Polyimide liftoff technology for high-density LSI metallization

A new liftoff technology, in which a metallization layer can be deposited at high temperatures, is developed to provide two-level highly packed interconnection metallization. A heat-resistant polymide, PIQ^®, is employed as the liftoff layer. The reverse pattern of the metallization is formed by reactive sputter etching of the PIQ layer around a thin Mo mask. After metallization layer deposition, lift-off is carried out by electrolytic etching of the Mo mask, thus removing the layer deposited on PIQ. A higher packing density of interlevel connection is also accomplished by adopting exposed via holes. Utilizing this technology, fine-featured smoothly tapered metallization patterns can be obtained almost irrespective of the underlying topology. The minimum pitches of the first level, second level, and via holes are 5, 7, and 7 µm, respectively. This technology does not appear to be detrimental to bipolar device characteristics. A 4096-bit high-speed bipolar memory LSI with two-level highly packed interconnection metallization was produced experimentally utilizing this liftoff technology.