ELSEVIER Microelectronic Engineering 41/42 (1998) 145-148 ~ CF~~)ELECTBONIC ENGINEERING Interferometric lithography - from periodic arrays to arbitrary patterns* S. R. J. Brueck, S. H. Zaidi, X. Chen and Z. Zhang Center for High Technology Materials, University of New Mexico, Albuquerque, NM 87106, USA Interferometric lithography is a simple technology for the production of very small features. Using I-line wavelength laser sources (364-um At-ion or 355-nm tripled YAG lasers) dense (1:1 line:space ratio) periodic structures at 0.125-1am have been demonstrated. Mix-and-match with optical lithography has been demonstrated. Imaging interferometric lithography, a true integration between optical and interferometric lithographies, poten- tially provides a route to arbitrary structures with a resolution up to 130 nm (dense patterns) at I-line and 65 nm at a 193 nm exposure wavelength. 1. INTRODUCTION The difficulties associated with extending tradi- tional optical hthography (OL) to future ULSI gen- erations are well known. Often, the resolution limi- tations of optical lithography are discussed in terms of the simplified relationship CD = ~,L/NA where ~. is the optical wavelength, NA the lens numerical aperture, and K1 a process-dependent parameter that describes some of the trade-offs between process latitude and resolution. For a 365-um I-line system with a 0.65-NA lens and an aggressive K1 of 0.5, this expression gives a resolution of ~ 280 tam. This resolution constraint is largely due to the spatial fre- quency limitationsimposed by the lens. When two coherent plane waves intersect at an angle of 20, bright and dark interference fringes are formed with a spacing (pitch) ofp = k/2sin0. Since angles of 0 ~ 90 ° are possible geometrically with plane waves, the limiting dense feature size that can be created with optical exposure (but without imag- ing lenses) is ~X/4. There is no fundamental limit on the CD that can be achieved, only on the pitch. Iso- lated structures as small as 50 nm have been demon- strated at I-line, limited not by the optics but only by the photoresist mechanical properties. 2. INTERFEROMETRIC LITHOGRAPHY Interferometric lithography1-3 (IL) employs these interference effects to expose periodic (line/space or via-hole array) patterns with resolution approaching this ultimate limit. For an exposure with two coher- ent laser beams incident on a wafer at angles ±0 with respect to the wafer normal, the normalized aerial exposure dose is D(x) = 1 + cos(2kxsin0) where k = 2~/~.. Sin0 ~ 1 is easy to achieve so the limiting critical dimension for a dense (1:1 line:space) struc- ture is CDr~ = k/4 ~ 90 nm at l-line (and 50 tam at a 193-nm exposure wavelength). The modulation transfer function [MTF = (Dm~,-Dtm)/(Dm.,,+DmO] for this exposure is unity, independent of the spatial frequency. Importantly, there is no z-dependence in the intensity pattern; the depth-of-field is effectively inf'mite, macroscopically limited only by laser coher- ence lengths and beam overlaps. Figure 1 shows a line/space pattern of dense 125-nm lines exposed using an At-ion laser source at 364 nm. Using addi- tional beams in a single exposure 4 and overlaying multiple exposures 5 allows these results to be ex- tended to 2D patterns such as via hole arrays and to other more complex but still repetitive patterns. 3. MIX-AND-MATCH ALIGNMENT These results can be further extended by multiple exposures in a mix-and-match scenario with optical lithography tools. The basic idea is to use interfer- ometric lithography to provide the high spatial fre- quency information and optical lithography to pro- vide overall non-repetitive features. Further devel- opment of an alignment capability will allow the extension of interferometric lithography to multi- level test structures, for example interconnect levels. *Partial support for tins work provided by DARPA, SEMATECH and Sandia National Laboratories 0167-9317/98/$19.00© Elsevier ScienceB.M All rights reserved. PII: S0167-93 ]7(98)00032-X