Semiconductor device manufacturing: process – Chemical etching – Liquid phase etching
Reexamination Certificate
2001-09-14
2003-02-11
Dang, Trung (Department: 2823)
Semiconductor device manufacturing: process
Chemical etching
Liquid phase etching
C438S750000, C438S752000, C438S312000
Reexamination Certificate
active
06518197
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-104859, filed Apr. 3, 2001, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device such as a diode or a transistor which utilizes SiGe.
2. Description of the Related Art
An SiGe mixed crystal film is used to form an electronic device in which the SiGe mixed crystal film is joined to Si, for example, a device which has a structure in which n-type Si, p-type SiGe, and n-type Si are sequentially joined to each other, that is, a heterojunction bipolar transistor. The transistor exhibits excellent high-frequency characteristics superior to a transistor with a structure using Si alone. Accordingly, the SiGe mixed crystal film is lately becoming widespread in integrated circuits for high frequency. The present inventors disclosed the following fact in the specification and the drawings of Jpn. Pat. Appln. No. 2000-044306 (hereinafter referred to as the prior patent application). That is, in a diode in which p-type SiGe is joined to n-type Si, recovery time required when the application of bias is changed from the normal direction to the reverse direction is shorter than that of a conventional Si diode, so that a high-speed operation can be realized.
As for the heterojunction transistor or diode utilizing the SiGe mixed crystal film having such characteristics, from the viewpoints of improving the yield and increasing the range of uses, manufacturers strongly demand that breakdown voltage characteristics rise. The diode structure near to a portion where a breakdown voltage is applied, that is, the structure in which p-type SiGe is joined to n-type Si, or the structure in which n-type SiGe is joined to p-type Si will be referred to as an SiGe/Si diode. In the SiGe/Si diode, a leakage current becomes a problem when reverse bias is applied on the pn junction boundary (interface)
A leakage current generated in a conventional SiGe/Si diode will now be described with reference to FIG.
1
.
FIG. 1
schematically shows a leakage current
17
generated when reverse bias is applied to an SiGe/Si diode
10
. When reverse bias is applied to the diode
10
, as shown in the diagram, a depletion layer
13
is formed in an area including a pn junction boundary
18
and an electric field concentrates on the depletion layer
13
. Portions
16
where the diode is exposed exist on the pn junction boundary
18
. A depletion layer
13
a
in each exposed portion
16
tends to be narrower than the depletion layer
13
excluding the layer
13
a.
Consequently, the degree of electric field concentration rises in the exposed portion
16
and the leakage current
17
generated in the diode remarkably depends on the substance characteristics of the exposed portion
16
.
Specifically, the substance characteristics of the exposed portion
16
controlling the leakage current
17
include a crystal defect, air discharge, and impurities in the exposed portion
16
. Among them, the crystal defect depends on a process of manufacturing semiconductor layers. Since attention is sufficiently paid to the quality management of the semiconductor manufacturing process so as to inhibit a crystal defect as much as possible, the crystal defect does not generally grow so severely that it cannot be ignored by itself.
However, effective countermeasure against impurities is not found so far, though the above two disadvantages have been effectively inhibited. As elements serving as impurities causing the leakage current
17
, metals such as Na, K, Fe, and Au and impurities such as hydrocarbon and the like which exist in the atmosphere or which are adhered upon cleaning with water are mentioned. When the exposed portion
16
is oxidized to form Ge oxide on the surface, the oxide also causes the leakage current
17
. Accordingly, to reduce the leakage current
17
, a process of suppressing crystal defect, suppressing air discharge, and sufficiently reducing harmful impurities causing the leakage current
17
is required in the exposed portion
16
.
In the conventional diode constructed by Si as a whole, generally, the exposed portion
16
on the pn junction boundary is subjected to a thermal oxidization treatment. That is, the surface of the diode is oxidized in an atmosphere of oxygen or water vapor at a high temperature of 900° C. or more. When the thermal oxidization treatment is performed, Si of the exposed portion
16
is oxidized to be insulated, so that the leakage current
17
is reduced. Although the thermal oxidization method is not effective against metal impurities, it is effective in an Si-based device. The thermal oxidization method is often used in the Si-based device so far.
When the conventional thermal oxidization method is applied to the SiGe/Si diode as it is, Ge is segregated on the interface between SiGe and an oxidized layer formed on the surface of SiGe. The segregation causes the leakage current
17
. Ge oxide (GeO, GeO
2
) has high conductive properties. The Ge oxide itself also becomes a cause of the leakage current
17
. Accordingly, to perform the thermal oxidization method to SiGe, searches and researches with much labor regarding oxidizing conditions that Ge is not segregated are needed. Effective means is not found so far and the problem is left outstanding.
The problems of the foregoing conventional technique are concerned with the heterojunction boundary of the SiGe/Si diode. The junction boundary of an SiGe/SiGe diode also has the similar problems.
BRIEF SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method for manufacturing a semiconductor device in which a leakage current is not generated on an SiGe/Si heterojunction boundary or an SiGe/SiGe junction boundary, particularly, an exposed portion thereof.
According to a first aspect of the present invention, there is provided a method for manufacturing a semiconductor device having a junction boundary where SiGe of a first conductivity type and Si or SiGe of a second conductivity type come in contact with each other, the method comprising the steps of: cleaning a portion, where the junction boundary is exposed on the surface, with a first solution containing hydrofluoric acid; and cleaning the portion with a second solution containing sulfuric acid.
When an exposed portion on the surface of the junction boundary of an SiGe/Si diode is left in the atmosphere, it is oxidized spontaneously. In this instance, as impurities, adsorption of impurities (such as hydrocarbon and the like) in the atmosphere and mixing of metal (Na, K) ions given on contact with the operator's bare hands are expected, and furthermore, an impurity such as Ge oxide (GeO
2
), which is formed by oxidizing Ge, is expected. The impurities cause a leakage current, resulting in deterioration of breakdown voltage characteristics of the semiconductor device.
The first aspect of the present invention is concerned with the method for effectively eliminating impurities on the surface layer. According to the first aspect of the invention, first immersing the SiGe/Si diode in the first solution containing hydrofluoric acid eliminates oxide formed on the exposed portion. When the oxidized layer has a thickness of several microns, it can be easily eliminated so long as time to immerse in the hydrofluoric acid solution is changed. Due to the cleaning treatment, the junction boundary is terminated with hydrogen on the surface of the exposed portion. In the process, however, hydrocarbon and metal impurities are not eliminated.
Subsequently, when the SiGe/Si diode is immersed in the second solution (solution containing sulfuric acid), the metal impurities and hydrocarbon dissolve in the solution to be removed from the surface layer. At that time, the surface layer is oxidized at a thickness of about 1 nm (10Å). In this instance, SiO
2
alone is formed and Ge
Dang Trung
Mitsubishi Heavy Industries Ltd.
Wenderoth , Lind & Ponack, L.L.P.
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