Andrew "bunnie" Huang's IRIS Boasts a "Nanometer Resolution" Focus Stage

Andrew "bunnie" Huang has released more details of the automated IRIS inspection system, designed to use bright infrared lighting to perform in-situ die photograph of unencapsulated silicon chips as a means of detecting hardware-level Trojans inserted during the fabrication process — showcasing a focus stage with "nanometer resolution."

"The depth of field of the 10x objective used on IRIS is estimated to be around 8.5 microns," Huang explains. "This means that I need to be able to control the distance of the chip to the lens in steps much finer than 8.5 microns. Note that depth of field typically decreases with increasing magnification, so if we want to support even higher magnifications, we would need even smaller focus steps."

IRIS, revealed in March last year, is Huang's approach to non-destructive imaging of physical silicon chips. When mounted unencapsulated with the back side of the silicon exposed, as many chips are, it's possible to shine a bright infrared light through the silicon and capture photographs of the hardware within with a lightly-modified inspection camera — giving rise to IRIS, the Infrared In-Situ inspection system.

Initially, IRIS was a manual affair — but earlier this year Huang showed off an automated version, capable of capturing multiple images that can be combined into a single high-resolution shot. To do so, however, requires fine focus — and the motion platform used for the build, Jubilee3D, only delivers a 10-micron resolution on the Z axis. The solution: a piezoelectric positioner.

"Unfortunately, piezoelectric actuators are expensive. They start around $1000, and go up from there if you need features like kinematic coupling," Huang laments. "So, I decided I’d try to build one from scratch, because it seemed like the rare case where solving an interesting problem with a one-off solution is also cheaper than buying an off-the-shelf unit."

Using the low-cost TDK PowerHap haptic actuator as a base, available for $20 a unit, Huang was able to design a aluminum plate that can be adjusted to vary the distance of the chip from the camera under the control of a Raspberry Pi RP2040 in steps measured in "tens of nanometers" — well within the eight-micron window required. By designing around the concept of "kinematic coupling," which reduces the number of contact points, it's also possible to remove and replace the upper part of the focus stage without trouble.

"The neat part is that at the end of the day, the whole thing cost a few hundred bucks, even counting all of the costs for one-off custom machining of parts and building circuit boards (it helps that I hand-soldered the whole thing myself)," Huang notes. "I’m not sure how this compares in performance to the thousand-dollar plus commercial alternatives, but it worked well enough and the software is tailored to exactly what I needed."

Huang has suggested that the focus stage will not be offered as a product, but has included the design in the IRIS hardware repository under the Strongly Reciprocal variant of the CERN Open Hardware License Version 2. More information is available on Huang's blog.