Fabricating t-gate mesfets employing double exposure, double develop

Abstract

A method for forming a T-gate for a MESFET device comprises a double exposure, double develop process. In a first exposure employing lithography a layer of PMMA is applied first to a substrate and spun to a desired thickness and then baked for a predetermined period. The gate pattern was aligned to the ohmic level and either E-beam written or exposed to deep UV radiation through a quartz mask. The wafer as treated was then spray developed using a mixture of MIBK and alcohol. After coating with a Novolak resist, the same gate mask was either realigned to the Ohmic level and exposed to mid-range UV radiation in the 400 nm range or alternatively E-beam written with a modified gate pattern to eliminate the T at the gate pad. The wafer was then spray developed again, this time using LSI developer. The second photo was overexposed in order to form a large opening through the top of the T while the first photo was underexposed to make the stem of the T as narrow as possible. After an oxygen plasma descum, the wafer was spray etched with a suitable solution and then rinsed. The resultant wafer possessed a T-shaped recess for the gate configuration which then was conventionally metallized to form MESFETS.