Multilayered Interconnections for VLSI
- PDF / 1,655,792 Bytes
- 12 Pages / 420.48 x 639 pts Page_size
- 64 Downloads / 285 Views
Multilayered Interconnections for VLSI Donald S. Gardner and Krishna Saraswat Integrated Circuits Laboratory Stanford University, Stanford, Calif. 94305
Abstract Interconnections are predicted to become the limit in performance and reliability at submicron dimensions. Layered structures are one possible solution to the problems of electromigration and hillocks. Aluminum alloys can improve the properties of pure aluminum but with a consistent increase in resistivity due to lattice distortions introduced by the alloy in solid solution. The rise in resistivity is minimal in layered films because aluminum layers act as parallel conductors. Titanium is the most effective refractory metal for eliminating hillocks in layered films; however vanadium, tantalum, and tungsten also reduce hillocks to a manageable level although high resistivity, the inability to dry etch, or lateral hillocks can become a problem. Layered films of aluminum with TiSi2 encounter problems with hillocks. These differences are partially explained by the increased strength of the aluminide TiA13 that forms compared to the other compounds. Barrier metals are needed for aluminum layered with refractory metals because of the formation of ternary compounds, and several metals have proven to be satisfactory. Such barriers, however, are required for even aluminum at submicron dimensions for stable low-resistivity contacts.
1
Introduction
In future VLSI technology, several materials and alloys will be combined to fabricate multilevel interconnections because performance will be so limited by the interconnections that different materials will be chosen for their various properties. Aluminum has long been used to form the metal interconnections; however, as device dimensions are scaled, the resulting increase in current density will lower reliability [1]. With multiple levels of aluminum, failures between levels also become a problem. The major problems with pure aluminum are electromigration, hillock formation causing electrical shorts between successive levels of aluminum, and the high solubility (0.5 atm% at 450'C) and diffusivity 0 of silicon in aluminum (diffusion length is 1.4 ym for 1 sec at 450 C) leading to poor contacts to shallow junctions. Aluminum-copper can reduce the problems characteristic of pure aluminum [2], but these films are difficult to dry etch, easily corrode after etching [3], are susceptible to longterm corrosion [4] and have bonding problems [5] In addition, hillocks are not completely eliminated. In this work, the term level will be used to describe conductors which are separated by an insulator whereas the term layer will be used to describe different conductors tiered together in one level of interconnections Hillocks are partly the result of the large difference between the thermal expansion coefficients of the metal and substrate [6] and have been observed in films of lead, tin, and
Mat. Res. Soc. Symp. Proc. Vol. 103. -1988 Materials Research Society
344
X
Layer N
U ,
... ...
97MEM/ Layer 2..
Al-SI
Layer I
...
Data Loading...
