Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Heterojunction
Reexamination Certificate
2002-11-19
2004-09-07
Wilczewski, Mary (Department: 2822)
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
Heterojunction
C257S018000, C257S015000, C257S055000
Reexamination Certificate
active
06787793
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION
This application is related to Japanese Patent Application No. 2001-377603 filed on Dec. 11, 2001, whose priority is claimed under 35 USC § 119, the disclosure of which is incorporated by reference in its entirety.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor device and its production process, more particularly, a semiconductor device using a semiconductor substrate into which strain is introduced by providing a SiGe film and a process of producing the semiconductor device.
2. Description of Related Art
For the purpose of improving the mobility of carriers (electrons or holes) passing through channel regions, there is known a technique of forming a strained SiGe pseudomorphic film on a Si substrate, relaxing the strain of the film caused by lattice mismatch between the film and the substrate by introducing misfit dislocation, and forming a Si film as a cap layer. The Si film is strained by the SiGe film having a larger lattice constant, thereby changing a band structure and improving the mobility of carriers.
For relaxing the strain of the SiGe film, there is known a technique of forming a SiGe film in a thickness of several &mgr;m to increase the resilience of the SiGe film, thereby relaxing the strained SiGe film. For example, Y. J. Mii et al. published the relaxation of a strained SiGe film by forming a SiGe film of about 1 &mgr;m thickness with a increasingly graded Ge concentration profile in Appl. Phys. Lett. 59(13), 1611(1991)].
Also, for relaxing the strain of a thin SiGe film, there is known a technique of producing misfit dislocation at a SiGe film/Si substrate interface by high-temperature annealing after implantation of ions such as hydrogen ions and thereby causing the slipping of stacking fault in a defect layer in the Si substrate. For example, D. M. Follstaedt et al. published the relaxation of strain by He ion implantation in Appl. Phys. Lett. 69(14), 2059(1996), and H. Trinkaus et al. published the relaxation of strain by H ion implantation in Appl. Phys. Lett. 76(24), 3552(2000).
As a technique of relaxing the strain of a thin SiGe film without implantation of ions such as hydrogen ions, Japanese Unexamined Patent Publication HEI 10(1998)-256169 proposed a technique of forming a Ge layer of 20 nm thickness on a Si substrate, forming thereon a SiGe cap layer of 1 nm or smaller thickness and annealing at 680° C. for 10 minutes, thereby relaxing the Ge layer.
Further, Sugimoto et al. published a technique of relaxing strain in the 31
st
workshop material, page 29 of the 154
th
committee “Semiconductor Interface Control Technology” of the Japan Society for the Promotion of Science. According to this technique, a first SiGe film and a first Si cap film are formed on a Si substrate in this order at a temperature as low as 400° C., annealed at 600° C. to generate a low-density misfit dislocation at a SiGe film/Si substrate interface. Subsequently, a second SiGe film is grown at a temperature as high as 600° C. Thereby, an undulation is generated on the surface of the growing SiGe film due to influence of strain fields caused by the misfit dislocation at the SiGe film/Si substrate interface. By compressive stress on troughs of the undulation, dislocation generation sites are newly introduced. Thereby the strain is relaxed while the second SiGe film is grown. According to this technique, threading dislocation in the first SiGe film caused by the misfit dislocation at the first SiGe film/Si substrate interface is reduced by forming the first Si cap film. Further, even if the first SiGe film is formed to have a high Ge concentration (30%), the second SiGe film can be relaxed about 90%.
In the above-mentioned relaxation technique by forming the thick SiGe film to increase the resilience of the SiGe film, an extremely large number of defects are generated in the SiGe film, because the thickness of the SiGe film exceeds a critical thickness for obtaining perfect crystal.
In the technique of high-temperature annealing after implantation of ions such as hydrogen ions, since only the first SiGe film and the first Si cap film form a heterostructure, threading dislocation caused by the misfit dislocation at the SiGe film/Si substrate interface reaches the surface in a high density (about 10
7
/cm
2
), which results in an increase in a junction leakage current after a semiconductor device is formed. Further, protrusions called crosshatches are produced by the threading dislocation and remaining resilience. In addition to that, if the Ge concentration of the SiGe film becomes high, large holes are liable to emerge owing to hydrogen ions at the SiGe/Si interface, and a very large surface roughness is likely to occur on the surface of the SiGe film.
Further, if the technique of Japanese Unexamined Patent Publication HEI 10 (1998)-256169 is applied to the technique of forming the SiGe film and the Si cap film on the Si substrate to relax the SiGe film, the relaxation ratio declines greatly where the strained SiGe film is thinner than the critical thickness. For example, according to the above 31
st
workshop material, if the same construction as disclosed by Japanese Unexamined Patent Publication HEI 10 (1998)-256169 is formed under SiGe film formation conditions of a substrate temperature of 400° C., a Ge concentration of 30% and a thickness of 100 nm or smaller, which is below the critical thickness, and is annealed at 600° C. for five minutes, the strained Si
0.7
Ge
0.3
film is relaxed only about 20%. Therefore, the Si cap film on the top cannot be strained sufficiently, and the carrier mobility cannot be raised to a targeted level.
In the technique of growing the second SiGe film on the first Si cap film/first SiGe film/Si substrate structure while relaxing strain, an undulation of great amplitude (rms: about 9 nm) remains on the surface of the second SiGe film owing to the influence of strained fields by a low-density misfit dislocation and due to film growth at a high temperature.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above-discussed problems. An object of the present invention is to provide a semiconductor device and its production process which can achieve a high strain relaxation degree and reduce the threading dislocation density even in a strained SiGe film having a high Ge concentration and a thickness not greater than the critical thickness, and, regarding a second SiGe film formed thereon, can suppress undulation therein, obtain as complete relaxation as possible and improve its smoothness.
The present invention provides a semiconductor device a semiconductor device comprising a first Si
1−&agr;
Ge
&agr;
film, a first cap film, a second Si
1−&bgr;
Ge
&bgr;
film (&bgr;<&agr;≦1) and a second cap film formed in this order on a substrate whose surface is, formed of silicon, wherein the first Si
1−&agr;
Ge
&agr;
film is relaxed to have substantially the same lattice constant as that of the second Si
1−&bgr;
Ge
&bgr;
film in a horizontal plane.
The present invention also provides a process of producing a semiconductor device which comprises a first Si
1−&agr;
Ge
&agr;
film, a first cap film, a second Si
1−&bgr;
Ge
&bgr;
film (&bgr;<&agr;≦1) and a second cap film formed in this order on a substrate whose surface is formed of silicon, the first Si
1−&agr;
Ge
&agr;
film being relaxed to have substantially the same lattice constant as that of the second Si
1−&bgr;
Ge
&bgr;
film in a horizontal plane,
the process comprising the steps of:
(a) forming a first Si
1−&agr;
Ge
&agr;
film on a substrate whose surface is formed of silicon;
(b) forming a first cap film on the first Si
1−&agr;
Ge
&agr;
film;
(c) annealing the resulting substrate to relax the first Si
1−&agr;
Ge
&agr;
film so that the lattice constant of the first Si
1−&agr;
Ge
&agr;
film becomes substantially the same as the lattice constant of a second Si
1−&bgr;
Ge
&bgr;
film w
Lewis Monica
Nixon & Vanderhye P.C.
Wilczewski Mary
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