Dry etching method for selectively etching silicon nitride existing on silicon dioxide
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention is in the field of selective etching of silicon nitrides overlying a substrate of SiO 2  and involves the use of an etchant gas including a fluorohydrocarbon and a substantial amount of CO 2 . 
     2. Description of the Prior Art 
     In conventional dry etching methods for etching silicon nitride films (usually designated Si 3  N 4  films) formed on an SiO 2  film in the process of fabricating semiconductor devices, the etchant gas is usually a mixture of tetrafluoro methane (CF 4 ) and 0 2 , sometimes containing an additional diluent gas such as argon. The apparatus for industrial practice of this dry etching method includes tunnel-type plasma etchers, parallel plate plasma etchers of the anode coupling type and chemical dry etching (CDE) of the separate discharge chamber type. When operating such dry etching apparatus using a conventional etchant, it is possible to etch the Si 3  N 4  film at a selective ratio (about 5:1) between the Si 3  N 4  and the underlying Si0 2  film. However, in each case the etching is accomplished in the manner of isotropic etching because the reaction mechanisms inherent in the etching apparatus and particularly the mechanisms of reactions which occur when radicals formed by dissociation of CF 4  attack the Si 3  N 4  and SiO 2  films. As a result, side etching occurs beneath the mask. 
     With the recent trend toward further miniaturization of semiconductor devices, it has become difficult to satisfy fully the requirements of the Si 3  N 4  films by conventional isotropic etching. It will soon become indispensable to accomplish anisotropic etching in exact conformity with the pattern dimensions of the mask by employing reactive ion etching (RIE). When etching a Si 3  N 4  film laid on a SiO 2  film by a conventional RIE technique, there is no alternative to using an etchant gas which also etches Si0 2 . Therefore, it is difficult to accomplish good selectivity of the etcl hing rate between the Si 3  N 4  film and the underlying SiO 2  film. Trifluoro methane (CHF 3 ) is well known as an etchant gas for reactive ion etching of SiO 2 . When, for example, O 2  is added to CHF 3  at a CHF 3  /O 2  ratio of 40/7 on a flow rate (sccm) basis to etch either Si 3  N 4  or SiO 2  at a pressure of 0.06 Torr and a radio-frequency power of 400 W (0.20 W/cm 2 ), the etching rate is 980 Å for Si 3  N 4  and 510 Å for SiO 2 . Thus, the Si 3  N 4  /SiO 2  selective ratio is only 1.9 though anisotropic etching is accomplished without significant undercutting. Since the controllability must also be taken into consideration, this method is deemed to be impractical for a process in which good selectivity of etching between a silicon nitride film and an underlying silicon dioxide layer is required. For example, the same method is not useful for etching Si 3  N 4  which exists as a selective oxidation mask on a thin pad of SiO 2 . 
     Recently, an etching gas consisting of difluoro methane (CH 2  F 2 ) was reported in the International Electric Device Meeting, 1983, under the title &#34;VLSI Device Fabrication Using a Unique, Highly-Selective Si 3  N 4  Dry Etching&#34;. This etchant attracted interest as a promising solution to the above described problem. However, some problems still remain unsolved as to the practical use of this etchant gas. For example, when this etchant gas is used under conditions of high selectivity, it often results in the production of a polymer film which is difficult to remove from the surface after etching. A considerable etching residue thus may adhere to the substrate surface after etching. It is probable that such phenomena are attributable to the fact that CH 2  F 2  has a lower C/F ratio than CF 4  and CHF 3  which are conventionally used for etching Si 3  N 4 , and that the etchant gas contains H 2  within the molecule. Because of the nature of CH 2  F 2 , a considerably carbon-rich condition is produced in the plasma of etchant gas so that polymers of fluorocarbons are likely to be deposited on the surface after etching. A probable cause of the existence of etching residue on the substrate surface is an accumulation of such polymers. In addition, accumulation of the polymers in the chamber of the etching device is considerable. Due to these considerations, it is practically impossible to perform stable etching operations with good reproducibility using this type of etchant. 
     With respect to the commonly used fluorine-containing etchant gases such as CF 4  and CHF 3 , it has been suggested to add a small amount of O 2  CO 2 , viz. about 5% in most cases and up to about 10% at the maximum, for the purpose of suppressing formation of fluorocarbon polymers. This technique is based on the thought that oxygen radicals formed in the plasma of the mixed gas were removed carbon by converting it into CO and/or CO 2  with the effect of increasing the F/C ratio in the plasma and thereby preventing deposition of the polymers. Additionally, the etch rate of Si 3  N 4  becomes higher when the etching gas contains a small amount of O 2  or CO 2 . However, when using such a mixed gas for etching of Si 3  N 4  on top of SiO 2 , the etch rate of the underlying SiO 2  also increases because of the suppression of the formation of the polymers which are effective to prevent etching of SiO 2 . Consequently, the selective ratio of etching between Si 3  N 4  and SiO 2  becomes very much lower than the desired or tolerable level. This deficiency is not fundamentally removed even when CO 2  is added to the etching gas instead of O 2   
     SUMMARY OF THE INVENTION 
     The present invention provides a dry etching method by means of which silicon nitride Si x  N y  existing on a substrate of SiO 2  can be etched at a sufficiently high selective ratio between Si x  N y  and SiO 2  without providing for deposition of an obstructive polymer film or any other phenomenon which provides difficulty for practical etching operations. 
     The present invention provides a dry etching method for selectively etching a silicon nitride existing on SiO 2  with an etchant gas containing a fluorohydrocarbon which contains at least one carbon atom and at least one fluorine atom, and in which the atomic ratio of F/C is less than 3:1. This fluorohydrocarbon or mixtures of such fluorohydrocarbons is admixed with CO 2  in an amount of 30-70% of the mixture on a flow rate basis. 
     The preferred examples of etchants according to the present invention are CH 2  F 2  and CH 3  F. 
     The basic feature of the invention is to add a considerable amount of CO 2  to a fluorohydrocarbon having a low F/C ratio. In a mixed etchant gas according to the present invention, the amount of CO 2  is made large enough to remove the fluorine radical F* formed in the plasma of the etchant gas by converting it into COF, thereby suppressing recombination of F* and resultant formation of CF 3   + . Since CF 3   +  acts as a strong etchant for SiO 2 , the suppression of its formation leads to a lowering of the etch rate of SiO 2 . In contrast, etching of Si 3  N 4  (a usual example of a silicon nitride Si x  N y ) is not significantly influenced by the disappearance of CF 3   +  since Si 3  N 4  is efficiently etched by other ions and radicals. Therefore, the Si 3  N 4  /SiO 2  selective ratio upon etching becomes high enough so that the intended selective etching can be realized. Furthermore, the etching method according to the present invention does not encounter the difficulties or problems of deposition or accumulation of polymers or fluorocarbons when CH 2  F 2  alone is used as the etchant gas. Accordingly, the etching method is fully practical in the fabrication of semiconductor devices and the like. 
     The addition of a large amount of CO 2  to a fluorohydrocarbon gas having a low F/C atomic ratio is based on an entirely different concept than the addition of CO 2  or O 2  to a conventional etching gas which has a higher F/C ratio. In the prior art, the principal purpose of the addition of O 2  or CO 2  was to capture carbon in the plasma of the etchant gas. For example, in the case of a mixture of CF 4  and about 5% O 2  used for etching either monosilicon or polysilicon, the primary effect of the added O 2  is to prevent the roughening of the silicon surface which results from a masking influence of carbon which falls and lies on the silicon surface if not captured. In addition, the etching rate of silicon increases since the F/C ratio increases as carbon is captured by oxygen. In the case of RIE of SiO 2  with CHF 3 , a small amount of CO 2  (less than 10%) is added instead of O 2 . Although O 2  is more effective for the prevention of formation of the obstructive polymers, the addition of O 2  tends to lower the selective ratio of etching between SiO 2  and the substrate material, Si, because too much carbon is captured by oxygen. For this reason, a small amount of the less effective CO 2  is added to CHF 3 . 
     In an etchant gas according to the present invention, a large amount of CO 2  is included for the purpose of preventing recombination of the fluorine radical in the plasma to thereby prevent formation of an etchant for SiO 2 . There is thus a clear difference in scientific approach between this concept and the presence of a small amount of CO 2  in conventional etchant gases. Furthermore, even if a large amount of CO 2  is added to an etchant gas high in F/C ratio, such as CF 2  or CHF 3 , it is not possible to secure the beneficial effects of the present invention. When using a fluorohydrocarbon having a low F/C ratio, such as CH 2  F 2  or Ch 3  F, the CF 3   +  which acts as a strong etchant for SiO 2  is formed exclusively by recombination of radicals in the plasma. Accordingly, the capture of fluorine radicals is very effective for reducing the etching rate of SiO 2 . However, in the case of CF 4  or CHF 3  the capture of free F* by CO* originating from CO 2 , or resultant suppression of the recombination reaction to form CF 3   + ,has little influence on the etch rate of SiO 2  since most of the dissociated CF 4  or CHF 3  turns into CF 3   + . The beneficial effect of the addition of a large amount of CO 2  on the Si 3  N 4  /SiO 2  selective ratio can be obtained only when the principal component of the etchant gas is a compound having a sufficiently low F/C atomic ratio. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A further description of the present invention will be made in conjunction with the attached sheets of drawings in which: 
     FIG. 1 is a graph showing the relationship of etch rate of Si 3  N 4  and SiO 2  and the flow rate of CH 2  F 2  used as the etchant gas for reactive ion etching; and 
     FIG. 2 is a graph showing the changes in etch rates of Si 3  N 4  and SiO 2  and in the Si 3  N 4  /SiO 2  selective ratio in relation to the amount of CO 2  in an etchant gas consisting of CH 2  F 2  and CO 2 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the present invention, CH 2  F 2  is the preferred etchant gas material. By using this gas under appropriate conditions it is possible to attain a sufficiently high etch rate for Si 3  N 4  while the etch rate for SiO 2  is very low. FIG. 1 shows variations in etch rates of Si 3  N 4  and SiO 2  when etched in a parallel plate type RIE apparatus using CH 2  F 2  alone as the etchant gas at various flow rates. The etching apparatus was operated at a pressure of 5 Pa, an rf power of 350 W and an rf power density of 0.28 W/cm 2 . Quartz was used as the lower electrode cover. As shown by curve (I), the etch rate of SiO 2  becomes sharply lower as the flow rate of CH 2  F 2  gas is increased. As shown by curve (II), the etch rate of Si 3  N 4  is very high when the flow rate of CH 2  F 2  gas increases up to about 15 sccm so that the Si.sub. 3 N 4  /SiO 2  selective ratio, curve (III), reached about 30 which is a very high value. However, when CH 2  F 2  gas is used alone there arise the aforementioned problems attributed to the formation of obstructive polymers. 
     FIG. 2 shows the variation in etch rate of Si 3  N 4  and SiO 2  when a variable amount of CO 2  was added to the CH 2  F 2  gas. In this case, the aforementioned RIE apparatus was operated at a pressure of 5 Pa, an rf power of 300 W and an rf power of power density of 0.24 W/cm 2 . The flow rate of the etchant gas containing a variable amount of CO 2  was a constant 10 sccm. As shown by curve (I), the etch rate of SiO 2  gradually decreased as the amount of CO 2  in the etching gas was increased, although an increase in etch rate was observed when the amount of CO 2  was increasing up to about 10-20%. The etch rate of Si 3  N 4 , represented by curve (II), was fairly high and was not greatly affected by the amount of CO 2  in the etchant gas. When the CO 2  amounted to 70% of the etchant gas, the etch rate of Si 3  N 4  was about 600 Å/min and the etch rate of SiO 2  was as low as 80 Å/min, so that the Si 3  N 4  /SiO 2  selective ratio represented by curve (III) reached a highly acceptable value of about 7. When the amount of CO 2  in the etching gas was 30-70%, neither accumulation of obstructive polymers nor existence of etching residue was detected. 
     In the plasma of the etchant gas, the recombination reaction to form a strong etchant for SiO 2  is represented by the equation: 
     
         CF.sub.2.sup.+ +F*→CF.sub.3.sup.+ 
    
     WHen the etchant gas is a mixture of 30-70% of CO 2  and the balance CH 2  F 2 , such a recombination reaction is effectively suppressed by the following reactions. First, a sufficiently large amount of CO* is formed by dissociation of CO 2 . 
     
         CO.sub.2 →CO*+O* 
    
     At the same time, F* is formed by dissociation reactions of CH 2  F 2  such as: 
     CH 2  F 2  →CH 2  F+F* 
     However, the F* is soon consumed in the following reaction. 
     
         CO*+F*→COF 
    
     Thus, there is always a lack of F* in the plasma, and the aforementioned recombination reaction does not become significant. 
     While etching of SiO 2  is suppressed by the above phenomena, Si 3  N 4  undergoes efficient etching with radicals and ions other than CF 3   +  since the binding energy of the Si-N bond, 50 kcal/mole, is considerably lower than the binding energy of the Si-O bond, 80 kcal/mole. These reasons probably explain the success in the good selective etching of Si 3  N 4  on SiO 2  by using an etching gas which has a low F/C ratio and contains a large amount of CO 2 . 
     In FIG. 2, it will be seen that the etch rate of SiO 2  increases when the amount of CO 2  is increasing up to 10-20%. Presumably this is because in this region of CO 2  content, the carbon capturing effect of O* formed by dissociation of CO 2  overcomes the F* capturing effect of CO*. 
     In FIG. 2, curve (IV) represents the degree of uniformity of the wafer on which the selective etching was performed. Accordingly, curve (IV) represents the degree of uniformity of etching of the Si 3  N 4  film. 
     In the region P indicated in FIG. 2, where the amount of CO 2  in the etchant gas is less than 20%, deposition of obstructive polymers was observed in the experiment. Therefore, this region is unsuitable for practical operations at least under the etching conditions employed in the experiment. 
     The curve (V) in FIG. 2 represents the etch rate of a conventional resist, OFPR 800. From curve (V) it will be understood that a tapered etching of Si 3  N 4  can be accomplished while maintaining a good selective ratio between the Si 3  N 4  and the underlying SiO 2  by first forming a tapered resist film to thereby utilize the low Si 3  N 4  /OFPR selective ratio. The increase in etch rate of the resist represented by curve (V) is caused by the action of 0* formed by dissocation of CO 2 . 
     It will be understood that various modifications can be made to the described embodiments without departing from the scope of the present invention.