Ion Beam Exposure of Polymer Resist PMMA

Abstract

Ion beam exposure characteristics of a positive polymer resist, polymethylmetacrylate (PMMA) have been investigated. The samples were irradiated with B § ions in the energy range of 20-160 keV, the ion dose varying between 8 x 10" and 5 • 10 '3 cm -2. In the energy range considered, beam sensitivity and resist contrast were found to be between 4-5 • 10 '2 cm --~ and 2.3-2.5, respectively. An empirical formula is proposed to approximate the characteristic dissolutiOn curves (thickness removed vs. dissolution time). The empirical values agree with the experimental results within an accuracy better than 12%. A procedure is described whereby the total number of scission chain fragments as a function of resist depth may be extracted from the experimental curves.Ion beam lithography is a novel technique which bypasses some of the drawbacl}s of electron beam lithography. In general, resists are more sensitive to ions than to electrons. This is due to a greater energy deposition per unit volume. Another advantage in ion beam lithography is its feeble lateral spread (100k) for heavy ions. Ion beams, as compared to electron beams, produce low energy electrons and negligible amount of high energy backscattered electrons. In the case of direct writing using an ion beam, pattern dimensions are limited only by the beam size. Indeed, pattern dimensions inferior to 1000~ (1, 2) have been obtained with a liquid-metal gallium ion source.Ion lithography can be carried out in two different ways: through direct writing and through the ion projection exposure technique as developed by Sacher Technik Wien (3). A liquid metal ion source (4-8) can be used for direct writing, as in electron beam lithograpy, but with reduced computing time owing to the absence of proximity effects and enhanced ion sensitivity. Another method of transferring patterns to resists is through direct contact masking of the resist; channeling (9) may also be used; typical mask to resist distances are 20 ~m.Komuro et al. (10) have investigated the ion beam sensitivity of positive PMMA and negative PDMS (polydimethylsiloxane) using He + ions (60-and 200 keV) and Ar ~ ions (150 and 250 keV). According to their measurements, the calculated Gel and Gn values are 1.7 and 0.9, respectively, for PMMA whose average molecular weight is 1.85 x 105. Gel and Gn are the radiation yields per 100 eV absorbed for main chain scission brought about by electronic and nuclear collisions, respectively. Hall et al. (11) have studied the behavior of positive and negative resists using high energy ions: hydrogen, helium, and oxygen at 1.5 MeV. They proposed a model to account for the variation of the energy distribution around the particle track in terms of the number of sites required for exposure. Ryssel et al. (12) presented the dissolution characteristics of both positive and negative resists exposed to argon, gallium, hydrogen, and helium ions.Karapiperis et al. (13) have used Monte Carlo computer simulations to calculate the electronic and nuclear energy losses of 60 keV H + ions i...