Direct-write electron beam lithography for submicron integrated circuit fabrication

A self-aligning direct-write electron beam lithography instrument has been developed for fabricating Gallium Arsenide (GaAs) integrated circuits. The electron beam is used to directly write the critical layers in these circuits. The main application is to write the gate layer in high electron mobility transistors (HEMTs). A single HEMT may contain several gale electrodes, each of which is up to 150 mu m wide, less than 0.25 mu m long and which must be aligned with submicron accuracy. A variety of devices have been successfully written an the instrument, which comprises a scanning electron microscope (SEM) that has been interfaced to a purpose-built pattern generator and image correlation system. The standard SEM stage has been motorized and is used to position each device within the field of view of the SEM. The pattern generator then scans the electron beam to obtain an image of the device. This image is correlated with a reference image and the precise location of the device is calculated and used for aligning the subsequent exposure. The active alignment system achieves excellent alignment, far exceeding the accuracy of the standard SEM stage. Not only does this obviate the need for expensive stage positioning systems, but it also compensates automatically for positional errors on the sample caused by mask tolerances. As the instrument uses the image of the device for correlation, no alignment marks are required on the sample. The system is fully automated and has been used successfully to write a variety of device geometries.