A sensor with an accuracy of less than the size of an atom has been designed by Cyprus-born doctoral candidate Stefanos Andreou from the University of Eindhoven in the Netherlands. Andreou has built a sensor with which deformations measuring less than the width of an atom can be measured.
According to an article on the University of Eindhoven website, chip machinery builder ASML may be able to use this technology to improve the precision of its machines. Researchers envision producing computer chips whose details measure no more than a handful of nanometers.
Even the slightest deformation of the wafers causes problems, explains Andreou. "These wafers are actually quite stiff, but because they are moved about at such great speed, they are subject to g-forces that slightly deform them. Measuring this deformation gives ASML the opportunity to compensate for it in some or other way, and opens up the possibility of producing even smaller chips." This prompted the Cypriot to dedicate his Ph.D. work to designing a special sensor, based on a glass fiber, able to measure these deformations of roughly one nanometer per meter.
The idea behind the sensor is that deviations in the frequency of laser light can be measured with extraordinary accuracy—a principle that is applied in what is known as a Fiber Bragg Grating—a glass fiber of sorts treated in such a way that it becomes opaque for a very specific color of light. This resonance frequency as it is called depends on the extent to which the fiber is stretched.
Consequently, a Fiber Bragg Grating (FBG), applied to the moving parts in the chip machine, can be used as a measure of the wafer's deformation, explains Andreou. Assisted by master's student Roel van der Zon, Andreou tested a measuring system based on this kind of FBG sensor in the lab. "In practice ASML would need dozens of these sensors, but that's no problem: they can be produced cheaply and weigh almost nothing."
Sophisticated stabilization techniques were needed to ensure that the laser light used—generated by a photonic chip produced by Smart Photonics, a spin-off of the Photonic Integration research group where Andreou conducted his research—had exactly the right frequency. But perhaps the greatest challenge was the fact that the sensor's resonance frequency depends not only the deformation, but also the temperature.
To compensate for inevitable temperature fluctuations, Andreou split the laser light used for the measurement into two components: "For each of these components, or polarization states, the fiber displays a different relationship between temperature and resonance frequency." This cancels out the effect of the temperature, making it possible to determine the deformation very accurately.