Abstract
The MISPIA project is an EC-funded project concerning the development of single photon Time-of-Flight camera systems for safety and security related applications. Its primary goal is the development of a smart camera system that can deliver high frame-rate accurate ranging information at either low-light levels or long ranges. The use of smart pixel structures in conjunction with single photon avalanche diodes (SPADs) bestows significant timing resolution advantages at the cost of severely reduced overall active area fill-factors (typically around 2-6% of the overall area of the smart pixel). The integration of high fill-factor microlens arrays with the SPAD arrays gives a simple method of increasing the effective fill-factor of the composite structures without the need for additional complex VLSI processing steps. The microlens arrays (Figure 1), which can be either refractive or diffractive in nature, are fabricated using a combination of photolithography and reactive ion etching to generate surface relief structures on fused silica substrates. The lens arrays can be integrated with the SPADs in one of two orientations – lens-down, where the patterned surface is placed closest to the SPAD surface and lens-up, where the un-patterned surface is closest to the SPAD surface. Lens-down configuration requires the use of a spacer arrangement to ensure that the focal plane of the microlens array coincides with the SPAD surface whereas lens-up configuration places the lens substrate directly in contact with the SPAD surface. We have performed the design and fabrication of microlens arrays (both diffractive and refractive) and in this paper we will present verification of the optical properties of the microlens arrays as well as a full tolerance analysis of the integration process. The tolerance of the microlenses to light arriving at non-normal incidence is of critical importance due to relatively small active area of the SPAD smart pixels and a detailed consideration of this factor will be given and the modifications to the lens designs needed to permit the use of this system in a large field-of-view mode will be discussed.
© 2013 IEEE
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