Formation of pn junction in deep silicon pores

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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
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? OUTLINE
1. Introduction 2. Experiment 3. Results 4.
Summary
X. Badel, KTH, Stockholm
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?Introduction
1. Introduction
Application dental X-ray imaging
... Requirement Spatial resolution10LP/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)
- 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
X. Badel, KTH, Stockholm
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? Experiment Pore formation
2. Experiment
Setup and other examples of electrochemical
etching
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 T600ºC P150
mTorr t1h30 Gases SiH4 and B2H6 Na 6.1019
cm-3 thickness 400 nm.
X. Badel, KTH, Stockholm
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2. Experiment
? Experiment Techniques for analyses
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
X. Badel, KTH, Stockholm
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? Results Doping by diffusion
3. Results
Diffusion 1 1150ºC, 1h45
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.
X. Badel, KTH, Stockholm
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? Results Doping by diffusion
3. Results
Diffusion 1 SIMS profiles at different
positions along the pore depth
- No B in the substrate (profiles c, g). Walls
fully doped. - B in pores lt B in a planar
wafer (about 5.1019 instead of 2.1020 cm-3).
X. Badel, KTH, Stockholm
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? Results Doping by diffusion
3. Results
Diffusion 2 1050ºC, 1h10. SIMS profiles at
different positions along the depth
- 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.
X. Badel, KTH, Stockholm
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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 Na6.1019cm-3 thickness400 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
X. Badel, KTH, Stockholm
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? Results Detector efficiency
3. Results
Ideal matrix Pore spacing 50 µm Pores as
deep as possible Trade-off on the wall thickness
Calculated efficiency for depth300 µm and
wall4.1 µm 60.
X. Badel, KTH, Stockholm
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? Summary
4. Summary
1. Diffusion - Transport of boron into the pores
is limited at high temperature (diffusion at
1150C for 1h45). - Doping improved in the case
of diffusion at lower temperature (1050C 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.
X. Badel, KTH, Stockholm