The Reliability of Aluminum/Tungsten Technology for VLSI Applications
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tungsten became a dominant technology and was integrated with the aluminumalloy-based technology used for global interconnects. Tungsten technology has several useful properties that has made it so successful: (1) tungsten CVD was proven to be a manufacturable method, (2) reliable tungsten via-filling schemes were developed when they were required, (3) tungsten-filled vias with planarized interlevel dielectrics allow denser layout since they can be stacked, (4) tungstenfilled vias were more reliable than Alfilled vias, and (5) tungsten is compatible with most other materials and processes
that had already been used in conventional l-/xm silicon technology during the early 1980s. Multilevel aluminum/tungsten structures seem to offer solutions for subhalf-micron technologies. However, they also have some inherent reliability problems that are reviewed in this article. To properly describe the reliability issues, the structures of the Al/W for first-level contact (plug) and upper-level via contact should first be presented (Figure 2). Via-contact structures can include tungsten only (Figure 2a) or can also include other metallic thin films such as TiW, TiN, and Ti. Typical structures with TiN, which is a common barrier layer, can be integrated in several ways with the Wcontact (Figures 2b, c, and d). Other structures may include both TiN and titanium (Figure 2c) or just tungsten (Figure 2d). In this article, we focus on the most basic and common effects of Al/W technology. However, it should be understood that the failure of plugs and via contacts is more than just an Al/W binary failure, it is a failure of the whole structure. The reliability of the aluminum/tungsten technology is reviewed under four categories: (1) material-related problems, (2) process and equipment issues, (3) electrical reliability—electromigration, and (4) layout issues and topography. Following, in the summary and conclusions section, the W/Al technology highlights and problems are summarized. Alternative technologies are briefly described and compared.
TiN
TiN
Figure 1. W plug process (a) with complete TiN cladding, and (b) with TiN only above and below the W.
MRS BULLETIN/NOVEMBER 1995
The Reliability of Aluminum/Tungsten Technology for VLSI Applications
TiN TiN
Figure 2. Typical AI/W contact structures (a) without barrier, (bj and (c) with barrier on the sidewails and the bottom, and (d) with complete TiN cladding.
Material-Related Problems The interaction of W with Al and its common alloyed material, that is, copper and silicon, is very limited at conventional processing temperatures. There are several thermodynamically stable W/Al intermetallic phases. As shown in the W-Al phase diagram,6 the first intermetallic to form in an A1W contact is WAI 12. This reaction becomes significant at about 525°C.7 The dominant diffusion species in the initial phase formation is aluminum. Typical W-CVD temperatures range from 350 to 500°C so WAli2 may form during deposition. The reaction is typically slow, and only a small portion of the meta
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