Method of producing bonded wafer

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart showing production steps of a bonded wafer through a smart cut method;

FIG. 2 is a schematic view illustrating an example of forming a nitride layer through an ion implantation;

FIG. 3 is a schematic view illustrating an example of forming a nitride layer through a nitriding heat treatment; and

FIG. 4 is a schematic view illustrating another example of forming a nitride layer through a nitriding heat treatment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention will be described concretely below. In FIG. 1 is shown production steps of a bonded wafer taking a smart cut method as a typical production example of a bonded wafer.

After a wafer 1 for active layer and a wafer 2 for support layer are previously provided, at least one face of the wafer 1 for active layer is first subjected to a thermal oxidation to form an insulating film 3 (silicon oxide film) on the surface thereof (FIG. 1(a)), ions of a light element such as hydrogen, helium or the like are implanted to a predetermined depth position of the wafer 1 for active layer to form an ion implanted layer 4 (FIG. 1(b)), and thereafter the wafer 1 for active layer is bonded to the wafer 2 for support layer (FIG. 1(c)), which is then separated into SOI substrate 5 and a remnant 6 by exfoliating at the ion implanted layer 4 through a heat treatment (usually about 400-600° C.) (FIG. 1(d)). The separated remnant 6 is recycled as a wafer for active layer, while the SOI substrate 5 is applied, for example, to a polishing step, at where an active layer 7 is thinned to obtain a final product.

In the invention, a nitride layer is formed in the active layer (SOI layer) in order to increase the stiffness (hardness) of the wafer for active layer. In this case, if the oxide film is existent on the surface of the wafer for active layer, the nitride layer is formed just beneath the oxide film in the wafer for active layer, or if the oxide film is not existent in the wafer for active layer, the nitride layer is formed in the extreme vicinity of the surface of the wafer for active layer.

For example, in case of using a method of introducing nitrogen ions through ion implantation, when the oxide film is existent in the wafer for active layer, as shown in FIG. 2, nitrogen ions are introduced so that a peak of nitrogen ion concentration implanted comes to any position below the oxide film 3 in the wafer 1 for active layer and shallower than the ion implanted layer 4 of hydrogen ions or the like for exfoliation, whereby a nitride layer 8 is formed. On the other hand, when the oxide film is not existent in the wafer for active layer, the nitride layer is formed by introducing nitrogen ions so that a peak of nitrogen ion concentration implanted comes to any position beneath the surface of the wafer for active layer and shallower than the ion implanted layer 4 of hydrogen ions or the like for exfoliation.

When the nitride layer is formed by the above ion implantation process, it is preferable that the thickness of the nitride layer is about 5-200 nm. When the thickness of the nitride layer is less than 5 nm, the satisfactory increase of the stiffness is not attained, while when it exceeds 200 nm, a non-nitrided active region does not remain after the subsequent thinning treatment. Also, the forming position of the nitride layer may be any region as far as it is shallower than the ion implanted layer for exfoliation irrespectively of the presence or absence of the oxide film, but us preferable just beneath the oxide film.

In order to form such a nitride layer, the implantation conditions of nitrogen ions may be adjusted to the following ranges.

Acceleration energy: 1-80 keV
Implantation dose: 1.0×10¹⁵-1.0×10¹⁷/cm²

More preferably, the acceleration energy is 3 keV and the implantation dose is 2.0×10¹⁵/cm², whereby the nitride layer having a thickness of about 10 nm can be formed in the vicinity of the surface of the wafer for active layer.

In case of using a method of forming a nitride layer by a nitriding heat treatment, when the oxide film is required in the wafer for active layer, a nitride layer 9 is first formed on the surface of the wafer 1 for active layer as shown in FIG. 3(a), and then an oxide film 10 is formed on a surface layer portion of the nitride layer through an oxidation heat treatment, a chemical vapor deposition process or the like as shown in FIG. 3(b). Thus, the nitride layer 9 is formed just beneath the oxide film 10.

Alternatively, as shown in FIG. 4(a), the oxide film 10 is formed on the surface of the wafer 2 for support layer, while the nitride layer 9 is formed on the surface of the wafer 1 for active layer, and then these wafers may be bonded to each other.

On the other hand, when the oxide film is not required in the wafer for active layer, it is enough to form a nitride layer having a desired thickness by subjecting the surface of the wafer for active layer to the nitriding heat treatment.

Even in this case, the thickness of the nitride layer is preferable to be about 5-200 nm.

In order to form such a nitride layer, the conditions of the nitriding heat treatment are preferable as follows.

Treating atmosphere: dichlorosilane+ammonia atmosphere
Treating temperature: 750-800° C.
Treating time: 10-20 minutes

In the invention, if it is intended to form the oxide film, at least one of the wafer for active layer and the wafer for support layer may be subjected to the oxidation heat treatment. In this case, the thickness of the oxide film is preferable to be not more than 150 nm in total.

In the invention, the bonding method is not particularly limited, but any of the conventionally known atmospheric bonding method, reduced-pressure bonding method and the like may be adapted advantageously, but a plasma bonding method is particularly preferable.

EXAMPLE 1

After two wafers having a diameter of 300 mm are provided, an oxide film having a thickness of 50 nm is formed on a surface of a wafer for active layer by an oxidation heat treatment. Then, nitrogen ions are implanted from the surface of the wafer for active layer provided with the oxide film under conditions of acceleration energy: 23 keV and implantation dose: 2.0×10¹⁵/cm²so that a peak of nitrogen ion concentration comes to a depth position of 60 nm from the surface of the wafer, whereby a nitride layer having a thickness of 10 nm is formed.

Then, hydrogen ions are implanted under conditions of acceleration energy: 40 keV and implantation dose: 5.0×10¹⁶/cm²so that a peak of hydrogen ion concentration comes to a depth position of 400 nm from the surface of the wafer provided with the oxide film, whereby an exfoliation layer is formed.

Thereafter, the wafer for active layer is bonded to another silicon wafer as a wafer for support layer by the usual bonding method (atmospheric bonding method), which is subjected to a heat treatment of 500° C. to exfoliate at the exfoliation layer (ion implanted layer). After the exfoliation, the state of generating voids and blisters on the resulting SOI substrate is examined by an appearance inspection.

EXAMPLE 2

After two wafers having a diameter of 300 mm are provided, an oxide film having a thickness of 50 nm is formed on a surface of a wafer for active layer by an oxidation heat treatment. Then, nitrogen ions are implanted from the surface of the wafer for active layer provided with the oxide film under conditions of acceleration energy: 80 keV and implantation dose: 2.0×10¹⁵/cm²so that a peak of nitrogen ion concentration comes to a depth position of 200 nm from the surface of the wafer, whereby a nitride layer having a thickness of 10 nm is formed.

EXAMPLE 3

After two wafers having a diameter of 300 mm are provided, a nitride layer having a thickness of 10 nm is formed on a surface of a wafer for active layer by a nitriding heat treatment. Then, an oxide film having a thickness of 50 nm is formed on the surface of the wafer for active layer provided with the nitride layer through a chemical vapor deposition process. Thus, the wafer for active layer is at a state of forming the nitride layer of 10 nm in thickness just beneath the oxide film.

Then, hydrogen ions are implanted under conditions of acceleration energy: 40 keV and implantation dose: 5.0 ×10¹⁶/cm²so that a peak of hydrogen ion concentration comes to a depth position of 400 nm from the surface of the wafer for active layer, whereby an exfoliation layer is formed.

EXAMPLE 4

After two wafers having a diameter of 300 mm are provided, nitrogen ions are implanted from a surface of a wafer for active layer under conditions of acceleration energy: 3 keV and implantation dose: 2.0×10¹⁵/cm²so that a peak of nitrogen ion concentration comes to a depth position of 10 nm from the surface of the wafer, whereby a nitride layer having a thickness of 10 nm is formed.