Processing of AlGaAs-GaAs Heterojunction Bipolar Transistors

摘要

The processing of mesa isolated GaAs-based heterojunction bipolar devices isdemonstrated. The heterojunction bipolar transistor (HBT) process is tailored for the equipment in an electron physics laboratory and clean room of class 1000. The development effort for HBT process is discussed in detail, including remarks on crystal orientation and formation of shallow air bridges for proper mesa entrance of wiring metal. Processing steps are illustrated with scanning electron microscope images. The most critical processing steps are for defining mesa isolation depths. The steps are based on slow citric acid wet chemical etching and monitored with a profilometer. Due to the deep mesa structures, the lithography requires planarizing type. This process includes a double-layer planarizing lithography for AZ 5214E resists, that allows two resist layers to be patterned separately on top of each other, which is utilized in defining shallow air bridges.