Apple Inc. (Cupertino, CA)

A back-illuminated single-photon-avalanche diode (SPAD) image sensor is an image sensor wafer that is stacked vertically over an electronic wafer. The sensor wafer has one or more SPAD regions that each SPAD region including an anode gradient layer an cathode region that is adjacent to the front of the SPAD region and an anode avalanche layer that is positioned over the cathode area. Every SPAD region is linked to a voltage source and an output circuit inside the circuit wafer by inter-wafer connectors. Deep trench isolation elements are employed to provide electrical and optical isolation between SPAD regions.

Images sensors can be found in a variety of electronic devices, such as digital cameras and mobile phones. They can also be found in copiers, medical imaging devices (e.g. time-of-flight cameras) as well as security systems and security systems. A variety of photodetectors are utilized to detect and react to light that is incident. A typical image sensor includes a number of them. Single-photon avalanche regions (SPAD) is a kind of photodetector you can use within an image sensor. A SPAD region is a sensitive area that can detect low levels (down to just a single photon) of light and the arrival times.

Monolithically-integrated SPAD image sensors typically include an array of SPAD regions and electrical circuitry for the SPAD regions. However, the fill-factor of the array may be restricted because the electrical circuitry for the SPAD regionsconsumes space on the semiconductor wafer. Additionally, it can be difficult to prevent contamination of the semiconductor wafer during fabrication of the monolithically-integrated SPAD image sensor. Metals and other contaminants can adversely impactthe performance of the SPAD image sensor, for instance by increasing noise in the SPAD image sensor.

In some instances there may be a trade-off between the efficiency of photon detection and the response to timing of the SPAD regions. Although a larger semiconductor wafer can increase the SPAD region’s efficiency in detecting photons but a larger wafer may decrease the response time or timing resolution of the SPAD regions. This is because the charge carriers have to travel through the thicker semiconductor wave. Furthermore, a more dense semiconductor wafer may cause higher breakdown voltage, which will increase the power consumption of the SPAD image sensor when the SPAD image sensor is working in Geiger mode.

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