Investigation of the Adhesion of Electroless Copper to Glass Substrates
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1113-F03-16
Investigation of the Adhesion of Electroless Copper to Glass Substrates
Xiaoyun Cui, David A. Hutt, Paul P. Conway Wolfson School of Mechanical and Manufacturing Engineering, Loughborough University, Loughborough, LE11 3TU, United Kingdom
ABSTRACT This paper reports the effect of a Pd/Sn catalyst treatment process on the adhesion of electroless copper deposited onto a glass substrate. Adhesion of the copper varied with catalyst treatment time: short or extended catalyst immersion times led to lower adhesion. In this work silanisation of the glass surface with (3-aminopropyl)-trimethoxysilane was used to provide a layer of functional molecules to assist the adhesion of the Pd/Sn catalyst. Surface analysis of the catalyzed glass was carried out by X-ray Photoelectron Spectroscopy (XPS) and together with Time-of-Flight Secondary Ion Mass Spectrometry, showed that the Pd/Sn structures changed with increasing immersion time in the catalyst bath. The Pd XPS core level peaks indicated that Pd(0) became more significant in the catalyst layer than Pd(II) with increasing immersion time. Tape peel testing was used to assess the adhesion of the coatings: thin layers adhered well to the glass, but for layers thicker than 160 nm tape tests removed large areas. The failure surfaces of copper layers peeled off the glass were also examined by XPS which indicated that the failure occurred between the copper and catalyst. INTRODUCTION In order to meet the demands for smaller and lighter weight electronic products, flip-chip technology, where bare die are interconnected directly to circuit boards (substrates), is being more widely implemented. However, the fabrication of the matching substrates results in manufacturing difficulties due to the reduction in metal line widths, and the need to produce accurately aligned microvias. Significant advances have been made in production techniques however, many substrate materials such as the traditional glass fiber epoxy composite show dimensional instability during manufacture that limit the accurate alignment of features. Glass is a potential material from which to manufacture substrates for flip-chip interconnect as it offers good dimensional stability, coefficient of thermal expansion similar to silicon and enables the viewing of buried features for accurate machining. In order to produce glass multilayer substrates, metallic films deposited onto the glass surfaces are required to form reliable electrical tracks. Electroless deposition is an attractive method due to its relatively low cost, established infra-structure and potential for high volume production [1]. Electroless copper has been widely used as an interconnect metallisation in integrated circuits and in the fabrication of micro-electro mechanical systems [2-3]. However, metallising the glass using electroless methods to prepare conductive tracks, is challenging due
to the weak adhesion between the glass and metallic layer [4-5]. An essential step is to activate the glass surface by pre-treating with a catalyst to
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