Single Photon Avalanche Diode for a Time-Gated Raman Spectrometer

Chemicals and bacteria in drinking water pose significant health risks for human being. To improve the safety of drinking water, sensitive, low-cost, rapid detection methods are required for water monitoring. Both electrical¹ and optical²techniques have been used in water quality monitoring. However, optical techniques are sometimes preferred for chemical and biological analysis due to their non-contacting and non-destructive properties. Since water is a weak Raman scatterer, Raman spectroscopy is an especially promising technique for water monitoring. Considering the low concentration of contaminants in water and the small Raman cross section, an ultra-sensitive detector for extremely low light level detection is necessary for the detection system.

An avalanche diode biased above the breakdown voltage (Geiger mode)^3,4 was designed for the detection system. When biased above breakdown, single photon generated carriers in the depletion region can trigger a self-sustaining avalanche multiplication process, diode in Geiger mode is also named as the single photon avalanche diode (SPAD). To reduce the cost and increase its integration with other electronic systems, this diode was fabricated with the standard CMOS process. Determined by the specific process, the peak detection efficiency is in the visible region, and fluorescence emission becomes the most significant background noise in this situation. Raman signal is usually emitted simultaneously with the excitation, while there is delay between excitation and fluorescence emission⁵. According to the temporal distribution shown in Fig 1, if synchronizing the detection window with laser excitation and shutting down the detector after the excitation, the fluorescence background can be successfully removed⁶. 

Figure 1 Temporal distribution of Raman and fluorescence

To suppress fluorescence background, the SPAD in this work is operated in the time gated mode. Unlike the free running mode which is always biased above breakdown, SPAD in time gated mode is only biased above its breakdown in a very narrow time window, only photons coincident with this window can be detected. This operation can not only remove fluorescence signal, but also reduce the dark count detection probability. Fig 2 shows the schematic and timing diagram of the proposed pixel circuit. Three on chip pulse generators were designed. Two of them are used to gate on and off the diode, and the third one is for time gated read out. Depending on the arrival time of photons, the out signal will have variable width, which can be used to extract the temporal distribution of photons. The chip has been measured in terms of dark noise, temperature dependence, and detection efficiency. In this presentation, both simulation and measurement results will be discussed.

Figure 2: Proposed pixel circuit (a): Schematic; (b) Timing diagram

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