KR20010000401A - field ionization vacuum gauge - Google Patents

Abstract

PURPOSE: A vacuum gauge for field ionization is provided to precisely measure a vacuum(10-6Pa-10-11Pa) from an ultra high vacuum to an extreme high vacuum, thereby applicable to experiments requiring such a vacuum degree at a low cost. CONSTITUTION: A vacuum gauge for field ionization includes an ionization(emitter) electrode(1) and a collector electrode(2) mounted in a vacuum sealed container facing to each other under the state of insulation, wherein the ionization electrode is formed of a metal for introducing electricity from the outside of the vacuum container and closely contacting the container for maintaining the vacuum state and the collector electrode is formed of a metal in the semi-spherical shape for collecting generated ion to measure an ion current. In order to cause the field ionization, a positive voltage is applied to more than one needle having a radius less than 0.1micron at an end part or doped with carbon atoms in the shape of a tube of several nanometers.

Description

Field ionization vacuum gauge {field ionization vacuum gauge}

In measuring the degree of vacuum, it is an object of the present invention to make it possible to accurately measure extremely high vacuum of 10 ^-11 Pa or less with a simple structure.

Ion gauges are most widely used to measure ultra-high vacuum. Ion gauges use filaments and grids to generate ions and measure the amount of ions generated by the collector. However, the device is not only complicated in structure, but also generates soft X-rays and secondary electrons when electrons from the filament hit the grid. Because of this, even if there are no gas particles at all, a minute current flows through the collector due to the X-ray photon effect, and this current corresponds to an ion current flowing at 10 ^-8 Pa. Therefore, the vacuum degree below 10 ^-9 Pa cannot be measured. do.

Various types of vacuum gauges are used to measure the amount of gas particles in a limited space, but the apparatus capable of measuring up to a very high vacuum region (10 ^-9 Pa-10 ^-11 Pa) that cannot be measured by ion gauge is very expensive. It is a complex device. The present invention proposes a device that can measure up to an extremely high vacuum region with a simpler structure than an ion gauge.

As a method of measuring the degree of vacuum above a high vacuum, it is common to find the amount of gas indirectly by ionizing gas particles in a space and measuring the ion current. The method of ionizing a gas in space includes a method of colliding particles having energy greater than the ionization energy of the gas to neutral gas molecules and a field ionization by applying a high electric field. Until now, most of the ionization methods belong to the former, and the latter are very limited in practical use due to the difficult conditions for achieving field ionization and very low ion current. The general theory of field ionization is as follows.

-Field Ionization Theory

If no external electric field is applied, the ground state energy level E _g of the electrons in the neutrons near the metal surface is far below the Fermi level E _f of the metal atom.

However, when an external electric field is applied, the energy level of the electron increases by eFx relative to the distance X between the neutral atom and the metal surface. At this time, the energy level E _g of the electron in the neutral atom is increased to the Fermi level of the metal at a certain critical distance Xc. Therefore, the neutral atom is ionized by tunneling electrons through the potential barrier between the neutral atom and the metal surface. field ionization) (FIG. 5).

This reduction in potential barrier occurs because of the attractive force between the electrons and the induced image charge distribution inside the metal. In the region less than the critical distance Xc, the ground state energy level of the neutrons falls below the Fermi level of the metal. In this case, since all energy states of the metal are full, the Pauli exclusion rate prevents the accepting of more electrons in the metal, which leads to a large potential barrier, which dramatically reduces the probability of ionization. Therefore, when X <Xc, ionization by tunneling of electrons does not occur.

The probability P for tunneling through the potential barrier in one dimension is WKB ¹⁾

Is given by

Where v (x) and E are the potential and total energy of each electron, m is the mass of the electron, and x1 and x2 are the coordinates at both ends of the potential barrier at the energy level E. In equation (31), when the energy is ev and the width of the barrier is expressed in cm,

It is expressed as

In the equation (1), the potential barrier V (x) ²⁾ is

Where the first term is the coulomb potential by the cation as far as xi from the tunneling electron and the metal surface, the second term is the potential of the electron due to the external electric field, and the third and fourth terms are the positive and negative images of the electron. Represents potential energy by

The critical distance at which ionization takes place, that is, the distance Xc at which the tunneling probability is maximized, is given by the following equation.

Where I is the ionization potential, Is the work function of the metal, a _a and a _i are the polarization degree of the gas atom and the ion, respectively.

The third term is the imaging potential of the former and the fourth term is the difference in polarization energy before and after ionization occurs.

The last two terms are I and Is very small compared to, and can be ignored, so the critical distance Xc is

Is given by

The maximum transmittance at the critical distance Xc can be approximated by assuming that the height of the barrier is (I-ΔV) instead of the potential barrier given by (3).

here

It is given as 'Schottky reduction'. ³⁾

Then, at Xc, the maximum transmission probability is integrated by substituting (I-ΔV) into (32)

Is given by

When the ionization electrode is tungsten and the gas is helium, the optimal electric field F for field ionization was calculated by Muller and Young ⁴⁾ , where the critical distance Xc was 4.6 Å. Therefore, P (Xc) = 2.35 × 10 ⁻³ .

The electric field size of 440 MV / cm is a huge figure that is generally unthinkable. However, if the electric field is inversely proportional to the radius of the tip of the ionization electrode, it is possible as long as the radius r of the tip of the ionization electrode is reduced.

According to the experiment, when a voltage of 10 kV was applied to one tungsten electrode having a radius r of 50 nm, the ion current by field ionization was about 1 nA when the vacuum degree was 10 ^-3 Pa. However, with this degree of current, it is unmeasurable as 10 ^-4 pA at a vacuum of 10 ^-10 Pa or less.

Thus, in the present invention, carbon nanotubes are used instead of tungsten electrodes. As shown in FIG. 3, carbon nanotubes have a structure in which the carbon atoms are arranged in the shape of a few nm tube, and it is very easy to arrange billions of electrodes at a time far beyond the critical distance (Xc). . Therefore, under the same conditions, field ionization occurs at a voltage much lower than that of a tungsten electrode, which is several kV, and the ion current is tens of millions of times, which can be measured at several nA when the vacuum degree is less than 10 ^-10 Pa.

Bhom, D .: Quantum theory (Prentice Hall, New York, 1951)

2) Muller E. W. and Bahadur, K .: Phys. Rev. 102, 624 (1956)

3) Nishikawa, O .: Butsuri, 37 (1982)

4) Muller E. W. and Young, R. D .: J. Appl. Phys., 32, 2425 (1961)

Fig. 1 and Fig. 2 show the ionization vacuum gauge

3 shows the structure of the ionization electrode

4 is a structure of carbon nanotubes

Fig. 5 shows the potential energy of electrons of a gas atom near a metal surface in a high electric field.

<Explanation of symbols for main parts of drawing>

(1) long ionization electrode

(2) collector

(3) carbon nanotubes

(4) vacuum electric feed through

(5) Frangipani

In the present invention, the field ionization vacuum gauge is composed of two parts, an ionizer and a collector, and the ionization electrode and the collector are installed to face each other in an insulated state in a container that is vacuum sealed. have. The ionization electrode is made of metal so that electricity can be introduced from outside the vacuum vessel and is in close contact with the vessel to maintain the vacuum. In addition, carbon nanotubes are coated on the ends of the ionization electrodes in order to facilitate long ionization. Like the ionization electrode, the collector is made of metal to measure the ion current from the outside, close to the container to maintain vacuum, and hemispherical to capture the generated ions. However, this form can be modified into various forms in consideration of the efficiency of the vacuum exhaust. The vacuum container of the long ionized vacuum gauge can be attached to the open side so that it can be attached well to the other vacuum container to be measured. It can also be attached. The power supply is a DC voltage, a positive voltage of hundreds to thousands of V is applied to the ionizing electrode, and an ammeter is attached to the collector to read the current. At this time, the collector can apply a negative voltage of tens of V to collect the positive ion well. It may be.

Particle accelerators requiring very high vacuum, surface analysis equipment such as AES RBS XPS, by measuring the vacuum degree from ultra high vacuum to ultra high vacuum (10 ^-6 Pa-10 ^-11 Pa) accurately using a simple gauge It can be applied inexpensively to space development experiments and many other ultra high and ultra high vacuum experiments.

Claims (2)

In the vacuum gauge for measuring the degree of vacuum, the field ionization principle is used, and a device in which a positive voltage is applied to at least one needle having a radius of less than 0.1 micron in order to cause field ionization. The device of claim 1, wherein the tip radius includes carbon nanotubes instead of needles less than 0.1 micron.