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 94720-7450
1Also
Physics Department, University of California, Berkeley, CA 94720-7300
Lawrence Berkeley National Laboratory
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LBNL Microsystems Laboratory
Thermco/Expertech 150mm furnaces
150 mm Lithography tool
LBNL Microsystems Laboratory – Class 10 Cleanroom
Lawrence Berkeley National Laboratory
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Silicon Semiconductor Detectors
Al Electrode
SiO2
p+ B - implant
High purity - Si
200 to 300 mm
(n-type)
hn (low energy)
-Absorbed in the
h+
e-
n+ contact
hn (high energy)
-Absorbed in the
active volume.
contact.
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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
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MSL Processed Silicon Detector Wafer
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Instrument Size
WIND 3-D Plasma and Energetic
Particle Experiment
Suprathermal Electron Telescope
Element (STEREO-IMPACT)
(UC Berkeley Space Sciences Lab)
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In-Situ Doped Polysilicon
Baseline Process – In-situ phosphorus doped polysilicon (ISDP).
It yields a thin (≤200Å), low leakage (~300 pA/cm2 @ 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.
Current (A)/cm2)
10
-10
W127 A3
Detector Area =0.09 cm2
8
6
4
2
10
-11
8
6
4
Pixel
2
10
-12
0
20
40
60
80
100
Bias (V)
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Thin backside n+ ohmic contact development
6 Mar 2002 O2
SIMS depth profile
P profiles
ISDP – in-situ doped
polysilicon
CONCENTRATION (atoms/cc)
1E+21
The thin backside n+ contact technology
developed at the MSL is an enabling
technology for
a) Photodiodes for medical applications
b) CCDs
c) Charged-particle detectors in space
FILE: F1576com
1E+22
1E+20
1E+19
~ 20nm ISDP
~ 10nm ISDP
1E+18
1E+17
1E+16
0
200
Lawrence Berkeley National Laboratory
400
600
800
1000
DEPTH (Angstroms)
8
In-Situ Doped Polysilicon Contact
35
Detected Energy(keV)
30
Energy lost by the
protons in the contact is
about 2.3 keV.
25
20
15
10
5
0
0
5
10
15
20
25
30
Incident Proton Energy (keV)
35
Lawrence Berkeley National Laboratory
Data taken by R. Campbell at
UC Berkeley’s Space Sciences
Laboratory
9
Electron Peak Centroid (Channel Number)
In-Situ Doped Polysilicon Contact
50
Energy lost by electrons
in the 200Å doped
polysilicon window is
about 353 eV.
40
30
20
10
0
0
2
4
6
8
10
Data taken by D. Larson at UC
Berkeley’s Space Sciences
Laboratory
Incident Electron Energy (keV)
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In-Situ Doped Polysilicon Contact
109
Ag - L (~3 keV)
55
3000
Mn - K (5.9 keV)
Detector Area = 0.09cm
300mm thick
2500
109
Counts
2000
Ag - K (22.1 keV)
109
1500
2
Ag - K (25.0 keV)
Data taken by D. Curtis
at UC Berkeley’s Space
Sciences Laboratory.
1000
500
0
0
20
40
60
80
100
Energy (keV)
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.
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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
Lawrence Berkeley National Laboratory
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
http://www.nasa.gov/mission_pages/themis/spacecraft/SST.html
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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.
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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)
Silicon Semiconductor Detector
(STE)
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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
Lawrence Berkeley National Laboratory
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
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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.
Active Layer – 10 mm
n+ P - Implant
Al Electrode
p+ B - implant
SiO2
Back Contact
675
mm
Handle Wafer
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Thin Silicon Alpha Spectrum
600
1.75 MeV
FWHM = 92 keV
500
3 MeV
FWHM = 35 keV
Counts
400
W23922-A6, 0.25 cm
2
12mm thick
Illuminated through the n-type contact
Counting Time = 1800 secs
Pk Time = 8ms
Bias = 10V
300
200
100
0
0
200
400
600
800
1000
Channel Number
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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).
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Silicon x-ray Transmission
100
Transmission
80
Silicon Transmission
60
50Å
100Å
200Å
500Å
1000A
40
20
0
200
400
600
800
1000
Energy (eV)
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Thin Contact Fabrication Techniques
Technique
Thickness (Å)
Compatible
with metal?
Amorphous Si
≥300
Yes
≤77
In-situ doped poly
200
No
84
Implant/Anneal
~1000
Yes
42
Implant/Laser
~700
Yes
54
MBE
≤100
Yes
≥92
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%Transmission
at 280eV
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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”.
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Implant/Low Temperature Anneal
6
4
2
)
-8
Leakage Current (A/cm
10
2
10
-9
6
4
2
-10
2
10
0.077 cm Pixel
6
2
0.924 cm Pixel
4
2
10
-11
0
20
40
60
Bias (V)
80
100
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 – 16.59 cm2.
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Implant/Low Temperature Anneal
Guibilato, et. al. NIM A
650(2011) 184
SOI Imager (Active Pixel Sensor)
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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
26
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Implant/Laser Anneal
-8
Leakage Current (A)/cm
2
10
4
2
-9
10
2
4
0.09 cm Pixels
2
-10
10
Pixel 1
Pixel 2
4
2
-11
10
0
20
40
60
80
100
Bias (V)
- 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.
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Chemical Etching/a-Si
-6
Current (A)/cm
2
10
10
10
10
10
-7
a-Si Contacts
W151 C2
W152 B5
W152 C2
-8
-9
-10
0
20
40
60
80
100
Bias (V)
- 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.
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Molecular Beam Epitaxy (MBE)
Contact Configuration
Incoming x-rays
Silicon cap layer
- Ideally a single monolayer of
electrically active dopant atoms
is desired.
- The silicon capping layer is
required to form a stable contact.
d-doping
layer
Silicon device
Front side
pattern/electronics
The Key:
- This is a deposited contact, so
the beginning surface defect
density must be low in order
to obtain low leakage current.
Pioneering work on d-doped contacts
was done by Nikzad’s group at JPL.
IEEE TED, 55, Dec. 2008
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Molecular Beam Epitaxy (MBE)
Load Lock
Buffer
Chamber
MBE Chamber
Base Pressure ~5x10-11 torr
Substrate
Sb or B
Knudsen Cell
e-beam gun
(silicon)
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Molecular Beam Epitaxy (MBE)
Typical
SVT Associates
Silicon MBE
System
Load-Lock
Deposition
Chamber
Substrate
Preparation
Chamber
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Thin Contact Fabrication Techniques
Technique
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
Thin contact, low leakage.
Process temperature too high,
expensive equipment.
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Silicon x-ray Transmission
MBE
100
Transmission
80
Silicon Transmission
60
50Å
100Å
200Å
500Å
1000A
40
20
Implant/Low Temperature Anneal
“Pizza Process”
0
200
400
600
800
1000
Energy (eV)
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Fine Pitch Germanium Strip Detector
1024 strips, 50 mm pitch, 5 mm length
1 mm thick detector
~ 30 pA / strip @ Vb = 55 V, T >100 K
Developed for time-resolved x-ray
absorption spectroscopy
J. Headspith, et al., Daresbury Lab
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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)
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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.
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