JPH04130719A - Diffusion method of impurity - Google Patents

Abstract

PURPOSE:To diffuse impurities to the whole surface of a wafer in a short time and uniformly without increasing the number of processes by a method wherein an impurity diffusion source is deposited and formed on the surface of a semiconductor substrate and the impurities are driven in and diffused to the inside of the semiconductor substrate from the impurity diffusion source. CONSTITUTION:A silicon oxide film 12 is formed on the surface of a single- crystal silicon wafer 11 by a thermal oxidation operation; after that, a polycrystalline silicon film 13 is formed by a low-pressure CVD method. Then, the silicon wafer 11 is put into a diffusion furnace; and a phosphorus diffusion source is formed. At this time, the concentration of oxygen gas is changed continuously while the wafer is being heated at a temperature of 800 deg.C or lower in order to prevent phosphorus from being diffused into the polycrystalline silicon film 13. Thereby, a phosphorus diffusion source 14 is formed. After that, the silicon wafer 11 is put into the diffusion furnace; it is heated at a temperature of 850 deg.C for 60 minutes; a drive-in diffusion operation is executed; and phosphorus impurities are introduced into the polycrystalline silicon film 13. Thereby, it is possible to obtain the silicon wafer 11 provided with the polycrystalline silicon film to which phosphorus has been diffused uniformly.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention is a so-called two-step process in which an impurity diffusion source is deposited on the surface of a semiconductor substrate, and then impurities are drive-in diffused from this impurity diffusion source into the inside of the semiconductor substrate. Diffusion methods, in particular, forming a glass film made of a mixed crystal of silicon oxide and impurity oxides such as phosphorus, boron, and antimony on a polycrystalline silicon film, and diffusing impurities from this glass film into the polycrystalline silicon film. The present invention relates to a diffusion method suitable for

(Prior Art) Conventionally, when implanting phosphorus into a polycrystalline silicon film formed on a silicon substrate via a silicon oxide film, phosphorus oxide and silicon, which serve as an impurity source containing a large amount of phosphorus, are added to the surface of the polycrystalline silicon film. A method is known in which phosphorus is driven into a polycrystalline silicon film by depositing an oxide mixed crystal and then performing a heat treatment in an inert gas. Such a method is employed, for example, to form a gate electrode of a MOS transistor. FIG. 4 is a flowchart showing the flow of sequential steps in conventional phosphorus diffusion, and FIG. 5 is a sectional view of a semiconductor device in the same sequential steps. First, as shown in FIG. 5A, a silicon oxide film 2 through which phosphorus is difficult to permeate is formed on the surface of a silicon wafer 1 by, for example, thermal oxidation, and then a polycrystalline silicon film 3 is formed thereon. The purpose is to diffuse phosphorus impurities into this polycrystalline silicon film 3. Next, as shown in the flowchart in FIG.
PH3), phosphorus oxychloride (POPI3), and oxygen gas are introduced together with an inert gas such as nitrogen gas or argon gas that acts as a carrier gas, and
As shown in FIG. 5B, the surface of the polycrystalline silicon film 3 is heated to a temperature of
, SiO□) layer 4 is deposited. Next, in order to prevent further growth of the impurity diffusion source 4, the drive-in of phosphorus is heated to a temperature of 850° C. in an inert gas atmosphere such as nitrogen gas or argon gas used as a carrier gas. Perform diffusion. After that, silicon wafer 1
is removed from the diffusion furnace, and the phosphorous glass layer 4 formed on the surface of the polycrystalline silicon film 3 is removed by wet etching to form a polycrystalline silicon film 5 in which phosphorus impurities are diffused, as shown in FIG. 5C. can get.

Such a conventional method of diffusing phosphorus impurities into a polycrystalline silicon film is described in, for example, Japanese Patent Laid-Open No. 1-204412. In this method, to form an impurity diffusion source, a silicon wafer is heated at a temperature of 600 DEG to 890 DEG C. to perform phosphorus doping, thereby preventing the grain size of the polycrystalline silicon film from increasing. Furthermore, a method in which a silicon oxide film is formed on the surface of a polycrystalline silicon film and phosphorus is doped through the silicon oxide film is also proposed in Japanese Patent Laid-Open No. 2-121327.

(Problems to be Solved by the Invention) In the conventional diffusion method described above, when trying to uniformly diffuse impurities over the entire surface of a silicon wafer in a short time, it is difficult to diffuse impurities uniformly over the entire surface of a silicon wafer. (P20s, 5102), therefore, phosphosilicate glass (silicon phos
phide glass) will be formed locally. This phosphosilicate glass remains unremoved even when the diffusion source is removed by etching, and therefore acts as a mask when dry etching the polycrystalline silicon film in a later step, so that etching residue is removed. There are drawbacks that occur.

If the impurity concentration of the diffusion source is lowered in an attempt to eliminate these drawbacks, no compound that acts as a mask will be generated as described above, and therefore no etching residue will be generated, but it will take a long time for the impurities to accumulate and diffuse. Also, the impurity concentration cannot be made uniform over the entire surface of the wafer, and the sheet resistance of the polycrystalline silicon film in which impurities are diffused is not uniform over the entire surface of the wafer. In addition, the method of forming a thin silicon oxide film on the surface of a polycrystalline silicon film and diffusing impurities such as phosphorus through this requires a step of forming the silicon oxide film and a step of removing it after diffusing the phosphorus. This has the drawback of making the manufacturing process more complicated and increasing costs.

Furthermore, conventional diffusion methods have problems with the reproducibility and uniformity of the impurity diffusion concentration, which is one of the causes of deterioration of device characteristics.

The purpose of the present invention is to eliminate the drawbacks of the conventional diffusion method described above, to diffuse impurities uniformly over the entire surface of a wafer in a short time without increasing the number of steps, and to create a compound that can be used as a mask during etching. It is an object of the present invention to provide a method for diffusing impurities that prevents the formation of impurities.

(Means and Effects for Solving the Problems) When diffusing impurities into a semiconductor substrate, the present invention forms an impurity diffusion source by depositing it on the surface of the semiconductor substrate so that the impurity concentration decreases when viewed in the depth direction. Then, impurities are drive-in diffused into the semiconductor substrate from this impurity diffusion source.

In such a diffusion method of the present invention, since the impurity concentration is lowest at the interface between the diffusion source and the semiconductor substrate, no compound that can serve as a mask is generated during selective dry etching of the semiconductor substrate. Furthermore, by arbitrarily controlling the thickness of the silicon oxide film generated at the interface with the semiconductor substrate, the time required for diffusion can be shortened, and the impurity concentration within the wafer surface can be uniformly controlled. .

(Embodiment) FIGS. 1 and 2 are a flowchart showing the sequential steps of an embodiment of the diffusion method according to the present invention, and a sectional view showing the state in which a diffusion source is formed together with its impurity concentration.

After forming a silicon oxide film 12 having a thickness of 300 nm on the surface of a single crystal silicon wafer 11 by thermal oxidation,
A polycrystalline silicon film 13 with a thickness of 4,500 yen is formed by low pressure CVD method.
Formed to the thickness of a person.

Next, the silicon wafer 11 is placed in a diffusion furnace to form a phosphorus diffusion source. In this example, phosphorus oxychloride ( Nitrogen gas bubbling POCl, ) was flowed at a flow rate of 4003 CCM, nitrogen gas acting as a carrier gas was flowed at a flow rate of 517 min, and at the same time oxygen gas was continuously decreased from 11005 CCCM to 20 SCCM in 30 min. By continuously changing the concentration of oxygen gas in this way, we can create a concentration profile in which the phosphorus concentration decreases continuously from 4% by weight to 0% by weight as seen from the surface in the depth direction, as shown in Figure 2. It was possible to form a phosphorus diffusion source 14 having the following properties. In the process of forming this phosphorus diffusion source 14, POCl3+0□□P2O5+ C12Si
As shown by the reaction formula +02-SiO2, P2O5 and SiO□ are produced, and the phosphorus diffusion source 14 is formed as phosphorus glass made of a mixed crystal of phosphorus oxide and silicon oxide. In this case, the concentration of P2O in the phosphor glass is lowest at the interface with the polycrystalline silicon film 13 by continuously decreasing the flow rate of oxygen gas, and the concentration gradually increases toward the surface. It can be formed as follows.

After forming the diffusion source 14 as described above, the silicon wafer 11 was placed in a diffusion furnace and heated at a temperature of 850° C. for 60 minutes to perform drive-in diffusion to introduce phosphorus impurities into the polycrystalline silicon film 13. The sheet resistance of the polycrystalline silicon film I3 in which phosphorus was diffused in this manner was 30±2 Ω/portion. Further, phosphorus was uniformly diffused over the entire surface of the silicon wafer 11.

The silicon wafer 11 having the polycrystalline silicon film in which phosphorus is uniformly diffused in this manner is taken out of the diffusion furnace, the diffusion source 14 made of phosphorous glass is removed by wet etching, and the polycrystalline silicon film is selectively etched by dry etching. It was removed. According to the diffusion method of the present invention, no precipitates or compounds were generated on the surface of the polycrystalline silicon film 13, and no etching residue was observed.

Comparative Example 1 After forming a thermal oxide film on the surface of a silicon wafer, phosphorus oxychloride (POCl) was used to form an impurity diffusion source.
The flow rate of nitrogen gas bubbling through the tube was fixed at 4003 CCM, and the flow rate of oxygen gas was fixed at 503 CCM. In this case, as shown in FIG. 3A, the compound 15 was locally precipitated at the interface between the polycrystalline silicon film 13 and the diffusion source 14 made of phosphorous glass. Furthermore, drive-in diffusion was performed for 10 minutes in a diffusion furnace, and the diffusion source 13
was removed by etching. Although the sheet resistance of the polycrystalline silicon film 13 was as low as 21±2 Ω/hole, a compound I5 such as phosphosilicate glass was formed on its surface by the reaction between the high concentration of phosphorus in the diffusion source and silicon. Ta.

Since this compound 15 acts as a mask for dry etching, needle-shaped etching residues 16 were formed after dry etching the polycrystalline silicon film 13, as shown in FIG. 3C.

Comparative Example 2 When forming an impurity diffusion source, the flow rate of nitrogen gas for bubbling phosphorus oxychloride was set to 400 SCCM, and the flow rate of oxygen gas was maintained constant at 8003 CCM. The phosphorus concentration in the diffusion source thus formed was a constant low value of 1.5% by weight. Next, the silicon wafer was placed in a diffusion furnace to perform drive-in diffusion of phosphorus, the impurity diffusion source was removed by etching, and the silicon wafer film was further removed by dry etching. In this case, no compound was observed to precipitate at the interface between the polycrystalline silicon film and the diffusion source, and no etching residue was generated. However, in order to lower the sheet resistance of the polycrystalline silicon film after phosphorus diffusion to about 30Ω, it takes a long time to dope phosphorus for 120 minutes and drive-in phosphorus diffusion for 120 minutes. The change in sheet resistance over time is 30±5Ω/
The mouth had deteriorated.

(Effects of the Invention) As explained above, according to the diffusion method of the present invention,
When forming a diffusion source on the surface of a semiconductor substrate such as a polycrystalline silicon film, the concentration of the impurity to be doped is continuously changed so that the concentration is lowest at the interface of the polycrystalline silicon film. A compound is no longer formed at the interface with the silicon film, and therefore, when dry etching the polycrystalline silicon film, the compound does not act as a mask and generate etching residue. Further, since the impurity can be contained in the entire impurity source at a high concentration, the impurity can be uniformly diffused into the semiconductor substrate in a relatively short time.

[Brief explanation of the drawing]

FIG. 1 is a flowchart showing the sequential steps of an embodiment of the diffusion method according to the present invention; FIG. 2 is a cross-sectional view showing a diffusion source formed in the silicon wafer together with an impurity concentration profile in the diffusion source; The figure is a cross-sectional view showing the process of forming an etching residue using the conventional diffusion method. Figure 4 is a flowchart showing the sequential steps of the conventional diffusion method. Figure 5 is the configuration of the semiconductor device in the same sequential steps. FIG. 11...Silicon wafer 12...Silicon oxide film 13...Polycrystalline silicon film 14...Diffusion source Fig. 2

Claims (1)

[Claims] 1. When diffusing impurities into a semiconductor substrate, an impurity diffusion source is deposited on the surface of the semiconductor substrate such that the impurity concentration decreases when viewed from the surface in the depth direction, and then the semiconductor is diffused from this impurity diffusion source. An impurity diffusion method characterized by drive-in diffusion of impurities into the inside of a substrate.