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Direct-write electron beam lithography automatically aligned with optical lithography for device fabrication

A scanning electron microscope (SEM) has been modified for direct-write electron beam lithography. The instrument has the capability to automatically align with the features patterned by optical lithography and exposure the features requiring the finest linewidth with the electron beam. The main application for the instrument is one the process line for fabricating high electron mobility transistors (HEMTs) in GaAs monolithic microwave integrated circuits (MMICs). The high frequency performance of the HEMTs is critically dependent on the length of the gate electrode and the placement of this electrode between the source and drain electrodes. All of the mask layers for a MMIC except the gate layer are exposed using optical lithography, as it provides the required linewidth with high throughput. The sample containing the partially fabricated HEMTs is loaded in the instrument and is positioned at each HEMT in sequence in order to acquire an image of the source and drain electrodes. This image is correlated with a reference image of the HEMT to determine its precise location for subsequent exposure of the gate electrode by the electron beam. The instrument is able to achieve an alignment accuracy of 80 nm and has been used to expose features with linewidths less than 100 nm. As images of the device are used for alignment, the instrument does not require alignment marks on the sample and is able to automatically compensate for positional errors caused by same stage and mask tolerances. As the full SEM functionality of the instrument is retained, it may also be used to inspect the results of the lithography.