Resolution Limits of PMMA Resist for Exposure with 50 kV Electrons

Abstract

An electron beam with a diameter below 1 nm has been used to measure the resolution of PMMA. Test patterns are written in 30 nm thick PMMA layers coated onto 60 nm thick membrane substrates. Both lateral scattering of electrons in the resist and backscattering from the substrate are negligible for these samples. Preliminary results obtained with 50 kV electrons are presented in this paper. The test patterns contain nominal linewidths down to 4.4 nm. The exposure distribution is determined by measuring the exposure dose needed to open up lines with widths smaller than the width of the distribution. Samples are examined in the scanning transmission microscope mode using the same beam used to write the patterns. The width [standard deviation (σ) assuming a normal distribution] of the exposure distribution has been measured to be between 11 and 14 nm. This spread function makes it possible to resolve lines on 45 nm centers in these very thin resist layers. Spreading is assumed to be set by straggling of low energy secondary electrons into the resist although effects that may be attributable to the finite molecular size of the PMMA have also been observed. It is pointed out that thin film substrates are not essential to obtain linewidths of 50 nm. Resolution and contrast on thick substrates are discussed and compared with the resolution and contrast obtainable with light optical systems.