IC packages are assembled into functional systems using printed circuit boards (PCBs), which are normally fabricated using semi-additive processes (SAPs) to avoid the costs and complexities of sequential depositions and etches. Advanced PCBs for high-density packaging (HDP) require copper (Cu) metal electrical interconnects for both horizontal traces as well as vertical micro-via holes (MVH), and cost-effective manufacturing requires that both fine-line traces and MVH be formed using the same high-volume manufacturing (HVM) processes. Dow Electronic Solutions’ nickel-free (Ni-free) electroless copper plating chemistry offers new solutions for the plating of fine patterns needed in advanced IC substrate HVM, with high productivity (0.6um/20min) and peel strength (>0.5kN/m on 100nm roughness).
Advanced PCBs are needed to meet the demands of IC packaging for mobile electronics and network infrastructure. Advanced IC chip assembly uses “flip-chip” technology to miniaturize and increase the performance of consumer mobile products, and the need to integrate increasing functionality into smaller product sizes leads to the need for denser PCB layouts featuring ever-finer Cu line-widths.
The manufacturable minimum line-width for PCB traces is limited by multiple parameters: capability of electroless copper plating, resolution of dry-film etch, etc. Of these, the electroless Cu capability is the most critical when pushing line-arrays to the smallest possible pitch, which today can be 2-3 microns in the most advanced layouts, and as small as 1 micron in research and development (R&D).
One established electroless deposition process for Cu involves co-deposition of nickel (Ni) for a high deposition rate, high peel strength, and smooth surface morphology. However, there are integration issues with use of electroless Ni-Cu associated with the need to “flash etch” the metal surface, which causes excessive side-etch and under-cuts when Ni is present. Co-deposition of Ni typically also requires a relatively slower and more expensive process.
Generally, the electroless Cu deposition reaction can be divided into two phases: first, a relatively slow initial nucleation of Cu on palladium metal seed-layer; and second, Cu-on-Cu autocatalytic reaction. When the first deposition is too fast, the deposited Cu crystals form micro-voids, which lead to poor peel strength over dielectric. When the second deposition is too slow, the throughput of the process limits productivity. Dow has found a novel bath additive that significantly improves the electroless Cu plating process for advanced PCBs, as shown in Figure 1. In the first 10 minutes, the additive functions as a deposition suppressor to slow the reaction rate and thereby obtain uniform coverage of both the dielectric surface and the palladium seed, while the latter deposition rate provides high productivity.
MVH Coverage and Fine-Pitch Lines
In general, essential bath components such as Cu tend to get depleted inside the MVH during plating, and this chemical depletion necessarily slows the plating reaction, especially on the MVH bottom. The conventional approach to improving Cu uniformity inside of MVH is to strongly agitate the bath to try to drive fresh solution into recessed features, although this approach becomes more difficult as MVH diameters decrease. Nominal sizes of MVH are 50µm in diameter and 35µm in depth, and we use such feature sizes for our experiments on sidewall coverage to determine the throwing power (TP) of the technology. The TP is defined as the ratio of the thicknesses of deposition on the sidewall compared with that on the top surface outside of the MHV.
The improved additive in Dow’s Ni-free electroless Cu chemistry is key to the uniform deposition in MVH. This product achieves >80% TP, compared with the <40% TP typically seen using Ni-Cu. We also studied the effects of different Cu concentrations in the baths and found no differences in TP when varying Cu over the relative range of 40-100%. These results show that this Ni-free Cu chemistry has a wide process-window for manufacturing, providing stable results over long bath lives. All the chemicals used in our studies—such as cleaner, catalyzer, and reducer—are Dow Electronic Solutions’ commercialized products.
Removing Ni from the metallization provides many benefits beyond the important advantage of TP. Peel strength and surface morphology are other key process attributes to consider, and we have found that they are not substantially compromised when using Ni-free electroless Cu.
In terms of peel strength, it has historically been challenging to obtain high peel strength on low roughness dielectrics. Although a smooth surface is needed to reduce interconnection delays and increase overall performance, achieving high adhesion on such smooth surfaces has been difficult. Based on our testing, using the improved additive showed excellent and stable peel strength around on various dielectric materials. (Figure 2) It should be noted that peel strength with Nickel used in the additive dropped to around 0.43kN/m. These results demonstrate that in terms of peel strength, use of the improved additive contributes to achieving the fine pattern design required for future designs.
Surface morphology is another key benefit of using Dow’s additive for Ni-free electroless Cu because it is easy to flash-etch. The images below show a Ni-free surface and the surface morphology having used Dow’s additive (Figure 3).
Nickel concentration needs to be controlled in other baths to be able to maintain a uniform concentration of Ni in the final film, so the Ni-free bath is easier to control. Ni-Cu films are relatively more difficult to flash-etch, so Cu films can be more easily surface-treated. The new Ni-free chemistry is environmentally friendly because it is also cyanide-free.
Without Ni, proper selection of functional additives can provide excellent plating performance in terms of deposition rate, peel strength on low-profile dielectric, uniform deposition inside MVH and Cu-Cu joint reliability. The Ni-free chemistry can be run with great bath stability in existing plating lines, with no change to tooling required. The improved TP inside MHV allows for relatively thinner top-side depositions, which enables the most fine-pitch trace patterning without concern of collapse due to aspect-ratios.