Using a nondestructive test to qualify corrosive specialty gas cylinders

Alan Zdunek, Tracey Jacksier, John Borzio, and Denis Rufin, A reliable supply of ultrapure electronic specialty gases is critical to maintaining the high tool uptimes necessary to maximize productivity and manufacturing yield in semiconductor fabs. The delivery of such gases poses special challenges, however, because of their highly corrosive and reactive nature. Halogen gases such as boron trichloride, for example, can hydrolyze in the presence of moisture and react with metal container surfaces, forming particles that compromise gas purity. And if such particles reach the point of use, the wafer surface can become contaminated. The first challenge suppliers of corrosive specialty gases face in improving the reliability of their products is to develop a delivery cylinder that remains nonreactive to the gases over many product fills, yet minimizes cost. Although the steel material used to form gas cylinders is itself corrosion resistant, that characteristic can be enhanced by various techniques. An electroless-nickel surface treatment has been found to be a good choice for corrosive gas delivery because it can be applied consistently to large numbers of cylinders. The resulting coating does not react readily in corrosive environments, and the procedure is economically attractive compared with other alternatives. Figure 1: Typical measurement results showing thickness uniformity of electroless-nickel cylinder coating. Figure 2: Micrographs illustrating the properties of electroless-nickel cylinder coatings: (a) cross section and (b) surface morphology at 200x magnification. Described in detail elsewhere, the electroless-nickel coating is a metallic, nickel-phosphide glass formed on the interior surface of the steel cylinder by an autocatalytic chemical reaction. The electroless nickel inherently passivates, forming a strongly bonded, low-porosity surface layer that resists undercutting and has a consistent thickness from top to bottom, as seen in Figure 1. The two scanning electron microscope (SEM) micrographs in Figure 2 also illustrate the coating's beneficial properties. The