Chemistry: electrical and wave energy – Apparatus – Coating – forming or etching by sputtering
Reexamination Certificate
1998-02-18
2002-09-17
Kalafut, Stephen (Department: 1745)
Chemistry: electrical and wave energy
Apparatus
Coating, forming or etching by sputtering
C204S298060, C204S298070
Reexamination Certificate
active
06451184
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a thin film forming apparatus, and more particularly belongs to the technical field of thin film forming processes suited for forming electrodes or protective films for semiconductor elements, electrodes or protective films for liquid-crystal devices, protective films for photomagnetic recording media, reflection preventive films or reflection enhancing films for optical articles, or the like by the use of such an apparatus.
2. Related Background Art
Sputtering is one of thin film forming processes.
Conventional reactive sputtering is a process in which a mixed gas of sputter gas and reactive gas is introduced into a reaction chamber and a compound or metallic target is sputtered to form a metallic compound thin film by the chemical reaction of target constituent atoms with the reactive gas. In the case of insulating compound targets, the process commonly has a low deposition rate because high-frequency power such as RF or microwaves is supplied to cause a glow discharge. In the case of metallic targets, the deposition rate can be improved because a DC voltage can be supplied to cause a glow discharge.
Even when the target is a metal, the reactive gas may react with the metallic target on the target surface, so that unauthorized metallic compounds are formed on the target surface. Usually, the sputtering yield with respect to the metallic compound is about 10% of the sputtering yield with respect to the metal, and hence the deposition rate may be lower in the reactive sputtering.
Even in the case of compound targets, the compound thin films formed actually have a higher metallic atom content than the content determined by the stoichiometric ratio, and hence the reactive gas must be added in order to form films having composition approximate to the stoichiometric ratio. Not to speak of the case of metallic targets, if the reactive gas is supplied at a low flow rate, the metallic compound thin film may be formed as a thin film having metallic atoms in a high content, and cannot possibly be a thin film that satisfies the stoichiometric ratio, resulting in poor thin film characteristics with respect to optical characteristics (such as refractive index and transmittance) and so forth.
Accordingly, some attempts to solve such technical problems are proposed.
FIG. 10
is a schematic view of a reactive sputtering apparatus disclosed in Japanese Patent Application Laid-Open No. 62-56570. Reference numeral
1
denotes a target;
2
, a substrate;
3
, a supply pipe for argon (Ar) serving as a sputter gas;
4
, a supply pipe for oxygen (O
2
) serving as a reactive gas;
9
, a reaction chamber;
12
, a target holder; and
7
, a substrate holder.
The above publication states that the use of the apparatus shown in
FIG. 10
makes the rate of sputtering larger and brings about an improvement in the characteristics of the oxides, since the sputter gas and the reactive gas are separately introduced, and since the sputtering takes place preferentially in the vicinity of the target and the oxidation reaction takes place preferentially in the vicinity of the substrate.
In reality, however, the sputter gas and the reactive gas mix at a zone between the target and the substrate to form a mixed plasma of the both. Especially when a thin film is formed on a large-area substrate, the discharging region between the substrate and the target is so large that the sputter gas and the reactive gas can be separately present with difficulty. Thus, the film quality and the rate of sputtering can not be so much improved as expected.
Meanwhile,
FIG. 11
is a schematic view of a reactive sputtering apparatus disclosed in Japanese Patent Application Laid-Open No. 6-41733. Reference numeral
1
denotes a target;
2
, a substrate;
3
, a supply pipe for argon (Ar) serving as a sputter gas;
4
, a supply pipe for oxygen (O
2
) serving as a reactive gas;
7
, a substrate holder;
8
, a power source;
9
, a reaction chamber;
12
, a target holder;
13
, a differential pressure plate;
14
, a high frequency power source;
15
, a coil;
16
, a magnet;
17
, a pipe for circulating a refrigerant; and
18
, an exhaust pump.
In this apparatus, an exhaust vent communicating with a vacuum pump is provided at an upper part of the reaction chamber
9
, and the differential pressure plate
13
is utilized to produce a difference in pressure between the reaction chamber upper part and the reaction chamber lower part so that the sputter gas and the reactive gas can be separated.
In the apparatus shown in
FIG. 11
, however, an opening
13
a
of the differential pressure plate
13
is larger than the size of the substrate
2
, so that, in reality, the sputter gas unwantedly passes through the opening
13
a
of the differential pressure plate
13
to flow to the substrate
2
side, resulting a large discharge region. Hence, also in such an apparatus, the rate of sputtering and the film quality can not be so much improved as to be expected. Also, since preliminary excitation of oxygen must be made by the high-frequency power source
14
, the apparatus must be complicated in constitution and moreover the inner walls of the reactive gas feed pipe
4
is sputtered because of the preliminary excitation to rather cause a greater difficulty that, e.g., substances constituting the reactive gas feed pipe, such as iron, is incorporated into the films to be formed.
In addition, the particles dislocated by sputtering may jump into the substrate to cause an excessive rise of substrate temperature.
Japanese Patent Application Laid-open No. 7-335553 discloses a reactive sputtering apparatus proposed in order to achieve an object different from the above apparatus. This apparatus is provided with a collimator between the target and the substrate in order to fill up contact holes of semiconductor devices.
In the conventional apparatus having a collimator, the collimator has so great an aspect ratio that the target constituent atoms broken away by sputtering are incident on the substrate surface at small angles to make it difficult to form uniform and large-area continuous thin films. Moreover, the surface of the collimator is also sputtered, so that collimator constituent atoms (iron atoms in the case where, e.g., the collimator is made of stainless steel) are included into the TiN thin films to be formed.
The specification and drawings of U.S. Pat. No. 5,415,753 and a publication “THE SECOND INTERNATIONAL SYMPOSIUM ON SPUTTERING & PLASMA PROCESS, 1993, pp.269-274” also disclose a reactive sputtering apparatus provided with a perforated plate between the target and the substrate and also so constructed that the sputter gas and the reactive gas are separately supplied.
In the above reactive sputtering apparatus, however, no sufficient studies are made on the electrical properties of and materials for the perforated plate, and the film quality can not be so much improved as to be expected.
Among materials for thin films, magnesium fluoride (MgF
2
) attracts notice as a material having a low refractive index (1.38), which, however, often causes absorption of light when films are formed by sputtering. Moreover, when magnesium fluoride films are formed by sputtering, the rate of film formation is said to be lower than that of vacuum deposition.
To cope with this problem, Japanese Patent Applications Laid-open No. 9-41132 and No. 9-41134 disclose a method in which the target is supported through a heat-insulating packing plate so that the target can be kept at a high temperature and a method in which a granular target of MgF
2
is used.
Any of these methods, however, have paid attention to improving the rate of sputtering or improving the rate of film formation, without any satisfactory achievement of improvement in film quality. Hence, after all, compound thin films having good optical or electrical characteristics have not been available.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a thin film forming apparatus, and a thin film forming process,
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Kalafut Stephen
Mercado Julian A.
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