JPS5943089B2 - Gallium diffusion method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for diffusing gallium into silicon wafers by an open tube method in a semiconductor manufacturing process.

In the semiconductor manufacturing process, gallium trioxide (Ga2
03) is used as a diffusion source to diffuse gallium onto silicon wafers in a hydrogen atmosphere using the open tube method. A holder consisting of:

Figure 1 is a side cross-sectional view showing the silicon wafer inserted into the slit of the holder, and Figure 2 is a plane view showing the surface state of the silicon wafer after gallium has been diffused using the holder shown in Figure 1. 3 are diagrams showing the relationship between the gallium concentration distribution and the junction depth in each part of the silicon wafer shown in FIG. 2. In the figure, a holder 1 has a plurality of arcuate slits 2 having approximately the same radius as the silicon wafer and spaced from each other at appropriate intervals. The depth of the slit 2 in such a holder is 2 to 3 layers n to ensure the retention of the silicon wafer, and the width is determined by the width of the silicon wafer to ensure sufficient diffusion over the entire surface of the silicon wafer. It is normal to set the thickness to be 0.5 to 1.0 m wider than the thickness. Therefore, when a silicon wafer is inserted into this slit 2, the silicon wafer 3 is tilted and its surface is aligned with the slit 2, as shown in FIG.
It is held in line contact with the upper edge 2a of. Gallium is diffused into the silicon wafer 3 in this state, but an oxide film is generated on the surface of the silicon wafer 3 during the diffusion, and when the surface of the silicon wafer 3 is observed after the diffusion is completed, it is found that The color of the oxide film is different between the part where the silicon wafer 3 was in contact with the upper edge 2a of the slit 2 and the part where it was not in contact, as shown in FIG.
In a portion 3a where the silicon wafer 3 was in contact with the upper edge 2a of the slit 2, a blackish linear pattern with a slight width appeared. In order to investigate the cause of this, for samples in which gallium was diffused for 86 hours at a temperature of 1230°C, the silicon wafer 3 was placed in contact with the portion 3a that was in contact with the upper edge 2a of the slit 2. For part 3b,
As a result of measuring the surface concentration of gallium in each region using a spreading resistance measuring device, the former ranged from 4.5 to 5.0.
×1o17atoms/cc, and the latter is 1.
It showed a value of 5 to 2.0×10 17 atoms/cc.
That is, the silicon wafer 3 is attached to the upper edge 2 of the slit 2.
It was found that the surface concentration of gallium was 2.3 to 3.3 times higher in the portion 3a that was in contact with the gallium a than in the non-contact portion 3b.

Furthermore, as a result of measuring the gallium concentration distribution in the thickness direction of the same sample using the same spreading resistance measuring device, the third
As shown in the figure, the bonding depth of the non-contact portion 3b is 86μ, while that of the portion 3a where the silicon wafer 3 was in contact with the upper edge 2a of the slit 2 is 96 to 100μ.
It was found that the diffusion was 10 to 14 microns deeper than in the non-contact portion 3b. In this way, when so-called spike diffusion with locally different junction depths occurs in one silicon wafer, in the case of a semiconductor manufactured using one wafer or one pellet, the blocking voltage characteristics become partially non-uniform, and the semiconductor This is not desirable when manufacturing. It has been found that the cause of such spike diffusion is based on the following phenomenon. first,
When diffusing gallium into the silicon wafer 3 using the open tube method, gallium trioxide (Ga2O3), which is a diffusion source, is used.
) is decomposed with hydrogen and this hydrogen is used as a carrier gas, but this carrier gas contains water vapor (H2O
), it is not necessary to drain the silicon wafer 3 when inserting the silicon wafer 3 whose surface has been cleaned with pure water into the slit 2 of the holder as a preparatory step for diffusion. From this point of view, this draining had not been done. Therefore, moisture adheres to the surface of the silicon wafer 3, and when the silicon wafer 3 is inserted into the slit 2 of the holder, this moisture accumulates inside the slit 2, and when gallium diffuses, the heat is absorbed. Therefore, it will evaporate. On the other hand, after repeated use for a long period of time, the holder for silicon wafer 3 has become contaminated with excess gallium trioxide (
Fine particles of Ga2O3) are deposited on the surface, and the fine particles attached to the inside of the slit 2 are mixed with the evaporated water and dispersed to the outside of the slit 2 when gallium is diffused into the silicon wafer. become. However, on the side where the silicon wafer is in contact with the upper edge 2a of the slit 2, the opening of the slit 2 is closed by the contact with the silicon wafer 3, as shown in FIG.
The fine particles of gallium trioxide (Ga2O3) are not dispersed outside the slit 2, but are concentrated in the contact area with the upper edge 2a of the slit 2 of the silicon wafer 3, and are concentrated in this area. A new diffusion source of gallium oxide (Ga2O3) is created, and at this contact point, in addition to the normal gallium diffusion into the silicon wafer 3, another local gallium diffusion takes place, which causes the silicon wafer 3 to The above-mentioned spike diffusion will occur in this part.

In the above-mentioned gallium diffusion method, since the silicon wafer after cleaning is set in the holder without draining, a large amount of moisture is brought into the diffusion device together with the holder, and abnormal gallium diffusion as described later does not occur. Figure 4 is a patent application filed in 1973.
2-87338 is a side sectional view showing an embodiment of the silicon wafer holder used in No. 2-87338.

In the figure, the holder 1' is approximately 1.
An appropriate number of concave grooves each having an arcuate cross section with a radius smaller than 1 mm is formed, and furthermore, the grooves have a radius approximately equal to that of the silicon wafer at right angles to the groove direction, and approximately 0 mm less than the thickness of the silicon wafer. A plurality of arcuate slits 5 each having a width of 3 to 0.4 mm are formed at appropriate intervals.

Next, as a preparation step for diffusion, the silicon wafer 3 whose surface has been cleaned with pure water is thoroughly drained using a dry spinner or infrared drying, and then this silicon wafer 3 is placed in the slit 5 of the holder. Insert it.
As mentioned above, the width of the slit 5 is slightly wider than the thickness of the silicon wafer 3, so when the silicon wafer 3 is inserted into the slit 5, the silicon wafer 3 becomes as shown in FIG. The silicon wafer 3 is held almost vertically, and even if it is tilted to one side, the tilt is slight, and gallium is diffused into the silicon wafer 3 in this state. Since the silicon wafer 3 is sufficiently drained after washing, moisture will not accumulate inside the slit 5, and if the holder is used repeatedly for a long period of time, excess gallium trioxide will be removed. (Ga2
Even if fine particles of O3) adhere to and accumulate on the surface of the holder, moisture does not evaporate inside the slit 5, so as explained in FIG. There is no possibility that gallium oxide (Ga2O3) fine particles are locally concentrated and accumulated on the surface of the silicon wafer 3, causing spike diffusion. In fact, when the gallium concentration distribution in the thickness direction of the silicon wafer 3 in which gallium was diffused in this manner was measured using a spreading resistance measuring device, almost no spike diffusion phenomenon was observed. Although it has become possible to prevent spike diffusion within the silicon wafer 3 as described above, the following problems may occur.

This problem is that when the temperature is raised from 300 to 400 degrees Celsius to the set temperature of 1230 degrees Celsius by gallium diffusion,
After the diffusion time is over, the temperature is lowered to 300-400℃ and the next rod is diffused.
When the OWN operation is repeated, minute pits (locally deep depressions or areas where the oxide film and silicon are etched) are generated on the surface of the silicon wafer 3 in the white area at the center as shown in Figure 5, and the silicon wafer 3 is damaged. This is a phenomenon in which the blocking voltage characteristic determined by the specific resistance of the rod becomes lower than the designed value, resulting in abnormal diffusion that sometimes results in a rod failure.

The present invention was made as a result of such investigation into the cause, and provides a gallium diffusion method that prevents abnormal diffusion of gallium into silicon wafers. Hereinafter, one embodiment of the present invention will be described in detail with reference to the drawings.

FIG. 6 is a schematic diagram of a conventionally used gallium diffusion device, and the device shown in FIG. 6 is used for gallium diffusion by an open tube method using gallium trioxide as a diffusion source.

In the figure, 6 is a quartz tube, 8 and 8' are gallium trioxide (G
a2O3), V is the holder shown in FIG. 4, 3 is the silicon wafer, 7 is the carrier gas inlet, 7' is the carrier gas outlet, and 9 is the inner wall of the quartz tube outlet. 8' shows gallium trioxide fixed to the re-formed inner wall 9.

In gallium diffusion by the open tube method using gallium trioxide as a diffusion source, gallium trioxide 8, which is the diffusion source, decomposes with hydrogen (H2) as shown in equation (1) at a temperature of 950°C or higher and becomes gallium (Ga). Hydrogen containing water is usually used as the carrier gas.

In addition, to specifically explain hydrogen containing water, in the example, hydrogen at a flow rate of 300 CC/Mln passes through pure water in a bubbler whose temperature is controlled at 20°C, water vapor is supplied, and the carrier gas used as.

When normal diffusion occurs, gallium trioxide 8' is reformed and adhered to the inner wall 9 of the quartz tube outlet through the decomposition and reformation reaction cycle of gallium trioxide.

When abnormal diffusion occurs, when observing the re-formed gallium trioxide 8" on the inner wall 9 of the quartz tube outlet, fine particles of metallic gallium are always attached. Figure 5 shows the state of abnormal diffusion of gallium at 200 This is a metallurgical micrograph of the silicon surface magnified twice, and the white part in the center is metallic gallium, and the silicon surface is deeply depressed by the adhesion of this metallic gallium.
When we looked at the number of rods in which the problem occurred since the start of the NGUPDOWN operation, we found that it occurred in the 4th to 50th rods. We investigated the cause of this and found that before the diffusion furnace starts operating, a carrier gas containing moisture at room temperature is fed into the quartz tube 6 for a long time as a preparatory step, so sufficient moisture adheres to the inner wall of the quartz tube 6. are doing. PA of diffusion furnace
When the MPINGUPDOWN operation is started, the moisture that adhered to the inside of the quartz tube 6 before the operation of the diffusion furnace gradually decreases, and the moisture brought in by Yariya gas alone causes a lack of moisture, causing imbalance in the reaction cycle of equation (1) above. It has been found that abnormal diffusion occurs between the 4th and 50th points, as metal gallium is produced.

The present invention has been made with attention to this point, and the seventh aspect of the present invention is
The figure is a schematic diagram of a gallium diffusion device for explaining one embodiment of the present invention.

In the figure, the same reference numerals as those in FIG. 6 are the same or corresponding parts, and the explanation will be omitted. In the figure, the holder 1/inside the quartz tube 6
Water vapor (H2
In order to replenish O), pure water (O) is placed in a small quartz boat 11.
Steam replenishment device 1 containing about 5cc of H2O) 12
The reason is that 0 is set.

By providing this water vapor replenishment device 10, it becomes possible to compensate for the lack of water brought in by the carrier gas and reliably prevent the formation of metallic gallium as described above.
It has become possible to prevent the occurrence of abnormal diffusion even when performing RAMPINGUPDOWN operations. Note that pure water (H2O
) 12 to the small quartz boat 11, the diffusion rod should be replenished each time.

It is also conceivable to replenish water from the carrier gas direction, but in this case, the amount of evaporation of gallium trioxide 8 will be reduced, making it suitable for manufacturing thyristors.
This is unsuitable because it is not possible to obtain a gallium surface concentration of 18 at0 ms/Cc. As explained in detail above, according to the present invention, by providing a simple steam supply device, it is possible to provide a highly reliable gallium diffusion method using an open tube method that prevents abnormal diffusion of gallium. .

[Brief explanation of the drawing]

Figure 1 is a side sectional view showing a silicon wafer inserted into the slit of a conventionally used holder, and Figure 2 is a side sectional view of a silicon wafer in which gallium was diffused using the holder shown in Figure 1. A plan view showing the surface condition, FIG. 3 is a diagram showing the relationship between gallium concentration distribution and junction depth in each part of the silicon wafer shown in FIG. 2, and FIG. 4 is a diagram based on Japanese Patent Application No. 528
A side sectional view showing an example of the silicon wafer holder used in No. 7338, and Fig. 5 is a metallurgical micrograph of the silicon surface showing the abnormal diffusion state of gallium magnified 200 times.
FIG. 6 is a schematic diagram of a conventionally used gallium diffusion device, and FIG. 7 is a schematic diagram of a gallium diffusion device for explaining one embodiment of the present invention. 1, 17... Holder, 2, 5... Slit, 3... Silicon wafer, 6...
・Quartz tube, 8,8′...Gallium trioxide, 7・
...Carrier gas inlet, 7'...Carrier gas outlet, 9...Inner wall of quartz tube outlet, 1
0...Steam supply device, 11...Quartz boat, 12...Pure water.

Claims (1)

[Claims] 1. In the gallium diffusion method into silicon wafers using the open tube method using gallium trioxide as the diffusion source, in addition to sending a carrier gas containing hydrogen and water vapor into the quartz tube from the diffusion source side to the silicon wafer side. A gallium diffusion method using an open tube method, which is characterized by providing a water vapor supply device near the rear of a holder for setting silicon wafers.