JP2000065702A - Low-voltage discharge-type plasma ion film forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a plasma ion film forming device capable of speedily forming films without damaging samples by lowering the discharge voltage and raising the plasma density in a plasma ion film forming device to form organic synthetic films or osmium films on the surfaces of samples. SOLUTION: By making a cathode 2 a hollow cathode-type and impressing D.C. voltage between the cathode 2 and an opposing anode 3, it is possible to obtain the same discharge current by half a voltage or less in comparison with conventional parallel electrode-type diode discharge. By confining generated plasmas in the cup-shaped cathode, the plasma ion density is raised. By utilizing this, it is possible to implement a plasma ion film forming device to create an organic synthetic film or metal osmium film on the surface of a sample S placed in the hollow cathode efficiently without damaging the ions of the sample.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a scanning electron microscope,
And sample pretreatment when using a transmission electron microscope. Specifically, in a scanning electron microscope, a conductive coating is formed on the surface of a poorly conductive conductor sample, in a transmission electron microscope, a sample support film is formed, or Used to create a surface replica film.

[0002]

2. Description of the Related Art A scanning electron microscope (SEM) requires that the surface of a sample be a good conductor of electricity. For this reason, a poorly conductive conductor sample such as a biological sample or an inorganic compound is observed after forming a thin metal film or a carbon film of several nm on the surface. Observation with a transmission electron microscope (TEM) requires a thin film that supports a fine sample, and observation of the surface of a massive sample is performed by molding the surface into a thin film (replica film).

[0003] In order to deposit a conductive film in a SEM, gold, platinum, carbon, or the like is deposited using a vacuum evaporation apparatus or an ion sputtering metal deposition apparatus, and observation is performed. Since a certain thickness is required to form a continuous film, the fine structure of the sample surface is concealed, and the deposited metal is crystallized to become coarse particles, confusing with the structure of the sample surface and hindering accurate judgment. ing.

As a support film in a TEM, an organic compound such as collodion is used as a thin film, but it is reinforced by performing carbon vapor deposition because of insufficient strength, but as a result, the film becomes thick and promotes interference of electron beams to provide high resolution. Obstruction of observation.

[0005]

As a countermeasure to these problems, an organic gas or an inorganic compound gas is decomposed and repolymerized by utilizing the energy of plasma discharge to form a dense thin film or metal on the substrate surface or the sample surface. A method of forming a film (CVD method) is used.

For example, as a supporting film for a transmission electron microscope sample, methylene and ethylene alone or a mixed gas of several m
a to several + Pa atmosphere, and 15
A plasma discharge is performed by applying a direct current of 00 V to 3000 V to form a dense thin film on a substrate (for example, a cleavage surface of rock salt) placed on a negative electrode (cathode), and the thin film is recovered by dissolving the rock salt in water. Used as a support membrane.

To form a conductive film on a sample for a scanning electron microscope, 6 to 8 Pa of osmic acid (OsO ₄ ) is used.
When an atmosphere of 1300 V is applied and plasma discharge is performed by applying a voltage of about 1300 V, osmic acid is dissociated, and a film of metal osmium is formed on the surface of the sample placed on the cathode to become conductive.

However, the disadvantage of this method is that since the discharge voltage is extremely high, positive ions in the plasma have a large destructive power, and the sample (substrate surface or sample surface) placed on the cathode is subjected to ion etching. The support film for TEM loses smoothness, and the surface morphology is damaged in SEM samples. It is an object of the present invention to solve this drawback and to develop a plasma ion CVD apparatus capable of forming a support film adapted to more strict observation or generating a conductive film.

[Means for Solving the Problems]

The impact force F of plasma ions is the product of the atomic weight M of positive ions and the velocity v of the atoms, F = １ ／ Mv ^{2. Since} the velocity v is proportional to the discharge voltage V, F = １ ／ M
It is replaced by V ^2. In both cases of forming an organic film by plasma ion polymerization and forming a conductive film on the sample surface, it is not possible to reduce the atomic weight of the atmosphere gas as a raw material. Can not. In particular, when depositing a substance having a large atomic weight such as osmium, it is essential to lower the discharge voltage.

However, a stable discharge cannot be maintained simply by lowering the voltage. In the case of an organic gas, discharge is continued if the atmospheric pressure is increased to several hundred Pa, but the plasma region is too close to the cathode electrode surface, so that a three-dimensional sample comes out of the plasma region and the ion density is insufficient. As a result, the film forming speed becomes slow, which is not practical.

Since the vapor pressure of osmic acid at room temperature is limited to about 5 Pa, it is impossible to increase the atmospheric pressure and it is difficult to reduce the starting voltage of glow discharge to 1000 V or less.

There is a hollow cathode method as a method for stably maintaining a glow discharge. When the cathode electrode is formed in a cylindrical shape, the generated plasma is confined therein, and the electrons in the plasma help dissociate undissociated gas molecules to promote the formation of plasma, thereby increasing the plasma density. As a result, the discharge is stably maintained at a voltage that is one half to one third of the discharge voltage between the plate electrodes. In addition, since the plasma density in the cylinder is high, the film forming speed is accelerated on the surface of the sample placed in the cylinder.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention resides in the manufacture of an apparatus for forming a target film on a substrate or a sample surface with a relatively low voltage discharge by holding the sample in the hollow cathode electrode.

FIG. 1 is a longitudinal sectional view of an apparatus for explaining details of the invention. Reference numeral 1 denotes a vacuum chamber as a reaction chamber, which enables high vacuum evacuation at an atmospheric pressure of 0.1 Pa or less by a vacuum evacuation device 7. The vacuum evacuation device 7 is connected to the vacuum chamber 1 via a gate valve 8, closes the gate valve 8 as needed, and adjusts the adjustment valve 10 on the bypass 9 to adjust the evacuation speed according to the gas supply amount. Atmospheric pressure is vacuum gauge 15
Monitor with. A hollow cathode type electrode 2 as a negative electrode is installed in the vacuum chamber 1 insulated from the bottom plate of the vacuum chamber, and a negative voltage is applied from outside the vacuum. An anode electrode 3 is installed opposite to the cathode electrode 2 insulated from the vacuum chamber 1, and a positive voltage is applied from outside the vacuum. The cathode electrode 2 and the anode electrode 3 prevent unnecessary discharge on the back and side surfaces of the electrode by the electrostatic shield electrode 4. The hollow cathode electrode 2 has a side wall having a height of about half the electrode diameter, and a sample table 5 is installed on the bottom of the electrode insulated. The vacuum chamber 1 may include one or a plurality of gas supply devices 11 for film formation depending on the purpose.
Is connected. The gas supply device includes a gas chamber 12, a stop valve 13, and a needle valve 14. The DC power supply 16 can apply any voltage up to 1000 V so as to be convertible.

【Example】

As an example, the hollow cathode electrode 2 of the inventor's apparatus has a diameter of 60 mm, a side wall height of 25 mm, a diameter of the sample mounting table 5 in the electrode of 50 mm, an interval between the cathode electrode 2 and the anode electrode 3 of 50 mm, and gas supply. The apparatus 11 has two systems, and a molecular pump is used for the vacuum evacuation apparatus 7 at 10 ⁻² Pa.
Is obtained. The vacuum chamber 1 has a diameter of 150 mm and a height of 150 mm.

The film forming procedure in the above configuration will be described. The sample S fixed to a substrate such as rock salt as a sample to be adhered or a sample holder (stub) for a scanning electron microscope is a sample stage 5
And evacuated to a pressure of about 10 ⁻² Pa by the vacuum exhaust device 7. Close gate valve 8 and bypass 9
The flow control valve 10 is adjusted to maintain high vacuum, and the stop valve 13 of the gas supply device 11 is opened to inject gas for film formation into the vacuum chamber 1. Adjust the needle valve 14 to maintain the atmospheric pressure at 8 to 10 Pa, apply a high voltage, and reduce the discharge current to 5 to 10 mA.
Adjust to. The voltage at this time is 600V to 400V
Therefore, it is one-half or less as compared with the conventional flat electrode.

Osmic acid has a limit of a vapor pressure of 3 to 4 Pa at room temperature. At this pressure, a stable discharge cannot be obtained even with a hollow cathode electrode. Therefore, when nitrogen (air is acceptable) or argon gas is added to 8 to 10 Pa, the discharge is stably maintained and a metal osmium film is formed on the sample surface. However, the rate of osmium film formation decreases in proportion to the content of osmate in the atmospheric gas.

【The invention's effect】

The principle of plasma film formation is that a gas state as a raw material is separated into positive ions and electrons by the energy of discharge to form a plasma state, and the positive ions collide with the sample surface on the cathode side to form a plasma on the sample surface. That is, a new polymer film is formed. At this time, since the sample is placed in an insulated state from the cathode electrode, electrons flow in regardless of the negative potential of the cathode. Since the positive ions collide with the sample negatively charged by the incoming electrons, the sample is damaged by the high voltage discharge. Since the intensity of this ion impact is proportional to the square of the ion velocity v (= discharge voltage V), the lower the discharge voltage, the less the sample is damaged. In the hollow cathode method of the present invention, when compared at the same atmospheric pressure, the discharge voltage is less than one half of that of the flat electrode, so the impact force received by the sample is less than one fourth, and the biological strength is weak and has a fine structure. Film formation proceeds without being destroyed even by a sample or the like.

The rate of film formation on the sample surface is proportional to the density of the plasma surrounding the sample. In the case of a hollow cathode type electrode, plasma is confined in a cup-shaped electrode, so that plasma electrons help ionize undissociated gas to increase the plasma density. Since the sample is in a state of being immersed in the high-density plasma, the deposition rate is also accelerated.

As described above, a plasma ion film forming apparatus in which a sample is placed in a hollow cathode electrode reduces the discharge voltage and increases the plasma density around the sample so that the sample is not damaged by ions and the film forming speed is reduced. There is a great effect on hastening.

[0021]

[Brief description of the drawings]

FIG. 1 is a schematic sectional view of an apparatus configuration showing an embodiment of the present invention.

[Explanation of symbols]

1 vacuum layer. 10 Exhaust speed control valve. 2 Hollow cathode electrode. 11 Gas supply device. 3 Anode electrode. 12 Film source gas container. 4 Electrostatic shield. 13 Stop valve. 5 Sample table. 14 Needle valve. S sample. 15 Vacuum gauge. P plasma. 16 DC power supply. 6 Insulator. V voltmeter. 7 Vacuum exhaust device. mA ammeter. 8 Gate valve. 9 Bypass.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) G01N 1/28 F

Claims (2)

[Claims] In a diode discharge plasma ion polymerization film forming apparatus, a negative electrode is formed as a hollow cathode type, and plasma is confined in the cathode electrode to enable plasma polymerization by discharge at a relatively low voltage. Membrane equipment. 2. The plasma ion deposition apparatus according to claim 1, wherein a sample stage is provided in a hollow cathode electrode.