Surface modification technique such as plasma treatment have been used to change the surface chemistry of polymers and improve adhesion. With an increase in the use of polyimides for microelectronic applications, the technique of ion sputtering to enhance Cu/polymide interface adhesion is receiving increased attention. For this study, the argon or oxygen-plasma modified and unmodified surfaces of biphenyl tetracarboxylic diangydride and phenylene diamine(BPDA-PDA) polyimide films was used as substrate.
First, in chapter 3, the deposition condition of copper thin films for using in microelectronics packaging was determined. Especially, the effects of Ar working pressure, substrate surface microroughness and macroroughness(different substrates) on the microstructure, and surface topography have been investigated in the Cu thin films sputter deposited
And effects of microstructure and surface morphology on the patterning of microcircuits by wet chemical etching have been studied. The surface roughness of polyimide was controlled by oxygen RF plasma treatment. Using 3 target DC magnetron sputtering unit, 2-5 μm thick Cu thin fims are deposited on the water-cooled substrates which are polycrystalline alumina, single crystal sapphire, slide glass, and oxygen plasma-modified polyimide. 50nm thick Cr film is used as an adhesion layer between Cu and ceramic substrates. In the Cu film deposited onto polymide substrates at the pressure of 5 mtorr, the surface is smooth and the columnar structure is not visible regardless of polyimide surface roughness. As the sputtering pressure increases, the columnar structure is developed and the structure contains more open boundaries. The polymide surface roughness enhances these effects. These phenomena can be explained in terms of atomic shadowing effect. Similarly, in ceramic substrates, the microstructure is more open boundaries as the Ar working pressure increases. And pores or microcracks in the column boundaries are frequently observed, which are in good agreement with the structure zone model proposed by Thornton. The morphology of Cu films deposited onto sapphire and slide glass substrates are similar at the same sputtering pressure, but the microstructure deposited on coarser polycrystalline alumina substrates contains more cracks and open boundaries, which are attributed to self-shadowing effects during film growth. The micropatterns of 25 μm in line width are successfully obtained by photolithography and wet chemical method regardless of sputtering conditions. Line morphology becomes smoother in the film formed at lower Ar working pressure. This effect is related to the open boundaries in the columnar structure. Line patterns are not much affected by grain or column size of the film.
Chapter 4 represents the correlation between the interaction of metal (Cu or Cr) with polyimide and adhesion. Adhesion and interfacial reaction at metal/polymer interface have been studied using Auger Electron Spectroscopy(AES). Cr and Cu thin films were deposited on the unmodified and in situ RF plasma-modified polyimide by conventional DC magnetron sputter. Adhesion was measured by 90° peel test and interfacial reaction was characterized by Auger electron spectra obtained from peeled metal strips. Adhesion of Cu film to polyimide is very weak regardless of RF plasma treatment. While peel strength of Cr to unmodified polyimide is barely measurable, but peel strength of Cr to RF plasma-modified polyimide is very high. Auger electron spectra analyses on the peeled metal surfaces show that Cu does not react strongly with polyimide, but Cr reacts with polymide to form carbide-like phase and RF plasma treatment increases the reactivity. These AES results are directly related with adhesion strength between metal and polymide.
In chapter 5, the effects of Ar or $O_2$ RF plasma treatment on the adhesion behavior of Cr films to polymide substrates have been investigated by using SEM(Scanning Electron Microscopy), XRD(Glancing Angle X-Ray Diffraction), AES(Auger Electron Spectroscopy), and 90° peel test. By applying RF plasma treatment of the polymide surface prior to metal deposition, the peel adhesion strength of Cu/Cr films sputtered onto the fully cured BPDA-PDA type polymide was highly increased from about 3 g/mm to 90~100 g/mm. Improved peel adhesion strength of Cr/polymide interfaces due to RF plasma treatment were attributed to the contributions from surface cleaning, Cr-polyimide bonding at the interface, and force required for plastic deformation of the film during peel adhesion test. While the surface topology change of the polyimide caused by RF plasma treatment makes a little contribution to the improved adhesion.