Bioh Kim

Factors affecting copper filling process within high aspect ratio deep vias for 3D chip stacking

Through-silicon-via (TSV) copper electrodes can provide shortest-length and highest-density connections with reduced signal delay and power consumption. The issues involved with making TSV processes manufacturable include: (a) via shape and angle control; (b) insulator, barrier, and seed deposition with good conformality and adhesion; (c) void-free via filling with copper; (d) metal removal by CMP; (e) wafer thinning with small total-thickness-variation and with no critical defects; (f) via rerouting in the case of stacking heterogeneous chips; and (g) cost-effective wafer bonding. Copper deposition within high aspect ratio vias is one of the key technologies. We investigated the impacts of varying deposition conditions on the copper filling within a high aspect ratio via (AR > 8). We found that reducing current crowding at the via mouth and mass transfer limitations at the via bottom by optimizing deposition conditions is critical in achieving void-free, bottom-up filling. Bath composition and waveform have a significant influence on the filling profile at a fixed average current density. This study found that the plating bath requires a high metal concentration and a strong super-conformal capability. For a wide range of bath compositions (i. e., inorganic and organic concentrations), pulse reverse waveforms with the proper waveform parameters are required for reducing overhang and assisting bottom-up filling within a high aspect ratio via. With increasing average current density and all other process variables fixed, deposit profiles change significantly, which leads to higher tendency of overhang at the via mouth. This is attributed to current crowding at the via mouth and mass-transfer limitations at the via bottom. With optimized deposition conditions, we demonstrated void-free, bottom-up filling with a variety of via dimensions. Currently we are working on achieving higher filling rates. Initial tests with a modified bath composition showed that we may achieve at least two times faster filling rate with direct current

Electrodeposition of Near-Eutectic SnAg Solders for Wafer-Level Packaging

The electrodeposition of near-eutectic SnAg solders for wafer level packaging was investigated with Shipley-Ronals EXP-0700 SnAg bath. Fundamental studies including polarization behavior, morphological transition, and compositional change were performed to investigate the alloy deposition mechanism. It was proven that the mass transfer limitation of silver ions at low potential drove the drastic change of morphology and composition with increasing current density. The morphological transition occurred through four stages (dendrites, suppression of dendrites, facets, and dendrites) and the content of silver in the deposit dropped with increasing current density. The deposition mechanism of SnAg alloys with this bath looks similar to that of SnAgCu alloys with an alkaline bath due to a similar polarization behavior. Pattern plating results including studs, mushrooms, and stacks can be summarized as fine-grained surface, high deposition rate, good uniformity, and repeatability. Two types of in situ stack plating, Cu/SnAg and Ni/SnAg, were successfully performed with no defects between two layers. Reflow tests conducted with near-eutectic SnAg solders on copper studs showed spherical shapes with smooth surfaces, where the average melting point of whole bumps in a wafer was 223.7°C, which is close to the melting point (221°C) of the eutectic composition.

Electrochemically Deposited Tin-Silver-Copper Ternary Solder Alloys

A study on film properties of electrochemically deposited tin-silver-copper (SnAgCu) alloys was performed with an alkaline bath. This research focused on the bath and process development for dendrite-free. near-eutectic SnAgCu alloy deposition through the investigation of cathodic polarization, morphological transition, and film composition. Effects of process parameters on surface morphology, film composition, and diffusion-limited current density (LCD) were also examined. Ternary alloys were obtained only when the current density was driven beyond the mass-transfer limitation of noble metals (silver and copper). which seemed to cause the transition of surface morphology and film composition with increasing current density. The morphological transition occurred through four stages (dendrites, suppression of dendrites, nodules, and columns/dendrites), and the content of noble metals in the film tended to drop with increasing current density. With increasing the concentration of noble metals, bath temperature, and agitation, the morphology at low current densities became increasingly dendrite-dominated and the content of noble metals in the film was enhanced at a fixed current density. The morphology of stage four was influenced by the ratio of the applied current density to the LCD, where the LCD was significantly influenced by the metal concentration, bath temperature, and agitation.

Electrical Waveform Mediated Through-Mask Deposition of Solder Bumps for Wafer Level Packaging

Electrical waveform mediated through-mask deposition of solder humps was investigated with several types of plating baths for wafer level packaging applications. The influence of varying duty cycle in the presence of additives on deposit properties including shape evolution within the cavity, abnormal growth, surface morphology, alloy composition, and thickness distribution was evaluated at a fixed, moderate frequency Waveform mediation with properly selected duty cycles (i) improved surface flatness and morphology of deposits when the shape ratio with dc deposition was less than 1, (ii) suppressed the probability of abnormal growth (nonhomogeneous growth such as large nodules), (iii) reduced grain sizes resulting in smoother surfaces, and (iv) modulated alloy composition at a given bath and process condition. With decreasing duty cycle, the thickness distribution within the feature, pattern, and workpiece also changed due to the increased influence of primary current distribution. The fraction of current flowing along the cavity edge, die edge (when the space between dice is much larger than the bump pitch), and wafer edge seems to increase with decreasing duty cycle.

Electrodeposition of ternary near-eutectic SnAgCu solders with an alkaline bath

The electrodeposition of near-eutectic SnAgCu solders was investigated with an alkaline bath. The polarization behavior, surface morphology and film composition were examined to understand the mechanism of ternary alloy deposition. Pattern plating was tested with several types of patterns and reflow tests were conducted. Ternary alloys were obtained only when the current density was driven beyond the mass transfer limitation of noble metals (silver and copper), which seemed to cause the transition of surface morphology and film composition with increasing current density. With increasing current density, the morphological transition occurred through 4 stages (dendrites, suppression of dendrites, nodules and columns/dendrites) and the content of noble metals in the film tended to drop. Process results obtained with patterned wafers seemed to be comparable to those of PbSn alloy plating from the viewpoints of surface morphology, shape evolution, thickness and compositional uniformity, deposition rate and process repeatability. Two types of in-situ stack plating were successfully performed; Cu/SnAgCu and Ni/SnAgCu. Reflow tests conducted with near-eutectic SnAgCu solders on copper studs showed proper shapes with smooth surfaces.

High rate electrodeposition of near-eutectic PbSn solders

A study on the improvement of deposition rate for near-eutectic PbSn solder plating was performed. This research focused on (1) the effect of process parameters on the diffusion-limited current density (LCD) to extend the operating zone, (2) the process optimization at 4 /spl mu/m/min with a modified bath, and (3) the trouble-shooting procedures for abnormal and non-conformal growths. The effect of waveform on surface morphology, growth pattern, film composition and thickness uniformity was also examined. The LCD was increased by increasing metal concentration, bath temperature and agitation, or by decreasing the concentration of methane sulfonic acid (MSA). The effect of additives on LCD was not significant. No significant change in morphology and growth pattern was observed up to approximately 10 /spl mu/m/min with a modified bath. Process results obtained at optimized conditions showed smooth surfaces, conformal growth and good repeatability of thickness and composition. The probability of abnormal growth was increased with increasing deposition rate, but it was prominently suppressed by increasing MSA concentration and temperature or by decreasing duty cycle. The growth conformality along the resist was improved at a fixed deposition rate by decreasing wafer rotation speed and duty cycle or by increasing MSA concentration.