736 Nuclear Instruments and Methods in Physics Research B39 (1989) 736-738 North-Holland, Amsterdam zyxwvutsr ION RESIST PROPERTIES OF THIN FILMS OF TRANSITION METAL OXIDES N. KOSHIDA, Y. ICHINOSE and K. OHTAKA Department of Electronic Engineering, Faculty zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA of Technology, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184, Japan Properties of thin amorphous films of WO,, MOO,, V,O, and a mixed WO, + MOO, compound as a high contrast, negative inorganic ion resist have been studied using - 10 keV Li+ and Na+ beams. With the latter two films, a significant improvement in the resist sensitivity is observed. The details of the exposure response curves of the four materials can be explained in terms of their original physical properties. The potential usefulness of this resist for both microfabrication and metallization in the submicron region is also presented. 1. Introduction To obtain a high-resolution ion resist, we have studied ion beam modification of thin amorphous films of WO, and MOO,. As previously reported [1,2], these films which were irradiated by a beam of sodium ions showed resist properties of extremely high contrast (y - 6) at ion doses of a threshold value. An insulator-metal transition also occurs at this threshold dose [3]. These effects can be interpreted as a result of a change in the chemical potential; this is due to ion implantation dop- ing accompanied by some structural or compositional change at the film surface [4]. In this paper, further studies for an improvement in the resist sensitivity are reported, with some new experimental results obtained from thin films of another transition metal oxide V,O,. 2. Experimental Thin films of WO,, V,O, and MOO, were formed by vacuum deposition onto In,O, coated glass substrates. The former two films were deposited by flash evapora- tion from a Be0 crucible, and the last one by resistance-heated evaporation from a tungsten boat. Thin films of WO, + MOO, (1 : 1 in mole ratio), formed by flash evaporation, were also studied to improve the resist sensitivity. In all cases, the substrate temperature was adjusted to the values in the region where the evaporated films were in the amorphous state. The film thickness was 100-200 nm. The films were irradiated by a beam of thermionic Li+ or Na+ at an accelerating voltage of about 10 kV. Subsequent development by chemical etching in al- kaline solutions was carried out in the same way as that described previously. Only for V,O,, development was done in an acidic solution also. ESCA spectra were also measured to characterize the effects of ion irradiation. 0168-583X/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) 3. Results and discussion Every Li+-irradiated film exhibits a negative resist property of high contrast at a threshold dose as in the previous Na + irradiation case. Typical exposure re- sponse curves of WO, and WO, + MOO, films are shown in figs. 1 and 2 as a function of the accelerating voltage I/. The contrast value determined from these curves is in between 6 and 7. It should be noted that the threshold dose Dthr defined as the ion dose which results in a 50% remaining thickness, shows a distinct voltage dependence: as V is increased D,, of WO, resist increases, whereas that of WO, + MOO, resist decreases with increasing V. De, behavior similar to figs. 1 and 2 was also observed with MOO, and V,O, films, respec- tively. Fig. 3 compares the Dth-k’ curves of four films. Apparently the resist sensitivity of WO, + MOO, and V,O, films can be significantly improved by an increase in T/, in contrast to that of the other two films. As previously reported [4], the mechanism of these resists is based on ion implantation doping and/or ion zyxwvut 100 10000 IRRADIATION DOSE ( pClcm* ) Fig. 1. Exposure response curves of WO, resist as a function of the accelerating voltage of Lit.