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

Large-area (3m2), UV-sensitive focal plane arrays are needed 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). 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 optical fill-factor. To solve the above problems, a large-area, back-illuminated silicon photomultiplier (BaSiPM) array technology has been developed. The BaSiPM technology will integrate SiPM pixel arrays, fabricated on domestic, large volume commercial CMOS fab, with wafer-scale thinning. 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 enhances UV response by ensuring that all photo-carriers are generated on the correct side of the junction for efficient avalanche multiplication. Placing the optical entry surface on the back of the wafer also improves the optical fill since it is 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 significantly reduces the mass per unit area of the focal plane array. Voxtel has successfully demonstrated the ability to perform wafer-scale back thinning fabrication for superior UV sensitivity. Three SiPM architectures (25 variations) have been characterized and studied in detail and their performance compared with commercially available SiPMs. The design of a large format focal plane design, including a mechanical model, mounting, and alignment will be developed using the proposed technology.