Monitoring semiconductor etching with light in real time
01 October 2012
University of Illinois researchers have a new low-cost method to carve delicate features on semiconductor wafers using light – and watch as it happens.
From left, graduate student Amir Arbabi, professor Gabriel Popescu, graduate student Chris Edwards and professor Lynford Goddard (photo by L Brian Stauffer)
Chip makers and semiconductor researchers need to very precisely control the dimensions of their devices. The dimensions of the components affect performance, speed, error rate and time to failure.
Semiconductors are commonly shaped by etching with chemicals. Etching errors, such as residual layers, can compromise further etching, as well as hampering device performance. Thus, researchers use time-consuming and costly processes to ensure precise etching – for some applications, to within a few nanometers.
The Illinois researchers’ new technique can monitor a semiconductor’s surface as it is etched, in real time, with nanometer resolution. It uses a special type of microscope that uses two beams of light to measure topography with great precision.
“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 electrical and computer engineering professor Gabriel 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 faster, lower in cost, and less noisy than the widely used methods of atomic force microscopy 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 an 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 plateaux. For curved surfaces or other shapes, semiconductor researchers use photochemical etching. Usually, light is passed though expensive glass plates (masks) that have distinct patterns of grey to moderate the amount of light passing through. 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 projects a greyscale image on the sample being etched. This allows the researchers to create complex patterns quickly and easily, and adjust them as needed.
“To create each mask is very expensive. That’s impractical for research,” Goddard said. “Because our technique is controlled by the computer, it can be dynamic. So you can start off etching one particular shape, midway through realise that you want to make some change, and then change the projector pattern to get the desired outcome.“
The researchers believe this technology can be applied beyond etching, to 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.
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