High-performance germanium-seeded laterally crystallized TFTs for vertical device integration

Increasing chip complexity and area has resulted in interconnect delay becoming a significant fraction of overall chip delay. Continued scaling of design rules will further aggravate this problem. Vertical integration of devices will enable a substantial reduction in chip size and thus in interconnect delay. We present a novel technique to achieve vertical integration of CMOS devices. Germanium is used as a seeding agent at the source and/or drain of thin film transistors (TFTs) to laterally crystallize amorphous silicon films, resulting in high-performance devices. This is achieved through the formation of large grain polysilicon with a precise control over the location of the grain. TFTs have been demonstrated offering substantial performance improvement over conventional unseeded polycrystalline TFTs, with demonstrated mobilities as high as 300 cm2/V-s. The process is fully CMOS compatible and has a low thermal budget. It is highly scalable to deep-submicron technologies and, with suitable optimization, should enable the production of high-performance, high density, vertically integrated ULSI.