In silicon-based IC fabrication, the process of physical vapor deposition (PVD) "fires" electrically accelerated gas ions through a high vacuum at an aluminum target to chip off aluminum ions, forming a 0.5 micron conductive aluminum film on the wafer. A sustained high vacuum is critical to maintain purity and facilitate a clear path for the aluminum ions during this "sputtering" process. The process requires a vacuum of 5.0 ´ 10-7 torr, less than one billionth of an atmosphere.
The challenge (back to top)
Vacuum pump failures threatened both production and product quality at a major Silicon Valley manufacturer of semiconductor equipment. The high vacuum pumps, valued at $50,000 each, required water cooling through a ½ inch cooling coil. Narrow passages and high heat fluxes caused corrosion products and microbiological slime from sulfate-reducing bacteria (SRB) to deposit in critical flow areas, reducing heat transfer and cooling water flow. These conditions subsequently caused vacuum pumps to overheat and fail. The tool manufacturer used a "hot list" to track the condition of troubled vacuum pumps. At the inception of the BetzDearborn program, the list included 30 pumps in danger of burnout due to overheating.
The tool manufacturer was faced with 128 ppm (mg/L) iron levels, SRB fouling, and the need for frequent and risky acid cleanings to restore cooling water flow.
Solution (back to top)
Working with BetzDearborn, the customer conducted a failure analysis to identify and document the causes and effects of the various water quality problems that led to vacuum pump failure. The analysis identified microbiological fouling and mild steel corrosion as a result of low inhibitor levels. Large amounts of a nitrite-based corrosion inhibitor were being added to the closed cooling loop to compensate for bio-oxidation of the inhibitor. At process bulk water temperatures of 60 to 80°F (16 to 27°C), the nitrite was an excellent food source for destructive microorganisms present in the cooling water.
The system was drained and disinfected to remove microbiological slime and deposition from surfaces.
BetzDearborn's CorrShield OR404 was added to protect this mixed metallurgy system. CorrShield OR404 protects systems without the use of microbiological nutrients, such as nitrite, and therefore does not promote biological fouling or require frequent recharging. CorrShield OR404 is also low in electrical conductivity, making it suitable for use in other, more electrically sensitive systems, and thereby reducing inventory requirements for separate treatment products.
Results (back to top)
Following the new treatment program, iron levels are now down to <1.0 ppm="" (mg/l),="" the="" "hot="" list"="" of="" failing="" vacuum="" pumps="" is="" empty,="" and="" corrosion="" rates="" are="" now="" regularly="" measured="" and="" maintained="" under="" 0.5="" mpy="" (0.0125="" mmpy).="">1.0>
Cost savings were achieved in several areas, including the elimination of labor intensive acid cleanings, the life extension of vacuum pumps, and reduced inhibitor product consumption.
Savings: $35/hr x 30 hr/wk x 52 wk/year = $54,600/year
It is estimated that the elimination of destructive acid cleanings increased pump life from five years to a normal life span of 10 years. Annual cost of capital for a $50,000 vacuum pump is estimated at $5,000 over its normal 10-year life:
Savings: $5,000/pump/year x 30 pumps = $150,000/year
Reduction resulting from the elimination of nitrite and resulting bio-oxidation:
Total cost savings = $206,600/year
Cost of BetzDearborn treatment program = $10,150
Total cost reduction = $196,450
For more information: Hal Roth, BetzDearborn 200 Witmer Road, Horsham, PA 19044-0998. Phone: 215-773-6183; Fax: 215-674-6611.