Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
1999-11-24
2004-08-10
Flynn, Nathan J. (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
Reexamination Certificate
active
06774435
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to gettering technique applied to a semiconductor wafer having an SOI (Silicon On Insulator) structure (hereinafter referred to as SOI wafer). Gettering is a process of removing from a device region heavy metal impurities such as Fe, Cu, Cr, Ni and Pt that have an adverse effect on the characteristics of a semiconductor device.
2. Description of the Background Art
Referring to
FIG. 58
, a bulk silicon wafer is usually formed by depositing on the back of a wafer formed of a silicon layer
1
a polycrystalline silicon layer
9
which serves as a gettering layer.
SOI wafer has been attracting attention as a silicon substrate of the next generation since SOI is effective in preventing the short channel effect and CMOS (Complementary Metal-Oxide Semiconductor) latchup that are found problematic as the semiconductor device is further decreased in size, and it achieves a high speed operation even in a low voltage region. Referring to
FIG. 59
, SOI wafer has a structure in which an oxide film
2
is placed between an upper silicon layer
1
where a semiconductor device is formed and substrate formed of a lower silicon layer
3
or the like. Oxide film
2
is referred to as a buried oxide film in some cases.
The gettering technique has an increasing importance for ensuring reliability of the semiconductor device, and thus improvement of the gettering ability is demanded.
However, the structure of the SOI wafer is not suitable for applying gettering thereto since oxide film
2
located between the upper silicon layer
1
and the gettering layer on the back prevents heavy metal from diffusing.
There is thus a problem of difficulty in executing gettering in the SOI wafer regardless of the demand for improvement of the gettering ability so as to enhance the reliability of the semiconductor device.
SUMMARY OF THE INVENTION
One object of the present invention is to provide an SOI wafer having improved gettering ability and a method of manufacturing thereof.
In order to achieve the object above, according to one aspect of the present invention, a semiconductor wafer has a gettering layer which includes a porous silicon layer.
The structure above is employed to enable a dangling bond portion of a vacancy within the porous silicon layer to function as a gettering site.
The semiconductor wafer according to the present invention preferably includes an insulating film.
The structure above is employed to implement an SOI structure having a silicon layer on the insulating film. As a result, the gettering ability can be enhanced in the wafer having the SOI structure (SOI wafer).
The semiconductor wafer according to the invention preferably has the gettering layer located above the insulating film.
By employing such a structure, gettering from the silicon layer is not interrupted by an oxide film serving as the insulating film since the gettering layer is located above the oxide film. Therefore, although it was difficult to execute gettering in the SOI wafer, the gettering ability can be enhanced in that way.
According to the invention above, preferably the porous silicon layer is in contact with the upper side of the insulating film.
By employing such a structure, the manufacturing process can be simplified since a step of forming a silicon layer between the oxide film and the porous silicon layer can be omitted.
The porous silicon layer serving as a gettering layer can be arranged away from the upper surface where the semiconductor device is located, and thus the influence of heavy metal on the semiconductor device can be reduced.
The semiconductor wafer according to the invention preferably includes a silicon layer which is in contact with the lower side of the gettering layer and in contact with the upper side of the insulating film.
By employing such a structure, the silicon layer and the oxide film can be bonded instead of the porous silicon layer and the oxide film and accordingly, bonding strength can be enhanced.
According to a preferred aspect of the invention, the semiconductor wafer has a two-layer structure constituted of a first gettering layer formed of a porous silicon layer and a second gettering layer formed of a polycrystalline silicon layer having a high concentration impurity region or a low concentration impurity region or formed of a layer containing crystal defect, and the two-layer structure is in contact with the upper side of the oxide film.
By employing such a structure as described above, gettering is carried out by a gettering layer having the two-layer structure and thus an SOI wafer having its gettering ability improved is achieved.
According to another aspect of the invention, the semiconductor wafer has the gettering layer located below the insulating film.
By employing such a structure as described above, the gettering ability can be applied to heavy metal which is diffused via the oxide film as an insulating film. Especially, if the oxide film has a relatively small thickness, the barrier function of the oxide film relative to the heavy metal is not fully exercised. In this case, the structure having the gettering layer under the oxide film is particularly advantageous. Accordingly, the gettering ability can be applied to heavy metal diffused in an SOI wafer having a thin oxide film.
According to still another aspect of the invention, the gettering layer in the semiconductor wafer has a two-layer structure constituting of a first gettering layer formed of a porous silicon layer and a second gettering layer formed of a polycrystalline silicon layer having a high concentration impurity region or a low concentration impurity region or formed of a crystal defect containing layer, and the gettering layer is in contact with the lower side of the oxide film.
By employing such a structure as described above, the gettering ability can be applied to heavy metal which is diffused in the SOI wafer having a thin oxide film. Further, the gettering ability can be enhanced since the gettering layer has the two-layer structure.
According to another preferred aspect of the invention, the gettering layer is formed of a low concentration impurity layer, and a polycrystalline silicon layer or a single crystal silicon layer which is in contact with the lower side of the low concentration impurity layer and has a thickness of about 0.01 &mgr;m to 3 &mgr;m. The polycrystalline silicon layer has a high concentration impurity region containing impurities of high concentration that define n-type or p-type such as phosphorus, arsenic, antimony, boron and indium or a low concentration impurity region containing impurities of low concentration such as those listed above. The single crystal silicon layer contains those impurities of high concentration. The gettering layer is in contact with the lower side of the insulating film.
By employing such a structure as described above, the polycrystalline silicon layer having the high concentration impurity region containing impurities of high concentration which define n- or p-type or the low concentration impurity region containing those impurities of low concentration, or the single crystal silicon layer containing those impurities of high concentration has a sufficiently small thickness of about 0.01 &mgr;m to 3 &mgr;m. The interface between layers functions as a gettering site. Therefore, particularly in this structure, the interface between those layers functions effectively as a gettering site since each layer is thin. In this way, this structure can improve the gettering ability.
The low concentration impurity region is placed between the oxide film and the polycrystalline silicon layer having the high concentration impurity region or the low concentration impurity region or the single crystal silicon layer containing those impurities of high concentration. Accordingly, the influence of impurities diffused in high-temperature process on the silicon layer constituting the semiconductor device can be prevented.
According to a further aspect of the inven
Iwamatsu Toshiaki
Matsumoto Takuji
Flynn Nathan J.
McDermott Will & Emery LLP
Quinto Kevin
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