S. W. Pang

LARGE AREA ION BEAM ASSISTED ETCHING OF GaAs WITH HIGH ETCH RATES AND CONTROLLED ANISOTROPY.

Ion beam assisted etching (IBAE) is a dry etching technique in which the sputter etching component of an argon ion beam and the chemical etching component supplied by a Cl2 gas flux are independently controlled. This technique has been used to obtain anisotropic etching of GaAs with minimal surface damage over areas of a few square millimeters. The results reported here are achieved with an improved IBAE system designed to etch considerably larger areas. The system accurately and uniformly delivers reactive gas flux to the sample giving uniform etching rates over the 2‐cm‐diam area exposed to the ion beam. When the sample is exposed to high reactive gas fluxes, equivalent to a pressure of 1×10−2 Torr, and 1 to 2 keV Ar+ ions at 1 mA cm−2, etching rates of 5 to 10 μm/min are obtained making etched through‐holes in GaAs wafers realizable. Control of the ion beam collimation and the reactive gas flux allow for accurate control of undercutting making submicrometer etched structures in GaAs with aspect ratios>...

Effects of ion species and adsorbed gas on dry etching induced damage in GaAs

Dry etching techniques which induce minimum damage on the etched surfaces are essential for submicrometer device fabrication. In this study, damage induced in GaAs by ion‐beam etching and ion‐beam‐assisted etching was found to be affected by ion energy, ion mass, and adsorbed gas on the sample surfaces during etching. The etching characteristics of Ne, Ar, and Xe ions with energies ranging from 250 to 2000 eV were studied. The effect of gas adsorption was investigated by using Cl2 (reactive gas for GaAs) and NO2 (nonreactive gas for GaAs) with gas fluxes equivalent to pressures between 2 and 50×10−4 Torr. Dry etching induced damage was evaluated by measuring electrical characteristics of Schottky diodes fabricated on the etched GaAs surfaces. Our results indicated that damage in GaAs can be minimized by reducing the ion penetration distance into the substrate by using low ion energy and heavy ion species, and by introducing adsorbed gas, such as Cl2 or NO2, on the sample surface as a protective layer.

Plasma‐deposited organosilicon thin films as dry resists for deep ultraviolet lithography

Organosilicon thin films have been formed by plasma enhanced chemical vapor deposition and employed as deep‐UV photoresists. Films 20–200 nm thick were deposited from liquid organosilicon sources onto carbon‐based planarizing layers and patterned in projection with a 193 nm excimer laser. At fluences below ∼5 mJ/cm2/pulse, exposure to 193 nm radiation induced oxygen incorporation into the film. Following either a wet or dry development step, negative‐tone imaging was achieved, with the remaining photooxidized film being highly resistant to the O2 reactive ion etching employed in the subsequent pattern transfer step. At higher fluences, ∼15 mJ/cm2, positive‐tone imaging by self‐development was obtained for single‐pulse exposures.

High-speed resonant-tunneling diodes made from the In0.53Ga0.47As/AlAs material system

New double-barrier resonant-tunneling diodes have been fabricated in the pseudomorphic In0.53Ga0.47As/AlAs material system that have peak current densities exceeding 1x105 A cm-2 and peak-to-valley current ratios of approximately 10 at room temperature. One of these diodes yielded oscillations up to 125 GHz, but did not oscillate at higher frequencies because of a large device capacitance. A device with a much lower capacitance is estimated to have a maximum oscillation frequency of 932 GHz and a voltage rise time of 1.5 ps in switching from the peak bias point to the valley bias point. Other reported In0.53Ga0.47As/AlAs diodes are analyzed and yield theoretical maximum oscillation frequencies over 1 THz and rise times as low as 0.3 ps.

Pattern transfer by dry etching through stencil masks

Anisotropic profiles and linewidths as small as 60 nm have been controllably achieved using pattern transfer by dry etching through stencil masks. This technique eliminates the conventional polymer resist and lithographic steps and could be useful in obtaining precisely reproducible submicrometer linewidths without variation due to run‐to‐run variability in resist processing. The stencil masks used in this study were 1‐μm‐thick SiNx membranes with transmission openings and are similar to those used for masked ion beam lithography. The dry etching techniques consisted of reactive ion etching, ion beam assisted etching, and hot jet etching. The profile and linewidth control depend on the divergence of the ion or reactive flux and the gap between the stencil mask and the sample.

Aluminum oxides as imaging materials for 193‐nm excimer laser lithography

Aluminum oxide films deposited over amorphous carbon(a‐C:H) planarizing layers have been investigated as imaging layers for 193‐nm excimer laser lithography. The AlOx films were deposited by ion beam deposition, e‐beam evaporation, or sputter deposition. The films have been analyzed by Auger electron spectroscopy and x‐ray photoelectron spectroscopy. Depending on the deposition conditions, AlOx films with compositions varying from metallic to fully oxidized Al can be formed. The optical appearance of these films varies from highly reflective for metallic Al to highly transparent for fully oxidized layers. Using pulsed 193‐nm radiation from an ArF excimer laser, the single‐pulse self‐development threshold energy is similar for films with different compositions when the film reflectivity is taken into account. It was found that pure Al films and highly oxidized Al films do not provide good adhesion on a‐C:H layers and have a tendency to peel during laser exposure. In contrast, films with intermediate compos...

Masked ion beam lithography for submicrometer‐gate‐length transistors

GaAs metal‐semiconductor field‐effect transistors (MESFETs) with 0.3‐μm gate length were fabricated using masked ion beam lithography (MIBL). Since MIBL allows submicrometer features to be patterned in thick layers of polymethylmethacrylate (PMMA), a new self‐aligned technique was developed. The submicrometer gate as well as the source and drain patterns were defined in PMMA by MIBL and were transferred to the underlying SiO2 film by reactive ion etching. The high aspect ratio structure, formed in the combined PMMA and SiO2 layers used to define the submicrometer gate, allows Ohmic metals to be deposited only in the source and drain regions and not in the gate region via shadow evaporation. The rest of the patterning for contact pads, device isolation, and T‐gate overlay was done by optical lithography. The device fabrication process for GaAs MESFETs is presented and the electrical characteristics of these devices are analyzed. The fabricated devices have 0.3‐μm gate length and a transconductance of 150 m...

SUB-100-NM-WIDE, DEEP TRENCHES DEFINED BY REACTIVE ION ETCHING.

In this study, reactive ion etching (RIE) has been used to define vertical trenches with dimensions considerably smaller than the lithographically defined width. Gratings with 320 nm period were defined using a masked ion beam exposure of PMMA, followed by lift‐off of a Ni mask. For shallow etching, tapered profiles were obtained. For more deeply etched structures, the tapered walls converged and formed sub‐100‐nm‐wide, highly anisotropic trenches. The trench formation is not a crystal orientation dependent effect and has been obtained in GaAs, SiO2, and nonstoichiometric silicon nitride (SiNx). The etched profiles and the final trench widths depend on the RIE conditions and the substrate materials. Ion divergence in the RIE reactor was measured using nitrocellulose and was found to increase with chamber pressure. The measured half‐angle of ion divergence was 0.7° at 2 mTorr and increased to 1.2° at 20 mTorr during RIE in CF4 at 500 V self‐induced dc bias voltage. The ion distribution inside these trenche...