SemiSerious

A couple of my colleagues were surprised to say the least this week when the devices they were working on turned out to be rather elaborate fakes. Or not - I’m not exactly sure.

SI was undertaking a reverse engineering project looking at a certain leading CMOS image sensor supplier’s device who is known to use a certain leading Asian fab to manufacture its devices. Ten units of the chip came from a parts distributor in Hong Kong.

At first glance, the devices appeared as expected. The package was standard-issue for image sensors with hermetically-sealed cover glass over the sensor die. Everything still appeared normal after removing the sensor die from the package. A typical SoC type of image sensor layout was clear with the pixel array occupying about 26% of this 38 square millimeter chip.

But that’s where our expectations diverged from reality.

After dropping one die into acid to remove all the interconnect layers and get a quick look at the active area patterning, the silicon appeared blank. The engineer then quickly beveled the IC in order to expose all of the active layers. Again, there was only whitespace deeper into the chip. Starting with yet another sample from the lot, our lab cross-sectioned through the device. This final step revealed only one metal level, color filter array, and microlenses - no transistors or lower levels of metallization.

A senior process engineer not assigned to the project suggested these devices could be mechanical samples. That seemed reasonable. If you wanted to test a new package type for instance, it might make sense to order a few duds built with only the BEOL processes. Even more believable would be a BEOL-only device for environmental testing since microlens arrays are notoriously sensitive to high temperatures. Increasing adoption of imagers into the automotive industry along with research reports of temperature-hardened microlenses adds some credibility to this idea. (For example, see a TI Japan and Tohoku University paper from IISW 2007 entitled, “A Wide Dynamic Range CMOS Image Sensor with Resistance to High Temperatures.”)

However, the devices we bought were not advertised as mechanical samples but as fully operational image sensors under a leading manufacturers part number. And it wasn’t just one or two devices. The whole lot of 10 appears to suffer the same lack of active circuitry. The distributor is from China, so I know what many of you are thinking - another counterfeit chip scandal is brewing. There’s been a lot made of counterfeiting activity in China. A quick search of that keyword at EETimes will give you dozens of stories to choose from if you want to dive deeper into this murky field.

My best guess at what happened in the present case is that a legitimate order was placed for mechanical samples. But a few “extras” were produced beyond what was required for the environmental testing. Some enterprising worker at the manufacturer decided not to waste the spares and found a discount IC distributor on the lookout for cheap devices.

The first step to uncovering a more complex scandal would be to identify the IC foundry of origin. I happen to know some engineers who have a lot of experience in this field. If they are able to determine that these devices were produced at one of the expected manufacturers main production lines, then the mechanical sample overrun theory would be supported. If not, then maybe this will be the beginning of a bigger story of a semi-serious counterfeiting operation based on chips that could pass only an external visual inspection. Was it was an outfit that only managed to steal one layers worth of design files? Or maybe someone reverse engineered only one level of a legitimate version of this manufacturers device.

This isn’t the first strange case I’ve seen of a partially processed, yet fully packaged chip getting into the distribution channel. It’s unfortunate my memory is so bad that I can’t remember more details of that device which is now buried somewhere deep in the Semiconductor Insights chip graveyard. What I do remember is similar though. Despite containing all the interconnect levels, the polysilicon was an unpatterned, featureless sheet, and there were no vias between metal lines. I guess we weren’t as focussed on counterfeit in those days because it never occurred to me at the time.

My conspiracy theory is that the bean counters took over the fab. Maybe this is the “New Economics of Semiconductor Manufacturing” as described in IEEE Spectrum. Why not save a couple bucks by just avoiding those costly front-end manufacturing steps altogether? After all, with image sensor ASP’s getting continuously hammered, what else can manufacturers do to keep making a buck?

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