formation of pn junction in deep silicon pores
DESCRIPTION
Formation of pn junction in deep silicon pores. By Xavier Badel, Jan Linnros, Martin Janson, John Österman Department of Microelectronics and Information Technology KTH, Stockholm. September 2002. OUTLINE. 1. Introduction 2. Experiment 3. Results 4. Summary. X. Badel, KTH, Stockholm. - PowerPoint PPT PresentationTRANSCRIPT
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
X. Badel, KTH, Stockholm
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%.