JPH0232781B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an evaporation source cell of a vapor deposition apparatus such as a molecular beam source cell for forming a thin film used in a semiconductor device, and to improvements in its arrangement.

Conventionally, molecular beam epitaxial growth (hereinafter abbreviated as MBE) has been used as a technique for epitaxially growing high-quality semiconductor crystal thin films. To implement this technology, a molecular beam source cell surrounded by liquid nitrogen temperature walls is placed in a container maintained at ultra-high vacuum, and the thin film to be grown as a crystal is placed inside this cell. A crystal source material (constituent element) is put in and heated to eject it as a molecular beam, which is then 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 thickness distribution, being thickest at the part directly in front of the cell's molecular beam outlet, 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 to exclusively 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 Figure 1, molecular beam source cell 1
A large number of molecular beam jet ports 1a are provided in a slender and inclined manner.

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

methods have been adopted. However, in these figures, 2 is a heater and 4 is a thermocouple. For (a), the growth rate is slow, for (b), the ejection port is elongated and the source material tends to get clogged therein, and for (c), the growth rate decreases due to changes in the amount of evaporated matter.
There are defects such as changes in the spatial distribution of molecular beams. The evaporation source cell of the present invention ultimately aims to improve the film thickness distribution by always keeping the spatial distribution of molecular beams constant regardless of the amount of evaporated material in the cell and the shape of the evaporated material. It is something to do.

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 substrate center 8c, but this cell is moved to the position 1a'. Center the molecular beam on the substrate 8c
He deliberately made it eject in the direction of position 9 in the diagram. In this case, when appropriately selecting the distance between 1a' and 8c and the distance between 8c and 9 with respect to the ejection angle θ and l/r, the distance between 8c and 9 should be within ±1% even for a 4 inch diameter substrate 8. It has been found that uniformity of film thickness can be ensured.

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 top surface if 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 law of cosines means that at a point P located at a distance R in a direction forming an angle θ with the cell axis F from the center A of the cell in Figure 5, the film thickness T that can be obtained on a unit area in a unit time is as follows. It is as shown in equation (1).

T=K1/R ² cos ⁿ θ …(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
When =2, n≒4, when l/r=4, n≒7,
When l/r=10, n≈10. (Varian company explanatory material VR-152 (Reprinted from
ELECTRONICS magazine; 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 aim at nonuniform projection according to the cosine law. This method uses projection as is to achieve a highly uniform film thickness. In this case, the following issues arise:

The MBE method requires an extremely high degree of vacuum,
For this reason, MBE devices require long periods of degassing during which a significant amount of the source material within the cell evaporates. It's finally 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.
The uniformity of the film thickness distribution on the substrate 8 obtained by a' also deteriorates.

The invention of the present application aims to solve this problem.
Furthermore, regardless of whether the cell is placed at 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. For this reason, the device is often constructed in such a way that, for example, while maintaining the relative position of the substrate 8 and cell 1 shown in FIG. 4, the entire structure is tilted at 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 molecular beam investment patterns make MBE equipment very difficult to handle;
I want to prevent change as much as possible.

The present invention aims to solve this problem.

An embodiment of the evaporation source cell used in the present invention is shown in FIGS. 6a and 6b.

Reference numeral 10 denotes a cell, which is roughly divided into a steam rectifier and a source material storage 30. 40 is sauce storage section 3
0, 44 is its upper surface (molten liquid surface when the source material is not sublimable), 6
0 is a spiral heater surrounding the cell, and 70 is a reflector.

The cell 10 includes a cylinder 21 with a flange 22 made of PBN (pyrolytic boron nitride), a cylinder 31 with a flange 32 (with a bottom), and three-part rings 50a and 50b with a U-shaped cross section that connect the two at the flange. , 50c. 80 is a thermocouple, and 90 is an insulating tube for the thermocouple.

The characteristics of the cell 10 of the present invention are that the steam rectifying section 20 is cylindrical and single as in this embodiment, and the depth L is larger than the opening diameter D1; The opening area of the source material storage section 30 connected to the steam rectification section 20 is larger than the opening area of the rectification section 20, that is, the opening diameter D2 is larger than D1, and the source material storage section 30 connected to the steam rectification section 20 has a larger opening area than the rectification section 20. The volume of the section 30 is increased.

This configuration produces the following effects. That is, one effect is [A] As long as the source material 40 is stored in the storage part 30, whether the upper surface (liquid level) is at the upper position 44 or the lower position 44a, The above-mentioned molecular beam projection pattern hardly changes, and therefore the value of n does not change significantly. This also applies when the cell 10 has an inclined structure and the upper surface (liquid level) of the source material is inclined as shown at 46 and is lowered as shown at 46a.

Another effect is [B], source material storage section 3
0 can have an extremely large volume. This large volume can be achieved by increasing the diameter D2 of the accommodating portion, and the external shape of the cell does not become very large. Moreover, it has the advantage that the effect of [A] increases as the diameter D2 becomes larger.

The pattern of evaporative projection is the top surface 44 of the source material.
This is a combination of a steam pattern that is directly evaporated and projected from the upper surface 44 and a steam pattern that is indirectly projected by reflecting the steam that is projected from the upper surface 44 on the inner surface of the steam rectifying section 20.

It is easy to agree that the larger the values of L and D2 are compared to D1, the more difficult it is for the composite pattern to change, but if you choose D1<L and D1<D2 as mentioned above, n will not be large. No change, cell 1 in Figure 4
It has been found that sufficient practical film thickness uniformity can be ensured on the substrate 8 over a long period of time by a'.

The cell of the present invention can be implemented by making D1 and D2 eccentric to each other as shown in FIG. 7, and as shown in FIG. You may go. If the cell material is other than PBN (for example, carbon), the 9th
It is also possible to have an integrated structure as shown in the figure.

Further, the opening shape of the cell is not limited to a circular shape.
Oval shape, round shape, as shown in Figure 10 a, b, c, d.
Square, trapezoid, etc. can be selected as required. Note that the value of D1 at that time should be the length of the opening as shown in the figure.

Since the molecular beam epitaxial growth apparatus of the present invention is as described above, it is extremely effective in keeping the vapor radiation pattern constant and producing reproducible MBE films and MBE films with excellent film thickness uniformity. be.
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 and 6b and Figures 7, 8 and 9 are illustrations of embodiments of the cell of the present invention. Figure 10 a, b, c, d
2 is a diagram showing an example of the opening shape of a cell. FIG. 10: Cell, 20: Steam rectification section, 30: Source material storage section.

Claims (1)

[Claims] 1. In a molecular beam epitaxial growth apparatus that grows a semiconductor crystal thin film on the surface of a substrate while rotating it around a predetermined rotation axis, comprising a cylindrical steam rectifying section and a source material storage section connected to the rectifying section and having an opening area larger than that and a larger volume, and storing a constituent material of the thin film to be grown as a source material. A molecular beam epitaxial growth apparatus characterized in that the evaporation source cell is arranged such that the axis of the cylindrical vapor rectifying section intersects with the substrate at a position off the center of rotation thereof.