CMOS Single-Photon Systems for Bioimaging Applications

Recent advances in neurobiology and medical imaging have put an increasing burden on conventional sensor technology. This trend is especially pronounced in time-correlated imaging and other high precision techniques, where timing accuracy and sensitivity is critical.

Techniques have been proposed to achieve high speed in conventional charge-coupled devices (CCDs) and CMOS active pixel sensor (APS) architectures [1-4]. Among some of the most successful techniques are ultra-fast low-noise electronic readout circuitries, on-pixel A/D conversion, and local analog electrical storage. However, in general a significant design effort and considerable experience is needed to achieve satisfactory results.

Alternatives to conventional CCDs and CMOS APS sensors are single photon counters (SPCs). Several types of SPCs have been known for decades. Among the most successful devices in this class are microchannel plates (MCPs) and photomultiplier tubes (PMTs) that have become the sensors of choice in many applications [5]. Even though they have been studied since the 1960s [6], silicon avalanche photodiodes (SiAPDs) have become a serious competitor to MCPs and PMTs only recently. In SiAPDs, carriers generated by the absorption of a photon in the p-n junction are multiplied by impact ionization; thus producing an avalanche. The resulting optical gain is usually in the hundreds. The main drawback of these devices, however, is a relatively complex amplification scheme and/or complex ancillary electronics. In addition, specific technologies are often required.