BACKGROUND OF THE INVENTION

[0001] The present invention relates to a method for fabricating a semiconductor device that can prevent resist collapse from being caused by resist patterning in a dry etching process.

[0002] A known method for fabricating a semiconductor device will be described with reference to the drawings.

[0003] FIGS. 3A through 3D show the cross-sectional structures of a semiconductor device in the order corresponding to process steps of the known method for fabricating the same.

[0004] First, as shown in FIG. 3A, a thermal oxide film 102 is formed in the upper part of a semiconductor substrate 101 made of silicon. Subsequently, the top of the formed thermal oxide film 102 is coated with a resist film. Thereafter, the resist film is patterned using a lithography method, thereby forming a resist pattern 103 .

[0005] Next, as shown in FIG. 3 B, dry etching is performed on the thermal oxide film 102 by using the resist pattern 103 as a mask. For example, when capacitively coupled plasma etching equipment is employed, etching conditions are as follows: tetrafluorocarbon (CF ₄ ) is supplied at a flow rate of 50 ml/min; trifluoromethyl (CHF ₃ ) is supplied at a flow rate of 30 ml/min; oxygen (O ₂ ) is supplied at a flow rate of 5 ml/min; the gas pressure is 5 Pa; the upper discharge power is 1000 W; and the lower discharge power is 1500 W. Here, the flow rate of each gas is the one in a standard state, i.e., under 0° C. and 1 atm.

[0006] In recent years, the processing precision of semiconductor devices has become finer, and thus smaller pattern sizes have also been required for resist patterns 103 used as masks for patterning a film to be etched. Therefore, the physical strength of the resist patterns 103 has become smaller (see, for example, International Publication Number WO98/32162 pamphlet).

[0007] In addition, even when a semiconductor device becomes finer, the thickness of a film to be etched hardly changes. This disables the thickness of a resist pattern 103 to become smaller, because it is necessary that the selectivity to the resist at dry etching is ensured. Thus, the value of the aspect ratio (the height to the line width) of the resist pattern 103 at pattern formation has been larger.

[0008] On the other hand, a dry etching process allows the resist pattern 103 to be etched not only in the vertical direction but also in the parallel direction with respect to the principal surface of the semiconductor substrate 101 . This results in the line width of the resist pattern 103 becoming smaller during etching. Furthermore, an influence of heat and ultraviolet radiation coming from plasma used for dry etching causes stresses associated with heat stresses and degradation in the resist pattern 103 . Consequently, as the processing precision becomes finer, the upper part of the resist pattern 103 collapses from the insufficient strength of the resist pattern 103 as shown in FIG. 3B, i .e., a so-called resist collapse 103 a occurs. The resist pattern 103 in which the resist collapse 103 a occurs serves as an etching mask as it is, so that a part of the thermal oxide film 102 below the resist collapse 103 a is prevented from being etched. Consequently, a pattern abnormality 102 a is formed in the thermal oxide film 102 as shown in FIG. 3C .

[0009] Therefore, as shown in FIG. 3 D, even when the resist pattern 103 is removed by performing ashing and cleaning processes, the pattern abnormality 102 a remains in the thermal oxide film 102 as it is.

[0010] As described above, the known method for fabricating a semiconductor device has the problem that a resist collapse 103 a occurs in a resist pattern 103 at the etching of a film to be processed.

SUMMARY OF THE INVENTION

[0011] The present invention is made to solve the above problem, and therefore an object of the present invention is to realize a fine pattern without causing resist collapse.

[0012] After various studies, the present inventors found that a patterned resist pattern exposed to a gas containing sulfur increases the strength of each of the sidewalls of the resist pattern.

[0013] To be specific, a resist pattern made of a commonly used organic material such as novolac and containing carbon was exposed to a gas made of, for example, sulfur dioxide, and the resultant resist pattern was measured by Auger Electron Spectroscopy (AES). Consequently, sulfur atoms were identified in each of the sidewalls of the resist pattern. Furthermore, it has been shown by X-ray Photoelectron Spectroscopy (XPS) that bonds between carbon (C) and sulfur (S) (hereinafter, referred to as C—S bonds) exist in each of the sidewalls. A compound containing C—S bonds has a relatively low vapor pressure, and therefore it remains in each of the sidewalls of the resist pattern without being eliminated therefrom. In addition, the energy of the C—S bond is 175 kcal/mol, which is larger than the value of the energy of the bond between carbon and carbon (C—C bond), that is, 144 kcal/mol, so as to increase the strength of each of the sidewalls of the resist pattern. As a result, the resist collapse can be prevented.

[0014] More specifically, a method for fabricating a semiconductor device according to the present invention comprises the steps of: forming a thin film made of an inorganic material; forming a resist film containing carbon on the thin film and thereafter patterning the formed resist film to form a resist pattern from the resist film; exposing the resist pattern to a gas containing sulfur; and performing dry etching of the thin film using as a mask the resist pattern exposed to the gas containing sulfur.

[0015] According to the method for fabricating a semiconductor device of the present invention, a compound containing C—S bonds is generated on each of the sidewalls of the resist pattern as described above. Therefore, the strength of each of the sidewalls of the resist pattern is increased. As a result, a resist collapse can be prevented from being caused in a fine resist pattern, thereby obtaining a desired fine pattern of the thin film made of an inorganic material.

[0016] A sulfur dioxide gas described in International Publication Number WO98/32162 pamphlet is employed for etching a thin film made of an organic material, and therefore this is different from the case of the present invention where the target to be etched is made of an inorganic material.

[0017] In the method for fabricating a semiconductor device of the present invention, it is preferable that the inorganic material contains silicon and an etching gas employed for the dry etching is a fluorocarbon gas.

[0018] In the method for fabricating a semiconductor device of the present invention, the gas containing sulfur is preferably sulfur dioxide.

[0019] In the method for fabricating a semiconductor device of the present invention, the gas containing sulfur is preferably in a plasma state.

[0020] In the method for fabricating a semiconductor device of the present invention, the step of exposing the resist pattern to the gas containing sulfur and the step of performing dry etching preferably constitute the same step. This eliminates the need for providing a process step for only exposing the resist pattern to the gas containing sulfur, thereby improving the throughput of the fabricating process.

[0021] In the method for fabricating a semiconductor device of the present invention, the line width of the resist pattern is preferably 200 nm or less.

[0022] In the method for fabricating a semiconductor device of the present invention, the value of the ratio of the height of the resist pattern to the line width thereof is preferably 2.8 or more.

[0023] In this way, when the resist pattern is fine and has a high aspect ratio, the effects of the present invention become more noticeable.