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 ASICs Active 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) ? Pixel Column Select Row Select Reset Photodiodes Reset Voltage Vdd Source Follower  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. S horter 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)  Fabrication and testing of pixel test structures.  Established formal collaboration with Marconi (EEV)  Exchange of designs / ideas, packaging, testing,  Back-thinningUnique 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)  Design / fabrication of 0.25  m CMOS 4kx3k 5  m pixel APS.  Full testing of front and back-illuminated 512 x 512 prototypes.

RAL APS test structure development Pixel DesignTest 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/cm 2 )  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 Detector Goals  4k x 4k pixel sensor  5  m 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 5  m pixel test structures  Transfer 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 I bias Column Select Switch Reset Switch Source Follower Input MOS RST SEL Column Output I bias 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. SchematicLayout

Solar Orbiter Detector - Chip Architecture PIXEL SELECTION LOGIC 4K x 3K PIXEL ARRAY DIFFERENTIAL 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 sensor 4,000 pixels at 3  m pitch Electron sensor Area sensor 4,000 * 3,000 = 12 Million pixels 5  m pitch 1 2 3

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 pixels In field-free region EUV Photon < 5  m

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 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 / 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 ?