Len Borucki

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.

Investigating Effect of Conditioner Aggressiveness on Removal Rate during Interlayer Dielectric Chemical Mechanical Planarization through Confocal Microscopy and Dual Emission Ultraviolet-Enhanced Fluorescence Imaging

The effect of conditioner aggressiveness is investigated in interlayer dielectric polishing on three types of pad. A method using confocal microscopy is used to analyze the effect of conditioner aggressiveness on pad–wafer contact. Results show that a more aggressive conditioner produces a higher interlayer dielectric polishing rate while at the same time a pad surface with fewer contacting summits and less contact area. It is found that the ratio of the contacting summit density to the contact area fraction is more important than either parameter measured separately since the ratio determines the mean real contact pressure. Modeling results based on contact area measurements agree well with experimental results. Moreover, it is found that a more aggressive disc also generates a thicker slurry film at the pad–wafer interface. This is in agreement with our general findings regarding pad asperity height distribution obtained using confocal microscopy.

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.

Effects of Disk Design and Kinematics of Conditioners on Process Hydrodynamics during Copper CMP

This study focuses on determining the effect of conditioner disk design, kinematics, and pressure on the slurry distribution under the wafer as measured by the slurry film thickness between the wafer and the pad during actual polishing. Film thicknesses are measured using dual emission UV-enhanced fluorescence, which for thickness measurement requires the slurry to be tagged with two different fluorescent dyes. Results indicate that the wafer is tilted toward the center of the pad and that the extent of wafer tilt is a strong function of conditioning disk pressure. Increasing the oscillation frequency of the conditioner disk or the rotation rate decreases the slurry film thickness and the film thickness increases with slurry flow rate.

Analysis of Frictional Heating of Grooved and Flat CMP Polishing Pads

A detailed model is described for the generation, transport, and exchange of thermal energy in rotary chemical mechanical polishing (CMP) tools. Frictional energy generated due to abrasion of the wafer by the pad and slurry particles is partitioned between the pad and the rotating wafer, with the majority going to the latter. The slurry at the same time provides a major cooling mechanism, drawing heat away while it is entrained under the wafer and then redistributing it over the pad by radial convection. The slurry flow component of the theory includes thermal effects due the presence of concentric grooves. The model accounts for the time and radial dependence of pad temperature measurements performed on flat and concentrically grooved pads at the leading and trailing edges of the wafer, at the pad center and pad margins, and on the wafer carrier. It suggests that the temperature increase on the wafer may be approximately twice the increase measured on the pad.

Physics of the Coefficient of Friction in CMP

The implications of a theory of lubricated pad asperity wafer contact are traced through several fundamental areas of chemical-mechanical polishing. The hypothesized existence of a nanolubrication layer underlies a high accuracy model of polish rates. It also provides a quantitative explanation of a power law relationship between the coefficient of friction and a measure of pad surface flattening. The theory may further be useful for interpreting friction changes during polishing, and may explain why the coefficient of friction is sometimes observed to have a temperature or velocity dependence.

Novel Slurry Injection System for Improved Slurry Flow and Reduced Defects in CMP

In commercial CMP tools, slurry is applied near the pad center. As the pad rotates, more than 95% of the fresh slurry flows directly off the surface due to bow wave formation and inertial forces without ever entering the pad-wafer interface, resulting in low slurry utilization [1]. Furthermore, some slurry that manages to go under the wafer stays on the pad, mixes with fresh slurry and re-enters the pad-wafer interface. This used slurry contains reaction products, foam and pad debris (due to pad conditioning) that cause wafer-level defects [2]. Such defect-causing by-products keep recirculating on the pad during polishing and accumulate near the retaining ring over time. Also, since large amounts of DI water are used between wafer polishes to rinse off the debris and reaction products, appreciable amounts of water may stay on the pad and inside the grooves. When fresh slurry is introduced to polish the next wafer, it mixes with the residual water and is diluted, resulting in lower material removal. As such, the current slurry application method does not provide efficient slurry utilization and leaves significant room for improving defect levels. Moreover, the constant sweeping of the conditioner arm during in-situ conditioning results in uneven slurry distribution and introduces additional challenges when it comes to carrier multi-zone pressure control for reduced within-wafer removal rate non-uniformity.

Thermal, Tribological, and Removal Rate Characteristics of Pad Conditioning in Copper CMP

High Pressure Micro Jet (HPMJ) pad conditioning system was investigated as an alternative to diamond disc conditioning in copper CMP. A series of comparative 50-wafer marathon runs were conducted at constant wafer pressure and sliding velocity using Rohm & Haas IC1000 and Asahi-Kasei EMD Corporation (UNIPAD) concentrically grooved pads under ex-situ diamond conditioning or HPMJ conditioning. SEM images indicated that fibrous surface was restored using UNIPAD pads under both diamond and HPMJ conditioning. With IC1000 pads, asperities on the surface were significantly collapsed. This was believed to be due to differences in pad wear rates for the two conditioning methods. COF and removal rate were stable from wafer to wafer using both diamond and HPMJ conditioning when UNIPAD pads were used. Also, HPMJ conditioning showed higher COF and removal rate when compared to diamond conditioning for UNIPAD. On the other hand, COF and removal rates for IC1000 pads decreased significantly under HPMJ conditioning. Regardless of pad conditioning method adopted and the type of pad used, linear correlation was observed between temperature and COF, and removal rate and COF.