3D-stacked BSI enables SPAD
Article By : Junko Yoshida
The key factor that has enabled STMicroelectronics to develop single photon avalanche diodes is the advancement of 3D-stacked backside illumination (BSI) technology.
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Conventional BSI adds a silicon wafer essentially for mechanical support. 3D stacked technology favors a processed wafer with digital circuits supplementing the upper CIS array.
The key technological step for 3D stack, as Yole sees it, is interconnecting the upper and lower part of the circuit. Currently through-silicon via (TSV) is the main technology deployed to connect 3D stacked wafers. However, copper-to-copper hybrid bonding is coming, Cambou noted. This could open the way to direct pixel connections.
He suspects that this is what ST has done. By moving the digital pixel into the secondary chip via 3D hybrid bonding technology, ST was able to pull off SPAD image sensors.
Figure 1: 3D stacked BSI vs. 3D Hybrid BSI (Source: Yole Développement).
With the development of SPAD imagers, Apple can theoretically introduce a 3D sensing camera small enough to fit into its next-generation smartphones.
But what, not theoretically, will Apple do? Cambou sees Apple applying the 3D sensing camera to a new user interface. For example, when the user looks into the phone, the phone will look into the user, with face-recognition technology. The embedded 3D sensing camera will be producing a virtual duplicate of the user, creating an avatar for Facebook, Snapchat and FaceTime, for example, he added.
Figure 2: 3D imaging/sensing is becoming a “single technology that can address many markets." (Source: Yole Développement).
It’s important to note that 3D imaging/sensing is becoming a “single technology that can address many markets,” Cambou said. As shown above, 3D vision stands at the center of current consumer electronics trends.
Cambou noted that a new 3D imaging/sensing technology is still at an early stage of market penetration. It will appear in many very different variants, he predicted. But globally, 3D sensing has two basic applications: “human interaction” and ‘machine sensing.”