1. Field of the Invention
The present invention relates in general to an etching method, and particularly to a method of etching a mask layer and protecting layer for metal contacting window.
2. Description of the Related Art
With the technology of integrated circuits evolving, device sizes have reached sub-micron, half-micron, deep sub-micron and very deep sub-micron levels. Because of reduced device size, there can be more IC devices in one chip. It also means that chips have better performance. Therefore, etching technology plays a very important role. Desired circuit patterns being presented accurately depends on etching technology. High density plasma (HDP) etching technology has been widely applied in etching silicon dioxide layer, silicon nitride layer, polysilicon layer and metal layer.
The prior art utilizing high density plasma to etch a thin film is shown in FIGS. 1A-1C. In FIG. 1A, a silicon oxide layer 12 and a thin film, such as polysilicon layer 14, are formed on a silicon substrate 10 in sequence. The thin film is designed in a pattern on the surface for the following etching. Then, a photoresist layer 16 is coated on the polysilicon 14 and defines the pattern through photolithography, as shown in FIG. 1B. The polysilicon 14 undergoes high density plasma etching to form a pattern 18 using the photoresist layer with the defined patterns, as shown in FIG. 1C. After etching, the developed photoresist 16 is removed. High density plasma etching is dry etching. Vertical side-walls are obtained by this technology because a protecting film (not shown) is formed on the side-walls of polysilicon during dry etching, such that the etching is anisotropic. Forming the protecting film depends strongly on the reaction gases, the etched material and the photoresist. If any of the factors is incomplete, the etching is isotropic.
However, in order to obtain patterns with higher resolution, a thin photoresist layer is required. Etching selectivity between a thin photoresist layer and an etched material is not good enough, however, with the result that the surface of the etched photoresist is not flat, as shown in FIG. 1C. Therefore, hard mask use in etching has been developed, as shown in FIGS. 2A-2E. In FIG. 2A, a silicon dioxide layer 21 and a polysilicon layer 22 are formed on a silicon substrate 20 in sequence. In 2B, a silicon dioxide layer 23 (or a silicon nitride) is formed on the polysilicon 22. A photoresist layer 24 is spin-coated over the silicon dioxide layer 23. After coating, photolithography is performed and the resulting structure is shown in FIG. 2C. Then, the silicon dioxide layer 23 is etched to form a hard mask 25 using the pattern defined photoresist 24 to be the mask. The photoresist 24 is removed, as shown in FIG. 2D. Finally, high density plasma etching is performed on the polysilicon layer 22 to form patterns 26 using the hard mask 25. However, because the protecting film cannot be formed, this etching results in an undercutting, as shown in FIG. 2E. The desired circuit can't be presented exactly.