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P4.12.1

Hardness depth profiling of ion-implanted polymer thin films Gunnar Suchaneck1, Bodo Wolf2, Margarita Guenther1 and Gerald Gerlach1 1 Dresden University of Technology, Institute for Solid State Electronics, 2 Dresden University of Technology, Institute for Crystallography and Solid State Physics D-01062 Dresden, Germany. ABSTRACT Hardness measurements in ion implanted polymers are complicated by the fact that the hardness of the material varies as a function of depth within the modified layer. This effect is induced by the distribution of deposited energy, which produces a depth-dependent variation in microstructure. We have used the depth-sensing nano-indentation technique to investigate the mechanical properties of thin films of ion-beam modified aromatic polymers deposited onto silicon substrates. The depth of the ion-modified surface layer was determined using the load variation technique from the hardness and elastic module depth profile and the depth dependence of the power law coefficient of the unloading curve.

INTRODUCTION

Films of aromatic polymers are extensively used in electronic applications. For instance, bimorphic gas sensors make use of gas absorption-induced swelling of polyimide and polyethersulfone thin films [1,2]. The polymer’s sensitivity to gas uptake is significantly increased by surface ion-beam modification [3,4]. On the other hand, the gas selectivity of the sensor is improved by the formation of micro-voids with a narrow size distribution. Ion beam radiation-induced cross-linking results in an increase of the hardness and elastic modulus of the modified layers. The elastic modulus of the polymer is directly proportional to the cross-link density (or inversely proportional to the average molecular weight between cross-links) [5]. However, the thickness of the modified by ion implantation surface layer is still an object of controversy [6,7]. While damage-depth profiling by means of a variable-energy positron beam results in a modification-depth of 700 to 800 nm (1015 B+cm-2 at 180 keV) what is in good agreement with SRIM (stopping and range of ions in matter) estimations [6], values determined by ellipsometry for the same irradiation conditions, (200 to 400 nm for the multilayer Cauchy model [6] and 400 to 450 nm for refractive index profiling [7]) are much lower. The latter value is in good agreement with hardness depth profiling [7]. Depth-sensing nano-indentation is a powerful technique for determining the hardness, i.e. the resistance to penetration, and the elastic modulus. In the case of ion-implanted polymer thin films, the linear energy transfer (LET) mechanisms can be correlated with surface hardness for ion implanted polymers [8]. Impinging ions lose their energy to target atoms mainly through inelastic collisions causing ionization and subsequent nuclear recoils causing atomic displacement. Considering both LET and nano-indentation hardness values as function of the irradiation depth of polystyrene irradiated with 2 MeV He+ ions up to a dose of 3.3⋅1015cm-2, it was f