Hillock-free integrated-circuit metallizations by Al/Al-O layering

Abstract

Hillocks which grow on aluminum integrated-circuit films during the contact alloying process present processing and reliablity problems. Layered Al/Al-O films, deposited by periodically introducing controlled amounts of oxygen into the aluminum-deposition chamber, are shown to provide dramatic improvements in post-anneal film-surface topography. Aluminum films, unlayered Al-O films, and layered Al/Al-O films were deposited on thermally-oxidized silicon wafers in an e-beam system to thicknesses of approximately 1 μm at a rate of 25 Å/s. The films were photolithographically patterned into arrays of 100×100-μm bond pads and 7-μm lines, and annealed in forming gas for 20 min at 530 °C. After these anneals, both aluminum films and unlayered Al-O films with oxygen concentrations less than approximately 6 at.% had surface protrusions (hillocks and/or whiskers) approximately 1 μm in height. Protrusion density was on the order of 105 cm−2. Al-O films with oxygen concentrations greater than 6 at.% displayed blistering and had resistivities greater than twice those of aluminum films. Layered Al/Al-O films with a structure consisting of four layers of 1000-Å Al/1000-Å Al-O topped with 2000 Å of Al had no protrusions with heights greater than 0.5 μm for oxygen concentrations in Al-O layers ranging from 1.5 to 6 at. %. Overall composite-film resistivities ranged from approximately 15–45% higher than aluminum films over this oxygen-concentration range. Both film surface roughness and the density of hillocks with heights less than 0.5 μm decreased with increasing oxygen concentrations. The thickness of the top Al layer was an important layering parameter; 1-μm hillocks appearing on bond pads of films with top Al layers either less than approximately 1500 Å or greater than approximately 3000 Å thick. However, the 7-μm lines were hillock free even for such non-optimum layering structures. Al/Al-O layering also yielded dramatic improvement in post-530 °C-anneal topography for films deposited at 4 Å/s in an S-Gun® magnetron sputtering system. The S-Gun layered films had a generally higher density of short ( ⩽ 0.5 μm) hillocks than the e-beam layered films.