Large-Area, UV-Optimized, Back-Illuminated Silicon Photomultiplier Arrays

Existing photocathode-based technologies for visible and UV instruments lack sensitivity, are bulky, and have limited reliability. Solid-state silicon photomultipliers (SiPMs) are efficient, light, and reliable, but the front-illuminated designs demonstrated to date have poor UV response, limited sensitive area, and limited optical fill-factor. In the proposed program, back-illuminated, back-thinned SiPMs optimized for UV response and scalable tiling over very large areas will be developed for observation of air showers from ultra-high energy cosmic rays (JEM-EUSO) as well as for visible-wavelength spectrographic and photometric instruments planned for future telescopes (OWL). Short-wavelength light is absorbed near the surface of a silicon detector, and moving the optical entry surface to the back side of the wafer will enhance UV response by ensuring that all photocarriers from UV photons are generated on the correct side of the junction for efficient avalanche multiplication. Placing the optical entry surface on the back of the wafer will also improve optical fill factor because it will no longer be necessary to shine light through the quench resistor network on the front surface of the detector. Lastly, back-thinning the detector wafer will significantly reduce the mass per unit area of the focal plane array. In Phase I, SiPMs will be back-thinned to demonstrate enhanced UV response, and edge-buttable SiPM arrays that make optimal use of a standard 22-mm CMOS reticle will be designed. In Phase II, large-area back-illuminated SiPMs will be fabricated and demonstrated.Voxtel anticipates that its technology will enter the program at TRL=3, finish Phase I at TRL=5 or 6 (goal), and exit the Phase II program at TRL=7.