Phat Photons and Nifty Nanoscience

S. R. J. Brueck Center for High Technology Materials, University of New Mexico, Albuquerque, NM

Progress in optical lithography has paced the enormous progress in integrated circuit technology. The ultimate possible limits of optical lithography are explored. The spatial frequency transmission bandwidth of free-space is 2/λ, leading to a dense (equal line/space) pattern at a critical dimension of λ/4 (or 50 nm for a λ of 200 nm) as is shown in Figure 1. Immersion provides another factor of ~ 1.5 down to a pitch of 33 nm at λ ~ 200 nm. Nonlinear processes, based on the chemistry of photoresist processing and pattern transfer, can further extend optics beyond the linear systems limits of single exposures. The conclusion is that there is no fundamental limit to the resolution of optical lithography; there are only process latitude and manufacturing (e.g. cost) issues.

Nanotechnology is of great current interest. For many applications, large numbers of nanostructures with a well-defined long-range order are required. One such example is the use of nanostructuring for semiconductor materials development. Both nanoheteroepitaxy (NHE) for the growth of highly lattice mismatched systems (e.g. GaN on Si); and selective MBE growth of InAs quantum dots on patterned GaAs substrates will be discussed. An example of NHE for GaN on Si is shown in Figure 2. Photonic bandgap materials with periodic arrays of nanoscale struc-tures (with or without aperiodic defects) are another exciting example of the physics accessible with current interferometric lithography capabilities.

Fig. 1: 108-nm pitch photoresist grating written at l =213 nm and NA = 0.986

Fig. 2: GaN grown on ~ 70 nm diameter Si seed.