A. S. Metel

Filling the Vacuum Chamber of a Technological System with Homogeneous Plasma Using a Stationary Glow Discharge

Experimental study of a glow discharge with electrostatic confinement of electrons is carried out in the vacuum chamber volume V ≈ 0.12 m3 of a technological system “Bulat-6” in argon pressure range 0.005–5 Pa. The chamber is used as a hollow cathode of the discharge with the inner surface area S ≈ 1.5 m2. It is equipped with two feedthroughs, which make it possible to immerse in the discharge plasma interchangeable anodes with surface area Sa ranging from ∼0.001 to ∼0.1 m2, as well as floating electrodes isolated from both the chamber and the anode. Dependences of the cathode fall Uc = 0.4−3 kV on the pressure p at a constant discharge current in the range I = 0.2−2 A proved that aperture of the electron escape out of the electrostatic trap is equal to the sum So = Sa + Sf of the anode surface Sa and the floating electrode surface Sf. The sum So defines the lower limit po of the pressure range, in which Uc is independent of p. At p < po the cathode fall Uc grows up dramatically, when the pressure decreases, and the pressure p tends to the limit pex, which is in fact the discharge extinction pressure. At p ≈ pex electrons emitted by the cathode and the first generation of fast electrons produced in the cathode sheath spend almost all their energy up to 3 keV on heating the anode and the floating electrode up to 600–800°C and higher. In this case the gas in the chamber is being ionized by the next generations of electrons produced in the cathode sheath, their energy being one order of magnitude lower. When Sa < (2m/M)1/2S, where m is the electron mass and M is the ion mass, the anode may be additionally heated by plasma electrons accelerated by the anode fall of potential Ua up to 0.5 kV.

Broad beam source of fast atoms produced as a result of charge exchange collisions of ions accelerated between two plasmas

Experimental study of a fast argon atom beam source is carried out and the study results are presented. The source comprises a 90-mm deep and 210-mm in diameter hollow cathode as well as a flat emission grid, both electrodes made of titanium. The study revealed main factors, which influence the zone diameter of homogeneous substrate etching by a broad beam of fast argon atoms, produced as a result of charge exchange collisions of ions, accelerated between a plasma emitter inside the hollow cathode and a secondary plasma in the working vacuum chamber, the plasmas being separated from each other with the grid. It is shown that at a distance from the grid, exceeding the resonant charge exchange length up to 4 times, elastic collisions have no appreciable impact on the spatial distribution of the etching rate in the vacuum chamber. The homogeneous etching zone diameter is mainly influenced by angular characteristics of accelerated particles in the grid plane. At a constant beam power up to 3–5 kW the diameter is rising with a decrease of their energy and with a corresponding increase of the beam current.

Broad beam sources of fast molecules with segmented cold cathodes and emissive grids

Experimental study of fast neutral atom and molecule beam sources with rectangular and circular cross-section of the beam up to 0.8 m2 is carried out and the study results are presented. The fast particles are produced as a result of charge exchange collisions between gas molecules and ions accelerated by potential drop between the plasma emitter of the beam source and the secondary plasma inside the processing vacuum chamber. As the emitter is used a glow discharge plasma, whose electrons are confined in an electrostatic trap formed by a cold hollow cathode and an emissive grid, which is negative both to the cathode and to the chamber. In order to prevent from breakdowns between the emitter and the cathode at a current in the cathode circuit up to 10 A as well as between the emitter and the grid at a voltage between them up to 10 kV the cathode and the grid are composed of isolated from each other segments, which are connected to power supplies through resistors. When resistance of the resistorR > U/I0, where U is the power supply voltage and I0 is the minimal current of stable vacuum arc for a given segment material, then transition from the glow discharge to the steady-state vacuum arc is totally excluded in spite of numerous breakdowns of microsecond duration due to contamination of the source electrodes during its operation with dielectric films and other stimulants of the arc.

A compact vapor source of conductive target material sputtered by 3-keV ions at 0.05-Pa pressure

A vapor source is developed its 80-mm-diameter and 15-mm-thick flat target being positioned on the bottom of a 120-mm-diameter and 70-mm-deep hollow cathode, isolated from the cathode and sputtered by 1–4-keV argon ions. A permanent magnet induces an axially symmetric heterogeneous magnetic field, the field induction on the target surface reaching 20 mT and the field lines of force being diverging from the target surface and crossing the cathode surface. The cathode bombardment by 1–3-keV secondary electrons emitted by the target results in an increase of the electron emission current in the cathode circuit and enables to reduce the argon pressure down to 0.05 Pa. It allows a collisionless transport of the sputtered metal atoms to a substrate thus keeping their initial energy amounting to tens of electronvolts. A higher energy of deposited atoms improves quality of coatings, for instance of Ti3SiB2 films, their deposition rate on a substrate distanced at 0.1–0.2 m from the target amounting to 10–20 µm/h at 1-A current in the target circuit and 3-keV energy of sputtering ions. This value is one order of magnitude higher in comparison with the target sputtering in a planar magnetron discharge by 300–500-eV argon ions at the same 1-A current in the target circuit.

Beams of fast neutral atoms and molecules in low-pressure gas-discharge plasma

Fast neutral atom and molecule beams have been studied, the beams being produced in a vacuum chamber at nitrogen, argon, or helium pressure of 0.1–10 Pa due to charge-exchange collisions of ions accelerated in the sheath between the glow discharge plasma and a negative grid immersed therein. From a flat grid, two broad beams of molecules with continuous distribution of their energy from zero up to e(U + Uc) (where U is voltage between the grid and the vacuum chamber and Uc is cathode fall of the discharge) are propagating in opposite directions. The beam propagating from the concave surface of a 0.2-m-diameter grid is focused within a 10-mm-diameter spot on the target surface. When a 0.2-m-diameter 0.2-m-high cylindrical grid covered by end disks and composed of parallel 1.5-mm-diameter knitting needles spaced by 4.5 mm is immersed in the plasma, the accelerated ions pass through the gaps between the needles, turn inside the grid into fast atoms or molecules, and escape from the grid through the gaps on its opposite side. The Doppler shift of spectral lines allows for measuring the fast atom energy, which corresponds to the potential difference between the plasma inside the chamber and the plasma produced as a result of charge-exchange collisions inside the cylindrical grid.

Non-self-sustained glow discharge with electrostatic confinement of electrons sustained by a fast neutral molecule beam

Experimental study of plasma produced at the nitrogen pressure 0.2–1 Pa in the chamber volume V ≈ 0.12 m3 as a result of injection into the chamber of a broad nitrogen molecule beam with 1–4 keV energy and 0.1–1 A equivalent current is carried out, and the study results are presented. Dependences of the plasma density distribution on the beam equivalent current Ib, energy Eb, and gas pressure p indicate a crucial role of fast molecules in gas ionization, and the probe characteristics reveal two groups of plasma electrons with the temperatures Te ∼ 0.4 eV and Te ∼ 16 eV. Immersion in plasma of an electrode isolated from the chamber and application to the electrode of a positive voltage U result in non-self-sustained discharge. When U changes from ∼0.5 to ∼1.5 V, the discharge current I rapidly rises to a certain value I*, and after that the rate of rise dI/dU drops by an order of magnitude. At U ∼ 10 V, the current I rises to I0 ≈ 1.5I*, and dI/dU once again drops by an order of magnitude. Current I0 specifies the number of electrons produced inside the chamber per second, and it grows up with Eb, Ib, and p. At U > 20 V, due to gas ionization by fast electrons emitted by the chamber and accelerated up to the energy ∼eU in the sheath between the plasma and the chamber walls, the current I rises again. When U grows up to ∼50 V, production of fast electrons with energies exceeding the ionization threshold begins inside the sheath, and the ionization intensity rises dramatically. At U > 150 V, contribution of fast electrons to gas ionization already exceeds the contribution of fast molecules, and the plasma density and its distribution homogeneity inside the chamber both grow up substantially. However, even in this case, the discharge is non-self-sustained, and only at U > 300 V it does not expire when the beam source is switched off.

Characteristics of a fast neutral atom source with electrons injected into the source through its emissive grid from the vacuum chamber

In order to increase the equivalent current of a fast neutral atom beam the cold hollow cathode of the beam source is bombarded with electrons extracted from the plasma produced in the vacuum chamber and accelerated in the sheath between the plasma emitter of the source and its emissive grid. The cold cathode bombardment by accelerated electrons raises its electron emission current by an order of magnitude and as a result voltage Uc between the anode and the cathode of the source diminishes more than two times. This allows of increasing several times the beam equivalent current or decreasing the working gas pressure. A slight decrease in the Uc with increasing the accelerating voltage U at an overall cutoff of the electrons from the chamber reveals the influence of secondary electrons emitted by the grid. Measurement of the beam current is discussed.

Glow discharge with electrostatic confinement of electrons in a chamber bombarded by fast electrons

A metal substrate is immersed in plasma of glow discharge with electrostatic confinement of electrons inside the vacuum chamber volume V ≈ 0.12 m3 filled with argon or nitrogen at pressures 0.005–5 Pa, and dependence of discharge characteristics on negative substrate potential is studied. Emitted by the substrate secondary electrons bombard the chamber walls and it results in electron emission growth of the chamber walls and rise of gas ionization intensity inside the chamber. Increase of voltage U between the chamber and the substrate up to 10 kV at a constant discharge current Id in the anode circuit results in a manifold rise of current I in the substrate circuit and decrease of discharge voltage Ud between the anode and the chamber from hundreds to tens of volts. At pressure p < 0.05 Pa nonuniformity of plasma density does not exceed ∼10%. Using the Child-Langmuir law, as well as measurement results of sheath width d between homogeneous plasma and a lengthy flat substrate dependent on voltage U ion current density ji on the substrate surface and ion-electron emission coefficient γi are calculated. After the current in circuit of a substrate made of the same material is measured, the γi values may be used to evaluate the average dose of ion implantation. The rate of dose rise at a constant high voltage U is by an order of magnitude higher than in known systems equipped with generators of square-wave high-voltage pulses. Application to the substrate of 10-ms-wide sinusoidal high-voltage pulses, which follow each other with 100-Hz frequency, results in synchronous oscillations of voltage U and ion current Ii in the substrate circuit. In this case variation of the sheath width d due to oscillations of U and Ii is insignificant and d does not exceed several centimeters thus enabling substrate treatment in a comparatively small vacuum chamber.

Equipment for deposition of thin metallic films bombarded by fast argon atoms

Deposition of thin metallic films on dielectric substrates using a source of metal atom flow combined with a flow fast argon atoms has been investigated and the investigation results are presented. The fast atoms are produced due to charge-exchange collisions in a vacuum chamber of argon ions, which are accelerated by potential difference between the hollow-cathode glow-discharge plasma and an emissive grid and enter the chamber through the grid. The metal atoms produced due to ion sputtering of a metallic foil placed on the inner surface of the hollow cathode enter the chamber through the same grid. Substrate pretreatment and pulse-periodic bombardment of the growing film by ∼1-keV argon atoms both ensure adhesion of copper to glass up to 2 × 107 Pa. The use of a hollow substrate holder, whose inner surface is also covered with the same foil, makes it possible to exclude losses of the depositing metal and allows recommendation of the equipment for beam-assisted deposition of precious metal films.

Gas discharge source of metal vapor and fast gas atoms

A source of metal atom flow coinciding in time and space with a flow of fast gas atoms has been studied and the study results are presented. The fast particles are produced due to charge exchange collisions of ions accelerated by potential difference between a plasma emitter inside the source and secondary plasma inside a process vacuum chamber. The emitter is the glow discharge plasma, whose electrons are confined in an electrostatic trap formed by a cold hollow cathode and an emissive grid the latter being negative both to the cathode and the chamber. The metal atoms are produced due to sputtering a target placed at the hollow cathode bottom by ions from the plasma emitter with energy up to 3 keV. Sputtered atoms cross the emitter, together with accelerated ions enter the chamber through the emissive grid and deposit on pieces placed therein. When a mixture of argon and nitrogen is used, the metal nitride coatings are being synthesized and interruptedly bombarded during the synthesis by atoms and molecules with energy variable from ∼10 to ∼300 eV.