Semiconductor device manufacturing: process – Introduction of conductivity modifying dopant into... – Diffusing a dopant
Reexamination Certificate
2001-06-13
2004-11-23
Niebling, John F. (Department: 2812)
Semiconductor device manufacturing: process
Introduction of conductivity modifying dopant into...
Diffusing a dopant
C438S479000, C438S569000
Reexamination Certificate
active
06821871
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device, a substrate treatment method, and a semiconductor manufacturing apparatus, and more particularly to an improved method for phosphorus diffusion, in which phosphorus is diffused into a silicon film, such as a polysilicon film or an amorphous silicon film, which is an under-layer formed on a substrate such as glass or a silicon wafer.
2. Description of the Related Art
The “diffusion step” in a semiconductor device manufacturing process is, for example, a step in which phosphorus, arsenic, boron, or another such impurity element is diffused in a silicon substrate. This phosphorus diffusion controls resistivity or sheet resistance by introducing phosphorus (P), which is an n-type impurity, through diffusion after the formation of a polysilicon film used as a resistor or wiring material or a gate electrode of a MOS transistor.
FIG. 4
is a simplified structural diagram of the diffusion apparatus used in the past to perform this phosphorus diffusion. The phosphine gas (or a mixed gas containing phosphine) that passes through a gas supply pipe
1
must be decomposed and activated. Methods that have been employed to decompose and activate this gas include heating in a gas preheating chamber
2
converting the gas into a plasma with a plasma generator
6
, and utilizing the energy of heating a silicon wafer W in a reaction tube
5
. The gas activated in the gas preheating chamber
2
or the plasma generator
6
is supplied from a gas introduction flange
3
to the reaction tube
5
, which consists of a quartz tube or the like. The wafer W, on whose surface is formed a silicon film such as a polysilicon film or an amorphous silicon film, is inserted into the reaction tube
5
and heated by a heater
4
. Phosphorus is diffused into the silicon film by allowing activated phosphine or a mixed gas containing phosphine to flow over this heated wafer W. The gas supplied to the reaction tube
5
is drawn out of an exhaust flange
7
by a vacuum pump
8
and exhausted through a gas exhaust pipe
9
.
In a phosphorus diffusion process such as this, a wafer on which a silicon film such as a polysilicon film or an amorphous silicon film has been formed is heated, after which phosphine or a mixed gas containing phosphine is allowed to flow over this substrate so that the phosphorus is diffused into the silicon film. In the past this flow of gas was timed such that the phosphine or mixed gas containing phosphine flowed after the temperature of the wafer W had been raised to the diffusion process temperature.
Unfortunately, the following problems were encountered with conventional methods in which the gas flowed after the temperature of the substrate had been raised to the process temperature.
(1) When the underlying film is polysilicon, it is difficult to diffuse the phosphorus. If the underlying film is an amorphous silicon film, though, the amorphous silicon ends up migrating to the polysilicon by the substrate temperature raising, after which it tends to be more difficult to diffuse the phosphorus.
(2) During the substrate temperature raising, an in-plane temperature distribution in this substrate temperature raising process results in uneven polysilicon conversion within the substrate plane, so the concentration in which the phosphorus is diffused within the substrate plane cannot be kept uniform.
The inventors discovered that the way phosphorus enters an amorphous silicon film is different from that with a polysilicon film, and that phosphorus has considerable difficulty entering a polysilicon film, and arrived at the present invention on the basis of the knowledge that the phosphorus should be diffused into an amorphous silicon film that has yet to make the transition to a polysilicon film, or while the film is in a mixed crystal state consisting of both amorphous and polycrystalline forms, as a result thereof, the condition is obtained in which the phosphorus can be easily doped by enhancing the rate-determining factor of polysilicon conversion utilizing respective properties of the elements, phosphorus of Group 5 and silicon of Group 4.
SUMMARY OF THE INVENTION
It is an object of the present invention to solve the above problems encountered with prior art and provide semiconductor device manufacturing method and a semiconductor manufacturing apparatus with which phosphorus diffusion is easy and the diffused phosphorus concentration can be easily controlled, which is accomplished by varying the timing at which the dopant gas is allowed to start flowing.
The first invention is a method for manufacturing a semiconductor device, comprising raising to a process temperature a temperature of a substrate on whose surface a silicon film has been formed and diffusing phosphorus in the silicon film, wherein the diffusing step includes a diffusing step which diffuses the phosphorus in the silicon film by exposing the substrate to phosphine or a mixed gas containing phosphine in the process in which the temperature of the substrate is raised to the process temperature.
The above-mentioned substrate is glass, a silicon wafer, or the like. The process temperature is the temperature required to introduce phosphorus (an impurity, or dopant) through diffusion into a certain region of a silicon film at the desired diffusion depth and concentration distribution. The gas that flows over the substrate may be phosphine gas alone or a mixed gas containing phosphine. The semiconductor device is a NOS transistor, a bipolar transistor, or the like.
With the first invention, in the substrate temperature raising process in which the temperature of the substrate is raised to the process temperature, the substrate is exposed to phosphine or a mixed gas containing phosphine before the temperature of the substrate actually is raised to the process temperature, so it is easier for the phosphorus to enter the silicon film than when the substrate is exposed to the gas after the temperature of the substrate being raised to the process temperature, which means that the diffusion of phosphorus into the silicon film is promoted and uniformity of the phosphorus concentration is enhanced.
With the first invention, while exposing the substrate to phosphine or a mixed gas containing phosphine, the phosphine or mixed gas containing phosphine can be allowed to flow over the substrate.
Alternatively, with the first invention, phosphine or a mixed gas containing phosphine may be introduced in the reaction chamber in which the substrate has been inserted, while exposing the substrate to the phosphine or mixed gas containing phosphine in this case, throughput is improved because the gas introduction step and substrate heating step are carried out in parallel.
Alternatively, with the first invention, while exposing the substrate to phosphine or a mixed gas containing phosphine, the phosphine or mixed gas containing phosphine can be allowed to be introduced into the reaction chamber and sealed therein after the substrate has been inserted in the reaction chamber but before the temperature of the substrate is raised, and the flow of the phosphine or mixed gas containing phosphine can be allowed to be halted during the raising of the temperature of the substrate. In this case, throughput is not as good since the gas introduction step and substrate heating step are carried out separately, but the same effect is obtained as when the gas introduction step and substrate heating step are carried out in parallel.
With the first invention, the mixed gas containing phosphine can be a mixed gas containing phosphine and nitrogen, a mixed gas containing phosphine and hydrogen, or a mixed gas containing phosphine and helium.
With the first invention, the process temperature is preferably 450 to 800° C. Doping will become the rate-determining factor if the polysilicon conversion temperature (over about 600° C.) is reached during the phosphorus diffusion, so a low-temperature process of about 450° C. is best. When out-diff
Imai Yoshinori
Karasawa Hajime
Nomura Hisashi
Takasawa Yushin
Yamaguchi Kenichi
Hitachi Kokusai Electric Inc.
Niebling John F.
Roman Angel
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