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 Si3N4 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.

Nanolithography with a high-resolution STEM

“…As a check of the STEM operation, a calibration run was made using 17-nm-thick PMMA. The exposure distribution for this PMMA produced tr = 15 nm, in nominal agreement with previous PMMA results [7]. This latter result indicates not only that the PC XT-driven STEM is producing reliable exposure distribution data, but also that t-BOC has approximately the same resolution as PMMA, even though its sensitivity can be six times higher.…”

“…The dose to barely expose a large area of resist was approximately 5X10" C/cm . Under similar exposure conditions (50 keV electrons and thin substrates), positive-mode PMMA has been found to require doses roughly six times higher for development [7].…”

“…The use of these materials as resists for device fabrication, however, has been limited by their high electron dose requirements and the difficulty encountered in transferring patterns in the resists cleanly to an underlying sutetrate. A compromise between resolution and sensitivity is achieved with resists such as PMMA processed in the positive mode, in which liftoff structures about 12 nm wide are possible [7,8]. Structures with linewidths of roughly 23 nm made using conventionally processed negative resists have appeared in the literature [9,10].…”

“…The exposure of the pattern was repeated at a variety of doses. Analysis of the dose required to expose Unes of diiferent widths permits the determination of the exposure distribution [7]. This exposure distribution can be used to evaluate resolution in the same way that the Airy distribution is used to determine the resolution of a diflfraction-limited optical microscope.…”

Morphological alteration of microwave disinfected acrylic resins used for dental prostheses

“…Moreover, polymer exposure to incident electrons generates electrostatic charging and dimensional variation of the PMMA substrate, due to the changes in molecular weight distribution. An electron beam can induce main-chain scissions in PMMA [24,25], lowering the average molecular weight. Almost all the scanning parameters were modified for every specimen, especially because of the ever-present electrostatic charging of the surface, requiring frequent contrast adjustment for every magnification order or newly investigated region.…”