CircuiTree - August 2008 - (Page 20)

[ Tech Talk ] Fine Lines in High Yield (Part CLV) Advances in Copper Plating (Part B) ast month’s Tech Talk covered several advances in copper plating, including environmental advances, and new applications such as copper pillar plating on wafers. But the focus was on bottom-up via ﬁll processes for plating microvias shut and the metallization of through silicon vias. The mechanism of bottom-up plating into blind vias was discussed for additive systems that feature accelerators, inhibitors, chloride, and optionally leveler. Additional studies on ﬁlling microvias and plated through-holes with plated copper, using other additive systems, and a method to minimize the degradation of additives is being covered in this article. Dow and Chen1 point out that a copper plating bath containing carrier, brightener, and level in a composition that was optimized for blind via bottom-up ﬁlling gave unsatisfactory results with plated throughholes. They developed a new composition for DC plating that gave good results with both blind vias and through-holes. The new composition was not disclosed in the article. More recently, Dow, Yen, and Chen2 introduced a novel plating bath composition that was optimized for through-hole ﬁlling. It does not contain a carrier such as polyethylene glycol nor does it contain an accelerator. Instead, it contains two strong inhibitors, one quaternary amine and one amide that both rely on chloride ions to be attached to the copper surface, in the case of the amide in the form of a protonated species. The authors point out that the adsorption of chloride to the copper surface is strongly potential dependent and this potential dependency in turn is chloride concentration dependent. Given the typical potential proﬁle on the copper surface and in the through-hole, more chloride is adsorbed on the surface and near the through-hole entrances than near the center of the through-hole. Thus, more inhibitor is anchored where more chloride is adsorbed and plating is more inhibited in these areas. Consequently, the highest plating rate is seen By Karl Dietz L near the center of the hole. Strong forced bath convection will level the concentration gradient of the adsorbed inhibitors, equalizing plating rates and leading to plating voids in the through-hole. Dyar and others3 introduced a novel lowcost blind via metallization/planarization process that uses an electrically mediated (pulse plating) technique. The only additives in the bath are polyethylene glycol and chloride. The electrically mediated waveform shows forward modulation, reverse modulation, and off-time. Complete copper ﬁlling Strong forced bath convection will level the concentration gradient of the adsorbed inhibitors, equalizing plating rates and leading to plating voids in the through-hole. of the blind vias with only minimal surface plating is achieved by alternating uniform (conformal) copper deposition with non-uniform copper etching whereby more copper is etched on and near the surface than at the bottom of the blind via. This is achieved by creating different types of diffusion layers during plating and etching cycles. During plating, the hydrodynamic diffusion layer is thin relative to the surface proﬁle in which case the diffusion layer follows the contours of the surface proﬁle in uniform thickness. Because plating is diffusion limited, the plating is conformal (i.e., of uniform thickness). This thin, uniform diffusion layer is formed when a high cathodic peak current is employed for a relatively short time. During etching, a thicker diffusion layer is created that has a ﬂat surface and therefore is thinner over the copper surface than over the microvia. Thus, the etch rate during the anodic cycle is faster on the surface and near the opening of the microvia than it is at the bottom of the via. This thicker diffusion layer is achieved by employing a low anodic peak current and a relatively long on-time. The alternating plating and etch cycles result in completely ﬁlled blind vias with little surface copper. Yeh, Dow, and others4 did a comprehensive study on the impact of different plating process parameters on microvia and through-hole ﬁlling, conﬁrming many of the ﬁndings mentioned in last month’s Tech Talk. The authors concluded that optimizing the microvia ﬁll as well as throughhole ﬁlling is difﬁcult because high copper/ low acid concentration favors the microvia ﬁll process whereas low copper/high acid is the preferred condition for through-hole plating. Low current density plating gave better bottom-up ﬁlling performance, but to shorten the plating cycle, the current density was doubled in the second phase of the plating cycle, yielding a reasonable tradeoff between productivity and microvia ﬁll percentage. The effects of different additive components were studied but the discussion of the results describes the additive system only as a new type. It was found that when insoluble anodes were used, which is often the case in periodic reverse pulse plating, the consumption of additives increases greatly. A method to reduce this undesirable side effect is described as follows.5 The insoluble anodes are typically made of titanium that is coated with mixed oxides. Oxygen is formed at the insoluble anodes during plating. This oxygen degrades (oxidizes) organic additives in acid copper plating baths. The organic additives are either cationic species or carry a positive charge due to protonation and therefore migrate to the negatively charged cathode. These additives also reach the anode because of bath agitation. Menard and Wurm reported much reduced additive consumption with a modiﬁed anode. The basic idea was to make it more difﬁcult for the additives to reach the anode by shielding the anode surface from the ﬂuid movement due to agitation. 20 August 2008 • circuitree.com http://circuitree.com

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CirciuTree - August 2008 Contents My Line Industry Review Tech Talk Flexible Thinking Creating Connections Between Electronics Design and Manufacturing Happy Thoughts View From the Middle Market Outlook Fein Lines IPCA Showcase Technical Product Spotlights Classified Ads Upcoming Events Ad Index Buyers Guide

CircuiTree - August 2008

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