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30 PCB007 MAGAZINE I MARCH 2020 residues in the target pad through modification of the etch module yielded process improve- ments that are now actively deployed in pro- duction. The first major improvement involves the use of a dual sump etching module to pro- vide more precise etch depth controls. The first stage removes the bulk of the carbon from the copper surface, and the second stage contains fresh etch chemistry not contaminated with carbon, which may be redeposited onto the surface. A second improvement incorporates technology found in copper reduction lines for maximizing the uniformity of the etch amount across the panel surface. Reducing variation in the etch depth across the panel surfaces helps to tightly control the total etch amount required to clean the target pads. The coefficient of variation in the etch is tightly controlled by chemistry concentration, spray bar design, and spray pressure param- eters (Figure 3). Advancements in Chemistry In terms of chemistry improvements, tradi- tional cleaner/conditioner and etchant chem- istries have been tested and designed with controlled etch capabilities in mind. Cleaners now feature an organic additive that selective- ly covers copper surfaces but is not attracted to dielectric surfaces. On the copper surfaces, the carbon black is adsorbed on the organic coating provided by this technology over the copper. When the panel enters the etch mod- ule, the carbon black is undercut by the etch and lifted off the surface. The organic coating is highly soluble in acid medium and is imme- diately removed by the microetching step. In another improvement, two-component microetches are being implemented that re- move the dried carbon and reduce the cop- per roughness. The copper surface is ideal for dry film adhesion, and testing has also shown a smoother target pad is better for microvia reliability. The target pad copper crystalline structure is ideally prepared as a substrate to form epitaxial growth of the electrolytic cop- per (Figure 4). Copper Grain Boundary Improvements This combination of specific improvements to key process modules and chemistry has re- sulted in a line capable of running thin foil buildup layers for advanced HDI/mSAP. Micro- via reliability is enhanced with a single inter- face of direct copper-to-copper bonding, form- ing a continuous metallurgical structure. The Figure 3: Uniform etching control through equipment and chemical improvements has allowed for the complete removal of carbon residues in target pads.