SystemPlus and Yole "deduced that silicon-on-insulator (SOI) wafers are being used in near-infrared (NIR) imaging sensors. They noted that SOI has played a key role in improving the sensitivity of NIR sensors — developed by STMicroelectronics — to meet Apple’s stringent demands.
Pierre Cambou, activity leader for imaging and sensors at Yole Développement, called the SOI-based NIR image sensors “a very interesting milestone for SOI.”
Apple’s adoption of ST’s NIR sensors marks the debut of SOI in mass production for image sensors, noted Cambou. “Image sensors are characterized by large surface due to the physical size of light. Therefore, this is a great market to be in for a substrate supplier” like Soitec, he added.
Yole and System Plus Consulting found inside ST’s NIR sensor “the use of silicon-on-insulator (SOI) on top of deep-trench isolation (DTI).” DTI is deployed to prevent leakage between photodiodes. Apple reportedly etched literal trenches between each one, then filled the trenches with insulating material that stops electric current.
Optically speaking, Cambou explained that SOI wafers are advantageous because the insulator layer functions like a mirror. “Infrared light penetrates deeper, and it reflects back to the active layer,” he noted. Electrically speaking, Cambou noted, SOI improves NIR’s sensitivity largely because it’s good at minimizing leakage within the pixel. The improved sensitivity provides good image contrast.
Asked if ST’s NIR sensors are using FD-SOI or SOI wafers, Cambou said that the research firms couldn’t tell.
Asked about surprises unearthed by the teardown, Cambou cited the size of ST’s NIR sensor chip. It measures 25mm2, and has only 1.4 megapixels due to the large 2.8-μm pixel size.
any clew about the dark current level of this IR sensor?
ReplyDeleteThe DC is probably high, but in this use case the IR sensor only works in short pulses. After all, Face ID should recognize the projected pattern even under the direct sun. The sunlight photocurrent is way larger than the DC, probably to a degree when DC does not matter.
DeleteWhy would you expect the dark current to be high?
DeleteBecause of BOX surface that is harder to passivate. May be I'm wrong, though.
DeleteMaybe the buttom silicon is biased to reduce the dark current. We shoudl check if there is a electric connection to the handling wafer.
DeleteIt's crazy to see that despite of huge financial and human investments, the semi-conductor chip makers capture so few value in front of the packaging industry!!
ReplyDeleteYangNI, your english is a little broken, are you asking why people aren't amazed at the technology and effort that went into the module packaging?
ReplyDeleteHe's talking about packaging cost is even higher than silicon
DeletePackaging costs dominate total cost of the items. This has been a common theme in electronics as silicon and processing gets cheaper the packaging does not.
DeleteTechInsites has a tiny bit of info about the IR Sensor and a Die shot.
ReplyDeleteTheir article also has over a dozen photos and some part numbers along with a cost breakdown: http://www.techinsights.com/about-techinsights/overview/blog/apple-iphone-x-teardown/ .