Dry etching method

Abstract: A dry etching method whereby an SiO₂ layer and an Si₃N₄ layer may be etched with high selectivity for each other. As etching gas, such sulfur fluorides as S₂F₂ are used which, when dissociated by electric discharges, will form SF x ⁺ as a main etchant for the SiO₂ layer or F* as a main etchant for the Si₃N₄ layer and release sulfur in plasma. When the SiO₂ layer is etched on the Si₃N₄ layer as an underlying layer via a resist mask, nitrogen atoms, removed from the underlying layer upon exposure thereof to plasma, will combine with sulfur in plasma to form on the exposed surface thereof such sulfur nitride compounds as polythiazyl (SN) x , which will, in turn, serve to achieve high selectivity for the underlying layer. The SiO₂ layer can also be etched via an Si₃N₄ mask patterned into a predetermined shape, in which case sulfur nitride compounds formed on the Si₃N₄ mask will serve to achieve high selectivity therefor.

Description: The present invention relates to a plasma etching method employed in such applications as production of semiconductor devices. More particularly, it relates to a plasma etching method whereby a silicon oxide layer and a silicon nitride layer may be etched with high selectivity for each other. The recent trend toward higher integration and performance of such semiconductor devices as VLSIs and ULSIs requires dry etching technologies for insulation films to achieve correspondingly higher anisotropy, higher etchrate, higher selectivity, lower pollution, and less damage with no compromise in these requirements. Conventionally, etching gases typified by CHF 3 gas, CF 4 / H 2 mixed gas, CF 4 / O 2 mixed gas, and C 2 F 6 / CHF 3 mixed gas have been widely used to etch an insulation film composed of silicon oxide (SiO x ; particularly, x = 2). All these etching gases are composed mainly of fluorocarbon based gas whose molecule has a C/F ratio (the ratio of the numbers of carbon atoms to that fluorine atoms in one molecule) of 0.25 or higher. The common functions of these gases include: (a) forming a C - O bond from a constituent element C on the surface of a SiO 2 layer and cleaving or weakening an Si - 0 bond, (b) forming CF n + (particularly, n = 3) as a main etchant for an SiO 2 layer, and (c) generating relatively carbon-rich plasma and thereby removing oxygen from SiO 2 in the form of CO or CO 2 while achieving a lower etchrate and higher selectivity for an underlying silicon layer as C, H, F, and other constituent elements contribute to deposition of carbonaceous polymers on the surface of the underlying layer. It is to be noted that the above mentioned H 2 and O 2 are used to regulate selectivity for an underlying silicon layer, in other words, the apparent C/F ratio of an etching reaction system by increasing or decreasing the quantity of F*. Basically, etching gases for an SiO 2 layer are also used to etch an insulation layer composed of silicon nitride (Si x N y particularly, x = 3 and y = 4). While the SiO 2 layer is etched mainly through an ion-assisted reaction, the Si x N y layer is etched at a higher rate through a radical reaction using F* as a main etchant. Such an etchrate difference is somewhat predictable from the descending order in binding energy of an Si - F bond (132 kcal / mole), Si - 0 bond (111 kcal / mole), and Si- N bond (105 kcal / mole). Incidentally, these binding energy values are cited from data shown in "Handbook of Chemistry and Physics" 69th Edition (1988) edited by R.C. Weast (published by CRC Press Inc. in Florida, U.S.A.) and may vary slightly according to any other calculation method. Meanwhile, the recent trend toward higher integration of semiconductor devices requires correspondingly higher selectivity for an SiO x layer and an Si x N y layer. For instance, an Si x N y layer formed on an SiO x layer as an underlying layer is etched in the LOCOS method where the Si x N y layer is patterned to define an element isolation region. This etching process requires extremely high selectivity for the underlying layer now that a pad oxide film (SiO 2 layer) is decreased in thickness to minimize bird's beak length. On the other hand, an SiO x layer formed on an SiN x layer as an underlying layer must be etched, for instance, in a contact hole forming process. In recent years, a thin Si x N y layer may be formed between the SiO x interlayer insulating film and the Si substrate for the purpose of-reducing damage to the substrate in an over-etching process. To achieve this purpose, this etching process also requires extremely high selectivity for the underlying SiN x layer. In principle, when achieving high selectivity for two laminated dissimilar material layers, it is preferable that there should be much difference in interatomic bond energy between the chemical bonds of the layers. In the case of an SiO x layer and an Si x N y layer, in particular, it is essentially difficult to achieve high selectivity between these two layers because of little difference in bond energy between the bonds thereof (Si - O bond and Si - N bond, respectively) and common bases of the etching gas thereof. Therefore, persistent efforts have been made in industrial sectors concerned to develop technologies for overcoming such difficulty. In fact, some technologies have been reported for etching an Si x N y layer formed on an SiO x layer with high selectivity between these two layers. For instance, the present inventor has disclosed in JP-A- 61 - 142744 (1986) one such technology whereby CH 2 F 2 or any other gas with a low C/F ratio are used as etching gas with CO 2 added at a molar ratio of 30 to 70%. More specifically, such gases with a low C/F ratio will form CF x + (particularly, x = 3) as an etchant for an SiO x layer only by recombining with F*. The quantity of CF x + thus formed will decrease when a great quantity of CO* is fed to capture F* for removal from an etching reaction system in the form of COF. As a result, an etchrate for the SiO x layer will also decrease. Meanwhile, an Si x N y layer is etched by other ions than CF x + and radicals, so that the etchrate thereof will remain almost unchanged even when a great quantity of CO 2 is fed to the etching reaction system. Thus, high selectivity will be achieved between the SiO x layer and the Si x N y layer. "Proceedings of Symposium on Dry Process", Vol. 88, No.7, p.86 - 94 (1987) has also reported another technology whereby an Si x N y layer formed on an SiO x layer is etched by FCl which will be formed in the gaseous phase by microwave discharge when NF 3 and Cl 2 are fed to a chemical dry etching apparatus. This technology is based on the fact that an Si- N bond with a 30% ionicity has a stronger covalency than an Si - O bond with a 55% ionicity. Namely, the Si x N y layer, whose chemical bond (Si - N bond) is similar in nature to the chemical bond (covalent bond) of single-crystal silicon, will be etched by F*, Cl*, and other radicals resulting from dissociation of FCl while the SiO x layer will almost never be etched by these radicals. Thus, high selectivity will also be achieved between the two layers. As mentioned above, some technologies have been reported for selectively etching an Si x N y layer formed on an SiO x layer. This is a natural consequence considering the difference in etchrate between the two layers. When the Si x N y layer is etched mainly through a radical reaction, exposure of the SiO x layer to plasma in the etching process will inevitably result in reduction of the etchrate. However, there are some difficulties with the conventional technologies. For instance, anisotropic etching is essentially difficult with the above-mentioned technology using FCl because it is based on a radical reaction. Conversely, no technology has been reported for selectively etching an SiO x layer formed on an Si x N y layer. This etching process etches the SiO x layer mainly through an ion-assisted reaction and involves even greater difficulty in achieving high selectivity between the two layers because radicals formed invariably in the etching reaction system will increase the etchrate upon exposure of the Si x N y layer to plasma. In fact, it is certain that this etching process will be needed in the future and is required to realize it as soon as possible. US-A-4 431 477 discloses a process for selectively etching polysilicon and silicon nitrides by using a gaseous mixture of nitrous oxide and a fluoro compound, preferably a halocarbon, wherein each halocarbon has at least one fluorine atom per molecule. Other fluoro gases include, for example, SF 6 and SF 4 . It is therefore an object of the present invention to provide a plasma etching method whereby an SiO x layer and an Si x N y layer may be etched with high selectivity for each other. The present invention provides a plasma etching method for selectively etching a silicon oxide layer formed on a silicon nitride layer supported on a substrate or masked with a silicon nitride layer patterned into a predetermined shape, which is characterized by using as an etching gas at least one compound selected from S 2 F 2 , SF 2 , SF 4 , and S 2 F 10 , optionally in admixture with at least one compound selected from H 2 , H 2 S, and silane compounds as additive gas capable of consuming halogen radicals, and by etching said silicon oxide layer while maintaining the substrate at a temperature lower than room temperature. This process uses four sulfur fluorides S 2 F 2 , SF 2 , SF 4 , and S 2 F 10 , which have been proposed by the present inventor as effective chlorofluorocarbon-free (CFC-free) etching as for an SiO x layer. These sulfur fluorides will be dissociated by electric discharges to form SF x + in plasma, which will, in turn, act as a main etchant for an SiO x layer. Si will be removed from the SiO x layer in the form of SiF x . The four sulfur fluorides have an important advantage over the conventionally well-known sulfur fluoride SF 6 in that they have a high S/F ratio (the ratio of the numbers of sulfur atoms to that of fluorine atoms in one molecule) and form free sulfur in plasma. The sulfur thus formed will adsorb on the surface of a target substrate (wafer) maintained at lower temperature than room temperature. Likewise, sulfur will adsorb on the surface of a target layer (SiO 2 layer) and form SO x (x = 2 or 3) with the assistance of high-energy incident ions striking on that surface for removal from the etching reaction system. Meanwhile, sulfur will be deposited on, and competitively removed through sputtering from, the surface of a resist mask and an underlying silicon layer, thus improving selectivity for both the resist mask and the underlying layer. Further, the sulfur will serve to protect pattern sidewalls, on which no incident ion will strike in principle. The sulfur deposits will sublime immediately upon heating of the substrate after etching and therefore avoid the danger of particle pollution. When the SiO x layer has almost been etched, the underlying Si x N y layer will be exposed to plasma while F* present in plasma will extract silicon atoms from the surface of the Si x N y layer to form dangling nitrogen bonds. The dangling nitrogen bonds present on the surface of the Si x N y layer has already been discussed in "Semiconductor -Plasma Process Technology" by Takuo Sugano, P.133 - 134 (published by Sangyo Tosho Co., Ltd.), and 1980 National Convention Record, IEE Japan, Vol. 5, S 6 - 2. According to the present invention, the dangling nitrogen bonds will combine with the sulfur formed in plasma to form various sulfur nitride based compounds, which, in turn, will serve to protect the surface of the Si x N y layer and achieve high selectivity for this layer. The most typical of the above-mentioned sulfur nitride based compounds is polythiazyl represented by a general formula (SN) x . The simplest process of forming polythiazyl is that the sulfur formed in plasma combines with the dangling nitrogen bonds to form thiazyl (N S), which has unpaired electrons analogous to those of an oxygen analog nitrogen oxide (NO) and polymerizes easily to form (SN) 2 , (SN) 4 , and (SN) x . (SN) 2 polymerizes easily at temperatures around 20°C to form (SN) 4 and (SN) x , and decomposes at temperatures around 30°C. (SN) 4 is a ring compound with a melting point of 178°C and a decomposition point of 206°C. The property and structure of (SN) x are described in detail in J. Am. Chem. Soc., Vol. 29, p.6358 - 6363 (1975). (SN) x is a polymerized compound that will remain stable up to temperatures around 208°C under atmospheric pressure and 140 to 150°C under reduced pressure. In a crystalline state, it is so structured that principal chains each composed of repetitive covalent bonds S - N - S - N - - - - are oriented in parallel to one another. Consequently, when deposited on a layer, sulfur nitride based compounds, typically (SN) x , will effectively inhibit F* and other radicals from invading the layer. Even when accelerated incident ions strike the layer, they will absorb or alleviate the impact of the ions on the layer by producing so-called sponge effects derived from changes in their bond angle, conformation, and other property. Besides sulfur, F* deriving from sulfur fluoride is also present in plasma, so that the resulting fluorine may combine with the (SN) x to form thiazyl fluoride. Further, when nitrogen based gas is added to regulate the quantity of F* formed in plasma, the resulting hydrogen may combine with the (SN) x to form hydrogen thiazyl. Under some conditions, the above-mentioned sulfur nitride based compounds may be S 4 N 2 (melting point: 23°C), S 11 N 2 (melting point: 150 - 155°C), S 15 N 2 (melting point: 137°C), S 16 N 2 (melting point: 122°C), and other cyclic sulfur nitride compounds containing much more sulfur atoms than nitrogen atoms, as well as S 7 NH (melting point: 113.5°C), 1,3-5 6 (NH) 2 (melting point: 130°C), 1,4-5 6 (NH) 2 (melting point: 133°C), 1,5-5 6 (NH) 2 (melting point: 155°C), 1,3,5-5 5 (NH) 3 (melting point: 124°C), 1,3,6-5 5 (NH) 3 (melting point: 131°C), S 4 (NH) 4 (melting point: 145°C), and other imide compounds composed of hydrogen atoms combined with nitrogen atoms of the above-mentioned cyclic sulfur nitride compounds. All these sulfur nitride based compounds with sulfur and nitrogen as the constituent elements thereof will be removed in the form of N 2 , NO x , SO x , etc. upon removal of the resist mask through O 2 plasma ashing without remaining on the wafer or causing particle pollution. The process of the invention in the embodiment of selectively etching a silicon oxide layer formed on a substrate and masked with a silicon nitride layer patterned into a predetermined shape achieves high selectivity on the same principle as mentioned above. In this process, the top surface of the Si x N y mask is exposed to plasma at the start of etching, whereupon F* present in plasma will remove silicon atoms from the surface of the Si x N y mask to form dangling nitrogen bonds and sulfur will combine with the dangling nitrogen bonds to form various sulfur nitride based compounds, typically (SN) x , which will, in turn, cover the surface of the Si x N y mask. Meanwhile, the SiO x layer will be etched at a high rate while free sulfur in plasma will serve to protect the pattern sidewalls. Conventionally, the SiO x layer used to be etched by high-energy incident ions and often cause dimension losses due to recession of the Si x N y mask. This problem can be solved by the present invention. In this process, the Si x N y layer formed on the target SiO x layer and covered with a resist mask must be etched to form the Si x N y mask. Such preliminary etching can be performed with high selectivity in the above-mentioned process. According to the present invention, once the Si x N y mask is formed, the resist mask is removed from the Si x N y layer and the SiO x layer is etched with the surface thereof covered only with the Si x N y mask. As a result, there is no danger of carbonaceous decomposition products being formed in the etching reaction system due to decomposition of the resist material and hence the advantage of reducing particle pollution. As is clear from the foregoing description, the present invention provides a plasma etching method whereby an SiO x layer and Si x N y layer may be etched with high selectivity for each other. More particularly, the present invention realizes an unprecedented process of selectively etching an SiO x layer formed on an Si x N y layer, thus opening up the possibility of developing novel semiconductor device structures, to say nothing of providing an effective CFC-free etching method. Accordingly, the present invention is particularly useful for such industrial applications as production of large-scale integrated high-performance semiconductor devices conforming to strict design rules. Figs. 1a - 1d are schematic sectional views showing the stages of a plasma etching method applied in one preferred embodiment to a contact hole forming process. Fig. 1a shows a stage at which a resist mask was formed on an SiO 2 inter-layer insulation film. Fig. 1b shows a stage at which the SiO 2 inter-layer insulation film was etched. Fig. 1c shows a stage at which a resist mask and sulfur and (SN) x deposits were removed through plasma ashing. Fig. 1d shows a stage at which an Si 3 N 4 base film was selectively removed from a contact hole. Figs. 2a - 2c are schematic sectional views showing the stages of a plasma etching method applied in another preferred embodiment to an etch-back process in which a sidewall is formed on both sides of a gate electrode formed on a gate insulation film having a so-called ONO structure. Fig. 2a shows the stage at which an SiO 2 layer was formed all over a wafer. Fig. 2b shows the stage at which the SiO 2 layer or a second SiO 2 insulation film was etched back or selectively etched, respectively. Fig. 2c shows the stage at which sulfur and (SN) x deposits were decomposed or sublimed for removal through heating. Figs. 3a - 3 c are schematic sectional views showing the stages of a plasma etching method applied in still another embodiment to a contact hole forming process. Fig. 3a shows the stage at which an Si 3 N 4 mask was formed on an SiO 2 inter-layer insulation film before it was etched. Fig. 3b shows the stage at which the SiO 2 inter-layer insulation film was etched. Fig. 3c shows the stage at which sulfur and (SN) x deposits were decomposed or sublimed for removal through heating. The following paragraphs describe some preferred embodiments of the present invention. Example 1 In the first example, a plasma etching method according to one aspect of the present invention is applied to a contact hole forming process in which an SiO 2 inter-layer insulation film is etched by S 2 F 2 / H 2 mixed gas. This process is described by referring to Figs. 1a - 1d. It is to be noted that Figs. 1a - 1d are drawn with a smaller aspect ratio than usual in order to provide schematic representations. Referring first to Fig. 1a, a silicon substrate (wafer) 1 carrying an impurity-diffused region 2 was provided on the surface thereof with an Si 3 N 4 base film 3 which was formed in the thickness of 10 nm, for instance, by low pressure chemical vapor deposition (LPCVD). Then, the Si 3 N 4 base film 3 was provided on the surface thereof with a SiO 2 inter-layer insulation film 4 which was formed in the thickness of 500 nm by normal pressure CVD. Further, the SiO 2 inter-layer insulation film 4 was coated on the surface thereof with chemical amplification negative type three-component photoresist SAL - 601 (brand name of Shipley Co., Inc.) and a resist mask 5 was so formed as to have an opening 5a by excimer laser lithography and alkaline development. The target wafer thus formed was set on a wafer supporting electrode provided on a magnetron reactive ion etching (RIE) apparatus and cooled to about - 50°C by an ethanol refrigerant which was fed from a cooling system, for instance, a chiller for circulation through a cooling pipe housed in the wafer supporting electrode. In this state, the SiO 2 inter-layer insulation film 4 was etched, for instance, with an S 2 F 2 flow rate of 50 SCCM, H 2 flow rate of 20 SCCM, gas pressure of 1.3 Pa (10 mTorr), and RF power of 1000 W (2 MHz). Referring next to Fig. 1b, there is schematically shown a mechanism of the above etching. In the drawing, the composition formulas encircled in a dotted line represent those chemical species which are deposited on, and simultaneously removed through sputtering from, the surface of the wafer while encircled in a continuous line are those chemical species which remain deposited on the surface of the wafer. The SiO 2 inter-layer insulation film 4 was etched by S*, F*, and other radicals formed in plasma as the reaction of these radicals was assisted by SF x + , S + , and other ions. Also present in plasma was free sulfur, which resulted from dissociation of S 2 F 2 by electric discharges and adsorbed on the surface of the wafer as it was cooled to lower temperature than room temperature. Likewise, sulfur adsorbed on the surface of the SiO 2 inter-layer insulation film 4 and combined with oxygen fed from that surface through sputtering to form SO x for removal from the etching reaction system. Thus, the sulfur present on the surface of the SiO 2 inter-layer insulation film 4 never reduced the etchrate thereof. By contrast, sulfur was deposited on, and simultaneously removed through sputtering from, the surface of the resist mask 5, thus reducing the etchrate thereof and improving selectivity for the resist mask 5. Meanwhile, sulfur deposited on pattern sidewalls, on which no incident ion struck in principle, and served to protect it, forming a contact hole 4a having a vertical wall. It is to be noted that H 2 is used as additive gas to increase the apparent S/F ratio of the etching reaction system and prevent F* from deteriorating selectivity for the base layer. More specifically, H 2 will be dissociated to form H*, which is capable of capturing part of F* for removal from the etching reaction system in the form of HF. Such S/F ratio regulation is extremely useful in preventing deterioration of anisotropy or selectivity for the underlying layer in an over-etching process where F* will become excessive. Referring further to Fig. 1b, when the SiO 2 inter-layer insulation film 4 was almost etched, the Si 3 N 4 base film 3 was exposed to the bottom of the contact hole 4a, whereupon nitrogen supplied from the Si 3 N 4 base film 3 combined with sulfur present in plasma to form a sulfur nitride based compound, that is, a polymerized thiazyl compound (SN) x in the drawing. (SN) x thus formed deposited on the Si 3 N 4 base film 3, reducing the etchrate thereof and achieving selectivity ratio of about 15 for the Si 3 N 4 base film 3. Referring next to Fig. lc, when the wafer was set on a plasma asher to remove the resist mask 5 in O 2 plasma, the sulfur deposited on the pattern sidewalls was burned for removal in the form of SO x while the (SN) x deposited on the surface of the SiO 2 inter-layer insulation film 4 were also burned or decomposed for removal in the form of N 2 , NO x , SO x , etc. Referring finally to Fig. ld, when the wafer was immersed in hot aqueous phosphate solution, the Si 3 N 4 base film 3 exposed to the bottom of the contact hole 4a was decomposed for removal. In the above example, the contact hole 4a was formed in an excellent anisotropic shape without causing damage to the impurity-diffused region 2 or particle pollution. Example 2 In the second example, a plasma etching method according to one aspect of the present invention is applied to a contact hole forming process in which an SiO 2 inter-layer insulation film is etched by S 2 F 2 / H 2 S mixed gas. In the second example, the same wafer as in the first example is used as an etching sample. The wafer was set on a wafer supporting electrode provided on an RF biased magnetically enhanced microwave plasma etching apparatus. In this state, the SiO 2 inter-layer insulation film 4 was etched, for instance, with an S 2 F 2 flow rate of 50 SCCM, H 2 flow rate of 15 SCCM, gas pressure of 1.3 Pa (10 mTorr), microwave power of 850 W, RF bias power of 200 W (400 kHz), and wafer temperature of -50°C. In the second example, etching proceeded by the same mechanism as in the first example, except that H 2 S was used as additive gas to supply sulfur, thereby promoting sulfur deposition. In the second example, too, the contact hole 4a was formed in an excellent anisotropic shape while achieving high selectivity for the Si 3 N 4 base film 3. Example 3 In the third example, a plasma etching method according to one aspect of the present invention is applied to an etch-back process in which a sidewall-composed of an SiO 2 layer is formed on both sides of a gate electrode formed on a gate insulation film containing an Si 3 N 4 film. This process is classified among manufacturing processes for MOS-FET having an LDD structure. This process is described by referring to Figs. 2a - 2c. Referring first to Fig. 2a, a silicon substrate 11 was provided on the surface thereof with a gate insulation film 16 having a so-called ONO (oxide film / nitride film / oxide film) structure. Then, the gate insulation film 16 was provided on the surface thereof with a gate electrode 17 which was formed through patterning from an n + type polysilicon layer. Masked with the gate electrode 17, the silicon substrate 11 was implanted with ions to form low-density-impurity-diffused regions 12 on the surface thereof. Further, an SiO 2 layer 18 was deposited all over the wafer by CVD. The gate insulation film 16 was composed of a first SiO 2 gate insulation film 13 (4 nm thick), Si 3 N 4 gate insulation film 14 (6 nm thick), and second SiO 2 gate insulation film 15 (2 nm thick) which were laminated in this order on the silicon substrate 11. The wafer was set on a wafer supporting electrode provided on an RF biased magnetically enhanced microwave plasma etching apparatus. In this. state, the SiO 2 layer 18 was etched back and the second SiO 2 gate insulation film 15 was etched, for instance, with an S 2 F 2 flow rate of 50 SCCM, gas pressure of 1.3 Pa (10 mTorr), RF power of 200 W (400 kHz), and wafer temperature of - 80°C. In the third example, etching proceeded by the same mechanism as in the first example, except that neither H 2 nor H 2 S was used as additive gas to consume F* and that the wafer was cooled to even lower temperature, thereby inhibiting reaction by F* and promoting sulfur deposition. Referring next to Fig. 2b, when etching was completed, the gate electrode 17 and the Si 3 N 4 gate insulation film 14 were exposed to plasma, whereupon sidewalls 18a were formed on both sides of the gate electrode 17. Unlike the SiO 2 layer 18, the gate electrode 17 ceased to be supplied with oxygen and had sulfur depositing on the surface thereof, thus greatly reducing the etchrate and achieving high selectivity therefor. Meanwhile, the Si 3 N 4 gate insulation film 14 also had sulfur nitride based compounds, typically (SN) x , depositing on the surface thereof, thus greatly reducing the etchrate and achieving a selectivity ratio of about 20 therefor. Referring finally to Fig. 2c, when the wafer was heated to about 130°C or higher after completion of etching, the sulfur and sulfur nitride based compound were removed from the surface of the wafer immediately. Namely, the sulfur was sublimed upon heating of the wafer to temperatures around 90°C and the sulfur nitride based compound was then decomposed for removal upon further heating of the wafer. In the third example, the sidewalls 18a were formed without causing damage to the underlying low-density-impurity-diffused regions 12. Example 4 In the fourth example,- a plasma etching method according to another aspect of the present invention is applied to a contact hole forming process in which an SiO 2 inter-layer insulation film was etched by S 2 F 2 / H 2 mixed gas via an Si 3 N 4 mask. This process is described by referring to Figs. 3a - 3c. Referring first to Fig. 3a, a silicon substrate (wafer) 21 carrying an impurity-diffused region 22 was provided on the surface thereof with an SiO 2 inter-layer insulation film 23 which was formed in the thickness of 1 µm, for instance, by normal pressure CVD. Further, the SiO 2 inter-layer insulation film 23 was provided on the surface thereof with Si 3 N 4 masks 24 patterned into a predetermined shape. The Si 3 N 4 masks 24 were so formed as to have an opening 24a by etching an Si 3 N 4 layer (100 nm thick) formed, for instance, by low pressure CVD via a chemical amplification photoresist mask formed by excimer laser lithography and alkaline development. The photoresist mask had been removed from the Si 3 N 4 layer through ashing. The target wafer thus formed was set on a wafer supporting electrode provided on an RF biased magnetically enhanced microwave plasma etching apparatus. In this state, the SiO 2 inter-layer insulation film 23 was etched, for instance, with an S 2 F 2 flow rate of 50 SCCM, H 2 flow rate of 20 SCCM, gas pressure of 1.3 Pa (10 mTorr), microwave power of 850 W, and RF bias power of 200 W (400 kHz). Referring next to Fig. 3b, when etching was started, the Si 3 N 4 masks 24, which had already been exposed to plasma, were coated immediately on the surface thereof with sulfur nitride based compounds, typically (SN) x , thus greatly reducing the etchrate and achieving a selectivity ratio of about 20 therefor. As a result, the SiO 2 inter-layer insulation film 23 was etched without causing dimension losses due to recession of the Si 3 N 4 mask while a contact hole 23a was formed without deteriorating the sectional form thereof. Meanwhile, the SiO 2 inter-layer insulation film 23 was etched into an anisotropic shape as sulfur deposited on the sidewalls thereof and served to protect them. Upon exposure of the underlying impurity-diffused region 22 to plasma, sulfur also deposited on the surface thereof, thus greatly reducing the etchrate, and achieving high selectivity in an over-etching process, therefor. Referring finally to Fig. 3c, when etching was completed, the wafer was heated to about 130°C or higher to remove the sulfur and sulfur nitride based compound. Conventionally, an SiO 2 layer used to be etched by high-energy incident ions, which, when sputtering on a resist mask, will cause recession thereof and resulting particle pollution. This problem can be solved by a plasma etching method according to another aspect of the present invention, whereby the SiO 2 layer may be etched via an Si 3 N 4 layer instead of the resist mask. The Si 3 N 4 layer thus substituting for the resist mask may be left unremoved as part of an insulation film. The etching gas may contain any additive gas other than H 2 and H 2 S, such as silane based gas for increasing the S/F ratio of gases in an etching reaction system and He, Ar, and other rare gases for producing sputtering, cooling, and dilution effects.

Claims: A plasma etching method for selectively etching a silicon oxide layer (4, 23) formed on a silicon nitride layer (3) supported on a substrate (1. 21) or masked with a silicon nitride layer (24) patterned into a predetermined shape, characterized by using an etching gas constituted by at least one compound selected from S 2 F 2 , SF 2 , SF 4 , and S 2 F 10 , optionally in admixture with at least one compound selected from H 2 , H 2 S, and silane compounds as additive gas capable of consuming halogen radicals , and a rare gas while maintaining the substrate (1, 21) at a temperature lower than room temperature. The plasma etching method as claimed in claim 1 wherein said substrate is composed of a silicon substrate (1), an Si 3 N 4 base film (3) formed on said silicon substrate, an SiO 2 inter-layer insulation film (4) formed on said Si 3 N 4 base film (3). and a resist mask (5) formed selectively on said SiO 2 inter-layer insulation film (4), said SiO 2 inter-layer insulation film (4) being etched via said resist mask (5) to form a connection hole on said SiO 2 inter-layer insulation film (4). The plasma etching method as claimed in claim 1 wherein said substrate is composed of a silicon substrate (11), a three-layer gate insulation film (16) formed on said silicon substrate (11), a gate electrode (17) formed on said gate insulation film (16), and an SiO 2 layer (18) covering said gate electrode (17) with said three-layer gate insulation film (16) having a first SiO 2 gate insulation film (13), a Si 3 N 4 gate insulation film (14), and a second SiO 2 gate insulation film (15), part of said SiO 2 layer (18) being etched back and part of said second SiO 2 gate insulation film (15) being etched to form a sidewall (18a) on both sides of said gate electrode (17). The plasma etching method as claimed in claim 1 wherein said substrate is composed of a silicon substrate (21), an SiO 2 inter-layer insulation film (23) formed on said silicon substrate (21), and an Si 3 N 4 mask (24) patterned into a predetermined shape and formed on said SiO 2 inter-layer insulation film (23), said SiO 2 inter-layer insulation film (23) being etched via said Si 3 N 4 mask (24) to form a connection hole on said SiO 2 inter-layer insulation film (23).

Inventors: KADOMURA SHINGO, KADOMURA, SHINGO
Applicants: SONY CORP, SONY CORPORATION
Application year: 1992
Main classification: H01L21
IPC codes: H01L21