Sum Huan Ng

Influence of Colloidal Abrasive Size on Material Removal Rate and Surface Finish in SiO2 Chemical Mechanical Polishing

This paper addresses the influence of nano-scale abrasive particle size in the polishing of thermally-grown silicon dioxide on 100 mm diameter, p-type, (100), single crystal silicon wafers. The abrasive particles are incorporated in a chemical slurry, which is used in chemical mechanical polishing (CMP). Polishing (material removal) rate was measured with six (6) slurries, each with a different mean abrasive particle diameter of 10, 20, 50, 80, 110 and 140 nm. The experimental results indicate that the material removal rate (MRR) is related to the particle size. Results confirm that there exists an optimum abrasive size (80 nm) with respect to material removal rate and surface finish, for a given set of experimental conditions. The variation of the MRR vs. particle size (on a log-log plot) varies as d3.4. In addition, the surface polished with 10 nm and 20 nm abrasives was stained by the slurries. For a pad surface roughness of 5.2 μm (Ra), the slurry containing the 80 nm particles resulted in the highest material removal rate and best surface finish. The effects of polishing conditions and abrasive concentrations are also investigated in this paper. The nano-film polishing model that relates the particle size to the roughness of the pad can explain the polishing mechanism. Presented at the 56th Annual Meeting in Orlando, Florida May 20–24, 2001

Investigation of hot roller embossing for microfluidic devices

Microfluidics for most bio-related diagnostic applications typically requires single usage disposable chips to avoid bio-fouling and cross-contamination. Individual piece-wise manufacturing of polymeric microfluidic devices has been widely employed in recent years. To significantly lower the manufacturing costs, one possible way is to improve the production yield of polymer microfluidic chips via the hot roller embossing method. This paper discusses the effects of varying the process parameters such as roller temperature, applied pressure and substrate preheating during hot roller embossing (according to a systematic set of experiment designs) and its influence on the corresponding mold to pattern fidelity in terms of normalized embossed depths on the poly(methylmethacrylate) (PMMA) substrate. Concurrently, pattern density studies on the mold were also conducted. Functional testing in terms of fluid flow and micromixing was carried out to evaluate the feasibility of using hot roller embossed PMMA substrates as microfluidic chips.

A Mixed-Lubrication Approach to Predicting CMP Fluid Pressure Modeling and Experiments

Chemical mechanical polishing (CMP) is a manufacturing process used to remove or planarize metallic, dielectric, or barrier layers on silicon wafers. During polishing, a wafer is mounted face up on a fixture and pressed against a rotating polymeric pad that is flooded with slurry. The wafer also rotates relative to the pad. The combination of load on the wafer fixture, relative speed of rotation, slurry chemistry, and pad properties influences polishing rates. Prior work has shown that an asymmetrical subambient pressure, which exceeds that expected from the applied load, can develop at the interface between the fixture and a plane pad. The spatial distribution of this pressure can be measured and then simulated using a specially designed fixture with water as the slurry. A mixed-lubrication approach to modeling the fluid pressure was developed by including the contact stress, frictional behavior, and fluid film thickness. For a given fixture/pad separation, the contact stress can be determined using a Winkler model approximation. The film thickness can be approximated as the distance from the fixture surface to the mean asperity plane. Once the fluid film thickness is known, the fluid pressure can be determined from the two-dimensional polar Reynolds equation using finite-differencing. The theoretical pressure solution was found to match the experimental pressures when the system of forces and moments were balanced. The iterative secant numerical method was employed to compute the appropriate fluid film thickness that accommodates a balanced system of forces and moments produced by the fluid/solid interactions. After the fluid pressure is determined from an initially assumed separation, all shear and normal forces are computed from the solid contact stress and hydrodynamic fluid pressure. The results agree with the experiments.

Tilt and Interfacial Fluid Pressure Measurements of a Disk Sliding on a Polymeric Pad

Previous experimental work has shown that negative fluid pressure does develop at the disk/pad interface during chemical mechanical polishing. However, these studies dealt with one-dimensional measurement and modeling. To better understand the problem, two-dimensional pressure mapping is carried out. In addition, the orientation of the disk is measured with a capacitive sensing technique. Results reveal a large negative pressure region at the disk/pad interface that is skewed toward the leading edge of the disk. The disk is also found to be leaning down toward the leading edge and toward the center of the pad. A mixed-lubrication model based on the Reynolds equation and taking into account the disk orientation angles has been developed. Modeling and experimental results show similar trends, indicating the tilting of the disk as a dominant factor in causing the negative pressure phenomenon.

Fluid Pressures and Pad Topography in Chemical Mechanical Polishing

Measurements performed on a benchtop polishing tool using plain commercial polyurethane chemical mechanical polishing pads and an instrumented, nonrotating stainless steel polishing head show both negative (suction) and positive fluid pressures under different regions of the head. Observations of the head attitude also show a pronounced bank of the head toward the center of the pad. By carefully characterizing the pad topography and surface mechanical properties and by the application of this physical data in a load- and moment-balanced hydromechanical theory, we show that it is possible to accurately account for the location and magnitude of fluid pressure observations and for head tilt data. The results highlight the influence of the pad radial surface height profile on fluid and solid contact pressures on plain pads.

Wafer-Bending Measurements in CMP

Wafer curvature, contact pressure, and film stresses have been a subject of interest to many researchers who are working on the modeling of within-wafer nonuniformity in chemical mechanical polishing (CMP). Wafer shape and film stresses prior to and after CMP have been measured before in order to correlate film stresses with removal rate distribution across the wafer. In this paper we describe measurements of wafer bending under static loading and in dynamic conditions. A finite element analysis is also carried out to model wafer bending under static loading.

Pad Soaking Effect on Interfacial Fluid Pressure Measurements During CMP

Pad Soaking Effect on Interfacial Fluid Pressure Measurements During CMP Prior work has shown that there exist a sub-ambient fluid pressure at the interface between a rigid flat and the polishing pad during chemical mechanical polishing (CMP). This sub-ambient fluid pressure can have a significant impact on the polishing process since its magnitude may be similar to the applied load, depending on conditions. Further results have shown that there is a relationship between pad soaking time and the magnitude of this sub-ambient fluid pressure. This paper addresses measurements of the pad soaking time versus the magnitude of the sub-ambient interfacial fluid pressure. Experiments utilized a Rodel IC1000 polishing pad made of foamed polyurethane with average void size of 30 to 50 microns. Pad soaking tests indicated that the weight of the pad increased with soaking time due to water absorption. There is a high rate of water absorption initially before the pad becomes saturated and the mass of the pad stabilizes. It is also observed that the pad material is impermeable to water and most of the water penetrated only the topmost layer of voids in the material. These experiments suggest that the water progressively softens the top layers of the pad during the soaking and causes the sub-ambient fluid pressure to increase in magnitude. A model of the sub-ambient fluid pressure increasing as the elastic modulus of the pad decreases is also suggested.

An Analysis of Mixed Lubrication in Chemical Mechanical Polishing

Pressure and shear flow factors (Patir and Cheng, 1978) were used to take into account the roughness of the pad surface in the modeling of the interfacial fluid pressure during chemical mechanical polishing. An attempt was made to explain the physical meaning of the flow factors in this particular application. Additionally, a parametric study was carried out to see the effect on the model after the incorporation of the flow factors. The pressure and shear flow factors were found to have a competing effect on the magnitude of the sub-ambient fluid pressure.

Effects of Nano-scale Colloidal Abrasive Particle Size on SiO 2 by Chemical Mechanical Polishing

This paper reports on the effect of colloidal abrasive particle size in the polishing of thermally grown silicon dioxide on 100mm diameter, P-type, (100), single crystal silicon wafers. The abrasive particle sizes were varied in six (6) slurries with pH values of 10.97 ± 0.08. The abrasive sizes were 10, 20, 50, 80, 110 and 140nm in diameter, and the slurry contained 30 weight percent abrasives. The experimental results indicate that the material removal rate (MRR) varies with the volume of the particle size. Results also confirm that there exists an optimum abrasive particle size with respect to material removal rate and surface finish. For a pad surface roughness of 5.2μm (Ra), the slurry containing 80nm particles resulted in the highest material removal rate and best surface finish. A nano-film model based on the pad roughness is used to explain the results.

Adhesive bonding of polymeric microfluidic devices

This paper investigates the use of adhesive bonding on polymeric microfluidic devices. Both pressure sensitive and UV curable adhesives have been studied. Characterization tests conducted include flow, pressure and 180° peel tests. All the five adhesives are able to pass the flow and pressure tests. Pressure sensitive adhesives generally perform better than UV curable adhesives in the peel tests due to the different substrates used. With careful application of the adhesives, UV curable adhesives show less sagging into the microfluidic channels as compared to pressure sensitive adhesives.