RU2316845C1 - Method for plasmochemical etching of semiconductor and insulating materials - Google Patents

Abstract

FIELD: electronics; semiconductor devices and methods for etching structures on their wafers.

SUBSTANCE: plasmochemical etching of material is conducted by way of acting on its surface with ion flow of plasma produced from plasma forming gas filling evacuated camber, electron beam being used to act upon plasma forming gas for plasma generation. Constant longitudinal magnetic field with flux density of 20-40 Gs is built on axis, plasma-generating gas pressure is maintained within chamber between 0.01 and 0.1 Pa, and electron beam at current density of 0.1-1 A/cm² ensuring ignition of beam-plasma discharge is used. Etching condition (energy and ion current density) can be controlled ether by modulating electron beam with respect to speed or by varying potential of discharge collector.

EFFECT: enhanced etching efficiency (speed) and quality of etching structures on semiconductor material surface: high degree of etching anisotropy preventing etching under mask, minimized material structure radiation defects brought in during etching.

2 cl, 1 dwg

Description

The invention relates to a technology for the production of semiconductor electronics devices, in particular to methods for etching structures on the surface of semiconductor and dielectric materials.

Currently, in the production of a wide range of semiconductor devices, from lasers to microcircuits of the microwave range based on silicon and semiconductor compounds AIII / BV, various plasma-chemical processes are widely used both for etching and cleaning the surface of semiconductors, dielectrics and metals, and their deposition on the surface of semiconductor structures. Processing materials using non-equilibrium _{(T e >> / T ion,} gas) plasma often provides greater process control and simultaneously reduced environmental impact compared to other materials processing techniques, and therefore represents a tremendous opportunity for the development of technologies of thin films and surface modification. For these purposes, RF and microwave discharges of two configurations are mainly used: an E-type discharge (capacitive) between two flat electrodes and an H-type discharge (induction) generated inside the contour coil of an RF generator. Technologies based on etching in discharges of this type are called RIE (reactive ion etching) and ICP (inductively coupled plasma), respectively [1].

The main disadvantages of plasma-chemical reactors based on RF discharges are the difficulty in controlling plasma parameters, the need to maintain a sufficiently high pressure of the working gas in the working volume to ensure the required etching rate, and as a result, weak anisotropy of the ion flux to the treated surface.

As an alternative or additional method of gas ionization in a plasma-chemical reactor, ionization by the electron flow of the kilovolt energy range is proposed. The advantages of this plasma source are: effective ionization (a smaller fraction of the energy input is spent on heating and optical excitation of the gas); independent control of ion and radical flows; the ability to control the spatial distribution of the ion flux.

Closest to the claimed method is a method of plasma chemical etching of semiconductor materials, in particular silicon, used in [2]. In this work, a plasma is created in a fluorine-containing gas medium (SF ₆ ) at a pressure of 50 mTorr by impact ionization of molecules by an electron beam with an electron energy of 2 keV and a low current density (up to 10 mA / cm ² ). The surface to be processed is placed in the chamber parallel to the direction of the beam in the immediate vicinity of the volume occupied by the beam.

The disadvantage of this method is that when used for ionization of low-density electron beams carrying out ionization by electron impact, the resulting plasma has a low electron temperature. As a result, the ion flux emerging from it has low energies and density, insufficient for effective etching of semiconductor materials, and to increase the efficiency, an RF voltage source must be used together with an electron-beam plasma source to provide an ion-accelerating potential on the surface of the semiconductor. It is also necessary to use a rather high pressure of the working gas (in [2] - 6.5 Pa), at which, due to the small mean free path of the ion in the gas, the ion flow has a weak directivity, which makes it difficult to etch the required structures on the surface of the semiconductor material. An increase in the directivity and etching efficiency is possible only with an increase in the RF voltage on the substrate, which leads to a significant increase in radiation defects due to an increase in the energy of ions bombarding the surface of the semiconductor.

The technical problem to be solved in the invention is to ensure the etching efficiency (required speed) and the high quality of etching of structures on the surface of semiconductor materials: a high degree of etching anisotropy, which excludes mask etching, and minimizing radiation defects of the material structure introduced during etching.

The solution of this problem and the technical result is achieved due to the fact that the plasma-chemical etching of a semiconductor or dielectric material is carried out by exposure to its surface of a stream of ions from a plasma formed from a working gas filling an evacuated chamber, and, to generate a plasma, an electron beam is applied to the working gas kilovolt range of energies. A longitudinal constant magnetic field with induction on the axis of 20–40 G is created in the chamber, the working gas pressure in the chamber is maintained at 0.01–0.1 Pa, and an electron beam with a current density of 0.1–1 A / cm ² , which provides beam ignition, is used - a plasma discharge and the generation of an ion stream in it with a density and energy that ensures effective etching. To increase the etching rate, one can make a change in the energy and density of the ion flux from the plasma either by modulating the electron beam in velocity or by changing the potential of the discharge collector.

SPR is generated at electron beam energies of 1-5 keV. At beam energies lower than 1 keV, the SPR is characterized by strong instability (high sensitivity to changes in the beam and gas parameters); The parameters of the ion flux required for effective etching can be obtained only at excessively high working gas pressures. At a beam energy above 5 keV, hard ultraviolet and X-ray radiation comes from the discharge region, leading to radiation defects in the structure of the semiconductor.

The range of magnetic field values is due to the requirement that the plasma electrons are magnetized (their Larmor radius is small compared to the transverse dimensions of the chamber, and the electron current mainly goes to the collector), and plasma ions are not magnetized (the Larmor radius is large compared to the transverse dimensions of the chamber )

The threshold beam current density for ignition of a discharge is determined by the condition

J = const · U ^3/2 · f (p) / HL,

where U is the electron energy, H is the magnetic field, L is the length of the chamber, af (p) is the function of pressure, which has a minimum at 0.01-0.03 Pa (in the specified range of beam parameters) (see, for example, [ 3]).

In accordance with this regularity, it is precisely the range of beam densities of 0.1-1 A / cm ² that provides ignition and maintenance of the SPR while observing the above conditions.

A diagram of an installation that implements the proposed method is shown in the drawing, where 1 is an electronic injector, 2 is a focusing coil, 3 is a vacuum chamber, 4 is a magnetic coil, 5 is a substrate with an etched structure, 6 is a substrate holder, 7 is a current sensor ions, 8 — discharge collector, 9 — microwave beam modulator, 10 — SPR plasma, 11 — electron beam, 12 — voltage source at the discharge collector.

Plasma 10 is formed in a vacuum chamber 3 — a cylinder with a diameter of 2R ₀ = 0.5 m and the same length. A longitudinal magnetic field with an induction of up to 0.5 mT in the chamber is created by coils 4. The source of the axial electron beam 11 is a Pierce diode gun with a flat hexabaride cathode, placed in a separate chamber, which is connected to the main chamber by a pressure differential tube. The parameters of the electron beam at the entrance to the plasma chamber: accelerating voltage U _b = 2 kV, current I _b - 300-500 mA, characteristic diameter 1 ÷ 1.5 cm. The gun’s power supply ensures its operation in pulsed mode with a pulse duration τ _b = 10-200 ms.

A discharge collector 8, a molybdenum disk with a diameter of 8 cm, is placed at the opposite wall of the plasma chamber.

The substrate holder 6 is mounted at a distance of 10-12 cm from the axis of the chamber so that the plane of the material being etched is parallel to the axis of the chamber. After chemical cleaning of the chamber, the processed structure 5 (a plate of semiconductor material with a resist mask applied) is installed on the substrate holder and the vacuum chamber is pumped out to a pressure of 2 MPa. After the inlet of the working gas (argon, fluorine-containing gas or their mixture in a controlled ratio), the chamber is additionally cleaned by ion bombardment and the sample is etched for a fixed time.

At various stages of processing, for example, when etching heterostructures, different densities and energies of the ion flux may be required. These parameters are changed by modulating the electron beam in velocity or by changing the discharge collector potential. In the first case, the ion energy decreases without a significant change in flux density. In the second case, the ion energy increases by an amount close to the collector potential, with a simultaneous increase in the flux density.

The technology of soft etching by Ar + ion streams with an average energy of 60-70 eV of pseudomorphic AlGaAs / InGaAs / GaAs (P-HEMT) semiconductor heterostructures with two-dimensional electron gas (2DEG) grown on GaAs substrates and used to fabricate microwave field-effect transistors has been tested.

Studies conducted on test samples of p-HEMT structures with Hall contacts showed that under these conditions there is no accumulation of radiation defects that worsen the parameters of a two-dimensional electron gas.

A variant of the technological process of manufacturing gate grooves of r-HEMT devices through a gap in the dielectric was also tested. Using electron beam lithography, narrow (0.1-0.5 μm) gaps were formed in a resist (PMMA950), through which selective etching was performed in a plasma of a Si ₃ N ₄ dielectric coating with a thickness of 80 nm to the GaAs semiconductor layer, where the etching process stopped. After removing the resist, the size of the etched grooves was measured using an atomic force microscope. The etching depth at the exposure time indicated above was 35 nm. There were no signs of etching inhomogeneity along a plate 60 mm in diameter. The presence of an etching effect without significant degradation of the parameters of the heterostructures (electron mobility 2DEG) is also shown, which indicates a low density of radiation disturbances and the possibility of using SPR in the manufacturing technology of heterostructured microwave NEMT devices.

In the proposed method due to the high density of the electron beam and the development of high-frequency instability, the plasma of the required density is formed at a working gas pressure that is 2 orders of magnitude lower than in the prototype method, which ensures high directivity of the ion flux and, accordingly, the quality of etching of structures under the mask.

The generated ion flux has the parameters required for effective etching (average energy and ion flux density) without the use of additional sources of RF voltage.

Information sources

1. G.F. Ivanovsky, V.I. Petrov. Ion-plasma processing of materials. M .: Radio and communications, 1986, p. 202-216.

2. K.D.Shatz and D.N. Ruzic. An electron-beam plasma source and geometry for plasma processing. Plasma Sources Science and Technology, 1993, v. 2 p. 100-105.

3. Artificial particle beams in space plasma. Sat under the editorship of B. Grannala. Per. from English M.: Mir, 1985, pp. 279-319.

Claims (2)

1. A method of plasma-chemical etching of a semiconductor and dielectric material by exposing the ion stream from a plasma formed from a working gas filling an evacuated chamber to its surface while creating a plasma by ionizing the working gas with an electron beam of a kilovolt energy range, characterized in that a longitudinal constant magnetic field with induction on the axis of 20-40 Gs, maintain a working gas pressure of 0.01-0.1 Pa and use an electron beam with a current density of 0.1-1 A / cm ² providing ignition beam-plasma discharge. 2. The method according to claim 1, characterized in that the change in the energy and density of the ion flux from the plasma is produced either by modulating the electron beam in speed or by changing the potential of the discharge collector.