E. D. Wolf

Chemically assisted ion beam etching of GaAs, Ti, and Mo

Investigations with argon and chlorinated ion beams have shown that the etch rates of GaAs and Ti are strongly dependent on the flux of molecular and atomic chlorine. Etch rate increases were observed with an increase in the chlorine flux from the background gas composition or from the impinging ion beam. At increased chlorine concentrations in the presence of ion bombardment, wall profiles were altered from an overcut slope to nearly vertical. In the etching of Mo, etch rates were limited by the removal of low volatility chlorides by ion‐assisted mechanisms.

DRY DEVELOPMENT OF ION BEAM EXPOSED PMMA RESIST.

The etching characteristics of polymethyl methacrylate (PMMA) exposed to 28Si+ ions and then dry developed by reactive ion etching in an oxygen plasma have been studied. It is shown that etch rates of resists exposed to doses greater than 1×1015 cm−2 (1.6×10−4 C/cm2) at 40 keV are much smaller than those of unexposed resists. A differential etch rate as high as 11 is demonstrated. This property of ion beam inhibited etching (IBIE) has been used to fabricate resist structures with submicrometer features using ’’see‐through’’ thin silicon film masks. With further developments of high brightness ion sources, IBIE may be a useful technique to realize high resolution negative tone images in resists.

High-power AlGaAs/GaAs single quantum well lasers with chemically assisted ion beam etched mirrors

We report the first use of chemically assisted ion beam etching to form laser mirrors on GaAlAs graded index separate confinement single quantum well heterostructures grown by metalorganic chemical vapor deposition. Over 80 mW cw optical power is obtained from the etched facet of uncoated 300‐μm‐long, etched/cleaved 60 μm stripe devices mounted p side up, and catastrophic failure occurs at a cw power as high as 205 mW. Differential quantum efficiencies for light emitted from the etched facet are 32% pulsed (27% cw) and the threshold current is 145 mA pulsed (150 mA cw).

Al‐Si contacts formed by ion irradiation and post‐annealing

Al‐Si contacts have been formed by implantation of As ions through Al‐Si interfaces followed by heat treatment at 400–500 °C for 10 min. The erosion of Si proceeds uniformly in contact areas at the sintering temperatures. Diodes using Al‐Si contacts produced by this technique have been fabricated on thin n+ layers in p‐type Si substrates with junction depths of 0.35 μm and contact areas of 5 μm2. The average leakage current per diode (with an approximate junction area of 14×26 μm2) is about 10−8 A, as compared to the leakage current of 10−5 A for diodes with Al‐Si contacts prepared by sintering of Al on Si at 420 °C. We attribute the improvement to the uniformity of Si erosion after the interfacial oxide has been dispersed by ion irradiation.

REACTIVE ION ETCHING FOR SUBMICRON STRUCTURES.

The etch resistance of PMMA was measured under various reactive ion etching conditions and compared with that of silicon dioxide, silicon and Shipley AZ 1350 resist. The resulting profiles transferred into the substrates masked with PMMA were also studied under various reactive ion etching conditions. This study showed that PMMA can be used as a masking resist for etching silicon dioxide using fluorine‐deficient etch gases. Linewidths with dimensions of 0.1 μm have been obtained in silicon dioxide with usable selectivity between PMMA and silicon dioxide.

Orientation dependent reactive ion etching of GaAs in SiCl4

The reactive ion etching characteristics of (100) GaAs in SiCl4 have been investigated. It was found that pronounced orientation dependent etching could be obtained under plasma conditions of low power density and moderate pressures (around 20 m Torr). At very low pressures, structures with vertical sidewalls were obtained. The structures obtained in the orientation dependent etching mode are similar to those obtained by wet etching and those observed in bromine plasma etching. Etch profiles were different along the [011] and [011] orientations and the relation of etch rates in the different planes follows the relation R〈100〉>R〈111〉>R〈100〉.

Profile control by chemically assisted ion‐beam and reactive ion beam etching

Investigations with broad‐beam argon and reactive fluorinated ion beams have shown that the etched wall profiles of silicon at submicrometer linewidths can be controlled by varying the ion energy, current, and partial pressure of XeF2. Inert argon and reactive ion beams generated from xenon difluoride produced overcut profiles resulting from predominantly physical etching mechanisms. With very low partial pressures of XeF2 in the background ambient, purely chemical etching of silicon is low while enhanced line‐on‐sight ion‐assisted etching can be used to produce vertical profiles. By increasing the background partial pressure of XeF2, undercut profiles were produced by purely chemical and chemically assisted ion beam mechanisms. Thus, various wall profiles were produced in silicon within the same broad‐beam ion etching equipment using the same two‐component gas system of Ar and XeF2.

Dry etching for submicron structures

Various drying etching techniques are briefly reviewed and application of some of these techniques for the fabrication of small structures in electronic and metallic materials are discussed. Experimental results from reactive ion etching, reactive ion beam etching, and chemically assisted ion beam etching will be shown for a number of substrate/etch mask systems at dimensions well below 1 μm linewidths.

Etch masks for microfabrication produced by electron beam sublimation of graphitic carbon deposited as semimetallic amorphous carbon thin films

High quality etch masks for nanometer plasma processing can be formed from thin films of semimetallic amorphous carbon that are deposited by electron beam sublimation of graphitic carbon. These films are amorphous, hard, semimetallic, and mirror‐reflective. These electron beam sublimation deposited (EBSD) semimetallic amorphous carbon (semimetallic a‐C) thin films can be routinely deposited up to at least 400 nm thick and patterned by SF6 plasma reactive ion etching (RIE) via standard photoresist masks. They are demonstrated to be excellent etch masks on gallium arsenide, silicon, and germanium substrates using chemically assisted ion beam etching (CAIBE), also known as ion beam assisted etching (IBAE), reactive ion beam etching (RIBE), and RIE. The carbon etch masks have fine grain, low chemical reactivity, low sputter rates, and high thermal stability. Finally, the EBSD semimetallic a‐C can be readily stripped by SF6 or O2 or H2 plasmas.

Lithography with silicon ions

Some applications of focussed ion beams are discussed with special emphasis on the lithography aspects. Experimental data on the ranges of H+, Be+ and Si++ in polymethylmethacrylate (PMMA) are presented. The sensitivity of PMMA to Si++ ions is measured to be 0.35 µC/cm2. It is shown by replication through a transmission mask that Si++ ions with low to moderate incident energies, such as found in focussed ion beam systems, can be used for submicrometer lithography in single and multi-layer resist systems.