Looks Like AuthenTec Descendants
Contributed by Dick James
It’s a matter of history now. The widely expected fingerprint sensor did show up in the iPhone, to generally good reviews, so of course we had to take a look at it.
Today, we take a technology perspective and dive into what looks like an AuthenTec descendant.
They were very efficient at taking out the sensor and separating it from the connector, so I’ve borrowed the iFixit images so that we can see the assembly.
The top image shows the sapphire disc and the stainless steel ring that has to be contacted to make it work. In the bottom image, we can see an NXP interface chip on the flextape.
Looking closer at our black-on-black sample (our phone was a black model), we can hardly see anything since both the sapphire touch-disc and the bezel are jet black, but if we separate the sapphire disc off the bezel, then it becomes more clear, and if we flip the disc, then it becomes even more so – we can see the outline of the actual sensor chip.
We can see from the scale in the background (in mm) that the sapphire disc is ~9.5 mm in diameter, so the chip is relatively huge, my tentative measurements had it coming in at ~6.6 mm x 6.1 mm, not quite square. At the top edge are the remnants of the gold bond wires connecting the chip to the outside world.
This confirms what we saw during the launch of the iPhone 5s; that the TouchID was a square touch sensor (capable of reading a fingerprint using a touch action, rather than swiping across a strip, as we’re used to in other sensors) – more costly, but a lot more convenient.
The Apple video describing the sensor also showed that. Here’s a screen shot which is a relatively accurate schematic of the assembly stack (the annotations are ours):
After taking the chip off the sapphire, we can finally get a look at the layout of the die.
The capacitors are arranged in an 88 x 88 array for a total of 7744 pixels, and are ~0.051 mm (~0.002”) square, which agrees with the 500 ppi shown above.
An unusual feature of the die is the dark areas at the top and bottom edges. It appears that the silicon has been partially etched away to provide a recessed shelf within the die area for the wire bonds. That allows the top surface of the chip to be bonded straight onto the sapphire disc, minimizing the finger-to-chip distance. Other styles of packaging could have been used, similar to image sensors, but they are likely more expensive than good old fashioned wire bonds.
The title states that we think that this sensor is a descendant of the AuthenTec sensors that we found in earlier phones; why would we think that?
First, Apple bought AuthenTec back in July last year. Second, if we look at the die markings on the Apple sensor, they look similar to those on the last AuthenTec sensor that we looked at.
Here’s the Apple sensor die marking:
And this is the one from an AuthenTec sensor in a Motorola Atrix phone from 2011:
No AuthenTec (or Apple) logo on the new chip, but the style and fonts are very similar. You could also say the same for the chip layout, looking at the Atrix sensor (even though it’s clearly a swipe, not a touch sensor); again the style of layout is very similar.
Have we made the case? I believe so! We’ll be taking a look at the way the sensor is put together in an upcoming blog.
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