Electroless Deposition of 4 μm High Ni/Au Bumps for 8 μm Pitch Interconnection

Abstract

We propose that electroless plating is a superb approach to preparing metallic bumps with an ultrafine pitch for the integration of a micro light-emitting diode (micro-LED). Electroless plating does not suffer from lift-off-related issues, which are ubiquitous in thermal evaporation. Besides, it can result in much more uniform bumps than electroplating because the bump height is not affected by the current distribution. This study reports ultrafine pitch Ni/Au bumps fabricated by electroless nickel immersion gold (ENIG) plating. Furthermore, cheap metals iron and nickel are selected to catalyze the electroless nickel process. The results indicate that uniform and consistent Ni/Au bumps can be obtained through the iron sheet and nickel layer method. Besides, no voids and impurities are found inside the bumps, which is beneficial for the following interconnection process. Moreover, the change in Ni bump height values with the electroless plating time is also provided.

Electroless deposition of nickel microbumps for fine-pith flip-chip bonding

“…Our previous research proposed using electroless plating to prepare nickel microbumps to interconnect ultra-high-density Micro-LED with a driving circuit board. The prepared bumps have a regular morphology, and there are no impurities and voids inside the bumps, which confirms that it is a feasible and practical approach [6]. Unfortunately, previous studies have not investigated the uniformity of bump arrays, and the existing studies on bump consistency have concentrated on large-feature-sizes and low-density bump arrays [3,4,7], leaving much space for exploration of small-dimension bump arrays with pith inside 10 μm.…”

23‐2: Invited Paper: Fabrication of Super Uniform Nickel Bumps Using Electroless Plating on Micro‐LEDs’ TFT Driver Substrates

“…Surfaces with contact angles less than 65° are hydrophilic. [13] So, the hydrophilicity of photoresist surface on the TFT substrate makes it easier to remove the hydrogen bubbles produced by redox processes [9] during the growth of bumps, and improves the uniformity of Ni bumps. Figure 4 is a schematic diagram of hydrogen bubbles generated by the redox reaction in photoresist micropores before and after plasma treatment.…”

“…Conversely, low-cost electroless [6][7][8] plating is a highly selective and conformal deposition technology that does not require a high vacuum environment or expensive equipment. It is a simple, time-cost-effective method enabling the large-scale growth of metal bumps, holding promise for expanding the scope of Micro-LED applications [9] . Notably, nickel micro-bumps prepared by electroless plating exhibit high wear and corrosion resistance, with dense, regularly shaped bumps resistant to deformation when subjected to external forces, making them an ideal material for Micro-LED bonding.…”