of Pit Contacts . . . . . . . . . . . . . . . . 47

4.3 Metallization of Pit Contacts . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

4.3.1 Ni Plated Pits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

4.3.2 Ni Sintered Pits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

4.3.3 Pd Activated Pits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

4.3.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

4.4 Ni Silicidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

4.4.1 Effect of Surface Activation . . . . . . . . . . . . . . . . . . . . . . . 59

4.4.2 Effect of Pd Sintering . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

4.4.3 Effect of Ni Plating Temperature . . . . . . . . . . . . . . . . . . . . . 64

4.4.4 Unreacted Ni . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66


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4.4.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

4.5 Cu Plated Layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

4.5.1 Without Ni Silicide . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

4.5.2 With Ni Silicide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

4.5.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

4.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

5 Conclusion and Future Work 75

Appendix

A SEM and EDS 79

Bibliography 81


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List of Tables

Table

2.1 Summary of Ni silicide phase transitions and sheet resistivity results from vari-

ous authors. All substrates are (100), c-Si . . . . . . . . . . . . . . . . . . . . 17

2.2 Phenomena observed for AEDs of Ni plated to (100) substrates and sintered at

various temperatures. Deposition 1 refers to 30-40 nm of NiP plated on EB-

Ni seed layers on n-type substrates [46], Deposition 2 refers to 30-40 nm of

NiP plated to Pd activated, n-type substrates [46] and Deposition 3 refers to

800-3000 nm of NiP plated directly onto p-type 1-10cm substrates [57]. . . . 19

2.3 Summary of processing parameters of various c-Si solar cells with planar, pho-

tolithographically defined, electrolessly plated contacts. . . . . . . . . . . . . . 23

2.4 Summary of processing parameters of various c-Si solar cells with buried, elec-

trolessly plated contacts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

3.1 IBBC solar cells processing sequence prior to metallization. . . . . . . . . . . 30

3.2 Silicidation test structure types. . . . . . . . . . . . . . . . . . . . . . . . . . . 34

3.3 Bath constituents of an ammonia-based, proprietary electroless Ni plating solu-

tion from Transene [22]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

3.4 Bath constituents of a proprietary electroless Cu plating solution from Enthone

[64]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

4.1 Summary of results from Section 4.3 . . . . . . . . . . . . . . . . . . . . . . . 59

4.2 Summary of the experiment. The results are shown in Figure 4.16. . . . . . . . 60


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4.3 Sheet resistivity variation with Ni sintering temperature. Measurements were

taken before the plating sequence (ie: this is the sheet resistivity of the phos-

phorus diffusion), and after the unreacted Ni had been removed (ie: this is the

resistivity of the phosphorus diffusion and the Ni silicide in parallel). . . . . . . 61

4.4 Pd sintering on boron diffused test structures. . . . . . . . . . . . . . . . . . . 64

4.5 Summary of results from Sections 4.4.1 and 4.4.3 . . . . . . . . . . . . . . . . 68

4.6 Ni plating and sintering parameters for three SSBC PGD test structures with

photolithographically defined contacts. . . . . . . . . . . . . . . . . . . . . . 71

4.7 Sheet resistivity measurements of the various layers formed on photolithograph-

ically defined contacts. Sample 2 was sintered at 300C and Sample 3 was

sintered at 400C. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72


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List of Figures

Figure

2.1 Schematic of the electroless plating sequence for IBBC solar cells. . . . . . . . 8

2.2 Adhesion of an AED of Ni plated to variously doped substrates, activated with

various Pd solutions. The Pd layer was sintered for 30 minutes at 200C prior

to the Ni plate. Adhesion as a function of (a) surface concentration and dopant

type for various Pd activation solutions and (b) Pd solution type for various Ni

plating times and temperatures. The substrates here are 4 .cm, p-type (after

[42]). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2.3 Adhesion of an AED of Ni plated to variously doped substrates, with and with-

out Pd activation. The activation solution is NH4F-based and the Pd deposit was

sintered for 30 minutes at 200C prior to the Ni plate. The surface concentra-

tion of the n-type substrates was 4-8 x 1015/cm3 and for p-type substrates, 1-2

x 1016/cm3. P+ denotes a boron diffused surface and N+ denotes a phosphorus

diffused surface both with surface concentrations >1020/cm3. All substrates

are (111). The graphs show adhesion of a 2 minute, AED of Ni plated from an

alkaline, hypophosphite bath at 90-95C onto (a) n-type surfaces and (b) p-type

surfaces (after [41]). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2.4 A summary of the most commonly reported phases of Ni silicide and their tem-

peratures of formation, [10], [16], [18], [28], [38], [39], [55], [58], [60], [61].

In general, x1 < x2 < x3 and the Si rich phase, formed at higher temperatures,

tends to agglomerate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17


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2.5 Schematic of various electroless plated contacts. Dotted lines indicate diffused

Si, light grey areas indicate the dielectric and dark grey areas indicate the plated

metal. Photolithographically defined contacts with a planar homogeneous emit-

ter, a planar selective emitter, and a buried selective emitter are shown in a), b),

and c), respectively. A scribed contact with a selective emitter is shown in d). . 22

3.1 Schematic of a SSBC solar cell showing 1) random pyramid textured, front

surface, diffused with 100 /sq. phosphorus emitter and passivated with SiO2,

2) laser scribed grooves, diffused to < 5 /sq with phosphorus, 3) random

pyramid textured rear surface with an aluminium back surface field, 4) Ni and

Cu plating in the front surface grid and 5) Ni and Cu plated over the entire rear

surface. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

3.2 Schematic of a IBBC solar cell showing 1) random pyramid textured, front sur-

face, diffused to 100 /sq. with phosphorus and passivated with SiO2, 2) saw

damage etched, rear surface, diffused with 100 /sq. boron emitter and passi-

vated with SiO2, 3) laser scribed grooves, diffused to< 5/sq with phosphorus,

4) laser scribed grooves, diffused to < 10 /sq. with boron and 5) Pd, Ni and

Cu plating in both rear grids. . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

3.3 Schemaitic of an elephant. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

3.4 The effects of O2 on electroless plated Ni when heated. The samples have

been Ni plated and (a) no sintered, (b) sintered at 200C without the use of an

elephant, (c) sintered at 400

C without the use of an elephant, (d) sintered at

400C without the use of an elephant and with the furnace N2 turned down. . . 38

3.5 Temperature profile of furnace used for Ni silicide formation . . . . . . . . . . 38

3.6 Three sections of a boron diffused sample that has been Ni plated. Section a)

shows the Ni plate intact, b) shows the Ni plate removed with RCA2 and c)

shows the Ni plate removed with SPM. . . . . . . . . . . . . . . . . . . . . . . 40


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3.7 A boron diffused wafer a) after Pd activation and b) the same wafer after clean-

ing. The left side of b) has been swabbed with a cotton bud soaked in IPA and

the right side has been SPM cleaned and then swabbed with a cotton bud soaked

in IPA. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

4.1 Ni and Cu plated 50 m deep groove on an IBBC solar cell showing the void at

the centre of the trench and 50 m of overplating. . . . . . . . . . . . . . . . . 46

4.2 SEM micrograph of (a) the plane view and b) the cross section of etched pits.

The pits were scribed with a laser wavelength of 1064 nm and hence end up

being 20 m across. The pits are spaced 50 m apart and hence the adjacent

pits are out of the frame of this image. Note the oxide overhang in the plane

view and the Si overhang in the cross section. The border with rounded corners

in the plane view corresponds to the Si overhang and there is no oxide overhang

in the cross section because the oxide was thinner to begin with and hence got

etched back to the edges. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

4.3 Injection-level dependence lifetime curves of the various contact types. . . . . . 48

4.4 Saturation current contribution from the diffused regions as a function of contact

type. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

4.5 SEM micrograph of poorly adhesive electroless Ni and Cu plated on the pits

of several fingers of an IBBC cell. The first and third fingers are phosphorus

diffused and the second and fourth fingers are boron diffused. . . . . . . . . . . 49

4.6 a) A busbar on a pit IBBC cell showing how the Cu plating has fallen out of

the pits and b) a close-up of the top left hand corner of the same busbar. The

numbers refer to the pit to the right of the number. . . . . . . . . . . . . . . . . 50

4.7 A finger on a pit IBBC cell showing the Cu plate detaching from the Ni plate in

the pit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

4.8 SEIs and EDS maps of the pits labeled Pit 1, Pit 2 and Pit 3 in Figure 4.6 b). . . 51


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4.9 An example of badly deglazed a) phosphorus and b) a boron diffused pits, plated

with electroless Ni. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

4.10 An example of well deglazed a) phosphorus and b) a boron diffused pits, plated

with electroless Ni. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

4.11 Pit contacts that have a) had no plating at all and b) been Ni plated at 90-95C

and the Ni removed in a standard RCA2 etch (no sintering). . . . . . . . . . . . 56

4.12 An a) phosphorus diffused and b) a boron diffused pit that has been Ni plated at

90-95C after sintering at 350C and removal of the excess Ni in RCA2 solution. 56

4.13 Ni plated onto the field oxide. The bottom left pit is plated with Cu but all other

pits and the areas between them were determined to be Ni by EDS mapping. . . 58

4.14 A well deglazed, Pd activated a) phosphorus and b) boron diffused pit, with Pd

activation and electroless Ni plating at 80C. . . . . . . . . . . . . . . . . . . . 58

4.15 The Pd activated a) phosphorus and b) boron diffused pits after sintering and

RCA2 to remove the unreacted Ni. . . . . . . . . . . . . . . . . . . . . . . . . 58

4.16 P-type substrates with SSBC PGD of 3

/sq. across the whole surface. The

samples had different Pd treatments prior to electroless Ni plating at 80C for

4 minutes. The samples were sintered for 10 minutes at the temperatures indi-

cated and the unreacted Ni removed in a 50% SPM. . . . . . . . . . . . . . . . 62

4.17 Boron groove diffused samples that have been Pd activated and the Pd sintered

for a) 10 minutes at 250C and b) 40 minutes at 200C. Both samples then

underwent SPM cleaning prior to Ni plating and at 300C. Finally, the unreacted

Ni was removed in SPM. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

4.18 Silicide formation on an a) IBBC phosphorus and b) boron test structure that

was Pd activated and the Pd sintered at 250C for 20 minutes. . . . . . . . . . 65

4.19 The three different test structures with three different surface treatments prior

to electroless Ni plating at 90-92C for 4 minutes. The samples were sintered

at 350C for 5 minutes and the unreacted Ni removed in a SPM. . . . . . . . . 66


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4.20 IBBC phosphorus diffused test structures with three different surface treatments

prior to electroless Ni plating at 90-92C for 4 minutes. The samples were

sintered at 300C for 5 minutes and the unreacted Ni removed in a SPM. . . . . 66

4.21 A photograph of Ni flaking off a test structure after sintering at 300C for 7

minutes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

4.22 Cu plated layers on phosphorus diffused IBBC test structures that were first

plated with Ni at a) 80C and b) 90C. . . . . . . . . . . . . . . . . . . . . . . 69

4.23 Cross sectional schematic of the photolithographically defined pattern used in

Section 4.5.2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

4.24 Plan view of two SSBC PGD test structures that were photolithographically

patterned and metallized. After the initial Ni deposition, both samples were

sintered at 300C for 10 minutes and the unreacted Ni was a) left on and b)

removed in an SPM before further processing. . . . . . . . . . . . . . . . . . . 71

A.1 X-ray spectrum generated on a Ni plated pit at low magnification (black spec-

trum) and at high magnification (grey spectrum). . . . . . . . . . . . . . . . . 80

A.2 X-ray maps of a Ni plated pit; a) the SEI image of the mapped area, b) the Si

signal, c) the Ni signal and d) the phosphorus signal. . . . . . . . . . . . . . . 80

A.3 X-ray maps of a pit where the Ni has been sintered and the unreacted Ni re-

moved with RCA2; a) the SEI image of the mapped area, b) the Si signal, c) the

Ni signal and d) the phosphorus signal. . . . . . . . . . . . . . . . . . . . . . . 80


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Chapter 1

Introduction

1.1 Motivation

Metallization is an important area of solar cell research because energy is extracted from

a solar cell through its metal contacts. A good metallization scheme is one that provides low

recombination, has low contact and series resistance and incurs low shading losses. The metal

contacts must also adhere well to the surface of the cell. Ideally, they will be easy to manufac-

ture, enable high through-put and incur minimal manufacturing costs.

Many different metallization schemes are employed in photovoltaic devices. Vacuum

evaporated, Ti-Pd-Ag contacts are utilised in high efficiency laboratory devices such as PERL

cells [74]. Many cells also use vacuum evaporated or sputtered Al contacts (see [3] for exam-

ple). Vacuum evaporation yields high purity, low resistivity contacts but they are expensive to

manufacture and are not suitable for use in high through-put production lines.

The majority of photovoltaic devices in the market today use screen printed contacts.

This is because the high throughput capabilities of this technique make it highly suitable for

use in commercial production lines. Screen printed contacts are applied to the cell surface by

forcing a metal paste through an emulsion screen using a squeegee. Screen printed contacts

incur substantial shading losses and have high bulk resistivity [31].

This thesis looks in detail at another metallization technique: electroless plating. In

order to electrolessly plate metal to a solar cell, the Si must be exposed in the contact areas and


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masked everywhere else. An electroless deposition of Ni is then plated to the contact areas and

sintered in a furnace to form a low resistance Ni silicide at the contact interface. Another layer

of Ni is then plated to the first layer as a strike. This ensures that the subsequent Cu plate, which

comprises the bulk of the metal contact, will initiate on the Ni surface. Cu is used as the bulk

material because of its low bulk resistivity. A thin layer of Ag is often plated over the Cu to

better facilitate soldering.

Electrolessly plated contacts have many advantages compared to screen printed contacts.

The maximum processing temperature is 400C whereas screen printed contacts are typically

fired at 800

C. Low temperature processing reduces the thermal budget and the probability

of contamination. Screen printed pastes incorporate solvents and frits [31] that result in printed

contacts having much higher series resistance than plated contacts. Electroless metallization

schemes utilise Ni silicide at the contact interface, which has the advantage of providing low

contact resistance [28]. Shading losses from screen printed contacts are also high because the

minimum line width is 120 m [72]. Electroless plated contacts can be much narrower be-

cause they are defined photolithographically [17], [69], or defined by laser or mechanical scrib-

ing [3], [70]. The main disadvantage of electroless metallization, compared to screen printing,

is that there are more steps in the processing sequence.

One example of photovoltaic devices that utilise electroless metallization schemes are

the buried contact solar cells. Buried contacts (BCs) are photovoltaic devices in which con-

tact is made to the cell by applying metal into grooves scribed into the wafer surface either

mechanically or with a laser. Burying the metal contact areas vertically inside the substrate

reduces the opaque area on the front surface, which reduces shading losses. The grooves are

scribed through the surface dielectric, which acts as a mask for a heavy diffusion in the contact

areas. This ensures low contact resistance between the metal and the semiconductor [66]. It

also means that the doping level in the emitter can be as low as 100 /sq, which results in good

utilisation of the solar spectrum [70].

There are three buried contact solar cell structures - the single sided buried contact


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3

(SSBC), the double sided buried contact (DSBC) and the interdigitated backside buried con-

tact (IBBC). SSBCs comprise a phosphorus emitter on a p-type substrate with emitter contact

grooves scribed through the passivating dielectric [70]. The base contact is realised with a thin

Al alloyed region over the entire rear surface. This results in voltage limitations for thin cells

with long diffusion lengths, because the rear surface recombination velocity high [32]. The

DSBC seeks to overcome this limitation by replacing the Al rear contact with a well-passivated,

light phosphorus diffusion on the rear surface [52]. This diffusion acts as a floating junction,

which has been shown to provide excellent surface passivation [32]. The base contact is re-

alised with a heavily, boron diffused, buried contact grid scribed through the floating junction.

It was found, however that diffusion induced, boron misfit dislocations, formed during this

heavy diffusion, severely limit the performance of these cells because they cause asymmetric

Schottky-Read-Hall (asymmetric-SRH) recombination in the bulk [15]. The IBBC solar cell

comprises an n-type base, lightly diffused boron emitter, lightly diffused phosphorus front sur-

face field and both heavy boron and phosphorus contact diffusions in an interdigitated pattern

on the rear surface [29]. N-type, IBBCs are not affected by asymmetric-SRH recombination

in the way that p-type DSBCs are, because the capture-cross section of holes is much lower

than that of electrons (see [13], for example). IBBCs also incur no shading losses because the

contacts are on the rear surface and efficiencies approaching 20% on 180 m thick, 1 cm FZ

wafers have been demonstrated with this cell structure [29].

A common way to increase the performance of high efficiency, c-Si solar cells is to

reduce the surface area of the heavily diffused contact regions by using points of contact instead

of lines or whole surfaces of contact [65], [74]. Point contact IBBC structures could potentially

be made for no extra cost, simply by changing the laser scribing code. Instead of creating a

groove in the wafer surface, the new code would create a line of pits, which would need to

be electrically connected by the metallization scheme. Cells with this type of grid design are

called pit contact IBBC solar cells. Realisation of pit contact IBBC solar cells by electroless

metallization is the subject of this thesis.


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1.2 Thesis Statement

The aim of this thesis is to develop a reliable electroless metallization scheme that en-

ables maximal current extraction from pit contact, IBBC solar cells. A reliable process is one in

which the same outcome is achieved for the same processing parameters, every time the process

is run. Developing a reliable process generally requires the effects of the first and second order

parameters of the process to be known and controllable.

The electroless metallization scheme of the IBBC solar cell is comprised of a sequence

of distinct processes. The most important ones are Ni plating of Si, Ni silicide formation and

Cu plating of Ni. It is necessary to understand the main parameters of each one in order to

gain control over the process as a whole. To achieve the main goal of producing an electrically

operational, pit contact, IBBC solar cell, four intermediate goals were identified:

1. To determine the main parameters affecting electroless Ni plating of heavily phospho-

rus and boron diffused silicon.

2. To develop a reliable method for simultaneously plating electroless Ni to heavily phos-

phorus and heavily boron diffused Si.

3. To develop a process that allows a silicide to form, from a layer of electrolessly plated

Ni, simultaneously on heavily phosphorus and heavily boron diffused Si.

4. To develop a method of electrolessly plating thick layers (> 50 m) of Cu to a heavily

diffused Si/silicide/Ni region.

1.3 Thesis Contributions

During the course of this thesis, the main parameters of each of the processes of the

IBBC, electroless metallization scheme and their interdependency were identified experimen-

tally. The main contributions of this work can be summarised as follows:


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5

Bath activation greatly affects the initiation of autocatalytic electroless depositions of

both Ni and Cu.

Ni plating bath temperature greatly affects silicide formation.

Pd activation of heavily phosphorus diffused (100) Si impedes silicide formation for

furnace sintering temperatures up to 450oC and sintering times of less than 10 minutes.

Incomplete silicidation can cause the unreacted Ni to peel away from the underlying

silicide.

Plating thick Cu layers to Ni plated layers on planar Si substrates causes adhesion fail-

ure at the Ni/Si interface unless the Ni layer is bonded to the substrate by silicidation.

An electroless plating sequence was developed that allowed adhesive layers of thick

Cu to be plated to planar, (100) test structures.

In addition to these findings, several processing techniques were developed during this

work to allow the parameters of each of the processes to be examined. Inclusion of the groove

diffusion dummy wafers into the metallization processing sequence, not only greatly improved

the platability of the samples, but allowed for a macroscopic view of the silicidation process.

It was shown that Ni silicide formation could be examined on planar dummy wafers by plating

and sintering a Ni layer and then removing the unreacted Ni in a selective wet chemical etch.

This allowed for a macroscopic view of the silicidation process and proved to be a much more

efficient way of examining problems with the metallization process than manufacturing batches

of cells.

The main objective of producing an operational, pit contact, IBBC solar cell was not

achieved during the course of this work. This thesis shows, however, that such a device is

unlikely to ever be realised by electroless metallization.


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1.4 Thesis Outline

This thesis has three main chapters. Chapter 2 presents a review of published work per-

taining to the electroless metallization of Si solar cells. Different types of electroless plating

are reviewed in Section 2.2, the silicidation of Ni is reviewed in Section 2.3 and a discussion

of the application of these is presented in Section 2.4. Chapter 3 describes the experimental

method used in this thesis. A review of the contact structures used in UNSW buried contact so-

lar cells is presented in Section 3.2 and the electroless plating, Ni sintering and metal cleaning

techniques used in this work are presented in Section 3.3. Chapter 4 describes and discusses

the experiments carried out during the course of this work. The potential benifits of pit contact

IBBC solar cells and the problems encountered in their metallization is presented in Section 4.2.

Section 4.3 describes an experiment that shows that Pd activation of heavily diffused boron pits

is necessary to avoid excessive Si migration during Ni sintering. Section 4.4 presents the results

of experiments that show how Pd activation inhibits Ni silicide formation on heavily phospho-

rus diffused surfaces and how unreacted Ni may be one potential source of poor adhesion in

the IBBC electroless metallization scheme. Section 4.5 presents evidence that a lack of Ni sili-

cide at the Ni/Si interface is another potential source of adhesion failure and shows that proper

silicidation in conjunction with the removal of the unreacted Ni after silicidation imporves the

adhesion of subsequent Cu plated layers. The main findings of this work are summarised in

Chapter 5 and options for future work are discussed.


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Chapter 2

Literature Review: Electroless Metallization

2.1 Introduction

This chapter presents a review of the literature that is relevant to the experimental work

carried out in this thesis. The main aim of this thesis is to improve the adhesion of electrolessly

plated layers to pit contact IBBCs. The electroless plating sequence utilised in IBBC solar cells

is broken down into its constituent parts and the literature pertaining to each part is reviewed

in order to gain an understanding of the mechanisms affecting each of the layers.

A schematic of the steps in the IBBC solar cell metallization sequence is shown in Figure

2.1. All oxide is removed from the contact areas leaving bare Si exposed (a). Activation of

the contact areas with Pd (b), is employed to better facilitate Ni plating and improve contact

adhesion. A thin layer of Ni is then plated to the contact areas (c) and sintered to form a silicide

at the Ni/Si interface (d). A second layer of Ni is then plated over the top of the first layer

(e) to better facilitate Cu plating. A thick layer of Cu is then plated to the second Ni layer (f).

Formation of the Ni silicide reduces the contact resistance between the metal and the Si and

anchors the metal to the cell. Cu is used as a low resistance current carrier because its bulk

resistivity is 4 times lower than that of Ni and it is much cheaper than lower resistivity metals

such as Ag or Au.

The literature pertaining to electroless metallization of IBBC has been reviewed and

grouped into three topic areas. The first is electroless plating. Plating mechanisms and the


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a) Si b) Pd c) Ni d) NiSi e) Ni f) Cu

Figure 2.1: Schematic of the electroless plating sequence for IBBC solar cells.

properties of each of the metal deposits are presented in Section 2.2. Section 2.2.1 discusses

how Pd activation can affect the adhesive properties of subsequent metal deposits. The effect

of plating bath parameters on the properties of electroless Ni and Cu depositions is presented in

Section 2.2.2.

The second topic to be reviewed is Ni silicide formation. After Ni has been plated to

an IBBC solar cell it is sintered to form Ni silicide at the contact interface. In Section 2.3 an

understanding of the mechanisms involved in Ni silicidation is sought from the prolific amount

of literature available pertaining to silicidation in CMOS devices. These are mostly concerned

with vacuum deposits of Ni sintered by rapid thermal processing (RTP). The few publications

that have considered the silicidation of AEDs of Ni by furnace processing are then reviewed to

gain some insight into the actual silicidation process occurring on IBBC devices.

The third topic is to be reviewed is the electroless metallization of c-Si solar cells in

general and buried contacts in particular. The purpose of this section is to highlight the benefits

of electroless metallization in comparison to other metallization schemes and to identify some

of the known problems associated with the process.

2.2 Electroless Plating

Electroless plating refers to the deposition of metal onto a substrate from an aqueous

medium without the use of an external bias potential to drive the reaction. There are two types

of electroless plating that are utilised in solar cell metallization and they proceed by different

mechanisms. The first is referred to as immersion plating or immersion activation. The plating


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9

rate from an immersion plating bath is typically low and the layers are generally used as a

seed layer for a subsequent autocatalytic deposit. Autocatalytic electroless depositions, (AEDs),

which plate a much greater rates, are used to plate the bulk of the conductor. The mechanisms

behind immersion plating and the type of immersion plating utilised in IBBC solar cells are

discussed in Section 2.2.2. The mechanisms behind autocatalytic electroless deposition and the

properties of Ni and Cu AED baths and deposits are discussed in Section 2.2.1.

2.2.1 Palladium Immersion Activation

Pd is often used as a seed layer for subsequent AEDs on non-catalytic substrates or

substrates where nucleation is minimal. For example, non-conductive materials, that would

otherwise not be able to be plated by an autocatalytic process, can first be activated with a thin

layer of Pd (see: [12], [23], [33] and [36] for example). The Pd layer, which acts as a catalyst

for many AED solutions, allows the non-conductive substrate to be metallized.

It has been shown that p-type Si has a lower electroless Ni nucleation density than n-type

Si [41]. This is due to the electronegativity of p-type Si being greater than that of n-type which

causes the plating rate to be much lower on p-type surfaces than on n-type [34]. The higher the

concentration of p-type dopants, the worse this problem becomes. Pd immersion is therefore

useful for activating heavily boron diffused Si, because it increases the Ni nucleation density.

The chemical equation for the reduction of Pd ions to Pd metal is given by Equation

2.1. Pd has a relatively high, positive redox potential which means that the forward reaction of

Equation 2.1 is more likely to proceed than the reverse reaction [24]. Pd ions will be reduced to

Pd metal on any substrate from which they can gain electrons. Pd has a much greater propensity

to plate out of solution than Ni, for example, which has a negative redox potential.

Pd 2+ + 2e Pd Eo = 0.83 V (2.1)

To plate Pd to insulating or otherwise non-catalytic substrates (such as printed circuit

boards), where the transfer of electrons is limited, the substrate is first sensitized with Sn then


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10

activated with Pd and the Sn removed at the end in a acceleration step (for examples see: [12],

[23], [33] and [36]). When the substrate is Si, however, a much simpler process is possible due

to fact that Si dissolves in HF according to Equation 2.2 [24].

Si+ 6F SiF62 Eo = 1.24 V (2.2)

When Si is immersed in a mixture of Pd and HF, a galvanic reaction proceeds where

anodic dissolution of the substrate (Equation 2.2) enables cathodic reduction of the metal ions

from the solution (Equation 2.1) by transferring electrons through the substrate [11], [43], [56].

There have been reports of Pd activation of Si in HF-free solutions but without discussion of

the plating mechanism [19], [47]. Several authors found that Pd activation of Si is not possible

without the inclusion of HF in the solution [11], [56]. It has been shown that Au, Cu and Pd are

all able to be immersion plated to p-type, (100) Si from sulfate solutions (with HF) and that the

HF concentration greatly affects the nucleation rate for the case of Cu [56]. One study found

that for c-Si and a-Si surfaces (of unspecified dopant concentrations) the HF concentration did

not affect the size and distribution of the Pd aggregates plated from a chloride-based solution but

the number of dissolution etch pits increased with increasing HF concentration [11]. Defects

on or near the surface (such as diffusions) have also been found to cause inhomogeneous Si

dissolution during Pd activation [69].

A comprehensive study of the effects of HF-based Pd activation on the adhesion of sub-

sequent AED layers has been carried out by Karmalkar et. al. [40] - [43]. Various substrates

were activated with Pd from various bath types, sintered to form a Pd silicide and swabbed with

iso-propyl alcohol to remove any residue, prior to AEDs of Pd and Ni. They found that the

adhesion of subsequent AEDs on Pd activated samples is not governed by the HF concentration

but by the Pd complex. The Pd-diamine complex was found to improve adhesion compared to

the Pd-chlorine complex while the Pd-tetramine complex inhibits subsequent plating [40]. This

means that too little NH4F or NH4OH for a given volume of PdCl2 will result in a high level of

chlorine complexes and the subsequent AED may be non adherent. Too much of these additives


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11

will convert all the diamine complexes to tetramine complexes which can result in the AED not

plating as fast or at all. The Pd bath used in the standard IBBC plating sequence is Bath Ffrom

[40].

Figures 2.2 and 2.3 show a summary of the results of Karmalkar et al. that are relevant

to the adhesion of plated layers activated by Pd immersion. Figure 2.2 a) shows a comparison

of activation baths on various surfaces. Diamine-based baths improve the adhesion of AEDs of

Ni in comparison to HF based ones for p-type surfaces but dont affect the adhesion on heavily

doped n-type surfaces. Figure 2.3 shows that diamine-based activation does not significantly

improve the adhesion of AEDs of Ni on n-type, (111) surfaces in comparison to no activation

but does improve adhesion on p-type surfaces, especially when heavily doped. This suggest

that the nucleation density of the Ni plate on heavily diffused boron surfaces is increased by Pd

activation - a phenomenon that is demonstrated experimentally in Section 4.3 of this work.

Interestingly, Pd immersion plating was found to improve the adhesion of AEDs of Ni

more than AEDs of Pd and adhesion of both metals was reduced if the immersion layer was

not sintered [43]. This is a potential cause of poor adhesion in the pit contact IBBCs and is

investigated in Section 4.4.2. Figure 2.2 b) shows that lowering the AED plating temperature

also reduces adhesion. This may be related to the results of Section 4.4.3 which shows that

silicide formation does not occur on unactivated samples plated at low temperatures. Figure

2.2 b) also shows that increasing the thickness of the Ni AED reduces the adhesion which is

probably due to increased stress in the deposits [5]. For this reason, a thinner Ni layer was used

in the experiments of Chapter 4 than in the standard IBBC plating sequence.

2.2.2 Autocatalytic Electroless Depositions

Cu is used in integrated circuits due to its low resistivity and superior resistance to

electromigration. It is a fast diffuser in Si however, and if located near a junction, can act as

a trap, decreasing the minority carrier lifetime and increasing junction leakage [67], [73]. For

this reason, Ni and Ni silicide is used as an interlayer between Cu contacts and Si substrates in

Si solar cells. Ni and Cu contacts can both be plated by AED onto Si, Pd and each other.


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a) b)

Figure 2.2: Adhesion of an AED of Ni plated to variously doped substrates, activated with

various Pd solutions. The Pd layer was sintered for 30 minutes at 200C prior to the Ni plate.

Adhesion as a function of (a) surface concentration and dopant type for various Pd activation

solutions and (b) Pd solution type for various Ni plating times and temperatures. The substrates

here are 4 .cm, p-type (after [42]).

a) b)

Figure 2.3: Adhesion of an AED of Ni plated to variously doped substrates, with and without

Pd activation. The activation solution is NH4F-based and the Pd deposit was sintered for 30

minutes at 200C prior to the Ni plate. The surface concentration of the n-type substrates was

4-8 x 1015/cm3 and for p-type substrates, 1-2 x 1016/cm3. P+ denotes a boron diffused surface

and N+ denotes a phosphorus diffused surface both with surface concentrations >1020/cm3.

All substrates are (111). The graphs show adhesion of a 2 minute, AED of Ni plated from an

alkaline, hypophosphite bath at 90-95C onto (a) n-type surfaces and (b) p-type surfaces (after

[41]).


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13

The mechanism that drives AED is much the same as that of immersion plating except

that the reduction of the metal ions is enabled via an oxidation reaction within the solution,

at a catalytic surface, rather than by dissolution of the substrate. Generically, the process is

represented by simultaneous anodic oxidation and cathodic reduction [6]:

Reducing Agent Reducing Agent z+ + ze (2.3)

Metal z+ + ze Metal (2.4)

Formaldehyde is employed as the reducing agent in autocatalytic electroless Cu plat-

ing [7]. The reduction of Cu2+ has been shown to be diffusion limited and the oxidation of

formaldehyde, to be electrochemically controlled [6]. The plating rate is therefore kinetically

limited by the oxidation step, and variation of diffusion mechanisms (increasing the metal ion

concentration or agitating, for example) will not change it. The catalytic nature of the substrate,

however, does affect the decomposition of formaldehyde. It has been shown that Ni cannot be

used as a catalyst for the oxidation of formaldehyde [6] so the deposition of Cu on Ni must pro-

ceed by another mechanism in these baths. A pure copper-formaldehyde-water bath needs to be

operated below a pH of 6 [8]. Copper plating baths typically contain complexants, stabilizers

and exaltants which stop the Cu ions precipitating in the bulk solution and change the initiation

conditions. These often allow the bath to be operated at higher pH values.

AEDs of Ni do not result in a pure Ni deposit on the catalytic surface but in an alloy

with a composition that is dependent on the reducing agent. The use of sodium hypophosphite

(NaH2PO2.H2O) as the reducing agent, yields deposits of NiP alloys while deposits of NiB

alloys can be obtained by using sodium borohydride (NaBH4) as the reducing agent [5], [49].

Both acid and alkali, Ni/NaH2PO2.H2O plating baths can be employed [49], [48], but here the

discussion is mostly confined to alkaline/Ni/NaH2PO2.H2O baths.

The resistivity of the a NiP alloy increases from 10 .cm to 120 .cm as the phos-

phorus content varies from 0 - 12 wt% (data from Enthone, Netherlands published in [37]). It

has been shown that the P content of Ni AEDs varies from 4.7% to 3.3% as the pH was varied


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from 8 to 12 [5]. In acid baths the same trend is observed but it is much more pronounced

and the P content is generally higher than in alkali baths [48]. The phosphorus content of the

deposit is also increased by increasing the plating temperature [62]. The plating rate is known

to be directly proportional to the bath temperature and increasing the pH will also increase the

plating rate [27], [48]. As with AEDs of Cu, AEDs of Ni are also kinetically limited by the

oxidation step and therefore dependent on the hypophosphite concentration [49], but not the

NiCl2 concentration [34].

The catalytic effect of the substrate also plays an important role in the AED of Ni. It

was found that NiP films plated from an alkaline, Ni-hypophosphate bath at 90

C onto n-type,

3-5 .cm samples were thicker on (100) surfaces than on (111) surfaces [9]. Differences in

the Ni plating rates on n- and p-type surfaces have also been reported [34], [40]. One study

found that this difference increased with increasing illumination but equal plating rates could

be obtained by the addition of 6g/L of ethyl-diamine-tetra-acetic acid (EDTA) into the plating

solution [34]. EDTA concentrations greater than this suppressed plating altogether on both

substrate types. When plating DSBC solar cells, however, it was found that proprietary, alkali-

based, Ni plating solutions containing EDTA were not able to plate the heavily boron diffused

grooves but that EDTA-free solutions [21] and maintaining high pH values and temperatures

[20] could overcome the problem. This was also found to be the case in this work.

2.3 Ni Silicidation

Ni silicide is used in IBBC solar cells for two reasons. The first is to reduce the contact

resistance at the metal/semiconductor interface. The contact regions of a BC cell, however, are

so heavily doped that the benefit of forming a Ni silicide, in comparison to a Ni/Si interface on

heavily doped Si, often does not outweigh the draw backs, such as the potential for shunting and

the extra processing steps. The second reason for forming a silicide on IBBC solar cells is to

promote adhesion of the subsequent, thick Cu layer. Cu can be plated directly to Si but it can not

be heated to obtain good adhesion because of its high diffusion coefficient in Si and potential


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to form traps. Ni, on the other hand, forms stable silicides when heated, which improves the

adhesion of the existing Ni layer and of subsequent plated layers. In this section, a review of

literature published on the theory and experimental results of Ni silicidation is presented, in

order to better understand and control Ni silicidation in IBBC solar cells.

2.3.1 Silicidation of Pure Ni Deposits

Ni silicide is used extensively in CMOS devices because of its low sheet resistivity, low

silicon consumption and ability to form narrow line widths without compromising resistance

(see [26], [28], and [54], for example). Silicide gates and interconnects in IC devices are gener-

ally formed by sputtering (see [26], [45], [38], [54] and [55], for example) or e-beam evapora-

tion (see [16], [18], [39], [61] and [68], for example) at pressures of less than 106 Torr and the

Ni silicide formed by rapid thermal processing (RTP). Ni is known to be the dominant mobile

species in a Ni-Si system such that the growth of silicide is limited by the diffusivity of metal

atoms through the silicide layer(s) to the Si interface (see [28], [50] and [60], for example). If

the supply of Ni stops (if it is all consumed for instance or if the silicide layer becomes suffi-

ciently thick for the diffusion of Ni to the Si interface to become negligible) and energy is still

supplied to the system then the phase of the silicide will change depending on the temperature,

as shown in Figure 2.4.

It is well established that for thin films of pure Ni on Si substrates, the first phase that

forms is the Ni rich Ni2Si phase which is stable up to 250-300C (for examples see: [10], [28],

[38], [55], [60]). For every micron of Ni consumed, 1.4 m of Ni2Si will form [38]. As the

temperature is raised and all the Ni is consumed the excess Ni in the Ni rich phase then diffuses

to the Si interface forming NiSi in the temperature range 300-700C (for examples see [10],

[16], [28], [38], [39], [55], [60], [61]). For every micron of Ni2Si reacted, 1.5 m of NiSi will

form [58]. If the temperature of the system is raised high enough the epitaxial, Si rich, NiSi2

phase begins to form (for examples see [10], [18], [28], [39], [55], [61]). The NiSi2 phase is

generally uneven in thickness and characterised by agglomeration of Si at the surface [39]. A

summary of this is shown in Figure 2.4.


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Table 2.1 presents a summary of sheet resistance measurements of Ni silicide layers

found in the literature. It shows that, as a thin layer of Ni on a Si substrate is heated from room

temperature to 900C, the resistivity of the film changes as the phase of the silicide changes. The

Ni2Si phase, which is formed below 300C, is less conductive than the NiSi phase, which has

sheet resistivity in the range 1.8 - 7 /sq. A 30 nm Ni film can be completely converted to the

low resistivity NiSi phase in the temperature range 400-700C. Sintering at temperatures higher

than this causes the sheet resistivity to increase, which is usually attributed to agglomeration of

the film during the NiSi to NiSi2 phase transformation. Similar sheet resistivities as those given

in table 2.1 for Ni silicide films were also observed in this work (see Sections 4.4.1 and 4.5.2).

As shown in Table 2.1, the affect of temperature on Ni silicide phase transformation has

been well characterised in the literature. While not as well characterised, dopant density at the

substrate surface has also been found to affect the phase transitions during Ni silicide growth.

One study found a significant difference in the sheet resistivity of Ni silicide films prepared on

As and B ion implanted, (100) substrates [38]. The silicide films were prepared by Ni sput-

tering and RTP. For sintering temperatures between 350

C and 550

C the sheet resistivity was

the same on both polarity samples (7 /sq) but for sintering temperatures between 260C and

330C, the sheet resistivity on As implanted wafers was 30 /sq. while on B implanted wafers it

was 70 /sq. This was attributed to a difference in the grain size which was found to be smaller

on B implanted samples. The NiSi phase was found to form at a lower temperature on the B

implanted samples because Ni diffusion through the Ni2Si layer is faster with a higher density

of grain boundaries. Another study observed the usual Ni2Si-NiSi-NiSi2 phase transitions on

3-8 .cm, (100), n-type substrates but when they were BF2+ implanted, the order of the phase

transformations was different, with the NiSi2 phase being found to form at much lower tem-

peratures [10]. Stabilisation of the Ni2Si phase and transition from Ni2Si - NiSi2 without the

formation of NiSi on samples with boron implanted through the Ni films has also been reported

[68]. Hence, the correlation between sheet resistivity and silicide phase that exists for undif-

fused samples is not the same for diffused samples. This factor has been taken into account in


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Figure 2.4: A summary of the most commonly reported phases of Ni silicide and their temper-

atures of formation, [10], [16], [18], [28], [38], [39], [55], [58], [60], [61]. In general, x1 < x2< x3 and the Si rich phase, formed at higher temperatures, tends to agglomerate.

TypeDoping

(.cm)

Thickness

of Ni (nm)

Sintering

Temperature

(C)

Phase

Sheet

Resistivity

(/sq.)

Source

N 3 - 8 30 200 - 300 Ni2Si 6 - 3.5 [10]

P 20 - 30 30 350 - 450 Ni2Si 30 - 13 [16]

N 3 - 8 30 400 - 600 NiSi 3.5 [10]

P 20 - 30 27 450 - 700 NiSi 2.5 [16]

P 1 - 10 30 500 - 700 NiSi 7 [18]P 20 - 30 30 400 - 700 NiSi 1.8 [61]

N 3 - 8 30 600 - 900 NiSi + NiSi2 3.5 - 6 [10]

P 20 - 30 27 700 - 800 NiSi2 2.5 - 10 [16]

P 1 - 10 30 800 NiSi2 110 [18]

P 20 - 30 30 700 - 800 - 1.8 - 3.5 [61] units are in .cm

Table 2.1: Summary of Ni silicide phase transitions and sheet resistivity results from various

authors. All substrates are (100), c-Si


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the experiments of this work.

Crystal orientation of the substrate has been found to affect the silicidation growth rate.

One study compared Ni silicide formation from 160 nm thick, vacuum evaporated Ni films on

2-10 .cm, (100) and (111) substrates [60]. The films were sintered in a vacuum in the temper-

ature range 200C - 325C. The Ni2Si phase (only) was formed on both substrate orientations,

as expected at this temperature. The growth rate, however was found to be significantly higher

on (100) substrates than on (111) substrates. These findings were confirmed by another study

which found that the growth rates of both Ni2Si and NiSi on 1-10 .cm, n-type substrates sin-

tered in the temperature range 300

C-370

C was nearly double on (100) substrates compared

to (111) substrates [50]. In this thesis silicide formation and the adhesion of subsequent AEDs

is observed on planar, (100) samples. In a pit contact IBBC solar cell, however, the contacts are

inverted pyramids with (111) facets. This means that when transferring the process from test

structures to cells, longer sintering times may need to be employed in order to form the same

thickness of silicide.

2.3.2 Silicidation of AEDs

It has been reported that there are considerable differences in the phase transformations

at low temperatures for AEDs of Ni compared to vacuum depositions. One author studied

the Ni-silicide phase transformations of 30-40 nm AEDs from acidic, NiSO4-NaH2PO2 based

baths at 70C on n-type, (100) substrates, sintered at various temperatures for 1 hour [46]. The

substrates were activated with either a seed layer of electron beam evaporated Ni (EB-Ni) or

SnCl2 sensitized Pd. Another reports a similar study of AEDs of Ni on p-type, 1-10 .cm Si

plated in alkaline, NiCl2-NaH2PO2 baths at 95C, sintered at various temperatures for 1 hour

[57]. Their findings are summarised in Table 2.2.

Table 2.2 shows that Pd activation tends to inhibit Ni silicide formation, a phenomenon

also observed elsewhere [47] and in this thesis (see Section 4.4). Epitaxial NiSi2, usually re-

ported to form above 600C, was found to form on the EB-Ni and Pd-activated samples at

temperatures of 300C and 400C, respectively [46]. This was attributed, by the authors, to


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Sintering

TemperatureDeposition 1 Deposition 2 Deposition 3

As Deposited

3 nm amorphous

layer at AED-Niinterface

1.5 nm amorphous

layer at AED-Niinterface

200C - -Ni3Si2 major

NiSi traces

300C20 nm unreacted NiP,

15 nm epi-NiSi2

Ni3P at surface, no

reaction at interface

400C

10 nm Ni3P

10 nm NiSi

20 nm epi-NiSi2

10 nm NiSi 20 nm,

epi-NiSi2

Ni3Si2 major

NiSi minor

500-700C NiSi only NiSi onlyNiSi major

Ni3Si2 minor

>800C NiSi and poly-NiSi2 NiSi and poly-NiSi2

NiSi2 major

NiSi minor

Ni3Si2 traces

Table 2.2: Phenomena observed for AEDs of Ni plated to (100) substrates and sintered at vari-

ous temperatures. Deposition 1 refers to 30-40 nm of NiP plated on EB-Ni seed layers on n-type

substrates [46], Deposition 2 refers to 30-40 nm of NiP plated to Pd activated, n-type substrates

[46] and Deposition 3 refers to 800-3000 nm of NiP plated directly onto p-type 1-10cm sub-

strates [57].

the presence of a Ni3P layer at the surface, reducing the metal concentration at the interface

and hence inhibiting the metal rich phase formation. It is possible that the presence of the Ni3P

layer is due to the presence of the amorphous interlayer (assumed to be oxide). Others, however,

have not found impurity containing phases. For example, Ni2B or NiBSi were not detected on

samples where boron was implanted through a pure Ni film [68] and the formation of a Ni3P

layer was not observed for Deposition 3 in Table 2.2. Instead, a Ni3Si2 phase, which has not

been previously reported, formed from AEDs of Ni varying in thickness from 800-3000 nm

[57]. The formation of the NiSi phase, at temperatures as low as 275C, has also been reported

for AEDs of Ni [4]. This silicide layer was formed on the (111) facets of a textured, (100),

p-type wafer with a 20-30 /sq. phosphorus diffusion. Sintering was carried out in a furnace in

air ambient. Although the authors attribute the low temperature formation of the NiSi phase to

the orientation of the plated surfaces, the preceding discussion indicates that the nature of the

Ni layer (pure Ni or NiP), and the dopant level at the surface, are probably also contributing


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factors in this case.

Clearly, silicidation of AEDs of Ni is not as well characterised or as predictable as the

silicidation of vacuum deposits of Ni. It is also clear that, impurities within the plated layer,

substrate orientation and doping level, dopant type and Pd activation all play an important role

in determining the characteristics of the resultant Ni silicide. Heavy doping levels, variable

impurity levels in the Ni plate, Pd activation and plating of (111) surfaces formed in (100)

wafers are all features of IBBC solar cells and should be taken into account during silicide

formation.

2.4 Electroless Metallization of Silicon Solar Cells

The electroless metallization techniques described in Sections 2.2 and 2.3 have been

applied in various combinations to various types of c-Si solar cells. In order to plate metals in

a contact pattern on the cell surfaces, the wafer must first be masked and the contact pattern

defined in the mask such that only the contact areas plate. Oxides or nitrides or a combination

of the two are generally employed in c-Si solar cells as anti-reflection coatings and for surface

passivation and they can also be used as plating masks. These dielectrics can be patterned in

various ways, the most common of which are photolithography and scribing.

Patterning a dielectric layer by photolithography involves the deposition of a temporary,

photosensitive layer onto the wafer surface and the patterning of this layer by exposure to light

through a detachable mask. The contact pattern is then developed in the photosensitive layer

which is resistant to certain etches that will dissolve the underlying dielectric. The contact

pattern is formed in the dielectric layer by immersing the wafer in such an etching solution.

The photosensitive layer is then removed. Photolithography can achieve fine line widths (on

the order of a few microns) but it is slow, costly and involves many processing steps. For this

reason it is generally used in conjunction with vacuum evaporated metals for high efficiency,

laboratory devices (see [74], for example).

Contact openings can also be made through the field dielectric by laser or mechanical


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scribing. Scribing processes not only remove the dielectric mask in the contact areas but usually

some of the underlying Si as well. Laser scribed contacts require alkaline etching prior to further

processing, to remove loose, ablated Si from the contact areas [70]. Laser and mechanically

scribed contacts are generally 30 m wide and 50 m deep [3], [37], and usually require

an additional diffusion step.

To make ohmic contact between a metal and a semiconductor, a minimum surface con-

centration of 1019 dopant atoms/cm2 is required at the interface [66]. For conventional furnace

diffusions, this usually corresponds to sheet resistances lower than about 20 /sq. Doping an

emitter to this level, to minimise contact resistance, would result in substantial recombination

losses in the emitter. The ideal emitter sheet resistivity, that minimises both series resistance

and shading losses, depends on the geometry and resistivity of the fingers [53]. In screen printed

solar cells, emitter sheet resistances of 40 - 60 /sq. are used which results in adequate con-

tact resistance to the wide, high resistivity printed pastes but substantial emitter recombination

losses [59]. If contact resistance was not an issue, better cell performance could be achieved by

going to higher emitter sheet resistivities and thinner, less resistive fingers. Such a cell can be

realised by the use of a selective emitter. A selective emitter has light emitter doping (typically

100 /sq.) over most of the cell surface but heavy doping (less than 20 /sq.) in localised

regions under the metal contacts.

Figure 2.5 shows the various types of solar cell contacts that can be made using pho-

tolithographically defined or scribed contact openings. Planar contacts with either homoge-

neous or selective emitters (Figure 2.5 a) and b), respectively) can be made using photolitho-

graphy. Three dimensional or buried contacts can be made using either photolithography in

conjunction with etching or laser or mechanical scribing (Figure 2.5 c) and d), respectively).

Emitters with buried contacts can be homogeneous but are usually selective, because a second

diffusion is required. There are various studies reported in the literature which utilise the dif-

ferent techniques of fabricating electroless contacts in silicon solar cells. A discussion of the

electrical and adhesion properties of such contacts are presented in Sections 2.4.1 and 2.4.2.


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22

a) b)

c) d)

Figure 2.5: Schematic of various electroless plated contacts. Dotted lines indicate diffused Si,light grey areas indicate the dielectric and dark grey areas indicate the plated metal. Photolitho-

graphically defined contacts with a planar homogeneous emitter, a planar selective emitter, and

a buried selective emitter are shown in a), b), and c), respectively. A scribed contact with a

selective emitter is shown in d).

2.4.1 Planar AED Contacts

Several devices with electroless contacts plated to photolithographically defined open-

ings in the field dielectric have been reported in the literature. A summary of the processing

used in these is presented in Table 2.3. All the cells are p-type with homogeneous, phosphorus

diffused emitters and all ADEs of Ni are in fact Ni-P.

One study compared cells with Ni plated contacts sintered by two different methods to

cells of the same structure with vacuum evaporated Ti-Pd-Ag contacts [47]. It found that the

adhesion and series resistance of the Ni plated contacts was poorer than the Ti-Pd-Ag contacts.

The Ni plated cells that were sintered by rapid thermal processing (RTP) had only marginally

poorer IV characteristics than the Ti-Pd-Ag ones. Cells with furnace sintered, Ni plated con-

tacts, however, exhibited IV characteristics consistent with a high concentration of recombina-

tion centres in the diode due to excessive Ni diffusion.

Another study compared cells with plated contacts to the same cells with Al sputtered

contacts and found the IV behaviour of both cell types to be the same [69]. The Ni sintering

time at 270C was varied and it was found that cells sintered for 40 minutes exhibited only


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Cell

T

ype

Plated

Surface

Sheet

Resistivity

(/sq.)

PdType

PdSinter

Time(min)

PdSinter

Tem

p(C)

NiSinter

Time(min)

NiSinte

r

Temp(

C)

NiSinter

Ambient

Silicide

Other

Meta

l

Source

P

lanar

111

30

PdCl-HCl

30

350

15

400

N2

Various

None

[47]

P

lanar

111

30

PdCl-HCl

30

350

RTP

RTP

RTP

Various

None

[47]

P

lanar

100

30

PdCl3-HF

30

290

20

270

Vacuum

NiSi

Cu+Sn

[69]

Te

xtured

111

20-30

-

-

-

5

260

Air

NiSi

Sold

er

[4]

P

lanar

111

14

-

-

-

15

240-400

N2

N/A

None

[3]

ElectrolyticCuwasthensolderdipped

T

able2.3:Summaryofprocessing

parametersofvariousc-Sisolarcellswithplanar,photolithographicallydefined,electrolesslyplatedcontacts.


43/104

24

a small degradation in Voc while Voc was reduced by 20% in cells that were sintered for 60

minutes. The cells that were sintered for 20 minutes [69] and the cells with RTP Ni contacts

[47] and the textured cells in Table 2.3 [4] all exhibit similar IV characteristics. This suggests

that the non-optimal IV curves observed by Lue for the furnace sintered cells are the result of

to high an sintering temperature (see Table 2.3).

Table 2.3 illustrates that it is possible to plate adhesive AEDs of Ni to planar (as opposed

to buried) surfaces and form a silicide at the Ni/Si interface on both (100) and (111) surfaces,

with and without Pd activation. Only one of the authors reported the adhesion of Ni to be a

problem on planar surfaces who observed that the Ni layer would flake off at a thickness of 1

m [3].

2.4.2 Buried AED Contacts

Table 2.4 lists the parameters of various types of electrolessly plated solar cells with

buried contacts and selective emitters. Cell No. 1 is an example of a buried contact produced by

photolithography and reactive ion etching (RIE), Cell No. 2 is an example of a buried contact

made by mechanical scribing and the rest are all laser scribed buried contacts. The RIEed

contacts had a 5 m x 5 m square cross section [17]. The mechanically scribed and etched

contacts were 40 m wide, 60 m deep and diamond shaped [3]. The laser scribed grooves of

Cell No. 4 are of a similar shape but the dimensions are approximately 25 m x 25 m [37].

The grooves on Cell No. 8 and Cell No. 9 are the same depth as the mechanically scribed ones

but only half as wide [15], [29].

One of the drawbacks of electroless plating of buried contacts is that the metals plate

faster at the corners of the grooves than inside them resulting in the grooves being closed off

before they can be filled [37]. This does not appear to be a problem for the small, square shaped

grooves of Cell No. 1 which were able to be completely filled with an adhesive AED of Ni [17].

Mechanically scribed grooves plated with electrolytic Ag (Cell No. 2) and small laser scribed

grooves plated with electroless Cu (Cell No. 4) or with non-optimised electrolytic Cu (Cell No.

5) all result in a thin layer if metal on the walls of the grooves and overplating at the groove


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25

Cell

No.

CellType

Contact

Dopant

S

heetResistivity(/sq.)

Activation

1

Etched

Photolith

ography

andRIE

As

Heavy

Chemicalroughening

2

SSBC

Mechanicallyscribed

P

14

-

3

EWT

Laser

scribed

P

Heavy

-

4

SSBC

Laser

scribed

P

Heavy

-

5

SSBC

Laser

scribed

P

Heavy

-

6

SSBC

Laser

scribed

P

cavitation erosion thesis

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