A Simple Two-Layer Aluminum Metal Process for VLSI

The use of two levels of metal interconnect lines in an integrated circuit chip layout is a very desirable feature that allows higher density and greater freedom in the placement of the active components. In spite of these benefits it has often been avoided in the design of integrated circuits. For many applications the cost of the extra processing steps is not justified. In the case of MOS technology, long diffusion runs can be successfully used. In silicon gate MOS the polycrystalline silicon itself provides, with some restrictions, a second level of signal interconnect lines. However other technologies, for example I^2L, need a second layer of low-resistance metal interconnect to effectively utilize the chip area. While conceptually simple, two-layer metal processes have proved to be quite difficult to implement. This paper describes a relatively simple two-layer metal process that is well suited to university laboratories and others with limited facilities.