JPH043663B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a method for uniformly adding phosphorus (P) to a polycrystalline silicon layer.
It relates to a method of adding impurities such as. Generally, polycrystalline silicon is formed by normal pressure or reduced pressure vapor phase growth using monosilane (SiH ₄ ) gas as a source gas. Conventionally, when doping this polycrystalline silicon layer with impurities, no consideration was given to the crystal grain size of the polycrystalline silicon layer.
This time, it has been found that when P is diffused and added to a polycrystalline silicon layer using, for example, POCl ₃ as a source, the doping concentration changes depending on the crystal grain size. The present invention takes this point into consideration, and if a polycrystalline silicon layer is formed under the condition that the crystal grain size is 100 nm or less, even if the POCl ₃ diffusion temperature is relatively low, 1000°C or less, within the wafer surface. Moreover, it provides a method that can maintain a uniform doping concentration between wafers.

Example: A polycrystalline silicon layer is grown by low pressure vapor phase epitaxy.
3500Å on 1000Å thick silicon dioxide (SiO ₂ )
Form. The source gas is _SiH4 based on nitrogen ( _N2 ) gas, the pressure is 1.0Torr, and the formation temperature is 500℃.
The temperature range was ~850°C. P was diffused into the polycrystalline silicon layer thus formed using POCl ₃ as a source. The diffusion temperature and time were 1000℃ and 25 minutes.
Two tests were conducted at 875°C for 20 minutes.

As shown in Figure 1, in the case of POCl ₃ diffusion at 1000℃, the value of the surface layer resistance ρ _s after diffusion is approximately 20Ω/
□, and the nonuniformity of ρ _s within the wafer surface and between wafers can be maintained within ±2%.

However, when POCl ₃ diffusion is performed at a low temperature of 875° C., as shown in FIG. 1, the non-uniformity of ρ _s is large, and the degree of non-uniformity greatly depends on the formation temperature of polycrystalline silicon. It can be seen that in order to suppress the non-uniformity within ±2%, the formation temperature must be selected to be 600° C. or lower or 700° C. or higher.

In order to investigate the cause of this, the following experiment was conducted. First, the crystal grain size of the polycrystalline silicon layer after low-pressure vapor phase growth and the crystal grain size of the sample in which POCl ₃ was diffused into polycrystalline silicon were observed using a transmission electron microscope. This crystal is shown in FIGS. 2 and 3. Immediately after polycrystal formation, as shown in FIG. 2, the crystal grain size depends on the formation temperature. On the other hand, the crystal grain size of the sample subjected to POCl ₃ diffusion is observed to increase at 1000°C due to the addition of a high concentration of impurity P, but at While it maintains almost 5000Å without depending on
In the case of POCl ₃ diffusion at 875°C, it is highly dependent on the polycrystal formation temperature. In 875°C diffusion, if the original crystal grain size is large, the growth of the grain size is inhibited, and the ρ _s value after diffusion becomes as high as 70Ω/□.

If we consider FIG. 1 and FIGS. 2 and 3 together, it will be understood that the non-uniformity of ρ _s is largely dependent on the crystal grain size immediately after polycrystal formation.

With the miniaturization of semiconductor devices in recent years, the temperature of POCl ₃ diffusion tends to be lowered, and in this case, it is important to pay particular attention to the crystal grain size.

Therefore, as is clear from FIGS. 1 to 3, the temperature for forming polycrystalline silicon is approximately 500 to 600.
℃ or 700 to 850℃, the crystal grain size after polycrystalline silicon formation is small, and an increase in the grain size after POCl ₃ diffusion is also observed _. is extremely small.

Therefore, approximately 500-600℃ or 700-850℃
Forming polycrystalline silicon at a temperature of .degree. C. and lowering the impurity diffusion temperature is extremely effective in forming semiconductor devices with high integration density.

For convenience, the above explanation is based on the case where phosphorus is diffused, but not only phosphorus but also B and A _s
The same results can be obtained when diffusing various impurities such as, and the method can also be applied to diffusing these impurities.

[Brief explanation of drawings]

1 to 3 are curve diagrams for explaining the principle of the present invention.

Claims (1)

[Claims] 1. A step of forming a polycrystalline silicon film by vapor phase reaction on a wafer kept at 500 to 550°C;
An impurity diffusion method comprising the step of diffusing an impurity into the polycrystalline silicon film.