A New Technique to Measure and Deliver Low Levels of Water Vapor into Atmospheric and Vacuum Processes

Abstract

As material processes move to the nanoscale, fine control of process chemistries becomes critical. Atomic Layer Deposition, Chemical Vacuum Deposition, and Selective Gate Oxidation, as well as fabrication of Carbon Nanotubes all need water vapor from 50 sccm (40mg/min) to below 1 sccm ( 0.8 mg/min). Water vapor can be used for oxidation, cleaning and annealing of the nanoscale devices and thin films.

At 0.8 mg/min, flow control of liquid water with a mechanical pump is difficult without pulsation. Because water is liquid at room temperature, a standard thermal mass flow controller for gas cannot be used. Instead bubblers are used. However, they do not provide consistent or repeatable delivery. They are affected by gas and liquid temperature, relative pressures as well as gas velocity, liquid height, thermal droop, and contamination build up.

RASIRC® has developed a new pervaporation device that selectively allows water to diffuse into a carrier gas stream. The liquid water never directly contacts the carrier gas, allowing for independent liquid and gas pressures. A nonporous, hydrophilic membrane provides very rapid diffusion of water vapor into the carrier gas stream. This allows for precise amounts of water vapor to be added to the carrier gas based solely on relative vapor pressures.

To validate the performance of the device at very low flow rates into both atmosphere and vacuum process pressures, a heated humidity probe was used. This device was able to measure flow rates of 1 to 200 sccm in both vacuum and atmospheric pressure. The experimental results were compared with the expected theoretical dew point to validate the procedure. The humidity probe was used to measure both long term stability of the device and the instantaneous response time. Experimental results were compared to the expected time needed to change the water vapor concentration in the test volume.