New method monitors, catalyzes semiconductor etch in real time

October 2, 2012 – Researchers at the U. of Illinois have devised a method to monitor a semiconductor surface as it is etched, in real time, with nanometer precision.

The new method, dubbed "epi-diffraction phase microscopy" (epi-DPM), is purely optical, and thus noncontact, so researchers can monitor the entire wafer at once instead of point-by-point. It’s faster, lower in cost, and less noisy than the widely used methods of atomic force microscopy or scanning tunneling microscopy, which can only compare before/after etch measurements, the researchers say.

In their work, a grayscale image is shined via projector onto the sample being etched, enabling creation of complex patterns quickly and easily, and the ability to adjust them as needed. "The idea is that the height of the structure can be determined as the light reflects off the different surfaces," stated electrical and computer engineering professor Lynford Goddard, who co-led the group with fellow 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."

A three-dimensional image of an etched GaAs semiconductor in a wet etch
solution, taken during etching with the new "epi-diffraction phase microscopy"
(epi-DPM) technique. The height difference between the orange and purple
regions is about 250nm. (Photo by Chris Edwards, Amir Arbabi, Gabriel
Popescu and Lynford Goddard)
   And here’s a video describing the process.

Besides monitoring the etching process, the light also catalyzes the etching process itself ("photochemical etching"), a process already used in place of chemical etching on curved features or other shapes. It eliminates the problem of using expensive masks to pattern light through by degrees — and requiring new masks for every tweak of the chip features to achieve correct patterning. "Because our technique is controlled by the computer, it can be dynamic. So you can start off etching one particular shape, midway through realize that you want to make some change, and then change the projector pattern to get the desired outcome," Goddard said.

Beyond semiconductor etching, the researchers see applications for real-time monitoring of other processes in materials and life sciences, e.g. observing carbon nanotubes self-assembly, error monitoring during large-scale computer chip manufacturing, or ensuring precise equipment calibration. Their work, funded by the National Science Foundation, appears in the Sept. 28 journal Light: Science and Applications. Here’s a snippet:

We present epi-diffraction phase microscopy (epi-DPM) as a non-destructive optical method for monitoring semiconductor fabrication processes in real time and with nanometer level sensitivity. The method uses a compact Mach–Zehnder interferometer to recover quantitative amplitude and phase maps of the field reflected by the sample. The low temporal noise of 0.6nm per pixel at 8.93 frames per second enabled us to collect a three-dimensional movie showing the dynamics of wet etching and thereby accurately quantify non-uniformities in the etch rate both across the sample and over time. By displaying a gray-scale digital image on the sample with a computer projector, we performed photochemical etching to define arrays of microlenses while simultaneously monitoring their etch profiles with epi-DPM.

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