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![Page 1: Formation of pn junction in deep silicon pores September 2002 By Xavier Badel, Jan Linnros, Martin Janson, John Österman Department of Microelectronics.](https://reader035.fdocuments.us/reader035/viewer/2022062515/56649cb75503460f9497d22c/html5/thumbnails/1.jpg)
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
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OUTLINE
1. Introduction
2. Experiment
3. Results
4. Summary
X. Badel, KTH, Stockholm
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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
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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
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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
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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
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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
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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.
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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
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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.
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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
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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
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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
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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%.