No abstract

Recently the use of Copper as a conductor material for Thin Film Transistor (TFT) structures has received increasing attention in the Flat Panel Display industry [1,2]. Since sputtering of thick Cu layers has serious disadvantages Flat Panel Display (FPD) manufacturers are considering wet chemical Cu deposition as a viable process alternative. Depending on the required film properties like conductivity and surface roughness either electroless or electrolytical Copper deposition can be applied.

The focus of the chip and IC substrate manufacturing industry for interposers is currently shifting from organic substrates to inorganic materials. Interposers overcome the dimensional mismatch between a die and an organic PWB substrate and need to buffer the differences in thermal expansion between these two materials. While inorganics like silicon and glass, have a low CTE value compared to organic material, glass has some significant advantages over silicon. These are namely material cost, availability in panel size and a better electrical insulation. In order to further increase the cost advantage, metallization could be done by either electroless or electrolytic copper plating as an alternative route to sputter technology. Both wet chemical processes are well established in PWB manufacturing and need adaptation to glass substrates. Compared to sputtering, 3D-features can be covered with a wet chemical treatment in a comparatively economical process. Filling of TGVs (Through Glass Vias) by electrolytic copper plating requires the presence of a conductive film which is created by applying electroless copper deposition of typically 300–500nm thickness. The general issue is poor adhesion to the smooth glass surfaces. To improve adhesion between glass and metal, various treatments were assessed in this study: Mechanical anchoring was achieved by etching the substrate (subtractive) as well as by depositing nano-sized silica particles (additive). Both treatments are in solution and 3D-features are accessible in contrast to a purely mechanical approach. Surface treatments without inducing additional roughness included the adsorption of functional metal-affine polyelectrolytes and silanization for conditioning which enhanced the adhesion of the copper layer to various degrees. To study the impact of chemical formulation on strain/stress development and relaxation in the film as a key impact factor for blister (adhesion) performance, the electroless copper layer growth was monitored by in-situ XRD methods during and after deposition.

Polymer substrates were chemically coated with copper using various electroless copper baths and the internal strain/stress, as well as the adhesion quality, in the resulting copper films were studied during and after deposition as a function of the deposit thickness and the operation parameters of the electroless bath. The appearance of internal compressive stress in the copper film correlates to the probability of buckle driven delamination failure (blistering). Based on a simple theoretical concept we derived limits of allowed compressive stress in the copper film without inducing this failure mode. Furthermore depth-resolved X-ray diffraction (XRD) measurements in up to 1 μm thick electroless films indicate an approximately linear internal stress profile from about +200 MPa tensile stress at the substrate/adsorbate interface to −100 MPa compressive stress at the surface of the deposit. This will be explained in terms of a possible composition gradient of nickel in the copper film.

Glass interposers offer a compelling alternative to silicon interposers with highest I/Os and excellent electrical performance, with potential for low cost from large panel processing. For sub-32nm IC nodes and 3D-IC packages at fine I/O pitch, organic substrates are reaching their limits in terms of I/Os, design rules and CTE mismatch. Glass offers the best combination of electrical insulation, dimensional stability, CTE match to Si ICs, and flat, smooth surfaces for ultra-fine line lithography. The biggest challenge in glass interposers is the formation and metallization of ultra-fine pitch through vias. The formation of small via diameters in thin glass substrate have been demonstrated. The focus of this paper will be on wet metallization of glass interposer with through via, and addressing the challenge of providing reliable adhesion on the copper-to-glass interface. Two main approaches are currently pursued in the wet-chemical metallization of glass interposers: the electroless and electrolytic copper deposition on a) bare glass with photo-structured or laser-ablated through vias and b) the deposition on an intermediate polymer surface layer, requiring glass metallization only on the via sidewalls. Therefore, in addition to the analysis and optimization of adhesion-improving techniques on different glass types, their performance on different polymer liners is also assessed. The techniques include surface conditioning with cationic polyectrolytes and deposition of silica. The effect of the above surface pretreatments on plated copper adhesion is analyzed and the results provide guidelines for reliable glass interposer TPV metallization.