JPH0228246B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method of controlling the amount of boron diffused into a semiconductor wafer, and particularly to a method of controlling the amount of boron diffused when boron is diffused using a polyboron film (PBF).

Conventionally, BN (boron nitride) has been used as a source for transistor base diffusion. However, BN has the disadvantage that crystal defects occur in the formed wafers due to its large heat capacity. Therefore, in order to eliminate this drawback, we incorporated boron into the organic polymer.
PBF is applied to the wafer using a spin-on method, and this
Attempts have been made to put a wafer coated with PBF into a diffusion furnace, heat it in an atmosphere of nitrogen gas and a trace amount of oxygen gas, and diffuse it.

However, with this method, it is not possible to control the amount of boron diffusion during diffusion, and in order to achieve a certain predetermined amount of diffusion, the wafer must be removed from the diffusion furnace when the sheet resistance value reaches a predetermined value. Ta.

Additionally, when applying PBF, unevenness in the wafer pattern causes uneven film thickness on the diffusion area, and if diffusion continues, protrusions will form on both ends of the bottom of the diffusion area, and the lateral spread will become uneven. There was a drawback. Therefore, this boron diffusion method using PBF has a problem that it is difficult to use for device formation such as base diffusion.

The purpose of this invention is to use PBF to obtain a wafer with few crystal defects, without protrusions on both ends of the bottom surface of the diffusion region, and with no non-uniform lateral spread, and furthermore, the sheet resistance can be easily controlled. An object of the present invention is to provide a method for controlling the amount of boron diffusion.

In order to achieve the above object, the method of controlling the boron diffusion amount of the present invention is such that the atmosphere in the diffusion furnace is made into nitrogen gas and a trace amount of oxygen gas in the first step, and the boron glass layer is left in the second step. The boron diffusion is controlled by switching to oxygen gas and performing stretching diffusion in the boron diffusion layer, and controlling the time of the first stage.

Hereinafter, the present invention will be explained in more detail with reference to Examples.

As one embodiment of the present invention, a case will be described in which base diffusion of a transistor is performed.

First, as shown in FIG. 1a, a silicon dioxide (SiO ₂ ) layer 2 is formed on a silicon (Si) substrate 1, and a base opening 3 is formed in this silicon dioxide layer 2 on a pattern of a wafer 4. , Figure 1b
Apply PBF5 as shown. This application is performed by a spin-on method.

FIG. 2 shows the state in which the wafer 4 is placed in the diffusion furnace. In FIG. 2, the diffusion furnace 11 is the heating section 1.
A quartz tube 13 is inserted into the heating section 12, and several dozen wafers 4 are stored in the quartz tube 13, arranged vertically on a boat 14. The quartz tube 13 is supplied with nitrogen gas from the side.
N ₂ , oxygen gas O ₂ , and water vapor H ₂ O are supplied, and the heating section 12 is configured to be temperature controllable. Of course, the diffusion furnace 11 itself shown here is already well known.

The wafer 4 placed in the diffusion furnace 11 is subjected to temperature control and gas control in accordance with the order shown in FIG.

The inside of the diffusion furnace 11 is initially maintained at 800°C.
A quartz tube 13 is placed in this diffusion furnace 11, and nitrogen gas N ₂ and oxygen gas O ₂ are sent to the quartz tube 13.
The PBF layer 5 of the wafer 4 is burned in the gas atmosphere, and becomes a boron glass layer 6 as shown in FIG. 1c.

In the first process pr1, when the temperature is further heated from 800°C to 900°C in the above gas atmosphere, boron diffuses from the boron glass layer 6 into the silicon substrate 1, forming a boron diffusion region 7. .
This first process pr1 is carried out for about 10 minutes to about 2 hours, and this time controls the boron concentration and therefore the sheet resistance Rs. If this time is made longer, the boron concentration will be higher, and the sheet resistance Rs will be lowered. Conversely, if the time is made shorter, the sheet resistance Rs will be increased.

Also, while the time of this process pr1 is small,
Since the supply of boron from the boron glass layer 6 is uniform, the depth of the boron diffusion region 7 is, for example, 300 mm.
Å, the bottom surface of the boron diffusion region 7 becomes uniform. This is shown in Figures 4a and 4b.
This is clear from the comparison. Fig. 4a shows the case according to the conventional method, and Fig. 4b shows the case according to the present invention. In each figure, part A shows a top view of the wafer, and part B shows the wafer taken along line C-C. Figure 3 shows a side view in perspective view. It can be seen that in FIG. 4a, the bottom surface of the boron diffusion region 7 is uneven, whereas in FIG. 4b, the bottom surface 7a of the boron diffusion region is much flat, that is, uniform. Furthermore, it has been confirmed that while the time of the process pr1 is short, the horizontal spread of the boron diffusion region 7 is also uniform. That is, as shown in FIG. 5, the line width of the boron diffusion region 7 indicated by the black line is constant, which indicates that the lateral spread of the boron diffusion region 7 is not non-uniform.

Next, in the second process pr2, the temperature is raised to 1100°C and the gas is switched from _N2 ⁺ trace _O2 to _O2 . In general, in the process of boron diffusion, the relationship between the degree of diffusion, that is, the sheet resistance Rs, and the proportion of oxygen gas (=O ₂ /N ₂ +O ₂ ×100%) is determined by experiment, as shown in Figure 6. It was found that O ₂ % and sheet resistance Rs are linear. In other words, if O ₂ is 100%, the sheet Rs will be close to infinity. Therefore, by switching from N ₂ + a trace amount of O ₂ to O ₂ , diffusion from the boron glass layer 6 is stopped and diffusion only within the boron diffusion region 7 proceeds. That is, by switching the gas atmosphere from N ₂ + O ₂ to 0 ₂ ,
The boron diffusion expressed by the error function from the boron glass layer 6 is terminated, and the process shifts to stretching diffusion expressed by a Gaussian distribution.

Subsequently, in the third process, while keeping the temperature at 1100°C, H ₂ O is sent into the quartz tube 13 to form a boron-diffused SiO ₂ film on the boron diffusion region 7, increasing the concentration of high-concentration boron glass. I am trying to remove it and eliminate its effects. And in process pr4,
Formed by converting gas back to N ₂ + O ₂ at the same temperature
Annealing of _two SiO layers is being performed.

Note that the atmospheric gas in the examples is nitrogen gas N ₂ in the first process, oxygen gas O ₂ + water vapor H 2 O, water vapor H ₂ O only, or nitrogen gas _{N 2 +} oxygen gas O ₂ in the third and subsequent processes. Large amounts etc. are applicable.

As described above, according to the present invention, in the first step, boron is supplied and diffused from the boron glass layer with the gas atmosphere being nitrogen gas and a trace amount of oxygen gas,
In the second stage, the boron glass layer is left in place, the oxygen gas is switched to, the boron supply from the boron glass layer is stopped, and the boron is only stretched and diffused within the boron diffusion region, and the time of the first stage is controlled. Therefore, by appropriately switching from the first stage to the second stage, it is possible to prevent the occurrence of protrusions at both ends of the bottom surface of the diffusion region and the occurrence of non-uniform lateral spread. can. Therefore PBF
This boron diffusion can be applied to element formation, such as base diffusion, and is said to cause fewer crystal defects.
A device wafer that effectively utilizes the characteristics of PBF can be obtained. Furthermore, since the diffusion of boron from the boron glass layer can be interrupted at any time, the sheet resistance can be easily controlled and its variation can be reduced.

[Brief explanation of drawings]

Fig. 1 is a cross-sectional view of a wafer at each stage in an embodiment of the present invention, Fig. 2 is a diagram showing a diffusion furnace used in carrying out the invention, and Fig. 3 is a diagram showing each step in an embodiment of the invention. FIG. 4a is a diagram for explaining the bottom surface state of the boron diffusion region formed by the conventional method, and FIG. 4b is a diagram for explaining the state of the bottom surface of the boron diffusion region formed by the method of the present invention. FIG. 5 is a wafer plan view for explaining the lateral spread of the boron diffusion region formed by the method of the present invention, and FIG. FIG. 3 is a diagram showing the relationship between oxygen concentration and sheet resistance in boron diffusion. 1: Silicon substrate, 4: Semiconductor wafer, 5: Polyboron film layer, 11: Diffusion furnace.

Claims (1)

[Claims] 1. Applying a polyboron film onto a semiconductor wafer, storing the applied semiconductor wafer in a diffusion furnace and heating it to form a boron glass layer on the semiconductor wafer to perform boron diffusion. In this method, the atmosphere in the diffusion furnace is changed to nitrogen gas and a trace amount of oxygen gas in a first step, and in a second step, the atmosphere in the diffusion furnace is changed to nitrogen gas and a trace amount of oxygen gas while leaving the boron glass layer. Switch to
A method for controlling the amount of boron diffusion, characterized in that the diffusion of boron is controlled by stretching and diffusing into the boron diffusion layer and controlling the time of the first step.