Maskless momentum

As semiconductor fabrication technology progresses, one of the growing problems is lithography. Semiconductors are created using optical techniques to expose photoresists. However, the wavelength of the light used to expose the photoresists is now much larger than the size of the features being created. If this problem isn’t solved, then progress to ever smaller nodes will be stalled. Needless to say, work is underway to solve the problem and – in some cases – this work has been going on for some time. The industry is keen to use current technology for as long as possible. Today, most lithography systems are based on 193nm laser light. But the time is rapidly approaching when the systems will no longer be of use – even with such tricks as immersion, which takes advantage of higher indices of refraction. Two schools of thought dominate the research. One is to use extreme ultraviolet (euv) light. Instead of the current 193nm wavelength, euv uses a light source with a wavelength of 13.5nm. At first sight, this would appear to solve the problem. But it hasn’t been as easy to develop the process as was originally believed. Now, companies like Intel and IBM, who had been looking to use euv in the near future, are putting developments on hold. But optical lithography also needs masks and, as feature size decreases, mask costs increase dramatically. The other approach being pursued is to do away with masks altogether. Instead of exposing photoresists to light, maskless lithography uses an electron beam to ‘write’ the circuitry directly onto a wafer coated with an appropriate resist. It’s a development of a process currently used to create low volume asics under the guise of direct writing. Belgian research institution IMEC has been working on euv for some time but, interestingly, is now looking at maskless lithography as one of its future options. Kurt Ronse, director of IMEC’s lithography department, noted the techniques are very different. “With euv, everything has to happen in a vacuum. It’s broadly similar to existing lithography, but it has to use mirrors instead of lenses because there are no lenses which are transparent to euv.”