E-beam lithography speeds up chip production

Half a century has passed and manufacturers still use the same technique in designing computer chips—photolithography. However, it is the very nature of light that limits the extent of chip miniaturization that photolithography can produce. If chips are to shrink even more, newer manufacturing methods need to be adopted.

Electron-beam (e-beam) lithography has long been used to make prototype chips, but has proven to be much slower than photolithography. Increasing its speed generally comes at the expense of resolution. Previously, the smallest chip featured that high-speed e-beams could resolve were 25nm across, barely better than the experimental 32nm photolithography systems that several manufacturers have demonstrated. However, in a forthcoming issue of the Microelectronic Engineering journal, researchers at MIT's Research Laboratory of Electronics (RLE) has presented a way to get the resolution of high-speed e-beam lithography down to just 9nm. Combined with other emerging technologies, the process could point the way toward making e-beam lithography practical as a mass-production technique.

The most intuitive way for manufacturers to keep shrinking chip features, which is easier said than done, is to switch to shorter wavelengths of light—known in the industry as extreme UV. "Because the wavelength is so small, the optics [are] all different," said Vitor Manfrinato, an RLE graduate student and first author on the new paper. "So the systems are much more complicated ... [and] the light source is very inefficient."

Visible-light, UV and e-beam lithography all use the same general approach. The materials that compose a chip are deposited in layers. Every time a new layer is laid down, it is covered with a material called a resist. Much like a piece of photographic paper, the resist is exposed—to either light or a beam of electrons—in a carefully prescribed pattern. The unexposed resist and the material underneath are then etched away, while the exposed resist protects the material it covers. Repeating this process gradually builds up 3D structures on the chip's surface.

The main difference between e-beam lithography and photolithography is the exposure phase. In photolithography, light shines through a patterned stencil called a mask, striking the whole surface of the chip at once. With e-beam lithography, on the other hand, a beam of electrons scans across the surface of the resist, row by row, which is a more time-consuming operation.

One way to improve the efficiency of e-beam lithography is to use multiple electron beams at once. However, MIT researchers are tackling the problem of how long a beam has to remain trained on each spot on the surface of the resist.

The fewer electrons it takes to expose a spot on the resist, the faster the e-beam can move, but lowering the electron count means lowering the energy of the beam. Low-energy electrons also tend to "scatter" more than high-energy electrons as they pass through the resist, spreading farther apart the deeper they go. To reduce scattering, e-beam systems generally use high-energy beams, but that requires resists tailored to larger doses of electrons.

The research group consisted of Manfrinato, a member of RLE's Quantum Nanostructures and Nanofabrication Group; Karl Berggren, group leader and the Emanuel E. Landsman (1958) Associate Professor of Electrical Engineering and Computer Science; and Henry Smith, professor of electrical engineering. Lin Lee Cheong and Donald Winston, graduate students of RLE and Huigao Duan, a visiting student also took part in the study. They used two techniques to improve the resolution of high-speed e-beam lithography. The first was to use a thinner resist layer, to minimize electron scattering, and the second was to use a solution containing ordinary table salt to 'develop' the resist, hardening the regions that received slightly more electrons but not those that received slightly less.

Pieter Kruit, a professor of physics at the Delft University of Technology in the Netherlands and co-founder of Mapper, a company that has built lithographic systems with 110 parallel e-beams, said that in addition to being faster, e-beam systems that deliver smaller doses of electrons are much easier to build. The larger the dose of electrons, the more energy the system consumes, and the more insulation it requires between electrodes. "That takes so much space that it's impossible to build an instrument," he noted.