Characterisation of Active Pixel Sensors

1. Characterisation ofActive Pixel Sensors Dima Maneuski 1st year PhD Student University of Glasgow

2. Outline • What is APS? What is CCD? • Vanilla APS • HEPAPS4 • What do we actually need to characterise? • Photon transfer technique • Experimental setup • Status on Prague activity

3. We live here in Kevin Building

4. Basic CCD camera Advantages: • Low noise; • High full-well capacity; • 100% fill factor; • High uniformity; • Mature technology; Disadvantages: • Slow readout; • Pixel blooming; • Specialised fabrication; • Low functionality;

5. Basic CMOS camera Advantages: • High Speed readout; • Random access; • On-chip intelligence; • Low power consumption; Disadvantages: • High read-out noise; • Reduced dynamic range; • Reduced uniformity; • Reduced fill-factor; • High cost;

6. Vanilla APS • 512 x 512 pixels – 25 mm square • Fill Factor ~ 85 % • Optional soft/hard/flushed • Analogue and digital output • up to 100 fps at 12 bit digital output • Flexible ROIs (up to 6 x 6 @ 20kHz in analogue output) • 100k –e full well capacity

7. Flushed Reset • Soft reset results in a lowered reset noise (kTC/2)1/2 instead of (kTC)1/2 • However, frames taken utilising soft reset are affected by image lag, where the current image is affected by the previous frame. • Using hard reset, image lag is overcome, but results in full (kTC)1/2 noise and reduced full well capacity. • By using a hard reset followed by a soft reset, it is possible to get the best of both reset methods. This is known as flushed reset.

8. Region of Interest • 12 bit digital output for full frame mode. • Region of Interest (ROI) readout for up to 6 ROI (6 x 6 pixels/ROI). • 20kHz analogue readout at 10 bits resolution for ROI. 520x520 – 4fps 200x200 – 28fps 50x50 – 432fps

9. HEPAPS4 • 1024 x 384 pixels – 15 mm square • 100 % efficiency for MIPs • 20um epi • Radiation hard ( > Mrad) (?)

10. Spectral Response Refers to the detected signal response as a function of the wavelength of light. Often expressed in terms of the Quantum Efficiency, a measure of the detector's ability to produce an electronic charge as a percentage of the total number of incident photons that are detected.

11. Dynamic Range A measure of the maximum and minimum intensities that can be simultaneously detected in the same field of view. It is often calculated as the maximum signal that can be accumulated, divided by the minimum signal which in turn equates to the noise associated with reading the minimum signal. It is commonly expressed in decibel scale. Camera Gain The camera gain is the conversion factor that relates the ADU value to the number of electrons collected by a pixel. Signal to Noise Ratio The comparison measurement of the incoming light signal versus the various inherent or generated noise levels, and is a measure of the variation of a signal that indicates the confidence with which the magnitude of the signal can be estimated.

12. Linearity non-linearity is based on the error between the best-fit straight-line to the original data of the camera input vs. the camera mean response at each illumination level, in ADU’s

13. Noise, noise, noise… • Read Noise: inherent output amplifier noise • Dark Noise: thermally induced noise arising from the camera in the absence of light • Shot Noise (Light Signal): noise arising out of the stochastic nature of the photon flux itself Sensor Photon noise, dark current, fixed pattern noise, and shot noise Electronics reset noise, quantization noise

14. Photon Transfer Curve • Measuring noise with noise • The camera is a system block with light as an input, and digital data as an output • The only noise introduced at the input is shot noise. • Any difference between the noise at the input and the noise at the output is sensor and/or electronics noise. Mean = S (signal) Standard deviation = S1/2 (Noise)

15. Photon Transfer Curve • Direct data • Read noise • Gain • Full well • Indirect data • Dynamic range • SNR • Linearity (if power of LED is known)

16. HEPAPS4 PTC

17. Vanilla

18. Prague activity • Calibration of MXRs for ATLAS • 55Fe 6 KeV • 241Am 50 KeV, 14 KeV • 252Cf 2 MeV • AmBe 4 MeV • Results in numbers • 16 MXRs calibrated with Am and Fe • 1 in progress, 4 left • 10 MXRs calibrated with Sr • 4 MXRs calibrated with neutrons

19. Thank you for attention!

20. University of Glasgow, Scotland 1st - 5th September 2008 The conference explores the scientific and technical developments of detector systems used in: Astronomy and space science; Astrophysics; Condensed matter studies; Industrial applications; Life sciences; Medical physics; Nuclear Physics, Particle physics and Synchrotron based science. National Organising Committee (subject to change) P.P. Allport, Liverpool R.L. Bates, Glasgow A.J. Bird, Southampton C.R. Cunningham, UK ATC, Edinburgh G.E. Derbyshire, STFC, RAL P. Evans, ICR, London R. Farrow, STFC, Daresbury W. Faruqi, MRC, Cambridge M. Grande, Aberystwyth P.R. Hobson, Brunel D.P. Langstaff, Aberystwyth P.J. Nolan, Liverpool D.J. Parker, Birmingham P.J. Sellin, Surrey A. Smith, MSSL, London R. Speller, UCL, London T.J. Sumner, IC, London S. Watts, Manchester psd8@physics.gla.ac.uk http://www.psd8.physics.gla.ac.uk

22. Potential barriers - + - + - + - + - + - + - + - + - + - + Radiation Metal layers Dielectric for insulation and passivation Polysilicon N+ N+ P+ N+ P-Well N-Well P-Well Concept first proposed in 1999, and published in NIM in 2001 (R. Turchetta et al.) P-epitaxial layer P-substrate