1. Thin Contact Development for
Silicon Detectors
C. Tindall, P. Denes, S. E. Holland, N. Palaio,
D. Contarato, D. Doering
Lawrence Berkeley National Laboratory, Berkeley, CA 94720
D.E. Larson, D.W. Curtis, S.E. McBride, R.P. Lin1
Space Sciences Laboratory, University of California Berkeley, Berkeley, CA
1Also Physics Department, University of California, Berkeley, CA

2. LBNL Microsystems Laboratory
Thermco/Expertech 150mm furnaces
150 mm Lithography tool
LBNL Microsystems Laboratory – Class 10 Cleanroom

3. Silicon Semiconductor Detectors
Al Electrode
SiO2
p+ B - implant
200 to 300 mm
High purity - Si
(n-type) h+ e-
n+ contact
hn (high energy)
Absorbed in the
active volume.
hn (low energy)
-Absorbed in the
contact.

4. CCD Project
LBNL Engineering Group – 200 fps CCDs for direct detection of low-energy x-rays
Amplifiers every 10 columns, metal strapping of poly, and custom IC readout

5. MSL Processed Silicon Detector Wafer

6. Instrument Size WIND 3-D Plasma and Energetic Particle Experiment
Suprathermal Electron Telescope
Element (STEREO-IMPACT)
(UC Berkeley Space Sciences Lab)

7. In-Situ Doped Polysilicon
Baseline Process – In-situ phosphorus doped polysilicon (ISDP).
It yields a thin (≤200Å), low leakage (~300 ambient temp) contact.
Deposition temperature is >600°C so it can not be used on devices with metal.
In LBNL’s PIN diode and CCD processes it is deposited before the metal.
W127 A3
Detector Area =0.09 cm2
/cm2)

8. Thin backside n+ ohmic contact development
SIMS depth profile
ISDP – in-situ doped
polysilicon
The thin backside n+ contact technology
developed at the MSL is an enabling
technology for
Photodiodes for medical applications
CCDs
Charged-particle detectors in space

9. In-Situ Doped Polysilicon Contact
Energy lost by the protons in the contact is about 2.3 keV.
Data taken by R. Campbell at
UC Berkeley’s Space Sciences Laboratory

10. Data taken by D. Larson at UC Berkeley’s Space Sciences Laboratory
In-Situ Doped Polysilicon Contact
Energy lost by electrons in the 200Å doped polysilicon window is about 353 eV.
Data taken by D. Larson at UC Berkeley’s Space Sciences Laboratory

11. Data taken by D. Curtis at UC Berkeley’s Space Sciences Laboratory.
In-Situ Doped Polysilicon Contact
Data taken by D. Curtis at UC Berkeley’s Space Sciences Laboratory.
Spectrum obtained by illuminating a PIN diode to a mixed 55Fe and 109Cd source. The detector has a 200Å in-situ doped polysilicon entrance contact.

12. MSL detectors on NASA space missions
Mars Atmosphere and Volatile Evolution (MAVEN)
- MAVEN will make definitive scientific measurements of present-day atmospheric loss that will offer clues about the planet's history.
- To date, the MSL has provided 36 thin window detectors for MAVEN.
16 detectors have been selected for flight as part of the Solar Energetic
Particle (SEP) Instrument.
- Launch: late 2013.
Prototype Detector Stack
Mock up of the SEP Instrument

13. MSL detectors on space missions
Charged particle detectors fabricated in the MSL by Craig Tindall
CINEMA – Understanding space weather
Solid State Telescopes (two for ions, two for electrons per spacecraft)
104 detectors delivered, 80 used in flight
THEMIS PIN Diode
Fabricated in the MSL

14. MSL detectors on NASA space missions
THEMIS Update
Launched in 2007, all major science goals were achieved by 2009
MSL detectors on all five spacecraft are still returning science data.
ARTEMIS – extended mission to study the interaction of the moon
with the solar wind. Two THEMIS spacecraft diverted to the moon.
These two “ARTEMIS” spacecraft are now in lunar orbit.

15. STEIN Detector (First Design)
Low Energy Threshold (1-2 keV)
~1 keV Energy Resolution
Sensitive to Electrons, Ions, and Neutrals (But Can’t Separate)
4 x 1 Pixel Array
Flight Heritage: STEREO Mission STE Instrument
(SupraThermal Electrons)
(STE)
Silicon Semiconductor Detector

16. STEIN Instrument Collimator
± 2000 V Field Separates Electrons, Ions, and Neutrals to ~20 keV
Particle Attenuator
(Blocks 99% of Particles)
Initial Version of the Instrument – Designed by Space Sciences Laboratory

17. MSL detectors on an NSF space mission
Cubesat for Ions, Neutrals and Magnetic Fields (CINEMA)
Mission consists of four “triple” cubesats, small satellites (10cm x 10cm x 30cm) Two will be made by UC Berkeley’s Space Sciences Laboratory and two by Kyung Hee University in South Korea.
Each cubesat contains a magnetometer and a Suprathermal Electrons, Ions and Neutrals (STEIN) instrument. STEIN contains a 30 pixel array of detectors with a thin entrance window.
First spacecraft has been delivered Launched: September 2012.
Cubesat Mock-up
STEIN Detectors and Readout ASIC

18. MSL detectors on NASA space missions
Solar Probe Plus (SPP) – Prototyping Phase
- Mission to study the sun close-up. The closest approach – 9.5 solar radii.
- Prototype detectors for the Low Energy Telescope in the EPI-HI instrument
are being fabricated in the MSL.
- Detectors with active volumes that are 10mm and 25mm thick are required.
- Launch – 2015.
675 mm
SiO2
p+ B - implant
Al Electrode
Handle Wafer
Back Contact
Active Layer – 10 mm
n+ P - Implant

19. Thin Silicon Alpha Spectrum

20. Other Thin Contact Techniques
Commercial silicon detectors (PIN diodes) are available with
contacts that are ≥500Å thick. (ion implantation)
Reported leakage currents are roughly 20nA/cm2.
A 500Å contact transmits only about 65% of 280eV photons into
the active volume of the detector.
A thinner contact is needed to get high efficiency
at 280eV (C - K edge).

21. Silicon x-ray Transmission

22. Thin Contact Fabrication Techniques
Thickness (Å)
Compatible with metal?
%Transmission at 280eV
Amorphous Si
≥300
Yes
≤77
In-situ doped poly
200 No 84
Implant/Anneal
~1000 42
Implant/Laser
~700 54
MBE
≤100
≥92

23. Implant/Low Temperature Anneal
- ISDP is a very useful process for making thin contacts.
However: a.) The deposition temperature ≥600°C so it
can’t be used on devices with metal.
b.) Integration with the CCD process is complex.
c.) Integration with CMOS processes used to make
active pixel sensors is impossible.
For applications that do not require the thinnest contact we
developed a much simpler alternative – ion implantation and
low temperature annealing – that does not damage the metal.
- Informally referred to as our “pizza process”.

24. Implant/Low Temperature Anneal
Our CCDs that utilize “pizza process” contacts for soft x-ray detection.
Leakage current ranges from about 600 pA/cm2 to several nA/cm2
at 100V bias and ambient temperature with this method.
The window thickness is about 1000Å of silicon.
Good uniformity. Used successfully with our largest CCD – cm2.

25. Implant/Low Temperature Anneal
Guibilato, et. al. NIM A 650(2011) 184
SOI Imager (Active Pixel Sensor)

26. Implant/Low Temperature Anneal
After Thinning
Before Thinning
After the
“Pizza”
Process
SOI Imager-2 (Active Pixel Sensor)
Battaglia, et. Al. NIM A 676 (2012) 50

27. Implant/Laser Anneal
Gives only a nominal decrease in the window thickness from 1000Å
to an estimated 700Å.
Requires a significant amount of stitching. Stitching only in one direction
works at some level. The yield is about 80%.
X-Y stitching doesn’t seem to give low enough leakage current, but our
testing of this is limited.
Bottom line – further testing needed to optimize the process. Most likely a
laser with a larger spot size would improve the result significantly.

28. Chemical Etching/a-Si
Surface is chemically etched, then a 300Å thick layer of a-Si
is sputtered onto the surface. It is essentially a room
temperature process.
The defects on the surface form the contact. One obtains the
same contact properties with or without the a-Si.
The contact thickness has not been measured.

29. Molecular Beam Epitaxy (MBE)
Contact Configuration
Ideally a single monolayer of electrically active dopant atoms is desired.
The silicon capping layer is required to form a stable contact.
Incoming x-rays
Silicon cap layer
d-doping layer
Silicon device
The Key:
This is a deposited contact, so
the beginning surface defect
density must be low in order
to obtain low leakage current.
Front side pattern/electronics
Pioneering work on d-doped contacts was done by Nikzad’s group at JPL.
IEEE TED, 55, Dec. 2008

30. Molecular Beam Epitaxy (MBE)
Buffer Chamber
Load Lock
MBE Chamber
Base Pressure ~5x10-11 torr
Substrate
Sb or B
Knudsen Cell
e-beam gun
(silicon)

31. Molecular Beam Epitaxy (MBE)
Typical
SVT Associates Silicon MBE System
Load-Lock
Deposition Chamber
Substrate Preparation Chamber

32. Thin Contact Fabrication Techniques
Advantages
Disadvantages
Amorphous Silicon
Room Temperature Process
Leakage current varies significantly from run to run,
n-type only.
Implant/Low Temp Anneal
Low temperature, low leakage, simple process, high yield.
Relatively thick contact.
Implant/Laser Anneal
Patterned side of the wafer is at room temperature.
Leakage current is somewhat variable, thicker than optimal.
MBE
Low temperature, low leakage, ultimately thin contact.
Complex equipment and process.
In-situ doped poly.
Thin contact, low leakage.
Process temperature too high for metalized devices.
Implant/Flash UV
Process temperature too high, expensive equipment.

33. Silicon x-ray Transmission
MBE
Implant/Low Temperature Anneal
“Pizza Process”

34. Fine Pitch Germanium Strip Detector
1024 strips, 50 mm pitch, 5 mm length
1 mm thick detector
~ 30 pA / Vb = 55 V, T >100 K
Developed for time-resolved x-ray absorption spectroscopy
J. Headspith, et al., Daresbury Lab

35. Detector Group at LBNL
Historical accomplishments with significant impact
on radiation detector technology:
One of the first groups to develop lithium-drifted Si detectors (early 1960’s)
One of two groups that originally developed high-purity Ge crystal growth (early 1970’s)
Fabrication technologies developed include: amorphous semiconductor contact, implanted contact, and surface passivation
Invented shaped-field point-contact Ge detector (1989)
Invented coplanar-grid technique for CdZnTe-based detectors (1994)

36. Summary Thin contacts are needed for imaging soft x-rays.
The techniques of most interest appear to be:
1.) implant/low temperature anneal or “pizza” process
2.) Molecular Beam Epitaxy (MBE)
Germanium may be useful for higher energies. We have produced
strip detectors with 50mm pitch for use at light sources.
Thin contacts also have application in other fields of science,
for example - space science.