JPH0265227A - Impurity diffusion into semiconductor substrate - Google Patents

Abstract

PURPOSE:To equalize the impurity concentration of respective semiconductor substrate by a method wherein the impurity is diffused on the semiconductor substrate after the temperature in a diffusion furnace has a temperature-gradient so that the in-furnace temperature on the gas exhaust side may be increased higher than that on the gas supply side. CONSTITUTION:A heating coil on the top side of a tube 3 is supplied with specified current to be increased in proportion to the downward distance from the top side so that the in-furnace temperature in a wafer mounting part may have a temperature-gradient slowly increasing from the gas supply side to the gas exhaust side. Since the impurity diffusion reaction starts from the wafer on the gas supply side, the reaction time of the wafer 4 on the gas supply side to the impurity gas is longer than that of the wafer 4 on the gas exhaust side taking the direction to increase the impurity concentration in the wafer 4. However, the reaction temperature on the gas supply side is lower than that of the gas exhaust side to increase the impurity diffusion rate on the gas exhaust side. Consequently, the impurity concentration in the wafer 4 can be equalized regardless of the wafer mounting positions.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for diffusing impurities into a semiconductor substrate, which comprises diffusing impurities into a semiconductor substrate placed in a diffusion furnace supplied with impurity gas.

[Conventional technology]

As a conventional method for diffusing impurities into a semiconductor substrate, for example, there is a method disclosed in Japanese Patent Application Laid-Open No. 63-42119.

According to this conventional example, a plurality of semiconductor substrates are arranged in a quartz port, and the quartz boat is inserted into a quartz furnace tube to supply impurity gas into the furnace tube maintained at a uniform temperature. POCl3 is used as the impurity gas, and this impurity gas is supplied from one end of the furnace tube and discharged from the other end.

[Problem to be solved by the invention]

Generally, the amount of impurities diffused into a semiconductor substrate is proportional to the length of reaction time between the semiconductor substrate and impurity gas and the height of the reaction temperature (temperature inside the diffusion furnace). In the above conventional method, while the temperature distribution inside the furnace is uniform at the location where the semiconductor substrate is installed, the diffusion reaction of impurities occurs from the semiconductor substrate on the impurity gas supply side. The reaction time between the semiconductor substrate and the impurity gas is longer than that for the semiconductor substrate on the gas discharge side. For this reason, the amount of diffusion of impurities differs between semiconductor substrates, resulting in a problem that semiconductor substrates having uniform characteristics cannot be obtained.

In order to solve such conventional problems, it is an object of the present invention to provide a method for diffusing impurities into semiconductor substrates, which can make the impurity concentration of each semiconductor substrate uniform.

[Means to solve the problem]

In order to achieve the above object, the present invention disposes a semiconductor substrate in a diffusion furnace, supplies an impurity gas from the gas supply side of the diffusion furnace, discharges the impurity gas from the gas discharge side, and then disposes the semiconductor substrate in a diffusion furnace. In a method for diffusing impurities into a semiconductor substrate, a temperature gradient is formed in the diffusion furnace so that the temperature inside the furnace on the gas discharge side is higher than the temperature inside the furnace on the gas supply side. This method is characterized by diffusing impurities.

[Effect]

According to the present invention, the temperature inside the diffusion furnace is low on the impurity gas supply side, while the temperature inside the diffusion furnace is high on the impurity gas discharge side.

Therefore, even if the contact time between the semiconductor substrate and the impurity gas on the impurity gas supply side is longer than that of the semiconductor substrate on the impurity gas discharge side, the impurity diffusion rate into the semiconductor substrate on the impurity gas discharge side is be larger than that on the side.

As a result, the impurity concentration of each semiconductor substrate in the diffusion furnace can be made the same, so that semiconductor substrates having uniform characteristics can be provided.

〔Example〕

Next, one embodiment of the present invention will be described.

FIG. 1 shows a joint cross-sectional configuration diagram of a vertical impurity diffusion device for implementing this embodiment.

In FIG. 1, an impurity diffusion device 30 includes a cylindrical quartz tube 3, which is a diffusion furnace, and a mounting table 2 (quartz quartz tube) provided inside the tube 3 and on which a plurality of semiconductor substrates (wafers) 4 are placed. boat), a boat support stand 5, an impurity gas supply pipe 12 for introducing impurity gas 1 into the gas supply section 6 provided at the top of the tube 3, and a gas discharge section provided at the bottom of the tube 3. It will be equipped with 7.

Further, on the outer periphery of the tube 3, a heating device 8 is provided around the tube.

This heating device 8 consists of a plurality of heating coils, each of which is connected to a power source (not shown).

Next, the operation of the above embodiment will be explained.

Impurity gas 1B generated from an impurity gas source (not shown),
It passes through the supply pipe 12 and reaches the gas supply section 6 at the top of the tube 3.

The impurity gas 1 that has reached the gas supply section 6 flows toward the gas discharge section 7 formed at the bottom of the tube 3.
move within.

When the impurity gas 1 moves downward within the tube 3, a reaction occurs in which the impurities in the gas adhere to the wafer 4 and then diffuse. At this time, the heating device 8 accelerates the diffusion reaction by heating the tube 3 and increasing the temperature inside the furnace.

When the tube 3 is heated by the heating device 8, a separately controlled current is supplied to each of the heating coils, so that the temperature inside the furnace can be controlled in the height direction within the furnace.

In this embodiment, a predetermined current is supplied to the heating coil on the top side of the tube 3, and the current flowing through the heating coil increases as it goes below the tube 3. A temperature gradient is formed such that the temperature inside the furnace gradually increases from the gas supply side to the gas discharge side. In other words, the furnace temperature on the gas supply side of the wafer placement part (top side of tube 3) is lower than the set temperature, while the furnace temperature on the gas discharge side of the wafer placement part (bottom side of tube 3) is lower than the set temperature. is higher than the set temperature.

Since the impurity diffusion reaction starts from the wafer on the gas supply side,
The wafer on the gas supply side takes longer to react with the impurity gas than the wafer on the gas discharge side, and the impurity concentration within the wafer tends to be higher. However, in the above embodiment, on the gas supply side (top side of the tube 3),
Since the reaction temperature is lower than that on the gas discharge side (the bottom side of the tube 3), the impurity diffusion rate is higher on the gas discharge side. Therefore, the impurity concentration within the wafer can be made uniform regardless of the placement position of the wafer.

The temperature gradient in the furnace can be appropriately determined depending on various conditions such as the number of wafers 4 placed and the size of the tube 1.
It is desirable to set the temperature to be ~20°C higher. If the temperature gradient is less than 1°C, the rate of impurity diffusion to the wafer on the gas exhaust side will be low; on the other hand, if it exceeds 20°C, the rate of impurity diffusion to the wafer on the gas exhaust side will be high (so that
This is because it is not possible to make the impurity concentration within the wafer uniform between the gas supply side and the gas discharge side.

In the embodiments described above, the wafer 4 is heated in the tube 3 to temperatures above 1000''C in most cases.The diffusion impurity for the silicon wafer is boron in the p-type.

N-type phosphorus is typical. In many cases, impurities are carried in the form of compounds to the silicon surface, which is led into a heated tube with an inert gas and oxygen.

When diffusing impurities, the above-mentioned temperature gradient is formed with respect to the set temperature, and this state is maintained for a necessary time. At this time, the attachment and diffusion of the impurity can be performed in the same process, or the diffusion process can be divided into two stages.The first stage is a process called pre-deposition, in which the impurity is diffused relatively shallowly into the silicon surface, and then, In the second stage, the supply of impurity gas may be stopped and oxidizing gas may be sent to generate an oxide film while performing diffusion.

Next, a specific experimental example will be explained.

An impurity diffusion experiment was conducted by installing a plurality of wafers from the gas supply side to the discharge side of the vertical impurity diffusion furnace described in FIG. 1 above. As shown in the cross-sectional view of Figure 2, the wafer used in this experimental example was an N-type substrate (11) of SiO□
It has a three-layer structure in which (10) is formed to a thickness of 500 angstroms and polycrystalline 5t (9) is further formed.

The impurity gas supplied to the tube 3 is POCl
3 was used. The temperature in the upper part of the furnace (gas supply side) was set to 845°C, which is 5°C lower than the set temperature, and the temperature in the lower part of the furnace (gas discharge side) was set to 845°C, which was 5°C lower than the set temperature.
The current supplied to the heating coil of the heating device 8 is controlled so as to achieve a high temperature of 855°C. That is, as shown in FIG. 3, the temperature inside the tube 3 (furnace reaction temperature) has a temperature gradient of 10° C. between the gas supply side and the gas discharge side.

Next, the diffusion impurity concentrations of the respective impurity diffusion wafers obtained in the present experimental example were compared. Sheet resistance was used as a factor for comparing the amount of impurities diffused into the wafer. The lower the sheet resistance, the higher the concentration of impurities diffused into the wafer, and the higher the sheet resistance, the lower the concentration of impurities diffused into the wafer. The actual impurity concentration can be estimated a little higher if it includes impurities that are not electrically activated.

FIG. 4 shows the sheet resistance of the wafer obtained in this experimental example. In FIG. 4, the vertical axis shows the sheet resistance of each wafer, and the horizontal axis shows the wafer mounting position (top to center to bottom) on the wafer mounting table (boat, 2) in the tube 3.
shows.

On the other hand, FIG. 5 shows the sheet resistance of each wafer when impurities were diffused into the wafer according to a comparative example in which the furnace temperature was fixed at 850° C. and no temperature gradient was applied.

According to FIG. 4, regardless of the wafer placement position within the tube 3, the inter-wafer and intra-wafer sheet resistance of each wafer placed at the top or bottom of the tube is 30±4.
It can be seen that there is no difference between each wafer or within a wafer, and that the impurity diffusion concentration between each wafer and within a wafer is the same.

On the other hand, according to the comparative example shown in Fig. 5, the sheet resistance of the wafer on the gas supply side (top of the tube) is 22 ± 2 Ω/mouth, but the sheet resistance of the wafer on the gas discharge side (bottom of the tube) is 3.
It can be seen that the impurity diffusion concentration decreases as the wafer placement position moves toward the bottom of the tube. It can also be seen that the sheet resistance varies within the wafer, and there is a difference in the impurity diffusion concentration. It can be seen that this sluggishness is particularly large on the gas exhaust side.

In contrast, according to the method of the present invention shown in FIG. 4, there is little variation in sheet resistance between and within wafers, and
It can be seen that there is no difference in impurity diffusion concentration. Therefore, according to the above embodiment, the impurity diffusion concentration of all the wafers installed in the tube 3 can be made uniform.

In the embodiment described in FIG. 1 above, the temperature inside the tube 3 can also be controlled by feed hacking. In this case, a plurality of temperature detection means are arranged inside the tube 3 from the top to the bottom, detecting the temperature inside the furnace, and controlling the current supplied to each heating coil to achieve an accurate temperature gradient. Therefore, a semiconductor substrate with uniform characteristics can be obtained.

Further, the heating method does not necessarily have to be a method using a coil, and various heating methods can be selected.

Further, in the present embodiment, the temperature gradient is formed linearly, but the temperature gradient is not limited to this, and it is also possible to form a temperature gradient having a predetermined curve.

〔Effect of the invention〕

As explained above, according to the impurity diffusion method into a semiconductor substrate according to the present invention, while the temperature inside the diffusion furnace is low on the impurity gas supply side, on the impurity gas discharge side,
Since a temperature gradient is formed so that the temperature inside the diffusion furnace becomes high, it is possible to make the impurity concentration of each semiconductor substrate within the diffusion furnace the same.

As a result, it is possible to provide a semiconductor substrate with uniform characteristics.

[Brief explanation of the drawing]

FIG. 1 is a vertical cross-sectional configuration diagram of a vertical impurity diffusion device for carrying out an embodiment of the present invention, FIG. 2 is a cross-sectional configuration diagram of a wafer used in an impurity diffusion experiment, and FIG. A schematic diagram explaining the temperature distribution formed in the diffusion experiment, FIG. 4 is a characteristic diagram showing the sheet resistance of the wafer obtained in the impurity diffusion experiment example according to the present invention, and FIG. 5 is the sheet resistance of the wafer according to the comparative example. FIG. In the figure, ■ is an impurity gas, 2 is a wafer mounting table (boat)
, 3 is a gas diffusion furnace (tube), 4 is a wafer, 5 is a boat support, 6 is a gas supply section, 7 is a gas discharge section, and 8 is a heating device.

Claims (1)

[Claims] (1) A semiconductor substrate is placed in a diffusion furnace, an impurity gas is supplied from the gas supply side of the diffusion furnace, the impurity gas is discharged from the gas discharge side, and the impurity is diffused into the semiconductor substrate. In the impurity diffusion method, impurities are diffused into the semiconductor substrate by forming a temperature gradient in the diffusion furnace so that the temperature in the furnace on the gas discharge side is higher than the temperature in the furnace on the gas supply side. Characteristic impurity diffusion method into semiconductor substrates.