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Copyright 2010 California Institute of Technology. Government sponsorship acknowledged.

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

National Aeronautics and

Space Administration

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.


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


MIDEX-ISTOS DetectorMIDEX-ISTOS

PI: Chris Martin, Caltech

Detector Requirement:

Six cylindrically-curved ultraviolet photon

counting detector array

Strategy:

Combine delta doping with avalanche

gain CCDs and AR coatings, along with

Curved FPA technology

Image Area

Storage

Area

Data Flow

Serial register

Extended serial register (50V)

Amplified data is sent to a photon counting

discriminator, eliminating read noise.

UV Photons

L3 functional diagram

8” wafer, 30 CMOS imagers


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

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igital N

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Low-energy Molecular Ion Detection

Ion Image Data – Background Image Data = Spectrum Data

Iron Pentacarbonyl (196amu)


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.


Vertebrate Eye Cephalopod Eye

500 Million years of back-illumination

Vertebrates and mollusks developed the camera eye independently.

Vertebrate retina is less efficient than Mollusk retina.

A Brief History of Back Illumination

DetectorReadout

Cross section of human retina


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.

Silicon Imager Architectures: CCD vs. CMOS

CCD

CMOS APS

Scientific CMOS imagers

are catching up with CCDs– Jim Janesick, 2009

Column Decoder

Row

Decoder

Pixel

Array

Column

Processing Circuitry

Analog

Output


Thinned CMOS Array

Back surface

front surface

Back Illuminated CMOS Imager

Thinning and back illumination can overcome limitations of silicon detectors:

• Quantum efficiency

• Fill factor

• Spectral range (especially in the blue-UV range)

100

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an

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11001000900800700600500400

Wavelength (nm)

Delta-doped (without AR coating)

Front illuminated (control)


101

102

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107

Abso

rpti

on L

ength

(nm

)

1 10 100 1000

Wavelength (nm)

UV Photon Absorption in Silicon

High purity silicon detector thickness

Soft X-ray EUV UV Vis. NIR

Delta-doping: 100% internal QE throughout the entire spectral range of silicon detectors

Shallow ion implant

Delta-doping thickness ~1 nm

Conventional silicon detector thickness

Conventional electrode thickness


http://rohlfing.physik.uni-osnabrueck.de/

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

• 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


Front surface

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Back surface -

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Trapping of photo-generated charge

Backside potential well Buried channel

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Front surface

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Back surface -

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-

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Back surface -

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Back Illumination and QEH

Quantum efficiency hysteresis – CCD response depends on prior illumination history

• Unacceptable – Hubble needs stability to 1% over 30 days…

• Passivation of surface defects is necessary to solve the problem.

In 1984, quantum efficiency hysteresis was discovered during thermal vacuum testing

of Wide Field Camera (WFPC-1).


UV Flood

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Back surface-

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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 the first delta-doped CCD.

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.


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 lumogenJohn 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

.


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)


Platinum Flash Gate

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Unlike MBE, surface charging methods provide poor control to environmental changes. Not suitable for high speed readout

Back surface

-

High work function metal induces surface charge

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

-

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

Chemisorption Charging

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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.


Ion Implantation and Laser Anneal

Hubble’s Wide Field Camera 3 uses ion-implanted CCDs.Quantum efficiency hysteresis is still a problem.

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

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Back surface

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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


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.


Bandstructure engineering for optimum performance

Atomic layer control over device structure

Delta doping

Low temperature process, compatible with VLSI, fully

fabricated devices (CCDs, CMOS, PIN arrays)

Conduct

ion b

and e

dge

(eV

)

Depth from surface (nm)

delta-doped potential well

width ~ 5 Å

Back Surface

2.0

1.8

1.6

1.4

1.2

1.0

0.81086420

Ec

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

Fully-processed devices are modified using Molecular Beam Epitaxy (MBE)

MBE growth

3 nm

Delta-doped layer(dopant in single atomic layer)

Native oxide1 nm

original Si

CCDfrontsidecircuitry


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

Delta doping produces ideal surface passivation in back-illuminated CCDs

Near Surface Electronic Potential

Delta-doped Imaging Detectors

2.0

1.8

1.6

1.4

1.2

1.0

En

erg

y (

eV

)

20151050

Depth from Surface (nm)

Ion implantUltrashallow implant

MBE

Delta-doped


Electric Field

0.0001

0.001

0.01

0.1

1

10

Ele

ctr

ic f

ield

(V

/nm

)

20151050

Depth from Surface (nm)

MBE

Ultrashallow ion implant

Ion implant

Delta-doped


Quantized States in Delta-doped Detectors

2.0

1.5

1.0

0.5

0.0

Ene

rgy (

eV

)

20151050

Depth (nm)

-1.0

-0.5

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0.5

1.0

Fermi level

Conduction band (bulk)

Quantized electronic "ground state"

Quantized valence band

Valence band (bulk)


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


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antu

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Wavelength (nm)

Delta-doped CCD(AR-coated)

Delta-doped CCD(uncoated)

Front-illuminated CCD

Quantum Efficiency of Delta-doped CCDs


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

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Wavelength (nm)

Si transmittanceAfter -doping18 mos. later 3 yrs later

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delta-doped CCD(no AR coatings)

UnmodifiedCCD

Delta-doped Loral, Reticon, SITe CCDs


Flat field 334 nm

E2v CCD ion implant/laser anneal Delta doped CCD

Flat field DDCCD, 300 nm

Uniformity


Delta doped CCDs and CMOS Arrays

A thinned (6-µm thick) 6”

wafer containing 30 CMOS

devices supported by a

quartz wafer. 100

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tum

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11001000900800700600500400

Wavelength (nm)

Back illuminated

Front illuminated

QE of a 1kx1k delta doped CMOS-

APS array

Thinned CMOS Array

Back surface

front surface

Delta-doped

p-channel CCD

LBNL 2k x 4k

Delta-doped p-channel

CCD, LBNL 1k x 1k

Delta-doped n-channel

CCD with structurally

supported membrane

------------- 1.00 mm

9x8 HYBRID DETECTOR

9x8 -doped diode array bump-bonded to APS readout

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700500300100Wavelength (nm)

-doped CCD

UnmodifiedCCD

WF/PCII

AR-coated

-doped CCD

H R D iode and P hotom ultip lier R esponse vs .

W ave length

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Illumination Wave length (nm)

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Photomultiplier (Arbitrary)

HR Diode Current (pA)

QE of delta doped CCDs

showing 100% internal QE

JPL Facilities for End-to-end Post-Fabrication Process

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

Bonding: Thermocompressionbonding or post MBE bonding is used for achieving flat, robust

membranes

Thinning: Excellent quality thinned CMOS and CCDs have been demonstrated.

Versatile approach makes it possible to work with various imaging arrays and technologies

Fully-processed arrays fabricated at outside foundries are obtained.

Chemical Mechanical Polishing (CMP)

AR Coatings and Filters Modeling capability and PECVD and sputtering system for deposition of filters and AR coatings


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.


Thinned 6” wafer with 32 CMOS Imagers

Full Wafer Processing with 8” MBE

Raft of 9 thinned CMOS Imagers

Mounted in 3” MBE

Storage Module

Cluster tool movie

8-inch Wafer Silicon MBE

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 after

Initial Hook-

up at JPL

MBE under

bake

MBE Installation in JPL’s

Microdevices Laboratory


Multiple 3-inch wafer

Single 8-inch wafer

Photographs of wafers heating inside MBE

6-inch wafer rotating inside the MBE


Conclusions• Delta-doped single photon detectors for UV astronomy

ISTOS Mission: Chris Martin, David Shimonovich, Patrick Morrissey, Shouleh Nikzad

• Silicon surface physics and passivationRequires atomic scale control of dopant profile

• Delta-dopingNanostructured surface by MBEStrong electric field (107 V/cm) and quantum exclusionProven performance

• FutureDelta-doped L3CCDs for UV photon countingDelta-doping at wafer level for high yield and throughput

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