2nd Trento Workshop

1
Fabrication of 3D-STC at IRST

• Sabina Ronchin

2
Outline

• Preliminary tests
• Process flow
• Characteristics of fabricated devices
• Conclusion and future developments

3
Preliminary tests
- lithography
as-deposited Photoresist tends to penetrate
inside the hole forming a thicker layer in
these region
after exposure and development residual
photoresist in the hole region
Important that resist can be defined in the
proximity of the hole. The resist trapped in
the hole is removed after the process
4
Preliminary tests - deposition
Poly and oxide deposition
oxide
top
bottom
hole
surface
poly
oxide
hole
poly
5
Preliminary tests oxide growth
Oxide Growth (500nm) in a 5mm diameter hole
top
bottom
oxide
metal
hole
oxide
Oxide thickness is uniform inside the hole
6
Preliminary tests metal sputtering
Aluminum does not penetrate inside the hole
contacts have to be realized on the wafer surface
aluminum
hole
7
Fabrication process

• initial oxide

• p-doping of back

• Isolation p-stop or p-spray

• masking for deep- RIE process

p- Isolation
oxide
TEOS mask
Si wafer
p doping
8
Fabrication process

• hole definition

• deep-RIE

holes
9
Fabrication process

• Column doping

• n-doped areas opening

• P-diffusion

P- diffused regions
10
Fabrication process

• Column Passivation
• Nitride deposition
• Oxide grown

11
Fabrication process

• Contact definition and opening

• Front and back metal deposition

• Front metal definition

• Sintering

metal
contacts
metal
12
Main features of fabrication process
n electrodes
p-type substrate
p back contact

• Single-Type-Column
• Etching and column doping
• performed only once

• Holes not etched through all the wafer

No need of support wafer.
Bulk contact is provided by a backside uniform p
implant (single side process)

• No hole filling

13
SEM photographs (cross section)of a detail of a
processed device

• Deep RIE used for digging holes
• (in collaboration with CNM, Barcelona,Spain)
• Wide superficial n diffusion around the hole
  to assure good contact
• Passivation of holes with oxide

metal
oxide
hole
contact
n diffusion
Hole depth 120µm
14
Layout example - strip detectors
metal
p-stop
hole

• AC and DC coupling
• Inter-columns pitch 80-100 ?m
• Two different p-stop layouts
• Holes Ø 6 or 10 ?m

Contact opening
n
15
Fabrication process information

• Substrate used for this production

• Si High Resistivity, p-type, lt100gt

• Surface isolation
• p-stop
• p-spray

• Sintering
• Standard _at_ 420C for FZ
• 380C for Cz to minimize thermal donor activation

16
Future developments
Conclusion

• A new type of 3D detector has been conceived
  which
• leads to a significant simplification of the
  process
• hole etching performed only once
• no hole filling
• no wafer bonding

• Si, FZ, p-type, lt111gt, with resistivity 1850
  Ohm-cm, 380 ?m thick
• Surface isolation by combined p-spray and p-stop
• Double Type Columns (p- and n-type)