New microscope for the next generation of microchips

Moore’s Law, hardly a law but undeniably a persistent trend, says that every year and a half, the number of transistors that fit on a chip roughly doubles. It’s why electronics – from smart phones to flat screens, from MP4 players to movie cameras, from tablets to supercomputers – grow ever more varied, powerful, and compact, but also ever less expensive. Whether the trend can continue until it runs up against immutable laws of nature, like the finite size of an atom, depends on how far scientists and technicians can push electronic technologies down into the nanoworld with better tools for using short-wavelength light.

 

Now scientists at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have partnered with colleagues at leading semiconductor manufacturers to create the world’s most advanced extreme-ultraviolet (EUV) microscope. Called SHARP (a succinct acronym for a long name, the Semiconductor High-NA Actinic Reticle Review Project), the new microscope will be dedicated to photolithography, the central process in the creation of computer chips.

 

 “EUV light is tricky to work with,” says Kenneth Goldberg of the Center for X-Ray Optics, “because every material absorbs it so strongly. So instead of glass lenses, EUV optical systems rely mainly on specialized mirrors with atomic-scale smoothness, topped by multilayercoatings for high reflectivity.” To maintain efficiency, the entire optical system has to be placed in a high-vacuum environment.

 

While the existing eight-year-old microscope at beamline 11.3.2, dubbed the AIT (for Actinic Inspection Tool), has unique imaging capabilities, the fast-moving nature of semiconductor technology means its future is limited. SHARP will exceed its performance in every metric: resolution, speed, uniformity of illumination, and coherence control. SHARP will enable forward-looking research years before commercial tools become available.

 

Within a few years, semiconductor devices will be measured in dimensions of 16, 11, or 8 nanometers, mere billionths of a meter. To mass-produce them, industry is pushing a photolithography process that uses EUV light with a wavelength of just 13.5 nanometers, 40 times smaller than visible light.

 

“At this short wavelength, we can print and image circuit patterns at nanometer length scales,” says Goldberg. “The new microscope will leverage years of cutting-edge EUV and soft-x-ray microscopy experience, experimental systems-engineering at CXRO, and EUV optics expertise developed as part of the lithography research programs here.” Goldberg says that the ALS, as one of the world’s brightest sources of EUV light, “is a great place to develop EUV lithography technologies.”

 

A special feature of the new microscope will be illumination coherence control. The ALS produces an EUV beam with laser-like coherence, ideal for many experiments. For microscopy, however, the image resolution can be improved by a factor of two by carefully re-engineering the illumination into a state called partial coherence. Microscopists have recognized the importance of partial coherence for years, and the synchrotron community is now catching up.

 

An angle-scanning mirror in the new microscope’s beamline illuminator will take the highly-coherent ALS light and steer it into patterns, like a mini-laser-light show, breaking and re-shaping the coherence properties. In this way, the SHARP microscope will replicate the properties of current and future tools for lithography production and research, giving researchers the most advanced look at what’s to come.