1 AY105 IR Detector Experiment This week we will move into the COO detector lab under the guidance of Roger Smith and Gustavo Rahmer, members of the Caltech Optical Observatories detector team. The aim of this experiment is to demonstrate infrared detector readout principals, behavioral problems and calibration techniques, with particular emphasis on how IR detectors differ from CCDs. The detector system we would normally use is busy being applied for its intended research purposes which cannot be interrupted this year so you will process data taken previously with this system and analyze it using IRAF. This is lab has been pressed for time in previous years so having the data on file already should help you get through more material. Since we are uncertain how are you will get there is a marker in this document showing a minimum expected set of tasks and then supplemental material which should be educational if you find time and have an interest. …the lab is for education and is not just a hurdle to jump. IR detectors in Astronomy We have already experimented with CCDs. How do IR detectors differ? Manufacture: Maximum detectable wavelength is that where the photon energy (h.c/λ) equals the energy required to excite an electron from the valence to conduction band of the semiconductor, the “bandgap”. Beyond this “cutoff wavelength” the material becomes transparent.2 Figure 1: Infrared photodiode arrays are made from low bandgap semiconductors, such as HgCdTe, which are connected vertically by Indium columns to underlying electronics built on Silicon using conventional CMOS transistor fabrication technology. Courtesy I.McLean In the1980’s some attempts were made to build CCDs using low bandgap (IR sensitive) material. However processing techniques are still not available, which allow fabrication of IR CCDs with adequate charge transfer efficiency. Manufacturers turned instead to using an array of photodiodes fabricated in IR sensitive material, connected to a readout circuit implemented in Silicon. 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. Figure 23 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 deposited on the underside of the photodiode array. The tops of the Indium bumps must be accurately coplanar and very clean so that when the bumps on the detector layer and 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 is possible to connect 4 million pixels with only a few hundred failures. A low viscosity epoxy is then wicked into the <10um wide spaces between the Indium columns and the detector layer is then polished and etched until it is only ~10um thick. This is a very complex and delicate process with yield problems at every step, so the top quality devices carry price tags in the $300-600K range, making IR detectors five to ten times as expensive as CCDs. Operation: See Figure 3. Before the exposure, the reset switch is closed, so that the photodiodes are reverse biased by a hundreds of millivolts. The CMOS transistor, which buffers the diode voltage has essentially zero gate leakage at the low temperature required for optimal photodiode performance, so the change in voltage on the photodiode is dominated by the electron-hole pair generation by photons. During the exposure the diode junction acts as a capacitor, which stores this photogenerated charge. The electric field induced by the reverse bias has driven all mobile charges (carriers) from the P-N junction (depletion region), which then acts as an insulator (dielectric) between the capacitor “plates”, which are formed by the conductive outer regions of the diode where mobile charges reside. The voltage across the diode capacitance is reduced by the accumulation of the photo-generated charge (and thermally generated “dark current”). Since the separation of the “capacitor plates” (width of the depletion region) is a function of accumulated charge, the voltage change is slightly non-linear, but we will ignore this for the moment. Figure 3: schematic representation of one pixel.4 The pixel architecture described is able to detect very faint fluxes only because the leakage currents in the reverse biased photodiodes and the fully depleted channel of the reset MOSFET are incredibly low. This is achieved by cooling to <150K. All mobile (conduction band) charge has been driven from the MOSFET channel or diode junction by the application of suitable voltages, so that the only source of spurious mobile charge is from excitation of valence electrons by lattice vibrations. At low temperatures, the probability of thermal excitations exceeding the bandgap energy becomes very small. The dependence of dark current on bandgap means that the IR detector material has much larger dark current than the Silicon reset transistor. This dark current sets the minimum detectable flux, which explains why these detectors are operated at such low temperatures. Impurities or lattice defects create localized intermediate energy levels producing higher dark current than would be expected for the bandgap, so much effort is invested in obtaining the best quality materials. With cooling, these reach a dark current floor at about 0.002 to 0.010e-/s. … just 10-21A. To record an image, all we need to do is reverse bias (“reset”) the diode, then disconnect any current path (open the reset switch). We then measure the rate of voltage change induced by the light incident on the diode. The circuitry to sense the accumulated charge is all within the pixel footprint. Unlike a CCD the image is not shifted across the detector surface. Since the image is read in situ, no shutter is required. The exposure time is the time between samples and not