JPS6152565B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for diffusing impurities into a semiconductor substrate, and particularly to a method for diffusing low concentration impurities.

Impurity diffusion methods for forming impurity regions in semiconductor substrates include vapor phase diffusion, two-step diffusion,
Examples include diffusion from doped oxide and diffusion by ion implantation. In particular, it is necessary to diffuse low-concentration impurities into a semiconductor substrate to form, for example, a so-called P-well region for a C-MOS (complementary metal oxide semiconductor) integrated circuit device. In order to form regions with good controllability, two-step diffusion methods and ion implantation methods are often used.

In the former method, the impurity boron is pre-deposited onto the substrate surface at low temperature for low concentration diffusion, and then a drive-in process is performed to introduce boron to the desired depth. The pre-deposition is poorly controllable and causes variations, making it impossible to obtain a good impurity region. In the latter ion implantation method, a low concentration impurity region can be obtained by implanting ionized boron into the substrate and then heating it to diffuse it to the desired depth. However, lattice defects occur on the semiconductor substrate surface and the ion implantation equipment is expensive. This also causes high costs.

An object of the present invention is to provide a method for diffusing impurities into a semiconductor substrate by which relatively low concentration impurities can be diffused with good controllability and at low cost.

In the diffusion method of the present invention, a layer of an impurity to be diffused into the substrate is deposited (pre-deposition) on one main surface of a semiconductor substrate, and this impurity is introduced into the substrate in an atmosphere containing water vapor. It is characterized by Silicon is preferably used as the substrate, and boron is preferably used as the impurity.

The present invention will be described below with reference to Examples.

First, a silicon semiconductor substrate (wafer) is prepared, and boron as a diffusion impurity is deposited (pre-deposition) on one main surface of the substrate under certain conditions. In this case, the pre-deposition can be carried out at a relatively easily controllable temperature, that is, about 800 DEG C. to 900 DEG C., rather than at a low temperature which is difficult to control as in the conventional method.

Next, a drive-in process is performed to introduce the predetermined boron into the silicon substrate, and at this time, water vapor is previously introduced into the reaction tube so that it is almost saturated. The pre-positioned silicon substrate is slowly inserted into the reaction tube having an atmosphere containing water vapor. At this time, a relatively fast oxidation phenomenon occurs in a relatively low temperature area near the inlet of the reaction tube. This is because the oxidation rate is faster in an atmosphere containing water vapor than in the conventional atmosphere of nitrogen, dry oxygen, or a mixture thereof.

The oxide film (SiO ₂ ) thus formed in the low-temperature area has the property of easily containing impurities even at low temperatures, so the boron pre-deposited in the low-temperature area is incorporated into the oxide film and is The boron that was shallowly diffused into the silicon substrate in the pre-deposition step is also introduced into the oxide film at the same time.

The silicon substrate is heated to a predetermined set temperature (approx.
After reaching the part (1000℃~1200℃), about 50~
By heat treatment for 2000 minutes, boron is driven in using the oxide film as an impurity diffusion source, and is diffused to a predetermined depth. At this time, since the amount of boron in the oxide film that is driven in and redistributed to the silicon substrate is small, low concentration diffusion of approximately 10 ¹⁵ to 10 ¹⁶ /cm ³ is possible. In addition, as mentioned above, since the temperature treatment was performed at a relatively well-controlled temperature instead of low temperature treatment during the pre-day position, the boron that had diffused into the substrate during the pre-day position was attached to the surface. Since boron is quickly incorporated into the oxide film together with boron, the impurity distribution of boron within the substrate becomes uniform.

The P-type region made of boron at a low concentration thus obtained can be used as a P-well for an N-channel MOS transistor of a C-MOS device. Note that the present invention is applicable not only to the formation of a P-well of a C-MOS device but also to the formation of various low concentration impurity regions.

According to the present invention, since an atmosphere containing water vapor is used during the drive-in process, rapid oxidation occurs in the low temperature part of the atmosphere, so that impurities diffused into the substrate during the pre-deposition and even more The deposited impurities will also be incorporated into the oxide film, so even if there is non-uniform deposition of impurities during pre-deposition or if impurities are diffused into the substrate, it will be possible to maintain low concentration and This results in uniform impurity diffusion.

In the above example, boron was used as an impurity, but this is because boron has a relatively small diffusion coefficient in silicon.

Claims (1)

[Claims] 1 A layer of boron to be diffused into the substrate was deposited on one main surface of a silicon substrate, and the silicon substrate was heated at 800 to 900°C in an atmosphere containing water vapor to incorporate the boron layer. 1. A method for diffusing impurities into a semiconductor, comprising forming an oxide film, heating the substrate at 1000 to 1200° C., and introducing the boron into the substrate using the oxide film as an impurity diffusion source.