CircuiTree - July 2008 - (Page 18)

[ Tech Talk ] Fine Lines in High Yield (Part CLIV) Advances in Copper Plating (Part A) t a 2004 IEEE symposium on VLSI circuits, Shekhar Borkar (director of circuit research for microprocessors, Intel) expressed that he didn’t see anything to replace copper. This remark referred to the use of copper in IC fabrication, in ﬁrst- and second-level packaging, and not to long-distance data transmission. Copper is displacing aluminum in IC fabrication, copper pillars are competing with solder bumps, and new plating processes and plating bath compositions allow plating of microvias shut in a bottom-up via-ﬁll process and enable the metallization of through silicon vias (TSVs). Extensive studies have shed light on the role of plating additives and other parameters that play a role in these via ﬁll processes. In addition, there have been advances in the rejuvenation of plating baths and in recycling of rinse waters from plating operations. for recycling rinse water from copper plating operations using electrical remediation. The system integrates electrowinning and ion exchange to remove copper and reduce total dissolved solids. The electrical remediation uses a forward pulsed waveform. By Karl Dietz Uziel Landau and others studied the mechanism by which plating additives enable bottom-up plating.4-6 Effect of Current Density on Via Filling Bottom-Up Via Fill Processes These processes can be used to create stacked vias in HDI boards and IC substrates. Added advantages include improved planarity, better thermal conductivity of the vias, and the ability to place solder bumps directly on top of microvia connections. This via ﬁll technology can be applied to blind microvias and also for ﬁlling through-holes.3 Using conventional plating baths and plating parameters, copper deposits faster on the board surface than in through-holes and blind vias, and vias show the familiar dog bone-shaped copper thickness distribution. So it is puzzling that there are conditions under which more copper is deposited in the hole than on the surface and vias can be ﬁlled from the bottom up. Early in the development of via ﬁll plating processes, it was found that the type of agitation, the copper to acid ratio, the current density, and the microvia aspect ratio all had an inﬂuence on plating quality. Figure 1 shows the beneﬁcial effect of impingementtype bath agitation. Figure 2 shows the effect of the copper concentration and current density on via ﬁlling. Figure 3 shows how the microvia aspect ratio, or via diameter for a given height, affects the percentage ﬁlled. Effect of Copper Sulfate Pentahydrate Concentration on Via Filling Figure 2 Effects of Copper Concentration and Current Density Via Filling (MCP-PAL) Material: RCC Plating Thickness: 25µm Current Density: 20 ASF Environmental Advances Traditional methods to remove organic contaminants from acid copper plating baths, such as active carbon or a combination of active carbon and hydrogen peroxide, were found to produce unsatisfactory results with periodic reverse pulsed plating baths so that the throwing power of the bath and the metallurgical properties of the copper deposit deteriorated with time. A new UV oxidation process (Enviolet®1) is removing organic contaminants more effectively as was demonstrated by total organic carbon and high-performance liquid chromatography analyses. Sulfur containing compounds are oxidized by this process to sulfate and organics are converted to CO2 or highly oxidized compounds that are electrochemically inactive. The UV radiation forms OH radicals from hydrogen peroxide that abstract hydrogen radicals from organic compounds that then react with oxygen to form peroxides that further oxidize to CO2. This method was found effective in rejuvenating or waste treating other plating baths (e.g., removal of ETDA from spent electroless copper plating baths). Reference 2 describes a novel method 18 July 2008 • circuitree.com Figure 3 Effect of Via Aspect Ratio (Source: PAL) Figure 1 Air Sparging Versus Jet Impingement The functions of plating additives are fairly well understood7,8: polyglycols such as polyethyleneglycol (PEG) act as inhibitors, small amounts of chloride enhance the inhibiting effect of PEG, bis(3-sulfopropyl)disodium-sulfonate (SPS), other disulﬁdes, thiourea, thiocarbamates act as accelerators, and quaternary nitrogen compounds act as levelers. In particular, Landau studied the role of PEG and SPS in the bottom-up via ﬁll process. He points out that at the beginning of the plating process there are hardly any additives in the via hole to be adsorbed on the hole wall and hole bottom surfaces because of the low liquid volume to surface area ratio in the via. PEG adsorbs faster than SPS but there is very little PEG to cover the hole walls and bottom. However, on the surface, the PEG supply is plentiful and the fast adsorbing PEG inhibits plating on the surface. In contrast, practically all additives that end up on the hole surfaces have to diffuse into the hole. Because SPS diffuses faster than PEG, it reaches the bottom of the hole ﬁrst and, once adsorbed, forms a bar- http://circuitree.com

Table of Contents Feed for the Digital Edition of CircuiTree - July 2008

CircuiTree - July 2008 Contents My Line Industry Review Tech Talk Flexible Thinking Toward a PCB Production Floor Metric for Go/No Go Testing of Lossy High-Speed Transmission Lines Intelligent Design 20-Year Retrospective Ask the Flexperts Environmentally Speaking BPA Growth Curves Considering Design Variants to Maximize Process Efficiency Market Outlook Technical Product Spotlights Classified Ads Upcoming Events Ad Index

CircuiTree - July 2008

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