JPH0793278B2 - Diffusion method of phosphorus into polycrystalline silicon film - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for diffusing phosphorus into a polycrystalline silicon film capable of uniformly diffusing phosphorus into the polycrystalline silicon film at an arbitrary concentration.

[Conventional technology]

The polycrystalline silicon film is used, for example, as a gate electrode or wiring of a MOS transistor. The phosphorus diffusion is to introduce phosphorus, which is an impurity, into an arbitrary region of the polycrystalline silicon film which is a target with a desired diffusion depth and concentration distribution by utilizing a diffusion phenomenon.

The diffusion of phosphorus into the polycrystalline silicon film will be described. This polycrystalline silicon film is formed on the oxide film (SiO ₂ ) formed on the silicon substrate by the CVD method (vapor phase growth method). And it is to diffuse the phosphorus which is an n-type impurity into the polycrystalline silicon film, but the conventionally POCL ₃
Phosphorus was diffused by supplying it to a diffusion furnace together with O ₂ and N ₂ gas. Next, a mode of phosphorus diffusion into the polycrystalline silicon film will be described. An oxide film P ₂ O ₅ containing phosphorus as an impurity is formed on the surface of the polycrystalline silicon film. The reaction formula is Next, P ₂ O ₅ reacts with silicon (Si), and phosphorus diffuses into the polycrystalline silicon film.

The reaction formula is The chemical reactions of the above formulas (1) and (2) are caused to occur in a furnace at 800 to 1000 ° C.

FIG. 5 shows the pattern of the above reaction.

Here, when phosphorus is diffused in the polycrystalline silicon film, it is necessary to uniformly diffuse phosphorus in order to make the resistance value of the polycrystalline silicon film uniform.

[Problems to be Solved by the Invention]

However, in the method of diffusing phosphorus into the polycrystalline silicon film as described above, the diffusion source P ₂ O ₅ is formed in the diffusion furnace under the high temperature condition of 800 to 1000 ° C. where the semiconductor wafer is installed. , P ₂ O ₅ is formed and phosphorus diffusion starts.

The distance between semiconductor wafers is usually 4 to 8 m.
Since it is m and the number of processed sheets is 50 to 100, there are the following problems when diffusing an impurity gas such as P ₂ O ₅ or the like. That is, the impurity gas diffuses from the periphery of each of the arranged semiconductor wafers toward the central portion, and the reactions are sequentially performed in the same direction. This is mainly because the distance between the semiconductor wafers is narrow.

Therefore, the concentration of phosphorus diffused to the peripheral portion of the polycrystalline silicon film becomes higher, while the concentration of phosphorus becomes lower toward the central portion. And the phosphorus concentration becomes non-uniform.

In order to cope with this, the diffusion time is lengthened to diffuse phosphorus into the polycrystalline silicon film until it is saturated, and phosphorus is diffused at a high concentration. According to this method, since phosphorus is diffused into the polycrystalline silicon film in a saturated state, it is possible to make the phosphorus concentration uniform from the peripheral portion to the central portion of the polycrystalline silicon film.

However, according to this method, phosphorus is diffused at a high concentration (diffused to the solid content limit value), so that phosphorus is segregated at the interface between the SiO ₂ film and the polycrystalline silicon film, resulting in a step after phosphorus diffusion. Undercutting occurs in the polycrystalline silicon film during the etching of the polycrystalline silicon film. When this undercut occurs, for example, in an insulated gate field effect transistor, the gate electrode (polycrystalline silicon film) and the source / drain end are separated from each other, which causes an offset state and causes a defect in the semiconductor device. There's a problem. Here, the undercut means an engraving in the vicinity of the gate oxide film of the polycrystalline silicon gate that occurs during dry etching.

On the other hand, if the diffusion time is shortened to avoid undercut, the phosphorus concentration diffused into the polycrystalline silicon film becomes nonuniform.

The present invention is proposed in order to solve the above problems, and provides a method for diffusing phosphorus into a polycrystalline silicon film, which is capable of uniformly diffusing phosphorus into the polycrystalline silicon film without causing an undercut. It is intended to be provided.

[Means for Solving the Problems]

In order to achieve the above-mentioned object, the present invention is applied on a silicon wafer.
In a method of diffusing phosphorus into a polycrystalline silicon film formed via a SiO ₂ film, in a phosphorus diffusion furnace kept at a low temperature,
At least POCl ₃ gas and O ₂ gas were introduced, P ₂ O ₅ was formed, and after depositing on the polycrystalline silicon film, the introduced gas was shut off and the phosphorus diffusion furnace was heated. In the method of diffusing phosphorus into the polycrystalline silicon film, and diffusing phosphorus into the polycrystalline silicon film formed on the silicon wafer via the SiO ₂ film, the method is as follows: in introducing P ₂ O ₅ was formed, after being deposited on the polycrystalline silicon film, blocking the introduction P ₂ O _5,
The phosphorus diffusion furnace was heated to diffuse phosphorus into the polycrystalline silicon film, which was a method of diffusing phosphorus into the polycrystalline silicon film.

[Action]

In this way, the P ₂ O ₅ formation step and the phosphorus diffusion step are separated, and the phosphorus diffusion furnace is set to a high temperature only when phosphorus diffusion is performed, so that phosphorus is diffused uniformly without causing undercuts. be able to.

〔Example〕

A first embodiment of the present invention will be described with reference to the diffusion device shown in FIG. In this embodiment, the external combustion furnace 1 is used to form an oxide film P ₂ O ₅ containing phosphorus as an impurity on the surface of the polycrystalline silicon film.
Was used.

In this diffusion device, a wafer support 7 is provided inside the phosphorus diffusion furnace 2 so that the silicon wafers 4 can be aligned and accommodated. Then, heaters 6 for the phosphorus diffusion furnace are arranged on both sides outside the phosphorus diffusion furnace 2 so that the inside can be heated. A gas introduction port 5 for supplying an impurity gas or the like is provided inside the phosphorus diffusion furnace 2, and an external combustion furnace 1 is provided between the gas introduction port 5 and the phosphorus diffusion furnace 2 as well as the outside thereof. On the other hand, an external combustion furnace heater 3 is provided.

First, a single crystal silicon wafer is carried into an oxidation furnace, and a 300 Å gate SiO ₂ film is formed on the surface of the silicon wafer under heating conditions of 1000 ° C. Next, this silicon wafer is taken out from the oxidation furnace and carried into a low pressure CVD furnace. Silane gas is supplied together with a carrier gas to the low pressure CVD furnace to form a polycrystalline silicon film of 3500 Å on the gate SiO ₂ film. In this case, the reaction formula for forming the polycrystalline silicon film is SiH ₄ → Si + 2H ₂
Is.

Next, the silicon wafer 4 is carried into the phosphorus diffusion furnace 2 to perform phosphorus diffusion. First, the external combustion furnace heater 3
Thus, the temperature in the external combustion furnace 1 is set to 900 ° C., and the temperature in the phosphorus diffusion furnace 2 is maintained at 400 ° C. And liquid POCl ₃
N _2: bubbling 0.4 L / min. O _2: 0.5 l /
It is supplied to the external combustion furnace 1 through the gas inlet 5 together with the amount. This process is continued for 1 hour. After that, the POCl ₃ gas and the O ₂ gas were shut off, the purge N ₂ was switched to 10 liters, and the phosphorus diffusion furnace 2 was heated to 850 ° C. for about 30 minutes to 30
Hold this state for a minute. In addition, it is desirable to provide a heater between the external combustion furnace 1 and the phosphorus diffusion furnace 2 so as to prevent P ₂ O ₅ from dew condensation so as to maintain the temperature.

In order to determine the phosphorus concentration distribution in which phosphorus has diffused in this manner, the sheet resistance as a factor correlated with this may be measured. The larger the sheet resistance, the higher the phosphorus concentration diffused in the silicon wafer 4.
The smaller the variation in sheet resistance, the more uniformly phosphorus is diffused in the silicon wafer 4. In this example, the sheet resistance was measured and found to be 35 ± 1 Ω /
It was a mouth. In this way, the variation in sheet resistance is ±
It was as small as 1 Ω / mouth, and it was found that phosphorus was uniformly diffused.

Next, the silicon wafer in which phosphorus is diffused is loaded into a reactive sputter etching apparatus and dry-etched. The silicon wafer according to this example is dry-etched, and as a result, an undercut is almost observed in the polycrystalline silicon film. I couldn't do it. FIG. 2 shows the state.

Next, a second embodiment of the present invention will be described with reference to the diffusion device shown in FIG. In this embodiment, the oxide film P ₂ O ₅ is formed by using the phosphorus diffusion furnace 2
I went there.

Unlike the case of the first embodiment, this diffusion device does not have an external combustion furnace. Other configurations are similar to those in the first embodiment.

First, a single crystal silicon wafer is carried into an oxidation furnace, and a 300 Å gate SiO ₂ film is formed on the surface of the silicon wafer under heating conditions of 1000 ° C. Next, this silicon wafer is taken out from the oxidation furnace and carried into a low pressure CVD furnace. Silane gas is supplied together with a carrier gas to the low pressure CVD furnace to form a polycrystalline silicon film of 3500 Å on the gate SiO ₂ film.

Next, it is carried into the phosphorus diffusion furnace 2 to perform phosphorus diffusion. First, liquid POCl ₃ is bubbled with N ₂ : 0.4 liter, and is supplied to the phosphorus diffusion furnace 2 together with O ₂ : 0.5 liter / min. In this case, the temperature condition is 750 ° C. and the temperature is maintained for 1 hour. Then POCl ₃
The gas and O ₂ gas were shut off, the purge N ₂ was switched to 10 liters, the phosphorus diffusion furnace 2 was heated to 850 ° C. over 10 minutes, and this state was maintained for 50 minutes.

The sheet resistance of the silicon wafer 4 was measured to determine the phosphorus concentration distribution due to such phosphorus diffusion.
It is 2Ω / port and the variation of sheet resistance is ± 2Ω /
It was mouth, and the phosphorus diffusion was found to be uniform.

Next, the silicon wafer 4 having the phosphorus diffused therein was carried into a reactive sputter etching apparatus and dry-etched.
No undercut was observed in the polycrystalline silicon film as in the example.

The effects according to the present invention described above become more clear by comparing with the conventional example. That is, when the sheet resistance of the silicon wafer was measured by the conventional method, it was 35 + 10 Ω / port. Thus, the variation in sheet resistance was as large as ± 10 Ω / port, and the diffusion state of phosphorus was non-uniform. Further, as a result of dry etching by the reactive sputter etching apparatus, the occurrence of undercut was remarkably recognized as shown in FIG.

The diffusion of phosphorus according to the conventional example will be described. First, a single crystal silicon wafer is loaded into an oxidation furnace and a 300Å oxide film (SiO ₂ ) is formed on the surface of the silicon wafer at 1000 ° C. Next, the silicon wafer was taken out from the oxidation furnace and depressurized CV
Carry it into the D furnace. Silane gas is supplied to the low pressure CVD furnace together with a carrier gas to form a polycrystalline silicon film having a thickness of 3500Å on the oxide film. Next, the silicon wafer is carried into a phosphorus diffusion furnace to perform phosphorus diffusion. In this case liquid POCl ₃
Bubbling N ₂ : 0.4 l / min and O ₂ : 0.5 l / min into the phosphorus diffusion furnace. In this case, the temperature of the phosphorus diffusion furnace was 850 ° C. and was maintained for 45 minutes.

〔The invention's effect〕

As described above, according to the present invention, the P ₂ O ₅ forming step and the phosphorus diffusion step are separated, so that the formed P ₂ O ₅ is stored in the phosphorus diffusion furnace kept at a low temperature for a long time (1 to 1). By supplying for 2 hours, phosphorus diffusion can be prevented and a sufficiently concentrated P ₂ O ₅ deposition can be secured from the peripheral portion to the central portion of the silicon wafer. Therefore, by raising the temperature of the phosphorus diffusion furnace and performing the phosphorus diffusion through the required reaction, the phosphorus can be uniformly diffused within the surface of the silicon wafer. In this way, phosphorus can be uniformly diffused in the surface of the silicon wafer and in the depth direction of the polycrystalline silicon film, and undercuts that occur during the formation of the polycrystalline silicon gate can be prevented.
Further, as the design rule becomes smaller, the gate oxide film becomes thinner, but the penetration of phosphorus due to it can be prevented by making the phosphorus concentration uniform in the plane of the silicon wafer and in the thickness direction of the crystalline silicon film.

[Brief description of drawings]

FIG. 1 is a schematic view of a diffusion device used in the first embodiment of the present invention, FIG. 2 is an enlarged view of the vicinity of a polycrystalline silicon film after dry etching, and FIG. 3 is a second embodiment of the present invention. FIG. 4 is an enlarged view of the vicinity of a conventional polycrystalline silicon film after dry etching, and FIG. 5 is an explanatory view showing a mode of phosphorus diffusion. 1 ... External combustion furnace 2 ... Phosphorus diffusion furnace 3 ... External combustion heater 4 ... Silicon wafer 5 ... Gas inlet

Claims (2)

[Claims] 1. A method of diffusing phosphorus into a polycrystalline silicon film formed on a silicon wafer via an SiO ₂ film, wherein at least POCl ₃ gas and O ₂ gas are introduced into a phosphorus diffusion furnace kept at a low temperature. Then, after forming P ₂ O ₅ and depositing it on the polycrystalline silicon film, the introduction gas is shut off and the phosphorus diffusion furnace is heated to diffuse phosphorus into the polycrystalline silicon film. A method of diffusing phosphorus into a polycrystalline silicon film, which is characterized by the above. 2. A method of diffusing phosphorus into a polycrystalline silicon film formed on a silicon wafer via an SiO ₂ film, wherein P ₂ O ₅ formed outside is introduced into a phosphorus diffusion furnace kept at a low temperature. After depositing on the polycrystalline silicon film, the introduction P ₂ O ₅ is blocked, the temperature of the phosphorus diffusion furnace is raised, and phosphorus is diffused into the polycrystalline silicon film. A method for diffusing phosphorus into a silicon film.