Insulating film formation method, semiconductor device, and...

Details Insulating film formation method, semiconductor device, and... Insulating film formation method, semiconductor device, and...

2002-01-04

2003-09-16

Cao, Phat X. (Department: 2814)

Semiconductor device manufacturing: process

Coating of substrate containing semiconductor region or of...

Insulative material deposited upon semiconductive substrate

C438S770000, C438S149000, C438S486000, C257S347000

Reexamination Certificate

active

06620744

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for forming an insulator film on a semiconductor, where a combination of the semiconductor and the insulator film is used in a FET (Field Effect Transistor) or a polycrystal silicon thin film transistor which has a MOS (Metal Oxide Semiconductor) structure. The present invention also relates to a semiconductor device fabricated using the method, and a production apparatus.
2. Description of the Related Art
FETs are widely used for LSIs. To improve the performance of LSIs, there is a demand for a satisfactory thin insulator film which can be formed at a low temperature, and with satisfactory semiconductor-insulator film interfacial quality.
Conventionally, single-crystal silicon is generally thermally oxidized at a temperature of 700° C. to 1000° C. In thermal oxidation, an oxidation reaction develops from a surface of a semiconductor (a surface of a semiconductor layer) and progresses inward. Therefore, an interface is produced between the semiconductor layer (semiconductor) and an oxide silicon film (gate insulator film) provided by the thermal oxidation of the semiconductor layer surface, i.e., the interface is provided inside the original semiconductor layer. Therefore, the interface is not substantially affected by a condition of the original surface, so that a very satisfactory interface can be advantageously obtained. However, the high temperature process is likely to warp a silicon wafer. Low temperatures suppress warp, but cause an oxidation rate to be rapidly reduced. Thus, a low temperature process is not practical.
An insulator film may also be produced by plasma CVD (Chemical Vapor Deposition), but it is difficult to obtain satisfactory interfacial quality. In this case, the most critical problem is that ion damage due to plasma is inevitable.
On the other hand, the recent development of large-size, high-definition, and high-performance liquid crystal display apparatuses require higher and higher-density TFTs (Thin Film Transistors). There is an increasing demand for TFTs of a polysilicon (poly-Si) film in place of conventional amorphous silicon film TFTs. A gate insulator film, which is crucially important for TFT's performance and reliability, is provided by plasma CVD. However, when plasma CVD is employed to form a gate insulator film, damage due to plasma is inevitable. In this case, particularly, a threshold voltage of the resultant transistor cannot be controlled with high precision, and reliability of the transistor may be lowered.
As often the case in poly-Si TFTs, a SiO
2
film may be formed by plasma CVD using TEOS (Tetra Ethyl Ortho Silicate) and O
2
gases. Such a SiO
2
film contains carbon atoms which are originally contained in a gas material. Even if the film is formed at 350° C. or more, it is difficult to reduce the carbon concentration to 1.1×10
20
atoms/cm
3
or less. In particular, when the film-forming temperature is as low as about 200° C., the carbon concentration in the film is increased by an order of magnitude up to 1.1×10
21
atoms/cm
3
. Therefore, it is difficult to reduce film-forming temperature.
In the case of plasma CVD using SiH
4
and N
2
O-based gases, an interface nitrogen concentration is as great as one atom % or more, so that an interface fixed charge density cannot be 5×10
11
cm
−2
or less. A functional gate insulator film cannot be obtained.
For the purpose of reducing ion damage due to plasma CVD so as to obtain a high-quality insulator film, oxidation methods, such as for example ECR plasma CVD and oxygen plasma, have been developed. However, since plasma is generated in the vicinity of a surface of a semiconductor, it is difficult to fully avoid ion damage.
Cleaning apparatuses using a light source, such as for example a low-pressure mercury lamp and an excimer lamp, have already been brought into mass production.
A method in which light is used to oxidize silicon at a low temperature of 250° C. has been studied. In this method, however, a film formation rate is as slow as 0.3 nm/min. At present, it is practically difficult to form an entire gate insulator film (J. Zhang et al., A. P. L., 71(20), 1997, P2964).
Japanese Laid-Open Publication No. 4-326731 discloses an oxidation method which is carried out in an ozone-containing atmosphere. As described below, however, in this method, ozone is produced using light, and the ozone is decomposed using light to produce oxygen atom radicals, i.e., the method comprises two reaction steps. Therefore, the method is poorly efficient, resulting in a low oxidation rate.
As described above, in the case of deposition (plasma CVD, etc.), a thick insulator film can be quickly formed on a semiconductor, but a surface of the original semiconductor remains as an interface between the semiconductor and the insulator film (gate insulator film), and ion damage is inevitable. Therefore, since interface trap density is increased, it is not possible to obtain satisfactory device characteristics.
When an insulator film is formed on a semiconductor using an oxidation method (e.g., an oxygen plasma oxidation method), an oxidation reaction develops from a surface of a semiconductor to the inside, and an interface between a semiconductor layer (semiconductor) and the insulator film is formed inside the original semiconductor layer. Therefore, the interface is not substantially affected by a condition of the original surface, so that a very satisfactory interface can be advantageously obtained. However, the high temperature process is likely to warp a silicon wafer. Low temperatures suppress warp, but cause an oxidation rate to be rapidly reduced. Thus, a low temperature process cannot produce an insulator film at a practical rate.
SUMMARY OF THE INVENTION
According to an aspect of the present invention, a method for forming an insulator film at a semiconductor temperature of 600° C. or less comprises the steps of forming a first insulator film by oxidizing a surface of a semiconductor in an atmosphere containing oxygen atom radicals, and forming a second insulator film on the first insulator film by deposition without exposing the first insulator film to outside air.
In one embodiment of this invention, the first insulator film forming step comprises generating the oxygen atom radicals by irradiating an atmosphere containing an oxygen gas with light having a wavelength of 175 nm or less.
In one embodiment of this invention, the first insulator film forming step comprises generating the oxygen atom radicals by irradiating an atmosphere containing an oxygen gas with light having a wavelength of 172 nm, the light emitted from a xenon excimer lamp.
In one embodiment of this invention, the first insulator film forming step comprises generating the oxygen atom radicals by irradiating an atmosphere containing an oxygen gas having a partial pressure of 0.05 torr to 50 torr with light having a wavelength of 172 nm, the light emitted from a xenon excimer lamp.
In one embodiment of this invention, the method further comprises, prior to the first insulator film forming step, the step of cleaning the surface of the semiconductor by irradiating the surface of the semiconductor with light having a wavelength of 175 nm or less in an atmosphere having substantially no oxygen.
In one embodiment of this invention, the first insulator film forming step comprises generating the oxygen atom radicals by plasma CVD, wherein there is a predetermined distance or more between a plasma generating site and the surface of the semiconductor.
In one embodiment of this invention, the first insulator film forming step comprises forming the first insulator film where a temperature of the semiconductor is in the range from 100° C. to 500° C.
In one embodiment of this invention, the first insulator film forming step comprises forming the first insulator film where the formed first insulator film has a thickness in the range from 0.5 nm to 20 nm.
In one embodiment of this invention, the first insulator film form

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