Dry-Etching Method

1. Field of the Invention

The present invention generally relates to a dry-etching method for making electronic parts such as semiconductor devices or the like, and especially to a dry-etching method for silicon nitride (Si.sub.3 N.sub.4).

2. Description of the Prior Art

In the conventional dry-etching method for silicon materials, especially for Si.sub.3 N.sub.4 film on SiO.sub.2 film or Si.sub.3 N.sub.4 film masked by SiO.sub.2 film, a mixed gas of carbon fluoride such as CF.sub.4 which contains several percent of O.sub.2 has been used. By making a high-frequency electric discharge in such etching gas, F radical reacts to Si and changes it into a volatile material, and etching process proceeds. Namely, an electron produced by the electric discharge reacts to CF.sub.4 and changes it into an F radical, an F ion and an electron. The reaction proceeds by the following reaction as follows:

Further, CF.sub.3 reacts with an O radical and number of the F radical increases. The reaction proceeds by the following reaction: ##EQU1## And the F radical reacts with Si, so the etching process proceeds as follows:

The conventional dry-etching method, however, has a problem in that it is impossible to selectively etch only Si.sub.3 N.sub.4 film, which is intended to be selectively without hardly etching the SiO.sub.2 base film, the SiO.sub.2 masking film or the photo-resist (hereinafter abridged as P.R) Namely, the combination Si and O in SiO.sub.2 is comparatively stable and hardly separated by the impinging of F radical. However, it is considered that the combination is separated by the impinging O radical, and the separated and retained Si combines with the F radical, and so the etching process proceeds. The reaction proceeds as follows: ##EQU2## Therefore, in the use of mixed gas of carbon fluoride and O, the selection ratio of Si.sub.3 N.sub.4 /SiO.sub.2 which is obtained is only 2 or 3 (which is defined as ratio of etching rate of Si.sub.3 N.sub.4 /etching rate of SiO.sub.2).

As an alternative method, a case may be considered that the etching gas contains only carbon fluoride in the absence O. However, such case has another problem of C adhering onto the surface of Si.sub.3 N.sub.4 film with the resultant substantial ceasing of the etching process.

On the other hand, another method is considerable, such that a part of F of carbon fluoride is replaced by bromine to selectively etch the Si.sub.3 N.sub.4 film hardly etching the SiO.sub.2 film. It is known that the selection ratio of Si.sub.3 N.sub.4 /SiO.sub.2 increases by adding several percent of such bromic-replaced gas within the mixed gas of carbon fluoride and O.sub.2 as mentioned above. The Br atom excited by plasma electric discharge is easy to combine with the O atom as follows:

In other words, the Br atom has an effect to suppress the O atom which contributes to the etching process. And responding to the suppression of function of the O atom, the number of F radical decreases and the etching rate of Si.sub.3 N.sub.4 also decreases, and furthermore, the etching of SiO.sub.2 is suppressed. As a result, the selection ratio of Si.sub.3 N.sub.4 /SiO.sub.2 rises.

FIG. 1 shows the characteristic curves of the etching rate of Si.sub.3 N.sub.4 and the selection ratio of Si.sub.3 N.sub.4 /SiO.sub.2 as a function of the concentration of CF.sub.3 Br (volume percent, namely equal mol %) under the condition of mixing CF.sub.3 Br gas into (CF.sub.4 gas containing 5 Vol % of O.sub.2). In this experiment, the Si.sub.3 N.sub.4 film and SiO.sub.2 film accumulated on the surface of silicon substrate were etched by using the P.R pattern as a mask under the condition that the total quantity of gas flow was 50 SCCM (standard Cubic Centimeter per Minute, which means the quantity of flow in standard state of 273.degree. K. and 760 mmHg.), the pressure of the gas was 0.4 Torr, and electric power of high frequency was 150 W (0.5 w/cm.sup.2). As shown in FIG. 1, responding to the increase of the concentration of CF.sub.3 Br, the etching rate of Si.sub.3 N.sub.4 decreases, and moreover the etching of SiO.sub.2 is suppressed, so the selection ratio of Si.sub.3 N.sub.4 /SiO.sub.2 increases as shown by chain line curve in FIG. 1. And the characteristic curve of the concentration of CF.sub.3 Br has a peak at the position over twelve percent. In this case, the etching rate of Si.sub.3 N.sub.4 was 960 angstrom per minute and the selection ratio of Si.sub.3 N.sub.4 /SiO.sub.2 was 6.1 and Si.sub.3 N.sub.4 /P.R was 4.0. As mentioned above, such prior art is known that the mixing of CF.sub.3 Br gas into CF.sub.4 +5% O.sub.2 makes the selection ratio of Si.sub. 3 N.sub.4 /SiO.sub.2 increase to some extent. But there is a strong demand for still higher selection ratio, with increasing the etching rate of Si.sub.3 N.sub.4.

Accordingly, the purpose of the present invention is to provide an improved dry-etching method for etching material of the Si group, without hardly etching SiO.sub.2 mask or SiO.sub.2 base or P.R mask. The method is especially suitable for dry-etching silicon nitride (Si.sub.3 N.sub.4) with a high selection ratio against P.R or SiO.sub.2 film.

A dry-etching method for etching material such as silicon nitride in accordance with the present invention has steps of:

setting an etched material of silicon nitride into a reaction chamber,

supplying a mixed gas as an etching gas, which is made of carbon fluoride, oxygen and another gas having such a molecular structure that a part of hydrogen of hydrocarbon is replaced by halogen, into the reaction chamber, and

giving high frequency electric power so as to make the mixed gas as plasma state and to etch the etched material.

The material from the silicon group includes, for example, single crystalline silicon (used for making a semiconductor device), polycrystalline silicon, or silicon nitride.

FIG. 1 is the graph of characteristic curves of the etching rate of Si.sub.3 N.sub.4 and a selection ratio of Si.sub.3 N.sub.4 /SiO.sub.2 and Si.sub.3 N.sub.4 /P.R as a function of concentration of CF.sub.3 Br under the condition of mixing the CF.sub.3 Br gas into the CF.sub.4 +5% O.sub.2 gas (CF.sub.4 gas containing 5 Vol % of O.sub.2 gas).

FIG. 2 is a graph of characteristic curves in accordance with the present invention of an etching rate of Si.sub.3 N.sub.4 and a selection ratio of Si.sub.3 N.sub.4 SiO.sub.2 and Si.sub.3 N.sub.4 /P.R as a function of concentration of CH.sub.3 Br under the condition of mixing the CH.sub.3 Br gas into the CF.sub.4 +10% O.sub.2 gas (CF.sub.4 gas containing 10 Vol % of O.sub.2 gas).

FIG. 3 is a cross sectional view of an etching apparatus for use in the present invention.

The following is detailed description of the preferred embodiment of the present invention.

FIG. 2 shows a characteristic curves of an etching rate of Si.sub.3 N.sub.4 and a selection ratio of Si.sub.3 N.sub.4 /SiO.sub.2 and Si.sub.3 N.sub.4 /P.R as a function of concentration of CH.sub.3 Br under the condition of mixing CH.sub.3 Br gas into CF.sub.4 +10% O.sub.2 gas (CF.sub.4 gas containing 10 Vol % of O.sub.2 gas).

FIG. 3 is a cross sectional view of an etching apparatus which is used in the embodiment in accordance with the present invention. The etching apparatus comprises a reaction chamber 1 which is to be filled with an etching gas kept in 0.4 Torr, an exhaust opening 2, a gas cylinder 3 filled with the etching gas of high pressure, a flow control valve 4, a power supplying electrode of high frequency of electric power 5, a grounded electrode 6, and an electric power source 7 for supplying high frequency electric power for the power supplying electrode 5. A substrate 8 which is to be etched by the etching gas is disposed on the surface of the power supplying electrode 5. In the embodiment of the present invention, the substrate 8 is Si wafer which has an Si.sub.3 N.sub.4 film or SiO.sub.2 film on its surface. In this embodiment, etching process is made under the condition that the total quantity of gas flow is 50 SCCM, the pressure of gas is 0.4 Torr and electric power of high frequency is 150 W (0.5 w/cm.sup.2). As shown in FIG. 2, responding to the increase of the concentration of CH.sub.3 Br, the etching rate of SiO.sub.2 quickly decreases as the concentration of CH.sub.3 Br increases, and as shown by the chain line at the position over six percent of the concentration of CH.sub.3 Br, the etching rate of Si.sub.3 N.sub.4 is 1120 angstrom per minute and the selection ratio of Si.sub.3 N.sub.4 /SiO.sub.2 is 13.5. And selection ratio of Si.sub.3 N.sub.4 /P.R is 6.7.

As mentioned in the prior art, mixing of CF.sub.3 Br into the mixed gas of CF.sub.4 +5% O.sub.2, the selection ratio of Si.sub.3 N.sub.4 /SiO.sub.2 increases. It is because Br reacts with O radical and suppresses the action of O radical which separates the combination of Si and O of SiO.sub.2. In the present invention, using the mixed gas containing CH.sub.3 Br, provides not only the above-mentioned Br but also H separated from CH.sub.3 Br reacts as,

by which the reaction of O radical is suppressed, so that the etching rates of SiO.sub.2 and P.R rises and the selection ratio of Si.sub.3 N.sub.4 /SiO.sub.2 or Si.sub.3 N.sub.4 /P.R sharply increase.

Accordingly, by using the dry-etching method in accordance with the present invention, namely by using the mixed gas of carbon fluoride, O.sub.2 and the gas having such a molecular structure that a part of H of hydrocarbon is replaced by halogen, the material of Si group such as Si.sub.3 N.sub.4 can be etched with high selection ratio of such materials against SiO.sub.2 mask, SiO.sub.2 base or P.R mask as a result of such selective etching that the fluoride hardly etches the SiO.sub.2 mask or base. In this embodiment, Br is used as halogen, but I has the same effect as Br.