Optimum process performance through better CMP slurry management

08/01/2003

By Budge Johl, Rodel Inc., Phoenix, Arizona
Rakesh K. Singh, BOC Edwards, Santa Clara, California

Overview

Within wafer fabs where processing is done largely through the manipulation of ions and photons, chemical mechanical planarization and its slurry seem somewhat industrial. Yet through the meticulous class of engineering that the IC industry is known for, this process fits well in today's advanced processing. But success is only achieved by carefully managing all the potentially troublesome facets of slurry handling, mixing, and distribution.

Chemical mechanical planarization (CMP) is one of the most complex and expensive steps in IC manufacturing, with a cost of ownership (COO) ranging from 60–80% for CMP consumables [1–3]. CMP defects affect manufacturability and reliability. Effective CMP slurry management is essential to meet interconnect challenges on the International Technology Roadmap for Semiconductors; improper slurry handling can adversely affect slurry health and device yield, and drive up process COO [4, 5].

CMP slurries are primarily aqueous-based complex suspensions containing silica, alumina, or ceria abrasive particles, and chemical additives such as oxidizers, polymers, pH stabilizers, dispersants, and surfactants. During use, slurries go through numerous handling steps from the slurry supply pail, drum, or tote to wafers, including mixing and distribution, humidification, pressure control in the global loop, and filtration [6]. CMP slurry characteristics ("slurry health") can significantly change during shipping, handling, blending, distribution, and filtration. Extensive slurry recirculation without use or replenishment may lead to agglomeration or large-particle formation and an increase in wafer defects [7, 8]. Therefore, measurement of large particles and reliable online slurry analysis are essential for achieving efficient CMP processing.

There have been many studies related to CMP slurry characterization to quantify the effects of extensive handling on slurry metrology parameters and colloidal stability [9–14], but only a few studies have focused on the performance of different blending and distribution methods based on pumps, vacuum-pressure dispense, cycling dispense valves, or load cell technology. Only a few have highlighted the advantages, limitations, and effects of these approaches on slurries containing different abrasive particles and chemistries [7, 8, 15, 16].

Slurry receiving and handling

Most CMP slurries are sensitive to extreme temperatures. For incoming slurry lots, one must ensure that the slurry container has not been exposed to low temperatures (e.g., ≤10°C). The minimum allowable temperature range for different slurries may vary. Even short exposure to freezing temperatures can cause irreversible agglomerations and gelling in some slurries. Any exposure can be easily verified by checking the freeze indicator or recorders shipped on slurry containers. Similarly, slurry exposure to high temperatures (e.g., ≥~45°C) must be avoided for consistent polishing performance.

Figure 1. A typical slurry-blending and distribution system. (Source: BOC Edwards)Click here to enlarge image

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Slurry container cleanliness must be ensured before opening the cap. Depending on dock time and long-term storage, slurry containers may have accumulated dust and other contaminants. The goal is to avoid the contamination of the slurry with these foreign particles while opening the container. Threads on the bunghole and cap should be carefully cleaned before closing partial pails or drums. These areas can have large particles of dry slurry with potential to contaminate the slurry in the container and cause defects during polishing.

Before new slurry containers are connected to a blending and distribution system, one must ensure that the system has been thoroughly cleaned and that all residual slurry particles have been removed. One component often overlooked is the drum dip tube. These tubes may have slurry dry-out in them if left in empty containers over time and not thoroughly cleaned from past use, and can contaminate the new slurry. Many fabs now keep backup, clean, ready-to-use dip tubes to meet this requirement without losing time to get a new slurry drum or tote online.

Slurry abrasive dispersion

Uniform dispersion of abrasive particles in slurry is essential for achieving consistent CMP performance. Even the most colloidally stable slurries may settle, over extended periods, from gravity. To ensure homogeneity of abrasive particles in slurry drums, most slurries are mixed with electric or air motor-driven stirrers or recirculated for 1–2 turnovers using a bellows or diaphragm pump. Mixing times vary depending on stirrer configuration, rotational speed, and mixing cycle. For initial abrasive dispersion or redispersion, after slurry has been left undisturbed over a significant period, and to maintain dispersion during use, the requirements of slurries can be vastly different depending on the settling characteristics of particles and slurry chemistries [14, 17].

Blending technologies

CMP slurries may be single-component (ready to use) or have two to three components that need to be blended with DI water or other additives. Slurries can be blended using a centralized blending and distribution system or a point-of-use (POU) blender at a CMP tool [18–21]. Both systems have their advantages and limitations. Metering pump-based systems may be used for batch blending of slurries and chemicals with high accuracy. These systems rely on high accuracy and repeatability of the stroke volume of bellows or other pumps and may be equipped with dynamic slurry metrology feedback mechanisms to monitor and control the slurry blend ratio.

Vacuum-pressure dispense systems work on the principle of volumetric blending (Fig. 1). In such systems, vacuum pulls a precise amount of chemicals from the slurry supply containers into the blend vessels using liquid level sensors. The chemical is subsequently transferred using high-pressure humidified nitrogen and dispensed to a slurry blend storage tank ("daytank"). Homogenization of the blend takes place in this container using appropriate stirrers. The dispense module of these systems uses vacuum to pull the chemical blend from the daytank into the dispense vessels and humidified nitrogen to transfer it to the global distribution loop for tool use [7]. The excess amount of blend ("return flow") is collected in the daytank and recirculated. A fresh slurry blend is periodically created by the system, depending on the level of blend in the daytank and slurry use.

Slurry distribution systems

CMP slurry blend recirculation in the global distribution loop can be accomplished using a pressure dispense approach, where high-pressure humidified nitrogen (≥90% RH) is used to dispense slurry in the loop [15]. Alternatively, slurry can be pumped in the global loop using a bellows or diaphragm pump. The latter approach requires more rigorous monitoring of large-particle agglomerates, which may develop as a result of repeated shearing due to pump handling, especially in shear-sensitive silica slurries.

During slurry distribution, it is important to control global loop flow and pressure. Less fluctuation in distribution loop pressure is the key to consistent slurry supply at the tool and uniform CMP processing. The distribution loop pressure can be controlled using various methods employing pressure reducers, small diameter tubing, pinch valves, etc. It is also possible to dynamically control pressure in the global loop using a feedback control PLC system [19].

As discussed earlier for slurry supply containers, to maintain slurry homogeneity and abrasive particle suspension, the slurry blend should be appropriately stirred in the daytank. Based on settling characteristics of slurries [14], stirring requirements for different slurry blends may vary from intermittent slow-speed mixing to high-speed continuous stirring with a large mixing element close to the bottom of the daytank.

Slurry humidification

Many slurry delivery systems and storage tanks use nitrogen to either blanket the top of the slurry or to apply pressure for dispensing slurry in a distribution system. Dry nitrogen has been found to remove moisture from the slurry, creating a ring of dry slurry particles in the dispense vessel, and to increase large-particle counts over time [15].

Humidified nitrogen is also used to blanket the slurry or slurry blend contained in the supply container and daytank. This eliminates the formation of a ring of dry slurry particles. It is recommended to keep nitrogen humidification in the range of 90–95% RH humidity for optimum performance.

Effect of extensive handling

During slurry use, slurry recirculates in supply containers, blend and dispense modules of the slurry delivery system, and the global distribution loop. Slurry systems typically recirculate slurry from a daytank or dispense module vessel to polishers, with the unused slurry returning to the daytank. Recirculation may cause shear to the slurry due to sudden changes in cross-sectional area and direction of the flow path, and pressure fluctuations in pumps, valves, and fittings causing premature aging of the slurry. Different slurries have different shear sensitivities, depending on their formulation. Particle shape, zeta potential, and particle concentration are key parameters. Fumed silica slurries seem to be most sensitive to shear and usually show varying degrees of shear agglomeration with all pumps. Solution-grown or colloidal silica slurries show much less shear sensitivity and may not show much shear agglomeration in pumps. Typical alumina and ceria abrasive slurries have even less sensitivity to shearing conditions.

Figure 2 shows large-particle-count results after an oxide-CMP-slurry accelerated aging and handling evaluation, where the slurry underwent 5000 turnovers in seven days [7]. Special attention was given to large-particle distribution because an increase in large particles can cause an increase in defects on a wafer during CMP. While most of the slurry health parameters showed no change, there were significant differences among the various dispense engines in the generation of large particles. In this test, an increase in large-particle counts could be detected as early as after one hour of handling in some dispense engines.

Figure 2. Large-particle distribution for dispense engines after 5000 turnovers [7].Click here to enlarge image

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In another study involving a different bellows pump, minimal changes in large-particle counts were detected after 1000 turnovers. Therefore, different pumps can have very different effects on slurry large-particle generation and other slurry health measurement parameters. The slurry-shearing characteristics of a pump can be a complex function of pump internal geometry and flow path, pump speed, and flow rate. The effects of extensive pump handling on different slurries have been presented earlier [8]. Vacuum-pressure dispense systems have been found to have less effect on slurry large-particle characteristics as a result of handling, especially in shear-sensitive slurries [7, 10, 11].

Slurry filtration

The main objective of filtration is to remove large particles and agglomerates and any other contaminants that may have been introduced from improper slurry handling or distribution systems that were not thoroughly cleaned. The goal is to remove those particles that may be defect-causing without changing slurry performance. Filtration of the slurry can be done at various locations, such as intake filtration of source containers, post-dilution filtration, global distribution loop or recirculation filtration, and POU filtration.

In general, CMP slurry filtration may reduce defects and improve process consistency [9, 22, 23]. Higher retention by a POU filter provides better defect reduction, depending on slurry, equipment, and process parameters. Filter lifetime is highly dependent on the slurry's abrasive characteristics distribution and filtration process, and can be monitored in most distribution loops using pressure drop across the filter.

Slurry metrology

Monitoring slurry health parameters is essential to maintaining a consistent and efficient CMP process [24–27]. Typical blend ratio monitoring and control parameters include pH, conductivity, total wt % solids, density or specific gravity, refractive index, and oxidizer level or component assay [28–30]. In addition, oxidation-reduction potential, viscosity, mean particle-size distribution, large-particle distribution or counts, and zeta potential can also be monitored to check for changes in slurry health.

Achieving efficient slurry management

CMP slurry management plays a critical role in uniform and repeatable local and global planarization of ICs. Slurry-handling and distribution systems must be capable of providing an uninterrupted supply of good-quality slurry over extended periods of time. Efficient slurry management can be achieved using appropriate slurry abrasive dispersion and homogenization, blending, distribution, storage tank agitation, humidification, global loop pressure and flow control, slurry metrology, and filtration. Finally, it is essential to pay attention to details during slurry receiving, handling, blending, and distribution to avoid slurry contamination, shear, and agglomeration, in order to achieve optimum performance in your CMP process.

Acknowledgments

We thank Mike Oliver, Mike Kulus, Ben Roberts, and Virginia Becker for reviewing our original manuscript. BOC Edwards is the trade name used by affiliate companies of the BOC Group plc.

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Budge Johl received his BA in biological sciences with an additional emphasis in chemistry from Chico State University. He is a semiconductor integration engineer managing the Analytical Slurry Characterization and Handling Labs at Rodel Inc., 3804 E. Watkins St., Phoenix, AZ 85034; ph 602/445-4493, fax 602/431-0200, [email protected].

Rakesh K. Singh received his BE in mechanical engineering, ME in design of process machines, an advanced diploma in management, and a PhD in fluid dynamics. He is manager of the R&D laboratory for the Chemical Management Division of BOC Edwards.