Optimization of Bi-layer Lift-Off Resist Process

Bi-layer lift-off metallization techniques offer significant advantages in resolution, removal, process simplicity, undercut control and yield over conventional single-layer lift-off processes. Because of its ease of application, long shelf life and lower tool cost, the polydimethylglutarimide (PMGI) bi-layer process has become an attractive method for the metallization of III-V compound semiconductor devices. The LOR/PMGI bilayer lifts-off cleanly when fabricating source, drain or T-shaped gate ohmic contacts for gallium arsenide (GaAs), GaN, InP and other compound semiconductor devices. Because of its excellent undercut control, LOR/PMGI can be removed in either conventional photoresist removers or in low temperature, metalion free aqueous alkaline developers. Because LOR/PMGI is used with such a wide variety of photoresist and process conditions, a comprehensive process optimization has never been implemented. Therefore, the purpose of this paper is to explore and optimize several key process parameters for controlling the critical dimensions (CD) of the MicroChem LOR/PMGI bi-layer process. LOR/PMGI coatings as thin as 0.20 μm, for the production of 0.45 μm metallic features, and as thick as 3.0 μm, for the production of thick metal depositions, will be investigated using common process controls, such as: LOR/PMGI bake time, bake temperature, TMAH development time and development method. By optimizing these process conditions, further improvements in resolution can be found.