Removing Metal Ions from Photoresist Solvents

Laura Peters, Senior Editor -- Semiconductor International, 7/1/1999

New filter and purifier technology improves the removal of metal ion contamination, such as sodium, iron and chromium, from photoresist solvents. The filter uses a chemically modified polyethylene porous membrane with ion exchange groups in the media to rapidly remove metal contam inants without the long residence time needed using standard methods. Pall Corporation (Port Washington, N.Y.) specifically designed the product to filter ionic contaminants from organic or mixed solvents used in resist manufacturing. At the recent SPIE Microlithography Conference, Dr. Dennis Capitanio of Pall Corp., Yoshiki Mizuno of Nihon Pall Ltd. (Tsukuba, Japan) and Dr. Joseph Lee of Pall RAI Corp. (Hauppauge, N.Y.) reported reduction of metal ion contamination from several hundred ppb to single-digit ppb levels in four solvents: ethyl lactate, ethyl 3-ethoxy propionate, propylene glycol monomethyl ether acetate and 2-heptanone. The SIA Roadmap calls for photoresist metal ion contamination levels below 5 ppb for #0.25 µm device manufacturing.

Metal ions pose an increasing contamination problem for the layer being fabricated as well as neighboring regions because resist ashing can cause mobile specie to diffuse, impacting electrical integrity and yield. In this study, purification filter capsules demonstrated the ability to reduce metal ion concentration from several hundred ppb to single-digit ppb in a single pass (Table).

The capsules contained 47 mm diameter membrane disks with PFA supports and shell. Solvents were pumped from a high-density polyethylene reservoir through PFA tubing into the filter capsule. An ICP-MS (inductively coupled plasma mass spectroscopy) instrument was used to measure ionic levels. Next, the ethyl lactate solvent was spiked with 2.61 ppm of sodium and 1.9 ppm of iron to determine the point of breakthrough for the purification filter capsule. The filter demonstrated high removal efficiency (>95%) on Na, Fe, Cr and Zn metal contaminants up to the breakthrough point (Figure). Results indicate a 1 milligram capacity for the 47 mm membrane disk, allowing users to scale up in production.

Fig. 1. The filter reduced ppm levels of metal ion contamination to single-digit ppb levels in a single pass.

For the complete paper presented at the SPIE Microlithography Conference, please visit Pall Corporation's website at www.pall.com/photoresist.