ST’s SPAD imager in iPhone 8?

Article By : Junko Yoshida

Apple's iPhone 8 is rumoured to bring with it a 3D camera. Now, there's reason to argue that STMicroelectronics' single photon avalanche diode (SPAD) is the technology enabler.

Apple’s upcoming iPhone 8 is rumoured to feature a “3D camera.” Now, there's reason to believe or at least argue that single photon avalanche diodes (SPAD) and STMicroelectronics might be what's making this possible for Apple.

Obviously, any connection that would identify Apple as an ST customer is one that ST wouldn't touch with a ten-foot diode. ST, ergo, is mum.

However, Yole Développement, a Lyon-based market research firm with strong focus on technology analysis, recently drew an almost unbroken line from dot to dot. Yole’s educated guess is that ST is bringing a brand new 3D (array) imager to Apple iPhone 8 — an innovation that will alter the phone’s user interface.

As Pierre Cambou, activity leader for Imaging & Sensor at Yole, tells it, one of the first clues is found in a technical paper ST discussed at IEDM 2016. The paper is entitled “Backside illuminated SPAD image sensor with 7.83μm pitch in 3D-stacked CMOS technology.”

Why is that so important?

Let’s start by explaining SPAD, or single photon avalanche diodes (SPAD), which capture individual photons with very high time-of-arrival resolution.

SPAD for 3D sensors

Cambou called SPAD “a completely novel approach to photonics sensing,” which until now was mainly based on the PIN photodiode. He believes SPAD will eventually disrupt PIN photodiode Time of Flight (ToF). Typically ToF uses standard CMOS image sensor (CIS) pixels. SPAD uses avalanche photodiodes to “count photons” in Geiger mode, Cambou explained.

The upside for the use of SPAD in imagers is that it would allow extreme sensitivity. The downside, however, is limited resolution, with pixel size typically in the order of 100µm, said Cambou.

SPAD is known to be “tricky,” for various reasons, he said, but mainly because “it requires high voltage and has a lot of digital handling in each pixel.”

While applying a SPAD pixel to image arrays has proven problematic, SPAD “found an interesting high volume application in the LG smartphones, as a range finder for the autofocus,” according to the Yole analyst.

The breakthrough ST is bringing to Apple’s new generation of iPhones, however, isn’t the SPAD-applied “sensor" of ~1 pixel, which ST delivered first to LG and by now to almost a third of the worldwide smartphone market.

Instead, it’s a breakthrough 3D imager (or “array”) that ST is presumably delivering to Apple.

Cambou stressed the important distinction between a “sensor” and an “imager” in the context where a SPAD pixel is applied.

A 3D sensor usually does distance ranging (camera autofocus) or proximity measurements (powering off the touch screen). The newest sensors come in small arrays of 3×3 or 4×4 pixels.

In contrast, the 3D imager — beyond 80×80 pixels — is a totally different animal, Cambou noted. What’s new here is that by using 3D hybrid stack technology (semiconductor process technology), ST has been able to scale it down to smartphone requirements, Cambou said.

Next: 3D-stacked BSI enables SPAD »

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