In 1986, Bendorz and Muller1 discovered that metallic oxide with a perovskite structure in the La-Ba-Cu-O system exhibited superconductivity at 20°K. In early 1987, Chu and his co-workers2 made a YBa2Cu3O7-x compound which had a Tc higher than the temperature of liquid nitrogen. Since these discoveries, the research and development of high-Tc superconductors has aroused unpecedented attention, not only from the scientific community, but also from various industries and governments. As these new superconducting materials are in general very "brittle" and difficult to machine, it is extremely difficult to fabricate them into thin wires. In addition, practical use of new high-Tc superconductors will require materials with very high critical current density. Thus thin film preparation is particularly important for many applications of superconductors, such as electronics for the computer industry. Films with high superconducting transition temperatures have been prepared by electron beam evaporation, organometallic chemical vapor deposition, DC sputtering, molecular beam epitaxy, and laser evaporation and deposition processes. For example, Rice et al.3 used electron beam and thermal evaporation to produce Ca-Sr-Bi-Cu-O superconducting films from CaF2, SrF2, Bi, and Cu targets. Berry et al.' produced a superconductive film by organometallic chemical vapor deposition. Hellman et al.5 used molecular beam epitaxy to produce a DyBa2Cu307, film on a SrTiO3 substrate. Lynds et al.6 used a Nd-YAG laser beam to do laser evaporation and deposition to prepare a YBa2Cu307-x thin film from targets of Y203, Bat CO3, and CuO. Kwok et al.7 applied a homogeneous excimer laser to obtain a film with the right stoichiometry of YBa2Cu307,. However, most of the films in the works cited above need to be annealed in an oxygen environment in order to produce superconductivity. Recently, several research groups obtained superconductive films of YBa2Cu307, by raising the temperature of the substrate and introducing oxygen into the chamber during the thin film preparation process.8 However, no superconductive films of the Bi-Sr-Ca-Cu-O or Tl-Sr-Ba-Cu-0 systems were obtained without oxygen annealing. Nevertheless, the right ratio of various metal elements is one of the key factors in achieving superconductivity. Superconductive films are usually obtained based on repeated trials and adjustments. Venkatesan et al.9 observed two distinct components during pulsed laser deposition of high-T, superconductive film. It was pointed out that the quality of the superconductive film depends on controlling the film stoichiometry. The role of introducing oxygen into the thin film preparation chamber has never been fully understood. Thus, the need for a real-time monitor for high-Tc thin film preparation is quite critical.