JPH0680496A - Knudsen cell for low temperature - Google Patents

Abstract

PURPOSE:To provide the subject Knudsen cell designed to prevent developing conditions hampering the vacuum maintenance in material filling, and also enabling the continuous operation of a molecular beam epitaxial device. CONSTITUTION:A reservoir 8 which can hold materials in is provided outside a vacuum chamber 3 and connected to the inside of the chamber 3 through an introducing pipe 5. A needle valve 7 and an original heater 16 are arranged at the intermediate sites of the pipe 5, respectively. This heater 16 serves as preventing materials from being stuck to the inner wall of the pipe 5. The reservoir 8 is equipped with each original vacuum exhauster and heater 9. The reservoir 8 can be filled with materials at the atmospheric pressure while maintaining the inside of the vacuum chamber at an ultra-high vacuum. The quantity of molecular beams is controlled by the needle valve 7.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improved knudosen cell used in a molecular beam epitaxy apparatus. This may be a molecular beam cell or simply K cell. The name of the inventor is sometimes called Knudsen cell or Knud cell. In any case, the material is put in a crucible and heated by a heater to evaporate or sublime the material to form a molecular beam.

A molecular beam epitaxy apparatus heats a material in an ultrahigh vacuum, so that what is evaporated or sublimated becomes a molecular beam. Materials for molecular beam epitaxy are various such as metal and nonmetal. The term "for low temperature" as used herein means that it applies to substances having a high vapor pressure even at low temperatures, such as organic materials, sulfur S, selenium Se, and chlorine Cl.

[0003]

2. Description of the Related Art FIG. 2 is a sectional view of a conventional Knudsen cell. This is not for low temperatures, but for materials with relatively low vapor pressure, such as metals. There is currently no suitable Knudsen cell for low temperature, low vapor pressure materials. The cell shown is provided as part of a molecular beam epitaxy system.

The Knudsen cell includes a crucible 1 containing the material and a heater 2 for heating the material, which are provided in the hole of the liquid nitrogen shroud 4. All of these are inside the vacuum chamber 3. Knu for each material
Because of the need for dozens of cells, a plurality of such knud dozen cells are provided inside the vacuum chamber. For this reason, the vacuum chamber 3 has a number of cylindrical Knudsen cell attachment ports 24.

In addition to the crucible 1 and the heater 2, the Knudsen cell has a reflector 11 for shielding the heat of the heater, and a column 12 for supporting the reflector 11, the crucible 1 and the heater 2.
Have. The column 12 is attached to an ultra-high vacuum flange 13 fixed to an opening of a Knudsen cell attachment port 24 of the vacuum chamber 3. Heater that supplies the electric power of the heater
The power line 14, the terminals of the thermocouple, etc. are taken out through the ultrahigh vacuum flange. As the crucible 1, for example, PBN is used. The heater may be a W or Ta coil or ribbon. The temperature of the heater is monitored by a thermocouple. The reflector is made by stacking several thin Ta plates. The radiant heat is confined in the vicinity of the crucible, which escapes to the outside. In addition, a shutter (not shown) is provided to open and close the opening of the crucible.

Inside the vacuum chamber 3, there is a manipulator (not shown) in addition to this, which supports the substrate fixed to the holder. The manipulator has a heater for heating the substrate and often has a rotating mechanism. This is because most epitaxial growth does not occur unless the substrate is heated. However, depending on the material, it may not be necessary to heat. Some also require cooling. The rotation of the substrate is for uniform epitaxial growth on the substrate. Further, a transport mechanism (not shown) is provided so that the sample holder can be automatically transported to and from the sample preparation chamber, analysis chamber, etc. at the preceding stage.

Such molecular beam epitaxy apparatus is already well known. After filling the Knudsen cell with material, the vacuum chamber is sealed and a vacuum is applied. By baking, the gas adhering to the inner wall of the chamber and internal structures is removed, and then a vacuum is applied to obtain 10 ^-10.
Ultra high vacuum of -10 ^-11 Torr is applied. The baking is, for example, to wind a heater around the entire vacuum chamber and cover it with a metal plate to heat the entire chamber. Since the substrate is transferred through another vacuum chamber, the ultra-high vacuum is maintained even when the substrate is replaced. The material filled in the crucible where the epitaxial growth is repeated is exhausted.
At this time, the vacuum in the vacuum chamber 3 is broken, and the crucible 1 is filled with the material. Since gas and impurities adhere to the wall surface once the atmospheric pressure is reached, it is necessary to repeat the operation of baking the whole and drawing a vacuum. It takes a lot of time to return to the original ultra-high vacuum. It takes 3 days to 1 week depending on the equipment and conditions.

The opening and closing of the chamber for filling the material varies depending on the type of epitaxial growth.
Sometimes, once a month, or once a week. It is better not to open and close the chamber as much as possible because it takes time to start up the vacuum and causes a drop in productivity.

In order to solve such a difficulty, a gas source
Scell is proposed. This introduces a material gas into a chamber from a gas cylinder placed outside the chamber. Since the material is supplied from the outside, the material is not depleted and the necessity of breaking the ultra-high vacuum is reduced. There is also a method in which the molecular beam cell is moved away from the center of the chamber to replenish the material without opening the vacuum chamber. (JP-A-63-310792, JP-A-64-79095)

[0010]

In a conventional molecular beam epitaxy apparatus which fills a knudsen cell inside a chamber with a material, it is necessary to break the vacuum in order to fill the material. Evacuating again is time consuming and undesirable. There is a further problem especially when a substance having a high vapor pressure even at a low temperature is used as a material. At the time of evacuation, the entire chamber is baked, but at this time, the substance having a high vapor pressure is heated and evaporates and sublimes to lose the amount. To avoid this, baking must be performed at a low temperature. There's nothing you can't do, but the increase in vacuum slows down.

A gas source cell which supplies a material as a gas from the outside does not have such a defect. However, materials that are gases at room temperature are limited. Since molecular beam epitaxy uses a simple substance as a material, it is possible to grow a thin film having excellent purity. There are even fewer materials that are gases alone.
Since many of hydrides, chlorides, halides, and organometallics are gaseous at room temperature, materials of these compounds may be used. However, some substances do not necessarily form gaseous compounds.

Further, since it is a molecular beam of a compound, it is different from a molecular beam of a simple substance. When the molecular beam of the compound is applied to the substrate, impurities are likely to be contained inside the thin film and it is difficult to control the composition. Further, in the case where the molecular beam cell is separated from the center of the chamber, the flight distance as a molecular beam becomes long, so that the energy and direction cannot be controlled well. The present invention is a knudsen cell that handles a material having a high vapor pressure even at a low temperature, does not hinder baking, does not need to open and close the vacuum chamber each time the material is replenished, and combines the functions of a cracker cell. Knu that has a precise control of molecular dose
The purpose is to provide dosencell. As a cracker cell, there is, for example, Japanese Utility Model Laid-Open No. 4-29664, but this is not easy to manufacture and use because of the large scale of the device.

[0013]

In the Knudsen cell of the present invention, a vacuum chamber is evacuated to an ultrahigh vacuum, a material is heated and vaporized, and a substrate in the ultrahigh vacuum is irradiated as a molecular beam for epitaxial growth on the substrate. A knudsen cell for a molecular beam epitaxy device, which is a reservoir provided outside a vacuum chamber and having a lid that can be opened and closed to accommodate a material.
And a heater for heating the material of the reservoir, an introduction pipe for connecting the reservoir and the vacuum chamber and introducing a molecular beam into the vacuum chamber, and an introduction pipe. , An opening heater provided in the vicinity of the outlet in the vacuum chamber, a needle valve with adjustable flow rate provided in the introduction pipe between the reservoir and the introduction pipe, and the introduction pipe. It has a connecting portion heater provided for heating the intermediate portion and a vacuum exhaust device for drawing the reservoir to a vacuum, and controls the temperature of the reservoir and the opening degree of the needle valve. It is characterized in that the molecular dose can be adjusted by carrying out.

[0014]

Since the reservoir is outside the vacuum chamber,
It is not necessary to open the vacuum chamber when filling the reservoir with material. Keep the needle valve closed and open the lid of the reservoir to fill the reservoir with material. After this, the reservoir was closed and the vacuum was evacuated.
Open the dollar valve to connect the reservoir to the vacuum chamber. Material vapor generated from the reservoir enters the vacuum chamber and becomes a molecular beam. Reservoir - Ba - heat - data and the ratio of the intermediate connection part of the introduction pipe - heat of motor and the tip opening - it controls the motor to independently control the temperature T _1, T _2, T ₃ of these portions You can The reservoir heater can adjust the temperature of the reservoir to control vapor generation and control the molecular dose. Further, the molecular dose can be directly controlled by adjusting the opening degree of the needle valve. The needle valve controls the molecular dose directly, but it can also be fully closed. In this respect, it can be said that it also has the function of a partition valve. However, if the needle valve does not have the function of fully closing, it is advisable to provide a partition valve in a part of the introduction pipe in series with the needle valve.

The connecting portion heater can prevent the material vapor from cooling and solidifying inside the introducing pipe and adhering to the inner wall. Since the reservoir was provided outside, there was a possibility that the material vapor would solidify on the way. The conventional one as shown in FIG. 2 is unnecessary. The opening heater has a cracking action when the vapor of the material is a mass of a large number of molecules and is not in a molecular state, by igniting the vapor to make a small molecular state.

Further, since the material is outside the vacuum chamber, when the vacuum chamber is baked, the heat for baking is not transferred to the material so that the material is evaporated and sublimated and the amount is not reduced. Especially, it is suitable for a material having a high vapor pressure at a low temperature.

[0017]

1 shows a schematic structure of a Knudsen cell according to an embodiment of the present invention. This Knudsen cell does not have a container filled with a material such as a crucible inside the vacuum chamber 3. Only the introduction pipe 5 for introducing the vapor of the material is inserted into the vacuum chamber 3.
The introduction pipe 5 communicates with the outside of the vacuum chamber 3 and is connected to the reservoir 8 at the outer end. External reserve
The bar 8 is a container that can be opened and closed and can store materials. A liquid nitrogen shroud 4 is provided inside the vacuum chamber 3, and the introduction pipe 5 is provided in the hole portion.
Is inserted.

There is an opening heater 6 around the introduction pipe 5. This is at a position corresponding to the heater 2 around the crucible 1 in FIG. 2, but the function is slightly different. Opening heater 6
A reflector 11 is installed to surround the. A pillar (not shown) set up on the ultra-high vacuum flange 13 has an opening
6, a reflector 11, a heater power line 21, a thermocouple 2
We support 0 mag. The introduction pipe 5 extends through the ultra-high vacuum flange 13 to the outside. A needle valve 7 is provided on the way, and the needle valve 7 is connected to the reservoir 8 through the needle valve 7. It is assumed that the needle valve has a linear relationship between the flow rate and the opening degree at an intermediate opening degree and can perform accurate flow rate adjustment.

A lid (not shown) for opening and closing the reservoir 8
Have their own. The lid can be opened to fill the material freely. The reservoir 8 also has its own reservoir heater 9 around the outer wall. This is to heat and evaporate and sublimate the material inside the reservoir 8. The thermocouple 17 can monitor the reservoir temperature T ₁ .

A connecting portion heater 16 is provided at an intermediate connecting portion of the introducing pipe 5. Although the present invention is directed to a material having a low melting point, some materials are still solid at room temperature, and some of them may be cooled and solidified in the middle portion of the introduction pipe 5. The connecting portion heater 16 is for preventing the material from being cooled here and adhering to the inner wall of the introduction pipe 5. Thermocouple 18 monitors its temperature. The reservoir heater 9, the opening heater 6, and the connecting portion heater 16 can operate independently of each other. Therefore, the reservoir temperature T ₁ , the intermediate temperature T _{2 of} the introduction pipe, and the introduction pipe T
The opening temperature of ₃ can be controlled independently. These temperature parameters can be appropriately determined by the difference in materials. Further, the reservoir 8 is connected to a vacuum exhaust device (not shown) via an exhaust pipe 15, and by this vacuum exhaust device, it is possible to exhaust to a vacuum independently of the vacuum chamber 3. A vacuuming valve 10 is provided in the middle of the exhaust pipe 15.

When the needle valve 7 is closed, the evacuation device is operated, and the evacuation valve 10 is opened, the reservoir 8 is opened.
Only the vacuum can be pulled. The material can be filled into the reservoir 8 by closing the needle valve 7, closing the vacuuming valve 10 and introducing atmospheric pressure into the reservoir 8 to open the lid. The needle valve 7 can be fully opened or fully closed. At the intermediate opening, the flow rate can be controlled with good reproducibility according to the opening. Although the leak port is not shown in the figure, it goes without saying that the vacuum valve 10 may also be used.

The operation of the above structure will be described.
It is not necessary to open the vacuum chamber 3 to fill this Knudsen cell with material. The needle valve 7 is closed and the vacuuming valve 10 is also closed to introduce atmospheric pressure and break the vacuum state of the reservoir 8. Open the lid of the reservoir 8. Fill the reservoir 8 with the required amount of material. During this time, vacuum chamber 3
Ultra-high vacuum is still maintained. The lid is closed and the evacuation valve 10 is opened to operate the evacuation device. The inside of the reservoir 8 is pulled to a high vacuum. The heater 9 for the reservoir is energized to heat it. The material of reservoir 8 is heated to initiate evaporation or sublimation. Open the needle valve 7. The vapor passes through the introduction pipe 5 and the vacuum chamber 3
From the tip of the introduction pipe 5 into the inside of the vacuum chamber 3 as a molecular beam.

When the melting point of the material is higher than room temperature, the connecting portion heater 16 has an effect of preventing the material from being cooled and solidified inside the introducing pipe 5 and adhering to the inner wall. It is unnecessary if it is a gas at room temperature.

The in-chamber heater 6 has the function of preventing the material from solidifying at the tip of the introduction pipe 5. Further, some materials may not be sufficiently activated by only the reservoir heater 9 and may only form a mass of many molecules. Immediately before introducing this into the vacuum chamber 3, it can be heated by the opening heater 6 and subdivided into molecules. That is, the cracking action can be performed by the opening heater 6. The opening heater 6 can be used not only for cracking but also for controlling the energy of the molecular beam. Since the liquid nitrogen shroud 4 is located in the vicinity of this, the tip opening of the introduction pipe 5 can be controlled to an arbitrary temperature by waiting for this.

There are two ways to control the molecular dose. One is a needle valve 7 and the other is a reservoir heater 9. The flow rate increases when the needle valve 7 is opened more, and decreases when the needle valve 7 is closed. A fully closed state is also possible. If it is not possible to fully close it, install a separate valve.
The control by the needle valve is direct and has little time delay. When the temperature of the reservoir heater 9 is raised, the molecular dose increases, and when the temperature thereof is lowered, the molecular dose decreases.
This is control by changing the pressure in the reservoir 8. Indirect and time delayed. In addition to this, of course, since there is a shutter (not shown) at the tip of the introducing pipe, the molecular beam can be blocked by opening and closing the shutter. Accurate control of the molecular dose is one of the features of the present invention. As described above, according to the present invention, when filling the material, the needle valve 7 is closed and the reservoir 8 and the vacuum chamber 3 are shut off. That is, the apparatus of the present invention has an advantage that the material can be filled while the vacuum chamber 3 is maintained at a high vacuum. This is effective in improving the productivity of molecular beam epitaxy.

There is another advantage. Although it is necessary to perform baking when starting up the vacuum of the vacuum chamber 3,
If a material having a high vapor pressure at a low temperature is present in the internal Knudsen cell, the material is lost by the baking, so that the baking cannot be sufficiently performed. However, in the case of the present invention, the vacuum valve 3 can be baked after the needle valve 7 is closed and the reservoir 8 and the vacuum chamber 3 are shut off. The heat of baking is not applied to the reservoir 8. Therefore, the baking temperature can be raised freely and the purpose of baking can be achieved well.

A very convenient Knudsen cell can be provided for materials with high vapor pressure at low temperatures. For example, it can be applied to S, Se, organic materials, and the like of Group VI.

The tip of the introduction pipe may thus be a simple opening. However, when there is too much difference between the vacuum degree inside the vacuum chamber and the vacuum degree of the reservoir 8, a narrow neck part or a perforated plate is installed at the tip of the introducing pipe to maintain the differential pressure. You may allow it.

[0029]

A reservoir for accommodating a material and turning it into vapor is provided outside the vacuum chamber, and both are connected by an introducing pipe so that the vapor generated by the reservoir is introduced into the vacuum chamber. I am trying to guide you. A needle valve is provided in the middle of the introduction pipe so that the flow of gas can be freely controlled. The molecular dose can be controlled by a reservoir heater and a needle valve. Further, a connecting portion heater 16 is provided at an intermediate portion of the introduction pipe 5 so that vapor of the material does not condense on the inner wall surface.

When filling the reservoir with the material, the needle valve 7 can be closed and the lid of the reservoir can be opened to supplement the material. It has no effect on the ultra high vacuum of the vacuum chamber. Once the vacuum chamber is once brought to atmospheric pressure, it takes a long time and labor of 3 days to 1 week to start up again, but the present invention can save such time and labor. Epitaxial growth can be continuously performed, and the productivity of molecular beam epitaxy can be enhanced.

Further, when the vacuum chamber is opened and the material is exchanged, harmful substances may be scattered into the atmosphere to deteriorate the working environment. However, the present invention avoids such a danger, and therefore, the improvement of working hygiene is called. It is also excellent in terms.

Furthermore, during baking of the vacuum chamber, since the reservoir is separated from the vacuum chamber, the heat of baking is not transferred to the material in the reservoir and does not evaporate. Do not reduce the weight. There is an advantage that baking can be performed freely, and loss of a material having a high vapor pressure at a low temperature can be prevented.

[Brief description of drawings]

FIG. 1 is a schematic sectional view of a Knudsen cell according to an embodiment of the present invention.

FIG. 2 is a schematic cross-sectional view of a Knudsen cell according to a conventional example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 crucible 2 heater 3 vacuum chamber 4 liquid nitrogen shroud 5 introduction pipe 6 heater 7 needle valve 8 reservoir 9 reservoir heater heater 10 vacuum drawing valve 11 reflector 12 column 13 ultra high vacuum Flange 14 Heater power line 15 Exhaust pipe 16 Connection part heater

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Takaminori Yamamoto, 47 Umezu Takaune-cho, Ukyo-ku, Kyoto City, Kyoto Prefecture Nissin Electric Co., Ltd. Electric Co., Ltd.

Claims (1)

[Claims] 1. A knudsen cell of a molecular beam epitaxy apparatus for drawing an ultrahigh vacuum in a vacuum chamber, heating and vaporizing a material to irradiate a substrate in the ultrahigh vacuum as a molecular beam to perform epitaxial growth on the substrate. , A reservoir provided outside the vacuum chamber, having a lid that can be opened and closed, and containing a material, and a reservoir for heating the material of the reservoir.
A heater for the bar, an inlet pipe for connecting the reservoir and the vacuum chamber to introduce the molecular beam into the vacuum chamber,
An opening heater provided near the outlet of the introduction pipe in the vacuum chamber, a needle valve with adjustable flow rate provided in the middle of the introduction pipe, and a heater for heating the middle portion of the introduction pipe. And a vacuum exhaust device for pulling the reservoir to a vacuum, and the molecular dose is adjusted by controlling the temperature of the reservoir and the opening of the needle valve. A low temperature Knudsen cell characterized by the above.