Yasushi Asaoka

The use of surface oxide and MBE-grown multi-layers as an inorganic resist for nano-patterning

Various kinds of semiconductor microstructures have been fabricated based on the conventional lithography where a mask pattern is projected on a photoresist using light, electron beam, or X-ray. However, this lithographic process has a potential difficulty in the control on nano-scale due to the resolution limits of the mask and photoresist to be patterned on the substrate. In order to overcome theses resolution limits for nano-scale patterning, we propose here a novel method using surface oxide and MBE-grown multi-layers as an inorganic resist. One particular advantage of employing these resist materials is a superior controllability in thickness and uniformity compared with the case of organic resist due to the different methodology of epitaxial growth. Functionality as a resist is developed by the direct irradiation of 30 keV focused Ga ion beam (FIB). The area irradiated performs as either negative or positive resist against the subsequent in-situ layer-by-layer etching, depending on the amount of Ga/sup +/ dose. The negative resist (etch-stop region) is produced by the FIB-induced oxidization of the surface, while the positive resist (etch-enhanced region) by the FIB-induced amorphization or sputtering of the bulk. We call this integrated multi-process as in situ maskless-nanolithography.

In situ control of the desorption process of GaAs surface native oxide by direct Ga beam irradiation

Summary form only given. The desorption mechanism of GaAs surface native oxide associated with thermal and chemical stability has been widely studied due to its fundamental and technological importance in fabricating optical and electrical devices by MBE . In our experiments, the surface oxide desorption process by irradiating Ga flux was investigated on GaAs[100] as a function of the amount of Ga deposited on the native oxide in an MBE chamber. The surface morphological change during the process was monitored by in-situ RHEED. The reduction of the oxide layer thickness was clearly observed. The process of the oxide thickness reduction was also shown in the gradual increase in RHEED specular intensity. A further irradiation of Ga brought about a decrease in the specular intensity. In order to compare the final surface morphology with the typical thermal desorption process, the Ga irradiated surface was exposed to As flux and the temperature increased. Ex-situ AFM images of the samples were taken. It was clearly observed that the surface irradiated by Ga had an absence of deep pits, which was not the case by conventional thermal desorption. The rms roughness value of the Ga irradiated surface is 0.7nm, while the conventional thermal desorption one is 1.5nm. Therefore the use of Ga flux during GaAs oxide desorption is an effective method to reduce the surface roughness, especially the density of pits, prior to MBE growth.