Using ultrapure steam for water-contamination control
03/01/2008
EXECUTIVE OVERVIEW
Multiple semiconductor and related processes-including diffusion, RTP, ALD, etch, and DUV lithography-currently use water vapor as part of manufacturing. Steam generated from de-ionized water is an excellent cleaning agent, but its key benefits have been historically limited, since it has not been practical either to generate ultrapure steam and/or keep it from entraining contaminants before it reaches the wafer to be cleaned. Data is presented that shows that ultrapure steam can be generated from a DI water source that is heavily contaminated with metals. By use of this new steam delivery and purification process, steam can be released for wafer cleaning from a water source that would be considered too dirty to use for wet cleaning.
As device sizes continue to shrink, the advantages of using a cleaning gas over liquid water grow more significant. They include the ability to rapidly move into and out of high aspect ratio nanometer structures, ease of chemical modification of the cleaning solution, solvation of contaminants, the thousand-fold reduction in chemical usage by moving from a liquid to a gas, and ease of drying.
Steam generated from de-ionized water is an excellent cleaning agent. It is extremely aggressive at absorbing ionic and hydrocarbon contaminants, can deliver large amounts of energy to the contact surface, and the waste product is water. These key benefits have been historically limited, since it has not been practical either to generate ultrapure steam and/or keep it from entraining contaminants before it reaches the wafer to be cleaned.
Steam cleaning of wafers would be ideal, since vapor phase delivery allows rapid penetration into high aspect ratio structures. It can easily be modified to follow existing wafer cleaning recipes. Steam is extremely aggressive at removing molecular contaminants from the wafer surface. In regard to particle filtration, liquid water can only be filtered to 0.1µm while gas can be filtered to 0.001µm. The contaminant carrying capacity of water is very large, being able to either dissolve or physically remove particulates from the condensed gas stream.
From a raw materials standpoint, the use of ultrapure steam as a replacement for a dip and dunk water process for cleaning could reduce water needs from liters per wafer to grams per wafer (1ml of water equals 1250sccm of water vapor). Existing wafer drying processes could easily be adapted to this cleaning technique and, finally, the by-products of the process could be easily handled by existing fab-wide waste management systems.
Serious constraints still prevent the adoption of steam cleaning of water as a replacement for single wafer spray cleaning-the key being the ability to control the delivery and purity of the steam. The ability of DI water to rapidly pick up metals becomes more pronounced as temperatures increase. RASIRC has previously demonstrated the ability to generate and control the delivery of 100% steam at greater than atmospheric pressures and flow up to 60slm. Purification results have shown the ability to deliver sub-10ppt for total metals and 1000× reduction in urea from DI water [1, 2].
Experimental
Initially, 1001.02 grams (g) of water was mixed with 10.12g of a trace metals calibration standard solution. This solution is a 5% nitric solution containing 100 parts-per-million by weight (ppmw) of each of the following metals: aluminum, antimony, arsenic, barium, beryllium, cadmium, calcium, chromium, cobalt, copper, lead, magnesium, manganese, molybdenum, nickel, potassium, selenium, silver, sodium, strontium, thallium, tin, titanium, vanadium, and zinc. Also added to the solution was 1.31g of a 5% nitric solution containing 1000ppmw of lithium. The final solution was used to present a 1ppmw challenge of each metal contaminant. The solution was then loaded into a RASIRC Steamer UHP 200.
 Metal removal test manifold. |
The figure is a schematic of the manifold used for this experiment. The steamer was set to a pressure of 140Torr above atmospheric pressure. The temperature of the process lines was maintained between 110°C and 115°C to prevent condensation of the steam. The pre- and post-purified steam streams were sent to separate condensers. The output from the two condensers was trapped in the collection bottles downstream. The shell water temperature was maintained at 5°C with a chiller.
Before collecting the samples, the test manifold was conditioned for 5 minutes. During this time, both bypass lines were open. After the conditioning step, the bypass valves were closed and the valves upstream of the condensers were opened. After trapping over 20g of water in each of the collection bottles, the system was turned off. The amount of source steam and purified steam collected was 32.22g and 21.75g, respectively. Twenty-three grams of the source water challenge were removed from the steamer. Metal concentrations in the three samples were measured with inductively coupled plasma mass spectroscopy (ICP-MS).
Results
The table shows the metal concentrations that were measured in the source water, source steam and purified steam. Values highlighted in yellow are those metals above the lower detection limit in the source steam and purified steam. There was a 99.9% reduction in metals from the source water to the purifier steam. There was a 78% reduction in metals when purifying the steam. Calcium was found to be the most penetrating ion with a carry through into the source steam of 6.3%; 1.3% of the Ca or 10ppt out of 782ppt was found in the purified steam. There was a higher concentration of chromium found in the purified steam. We believe this may have leached from the Teflon housing. Further testing will be conducted to address this issue.
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Conclusion
Twenty-six different metals were added to the challenge solution to generate a 20,000ppb metallic challenge. Conversion of water to purified steam reduced total metals by 99.9%. Calcium was found to be the most penetrating ion with a carry through in the source steam of 6.3% and 1.3% of the challenge in purified steam.
References
- Jeffrey Spiegelman, Russell Holmes, “Urea and Ammonia Removal from De-ionized Water via Steam Purification,” SPWCC 2007.
- Jeffrey Spiegelman, Russell Holmes, “Alternative Method and Device to Purify and Deliver Water Vapor,” SPWCC 2006.
Acknowledgment
Teflon is a registered trademark of DuPont.
Jeffrey Spiegelman received his BS and MS degrees from the University of California at San Diego and is the president and founder of RASIRC, 11760 Sorrento Valley Road, Suite E, San Diego, CA 92121 United States; ph 858/259-1220, e-mail [email protected].