The Reliability of Aluminum/Tungsten Technology for VLSI Applications

Abstract

Interconnects technology and back-end processing moved to the center stage of very large-scale integration (VLSI) technology in the mid-1980s. At that time, the critical dimensions dropped below 1 μm while the chip size and complexity increased to a level where interconnects were recognized to be a limiting factor. As dimensions decreased, the step coverage of sputtered aluminum inside contacts and via-contact holes decreased and alternative technologies were studied. The increasing cost of ownership (COO) of single-wafer Al sputtering processes also supported the search for alternative technologies, such as tungsten chemical vapor deposition (CVD) for via contacts and plugs (Figure 1). Only recently have all the W CVD process steps been optimized to lower cost without loss of reliability and/or performance. The development of cluster tool technology and multiwafer process modules also allowed reliable and cost-effective utilization of the W/Al technology.Tungsten technology for VLSI circuits became complementary to that of aluminum. Tungsten thin-film resistivity ρw = 7–8 μΩ cm is much higher than that of aluminum ρAl = 3–4 μΩ cm, introducing large W interconnect resistance-capacitance (RC) delays compared to Al. Therefore, tungsten is not favorable for high-speed global-interconnect schemes. However, tungsten is suitable for local interconnects where the impedance of the driving transistors is dominant and the RC interconnect delay is less significant. Tungsten is also suitable for contact filling, in which the via resistance is negligible. For these applications, tungsten became a dominant technology and was integrated with the aluminumalloy-based technology used for global interconnects.