Today, Feb. 21st, 2011, the International Solid-State Circuit Conference started. On this first day, there were two interesting presentations about image sensors :
Bart Dierickx (Caeleste) presented a paper : “Indirect X-ray Photon-Counting Image Sensor with 27 T Pixel and 15 electrons Accurate Threshold”. The paper started with the explanation of direct and indirect detection. In the case of indirect detection the X-ray is absorbed in a scintillator, and is generating a cloud of visible photons which are detected by the underlying imager. In the (test-)chip presented, the pixels are based on a photodiode that is detecting the incoming cloud of scintillator-generated photons. After absorbing the latter, the photodiode generates a pulse corresponding to 100 up to 500 electrons. The in-pixel circuitry creates out of the burst signals of the photodiode a digital pulse train which are counted in the pixel itself. The nice thing about this counter it its implementation as an analog part. This consumes only a very small part of the pixel real estate. Unfortunately this was only a short paper and the presenter could not go into detail about the performance of the analog counter. Would have been interesting to hear something about linearity and other performance parameters.
The paper concluded with measurement results and a look into the future. It clearly showed the ability of detecting single X-ray photons. Apparently the next focus will be on the shrinkage of the pixels, increase performance, as well as increasing functionality. (Where did I heard this before ?)
The second imager paper was delivered by Suat Ay (University of Idaho, ID) : “A 1.32 pW/frame.pixel 1.2 V CMOS Energy-Harvesting and Imaging (EHI) APS Imager”. Quite funny idea in which the imager itself tries to collect the energy that it needs to support its own operation. That captured energy is the incoming light itself. Primary application is an artificial retina.
The pixel used is based on a 3T architecture, with a double photodiode : the first (classical) one is used for the integration of the incoming light information, and the second one is operating in the solar cell mode. In the imaging mode, the solar cell diode is connected in parallel to the first diode to increase the sensitivity. In the design/lay-out, the imager photodiode is a p+/n-well diode, while the energy harvesting diode is an n-well/p-sub diode. So they are optimally stacked in the pixel to allow a maximum fill-factor for both. During the presentation the author showed very nice sheets to illustrate the switching between the two modes of operation : imaging and energy harvesting. Unfortunately these sheets are not published in the conference proceedings.
As can be understood, the amount of generated power will be small, so has to be the amount of power consumption by the imager itself. In imaging mode, the device consumes 14 mW at full speed (7.5 fr/sec, 54 x 50 pixels, 10 bit SAR ADC, digital timing off-chip), in energy harvesting mode the device consumes 6.7 mW and is able to harvest 2 mW of energy and to store this on an external capacitor. These numbers are valid under normal daylight. This means that the device is able to harvest the energy with an efficiency of 9 %. The imager is made in 0.5 mW technology and the pixels are 21 mm x 21 mm. Some other data not shown in the proceedings : saturation level of 0.7 V, full well of about 400,000 electrons, floating diffusion capacitance of 91 fF, noise level of 460 electrons (mainly kTC noise of the 3T pixel with a large capacitance and operated in non-correlated double sampling) and a responsivity to light of 0.4 V/lux.s. Despite the large noise floor and the very low supply voltage of 1.2 V the sensor is able to deliver a dynamic range of 58 dB.
Albert, 21-02-2011.