Paper TF+SS-WeM6
A Comparison of Water Delivery Methods for Atomic Layer Deposition

Wednesday, October 21, 2015, 9:40 am, Room 114

Water is frequently used as an oxygen source for atomic layer deposition (ALD) of metal oxides. However, water exhibits a relatively high vapor pressure at room temperature and readily adsorbs on reactor surfaces. These characteristics can make it difficult to reproducibly control water delivery during ALD, particularly when small quantities are desired. The focus of this work is characterizing and comparing three different methods of water vapor delivery in an effort to identify techniques for ensuring reproducible delivery of water vapor quantities. For this investigation, three methods of water injection were compared. The first method utilizes a needle valve between the water reservoir and the water injection valve (the valve that controls the water allowed into the delivery line to the reactor). This method is commonly employed for research grade reactors as the use of an adjustable orifice permits the water flow rate to be varied. However, control of small water quantities can be difficult due to the buildup of water vapor between the needle valve and the water injection valve. The second method overcomes this control limitation by utilizing a pump to vent water vapor from the volume between the needle valve and injection valve prior to injection into the delivery line. This method provides a high degree of control at the cost of added complexity and expense. The third method utilizes a laser-drilled orifice in a VCR gasket as the flow-controlling orifice. This method is simple and inexpensive, however, water flow rates cannot be adjusted without changing the orifice. The performance of these three water injection methods was characterized using an optical water mass flow meter (MFM) that has been developed to rapidly quantify water vapor. This MFM was based upon a wavelength modulation spectroscopic technique utilizing a near-infrared diode laser. This MFM permits quantitative comparison of the performance of these three water injection methods, allowing a potential user to identify method suited to a particular application.