A compatible technique for the formation of tantalum thin-film resistors on silicon integrated circuits

Sikina, Casey, et al have indicated that a tantalum thin-film resistor complement on silicon integrated circuits will yield increased performance and lower cost for most microcircuits now fabricated using the silicon planar process alone. This paper describes a process in which tantalum is used as the material for the formation of film resistors on silicon integrated circuits. Tantalum was selected for this purpose over other thin-film materials for the following reasons: 1. It has a wide and desirable range of available bulk resistivities when used in conjunction with a specialized sputtering technique. 2. It has the ability to form a chemically inert, stable, and self-limiting surface oxide either by thermal or anodic means. 3. A relatively low thermal coefficient of resistance is associated with tantalum films in the required sheet resistivity range. Solutions are described to two of the basic problems encountered in the development of the process. The first problem involves the inability of sputtered tantalum to form a satisfactory low-resistance ohmic contact to silicon contacts. The solution is to deposit and alloy an aluminum contacting finger to the silicon contact before deposition of the resistor tantalum. The second problem is the surface damage and potential penetration of the protective SiO2by the tantalum etching solution. A technique is described for protecting the SiO2with a thin metal oxide layer which, for the required etching time, is impervious to the tantalum etching solution.