Abstract:
An etching method includes: applying a radiation to an etching aqueous solution; and etching a material to be etched by using the etching aqueous solution irradiated with the radiation.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims the benefit of priority from Japanese Patent Application No. 2011-55988 filed on Mar. 14, 2011, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments relate to an etching method, a method for manufacturing a semiconductor device, and an etching device. 
     BACKGROUND 
     As a transistor element is miniaturized, a gate insulating film becomes thinner. 
     As the gate insulating film becomes thinner, a leakage current may increase and the reliability of operation of the transistor element may be reduced. 
     Related art is disclosed in Japanese Laid-open Patent Publication No. 2009-177007 and the like. 
     SUMMARY 
     According to an aspect of the embodiments, an etching method includes: applying a radiation to an etching aqueous solution; and etching a material to be etched by using the etching aqueous solution irradiated with the radiation. 
     Additional advantages and novel features of the invention will be set forth in part in the description that follows, and in part will become more apparent to those skilled in the art upon examination of the following or upon learning by practice of the invention. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1A  and  FIG. 1B  illustrate an exemplary etching device. 
         FIG. 2  illustrates an exemplary relationship between a chemical bond and a binding energy of an etching aqueous solution. 
         FIG. 3  illustrates an exemplary facilitation mechanism of generation of active species. 
         FIG. 4  illustrates an exemplary relationship between a chemical bond and a binding energy of a layer to be etched. 
         FIG. 5  illustrates an exemplary relationship between a distance from a catalyst layer and an ultraviolet intensity. 
         FIG. 6  illustrates an exemplary etching device. 
         FIG. 7A  and  FIG. 7B  illustrate an exemplary etching device. 
         FIG. 8  illustrates an exemplary etching device. 
         FIG. 9  illustrates an exemplary etching device. 
         FIG. 10  illustrates an exemplary average etching rate and in-plane uniformity. 
         FIG. 11  illustrates an exemplary emission spectrum. 
         FIG. 12  illustrates an exemplary etching rate. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A high dielectric material is used as a material for forming a gate insulating film. Since the gate insulating film having a high dielectric constant is used, the thickness of the insulating film is increased while the electrical characteristics are maintained so that a leakage current may be reduced. 
     Examples of the high dielectric materials for forming the gate insulating film may include aluminum oxide and hafnium oxide. 
     In the manufacturing process of a transistor element, for example, a high dielectric film provided with a patterned resist layer is etched and a gate insulating film is formed. 
     For example, wet etching by using an etching aqueous solution may be employed. The etching selectivity between the high dielectric film and a layer under the high dielectric film may be given to the etching aqueous solution by pH adjustment, addition of a catalyst, or the like with respect to the etching aqueous solution. The high dielectric film is wet etched and the gate insulating film is produced. 
     Regarding the wet etching, the etching rate of the high dielectric film with the etching aqueous solution is small and, therefore, formation of a gate oxide film may take much time. 
     The high dielectric film may be etched through dry etching using plasma or the like. The etching rate of dry etching of the high dielectric film may be larger than that of wet etching. Regarding the dry etching, the etching selectivity between the high dielectric film and a layer under the high dielectric film is low and, therefore, the layer under the high dielectric film may be etched together with the high dielectric film. 
       FIG. 1A  and  FIG. 1B  illustrate an exemplary an etching device.  FIG. 1A  is a plan view and  FIG. 1B  is a sectional view of a section taken along the line IB-IB illustrated in  FIG. 1A . 
     An etching device  10  illustrated in  FIG. 1A  and  FIG. 1B  includes radiation sources  12  to apply a radiation  11  to an etching aqueous solution  20  and an etching bath  13  to hold the etching aqueous solution  20  irradiated with the radiation  11 . The etching device  10  may be used for the purpose of fabricating a semiconductor device. 
     A layer to be etched  30  etched by the etching device  10  may be, for example, a high dielectric film provided with a patterned resist layer. The layer to be etched  30  is formed on a semiconductor substrate  31 . The shapes of both the layer to be etched  30  and the semiconductor substrate  31  may be a circle in plan view. 
     Examples of materials for forming the high dielectric film include aluminum oxide, hafnium oxide, zirconium oxide, and tantalum oxide. 
     In the etching device  10 , the radiation  11  is applied to the etching aqueous solution  20 , and an active species to functionalize the surface of a portion, which is exposed at an opening portion of the resist layer, of the layer to be etched  30  with a hydroxyl group (OH) is generated from the etching aqueous solution  20 . Hydroxyl groups are formed on the surface of the layer to be etched  30  by the active species generated from the etching aqueous solution  20 . The layer to be etched  30  having a surface functionalized with the hydroxyl group is dissolved by the etching aqueous solution  20  easily and, thereby, the etching rate is improved. The radiation  11  is continued to be applied to the etching aqueous solution  20 , so that active species are generated continuously from the etching aqueous solution  20 . Likewise, a fresh surface of the layer to be etched  30 , which has been exposed to the etching aqueous solution  20  through etching of the layer to be etched  30 , is also etched by the active species after a hydroxyl group is formed. 
     The radiation source  12  is disposed opposing to the layer to be etched  30 . The radiation emitted from the radiation source  12  gives energy to a water molecule serving as a solvent of the etching aqueous solution  20  or a molecule forming a solute of the etching aqueous solution  20 , so as to cut bonds in these molecules and generate active species. Examples of the radiation  11  emitted from the radiation source  12  may include electromagnetic waves, e.g., ultraviolet rays, corpuscular beams, e.g., electron beams, and the like. 
     As for the radiation source  12  of the etching device  10  illustrated in  FIG. 1A  and  FIG. 1B , an ultraviolet lamp to emit ultraviolet rays may be used. The etching device  10  includes two ultraviolet lamps as the radiation sources  12 . An electric power may be fed to the radiation source  12  from a power supply, although not illustrated in the drawing. 
     The etching aqueous solution  20  is held in an etching bath  13 . A semiconductor  31  with the layer to be etched  30  is disposed in the inside of the etching bath  13 , and the layer to be etched  30  is immersed in the etching aqueous solution  20 . In the etching bath  13 , the portion, which is exposed at the opening portion of the resist layer, of the layer to be etched  30  is etched with the etching aqueous solution  20  irradiated with the radiation  11 . 
     The etching bath  13  includes a stage  17  to place the semiconductor substrate  31  with the layer to be etched  30 . The semiconductor substrate  31  is fixed to the stage  17  through adsorption. The stage  17  is rotated by a rotation unit, although not illustrated in the drawing. The surface of the layer to be etched  30  may be etched uniformly by rotating the layer to be etched  30  in the etching aqueous solution  20 . 
     The etching bath  13  includes a liquid feed pipe  18  to feed the etching aqueous solution  20  from the outside to the inside and a liquid discharge pipe  19  to discharge the etching aqueous solution  20  in the inside to the outside. A fresh etching aqueous solution  20  is fed into the etching bath  13  from the liquid feed pipe  18  and, in addition, etching products generated through etching of the layer to be etched  30  or an exhausted etching aqueous solution  20  is discharged from the liquid discharge pipe  19  to the outside. 
     The etching bath  13  includes radiation windows  15  to transmit the radiation  11  at portions opposing to the radiation sources  12 . 
     The material for forming the radiation window  15  may be a material which transmits the radiation  11  and, in addition, which is not etched with the etching aqueous solution  20 . In the case where ultraviolet rays are used as the radiation  11 , for example, magnesium fluoride, calcium fluoride, or quartz glass may be used as the material for forming the radiation window  15 . As for the quartz glass, synthesized quartz glass may be used. 
     A catalyst layer  16   a  including a catalyst, which facilitates generation of active species to functionalize the surface of the layer to be etched  30  with a hydroxyl group from the etching aqueous solution  20 , is disposed inside the radiation windows  15 . The etching device  10  applies the radiation  11  to the etching aqueous solution  20  through the catalyst layer  16   a . The surface of the catalyst layer  16   a  may be in contact with the etching aqueous solution  20  held in the etching bath  13 . 
     As illustrated in  FIG. 1A , the catalyst layer  16   a  may be disposed in such a way as to cover the layer to be etched  30 . In  FIG. 1A , the shape in plan view of the catalyst layer  16   a  may be substantially equal or analogous to the shape of the layer to be etched  30 , for example, the shape of a circle. The catalyst layer  16   a  scatters the applied radiation  11  to the direction parallel to the layer and, therefore, the catalyst layer  16   a  facilitates generation of the active species. The active species may be generated in the plane direction of the layer to be etched  30  uniformly. 
     A catalyst layer  16   b  including a catalyst, which facilitates generation of the active species to functionalize the surface of the layer to be etched  30  with a hydroxyl group from the etching aqueous solution  20 , is disposed inside the etching bath  13  except the portions of the radiation windows  15  and the stage  17 . The surface of the catalyst layer  16   b  may be in contact with the etching aqueous solution  20  held in the etching bath  13 . 
     As illustrated in  FIGS. 1A and 1B , the catalyst layer  16   b  is disposed in such a way as to surround the layer to be etched  30 . Therefore, the active species are generated throughout the etching aqueous solution  20  and the resulting active species are fed to the layer to be etched  30 . 
     The etching device  10  applies the radiation  11  to the catalyst layer  16   a , the catalyst layer  16   b , and the etching aqueous solution  20 . 
     The catalyst for forming the catalyst layer  16   a  or the catalyst layer  16   b  may be titanium oxide or titanium oxide including at least one of antimony, chromium, and nickel. 
     The catalyst for forming the catalyst layer  16   a  or the catalyst layer  16   b  may be strontium titanate or strontium titanate including at least one of antimony, chromium, and nickel. 
     The catalyst layer  16   a  and the catalyst layer  16   b  may be formed by using substantially the same catalyst or be formed by using different catalysts. 
     The etching bath  13  may include an opening and closing unit, although not illustrated in the drawing, to open and close the radiation windows  15 . The semiconductor substrate  31  including the layer to be etched  30  is taken into or taken out of the etching bath  13  by opening or closing of radiation windows  15  with the opening and closing unit. 
     The etching device  10  includes reflection portions  14  to reflect the radiation  11  applied from the radiation sources  12  and apply the radiation  11  to the etching aqueous solution  20 . 
     The reflection portion  14  has a concave shape with an opening portion open toward the radiation window  15  and the radiation source  12  is disposed in the inside. 
     The reflection portion  14  is formed by using a material which reflects the radiation  11 . For example, in the case where ultraviolet rays are used as the radiation  11 , the reflection portion  14  may include a metal plate of aluminum or the like. In the case where the radiation source  12  has directivity with respect to the direction of emission of the radiation  11 , the etching device  10  may not include the reflection portion  14 . 
     As illustrated in  FIG. 1B , the radiation  11  including ultraviolet rays is emitted in all directions from the radiation source  12  including the ultraviolet lamp. The radiation  11  emitted toward the opening portion of the reflection portion  14  is passed through the radiation window  15  and is applied to the etching aqueous solution  20  in the etching bath  13 . The radiation  11  emitted in directions other than the direction toward the opening portion of the reflection portion  14  is reflected at the reflection portion  14  and is passed through the opening portion and the radiation window  15 , so as to be applied to the etching aqueous solution  20  in the etching bath  13 . Most of the radiation  11  passed through the radiation windows  15  is passed through the catalyst layer  16   a  and is applied to the etching aqueous solution  20 . Generation of the active species is facilitated in the etching aqueous solution  20  in contact with the catalyst layer  16   a.    
     The radiation  11  passed through a portion not provided with the catalyst layer  16   a  of the radiation windows  15  is not absorbed by the catalyst layer  16   a  and, therefore, may have high radiation intensity. The radiation  11  passed through a portion not provided with the catalyst layer  16   a  of the radiation windows  15  is passed through the etching aqueous solution  20  and reaches the catalyst layer  16   b . Generation of the active species is facilitated in the etching aqueous solution  20  in contact with the catalyst layer  16   b.    
     The etching aqueous solution  20  held in the etching bath  13  of the etching device  10  may be selected in accordance with, for example, the material for the layer to be etched  30  or the layer under the layer to be etched  30 . For example, in the case where the layer to be etched  30  is a high dielectric film, examples of the etching aqueous solution  20  include a potassium hydroxide (KOH) aqueous solution, a sodium hydroxide (NaOH) aqueous solution, a hydrofluoric acid (HF) aqueous solution, an ozone (O 3 ) aqueous solution, a hydrogen peroxide (H 2 O 2 ) aqueous solution, and a tetramethylammonium hydroxide (TMAH) aqueous solution. 
     The selectivity between the high dielectric film and the layer under the high dielectric film may be given to the etching aqueous solution by performing pH adjustment of the etching aqueous solution or adding a catalyst to the etching aqueous solution. 
     Examples of the active species, which is generated from the etching aqueous solution  20  irradiated with the radiation  11  and which functionalizes the surface of the layer to be etched  30  with a hydroxyl group, may include a hydroxyl radical (OH.) generated from a water molecule (H 2 O) of the etching aqueous solution  20 . The hydroxyl radical (OH.) may be generated from potassium hydroxide (KOH), sodium hydroxide (NaOH), or tetramethylammonium hydroxide (TMAH), which has a hydroxyl group, or hydrogen peroxide (H 2 O 2 ). 
     Examples of active species may include active oxygen, e.g., super oxide anion (.O 2   − ). The super oxide anion (.O 2   − ) may be generated from, for example, an ozone (O 3 ) aqueous solution, a hydrogen peroxide (H 2 O 2 ) aqueous solution, or dissolved oxygen (O 2 ) in an etching aqueous solution. 
       FIG. 2  illustrates an exemplary relationship between a chemical bond and a binding energy of an etching aqueous solution. 
     For example, regarding the water molecule (H 2 O), the binding energy between H and OH is 4.6 eV. A radiation (ultraviolet ray), which is an electromagnetic wave with a wavelength of 268 nm, is applied to a water molecule (H 2 O), and a hydroxyl radical (OH.) is generated through direct transition. The radiation may be corpuscular beams, e.g., an electron beam, having incident energy of 4.6 eV. Regarding the direct transition, the energy held by the radiation may be used for cutting a chemical bond. 
     An electromagnetic wave having energy larger than the binding energy between H and OH (4.6 eV) and a wavelength smaller than 268 nm may be applied to the etching aqueous solution and a hydroxyl radical (OH.) may be generated through indirect transition. As for the radiation, corpuscular beams, e.g., an electron beam having incident energy larger than 4.6 eV may be used. Regarding indirect transition, the radiation may have energy larger than the binding energy of the chemical bond to be cut in order to generate the active species. Regarding indirect transition, for example, the energy held by the radiation may be absorbed at some other place and, thereafter, a part of the absorbed energy may be used for cutting a chemical bond in order to generate the active species. 
       FIG. 3  illustrates an exemplary facilitation mechanism of generation of active species. In  FIG. 3 , generation of the active species is facilitated by the catalyst. As for the catalyst to form the catalyst layers  16   a  and  16   b , titanium oxide (TiO 2 ) may be used. 
     As illustrated in  FIG. 3 , a radiation (energy hν) is applied to titanium oxide. A pair of hole and electron is generated from two molecules of titanium oxide which have received the energy, so that a titanium oxide molecule having a hole (TiO 2 (h + )) and a titanium oxide molecule having an electron (TiO 2 (e − ) are formed, where h represents the Planck constant and ν represents a frequency of the radiation. 
     The titanium oxide molecule having a hole (TiO 2 (h + )) and a water molecule (H 2 O) are reacted, so that a titanium oxide molecule (TiO 2 ), a hydrogen ion (H + ), and a hydroxyl radical (OH.) are generated. 
     In the case where titanium oxide is present, the activation energy may be reduced as compared with that in the case where a hydroxyl radical (OH.) is generated from only a water molecule (H 2 O). Consequently, generation of the hydroxyl radical (OH.) may be facilitated. 
     The etching device  10  applies the radiation  11  to the etching aqueous solution  20  and, thereby, a solute of the etching aqueous solution  20  is dissociated. The dissociated solute facilitates etching of the layer to be etched  30 . 
     As illustrated in  FIG. 2 , for example, regarding a hydrofluoric acid (HF) aqueous solution, the binding energy between H and F in hydrofluoric acid (HF) serving as a solute is 5.6 eV. The hydrofluoric acid (HF) aqueous solution may be dissociated into H and F through direct transition by application of a radiation (ultraviolet ray), which is an electromagnetic wave with a wavelength of 220 nm, to the hydrofluoric acid (HF) aqueous solution. Corpuscular beams, e.g., an electron beam, may be used for the dissociation of the solute. The solute may be dissociated through indirect transition. 
     In the etching device  10 , the radiation  11  may not be applied to the layer to be etched  30 . When the radiation  11  is applied to the surface of the layer to be etched  30 , the chemical bond of the layer to be etched  30  may be cut so that a dangling bond may be formed or the surface may be damaged. For example, when a dangling bond is present on the surface of a high dielectric film serving as a gate insulating film of a transistor element, a surface level due to the dangling bond is formed. The surface level serves as a center of recombination of carrier and the electrical characteristics of the gate insulating film may be degraded. 
       FIG. 4  illustrates an exemplary relationship between a chemical bond and a binding energy of a layer to be etched. 
     As illustrated in  FIG. 4 , for example, the binding energy between Al and O in aluminum oxide is 5.3 eV and, therefore, Al and O may be dissociated by being irradiated with a radiation corresponding to an electromagnetic wave, for example, ultraviolet rays, with a wavelength of 234 nm. The binding energy between Hf and O in hafnium oxide is 8.3 eV and, therefore, Hf and O may be dissociated by being irradiated with a radiation corresponding to an electromagnetic wave, for example, ultraviolet rays, with a wavelength of 149 nm. 
     In the etching device  10 , the distance L between the catalyst layer  16   a  and the layer to be etched  30  is specified in such a way that the radiation  11  is not applied to the layer to be etched  30 . 
     The radiation  11  is absorbed by the etching aqueous solution  20 . Therefore, when the distance L between the catalyst layer  16   a  and the layer to be etched  30  is large, arrival of the radiation  11  at the layer to be etched  30  may be reduced. 
     The distance L between the catalyst layer  16   a  and the layer to be etched  30  may be small since the active species generated on the surface of the catalyst layer  16   a  move to the surface of the layer to be etched  30  promptly. 
     In the etching device  10 , the absorptance of the radiation  11  by the catalyst layer  16   a  is adjusted and the etching aqueous solution  20  absorbs the radiation  11 . Consequently, application of the radiation  11  to the layer to be etched  30  may be reduced. 
     For example, the distance L between the catalyst layer  16   a  and the layer to be etched  30  may be set at the position where the radiation  11  is attenuated sufficiently through absorption by the etching aqueous solution  20 . The position where the radiation  11  is attenuated sufficiently through absorption by the etching aqueous solution  20  may corresponds to the position where the radiation  11  may not cut the chemical bond of the layer to be etched  30 . The intensity of the radiation  11  at which the chemical bond of the layer to be etched  30  may not be cut may be, for example, the ultraviolet intensity measured with an illuminometer may be less than or equal to the lower limit value of measurement. 
       FIG. 5  illustrates an exemplary relationship between a distance from a catalyst layer and the ultraviolet ray intensity. The ultraviolet ray may be a radiation  11 . 
     As illustrated in  FIG. 5 , in the etching device  10 , the surface of the layer to be etched  30  is disposed at the position where the ultraviolet ray is attenuated sufficiently through absorption by the etching aqueous solution  20 . 
     The amount of absorption of the radiation  11  by the catalyst layer  16   a  may be larger than the amount of absorption of radiation by the etching aqueous solution  20  in the portion between the catalyst layer  16   a  and the layer to be etched  30 . The amount of absorption of the radiation  11  by the catalyst layer  16   a  may be larger than the amount of absorption of water by the etching aqueous solution  20 . The amount of absorption of the radiation by the etching aqueous solution  20  between the catalyst layer  16   a  and the layer to be etched  30  may be reduced. Since the distance L between the catalyst layer  16   a  and the layer to be etched  30  is reduced, the active species generated in the catalyst layer  16   a  may be fed into the plane of the layer to be etched  30  sufficiently. The amount of absorption of the radiation  11  by the catalyst layer  16   a  may be increased by selection of the material for the catalyst layer  16   a  or adjustment of the thickness of the catalyst layer  16   a.    
     The etching rate of the etching device  10  may be improved. When the etching device  10  performs etching of a high dielectric film with the etching aqueous solution  20 , the time for forming a gate oxide film of the transistor element is reduced. 
     In the etching device  10 , the surface of the layer to be etched  30  is etched uniformly because the radiation  11  is applied to the layer to be etched  30  in a face-to-face manner. The catalyst layer  16   a  is disposed in such a way as to cover the whole layer to be etched  30  and, in addition, the catalyst layer  16   b  surrounds the layer to be etched  30 , wherein the catalyst layers  16   a  and  16   b  are provided respectively on an upper inner-wall of the etching bath, and a side inner-wall of the etching bath and a bottom inner-wall of the etching bath, as illustrated in  FIGS. 1A-1B . Therefore, functionalization with the hydroxyl group is performed while the active species are fed to the surface of the layer to be etched  30  uniformly and sufficiently. Consequently, the surface of the layer to be etched  30  is etched more uniformly. 
       FIG. 6  illustrates an exemplary etching device.  FIG. 6  is a sectional view of the etching device. 
     The etching device  10  illustrated in  FIG. 6  does not include the catalyst layer  16   a  nor catalyst layer  16   b . The structure illustrated in  FIG. 6  may be substantially the same or similar to the structure of the etching device illustrated in  FIGS. 1A and 1B . 
     A catalyst layer is not disposed inside the radiation window  15 . Therefore, the radiation  11  is applied to the etching aqueous solution  20  and, in addition, the material to be etched. The radiation  11  is not absorbed by a catalyst layer. The amount of generation of active species increases because the radiation  11  activates the etching aqueous solution  20  directly. 
     For example, the layer to be etched  30  is removed from the semiconductor substrate  31  through etching and, thereby, the semiconductor substrate  31  may be regenerated. The layer to be etched  30  is removed. Therefore, a dangling bond may be formed in the layer to be etched  30  irradiated with the radiation  11  or the layer to be etched  30  may be damaged. A resist layer may not be formed on the layer to be etched  30 . Alternatively, a resist layer may be removed. 
       FIG. 7A  and  FIG. 7B  illustrate an exemplary etching device.  FIG. 7A  is a plan view of the etching device.  FIG. 7B  is a sectional view of a section taken along the line VIIB-VIIB illustrated in  FIG. 7A . 
     The etching device  10  includes a liquid feed pipe  18  which transmits the radiation  11  and which feeds the etching aqueous solution  20  into the etching bath  13 . The radiation sources  12  apply the radiation  11  to the etching aqueous solution  20  by passing the radiation  11  through the liquid feed pipe  18 . The other configurations may be substantially the same or similar to the configurations illustrated in  FIGS. 1A and 1B . 
     The liquid feed pipe  18  transmits the radiation  11  and, in addition, may include a material which is not etched with the etching aqueous solution  20 . When ultraviolet rays are used as the radiation  11 , examples of the materials for the liquid feed pipe  18  may include magnesium fluoride, calcium fluoride, and quartz glass. The quartz glass may be synthesized quartz glass. 
     The shape of the liquid feed pipe  18  of the etching device  10  illustrated in  FIG. 7A  may be a circular cylinder. The liquid feed pipe  18  is disposed in such a way as to be located at the center of the circular layer to be etched  30  and is piped to the etching bath  13 . 
     In the etching device  10 , two portrait radiation sources  12  are disposed oppositely with the liquid feed pipe  18  therebetween. Each radiation source  12  is disposed along the side surface of the portrait liquid feed pipe  18 . The radiation  11  emitted from the radiation source  12  is applied to the liquid feed pipe  18  directly or after being reflected at a reflection portion  14 . The radiation  11  passed through the liquid feed pipe  18  is applied to the etching aqueous solution  20  in the liquid feed pipe  18 . 
     In the liquid feed pipe  18 , active species to functionalize the surface of the layer to be etched  30  with the hydroxyl group are generated from the etching aqueous solution  20  irradiated with the radiation  11 . 
     As illustrated in  FIG. 7B , the etching aqueous solution  20  is fed to the liquid feed pipe  18  from above. The etching aqueous solution  20  irradiated with the radiation  11  in the liquid feed pipe  18  flows together with generated active species from the bottom of the liquid feed pipe  18  toward the layer to be etched  30  in the etching bath  13 . The radiation  11  may not be applied to the layer to be etched  30  directly. 
     In the etching bath  13 , the layer to be etched  30  is fixed to the stage  17  through adsorption with the semiconductor substrate  31  therebetween. The etching aqueous solution  20  is fed to the layer to be etched  30  together with the active species from the liquid feed pipe  18  disposed above. The surface of the portion, which is exposed at an opening portion of a resist layer, of the layer to be etched  30  is functionalized with the hydroxyl group and, in addition, is etched. 
     The etching bath  13  includes liquid discharge pipes  19 , which discharge the etching aqueous solution  20  from the inside, at the individual end portions opposite to each other. Etching products generated through etching of the layer to be etched  30  or an exhausted etching aqueous solution  20  is discharged from the liquid discharge pipe  19  to the outside. 
     The distance M between the liquid feed pipe  18  and the surface of the layer to be etched  30  may refer to a distance between the lower end portion, which is coupled to the etching bath  13 , of the liquid feed pipe  18  and the surface of the layer to be etched  30 . 
     The radiation  11  may not be applied to the layer to be etched  30  directly, but a part of the radiation  11  may enter the etching bath  13  in the vicinity of the lower end portion of the liquid feed pipe  18 . The distance M between the liquid feed pipe  18  and the surface of the layer to be etched  30  may be specified in such a way that the radiation  11  is not applied to the layer to be etched  30 . 
     The distance M between the lower end portion of the liquid feed pipe  18  and the surface of the layer to be etched  30  may be small since the active species generated in the liquid feed pipe  18  move to the surface of the layer to be etched  30  promptly. 
     In the etching device  10 , the etching aqueous solution  20  absorbs the radiation  11  and, therefore, the layer to be etched  30  may not be irradiated with the radiation  11 . 
     For example, the distance M between the liquid feed pipe  18  and the surface of the layer to be etched  30  may be set at the position where the radiation  11  is attenuated sufficiently through absorption by the etching aqueous solution  20 . 
     The etching bath  13  may include an opening and closing unit, although not illustrated in the drawing, to open and close a part of the etching bath  13 . The semiconductor substrate  31  including the layer to be etched  30  is taken into or is taken out of the etching bath  13  by the opening or closing unit. 
     The etching device  10  reduces application of the radiation  11  to the layer to be etched  30 . The etching rate of the etching device  10  may be improved. 
     A catalyst layer may be disposed inside the liquid feed pipe  18  of the etching device  10  illustrated in  FIGS. 7A and 7B . 
       FIG. 8  illustrates an exemplary etching device. 
     The inside diameter of the liquid feed pipe  18  of the etching device  10  may be substantially equal to the diameter of the layer to be etched  30 . The etching aqueous solution  20  is fed together with the active species to all over the surface of the layer to be etched  30  from the liquid feed pipe  18  disposed above. 
     Regarding the etching device  10  illustrated in  FIG. 8 , the etching rate of the surface of the layer to be etched  30  may become uniform. 
     The inside diameter of the liquid feed pipe  18  may be larger than the diameter of the layer to be etched  30 . 
       FIG. 9  illustrates an exemplary etching device. 
     The etching device  10  illustrated in  FIG. 9  includes a plurality of liquid feed pipes  18 . Each liquid feed pipe  18  is provided with radiation sources  12  opposite to each other. Each radiation source  12  is provided with a reflection portion  14 . The etching device  10  illustrated in  FIG. 9  includes three liquid feed pipes  18 . The individual liquid feed pipes  18  are disposed at positions which divide the layer to be etched  30  into 3 equal parts in the circumferential direction. The etching aqueous solution  20  is fed uniformly together with the active species to the layer to be etched  30 . 
     The etching device  10  illustrated in  FIG. 9  includes a plurality of liquid feed pipes  18  and, therefore, the etching aqueous solution  20  including the active species is fed to the layer to be etched  30 . Functionalization of the surface of the layer to be etched  30  with the hydroxyl group may be further facilitated and the etching rate may be improved. In the etching device  10 , the etching aqueous solution  20  is fed uniformly together with the active species to the layer to be etched  30  from the plurality of liquid feed pipes  18 . Consequently, the layer to be etched  30  is etched with good in-plane uniformity. 
     The etching rate of the layer to be etched and the in-plane uniformity of etching of the etching device illustrated in  FIGS. 1A and 1B  are measured. 
     As for the material for forming a layer to be etched, aluminum oxide was used. The layer to be etched having a thickness of 40 nm was formed on a silicon substrate having a diameter of 3 inches. A resist layer was not formed on the layer to be etched. As for an etching aqueous solution, a hydrofluoric acid aqueous solution having a concentration of 10 percent by mass was used. As for a radiation source, an ultraviolet lamp with a wavelength of 172 nm was used. The ultraviolet irradiance at a position of an outside surface of a radiation window of the etching device was 10 mW/cm 2 . The distance between the catalyst layer  16   a  and the layer to be etched  30  was 2 mm. The etching time was 5 minutes. The layer to be etched was etched with the etching device (Etching  1 ). 
     As for an etching aqueous solution, a potassium hydroxide aqueous solution having a concentration of 6 percent by mass was used, and other etching conditions were specified to be substantially the same as those in Etching  1  (Etching  2 ). 
     As for an etching aqueous solution, a TMAH aqueous solution having a concentration of 2.38 percent by mass was used, and other etching conditions were specified to be substantially the same as those in Etching  1  (Etching  3 ). 
     As for the material for a layer to be etched, hafnium oxide was used, and other etching conditions were specified to be substantially the same as those in Etching  1  (Etching  4 ). 
     Ultraviolet rays were not applied to an etching aqueous solution, and other etching conditions were specified to be substantially the same as those in Etching  1  (Etching  5 ). 
     Ultraviolet rays were not applied to an etching aqueous solution, and other etching conditions were specified to be substantially the same as those in Etching  2  (Etching  6 ). 
     Ultraviolet rays were not applied to an etching aqueous solution, and other etching conditions were specified to be substantially the same as those in Etching  3  (Etching  7 ). 
     Ultraviolet rays were not applied to an etching aqueous solution, and other etching conditions were specified to be substantially the same as those in Etching  4  (Etching  8 ). 
     The thicknesses of the layers to be etched in Etching  1  to Etching  8  were measured with an ellipsometer. The measurement of the thickness was performed at nine points in total of the center of the circular layer to be etched and positions located at one-half of radius and in the vicinity of the edge, where center angles were 0 degrees, 90 degrees, 180 degrees, and 270 degrees. The etching rate was specified to be the value obtained by dividing the amount of decrease in thickness at each of the nine measurement points by the etching time. The average etching rate was specified to be the average value of etching rates at the nine measurement points. 
     The in-plane uniformity of etching rate was determined by using the etching rates at the nine measurement points. The in-plane uniformity was calculated as a difference between the maximum value and the minimum value of the etching rates at the nine measurement points. 
       FIG. 10  illustrates an exemplary average etching rate and in-plane uniformity.  FIG. 10  indicates the average etching rate and the in-plane uniformity in each of Etching  1  to Etching  8 . 
     As illustrated in  FIG. 10 , the average etching rates in Etching  1  to Etching  4  are larger than the average etching rates in Etching  5  to Etching  8 , and the uniformity is equal to or better than that in Etching  5  to Etching  8 . 
     An illuminometer was disposed at the position of the layer to be etched in the same manner as in Etching  1 , and the ultraviolet intensity at the position of the surface of the layer to be etched and the emission spectrum of the active species were measured.  FIG. 11  illustrates an exemplary emission spectrum. 
       FIG. 11  illustrates the ultraviolet intensity at the position of the surface of the layer to be etched and the emission spectrum of the active species. 
     As illustrated in  FIG. 11 , at a wavelength of 172 nm, the ultraviolet intensity was lower than or equal to the lower limit value of measurement and, therefore, ultraviolet rays did not reach the surface of the layer to be etched, so that ultraviolet rays were not applied to the surface of the layer to be etched. At a wavelength of 305 nm, a peak of emission spectrum of hydroxyl radical (OH.) serving as an active species was detected, so that the hydroxyl radical (OH.) reached the surface of the layer to be etched. 
     The etching rate of the layer to be etched was measured by using the etching device illustrated in  FIGS. 7A and 7B . 
     As for the material for forming a layer to be etched, aluminum oxide was used. The layer to be etched having a thickness of 40 nm was formed on a silicon substrate having a diameter of 3 inches. The inside diameter of the liquid feed pipe was 4 cm. As for an etching aqueous solution, a hydrofluoric acid aqueous solution having a concentration of 10 percent by mass was used. As for a radiation source, an ultraviolet lamp with a wavelength of 172 nm was used. The ultraviolet irradiance on the outside surface of the liquid feed pipe of the etching device was 10 mW/cm 2 . The distance between the lower end portion of the liquid feed pipe and the surface of the layer to be etched was 2 mm. The etching time was 5 minutes. The layer to be etched was etched with the etching device (Etching  11 ). 
     As for an etching aqueous solution, a potassium hydroxide aqueous solution having a concentration of 6 percent by mass was used, and other etching conditions were specified to be substantially the same as those in Etching  11  (Etching  12 ). 
     As for an etching aqueous solution, a TMAH aqueous solution having a concentration of 2.38 percent by mass was used, and other etching conditions were specified to be substantially the same as those in Etching  11  (Etching  13 ). 
     As for the material for a layer to be etched, hafnium oxide was used, and other etching conditions were specified to be substantially the same as those in Etching  11  (Etching  14 ). 
     Ultraviolet rays were not applied to an etching aqueous solution, and other etching conditions were specified to be substantially the same as those in Etching  11  (Etching  15 ). 
     Ultraviolet rays were not applied to an etching aqueous solution, and other etching conditions were specified to be substantially the same as those in Etching  12  (Etching  16 ). 
     Ultraviolet rays were not applied to an etching aqueous solution, and other etching conditions were specified to be substantially the same as those in Etching  13  (Etching  17 ). 
     Ultraviolet rays were not applied to an etching aqueous solution, and other etching conditions were specified to be substantially the same as those in Etching  14  (Etching  18 ). 
     The average etching rates of the layers to be etched in Etching  11  to Etching  18  were determined in the same manner as that in Etching  1  to Etching  8 . 
       FIG. 12  illustrates an exemplary average etching rate. The etching rates illustrated in  FIG. 12  indicate the average etching rates in Etching  11  to Etching  18 . 
     As illustrated in  FIG. 12 , the average etching rates in Etching  11  to Etching  14  are larger than the average etching rates in Etching  15  to Etching  18 . 
     Example of embodiments of the present invention have now been described in accordance with the above advantages. It will be appreciated that these examples are merely illustrative of the invention. Many variations and modifications will be apparent to those skilled in the art.