JPS6231814B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for diffusing gallium into a semiconductor substrate, and more particularly to a method for diffusing gallium into a semiconductor substrate by thermally diffusing gallium into a semiconductor substrate using elemental gallium as a diffusion impurity.

Boron is widely used as an impurity for forming p-type impurity regions in semiconductor substrates. However, since boron has an extremely slow diffusion rate in a semiconductor substrate, it takes a long time to form a deep junction, which is very disadvantageous in terms of cost.

Therefore, gallium, which has a high diffusion rate in the semiconductor substrate, is used to form deep junctions. In particular, in order to form a bond with high voltage resistance and long lifetime,
The gallium source must be of high purity, and elemental gallium is a satisfactory diffusion source in this respect. However, since elemental gallium has a significantly low vapor pressure at low temperatures and is easily oxidized, it has been difficult to uniformly diffuse a desired amount of impurities. Therefore, when elemental gallium is used as a diffused impurity source, the impurity source and the semiconductor substrate must be confined in a high-temperature soaking region that does not contain oxygen. Due to these circumstances,
Conventionally, gallium diffusion using elemental gallium as an impurity source has been carried out by a closed tube method. In this method, a diffusion impurity source and a semiconductor substrate are placed in a quartz diffusion tube, the tube is vacuumed or filled with an inert gas, the quartz diffusion tube is melt-sealed, and then diffusion heat treatment is performed. However, in this case, after diffusion,
Since the quartz tube must be cut to remove the semiconductor substrate, the quartz diffusion tube cannot be reused and is extremely uneconomical.

In order to solve this problem, a method called the vacuum open tube method has been proposed. In this method, a diffused impurity source and a semiconductor substrate are placed in a diffusion tube, the inside of the tube is evacuated using a vacuum evacuation device to maintain a vacuum state, and the impurity source and semiconductor substrate inside the diffusion tube are placed in a high-temperature soaking area. Diffusion heat treatment is applied. This vacuum open tube method eliminates the problems of the closed tube method because the diffusion tube is not melt-sealed. However, in actual application to diffusion, the material of the diffusion tube becomes a problem. Currently, quartz is preferred as a material for the diffusion tube in terms of purity, but since quartz reacts with gallium vapor, there is a problem that quartz diffusion tubes become weak in strength and easily break. Particularly in a reduced pressure state below atmospheric pressure, a strong force is applied to the quartz diffusion tube, and the life of the tube itself is extremely shortened.

Therefore, an object of the present invention is to provide a method for diffusing gallium into a semiconductor substrate, which allows a desired amount of impurities to be diffused with good uniformity.

Another object of the present invention is to provide a method for diffusing gallium into a semiconductor substrate that has a long life when quartz is used as a diffusion tube.

The present invention is characterized by placing a semiconductor substrate together with elemental gallium in a semi-closed container,
The container is inserted into a diffusion tube whose gas pressure can be adjusted, and the inside of the tube is replaced with an inert gas to achieve a predetermined gas pressure, and diffusion heat treatment is performed.

The opening area of a semi-closed container is preferably 2 to 10% of the cross-sectional area of the container. Further, the gas pressure of the inert gas in the diffusion tube is preferably 10 Torr to atmospheric pressure.

Hereinafter, the present invention will be explained based on the drawings.

FIG. 1 shows an embodiment of the invention.

As shown in Figure 1, the outer diameter is 65 mm with one side closed.
A silicon boat 3 containing 50 mg of elemental gallium 2 as a diffused impurity source is placed in a quartz container 1 with an inner diameter of 61 mm and a length of 500 mm, and a silicon boat 3 with a diameter of 50 mm and a thickness of 500 μm and 20 wafers placed on a quartz wafer holder 4 as a semiconductor substrate. Silicon wafer 5, silicon boat 3 containing 50 mg of elemental gallium 2 are placed in this order, and the outer diameter
It was capped with a 60 mm quartz inner stopper 6. This quartz container 1
is inserted into the quartz diffusion tube 7 shown in _FIG.
Using the gas supply device 10 using gas, the operation of replacing the gas and changing the atmosphere inside the tube to an N ₂ gas atmosphere and then exhausting the tube was repeated three times. and 10 ^-6 Torr
The temperature was raised to 600℃ using the heating heater 11 while continuously exhausting at a pressure of
N ₂ gas was gradually filled to bring the inside of the tube to atmospheric pressure, and the temperature was maintained at 600°C for 10 minutes. From this state, the temperature was increased to a diffusion temperature of 1150°C, maintained at the temperature for 3 hours, and then cooled, and the container 1 was taken out from the diffusion tube 7 at 600°C.

FIG. 2 shows the relationship between the aperture ratio of the container 1 and the sheet resistance of the silicon wafer 5 after diffusion.

Here, the opening ratio is the ratio of the opening area of the semi-closed container 1 to the cross-sectional area of the container 1, expressed as a percentage.

If the opening ratio is larger than 10%, the effect of trapping gallium vapor will be reduced, and if it is smaller than 2%, it will be difficult to replace the gas inside the container and adjust the pressure.
This causes oxidation of the diffused impurity source, and the sheet resistance, which is a measure of the amount of impurities in the diffused layer, does not reach the desired value ρ _SO .

In such a semi-sealed state, the inert gas pressure during the diffusion heat treatment must be at least 10 Torr to atmospheric pressure. This is because when the gas pressure is 10 Torr or less, the gallium vapor flows well at the opening, and the gallium vapor inside the container 1 flows out of the container 1 and reacts with the diffusion tube 7 outside, causing the vapor pressure inside the container 1 to decrease. is not high enough, and as shown in Figure 3, the sheet resistance of the silicon wafer after diffusion is high enough to reach the desired value ρ _SO
This is to prevent this from happening. In addition, if the gas pressure is over 10 Torr, the flow of steam at the opening will be poor.
The vapor pressure within the container 1 becomes sufficiently high, and a desired sheet resistance value can be obtained with good uniformity.

Note that the above aperture ratio and inert gas pressure are numerical values that can be similarly applied even if the desired sheet resistance is changed.

This is because the conditions under which gallium vapor escapes from the container remain the same.

Further, it is desirable that the diffusion process be performed in accordance with the procedure shown in FIG. The reason why backfilling and evacuation are repeated several times with an inert gas at room temperature is to reduce the amount of residual oxygen in the container 1 as much as possible and prevent oxidation of the impurity source. In addition, the low-temperature (500 to 800°C) heat treatment at a temperature lower than the diffusion temperature during diffusion heat treatment is performed in a continuous evacuation vacuum state, which creates a kind of dry firing effect and eliminates waste in the system at the diffusion temperature. This is to avoid the influence of impurities.

That is, the heat treatment is performed from room temperature to the diffusion temperature in an inert gas atmosphere at a relatively low degree of vacuum or atmospheric pressure, but during this time, residual oxygen in the container 1 and oxygen and moisture adsorbed in the container 1 are As the temperature rises, it is released and elemental gallium is oxidized, resulting in poor sheet resistance uniformity. Therefore, in order to sufficiently remove the released oxygen and moisture and further improve the uniformity of sheet resistance, low-temperature heat treatment is performed while the interior of the diffusion tube is continuously evacuated.

Incidentally, the variation in sheet resistance when low-temperature heat treatment was not performed was approximately ±10%.

As described above, according to the present invention, gallium can be uniformly diffused into a semiconductor substrate to a desired value. According to the measurement results, the variation in sheet resistance of the diffusion layer was within ±3% within the same rod.

Furthermore, according to the present invention, the container 1 and the diffusion tube had a long lifespan and could be used repeatedly more than 10 times.

Incidentally, according to the conventional method, the diffusion tube had to be replaced every 3 to 4 times.

Note that the present invention is not limited to the above example, and it is possible to use germanium or the like as the semiconductor substrate, and the same effect can be obtained even if tantalum or the like is used as the container.

Furthermore, similar effects can be obtained by using argon gas or the like as the inert gas.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of a diffusion furnace showing an embodiment of the present invention, and FIG. 2 is a diagram showing the relationship between the aperture ratio of a container used for diffusion in the present invention and the sheet resistance of a diffusion layer in a silicon wafer. FIG. 3 is a diagram showing the relationship between the pressure inside the diffusion tube and the sheet resistance of the diffusion layer in the silicon wafer, and FIG. 4 is a diagram showing a diffusion program suitable for implementing the present invention. DESCRIPTION OF SYMBOLS 1... Container, 2... Elemental gallium, 3... Silicon boat, 4... Quartz wafer holder, 5... Silicon wafer, 6... Quartz inner plug, 7... Quartz diffusion tube, 8... Opening/closing port, 9... Vacuum exhaust device, 10 ...Inert gas supply device, 11...Heating heater.

Claims (1)

[Claims] 1. Elemental gallium is used as a diffused impurity source and the ratio of the opening area to the cross-sectional area is 2 along with the semiconductor substrate.
~10% in a reaction vessel, the reaction vessel is placed in a quartz diffusion tube where the gas pressure can be adjusted, and heat treatment is performed at a temperature of 500 to 800°C to remove the oxygen adsorbed in the reaction vessel. Alternatively, remove moisture, fill the quartz diffusion tube with an inert gas at a pressure of 10 Torr to atmospheric pressure, further raise the temperature of the reaction vessel to maintain it at the gallium diffusion temperature, and then cool the reaction vessel. Characteristic method of gallium diffusion into semiconductor substrates.