Ultramicroscopy 23 (1987) 283-290 North-Holland, Amsterdam 283 LASER PHOTOCHEMICAL REACTIONS * D.J. EHRLICH, M. ROTHSCHILD and J.G. BLACK Lincoln Laboratoo'. Massachusetts Institute of Technoloo'. Lexington, Massachusetts 02I 73, USA Received 26 January 1987; received at editorial office 15 June 1987 Laser-driven chemical reactions have been developed for thin-film microelectronics fabrication. A class of these processes relies on photochemical interactions between chemical species on surfaces and a beam of localized laser energy. This paper briefly describes the nature of such reactions and expands upon principal examples of both deposition and etching by photochemical processes. The special role of surface modifications by laser-excited reactions is emphasized. I. Introduction Recently an interest has arisen in laser-driven surface reaction and thin-film modifications. This is motivated in part by applications in microfabri- cation technology, particularly as methods for non-lithographic direct construction of microelec- tronic devices without the use of (indirect) photo- resist and pattern transfer multi-step cycles. Fur- ther equally strong motivations stem from new information which is emerging from these studies on physical and chemical surface kinetics and photochemistry at interfaces. Laser microchemical methods for thin film de- position, etching and doping rely on UV- and visible-laser-controlled reactions induced in a scanned focus or projected image. Vapor, ad- sorbed, and occasionally liquid phases are active in the surface and external homogeneous chem- istry used to drive and to localize these processes and also in the transport of reactive species into the microscopic reaction zones. One class of de- position and etching reactions with few conven- tional analog'~ is driven photochemically by selec- tive bond scission and without the spurious gener- * This work was sponsored by the Department of the Air Force, in part under a specific program supported by the Air Force Office of Scientific Research, by the Defense Ad- vanced Research Projects Agency, and by :.he Army Re- search Office. ation of heat. In this paper we will briefly survey recent research in this last class. More detail on these reactions and their applications can be found in the references cited. In particular a general review of laser microchemical processing can be found in reference [1]. 2. Laser photodeposition: the general case "lhe most widely studied case of metal deposi- tion on arbitrary subst~ates is UV-laser photode- position. These processes have been developed for both large-area and direct-write (localized-area) applications and are based on the photolysis of metal- or semiconductor-bearing molecules such as metal (or semiconductor) alkyls, metal chlo- rides and metal carbonyls. Volatile alkyl com- pounds are particularly favored, because they are available readily in electronic grade purity. De- position is driven by one or several sequential single-photon absorptions in these molecules. Both homogeneous photolysis in the vapor ohase and photolysis of surface-adsorbed molecules can be important. The relative importance of the hoqlo- geneous and surface contributions varies consider- ably, depending on the UV wavelength, molecular spectrum, pressure and temperature. Adsorbed- phase photolysis is often important in "'pre- nucleating" initial condensation and film growth. 0304-3991/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)