Astronomical Observing Techniques 2017 Lecture 10: Silicon Eyes 2

Transcription

1 Astronomical Observing Techniques 2017 Lecture 10: Silicon Eyes 2 Christoph U. Keller keller@strw.leidenuniv.nl

2 Content 1. Quantum Efficiency 2. CCD Focal Plane Architectures 3. Readout Noise 4. CCD Data ReducGon 5. CMOS devices 6. IR Arrays 7. Bolometers 8. MKIDS Astronomical Observing Techniques 2017, Lecture 10: Detectors 2 2

3 Quantum Efficiency Quantum efficiency: average number of electrons per incident photon Influenced by electrode absorpgon surface reflecgon deplegon depth electric field strength Silicon solar cell efficiency from en.wikipedia.org/wiki/quantum_efficiency Astronomical Observing Techniques 2017, Lecture 10: Detectors 2 3

4 CCD Quantum Efficiency front illuminagon: poly-silicon gate electrodes absorb in the blue blue-enhanced: holes in poly-silicon gate electrodes backside illuminated: light enters on backside of CCD Astronomical Observing Techniques 2017, Lecture 10: Detectors 2 4

5 Backside IlluminaGon back-illuminagon: thin silicon, photo-electrons reach potengal wells electric field gradient moves charges: increase doping in regions close to silicon surface increases blue sensigvity where electrons are generated close to silicon surface minimize reflecgon of light from back surface with SiO layer Astronomical Observing Techniques 2017, Lecture 10: Detectors 2 5

6 Frame Transfer OperaGon transfer needs to be done quickly to prevent disturbance by light falling on the image secgon during read-out during readout, all CCD cells in image array are again in integragon mode Astronomical Observing Techniques 2017, Lecture 10: Detectors 2 6

7 Focal Plane Architecture astronomy: full-frame and frame-transfer arrays interline-transfer arrays in commercial CCD cameras frame-transfer CCD has photosensigve array and memory array full-frame device lacks storage secgon shu[er interrupts illuminagon during readout Astronomical Observing Techniques 2017, Lecture 10: Detectors 2 7

8 Binning Astronomical Observing Techniques 2017, Lecture 10: Detectors 2 8

9 Readout Noise main origins: on-chip amplifier translagng charge (electrons) to analog voltage wires between on-chip amplifier and analogto-digital converter acgng as antennae depends strongly on readout frequency signal-to-noise improved by binning Astronomical Observing Techniques 2017, Lecture 10: Detectors 2 9

10 EM-CCD Electron-MulGplying CCD solid-state electron mulgplier digitalimaging/concepts/ emccds.html provides readout noise <<1 electron at 10MHz pixel readout rate requires cooling to <-80C Astronomical Observing Techniques 2017, Lecture 10: Detectors 2 10

11 Bias Astronomical Observing Techniques 2017, Lecture 10: Detectors 2 11

12 Dark Current Astronomical Observing Techniques 2017, Lecture 10: Detectors 2 12

13 FlaPield Detector response (QE, geometrical pixel size) varies slightly from pixel to pixel à image has structure, even with flat illumination Astronomical Observing Techniques 2017, Lecture 10: Detectors 2 13

14 Common FlaPield Methods 1. Dome flats: illuminate a white screen within the dome (can be done during the day, but may introduce spectral artefacts) 2. Twilight flats: observe the twilight sky at two Gmes during sunrise or sunset (high S/N but Gme is oden too short to get flaeields for all instrument configuragons) 3. Self-calibraGon: use the observagons themselves (e.g. average all data) kelller@strw.leidenuniv.nl Astronomical Observing Techniques 2017, Lecture 10: Detectors 2 14

15 Typical Array Detector Data ReducGon science frame signal S, exposure Gme t S dark frame signal D, exposure Gme t D bias frame signal B, zero exposure Gme flat field frame F, exposure Gme t F corrected (calibrated) image given by S = S t s t D F t F t D ( D B) B ( D B) B F-(t f /t d) (D-B)-B oden normalized such that median of S = median of S kelller@strw.leidenuniv.nl Astronomical Observing Techniques 2017, Lecture 10: Detectors 2 15

16 CCD Data ReducGon Raw Bias + Dark Current Reduced Flaeield kelller@strw.leidenuniv.nl 16

17 CMOS and CCD Complementary Metal Oxide Semiconductor (CMOS) Charge Coupled Device (CCD) Astronomical Observing Techniques 2017, Lecture 10: Detectors 2 17

18 CCD Camera Astronomical Observing Techniques 2017, Lecture 10: Detectors 2 18

19 CMOS Camera Astronomical Observing Techniques 2017, Lecture 10: Detectors 2 19

20 CMOS vs. CCD CMOS advantages over CCD: standard semiconductor processing low power consumpgon ( 1% of CCD) random access to regions of interest blooming and streaking much reduced compared to CCDs addigonal electronics can be integrated on chip and in pixel (smart sensor) non-destrucgve readout CMOS disadvantages: small geometric fill factor (microlenses can help) typically larger read noise kelller@strw.leidenuniv.nl Astronomical Observing Techniques 2017, Lecture 10: Detectors 2 20

21 Infrared Arrays ConstrucGon 1. Produce a grid of readout amplifiers 2. Produce a (matching mirror image) of detector pixels 3. Deposit Indium bumps on both sides 4. Squeeze the two planes together è hybrid arrays 5. The Indium will flow and provide electrical contact kelller@strw.leidenuniv.nl Astronomical Observing Techniques 2017, Lecture 10: Detectors 2 21

22 MulGplexers MulGplexing: Pixel signals à SequenGal output lines MulGplexer (MUX) Tasks: address a column of pixels by turning on their amplifiers pixels in other columns with power off will not contribute to signal Signal at photodiode à gate T 1 Readout uses row driver R 1 and column driver C 1 to close the switching transistors T 2, T 3, T 4 Power to T 1 à signal to the output bus Reset: connect V R via T 5 and T 3. kelller@strw.leidenuniv.nl Astronomical Observing Techniques 2017, Lecture 10: Detectors 2 22

23 Example: The Teledyne HAWAII-2RG Can also be combined to a 2x2 mosaic kelller@strw.leidenuniv.nl Astronomical Observing Techniques 2017, Lecture 10: Detectors 2 23

24 Single Sampling IR Array Read Out Modes Reset-Read-Read most simple approach does not remove ktc noise measures the absolute signal level (MulGple) Fowler Sampling Resets, reads and reads pixel-by-pixel Signal = Read(2) Read(1) best correlation, no reset noise but requires frame storage reduced dynamical range (saturation!) Sample-up-the-ramp FiZng similar to reset-read-read but each read is repeated m times Signal = mean(read2) mean(read1) Reduces readout noise by m over RRR m equidistant reads during integration linear fit à slope reduces readout noise by m particularly useful in space (cosmics!) kelller@strw.leidenuniv.nl Astronomical Observing Techniques 2017, Lecture 10: Detectors 2 24

25 Dithering / Ji]ering 1. Observe the same field with many exposures, each offset by a small amount 2. Combine the image e.g., via median filtering kelller@strw.leidenuniv.nl Astronomical Observing Techniques 2017, Lecture 10: Detectors 2 25

26 Chopping / Nodding Chopping: switch between target and sky at a few Hz Nodding: offset to the sky posigon to repeat the chopping sequence inverted kelller@strw.leidenuniv.nl Astronomical Observing Techniques 2017, Lecture 10: Detectors 2 26

27 CCDs vs IR Arrays CCDs: destrucgve reads charges are physically shided to the output line shu[er determines exposure Gme IR arrays: non-destrucgve reads readout requires sophisgcated mulgplexer circuit mulgplexer readout addresses individual pixels directly read/reset determines exposure Gme Astronomical Observing Techniques 2017, Lecture 10: Detectors 2 27

28 Detector Artefacts: Bad Pixels dead, hot and rogue pixels, rows, columns bias and dark correcgon help somewhat can oden us dead-pixel map and replace with median of surrounding pixels Astronomical Observing Techniques 2017, Lecture 10: Detectors 2 28

29 Detector Artefacts: Latent Images mostly seen in hybrid (IR) arrays avoid overexposure wait addigonal resets anneal (warm detector) Astronomical Observing Techniques 2017, Lecture 10: Detectors 2 29

30 Detector ArGfacts: Cosmic Rays cosmic ray pargcles free electrons in detector remove with median filtering of several exposures Astronomical Observing Techniques 2017, Lecture 10: Detectors 2 30

31 Detector Artefacts: Fringing detector l 2 l 1 If the phase difference between l 1 and nl 2 is an even mulgple of π construcgve interference occurs. If an odd mulgple destrucgve interference occurs à fringes = wave pa[ern. kelller@strw.leidenuniv.nl Astronomical Observing Techniques 2017, Lecture 10: Detectors 2 31

32 Detector Artefacts: Blooming Astronomical Observing Techniques 2017, Lecture 10: Detectors 2 32

33 Detector Artefacts: Cross-Talk electronic interference between channels that are read out simultaneously Astronomical Observing Techniques 2017, Lecture 10: Detectors 2 33

34 Basic Bolometer detector: heat capacity C, connected via thermal link with thermal conductance G to heat sink at temperature T 0 incoming photons increase temperature of detector by T 1 thermal conductance to heat sink transfers power GT 1 astronomical signal changes detector energy by dq/dt heat capacity C=dQ/dT dt1 total power absorbed by detector is: P T ( t) = GT1 + C dt kelller@strw.leidenuniv.nl Astronomical Observing Techniques 2017, Lecture 10: Detectors 2 34

35 Principle of Bolometer ConstrucGon measure voltage across thermometer voltage depends on resistance resistance depends on temperature temperature depends on photon flux Bolometers are especially for the far-ir/sub-mm wavelength range! Astronomical Observing Techniques 2017, Lecture 10: Detectors 2 35

36 QE and Composite Bolometers Si bolometers with high impurity concentragons can be very efficient absorbers But: QE is too low. SoluGon: enhance absorpgon with black paint but this will increase the heat capacity A high QE bolometer for far-ir and sub-mm would have too much heat capacity à composite bolometers The heat capacity of the blackened sapphire plate is only 2% of that of Ge. kelller@strw.leidenuniv.nl Astronomical Observing Techniques 2017, Lecture 10: Detectors 2 36

37 Etched Bolometers The bolometer design has been revolugonized by precision etching techniques in Si Thermal Gme response ~ C/G à small structures minimize the heat capacity C by reducing the volume of material. kelller@strw.leidenuniv.nl Astronomical Observing Techniques 2017, Lecture 10: Detectors 2 37

38 Cryogenic Temperatures Four standard cooling opgons: 1. 4 He dewar (air pressure) à T=4.2K 2. 4 He dewar (pumped) à 1K<T <2K 3. 3 He (closed-cycle) refrigerator à T~0.3K 4. adiabagc demagnegzagon refrigerator à T ~ 0.1K Simplest solugon is to use a two-stage helium dewar (here: model from Infrared Laboratories, Inc.) kelller@strw.leidenuniv.nl Astronomical Observing Techniques 2017, Lecture 10: Detectors 2 38

39 Bolometers an Overview The single pixel Ge:Ga bolometer invented in 1961 by Frank Low Herschel / PACS bolometer: a cut-out of the 64x32 pixel bolometer array assembly LABOCA the multi-channel bolometer array for APEX operating in the 870 µm (345 GHz) atmospheric window The signal photons are absorbed by a thin metal film cooled to about 280 mk The array consists of 295 channels in 9 concentric hexagons The array is under-sampled, thus special mapping techniques must be used kelller@strw.leidenuniv.nl Astronomical Observing Techniques 2017, Lecture 10: Detectors 2 39

40 Performance Comparison Bolometer ó Heterodyne Receiver Case 1: Bolometer operagng at BLIP and heterodyne receiver operagng in the thermal limit (hν«kt) è the bolometer will perform be[er This is always true, except for measurements at high spectral resolucon, much higher than the IF bandwidth. Case 2: detector noise-limited bolometer and a heterodyne receiver operagng at the quantum limit (hν»kt). è the heterodyne receiver will outperform the bolometer. In the case of narrow bandwidth and high spectral resolucon the heterodyne system will always win. kelller@strw.leidenuniv.nl Astronomical Observing Techniques 2017, Lecture 10: Detectors 2 40

41 MKIDS Physical Principle KID = KineGc Inductance Detector MKID = Microwave KID kelller@strw.leidenuniv.nl Astronomical Observing Techniques 2017, Lecture 10: Detectors 2 41

42 MKIDS ConstrucGon Astronomical Observing Techniques 2017, Lecture 10: Detectors 2 42

43 MKIDS OperaGng Principle Astronomical Observing Techniques 2017, Lecture 10: Detectors 2 43