Highly parallel SPAD detector for time-resolved lab-on-chip

Fluorescence lifetime detection is widely used in molecular biology to monitor many cell parameters (such as pH, ion concentrations, etc.) and for an early diagnosis of many pathologies. In a typical fluorescence lifetime experiment a pulsed laser is used to excite the fluorescent dyes and the emitted light is revealed by means of high sensitivity detectors, typically: intensified CCD, PMTs or Single-Photon Avalanche Diodes (SPADs). In this contribute we present a SPAD detector module fabricated in a 0.35 mu m High Voltage CMOS technology to be used within a lab-on-chip system consisting of a micro-reactor array for bioaffinity assays based on fluorescence markers. The detector module, having a total area of 600 x 900 mu m2, can be arranged to build a small pixel array to be directly coupled to the micro-reactors. No emission filters are needed, since the ultra-short laser pulse is cut off in the time domain. The module consists of a 10x10-SPAD array, where each SPAD cell is equipped with dedicated active quenching and recharging circuit. Each cell has a pitch of 26 mu m with a fill factor of 48%. The SPADs have been binned in order to realize a large photosensitive area detector exhibiting a reasonably low dark count rate (DCR) and reduced dead time, as required in a fast measurement system. A memory has also been implemented in order to enable only low DCR SPADs, so that a total DCR of about 100kHz can be achieved for the whole photosensitive area. The digital output generated by the SPAD array is sent to a time-discriminator stage which allows a time-gated detection of the incident light. Two time-windows have been implemented in this architecture. Their time width is controlled by an on-chip digital PLL locked to the external laser clock whereas the width of the time-windows can be set within the range 500ps-10ns with a resolution of 500ps. Photons detected within each time window are then counted by two 10-bits digital counters. Time-interleaved operation has been implemented to read out the pixel data in parallel with the photon detection phase.