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

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

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.

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

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

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. W127 A3 Detector Area =0.09 cm2 /cm2)

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

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

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

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.

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

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

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 mm n+ P - Implant

Thin Silicon Alpha Spectrum

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

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

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”.

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 – 16.59 cm2.

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

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

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.

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.

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

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)

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

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.

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

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

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)

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.