Hydrogen is being used in many semiconductor manufacturing steps as a reactant or as a carrier gas. Examples of these process steps are silicon epitaxy, doping of boron, and Ž . chemical vapor deposition CVD of tungsten metal. In many Ž . of these processes, ultrahigh-purity UHP hydrogen is required to ensure the desirable properties of the semiconductor product.
UHP H can be generated in different ways. In places 2 where liquid H is readily available, UHP H can be ob-2 2 tained by '' vapor withdrawal'' in that only the vapor phase of the H is taken from a liquid H storage tank. No purifier is 2 2 Ž . needed. Homyak et al. 1995 demonstrated that vapor H 2 gas from a liquid source contained impurities with concentra-Ž . tions well below parts-per-billion ppb levels. Presumably, all impurities were trapped in the liquid H , since H has the 2 2 Ž . lowest boiling point except for helium of all gases. In areas where only pipeline gaseous H is available, such as most of 2 Asia, purification is required to remove impurities such as N , O , CO, CO , CH , and H O. 2 2 2 4 2 One common commercial H purifier uses a Ag-Pd alloy 2 membrane. This precious-metal-based membrane purifies H 2 by a mechanism in which the H transports through the non-2
porous membrane as protons. After diffusing through the Ž . membrane, the protons recombine with electrons to form H molecules. This mechanism should exclude impurities 2 from permeating through the membrane. The H permeabil-2 ity of the Ag-Pd membrane increases with increasing temperature. For the membrane to be cost-effective, the operating temperature for the membrane is usually between 350 and 450ЊC.
Recently, in an effort to qualify a commercial Ag-Pd membrane purifier for a customer in the electronics industry, we discovered that CO and CO in the feed hydrogen can result 2 in traces of CH in the purified product. The formation of 4 CH , even in trace levels, disqualifies the product for the 4 customer. This specific customer requires a CH concentra-4 Ž . tion to be less than 10 ppb by volume ppbv in hydrogen. For CH to form, carbon has to be present in some form. We 4