Semiconductor Etching Monitored with Light
CHAMPAIGN, Ill., Oct. 1, 2012 — An inexpensive, completely optical technique uses a special type of microscope to simultaneously etch features onto a semiconductor wafer’s surface while monitoring the entire process in real time with nanometer precision.
Semiconductors are commonly shaped by chemical etching. Chip makers and researchers need to control the dimensions of their devices very precisely, because errors affect performance, speed, error rate and time to failure.
A team led by electrical and computer engineering professors Gabriel Popescu and Lynford Goddard at the University of Illinois at Urbana-Champaign has developed a microscopy method that uses two light beams to precisely image and sculpt the topography of a semiconductor’s surface with high precision.
A 3-D image of the University of Illinois logo etched into a gallium-arsenide semiconductor, taken during etching with a new microscopy technique that monitors the etching process on the nanometer scale. The height difference between the orange and purple regions is approximately 250 nm. Courtesy of Chris Edwards, Amir Arbabi, Gabriel Popescu and Lynford Goddard.
“The idea is that the height of the structure can be determined as the light reflects off the different surfaces,” Goddard said. “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.”
Techniques currently used — scanning tunneling microscopy and atomic force microscopy (AFM) — are not capable of monitoring the etching in progress but can only compare it before and after. The new method is faster, less expensive and less noisy. It is also purely optical, enabling noncontact monitoring of the entire semiconductor wafer at once rather than 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.”
Illinois researchers, from left, graduate student Amir Arbabi, professor Gabriel Popescu, graduate student Chris Edwards and professor Lynford Goddard — use a special microscope to simultaneously etch tiny features in semiconductor wafers and monitor the process in real time. Photo by L. Brian Stauffer courtesy of UIUC.
Besides monitoring the process, the light acts as a catalyst for the etching process itself, called photochemical etching. With the old method, light was shone through glass plates called masks that were patterned to let light through by degrees. The new method uses a projector to shine a gray-scale image onto the sample being etched; a computer allows the team to create complex patterns quickly and easily, and to adjust them simply by changing the pattern.
This method holds promise for real-time monitoring of the self-assembly of carbon nanotubes, or for error monitoring during large-scale production of computer chips, the researchers said. It may also help chip makers decrease processing time and costs by allowing them to continuously calibrate their equipment.
The work appeared Sept. 28 in Light: Science & Applications. doi: 10.1038/lsa.2012.30
For more information, visit: www.illinois.edu
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