Printed Circuit Board

Semiconductor products can be counterfeited. That’s an unfortunate fact. But because they control the operation of vital electronics, the failure of a single counterfeit semiconductor component can have catastrophic consequences, presenting important risks to our health – and via security applications, risks to our safety, too.

Detection of counterfeit and tampered ICs has become a major challenge. This is true whether authentication is required over the lifetime of the product or for a short-period of time (for example, during transitions such as transportation from one facility to another). In the latter case, optical (and, more particularly, excitonic) techniques based on optical transitions in semiconductors present an attractive possible solution.

By way of review, an exciton is a bound state of an electron and an electron hole (i.e. the lack of an electron at a position where one could exist in an atom or atomic lattice) that can transport energy without transporting a net electric charge. An exciton can form when a material absorbs a photon of higher energy than its bandgap. This excites an electron from the valence band into the conduction band, which in turn leaves behind a positively-charged electron hole.

Recently, self-erasing chips developed at the University of Michigan have shown promise in helping to stop electronics counterfeiting. They rely on a new material that temporarily stores energy, changing the color of the light it emits. The chip self-erases in a matter of days, or it can be erased on demand with a flash of blue light.

“It’s very hard to detect whether a device has been tampered with. It may operate normally, but it may be doing more than it should, sending information to a third party,” said Parag Deotare, assistant professor of electrical engineering and computer science at the university.

In operation, nanoscale strain engineering is utilized to create a self-erasable and rewritable platform for anti-tamper hardware. The reversible structural change between isomers in azobenzene molecules is utilized to strain an overlying tungsten diselenide monolayer, thereby affecting its optical bandgap.

Azobenzene is a widely-applied photoisomerizing molecule – that is, the behavior in which a structural change between isomers is caused by photoexcitation. This kind of molecule shrinks in reaction to UV light, and can be efficiently and reversibly switched between two structurally-different forms upon exposure to the different wavelengths of light.

When the azobenzene is exposed to UV light, it shrinks, stretching out the tungsten diselenide atoms above it. That, in turn, changes the wavelength of the light they emit. These self-erasing chips are built from a three-atom-thick layer of semiconductor material. The semiconductor itself is somewhat murkily described as “beyond graphene,” according to Deotare, as it has many similarities with that nanomaterial. He points out, however, that it can also do something graphene can’t: it emits light in particular frequencies.

The azobenzene molecules are utilized to tug on an overlying tungsten diselenide layer, thereby affecting its optical bandgap. A large strain is generated that results in dramatic shift in photoluminescence wavelength (it gets slightly longer) thereby changing the excitonic emission wavelength and causing it to emit light.

This strain can be rapidly relaxed under exposure to visible light or can be retained up to seven days under dark condition – a time that can be shortened with exposure to heat and light, or lengthened if stored in a cold, dark place. As a result, a self‐erasable and rewritable platform is created that responds to environmental changes (light/temperature) to detect tampering of hardware system

Whatever has been written on the chip, be it an authentication bar code or a secret message, disappears when the azobenzene stops stretching the semiconductor. Alternatively, it can be erased all at once. Once erased, the chip can record a new message or bar code. If the lifespan of these bar codes was extended, they could be written into devices as authorization keys.

With a self-erasing bar code printed on the chip inside the device, the owner could get a hint if someone had opened it – for instance, to secretly install a listening device. Or, a bar code could be written and placed on integrated circuit chips or circuit boards to prove that they hadn’t been opened or replaced on their journeys.

Reading the message requires the right kind of light. Che-Hsuan Cheng, a doctoral student in material science and engineering in Deotare’s group and the first author on the study published in “Advanced Optical Materials,” is most interested in its application as self-erasing invisible ink for sending secret messages. The researchers point out that the results also open avenues for varied applications in information storage, providing time-sensitive, self-destructive memories.

In practice, a user can write a message or draw a symbol into the azobenzene using UV light, and when a certain wavelength of light is shone over the tungsten diselenide, that same shape will appear to glow on the surface. If the azobenzene is exposed to visible light, it will relax again, wiping the message completely.

The idea, the research team says, is that if a chip is tampered with, the tampering would inadvertently wipe a symbol or other message when they open the box. A user could easily spot any tampering by checking for that symbol.

The platform has promising applications in anti-tamper hardware following further research on optimizing azobenzene films for faster and more sensitive response. The researchers believe that such an opto-excitonic platform can further benefit applications such as self-destruction of sensitive data over time, and to create optical “scratch pads” by using sensitive light detectors.

Next steps for the research include extending the amount of time that the material can keep the message intact for use as an anti-counterfeit measure. The University of Michigan has applied for patent protection and is seeking commercial partners to help bring the technology to market.

Of course, there is another route to assurance of authenticity, and it is one that bears repeating: the only sure-fire way to ensure that semiconductor components are genuine and have optimal quality and reliability levels is to buy them exclusively through authorized sources. The authorized supply chain for semiconductor components ensures that this supply chain is not contaminated by counterfeits.

Statements of fact and opinions expressed in posts by contributors are the responsibility of the authors alone and do not imply an opinion of the officers or the representatives of TTI, Inc. or the TTI Family of Companies.


Murray Slovick

Murray Slovick

Murray Slovick is Editorial Director of Intelligent TechContent, an editorial services company that produces technical articles, white papers and social media posts for clients in the semiconductor/electronic design industry. Trained as an engineer, he has more than 20 years of experience as chief editor of award-winning publications covering various aspects of consumer electronics and semiconductor technology. He previously was Editorial Director at Hearst Business Media where he was responsible for the online and print content of Electronic Products, among other properties in the U.S. and China. He has also served as Executive Editor at CMP’s eeProductCenter and spent a decade as editor-in-chief of the IEEE flagship publication Spectrum.

View other posts from Murray Slovick. View other posts from Murray Slovick.

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