1. Field of the Invention
The present invention relates to a film formation method and apparatus for a semiconductor process for forming an insulating film on a target substrate, such as a semiconductor wafer. The term “semiconductor process” used herein includes various kinds of processes which are performed to manufacture a semiconductor device or a structure having wiring layers, electrodes, and the like to be connected to a semiconductor device, on a target substrate, such as a semiconductor wafer or a glass substrate used for an FPD (Flat Panel Display), e.g., an LCD (Liquid Crystal Display), by forming semiconductor layers, insulating layers, and conductive layers in predetermined patterns on the target substrate.
2. Description of the Related Art
In manufacturing semiconductor devices for constituting semiconductor integrated circuits, a target substrate, such as a semiconductor wafer, is subjected to various processes, such as film formation, etching, oxidation, diffusion, reformation, annealing, and natural oxide film removal. US 2006/0286817 A1 discloses a semiconductor processing method of this kind performed in a vertical heat-processing apparatus (of the so-called batch type). According to this method, semiconductor wafers are first transferred from a wafer cassette onto a vertical wafer boat and supported thereon at intervals in the vertical direction. The wafer cassette can store, e.g., 25 wafers, while the wafer boat can support 30 to 150 wafers. Then, the wafer boat is loaded into a process container from below, and the process container is airtightly closed. Then, a predetermined heat process is performed, while the process conditions, such as process gas flow rate, process pressure, and process temperature, are controlled.
In order to improve the performance of semiconductor integrated circuits, it is important to improve properties of insulating films used in semiconductor devices. Semiconductor devices include insulating films made of materials, such as SiO2, PSG (Phospho Silicate Glass), P—SiO (formed by plasma CVD), P—SiN (formed by plasma CVD), and SOG (Spin On Glass), Si3N4 (silicon nitride). Particularly, silicon nitride films are widely used, because they have better insulation properties as compared to silicon oxide films, and they can sufficiently serve as etching stopper films or inter-level insulating films. Further, for the same reason, carbon nitride films doped with boron are sometimes used.
Several methods are known for forming a silicon nitride film on the surface of a semiconductor wafer by thermal CVD (Chemical Vapor Deposition). In such thermal CVD, a silane family gas, such as monosilane (SiH4), dichlorosilane (DCS: SiH2Cl2), hexachlorodisilane (HCD: Si2Cl6), or bistertialbutylaminosilane (BTBAS: SiH2(NH(C4H9))2), is used as a silicon source gas. For example, a silicon nitride film is formed by thermal CVD using a gas combination of SiH2Cl2+NH3 (see U.S. Pat. No. 5,874,368 A) or Si2Cl6+NH3. Further, there is also proposed a method for doping a silicon nitride film with an impurity, such as boron (B), to decrease the dielectric constant.
In recent years, owing to the demands of increased miniaturization and integration of semiconductor integrated circuits, it is required to alleviate the thermal history of semiconductor devices in manufacturing steps, thereby improving the characteristics of the devices. For vertical processing apparatuses, it is also required to improve semiconductor processing methods in accordance with the demands described above. For example, there is a CVD (Chemical Vapor Deposition) method for a film formation process, which performs film formation while intermittently supplying a source gas and so forth to repeatedly form layers each having an atomic or molecular level thickness, one by one, or several by several (for example, Jpn. Pat. Appln. KOKAI Publications No. 2-93071 and No. 6-45256 and U.S. Pat. No. 6,165,916 A). In general, this film formation method is called ALD (Atomic layer Deposition), which allows a predetermined process to be performed without exposing wafers to a very high temperature.
For example, where dichlorosilane (DCS) and NH3 are supplied as a silane family gas and a nitriding gas, respectively, to form a silicon nitride film (SiN), the process is performed, as follows. Specifically, DCS and NH3 gas are alternately and intermittently supplied into a process container with purge periods interposed therebetween. When NH3 gas is supplied, an RF (radio frequency) is applied to generate plasma within the process container so as to promote a nitridation reaction. More specifically, when DCS is supplied into the process container, a layer with a thickness of one molecule or more of DCS is adsorbed onto the surface of wafers. The superfluous DCS is removed during the purge period. Then, NH3 is supplied and plasma is generated, thereby performing low temperature nitridation to form a silicon nitride film. These sequential steps are repeated to complete a film having a predetermined thickness.
When an insulating film as one of those described above is formed and then another thin film is formed thereon, contaminants such as organic substances and particles may have stuck to the surface of the insulating film. Accordingly, a cleaning process is performed to remove the contaminants, as needed. In this cleaning process, the semiconductor wafer is immersed in a cleaning solution, such as dilute hydrofluoric acid, to perform etching on the surface of the insulating film. Consequently, the surface of the insulating film is etched by a very small amount, thereby removing the contaminants.
Where such an insulating film is formed by CVD at a higher process temperature of, e.g., about 760° C., the etching rate of the insulating film during the cleaning process is very small. Accordingly, the insulating film is not excessively etched by cleaning, and thus the cleaning process is performed with high controllability in the film thickness. However, where a thin film having a low heat resistance is present as an underlayer, a thermal CVD process at high temperature is unsuitable.
On the other hand, where such an insulating film is formed by ALD film formation at a lower process temperature of, e.g., about 400° C., the etching rate of the insulating film during the cleaning process is relatively large. Accordingly, the insulating film may be excessively etched by cleaning, and thus the cleaning process entails lower controllability in the film thickness.
Further, a silicon nitride film may be used as an etching stopper film or inter-level insulating film. In this case, the etching rate of the silicon nitride film must be very low. However, the conventional film formation method cannot satisfy this requirement.