JPH0634687A - Surface potential measuring device - Google Patents

Abstract

PURPOSE:To measure the surface potential easily while performing specimen treatment such as heat treatment at a high temperature and exposure to a special atmosphere by changing the dielectric constant of a space which is sandwiched by a reference electrode and a specimen electrode. CONSTITUTION:A rotating chopper 1 is fed into and out of a space repeatedly as a screening plate for changing the dielectric constant of a space which is sandwiched by both electrodes 2 and 3 without moving the reference electrode 2 and the specimen electrode 3 forming a capacitor. The chipper 1 is formed by hardening the powder of barium titanate to a thickness of approximately 2mm with an adhesive and is mounted to a phase-synchronous type motor 4 for drive and is rotated at approximately 300rpm, thus changing the electrostatic capacity between the electrodes 2 and 3 from approximately 0.2pF to approximately 100pF at a frequency of 30Hz. Also, a specimen heating mechanism 11 for purifying the electrode surface by heating at approximately 2000 deg.C is provided, thus reducing the limitation regarding the method for supporting the electrodes 2 and 3 and facilitating heat treatment and specimen replacement.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface potential measuring device, and more particularly to a surface potential measuring device which requires sample treatment such as heat treatment and exposure to a corrosive atmosphere, and sample exchange.

[0002]

2. Description of the Related Art A method of changing the capacitance of a capacitor composed of a reference electrode and a sample electrode by mechanically oscillating a reference electrode (Kelvin method) is reviewed by Scientific Instrument No. 4
7 (1976), pages 840-842.

[0003]

In the above prior art, the capacitance of the capacitor formed by the reference electrode and the sample electrode is changed by periodically changing the distance between both electrodes. Due to this capacitance change, an AC electromotive voltage proportional to the difference in surface potential between the reference electrode and the sample electrode is generated, and the difference in surface potential between the reference electrode and the sample electrode is obtained from the magnitude of this electromotive voltage. In this technique, it is necessary to move at least one of the reference electrode and the sample electrode in order to change the distance between the two samples. Therefore, there is a problem in that the mechanism for heating the electrodes and exchanging the sample is limited in weight, size, and the like.

An object of the present invention is to provide a surface potential measuring device which can alleviate restrictions on the method of supporting the reference electrode and the sample electrode and can easily carry out heat treatment and sample exchange.

[0005]

The above object is achieved by changing the dielectric constant of the space sandwiched between both electrodes without moving either the reference electrode or the sample electrode forming the capacitor.

One method of changing the dielectric constant is to repeatedly insert and remove the shielding plate in the space. The shielding plate may be made of any insulating material, but in order to perform measurement with high sensitivity, it is preferable to use a shielding plate made of a material having a large dielectric constant, that is, a relative dielectric constant with respect to air (or vacuum). Good. The shield plate can be put in and out by rotating the shield plate between the reference electrode and the sample electrode by making the shield plate into a notched disk shape (chopper), or by attaching a shield plate to the tip of a rod to form the pendulum like the reference electrode. It may be shaken between the sample electrode and the sample electrode.

As another method of changing the dielectric constant, the space between the reference electrode and the sample electrode may be alternately filled with two kinds of gases or liquids having different dielectric constants. For example, a gas or a liquid having a dielectric constant different from that of the atmosphere may be intermittently discharged into the space filled with the atmosphere. Further, two kinds of gases or liquids having different permittivities may be alternately discharged into the space.

[0008]

[Function] The capacitance of a capacitor composed of both electrodes depends on the area where both electrodes face each other, the distance between the electrodes, and the dielectric constant of the substance (including vacuum) that fills the space sandwiched between the electrodes. Decided. Therefore, if the dielectric constant is changed, it is possible to obtain the same effect as the method of changing the inter-electrode distance as shown in the conventional technique. When a pair of electrodes with different surface potentials are electrically connected by a conductor and face each other, an electric field corresponding to the surface potential difference between both electrodes is generated in the space sandwiched between the electrodes, and an electric charge corresponding to this electric field is applied to both electrodes. Accumulates on the electrode surface. In this state, if the capacitance of the capacitor composed of both electrodes is changed, the amount of charge accumulated on the electrode surface changes, but this change is caused by the conductor electrically connecting both electrodes. Done. Therefore, if a resistor is provided in the middle of the conductor, an AC electromotive voltage proportional to the surface potential difference between both electrodes is generated at both ends of the resistor in synchronization with the change in the dielectric constant.

For this reason, as a shield plate that can change the electrostatic capacity of a capacitor composed of both electrodes,
An insulating material having a dielectric constant different from that of the substance in the space sandwiched between the electrodes may be used. In this case, by inserting the insulating shield plate between both electrodes, the apparent dielectric constant of the space between both electrodes changes and the capacitance of the capacitor changes. In this case, the electromotive voltage is changed in accordance with the ratio of the dielectric constant of the shield plate itself, that is, the condition in which the shield plate is not inserted, that is, the atmosphere in which the sample electrode is exposed, such as the atmosphere or vacuum, and the dielectric constant. Occurs. Therefore, it is better that the ratio of the dielectric constant between the atmosphere and the shield plate is larger, and it is generally preferable to manufacture the shield plate using a material having a high relative dielectric constant.

[0010]

【Example】

<Example 1> As an example of the present invention, a surface potential (work function) measuring device for a metal single crystal will be described with reference to Figs.

FIG. 1 schematically shows the structure of a mechanism for using a rotating chopper 1 as a shield plate to insert and withdraw between a reference electrode 2 and a sample electrode 3. In this example, all of the chopper 1, the reference electrode 2, the sample electrode 3, and the chopper driving motor 4 shown in FIG. 1 were installed in an ultrahigh vacuum container having a vacuum degree of 10 nPa. As the chopper 1, a barium titanate powder is used which is hardened in the shape of a chopper as shown in FIG. 2 having a thickness of 2 mm by using an adhesive, and the chopper is attached to a phase-synchronous drive motor 4 at 300 rpm. It was rotated at the number of rotations. Both the reference electrode 2 and the sample electrode 3 are disc-shaped with a diameter of 10 mm and a thickness of 2 mm.
The distance between the electrodes was fixed to be 3 mm. As a result, the capacitance between the reference electrode 2 and the sample electrode 3 can be changed from about 0.2 pF to about 100 pF at a frequency of 30 Hz. Further, in order to clean the electrode surface by heating at about 2000 ° C., both of the electrodes are provided with a sample heating mechanism 11 by irradiating an electron beam of about 1 keV and 500 mA from the back surface. Details of the heating mechanism are shown in FIG. This heating mechanism electrically heats the tungsten filament 12, accelerates electrons with a high voltage applied between the filament and the electrode, and irradiates the electron beam from the back surface of the electrode. In order to increase the electron beam irradiation efficiency, the space between the filament 12 and the electrode is covered with a cylindrical shield plate 13 made of a tantalum plate having the same potential as the filament. Since the electric current flowing through the filament 12 is about 5 A, the heating lead wire 14 for supplying the electric current has to be relatively thick, which is an obstacle to the vibration of the electrode required by the conventional method. Becomes

The reference electrode 2 and the sample electrode 3 of the apparatus shown in FIG. 1 are connected via a detection resistor 5 of 1 kΩ, and these are connected to each other as shown in FIG.
It was connected to the control device having the structure shown in FIG. This control device is composed of a capacitor 6 and a lock-in amplifier 7, and extracts only an AC component of an electromotive voltage generated at both ends of a resistor 5 connected to the reference electrode 2 and the sample electrode 3 by a lock-in. Amplify by amplifier 8,
Get its amplitude value. The output of the lock-in amplifier 8 is detected by the DC voltmeter 10 and applied to the sample electrode 3 as a bias voltage. When the chopper 1 is rotated in a state where there is a difference in surface potential between the reference electrode 2 and the sample electrode 3, an AC electromotive voltage having an amplitude which is synchronized with the rotation of the chopper and has an amplitude proportional to the surface potential difference between both electrodes is generated by the resistance 5 Occurs at both ends of. At the output of the lock-in amplifier 8, a DC electromotive voltage proportional to the amplitude of this AC electromotive voltage is generated. This control device uses the bias voltage output from the lock-in amplifier 8 to
The surface potential difference between both electrodes is corrected, and the steady state is reached. In this steady state, the sum of the surface potential of the sample electrode 3 and the bias voltage is equal to the surface potential of the reference electrode 2. Since the surface potential difference between both electrodes is corrected to the same potential, the bias voltage required for the correction represents the surface potential difference between the reference electrode 2 and the sample electrode 3.

As the reference electrode 2 and the sample electrode 3, a measuring device was constructed using a tungsten (110) single crystal. In this device, when the measurement was performed without cleaning both electrodes, the surface potential difference was not detected because the surfaces of both electrodes were polluted by the air to the same extent. Next, the reference electrode 2 and the sample electrode 3 are heated at about 2000 ° C. for 10 minutes to clean the electrode surfaces, and then 10
Oxygen gas was introduced so that the pressure became 0 μPa, and the mixture was allowed to stand for 10 minutes to be saturated and adsorbed on the cleaned surfaces of both electrodes. The oxygen gas introduced at the saturated adsorption was evacuated, and only about 200 of the sample electrode 3 was evacuated in an ultrahigh vacuum with a vacuum degree of 10 nPa.
Surface cleaning was performed by heating at 0 ° C. for 10 minutes. At this time, in order to prevent the reference electrode 2 from being heated by the radiant heat from the sample electrode 3, a heat shield plate (not shown) was inserted between the two electrodes instead of the chopper. When the temperature of the sample electrode 3 dropped to room temperature, the surface potential difference between the reference electrode 2 and the sample electrode 3 was measured and found to be 2.3 eV. Since the work function of the clean surface of the tungsten (110) single crystal used for the reference electrode 2 is known to be 4.6 eV,
It was found that the work function of the sample electrode 3 at this time was 6.2 eV. FIG. 5 shows the time change of the surface potential difference between the reference electrode 2 and the sample electrode 3 when oxygen is introduced so that the degree of vacuum is 1 μPa in this state. The noise component included in the change in surface potential difference during this measurement was about 1 mV, and the measurement reproducibility of the surface potential difference was 10 meV or less.

Although an example in which a tungsten single crystal is used as the reference electrode 2 is shown here, any material having a known surface potential may be used, and in addition to tungsten, refractory metals such as tantalum and molybdenum may be heated and washed to obtain a reference electrode. Even when used as the electrode 2, the same measurement was possible. Also, lead titanate,
It was possible to measure even if the chopper 1 was formed using the powder of strontium titanate.

<Embodiment 2> FIG. 6 shows an embodiment of a surface potential measuring device using a bimorph type piezoelectric actuator. A shield plate 16 made by cutting a single crystal of titanium dioxide to a thickness of 2 mm with a reference electrode 2 and a sample electrode 3 facing each other at a distance of 3 mm, and a bimorph type piezoelectric plate having both fixed ends and free ends. The actuator 17 was attached to the free end of the actuator 17 and was driven so that the space between the electrodes could be moved in and out. The measurement circuit shown in FIG. 7 was connected to this, and the surface potential of the tungsten crystal accompanying oxygen adsorption was measured in the same manner as in Example 1, and the same result as in Example 1 was obtained. The same measurement was possible even if crystals of tungsten trioxide, tin dioxide and lead oxide were used as the shielding plate 16.

<Embodiment 3> FIG. 8 shows an embodiment of a surface potential measuring device using a nozzle beam at atmospheric pressure. The reference electrode 2 and the sample electrode 3 are opposed to each other at an interval of 5 mm to form a capacitor, and the condenser is installed in a measurement duct 19 in which nitrogen is flowed at a flow rate of 10 m / sec. did. The reference electrode 2 and the sample electrode 3 each having a diameter of 10 mm and a thickness of 3 mm are opposed to each other at an interval of 2 mm, and another gas that can be intermittently connected by the electromagnetic valve 21 can be flown between the two electrodes through the nozzle 20. . By intermittently ejecting a gas having a dielectric constant different from that of nitrogen from the nozzle 20 while flowing nitrogen between both electrodes, it is possible to change the capacitance of the capacitor formed by both electrodes.

A sample electrode 3 was used as the reference electrode 2 with a polycrystalline gold film having a thickness of 1 μm formed on the surface of stainless steel.
As a result, the apparatus shown in FIG. 8 was constructed using mirror-polished chrome crystals. In this state, the reference electrode 2 and the sample electrode 3
9, the control circuit shown in FIG. 9 is connected to the nozzle 20, and steam is used as a gas to be ejected from the nozzle 20.
It was ejected for 2 seconds. The ejection amount was controlled so as to be 3 to 5 times the flow rate of nitrogen between both electrodes so that the gas between both electrodes was sufficiently replaced during ejection. As a result, the electrostatic capacitance between both electrodes could be changed from about 0.2 pF to about 50 pF. With this device, in order to check the corrosion state of the chromium surface due to moisture, the amount of water vapor injection was adjusted so that the average humidity was 90%, and the change in surface potential was examined.
It was possible to measure the change from 4.2 eV to 4.5 eV.

[0018]

EFFECTS OF THE INVENTION By using the present invention, it becomes possible to easily measure the change in surface potential accompanying sample treatment such as heat treatment at high temperature or exposure to a special atmosphere.

[Brief description of drawings]

FIG. 1 is a perspective view of a surface potential measuring device using a dielectric chopper.

FIG. 2 is a plan view of a chopper.

FIG. 3 is an explanatory view of the principle of the sample heating device.

FIG. 4 is a block diagram of a surface potential measurement control circuit using a chopper.

FIG. 5 is a characteristic diagram of measurement results.

FIG. 6 is a perspective view of a surface potential measuring device using a piezoelectric actuator.

FIG. 7 is a block diagram of a surface potential measurement control circuit using a piezoelectric actuator.

FIG. 8 is an explanatory diagram of a surface potential measuring device using a nozzle beam.

FIG. 9 is a block diagram of a surface potential measurement control circuit using a nozzle beam.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Chopper, 2 ... Reference electrode, 3 ... Sample electrode, 4 ... Chopper drive motor, 11 ... Sample heating mechanism.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Emiko Yamada 1-280, Higashi Koikekubo, Kokubunji, Tokyo Inside Hitachi Central Research Laboratory (72) Inventor Susumu Sasaki 1-280 Higashi Koikeku, Tokyo Kokubunji City Hitachi, Ltd. Central Research Center

Claims (13)

[Claims] 1. A capacitor is formed by making a fixed reference electrode and a sample electrode of a measurement target, which are also fixed, face each other,
In an apparatus for measuring the electromotive voltage generated by changing the capacitance of the capacitor and measuring the surface potential difference between the sample electrode and the reference electrode, the device is sandwiched between the reference electrode and the sample electrode. A surface potential measuring device having a mechanism for changing the permittivity of the space. 2. The surface potential measuring device according to claim 1, further comprising a mechanism for putting a shield plate made of an insulating material in and out of a space sandwiched between the reference electrode and the sample electrode. 3. The surface potential measuring device according to claim 2, wherein the shielding plate is made of a material containing an oxide. 4. The surface potential measuring device according to claim 2, wherein the shielding plate is made of an oxide material containing at least one element of titanium, terbium, tungsten, lead, tin and lanthanum. 5. The surface potential measuring device according to claim 2, wherein the shielding plate contains at least one of titanium dioxide, tungsten trioxide, barium titanate, lead oxide, lead dioxide and tin dioxide. 6. The surface potential measuring device according to claim 2, 3, 4 or 5, wherein the rotating chopper is used to insert and remove the shielding plate between the reference electrode and the sample electrode. 7. The surface potential measuring device according to claim 1, 2, 3, 4, 5 or 6, wherein the reference electrode is tungsten, tantalum or molybdenum. 8. The surface potential measuring device according to claim 1, 2, 3, 4, 5 or 6, wherein at least the surface of the reference electrode facing the sample electrode is covered with gold. 9. The surface potential measuring device according to claim 1, further comprising a mechanism capable of intermittently filling a space between the reference electrode and the sample electrode with a gas having a dielectric constant different from that of a vacuum. 10. The surface potential for changing the dielectric constant of the space by alternately discharging two kinds of gas or liquid having different dielectric constants into the space between the reference electrode and the sample electrode according to claim 1. measuring device. 11. The surface potential measuring device according to claim 9, wherein the gas or liquid is discharged into the space using a nozzle. 12. The surface potential measuring device according to claim 9, 10 or 11, wherein the gas contains at least one of steam, hydrogen sulfide, methyl chloride, carbon monoxide and nitric oxide. 13. Claims 1, 2, 3, 4, 5, 6, 7,
8, 9, 10, 11 or 12, at least the surface of the sample electrode facing the reference electrode is
A surface potential measuring device having a structure which is exposed to any one of a vacuum atmosphere having a pressure of 100 μPa or less.