C. Tindall, P. Denes , S. E. Holland, N. Palaio , D. Contarato , D. Doering

Lawrence Berkeley National Laboratory1

C. Tindall, P. Denes, S. E. Holland, N. Palaio, D. Contarato, D. Doering

Thin Contact Development for Silicon Detectors

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


Thermco/Expertech 150mm furnaces 150 mm Lithography tool

LBNL Microsystems Laboratory

LBNL Microsystems Laboratory – Class 10 Cleanroom


Silicon Semiconductor Detectors

High purity - Si

(n-type)

200 to 300 mm

SiO2

n+ contact

p+ B - implant

Al Electrode

e-

h+

hn (high energy)- Absorbed in the active volume.

hn (low energy)-Absorbed in the contact.


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


MSL Processed Silicon Detector Wafer

Lawrence Berkeley National Laboratory

Instrument Size

WIND 3-D Plasma and EnergeticParticle Experiment

Suprathermal Electron TelescopeElement (STEREO-IMPACT)

(UC Berkeley Space Sciences Lab)

6


10-12

2

4

68

10-11

2

4

68

10-10

Cur

rent

(A)

100806040200Bias (V)

Pixel

In-Situ Doped Polysilicon

W127 A3Detector Area =0.09 cm2

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.

/cm

2 )


Thin backside n+ ohmic contact development

1E+16

1E+17

1E+18

1E+19

1E+20

1E+21

1E+22

0 200 400 600 800 1000

DEPTH (Angstroms)

CO

NC

ENTR

ATI

ON

(ato

ms/

cc)

~ 10nm ISDP

~ 20nm ISDP

6 Mar 2002 O2 FILE: F1576com

P profiles

The thin backside n+ contact technologydeveloped at the MSL is an enabling technology fora) Photodiodes for medical applicationsb) CCDsc) Charged-particle detectors in space

SIMS depth profile

ISDP – in-situ dopedpolysilicon


In-Situ Doped Polysilicon Contact

35

30

25

20

15

10

5

0

Det

ecte

d E

nerg

y(ke

V)

35302520151050Incident Proton Energy (keV)

Energy lost by the protons in the contact is

about 2.3 keV.

Data taken by R. Campbell atUC Berkeley’s Space Sciences

Laboratory



50

40

30

20

10

0

Ele

ctro

n P

eak

Cen

troid

(Cha

nnel

Num

ber)

1086420

Incident Electron Energy (keV)

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



3000

2500

2000

1500

1000

500

0

Cou

nts

100806040200

Energy (keV)

109Ag - L (~3 keV)

55Mn - K (5.9 keV)

109Ag - K (22.1 keV)

109Ag - K (25.0 keV)

Detector Area = 0.09cm2

300mm thick

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.

Data taken by D. Curtis at UC Berkeley’s Space Sciences Laboratory.


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.

Mock up of the SEP InstrumentPrototype Detector Stack


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

http://www.nasa.gov/mission_pages/themis/spacecraft/SST.html

THEMIS PIN DiodeFabricated in the MSL


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.


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


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


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


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 mmn+ P - Implant


Thin Silicon Alpha Spectrum

600

500

400

300

200

100

0

Cou

nts

10008006004002000Channel Number

W23922-A6, 0.25 cm2

12mm thick Illuminated through the n-type contact

Counting Time = 1800 secs Pk Time = 8ms

Bias = 10V

1.75 MeVFWHM = 92 keV

3 MeVFWHM = 35 keV


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).


Silicon x-ray Transmission

100

80

60

40

20

0

Tran

smis

sion

1000800600400200Energy (eV)

Silicon Transmission

50Å 100Å 200Å 500Å 1000A


Thin Contact Fabrication Techniques

Technique Thickness (Å)Compatible with metal?

%Transmission at 280eV

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


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”.



- 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.

10-11

2

46

10-10

2

46

10-9

2

46

10-8

Leak

age

Cur

rent

(A/c

m2 )

100806040200

Bias (V)

0.077 cm2 Pixel

0.924 cm2 Pixel

Our CCDs that utilize “pizza process” contacts for soft x-ray detection.



25SOI Imager (Active Pixel Sensor)Guibilato, et. al. NIM A

650(2011) 184



26

After Thinning Before Thinning

After the “Pizza” Process

SOI Imager-2 (Active Pixel Sensor)Battaglia, et. Al. NIM A 676 (2012) 50


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.

10-11

2

4

10-10

2

4

10-9

2

4

10-8

Leak

age

Cur

rent

(A)/c

m2

100806040200

Bias (V)

Pixel 1 Pixel 2

0.09 cm2 Pixels


Chemical Etching/a-Si

10-10

10-9

10-8

10-7

10-6

Cur

rent

(A)/c

m2

100806040200

Bias (V)

W151 C2 W152 B5 W152 C2

a-Si Contacts

- 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.


- Ideally a single monolayer of electrically active dopant atoms is desired.

- The silicon capping layer is required to form a stable contact.

Molecular Beam Epitaxy (MBE)

Contact Configuration

Incoming x-rays

Silicon cap layer

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



Load Lock Buffer Chamber

MBE ChamberBase Pressure ~5x10-11 torr

e-beam gun(silicon)

Sb or B Knudsen Cell

Substrate



Deposition Chamber

Load-Lock

Substrate Preparation Chamber

Typical SVT Associates

Silicon MBE System


Thin Contact Fabrication Techniques

Technique Advantages DisadvantagesAmorphous 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.


Silicon x-ray Transmission

100

80

60

40

20

0

Tran

smis

sion

1000800600400200Energy (eV)

Silicon Transmission

50Å 100Å 200Å 500Å 1000A

Implant/Low Temperature Anneal“Pizza Process”

MBE


Fine Pitch Germanium Strip Detector

Developed for time-resolved x-ray absorption spectroscopyJ. Headspith, et al., Daresbury Lab

1024 strips, 50 mm pitch, 5 mm length1 mm thick detector~ 30 pA / strip @ Vb = 55 V, T >100 K


Detector Group at LBNL

• 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)

Historical accomplishments with significant impact on radiation detector technology:


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.

C. Tindall, P. Denes , S. E. Holland, N. Palaio , D. Contarato , D. Doering

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Transcript of C. Tindall, P. Denes , S. E. Holland, N. Palaio , D. Contarato , D. Doering