ELSEVIER Microelectronic Engineering 46 (1999) 287-290 Process Optimisation of DUV Photoresists for Electron Beam Lithography S. Thoms and D. Macintyre Nanoelectronics Research Centre Department of Electronics and Electrical Engineering, The University of Glasgow, Glasgow G12 8QQ, UK MI~OEI.ECrRONIC ENGINEERING This paper describes work carried out to optimise two DUV photoresists for high resolution electron beam lithography using experimental design techniques. Shipley UV5 XMpositive tone resist and UVN2 negative tone resist have been studied. Screening experiments were initially carried out to identify significant factors influencing processing. For UV5 a Box-Behnken design was carried out and used to produce response surface plots. In-air delay effects were significant for UVN2. Optimised procedures were used to produce 70 nm lines and spaces in 300 nm thick layers of UV5 and discrete lines of 60 nm in 300 nm thick films of UVN2. 1. INTRODUCTION In recent years chemically amplified DUV resists have gained increasing importance in electron beam lithography largely due to their much improved sensitivity and dry etch resistance relative to PMMA [1, 2]. DUV resists have less obvious merits in that they are now widely used in the semiconductor industry with the result that they are readily sourced and meet present day expectations on health and safety. Many factors influence the electron beam performance of chemically amplified resists for instance softbake (SB) time and temperature; post exposure bake (PEB) time and temperature; developer time, temperature and concentration; and various exposure parameters such as dose, beam voltage and spotsize. Experimental design techniques help to reduce the number of runs needed to optimise processes which can be influenced by more than one factor. This paper describes the use of these techniques to optimise Shipley UV5 TM and UVN2 chemically amplified resists for high resolution electron beam lithography applications. 2. EXPERIMENTAL METHOD Statistical experimental design techniques are well established and Montgomery [3] gives a good account of the methodology available. Initially for each resist a number of factors were shortlisted for study and screening tests were carried out to determine which were the most significant. Fractional factorial designs using five or six factors were used here. Further work for each resist varied depending on the initial results. Silicon substrates were used for all the experiments and were treated with HMDS prior to the application of resist. A vacuum hot plate with temperature control to within I°C was used for all 0167-93t7/99/$ - see front matter PII: S0167-9317(99)00083-0 bake steps. The exposure system used was a Leica Microsystems EBPG5-HR100. All pattern development was carried out using Shipley CD26 developer and immediately followed the PEB. Resist patterns were inspected using a Hitachi S-900 SEM. Two types of test pattern were used to evaluate the resists. The first was an exposure dose wedge with 20 p.m wide lines which was used to make sensitivity measurements. The second pattern type comprised sets of 2 mm long lines which were suitable for cleaving. Each set of lines was written with a range of doses. The widths and spaces used with each resist were different. The measured responses used to evaluate the resists were: the process window (defined as the number of dose values for which the pattern was resolved and fully cleared); the resist profile; the LDS (defined as the slope obtained by plotting linewidth against log dose); the minimum fully resolved linewidths; and linewidths at a fixed dose. 3. EXPERIMENTS ON UV5 The initial screening test used the factors shown in Table 1. The ranges were as shown and are centred around Shipley's recommended processing conditions. Four centre runs were included to look for curvature and to provide a measure of process variance. The test pattern contained 50 nm lines with 200 nm spacings written at doses ranging from 16 to 400 ~C cm-2. The process window was measured for each substrate and the contrast of each factor on the process window was calculated. These are shown in tabular form in Table 1 while Figure 1 presents the data graphically as an analysis of means (ANOM) test [4]. In this the magnitude of the contrasts are shown together with calculated 0.01 and 0.05 © 1999 Elsevier Science B.V. All rights reserved.