Progress Towards Active Pixel Sensor Detectors for Solar Orbiter

Dr Nick Waltham

Head of Imaging Systems Division,

Space Science & Technology Department, Rutherford Appleton Laboratory

CCDs for Solar Physics . . .

CCDs for Solar Physics e.g. SOHO, SMEI, SOLAR-B, STEREO . . . Future missions e.g. SDO

But inherent limitations . . . External drive electronics needed. Solar physics community always want more

pixels, more channels, faster readout. But with smaller/lower size, mass, and power !

And for Solar Orbiter . . . Radiation damage !

Protons, neutrons, etc Loss of charge transfer efficiency !

Pixel Size ? Smaller pixels yield a smaller instrument.

STEREO CCD

Solar Mass Ejection Imager (SMEI)

RAL - Current programme . . .

SMEI (Solar Mass Ejection Imager) CCD camera

Birmingham University, UCSD, AFRL, Primary requirement - high dynamic range. Await launch

STEREO/SECCHI solar science mission

CCD camera design for 4 instruments with NRL and Birmingham.

Key requirements - high speed readout, high dynamic range, yet reduced size, mass, and power.

Now feeds into camera designs for SDO AO proposals.

R&D Activities

ASIC based CCD camera readout electronics. CMOS Active Pixel Sensors.

Solar Mass Ejection Imager (SMEI)

CCD ASICsActive Pixels

What are we doing about it ?

An alternative sensor technology to CCDs . . .

Development of

Science-grade

CMOS Active Pixel Sensors !

What is an Active Pixel Sensor (APS) ?

RampGen

PixelArray

Counter

FiniteState

Machine

ColumnAmplifiers

ColumnComparators

Digital Output

Digital Latches

Digital Store

Pixel

Column Select

RowSelect

Reset

Photodiodes

Reset Voltage Vdd

SourceFollower

Image sensor with pixels. Wavelength coverage same as a CCD. Difference charge sensed inside pixel .

Advantages . . . CMOS allows on-chip readout circuitry. Low mass, low power cameras. Smaller pixel size.

Shorter optics / smaller instrument.

Charge sensed inside pixel . . .

No charge transfer.

Greater radiation tolerance.

What are we doing about it . . . Science-Grade APS development Program.

RAL APS Development Programme 1999 Design / modelling of pixel test structures (0.5 and 0.7 m CMOS).

2000 Fabrication and testing of pixel test structures. Established formal collaboration with Marconi (EEV)

Exchange of designs / ideas, packaging, testing, Back-thinning Unique capability !

Design of 512 x 512 pixel sensor (0.5 m CMOS)

2001 Fabrication of 512 x 512 pixel prototype sensors. Birmingham join collaboration (Helen Mapson-Menard, Chris Eyles). EEV thinning test structures and 512 x 512 prototypes. First tests of 512 x 512 prototypes. Planning for Solar Orbiter (move to 0.25 m CMOS).

2002 Design / fabrication of 0.25 m CMOS 4kx3k 5m pixel APS. Full testing of front and back-illuminated 512 x 512 prototypes.

RAL APS test structure development

Pixel Design Test Structure chip with four 16 x 16 pixel arrays

Test Results

QE ~35% 10um active thickness Noise estimate < 50 electrons rms Calculated node capacitance = 18 fF Overall responsivity = 6 V per electron Peak output signal = 1 volt (167k electrons) Dark signal = 140 mV per second (0.6 nA/cm2) Fixed pattern noise = 10mV pk-pk

Quadradot Pixel Quantum Efficiency

Helen Mapson-Menard Chris Eyles (Birmingham)

512 x 512 Pixel Prototype Pixel Active Pixel Sensor

6 inch wafer of 512 x 512 sensors

Wire-bonded to an evaluation PCB

Individual

512 x 512 pixel

APS chip

512 x 512 Pixel Prototype Pixel Active Pixel Sensor

Two test Images

512 x 512 Pixel Prototype Pixel Active Pixel Sensor

Note Dynamic range and Anti-blooming Performance !

So where next ?

ESA’s Solar Orbiter

Driven by the proposal of a Spectrometer on Solar Orbiter

Detector Requirements

EUV sensitive

Large format (4k x 4k pixels)

Small (5 m pixels)

Science-grade linearity, low-noise, good uniformity

Radiation hard

Low power

Solar Orbiter Detector

Foveon's 16.8-million-pixel APS

RampGen

PixelArray

Counter

FiniteState

Machine

ColumnAmplifiers

ColumnComparators

Digital Output

Digital Latches

Digital Store

Detector Goals 4k x 4k pixel sensor 5m pixel size 14 bit dynamic range EUV Sensitive 4-transistor CDS pixel 0.25 m rad-hard

CMOS process

Development Programme Refine requirement specification Develop CAD models and simulations Define architectural design Design, fabricate, and test 5m pixel test structures Transfer 14-15 bit ADC to 0.25 m CMOS Investigate back-thinning with Marconi

Demonstrate feasibility in time for the AO !

It can be done !

Solar Orbiter Detector - Photodiode pixels.

Standard APS pixel SO 4T pixel

Column Output

Ibias

Column Select Switch

Reset Switch

Source Follower

Input MOS

RST

SEL

Column Output

Ibias

Transfer Gate

RST

SEL

TX

Simplest architecture.

No kTC or Fixed-Pattern noise reduction.

kTC and Fixed-Pattern noise reduction

possible by differential readout of dark

and signal level.

Solar Orbiter Detector - Photodiode pixels.

4-MOS transistor pixel. PMOS transfer gate for kTC / FPN noise reduction and increased dynamic range.

Schematic Layout

Solar Orbiter Detector - Chip Architecture

PIXEL SELECTION LOGIC

4K x 3K PIXEL ARRAYDIFFERENTIAL ANALOGUE OUTPUT

DIFFERENTIAL AMPLIFIER

A

Solar Orbiter Detector - Progress

1. Area sensor: ESA’s Solar Orbiter for

EUV imaging of Sun 4Kx3K, noise

~ 10 e- rms, DR ~ 12+ bits

2. Linear sensor: TOPSAT-2

BNSC Earth Observation 1 m

ground resolution, colour, on-chip 10-

bit ADCs, ~ 1500 fps

3. Electron sensor: HEP Vertex (Linear

Collider, RHIC upgrade, biomed)

intelligence in the pixel, fast readout

(50MHz/row), low noise (~ 10 e- rms),

data sparsification, radiation

resistance

0.25 m CMOS manufactured by TSMC

IC design / manufacturing sharing reduction of costs

Linear sensor4,000 pixels at 3 m pitch

Electron sensor

Area sensor

4,000 * 3,000 = 12 Million pixels

5 m pitch

1

23

Solar Orbiter Detector - Progress

4k x 3k Sensor – Predictions from CAD simulation work

• Full Well Capacity ~ 46k electrons (5 m pixel)

• Readout Noise ~ 10 electrons rms

• Dynamic Range ~ 12+ bits

• Fill Factor ~ 30% (but be careful !)

• Readout rate ~ 1 MHz (through one port only)

• Readout time ~ 12 seconds (full-frame, no windowing)

Solar Orbiter Detector - Future

Future Possibilities

Chips of any format up to 4k x 4k (5 m) pixels. e.g.

4k x 512 (5 m) pixel strips on selected bands.

2k x 2k (10 m) pixels.

or you could have odd-shaped pixels (10 x 20 m).

CMOS allows you to design what you want !

Multi-port readout for higher frame rate.

Windowed readout.

Anything else wanted – besides EUV sensitivity ?

EUV sensitivity – Backthinning, or FIB, or ?

Two different approaches to enhance the EUV sensitivity of silicon APS.

1) Backthinning ( back-illumination): with Marconi Applied Technology

2) Front etching of the oxide by Focused Ion Beam

Front vs Rear illumination

Charge diffusion to adjacent pixelsIn field-free region

EUV Photon

EUV Photon

< 5m

Solar Orbiter Detector - Future

Development Programme

4k x 3k pixel Array Manufacture (Delivery Dec 02).

Array Packaging (Jan 03).

First light (Feb 03).

Testing of 512 x 512 pixel prototypes (front and rear illuminated). E2V are delivering thinned sensors this week.

Transfer of 14-15 bit ADC to 0.25 m CMOS in second phase.

Thinning of 4k x 3k pixel array and EUV testing.

Summary

Requirement

EUV Sensitivity

4k x 4k pixels

5 m pixel size

Readout < 1 second

Low power

Cooling

Radiation hardness

Availability

Science-grade CCD

Yes

Yes (for SDO ?)

No

Not easily

No

-80 °C

Poor (CTE degradation)

Soon (12 m pixels)

? (5 m pixels)

APS

Probably

Yes

Yes

Yes / Windowed readout

Yes

-80 °C

Good (no CTE involved)

(2003 – 2006) ?

Solar Orbiter Detector - Alternatives

Have considered naked CMOS APS so far.

Are there alternatives ?

Diamond / Bold ? Progress towards a large format array ?

Smallest pixel ? / Bump bonding ?

CID Radiation hard but limited availability, small range of

formats, large pixels, very high read noise.

Intensified APS – like in CDS ?