N. N. Efremow

A new surface electron-emission mechanism in diamond cathodes

An electron-emission mechanism for cold cathodes is described based on the enhancement of electric fields at metal–diamond–vacuum triple junctions. Unlike conventional mechanisms, in which electrons tunnel from a metal or semiconductor directly into vacuum, the electrons here tunnel from a metal into diamond surface states, where they are accelerated to energies sufficient to be ejected into vacuum. Diamond cathodes designed to optimize this mechanism exhibit some of the lowest operational voltages achieved so far.

Comparison of electric field emission from nitrogen‐doped, type Ib diamond, and boron‐doped diamond

Field emission of electrons from boron‐ and nitrogen‐doped diamond is compared. Emission from boron‐doped diamond requires vacuum electric fields of 20–50 V μm−1, while nitrogen‐doped, type Ib diamond requires fields of 0–1 V μm−1. Since boron‐doped diamond is very conductive, very little voltage drop occurs in the diamond during emission. Nitrogen‐doped diamond is insulating, so during emission a potential of 1–10 kV appears in the diamond. This potential is a function of the back contact metal‐diamond interface. A roughened interface substantially reduces the potential in the diamond and increases emission. The electrons are often emitted from the nitrogen‐doped diamond as beamlets. These beamlets leave the surface of the diamond at angles up to 45° from the substrate normal. Although the vacuum field is small, these electrons have energies of several kV. It is unknown whether the electrons are accelerated to these energies in the bulk of the diamond, or at high electric fields near the emitting surface.

Ion-beam-assisted etching of diamond

The high thermal conductivity, low rf loss, and inertness of diamond make it useful in traveling wave tubes operating in excess of 500 GHz. Such use requires the controlled etching of type IIA diamond to produce grating like structures tens of micrometers deep. Previous work on reactive ion etching with O2 gave etching rates on the order of 20 nm/min and poor etch selectivity between the masking material (Ni or Cr) and the diamond. We report on an alternative approach which uses a Xe+ beam and a reactive gas flux of NO2 in an ion‐beam‐assisted etching system. An etching rate of 200 nm/min was obtained with an etching rate ratio of 20 between the diamond and an aluminum mask.

A High‐Yield Photolithographic Technique for Surface Wave Devices

In the fabrication of surface wave devices, standard photoresist‐chemical etching techniques often provide a very low yield even at moderate resolutions, frequently cause a degradation of the substrate finish, and in many cases are incompatible with substrates of interest. A photolithographic technique is described in which metalizing is done after a photoresist image is produced on the substrate, thereby circumventing most of the problems inherent in chemical etching. It is shown by means of electron micrographs of photoresist profiles that intimate mask‐substrate contact is essential. Devices with line widths of 1 µm can be produced with high yield.

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.

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.

A simple technique for modifying the profile of resist exposed by holographic lithography

Holographic lithography is a rapid and convenient method of exposing low distortion periodic and quasi‐periodic patterns over large areas. Spatial periods below 0.2 μm are readily achieved using commercial He:Cd lasers. However, many substrates of interest have significant reflectivity and, because the phase change on reflection is close to π, an intensity minimum occurs at the resist–substrate interface, leading to rounded ’’overcut’’ resist profiles. Such profiles are unsuitable for liftoff and undesirable for anisotropic dry etching. Moreover, the linewidth‐to‐period ratio is difficult to control and exposure latitude is limited. If a dielectric film is placed over a reflecting substrate, the intensity at the resist–dielectric interface can be maximized by proper choice of the optical thickness of the dielectric. We have developed a simple, operational, nondestructive method for precisely determining the angles of laser incidence that correspond to the condition of maximum interface intensity. The meth...

Instrumentation for conformable photomask lithography

A vacuum frame and a multiple mask alignment system for conformable photomask lithography are described. These instruments greatly improve the convenience and adaptability of conformable photomask lithography and may permit the advantages of improved resolution, dimensional control, photoresist profile control, and reduced mask damage to be realized in routine photolithographic production.

Anisotropic etching of Al by a directed Cl2 flux

A new Al etching technique is described that uses an ion beam from a Kaufman ion source and a directed Cl2 flux. The ion beam is used primarily to remove the native oxide to allow the Cl2 to spontaneously react with the Al film forming volatile Al2Cl6. By controlling both the flux equivalent pressure of Cl2 and the ion beam current, this etching technique makes possible the anisotropic etching of Al with etch rates from 100 nm min−1 to nearly 10 μm min−1 with a high degree of selectivity.