JPH02199836A - Formation of extremely thin oxide film - Google Patents

Abstract

PURPOSE:To form an extremely thin oxide film whose interface and film quality are good by a method wherein extreame ultraviolet rays which can be absorbed by SiO2 are applied, a volume of a thermal oxide film is contracted, its density is enhanced and its film thickness is reduced. CONSTITUTION:After a pretreatment, a thermal oxidation operation is executed in a dry acid atmosphere; an oxide film whose film quality and interface are uniform is formed on a whole substrate. Then, a steam oxidation operation in executed in succession; a characteristic of an interface and a film thickness of an extremely thin oxide film are decided approximately by this steam oxidation operation. Then, this substrate is irradiated with an excimer laser beam; its film thickness is reduced; its density is enhanced.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) Thin oxide films are used as gate oxide films and capacitor oxide films in very large scale integrated circuits (LSIs), and play an important role in transistor cells, which are the constituent elements of LSIs.

(Prior art) In order to form an oxide film, low pressure vapor deposition method, sputtering method,
Various methods have been developed, including vapor deposition, ion beam deposition, and thermal oxidation.

Among these methods, the thermal oxidation method is said to be optimal for forming a thin oxide film used in VLSI.

The reason why thermal oxidation is used as a method for forming thin oxide films is that the interface between the oxide film and silicon has the best properties, the oxide film itself has excellent properties, and The three points are that the thickness can be controlled relatively easily. All other methods except the thermal oxidation method produce 5i0 from the gas phase.
The basic method is to supply and deposit 2 onto the silicon surface. At this time, the interface between the oxide film and the silicon is formed on the surface of the silicon. Therefore, many dangling bonds are left at the interface between the oxide film and silicon, which have different atomic bonding styles. Further, the structure of the oxide film is such that oxygen atoms are coordinated at the vertices of a regular tetrahedron centered on silicon, and a polytype is formed depending on the degree of freedom of the apex bonds. An oxide film grows by the diffusion of oxygen atoms into a single silicon product, and compared to thermal oxidation, which has relatively little freedom in forming polytypes, the method in which 5i02 is deposited has less freedom in space. The appearance of polytypes due to large size cannot be avoided. Furthermore, in order to control the thin oxide film, it is necessary that the oxide film growth rate be very slow. The thermal oxidation method can achieve a very slow growth rate suitable for controlling oxide film thickness, and also provides good thickness uniformity.

(Problems to be Solved by the Invention) However, it is extremely difficult to create an ultra-thin oxide film of several tens of angstroms to a hundred angstroms in ultra-super LSI even by conventional thermal oxidation methods. When the thickness is on the order of an extremely thin oxide film, the presence of a so-called natural oxide film that exists before thermal oxidation cannot be ignored, partly due to the low formation temperature. The properties of the interface become dependent on the pretreatment conditions rather than the oxidation conditions. Furthermore, if the oxidation temperature is lowered to better control the thickness than when forming a thin oxide film, the interface becomes rough and the density decreases (film quality deteriorates).
I do things. The present invention solves the problems in forming an ultra-thin oxide film by such a thermal oxidation method and provides a method for forming an ultra-thin oxide film having good interface and film quality.

(Means to solve the problem) Irradiate extreme ultraviolet light that Si02 absorbs to shrink the volume of the thermal oxide film to improve its density, and reduce the film thickness to make it as thin as necessary. Obtain an oxide film.

(Function) The thermal oxide film having the Si02 composition locally contains many polytypes. The series from tridymite to quartz to conisite all have Si2 tetrahedrons as their structural units, and are constructed by connecting vertices.

The density changes by more than 30% depending on the connection mode, and the density varies by more than 0.7. The properties of the thermal oxide film are average properties of these polytypes, and as the proportion of low-density tridymite increases, the film quality will deteriorate. Therefore, in order to improve the film quality of the thermal oxide film, it is necessary to increase the proportion of conisite. In order to generate such a polytype phase change, the bonds between silicon and oxygen can be broken under compressive stress. As a result, the thermal oxide film mainly consists of conisite, and the film quality is improved.

To break the bond between silicon and oxygen, extreme ultraviolet light is irradiated. The absorption edge of quartz is about 8 eV, but absorption starts at about 5 eV for polytype. Such ultraviolet light generates electrons and holes in the thermal oxide film,
A hole corresponds to a state in which the bond between a silicon atom and an oxygen atom is broken. Evaporation from the surface is used to generate compressive stress. The surface of the thermal oxide film consists of four-membered rings, six-membered rings, human rings, and ten-membered rings made of silicon atoms and oxygen atoms. Of these, the most stable is the personnel circle. When the bonds between silicon atoms and oxygen atoms in an unstable structure are broken by extreme ultraviolet light, the surface tends to form a human ring, and the excess silicon and oxygen are converted into SiO or O
Release as 2. By utilizing this fact, it is possible to generate compressive stress and to control the film thickness on the order of angstroms.

(Example) A case using a silicon (100) substrate will be described.

Hydrochloric acid Branson washing was performed as pretreatment. The same applies to ammonia Branson treatment and hydrofluoric acid treatment. Since the method for forming an ultra-thin oxide film according to the present invention does not depend on pretreatment conditions, it is sufficient to prevent contamination of impurities. After pretreatment, the substrate is first oxidized in a dry oxygen atmosphere to form an oxide film with similar film quality and interface over the entire substrate. Oxidation temperature is 850°C
One oxidation time was 10 minutes. Since most of the impurity distribution has already been set at the time of gate oxidation, it is desirable that the oxidation temperature be as low as possible. Not only must it be low, but it must also be short.

The quality of the ultra-thin oxide film is almost determined by these dry oxidation conditions. Next, steam oxidation was performed continuously. Oxidation temperature: 800°C, oxidation time: 1 minute, atmosphere: H2O
:02=2:1. The atmosphere was an internal combustion type.

The characteristics and thickness of the interface of the ultra-thin oxide film are largely determined by this steam oxidation.

The thickness of the thermal oxide film thus formed was approximately 150 Å, as measured by an ellipsometer.
The density was 2.4. This substrate was irradiated with excimer laser light in the arrangement shown in FIG. The light source is K
They were rF and ArF. In the case of KrF, the irradiation conditions were: repetition rate 90H2, pulse width 12nsec, light intensity 2.6W/
cm2, for ArF, repeat 90H2, pulse width 10
nsec, and the light intensity was 1.0 W/cm2. The irradiation time was 15 minutes, and the substrate temperature was varied between room temperature and 500°C.

The atmosphere was argon or oxygen. As a result of such excimer laser light irradiation, in the case of KrF, the film thickness is approximately 1
00 angstroms, and the density was 2.8.

In addition, in the case of ArF, the film thickness is about 80 angstroms,
The density was 2.85. In the case of ArF excimer laser, the light intensity is lower than that of KrF excimer laser, but since the wavelength is shorter, it is considered that polytype phase change and evaporation reaction occur effectively. When the substrate temperature was low, there was no change in the quality or thickness of the ultra-thin oxide film due to the atmosphere. but,
At temperatures above about 400° C., the decrease in film thickness became more severe in the case of an argon atmosphere, and the decrease in the film thickness was suppressed in the case of an oxygen atmosphere. Furthermore, the film quality was also better as the substrate temperature was lower.

In the above embodiments, a (100) Si substrate was used, but the present invention can also be effectively applied to Si substrates with other plane orientations such as (111) and (IIO).

(Effects of the Invention) As described above, according to the method for forming an ultra-thin oxide film according to the present invention, a thermal oxidation method is used to form a very thin oxide film with the same interface properties and good film quality as those obtained when the oxidation temperature is high. Since it is possible to form a film, it can be applied to a technology for forming gate oxide films in VLSIs.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing an embodiment of the method for forming an ultra-thin oxide film by irradiating extreme ultraviolet light according to the present invention. 1. Excimer laser 2. Silicon substrate 3 with thermal oxide film formed, substrate holder 4, substrate holder driving device

Claims (1)

[Claims] A method for forming an ultra-thin oxide film, characterized by reducing the thickness of the oxide film and increasing its density by irradiating the oxide film formed by a thermal oxidation method with extreme ultraviolet light.