Abstract: Factors affecting the reliability of metallization systems for semiconductor devices
Authors: Wilson Richard W. 
Published: July 2002

Thin film interconnection metallization and passive thin film components of semiconductor devices are subject to four primary modes of failure. They are (i) metallurgical reactions, (ii) electro chemical corrosion, (iii) electromigration, (iv) process related imperfections.

The use of alternative metallization systems for the commonly used aluminum is desirable for devices operating at high−current densities or in nonhermetic applications. If these alternate metallization systems consist of more than one material, then not only must the metallurgical stability between the metallization and the silicon be investigated, but also the effect of interdiffusion of the film constituents themselves. Such investigations conducted on an extensive combination of materials show that gold combined with tungsten as a barrier and with titanium as an adhesive produces excellent results with respect to processing and operation on complex integrated circuits and microwave power transistors.

Conductors and passive components on semiconductor devices are subject to electrochemical corrosion if moisture is present on the surface of the device. Tests made under conditions of temperature, humidity, and electrical bias are used to determine the stability of device metallization for nonhermetic applications. Electrochemical corrosion, under these conditions, is a rapid mode of device failure because of the close proximity and minuteness of the conductor lines or passive components. Passivation with deposited glass films afford some degree of protection to the device but exposed wire bonding pads, pinholes, and cracks in the glass and glass composition significantly affect the degree of passivation afforded by these films.

The complexity of integrated circuits in system function and in component parts is ever increasing. However, because of yield and assembly limitations such complexity cannot be paralleled with a corresponding increase in die size. As a result, conductors are being reduced in size, are required to operate at higher temperatures, and at current densities exceeding 106 A/cm2. The result of these stresses is to accelerate the migration of the metallic films with the consequent formation of voids and hillocks in the conductor metallization. Micrographs taken with the scanning electron microscope, both in still and motion form, show that the voids reduce the cross sectional area of conductors, contributing to conductor burn out, and the hillocks grow outward from the film surface which could ultimately cause shorts even through overlying dielectric layers. This migration of metal atoms is largely along grain boundaries and effective methods of inhibitions are (a) use of a metallization material with the smallest self−diffusion coefficient, (b) use a large grain size film structure and, (c) the addition of impurities to the film to reduce the grain boundary effect. Process−related failure mechanisms extend from an over−etch condition to inadequate process control, through poor film adhesion, to excessive thinning or discontinuities in the film because of inadequate coverage over the surface of the substrate. This latter factor is the primary concern of the thin film technologist, in that adequate conductor film continuity must be maintained over a surface which is severely contoured by prior etching of the silicon and overlying dielectric and metal films. Since these steps cannot be eliminated nor even reliability tapered for best film coverage, it is necessary to compensate for this by employing special procedures during the film deposition.

Elevated substrate temperature, along with appropriate substrate nutation, can enhance the step coverage of aluminum films. However, for refractory metals, or if high−substrate temperature would be detrimental to device performance, deposition by the appropriate mode of sputtering can be used to an advantage.

SEM micrographs are useful in demonstrating the results of a particular process, and along with appropriate temperature stress testing, are used to define the conditions for a successful metallization system and film depostion procedures.