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To: Skeeter Bug who wrote (43521)	3/6/1999 7:13:00 PM
From: DJBEINO	Respond to of 53903

Hyundai, Harvard team to leverage excess memory capacity for low-cost imaging devices -- DRAM process modified for CMOS image sensors Margaret Quan Ichon, Korea - Working with engineers at Hyundai Electronic Industries, a Harvard University professor has found a way to put excess DRAM fab capacity to work manufacturing single-chip CMOS image sensors. Electrical engineering professor Woodward Yang and the Hyundai team developed an 800- x 600-pixel sensor based on the modified process. They also created a back-end chip set for such applications as digital cameras. The camera chip set would involve three components: an ASIC for image compression, the DRAM-based sensor and a Universal Serial Bus (USB) transceiver chip. The CMOS image sensor is manufactured in Hyundai's 0.5-micron process. Hyundai is producing volume runs of the sensors in 800 x 600, 640 x 480 and 400 x 300 sizes. The main applications for the image sensors will be PC video cameras and still cameras with USB connectivity. Yang believes the chips could find their way into small cameras embedded in laptop computers and handheld devices as well as such security equipment as fingerprint-recognition systems. Yang presented a paper describing the work at the International Solid-State Circuits Conference last month. He said he and the team at Hyundai used a novel analog/digital converter scheme as well as advanced features and process optimization to create the sensors. The devices are said to feature low-voltage 4T NMOS pixel design, integrated A/Ds, integrated control and on-chip color filters. They run off a 3.3-V supply and consume 50 mW. The process modification made sense, Yang said, because DRAMs and CMOS image sensors share so much in common. Both technologies need to be managed for low-cost and high-yield production. Both DRAMS and image sensors store their charge in regular arrays. A DRAM's charge is produced during a write operation; an image sensor's charge is "written" as photons strike the surface of the array. Further, managing a process for the long refresh times required by DRAMs is the same as managing for minimum "dark" current, or leakage current through a device's p-n junctions. Leakage, often caused by light and temperature defects, shortens refresh time. Yang said the CMOS image sensors he produced in Hyundai's DRAM fab have what he believes is the lowest dark current yet achieved in a CMOS image sensor: less than 25 pA/cm2. He emphasized the importance of "the ability to control the whole process, because a CMOS image sensor is not a pure circuit design; it is really a system-on-a-chip." A full imaging system, he said, involves the lens and optical system, A/D conversion, pixel circuitry, color filtering and microlenses-features not found in traditional CMOS devices. Yang said many semiconductor companies have underestimated the complexity of the full task and do not understand how sensor performance scales. A key feature that lets the design take advantage of the basic DRAM process is the digital double sampling applied to each pixel through a bank of 800 A/D converters. Some form of signal conditioning is always required with CMOS imagers to alleviate fixed-pattern noise, which occurs because of slight variations among individual charge-storage capacitors at each pixel. Including the A/D conversion step on the same chip can eliminate fixed-pattern noise, Yang said. On-chip sensor- and timing-control features include 12,000 gates of standard-cell logic, on-chip generation of all control signals for the sensor array, and the A/D converter bank. Programmable registers allow charge integration during the sensing interval. Yang undertook the work at Hyundai three years ago, when his interest in CMOS image-sensor technology combined with his desire to help Hyundai find new uses for its DRAM fabs. Spending time away from teaching, he conducted research at Hyundai. His expertise includes VLSI design, chip design, image processing, computer vision, merged-memory logic, CMOS image sensors and CCD technology. Yang said a senior vice president at Hyundai who knew of Yang's technical expertise asked him to pursue an alternative device that could be manufactured in a DRAM fab. The small group of original researchers Yang assembled came up with very promising results and is now an entire department at Hyundai. Though CMOS image-sensor quality doesn't match that of CCDs, Yang said the group will be able to achieve CCD-image quality with the modified DRAM process in one to two years. He said he's confident the work will yield a low-cost, efficient means for sensor production. Copyright ® 1999 CMP Media Inc. techweb.com