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Detector II: IR arrays, bolometers Ay122a: Astronomical Measurements and Instrumentation, fall term 2015-2016 D. Mawet, Week 6, November 13, 2015 Radiation Discovery of infrared radiation The existence infrared wasby f infrared was of discovered by William el indiscovered 1800. Herschel Herschel in 1800. Herschel emperature of sunlight of measured the temperature of passing a colors prism and sunlightthrough of different passing through a prism and temperature increased just thatthe the temperature red found end of visible range. increased just outside of the d that these “calorific rays” red end of the visible range. He ted,later refracted andthese absorbed showed that light. “calorific rays” could be reflected, refracted and absorbed just like visible light. ixels of the detector array are attached to the pixels of the ROIC via indium l. Since the ROIC multiplexes the signals from each pixel to the off-chip ed to as a “multiplexer”, although multiplexing (signal transfer) is just one of Summary of digital imaging steps e (left) and the 6 steps of digital imaging. Drawing on left is courtesy of Laser Focus Anti-reflective coatings R1=(n0-n1)2/(n0+n1)2 R2=(n1-ns)2/(n1+ns)2 Conditions for destructive interference Phase: Φ1-Φ2 = π Amplitude: R1 = R2 => e = λ/4n => n1 = sqrt(n0 . ns) Rayleigh condition Charge generation via photoelectric effect hν An incoming photon excites an electron from the the valence band to the conduction band: hν > Eg conduction band e- Eg Eg = energy gap of material Critical wavelength: λc (μm) = 1.238 / Eg (eV) valence band Material name Symbol Eg (eV) λc (μm) Op. Temp. (*) Silicon Si 1.12 1.1 163-300 Indium Gallium Arsenide InGaAs 0.7 1.7 77-200 Mer-Cad-Tel HgCdTe 1.00-0.09 1.24-14 20-80 Indium Antimonide InSb 0.23 5.5 30 Arsenic doped Silicon Si:As 0.05 25 4 (*) to keep dark current low (thermal electrons) E Introductory notes on photovoltaic IR Detectors • Single pixel IR detectors have long used the photovoltaic effect • Diode is formed at the junction between a p- and n- doped semiconductor • This pn junction generates an internal electric field to separate the photon generated electron-hole pairs • Migration of holes and electrons changes the electric field, hence there is a voltage change across the junction which can be measured Intrinsic vs extrinsic IR detector material • Intrinsic photoconductivity = natural photoconductivity of pure semiconductor material • Extrinsic photoconductivity = synthesized photoconductivity of doped semiconductor material • The gap energy Eg is replace by the impurity energy Ei : • • e- gets elevated from the valence band to an impurity level to create a hole • Alternatively, e- may be freed by elevating it from an impurity level to the conduction band Either the hole or the electron can then move through the material in response to the electric field in the detector. Examples of extrinsic silicon detector spectral response A. Rogalski / Infrared Physics & Technolo Fig. 12. Examples of extrinsic silicon detector spectral response. Shown are Si:In, Si:Ga, and Si:As bulk detectors and a in A detec have IR b lm p are a plica (Si:S FPA have low devic basis array Pros and cons of extrinsic detectors • Pros: • • Can operate at much longer wavelength, because it takes less energy to free a charge carrier from an impurity atom than from an atom of the semiconductor crystal material Cons: • Extrinsic photoconductivity is far less efficient because of limits in the amount of impurity that can be introduced into the semiconductor without altering the nature of the impurity states. ong-wave IR (8-10 μm) and very long-wave IR (up to 18 μm). The hybrid CMOS architecture enables pixels fill factor and high QE. The quality of the HgCdTe material continues to improve and the JWST specificati current of less than 0.01 electrons per pixel per second (at 37 K operating temperature) is being achieved fo μm cutoff H2RG arrays. Tuning the bandgap/cutoff of Hg Cd Te detectors 1-x of the x cadmium fraction, x. This p presents the bandgap and cutoff wavelength of Hg1-xCdxTe as a function ed from the equation presented by Hansen et al4, where x is the cadmium fraction and T is the temperatu es Kelvin. E g = −0.302 + 1.93x − 0.81 x + 0.832 x + 5.35 × 10 T (1 − 2 x ) 2 3 −4 Fig. 3: Bandgap and cutoff wavelength of Hg1-xCdxTe as a function of the cadmium fraction, x. o the temperature dependence of the bandgap, it is important to define the temperature of operation as we the signal to the edge of the array. The analog-to-digital conversion associated focal plane electronics. The pixels of the detector array are a interconnects, one indium bump per pixel. Since the ROIC multiplexe electronics, the ROIC is sometimes referred to as a “multiplexer”, althou the functions provided by the ROIC. Most IR detectors are hybrid CMOS ROIC = readout integrated circuit, converts the charge to Fig. 2: Hybrid CMOS image array architecture (left) and the 6 steps of digital im World. complimentary metal-oxide-semiconductor. voltage with an amplifier in CMOS eachdenotes pixel, and transfers the signal to the edge of high theperformance array IR arrays involves three key tec The fabrication of large, 1. Growth and processing of the HgCdTe detector layer Note about CMOS • Complementary metal–oxide– semiconductor (CMOS) • Refers to a manufacturing technique • “metal–oxide–semiconductor” is a reference to the physical structure of certain field-effect transistors, having a metal gate electrode placed on top of an oxide insulator, which in turn is on top of a semiconductor material. CMOS Field Effect Transistor FET The readout circuit (to be described below) requires just three CMOS transistors per pixel (and associated electrical traces), which occupy only a fraction of the area of a typical 18µm wide pixel. This leaves space for the relatively large electrical contact pad that provides the interconnect path to the diode array, lying in the plane above it. Indium bump interconnects The technology for making the vertical connection between the dissimilar materials in the light sensing and signal processing layers is the key to IR detector manufacture. The contact is constructed by depositing a thick layer of Indium on each pad, one per pixel, of the readout IC (through an etched photo-resistive mask). Matching “Indium bumps” are Figure 2 deposited on the underside of the photodiode array. The tops of the Indium bumps must be accurately coplanar and very The cleantechnology so that whenfor themaking bumps on detector layer and the silicon layer are preciselymaterials aligned then squeezed thethe vertical connection between the dissimilar in the together, a cold weld is formed making a permanent electrical and mechanical connection – one per pixel. Currently it is light sensing and signal processing layers is the key to IR detector manufacture. The possible to connect 4 million pixels with only a few hundred failures. A low viscosity epoxy is then wicked into the contact is constructed by depositing a thick layer of Indium each pad, one per pixel, <10um wide spaces between the Indium columns and the detector layer ison then polished and etched until of it is only thethick. readout IC (through an etched photo-resistive mask). Matching “Indium bumps” are ~10um deposited on the underside of the photodiode array. The tops of the Indium bumps must This be is aaccurately very complexcoplanar and delicate process yield at every step, so the devices carry price tags and very with clean soproblems that when the bumps on top thequality detector layer and in the $250-500K range, making IR detectors five to ten times as expensive as CCDs. the silicon layer are precisely aligned then squeezed together, a cold weld is formed making a permanent electrical and mechanical connection – one per pixel. Currently it Note about multiplexer (MUX) Since the ROIC multiplexes the signals from each pixel to the off-chip electronics, the ROIC is sometimes referred to as a “multiplexer”, although multiplexing (signal transfer) is just one of the functions provided by the ROIC (see below). lexing is done in the detector material itself than an external readout circuit. The basic element of monolithic array is a metal–insulator–semiconctor (MIS) structure as shown in Fig. 2(c). A IS capacitor detects and integrates the IR-gented photocurrent. Although efforts have been de to develop monolithic FPAs using narrowp semiconductors, silicon-based FPA technol- technology, which has matured to a level practical use. Hybrid FPAs detectors and multiplexers a fabricated on different substrates and mated wi each other by the flip-chip bonding (Fig. 3) loophole interconnection. In this case we can o timise the detector material and multiplexer ind pendently. Other advantages of the hybrid FPA Monolithic vs hybrid focal plane arrays (FPA) Monolithic FPA Fig. 2. Monolithic IR FPAs: (a) all-silicon; (b) heteroepitaxy-on-silicon; (c) non-silicon (e.g., HgCdTe CCD) (after Ref. [14]). Monolithic vs hybrid focal plane arrays (FPA) Hybrid FPA Fig. 2. Monolithic IR FPAs: (a) all-silicon; (b) heteroepitaxy-on-silicon; (c) non-silicon (e.g., HgCdTe CCD) (after Ref. [14]). Fig. 3. Hybrid IR FPA with independently optimised signal detection and readout: (a) indium bump technique, (b) loophole technique. Fabrication of IR arrays The fabrication of large, high performance IR arrays involves three key technologies: 1. Growth and processing of the detector layer (e.g. HgCdTe) 2. Design and fabrication of the CMOS ROIC 3. Hybridization of the detector layer to the CMOS ROIC Molecular beam epitaxy (MBE) 3” MBE at Teledyne 10” MBE at Teledyne Fig. 4: Two of the molecular beam epitaxy (MBE) machines that Teledyne uses for growing high quality HgCdTe detectors. The machine on the left can hold 3-inch wafers, the machine on the right can hold 10-inch wafers. Since HgCdTe is a direct bandgap semiconductor, it is a very efficient absorber of light. The absorption depth of the photons in HgCdTe, i.e. the distance over which 1-e-1 (63%) of the light is absorbed, is shown in Fig. 5. For high quantum efficiency, the thickness of the HgCdTe detector layer should be at least 3 absorption depths, so that at least tion. In addition, this multiplexer can be programmed to a Unit Cellof(UC) forforDirect Readout (DRO)enabling very sh subsection pixels continuous readout, Reset Voltage Read Enable Clock Reset Enable Clock To Output FET and Current Supply UNIT CELL Integrating Node Unit Cell Source Follower FET with Inverter Detector Diode Detector Substrate Voltage Unit Cell Source Follower FET Drain FET = field effect transistor Detailed operation • Reset switch is closed to set the photodiode in reverse biased by Vr ~ -100 mV mode • Reverse bias => depletion region widens => diode is high-resistance insulator • Thermal leakage is negligible at low temperature for both the photodiode and MOSFET • Change in voltage dominated by electron-pair generation, which drops the voltage by ΔV Correlated Double Sampling (CDS) The exposure time is the time between samples and in notathe time since reset. Figure 4: Sample timing for the last pixel CDS frame. Signal arriving prior to the first sample is ignored (subtracted from the final sample), so the exposure duty cycle is not 100% but approaches it when the times are asonableexposure to ask why twolong. samples are necessary. The most obvious effect of th ample is to remove the DC offsets which are intrinsic to the readout circuit and Fowler N sampling (Al Fowler is a NOAO engineer) Fowler sampling Figure 9: Comparison of CDS timing and Fowler Sampling, Fowler sampling is a simple variant for improving the read noise. Since the signal is read non-destructively, multiple samples at the beginning and end of the exposure can be averaged to reduce the effective read noise. (The detector readout software will do this Classical Sampling”way involves nothing signal calculation in “Correlated real time). Double An alternative to think of more this than is assampling a set the of partially after reset andpairs, subtracting the final value. Anyexposure signal accumulated outside overlapping CDS which this are from averaged. The effective time is the difference these two samples is invisible, exceptand for its linearityfirst andand dynamic range. between the Nth sample in each group noteffect time on between last sample. 2D Multiplexor Only one row-enable is active at amultiplexor time: forevery pixel the selected row is Figure 5:line Schematic layout of a readout an infrared detectorin array. connected to a different column bus. At the edge of the array, each column bus is pixels areto accessed via a 2D multiplexor. Each pixel output has a single MOSFET connected via The a switch the output buffer. To raster through the pixels one enables which is driven hard on or off, to act as a switch. Outputs of all switches in the same a row then sequentially selects then thein each next row. column are connected to a bus. columns, The control lines for therepeats pixel-select for switches row are ganged together. Only one row-enable line is active at a time: every pixel in the selected row is connected to a different column bus. At the edge of the array, each Readout time From 3 to 10μs are typically needed to access each pixel. To reduce the time needed to raster through all pixels the columns are subdivided into groups each served by a separate output buffer/channel. A typical 2048x2048 H2RG has 32 outputs each serving 64 consecutive columns. Note that since the exposure time is defined by the times at which the pixels are read, then the exposure for the last pixel is displaced from the first by the time it takes to scan through whole array. (~10μs*2048*2048/32 = 1.3s) Pros of on-pixel integration • Electronic shuttering: Exposure time is the time between initial and final reads, not time since reset. Charge accumulated between reset and first sample has no effect on noise but can consume dynamic range and affect linearity. • Since readout is non-destructive, noise reduction is possible by combining multiple samples. (Fowler Sampling, Sample Up the Ramp). However the improvement isn’t as good as √N due to temporal correlations: there is significant noise power or systematic drift on frame-to-frame timescales. Cons of on-pixel integration • CDS occurs across the exposure time. Consequently IR detectors must be DC coupled and are at least 1000 times more sensitive than CCDs to electronic drifts and temperature changes. • Because each pixel has a different signal path, there is no “overscan” (as in a CCD) to provide an accurate zero-point reference. The next best thing is to use dark pixels in the image area, or unconnected pixels around the edges to mimic the zero- point drift of the image pixels. These “reference pixels” don’t tell us where zero is but do tell us how much it has changed. Cons of on-pixel integration cont’d • Read noise increases with exposure time. With good electronics, the read noise is reduced to the 1/f noise in the pixel buffer transistor and detector material. • The charge to voltage conversion is non-linear. The signal is accumulated on the detector diode capacitance. The reverse bias applied by the reset is discharged by the photocurrent causing the width of the depletion region to be reduced, so the diode capacitance increases: the voltage change of for a given charge increment drops. • When observing bright sources or in high background, substantial charge can be accumulated between the reset and first sample, eating into the apparent dynamic range. Notes on IR arrays (vs CCDs) • Many parameters vary considerably from pixel to pixel. (e.g. dark current, QE, noise, temperature sensitivity). • Dark current is higher and is more steeply dependent on temperature. In the best IR detectors this is just due to the lower bandgap, but it is often the case that imperfect surface passivation during manufacture degrades the dark current and causes large dark current variations from pixel to pixel. • Dark current takes several hours to fully stabilize (!!! major problem at observatories) after a perturbation such as a temperature or bias voltage change (eg cycling power). Example of IR arrays IR Arrays interfaces directly with the H1RG and H2RG and provides all of the functionality required from focal plane electronics (FPE). (System for Image Enhancement,Orbiter Control has and Retrieval) shownand in Fig. 11, Additionally, The the SIDECAR CRISM spectrometer in theDigitization, Mars Reconnaissance gone intoASIC, operation, is performing 9 provides significant in the size, weight power of the . very well. These arraysreduction are 640×480 pixels, andand both IR (λ = FPE 2.5 µm) and visible silicon PIN arrays are operating in co the CRISM instrument. Also, the 1024×1024 pixel (λco = 4.8 µm) IR array in the Deep Impact mission continues to The SIDECAR contains a programmable microprocessor, bias generators, clock generators, amplifiers and analog-toperform wellconverters since launch in 2005, and36the spacecraft added another cometinrendezvous in choice 2010 as mission. digital (ADCs). Up to analog inputs can be accommodated parallel, with of the 500EPOXI kHz, 16-bit IR arrays from Teledyne ADC or 10 MHz, 12-bit ADC (36 ADCs operate in parallel). The SIDECAR presents a digital interface to instrument In thiselectronics, paper, weand provide a summary of thedifferential technologies, and ongoing developments for IRseveral sensors at TIS. A with LVDS (low voltage signal)products communication, the SIDECAR can be placed meters companion paper, entitled “Teledyne advanced silicon CMOS sensors for x-ray to near-IR”, from the instrument electronics. AllImaging operationSensors: of the SIDECAR is fully programmable via LVDS communication lines.presented 3 in the visible sensors section of this conference, reports on silicon-based imaging sensors of TIS . The SIDECAR ASIC has been selected for use in 3 of the 4 instruments of the JWST. Two features of the SIDECAR were important factors in its selection for JWST: 1. Low power operation: For JWST operation - 4 ports continuously read at 100 kHz pixel rate and 16 bit digitization - the SIDECAR uses 11 mW power at 37K. The low power operation enables JWST to place the SIDECAR within the very cold (37K) instrument module which is located 4 meters cable length from the electronics located in the warm section of the observatory. 2. Low noise performance: The SIDECAR noise is negligible when compared to the H2RG readout amplifier, so that the total noise of the H2RG-SIDECAR system is set by the low noise H2RG operation. The SIDECAR ASIC was also selected for the repair of the Advanced Camera for Surveys (ACS) instrument in the Hubble Space Telescope. In this system, the SIDECAR will be used to operate two 4K×2K CCDs. The ACS Repair will take place during HST Servicing Mission 4, which is scheduled for October 2008. For the ACS Repair, a new hermetically sealed wasinfrared developed for the SIDECAR SIDECAR Fig. 1:spaceflight Examples package of Teledyne imaging sensors (left toASIC. right): The WFC3 1K×1K ASIC (HST),packaging is shown in Fig. 11. CRISM 640×480 (Mars Reconnaissance Orbiter), H2RG-18 2K×2K (JWST and ground-based astronomy) Copyright 2008 Society of Photo-Optical Instrumentation Engineers This paper was published in the Proceedings of the SPIE Conference on Astronomical Instrumentation (2008, Marseille, France) and is made available as an electronic reprint with permission of SPIE. One print or electronic copy may be made for personal use only. Systematic or multiple reproduction, distribution to multiple locations via electronic or other means, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibited. page 2 of 14 Fig. 11: SIDECAR ASIC and its packages. The ground-based astronomy package (left), the Hubble Space Telescope hermetically sealed package (center), and the James Webb Space Telescope cryogenic package (right). Both the H2RG and the SIDECAR have undergone environmental testing in spaceflight packages and have been Typical raw image So, here’s what we hav Raw K-band image of field shows stars, but also substantial sky signal • Ra sh su • Sk fie Sky signal intensity varies over field: – Large-scale variations: Illumination Quantum efficiency variations – Small-scale variations: Pixel-to-pixel variations – – Array defects • • High dark current pixels • These can be corrected by appropriate calibration images: Dark frames (bias) Flatfield images Ar Hi (m Th ap – – 19 July 2010 NOAO Gemini Data Wo Let’s try this CCD-like recipe When we try this (CCD style)… Obtain science images • Obtain science images Obtain calibration images images • Obtain calibration – Dark at same integration Dark frames atframes same integration time time – Flatfield images of uniform target Flatfield imagesdark of uniform • Subtract frame target from science images Subtract dark frame from science images • Divide dark-subtracted images by Divideflatfield dark-subtracted images by flatfield • Image of science field with uniform => Image of science field with uniform sky sky level level • Subtract (constant) sky level from image Subtract (constant) sky level from image • But, here is what we get….. But, here whatbut westill get..... – is Better, see substantial sky variations – Better, but still see substantial sky variations Small flatfield errors on sky still larger than faint science targets Small flatfield errors on sky still larger than faint science targets 19 July 2010 NOAO Gemini Data Workshop 15 The sky background is the problem Since the sky is the problem… • Subtract out the sky (or as much as possible) the flatfield Subtract out the skybefore (or as much as correction possible) before the flatfield correction • Obtain two images of field, move Obtain two images of field, move telescope between telescope between • Subtract two images Subtract two images – Eliminate almost all sky signal – Eliminate almostout all dark sky signal – Subtracts current, maverick pixels – Subtracts out dark current, maverick • Divide by flatfield image pixels • Result has almost no sky structure Divide by flatfield image Result has almost no sky structure Subtracting sky minimizes Subtracting sky minimizeseffects effectsofofflatfield flatfielderrors errors (but noise increased by 1.4) (but noise increased by 1.4) 19 July 2010 NOAO Gemini Data Workshop 16 Typical sequence for IR imaging • • Multiple observations of science field with small telescope motions in between (dithering) • Sky background limits integration time • Moving sources samples sky on all pixels • Moving sources avoids effects of bad/noisy pixels Combine observations using median filtering algorithm • Effectively removes stars from result => sky image • Averaging reduces noise in sky image • Subtract sky frame from each science frame => sky subtracted images • Divide sky subtracted images by flatfield image • • Dome flat using lights on – lights off to subtract background • Sky flat using sky image – dark image using same integration time • Twilight flats – short time interval in IR Shift and combine flatfielded images • Rejection algorithm (or median) can be used to eliminate bad pixels from final image Illustration Here’s what it looks like…. Sky frame Median Subtract sky, divide each by Flatfield 19 July 2010 NOAO Gemini Data Workshop 18 Shift and add Shift and combine images NGC 7790, Ks filter 3 x 3 grid 50 arcsec dither offset • NGC 77 • 3 x 3 gr • 50 arcs Bad pixe From com Bad pixels eliminated From combined image Higher n than in c combine Higher noise in corners than in center (fewer combined images) 19 July 2010 NOAO Gemini Data Workshop Summary: data reduction strategy for IR detectors SOURCE OBSERVATIONS (DITHERED) SKY OBSERVATIONS (DITHERED) [MEDIAN] [MEDIAN] SKY [―] DOME FLATS ON SKY SUBTRACTED IMAGES DARKS [―] DOME FLATS OFF [―] FLAT [/] SKY SUBTRACTED, FLATTENED IMAGES SHIFT, ALIGN SOURCES [MEDIAN] AVERAGED IMAGE Mid-infrared strategy (nodding & chopping) • • • Sky background at 10 μm is 1,000-10,000 greater than in K band • Detector wells saturate in very short time (< 50 ms) • Very small temporal variations in sky >> astronomical source intensities Mid-infrared strategy Read array out very rapidly (20 ms), coadd images • Sky background at 10 µm is 103 – 104 greater than in K band Sample sky at– high rate (~ 3 Hz) by chopping secondary mirror (15 arcsec) Detector wells saturate in very short time (< 50 ms) • • • – Very temporal variations in sky >> astronomical source intensities Synchronize withsmall detector readout, build up “target” and “sky” images But • Read array out very rapidly (20 ms), coadd images tilting of secondary its secondary own offset signal • Sample sky at highmirror rate (~ 3introduces Hz) by chopping mirror (15 arcsec) Synchronize with detector readout, build up “target” and “sky” images Remove offset– by nodding telescope (30 s) by amplitude of chop motion – But tilting of secondary mirror introduces its own offset signal • Removeof offset by nodding telescope (30 s) with by amplitude of chop motioncycle Relative• phase target changed by 180° respect to chop – Relative phase of target changed by 180° with respect to chop cycle phase of offset unchanged – Relative phase ofsignal offset signal unchanged – Subtraction adds signal from target, subtracts offset • Relative • Subtraction adds signal from target, subtracts offset • http://www.gemini.edu/sciops/instruments/t-recs/imaging chop 19 July 2010 …… nod NOAO Gemini Data Workshop 22 Bolometers • Measure the energy from a radiation field, usually by measuring a change in resistance of some device as it is heated by the radiation • Mainly used in FIR/sub-mm/microwave regime • Sensitivity is measured through the Noise Equivalent Power • Measure the energy(NEP): from atheradiation field, which produces S/N=1 at the output power absorbed usually by measuring(units a change W/Hz0.5)in Bolometers resistance of some device as it is heated by the radiation • • Typically use a semiconductor Mainly used in FIR/sub-mm/microwave resistance thermometer, and a regime metal coated dielectric as the • Sensitivity is measured absorber through the Noise Equivalent Power (NEP): the power absorbed which produces S/N=1 0.5 at the output (units W/Hz ) • Typically use a semiconductor resistance thermometer, and a metal coated dielectric as the absorber Examples of bolometers Semiconductor bolometers from SCUBA “Spiderweb” bolometer Components of a Bolometer Components of a bolometer • • • • • Absorber with heat capacity C • Heat sink heldC at fixed temperature Absorber with heat capacity T0 • Small thermal conductance G Heat sink held at fixed temperature T0sink between absorber and heat • Load resistor RL • ThermometerG w. resistance R Small thermal conductance • Constant current supply generating between absorber and heat sink bias current I • Device to measure voltage changes Load resistor RL • Thermometer w. resistance R • Constant current supply generating bias current I • Device to measure voltage changes Schematic of a bolometer Sources • http://spiff.rit.edu/classes/phys445/lectures/ccd1/ccd1.html • Observational astrophysics, 2nd edition, P. Lena • S. G. Djorgovski (Caltech, Ay122a, 2012) • R. Smith (Caltech Ay 105 notes) • J.W. Beletic notes (optics in astrophysics, R. & F.C. Foy editor, NATO Science Series) • George Rieke 2007, Ann. Rev. Astr. Ap. 45, 77. • An introduction to IR detectors (D. Joyce, NOAO)