Production process of SOI substrate

Details Production process of SOI substrate Production process of SOI substrate

1998-09-21

2001-11-13

Chang, Joni (Department: 2825)

Semiconductor device manufacturing: process

Formation of semiconductive active region on any substrate

On insulating substrate or layer

C438S471000

Reexamination Certificate

active

06316337

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a production process of an SOI (Si on Insulator) substrate, which has a promising prospect as a substrate material for next-generation LSIs and carries a semiconductor layer on an insulator.
2. Description of the Prior Art
As a process for forming on an insulator an SOI structure having a semiconductor active layer therein, there is the SIMOX (Separation by Implanted Oxygen) process as disclosed, for example, in J. Mater. Res., 8(3), 523-534 (1993), in which O
+
ions are implanted at a high dose in an Si substrate, followed by heat treatment at elevated temperature so that a continuous oxide film (SiO
2
film) is formed inside the substrate.
This process, while permitting rather easy provision of an SOI substrate, is accompanied by a drawback in that crystal defects remain in an upper Si active layer which is a device-forming region. The density of residual crystal defects depends upon the dose of implanted oxygen ions, so that a high dose leads to many residual crystal defects compared with a low dose. For a reduction in crystal defects, it is therefore necessary to control the dose of implanted oxygen ions low. From the viewpoint of achieving a reduction in production cost, a low dose is also desired as the time for ion implantation can be shortened. However, when oxygen ions are implanted at a low dose of 3.5×10
17
ions/cm
2
or lower and 180 KeV, for example, the subsequent high-temperature heat treatment cannot form any continuous oxide film but results in the formation of paths for current leakage, thereby failing to obtain good device characteristics.
Namely, the SIMOX process has the demerit that crystal defects remain in an upper Si active layer which is a region where devices are formed, although it has the merit that an SOI substrate can be obtained rather easily. Further, the implantation of oxygen ions at a low dose with a view to reducing crystal defects and production cost is accompanied by the drawback that the subsequent high-temperature heat treatment cannot form any continuous oxide film but results in the formation of paths for current leakage, thereby failing to obtain good device characteristics.
An object of the present invention is to overcome the above-described problems of SIMOX and to provide a production process for obtaining at low cost an SIMOX substrate with reduced crystal defects.
SUMMARY OF THE INVENTION
According to the present invention which has attained the above-described object, there is provided a process for the production of an SOI (Si on Insulator) substrate, including a step of forming a buried silicon oxide layer by oxygen ion implantation, which comprises the following steps: implanting O
2
+
ions in a silicon substrate and subjecting the thus-implanted silicon substrate to heat treatment at a temperature of from 1,200° C. to 1,410° C., both inclusive, in an atmosphere having an oxygen content of from 0.1% to 1%, both inclusive.
A description will hereinafter be made about reasons for which the present invention has made it possible to form a continuous oxide film even at a low dose and hence to produce at low cost an SIMOX substrate with reduced crystal defects.
At first, the present inventors studied in detail the mechanism of formation of an SOI structure by the conventional SIMOX process. As a result, it has been confirmed that the setting of acceleration energy and a dose at 180 KeV and about 1.2×10
18
ions/cm
2
or higher as ion implantation conditions leads to the formation of a continuous oxide film even in a form shortly after the ion implantation and the subsequent high-temperature heat treatment results in further gathering of surrounding oxygen atoms to form a SOI structure. It has also been corroborated that at a low dose falling below 1.2×10
18
ions/cm
2
, on the other hand, oxide islets exist scattering in an initial stage of heat treatment after ion implantation, continuation of the heat treatment results in growth of some of the oxide islets and disappearance of some of the oxide islets, and combinations of the thus-grown oxide islets themselves leads to the formation of a continuous oxide film.
According to a further detailed investigation, it has been found that, when heat treatment is applied subsequent to O
+
ion implantation, the distribution of oxide islets which existed scattering in the initial stage of the heat treatment develops two peaks at different depths from the surface. Namely, it has been confirmed that the position of the shallower peak coincides with the position of a disruption peak at which Si crystals have been disrupted most severely by the ion implantation and the position of the deeper peak coincides with the position of a peak at which the concentration of ion-implanted oxygen is highest. It has also been corroborated that, when the heat treatment is continued further, the oxide islets at the disruption peak disappear and the oxide at the concentration peak grows to form a buried continuous oxide film at the position of the concentration peak but that in the course of the formation of the buried continuous oxide film, crystal defects occur connecting the oxide islets at the position of the damage peak with the oxide islets at the position of the concentration peak. It has also been confirmed that these crystal defects remain even after the disappearance of the oxide islets at the position of the disruption peak and that the residual crystal defects occurred as described above amount to a large majority of crystal defects remaining in an SIMOX substrate. Further, it has also been found that, when the oxide islets disappear at the position of the disruption peak, some oxygen diffuses out of the substrate from a surface thereof. It has hence become evident that the implanted oxygen is not effectively utilized in its entirety for the formation of the buried oxide film.
As mentioned above, drawbacks such as the occurrence of crystal defects and an increased ion implantation cost due to the non-efficient utilization of implanted oxygen are attributed to the oxide islets formed at the position of the damage peak. The process in which oxygen contained in Si crystals is caused to become oxide islets by the heat treatment is promoted as the oxygen concentration increases and also as the disruption of crystals becomes severer. In the case of the formation of oxide islets in the above-mentioned SIMOX process, for example, the formation of oxide islets is promoted due to the considerable disruption of crystals although the concentration of oxygen is low there. At the position of the concentration peak, on the other hand, the formation of oxide islets is also promoted due to a high oxygen concentration although the disruption of crystals is slight there.
Here, the present inventors have found that the distribution of disruption to crystals upon implantation of ions in Si crystals varies depending on the mass of the implanted ions. When ions having a lighter mass than Si atoms, like O
+
ions, are implanted in Si crystals, a disruption peak occurs at a position shallower than the concentration peak of implanted ions. When O
2
+
ions are implanted, on the other hand, a disruption peak substantially coincides with a concentration peak or a disruption peak occurs at a position slightly deeper than a concentration peak. This can be attributed to the fact that the mass number of an O
2
molecule is 32 which is a value close to 28, the mass number of Si. Accordingly, when O
2
+
ion implantation is conducted instead of conventional O
+
ion implantation, the introduction of crystal defects is reduced and, even if crystal defects occur, they are formed at a position deeper than an embedded oxide film so that no deleterious effect is applied to an active surface layer where devices are to be formed. Even if oxide islets are formed at a damage peak, extra oxygen do not diffuse out from the surface upon disappearance of the oxide inlets, but are captured on oxide islets occu

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