The Rutherford Appleton Laboratory(RAL)in Oxfordshire has developed a high-resolution wafer-scale image sensor for medical imaging,that can be butted on three sides for 2xN applications.
The sensor is 120x145mm,including a 139.2x120mm array,"and effectively uses an entire 200mm silicon wafer in its production,"said the Science and Technology Facilities Council(STFC),which runs RAL.
X-rays cannot easily be focussed,so the sensor or film needs to be at least as large as the area to be imaged.
"Mammography applications require a sensor that is approximately 290x240mm,and it has to be even larger for chest radiography.For full body scanning for security purposes,an even more extensive sensor area can be necessary,"said STFC."A sensor measuring 139x120mm is usually adequate for extra-oral panoramic dental imaging.
Being buttable on three sides,it allows 2x2 and 2xN sensors to be made,while using relatively cheap 200mm wafers allows some control over costs.
"Traditionally CMOS imagers have the required electronic circuitry implemented on two sides of an imaging array to address the individual sensor pixels,"said STFC."To achieve this three-side buttable design,the STFC CMOS Sensor Design Group developed some innovative circuit IP[intellectual property]to implement pixel readout and row-address drive on just one edge of each sensor,with extra circuitry embedded in the actual pixel array."
Resolution on each sensor is 2,800x2,400(6.7Mpixel),on a 50μm pitch.
Pixels are three-transistor with a partially-pinned photodiode"offering charge-binning capability to deliver high signal-to-noise characteristics,"said STFC."This means the sensor can offer 40frame/s at full resolution,and binned images can be read at an increasingly faster rate."
The array has 32 analogue outputs,and a programmable region-of-interest readout.
Pixel-addressing IP is almost entirely analogue circuitry,with only a small amount of on-chip digital logic.
Californian EDA firm Tanner provided the analogue design tools,specifically Tools Pro in conjunction with HiPer Verify.
The sensor is implemented on a 180/350nm dual-gate CMOS image sensor process from TowerJazz,a foundary which also consulted on the design.
"TowerJazz'technology process enables the customisation of pixels,offering excellent dark current,low noise and dynamic range performance,"said STFC.
