To craft some of the most complex semiconductors, manufacturers etch pre-defined patterns into wafers, carving out structures layer by layer. The processes can be time-consuming and are executed blindly, leaving few opportunities to monitor the etching or make any necessary adjustments.
Now, researchers at the University of Illinois at Urbana-Champaign have developed a technique to watch and control the etching of semiconductors as it is happening, with a height resolution at the scale of nanometers--billionths of a meter.
The researchers describe the technique in the Sept. 28, 2012, issue of Light: Science & Applications, an open-access, peer-reviewed publication from the Nature Publishing Group.
In that paper, the researchers explain how they combined real-time observations from epi-illumination diffraction phase microscopy (epi-DPM) with photochemical etching techniques to manufacture gallium arsenide micro-lenses as a proof-of-concept for the techniques.
"The instrumentation we are developing will allow engineers to more thoroughly understand the dynamics of their fabrication processes and make fine adjustments to the processing conditions in real-time," says principal investigator and University of Illinois at Urbana-Champaign engineering professor Lynford Goddard, a National Science Foundation CAREER grantee.
The technique--which incorporates an optical microscope, a projector, a Nd:YAG frequency doubled green laser, a digital camera and a series of mirrors, lenses and filters--allowed the engineers to observe interference patterns as light bounced off of a semiconductor sample, revealing surface details as small as 2.8 nanometers in height.
As the camera captures interference images, software converts them to topographic height maps in real time. Each image is stable, shifting as little as 0.6 nanometers in height, per pixel, from frame to frame. The new process allows researchers to not only watch an etching underway, but to instantly make adjustments using a digital projector.
A National Science Foundation Major Research Instrumentation grant supported the research with matching funds from the University of Illinois.
"This optical non-invasive, non-destructive technique can monitor the dynamics of semiconductor fabrication processes in real time with nanoscale resolution," adds Leon Esterowitz, the NSF program officer who oversaw Goddard's instrumentation grant. "This 3-D technique should significantly reduce processing time, improve control of device properties, and reduce fabrication costs for a wide variety of semiconductor devices."
Currently, semiconductor manufacturers lose time and material calibrating their equipment and fabrication processes on dummy wafers before etching a retail product, and then have to do a post-etching check of the chips to ensure that the calibration during production was consistent.
The new ability to watch and control the etching in real time eliminates both the pre- and post-inspection steps. Additionally, because the technique uses optical microscopy, the semiconductor is not damaged by the illuminating source, as it would be with other electron microscope methods and techniques such as scanning electron microscopy or focused ion beaminspection.
"The exceptional stability and accuracy of our method will help to address some grand challenges in the semiconductor manufacturing industry," adds Goddard. "Besides enabling adaptive process control, we have begun to adapt the method to find isolated defects in patterned semiconductor wafers. Finding those device-killing defects can improve the overall yield during processing and reduce the cost of consumer electronics."