Orthogonal-Transfer Charge-Coupled Devices and Low-Noise Charge-Coupled Devices Readout Circuits* Barry E. Burke Title Slide. *The MIT Lincoln Laboratory portion of this work was performed under a Collaboration Agreement between MIT Lincoln Laboratory and The University of Hawaii, Institute for Astronomy (IfA). Opinions, interpretations, conclusions, and recommendations are those of the authors, and do not necessarily represent the view of the United States Government. *The MIT Lincoln Laboratory portion of this work was performed under a Collaboration Agreement between MIT Lincoln Laboratory and The University of Hawaii, Institute for Astronomy (IfA). Opinions, interpretations, conclusions, and recommendations are those of the authors, and do not necessarily represent the view of the United States Government. 1 1

Outline Review of Orthogonal-Transfer Charge-Coupled Devices (OTCCD) Development of the orthogonal transfer array (OTA) Low-noise CCD readout circuits Summary Outline.

Conventional vs. Orthogonal-Transfer CCDs The conventional three-phase Charge-Coupled Devices (CCD) shown on the left is the basis for most of the imagers that we have made. It is fabricated with a triple-poly process which we have used for many years. The Orthogonal-Transfer Charge-Coupled Devices (OTCCD), shown on the right, is a newer device that goes back about 12 years and features the capability for charge transfer in all directions. It is a four-phase device, and we use a four-poly process to fabricate it.

Application Areas Compensation of platform motion Video out Compensation of platform motion Imaging from unstable and/or moving platforms TDI (time delay and integrate) with variable scan direction Compensation of scene motion Ground-based astronomy Output register Frame store Imaging area The OTCCD can noiselessly compensate for scene motion across sensor during image integration. This imager architecture is useful for compensating platform or scene motion. The application for which we have developed the device is in the area of ground-based astronomy, where it can be used to compensate wavefront tilt due to atmospheric turbulence and winds. In effect, it performs an electronic tip/tilt correction. OTCCD can noiselessly compensate for scene motion across sensor during image integration

Star-cluster imagery (M71) With motion compensation, =0.50” Application of OTCCDs in Astronomy Star-cluster imagery (M71) Use OTCCD to remove blurring due random motion of star images (electronic tip-tilt) No motion compensation, =0.73" In 1996 we built a 512x512-pixel frame-transfer device with OT pixels in the imaging section and three-phase pixels in the frame store. This was used to demonstrate adaptive imaging at the Michigan-Dartmouth-MIT observatory on Kitt Peak. A suitably bright star on the frame store was selected as the guide star, and a small sub-array of pixels around it were read out at sufficient rates to provide guiding to the OT pixels. The data on the right show that the resolution (FWHM) improved to 0.50 arc sec with active guiding from an unguided value of 0.73 arc sec. In addition, the guiding improved the signal/noise by 70%. After this work, we proceeded to build larger format devices, including a 2048x4096 device. With motion compensation, =0.50” SNR increase: 1.7

Outline Review of Orthogonal-Transfer Charge-Coupled Devices (OTCCD) Development of the orthogonal transfer array (OTA) Low-noise CCD readout circuits Summary Outline.

Pan-STARRS (Panoramic Survey Telescope and Rapid Response System) Four 1.8-m telescopes viewing same sky sector 3˚ FOV, 24 mv sensitivity High-cadence, wide-field surveys Detect variable or moving objects 1.4-Gpixel CCD focal-plane array on each telescope The Pan-STARRS program has as its goal the construction of four 1.8-meter telescopes, usually viewing the same sector of the sky. The system is designed for rapid, wide-field surveys of the sky, with particular emphasis on detecting variable and moving objects (such as asteroids and comets). Each of the telescopes will have a focal plane of 64 CCDs comprising 1.4 Gpixels whose design is described in subsequent slides. These CCDs are of a new type based on the OTCCD. Currently we are working on supplying the CCDs for the focal-plane array to be deployed on the first of the telescopes shown on the right. Proposed Pan-STARRS telescope configuration Gigapixel focal-plane array (64 CCDs) First Pan-STARRS telescope on Haleakala (PS1)

Orthogonal-Transfer Array 2.38 arc min Wide field-of-view (FOV) imaging Wavefront tilt decorrelates over FOV > few arc minutes Need 2D array of OTCCDs, each independently clocked to track local wavefront tilt (“rubber focal plane”) OTA is a new CCD architecture Requires on-chip switching logic More complex layout and processing than conventional CCDs In wide-field-of-view imaging the wavefront tilt decorrelates over angular distances greater than a few arc minutes. Thus an OTCCD larger than this angle cannot be effectively used for tip/tilt correction. What is needed is an array of independent OTCCDs, and this is origin of the orthogonal transfer array or OTA. Such a device is more complex than a simple CCD, as it requires on-chip switching logic and extensive metal control lines.

Orthogonal Transfer Array OTA: 88 array of OTCCD cells OTA cell with I/O control Four-phase OTCCD pixels New device paradigm 2D array of independent OTCCDs Independent clocking and readout of OTCCDs Advantages Enables spatially varying tip-tilt correction Isolated defective cells tolerable (higher yield) The key features of the OTA are all portrayed in the drawing shown here. The OTA, as currently designed, consists of an 88 array of cells, each cell being a roughly 600600-pixel OTCCD. What distinguishes this device from other CCDs is that the parallel clocking and video output of each cell is under the control of a logic block along the left edge of the cell. Each logic block can be addressed and controlled from off chip. At the right is a drawing of the four-phase gate structures of the OTCCD. There are actually two designs, termed type 1 and type 2, and both are being investigated for this application. Because each cell can be clocked independently, one can apply a different tip-tilt correction to each cell and thus apply the device to wide-field imaging. Moreover, there is an important advantage to this architecture compared to a conventional CCD consisting of an unbroken array of pixels, namely, the ability to disable defective cells that could, in a conventional device, shunt the clock signals or inject large currents of minority carriers into the substrate.

OTA Operation Subset (4 – 5) of cells chosen to image guide stars Map of wavefront tilt constructed from guide-star data and applied to science cells Four redundant views of every patch of sky used to fill gaps due to Guide-star cells Dead cells Cosmic rays Dead areas between cells and devices One question is where is guide-star information obtained. The planned operation of the device consists of first selecting roughly five bright stars that can be used for guiding, with the remaining cells dedicated to acquiring science imagery. Then by reading small subarrays around each guide star at 10-30 Hz a spatial map of the wavefront tilt can be constructed. Then each science cell is updated with a one or two pixel shift until the end of the integration time and the image is read out. Because there are four views of the sky any missing data due to the guide-star cells or defective cells can be filled in with data from the other focal planes. This reduces the focal-plane costs because they can use devices with dead cells. And finally this approach is much faster than the conventional practice in astronomy where a single focal plane must record multiple shifted frames.

Device Fabrication Four OTAs on 150-mm wafer (die size 49.550.1 mm) Four-poly, n-buried-channel process Fabricated on 5 000 Ω·cm float-zone silicon wafers Back-illuminated devices thinned to 75 µm 150-mm wafer with four OTAs The device was designed to be sufficiently large so that four die would just fit on a 150-mm wafer. This translates into a die size of about 50x50 mm and brings the corners of the die less than 5 mm from the wafer edge. The OTCCD process uses four levels of polysilicon and an n-buried-channel process. We use 5000 Ω-cm wafers to enable deep depletion and therefore high quantum efficiency in the near infrared. We thin the completed devices to 75 µm in our back-illumination process. Photo of pixel array

Sample Imagery First devices were fully functional but with some issues (noise, logic “glow”) Device redesign resolved issues with prototype devices Redesigned devices have been fabricated and most of them packaged The first, or prototype, devices performed all the basic functions expected but had two problems that needed addressing. Logic “glow” was a low-level spurious charge that originated from high-field regions in the logic. This was resolved in the redesigned device with both device and circuit fixes. The noise was >10 e- in the prototype devices, while in the redesigned device the noise levels less than 6 e-. The program goal is 5 e-. The image on the left is an early wafer-probe image from a cell in which the parallel clocks were programmed to shift charge while a small fixed spot illuminated a pixel near the center. The image on the right is from a packaged OTA at -70˚C. Image from OTA cell with fixed light spot and CCD gates clocked Image from back-illuminated OTA 10-µm pixel, 22.6 Mpixels

Substrate Bias Substrate bias enables thick, fully depleted devices: High quantum efficiency, 800-1000 nm Small charge point-spread function An important feature of the redesigned device is the ability to bias the back side of the device independently from the circuitry on the front. This feature was tried as a design split on the prototype lot, and we found that we could use it to make the devices thicker than our standard 45 µm while retaining low spreading of the charge (small charge point-spread function).

Quantum Efficiency Back-surface p+ using ion-implant/laser anneal Two-layer anti-reflection coating with reflection null at 850 nm for reduced fringing Thicker device clearly superior beyond 800 nm In the prototype devices we thinned some wafers with the substrate-bias feature to 75 µm. The quantum efficiency is significantly enhanced in the I band relative to a 45-µm device, and as a result all the devices for Pan-STARRS are thinned to 75 µm. The back surface treatment for these devices consists of an ion implant followed by a pulsed laser anneal, which is then capped by a two-layer anti-reflection coating optimized for the visible and near infra-red. This coating and the relatively thick device virtually eliminates fringing from sky glow.

OTA Focal Planes TC3 focal plane assembled from 16 prototype devices; on-sky tests in February GPC1 assembled from lots 2 and 3 (summer 2007) On the left is a depiction of the gigipixel focal plane for the Pan-STARRS telescopes. The focal plane consists of 64 OTAs closely abutted together. In this drawing one can see the flexprints, one end of which are attached to the PGA on the bottom of the package. This focal plane will be assembled out of devices that we are currently packaging. This assembly will get under way this spring. Also illustrated here on the right is a prototype array called TC3 which consists of 16 devices from the prototype lot. This focal plane will be used for shakedown testing of the first Pan-STARRS observatory starting in mid-February.

Outline Review of Orthogonal-Transfer Charge-Coupled Devices (OTCCD) Development of the orthogonal transfer array (OTA) Low-noise CCD readout circuits Summary Outline.

CCD Output Circuits Our standard CCD output circuit for many years has been the floating diffusion output using a buried-channel MOSFET. A plan view of a typical layout is shown on the left. In this case, the charge-collection node is connected to the sensing MOSFET via a metal interconnect. We have explored ways of reducing the parasitic capacitance of this circuit and thereby increase the conversion gain (volts/electron). The approach on the right shows a structure in which the function of charge collection and sensing are merged. Here the n+ collection diode becomes the gate of a p-channel JFET as well.

Output Circuit Comparison Because the JFET version is more compact and eliminates components of parasitic capacitance, its conversion gain or responsivity is about 50% higher than that of the MOSFET version. In addition, the JFET has a lower noise spectral voltage than the MOSFET, especially at low frequencies. Sense-node capacitance is lower (higher responsivity) for JFET than MOSFET Noise spectral voltage is lower for JFET than MOSFET

Noise Comparison Best MOSFET noise vs. preliminary JFET noise We have incorporated this circuit into a 160x160, 20-port CCD for an adaptive optics application. On the left is a chart showing our best noise data for a MOSFET output circuit vs. pixel rate, and some preliminary data for the JFET output. The JFET clearly out-performs the MOSFET, though we have not explored the full range of pixel rates. Best MOSFET noise vs. preliminary JFET noise 2000-fps, 160160-pixel imager, 20 ports with JFET output circuits

Summary OTCCD developed for astronomical applications but has a potentially much broader range of uses OTA development for Pan-STARRS is new OTCCD concept, also with other applications Recent work with JFETs shows noise levels better than BCMOSFETs and nearing 1 e- In conclusion, the OTCCD has found an application in ground-based astronomy, but we feel it has potential for many other uses for image-motion compensation. Likewise, the OTA is a natural extension of the OTCCD concept to wide-field astronomical imaging, but may also find use in applications where image motion across the field of view is not uniform. Finally, the pJFET is performing substantially better than the buried-channel MOSFET, at least over the limited frequency range of current tests. Our best results lie close to the 1 e- level at pixel rates up to 500 kHz.