Direct measurement of the effect of substrate photoelectrons in x-ray nanolithography

We measure the dissolution rate of poly(methylmethacrylate) (PMMA) as a function of height above substrates of silicon and thin films suitable as bases for gold electroplating: a "thick-gold" film (10 nm Ti/10 nm Au), and a "thin-gold" film (10 nm Ti/1.8 nm Au). For the thick-gold film, a dramatic increase is seen in the PMMA dissolution rate starting approximately 50 nm above the substrate. This is attributed to increased x-ray absorption and photoelectron generation in the thick gold. An increased dissolution rate is not seen for the other two substrates. Our measurements are compared with simulations and with our experience in replicating sub-50-nm device structures. We point out two consequences of this increased exposure due to substrate-generated electrons. The first is an increased exposure rate near the resist/substrate interface which can lead to development at the interface in the dark areas. The second is an increase in exposure near a resist/substrate interface at the boundary between shadowed and unshadowed regions, leading to an undercut in the resist profile. (C) 1997 American Vacuum Society.