JPH0577135B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a niobium carbonitride field emitter exhibiting highly stable current characteristics. Field emitters can be used as high-intensity, coherent point light sources, such as in low-acceleration scanning electron microscopes,
It is important as an electron source for analytical electron microscopes, etc.

Conventional technology Conventionally, W metal has been put into practical use as a field emitter, but this W field emitter has problems with current stability, as the current significantly attenuates over time and has large 1/f noise. It excludes wide application.

Field emitters made of single crystal titanium carbide are also known, but they have problems with current stability.

The present inventors previously conducted research to solve these problems, and found that niobium carbonitride (NbCxNy)
0.7≦x+y≦1) Using a single crystal, this single crystal emitter is heat-treated with oxygen gas, hydrocarbon gas, and hydrogen sulfide alone or in combination at 1000 to 1200°C, and then heated in an ultra-high vacuum. below
By applying a strong electric field of 10 ⁸ V/cm or more,
We have developed a niobium carbonitride field emitter with good current stability and good electron emission characteristics. (Patent Application No. 62-113334) However, if the radiation current is increased to 10 μA or more to achieve high brightness, the current stabilization time becomes short, and in order to achieve stability, a vacuum of 1 × 10 ^-11 Torr is required. There was a problem that it was desirable to have.

Problems to be Solved by the Invention The present invention was made in order to solve the problem of this niobium carbonitride field emitter, and it is stable for a long time even if the emission current is increased to 10 μA or more, and is 2×10 The purpose of the present invention is to provide a method for obtaining a niobium carbonitride field emitter that exhibits extremely stable current characteristics even at ^-10 Torr.

Means for Solving the Problems In order to achieve the above object, the present inventors continued their research and found that a niobium carbonitride single crystal emitter was developed.
After being exposed in C ₂ H ₄ or other hydrocarbon gas for more than 1000 lang mire (1 lang mire = 1 x 10 ^-6 Torr sec) under heating at 900 to 1000°C, and then further heated in oxygen gas, When evacuated to a vacuum and applied a strong electric field of 10 ⁸ V/cm or more, the emission pattern changes, showing extremely stable current characteristics even at 2 × 10 ^-10 Torr, and emitting current as large as 10 μA. We found that it is stable for a long time. The present invention was completed based on this knowledge.

Niobium carbonitride single crystal emitter at 900~1000℃
Exposure in ethylene or other hydrocarbon gas for 1000 rangmire or more under heating ^of A method for producing a niobium carbonitride field emitter is provided.

Niobium carbonitride single crystal emitter used in the present invention (hereinafter referred to as NbCN emitter)
For example, 0.2×0.2× cut out from a single crystal rod
The tip of a 3 mm ³ rectangular parallelepiped was electrolytically polished to approx.
The tip diameter is 0.1 μm, and this is heated to 1700 μm in ultrahigh vacuum.
Flash heat at 1900℃.

This heating makes the surface clean and makes the tip of the chip a polyhedron covered with (100) and (111) planes. For example, in the case of an emitter whose emitter axis is in the <110> direction, the chip shape is a polyhedron as shown in FIG. The emission pattern from this clean surface is shown in FIG. The shaded area indicates the area where the electron beam was heated.

The resulting NbCN<110> emitter chips are heated, for example, 1× in ethylene or other hydrocarbons.
Under 10 ^-6 Torr, at temperatures ranging from 900 to 1000℃
Heat for 1000 seconds or more (exposure for 1000 degrees or more). Next, after vacuum evacuation, oxygen gas was introduced and 10 ^-6
Heat at the same temperature under Torr for at least 1 second, preferably 20 seconds (exposure of at least 1 lang mire, preferably 20 lang miurs). After such heat treatment, it is evacuated to vacuum and an electric field of 10 ⁸ V/cm or more is applied.
As a result, the emission pattern changes as shown in FIG. 2 to FIG. 3, and the radiation current is stabilized.

The heating temperature in the gas mentioned above must be in the range of 900 to 1000°C; below 900°C, spike noise and step noise will occur;
When the temperature exceeds .degree. C., drift occurs in addition to the above-mentioned noise. Short-term noise is ±0.2% over this temperature range
Below, the drift is less than ±0.1%/hr, resulting in extremely good current stability.

The heating time in a hydrocarbon gas of 1×10 ^-6 Torr must be 1000 seconds or more (1000 langurs or more). Figure 4 shows the relationship between heating time and stable current emission time (ethylene gas P=1×10 ^-6 Torr, emission current 10 μA).

As the diagram shows, 1000 seconds (1000 lang miur)
If it is less than 1, the stable current emission time is short. In addition, heating (exposure) in hydrocarbon gas only results in a small stable current and a short radiation time (1×
(10 ^-10 Torr, radiation current 6μA, about 20 minutes), but if heat treatment is further performed with oxygen gas, a stable current of 10μA can be obtained for over 1 hour.

Hydrocarbon gases include hydrocarbons such as methane in addition to ethylene.

Thus, in the present invention, the emitter surface is heated (exposed) under specific conditions and then a strong electric field is applied, and the effect is as follows.

(1) Heating (exposure) of the emitter surface By heating a NbCN chip with a clean surface in a hydrocarbon gas such as ethylene at a temperature range of 900 to 1000℃, a graphite film can be formed on the chip surface. can. A part of this graphite film surface layer moves to the tip of the chip by applying a strong electric field of the order of 10 ⁸ V/cm in a later step.
1/2 to 1/3 of the radius of curvature of the tip on a clean surface
It has a small radius of curvature, leading to the cutting edge of the central part of the chip. In other words, a part of the graphite film is moved to the tip of the chip by applying a strong electric field. If the heating temperature is less than 900°C, a sufficient graphite film cannot be formed on the chip surface, and if the heating temperature exceeds 1000°C, the graphite film formed will become too thick and the chip will not be able to form even when a strong electric field is applied. This is not preferable because it makes it difficult for the graphite film to move toward the tip.

It is known that on the surface of NbCN, many atomic vacancies (atomic holes) exist at anion sites (positions where carbon and nitrogen atoms should enter). On such a surface, surface atoms are easily mobile, and surrounding residual gas is also likely to be adsorbed by these atomic vacancies. The former causes short-time current noise, and the latter causes long-term noise (drift) due to an increase in work function due to gas adsorption. Such current noise can be eliminated by forming a graphite film on the chip surface. This is because the graphite film has a graphite structure, and this surface is extremely chemically inert, and furthermore, the covalent bonds between carbons are extremely strong, so there are almost no atomic vacancies. be.

The tip of the NbCN<110> chip has a polyhedral shape consisting of (100) and (111) planes (Fig. 1). The degree of current stability depends on whether a graphite film is formed on either the (100) or (111) plane of the chip tip due to exposure to hydrocarbon gas. Heating for less than 1000 seconds (exposure of less than 1000 langmire) under hydrocarbon gas pressure (1 x 10 ^-6 Torr) produces a graphite film only on the (111) plane.Heating for more than 1000 seconds (exposure of 1000 langmiure or more exposure),
Graphite films can be formed on both the (111) and (100) planes. The more the chip surface is covered with graphite film, the better the current stability will be.

Therefore, by forming a graphite film on the chip surface, a stable radiation current can be obtained.

After forming a graphite film on the chip surface, the chip is heated in oxygen gas (oxygen exposure).
This is to remove unreacted carbon atoms adsorbed on the graphite film by oxidation. Adsorbed atoms on the graphite film are undesirable because they cause current noise. Exposure in oxygen gas of 1 lang mire or more and 20 lang miurs or less is sufficient.

(2) Application of strong electric field After forming graphite on the chip surface, by applying a strong electric field of the order of 10 ⁸ V/cm under vacuum,
Move a part of the graphite membrane to the tip of the chip to create the leading edge in the center of the chip. In a chip having such a shape, the largest electric field is applied to the tip end, so electrons are mainly emitted from this part, and electrons parallel to the long axis direction of the chip are emitted.
FIG. 3 shows the emission pattern at this time.

Therefore, by applying a strong electric field, it is possible to reduce the radius of curvature of the tip of the chip and change the emission pattern from that shown in FIG. 2 to that shown in FIG. 3.

The reason why the strength of the strong electric field is set to 10 ⁸ V/cm or more is that the movement of the graphite film described above requires the application of a strong electric field of the order of 10 ⁸ V/cm or more. Application of an electric field weaker than this does not cause the graphite film to move toward the tip of the chip.

In addition, the following effects can also be obtained. That is,
The radius of curvature of the tip of the field emitter chip is usually formed to be 0.1 μm, but this is done by applying a voltage of 1000 to 2000 V between the electrodes, assuming that the distance between the tip of the chip (cathode) and the anode is 1 cm. Ten
Corresponds to drawing a current of ~20μA. The electric field applied to the chip at this time is on the order of 10 ⁸ V/cm. In other words, regardless of the radius of curvature of the chip tip, applying an electric field of the order of 10 ⁸ V/cm
A current of ~20 μA can be extracted. Moreover,
The leading edge of the chip is made of graphite film, allowing for stable extraction of current.

Example A NbC _0.95 N _0.01 <110> emitter with a tip diameter of 0.1 μm was heated at 1800°C under ultra-high vacuum (2×10 ^-10 Torr).
The surface was then flash heated to obtain a clean surface. Ethylene gas was introduced into this system and heated at 980°C for 25,000 seconds under a pressure of 1×10 ^-6 Torr (25,000 Langmiure exposure).
did. Next, after vacuum evacuation, oxygen gas was introduced and 1×
After heating at 980°C for 20 seconds under a pressure of 10 ^-6 Torr (20 lang mile exposure), the emitter is evacuated to an ultra-high vacuum and an electric field of 10 ⁸ V/cm or higher is applied to the emitter to create a second emitter pattern. The figure has been changed to Figure 3.

The obtained field emitter current noise is
At a vacuum level of 2×10 ^-10 Torr, the radiation current was below ±0.2%, the drift was below ±0.1%/hr, and a radiation current of 10 μA could be stably radiated for over 1 hour.

Effects of the Invention According to the method of the present invention, even under a vacuum of 2×10 ^-10 Torr, the current noise is ±0.2% or less, and the drift is ±0.1%/
hr or less and can stably emit a total current of 10 μA for more than 1 hour,
Moreover, it has an excellent effect of being able to obtain this with good reproducibility.

[Brief explanation of the drawing]

Figure 1 shows the tip tip shape of the NbC _0.95 N _0.01 <110> field emitter after flash heating at 1800°C, Figure 2 shows the emission pattern from the emitter after flash heating at 1800°C, and Figure 3 shows the present invention. The emission pattern of the field emitter obtained by the method shown in FIG. 4 is a diagram showing the relationship between the heating time in ethylene gas and the stable current emission time.

Claims (1)

[Claims] 1 Niobium carbonitride single crystal emitter 900~1000
℃, exposed in ethylene or other hydrocarbon gas for 1000 rangmire or more, then further exposed in oxygen gas for 1 to 20 langmire, evacuated to vacuum, and exposed to a strong electric field of 10 ⁸ V/cm or more. 1. A method for producing a niobium carbonitride field emitter, characterized in that a niobium carbonitride field emitter is applied.