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EETE JULAUG 2012

DESIGN & PRODUCTS MEDICal ElECTRONICS Wafer-scale CMOS X-ray imaging for medical applications By Paul Double and Dr. Renato Turchetta TheRe is an incReasing interest in the use of solid-state- quirements for based X-ray medical imaging and detection systems in the mammography replacement of conventional diagnostic imaging techniques. applications. One of these technologies is wafer-scale cMOs-sensor based additionally, imaging, which can bring key advantages in terms of perfor- any 2xn sensor mance such as high resolution, high dynamic range and low- arrangements noise capabilities. Additionally, it can offer significant system are possible, cost advantages for X-ray imaging applications, although it can thus making come with an initial penalty in terms of design complexity in the the device ideal development of the cMOs sensor. for applications that demand Image sensor target area even larger area Because of the absence of a lens for cMOs-imaging-based coverage, such X-ray applications, the size of an image sensor has to match as chest imag- the size of the target area. a sensor measuring 139x120mm, ing or security Fig. 1: A three-sided ‘buttable’ sensor design for example, is usually adequate for some medical applications scans. with readout circuitry designed on one single such as extra-oral panoramic dental imaging. however it is not Traditionally edge allows for the sensors to be ‘tiled’ sufficient for most medical applications such as mammography, cMOs imag- together in a 2x2 arrangement. which requires a sensor that is approximately 290x240mm in ers have the size. For chest radiography or other applications such as full required electronic circuitry implemented on two sides of an im- body scanning for security purposes, an even more extensive aging array to address the individual sensor pixels. To achieve sensor area is likely to be necessary. Therefore imaging applica- this three-side ‘buttable’ design, the design group developed tions such as X-rays will require a field of view in excess of that innovative electronic circuitry intellectual property to implement covered by a single-wafer based imager – even if the sensor the necessary pixel readout and row-addressing driver func- is manufactured using 300mm wafers, the largest wafer size tions on just one edge of each sensor – see figure 1 - with extra commercially available today. Therefore, several sensors need circuitry embedded in the actual pixel array; while, crucially, also to be ‘tiled’ to meet application needs. although this approach maintaining a high degree of image quality. can generate ‘dead’ or ‘low-sensitivity’ lines, such single-line defects can be tolerated in most applications. at the edge of Design flexibility and binning the wafers, the goal is to lose not more than one line of pixels, The full-custom-design sensor, which offers a focal plane of so that tiled versions will still generate high-quality images. 139.2x120mm, features 6.7-million (2800x2400) pixels on a 50-micron pitch and 32 analogue outputs. it also features A three-sided ‘buttable’ sensor design low noise, a high dynamic range and a programmable region- Based at the science and Technology Facilities council’s of-interest readout. each pixel is constructed from a basic (sTFc) Rutherford appleton Laboratory at harwell in the UK, the three-transistor (3T) base with a low-noise partially pinned cMOs sensor Design group has been designing full-custom photodiode, offering ‘charge-binning’ capability to deliver its image sensors for scientific applications since the 1990s. It high signal-to-noise characteristics. Pixel binning essentially has developed a high-resolution and radiation-hard wafer- combines a cluster of adjacent pixels into a single pixel. For ex- scale digital cMOs image sensor prototype aimed at use in ample, in 2x2 binning, an array of four pixels becomes a single X-ray medical imaging and more specifically mammography larger pixel, reducing the overall number of pixels available on and digital tomosynthesis, the advanced diagnostic technique the sensor. The sensor can offer a very high frame rate of 40 that is used to generate 3D representations of patients or other frames per second at full resolution and ‘binned’ images can be scanned objects. a unique feature of this sensor is that it has read at an increasingly faster rate. The high frame rate makes sensing pixels right up to the edges on three sides. This allows the sensor suitable for applications, such as digital tomosynthe- multiple sensors, manufactured on cost-effective 200mm silicon sis, that demand fast acquisition of multiple images. wafers, to be ‘butted’ or ‘tiled’ together in a 2x2 arrangement to The design has a high degree of built-in flexibility: binning form a significantly larger imaging area and to meet the re- can be achieved on the sensor in both directions in steps of two or four. Binning also changes the effective pitch of the image Paul Double is Managing Director of eDa solutions - to 100 or 200 microns from the fundamental 50-micron pixel www.eda-solutions.com – he can be reached at pitch. Binning in the row direction also increases speed by at pauldouble@eda-solutions.com least a factor proportional to the binning factor. additionally, Dr. Renato Turchetta is cMOs sensor Design group Leader in Region-Of-interest (ROi) readout can be implemented. The ROis the Rutherford appleton Laboratory, science and Technology can be programmed and the position of up to six ROis can be Facilities council (sTFc) - www.stfc.ac.uk – he can be reached stored on the sensor. high dynamic range can be achieved by at renato.turchetta@stfc.ac.uk reading the sensor multiple times. The timing information is also 28 Electronic Engineering Times Europe July/August 2012 www.electronics-eetimes.com


EETE JULAUG 2012
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