Membrane Purifies Wet Etch and Clean Processes
Maria A. Lester, Associate Editor -- Semiconductor International, 5/1/2003
Front-end-of-line (FEOL) and back-end-of-line (BEOL) wet chemical cleaning processes are critical to the fabrication of semiconductor devices. As linewidths continue to shrink, maximizing the purity of chemicals used in wet cleans is a prerequisite for maintaining and improving chip yield. The incoming and in-process purity of chemicals used in wet cleaning greatly impacts the surface contamination of wafers. While incoming ultrapure chemicals have reduced ion levels, exposure to metallics has persisted during wet processing.
One way of effectively minimizing ionic contamination is to continuously purify the chemicals in use. In this approach, the metallic ions are removed by binding them to the complexing agents immobilized on membrane filters that can be plumbed in-line in the wet tools.
During the manufacture of microelectronic devices, silicon wafers are exposed to high-purity water more frequently than any other liquid chemicals, and can require >1100 gal of ultrapure rinse water to process a 200 mm wafer. However, it is difficult to maintain high purity below 1 ppt out of the central system during the distribution to points of use for wafer surface cleaning. The process of distributing high-purity DI water often introduces particles and trace ionic contamination that can leach out from the plumbing components and process equipment. The ITRS 2001 guidelines call for a total metal contamination level on the wafer surface of <7×109 atoms/cm2 for 130 nm devices.
Some metal ions with higher electrochemical potential than silicon — especially copper or silver — can plate out on the wafer surface even under extremely low contamination levels. Mykrolis Corp. (Billerica, Mass.) has developed Protego, a point-of-use dissolved ion purifier/filter that removes low-level metal contamination from ultrapure water, organic solvents and other chemicals used in the microelectronics industry. The in-line purifiers are made of high-binding-constant membranes (containing sulfonic acid or chelating groups) that remove ions continuously from process chemicals either in a once-through or recirculation flow mode. They offer consistently low ionic levels in the bath, extended bath life, and additional advantages over other purifiers. Methods of maintaining ionic purity include spiking the baths with chelators, intermittent off-line purification or periodic replacement with new batches of ultrapure chemicals.
The purifier/filter devices, assembled as 10 in. cartridges and disposable capsules, use an "ion exchange membrane" in a pleated form. The ion exchange membrane is prepared using the radiation-induced grafting polymerization method, which introduces ion exchange groups directly and covalently onto the surface of a microporous membrane.
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| 1. Ion removal experiment on membrane shows copper removal efficiency. (Source: Mykrolis, printed by permission of SPWCC)
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In-house challenge tests were conducted to determine ion removal efficiency (single and multiple) and capacity. Experimental results at single pass demonstrated a higher removal rate for copper (Fig. 1) and several critical metals (Fig. 2). The ion removal experiments on membrane coupons showed up to 99% metal removal. The results also showed that ion removal capability is retained fairly well at high flow rates. Results of the Protego device field evaluation indicated that, overall, the filter removed most metals to 1 ppt or less.
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| 2. Experimental results demonstrated up to 99% metal ion removal. (Source: Mykrolis, printed by permission of SPWCC)
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Purification and filtration technologies have been developed to achieve the stringent purity levels required of liquid chemicals in semiconductor manufacturing. Although shrinking geometries continue to drive this innovation, other process-related needs that will provide the impetus for purification are:
- Use of dilute chemistry requiring more stringent ionic specs to prevent metal deposition on wafers.
- More focus on waste DI reclaim/recycle applications for future lithography resource conservation.
- To decrease fab ultrapure water use from 6-8 gal/in.2 in 2001 to 4-6 gal/in.2 in 2005, and 3-5 gal/in.2 beyond 2008.
For additional information on clean processing, go to www.semiconductor.net/clean
