National Aeronautics and Space Administration

Delta doping Technology for UV Photon Counting Detector Arrays Michael Hoenk Jet Propulsion Laboratory, California Institute of Technology

KISS Single Photon Counting Detectors Workshop California Institute of Technology Pasadena, California

The work described here was carried out by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. Copyright 2010 California Institute of Technology. Government sponsorship acknowledged.

Outline 

Motivation – UV photon counting detectors



Silicon surface physics and back surface passivation



Delta-doping – nanostructured silicon by MBE



Delta-doping with high throughput and high yield

Copyright 2010 California Institute of Technology. Government sponsorship acknowledged.

MIDEX-ISTOS Detector MIDEX-ISTOS PI: Chris Martin, Caltech

Data Flow Storage Area

L3 functional diagram

UV Photons Serial register Extended serial register (50V) Amplified data is sent to a photon counting discriminator, eliminating read noise.

Detector Requirement: Six cylindrically-curved ultraviolet photon counting detector array Strategy:

Image Area

8” wafer, 30 CMOS imagers

Combine delta doping with avalanche gain CCDs and AR coatings, along with Curved FPA technology

Copyright 2010 California Institute of Technology. Government sponsorship acknowledged.

Low-energy Molecular Ion Detection

5000-10000dn shown – 60s exposure /80amu centered

Iron Pentacarbonyl (196amu) Signal (Digital Number)

30000

56

25000

20000

84 15000

10000

54

5000

68/70

0 0

200

400

600

800

1000

1200

Row Number

Ion Image Data – Background Image Data = Spectrum Data

Copyright 2010 California Institute of Technology. Government sponsorship acknowledged.

Silicon Imaging Arrays “CCDs were born in the Si-SiO2 revolution and created their own revolution in widespread imaging device applications.” -- George Smith, co-inventor of CCDs* and Nobel Laureate

George E. Smith, “The invention and early history of the CCD,” Nuclear Instruments and Methods in Physics Research A, 607: 1-6, 2009. Copyright 2010 California Institute of Technology. Government sponsorship acknowledged.

A Brief History of Back Illumination 500 Million years of back-illumination Cross section of human retina

Vertebrate Eye

Cephalopod Eye

Vertebrates and mollusks developed the camera eye independently. Vertebrate retina is less efficient than Mollusk retina.

Readout

Detector

Copyright 2010 California Institute of Technology. Government sponsorship acknowledged.

Silicon Imager Architectures: CCD vs. CMOS Charge-coupled Device (CCD) Serial readout device with charge transfer and one (or few) readout amplifiers.

CMOS Imaging Array Parallel readout with few charge transfers and one readout amplifier per pixel.

Pixel Array

CCD Row Decoder

Column Processing Circuitry

Analog Output

Column Decoder

CMOS APS

Scientific CMOS imagers are catching up with CCDs – Jim Janesick, 2009 Copyright 2010 California Institute of Technology. Government sponsorship acknowledged.

Back Illuminated CMOS Imager Thinned CMOS Array

front surface

Back surface

Quantum Efficiency (%)

100

Delta-doped (without AR coating)

80 60

Front illuminated (control) 40 20 0 400

500

600

700

800

900

1000

1100

Wavelength (nm)

Thinning and back illumination can overcome limitations of silicon detectors: • Quantum efficiency • Fill factor • Spectral range (especially in the blue-UV range) Copyright 2010 California Institute of Technology. Government sponsorship acknowledged.

UV Photon Absorption in Silicon

Absorption Length (nm)

Soft X-ray

10 10 10 10 10 10 10

EUV

UV

Vis. NIR

7 6

High purity silicon detector thickness

5 4

Conventional silicon detector thickness

3

Conventional electrode thickness 2

Shallow ion implant 1

1

10

100

1000 Delta-doping thickness ~1 nm

Wavelength (nm)

Delta-doping: 100% internal QE throughout the entire spectral range of silicon detectors Copyright 2010 California Institute of Technology. Government sponsorship acknowledged.

Between Physics and Chemistry Si-SiO2 Surface States and Charging mechanisms • Fast states / surface traps – QE hysteresis • Acceptor-like – Neutral when empty, negative when filled • Donor-like – Positive when empty, neutral when filled

• Slow states • Surface charging • e.g., oxygen ions generated in UV flood

• Fixed oxide charge http://rohlfing.physik.uni-osnabrueck.de/

• Radiation damage • e.g., positive charge from exposure to ionizing radiation (including UV light) flattens bands

Surface Passivation • Reduce surface state density • Control surface potential Copyright 2010 California Institute of Technology. Government sponsorship acknowledged.

Back Illumination and QEH In 1984, quantum efficiency hysteresis was discovered during thermal vacuum testing of Wide Field Camera (WFPC-1). Back surface

+ + -- + + + + +

-

Trapping of photo-generated charge

Front surface

-

- - - + + + +

Backside potential well

Buried channel

Quantum efficiency hysteresis – CCD response depends on prior illumination history • Unacceptable – Hubble needs stability to 1% over 30 days… 2010 California of Technology. Government sponsorship acknowledged. • Passivation ofCopyright surface defects isInstitute necessary to solve the problem.

UV Flood JPL developed the ―UV flood‖ to stabilize the response (Jim Janesick and Tom Elliott). The Hubble Space Telescope optics were redesigned to allow a ―UV flood‖ in space. Back surface

-

+ + + + + + + + + + +

During the mid- to late- 19800’s, WFPC-1 was plagued with stability problems To help solve this problem, JPL formed a tiger team, which included MDL scientists. Hubble launched in 1990 with WFPC-1 on board. NASA installed WFPC-2 in 1993, with front-illuminated, lumogen-coated CCDs. In 1992, JPL’s Microdevices Laboratory demonstrated theGovernment first delta-doped Copyright 2010 California Institute of Technology. sponsorshipCCD. acknowledged.

Quantum Efficiency Hysteresis in WF/PC 1 & 2

.WF/PC1 (1983-1992) Massive UV flood at 250 nm through light pipe

WF/PC2 (1983-1992) Flash gate, biased flash gate WF/PC2 (1992-2009) Front illuminated Loral CCDs with lumogen John T. Trauger, “Sensors for the Hubble Space Telescope Wide Field and Planetary Cameras (1 and 2),” in CCDs in astronomy: Proceedings of the Conference, Tucson, AZ, Sept. 6-8, 1989 (A91-45976 19-33), San Francisco, CA, Astronomical Society of the Pacific, 1990, p. 217-230 Copyright 2010 California Institute of Technology. Government sponsorship acknowledged.

The Evolution of CCD Surface Passivation on Hubble

• Unpassivated • Doping by controlled thinning (early WF/PC 1)

• Surface charging • UV flood (late WF/PC 1) • Platinum flash gate (WF/PC 2 – never flown) • Chemisorption charging (ACS HRC)

• Unthinned CCD • Front-illuminated with phosphor (WF/PC 2)

• Surface doping • Ion implantation (WFC3) Copyright 2010 California Institute of Technology. Government sponsorship acknowledged.

Platinum Flash Gate High work function metal induces surface charge

• Vulnerable to environmental contaminants (notably hydrogen) • Encapsulation required for stability • Passivation layer doubles as antireflection coating -

Back surface

+ - + + + - + + - + + - + + + -

Unlike MBE, surface charging methods provide poor control to environmental changes. Not suitable for high speed readout Copyright 2010 California Institute of Technology. Government sponsorship acknowledged.

Chemisorption Charging Metal catalyst binds O2- ions on the surface. Encapsulation / AR coating stabilizes the charge.

-

+ + + + + + + + + + +

High pressure oxide Catalytic metal (1 nm Ag) Protective coating (10-20 nm HfO2)

“Chemisorption charging can be a permanent charging process if there are no other processes which either chemically react to remove the oxygen or contribute significant positive charge to produce a net positive charge on the surface.” —Michael Lesser, ―CCD backside coatings optimized for 200-300 nm observations,‖ SPIE Proc. 4139: 8-15, 2000. Copyright 2010 California Institute of Technology. Government sponsorship acknowledged.

Ion Implantation and Laser Anneal Doping the surface introduces fixed charge into the silicon lattice • Dopant profiles are generally broad – thinner is better, but problems with traps • Damaged lattice creates traps & dark current • “Brick wall” pattern in flat field images from laser anneal process

Back surface

+ + + - + + + + + - + + + -

Hubble’s Wide Field Camera 3 uses ion-implanted CCDs. Quantum efficiency hysteresis is still a problem. Copyright 2010 California Institute of Technology. Government sponsorship acknowledged.

The Evolution of Detector Efficiency on Hubble

Mark Clampin, “UV-Optical CCDs”, Proceedings of the Space Astrophysics Detectors and Detector Technologies Conference, STScI, Baltimore, June 26-29, 2000.

“The WFPC2 CCDs are thick, front-side illuminated devices made by Loral Aerospace. They support multi-pinned phase (MPP) operation which eliminates quantum efficiency hysteresis. They have a Lumogen phosphor coating to give UV sensitivity.” http://www.stsci.edu/instruments/wfpc2/Wfpc2_hand4/ch1_introduction2.html Copyright 2010 California Institute of Technology. Government sponsorship acknowledged.

Quantum Efficiency Hysteresis in WFC3

Wide Field Camera 3 • •

Instabilities on the order of a few percent Mitigated by on-orbit flooding with visible light to fill surface traps

Collins et al. SPIE proceedings 7439A-10, San Diego, CA, August 2009. Copyright 2010 California Institute of Technology. Government sponsorship acknowledged.

Delta doping Bandstructure engineering for optimum performance

Atomic layer control over device structure

Delta-doped layer (dopant in single atomic layer)

Low temperature process, compatible with VLSI, fully fabricated devices (CCDs, CMOS, PIN arrays) original Si

Conduction band edge (eV)

Hoenk et al., Applied Physics Letters, 61: 1084 (1992)

2.0

delta-doped potential well width ~ 5 Å

1.8

Native oxide 1 nm

1.6

CCD frontside circuitry

1.4 MBE growth 3 nm

Ec

1.2 1.0 0.8

0

2 4 6 8 Depth from surface (nm) Back Surface

10

Fully-processed devices are modified using Molecular Beam Epitaxy (MBE) Copyright 2010 California Institute of Technology. Government sponsorship acknowledged.

Delta-doped Imaging Detectors Delta-doped 2.0

Energy (eV)

1.8

Near1.6 SurfaceMBE Electronic Potential Ultrashallow implant

1.4

Ion implant

1.2 1.0 0

Hoenk et al., Applied Physics Letters, 61: 1084 (1992)

5

10 15 Depth from Surface (nm)

20

Delta doping produces ideal surface passivation in back-illuminated CCDs Copyright 2010 California Institute of Technology. Government sponsorship acknowledged.

Electric Field 10 Delta-doped

Electric field (V/nm)

1

MBE

0.1 Ultrashallow ion implant 0.01 Ion implant 0.001

0.0001

0

5

10 15 Depth from Surface (nm)

20

Copyright 2010 California Institute of Technology. Government sponsorship acknowledged.

Quantized States in Delta-doped Detectors 1.0 2.0

Quantized electronic "ground state" 0.5

Energy (eV)

1.5 Conduction band (bulk) 1.0

0.0 Quantized valence band

0.5

-0.5 Valence band (bulk)

0.0 0

Fermi level 5

10

15

-1.0 20

Depth (nm)

Copyright 2010 California Institute of Technology. Government sponsorship acknowledged.

Quantum Confinement in Delta-doped Surface • Quantized states in delta-doped surface – Holes form 2DEG with quantized subbands • High conductivity surface • Quantum-confinement enhanced potential and fields – Strongly peaked potential at delta-layer

– Electrons • Width of potential well ~1.5nm • Electrons isolated from surface by high fields • Quantized subbands with few quasi-bound states

• Passivation by Delta-doping – MBE creates abrupt interfaces, electric field ~107 V/cm – Buried electronic “surface” confines electrons to bulk – Surface dark current suppressed by quantum confinement Copyright 2010 California Institute of Technology. Government sponsorship acknowledged.

Quantum Efficiency of Delta-doped CCDs

100

Quantum Efficiency (%)

Delta-doped CCD (AR-coated)

80

60

Delta-doped CCD

40

(uncoated)

Front-illuminated CCD 20

0

0

100

200

300

400

500

600

700

Wavelength (nm) Copyright 2010 California Institute of Technology. Government sponsorship acknowledged.

Delta-doped CCD Stability and Reproducibility

• Near 100% internal QE is measured years after the MBE modification of the CCDs • Reproducible and compatible with different formats and CCD manufacturing processes

• No hysteresis is observed in delta-doped CCDs • Stable response over several years • Precision photometric stability measured by the Kepler group

Quantum Efficiency (%)

100 80

Delta-doped Loral, Reticon, SITe CCDs

60

delta-doped CCD (no A R coatings)

40 Unmodified CCD

20 0

300

500 Wavelength (nm)

700

100 80 60 40

Si transmittance After -doping 18 mos. later 3 yrs later

20 0

300

400 500 Wavelength (nm)

600

Copyright 2010 California Institute of Technology. Government sponsorship acknowledged.

Uniformity Flat field 334 nm

E2v CCD ion implant/laser anneal

Flat field DDCCD, 300 nm

Delta doped CCD

Copyright 2010 California Institute of Technology. Government sponsorship acknowledged.

Delta doped CCDs and CMOS Arrays Thinned CMOS Array

front surface

Delta-doped p-channel CCD, LBNL 1k x 1k

Delta-doped n-channel CCD with structurally supported membrane

80

-------------

1.00 mm

9x8 HYBRID DETECTOR

9x8 -doped diode array bumpbonded to APS readout

AR-coated -doped CCD

100

80

Back illuminated

60

40

Front illuminated

H R D io de an d P h o tom u ltip lier R e s p on s e v s . W a ve len g th

60 -doped CCD 40 Unmodified CCD

20 WF/PCII 0

100

300 500 Wavelength (nm)

20

60 R es po n se (A rb - p A )

Quantum Efficiency (%)

100

A thinned (6-µm thick) 6” wafer containing 30 CMOS devices supported by a quartz wafer. Quantum Efficiency (%)

Delta-doped p-channel CCD LBNL 2k x 4k

Back surface

700

50

Photom ultiplier (Arbitrary)

40

HR Diode Current (pA)

0 400

500

600

700

800

900

1000

1100

Wavelength (nm)

30 20

QE of a 1kx1k delta doped CMOSAPS array

10 0 200

300 Illu min ation Wa v e le n g th (n m)

400

QE of delta doped CCDs Copyright showing 100% internal QE 2010 California Institute of Technology. Government sponsorship acknowledged.

JPL Facilities for End-to-end Post-Fabrication Process Fully-processed arrays fabricated at outside foundries are obtained.

Bonding: Thermocompression

bonding or post MBE bonding is used for achieving flat, robust membranes

Delta doping MBE is used to grow a deltadoped layer of Si on the backside of fully processed silicon arrays .Response of CCD Si imager is enhanced to the theoretical limit.

Thinning: Excellent quality thinned CMOS and CCDs have been demonstrated. AR Coatings and Filters Modeling capability and PECVD and sputtering Chemical Mechanical Polishing system for deposition of filters and AR (CMP) coatings Versatile approach makes it possible to work with various imaging arrays and technologies

AR Coating Systems AJA International 18‖UHV chamber (ATC-Orion 1800) with 4- three inch guns

Deposition uniformity with A330-XP (5) Source Cluster Flange featuring in-situ tilt focused on rotating, 6.0" diameter, Si Wafer. SiO(2) deposition shows +/- 1.17% uniformity. Copyright 2010 California Institute of Technology. Government sponsorship acknowledged.

Full Wafer Processing with 8” MBE

Raft of 9 thinned CMOS Imagers Mounted in 3” MBE

Thinned 6” wafer with 32 CMOS Imagers Copyright 2010 California Institute of Technology. Government sponsorship acknowledged.

8-inch Wafer Silicon MBE

Storage Module

Cluster tool movie With large size wafer capacity and multiple wafer processing, high throughput processes is enabled and delta doping a lot run can be achieved in short period of time

MBE Installation in JPL’s Microdevices Laboratory

MBE after Initial Hookup at JPL

MBE under bake

Photographs of wafers heating inside MBE

Multiple 3-inch wafer

6-inch wafer rotating inside the MBE

Single 8-inch wafer Copyright 2010 California Institute of Technology. Government sponsorship acknowledged.

Conclusions

• Delta-doped single photon detectors for UV astronomy ISTOS Mission: Chris Martin, David Shimonovich, Patrick Morrissey, Shouleh Nikzad

• Silicon surface physics and passivation Requires atomic scale control of dopant profile • Delta-doping Nanostructured surface by MBE Strong electric field (107 V/cm) and quantum exclusion Proven performance • Future Delta-doped L3CCDs for UV photon counting Delta-doping at wafer level for high yield and throughput Copyright 2010 California Institute of Technology. Government sponsorship acknowledged.