BACKGROUND OF THE INVENTION
1. Field of the invention
This invention relates to a remover for a ruthenium containing metal which is effective for removing an undesired ruthenium containing metal adhering to, for example, a semiconductor substrate and the use thereof.
2. Description of the related Art
Recently, a high dielectric-constant film such as Ta2O5 has been used in place of a conventional silicon oxide or nitride film as a capacitance film for a DRAM or FeRAM. Such a high dielectric-constant film permits us to ensure a required accumulated capacitance in a small occupied area and to improve a degree of memory-cell integration.
When using such a high dielectric-constant film and polysilicon as an electrode material sandwiching a capacitance film, oxygen is liberated from the high dielectric-constant film during heating a semiconductor device to oxidize the electrode material. There, therefore, exists a dielectric film (silicon oxide film) having a lower dielectric constant than the high dielectric-constant film between the electrode materials, leading to reduction in a capacitance. Thus, when using a high dielectric-constant film, it is important to select a material which does not become an insulating film due to oxidation as an electrode material sandwiching a capacitance film. It is because once a part of an electrode becomes an insulating film by oxidation, it constitutes a part of a capacitance film, resulting in a reduced capacitance. Ruthenium has recently attracted attention as an electrode material meeting the above requirement. Ruthenium is preferable because it retains conductivity even after oxidation, thus does not cause capacitance reduction and is inexpensive.
However, forming an electrode using ruthenium may lead to peeling of a ruthenium containing metal such as ruthenium and ruthenium oxide adhering to the end face or the rear face of a silicon substrate. The peeled metal may adhere to a device forming area or may cause cross contamination between devices or wafers via a carrying system. Recently, a procedure such as forming an electrode film within a narrow hole has been frequently employed for reducing an occupied area for a capacitor. It requires even forming of a thin ruthenium film, so that it is often essential to use CVD exhibiting good coverage as a deposition method, where adhesion of the ruthenium containing metal to the end and/or the rear faces of the silicon substrate becomes more prominent.
A ruthenium containing metal is known as a so-called lifetime killer to a semiconductor device. In particular, it may cause a variety of problems; for example, it adversely affects device operation due to reduction in carrier mobility and varying a threshold voltage of a transistor with time. A ruthenium containing metal is diffused at a higher rate in a silicon substrate than platinum also known as a lifetime killer. A trace amount of the ruthenium containing metal remaining on the silicon substrate surface may give prominent adverse affects on device properties. As described above, an undesired ruthenium containing metal remaining on a silicon substrate surface may deteriorate reliability of a device.
Therefore, when using ruthenium as an electrode material, it is important to remove an undesired ruthenium containing metal by treatment with an etchant. There have been, however, no enchants capable of dissolving and removing a ruthenium containing metal. For example, aqua regia used for forming a platinum electrode may not be used as a remover for a ruthenium containing metal due to its insufficient dissolving ability.
For effectively removing a ruthenium containing metal, a remover for ruthenium must not only dissolve the ruthenium containing metal but also effectively preventing the dissolved ruthenium containing metal from re-adhering to a silicon substrate.
SUMMARY OF THE INVENTION
In the light of the above situation, an objective of this invention is to provide a remover for a ruthenium containing metal which can adequately dissolve and remove a ruthenium containing metal such as ruthenium and ruthenium oxide and can satisfactorily prevent re-adhesion of the dissolved ruthenium containing metal, and the use thereof.
This invention provides a remover for a ruthenium containing metal, comprising (a) a cerium (IV) nitrate salt and (b) at least one acid selected from the group consisting of nitric acid, perchloric acid and acetic acid.
The remover of this invention exhibits prominent performance of removing a ruthenium containing metal owing to synergism in the combination of components (a) and (b) and can satisfactorily prevent re-adhesion of the dissolved ruthenium containing metal.
This remover may be used for washing a substrate to which a ruthenium containing metal adheres or for etching a ruthenium film formed on a substrate.
The remover of this invention is particularly suitable for removing ruthenium adhering to a semiconductor device (by washing or etching). As described above, ruthenium is known as a so-called lifetime killer to a semiconductor device. It may give serious damage to device performance when remaining on the surface of the semiconductor device. The remover according to this invention may efficiently remove a ruthenium containing metal and prevent re-adhesion so that it is suitable for removing the ruthenium containing metal on such a semiconductor substrate.
The remover of this invention is particularly effective when used for washing a semiconductor substrate in which a ruthenium containing metal adheres to an area other than a device forming area. For example, it may be significantly effective when used for removing by washing a ruthenium containing metal adhering to an area other than a device forming area after depositing a ruthenium film in the device forming area on the semiconductor substrate. In such washing, the ruthenium containing metal adhering the area other than the device forming area is mainly made of ruthenium oxide. The remover of this invention may exhibit good performance of removal and re-adhesion for not only ruthenium but also ruthenium oxide. Thus, it may be suitably used for the above washing. For such washing, a particularly higher level of performance is required for preventing re-adhesion of the dissolved and removed ruthenium containing metal in comparison with, for example, etching. The remover of this invention exhibiting excellent performance of preventing re-adhesion may be suitably used in the above washing. The term “an area other than a device forming area” includes end and rear faces of a semiconductor substrate and also peripheral areas in the device forming area.
As described above, the remover of this invention is characterized in that a cerium (IV) nitrate salt is combined with a particular acid.
For a composition in which a cerium (IV) nitrate salt and an acid is combined, JP-B 7-7757 and JP-A 11-131263 have described that used as an etchant for preparing a chrome mask. When preparing the chrome mask, it is necessary to etch a chromium film such that its cross section becomes tapered. It is known that such a taper shape may be suitably formed by conducting wet etching using a composition having the above combination after forming a resist mask on a chromium film because chromium is dissolved by the action of a cerium (IV) nitrate salt while nitric acid peels the resist mask and the chromium film.
These publications have, however, described etching chromium and there are no description for action to a ruthenium containing metal.
As described above, it has been unknown that a combination of a cerium (IV) nitrate salt and a particular acid exhibits excellent performance in removing a ruthenium containing metal and may effectively prevent re-adhesion of the removed ruthenium containing metal. This invention is based on the observation.
This invention also provides method of using a remover for a ruthenium containing metal wherein a substrate is washed with a liquid containing at least one of hydrofluoric acid, nitric acid, perchloric acid and oxalic acid for removing the residual remover after removing with the above remover.
This process may allow us to effectively remove a residual remover, leading to washing with higher cleanliness.
This invention also provides a process for removing a ruthenium containing metal comprising the steps of depositing a ruthenium film in a device forming area on a semiconductor substrate; and spraying a remover containing (a) a cerium (IV) nitrate salt and (b) at least one acid selected from the group consisting of nitric acid, perchloric acid and acetic acid on a given area on the semiconductor substrate while rotating the substantially horizontal semiconductor substrate to remove a ruthenium containing metal adhering to an area other than the device forming area.
The removing process may more effectively remove a ruthenium containing metal.