JPS5949684B2 - Pretreatment method for process tube for diffusion - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a pretreatment method for a diffusion process tube used for diffusing impurities and the like into a semiconductor wafer through heat treatment.

A diffusion process tube (hereinafter referred to as a tube) that holds a semiconductor wafer while impurities and the like are diffused into the semiconductor wafer through heat treatment.

) is generally 1000 to 1000 to ionize impurities.
It is maintained at a high temperature of 1275°C.

For this reason, chemically and thermally stable quartz tubes are commonly used. However, since the quartz tube has a relatively low softening point temperature of about 1200° C., it has the disadvantage that it can be used as a furnace core material or a quartz boat member.
To improve these drawbacks, the use of tubes made of silicon, especially polycrystalline silicon, has been proposed in the Journal of Electrochemical Society.
eIe-Ctrochemical Society) Volume 121, No. 8, Published August 1974. Such polycrystalline silicon tubes are
Usually CVD (Chemical Vapor Depos)
tion) method with high purity and without dopants. However, even if such a polycrystalline silicon tube is used, not only do P-type or N-type doping impurities and heavy metals such as chromium, copper, and gold, which are so-called lifetime killers, adhere to the tube itself. All contaminants present must be removed.
By the way, the conventional tube pretreatment method was as follows.

First, the temperature is raised to 1000° C. while purging (purifying) with a rare gas such as helium or alco, and then heated with hydrogen gas for about 1 hour. Thereafter, heat treatment was performed at the working temperature while purging with hydrogen gas containing 5% hydrochloric acid gas. In addition, other conventional methods use 69% nitric acid 9 and 49% by volume.
The tube was cleaned with a mixture of 1% hydrogen fluoride and rinsed with deionized water. In the former case, there are disadvantages in that an expensive rare gas and a specially constructed gas doping device must be used. Further, in order to obtain good pretreatment results, highly purified hydrochloric acid gas is required, but it is difficult to obtain such highly purified hydrochloric acid gas. On the other hand, in the latter case, as shown in FIG. 1, polyethylene caps 2 are attached to both ends of a cylindrical tube 1, and the above-mentioned liquid mixture is introduced into the tube 1 through one opening 2a of the cap 2. This method performs a so-called etching process, and does not require expensive equipment.

However, for example, the dimensions of the tube 1 for processing a square semiconductor wafer with a diameter of about 50n are 85φ x 75φ x
It is about 180011 and its weight is 5-10k.
However, it has the drawback that good pretreatment results cannot be obtained due to the following reasons.

That is, the etching process for tube 1 is 3Si+4HN.
The chemical reaction 03+18HF→3H, SiF6+4N0+8H20 consumes the tube 1, which may damage the tube 1, and also has the disadvantage that a uniform etching effect cannot be obtained due to its size. When a semiconductor wafer is heat-treated using a tube pretreated by such a conventional method, the impurity doping concentration of the semiconductor wafer will vary greatly depending on the diffusion patch rod, as shown in Table 2, as described below. Such inconveniences often occur.

That is, Table 1 shows N-type high resistivity silicon (FZ (FlO
Resistivity 180-220Ω by the atingZOne) method
C! IL) semiconductor wafer surface resistivity (V/I (Ω・
Cln)) were measured at the top, middle and bottom, respectively.

Next, tube 1 is pretreated with the above-mentioned mixed solution,
Semiconductor wafers as shown in Table 1 are heated at a furnace temperature of 1250°C.
Table 2 shows the results of heat treatment for 30 hours using oxygen and nitrogen gases at 11/min each.

As is clear from what has already been said, the reason for the variation shown in Table 2 is that the diffused dopant is once diffused into the tube 1 itself by heat treatment, that is, migrated, and the subsequent heat treatment, for example, Diffusion patch rod number 4
In the same number 5, it is determined that the diffusion occurs from the tube 1 to the semiconductor wafer. Therefore, the present invention has been made to solve the above-mentioned drawbacks of the conventional methods. An object of the present invention is to provide a pretreatment method that more fully removes contaminants present on the surface of a silicon tube.

An object of the present invention is to provide a pretreatment method for removing contaminants and then coating the surface of a silicon tube with a thermally and chemically stable substance to prevent further contamination of the tube itself.

In order to achieve this objective, the present invention provides a pretreatment method for a silicon process tube for diffusion processing a semiconductor wafer, in which the silicon process tube is cleaned with hydrofluoric acid and aqua regia. After that, an oxide film is further formed on the surface of the silicon process tube that constitutes a space in which a semiconductor wafer is subjected to a diffusion process.

Hereinafter, one embodiment of the present invention will be described in accordance with the order of the pretreatment steps, with reference to FIG. 1 again.

First, in the first step, the inside of the silicon tube 1 is filled with a mixed solution of 1 volume of hydrogen fluoride and 3 volumes of deionized water, that is, dilute hydrofluoric acid.

As a result, the silicon oxide present on the surface of the tube 1 is dissolved, and the contaminants present on the silicon oxide are separated from the surface of the tube 1. The time required for this step is, for example, 20 minutes. In the second step, the inside of the tube 1 is washed with deionized water for about 20 minutes.

As a result, contaminants separated from the surface of the tube 1 are discharged to the outside of the tube 1. Note that this cleaning will not damage the tube 1 itself. In the third step, the tube 1 is filled with a mixed solution of 1 volume of nitric acid and 3 volumes of hydrochloric acid, that is, aqua regia, for example, for 1 hour.

This removes heavy metals that remained as contaminants that could not be removed in the second step. For example, copper is Cu+4HN03→CU(NO3)2+2H20+2N
02 reaction, and gold is dissolved in NNO3+3HC
1→2H20+NOCl+Cl2 and NOCI-l-C
12+HCl+Au→HAuCl4+NO, both of which are dissolved in a well-known reaction.

In the fourth step, the inside of the tube 1 is sufficiently washed with deionized water in the same manner as in the second step.

After the above process, the tube is subjected to a diffusion process on the next semiconductor wafer. In the fifth step, as shown in FIG. 2, a sufficiently clean quartz cap 3 is connected to one end of the tube 1, and through the opening 3a of the cap 3,
Introducing oxygen gas into the tube 1 from the direction of the arrow shown in the figure,
The tube 1 is heated to 1100-1250°C using a heating coil 4 wound around the outside of the tube 1.
A process for forming an oxide film on the inner surface of the tube 1, that is, on the surface of the tube 1 constituting a space in which the semiconductor wafer is diffused, is performed.

This suppresses the diffusion dopant from entering the tube 1 during the diffusion process. Using tube 1 with such preprocessing performed, N
A semiconductor wafer made of high resistivity silicon is heated at a furnace temperature of 1250°C.
Table 3 shows the results of heat treatment for 30 hours in an atmospheric gas containing oxygen and nitrogen at 11/min each.

According to Table 3, compared to the conventional method, the surface resistance (V/I
It has been shown that the variation in (Ω·CTn)) is sufficiently small for each diffusion patch rod. As explained above, the present invention removes contaminants undesirable for semiconductor wafers existing on the surface of a silicon tube with hydrofluoric acid and aqua regia, and then forms an oxide film inside the tube. By doing so, it is possible to prevent contaminants from diffusing into the tube itself, thereby maintaining and improving the cleanliness of the tube, and by repeatedly using the tube, contaminants can be removed from the tube. The inconvenience caused by back-diffusion onto the wafer can be eliminated, and the effect of contaminating the semiconductor wafer can also be reduced.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view showing an outline of the tube, and FIG. 2 is a diagram illustrating one step of pretreatment according to the present invention. 1...tube, 2,3...cap, 4...coil.

Claims (1)

[Claims] 1. In a pretreatment method for a silicon process tube for diffusion processing a semiconductor wafer, after cleaning the silicon process tube with hydrofluoric acid and aqua regia,
A method for preprocessing a diffusion process tube, further comprising forming an oxide film on the surface of the silicon process tube constituting a space for diffusing a semiconductor wafer.