The processing of semiconductor devices within a high vaccum or controlled ambient environment has received much attention over the past few years. The motivations fro in situ processing include avoidance of contamination or oxidation in the atmosphere, improved device yields, and process simplification. The realization of in situ processing ranges from cluster tool processing for silicon very large scale integrated (VLSI) devices to all ultrahigh vaccum processing compatible with molecular beam epitaxy for III–V optoelectronic devices. This paper will review in situ processing methods with particular emphasis on combinations of fine-scale patterning and crystal growth in the III–V materials.

In situ processing of semiconductors

Patterning of GaAs at the sub-100 nm size has been demonstrated using in situ electron beam (EB) lithography processes. Patterning is carried out by EB exposure of an ultrathin oxide layer on GaAs which is used as a mask material. The patterns are transferred into GaAs by Cl2 gas etching. A high-brightness Schottky electron gun is used in the exposure process. The size of the etched feature is as small as 50 nm, which is equal to the diameter of the electron beam. The results show that in situ EB lithography processes using an ultrathin oxide mask are very promising for fabricating nanometer-scale structures.

Sub-100 nm Patterning of GaAs Using In Situ Electron Beam Lithography

Buried GaAs/AlGaAs single quantum‐well structures have been fabricated for the first time by in situ electron beam (EB) lithography. The process includes the molecular beam epitaxy of a GaAs/AlGaAs single quantum well, electron‐beam direct writing, Cl2 gas etching, and overgrowth of an AlGaAs layer. A thin GaAs oxide layer was used as the etching mask, which was selectively formed on a clean GaAs surface by EB irradiation under an O2 ambient. Subsequent Cl2 gas etching resulted in the formation of isolated quantum wells. Prior to the overgrowth, thermal cleaning with atomic hydrogen was employed for removing the oxide mask. The cathodoluminescence image of the buried quantum well demonstrates the high quality of the resultant structure formed by in situ EB lithography.

In situ patterning and overgrowth for the formation of buried GaAs/AlGaAs single quantum‐well structures

New damage-free patterning method of a GaAs oxide mask and GaAs selective growth using the metalorganic molecular beam epitaxy method

The electron beam (EB)-induced modification of GaAs oxide for the in situ lithography process has been investigated. The oxide formed on an as-grown GaAs surface is irradiated with an electron beam at doses in the range of 1.5×1017 to 4×1019 electrons/cm2 at beam energies of 10-25 keV. Subsequent Cl2 gas exposure results in selective etching only in the irradiated area. EB-induced modification of the GaAs oxide is found to involve a partial removal of oxygen by in situ Auger measurements. The threshold dose for the etching depends on electron energy, whereas it is independent of the wafer temperature between 25 and 100°C during EB-irradiation. These results indicate that the modification is not a thermally activated process, but a direct interaction of the electron beam with the oxide.

Role of an electron beam in the modification of a GaAs oxide mask for in situ EB lithography

We have developed a low energy focused ion beam (FIB) system by employing retarding field optics. The system is equipped with a gas jet and is used for ion beam assisted etching or deposition. To evaluate the usefulness of low energy FIB, we have been investigating defects in GaAs induced by irradiation of low energy Ga+ FIB and Ar+ unfocused beams. Defets induced by irradiation at 300 K were observed even at a depth of >2000 ?, which is much deeper than the ion range. However, it was observed that the deep distribution was suppressed by irradiation at low temperature (100 K.

Ryou Mimura

Papers

Low Energy Focused Ion Beam System and Application to Low Damage Microprocess

Low Energy Focused Ion Beam System and Application to Low Damage Microprocess : Etching and Deposition Technology