1. Field of the Invention
This invention relates to a dry etching method adapted for fine processing of manufacturing a semiconductor device, and particularly to a method for preventing regression of a resist mask formed on an SiON based antireflection film so as to improve anisotropy.
2. Description of Related Art
In order to realize large scale integration of semiconductor devices, the minimum processing size of the circuit pattern formation has been rapidly diminished. For instance, the minimum processing size of the 16M DRAM of approximately 0.5 .mu.m (half micron), the minimum processing size of the 64M DRAM of 0.35 .mu.m (sub-half micron), and the minimum processing size of the 256M DRAM of 0.25 .mu.m (quarter micron) are required.
This increasingly fine processing depends largely upon a technique of photolithography to form a mask pattern. Visible to near ultraviolet rays, such as g rays having a wavelength of 436 nm or i rays having a wavelength of 365 nm, of a high pressure mercury lamp are used for the current 0.5-.mu.m class processing, and far ultraviolet rays, such as KrF excimer laser lights having a wavelength of 248 nm, are used for 0.35 to 0.25-.mu.m class processing. In the photolithography technique for forming a fine mask with a ray width of not greater than 0.4 .mu.m, an antireflection film to weaken a reflected light from an underlying material layer is substantially required for preventing reduction in contrast and resolution due to halation and standing wave effect.
As the component material of the antireflection film, amorphous silicon, TiN and TiON are conventionally used. However, since it has been shown that SiON (silicon oxide nitride) exhibits satisfactory optical properties in the far ultraviolet region, application of SiON to the excimer laser lithography is proposed. It is exemplified by a process of fine gate processing with an SiON film restraining the reflectivity of a W (tungsten)--polycide film or an Al (aluminum) based material film.
Meanwhile, after the patterning of the resist mask by such photolithography is finished, the antireflection film is etched in the subsequent etching process.
In this case, such a problem is now being apparent that the anisotropic shape of the underlying material layer may be deteriorated by oxygen discharged from SiON in the etching process, particularly in overetching. This problem is explained with reference to FIGS. 1 to 4.
FIG. 1 shows a state of a wafer prior to the etching, in which a gate SiO.sub.x film 22, a W-polycide film 25 and an SiON antireflection film 26 are sequentially stacked on an Si substrate 21, with a resist mask 27 patterned in a predetermined shape being formed thereon. The W-polycide film 25 is composed of, from the bottom, a polysilicon layer 23 containing impurities and a WSi.sub.x (tungsten silicide) layer 24 which are sequentially stacked.
If the W-polycide film 25 is etched using a Cl.sub.2 /O.sub.2 mixed gas, the etching is promoted by a formation of etching reaction products, such as SiCl.sub.x and WClO.sub.x. On the other hand, a carbon based polymer derived from decomposition products provided by forward sputtering of the resist mask is deposited to form a sidewall protection film 28 on the sidewall surface of the pattern. If the wafer temperature is sufficiently low, SiCl.sub.x of relatively low vapor pressure among the etching decomposition products can be a component of the sidewall protection film 28.
As a result, a gate electrode 25a of anisotropic shape is formed at the end of just etching, as shown in FIG. 2. In FIG. 2, materials after the etching are denoted by their respective original numerals plus subscripts "a".
However, if the overetching follows the just etching, regression of the edge of the resist mask 27 causes the SiON antireflection film 26a to have its end surface tapered to be easily exposed, as shown in FIG. 3. SiON, having an element composition ratio of approximately Si:O:N=2:1:1, is richer in Si than SiO.sub.2 is. Consequently, SiON has low durability to a Cl based plasma, and easily discharges active O* when its exposed end surface is etched. Then, O* removes the sidewall protection film 28 in the form of CO.sub.x, to lower sidewall protection effects. In addition, since the W-polycide film 25 to be etched is reduced in the overetching, a relatively excessive amount of O* is present in the etching gas.
As a result, a gate electrode 25b having an undercut is formed, as shown in FIG. 4. The material layers having the undercut denoted by their respective original numerals plus subscripts "b". The undercut is generated most conspicuously in the WSi.sub.x layer 24b. Since O* sputtered out from the end surface of the SiON antireflection film removes W atoms in the form of WClO.sub.x, the etchrate in the WSi.sub.x 24a is increased.
As the anisotropic shape of the gate electrode is thus deteriorated, serious problems rise, such as, the metallization resistance falling off the designed value and difficulty in forming the sidewall to attain an LDD structure.
The deterioration in the anisotropic shape in the overetching is not limited to the above-described SiON antireflection film, but is a phenomenon which may be generated in cases where an antireflection film capable of easily discharging oxygen is used and where conductive material layers of Al based metallization and the like other than the W-polycide film are used as etching targets.