JPH0136977B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to improvements in molecular beam source cells for forming thin films used in semiconductor devices.

Conventionally, molecular beam epitaxy (hereinafter referred to as MBE) has been used as a technology for epitaxially growing high-quality semiconductor crystal thin films. A molecular beam source cell surrounded by liquid nitrogen-temperature walls is placed in a container, and the source material (constituent elements) of the thin film crystal to be grown is placed in this cell, and the molecules are grown by heating. It is ejected as a line and deposited on a substrate maintained at an appropriate temperature to grow a single crystal thin film.

By the way, when a thin film is formed using the MBE method,
The thin film has a thin thickness distribution, being thickest at the part directly facing the molecular beam ejection port of the cell, and decreasing from there toward the periphery.

Conventionally, in order to reduce this distribution and grow a highly uniform crystal thin film, measures have been taken exclusively to project a uniform molecular beam with a small intensity distribution onto a substrate. For example, (a) increasing the distance between the substrate and the molecular beam source cell;

(b) As shown in FIG. 1, a large number of molecular beam ejection ports 1'a in the molecular beam source cell 1' are elongated and inclined.

(c) As seen in FIGS. 2 and 3, a molecular beam scatterer 5 is installed in the cell 1' near the molecular beam outlet 1'a.

methods have been adopted. However, in these figures, 2 is a heater and 4 is a thermocouple.

However, the uniformity of the film thickness distribution obtained by these methods cannot be achieved with the currently most commonly used 2-inch diameter film.
On a GaAs substrate, the height is ±5%, which is not necessarily sufficient. The ultimate objective of the molecular beam source cell of the present invention is to improve this.

The inventor of the present application took a different measure from the conventional method in order to obtain a uniform film thickness distribution. This will be explained with reference to FIG.

Conventionally, a configuration has been adopted in which a molecular beam is ejected from the cell 1 toward the center 8C of the substrate 8 rotating around an axis 8a passing through the center 8C of the substrate.
Move this cell to position 1a and direct the molecular beam to the center of the substrate at 8
He purposely made it deviate from C and eject toward position 9 in the diagram. In this case, when appropriately selecting the distance between 8C and 9 with respect to the distance between 1a and 8C, the ejection angle θ, and l/r, for example, the 4-inch diameter substrate 8
It has been found that uniformity of the film thickness within ±1% can be ensured even with respect to the film thickness.

However, in the above, l is the distance from the inlet A of the cell to the center of the liquid surface of the source material (or the center of the upper surface when the source material is sublimable), and 2r is the inner diameter of the cell.

In this case, it is known that the molecular beam projection density is approximated by the so-called cosine law. The cosine law refers to the angle θ between the cell center A and the cell axis F in Figure 5.
At a point P located at a distance R in the direction of , the film thickness T that can be obtained on a unit area per unit time is expressed by the following equation (1).

T=K 1/R ₂ cosnθ ………(1) where K is a proportionality constant. n is a value that changes depending on l/r, for example, when l/r=0, n≒1, l/r=
2, n≒4, l/r=4, n≒7, l/
When r=10, n≈10. (Varian company explanatory material VR-152 (Reprinted from ELECTRONICS
(Magajine, August, 1980) Therefore, the inventor of the present application is clearly different from the above-mentioned molecular beam projections (a), (b), and (c), which aim for "uniform projection," and aims at nonuniform projection according to the cosine law. A high degree of film thickness uniformity was achieved by utilizing the film as is. In this case, the following issues arise:

The MBE method requires an extremely high degree of vacuum,
For this reason, in the MBE device, degassing is performed for a long time, during which time a considerable amount of the source material in the cell evaporates. It's finally back up and running again
MBE equipment strives to operate for a long time in order to increase its operating rate, so the amount of evaporation of the source material in the cell during that time is also large, and the above-mentioned l/l/
The value of r and therefore n changes greatly from the beginning to the end.

When the value of n changes greatly, the pattern of molecular beam projection changes, so that cell 1 in Figure 4 mentioned above changes.
There is a possibility that the uniformity of the film thickness distribution on the substrate 8 obtained by a is also deteriorated.

The invention of the present application aims to solve this problem.

Furthermore, regardless of whether the cell is placed in the position 1a in FIG. 4, in general, MBE apparatuses extremely dislike impurity particles falling onto the substrate or into the cell in the form of dust. Therefore, for example, the device is often constructed in such a way that the substrate 8 and the cell 1 shown in FIG. 4 are kept in their relative positions, but the entire structure is tilted at an angle of 45 degrees with respect to the earth's axis.
At this time, the axis of the cell is greatly tilted with respect to the earth's axis, so the liquid level of the source material is significantly tilted with respect to the axis F of the cell.

When the liquid level is tilted, especially when the cell has a large diameter and the liquid level is close to the inlet A, the molecular beam projection pattern becomes asymmetrical with respect to the axis, and this can be approximated by equation (1). However, the shape of the pattern changes dramatically as the liquid level drops. It is clear that changes in the molecular beam projection pattern make the MBE system extremely difficult to use, and we want to prevent this change as much as possible.

The present invention aims to solve this problem.

FIG. 6a shows an embodiment of the molecular beam source cell of the present invention.

10 is the cell mantle, 11 is the cell body.
Made from PBN (pyrolytic boron nitride). 10,
11 constitute a cell. The former is fixed;
The latter is interpolated into it and moves up and down. A source material 12 is introduced into the latter, becomes liquid at a high temperature, and evaporates or sublimates to become vapor. 13 is the liquid level (in the case of sublimation, the top surface of the solid).

14 is a spiral heater surrounding the cell; 1
5 and 16 are reflectors, 17 is an insulator for leading out heater wires 18 is a guide ring made of PBN, 2
0 is a thermocouple, 21 is an insulator tube for the thermocouple which also serves as a drive rod for moving the cell body 11 up and down, 22
3 is a drive rack for the insulator tube 21, the upper end of which is locked to the protrusion 21a of the insulator tube 21, and the lower end is fixed to the free end 41 of the bellows 40; 30 is the wall of the vacuum container; 31 is a flange to which the bellows 40 is fixed. . 32 is a vacuum sealing material, 33 is a male screw attached to the flange 31, and 34 is a female screw to be screwed therein. When the female screw 34 is turned, the bellows 40 expands and contracts, and the drive rack 22
The cell body 11 moves up and down via the insulator tube 21.
42 is a lead wire of the thermocouple 20.

In the molecular beam source cells 10 and 11 of the present invention, the source material 12 is introduced into the cell body 11, and as evaporation progresses, the uppermost surface 13 thereof descends inside the cell body 11. For the downward movement, turn the female screw 34 to connect the cell body 11 to the cell mantle tube 1.
The distance l from the cell inlet A to the liquid level of the source substance can be kept constant at all times.
The situation is shown in Figures 6b and 6c. The above-mentioned compensation operation is performed by checking in advance the extent to which the top surface of the material decreases with respect to heating time and heating temperature.

It is obvious that each of the technical problems described above can be solved by the cells 10 and 11, and no explanation is required.

In addition, the cell jacket tube 10 and the cell main body 11 of the present invention
The shape and configuration of is not restricted to this embodiment. In particular, a wide range of application variations are possible in the method of compensation drive. A configuration in which a tubular body corresponding to the cell jacket tube 10 is inserted into the cell body 11, which is a source material storage container, is also possible, or the cell body may be fixed. In short, the cell of the present invention can be modified in various shapes and configurations while respecting the gist of the invention as long as the cells are inserted into each other and the tubular body and the source material storage container are capable of adjusting the insertion depth. can do. Cell materials other than PBN can also be freely selected.

Since the molecular beam source cell of the present invention is as described above, the pattern of molecular beam radiation is constant and reproducible. MBE film: It is extremely effective in producing an MBE film with excellent film thickness uniformity. The present invention greatly contributes to the production of high-performance semiconductor devices, and can be said to be an industrially useful invention.

[Brief explanation of drawings]

Figures 1, 2, and 3 are diagrams of conventional molecular beam source cells.
Figure 4 is a diagram illustrating a method for producing an MBE film with highly uniform film thickness distribution. Figure 5 is a diagram of the molecular beam radiation of the cell. Figures 6a, b, and c are diagrams of embodiments of the cell of the present invention. 10: Cell mantle (tubular body), 11: Cell body (source material storage container).

Claims (1)

[Claims] 1. In a cell for a molecular beam source, the cell is composed of a tubular body and a source substance storage container, both of which are heated almost equally, are inserted into each other, and whose insertion depth can be adjusted. A molecular beam source cell characterized by: