Particle beams, in their broadest context, have become the most common tools in the kit of the IC process engineer, In this paper the general characteristics of particle-beam technologies are systematically classified and critical applications issues are outlined. The uses of ion, electron, and photon beams for various processes (e.g., thin-film deposition, lithography, etching, doping) are reviewed and critical issues are highlighted. The feasibility of integrating several advanced beam processes to achieve a totally in situ process is discussed. Particulate contamination in "clean"- room environment and under vacuum conditions is addressed and related to chip defects and yield. The relative advantages, in terms of facility and operating cost, flexibility, device performance, of an in situ process are considered.