Abstract:
A method of forming a photoresist pattern having a uniformly fine line width, and a method of manufacturing a semiconductor device using such a photoresist pattern as a mask, include the step of forming an anti-reflective coating (ARC) using only a hydrocarbon based gas. A highly reflective layer is formed on a semiconductor substrate on which an underlayer is disposed. Using only a hydrocarbon based gas, the ARC is formed on the highly reflective layer. A photoresist layer is formed on the ARC, and is exposed and developed to form a photoresist pattern on the ARC. The ARC and the highly reflective layer under the photoresist pattern are etched using the photoresist pattern as a mask. Thereafter, the photoresist pattern and the ARC are simultaneously removed. The ARC is of an amorphous silicon film having high etching selectivity and being easily removed along with the photoresist pattern.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of manufacturing a semiconductor device. More particularly, the present invention relates to a method of forming, on a semiconductor substrate, a photoresist pattern used for etching a highly reflective layer. 
     2. Description of the Related Art 
     As the integration and performance of semiconductor devices continues to increase, a higher level of technology is required to form the fine patterns necessary to produce such highly integrated and high performance semiconductor devices. The fine patterns of semiconductor devices are generally formed through a photolithography process. However, during photolithography, light reflected to a photoresist layer from a layer disposed beneath the photoresist layer causes the following problems. 
     First, it is difficult to produce a fine line width because a ripple is generated in the profile of the photoresist pattern due to a standing wave generated by the interference of light waves propagating in the photoresist layer. Although the ripple is removed during post-exposure baking, the photoresist pattern is undercut or is deformed so as to have the shape of a tail. 
     Second, although constant exposure energy is used, a swing effect occurs in which the amount of light absorbed by the photoresist layer varies according to the thickness of the photoresist layer. The swing effect also occurs due to the interference of light waves in the photoresist layer. The swing effect makes it difficult to produce a fine line width within a required range. 
     Third, notching or bridging is produced in the photoresist pattern by light reflecting from the region of a step in an underlayer, i.e., a layer produced beneath the photoresist pattern during the manufacture of the semiconductor device. 
     FIGS. 1 and 2 illustrate a conventional manufacturing process in which these problems are likely to arise. 
     Referring first to FIG. 1, an underlayer  52  having a step difference is formed on a semiconductor substrate  50 . A highly reflective layer  54  having a high refractive index, such as a transparent insulating film, is formed on the underlayer  52 . A photoresist layer  56  is coated on the highly reflective layer  54 . Light  62  produced by exposure equipment (not shown) is passed through a mask  60 , having a light blocking layer  58 , in order to irradiate selected portions of the photoresist layer  56 . 
     Referring now to FIG. 2, after the photoresist layer  56  is exposed through the process shown in FIG. 1, the photoresist layer  56  is developed to form photoresist patterns  64  and  66 . The photoresist pattern  64  formed over a region of the underlayer  52  in which there is no step difference has a fine line width that is relatively uniform. However, the photoresist pattern  66  formed from that part of the photoresist layer  56  overlying the step difference in the underlayer  52  has a deformed line width pattern. This deformation is produced due to the interference of light waves  62  irradiating the highly reflective layer  54 . When the deformation becomes severe, a notching or bridging defect, which is a critical defect in the fine line width pattern, is produced. 
     A conventional process has employed an anti-reflective coating (ARC) to combat these problems. FIG. 3 illustrates a conventional method of manufacturing a semiconductor device using such an anti-reflective coating (ARC). 
     In FIG. 3, reference numeral  68  denotes an ARC formed between the photoresist layer  56  and the highly reflective layer  54 . In this case, the light reflected from the underlayer  54  and the ARC  68  to the photoresist layer  56  consists of the light ê ; 1 , reflected from the interface between the ARC  68  and the photoresist layer  56 , and the light ê ; 2 , reflected from the interface between the highly reflective layer  54  and the ARC  68 . The ARC can reduce the amount of reflected light ê ; 1 +ê ; 2  reaching the photoresist layer  56  by ensuring that the phase difference between ê ; 1  and ê ; 2  is 180° so as to give rise to destructive interference, or by absorbing almost all of the reflected light ê ; 1  or ê ; 2 . In the former case the ARC is referred to as an interference type of ARC and in the latter case as an absorption type of ARC. A hybrid type of ARC having some of the characteristics of an interference type of ARC and an absorption type of ARC has also been developed. 
     The forming of such ARCs by plasma enhanced chemical vapor deposition (PECVD) using a gas mixture of hydrocarbon and helium has been disclosed in U.S. Pat. No. 5,569,501 issuing on Oct. 29, 1996 and entitled “Diamond-like Carbon Films Form Hydrocarbon Helium Plasma”. In the PECVD method, an amorphous carbon layer is formed by controlling the temperature only under the substrate in a plasma chamber. However, this process is problematic in that the helium used as a carrier gas damages the anti-reflective coating during the generation of plasma or otherwise acts to limit the quality of the anti-reflective coating which can be produced. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method of forming a photoresist pattern, having a fine uniform line width that is free of bridging or notching, on a semiconductor substrate on which a stepped highly reflective layer has been formed. 
     To achieve this object, the method includes a step of forming an antireflective coating (ARC) using only a hydrocarbon based gas on the highly reflective layer. The hydrocarbon based gas is preferably methane, ethane, propane, butane, acetylene, propene, or n-butane gas. The photoresist pattern is then formed on the ARC. 
     Another object of the present invention is to provide such a method of forming a photoresist pattern, in which the anti-reflective coating (ARC) has excellent etching selectivity, is economical to produce, and can be easily removed once the photoresist pattern has been formed. 
     To achieve this object, the ARC is formed by plasma enhanced chemical vapor deposition (PECVD), during which the temperature on and under the substrate is controlled. In this case, an amorphous carbon ARC layer is produced from the hydrocarbon based gas. The amorphous carbon film preferably has a refractive index of 1.2 to 2.5, and an extinction coefficient of 0.2 to 0.8. 
     The semiconductor substrate is preferably a single crystal silicon substrate, an SOI substrate, an SOS substrate, or a gallium arsenide substrate. The highly reflective layer is typically formed of W, WSi x , TiSi x , Al, or an Al alloy. 
     To assist the ARC in preventing excessive light from reflecting to the photoresist from the highly reflective layer, an insulating film can be interposed between the highly reflective layer and the photoresist. The insulating film may be formed between the highly reflective layer and the ARC or between the photoresist and the ARC. The insulating film is preferably formed of polysilicon oxide, thermally grown silicon oxide, or SiON. 
     The method of the present invention may also include a step of adding at least one additive selected from the group consisting of oxygen, tin, lead, silicon, fluorine, and chlorine to the amorphous carbon film. The density of the ARC is preferably increased by subjecting the ARC to an annealing process, or a plasma process, an E-beam process, and a curing process. 
     According to the present invention, once the photoresist pattern is formed, the ARC and the highly reflective layer can be etched using the photoresist pattern as a mask. The photoresist pattern and the ARC can be simultaneously removed from the resultant because the ARC is of an organic material. 
     The ARC is preferably formed to a thickness of 150 to 10,000 Å. A mixture of oxygen and argon gases is preferably used for etching the ARC. The highly reflective layer may be sequentially or simultaneously removed with the ARC. 
     According to the present invention, using only a hydrocarbon based gas in forming the ARC keeps the manufacturing costs low compared to when using several thin film forming gases in forming the ARC. Secondly, because the ARC and the photoresist pattern can be removed simultaneously, it is not necessary to perform an additional dry etching step to remove the ARC, and the production process is simplified. Third, the present invention makes it relatively easy to form the ARC and, through the use of only a single gas to form the ARC, minimizes the number of process variations which could unduly influence the formation of the ARC. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof made with reference to the attached drawings, of which: 
     FIGS. 1 and 2 are sectional views, respectively, of a semiconductor substrate on which a highly reflective layer is formed according to the prior art, and showing the resulting deformation in the photoresist pattern; 
     FIG. 3 is a similar sectional view but showing the use of an anti-reflective coating (ARC) according to the prior art; 
     FIG. 4 is a sectional view of a semiconductor substrate, and shows a first embodiment of a method of forming a photoresist pattern using an ARC according to the present invention; 
     FIGS. 5 and 6 are sectional views similar to FIG. 4, respectively, and showing steps of etching the highly reflective layer using the ARC according to the method of the present invention; 
     FIG. 7 is a sectional view of a semiconductor substrate, and shows a second embodiment of a method of forming a photoresist pattern using an ARC according to the present invention; 
     FIG. 8 is a view similar to that of FIG. 7 but showing a modified version of the second embodiment of a method of forming a photoresist pattern using an ARC according to the present invention; 
     FIG. 9 is a graph showing the refractive index characteristic with respect to the temperature of an ARC formed according to the present invention; 
     FIG. 10 is a graph showing the extinction coefficient characteristic with respect to the temperature of an ARC formed according to the present invention; 
     FIG. 11 is a graph showing the thickness according to the change of the deposition time of an ARC formed according to the present invention; 
     FIG. 12 is a graph showing the changes in the refractive index and the extinction coefficient according to changes in thickness of an ARC formed according to the present invention; 
     FIG. 13 is a graph showing an X-ray diffraction surface analysis of an ARC formed according to the present invention; 
     FIGS. 14 and 15 are graphs showing simulation results of the refractive index and the reflexibility according to the extinction coefficient of an ARC formed according to the present invention when a highly reflective film is formed of aluminum; and 
     FIGS. 16 and 17 are graphs showing simulation results of the refractive index and the reflexibility according to the extinction coefficient of an ARC formed according to the present invention when the highly reflective film is formed of a silicon film. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 4, an underlayer  102  such as a transistor or a bit line is formed on a semiconductor substrate  100 . The underlayer  102  can be any type of layer in which a step is formed during the process of forming the lower portion of the semiconductor device. The semiconductor substrate  100  may be a single crystal silicon substrate, a silicon on insulator (SOI) substrate, a silicon on sapphire (SOS) substrate, or a gallium-arsenide substrate. A highly reflective layer  104  to be etched or implanted with ions is formed on the stepped underlayer  102 . W, WSi x , TiSi x , Al, and Al alloy are representative examples of the materials used to form the highly reflective layer  104  and having a high refractive index sufficient to cause the photoresist to be deformed during etching. 
     An ARC  106  is formed by plasma enhanced CVD (PECVD). For this technique, various reaction mechanisms in the form of liquid and gas, such as methane, ethane, propane, butane, acetylene, propene, and n-butane gas, can be injected into the chamber. However, in the present invention, methane gas CH 4  is preferably used. Also, in the prior art, helium is used as a carrier gas for generating plasma. However, in the present invention, the ARC  106  is formed of an amorphous carbon film using only the methane gas CH 4 . That is, no carrier gas is used. 
     The amorphous carbon film has an excellent etching selectivity with other layers and is easily removed since its characteristics are similar to that of the photoresist. In the prior art, the ARC is removed by dry etching. However, in the present invention, the ARC can be simply removed using only a cleaning process performed subsequent to the ashing process and the H 2 SO 4  stripping process which are performed to remove the photoresist pattern. Forming the ARC using only the relatively inexpensive methane gas CH 4  also has the advantage of keeping manufacturing costs to a minimum. The ARC can be provided with the desired refractive index, composition, and chemical characteristics by using additives such as oxygen, tin, lead, silicon, fluorine, and chlorine. 
     When an ARC is formed by PECVD, the temperature in the chamber affects the characteristics of the layer. In the present invention, it is possible to impart desired characteristics to the ARC by controlling the temperature to between 0 and 400° C., desirably 200° C. at the upper and lower parts of the substrate. Here, the upper parts means the showerhead and the lower parts means substrate of chamber. 
     Specific conditions for initially forming an ARC  106  of an amorphous carbon film according to the present invention are as follows. First, 250±10 sccm of methane gas CH 4  is injected into the reaction chamber. The RF power is controlled to 150±10 W. Under such conditions, an amorphous carbon film having a refractive index of 1.0 and an extinction coefficient of 0.05 in the band of a deep UV (DUV; 248 nm) wavelength can be formed. An ARC is generally effective when its refractive index is between 1.2 and 2.5 and its extinction coefficient is between 0.2 and 0.8. Here, the term extinction coefficient refers to a indicator of the degree to which light can pass through a layer. The amorphous carbon film, i.e., the ARC  106  can be formed in a thickness of between 100 Å and 10,000 Å depending upon the etching selectivity characteristic of the highly reflective layer  104 , i.e., the underlayer. In general, when the highly reflective layer is an insulating film, the highly reflective film is made thin. When the highly reflective film is a conductive layer, the highly reflective film is made thick. 
     To increase the refractive index and extinction coefficient of the ARC, the density of the ARC layer  106  is increased by performing an in-situ annealing process after initially forming the ARC  106 , or by performing an additional RF plasma process, an E-beam process, and a curing process. 
     A photoresist layer is coated on an ARC  106  formed of the amorphous carbon film according to the present invention. A photoresist pattern  108  is formed from the photoresist layer using light  114  having a wavelength of no more than 450 nm, such as 436 nm, 365 nm, 248 nm, or 193 nm, and a mask  110  having a light shield layer  112 . Because the ARC  106  will prevent the photoresist layer from experiencing the standing wave and swing effects, the resulting photoresist pattern  108  has an excellent shape. In this case, a positive photoresist, a negative photoresist, or i-line and deep UV (DUV) photoresists can be used. 
     Referring now to FIG. 5, the photoresist pattern  108  is formed using light having a wavelength of less than 450 nm and a mask having a light shield layer. The ARC  106  and the highly reflective layer  104  under the photoresist pattern  108  are then etched using the photoresist pattern  108  as a mask. In this case, the ARC  106  and the highly reflective layer  104  can be sequentially or simultaneously etched. 
     When the ARC  106  and the highly reflective layer  104  are respectively etched, the ARC can be etched under the same conditions as those in which a SiO 2  layer is generally etched. The highly-reflective layer  104  is etched using a process suited to the characteristics of the highly reflective layer. 
     When the highly reflective layer is a SiO 2  layer or a SiN layer, it is possible to simultaneously etch the ARC  106  and the highly reflective layer  104 . In the present embodiment, the ARC  106  and the highly reflective layer  104  are dry etched. When the DUV type of photoresist layer is used and the highly reflective layer  104  is an SiO 2  layer, it is possible to simultaneously remove the ARC  106  and the highly reflective layer  104  by controlling the power to within a range of 0 to 2,000 W, the pressure to within a range of 0 to 500 mTorr, and the etching time after etching gases of about 0 to 50 sccm of oxygen, about 0 to 1,000 sccm of argon, and about 0 to 50 sccm of methane CH 4  have been introduced. When the highly reflective layer  104  is a SiN layer, it is possible to simultaneously remove the ARC  106  and the highly reflective layer  104  by controlling the power to within a range of 0 to 2,000 W, the pressure to within a range of 0 to 500 mTorr, and the etching time after etching gases of about 0 to 50 sccm of oxygen, about 0 to 1,000 sccm of argon, about 0 to 50 sccm of CO, and about 0 to 100 sccm of CHF 3  have been introduced. Note, although the etching conditions have been described in detail when a SiO 2  layer and a SiN layer are used as the highly reflective layer, such etching conditions can differ depending on the characteristics of the thin film used as the highly reflective layer  104 . 
     Also, although the consumption (A) of the photoresist pattern  108  occurs during the etching of the ARC  106  and the highly reflective layer  104 , the portion of the ARC  106  formed under the photoresist pattern  108  remains. Therefore, the ARC  106  can be used as a mask in a subsequent etching or ion implanting process. The ARC  106 , that is, the amorphous carbon film formed using only the hydrocarbon based gas, has an etching selectivity of 0.7 with an i-line photoresist, at least 0.5 with a DUV photoresist, and at least 5 with a SiO 2  film. 
     As shown in FIG. 6, the ARC  106  can be removed along with the photoresist pattern  108  without using a dry etching process. Here, the photoresist pattern can be removed by an ashing process using O 2  plasma, or an H 2 SO 4  stripping process. 
     In the prior art, the ARC  106  must be removed by performing a dry etching process in addition to that used to remove the photoresist pattern. However, the ARC  106  according to the present invention, that is, the amorphous carbon film formed using only the hydrocarbon based gas, is an organic material. Therefore, the ARC  106  can be removed in the process of removing the photoresist pattern  108 . The present invention thus contributes to simplifying the processes of manufacturing the semiconductor device and to reducing the throughput time by doing away with the need for a dry etching step. 
     Referring now to the embodiment of FIG. 7, a transparent or semitransparent insulating film  116  is formed below the photoresist layer and between the highly reflective film  104  and the ARC  106 . The transparent or semi-transparent insulating film  116  can be formed of polysilicon oxide, polysilicon oxide to which impurities are added, thermally grown silicon oxide, or SiON. The insulating film  116  changes the phase difference between waves reflected to the photoresist layer in a manner that enhances the effect of the ARC. 
     In this embodiment, the insulating film  116  is etched instead of the highly reflective layer  104  formed of conductive metals having a high reflexibility, such as W, WSi x , TiSi x , Al, or an Al alloy. Therefore, either the conductive metal layer or the insulating film can be the layer which is etched when using the ARC  106 . Since the other layers and the steps of forming these layers are the same as those described in connection with the first embodiment, descriptions thereof will be omitted. 
     As shown in FIG. 8, the transparent or semi-transparent insulating film  116  can be formed on the ARC  106 . Because the materials and the steps of forming the other layers are the same as those described in connection with the first embodiment, detailed descriptions thereof will be omitted. 
     Hereinafter, characteristics of an amorphous carbon film using only the hydrocarbon based gas formed, according to the present invention, will be described in detail. 
     FIG. 9 is a graph of the refractive index with respect to the temperature of the ARC. 
     Referring to FIG. 9, the temperature (° C.) of a plate to which the ARC according to the present invention was attached is plotted along the X axis, and the refractive index (n) is plotted along the Y axis. When the temperature was raised from 140° C. to 260° C., the refractive index was maintained within the desired range of from 1.2 to 2.5. Accordingly, an ARC having such a refractive index can exist as the various processes of forming the semiconductor device are carried out. 
     FIG. 10 is a graph of the extinction coefficient characteristic with respect to the temperature of the ARC. 
     Referring to FIG. 10, the temperature (° C.) of a plate to which the ARC according to the present was attached is plotted along the X axis whereas the extinction coefficient (k) is plotted along the Y axis. When the temperature was raised from 140° C. to 260° C., the extinction coefficient was maintained within the desired range of 0.2 to 0.8. Accordingly, an ARC having such an extinction coefficient can also exist as the various processes of forming the semiconductor device are carried out. 
     FIG. 11 is a graph of the thickness of the ARC according to the length of the deposition time of the ARC. 
     Referring to FIG. 11, the time (sec) during which the thin film was deposited using PECVD is plotted along the X axis whereas the thickness (Å) of the film is plotted along the Y axis. As shown in the graph, the thickness of the ARC increases in almost direct proportion to the deposition time. This shows that an amorphous carbon film of a desired thickness can be easily formed by controlling the deposition time. 
     FIG. 12 is a graph illustrating changes in the refractive index (n) and the extinction coefficient (k) with respect to changes in the thickness of the ARC according to the present invention. 
     Referring to FIG. 12, the thickness (Å) of the ARC is plotted along the X axis, and the extinction coefficient (k) and the refractive index (n) are plotted along the Y axes. In general, when an ARC is used in the manufacturing of a semiconductor device, its reflectivity depends on its thickness. Accordingly, the critical dimension (CD) varies. Therefore, in the prior art, it was of critical importance to form the ARC at a specific thickness. However, the ARC according to the present invention, that is, the amorphous carbon film formed using only the hydrocarbon based gas, has a uniform refractive index and extinction coefficient so long as the thickness thereof is not less than 150 Å. Accordingly, the ARC according to the present invention allows for much more tolerance in the processes of manufacturing a semiconductor device than does the ARC produced according to the prior art. 
     FIG. 13 is a graph showing the surface analysis of the ARC according to the present invention, using X-ray diffraction (XRD). 
     Referring to FIG. 13, the incident angle (2θ) is plotted along the X axis and the intensity (cps) of the incident light is plotted along the Y axis. ARCs which were formed to thicknesses of 100 Å and 300 Å according to the present invention were used as samples. In order to obtain a surface analysis result, the value of the intensity of light was obtained by fixing the samples and varying the angle of incidence of X-ray light (2θ) from 20 through 60 degrees. As indicated in the graph, the ARC according to the present invention has an amorphous structure. Therefore, the ARC has a relatively stable extinction coefficient and refractive index with respect to the light source used during exposure. 
     FIGS. 14 and 15 are graphs showing simulation results of the reflectivity according to the refractive index and the extinction coefficient of the ARC when the highly reflective layer is formed of Al. 
     FIG. 14 is a graph showing reflexibility when the refractive index of the ARC is fixed at a value of 1.81 and the thickness (the X axis of the graph) and the extinction coefficient (the legend of the graph) of the ARC are changed. FIG. 15 is a graph showing the reflexibility when the thickness of the ARC is fixed to a value of 300 Å, and the refractive index (the Y axis of the graph) and the extinction coefficient (the X axis of the graph) are changed. As shown in the graphs, the ARC has a reflexibility of less than 5% when the refractive index is between 1.6 and 2.4 and when the extinction coefficient is between 0.4 and 1.1. Therefore, the ARC according to the present invention is most effective as a mask in etching the highly reflective layer when its extinction coefficient is between 1.2 and 2.5 and its extinction coefficient is between 0.2 and 0.8. 
     FIGS. 16 and 17 are graphs showing simulation results of the reflexibility according to the refractive index and the extinction coefficient when the highly reflective film is formed of a silicon layer. 
     In FIGS. 16 and 17, the parameters measured are the same as those in FIGS. 14 and 15. In this case, however, in which a highly reflective layer of silicon was used, the ARC has a reflexibility of less than 5% when its refractive index is between 1.6 and 2.4 and its extinction coefficient is between 0.4 and 1.1. 
     Although the present invention has been described in detail above in connection with certain preferred embodiments thereof, the present invention is not limited to such embodiments. For example, PECVD has been disclosed as the preferred process for forming the ARC. However various other methods, such as a sputtering method, a chemical vapor deposition (CVD) method, and a laser ablation method can be used. Therefore, all such various changes and modifications are seen to be within the true spirit and scope of the present invention as defined by the appended claims.