A comparison of tungsten film deposition techniques for very large scale integration technology

Abstract

Tungsten films have long been considered as a metallization candidate for integrated circuits because of their relatively low electrical resistivity, freedom from electromigration and mid-range work function. Initial work in the early 1970s employed electron gun evaporation at high substrate temperatures in order to improved adhesion, to lower resistivity and to control the high stress present in refractory metal films. Planar diode sputtering was not widely used because of radiation damage caused by the high target voltage and the slow deposition rate. Progress in magnetically enhanced plasma technology led to the development of magnetron sputtering, and this resulted in a higher deposition rate at much lower ion energies. The high stress ever present in refractory metal films has almost been eliminated by the use of a high sputtering pressure. By contrast with evaporation, films with reasonably low resistivity in the range 10-11 μΩ cm can be routinely deposited by magnetron sputtering onto unheated substrates with excellent adhesion and reproducibilty. Successful application of magnetron-sputtered tungsten films in metal/oxide/semiconductor (MOS) devices was reported recently. In the meantime new applications of tungsten films prepared by chemical vapor deposition in very large scale integration technology are emerging. The selective tungsten deposition process, in which the desired film grows on silicon and other metals and alloys, but not on oxides and nitrides, appears to be very attractive for contact metallization in MOS devices because of the process simplicity, although there seem to be some problems associated with this process: encroachment and tunneling. The same selective tungsten films are also ideally suited to via-hole filling, contact metallization and upper level wiring. The non-selective chemically vapor- deposited tungsten films may also find applications in upper-level wiring because of their resistance to electromigration. © 1987.