Jung-Hoon Chun

The recoiling of liquid droplets upon collision with solid surfaces

Although the spreading behavior of liquid droplets impacting on solid surfaces has been extensively studied, the mechanism of recoiling which takes place after the droplet reaches its maximum spread diameter has not yet been fully understood. This paper reports the study of the recoiling behavior of different liquid droplets (water, ink, and silicone oil) on different solid surfaces (polycarbonate and silicon oxide). The droplet dynamics are experimentally studied using a high speed video system. Analytical methods using the variational principle, which were originated by Kendall and Rohsenow (MIT Technical Report 85694-100, 1978) and Bechtel et al. [IBM J. Res. Dev. 25, 963 (1981)], are modified to account for wetting and viscous effects. In our model, an empirically determined dissipation factor is used to estimate the viscous friction. It is shown that the model closely predicts the experimental results obtained for the varying dynamic impact conditions and wetting characteristics. This study shows tha...

Instability of a liquid jet emerging from a droplet upon collision with a solid surface

A linear perturbation theory is developed to investigate the interface instabilities of a radially-expanding, liquid jet in cylindrical geometries. The theory is applied to rapidly spreading droplets upon collision with solid surfaces as the fundamental mechanism behind splashing. The analysis is based on the observation that the instability of the liquid sheet, i.e., the formation of the fingers at the spreading front, develops in the extremely early stages of droplet impact. The shape evolution of the interface in the very early stages of spreading is numerically simulated based on the axisymmetric solutions obtained by a theoretical model. The effects that factors such as the transient profile of an interface radius, the perturbation onset time, and the Weber number have on the analysis results are examined. This study shows that a large impact inertia, associated with a high Weber number, promotes interface instability, and prefers high wave number for maximum instability. The numbers of fingers at the spreading front of droplets predicted by the model agree well with those experimentally observed.

Evolution of Copper-Oxide Damascene Structures in Chemical Mechanical Polishing

Nonplanarity arising from the chemical mechanical polishing of Cu-oxide damascene structures results in the exposure field (die-size) being partially out of focus in subsequent lithography process. Thus the corresponding mechanisms of within-die polishing must be determined and the within-die nonplanarity due to polishing needs to be minimized to increase the process yield In this paper, contact mechanics models were developed to explain the role of pattern geometry on the variation of material removal rate. The effects of Cu linewidth, area fraction, and the elastic properties of the polishing pad on pad displacement into low features were examined to focus on the mechanical aspects of the process. The pressure distribution On the high features was determined and the rate of pattern planarization was quantified. Experiments on patterned Cu wafers were conducted to verify the model. Based on these results. The planarization and polishing behavior and the within-die nonplanarity due to the variation of pattern geometry were discussed.

Droplet-based manufacturing

Summary Droplet-based manufacturing (DBM) processes are promising production techniques for metallic pans and coatings with unique properties. In DBM processes, molten metal droplets are produced and deposited onto a substrate where they solidify to form near-net-shape pans. The quality of parts formed using such processes depends on the precise control of droplet size, flux, velocity and temperature. This paper presents an experimental study on the effect of droplet impact states and mass flux of the spray on deposit characteristics, using a unique DBM apparatus that generates molten metal droplet sprays with precisely controlled droplet size, mass flux and thermal state.

Spray deposition of a Sn-40 Wt Pct Pb alloy with uniform droplets

An apparatus capable of producing and depositing uniform droplets (100 to 200 μm in diameter) was developed and used to study the relationship between spray deposition parameters and the microstructures of Sn-40 wt pct Pb alloy spray deposits. The sprays used in the study consisted of uniform droplets, either 103 or 178 μm in diameter, that were in identical thermal and solidification states as they impacted the substrate. The thermal and solidification states of the uniform droplets were determined as a function of the flight distance (the distance from the metal pouring orifice) by model calculations and calorimetric measurements assuming equilibrium solidification. Although a fair agreement was noted between the model and the calorimetric measurements at small flight distances, corresponding to large liquid fractions, the calorimetric measurements indicated 10 to 20 pct higher liquid fractions at larger flight distances. The resultant microstructures comprised either a mixture of the Pb-rich and Sn-rich phases, both in an equiaxed morphology, or a lamellar eutectic structure with a small amount of the Pb-rich primary phase in a coarse spherical morphology. The mostly lamellar eutectic structure resulted from an excessive enthalpy flux and/or slow heat extraction from the deposit. Fine, equiaxed, two-phase microstructures and high deposit density resulted from optimal combinations of droplet enthalpy, deposition rate, droplet size, and deposit cooling rate which gave short local solidification times.

Effects of droplet thermal state on deposit microstructure in spray forming

Spray forming with mush uniform-droplets deposited onto a solidified deposit surface produced a fine, equiaxed microstructure in a Sn-5wt.%Pb alloy. The equiaxed grains are those of the Sn-rich phase and were delineated with thin films of the Pb-rich phase along the grain boundaries. No splat or spray boundaries were noted. The equiaxed microstructure evolved mainly from fragmented and randomly oriented crystals which originally formed in the droplets before impact. The lack of splat boundaries was explained by the local epitaxial growth of the grains in the prior splat and the oxide-free deposit surface maintained during the experiment. Use of mush droplets is the single most important requirement for the formation of equiaxed microstructures in a spray deposit. The thermal state of the deposit surface may play only a minor role. Molten droplets produced a rapidly solidified columnar microstructure by the epitaxial growth of the Sn-rich phase. No splat boundaries were produced. The primary Sn-rich phase columnar grains were contiguous through the entire deposit thickness of about 20 mm. A schematic process-microstructure map for the spray forming process is used to show the entire process window and expected microstructures for spray forming with uniform-droplet sprays.

Spreading and solidification of a molten microdrop in the solder jet bumping process

This work develops a model to predict the spreading and solidification of solder droplets deposited on a solid pad in the solder jet bumping process. The variational principle is employed to solve the fluid flow and the semi-solid phase is modeled as a non-Newtonian slurry. This modeling greatly saves the computational expenses of conventional numerical procedures. The simulations reveal that the substrate temperature is the single dominant controlling parameter that determines the final bump diameter (or height) when the substrate possesses a high effusivity. When the effusivity of a substrate is relatively low, both the substrate temperature and the droplet temperature at impact play important roles in determining the final bump diameter. Our model can be used in designing the experimental conditions to find the optimal process conditions for a desired bump geometry.

Mechanisms of the Chemical Mechanical Polishing (CMP) Process in Integrated Circuit Fabrication

Abstract A contact mechanics model that describes the polishing mechanisms of copper-patterned silicon wafers in the fabrication of ultra-large-scale integrated (ULSI) circuits is presented. The model explains the die-scale variation of material removal rates due to pattern geometry, and predicts results that are in agreement with experimental observations. When the width of the copper interconnect is less than 0.5 μm, dishing of interconnects is less than 20 nm and thus does not contribute much to surface nonuniformity. However, because the overpolishing rate varies with the copper area fraction, it may contribute to die-scale nonuniformity.

The Role of Pad Topography in Chemical-Mechanical Polishing

In this paper, the role of pad topography on material removal rate (MRR) in chemical-mechanical polishing (CMP) is investigated. First, based on the mechanics of pad/particle and particle/wafer sliding contacts at an asperity of the polishing pad a new MRR model is developed. The model is then extended to multi-asperity contacts, taking into account the statistics of the asperity heights. The single-asperity model reveals that the removal rate at relatively low pressure strongly depends on the pressure and the area at a sliding asperity contact. However, removal rate per asperity becomes independent of the contact pressure once it exceeds a critical value, which is determined by the asperity hardness and the particle concentration. Material removal by multi-asperity sliding contacts increases due to the increase in real contact area, provided a large number of asperity contacts at pressures greater than the critical. The plasticity index is identified as a key parameter that determines the contact area ratio and proportion of asperities in contact at pressures greater than the critical, thus the overall MRR. The model suggests that MRR in CMP can be greatly increased by controlling the surface topography of the pads. Results of polishing experiments on Cu thin films validate the model.

Effect of Slurry Selectivity on Dielectric Erosion and Copper Dishing in Copper Chemical-Mechanical Polishing

Abstract A formidable challenge in the present multi-step Cu CMP process, employed in the ultra-large-scale integration (ULSI) technology, is the control of wafer surface non-uniformity caused by dielectric erosion and Cu dishing. A definitive understanding of the causes of material loss and a physical model for non-uniformity in Cu CMP are thus required. This paper examines the effects of slurry selectivity on dielectric erosion and Cu dishing, in both single- and multi-step Cu CMP processes, in terms of several geometrical and physical parameters. Furthermore, optimal slurry selectivities to mitigate dielectric erosion and Cu dishing in both single- and multi-step polishing are suggested.