NIST researchers in the US have uncovered a potentially serious optical problem affecting designs for future generations of semiconductor manufacturing equipment using ‘deep ultraviolet’ light.
The ‘Moore’s Law’ phenomenon – the doubling of chip complexity with each generation – has been possible largely because of continual advances in lithography, allowing manufacturers to image and process integrated circuits with smaller and smaller dimensions.
Reaching very small dimensions requires using very short-wavelength light. Current state-of-the-art production processes use deep ultraviolet lasers at 248 and 193 nanometres to image circuits with critical features as small as 130 to 150 nanometres. The next major steps for lithography are expected to be systems using 157-nanometer light, ultimately achieving feature sizes in the range of 70 nanometres.
Some of the optics for 193-nanometer and all of the optics for 157-nanometer lithography are made of calcium fluoride, one of only a few materials that are transparent at 157 nanometres. NIST physicist John Burnett recently showed that calcium fluoride is inherently birefringent in the deep ultraviolet – meaning that the crystal refracts light differently depending on the polarization of the light. NIST physicists Eric Shirley and Zachary Levine confirmed these results theoretically.
The practical import for designers is that a calcium fluoride lens will not focus properly without, at the very least, careful control of the light as it enters the lens and a design that accounts for this intrinsic birefringence. This is a difficult problem given that there are about 20 such lens elements in a typical ‘stepper’ or optical lithography lens. The modelling software used to design such systems is being modified to account for the effects of birefringenc, but it is not expected to be ready until October.
NIST researchers are pursuing potential solutions to the problem involving mixed crystals to compensate for the birefringence effect. Details of Burnett’s findings were first made public at the International SEMATECH-sponsored International Symposium on 157-nanometer Lithography.