Abstract:
The present invention includes a system and method for characterizing the wavelength of a beam of radiation using a known reference transition. The invention comprises a vapor including a material having a transition which absorbs radiation of a known wavelength, the vapor being contained in a container. The container includes an optical path along which the beam of radiation can propagate through the vapor. A dispersive optical element is aligned along the optical path. A detector is aligned along the optical path after the dispersive element. Wavelength information about the beam of radiation is determined from the position of a dip in the detector signal that is correlated to the known reference transition. This method is most useful when the laser bandwidth substantially exceeds the transition bandwidth.

Description:
CONTINUING APPLICATION DATA 
     The present application is a continuation-in-part of United States Patent Application Ser. No. 08/896,384, filed Jul. 18, 1997. Now U.S. Pat. No. 5,978,391, issue date Nov. 2, 1999. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to systems and methods for measuring the wavelength and wavelength shifts of electromagnetic radiation, and more specifically to systems and methods for characterizing the wavelength and wavelength shift of a beam of laser radiation. 
     DESCRIPTION OF RELATED ART 
     Lasers such as excimer lasers are employed in a wide range of applications and uses. During many of these applications or uses the output wavelength of the laser must be actively controlled and measured during the operation cycle of the laser. For example, when lasers are used in the fabrication of integrated circuits, the wavelength output by the laser must be well characterized. 
     Conventional fabrication techniques for integrated circuits use a laser beam to selectively expose an optically sensitive material. Portions of the material not exposed by the laser beam are subsequently etched away. Additional fabrication steps are employed to achieve a circuit matching the shape of the exposed material. The short wavelength of an excimer laser operating in the deep ultraviolet region provides the potential for very sharp edge definition. However, when exposing the optically sensitive material using the laser beam from the excimer laser, the wavelength of the laser beam must be precisely controlled or characterized to ensure that it matches the wavelength for which the optics of the system were designed. A variation in the wavelength of the laser beam can cause the beam to be defocused at the surface of the optically sensitive layer, thereby losing the advantage of the short wavelength and resulting in an imprecise rendering of the integrated circuit. This can adversely affect the electrical characteristics of the resulting integrated circuit, resulting in a poor quality or inoperable circuit. 
     Accordingly, precise regulation or monitoring of the output wavelength of the excimer laser is important in the fabrication of integrated circuits using excimer lasers. The output of some excimer lasers which are line narrowed can be characterized using an atomic wavelength reference approach described in U.S. Pat. No. 5,450,207, issued Sep. 12, 1995. However, this approach does not work as well for lasers having bandwidths substantially greater than that of the wavelength reference. Therefore what is needed is a system and method to characterize the wavelength of a beam of radiation which has a bandwidth too large to be characterized by traditional means. 
     SUMMARY OF THE INVENTION 
     The present invention comprises a system and a method useful for characterizing the wavelength and shifts in the wavelength of a tunable excimer laser suitable particularly for partially-line-narrowed systems having bandwidths on the order of 20 to 100 picometers. According to one embodiment of the present invention a beam of radiation output from a laser is sent through a hollow cathode lamp containing a vapor of an absorbing material such as an atom, a molecule, or an ion. The absorbing material has one or more transitions which absorb radiation within the output spectrum of the laser. The transitions in the material in the vapor are fixed in wavelength. Materials can be chosen having transitions at very well known wavelengths which can easily be found in reference books. 
     After the beam of radiation emerges from the hollow cathode lamp it is then sent into a spectrometer which has a detector array placed at its output. The path taken by the beam of radiation to the detector array defines an optical path for the beam of radiation through the system. The absorption of a portion of the wavelength spectrum of the beam of radiation in the vapor provides a fixed wavelength reference point, referred to as an absorption feature, from which the wavelength or changes in the wavelength of the beam of radiation output by the laser can be measured. 
     At the output of the spectrometer the wavelength spectrum of the laser beam will be spread out along a dimension of the detector. The wavelength position of the absorption in the vapor is set by physical properties of the transition and will remain fixed even if the wavelength spectrum of the output of the laser shifts. Therefore, as the laser output spectrum varies, the absorption feature due to the transition in the vapor will provide a fixed reference point against which to characterize movements in the laser&#39;s output spectrum. 
     For a typical partially-line-narrowed excimer laser, the width of the spectrum of the laser will be significantly larger than the width of the transition in the vapor when the two widths are measured by any standard parameter such as full width of the half maximum. The bandwidth of the laser should not be so large as to make the detector impractical. 
     If two or more absorption transitions at known wavelengths are present in the vapor within the spectrum of the laser output, then an absolute measurement of the wavelength of the laser and the shift in the laser&#39;s output wavelength spectrum is possible. Two absorption transitions with a known separation in wavelength provide a wavelength versus distance calibration for the detector. This calibration can be used to measure the amount by which the output wavelength spectrum of the laser shifts. The two absorption transitions can be from different atomic, molecular or ionic materials in the vapor or in two different vapor chambers, or from spectral shifts in a transition such as an isotope shift. It is noted that the calibration of wavelength versus distance at the detector can also be determined from the physical layout of the system. If this calibration is known then only one absorption line is required to determine the amount by which the output wavelength spectrum of the laser shifts. Another embodiment of the present invention includes a system and method for characterizing the wavelength of a beam of radiation using a known reference transition. The invention comprises a container, a vapor including a material having a transition which absorbs radiation of a known wavelength, the vapor being contained in the container. The container includes an optical path along which the beam of radiation can propagate through the vapor. A dispersive optical element is aligned along the optical path. A detector is aligned along the optical path after the dispersive element. 
     In still another embodiment of the invention the detector generates an output signal and the output signal includes an absorption feature. The wavelength of the beam of radiation is characterized by comparing the output signal to the absorption feature. In one embodiment of the invention, the output signal includes a first signal corresponding to the spectrum of the beam of radiation absent the absorption feature. The output signal also includes a second signal which corresponds to the spectrum of the beam of radiation after it has passed through the vapor including the material. The second signal contains the absorption feature. The beam of radiation is then characterized by comparing the first signal to the second signal. 
     In yet another aspect of the invention, the container includes a hollow cathode lamp and the material in a cathode in the hollow cathode lamp includes iron. In another embodiment, the material in the hollow cathode includes platinum. In still another aspect of the invention the beam of radiation has a bandwidth larger than a bandwidth of the transition. The beam of radiation may have a bandwidth for example more than twenty or more than a hundred times larger than a bandwidth of the transition. 
     In another embodiment of the invention the dispersive element comprises a diffraction grating. In yet another embodiment of the invention, the dispersive element comprises one or more prisms. In still another aspect of the invention, a slit is placed along the optical path so that the beam of radiation encounters the slit before the beam of radiation encounters the dispersive element. In yet another embodiment of the invention, the detector is a photodiode array. 
     Another embodiment of the invention includes a collimating device placed along the optical path between the dispersive element and the detector. In one aspect of this embodiment, the collimating device includes a lens. 
     Another embodiment of the invention comprises a laser generating the beam of radiation. In still another aspect of the invention, the laser comprises an excimer laser, and in yet another aspect of the invention the excimer laser comprises a gain medium containing Krypton Fluoride. In another aspect, the excimer laser comprises a gain medium containing Argon Fluoride. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 is a schematic drawing showing one embodiment of the present invention in which a beam of radiation is sent though an absorbing vapor. The radiation is diffracted by a grating and impinges on a detector. 
     FIG. 2 is a schematic drawing showing the relative orientation of a slit and a grating in one embodiment of the invention. 
     FIG. 3A is a graph depicting output of a detector showing the spectrum of a beam of radiation before absorption of a portion of the beam of radiation by a transition in the vapor cell. 
     FIG. 3B is a graph depicting output of a detector showing the spectrum of a beam of radiation and an absorption dip due to absorption of a portion of the beam of radiation by a transition in the vapor cell before the wavelength of the beam of radiation has been shifted. 
     FIG. 3C is a graph depicting output of a detector showing the spectrum of a beam of radiation and an absorption dip due to absorption of a portion of the beam of radiation by a transition in the vapor cell after the wavelength of the beam of radiation has been shifted. 
     FIG. 4 depicts an embodiment of the invention in which a computer system receives and processes an output signal from the detector. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 depicts one embodiment of the present invention, system  100 . In this embodiment of the invention, beam  102  of radiation having a bandwidth of for example 20 to 100 picometers, enters vapor chamber  104 . Beam  102  of radiation can be any type of electromagnetic radiation from any type of source, including but not limited to a laser, an excimer laser, a Krypton Fluoride excimer laser, an Argon Fluoride excimer laser, a non-linear generation device, or a discharge lamp. Vapor chamber  104  can be any type of chamber containing an absorbing vapor medium. Vapor chamber  104  may be but is not limited to a hollow cathode lamp, a heat pipe, or any other open or enclosed container. In some cases it may be useful that the absorbing vapor is an atomic or molecular beam in a vacuum chamber, to reduce doppler broadening and thus obtain a narrower width of the absorbing feature. The vapor chamber  104  may be placed in system  100  at any point along optical path  105 . For example, vapor chamber  104  could be placed in beam of radiation  102  after slit  106  or after dispersive element  108 . 
     After beam  102  of radiation exits vapor chamber  104  it impinges on slit  106 . Beam  102  of radiation is depicted as being focussed into slit  106 , however the beam  102  of radiation may be passed through slit  106  without being focussed, or slit  106  may be removed from system  100  in various embodiments. 
     After beam  102  of radiation passes through slit  106  it passes through collimation device  112  and hits dispersive element  108 . Collimation device  112  is optional, and can be put in beam  102  of radiation at any point along its path if desired due to design consideration. Collimation device  112  can be a lens, a system of lenses, a mirror, a system of mirrors, and may include refractive or reflective properties of dispersive element  108 . For example, dispersive element  108  may be a curved grating which in combination with collimation device  112  acts to collimate beam  102  of radiation. In other embodiments of the invention, optical path  105  passes through collimation device  112  only once. 
     Dispersive element  108  can be a grating, one or more prisms, a system including gratings and prisms, or any other system which disperses radiation by wavelength. If slit  106  is present in system  100  it should be oriented so that the longer dimension of slit  106  is substantially perpendicular to the direction in which the radiation is dispersed by wavelength by dispersive element  108 . This relationship is depicted in FIG. 2 for slit  106  and grating  202 . The longer dimension of slit  106  is shown substantially parallel to the grooves on grating  202 , and therefore substantially perpendicular to the direction in which the radiation is dispersed by wavelength by grating  202 . 
     In FIG. 1, the beam of radiation is dispersed in a transverse pattern according to wavelength by dispersive element  108 , passes through collimation device  112 , and then is detected by detector  110 . Detector  110  detects the dispersed beam of radiation  102  according to the pattern. Detector  110  can be any type of system or device which is sensitive to radiation in beam of radiation  106 , including but not limited to a photodiode array, a CCD array, or film. Examples of the output of detector  110  are depicted in FIGS. 3A-3C. FIG. 3A shows the output from detector  110  for a typical spectrum for a partially-line-narrowed excimer laser before a portion has been absorbed in the vapor. When the wavelength of the laser is tuned or shifts for any reason it is difficult to reliably determine which direction or by how much the wavelength of the laser has shifted since there is no fixed reference point if a wavelength reference system is not employed. 
     The absorption transition in the vapor provides a fixed wavelength reference with a bandwidth of for example 0.8 to 1.2 picometers, against which shifts in the wavelength of the laser&#39;s broader output spectrum (e.g. 20 to 100 picometers) can be measured. FIGS. 3B and 3C show examples of the output of detector  110  when a portion of the laser output has been absorbed in the vapor. In FIG. 3B the absorption feature representing absorption of part of beam of radiation  102  by the absorbing material in vapor chamber  106  appears as a dip in the spectrum. When the wavelength of the laser shifts, the spectrum detected by the detector shifts in wavelength while the feature due to the transition remains fixed at the wavelength of the transition. FIG. 3C depicts the output of detector  110  after the laser&#39;s output spectrum has shifted. The fixed absorption feature appears in a different portion of the spectrum and the shift in wavelength of the laser&#39;s output spectrum becomes apparent. 
     FIG. 4 depicts yet another embodiment of the invention, system  400 . In this embodiment, Krypton Fluoride excimer laser  402  generates output beam of radiation  404  along optical path  406 . Beam pick-off  408  reflects a small portion of output beam  404  of radiation, beam  410  of radiation and directs it into hollow cathode lamp  412 . Beam pick-off  408  may be any device or optical system which will direct a portion of laser beam  406  into hollow cathode lamp  412 , including but not limited to a partially reflecting mirror, or an uncoated optical element. In other embodiments of the invention, the portion of output beam of radiation  404  idrected to hollow cathode lamp  412  is the portion transmitted through beam pick-off  408 . In these embodiments beam pick-off  408  may reflect less of output beam  404  of radiation than it transmits. 
     Hollow cathode lamp  412  includes a transparent (over the bandwidth of interest) entrance window  414  and a transparent exit window  416  each tilted at an angle to avoid unwanted reflection, for example  10  degrees, from the path of beam  410  of radiation. Entrance window  414  and exit window  416  form portions of a sealed chamber  418  having anode  420  and cathode  422  mounted therein. Cathode  422  is a generally tubular or annular cathode aligned with beam of radiation  410  such that beam  410  is transmitted through an interior aperture of cathode  422 . Anode  420  and cathode  422  are electrically connected to current control mechanism  424  which allows a selected amount of electrical current to be conducted through anode  420  and cathode  422 . In one embodiment of the invention, cathode  420  is fabricated from iron and chamber  418  is filled with a neon filler gas. In another embodiment, cathode  420  is platinum and chamber  418  is filled with neon. 
     In use, a pre-selected amount of current is conducted through anode  420  and cathode  422  causing vaporization of a portion of the material forming cathode  422 . The vaporous material fills the interior of cathode  422  through which beam  410  of radiation is transmitted. The vaporous material absorbs a portion of beam  410  of radiation at a wavelength determined by characteristics of the cathode material. For the embodiment of the invention wherein the cathode is fabricated from iron, a wavelength of maximum absorption occurs at a wavelength equal to 248.3271 nm with a FWHM width of about ±0.3 picometers, with little or no absorption occurring at wavelengths remote relative to the laser bandwidth from the wavelength of maximum absorption. Other absorption wavelengths exist, such as at about 248.4187 for iron, and can also be used. For a platinum cathode, maximum absorption occurs at 193.4369 nm. 
     Beam  410  of radiation, after possible partial absorption within chamber  418 , exits hollow cathode lamp  412  through exit window  416  and then passes through slit  426 . Beam  410  of radiation then goes through collimation device  428  and impinges on dispersion device  430 . In one embodiment of the invention, dispersion device  430  is a grating. Suitable gratings include ruled gratings and holographic gratings. 
     When beam  410  of radiation is reflected from dispersion device  430 , it is dispersed in wavelength. Beam  410  of radiation then passes again through collimation device  428  and then hits detector  432 . In one embodiment of the invention, detector  432  is a linear array of detection devices such as an array of charge coupled devices (CCD array); however any array of detection devices sensitive to beam of radiation  432  can be used. Also, a scanning single detector could be applied. 
     Detector  432  is coupled to computer system  434 . Computer system  432  generically represents any type of computer system, such as a microprocessor-based system, a mainframe system, or any other type of general or special purpose computing system which includes an interface, a processor, and a memory. The processor can be any type of processor such as a microprocessor, dedicated logic, a digital signal processor, a programmable gate array, a neural network, or a central processor unit implemented in any other technology. 
     The output of detector  432  is sent to computer system  434  and stored in memory. Computer system  434  may display the output on optional display  436 . In an embodiment of the invention not shown, computer system  434  is coupled to current source  424  and controls the amount of current that current source  424  supplies to hollow cathode lamp  412 . In this embodiment, computer system  434  can control the amount of vapor in hollow cathode lamp  412  that beam of radiation  410  passes through, by changing the current. 
     In one aspect of the invention, computer system  434  reduces the current from current source  424  until there is only very slight absorption or no absorption at all of beam of radiation  410  in hollow cathode lamp  412 . Computer system  434  then stores the output from detector  432 . This output may be called a baseline signal. Then computer system  434  increases the current from current source  424  until there is more absorption of beam  410  of radiation in hollow cathode lamp  412 . Computer system  432  then again stores the output form detector  432 . This signal may be called the absorption signal. Computer system  432  then subtracts the baseline signal from the absorption signal resulting in the absorption feature becoming more prominent. A baseline signal may be taken as often as desired, and for example may be taken more often for a laser which has higher output power fluctuations. 
     The foregoing description of embodiments of the present invention are presented for the purposes of illustration and description only. They are not intended to be exhaustive or to limit the invention to the forms disclosed. Many modifications and variations will be apparent to practitioners skilled in the art. It is intended that the scope of the invention be defined by the following claims and their equivalents.