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Masks Team Up to Beat Light's Limits

Photonics Spectra
Feb 2000
Aaron J. Hand

The wavelength of exposure light is an inherent limitation in optical lithography. Semiconductor manufacturers are making use of shorter-wavelength lasers, larger numerical apertures and resolution-enhancement techniques to print today's tiny circuits, but these methods are still ultimately limited by the light's wavelength.

Scientists at Lucent Technologies' Bell Laboratories have developed a photolithography technique that overcomes that limitation. The two-step etch process -- which uses standard lithographic tools -- is bound only by processing capabilities. Using a stepper with good alignment capabilities, the process could produce lines about 1/10 the wavelength of the light source, said Stanley Pau, a technical member of Bell Labs' Optical Lithography and Imaging Materials Research group.

The technique uses multiple exposures with different masks to print a single layer, the final pattern appearing only where the larger mask patterns overlap. Depending on the complexity of the final pattern design, manufacturers may use only two masks or several masks.

In experiments detailed in the January issue of the Journal of Vacuum Science and Technology B, Pau and his fellow researchers used four binary reticles made by DuPont Photomasks Inc. of Round Rock, Texas, with an Alta 3000 laser tool from Etec Systems Inc. of Hayward, Calif. The smallest feature on the masks was 1 µm. They transferred the pattern onto 6-in. wafers with an XLS 7800 lithographic tool from Ultratech Stepper Inc. of San Jose, Calif., which uses an exposure wavelength of 248 nm, magnification of 4× and a numerical aperture of 0.53. With this setup, the engineers were able to produce lines as narrow as 25 nm and approximately 400 nm deep.

Patterns printed with multiple-exposure lithography are defined by overlapping two or more mask patterns.

Although multiple-exposure lithography makes use of existing technologies and lower-cost masks, it does increase complexity. Because the amount of overlap determines the size of the features, alignment is extremely important. Also, the final printed pattern is very different from the patterns that appear on the individual masks. If the final pattern is simple or repetitive, you may be able to figure it out by hand, Pau said. "But if you have 20 million transistors, you need a new type of CAD [computer-aided design] tool."

If this technique catches on, the design software developers will undoubtedly provide the necessary tools, Pau said. But the method is best suited to such devices as low-density logic circuits, where smaller transistor gates are more important than dense circuitry. It is also well-suited to research, he noted, because scientists can use old systems to print smaller critical dimensions than ever before.

The researchers have filed a patent and are developing devices using the multiple-exposure technique.

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