Nuclear Instruments and Methods in Physics Research B19/20 (1987) 865-871 865 North-Holland, Amsterdam MOLECULAR WEIGHT DISTRIBUTION AND SOLUBILITY CHANGES IN ION-BOMBARDED POLYSTYRENE Orazio PUGLISI and Antonio LICCIARDELLO Dipartimento di Scienze Chimiche dell'Universit~ di Catania, viale A. Doria 6, 95125 Catania, Italy Lucia CALCAGNO and Gaetano FOTI Dipartimento di Fisica, Corso Italia 57, 95129 Catania, Italy Among the chemical and physical modifications induced by ion bombardment on polymers, solubility changes have attracted considerable attention owing to the technological interest for microlithography purpose. The solubility changes are due to occurrence of cross-linking and scission at molecular level with heavy modification of the molecular weight distribution too. The molecular weight distribution of implanted polystyrene shows considerable changes upon bombardment. These changes have been followed on bombarded nearly-monodisperse PS samples. The use of samples with known molecular weight distribution allows to apply the Gel Theory for determining the chemical yields. This method is a direct, relatively simple tool for following the chemical modifications in bombarded polymers. 1. Introduction High energy ion bombardment of organic materials produces several chemical and physical modifications in the bombarded area [1]. Although these effects are strictly correlated with each other only the physical effects have been extensively investigated while little work has been done on the chemical modification. The reason for this might be that the beam-induced chem- ical reactivity is generally considered an unwanted phe- nomenon, often referred to as "radiation-damage". In addition, there is the intrinsic difficulty of working with very small sample amounts which renders problematical the identification of the final products. Beam effects on polymer systems represent a good example of the possibility of exploiting this chemical phenomenology for "synthesizing" new material with different physical properties [2,3]. Here, the ion bom- bardment induces the formation of a new polymeric material with enhanced (positive-tone) or decreased (negative-tone) solubility in a solvent. This phenomenon can be exploited for microlithography purpose in order to obtain lithographic features of sub-micron dimension for VLSI devices [4,5]. Among the chemical effects occurring in ion- bombardment polymers, those generated at low fluence (10x2-1013 ions/cm 2) are attractive because at higher fluences the overlap of the reacted regions renders the system too complicated for the chemical mechanism being identified. Polystyrene (PS) has been chosen by us as a "proto- type" polymer because of the volume of literature (about conventional effects) on this system and this renders easier the comparison between beam-effects and those induced by conventional (thermal or photo-chemical) treatments. In addition PS is nowadays available as linear mac- romolecule of known molecular weight (MW). These samples are obtained by means of "living" polymeriza- tion which yields PS samples with very narrow molecu- lax weight distribution (MWD). These polymers are nearly mono-disperse (of uniform length) and are suit- able for study of the MWD change induced by ion bombardment. Moreover, the use of polymer samples with known MWD allows one to use the Inokuti's Gel Theory [6] for obtaining the number of cross-links be- tween the macromolecular chains. When MWD is known and sufficiently narrow for approximating the size dis- tribution to the particular case of polymer chains of uniform length, the Inokuti's theory can be replaced by simpler expressions first obtained by Flory many years ago [7]. This paper reports a study of the effects induced on PS by He + ion bombardment at 100 keV energy. Par- ticular focus is on the MWD changes induced by ion beam on nearly monodisperse PS samples. The aim is that of linking the macroscopic modification induced by the beam on PS, to the modifications occurring at molecular level. 2. Experimental The MWD determination of the polymer samples before and after bombardment was determined by 0168-583X/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) VI. INSULATORS