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INVENTION
Patent of the Russian Federation RU2088056
ATOMIC HYDROGEN GENERATOR
The name of the inventor: Kagadey VA; Proskurovsky DI; Troyan O.E.
The name of the patent holder: State Scientific-Production Enterprise "NIIPP"
Address for correspondence:
Date of commencement of the patent: 1993.05.07
Usage: in microelectronics technology for generation of atomic hydrogen based on a gas discharge of direct current. SUMMARY OF THE INVENTION: An increase in the efficiency of obtaining atomic hydrogen is achieved by combining in a single device various mechanisms of dissociation of molecular hydrogen, as well as an increase in the concentration of electrons in the plasma of a gas discharge. The discharge current is 1 - 2 A, the combustion voltage -50 - 100 V, the hydrogen pressure in the discharge is more than 4 · 10 -2 Torr. The generator contains a device that creates a magnetic field for burning a Penning gas discharge in a discharge chamber. To increase the volume of the generation region with a dense plasma, the hollow cathode penetrates into the cavity of the cylindrical anode. As a result, a magnetron discharge is further initiated between the outer surface of the hollow cathode and the inner surface of the anode.
DESCRIPTION OF THE INVENTION
The invention relates to microelectronics technology, namely to devices for producing reactive particles, and more particularly to atomic hydrogen generators.
Generators of reactive particles are widely used in the manufacture of microelectronic products. Thus, the treatment of Si, Ge, GaAs, InP, and other semiconductors with atomic hydrogen is successfully used to purify substrates in order to obtain an atomically clean ordered surface, for passivation of carriers and defects lying at deep levels [1 3]
An atomic hydrogen generator is known [4] in which a tungsten filament heated to 2000 ° C is used. Molecular hydrogen introduced into a vacuum chamber thermally dissociates on a heated filament. At the same time, the yield of atomic hydrogen is about 0.1% of the total amount of gas supplied. The disadvantage of this generator is the low productivity and efficiency of obtaining atomic hydrogen.
An atomic hydrogen generator based on a microwave gas discharge is known under conditions of electron cyclotron resonance [5]. The productivity and efficiency of obtaining atomic hydrogen in such sources are high. The main drawback of this generator is its high cost and complexity.
An atomic hydrogen generator is known that is closest to the proposed technical solution and was chosen by us as a prototype [6] in which the generation of atomic hydrogen occurs in a discharge of direct current. The discharge chamber consists of a hollow water-cooled cathode 1 and a disk anode 2 with an emission hole 4 2.5 mm in diameter separated by a cylindrical insulator 3. The discharge voltage U p is 600 V, and the discharge current I p 0.1 A. The hydrogen pressure in the discharge chamber is about P 3 · 10 -1 Torr. The advantage of this generator is its simplicity. The main drawbacks of the generator are as follows.
1. Low productivity of obtaining atomic hydrogen due to a small discharge current.
2. High discharge voltage, which leads to inefficient operation of the generator. The large ion energy in the discharge promotes erosion of the cathode and increases the probability of radiation damage to the substrate by protons.
3. High hydrogen pressure in the discharge chamber and, as a consequence, in the treatment zone.
It is an object of the present invention to improve the construction of a generator to increase the productivity and efficiency of producing atomic hydrogen. The increase in the yield of atomic hydrogen from the generator and, consequently, the growth of its concentration in the treatment zone lead to an increase in efficiency and a reduction in the time of the technological operation. In addition, it makes it possible to clean the substrate surface under the conditions of a standard process vacuum (~ 10 -6 Torr), whereas at low concentrations of atomic hydrogen it is necessary to maintain a vacuum at a level (10 -7 10 -9 Torr) [7]
The goal is achieved by the fact that the proposed generator contains a device that creates a magnetic field that fires a Penning gas discharge in a discharge chamber formed by a thermally insulated thin-walled hollow cathode, a cylindrical anode and a reflective / flat / cathode arranged in series and having a common symmetry axis, made of a magnetic material And having an emission hole. In order to increase the volume of the generation region with a dense plasma, the hollow cathode penetrates into the cavity of the cylindrical anode. As a result, a magnetron discharge is further initiated between the outer surface of the hollow cathode and the inner surface of the anode.
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The construction of the proposed generator is shown in Fig. 2 and is an axisymmetric system consisting of a thin-walled heat-insulated cathode 1, a cylindrical anode 2, a flat cathode 4 and a device creating a magnetic field 5. The electrodes 1, 2, 4 are separated by cylindrical insulators 3. The emission hole 6 is made in a flat cathode. A thin-walled insulated hollow cathode holder, an anode and a flat cathode have forced water cooling. The generator is powered by a high-voltage direct current source. Gas supply is carried out through a leak, which allows to regulate the gas pressure in the discharge chamber.
The design of the device is developed on the following known facts and experimentally established by the authors. The decomposition of molecular hydrogen into atomic hydrogen can occur through thermo- or photodissociation, as well as dissociation by electron impact. The latter process is most effective, so dissociation in a gas discharge is the preferred method for obtaining atomic hydrogen. The growth of the electron concentration in the plasma, i.e. Discharge current, is the main mechanism that allows a significant increase in the yield of atomic hydrogen. The increase in the lifetime of electrons (ie, the mean free path before reaching the electrode or recombination) and the choice of their optimal energy (ie, the burning voltage of the discharge) and leads to an increase in the concentration of atomic hydrogen in the plasma. The presence of heated parts in the discharge zone and emission by a photon discharge further increase the degree of dissociation of hydrogen.
Experimentally, the authors found that the proposed geometry of the discharge chamber leads to an increase in the productivity and efficiency of obtaining atomic hydrogen by means of all the above mechanisms. An increase in the electron concentration in the plasma of a gas discharge occurs due to the appearance of thermionic emission of electrons from a thin-walled insulated hollow cathode. Under the action of the discharge current flowing through it, it self-heating to high temperatures (~ 2000 ° C). The emission of electrons into the plasma leads to an increase in the discharge current to several amperes and to a decrease in the discharge voltage to U p ~ 50-100 V (see the current-voltage characteristic of the discharge shown in Fig. The introduction of a part of the hollow cathode into the cavity of the cylindrical anode contributes to the formation of an additional volume with a dense plasma in which the effective generation of atomic hydrogen occurs. This plasma is excited between the outer side of the hollow cathode inserted into the anode cavity and the inner side of the cylindrical anode due to the initiation of a magnetron discharge between these electrodes. In addition to directly increasing the discharge current and, consequently, the amount of dissociated hydrogen, the magnetron discharge leads to an additional heating of the heat-insulated hollow cathode. The increase in the lifetime of electrons in the discharge is due to the elongation of their trajectory in crossed magnetic and electric fields and the oscillation of electrons between the two cathodes. The thermodissociation of hydrogen on a heated, thin-walled cathode that is thermally insulated in the floor, and additional photodissociation, which results from an increase in the plasma density and, consequently, the emission of the discharge, and leads to an increase in the degree of dissociation of molecular hydrogen. Estimates show that the amount of atomic hydrogen relative to the total number of molecular hydrogen introduced into the chamber can reach ten percent.
Thus, the proposed generator allows to increase the discharge current relative to the prototype device by at least 30 times, while the power consumption increases only by 5 times. Considering that the discharge current is directly related to the yield of atomic hydrogen, it can be concluded that the proposed device significantly improves the productivity of obtaining atomic particles. Assuming that the efficiency of obtaining atomic hydrogen is equal to the ratio of the discharge current to the electric power put into the discharge, it can be shown that the proposed device makes it possible to increase the efficiency of obtaining atomic particles by several times.
Reduction of the burning voltage of the discharge by ~ 6 times compared with the prototype device allows to increase the service life of the electrodes of the discharge cell by reducing their sputtering with hydrogen ions, significantly reduces the probability of radiation damage to the substrate under treatment by ions discharged from the discharge and increases the efficiency of the device.
In the proposed device, the discharge function is provided in a wide range of hydrogen pressures in the discharge chamber from P ~ 4 × 10 -1 Torr (which is realized in the device chosen for the prototype) to P ~ 4 · 10 -2 Torr, which is below the minimum pressure, at Which can work as a prototype. Reduction of the minimum pressure of hydrogen in the discharge leads to a significant increase in the flexibility of the technological process for processing semiconductor substrates and allows it to be carried out at a pressure in the treatment zone P ~ 1 · 10 -4 Torr. According to the published data [1, 4], this value is close to the optimum pressure for purifying the surface of semiconductors. In addition, the operation of the generator at a gas pressure in the discharge P ~ 4 · 10 -2 Torr leads to a reduction in the flow rate of hydrogen.
The control of the productivity of the atomic hydrogen generator and, consequently, the parameters of the technological process of processing the semiconductor substrate is made by changing the discharge current and the amount of hydrogen flow entering the discharge chamber.
A thin-walled heat-insulated cathode is made of refractory metals that react weakly with hydrogen (for example, Re, W). The thickness of the walls of the cathode is determined, on the one hand, by its mechanical strength, and, on the other hand, by the instantaneous effective heating of the cathode by the discharge current flowing through it. Experiments have shown that a thickness d of 100 μm completely satisfies these requirements.
The holder of a thin-walled insulated cathode and a flat cathode, for the purpose of concentrating the magnetic field in the combustion region of the discharge, are made of magnetic metals. For example, you can use St3, 30X13, etc. The cylindrical anode is made of a non-magnetic material. Best for this purpose is stainless steel 12H18N10T. The magnitude of the magnetic field must, on the one hand, effectively increase the trajectory length of the electron in the plasma, and on the other, be no greater than the value above which the plasma density practically does not change, and the costs of obtaining such a magnetic field increase. The most simple way to create a magnetic field is to use permanent magnets. Magnets based on the samarium and cobalt alloy, providing a magnetic induction of 0.1 0.12 T, meet the above requirements. The size of the emission aperture in a flat cathode is determined by the pressure drop that must be obtained between the discharge chamber and the processing zone of the semiconductor substrate. Usually this size is 1 - 3 mm.
Below, to illustrate the effects produced by the input characteristics, an example is given, described with reference to the drawings.
Example
The production of atomic hydrogen was carried out using the generator shown in FIG. 2. A thin-walled, heat-insulated hollow cathode 1 made of tungsten foil 100 μm thick penetrated into the cavity of the cylindrical anode 2 by a length equal to half the length of the anode. When the voltage was applied to the electrodes in the region 7, the Penning reflective discharge with a hollow cathode flashed, and in the region 8 a magnetron discharge. After switching on the power source of the generator, heating of the hollow cathode to the maximum temperature and the output of the generator to the regime occurred in a time of 1 3 s. The characteristic current-voltage characteristic of the discharge is shown in Fig. 3. From a comparison of the operating modes of the prototype device and this generator it can be seen that the use of the proposed device makes it possible to increase the discharge current by a factor of ~ 30, while reducing the combustion voltage by a factor of ~ 6. The consumed electric power thus increased only in 5 times. The existence of a falling portion of the VAC is due to the thermoemission of electrons from a thin-walled insulated cathode whose temperature increases with increasing discharge current.
CLAIM
An atomic hydrogen generator based on a gas discharge comprising a discharge chamber with an emission port containing a system of electrodes with a common axis of symmetry including an anode and a hollow cathode, characterized in that it comprises a device that creates a magnetic field that ensures the combustion of a Penning discharge in the chamber , An additional reflective cathode made of a magnetic material, the anode being cylindrical and located between the hollow and reflective cathode, the emission aperture is made in a reflective cathode, and the hollow cathode is made thin-walled and thermally insulated and partially introduced into the cavity of the cylindrical cathode.
print version
Date of publication 04.01.2007gg
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