Semiconductor device manufacturing: process – Chemical etching – Liquid phase etching
Reexamination Certificate
2002-12-04
2003-12-23
Lebentritt, Michael S. (Department: 2824)
Semiconductor device manufacturing: process
Chemical etching
Liquid phase etching
C438S758000, C438S769000, C438S778000, C438S784000, C438S785000
Reexamination Certificate
active
06667246
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method for manufacturing a semiconductor device, and more particularly to a method for wet-etching a metal oxide film to be a gate insulating film having a high dielectric constant.
A silicon oxide film (SiO
2
film) has been used as a gate insulating film of an MIS semiconductor device. On the other hand, the degree of integration of semiconductor integrated circuits has been increased significantly in recent years. When a super thin silicon oxide film having a thickness of about 2 nm or less is used as a gate insulating film, the gate-leak characteristics deteriorate due to a direct-tunneling effect, etc., whereby it is difficult to realize an LSI having a low power consumption.
A high dielectric constant (i.e., high-k) metal oxide film made of an oxide of a metal such as hafnium is expected as a next-generation gate insulating film that replaces a silicon oxide film. For example, when a metal oxide film of hafnium (HfO
2
film), having a relative dielectric constant of about 20, is used as a gate insulating film, an HfO
2
film can have a capacitance equal to or greater than that of an SiO
2
film even if the SiO
2
-equivalent thickness of the HfD
2
film is 2 nm or less. Moreover, by using an HfO
2
film as a gate insulating film, it is possible to realize a transistor in which the leakage current is reduced by three orders of magnitude or more from that when an SiO
2
film is used.
An HfO
2
film is normally deposited by using a sputtering method, a CVD (chemical vapor deposition) method, or the like. An HfD
2
film immediately after deposition (i.e., an as-deposited HfO
2
film) is easily dissolved by a dilute hydrogen fluoride (DHF) solution. Therefore, a DHF solution can be used as an etching liquid (hereinafter referred to also as “chemical solution”) for wet-etching an HfO
2
film. However, when a deposited HfO
2
film is annealed, the solubility of the HfO
2
film to a DHF solution decreases (J. J. Chambers, et al., Effect of Composition and Post-Deposition Annealing on the Etch Rate of Hafnium and Zirconium Silicates in Dilute HF, The Electrochemical Society 200th Meeting, San Francisco, U.S.A., September 2001, abs. #1434). It is believed that this is due to the alteration of the surface of the HfO
2
film through the post-deposition annealing process.
However, in many cases where an HfO
2
film is to be used as a gate insulating film, one may desire to remove the HfO
2
film by wet-etching after the post-deposition annealing process.
In view of this, the present inventors examined how the thickness of an annealed HfO
2
film changes when immersed in various chemical solutions in search for chemical solutions with which an annealed HfO
2
film can be removed by wet-etching. The results are shown in
FIG. 1
, in which “Time” denotes the amount of time for which an HfO
2
film is immersed in a chemical solution, and “&Dgr;” denotes the change in the thickness of an HfO
2
film. Herein, “&Dgr;” being a negative value means that the thickness of the HfO
2
film increased. Chemical solution names “AF”, “UPS” and “PS etching liquid” denote “ammonium fluoride”, “hydrogen peroxide solution” and “polysilicon etching liquid”, respectively. The concentration of each chemical solution is shown in % by volume except for KOH. The concentration of “HF+NH
4
OH” being 1% means that the stock (undiluted) solution as shown in the remarks column was diluted to 1% by volume with pure water. The mixing ratio of “PS etching liquid” shown in the remarks column is a volume ratio. Finally, “peeling” in the remarks column means that an underlying layer of an HfO
2
film was lifted off.
As is apparent from
FIG. 1
, in addition to DHF solutions, an annealed HfO
2
film is not substantially dissolved in any of various chemicals commonly used in semiconductor device manufacturing processes such as hydrogen fluoride (HF) solutions of various concentrations. Thus, it is difficult to remove an annealed HfO
2
film by wet-etching with these various chemical solutions. This is believed to be for the following reason. Typically, a metal oxide film such as an HfO
2
film transitions from an amorphous state into a monoclinic crystal state by the post-deposition annealing process. Due to this transition, a passive film of HfO
2
is formed on the surface of the HfO
2
film, whereby it is difficult to remove the HfO
2
film by wet-etching.
It is known in the art that a passive film of HfO
2
can only be dissolved by a highly oxidative acid such as hot concentrated sulfuric acid. However, it is not practical to use such an acid in a semiconductor device manufacturing process, and it is therefore very difficult to remove an annealed HfO
2
film by wet-etching. Thus, it is expected that the complete removal of an HfO
2
film will be a significant problem in using an HfO
2
film in a transistor. Moreover, it is believed that oxide films of refractory metals, other than HfO
2
, such as zirconium (Zr), lanthanum (La), tantalum (Ta), aluminum (Al), etc., show a similar tendency.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above, and has an object to make it possible to reliably remove, by wet-etching, an insulative metal oxide film whose surface has been altered through an annealing process, or the like.
The present inventors have conducted a continuous process of trial and error aiming to achieve the object set forth above, and have found that an annealed HfO
2
film can be easily removed by wet-etching using a DHF solution, or the like, by exposing the annealed HfO
2
film to a plasma before wet-etching the annealed HfO
2
film. It is believed that the exposure of an HfO
2
film to a plasma gives a plasma damage to a surface portion of the HfO
2
film to a depth of about 1 to 3 nm, thereby forming a damaged layer, which is less resistant to wet-etching.
FIG. 2
shows the results of an examination obtained by the present inventors on the plasma treatment time (the amount of time for which a plasma treatment is performed before wet-etching) dependence of the amount of an annealed HfO
2
film that is wet-etched with a DHF solution. Herein, a mixed gas of an HBr gas, a Cl
2
gas and an O
2
gas, and a mixed gas of an HBr gas and a Cl
2
gas, were used as plasma gas species. As shown in
FIG. 2
, the use of either plasma gas species allows an HfO
2
film to be wet-etched. In view of this, the present inventors believe that the effect provided by the plasma treatment before wet-etching, i.e., the effect of forming a damaged layer in an annealed HfO
2
film, can be obtained irrespective of which plasma gas species is used. Note however that in a case where, for example, an HfO
2
film is used as a gate insulating film, and a damaged layer is formed in the HfO
2
film (a portion thereof that is located outside a gate electrode) successively after forming the gate electrode by dry-etching, it is preferred to use a plasma gas species with which side-etching of the gate electrode is prevented.
Specifically, a wet-etching method of the present invention, which has been made based on the findings set forth above, includes: a first step of annealing a substrate with a metal oxide film deposited thereon; a second step of exposing a surface of the annealed metal oxide film to a plasma; and a third step of removing, by wet-etching, at least a surface portion of the metal oxide film, which has been exposed to the plasma.
According to the wet-etching method of the present invention, the substrate with the metal oxide film deposited thereon is annealed, and then the surface of the metal oxide film is exposed to a plasma, after which at least a surface portion of the metal oxide film is removed by wet-etching. Specifically, the surface of the metal oxide film, which has been altered through the annealing process to be resistant to wet-etching, is exposed to a plasma. Thus, a damage layer that is less resistant to wet-etching is formed at least in a surface portion of the metal oxide film, Therefore, it is possible to reliably r
Hayashi Shigenori
Kubota Masafumi
Mitsuhashi Riichiro
Lebentritt Michael S.
Matsushita Electric - Industrial Co., Ltd.
McDermott & Will & Emery
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