Abstract:
This disclosure concerns a manufacturing method of a semiconductor device comprising an etching process using an etching solution having ozone dissolved by 10 ppm or more into a liquid containing H 2 SO 4  by 86 wt % to 97.9 wt %, HF by 0.1 wt % to 10 wt %, and H 2 O by 2 wt % to 4 wt %.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2006-29639, filed on Feb. 7, 2006 the entire contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a manufacturing method of a semiconductor device and an etching solution.  
         [0004]     2. Related Art  
         [0005]     At the time of etching silicon or polysilicon in an RIE (Reactive Ion Etching) process, a mixed gas of a halogen gas (Br gas and a CI gas, for example) and oxygen is used as an etching gas. In this case, a SiO 2  deposit is formed.  
         [0006]     At the time of etching a silicon oxide film or a silicon nitride film in the RIE process, a CF (fluorocarbon) gas is used as an etching gas. In this case, a CF deposit is formed.  
         [0007]     Therefore, when a lamination film of a polysilicon film and a silicon nitride film is continuously etched by RIE, both a SiO 2  deposit and a CF deposit are adhered to the etched result as etching residues. After this RIE, it is necessary to continuously execute a remove processing of the SiO 2  deposit and a remove processing of the CF deposit. For example, it is necessary to carry out a process as a combination of an oxidation process of the CF deposit by means of a dry ashing, an SPM (sulfuric acid and hydrogen peroxide mixture) process, an alkali process, a DHF (diluted hydrofluoric acid) process, a VPC (vapor HF) process, and the like. When these processes are combined, the number of steps of processes after the RIE increases. Further, the DHF process etches a gate insulation film and a silicon oxide film of STI (Shallow Trench Isolation) and the like. Therefore, the use of the DHF process after forming the STI and the gate insulation film is not preferable.  
       SUMMARY OF THE INVENTION  
       [0008]     A manufacturing method of a semiconductor device according to an embodiment of the present invention comprises an etching process using an etching solution having ozone dissolved by 10 ppm or more into a liquid containing H 2 SO 4  by 86 wt % to 97.9 wt %, HF by 0.1 wt % to 10 wt %, and H 2 O by 2 wt % to 4 wt %.  
         [0009]     A etching solution used in a manufacturing method of a semiconductor device according to an embodiment of the present invention, the etching solution comprises ozone dissolved by 10 ppm or more into a liquid containing H 2 SO 4  by 86 wt % to 97.9 wt %, HF by 0.1 wt % to 10 wt %, and H 2 O by 2 wt % to 4 wt %. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a schematic diagram showing an etching solution  5  used in the etching process according to an embodiment of the present invention;  
         [0011]      FIG. 2  is a cross-sectional diagram showing a gate structure after etching a lamination film below a BSG film  80  by RIE using the BSG film  80  as a hard mask;  
         [0012]      FIG. 3  shows a gate structure free from the etching residue  90  and the BSG film  80 ;  
         [0013]      FIG. 4  is a graph showing a relationship between an ozone content of the etching solution  5  according to the present embodiment and a temperature of this liquid; and  
         [0014]      FIG. 5  is a graph showing a relationship between contents of sulfuric acid, hydrofluoric acid, and water and time in the etching process according to the present embodiment. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]     Embodiments of the present invention will be explained below with reference to the accompanying drawings. The present invention is not limited to the embodiments.  
         [0016]      FIG. 1  is a schematic diagram showing an etching solution  5  used in the etching process according to an embodiment of the present invention. The etching solution  5  is a liquid having ozone (O 3 ) equal to or more than 10 ppm dissolved in a liquid containing sulfuric acid (H 2 SO 4 ) of 86 wt % to 97.9 wt %, hydrofluoric acid (HF) of 0.1 wt % to 10 wt %, and water (H 2 O) of 2 wt % to 4 wt %. This liquid is hereinafter also called SFOM. The above wt % means weight percent. The total weight percent of H 2 SO 4 , HF and H 2 O is 100 wt %.  
         [0017]     An etching solution (SFM) containing sulfuric acid and hydrofluoric acid as main components and water of 2 wt % to 4 wt % can etch the SiO 2  deposit at a high selection rate (100 to 300 or more) to a thermal oxide film. The SiO 2  deposits include SiOx, SiOxBry, SiOxFy, SiOxCy, SiOxBryCz, and SiOxBryCzFα (where x, y, z, and α are positive numbers). On the other hand, when an etching solution having a relatively high water content rate is used, a selection rate (a selectivity) of the SiO 2  deposit to the thermal oxide film becomes 100 or smaller. In other words, when the water content rate is set as low as 2 wt % to 4 wt %, the etching solution according to the present embodiment can selectively remove the SiO 2  deposit at a high selection rate.  
         [0018]     In general, commercially available sulfuric acid contains H 2 SO 4  of 96 wt % to 98 wt %, and H 2 O of 2 wt % to 4 wt %. Therefore, when commercially available anhydrofluoric acid is mixed into commercially available sulfuric acid, this etching solution can be prepared easily.  
         [0019]     In the present embodiment, ozone is further dissolved by 10 ppm or more into this etching solution, thereby generating the etching solution  5 . Because ozone is strongly oxidative, ozone can oxidize the CF deposit, and the hydrofluoric acid etches the oxidized CF deposit. CF deposits are CxFy and CxFyOz, for example (where x and y are positive numbers). In order to effectively oxidize the CF deposit, it is preferable to dissolve ozone into the etching solution by 10 ppm or more. Unlike ozone concentration in pure water, the ozone concentration in sulfuric acid has high saturation solubility. Therefore, ozone can be dissolve by about 10 ppm in a high temperature state of 150° C. At a lower temperature than this, solubility increases, and dissolved ozone concentration inevitably becomes high.  
         [0020]      FIG. 2  is a cross-sectional diagram showing a gate structure after etching a lamination film below a BSG film  80  by RIE using the BSG film  80  as a hard mask. The structure shown in  FIG. 2  is formed as follows. First, a tunnel gate insulation film  20  is formed on a silicon substrate  10 . The tunnel gate insulation film  20  is a silicon oxide film, for example. A material of a floating gate  30  is deposited on the tunnel gate insulation film  20 . The floating gate  30  is composed of polysilicon, for example. An ONO film  40  is deposited on the material of the floating gate  30 . The ONO film  40  is a lamination film of a silicon oxide film, a silicon nitride film and a silicon oxide film (SiO 2 —SiN—SiO 2 ), or a silicon oxynitride (SiON) film. A material of a control gate  50  is deposited on the ONO film  40 . The material of a control gate  50  is composed of polysilicon, for example. A silicide film  60  is formed on the material of the control gate  50 . The silicide film  60  can be any one of metal silicides including titanium silicide (TiSi), nickel silicide (NiSi), cobalt silicide (CoSi), tantalum silicide (TaSi), palladium silicide (PdSi), and platinum silicide (PtSi). A silicon nitride film  70  is deposited on the silicide film  60 . A BSG film  80  is deposited on the silicon nitride film  70 . The BSG film  80  is patterned into a gate electrode pattern using a photolithography technique.  
         [0021]     The silicon nitride film  70 , the silicide film  60 , the control gate  50 , the ONO film  40 , and the floating gate  30  are sequentially etched by RIE, using the patterned BSG film  80  as a hard mask. When the silicon nitride film  70  is etched by RIE using a gas containing CF in this etching process, a CF deposit is generated. When the control gate  50  is etched by RIE using a gas containing Br, an SiO 2  deposit is generated. When the ONO film  40  is etched by RIE using a gas containing CF, a CF deposit is generated. When the floating gate  30  is etched by RIE using a gas containing Br, an SiO 2  deposit is generated. As a result, an etching residue  90  is formed as a lamination on the sidewall of the gate structure as shown in  FIG. 2 .  
         [0022]     In order to remove the etching residue  90  formed as the lamination film, the gate structure shown in  FIG. 2  is immersed into the etching solution shown in  FIG. 1 . Because the etching solution  5  according to the present embodiment contains hydrofluoric acid, the SiO 2  deposit is etched. The etching solution  5  contains sulfuric acid as a main component, and contains water by only 2 wt % to 4 wt %. Therefore, this etching solution  5  etches the SiO 2  deposit while suppressing the etching of the tunnel gate insulation film  20  formed by a thermal oxide film or the ONO film  40  containing the thermal oxide film. Because the etching solution  5  contains sulfuric acid and ozone, a CF deposit can be oxidized, and this can be etched. In this way, the etching solution  5  can selectively remove the SiO 2  deposit and the CF deposit in the same process while suppressing the etching of the thermal oxide film.  
         [0023]     Further, because the BSG film  80  used as the hard mask is also an SiO 2  deposit, the etching solution  5  can also remove the BSG film  80  in the same process. As a result, a gate structure free from the etching residue  90  and the BSG film  80  can be obtained as shown in  FIG. 3 .  
         [0024]     The etching solution  5  according to the present embodiment has sulfuric acid (H 2 SO 4 ) as a main component. However, peroxosulfuric acid is considered to be generated due to the dissolving of ozone. Even when a part of usual sulfuric acid is peroxosulfuric acid, the effect of the present embodiment is not lost.  
         [0025]      FIG. 4  is a graph showing a relationship between an ozone content of the etching solution  5  according to the present embodiment and a temperature of this liquid. For comparison, a relationship between ozone content of ozone water and a temperature of this liquid is also shown. A line L 0  shows ozone content of conventional ozone water, and a line L 1  shows ozone content of the etching solution  5 . Both lines show saturation solubility at normal pressure. Ozone water is a liquid having ozone dissolved in pure water.  
         [0026]     It can be understood from the line L 0  that, in order to dissolve a large amount of ozone into pure water, the temperature of the pure water must be maintained at a low level. For example, when a liquid temperature is about 55° C., an ozone content rate in the pure water is only 0.5 ppm. The etching solution containing only such a small amount of ozone cannot effectively oxidize the CF deposit. Therefore, in order to dissolve a large amount of ozone into the etching solution containing water as a main component or containing much water, the etching solution needs to be kept at a low temperature. For example, in order to maintain the ozone content rate at or above 10 ppm, the liquid temperature needs to be set to about 30° C. or below. However, keeping the etching solution at a low temperature lowers the etching rate.  
         [0027]     On the other hand, it can be understood from the line L 1  that the etching solution  5  (SOFM) according to the present embodiment can maintain a high ozone content (10 ppm or above) over a wide range of temperature (0 to 150° C.). Particularly, when a temperature of the etching solution  5  is within a range from a room temperature (about 20° C.) to 150° C., the etching solution  5  can dissolve more ozone than the etching solution containing much water. For example, when a liquid temperature is 73° C., the ozone content is 40 ppm, and when a liquid temperature is 110° C., the ozone content is 25 ppm. Further, when a liquid temperature is 135° C., the ozone content is 10 ppm or more. Therefore, the etching solution  5  can sufficiently oxidize the CF deposit efficiently while increasing the etching rate of the etching residue  90 .  
         [0028]     The etching solution  5  (SOFM) can dissolve ozone of about 30 ppm at 55° C., for example. This etching solution  5  etches the thermal oxide film (SiO 2 ) at an etching rate of about one angstrom/minute. The etching solution  5  also etches the BSG (boron concentration=2*10 20  atoms/cm 3 ) formed by an LP-CVD (Low Pressure-Chemical Vapor Deposition) at an etching rate of about 3,000 angstrom/minute. In other words, a selection rate of the BSG to the thermal oxide film is about 3,000. Therefore, it is understood that the etching solution  5  can selectively etch the BSG film and the like from the thermal oxide film.  
         [0029]      FIG. 5  is a graph showing a relationship between contents of sulfuric acid, hydrofluoric acid, and water and time in the etching process according to the present embodiment. A temperature of the etching solution is 50° C. In this case, ozone concentration is determined as about 45 ppm, based on the liquid temperature and the saturation solubility. In the actual etching process, concentration of each of sulfuric acid, hydrofluoric acid, water, and ozone is measured with a concentration monitor. In order to keep these concentrations at a constant level, a predetermined chemical is automatically replenished.  
         [0030]     The etching solution  5  according to the present embodiment has a high water absorption rate because it contains sulfuric acid as a main content. Therefore, in order to suppress the increase of water content, replenishment of sulfuric acid and hydrofluoric acid becomes necessary. As a chemical for replenishing sulfuric acid, commercially available sulfuric acid containing 98% sulfuric acid and 2% water can be used. As a chemical for replenishing hydrofluoric acid, sulfuric acid/hydrofluoric acid (HF/H 2 SO 4 /H 2 O=10:88:2) manufactured by DAIKIN INDUSTRIES, Ltd., for example, can be used. With this arrangement, content of sulfuric acid, hydrofluoric acid, and water can be controlled at constant rates, respectively, as shown in  FIG. 5 . The content of ozone is determined based on the liquid temperature and the saturation solubility as described above. Therefore, the content of ozone can be maintained constant by itself, by keeping the liquid temperature of the etching solution  5  at a constant level and by exposing the liquid surface of the etching solution  5  in the ozone atmosphere at normal pressure as shown in  FIG. 1 .  
         [0031]     In the present embodiment, the etching process using the etching solution  5  can be a batch type or a wafer-feed type.  
         [0032]     The etching process according to the present embodiment can be applied to formation of a gate structure of a NAND flash memory, for example. Consequently, a product generated by etching (deposit) according to the RIE can be removed without giving damage to a tunnel gate oxide film and a shallow trench isolation (STI) oxide film. The etching process according to the present embodiment can be also performed after carrying out a resist ashing such as a dry ashing process.