9

Nanosize Diamond Particles for Copper CMP

Working with David Cerutti at GE Superabrasives and Donald Buckley at GE-CRD, Professor Yuzhuo Li and his graduate students/Visiting Scholars Earl Tyre, Jason Keleher, Xiaojing Shi and Yong Lin took a totally new approach to Cu CMP. Instead of using conventional abrasive particles such as alumina or silica, they evaluated various sizes of diamond particles, including polycrystalline and monocrystalline diamond, for their potential application in copper CMP. The idea of using diamond in CMP slurries comes from the fact that diamond slurries are being successfully used in other electronics applications where conventional abrasives fail. It is believed that diamond's unique ability to disperse in aqueous suspensions in a broad range of chemistries coupled with very uniform size and shape control, give excellent material removal and surface finish results. Because of these features, diamond solids concentrations can be lower than conventional abrasives and achieve similar or better results with much lower residual particulate contamination. Professor Li's lab has demonstrated that the slurries made with diamond particles could be economically competitive. Using a carefully selected particle concentration and matching chemistry, excellent surface quality can be obtained on copper/tantalum/oxide surfaces. In the presence of balanced chemistry, diamond slurries have yielded excellent results in terms of material removal rate, WIWNU, surface quality, dishing, and erosion with blanket/pattern wafers. comparable to specialty alumina and silica slurries. The results were presented for the first time at the Fifth International CMP Symposium held in Lake Placid during the month of August

Point of Use Filtration System for Copper CMP Slurry

The abrasive particles in slurries used for Chemical Mechanical Polishing (CMP) typically have a well-defined particle size distribution. Within the distribution there are some large (> 1mm) particles and agglomerations due to manufacturing processes, usage systems, and transportation or storage conditions. These oversized entities affect the surface defectivity of the wafer during the CMP process. Professor Yuzhuo Li and his group are addressing this problem. One method for reducing oversized particles is Point of Use (POU) filtration. POU filtration is positioned next to or within a polisher to provide the last line of defense to remove defect-causing particles from the polishing slurry. Prior to POU filtration, several filtration options are often considered to protect the POU filter from premature clogging, optimize filtration performance, minimize total filtration costs, and maximize tool uptime. These filtration opportunities include a recirculation loop in the slurry delivery system, a post mixing/blending before transfer to the day tank, and transferring from shipping drum to the slurry delivery system.

Working with Dr. Michael Tseng at CUNO Incorporated, Professor Yuzhuo Li and his graduate student John Westbrook and undergraduate student Krista Manno have studied the effectiveness of a POU system for the filtration of Alumina slurries in copper CMP. More specifically, the direct impact of the filtration system on the copper material removal rate and surface quality has been measured. In addition, particle size distribution and over particle counting are also compared before and after filtration. They will use the filtration system to study the impact of an over sized particle on scratching in Copper CMP.

Polishing of High-Density Storage Hard Drive Discs

Professor Yuzhuo Li and collaborators are studying the factors that affect the smoothness of poilshed disks. The fast development of computer industries requires fast-running CPU as well as high-speed high-density storage media. The storage density reached 52.5 gigabits per square inch at the beginning of this year. The storage density is to reach 80-100 gigabits per square inch in two years. This requires the development of methods to prepare a very smooth substrate surface for dot packing layers of magnetic particles. The current method to obtain a smooth surface for a hard drive disc, which has an aluminum substrate and on top a layer of Ni-P before other thin layers of magnetic materials, involves a polisher, a pad, and a slurry (containing abrasive particles and chemicals). The basic principles are the same as CMP of IC chips and other microelectronic devices. The surface quality and smoothness of the Ni-P surface will control the finishing quality of the final layer of magnetic materials. Working with Mark Mayton, Dan Dunn and Ki Sohn at Ferro Microelectronics, Professor Yuzhuo Li and Research Assistant Professor Gary Yu have investigated the factors affecting the material removal rate, surface quality, and smoothness of the polished discs. They found that during the polishing of the discs, the chemistry of the slurries and the mechanical effect must be balanced to reach the best surface finish. As a result, they have developed a qualitative model for the development of future useful slurries.

Effect of Surfactants on CMP
With postdoctoral research associate Ashok Babel, Professor Raymond Mackay is examining the effect of surfactant additives to CMP polishing slurries based on the various outcomes of the polishing process. Work has primarily centered on the polishing of copper with alumina slurries. Results have shown that, with the proper choice of surfactant, the slurry can be stabilized over a wide pH range, polishing rates can be maintained or even increased, and surface finish significantly improved. Qualitative examinations also show that the presence of surfactant enhances particle removal (post-CMP clean). Current studies are focused on extending these results to the polishing of glass and other silica-based substrates using ceria slurry.