| Liquid Flow-Through Core Post-CMP Brush Scrubber Designs and the Influence of PVA Brush Pore Structure |
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VMIC - September 6-10, 1999 Kristan G. Bahten, Dan McMullen, Yassin Mehmandoust Rippey
Corporation 2Oliver
Design Executive Summary PVA brush scrubbing technology is the most common form of cleaning in post-CMP applications1. Although there have been numerous papers published on the science of brush scrubbing, few have addressed the brush itself. During semiconductor manufacturing, the PVA brush is one of the few solid objects, which comes into direct contact with the face of the wafer. Cleaning performance during brush scrubbing is not only dependent on the chemistries used and the tool design, but also on the physical properties of the brush material2. Data will be presented to demonstrate the performance differences and advantages of flow-through technology, flow dynamics of flow-through cores and the benefits of disposable brush sleeves. Also discussed will be how those improvements will potentially impact the economics of manufacturing. Extended Abstract The current technology for the introduction of cleaning solutions, in post-CMP brush scrubbing operations in the semiconductor and silicon manufacturing industries, is the use of flow-through brush mandrel (brush core) techniques. In this approach, a hollow core with perforations forms the brush support. DI water or cleaning chemistries are delivered onto the PVA brush through the core. The solution then flows from the inside, through the brush, and is evenly distributed to the surface being cleaned. Not only will this provide a better distribution of the cleaning solution, but also improves performance and increases brush life. Liquid flow through the brush helps to reduce particle build up (brush loading) on the brush, thereby extending the useful life-time of the brush and reducing the cost of ownership for the scrubbing process. This paper presents liquid flow dynamics data and advantages between flow-through technology compared to spray or drip techniques, and the influence of material structure of the PVA brush material. Some of the particle and brush flow data generated for this paper was obtained on disk media utilizing an Oliver Design disk scrubber. Pore Structure The manufacturing techniques used for pore formation in the PVA sponge material can have a significant effect on the flow performance and characteristics. Common methods for sponge manufacturing are based on gas formation or gas injection (foaming technique). The Rippey/Kanebo PVA sponge material uses a unique technique of reactive phase separation for the formation of pores. This method consists of adding a pore-forming agent to the liquid polymer mixture. As the polymer is converted to its non-soluble state, the pore-forming agent assist is the stabilization of the developing insoluble polymer network and contributes to exclusion volume of the pores. The resulting differences in pore structure/distribution are shown below (Figure 1). Figure 1. SEM’s of Rippey Microclean vs. Air Formed Product Internal Pore Structure
As can be seen from the above photographs of the internal pore structure, the reactive phase separation techniques produce a highly consistent and open cell structure. The advantages of this method are tighter control of porosity, higher consistency and a much more open cell structure. The uniform and open architecture will result in higher flow and lower backpressure with a uniform flux over the surface. Air or gas formation (foaming) produces a higher concentration of closed cell pores and a less consistent pore distribution due to the difficulty in controlling the distribution of air bubbles. Core Design and Flow-through The unique pore structure created by reactive phase separation is most beneficial when using flow-through techniques. Flow-through brush cores can offer distinct advantages over solid core designs. With liquid or cleaning solutions flowing through from the inside, the brush is continually rinsed from the inside out. This technique will substantially reduce the effect of brush loading as demonstrated in Figure 2. This experiment was conducted on an Oliver Design dual rail disk scrubber equipped with a new flow-through core. The brushes were dipped into diamond slurry prior to scrubbing polished disk media. The experiment was conducted both with and without flow-through. The DI water flow to the cores was set at 0.95 gpm for a set of four brushes. This experiment measured the number of runs before the slurry was removed and particle adders returned to baseline. Flow performance Some preliminary experiments have been conducted on the flow characteristics of the PVA brush when used in conjunction with the flow-through core. The total flow through the core
was measured at varying rotational velocities (0 – 1000 rpm) for 15 seconds, both with and without brushes installed. It should be noted that some of the rpms are much higher than commonly used in post-CMP cleaning. The results are detailed in Figure 3. The initial line pressure feeding the cores was 25psi with a flow of 0.95 gpm. With no rotation, the total flow measured was the approximately the same for cores with and without brushes. This would indicate that at relatively low flow (a flux of 0.578 ml/cm2 • sec) the brush offers little resistance. What was not expected from this experiment was a drop in flow with brushes installed as the rotational velocity increased. As with the core itself, we had expected an increase in flow from the added centrifugal force. This is only preliminary data, but it appears to indicate that the brush has higher backpressure as the rotational velocity increases. This could be caused by several possible factors, which would need further investigation:
The brush takes only about two seconds to reach full speed after which the tangential force would terminate. Turbulent flow is unlikely due to the low flux rate and small pore size. Work is in progress to gain a better understanding of the factors involved and their influence. References
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