Optical method could improve semiconductor etching process

A method of carving features onto semiconductor wafers using light and observing the process as it happens has been developed at Illinois University.

‘You can use light to image the topography and you can use light to sculpture the topography,’ said electrical and computer engineering professor Gabriel Popescu in a statement. ‘It could change the future of semiconductor etching.’

Chip makers and semiconductor researchers need to very precisely control the dimensions of their devices as the scope of the components affects performance, speed, error rate and time to failure.

Semiconductors are commonly shaped by etching with chemicals, but etching errors, such as residual layers, can affect the ability to further process and etch as well as hamper device performance. Consequently, researchers use time-consuming and costly processes to ensure precise etching that, for some applications, are accurate to within a few nanometres.

The Illinois team’s new technique can reportedly monitor a semiconductor’s surface as it is etched, in real time, with nanometre resolution. It uses a microscope that uses two beams of light to very precisely measure topography.

‘The idea is that the height of the structure can be determined as the light reflects off the different surfaces,’ said electrical and computer engineering professor Lynford Goddard, who co-led the group with Popescu. ‘Looking at the change in height, you figure out the etch rate. What this allows us to do is monitor it while it’s etching. It allows us to figure out the etch rate both across time and across space, because we can determine the rate at every location within the semiconductor wafer that’s in our field of view.’

The new method is claimed to be faster, lower in cost and less noisy than the widely used methods of atomic force microscopy (AFM) or scanning tunnelling microscopy, which cannot monitor etching in progress but only compare before and after measurements.

In addition, the new method is purely optical, so there’s no contact with the semiconductor surface and the researchers can monitor the whole wafer at once instead of point by point.

‘I would say the main advantage of our optical technique is that it requires no contact,’ Popescu said. ‘We’re just sending light, reflected off the sample, as opposed to AFM where you need to come with a probe close to the sample.’

In addition to monitoring the etching process, the light catalyses the etching process itself, called photochemical etching.

Traditional chemical etching creates features in steps or plateaus. For curved surfaces or other shapes, semiconductor researchers use photochemical etching.

Usually, light shines through very expensive glass plates (masks) that have distinct patterns of grey to let light through by degrees. A researcher must purchase or make a mask for each tweak of a pattern until the correct pattern of features is achieved.

By contrast, the new method uses a projector to shine a greyscale image onto the sample being etched, allowing researchers to create complex patterns quickly and easily and to adjust them as needed.

The researchers envision this technology applied beyond etching to the real-time monitoring of other processes in materials science and life science — for example, watching carbon nanotubes self-assemble or error monitoring during large-scale computer chip manufacturing. It could help chip manufacturers reduce costs and processing time by ensuring that equipment stays calibrated.

The National Science Foundation supported this work, published on 28 September in the journal Light: Science and Applications.