Large Area Devices at Semiconductor Technology Associates, Inc.

Transcription

1 Large Area Devices at Semiconductor Technology Associates, Inc. Kasey Boggs and Richard Bredthauer Semiconductor Technology Associates, Inc Paseo Espada #1004, San Juan Capistrano, CA ABSTRACT Recent discoveries show new promise for a formerly assumed extinct technology, CCDs. A primary limitation to the implementation of new ground-based astronomy measurement techniques is the inaccuracy of navigation and targeting due to error in the celestial frame of reference. This celestial frame of reference is relied upon for satellite attitude determination, payload calibration, in-course missile adjustments, space surveillance, and accurate star positions used as fiducial points. STA will describe the development of an ultrahigh resolution CCD (up to the maximum limit of a 150 mm wafer) that integrates high dynamic range and fast readout that will substantially decrease the error in the celestial reference frame. STA will also discuss prior and ongoing experience with large area CCD focal-plane arrays which include innovative design and fabrication techniques that ensure performance and yield. Key words: Ultrahigh resolution CCD, Large Area Imager, Orthogonal Transfer Array 1. INTRODUCTION Semiconductor Technology Associates (STA) has been an independent company providing custom CCD imagers for over 7 years. The founder and president, Dr. Richard Bredthauer, has been involved with CCD development since the early 1970 s. His experience includes design and fabrication of imagers for Ford Aerospace, Loral Fairchild Image Sensors, and Lockheed Martin. Under Dr. Bredthauer s guidance, STA has been on the forefront of CCD design and process technology, which has resulted in the development of many devices ranging from high resolution and high-speed imagers to small low noise guider devices. Recent growth at STA has lead to the development of a full space qualification characterization laboratory, where all devices undergo intense testing to ensure quality prior to delivery. STA is currently working to push technology into a new regime with the development of an ultrahigh resolution CCD (up to the maximum limit of a 150 mm wafer). K.boggs@sta-inc.net; phone ; fax

2 2. CURRENT LARGE AREA CCD DEVELOPMENTS STA has been dedicated to the advancement of CCD technology, and the delivery of high quality imagers. The following table lists some of the past CCDs developed at STA: CCD Part Number Format (pixels) Pixel Pitch Imaging Area (mm) Features STA0500A 4k x 4k 15 um 61x61 4 low noise outputs, MPP mode,2 side buttable,2/wafer STA0700A 2k x 4k 15 um 30.5x60 3 side buttable,4/wafer STA0720A 2k x 2k 15 um 30.7x low noise outputs, 2 high speed outputs, MPP mode STA0880A 2k x 2k 14 um 28.6x high speed 20MHz STA1000A 4k 4k 12 um 64 Orthogonal Transfer, NMOS 5.8x5.8 logic, 3 side buttable, 3/wafer STA1042A 2.6k x 4k 12 um 31.8x48 Low noise, 3 side buttable STA1100A 4k x 8k 8 um 32.3x MHz, Frame store option, 3/wafer STA1200A 3.4k x 2.6k 18 um 61.5x46.8 Full Frame, Cinematography Chip Table 1. Previously Developed STA Large Format CCDs Along with commercially available imagers, STA also specializes in custom CCD development for space-based imagers. The following describes past space based imagers, as well as the STA0500A and the STA1000A, two of the most popular devices currently available from Semiconductor Technology Associates. 2.1 Space Based Imagers Figure 1. Kepler 2k x 1k and FAME 2k x 4k Mature silicon process technology has made possible very high performance CCDs for space based applications. A space based planet finder for the NASA Kepler Discovery mission requires a 2200x1024 pixel imager with 27 micron pixels 1. A large focal plane of 46 backside illuminated imagers fills the requisite optical field. An initial prototype manufactured by STA is shown Figure 2.

3 A somewhat larger 2048x4064 imager with 15 micron pixels was produced for the Naval Observatories FAME program. This device incorporated a novel input serial register at the top of the CCD to allow independently metered background charge to be injected into each column. The concept is to mitigate radiation damage effects over long term space missions by filling the radiation induced traps. STA has recently delivered space qualified imagers to Ball Aerospace for the SBSS camera system. This project was completed with delivery of 3 flight grade detectors in 16 months. The excellent yield and performance of these devices demonstrates the significant improvement in silicon material quality and processing cleanliness. 2.2 STA0500A 4k Imager STA utilizes 150mm wafers for CCD fabrication. This along with a mature process has improved device yield, as well as the cosmetic quality of the imager. In conjunction with the Imager Technology Laboratory (ITL) at the University of Arizona 3, we have designed a new 4kx4k imager incorporating new process technology and increased wafer size. Figure 2 demonstrates the advantages of this approach. With production costs of a 100mm wafer approximately equal to that of a 150mm wafer, the ability to have two 4kx4k devices on a single wafer results in a halving of each device s fabrication cost. This also reflects in higher device yield. DC yields of the STA0500A routinely exceed 60%, while the overall yield of Astronomy grade CCDs on smaller wafers is 30%. Figure 2. STA0500A Wafer Delineation

4 Figure 3. Backside thinned 4k CCD for Carnegie Magellan Telescope Once a fabrication run of 24 wafers is completed, STA performs DC parametric tests and nominal functional tests. The best wafers are selected and shipped to ITL for further low temperature testing and selection of candidates for thinning. Figure 3 shows a completed 4k in its dewar ready for mounting. The results for 4k thinned devices has been excellent. Output noise is routinely ~2.8 electrons noise at 50kHz for the four outputs. HCTE and VCTE are > For multi-pinned phase mode of operation full well charge capacity is > 80,000 electrons. Dark current in non-mpp mode is ~ 10 elec/pixel/hour at -100C. Measured QE SN % 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% University of Arizona Roy A. Tucker Temp = -115C Wavelength (nm) Figure 4. 4K QE at -115C optimized for blue response Maximum quantum efficiency is obtained by backside thinning and applying a suitable anti-reflection coating for the optical region of interest. Figure 4 shows the QE curve for a blue optimized 4k CCD. The 4k pictured in Figure 3 was recently mounted on the Magellan Telescope in Las Campanas, Chile. Figure 5 below represents sample images taken with this camera system, exemplifying the high quality imagery of the STA0500 4k device.

5 Figure 5. Images from an STA0500A on the Magellan Telescope (Courtesy of Greg Burley, OCIW) 2.3 STA1000A Orthogonal Transfer CCD As silicon process technology has improved, new process technology has provided additional device capabilities. An example is the orthogonal transfer CCD 2. This device requires 4 levels of polysilicon to achieve charge transfer in arbitrary directions. The additional complexity enables performance of astronomical tip-tilt correction directly within the CCD. In addition, on chip logic selects individual sub-arrays for readout. 490x484 pixel OTCCD OTCCD Pixel OTCCD SubArray 8x8 Array of OTCCDs Figure 6. Orthogonal Transfer CCD configuration Pictured in Figure 6 the focal plane consists of an 8x8 array of sub-arrays, each with 480x micron pixels. The CCD has 8 separate outputs. A column of 8 sub-arrays can be read out one by one through one of the outputs by setting the appropriate control lines 5. An important advantage of 64 separate sections for a large CCD is the inherent fault tolerance. If a section has a serious defect it can be deselected with a control signal.

6 The benefit is a higher effective yield. A catastrophic defect in a normal 4kx4k CCD would eliminate the whole device. In an Orthogonal Transfer CCD this would mean only a single missing section of 64, and the device could still be utilized. STA recently received delivery of the second lot run of the OTA CCD. The wafer layout is shown below in Figure 7. Minor adjustments were made to the original design for enhanced performance. Figure 7. STA1000A Wafer Delineation STA is currently in the process of testing the devices, and with much success we have first light images of the STA1000A CCD as seen in Figure 8. The overall DC yield was extremely high, and as exemplified in the image below the cosmetic quality of the device is quite good. These devices are currently undergoing extensive characterization, and those results with be available upon completion. STA has the only commercially available OTA, adding depth to our extensive list of scientific imagers. Figure 8. STA1000A 64 Cell First Light Image

7 3. FUTURE DEVELOPMENTS AT STA The next step in the evolution of large area CCDs is a single device that spans an entire 150mm wafer. STA is now working on such a device for the Naval Observatory. We have completed a phase I SBIR studying the various technology and design tradeoffs to produce such a device. The CCD will consist of 10,600x10,600 pixels. To read an image frame greater than 100 million pixels in 10 seconds will require a minimum of 16 outputs. A pixel size of 9 microns will provide a full 14 bit dynamic range with a readout noise <5 electrons rms. This size pixel also yields a 9.54x9.54 cm image area. STA has already proven the capability to yield high resolution imagers with the STA1100A CCD. As noted in Table 1, this devices contains a 4k x 8 k array of 8 micron pixels. Various process adjustment were made to improve the performance and yield of this device, and these technical improvements will be utilized in the fabrication of the 10.6k x 10.6k CCD. For improved CTE and enhanced quantum efficiency the CCD will be fabricated on very high resistivity silicon wafers. The use of this high resistivity material also allows for full depletion of the CCD, which results in improved modulation transfer function and lateral diffusion. During Phase II of the program we will fabricate several runs of the CCDs. The best candidates will be thinned at ITL and backside AR coated to meet the QE requirements of the Navy. The devices will be packaged and mounted in a dewar with an Astronomical Research Cameras, Inc Generation 3 CCD controller. The camera system will be mounted on a telescope for extensive evaluation and demonstration of the CCD s ability to provide accurate star positions for celestial navigation. 4. CONCLUSIONS As camera mosaics increase in size the desirability of large scale imagers is becoming more and more evident. These STA Ultrahigh Resolution CCD s will drive down the price of astronomy grade detectors by greatly decreasing the number of devices necessary for large mosaics. Also when attempting to populate a multi-gigapixel array, less devices implies a simplification in drive electronics. This paper exemplifies STA s efforts to revolutionize the ground and space-based astronomy market, thus paving the road for the future of CCD imaging. REFERENCES: 1. Philbrick, R., Geary, J.,Dunham, E., Koch, D., 95 Million Pixel Focal Plane for use on the Kepler Discovery Mission, 2004, P.Amico (ed.), Scientific Detectors for Astronomy, Kluwer Academic Publishers, p Tonry, J.L. Luppino, G.A., Kaiser, N., Burke, B., Jacoby, G.H., Giga-Pixels and Sky Surveys, 2004, P.Amico (ed.), Scientific Detectors for Astronomy, Kluwer Academic Publishers, p 395

8 3. M. Lesser, R. Bredthauer, Development of a 4096x micron Pixel Scientific CCD, The Proceedings of the International Conference on Scientific Optical Imaging, Georgetown, Grand Cayman, Dec 2- Dec. 5, Lesser, M. P. Very Large Format Back Illuminated CCDs, Scientific Detectors for Astronomy, Kluwer Academic Publishers, 2004, p B. Burke, J. Tonry, M. Cooper, G. Luppino, G. Jacoby, R. Bredthauer, K. Boggs, M. Lesser, P. Onaka, D. Young, P. Doherty, and D. Craig. The Orthaogonal-Transfer Array: A New CCD Architecture for Astronomy. Proc. SPIE volume 5499, June 2004.