Abstract:
Systems and methods are disclosed for reducing contaminants that can accumulate in a charged-particle-beam (CPB) microlithography system during use of the system for CPB microlithography. In general, the disclosed systems utilize a photocatalytic layer disposed on the walls of a vacuum chamber enclosing a CPB optical system of the microlithography system or on the surfaces of one or more components of the CPB optical system in the vacuum chamber. When exposed to a particular radiation, the photocatalytic layer reacts with a reactant, such as water vapor or oxygen, to create hydroxy radicals and/or superoxide ions. The hydroxy radicals and superoxide ions decompose the hydrocarbon-type contaminants and produce volatile reaction products that can be exhausted from the vacuum chamber. The systems and methods improve throughput of the CPB microlithography system because contaminants in the vacuum chamber are decomposed and removed in situ without having to disassemble and clean the microlithography system.

Description:
FIELD  
         [0001]    This disclosure pertains to microlithography (projection-transfer of a pattern, defined on a reticle, onto a suitable substrate) and related technologies. Microlithography is a key technology used in the manufacture of microelectronic devices such as integrated circuits, magnetic-recording heads, displays, and micromachines. More specifically, the disclosure pertains to charged-particle-beam (CPB) microlithography and to systems and methods for reducing certain contaminants that accumulate in a vacuum chamber housing certain components of a CPB microlithography system.  
         BACKGROUND  
         [0002]    As the limitations of optical microlithography have become more apparent, microlithography systems utilizing a charged particle beam (e.g., an electron or ion beam) or an X-ray light source have been developed. These new systems are capable of forming smaller pattern features than conventional optical microlithography systems. Microlithography systems that utilize an electron beam as the lithographic energy beam have the advantage that the electron beam itself can be focused narrowly, thereby resulting in improved pattern-transfer resolution.  
           [0003]    A conventional electron-beam microlithography system draws a circuit pattern by narrowly focusing the electron beam, inside a vacuum chamber, on a lithographic substrate coated with a suitable resist and scanning the focused beam across the substrate. This process of using a focused electron beam to draw the pattern line-by-line is time-consuming. Thus, conventional electron-beam drawing systems suffer from extremely low throughput compared to optical microlithography systems that use demagnifying projection-optical systems to transfer entire circuit patterns in one exposure “shot.” 
           [0004]    Accordingly, a divided-reticle electron-beam microlithography technique has been developed whereby a pattern is defined on a reticle that is divided into multiple subfields each defining a respective portion of the pattern and being exposed individually. The reticle is positioned in a vacuum chamber and irradiated by an electron beam directed by an illumination-optical system. As the electron beam passes through the irradiated portion of the reticle, the beam acquires an aerial image of the pattern and thus becomes a “patterned beam” propagating downstream of the reticle. The patterned beam then passes through the projection-optical system of the electron beam microlithography system, in which the patterned beam is demagnified and focused onto the surface of a downstream substrate. The area of the substrate that can be exposed with a single exposure shot is relatively large (e.g., 0.25 mm×0.25 mm), resulting in dramatically increased throughput compared to a conventional electron-beam drawing system.  
           [0005]    During pattern transfer using any type of CPB microlithography system, however, the charged particle beam may react with certain contaminants inside the vacuum chamber, resulting in decreased accuracy with which the charged particle beam can transfer the pattern. The contaminants may include residual or generated gas (e.g., from the resist, etc.) or any of various substances adsorbed onto the walls of the vacuum chamber or onto the surfaces of optical-system components inside the vacuum chamber. For instance, hydrocarbon-type contaminants may adhere to the inner walls of the vacuum chamber and to components inside the vacuum chamber. These accumulated contaminants may cause localized electrical charge-ups that perturb the trajectory of the charged particle beam in the CPB optical system.  
           [0006]    Conventionally, contaminant deposits are removed from the various parts of the vacuum chamber and components therein by disassembling the microlithography system, removing each contaminated component, and individually cleaning the removed component. Because the CPB microlithography system cannot operate while the contaminated components are being cleaned, this conventional cleaning process causes a substantial decrease in the throughput of the system.  
         SUMMARY  
         [0007]    In view of the shortcomings of the prior art as summarized above, the present disclosure provides, inter alia, systems and methods for reducing contaminants that can accumulate or that have accumulated during charged-particle-beam (CPB) microlithography. In general, the disclosed systems utilize a photocatalytic layer disposed on the walls of the vacuum chamber and/or on the surfaces of components located inside the vacuum chamber. Upon being exposed to certain types of radiation, the photocatalytic layer reacts with a reactant, such as water vapor and/or oxygen, to create hydroxy radicals and/or superoxide ions. Inside the vacuum chamber, hydroxy radicals and superoxide ions decompose hydrocarbon-type contaminants that have accumulated in the vacuum chamber. As a result, the frequency with which components must be removed from the vacuum chamber for cleaning is reduced substantially. Therefore, the throughput of the CPB microlithography system is improved because contaminants in the vacuum chamber can be decomposed in situ and removed without having to disassemble any part of the system.  
           [0008]    In a first aspect of the invention, CPB microlithography systems comprising a vacuum chamber and a CPB optical system situated inside the vacuum chamber are provided. In an exemplary embodiment of such a system, a photocatalytic layer is disposed on at least a portion of the inner walls of the vacuum chamber. The photocatalytic layer may comprise, for example, one or more of titanium oxide, zinc oxide, and cadmium sulfide. A radiation source may be situated inside the vacuum chamber and configured to irradiate the photocatalytic layer with, for example, ultraviolet radiation. The system also may include a tank filled with water vapor or oxygen, with a gas-introduction tube fluidly connecting the tank with the interior of the vacuum chamber. Thus, water vapor and/or oxygen may be released into the vacuum chamber during irradiation of the photocatalytic layer in order to generate the hydroxy radicals and/or superoxide ions that decompose hydrocarbon-type contaminants inside the vacuum chamber. The system may further include an exhaust pump and an exhaust tube fluidly connecting the exhaust pump with the vacuum chamber.  
           [0009]    In another embodiment of a CPB microlithography system, at least one of the components of the CPB optical system inside the vacuum chamber is coated with the photocatalytic layer. For example, any one of the multiple beam-shaping or beam-focusing components in the illumination-optical system or the projectionoptical system of the CPB optical system may be coated with the photocatalytic layer. Alternatively, at least a portion of the substrate stage of the CPB microlithography system may be coated with the photocatalytic layer.  
           [0010]    According to another aspect of the invention, methods are provided for decomposing hydrocarbon-type contaminants in a CPB microlithography system. An embodiment of the method includes coating at least a portion of the inner walls of the vacuum chamber with a photocatalytic layer. As discussed above, the photocatalytic layer may comprise one or more of titanium oxide, zinc oxide, and cadmium sulfide. A reactant, such as water vapor or oxygen, is introduced into the vacuum chamber at appropriate times. The photocatalytic layer is irradiated with radiation, such as ultraviolet (UV) radiation, which causes the photocatalytic layer to react with molecules of the reactant and create hydroxy radicals and/or superoxide ions that decompose the contaminant deposits to volatile reaction products. The method further may include exhausting the volatile reaction products out of the vacuum chamber via an exhaust tube. The method further may include simultaneously irradiating the photocatalytic layer with the charged particle beam used in the CPB microlithography system.  
           [0011]    In another embodiment of the method, at least one of the components of the CPB optical system is coated with the photocatalytic layer. Alternatively or in addition, at least a portion of the substrate stage of the CPB microlithography system also may be coated with the photocatalytic layer.  
           [0012]    The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is an elevational view of a charged-particle-beam (CPB) microlithography system according to a first representative embodiment.  
         [0014]    [0014]FIG. 2 depicts key reactions of titanium oxide (TiO 2 ) with gaseous oxygen and water vapor that generate superoxide ions and hydroxy radicals. 
     
    
     DETAILED DESCRIPTION  
       [0015]    The invention is described below in the context of representative embodiments and examples that are not intended to be limiting in any way. Furthermore, the embodiments are described below in the context of use with an electron beam as an exemplary charged particle beam. It will be understood that the general principles disclosed herein are equally applicable to use of an alternative charged particle beam, such as an ion beam.  
         [0016]    The system of FIG. 1 includes an electron gun  1  disposed in an upper part of the electron-beam microlithography system  17 . The electron gun  1  generates an electron beam  2  that propagates through an illumination-optical system  3 . The illumination-optical system  3  typically includes multiple condensers, deflectors, magnetic lenses, beam-forming apertures, etc. For illustrative purposes, however, the illumination-optical system  3  is shown as comprising only a single component. The illumination-optical system  3  shapes and focuses the electron beam  2  into an “illumination beam” that is directed onto a selected region, or “subfield,” of the patterned reticle  4  that is to be transferred.  
         [0017]    The reticle  4  is mounted to a reticle chuck (not shown, but well understood in the art) provided on the upstream-facing surface of a reticle stage  5 . The reticle  4  may be affixed to the reticle chuck via electrostatic adhesion or other suitable means. As the illumination beam passes through the irradiated region of the reticle  4 , the beam acquires an aerial image of the illuminated region, and thus becomes a “patterned beam” propagating downstream of the reticle. The patterned beam then passes through a projection-optical system  6 , which typically includes multiple condenser lenses, aberration-correction lenses, coils, deflectors, and a contrast aperture  7 . For illustrative purposes, however, the projection-optical system  6  is shown as comprising only a single component.  
         [0018]    The substrate (e.g., semiconductor wafer)  8  is mounted on a “wafer chuck” (not shown, but well understood in the art) located on an upstream-facing surface of a substrate stage  9 . The substrate  8  may be affixed to the wafer chuck via electrostatic adhesion or other suitable means. The projection-optical system  6  deflects and focuses the patterned beam onto a predetermined region on the substrate  8 . A detector  15 , which detects an alignment mark on the substrate stage  9 , is used to position the substrate  8  accurately at a predetermined position for exposure.  
         [0019]    All the parts of the illumination-optical system  3  and the projection-optical system  6  are housed inside a vacuum chamber  10 . During microlithographic exposure, the interior of the vacuum chamber  10  is maintained at a suitable vacuum.  
         [0020]    The basic principle of removing contaminant deposits from inside an electron-beam microlithography system using titanium oxide (TiO 2 ) is explained with reference to FIG. 2. FIG. 2 shows the energy-band structure of TiO 2 , which is an exemplary photocatalytic material that reacts with and decomposes certain contaminants whenever the TiO 2  is irradiated with ultraviolet light (UV)  21 . Titanium oxide is an n-type semiconductor having a band gap of about 3.2 eV, which is equivalent to radiation having a wavelength of about 380 nm. Thus, whenever radiation having a wavelength shorter than 380 nm is absorbed by titanium oxide, an electron  24  is excited from a valence band  22  to a conduction band  23 , creating a hole  25  in the valance band  22 . The hole  25  oxidizes a water molecule  26  inside the vacuum chamber and creates a hydroxy radical (.OH)  27 . The electron  24  in the conduction band  23  also ionizes an oxygen molecule  28  and creates a superoxide ion (O 2   − )  29 . The resulting hydroxy radicals  27  and superoxide ions  29  react with and decompose hydrocarbon-type contaminants with which the radicals and superoxide ions come into contact. Thus, if water vapor or oxygen is introduced into a vacuum chamber coated with titanium oxide that is being irradiated with UV radiation, any hydrocarbon-type contaminants inside the vacuum chamber and contacted by the hydroxy radicals and superoxide ions will be decomposed to volatile reaction products.  
         [0021]    Referring again to FIG. 1, a photocatalytic layer  16  comprising titanium oxide is formed as a film on selected regions of the surface of the inner wall of the vacuum chamber  10  and on the surface of various components inside the vacuum chamber  10  (e.g., on surfaces of the contrast aperture  7 , the substrate stage  9 , etc.). The photocatalytic layer  16  is not limited to a layer comprising titanium oxide, but may comprise any substance or combination of substances exhibiting the photocatalytic effect described above with reference to FIG. 2. For example, any of various photocatalysts such as zinc oxide (ZnO), cadmium sulfide (CdS), etc., may be used. Moreover, the titanium oxide in the photocatalytic layer  16  may be combined with a metal (e.g., platinum (Pt), chromium (Cr), etc.).  
         [0022]    To irradiate the photocatalytic layer  16 , a radiation source  11  is disposed inside the vacuum chamber  10  (or at least situated and configured to direct radiation into the vacuum chamber onto the photocatalytic layer  16 ). The radiation source  11  irradiates the inner walls of and the various components inside of the vacuum chamber  10  with, for example, ultraviolet light. The radiation source  11  may comprise any suitable light source, such as a mercury lamp, KrF excimer laser, XeCl excimer laser, nitrogen laser, etc. A gas-introduction tube  13  fluidly couples the vacuum chamber  10  with a tank  12  desirably located outside the vacuum chamber. The gas-introduction tube  13  is configured to introduce water vapor and/or oxygen into the interior of the vacuum chamber  10  from the tank  12 . Individual tanks  12   a ,  12   b  may be used for introducing water vapor and oxygen, respectively, separately into the vacuum chamber  10 .  
         [0023]    During operation, the amount of contaminants adhering to the walls and components of the vacuum chamber  10  may be measured by a detector (not shown). Whenever a certain threshold contamination level is reached, microlithography normally performed using the system is halted and water vapor and/or oxygen are introduced into the interior of the vacuum chamber  10 . The radiation source  11  is energized to produce radiation (e.g., ultraviolet radiation) that irradiates the inner walls and components of the vacuum chamber  10 . The photocatalytic layers  16 , upon receiving the radiation, convert the water vapor and oxygen into hydroxy radicals and superoxide ions that decompose the hydrocarbon-type contaminants into volatile reaction products. The reaction products are then exhausted from the vacuum chamber  10  by an exhaust pump  14  and an exhaust tube  18  fluidly coupling the interior of the vacuum chamber  10  and the exhaust pump  14  together. Thus, the contaminant deposits are removed quickly from the vacuum chamber  10  without having to disassemble the microlithography system  17 . Because the microlithography system  17  can be restarted immediately after exhausting the volatile reaction products, the system described above results in an overall increase in the throughput of the system.  
         [0024]    In an alternative embodiment of the system described above, the electron beam  2  continues to operate during irradiation of the vacuum chamber  10  by the radiation source  11 . In this alternative embodiment, the electron beam  2  also converts molecules of water vapor and/or oxygen into hydroxy radicals and superoxide ions that help decompose the contaminants inside the vacuum chamber  10 . Use of the electron beam  2  together with the radiation generated from the radiation source  11  increases the decomposition efficiency and shortens the time required to remove the contaminants from the vacuum chamber  10 .  
       FIRST WORKING EXAMPLE  
       [0025]    The following working example is provided to exemplify certain features of the disclosed embodiments. The scope of the invention should not be limited to those features exemplified.  
         [0026]    In this example, a contrast aperture  7  was coated with a photocatalytic layer  16  and positioned inside a vacuum chamber  10  of an electron-beam microlithography system  17 . The photocatalytic layer  16  comprised a film of titanium oxide having a thickness of 2000 Angstroms. The electron-beam microlithography system  17  was operated until hydrocarbon-type contaminants had accumulated on the surface of the contrast aperture  7 . These contaminants exhibited charge-up, which caused visually detectable perturbation of the trajectory of the electron beam  2 . For decomposing the contaminant deposits water vapor was introduced into the vacuum chamber  10  from a tank  12  via a gas-introduction tube  13  at a pressure of 600 Pa. A mercury lamp located inside the vacuum chamber was energized, and produced UV radiation of 254 nm that irradiated the contrast aperture  7 . The irradiated photocatalytic layer  16  on the contrast aperture produced superoxide ions and hydroxy radicals that reacted with the contaminants adhering to the contrast aperture  7 . The reaction products were volatile and readily removed. As a result of this in situ cleaning, perturbations of the electron beam were eliminated.  
       WORKING EXAMPLE 2  
       [0027]    In this working example, an alignment mark on the substrate stage  9  was coated with a photocatalytic layer  16  and positioned inside a vacuum chamber  10  of an electron-beam microlithography system  17 . The photocatalytic layer  16  comprised a film of titanium oxide having a thickness of 2000 Angstroms. The electron-beam microlithography system  17  was operated until hydrocarbon-type contaminants had accumulated on the surface of the alignment mark. The contaminant deposit on the alignment mark made it difficult for a detector  15  to detect the alignment mark, making accurate positioning impossible. To decompose the contaminant deposit, water vapor at a pressure of 400 Pa was introduced into the vacuum chamber  10  from a tank  12  via a gas-introduction tube  13 . A nitrogen laser was energized, which irradiated the alignment mark with radiation having a wavelength of 337 nm. The irradiated photocatalytic layer  16  produced hydroxy radicals that reacted with the contaminant deposit adhering to the alignment mark. The volatile reaction products were removed. As a result of this in situ cleaning, the detector  15  again could perform high-precision positioning of the substrate stage  9 .  
         [0028]    Whereas the invention has been described in connection with representative embodiments and examples, the invention is not limited to those embodiments and examples. On the contrary, the invention is intended to encompass all modifications, alternatives, and equivalents as may be included within the spirit and scope of the invention, as defined by the appended claims.