Patent Publication Number: US-6903354-B2

Title: Extreme ultraviolet transition oscillator

Description:
BACKGROUND 
   The present application describes systems and techniques relating to extreme ultraviolet light sources, for example, extreme ultraviolet light sources used in extreme ultraviolet light lithography or interferometry. 
   Traditionally recognized extreme ultraviolet (EUV) light sources include synchrotrons and plasma-based sources. A synchrotron is an annular particle accelerator that can produce synchrotron radiation or a free electron laser using wiggler magnets. Synchrotrons are generally large and expensive high energy devices, which may be able to radiate in the EUV region. 
   Plasma-based sources of EUV light include electric discharge plasma sources and laser discharge plasma sources. Typically, a jet of xenon gas is excited into a plasma using a laser or electricity. The plasma then provides atomic spectral radiation in the EUV range. Plasma-based EUV sources generate high energy ions, which tend to sputter any materials that fall in a line of sight of the plasma. Additionally, plasma-based EUV sources tend to have low energy efficiency and use extensive cooling systems. 

   
     DRAWING DESCRIPTIONS 
       FIG. 1  illustrates transition radiation. 
       FIG. 2  illustrates an example extreme ultraviolet transition oscillator. 
       FIG. 3  illustrates another example extreme ultraviolet transition oscillator. 
       FIG. 4  illustrates an approximated power spectra of an extreme ultraviolet transition oscillator. 
       FIG. 5  illustrates an extreme ultraviolet light system. 
       FIG. 6  illustrates a calculated relativistic factor for an electron beam used in the system of FIG.  5 . 
   

   Details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features and advantages may be apparent from the description and drawings, and from the claims. 
   DETAILED DESCRIPTION 
   The systems and techniques described here relate to extreme ultraviolet (EUV) light sources. A transition oscillator may be built using thin film deposition techniques such that, when a mildly relativistic electron beam passes through the transition oscillator, a coherent superposition of transition radiation is produced at EUV wavelengths. Thus, the transition oscillator may be used as a compact, high brightness, EUV illumination source in various EUV applications, such as EUV lithography and EUV interferometry. The EUV illumination source may be built into a portable device that may be used easily at various locations, such as a compact interferometer that may be used to test a set of optics on-site or with an EUV lithography tool that may ship from an exposure tool supplier to a semiconductor manufacturing facility. The EUV illumination source may provide high brightness EUV radiation with high stability, low cost of ownership, and without debris formation. 
     FIG. 1  illustrates transition radiation. When a mildly relativistic beam of electrons (e.g., 1 MeV or more) crosses the interface between two mediums with different dielectric or magnetic properties, forward directed radiation is created that is called transition radiation. This transition radiation becomes more forwardly directed as the electron beam becomes more relativistic. As the energy level of the electron beam increases, the energy of the transition radiation concentrates to a smaller solid angle. 
   A graph  100  illustrates transition radiation generated when a 1 MeV electron beam passes from a first material into a second material or vacuum. The peak intensity generally occurs at an angle of 18.4 degrees from the direction of the electron beam direction. A graph  150  illustrates transition radiation generated when a 10 MeV electron beam passes from a first material into a second material or vacuum. The peak intensity at this e-beam energy generally occurs at an angle of 2.78 degrees from the direction of the electron beam direction. 
     FIG. 2  illustrates an example extreme ultraviolet transition oscillator  200 . The EUV transition oscillator  200  may be part of an apparatus used to generate extreme ultraviolet light by producing coherent transition radiation. The EUV transition oscillator  200  may include multiple metal layers  210  interleaved with multiple dielectric layers  220 . The metal and dielectric layers  210 ,  220  have a thickness selected to produce coherent transition radiation in an extreme ultraviolet wavelength region when an electron beam  250  passes through the metal layers  210  and the dielectric layers  220 . The thickness is selected based on the desired wavelength of the coherent transition radiation, such as a per-layer optical thickness of 13 nm. 
   The layers  210 ,  220  may be deposited on a substrate using thin film deposition techniques. A first layer of the thin films deposited on the substrate may be exposed, such as by back-etching, to create a free-standing multi-layer stack, which may then be placed in the path of an electron beam. The substrate may be a silicon wafer, and the manufacturing process may also provide a supporting frame for the multi-layer stack. The metal layers may be thin films of molybdenum. The dielectric layers may be thin films of silicon. 
   The metal layers may have a per-layer thickness of between 5 and 50 nanometers, and the dielectric layers may also have a per-layer thickness of between 5 and 50 nanometers. The per-layer thickness may be selected based on a desired wavelength for the transition radiation and the materials being used. The metal layers may be selected to have a per-layer thickness of one wavelength, which causes the transition radiation off the front and back surfaces of a metal layer to add up coherently (i.e., constructive interference). A one wavelength separation from the back surface of one metal layer to the front surface of the next metal layer may also be used. Other non-regular thicknesses may be used to shape the power spectra, to reduce sensitivities to manufacturing/heating tolerances, and to optimize power output in absorbing materials. 
   By using thin film deposition techniques to produce the metal and dielectric layers, precise control over the thickness of the layers and the spacing of the layers is provided. Thin film deposition techniques are used widely in the semiconductor industry, such as the thin film deposition techniques used to create Bragg reflectors, and are an efficient and cost effective way to produce transition oscillators as described herein. 
     FIG. 3  illustrates another example extreme ultraviolet transition oscillator  300 . The EUV transition oscillator  300  may be part of an EUV illumination apparatus, just as the EUV transition oscillator  200  from  FIG. 2 , and may be built in a similar manner. The EUV transition oscillator  300  may include multiple interleaved metal layers  310  and dielectric layers  320 , which have a thickness selected to produce coherent transition radiation in an extreme ultraviolet wavelength region when an electron beam  350  passes through the layers. 
   The dielectric layers  320  act as spacers that define regions of vacuum  330  between the metal layers  310  in areas between the spacers. The dielectric layers may define multiple regions of vacuum per layer, as shown. This structure may provide a constant separation between the metal layers and additional strength and stability. Alternatively, the metal layers of the EUV transition oscillator  300  may be held together on the edges alone, such as by a supporting frame built around the transition oscillator  300 . 
   As before, the layers  310 ,  320  may be deposited on a substrate using thin film deposition techniques. This depositing may involve applying layer patterns lithographically. The dielectric layers may be hollowed out using etching techniques to create the regions of vacuum. For example, the dielectric layers may include a material having properties of etch selectivity and thermal stability, such as silicon carbide. A series of holes may be drilled through the multi-layer thin film stack (e.g., a contact array may be etched through the stack), and then the dielectric material may be etched out from between the metal layers using an etch process that is selective for one material over the other. 
   A first layer of the thin films deposited on the substrate may be exposed to create a free-standing multi-layer metal-vacuum stack, which may then be placed in the path of an electron beam. The electron beam may be one millimeter in diameter. Thus in some implementations, the electron beam would still pass through some of the dielectric material left in the dielectric layers to provide stability to the EUV transition oscillator  300 . The hollowed out dielectric layers may have widths that are larger than the metal layers, such as a per-layer thickness of two wavelengths. 
   These multi-layer metal-dielectric and metal-vacuum transition oscillators may be included in an apparatus or system that passes a mildly relativistic electron beam through the multi-layer stack. The multi-layer structure provides a high degree of control over the relative phase of transition radiation from successive interfaces so that coherent superposition at a design wavelength is achieved. 
   Increasing the number of total layers, or periods, in the multi-layer structure increases the power and intensity of the resulting transition radiation, but with generally diminishing returns. The transition oscillators  200 ,  300  may have approximately one hundred periods, such as at least forty-five metal layers and at least forty-five dielectric/vacuum layers, which results in about a fifty percent transmission. Additional layers increase absorption of the transition radiation generated by the previous layers. Thus, there is likely an upper bound for radiated power of the structure based on the thermal limits of the materials being used. 
   Power in the range of tens of Watts may be generated. Using an approximately one millimeter electron beam diameter, a high brightness [Watts/(cm^2 area×nm bandwidth×strRad] EUV light source may be created. 
     FIG. 4  illustrates an approximated power spectra of an extreme ultraviolet transition oscillator. A graph  400  shows power graphed against wavelength. Additional conclusions regarding the potential power of an apparatus using these systems and techniques may be obtained by additional analysis of the spectra, efficiency and thermal limits involved. 
     FIG. 5  illustrates an extreme ultraviolet light system  500 . The system  500  includes a beam source  510 . The beam source  510  may be a compact linear accelerator (e.g., a 10 MeV electron pump source), such as a Varian 10 MeV Linac provided by Varian Associates Corporation of Palo Alto, Calif. The system  500  includes a vacuum chamber  580  and a transition oscillator  530 , such as described above. The vacuum chamber  580  can be an ultra-high vacuum tube system with the multi-layer stack suspended inside. The system  500  also includes a beam diverter  550  and a beam dump  560 . 
   The beam source  510  generates a relativistic electron beam  520 , such as a beam of 10 MeV electrons with a focal spot size of one millimeter. The electron beam  520  passes through the multi-layer stack, which generates EUV illumination  540 . The metal layers of the multi-layer stack may have an electron plasma frequency near 20 eV, and the multi-layer stack may radiate in the extreme ultraviolet wavelength region at about 13.5 nm. The beam  520  then passes through, and is redirected by, the beam diverter  550  (e.g., a bending magnet) to fall on the beam dump  560 . 
   The beam dumb  560  may be configured as Faraday cup and may also electromagnetically slow the beam to recapture energy prior to absorbing the current. The beam dumb may be constructed with using a combination of Carbon and metals or other materials. Active cooling may be required. 
   The total thickness of the multi-layer stack is substantially smaller than the electron mean free path so very little energy from the electron beam  520  is deposited in the multi-layer stack by way of scattering. The illumination  540  is created at the transition oscillator  530  and provided to an exit aperture. Almost all of the unwanted energy is in the form of the beam  520 , which can be dumped far away from the transition oscillator  530 . Thus, the heating issues for the system  500  are generally limited to heating caused by the partial absorption of the transition radiation by the transition oscillator  530 . 
   The system  500  may also include an EUV tool  570 . The EUV tool  570  may include various components that are reflective and coated with distributed quarter-wave Bragg reflectors. The illumination  540  may be provided to the EUV tool  570  for various purposes, such as for lithography or actinic interferometric measurement of EUV mirrors. 
   The EUV tool  570  may be an EUV interferometer or an EUV lithography tool. The EUV interferometer may be a tool for testing optical systems. The EUV lithography tool may be an integrated circuit (IC) manufacturing system used to transfer circuit patterns from a mask to a silicon substrate. 
   The IC manufacturing system may include a low-expansion reflective reticle attached to a scanning reticle stage that moves the mask across the EUV illumination, which may be redirected with optics. The IC manufacturing system may also include a scanning wafer stage, which holds a semiconductor wafer coated with EUV-sensitive photoresist, that synchronously scans the wafer across the reflected EUV illumination with nanometer precision. The two scanning stages may be magnetically levitated. The IC manufacturing system may also include a reflective optical system with aspheric components to produce a reduction of the mask image that illuminates the wafer. 
     FIG. 6  illustrates a calculated relativistic factor for an electron beam used in the system of  FIG. 5. A  graph  600  shows β≡v/c and γ≡ 1/√{square root over (1−β     2     )} by electron energy in KeV.    
   The various implementations described above have been presented by way of example only, and not limitation. Other embodiments may be within the scope of the following claims.