New lithography makes better chips

New high-resolution lithographic techniques developed by researchers at the University of Wisconsin-Madison could allow chipmakers to write semiconductor features with dimensions as small as 20 nanometers.

New high-resolution lithographic techniques developed by researchers at the University of Wisconsin-Madison’s College of Engineering’s Centre for NanoTechnology and Synchrotron Radiation Centre could allow chipmakers to write semiconductor features with dimensions as small as 20 nanometers.

Smaller features would lead to more transistors on a chip, which in turn would create faster microprocessors.

Chemistry Professor James Taylor, researcher Lei Yang and Electrical and Computer Engineering Professor Franco Cerrina developed the new techniques, dubbed bright peak, enhanced X-ray phase-shifting mask (BPEXPM).

‘With this bright peak technology, you could write a 100 nanometer mask feature and wind up with a 20 nanometer wafer feature,’ said James Taylor, John Bascom Professor of Chemistry, Emeritus. ‘That is about 2010 on the semiconductor industry roadmap. It’s really exciting. The masks are not that difficult to create. We have undergraduates that can create a 250 nm mask without too much trouble. Using bright peak masks they would get 50 nanometer wafer features.’

Features are imaged onto silicon wafers via lithography. To make a modern microprocessor, chipmakers use lenses to focus ultraviolet-light wavelengths of 248 or 193 nanometers on a mask containing one level of the circuit pattern. The mask is projected over a silicon wafer covered with photoresist, a light sensitive material. Light shining through the mask produces pattern features as small as 100 to 110 nanometers in the photoresist. The unexposed portion is washed from the chip.

But to push microchip fabrication below 100 nanometers, researchers need new technical solutions. For example, working with shorter wavelengths allows for smaller chip features, but quartz lenses intended to focus and reduce the circuit pattern onto the silicon absorb much of the light. So the next generation of lithographic techniques must work with special lens materials or without lenses.

Centre for NanoTechnology scientists have developed new techniques to pattern on the scale of 50 nm and smaller using X-rays. It was in exploring the limits of X-ray lithography that researchers developed the bright peak technology.

‘There are several factors that play into the fabrication of smaller features, one of which is diffraction,’ said McFarland-Bascom Professor and NanoTechnology Centre Director Franco Cerrina. ‘We can control diffraction using phase-shifting techniques and turn it to our advantage. This works well in optical lithography, and even better in X-rays.’

BPEXPM technology is said to go beyond one-dimensional resolution and leapfrogs development on the semiconductor industry roadmap by positioning clear, adjacent phase-shifting features to take advantage of constructive interference that occurs at the edges. The interference is bent toward the centre of the structure where it combines to form a bright peak. The sharpness of the peak determines the final site of the image. The result is a wafer feature five to six times smaller than the opening written in the mask.

‘The maximum intensity and the sharpness of the peak depends on the thickness of the mask material or phase angle, the wavelength of the light used, and the width of the opening,’ said Taylor. ‘We are doing research now to determine how to best make the mask, which phase shifter materials to use, what resist materials produce the best resolution and sensitivity, and the long-term stability of the membrane and the phase shifter.’