Formation of pn junction in deep silicon pores

September 2002

By Xavier Badel,

Jan Linnros, Martin Janson, John Österman

Department of Microelectronics and Information Technology

KTH, Stockholm

OUTLINE

1. Introduction

2. Experiment

3. Results

4. Summary

X. Badel, KTH, Stockholm

Introduction 1. Introduction

n - s ilico n

C s I:T l

p + s i lic o n

X - ra y

n -

CsI

:Tl

E c

E v

B u lk c o n ta c t

p + c o n ta c ts

p +

Application: dental X-ray imaging ...

Requirement: Spatial resolution=10LP/mm; Low X-ray dose...

Detector principle: silicon based detector with CsI columns

Challenging process: Form pn junctions in pore walls.X. Badel, KTH, Stockholm

Experiment: Pore formation 2. Experiment

DRIE: Electrochemical Etching:

- Photolithography

- 10s Etching (SF6 plasma)

- 10s Passivation (C4F8 plasma)

- Etch rate: 2 m/min

- n-type silicon (Nd = 1.1014 cm-3)X. Badel, KTH, Stockholm

- Initial patterned surface: inverted pyramids

- Dissolution of n-type silicon

(Nd = 1013 cm-3) involving holes and aqueous HF

- Etch rate: about 0.5 m/min

Experiment: Pore formation 2. Experiment

Setup and other examples of electrochemical etching:

S i

Electroly te

300 WH alogen Lam p

A l g rid

Pt E

lect

rode

PC con tro led Pow er Supp ly

IV

M eta llic ring

X. Badel, KTH, Stockholm

2. Experiment Experiment: Doping methods

Boron diffusion from a solid source:

- diffusion 1 at 1150ºC for 1h45’ : Na = 2.1020 cm-3; thickness =6 m.

- diffusion 2 at 1050ºC for 1h10’ : Na = 3.1019 cm-3; thickness =2 m.

LPCVD of boron doped poly-silicon:T=600ºC; P=150 mTorr; t=1h30’; Gases: SiH4 and B2H6;Na = 6.1019 cm-3; thickness = 400 nm.

0 2 4 6 81E14

1E15

1E16

1E17

1E18

1E19

1E20

Diffusion 2

LPCVD

Diffusion 1

Bo

ron

co

nce

ntr

atio

n (

cm-3)

Depth (microns)X. Badel, KTH, Stockholm

2. Experiment Experiment: Techniques for analyses

X. Badel, KTH, Stockholm

SEM: Scanning Electron microscopy

SCM: Scanning Capacitance Microscopy

2D imaging of the doping

Principle: measure dC/dV (related to the doping) via a probe scanning the surface.

SSRM: Scanning Spreading Resistance Microscopy

2D imaging of the doping

Principle: measure the current (related to the resistance/doping).

SIMS: Secondary Ion Mass Spectrometry

Dopant profiling in planar samples and through the wall thickness

Results: Doping by diffusion 3. Results

Diffusion 1: 1150ºC, 1h45’

Profile along A

A

5 µm

AFM

SSRM

X. Badel, KTH, Stockholm

Thickness at the pore bottoms: 3 m.

Thickness on a planar wafer (SIMS): 6 m.

Transport of boron down to the pore bottom may be limited.

Results: Doping by diffusion 3. Results

Diffusion 1: SIMS profiles at different positions along the pore depth:

X. Badel, KTH, Stockholm

- No B in the substrate (profiles c, g). Walls fully doped.

- [B] in pores < [B] in a planar wafer (about 5.1019 instead of 2.1020 cm-3).

n - ty p e s u b s tra te

b o ro n d o p e d re g io n

c , g : su b s t ra te

d , i : b o tto m

e : m id d le

f , h : to p

boro

n do

ped

regi

on

0 1 2 3 4 5 61E15

1E16

1E17

1E18

1E19

1E20dei f

h

gcB

oron

con

cent

ratio

n (c

m-3)

Depth (microns)

Substrate: c g

Pore bottoms: i d

Pore middle: e

Pore tops: f h

Results: Doping by diffusion 3. Results

Diffusion 2: 1050ºC, 1h10’. SIMS profiles at different positions along the depth:

X. Badel, KTH, Stockholm

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.01015

1016

1017

1018

1019

1020

Bor

on

con

cent

ratio

n (c

m-3)

Depth (microns)

Pore tops: m p

Pore middles: n o

Pore bottoms: k l

Substrate: j

n - ty p e su b s tra tej : su b s t ra te

k , l : b o tto m

n , o : m id d le

m , p : to p

b o ro n d o p e d la y e rs

- [B] in pores [B] in a planar sample; no significant variation along pore depth.

- Boron atmosphere in the pores maybe more uniform at 1050ºC than at 1150ºC.

- Boron layers on each side of the walls.

Results: Doping by LPCVD 3. Results

On a DRIE matrix:

On a EE matrix, close to a defect: - Deposition on the DRIE matrix seems to be conformal.

- Deposition is disturbed by defects of the walls.

- SIMS measurement on a planar wafer:

Na=6.1019cm-3; thickness=400 nm.

X. Badel, KTH, Stockholm

Results: Doping by LPCVD 3. Results

SCM at a pore bottom of a DRIE matrix after deposition:

typical signature of a pn junction

SCMAFM

A

Profile along A

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Results: Detector efficiency 3. Results

Calculated efficiency for depth=300 µm and wall=4.1 µm : 60%.

X. Badel, KTH, Stockholm

“Ideal” matrix: Pore spacing = 50 µm; Pores as deep as possible;

Trade-off on the wall thickness:

0 2 4 6 8 100.0

0.2

0.4

0.6

0.8

1.0

Eff

icie

ncy

Wall thickness (microns)

Active area Absorbed photons (550 nm) Total efficiency

CsI(Tl)

CsI(Tl)Si

B: poly-Si

CsI

(Tl)

Si

B: poly-Si

Summary 4. Summary

X. Badel, KTH, Stockholm

1. Diffusion

- Transport of boron into the pores is limited at high temperature (diffusion at 1150°C for 1h45’).

- Doping improved in the case of diffusion at lower temperature (1050°C for 1h10’).

- p+/n/p+ structure in the walls revealed by SIMS, SEM and SSRM.

2. LPCVD

- Homogeneous coverage of the pore walls.

- Presence of the pn-junction revealed by SCM.

3. Next

- Need of contacts on the p+ layers for I-V characterization and final detector.

- Expected efficiency of about 60%.