The present invention relates to methods for fabricating a semiconductor device, and more particularly relates to a method for fabricating an MISFET or an MIS capacitor which includes a high dielectric gate insulating film.
When a known gate insulating film made of a silicon oxide film is formed, a silicon oxide film can be obtained by directly oxidizing a silicon wafer that is to be a substrate. In contrast, in the case of forming a high dielectric gate insulating film, a high dielectric constant film can not be obtained by simply oxidizing a silicon substrate because a metal material for a high dielectric constant film is not contained in the substrate. For this reason, methods in which a high dielectric constant film is deposited on a substrate using CVD (chemical vapor deposition), sputtering, molecular beam epitaxy, laser ablation or like techniques have been used. Particularly, if sputtering among the above-described deposition techniques is used, a high dielectric constant film with excellent electric properties can be obtained. More specifically, in a sputtering process, a single-element metal target is used as a material and therefore it is possible to prevent an impurity such as an organic substance or water from entering into a deposited film and also to densify the deposited film (see, for example, Byoung Hun Lee et al., Ultrathin Hafnium Oxide with Low Leakage and Excellent Reliability for Alternative Gate Dielectric Application, IEDM 1999, pp. 133-136), compared to the case where CVD is used.
When a high dielectric constant film (more specifically, a high dielectric metal oxide film) is deposited by sputtering, sputtering is performed using a metal target in an atmosphere containing argon and oxygen, or using a metal oxide target in an argon atmosphere. More specifically, in either case of sputtering and CVD, when the process step of depositing a metal oxide film is performed, an activated oxidation species is generated while a silicon substrate as a underlayer for deposition of a metal oxide film is oxidized by the oxidation species. If a silicon substrate is oxidized, a silicon oxide film with a low dielectric constant is formed under a metal oxide film with a high dielectric constant, resulting in reduction in the capacitance value of a capacitor structure. Therefore, in depositing of a high dielectric metal oxide film by sputtering, it is required to suppress oxidation of a silicon substrate by active oxygen or the like.
Then, as a method for suppressing oxidation of a silicon substrate, for example, there has been proposed a method in which before depositing a high dielectric metal oxide film (specifically, a hafnium oxide film), a metal film (specifically, a hafnium metal film) including the same metal as that contained in the metal oxide film is deposited (see, e.g., Byoung Hun Lee et al., Ultrathin Hafnium Oxide with Low leakage and Excellent Reliability for Alternative Gate Dielectric Application, IEDM 1999, pp. 133-136). Moreover, a method in which before depositing a high dielectric metal oxide film, a metal oxide film with a low oxygen content is deposited has been also proposed (see, e.g., Shui-Hsiang Su et al., A Two-Step Deposition Technology for High Dielectric Constant HfO2 Thin Films, Electrochemical and Solid-State Letters 4(9) F18-F19 (2001)). In these methods, a metal layer or a metal oxide layer having a low concentration of oxygen is deposited between a high dielectric metal oxide film and a silicon substrate, thereby making the metal layer or the like absorb oxygen diffused from the high dielectric metal oxide film side to the silicon substrate side. In this manner, oxidation of a silicon substrate is suppressed.
In the process step of depositing a high dielectric constant film, the above-described metal film or the like is useful as an oxygen absorption layer. However, if the metal film or the like is not sufficiently oxidized at the end of the deposition of the high dielectric constant film, oxygen deficiencies will occur in a metal oxide film formed by oxidizing the metal film or the like. With such oxygen deficiencies existing in the metal oxide film, dangling bonds of metal atoms are created, resulting in increased leakage current in a capacitor structure, or causing difficulties in adjustment of the flat band voltage or the threshold voltage thereof. Therefore, when a metal film or the like is used as an oxygen absorption layer, the thickness of the metal film or the like is required to be set so that oxidation of the silicon substrate is suppressed and no non-oxidized metal portion is left at the end of the deposition of the high dielectric constant film. That is to say, strict conditions are required for the thickness of the metal film or the like as an oxygen absorption layer.
When some other technique than sputtering is used in forming a high dielectric gate insulating film, there also arises a problem of reduction in the capacitance value of a gate insulating film due to oxidation of a silicon substrate. For example, assume that a high dielectric metal oxide film is deposited by CVD. Although it is also needed to suppress oxidation of a silicon substrate, a metal oxide film is, in general, deposited in an oxidation atmosphere and thus oxidation of a silicon substrate can not be avoided. When a metal oxide film is deposited by CVD, the amount of active oxygen or the like to be generated is smaller than that when sputtering is used, and thus the amount of oxidized part of the silicon substrate is slightly reduced. However, there is still a problem of reduction in the capacitance value thereof.
As has been described, in a deposition technique such as sputtering in which a high dielectric constant film is deposited while a silicon substrate is exposed to an oxygen atmosphere, it is very difficult to prevent oxidation of the silicon substrate. This is an essential problem in the deposition of a metal oxide film.