RASIRC Water Vaporizer
Direct Injection of Ultra Pure Water Vapor into Vacuum
Water vapor has been shown to have significant effect on thin film layers in ALD, wafer bonding, carbon nanotubes, MOCVD, molecular surface cleaning, and sputtering processes. The ability to deliver water vapor free from atmospheric contaminants is critical to film integrity.
Water is frequently used as a source of atomic oxygen. The use of water as a precursor has economic and safety benefits compared to other oxide sources. However the controlled delivery of pure water vapor is challenging. Direct flow control of the water into vacuum is difficult due to the expansion of 1 gram of water to 1,000,000 cc of gas at room temperature and 1 mbar pressure. Water vapor typically condenses unless it is added to a carrier gas stream or directly injected into vacuum.
The RASIRC Vaporizer is designed to control the delivery of water vapor into vacuum processes. The vaporizer uses a non-porous membrane that selectively allows water molecules to pass through it, but rejects other gas species.
The non-porous ionic perfluoropolymer membrane excludes particles, micro-droplets, volatile gases, and other opposite charged species from being transferred and ensures that only water vapor is added. The membrane is highly selective, also preventing most process gases from crossing over into the water source.
The membrane is supported by stainless steel internal support. The exterior of the membrane is encased in a PFA/PTFE shell to allow deionized (DI) water to be contained and/or circulated over the membrane. This eliminates the need to disconnect the vaporizer for water addition. The membrane rejects dissolved gases in the DI water, eliminating the need to degas water to remove gases such as oxygen, nitrogen, and CO2.
The flow rate of water vapor across the membrane is a function of the surface area of the membrane and the water temperature. The mass flow rate is controlled by varying the water temperature. With ampoules, water delivery is controlled through a load lock that allows only discrete delivery. Bubblers require a carrier gas. The Vaporizer can provide continuous delivery of 100% water vapor into vacuum without a carrier gas.
With bubblers and ampoules, the water supply is directly exposed to vacuum. The DI water must be degassed before use, to remove residual oxygen and nitrogen in the water. Degassing can take up to 48 hours to reach baseline acceptable levels, preventing tool use. Refill requires breaking the vacuum system integrity. In contrast, the Vaporizer uses an external water source so water filling, recirculation and draining can be accomplished without breaking the vacuum of the tool. This eliminates service related to change out, degassing, and vacuum leak checking needed with other delivery techniques.