ABSTRACT
J. ZEKONYTE, V. ZAPOROJTCHENKO, S. WILLE, U. SCHUERMANN and F. FAUPEL* Technical Faculty, Chair for Multicomponent Materials, Kiel University, Kaiserstrasse 2, 24143 Kiel, Germany
1. INTRODUCTION
The metallization of polymers with noble metals has been of interest in recent years, not only from a fundamental point of view, but also for different applica-
tions [1,2]. Due to high conductivity and low electromigration, copper, for example, is the best choice for microelectronic applications. However, due to the chemical inertness of noble metals, surface treatment of polymers is necessary before metal deposition in order to improve the metal/polymer adhesion. It is evident that the early stages of metal/polymer interface formation during metallization play a crucial role in this process. This paper provides new results concerning metal/polymer interface formation by evaporation of a noble metal on polymers after surface treatment with low-energy ions (1 keV). The early stages of metallization are far from thermodynamic equilibrium conditions, since isolated metal atoms impinge on the polymer surface. Therefore, various competing processes (adsorption, surface diffusion and nucleation after encountering of metal adatoms, metal atom re-emission into vacuum, etc.) have to be taken into account to understand how the metal/polymer interface is formed. Metals and polymers are very dissimilar materials. The cohesive energy of metals is typically two orders of magnitude higher than that of polymers. Furthermore, the interaction between moderately reactive metals and polymers is generally very weak in comparison to the strong metal-metal binding forces. As a consequence, these metals exhibit a strong tendency to aggregate. They do not wet polymer surfaces but form 3D clusters during vacuum deposition which finally coalesce and form a continuous film at high metal coverage. We have demonstrated [3, 4] that the tendency of metals of low reactivity not to wet polymer surfaces is accompanied by a very low sticking or condensation coefficient of metal atoms, which varies by several orders of magnitude depending on the metal-polymer interaction. There are two extreme cases for metal nucleation and film growth on a polymer surface: the socalled random nucleation and preferred nucleation. In random nucleation a metal cluster follows from a nucleus which is formed by random encounters among metal atoms. In preferred nucleation metal atoms are trapped at preferred sites which can be created on a polymer surface, for example, by ion irradiation. Some details on nucleation and growth of noble metals on non-treated polymer surfaces, as well as some new techniques for investigation of these phenomena, have been presented elsewhere [3-5]. The strong metal/polymer adhesion in the case of more reactive metals such as Cr, Al or Ni, as a rule, is accompanied by higher condensation coefficients and nucleation densities compared to noble metals. These tendencies should be taken into account in the study of the influence of ion irradiation on adhesion properties of noble metals on polymer surfaces. Recently [6, 7], we have shown that ion irradiation leads to enhancement of adsorption probability of metals on polymers. In this paper, a number of new details are discussed, in relation to the earlier studies. Chemical processes occurring in the outermost layers ( 0 - 5 nm) of the polymer surface during ion irradiation with a low ion dose between 1012 and 1016 ions/cm2 are taken into account.
