Abstract:
According to an aspect of the present invention, there is provided a method for correcting a defect in an EUV mask, the method including: preparing an EUV mask including an absorption layer and an anti-reflection layer forming a pattern; recognizing a defect region in the pattern; defining a first region and a second region on the defect region, the second region extending from a desired pattern edge by a given distance, the first region being defined on the rest; removing the first region of the anti-reflection layer and the absorption layer by irradiating a beam in a first atmosphere; removing the second region of the anti-reflection layer and the absorption layer by irradiating the beam in a second atmosphere; and oxidizing an exposed side surface of the desired pattern edge of the absorption layer.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from Japanese Patent Application No. 2008-279909 filed on Oct. 30, 2008, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     An aspect of the present invention relates to a defect correction method for an EUV mask. 
     2. Description of the Related Art 
     At present, transmission-type photomasks are used as exposure masks in manufacture of semiconductor integrated circuits. With those photomasks, reduction exposure is performed using mainly ultraviolet light of 248 nm or 193 nm in wavelength. In many cases, the optical magnification of the reduction exposure is set at ¼. Such transmission-type photomask will continue to be used for manufacturing memory devices whose half pitch is longer than about 30 to 40 nm, in view of their physical properties and manufacturing costs. 
     On the other hand, to manufacture further-miniaturized devices, other methods are being studied. One method is an extreme ultraviolet (EUV) photo-exposure technology. Photomasks (hereinafter referred to as EUV masks) in the EUV photo-exposure technology are used in reflection-type projection optical systems and thus different in structure from the transmission-type photomasks. The optical magnification in the EUV photo-exposure technology will remain ¼, as with transmission-type photomasks. To manufacture EUV masks, a technique for manufacturing finer mask patterns than with transmission-type photomasks is required. 
     In a manufacturing process of the EUV masks, defects may be formed on a mask pattern. To correct the defects formed on the mask pattern, a correction apparatus using a focused ion beam (FIB) or an electron beam (EB) is used. The correction apparatus of focused ion beam (FIB) is insufficient in beam resolution. The correction apparatus of electron beam (EB) has a high resolution, but etching on individual correction portion takes long time. To increase the etching speed when using the correction apparatus of electron beam (EB), it is attempted to use a highly reactive gas. However, in this case, etching proceeds excessively to disable a highly accurate correction (refer to JP-2004-537758-T, for example). 
     SUMMARY OF THE INVENTION 
     According to an aspect of the present invention, there is provided a method for correcting a defect in an EUV mask, the method including: preparing an EUV mask including: a substrate; an absorption layer formed on the substrate; and an anti-reflection layer formed on the absorption layer, the absorption layer and the anti-reflection layer forming a pattern; recognizing a defect region in the pattern, the defect region being continuous from a desired pattern edge of the pattern; defining a first region and a second region on the defect region, the second region extending from the desired pattern edge by a given distance, the first region being defined on a rest of the defect region; removing the first region of the anti-reflection layer and the absorption layer by irradiating a beam thereonto in a first atmosphere; removing the second region of the anti-reflection layer and the absorption layer by irradiating the beam thereonto in a second atmosphere; and oxidizing an exposed side surface of the desired pattern edge of the absorption layer. 
     According to another aspect of the present invention, there is provided a method for manufacturing a semiconductor device, the method including: preparing an EUV mask in which a defect has been corrected by use of the above-described method; and manufacturing a semiconductor device by use of the EUV mask. 
     According to still another aspect of the present invention, there is provided a defect correction apparatus for an EUV mask, the defect correction apparatus including: a chamber; a stage on which an EUV mask is loaded, the EUV mask including: a substrate; an absorption layer formed on the substrate; and an anti-reflection layer formed on the absorption layer, the absorption layer and the anti-reflection layer forming a pattern; a gas supplying device configured to supply an etching gas and an oxidizing gas through a nozzle disposed inside the chamber; a beam irradiation device configured to irradiate a beam onto the EUV mask loaded on the stage; and a controller configured to: recognize a defect region that is continuous from a desired pattern edge of the pattern; define a first region and a second region on the defect region, the second region extending from the desired pattern edge by a given distance, the first region being defined on a rest of the defect region; control the gas supplying device to supply at least the etching gas inside the chamber; control the irradiation device to irradiate the beam onto the first region; control the gas supplying device to supply at least the etching gas inside the chamber; control the irradiation device to irradiate the beam onto the second region; and control the gas supplying device to supply the oxidizing gas inside the chamber so that an exposed side surface of the absorption layer is oxidized. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are a plan view and a sectional view, respectively, illustrating an EUV mask. 
         FIGS. 2A and 2B  are a plan view and a sectional view, respectively, illustrating a defect region. 
         FIGS. 3A and 3B  are plan views illustrating an etching-correction of a defect region by an EB correction apparatus using a XeF 2  gas. 
         FIGS. 4A and 4B  are a plan view and a sectional view, respectively, illustrating a determination of a primary irradiation region and a secondary irradiation region. 
         FIGS. 5A-5C  illustrate a defect correction process for EUV mask according to a first embodiment. 
         FIGS. 6A-6C  illustrate the defect correction process for EUV mask according to the first embodiment. 
         FIG. 7  illustrates the defect correction process for EUV mask according to the first embodiment. 
         FIG. 8  is a flowchart of the defect correction process for EUV mask according to the first embodiment. 
         FIGS. 9A-9C  illustrate a defect correction process for EUV mask according to a second embodiment. 
         FIGS. 10A-10C  illustrate the defect correction process for EUV mask according to the second embodiment. 
         FIG. 11  is a flowchart of the defect correction process for EUV mask according to the second embodiment. 
         FIG. 12  is a flowchart of a modification example of the defect correction process for EUV mask. 
         FIGS. 13A and 13B  illustrate an exemplary procedure of determining a primary irradiation region and a secondary irradiation region. 
         FIGS. 14A and 14B  illustrate another exemplary procedure of determining a primary irradiation region and a secondary irradiation region. 
         FIGS. 15A and 15B  illustrate still another exemplary procedure of determining a primary irradiation region and a secondary irradiation region. 
         FIGS. 16A and 16B  illustrate still another exemplary procedure of determining a primary irradiation region and a secondary irradiation region. 
         FIG. 17  illustrates an example of a defect correction apparatus. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention will be hereinafter described with reference to the drawings. 
     First Embodiment 
     First, a defect correction method for an EUV mask according to a first embodiment will be described with reference to  FIGS. 1A to 8 .  FIG. 1A  is a plan view illustrating an EUV mask, and  FIG. 1B  is a sectional view taken along line A-B in  FIG. 1A . 
     A mask pattern is formed by subjecting a mask blank for EUV exposure on steps of electron beam drawing, resist development, etching, cleaning, etc.  FIGS. 1A and 1B  illustrate the thus formed mask pattern. In the embodiment, the mask blank for EUV exposure has such a structure that a reflection layer  102 , a capping layer  103 , a buffer layer  104 , an absorption layer  105  and an anti-reflection layer  106  are laminated in this order on a low-thermal-expansion mask substrate  101 . 
     For example, the reflection layer  102  is a multilayer in which molybdenum and silicon thin films are alternately laminated, the capping layer  103  is a silicon film, and the buffer layer  104  is a chromium-based material film. When a mask blank is formed to not have a buffer layer  104 , the capping layer  103  may be a ruthenium film. For example, the absorption layer  105  is made of tantalum boron nitride or tantalum nitride and the anti-reflection layer  106  is made of tantalum boron oxide or tantalum oxide. 
     Tantalum, which is used in the absorption layer  105  and the anti-reflection layer  106 , is prone to oxidation. In a state where the pattern has been formed, an oxide coating  107  is formed on the side surface of the absorption layer  105  through a reaction with oxygen in the air. On the other hand, the anti-reflection layer  106  is not affected by the air since the anti-reflection layer  106  itself is an oxide film. 
     The patterned mask is subjected to a defect inspection, such as a data comparison inspection and a pattern comparison inspection using a shape inspection instrument. A description will be made of a case that a defect region  108  as shown in  FIGS. 2A and 2B  has been detected by the defect inspection. For example, such a defect region  108  is generated because of dust sticks to the mask during procedure for forming a pattern. Masks from which no defect is detected are brought to the next step skipping a correction step. 
     As a photomask defect correction apparatus, for example, a correction apparatus using a focused ion beam (FIB) (hereinafter referred to as an FIB correction apparatus) or a correction apparatus using an electron beam (EB) (hereinafter referred to as an EB correction apparatus) is used. Generally, an FIB correction apparatus of a gallium ion beam that has been practically used is not sufficient in a resolution to correct fine patterns formed in EUV masks. In view of the resolution, at present, EB correction apparatus is suitable for EUV masks. EB correction apparatus has a sufficient resolution even for a line-and-space pattern of about 100 nm in pitch. 
     Irrespective of whether FIB correction apparatus or EB correction apparatus is used, correction is performed by applying a beam to the defect region  108  in a vacuum chamber while supplying an etching gas that is highly reactive to the subject film. Where the defect region  108  is etching-corrected by an FIB correction apparatus, the sputtering action of an ion beam itself contributes greatly, and the etching gas merely assists the beam sputtering. Therefore, a low-reactivity gas such as a chlorine gas or an iodine gas can be used as the etching gas. 
     On the other hand, where the defect region  108  is etching-corrected by an EB correction apparatus, the sputtering action of an electron beam itself is very weak, and the etching gas exited as a reaction species by the electron beam reacts to the subject film in the defect region  108 . Therefore, when a low-reactivity gas is used, etching takes a long time, and throughput is reduced. In view of this, in many cases, EB correction apparatus employ a fluorine-based gas which exhibits relatively high reactivity. In the embodiment, a xenon difluoride (XeF 2 ) gas is used as the fluorine-based gas in the correction apparatus. 
     As described above, in the EUV mask, the absorption layer  105  is made of tantalum boron nitride or tantalum nitride (TaN). These materials have a property that they react directly to an etching gas XeF 2  and dissolve. Therefore, if these materials are etched using XeF 2 , etching proceeds excessively and hence the defect region  108  cannot be corrected appropriately (see  FIGS. 3A and 3B ). Further, an undercut occurs may be caused in a mask pattern edge because only the absorption layer  105  is etched isotropically while the anti-reflection layer  106  of tantalum oxide is not etched. 
     In the defect correction method for an EUV mask according to the first embodiment, the correction by irradiation with an electron beam is performed in two steps so that the above problems are solved. In the embodiment, a step of loading an EUV mask into the EB correction apparatus, a step of moving the position of attention to defect-detected coordinates, a step of capturing an image in an area of several micrometers including the defect, a step of determining a correction region on the basis of the image information, and other steps are executed. The EB correction apparatus is capable of obtaining a SEM (scanning electron microscope) image consists of pixels having a dot pitch of 1 nm, and the correction can be performed on a dot-by-dot basis. 
     First, in a defect region  108 , a region (hereinafter referred to as a primary irradiation region)  109  to be corrected by first electron beam irradiation and a region (hereinafter referred to as a secondary irradiation region)  110  to be corrected by second electron beam irradiation are determined (see  FIGS. 4A and 4B ). A region that is within a distance d from the edge of a target pattern is employed as the secondary irradiation region  110 , and a region that is distant from the edge of the target pattern by more than the distance d is employed as the primary irradiation region  109 . For example, a side etching amount that is a retreat amount of the absorption layer  105  from the edge of the anti-reflection layer  106  when the isotropic etching is performed is previously measured by an experiment, a simulation, or the like, and the distance d is set based on the side etching amount. 
     After the primary irradiation region  109  and the secondary irradiation region  110  have been set, as shown in FIG.  5 A, only the primary irradiation region  109  is irradiated with an electron beam  113  while a XeF 2  gas  112  is supplied from a gas nozzle  111  inside a vacuum chamber. Since the anti-reflection layer  106  is made of tantalum oxide, it reacts to only the XeF 2  gas  112  which is irradiated with the electron beam  113  to form a reaction species. As a result, that portion of the anti-reflection layer  106  which is located in the primary irradiation region  109  is etched away as shown in  FIG. 5B . 
     The etching is continued from the state of  FIG. 5B , whereby the absorption layer  105  is etched isotropically as shown in  FIG. 5C . Whether the etching has reached the end point (the buffer layer  104  located under the absorption layer  105 ) may be judged based on whether a preset time has elapsed. Alternatively, whether the etching has reached the end point may be judged by detecting back-scattered electrons. The preset etching time may be set by calculating an etching rate from a gas pressure and beam conditions. 
     Since the etching rate of the absorption layer  105  is high, the gas pressure for etching the absorption layer  105  may be set lower than the gas pressure for etching the anti-reflection layer  106 . In either case, as soon as the downward etching has reached the buffer layer  104 , the supplying of the XeF 2  gas is stopped to prevent further downward progress of the etching. Since the side etching amount of the absorption layer  105  is substantially equal to the distance d that is previously set when determining the primary irradiation region  109  and the secondary irradiation region  110 , the shape of the defect region  108  can be made close to the shape of the target pattern. 
     After the isotropic etching of the absorption layer  105 , to prevent further side etching of the absorption layer  105 , an oxide coating  115  is formed on a side surface thereof by introducing an oxidizing gas  114  into the chamber (see  FIG. 6A ). For example, an O 2  gas is used as the oxidizing gas  114  in the embodiment. 
     Then, the anti-reflection layer  106  in the secondary irradiation region  110  is etched (see  FIG. 6B ). In this step, as in the step of  FIG. 5A , the etching can be performed accurately because the anti-reflection layer  106  reacts to only a XeF 2  gas  112  which is irradiation with an electron beam  113  to form a reaction species, and the absorption layer  105  is not etched because it is covered with the oxide coating  115 . 
     Then, the absorption layer  105  in the secondary irradiation region  110  is irradiated with the electron beam  113 , whereby a shape shown in  FIG. 6C  is obtained. In this step, for example, an etching end point is recognized by capturing back-scattered electrons. Since the absorption layer  105  to be etched away is very small, by irradiating a beam dose only for the anti-reflection layer  106  and then irradiating a light beam dose for the absorption layer  105 , the vertical shape as shown in  FIG. 6C  can be obtained finally. While adjusting the beam dose, the flow rate of the etching gas may be reduced. 
     In this step, an undercut would be formed again if the etching were performed excessively. Therefore, for example, the gas pressure may be lowered after the portion of the anti-reflection layer  106  has been etched away. For example, as the end point control, increasing of the etching gas flow rate in the step of etching the anti-reflection layer  106  and decreasing of the etching gas flow rate in the step of etching the absorption layer  105  are also effective. These measures may be combined as appropriate. 
     When the etching has reached an end point, the process gas is switched from the XeF 2  gas  112  to an oxidizing gas  114 . As a result, an oxide coating  116  is formed on a side surface that has exposed through the etching (see  FIG. 7 ). The oxide coating  116  serves as a side surface protective member and prevents influence of an etching gas when another defect region that is located within several millimeters from the corrected defect region is corrected likewise. 
       FIG. 8  is a flowchart of the above-described defect correction process for EUV mask according to the embodiment. In the embodiment, an electron beam is used to excite an etching gas, a XeF 2  gas is used as the etching gas, and an O 2  gas is used as the oxidizing gas. Alternatively, an ion beam may be used. An etching process using a chlorine gas or an iodine gas may be employed. Water (H 2 O) may be used as the oxidizing agent. An oxidizing gas other than the O 2  gas and the H 2 O gas may be used as long as it provides the same effect. 
     In the method according to the embodiment, a defect is sequentially collected by determining a primary irradiation region  109  and a secondary irradiation region  110  at the same time. In that exemplary procedure, the defect correction process is executed fast. As another exemplary procedure, after completion of processing in the primary irradiation region  109 , a secondary irradiation region  110  may be re-set by taking a SEM image again. In this case, since correction errors in the primary irradiation region  109  can be recognized and a secondary irradiation region  110  can be re-set accordingly, the final correction accuracy can thus be increased. 
     As described above, the defect correction method according to the first embodiment enables accurate defect correction on an EUV mask. 
     Second Embodiment 
     Next, a defect correction method according to a second embodiment will be described with reference to  FIGS. 9A to 11 . Although a primary irradiation region and a secondary irradiation region are set in the same manner as in the above-described first embodiment, they are set at a different ratio than in the first embodiment because of differences in the etching process. The second embodiment will be described by giving the same reference symbols to the same layers etc. as in the first embodiment. 
     After the primary irradiation region  109  and the secondary irradiation region  110  have been set, as shown in  FIG. 9A , only the primary irradiation region  109  is irradiated with an electron beam  113  while a XeF 2  gas  112  is supplied from a gas nozzle  111  inside a vacuum chamber. Since the anti-reflection layer  106  is made of tantalum oxide, it reacts to only the XeF 2  gas  112  which is irradiated with the electron beam  113  to form a reaction species. As a result, the anti-reflection layer  106  in the primary irradiation region  109  is etched away as shown in  FIG. 9B . 
     The etching is continued from the state of  FIG. 9B . In the embodiment, as shown in  FIG. 9C , a mixed gas of an O 2  gas and a XeF 2  gas is used as a process gas. O 2  has is used as an oxidizing gas. As a result, a proper etching rate is attained because the process proceeds in such a manner that the absorption layer  105  is oxidized and etched repeatedly. For example, the ratio of the oxidizing gas  114  is increased as the etching proceeds so that only the oxidizing gas  114  is supplied after the etching has reached an end point. Whether the etching has reached the end point may be judged based on a preset time or judged by detecting back-scattered electrons. The preset etching time may be set by calculating an etching rate from a gas pressure and beam conditions. 
     When etching the absorption layer  105 , a side etching occurs to a certain extent, and a side etching amount depends on the ratio of the oxidizing gas  114 . Therefore, for example, the side etching amount is previously obtained through an experiment. As in the first embodiment, a primary irradiation region  109  and a secondary irradiation region  110  are set according to the predicted side etching amount. The side etching amount in the second embodiment would be different than in the first embodiment. 
     Then, the anti-reflection layer  106  in the secondary irradiation region  110  is etched (see  FIG. 10A ). At a start, for example, a process gas is formed only by the XeF 2  gas (100%). The oxidizing gas  114  starts to be added immediately before the anti-reflection film  106  in the secondary irradiation region  110  is removed, and its ratio is thereafter increased gradually (see  FIG. 10B ). After the anti-reflection film  106  in the secondary irradiation region  110  has been removed, the only remaining step is to cut away the trailing portion of the absorption layer  105 . A vertical edge can be formed stably by increasing the ratio of the oxidizing gas  114  and thereby lowering the etching rate gradually. According to this procedure, for example, the probability that the etching proceeds laterally breaking an oxide coating  115  that is already formed on the side surface. At the start, the process gas may be formed not only by the XeF 2  gas (100%), and for example, the process gas may include several to several tens percent of the oxidizing gas. 
     When the etching has reached an end point so that a vertical shape of the absorption layer  105  has been obtained, the process gas is switched from the XeF 2  gas  112  to a 100% oxidizing gas  114  (see  FIG. 10C ) to form an oxide coating  116  on the side surface in order to maintain the vertical shape. The oxide coating  116  serves as a side surface protective member and prevents influence of an etching gas when another defect region that is located within several millimeters from the corrected defect region is corrected likewise. 
       FIG. 11  is a flowchart of the above-described defect correction process for EUV mask according to the embodiment. In the embodiment, an electron beam is used to excite an etching gas, a XeF 2  gas is used as the etching gas, and an O 2  gas is used as the oxidizing gas. Alternatively, an ion beam may be used. An etching process using a chlorine gas or an iodine gas may be employed. Water (H 2 O) may be used as the oxidizing agent. An oxidizing gas other than the O 2  gas and the H 2 O gas may be used as long as it provides the same effect. 
     In the method according to the embodiment, a defect is sequentially collected by determining a primary irradiation region  109  and a secondary irradiation region  110  at the same time. In that exemplary procedure, the defect correction process is executed fast. As another exemplary procedure, after completion of processing in the primary irradiation region  109 , a secondary irradiation region  110  may be re-set by taking a SEM image again. In this case, since correction errors in the primary irradiation region  109  can be recognized and a secondary irradiation region  110  can be re-set accordingly, the final correction accuracy can thus be increased. 
     As described above, the defect correction method according to the second embodiment enables accurate defect correction on an EUV mask. 
     The invention is not limited to the above embodiments and various modifications are possible without departing from the spirit and scope of the invention. 
     Although two of beam irradiation regions are set on a defect region in the first and second embodiments, three or more of beam irradiation regions may be set. For example, from a distal side to a proximal side of a defect region that protrudes from a desired pattern edge, first to third irradiation regions may be defined in this order. In this case, as shown in  FIG. 12 , the beam irradiation is performed for each region in the etching gas atmosphere. 
     Although the defect that protrudes from an edge of the desired pattern is corrected as a correction object in the first and second embodiments, other shape of defect also can be corrected. For example, a defect may be formed in the line and space pattern as shown in  FIG. 13A , a defect may be formed on a corner of the pattern as shown in  FIG. 14A , a defect may be formed in the hole pattern as shown in  FIG. 15A , and a defect may be formed in the DRAM (Dynamic Random Access Memory) pattern as shown in  FIG. 16A . 
     On the defect  108 , a boundary is defined so as to be far from any edge of the desired pattern by at least the distance d. And, the secondary irradiation region  110  is defined on a portion in the defect  108  between the boundary and the edges, and the primary irradiation region  109  is defined on the rest portion. 
     After the primary irradiation region  109  and the secondary irradiation region  110  are defined as shown in  FIGS. 13B ,  14 B,  15 B and  16 B, the method according to the first and second embodiments is applied to correct the defect  108 . 
     According to the first and second embodiments, a defect in an EUV mask can be corrected. By using such defect-corrected EUV mask, a semiconductor device can be accurately manufactured. 
     An example of a defect correction apparatus that is used to perform the defect correction method according to the first and second embodiments is shown in  FIG. 17 . The defect correction apparatus shown in  FIG. 17  includes a chamber  201 , a gas supplying device  203 , a beam irradiation device  205  and a controller  206 . 
     In the chamber  201 , a stage  202  on which an EUV mask  200  as a defect correction object is loaded. The gas supplying device  203  includes a nozzle  204  that is disposed inside the chamber  201  and supplies an etching gas and an oxidizing gas into the chamber  201 . The beam irradiation device  205  is capable of irradiating an electron beam onto the EUV mask  200  loaded on the stage  202 . The controller  206  controls the gas supplying device  203 , the beam irradiation device  205 , etc. to perform the defect correction method according to the first and second embodiments. 
     According to an aspect of the present invention, there is provided a defect correction method in which a defect in an EUV mask is accurately corrected.