Ponente
Descripción
We have developed a large area $(8 \times 9\,\mathrm{mm}^2)$ digital SiPM array with a high fill factor $(77\%)$ for light detection in rare event search experiments with liquid noble gases. Digital SiPMs combine SPADs and CMOS logic on the same silicon substrate so that the SPAD hits can be processed on-chip and the chip output signals are purely digital. This reduces power consumption and system complexity. Our chip consists of a large pixel matrix of $16\times 60$ so-called macropixels of $240 \times 291\,\mu\mathrm{m}^2$ and a narrow band of synthesized, data-driven readout logic located at its bottom. Each macropixel contains 9 SPADs and some CMOS logic which allows for disabling each SPAD in case its noise rate is too high, and a logical OR combining all SPAD signals to create a common macropixel hit signal. The hit signal is stored in a flipflop, so that multiple coincident hits in the matrix are possible. A readout logic in the periphery searches the matrix for hits and writes their X- and Y-address as well as an associated column-wise time information $(\Delta T = 10\,\mathrm{ns})$ into a FIFO. The FIFO data is injected into a serial data stream such that up to 64 chips can be read out through one digital signal in a serial chain. In total, a chip chain requires only seven signals: 3 analogue signals for power, ground and SPAD bias and 4 digital signals for clock, command and serial input and output. The SPADs are of excellent quality, offering a dark count rate of $0.02\,\mathrm{Hz/mm}^2$ at liquid xenon temperature $(T = 165\,\mathrm{K})$. The quantum efficiency at blue light is about $40\%$ and the manufacturer is currently optimizing it for deep VUV light. The number of SPADs with increased noise at $T = 165\,\mathrm{K}$ is about $10\%$, so that turning those off results in a moderate loss in active area.
