CHAMPAIGN, Ill. – A nondestructive optical technique that simultaneously etches features onto a semiconductor wafer’s surface while monitoring the entire process in real time with nanometer precision could change the future of semiconductor etching.
Semiconductors are commonly etched chemically. Chip makers and researchers must precisely control the dimensions of their devices to avoid etching errors such as residual layers, which can affect performance, error rate, speed and time to failure.
The microscopy method, developed at the University of Illinois at Urbana-Champaign, uses two light beams, one from a 532-nm frequency-doubled Nd:YAG laser to monitor, and the other from a color projector to sculpt the topography of a semiconductor’s surface with high precision. The work appeared in Light: Science & Applications
“The idea is that the height of the structure can be determined as the light reflects off the different surfaces,” electrical and computer engineering professor Lynford L. Goddard said in a university release. “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 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.
Current techniques used, such as scanning tunneling microscopy and atomic force microscopy, cannot monitor the etching in progress but can only compare it before and after. The new, low-noise method – which is inexpensive and fast – is purely optical, enabling noncontact monitoring of the entire semiconductor wafer at once rather than point by point.
Besides monitoring the process, the light acts as a catalyst for the process, called photochemical etching. Conventionally, light is shone through glass plates or masks that are patterned to let light through or to completely block it and thereby expose photoresist in a binary pattern before a separate etch step is performed. In the new technique, an ordinary digital projector shines a gray-scale image onto the sample being etched; complex patterns can be created quickly and easily and can be adjusted using a computer.
The monitoring process also can be adapted to etch chips, Goddard told Photonics Spectra
“It can and has been adapted to etch the chips themselves,” he said. “As a proof-of-concept example, we used the light from a digital projector to etch an array of microlenses. We simply drew a gradient shaded circle in Matlab and pasted it into a microlens array pattern in PowerPoint and displayed that image onto the sample in the etch solution to perform photochemical etching.”
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. It also may help chip makers decrease processing time and costs by allowing them to continuously calibrate their equipment.
Researchers at the University of Illinois used a special microscope to simultaneously etch tiny features in semiconductor wafers and monitor the process in real time. From left, graduate student Amir Arbabi, professor Gabriel Popescu, graduate student Chris Edwards and professor Lynford Goddard.
“Monitoring self-assembly of carbon nanotubes could start very soon,” Goddard said. “Error monitoring during large-scale production would be about five years, since there would need to be many proof-of-concept experiments done first in a research environment before a large manufacturer would let us put a new tool into their fab line.”
Besides refining their work on photochemical etching, the team plans to work on “imaging the self-assembly of nanotubes, the dissolution of biodegradable electronics, surface wetting and evaporation, the expansion of deformation of materials and finding isolated defects in patterned semiconductor wafers,” doctoral candidate Chris Edwards told Photonics Spectra
Electrical and computer engineering professor Gabriel Popescu and graduate student Amir Arbabi contributed to the results, which were supported by a National Science Foundation award and matching funds from the University of Illinois.
What’s involved with monitoring the etching process? We asked assistant professor Lynford L. Goddard of the University of Illinois at Urbana-Champaign to explain.
“For monitoring the etch, we first need to create a spatially coherent field. The phases at different positions in the plane perpendicular to the beam need to have a constant relation. We do this by coupling the light into a single-mode fiber and collimating the output. Next, we attenuate the laser to a desired intensity level with a neutral density filter. Next, we pass the laser light through a rotating piece of sanded polycarbonate so that we randomize the laser speckle pattern. The collimated light enters the back port of the microscope and is imaged onto the sample as a collimated beam. The light reflects off the sample and is imaged on an output port of the microscope. There, we place a grating with 300 grooves per millimeter so that we create copies of the reflected field. We use a lens to perform an optical Fourier transform. In the Fourier plane, we filter the copies of the reflected field. Next, we use a second lens to invert the Fourier transform and interfere the two remaining beams at the CCD camera. Finally, we perform a Hilbert transform in software to recover the phase and amplitude of the reflected field from the recorded interferogram and convert the phase signal into a height image.”