Utilizing Saturated and Unsaturated Halocarbon Gases in Plasma Etching to Increase Etch of SiO.sub.2 Relative to Si

This invention relates to plasma etching and more particularly to plasma etching with a gaseous mixture containing an unsaturated halocarbon and a saturated halocarbon.

It is a primary object of this invention to provide an improved gas system for plasma etching.

It is a further object to provide a gas system for plasma etching that etches SiO.sub.2 at a faster rate than Si.

It is another object of this invention to provide a hydrogen free gas system for plasma etching.

It is still another object of this invention to provide a two gas system for plasma etching.

It is yet still another object of this invention to provide a gas system suitable for easy adjustment of the halogen/carbon ratio.

Plasma etching has been recently evaluated for semiconductor processing since it promises a number of advantages over wet chemical methods in terms of terms of improved yield and resolution at lower cost. Specifically, plasma etching is used to form a window through silica (SiO.sub.2) films on silcon (Si) substrates. In this type of etching it is desirable for the etchant to have a high etch rate in SiO.sub.2 relative to the etch rate of Si substrates. Most etchants used in plasma etching, however, etch SiO.sub.2 at a rate that is not substantially different from the etch rate of Si.

One approach to increasing the relative etch rate of SiO.sub.2 to Si was described in the patent to Heinecke U.S. Pat. No. 3,940,506. Heinecke modified the gas supplied to the plasma so as to reduce the concentration of fluorine atoms. One way Heinecke achieved this reduced concentration of fluorine atoms was to add hydrogen to the supply of carbon tetrafluoride. Another approach was to add hydrogen in the form of a chemical compound such as ammonia. Heinecke stated that the presence of a hydrogen reduced the etch rate of Si much more than the etch rate of SiO.sub.2.

In Solid State Electronics, 1975, Vol. 18, pp 1146-1147 (printed in Great Britain) Heinecke describes the use of plasma etching with C.sub.3 F.sub.8 as the etchant in order to increase the relative etch rate of SiO.sub.2 /Si. In Solid State Electronics, 1976, Vol. 19, pp 1039-1040 (printed in Great Britain) Heincke describes the use of a single gas such as C.sub.4 F.sub.8, C.sub.5 F.sub.12, C.sub.2 F.sub.6 and CHF.sub.3 as a single gas etchant to be used in plasma etching. In the Japan J. Appl. Phys. Vol. 16 (1977), No. 1, Matsuo et al describes the use of a mixture of CF.sub.4 and C.sub.2 H.sub.4 for plasma etching. Matsuo reported that this gas mixture etches SiO.sub.2 faster than Si.

The FIGURE is a figure illustrating the relative etch rates of SiO.sub.2 and Si in a C.sub.3 F.sub.6 --C.sub.2 F.sub.6 mixture.

For a further understanding of the invention, and the objects and advantages thereof, reference will be had to the following description and the accompanying drawings, and to the appended claims in which the various novel features of the invention are more particularly set forth.

Accordingly, the invention includes the use of etching with a plasma from a gaseous mixture containing an unsaturated halocarbon and a saturated halocarbon. The preferred halogen is fluorine. A preferred embodiment is a gas mixture containing C.sub.3 F.sub.6 and C.sub.2 F.sub.6 gas.

We first describe a description of the processes embodying the invention in preferred forms.

An apparatus suitable for performing the etching process consists essentially of four items; a bell jar system, a gas train for leaking gases or mixtures of gases into the bell jar, an RF generator for sustaining a plasma within the bell jar, and a vacuum pump. Any conventional plasma generating system may be employed and the electrode may be either within the system or a coil surrounding the outside of the system. The excitation power is delivered by an r.f. generator that capacitively applies the power to the plasma. A power of 50 to 150 watts, that is between 1/2 and 11/2 watts per square centimeter is an example thereof. The frequency of the applied voltages is of the order of 13.56 MHz. Direct current may also be used. Both the power and the frequency can be varied over broad ranges as is well known to those skilled in the art.

In accordance with this invention, the plasma etching is effected by passing a mixture of gases through the etching chamber. The mixture of gases include an unsaturated halocarbon and a saturated halocarbon. While both the saturated and unsaturated halocarbon may contain fluorine, bromine and/or iodine, the preferred gaseous components contain fluorine or mixtures of fluorine with another halide. In other words, the unsaturated halocarbon may contain only one halogen or it may contain a mixture of two or more halogens, for example, C.sub.3 F.sub.6, C.sub.3 F.sub.4 Cl.sub.2, C.sub.3 F.sub.2 Cl.sub.2 Br.sub.2, and so forth. Similarly, the saturated halocarbon may contain only one halogen or a mixture of two or more halogens, for example, CF.sub.4, CF.sub.3 Cl, CF.sub.2 ClBr, and so forth. Specific examples of preferred unsaturated halocarbons are C.sub.3 F.sub.6 and C.sub.2 F.sub.4. Examples of preferred saturated halocarbons are CF.sub.4 and C.sub.2 F.sub.6.

Preferably the gaseous mixture contains a single unsaturated halocarbon and a single saturated halocarbon. While it is possible to use more than the two gases set forth above, the halogen/carbon ratio is easier to adjust in a two component system, however, more than two gases may be used if desired. The preferred halogen/carbon ratio needs to be determined experimentally for each particular system in order to find the preferred range since it may depend on the apparatus used. In the applicants' apparatus with unsaturated halocarbons containing fluorine, the preferred halogen/carbon ratio is between 2 and 3.

In a plasma etching system in which the total pressure was 20 millitorr, the unsaturated gas C.sub.3 F.sub.6 was mixed with the saturated gas C.sub.2 F.sub.6 to provide a flow rate of about 2.6 cm.sup.3 /min. The generator delivered 13.5 MHz rf power which resulted in a 100 watts being applied. The excitation electrode area was 185 cm.sup.2. The data set forth below show a comparison between the etch rate of SiO.sub.2 and Si in A/minute as the percentage of the unsaturated component C.sub.3 F.sub.6 is increased in the gaseous mixture. The etch rate with 0% C.sub.3 F.sub.6 was 7.3 for Si and 10.0 for SiO.sub.2. At a concentration of 5.9% C.sub.3 F.sub.6 the etch rate of Si had gone down to 2.0 whereas the etch rate of SiO.sub.2 was 9.3. For concentrations of 8.4 to 13.0, the Si had a negative etch rate, that is, a polymer formation occurred on the wafer. The data are illustrated in FIG. 1 which clearly show the faster etch rate of SiO.sub.2 over Si for the concentration range set forth.

A number of different combinations of unsaturated halocarbon and saturated halocarbon may be used in the etchings of Si wafers.

A mixture of C.sub.3 F.sub.6 and CF.sub.4 was made under a total pressure of 20 millitorr. The gas flow rate was 1.6 cm.sup.3 /min. A power of 50 watts was applied at 13.5 MHz. The SiO.sub.2 excitation electrode area was 185 cm.sup.2. The following tabulation set forth the data obtained.

In this system a gaseous mixture containing 20.9% C.sub.3 F.sub.6 had an SiO.sub.2 etch rate of 8.5 compared to a Si etch rate of 2.1 at this concentration.

A mixture of C.sub.2 F.sub.4 and CF.sub.4 having a total gas flow rate of 1.3 cm.sup.3 /min was used. The total pressure in the vacuum system was about 20 millitorr. A 100 watts of power was applied at 13.56 MHz. The SiO.sub.2 excitation electrode area was 100 cm.sup.2. The following tabulation sets forth the data obtained. The etch rate of SiO.sub.2 was 21.5 and the silcon etch rate of 0.5 occurred when 45.8% C.sub.2 F.sub.4 was used in the mixture.

While I have illustrated and described the preferred embodiment of my invention, it is understood that I do not limit myself to the precise compositions herein and the right is reserved to allow changes and modifications coming within the scope of the invention as defined in the appended claims.