ELECTRONICS

Hot DI Water

The use of hot ultrapure water rinses raises the cost of ultrapure water by 15 to 20 percent. However, this cost is warranted by many of the advantages. The trend toward higher temperature began some ten years ago. Now, the advantages of raising the ultrapure water temperature from 40°C to 80°C have induced many semiconductor manufacturers to incorporate this technology. This has in part been driven by higher circuit densities. At 80°C high purity water is a very good solvent for cleaning semiconductor wafers. It is an attractive replacement for chlorofluorocarbons. Chlorofluorocarbons are good for rinsing because they are inert and do not leave residue. However, CFCs are ozone depleting and have been slated for phase out. Another driving factor for the use of hot DI water is that it permits the wafers to be exposed to less water. This lowers the likelihood of water born contaminants that can cause defects. Hot water offers faster rinses and the ability to better clean down in a trench that is often less than one micron wide and three microns deep.

There are two options in providing heated DI water. One is the point of use approach and the other is the central heating system. The advantages of a point of use system are that variable temperatures are available at each end user point. Also a separate high purity water distribution system is not required. Another obvious advantage is that one point of use system can fail but the others will continue processing. The disadvantages of the point of use system include the higher overflow rate that is necessary to maintain a ready source of high purity water at a specific temperature. Point of use systems heated electrically have higher operating costs and poor energy efficiency if run continuously. Point of use systems may require additional components such as point of use filters, resistivity monitors, and particle counters to ensure that the heater has not degraded the water quality. Centralized systems can either be single pass or recirculating systems. A recirculating system is shown in Figure VI-6.

The decision about whether or not to use point of use or central heating is frequently decided by the flow capacity needed in the fabrication facility. One observer states that when a semiconductor plant begins using 60 to 80 gpm of ultrapure water, the tendency is to turn to steam powered DI central heating. For most point of use systems, the heater is the chief expense, but with central hot DI other costs include the distribution piping, pumps, and equipment to mix and adjust the temperature of the hot water at the point of use. PDF piping itself is not cheap. One to two inch diameter piping can run $75 to $85 per installed foot. The price of electric heaters is approximately $5000 per gpm as contrasted to $1,000 to $2,000 per gpm for steam powered heaters.

Point of use filters are used to remove contamination added by the distribution piping system. A typical point of use filter would be a cartridge filter rated at less than 0.05 micron absolute. Serial filtration is recommended from the aspect that no filter is one hundred percent efficient. A service life of up to two years is projected for point of use filters.