High-temperature-stable Si₃N₄ dummy T-gate and lift-off mask

A new technique is described that allows fabrication of high-temperature-stable and chemically inert Si3N4 single-material T-structures. Different layers of Si3N4 films are deposited with PECVD technique using different growing conditions. A high etch-rate selectively in BHF was observed between PECVD-grown films using high (RF) and low (LF) plasma excitation frequency. LF-PECVD films can be grown direct from nitrogen and silane, incorporate much less hydrogen, and etch up to 20 times slower in BHF than RF-PECVD films grown using ammonia and silane gas sources. T- or lift-off-type shapes can be achieved by depositing an LF-grown film on top of an RF film with subsequent patterning by lithography and RIE followed by a BHF step for selectively etching the RF layer for undercutting. Application of this new technique is described for fabrication of fully self-aligned submicron MESFET's in a dummy-gate process. © 1986.