Droplet Bouncing Behavior in the Direct Solder Bumping Process

This paper presents the results of an ongoing effort to develop a direct solder bumping process for electronics packaging. The proposed process entails delivering molten droplets onto specific locations on electronic devices to form solder bumps. This study is focused on investigating droplet deposition behaviors that affect solder bump characteristics such as final bump volume, shape, and adhesion strength. The occurrence of droplet bouncing has a strong influence on these characteristics. The potential for a droplet to bounce in the absence of solidification was modeled in discrete stages based on energy conservation. Wetting and target surface roughness were identified as the critical parameters affecting bouncing. The experimental results showed that improvements in wetting and decreases in surface roughness retard bouncing. These observations agreed well with the trends predicted by the energy conservation based model. The knowledge acquired in this study is expected to contribute to the development of an efficient solder bumping process.Singapore-MIT Alliance (SMA

Modeling Dielectric Erosion in Multi-Step Copper Chemical-Mechanical Polishing

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, which primarily is due to dielectric erosion and Cu dishing. In contrast with the earlier experimental and semi-theoretical investigations, a systematic way of characterizing and modeling dielectric erosion in both single- and multi-step Cu CMP processes is presented in this paper. Wafer- and die-level erosion are defined, and the plausible causes of erosion at each level are identified in terms of several geometric and physical parameters. Experimental and analytical means of determining the model parameters are also outlined. The local pressure distribution is estimated at each polishing stage based on the evolving pattern geometry and pad deformation. The single-step model is adapted for the multi-step polishing process, with multiple sets of slurry selectivities, applied pressure, and relative velocity in each step. Finally, the effect of slurry-switching point on erosion was investigated for minimizing dielectric erosion in the multi-step Cu CMP. Based on the developed multi-step erosion model, the physical significance of each model parameter on dielectric erosion is determined, and the optimal polishing practices for minimizing erosion in both multi-step and single-step polishing are suggested.Singapore-MIT Alliance (SMA

The Effect of Pad-asperity Curvature on Material Removal Rate in Chemical-mechanical Polishing

In chemical-mechanical polishing (CMP), surface asperities of the polishing pad play a key role, for they transmit normal force and impart tangential motion to the hard, nano-scale abrasive particles in the slurry. It has been shown recently, however, that the soft pad asperities themselves often generate micro-scale scratches on the surfaces being polished. To mitigate scratching by pad asperities, therefore, topography control by flattening pad asperities has been proposed and experimentally validated. In this study, the effects of asperity-flattening on pad topography and the material removal rate are investigated. It is found both theoretically and experimentally that even at a relatively high pressures only the tallest of the asperities are flattened and the ratio of asperity radius-to-standard deviation of heights is increased, but the average roughness itself is little affected. Specifically, surface profiles of new and asperity-flattened pads indeed show that the average roughness of about 5 μm is changed less than ten percent. Concurrently, the material removal rate is increased by about 30 percent due in part to the increased real area of contact––the result of increased asperity radius of curvature and decreased standard deviation of asperity heights.Samsung (Firm

Pad Scratching in Chemical-Mechanical Polishing: The Effects of Mechanical and Tribological Properties

In chemical-mechanical polishing (CMP), even the soft pad asperities may, under certain conditions, generate scratches on the relatively hard surfaces being polished. In the present study, contact mechanics models of pad-induced scratching are formulated, and the effects of the hardness of the surface layers and of pad asperities as well as the interfacial friction are elucidated. Additionally, scratch-regime maps are proposed to provide criteria for scratching hard surface layers by the softer pad asperities. Furthermore, scratching indexes are introduced to predict the proportion of asperities in contact that are likely to scratch. The contact mechanics models of scratching have been validated by sliding experiments with two commercial CMP pads (Pad A and IC1000) and various thin-films (Al, Cu, SiO[subscript 2], Si[subscript 3]N[subscript 4], TiN and three low-k dielectrics) using deionized water as a “lubricant.” Both the theoretical models and the experimental results show that the number of scratches increases as the scratching index exceeds 0.33. Al and Cu layers are found to be more susceptible to pad scratching due to their low hardness and high interfacial friction. The scratch-regime maps provide practical guidelines for mitigating pad scratching in CMP.Samsung (Firm

A Multi-scale Model for Copper Dishing in Chemical-Mechanical Polishing

The present success in the manufacture of multi-layer interconnects in ultra-large-scale integration is largely due to the acceptable planarization capabilities of the chemical-mechanical polishing (CMP) process. In the past decade, copper has emerged as the preferred interconnect material. The greatest challenge in Cu CMP at present is the control of wafer surface non-uniformity at various scales. As the size of a wafer has increased to 300 mm, the wafer-level non-uniformity has assumed critical importance. Moreover, the pattern geometry in each die has become quite complex due to a wide range of feature sizes and multi-level structures. Therefore, it is important to develop a non-uniformity model that integrates wafer-, die- and feature-level variations into a unified, multi-scale dielectric erosion and Cu dishing model. In this paper, a systematic way of characterizing and modeling dishing in the single-step Cu CMP process is presented. The possible causes of dishing at each scale are identified in terms of several geometric and process parameters. The feature-scale pressure calculation based on the step-height at each polishing stage is introduced. The dishing model is based on pad elastic deformation and the evolving pattern geometry, and is integrated with the wafer- and die-level variations. Experimental and analytical means of determining the model parameters are outlined and the model is validated by polishing experiments on patterned wafers. Finally, practical approaches for minimizing Cu dishing are suggested.Singapore-MIT Alliance (SMA

A Mechanical Model for Erosion in Copper Chemical-Mechanical Polishing

The Chemical-mechanical polishing (CMP) process is now widely employed in the ultralarge scale integration chip fabrication. Due to the continuous advances in semiconductor fabrication technology and decreasing sub-micron feature size, the characterization of erosion, which affects circuit performance and manufacturing throughput, has been an important issue in Cu CMP. In this paper, the erosion in Cu CMP is divided into two levels. The wafer-level and die-level erosion models were developed based on the material removal rates and the geometry of incoming wafers to the Cu CMP process, including the Cu interconnect area fraction, linewidth and Cu deposition thickness. Experiments were conducted to obtain the selectivity values between the Cu, barrier layer and dielectric, and the values of within-wafer material removal rate ratio, β, for the validation of the new erosion model. It was compared with the existing models and was found to agree better with the experimental data.Singapore-MIT Alliance (SMA

Recent Progress in Droplet-Based Manufacturing Research

This article reports the recent progress of re-search made in the Droplet-Based Manufacturing Laboratory at MIT. The study has been focused on obtaining a fundamental understanding of microdroplet deposition and applying the technology to various practical applications. Specific scientific contributions include the development of an analytical model for droplet splashing/recoiling, an in situ droplet size control methodology, and a study of microstructure design for spray forming. The research per-formed in the lab provides both fundamental knowledge base and practical process developments for a range of manufacturing applications, including electronics packaging, spray forming and freeform fabrication.Singapore-MIT Alliance (SMA

Prediction Data Processing Scheme using an Artificial Neural Network and Data Clustering for Big Data

Various types of derivative information have been increasing exponentially, based on mobile devices and social networking sites (SNSs), and the information technologies utilizing them have also been developing rapidly. Technologies to classify and analyze such information are as important as data generation. This study concentrates on data clustering through principal component analysis and K-means algorithms to analyze and classify user data efficiently. We propose a technique of changing the cluster choice before cluster processing in the existing K-means practice into a variable cluster choice through principal component analysis, and expanding the scope of data clustering. The technique also applies an artificial neural network learning model for user recommendation and prediction from the clustered data. The proposed processing model for predicted data generated results that improved the existing artificial neural network–based data clustering and learning model by approximately 9.25%