Single-crystal – oriented-crystal – and epitaxy growth processes; – Processes of growth with a subsequent step acting on the...
Reexamination Certificate
1998-11-12
2001-12-04
Kunemund, Robert (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Processes of growth with a subsequent step acting on the...
C117S020000, C117S054000, C117S084000, C117S088000, C117S935000, C438S471000
Reexamination Certificate
active
06325848
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor substrate and a fabrication method thereof and more particularly, to a single-crystal silicon (Si) substrate having a low oxygen-concentration layer and a fabrication method of the substrate.
2. Description of the Prior Art
In recent years, integration and miniaturization of the semiconductor integrated circuit devices (ICs) using a single-crystal Si substrate have been progressing more and more. Under such the circumstances, the need to accurately control the concentration and profile of impurity-doped regions such as well regions and source/drain regions has been becoming stronger and stronger.
On the other hand, the impurity-doped regions have been usually formed by an ion-implantation process.
To accurately control the concentration and profile of the impurity-doped regions, the dopant atoms introduced into the Si substrate by the ion-implantation process are required not to cause unwanted diffusion in subsequent fabrication processes for the ICs.
It is popular that the dopant atoms introduced by the ion-implantation process are electrically inactive. Therefore, to make the ion-implanted substrate suitable for the ICs, the dopant atoms usually need to be electrically activated by a heat-treatment or rapid annealing process.
To make it sure that the introduced dopant atoms are electrically activated, the heat-treatment or annealing process has been typically performed at a temperature as high as 800° C. or higher for a short time (e.g., several seconds) or at a temperature as high as approximately 700° C. for a long time (e.g., several minutes).
However, if the annealing process is performed at 800° C. or higher, there is a problem that the introduced dopant atoms tend to diffuse in the substrate, resulting in unwanted change of the concentration and profile of the impurity-doped region. On the other hand, if the annealing process is performed at 700° C. or lower, there is a problem that the annealing process necessitates a long processing time and as a result, this annealing process is not practical for actual IC fabrication.
Moreover, there is another problem that the substrate tends to be contaminated during the annealing process due to the long processing time.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide a single-crystal Si substrate that makes it possible to electrically activate an introduced impurity by an annealing process at a temperature lower than 700° C. for a short time, and a fabrication method of the substrate.
Another object of the present invention is to provide a single-crystal Si substrate that makes it possible to accurately control the concentration and profile of an introduced impurity by an ion-implantation process, and a fabrication method of the substrate.
Still another object of the present invention is to provide a single-crystal Si substrate that is difficult to be contaminated during an annealing process for an introduced impurity by an ion-implantation process.
The above objects together with others not specifically mentioned will become clear to those skilled in the art from the following description.
According to a first aspect of the present invention, a silicon substrate is provided, which is comprised of a single-crystal Si base layer and a single-crystal Si low oxygen-concentration layer formed on the base layer. The base layer has a first oxygen concentration and the low oxygen-concentration layer has a second oxygen concentration lower than the first oxygen concentration.
According to a second aspect of the present invention, a fabrication method of the silicon substrate according to the first aspect is provided, which is comprised of the following steps (a) and (b).
(a) A single-crystal Si base material with a flat main surface is prepared. The base material has a first oxygen concentration.
(b) A single-crystal Si epitaxial layer is grown on the main surface of the base material in such a way that the epitaxial layer has a second oxygen concentration lower than the first oxygen concentration.
According to a third aspect of the present invention, another fabrication method of the silicon substrate according to the first aspect is provided, which is comprised of the following steps (a′) and (b′).
(a′) A single-crystal Si base material with a flat main surface is prepared. The base material has a first oxygen concentration. This is the same as the above step (a).
(b′) The base material is heat-treated to cause outward diffusion of oxygen existing in the base material through the main surface of the base material, thereby forming a low oxygen-concentration layer extending along the main surface of the base material in the base material. The remaining base material serves as a single-crystal Si base layer having the first oxygen concentration. The low oxygen-concentration layer has a second oxygen concentration lower than the first oxygen concentration.
DETAILED DESCRIPTION OF THE INVENTION
As described above, the silicon substrate according to the first aspect of the present invention is comprised of a single-crystal Si base layer and a single-crystal Si low oxygen-concentration layer formed on the base layer. The base layer has a first oxygen concentration and the low oxygen-concentration layer has a second oxygen concentration lower than the first oxygen concentration.
With the silicon substrate according to the first aspect of the present invention, the low oxygen-concentration layer whose oxygen concentration is lower than that of the base layer is formed on the base layer. Therefore, when an impurity is introduced into the low oxygen-concentration layer by an ion-implantation process in order to selectively change the electrical conductivity of the low oxygen-concentration layer, and then, the substrate is subjected to an annealing or heat-treatment process to electrically activate the introduced impurity, crystal defects existing in the low oxygen-concentration layer (which are induced by the ion-implantation process) are readily repaired.
As a result, the annealing or heat-treatment process is able to be performed at a temperature lower than 700° C. for a short time. This temperature lowering of the annealing or heat-treatment process leads to decrease of a diffusion length of the introduced impurity in the low oxygen-concentration layer. Accordingly, the concentration and profile of the introduced impurity into the low oxygen-concentration layer is able to be controlled accurately.
The mechanism of the impurity diffusion during the annealing process is not yet understood. However, it is assumed that the diffusion rate of the introduced impurity is lowered with the decreasing oxygen concentration, because the diffusion rate of crystal defects induced by the ion-implantation process is increased and the crystal defects tend to be restored or eliminated even at a low annealing temperature.
In a preferred embodiment of the silicon substrate according to the first aspect of the present invention, the second oxygen concentration of the low oxygen-concentration layer is equal to 1×10
17
atoms/cm
3
or lower. This is due to the following reason:
In a typical single-crystal Si ingot fabricated according to the well-known Czochralski (CZ) method, the first oxygen concentration of the single-crystal Si base layer is usually equal to approximately 0.8×10
18
atoms/cm
3
to 2.0×10
18
atoms/cm
3
. Therefore, if the second oxygen concentration of the lower oxygen-concentration layer is equal to 1×10
17
atoms/cm
3
or lower (which is low enough with respect to the first oxygen concentration), the advantage of the present invention is ensured.
In another preferred embodiment of the substrate according to the first aspect of the present invention, the low oxygen-concentration layer has a thickness ranging from 5 &mgr;m to 20 &mgr;m. If the low oxygen-concentration layer has a thickness less than 5 &mgr;m, oxygen (O) atoms existing in the base layer tend
Kunemund Robert
NEC Corporation
Young & Thompson
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