Laser bonding characteristics of sputtered titanium on glass with polymeric films

For miniaturized biomedical devices, laser joining of dissimilar materials offer excellent potential to make precise joints. An important system for consideration is Ti coated glass joined with biocompatible polymers. It is known that thin film characteristics (such as Ti thin film on glass) are strongly influenced by surface morphology and film thickness. In this investigation we studied sputtered Ti coated glass and examine the bonding characteristics of two polymers, they are polyimide and Teflon fluorinated ethylene-propylene (FEP) Polymer. Comparison of sputtered method to that of cathodic arc physical vapor deposition (CA-PVD) are made and found that sputtered deposition technique provides better film quality and better bond strength over CA-PVD technique. We have clearly observed formation of macro-droplets for CA-PVD for the various film thicknesses of titanium (50∼400nm), which is undesirable for good bonding. It has been observed that higher thickness and roughness of film significantly improves bond quality. For stronger bond with higher thickness mixed mode failure are observed- cohesive failure of polymer, interface failure of Ti/glass and failure on the glass itself. On the other hand for weak bond with thinner film mostly interface failure observed for this system of Ti coated glass/imidex. For thicker film chemical bond of Ti-C and Ti-O are observed. Also presence of imidex on Ti surface is confirmed by the N spectrum of XPS. We investigated the role of both surface characteristics and chemical bonding for laser joints using advanced techniques such as Optical Microscopy, Atomic Force Microscopy (AFM), X-ray Photoelectron Spectroscopy (XPS), Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS) and Auger Electron Spectroscopy (AES), Positron Annihilation Lifetime Spectroscopy (PALS). A comprehensive three dimensional (3D) transient model for thermal analysis using the finite element (FE) code ABAQUS has been used. A comparison with experimental result shows that above glass transition temperature, finite element generate same bond widths as observed experimentally for 200nm and 400nm Ti coated glass/imidex system. However, it failed to generate the same bond width as experimentally obtained for 50 nm thick Ti coated glass/imidex system. This is due to the lower thermal conductivity and elevation of surface temperature of the thinner film compared to other two thicknesses of the film. For the case of Ti coated glass/TeflonRTM FEP, higher bond strength with thicker film are obtained. No detectable degradation of polymer is observed in terms of free volume of voids for this system of TeflonRTM FEP.