Inspecting the Ultra Small


The Numerical Aperture Increasing Lens (NAIL) technique developed by M. Selim Ünlü and colleagues has demonstrated a nearly five-fold improvement over current optical technology. Figure A shows a frontside image in which the semiconductor circuit is partially obscured by its own dense, multilayer metallic structure. Backside imaging through the silicon substrate with a conventional near-infrared microscope in Figure B reveals the obscured features, but results in degraded resolution. The same structures imaged with the patented NAIL technique, Figure C, show significant improvement in both resolution and visibility.
Image courtesy of M. Selim Ünlü

Is there a limit to how much data you can fit in your PalmPilot? The answer may lie at the nanoscale. Over the past decade, as PCs, iPods, and other consumer electronic devices have packed more and more information into faster, higher-density chips, the smallest feature size used in semiconductor circuit fabrication has shrunk by a factor of three. While manufacturers aim to deliver chips free of processing faults and semiconductor defects, today’s nanoscale feature sizes are making semiconductors impossible to inspect with conventional optical imaging methods.

Since 2000, however, M. Selim Ünlü, CNN associate director and professor of electrical and computer engineering, has been developing and refining a spherical microlens which enables the inspection of semiconductor circuits at a much higher resolution using existing microscopes. Funded by the Defense Advanced Research Projects Agency, the U.S. Air Force, and the NSF, this research effort has consisted of several projects in the Optical Characterization and Nanophotonics Laboratory in collaboration with CNN Director Bennett Goldberg and Professor of Electrical and Computer Engineering Anna Swan. The group’s findings could enable the next generation of microprocessors.

Semiconductor circuits are fabricated in dense, multilayer metallic/dielectric structures using very short, ultraviolet (UV) wavelengths of light to “write” under-100-nanometer-wide features onto the surfaces of each layer. To inspect and analyze a circuit, engineers either collect light from it or excite it with light and monitor the electrical response, focusing the light to a point comparable to the smallest feature size. Since top inspection is prevented by the dense metallic multilayer structure, backside imaging through the silicon substrate is necessary. But silicon doesn’t transmit visible or UV light to buried features, so engineers must rely on longer, infrared wavelengths that exceed one micron (1,000 nanometers)—making the focal point far greater than the feature sizes.

“I dare you to touch the Pentium chip in your computer!”

“Feature resolution scales with wavelength, so whatever you’ve written with UV photolithography, you cannot ‘read’ it anymore at wavelengths allowed by the silicon substrate,” Ünlü explains. “But we’ve demonstrated in the past five years that you can regain some of this resolution using our microscopy technique on the backside of the circuit.”

In Ünlü’s most recent paper, his group demonstrated a resolution of 200 nanometers with their novel microlens in wide-field imaging, a nearly five-fold improvement on current optical technology. The group has licensed the new technology to Hamamatsu, a microscopy firm that builds instruments for semiconductor analysis.

Ünlü next plans to use his advanced inspection technique to perform high-resolution thermal imaging of semiconductor chips. “One of the bottlenecks of semiconductor chips is that as you shrink a device, you increase the clock speed and generate more heat in its operation,” says Ünlü. “I dare you to touch the Pentium chip in your computer!” By obtaining a high-resolution heat distribution map of a chip, Ünlü hopes to help engineers to better identify and reduce semiconductor hotspots.