There are some methods, based on conventional art, for forming a film on a surface of an object, making use of a gaseous phase in which substances are permitted to move in a gas-like manner on an atomic or molecular basis. Such methods include chemical vapor deposition, which is hereinafter referred to as CVD, and physical vapor deposition, which is hereinafter referred to as PVD.
For example, PVD methods include vacuum vapor deposition, sputtering, and the like. Sputtering, which generally involves use of expensive devices, can form a film of high quality having good uniformity in quality and thickness. Therefore, sputtering is widely applied to display devices, such as liquid crystal displays.
In a CVD method, a material gas is introduced into a vacuum chamber and one, two or more gases are decomposed or reacted with each other on a substrate, using thermal energy to grow a solid film.
In this case, in order to promote reactions in film formation or to decrease reaction temperatures, plasma or catalyst reactions are often used in combination with the CVD method.
Of these CVD methods, one using a plasma reaction is referred to as a plasma enhanced CVD (PECVD) method, and one using catalyst reaction is referred to as a Cat-CVD method.
Use of the CVD methods mentioned above decreases defects in the films after being formed. Therefore, the CVD methods are applied, for example, to processing steps of fabricating semiconductor devices (e.g., steps of forming a gate insulating film).
In recent years, attention is focused on atomic layer deposition (ALD) methods as film-forming methods, hereinafter referred to as ALD methods.
In an ALD method, films are formed one by one on an atomic basis by chemical reaction of substances on a surface where the substances have been adsorbed. The ALD method is classified into the CVD method category.
In a so-called CVD method (typical CVD method), one or a plurality of gases are concurrently used for reaction on a substrate to grow a film. In contrast, in an ALD method, a highly active gas, which is also called a precursor (first precursor), and a reactive gas (called a precursor (second precursor) in the ALD method) are used in an alternate manner. Thus, the ALD method is a special film-forming method with which films are grown one by one on an atomic basis by adsorption and subsequent chemical reaction on a substrate surface.
The film-forming method based on the ALD method is specifically performed as follows.
First, using a so-called self-limiting effect, unreacted precursor is discharged after completing adsorption of one layer of precursor on a substrate (first step). The self-limiting effect is a phenomenon in which gas adsorption is no longer caused once a surface-adsorbing substrate is covered with a specific gas.
Then, a reactive gas is introduced into a chamber to oxidize or reduce the precursor to form one layer of film having a desired composition, followed by discharging the reactive gas (second step).
In the ALD method, the first and second steps are taken to be one cycle. The cycle is repeatedly performed to grow films on the substrate.
Thus, in the ALD method, films are two-dimensionally grown. The ALD method causes fewer defects in a film after being formed, compared with not only conventional vacuum vapor deposition, sputtering, and the like, but also with generally used CVD methods.
Therefore, the ALD method is expected to be applied to various fields such as of packaging for food products, pharmaceutical products, and the like, and electronic components.
As one ALD method, plasma is used for activating reactions in a step of decomposing the second precursor for reaction with the first precursor adsorbed on a substrate. This method is called plasma enhanced ALD (PEALD), or simply, plasma ALD.
The technique of the ALD method was proposed by Dr. Tuomo Sumtola of Finland in 1974. Typically, the ALD method, which provides high quality and high density films, is being actively applied to fabrication of semiconductor devices (e.g., steps of forming a gate insulating film). Mention has also been made accordingly in the International Technology Roadmap for Semiconductors (ITRS).
The ALD method, when compared with other film-forming methods, causes no shadowing effect, which is a phenomenon in which sputtering particles obliquely incident on a surface of a substrate cause unevenness in a film after being formed. Thus, the ALD method enables film formation as long as there is a gap into which a gas can enter.
Therefore, the ALD method is expected to be applied to coating of lines or holes on a substrate having a high aspect ratio of depth to width, or to MEMS (micro electro mechanical systems) related techniques used for coating three-dimensional structures.
However, the ALD method also suffers from problems. The problems include, for example, the necessity of using special materials, and cost increase due to the use of special materials, and the like. The biggest problem is that the film-forming speed is slow. The film-forming speed of the ALD method is very slow by a factor of about ⅕ to 1/10 compared with that of typical vacuum vapor deposition, sputtering, or the like.
Substrates on which films are formed by means of the ALD method mentioned above include, for example, small plate-like substrates, such as wafers and photomasks, inflexible substrates with a large area (e.g., glass substrate), and flexible substrates with a large area, such as films.
In mass production facilities for forming films on these substrates, there are proposed various methods of handling substrates, depending on cost, ease of handling, and quality of films to be formed, and the like, and the proposals are being put into practice.
For example, film-forming devices used in the case of forming a film on a wafer include single wafer film-forming devices or batch film-forming devices. In a single wafer film-forming device, one wafer is conveyed into a chamber of the device to form a film, followed by replacing the formed wafer with an unprocessed wafer, which is again followed by performing the film-forming treatment. In a batch film-forming device, a plurality of wafers are collectively placed in a chamber, followed by performing the same film-forming treatment with respect to all of the wafers.
Film-forming devices used in the case of forming a film on a glass substrate include in-line film-forming devices. In an in-line film-forming device, glass substrates are sequentially conveyed to a part serving as a film-forming source, with concurrent formation of a film.
Film-forming devices used in the case of forming a film on a flexible substrate include coating film-forming devices adopting so-called roll-to-roll processing. In a coating film-forming device, a film is formed while a flexible substrate is unrolled from a roller, and the flexible substrate is taken up by another roller.
The coating film-forming devices also include web coating film-forming devices for continuously forming a film, with the substrates targeted for film formation being conveyed on a flexible sheet or on partially flexible trays that can continuously convey the substrates.
The film-forming method and the substrate handling method of any of the film-forming devices can be combined, however, a film-forming device providing a combination that achieves a highest film-forming speed is typically used, taking account of cost, quality, ease of handling, and the like.
There are widely known laminates of conventional art, in which an atomic layer deposition film is formed on an outer surface of a substrate by means of the ALD method. For example, such a laminate is used as a gas barrier film having high gas barrier properties.
PTL 1 discloses a technique in which an atomic layer is vapor-deposited by the ALD method to form a barrier layer on a surface of a plastic film. According to this technique, the vapor-deposited atomic layer is formed by the ALD method, realizing a gas barrier film having good barrier properties.