JPH0152890B2 - - Google Patents

Description

[Detailed Description of the Invention] [Summary] A shutter shaped so that the heat rays emitted from the molecular beam source is reflected back to the molecular beam source and does not bounce back is provided in front of the ejection port of the molecular beam source cell, This invention relates to a molecular beam epitaxial growth apparatus in which the beam intensity of a molecular beam source does not fluctuate due to opening and closing of the shutter. It relates to a shutter provided in front of the spout of a cell (container).

As is well known, when manufacturing semiconductor devices, an epitaxy method is used in which a semiconductor film is epitaxially grown along a crystal substrate, and this is the most basic technology for semiconductor manufacturing.

Among such epitaxy methods, molecular beam epitaxy (MBE) has recently become known.
This molecular beam epitaxy method is a method of vapor deposition under ultra-high vacuum (below 10 ^-11 Torr), and since a clean crystal substrate surface can be maintained, epitaxial growth can be performed at low temperatures, and film thickness and It is characterized by the ability to precisely control impurity distribution at the single atomic level of several tens of angstroms.

Furthermore, a major feature of the MBE method is that heterojunctions and graded heterojunctions of various elements or compound elements can be easily obtained, and it is becoming widely used for the epitaxial growth of compound semiconductors such as GaAs.

Although the MBE method has many advantages, it is difficult to form epitaxial growth layers with high precision unless the evaporating molecular beam source is constantly controlled under certain conditions.

[Conventional technology]

Fig. 2 shows an outline of the main parts of the molecular beam epitaxial growth apparatus. 1 is an ultra-high vacuum processing chamber, 2 is a growth substrate, 3 is a molecular beam source cell, 4 is a cooling partition (liquid nitrogen shroud), 5 is a shutter, 6 is a cryopump, and 7 is a vacuum exhaust port.

When performing molecular beam epitaxial growth on the growth target substrate 2 using such a growth apparatus, the shutter 5 above the cell 3 storing the desired molecular beam source is opened, and the molecular beam melted by the heater is released. The source is evaporated (spouted) and epitaxially grown on the substrate to be grown. In the figure, a large number of cells are provided for one growth substrate 2, but this is because, for example, junctions of various compositions are stacked one after another on a GaAs substrate, and these cells are switched by opening and closing the shutter. .

FIG. 3 shows a schematic cross-sectional view of the molecular beam source cell 3 and shutter 5 in such a molecular beam epitaxial growth apparatus. It consists of a molecular beam source (molecular beam material) 33 and a temperature sensor 34, and the other members in the figure are the same as in Figure 2, 4 is a liquid nitrogen shroud (cooling partition), 5 is a shutter, and 51 is a shutter operating rod. be. The figure shows the state where the ejection port is shielded by the shutter,
When the shutter 5 is opened, the molecular beam evaporates from the liquid surface and lands on the surface of the substrate to be grown, directly in front of it, and epitaxial growth is performed.

The size of the cell storing this molecular beam source is, for example, a conical crucible 31 with a diameter of 20 mm and a length of about 100 mm, made of sintered boron nitride (PBN), and a heater 32 made of tantalum wire or the like. is used. In addition, shutter 5 is made of tantalum (Ta),
It is made of heat-resistant metals such as tungsten (W) and molybdenum (Mo), and although a concave shutter is shown in the figure, flat plate shutters are also used.

[Problem that the invention seeks to solve]

By the way, in the diagram of the cell 3 and the shutter 5 shown in FIG. 3, the ejection port is shielded by the shutter, but in this state, the molecular beam source 33, for example,
It is heated to about 1,200℃ to melt it and is controlled at a constant temperature. When the jet nozzle is shielded by this shutter, the heat rays (thermal radiation) emitted from the molecular beam source are reflected by the shutter 5 and are reflected back to the molecular beam source 33 in the crucible 31, thus reducing the temperature of the crucible. It is in a stable and controlled state. Dotted arrow lines in the figure indicate thermal radiation lines.

When the shutter 5 is opened in such a stable state, the thermal radiation that was previously reflected by the shutter to the molecular beam source 33 in the crucible 31 is suddenly emitted to the outside, and as a result, the surface temperature of the molecular beam source 33 increases. decreases rapidly, and the beam intensity of the molecular beam becomes weaker. In other words, when the shutter is shielded, heat is returned by thermal radiation and controlled with low energy, but when the shutter opens, the radiant heat energy dissipates, so the control system including the temperature sensor has to follow suit. As a result, the surface temperature of the molecular beam source 33 suddenly decreases.

Then, the temperature gradually returns to the set temperature with a certain time constant (a time constant that depends on the heat capacity of the crucible, the capacity of the heater, etc.). However, this phenomenon is an extremely important problem, as temperature changes cause variations in the composition ratio and film thickness of the grown film.

Furthermore, to explain this using the growth method of a single heterojunction crystal structure for HEMT as a specific example, the fourth
The figure is a cross-sectional view of a GaAs crystal. On a semi-insulating GaAs substrate 21, a GaAs layer 22, an AlGaAs layer 23, a GaAs
This is a structure in which layers 24 are grown sequentially. In this structure, the GaAs layer 22 is placed at the heterojunction interface between the GaAs layer 22 and the AlGaAs layer 23 due to the difference in electron affinity between the two layers.
An electronic layer (indicated by a dotted line) is formed in the wafer, which contributes to high-speed operation. This electron layer is called a two-dimensional electron gas layer because it cannot move in the vertical direction of the crystal layer but can move freely in the parallel direction, but the electron channel width is at most about 80 to 120 Å, and the heterojunction The interface must be controlled to this extent.

Now, when this crystal growth is performed using the conventional shutter 5, from the GaAs layer 22
When switching to the AlGaAs layer 23, the shutter of the aluminum (Al) molecular beam source cell is opened to evaporate the aluminum. On the other hand, gallium (Ga)
The shutter of the arsenic (As) molecular beam source is kept open following the growth of the GaAs layer 22. At the time the shutter of this Al molecular beam source cell is opened, the temperature of the Al molecular beam source is 1390℃ to 1370℃
As a result, the beam intensity of the Al molecular beam source drops sharply.

For example, the state in which the composition ratio x value of the AlxGaxAs crystal was set to 0.3 is reduced to x=0.26. Then, after about 5 minutes have passed, the steady state is returned, but during that time the thickness of the grown layer has grown to about 1000 Å, and the x value of the grown layer has changed from 0.26 to 0.3.
On the other hand, since the thickness of the AlGaAs layer 23 of the GaAs crystal described above is approximately 300 to 500 Å at most, the growth of the AlGaAs layer 23 will be completed before the composition ratio x returns to a steady state, and therefore the AlGaAs layer 23 will have a predetermined thickness. Unable to obtain mixed crystal ratio.

Such fluctuations in the composition ratio have a large adverse effect on the operating characteristics, and the present invention proposes a molecular beam epitaxial growth apparatus that eliminates these fluctuations.

[Means for solving problems]

The purpose of this is to provide a shutter with a heat radiation curved surface that does not reflect the heat rays emitted from the molecular beam source back to the molecular beam source, and a heat ray emission opening, such as a curved horn shape. This problem can be solved by using a molecular beam epitaxial growth apparatus equipped with a shutter having an opening at its tip.

[Effect]

That is, in the present invention, a shutter is provided in front of the ejection port of the molecular beam source cell to prevent the heat rays emitted from the molecular beam source from returning to the molecular beam source, so that the temperature of the molecular beam source is reduced by opening and closing the shutter. Fluctuations are eliminated, making it easier to keep the beam intensity of the molecular beam source constant.

〔Example〕

Hereinafter, embodiments will be described in detail with reference to the drawings.

FIG. 1 shows a partial sectional view of a molecular beam source cell and a shutter of a molecular beam epitaxial growth apparatus according to the present invention, and numeral 15 is an example of the shape of the shutter according to the present invention. Other members are the same as those in FIG. 3 and are given the same symbols. As shown in the diagram, the shutter in this example has a downwardly curved horn (Horn:
It is angular) in shape and has an opening at the tip.

With this shape, the thermal radiation (indicated by the dotted line) is not reflected by the molecular beam source 33 and bounces back, but reaches the tip of the horn while being reflected within the shutter, and is emitted from the opening at the tip. Moreover, if the shutter is shielded with such a shape, the ejected beam from this aperture will not hit the growth substrate directly in front of it, and if there is ejected material, it will fall below the aperture.

If such a shutter is provided, even if the molecular beam source cell is shielded by the shutter 15, thermal radiation will not be reflected to the molecular beam source 33, and in an open state except for the shutter, thermal radiation will not be reflected. Since it is not reflected, the heating temperature of the molecular beam source 33 is kept constant regardless of whether the shutter is opened or closed.

Therefore, the temperature difference in the molecular beam source due to opening and closing of the shutter becomes negligible, which stabilizes the beam intensity, reduces fluctuations in the composition ratio and film thickness of the growing crystal, and increases the height of the grown crystal. Quality can be improved, and the quality of the GaAs crystal for HEMT described above is also significantly improved.

〔Effect of the invention〕

As is clear from the above description, according to the molecular beam epitaxial growth apparatus using the shutter according to the present invention, the quality of the grown crystal is significantly improved, and the performance of the semiconductor device can be improved.

[Brief explanation of drawings]

Figure 1 is a partial sectional view of the molecular beam source cell and shutter of the molecular beam epitaxial growth apparatus according to the present invention, Figure 2 is an overall schematic diagram of the molecular beam epitaxial growth apparatus, and Figure 3 is a conventional molecular beam source. FIG. 4 is a partial cross-sectional view of the cell and shutter, and is a cross-sectional view of a GaAs crystal to explain the problem. In the figure, 1 is an ultra-high vacuum processing container, 2 is a growth substrate, 3 is a molecular beam source cell, 4 is a cooling partition, 5,
15 is a shutter, 6 is a cryopump, 7 is a vacuum exhaust port, 31 is a conical crucible, 32 is a heater,
33 is a molecular beam source, 34 is a temperature sensor, and 51 is a shutter operating rod.

Claims (1)

[Claims] 1. Equipped with a molecular beam source cell that ejects hot rays from the ejection port in an oblique direction, and a shutter provided in front of the ejection port, and the shutter has a horn shape with a wide opening diameter toward the ejection port. A heat ray emitting device having an inner surface, the inner surface being a heat ray radiation curved surface that does not reflect the heat rays emitted from the molecular beam source back to the molecular beam source, and directed downward on the side away from the jet nozzle. 1. A molecular beam epitaxial growth apparatus characterized by having an aperture for use.