Abstract:
A cell for a vacuum ultraviolet plasma light source, the cell having a closed sapphire tube containing at least one noble gas. Such a cell does not have a metal housing, metal-to-metal seals, or any other metal flanges or components, except for the electrodes (in some embodiments). In this manner, the cell is kept to a relatively small size, and exhibits a more uniform heating of the gas and cell than can be readily achieved with a hybridized metal/window cell design. These designs generally result in higher plasma temperatures (a brighter light source), shorter wavelength output, and lower optical noise due to fewer gas convection currents created between the hotter plasma regions and surrounding colder gases. These cells provide a greater amount of output with wavelengths in the vacuum ultraviolet range than do quartz or fused silica cells. These cells also produce continuous spectral emission well into the infrared range, making them a broadband light source.

Description:
[0001]    This application claims all rights and priority on prior pending U.S. provisional patent application Ser. No. 61/308,206 filed 2010 Feb. 25 and PCT patent application serial number US2011/025198 filed 2011 Feb. 17. This invention relates to the field of integrated circuit fabrication. More particularly, this invention relates to plasma light sources that emit broadband radiation including the vacuum ultraviolet range. 
     
    
     FIELD 
     Introduction 
       [0002]    Plasma discharge is used as a light source in a variety of different applications, such as in the inspection and metrology of integrated circuits. The most common commercially available vacuum ultraviolet (VUV) light source is a low pressure deuterium discharge lamp, which exhibits a relatively high radiant output at wavelengths from about 120 nanometers to about 160 nanometers, but a relatively low radiant output at wavelengths greater than about 170 nanometers. Since for many applications it is desirable to use broadband radiation that spans vacuum ultraviolet, ultraviolet, visible and near infrared ranges, for those applications it is currently necessary to combine the output of a deuterium lamp with the output from another lamp such as a xenon arc lamp or quartz-halogen lamp to cover the whole wavelength range. 
         [0003]    A hot, high-pressure xenon plasma can emit radiation covering the vacuum ultraviolet through near infrared wavelength ranges. However conventional xenon arc lamps are enclosed in fused silica envelopes. Hot fused silica does not transmit wavelength shorter than about 180 nanometers, so there is little useful output from the light source in the vacuum ultraviolet part of the spectrum. 
         [0004]    Commercially available vacuum ultraviolet deuterium lamps overcome the problem of the poor vacuum ultraviolet transmission of fused silica by using a small magnesium fluoride output window that is fused or bonded onto the end of a snout in the fused silica envelope. Magnesium fluoride transmits light from about 115 nanometers to about 8 microns in wavelength. Bonding between the magnesium fluoride window and fused silica is mechanically weak. A deuterium lamp operates at low pressure, so the force on the window is a compressive force from the outside due to atmospheric pressure. If the deuterium lamp is operated in a vacuum, then the force on the window is a weak outward force due to the low pressure gas in the lamp. In either case the pressure difference between inside and outside of the envelope will not exceed one atmosphere. The force on the window of a deuterium lamp is always low enough that a properly formed bond between the window and envelope does not fail. 
         [0005]    In order to obtain high brightness emission from a xenon plasma over a broad range of wavelengths, it is necessary that the xenon be at high pressure, typically about ten to thirty atmospheres. Such a high pressure precludes the use of a magnesium fluoride window in the fused silica envelope because the bond between the window and the envelope cannot reliably withstand the outward forces caused by the high pressure difference between inside and outside the envelope. 
         [0006]    What is needed, therefore, is a light source that reduces problems such as those described above, at least in part, while providing high brightness radiation over a broad spectral range including vacuum ultraviolet, ultraviolet and visible wavelengths. 
       SUMMARY OF THE CLAIMS 
       [0007]    The above and other needs are met by a cell for a vacuum ultraviolet plasma light source, the cell having a closed sapphire tube containing at least one noble gas. 
         [0008]    Such a cell does not have a metal housing, metal-to-metal seals, or any other metal flanges or components, except for the electrodes (in some embodiments). In this manner, the cell is kept to a relatively small size, and exhibits a more uniform heating of the gas and cell than can be readily achieved with a hybridized metal/window cell design. These designs generally result in higher plasma temperatures (a brighter light source), shorter wavelength output, and lower optical noise due to fewer gas convection currents created between the hotter plasma regions and surrounding colder gases. These cells provide a greater amount of output with wavelengths in the vacuum ultraviolet range than do quartz or fused silica cells. These cells also produce continuous spectral emission well into the infrared range, making them a broadband light source. 
         [0009]    In various embodiments according to this aspect of the invention, the cell is formed exclusively of sapphire or other VUV-transmissive material. In some embodiments the at least one noble gas includes a mixture of xenon with at least one of argon, krypton, neon and helium. In some embodiments mercury is added to the at least one noble gas. In some embodiments electrodes extend through the tube into the cell, where the electrodes are hard-sealed to the sapphire of the tube. In some embodiments the sapphire tube is closed by means of two end caps that are diffusion bonded to the tube. In some embodiments at least one of the end caps is formed of a more pure grade of sapphire than the tube. In some embodiments one of the end caps is coated with an anti-reflective coating so that a laser light directed into the cell through the coated end cap exhibits a reduced degree of reflectance. 
         [0010]    In some embodiments, the light, instead of entering and/or leaving via the end caps, enters and/or leaves through the sidewalls of the tube. In some embodiments the sapphire tube has a flat window formed therein, wherein the flat window is formed of a more pure grade of sapphire than the tube. In some embodiments one region of the surface of the cylinder is coated with an anti-reflective coating. In other embodiments a different region of the surface of the cylinder is coated with a reflective coating. In some embodiments, an anti-reflective coating is coated on the outside of a flat window. 
         [0011]    According to another aspect of the invention there is described a vacuum ultraviolet plasma light source having a cell having a closed sapphire tube containing at least one noble gas, means for initiating a plasma within the cell, and means for sustaining a plasma within the cell, thereby creating a vacuum ultraviolet light. 
         [0012]    In various embodiments according to this aspect of the invention, the means for initiating the plasma within the cell includes at least one of a direct current potential applied by electrodes extending into the cell, an alternating current potential applied by electrodes extending into the cell, a pulsed laser directed into the cell, a continuous laser directed into the cell, microwaves directed into the cell, radio frequency electromagnetic radiation directed into the cell, ionizing radiation of gamma rays directed into the cell, ionizing radiation of X-rays directed into the cell, ionizing radiation of alpha particles directed into the cell, and ionizing radiation of beta particles directed into the cell. 
         [0013]    In some embodiments the means for sustaining a plasma within the cell includes at least one of a direct current potential applied by electrodes extending into the cell, an alternating current potential applied by electrodes extending into the cell, a pulsed laser directed into the cell, and a continuous laser directed into the cell. Some embodiments include an aperture formed in a light-stop, where the aperture passes only a desired portion of the vacuum ultraviolet light. In some embodiments the means for sustaining the plasma comprises a laser light source coupled to a fiber optic for directing a laser beam into the cell. 
         [0014]    According to yet another aspect of the invention there is described a spectrographic instrument having a selection of the components described above. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    Further advantages of the invention are apparent by reference to the detailed description when considered in conjunction with the Figures, which are not to scale so as to more clearly show the details, wherein like reference numbers indicate like elements throughout the several views, and wherein: 
           [0016]      FIG. 1  is a cross-sectional view of a light source according to a first embodiment of the present invention. 
           [0017]      FIG. 2  is a functional block diagram of a broadband spectroscopic ellipsometer according to a first embodiment of the present invention. 
           [0018]      FIG. 3  is a cross sectional view of a light source according to a second embodiment of the present invention. 
           [0019]      FIG. 4  is a top plan view of a light source according to the second embodiment of the present invention. 
           [0020]      FIG. 5  is a cross-sectional view of a light source according to a third embodiment of the present invention. 
           [0021]      FIG. 6  is a top plan view of a light source according to the third embodiment of the present invention. 
           [0022]      FIG. 7  is a cross-sectional view of a light source according to a fourth embodiment of the present invention. 
           [0023]      FIG. 8  is a cross-sectional view of a light source according to a fourth embodiment of the present invention. 
           [0024]      FIG. 9  is a functional block diagram of a broadband spectroscopic ellipsometer according to another embodiment of the present invention. 
           [0025]      FIG. 10  is a functional block diagram of a broadband spectroscopic ellipsometer according to yet another embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    One aspect of some embodiments of the present invention is a sealed plasma discharge cell formed entirely of glass that can contain pressures of up to about fifty atmospheres of an appropriate gas, such as helium, neon, argon, krypton, and xenon. The term “glass” as used herein has a specific definition, which is that the material is optically transmissive within the desired wavelength range. However, it does not denote that the material is necessarily formed of silica, or that the material is amorphous. In most embodiments, the material from which the cell is formed is in a crystalline state. 
         [0027]    With reference now to  FIG. 1  there is depicted a schematic illustration of one embodiment of a bonded sapphire plasma discharge cell  100 . The cell  100  has sidewalls  102 , such as formed of a cylinder of sapphire with a diameter of less than about two centimeters. An end-plate  104  is disposed at one end of the cell wall  102 , and is diffusion bonded to the cell wall  102 . An end-plate  106  is disposed at the other end of the cell wall  102 , and is also diffusion bonded to the cell wall  102 . In some embodiments the end-plates  104  and  106  are also formed of sapphire, and in other embodiments they are formed of other materials. In some embodiments, the entire volume of the cell  100  is about two cubic centimeters. 
         [0028]    The cell  100  is filled with a gas  118 . In one embodiment the gas  118  is xenon at an initial room-temperature-pressure of between about two and about fifty atmospheres. In other embodiments, other noble gases or mixtures of noble gases at such pressures are used. In some embodiments, mixtures of xenon and argon, xenon and krypton, and xenon and neon are used. Trace amounts of other elements such as mercury are added in some embodiments to more efficiently excite the atomic energy levels of interest. 
         [0029]    Electrodes  108  are positioned to either provide a direct current potential to a central spot  112  within the cell  100 , so as to sustain a plasma discharge  114  within the cell  100 , or to “start” or ionize the plasma  114  when using an alternate energy source such as a laser beam  110 , or application of radio frequency energy. Regardless of how it is initiated or sustained, the plasma  114  produces an output light  116  having the properties as desired and described herein. The electrodes  108  are sealed to the side walls  102  of the cell  100  such as with a hard-sealing technique. 
         [0030]      FIG. 2  depicts an ellipsometer  200  that uses the broadband light source cell  100  described above. The output beam  116  is passed through a variety of optics, including in some embodiments a polarizer  202 , and is reflected off of a substrate  204 . In some embodiments the beam  116  passes through an analyzer  206 , before being received by a spectrograph  208 . More details of an ellipsometer that can use this broadband light source can be found in U.S. Pat. No. 6,734,967. It should be understood that the ellipsometer optics shown in  FIG. 2  do not all lie in one plane. Some of the optical elements lie above or below the plane of the page and are shown in projection with the light passing in front of, or behind, those elements. 
         [0031]      FIG. 3  is a cross-sectional view of another embodiment of the present invention.  FIG. 4  is a top plan view of the same embodiment. In this embodiment the sealed cylinder  102  is formed of sapphire or some other material that is transmissive to vacuum ultraviolet wavelengths to at least some extent. The cylinder  102  has a diameter of about one centimeter, though larger and smaller diameters would also be acceptable. The length of the cylinder  102  is between about one centimeter and about ten centimeters, though longer and shorter lengths would also be acceptable. 
         [0032]    In some embodiments, laser light  110  is delivered by a fiber optic  304  from a laser source  302 . Light  110  emitted from the end of the fiber optic  304  is focused by a lens  306  to a point  112  near the center of the cylinder  102  to maintain a hot plasma  114  in the gas  118 . In some embodiments the wavelength of the laser light  110  is in the infrared range, such that it is only weakly absorbed by the gas atmosphere  118 , but is strongly absorbed by the hot plasma  114 . For xenon gas, wavelengths of between about 970 nanometers and about 975 nanometers are used in one embodiment. Wavelengths of about 515 nanometers, about 523 nanometers, about 527 nanometers, or about 532 nanometers are used in other embodiments. In one embodiment, the power of the laser  302  is in the range of from about twenty watts to about two-hundred and fifty watts. In one embodiment, the power of the laser  302  is between about fifty watts and about sixty watts. In one embodiment, the laser  302  consists of at least one diode laser coupled to the same fiber optic  304 . In another embodiment a fiber laser  302  is used. In another embodiment a gas laser  302  is used. In another embodiment a diode-pumped solid-state laser  302  is used. 
         [0033]    The lens  306  can be implemented in many different ways. It some embodiments the lens  306  is a singlet, doublet, or triplet lens. In some embodiments it is comprised of one or more curved mirrors. In some embodiments it is a combination of minors and lenses. In some embodiments, flat minors are used to change the direction of the light to allow the fiber optic  304  to be conveniently located. Any or all of the mirrors or lenses directing the laser beam  110  in different embodiments have coatings to optimize the transmission of the laser wavelength from the fiber optic  304  to the plasma  114 . When minors are used, one or more minors can be coated to maximize the reflection of the wavelength of the laser light  110 . When lenses are used, one or more lens surfaces can be coated with anti-reflection coatings to maximize the transmission of the laser light  110 . 
         [0034]    The plasma  114  emits broad-band radiation  116  spanning wavelengths from the vacuum ultraviolet to the near infrared, in all directions. For example, the wavelengths emitted may include a range of from about 155 nanometers to about one thousand nanometers. Some of the emitted light  116  passes through an output port  312 . The light  116  passing though the output port  312  can be used in a metrology instrument, such as the ellipsometer  200  depicted in  FIG. 2 . The light  116  can also be used in a reflectometer. U.S. Pat. No. 5,747,813 describes a small-spot broad-band spectroscopic reflectometer that might advantageously use the light source described herein. The light  306  can also be used in an inspection system that detects reflected or scattered light. 
         [0035]    In some embodiments, two electrodes  108  are installed along the length of the cell  102 , with a gap disposed between them, near the point  112  where the laser light  110  is focused. The gap in some embodiments is between about one millimeter and five millimeters in length, though shorter and longer gaps can also be made to work. Gas-tight seals are formed between the electrodes  108  and the material of the cell  102 . The electrodes  108  in some embodiments are either brazed or soldered to the material of the cell  201 . An electrical discharge (such as a spark or arc) is used in some embodiments to create an initial plasma that absorbs the laser light  110  more efficiently than the neutral gas  118 . A brief pulse of a voltage between about one kilovolt and fifty kilovolts is used in some embodiments to create a short-lived electrical discharge. Once the plasma  114  starts absorbing the laser light  110 , the plasma  114  becomes self-sustaining and the discharge is no longer needed. The electrical discharge in some embodiments is a series of pulses repeated every few milliseconds until a self-sustaining plasma  114  is created. The electrical discharge can be either direct current or alternating current. The repetition rate of the pulses in some embodiments is between about one megahertz and one hertz, though lower or higher rates may be used. 
         [0036]    Alternate embodiments of the light source  100  disclosed herein do not use an electrical discharge to create the initial plasma  114 , but instead use a pulsed laser, microwaves, radio frequency electromagnetic radiation, or ionizing radiation such as gamma rays, X-rays, alpha particles, or beta particles. In various embodiments, such a source of ionizing radiation is disposed either within the gas mixture  118 , or outside the cell  102 . 
         [0037]    In one embodiment, the cylinder  102  lies approximately horizontally. In one embodiment, the laser light  110  is focused from below the horizontal, as illustrated in  FIG. 3 . In one embodiment, the laser light  110  is aimed from about thirty degrees below the horizontal. In another embodiment, the laser light  110  is focused from above the horizontal, such as about thirty degrees above the horizontal. In another embodiment, the laser light  110  comes from near the horizontal. In one embodiment the laser light  110  is directed from one side as illustrated in  FIG. 6  in top-plan view. In any of these embodiments the relative angle between the laser light  110  and the output port  312  may be chosen so as to avoid the laser light  110  being directly transmitted through the output port  312 . 
         [0038]    In some embodiments, the cylinder  102  has an anti-reflection coating  308 , as depicted in  FIGS. 3 and 8 . The coating  308  is selectively coated on part of the surface of the cylinder  102  to enhance the transmission of the laser light  110  through the material. Since such a coating  308  is likely to not be transmissive at vacuum ultraviolet wavelengths, in some embodiments the coating  308  is omitted from the region of the cylinder  102  through which the emitted light  116  passes. 
         [0039]    In some embodiments as depicted in  FIG. 3 , the cylinder  102  has a coating  310  designed to reflect the laser light  110  that passes through the plasma  114 , back into the gas  118 . Since such a coating  310  is likely to not be transmissive at vacuum ultraviolet wavelengths, in some embodiments the coating  310  is omitted from the region of the cylinder  102  through which the emitted light  116  passes.  FIG. 7  depicts the cell  102  without the coatings  308  and  310 . 
         [0040]    Although the cell  102  is depicted as a cylinder, other shapes for the cell  102  are also contemplated herein. For example, in various embodiments the cell  102  is a sphere or an oblate spheroid. 
         [0041]      FIG. 5  depicts a cross-sectional view of another embodiment of the present invention, where the cell  201  has a flat output window  106 . In some embodiments the output window  106  is made of the same material as the cell  102 . In other embodiments it is made of a different material. Because most of the cell  102  does not need to transmit vacuum ultraviolet wavelengths, the cell  102  in some embodiments is made of material that is not transmissive (or is poorly transmissive) at vacuum ultraviolet wavelengths, such as quartz or fused silica, or is made of a less pure grade of the same material as the window  106 . In some embodiments the window  106  is made of a highly pure grade of sapphire and the rest of the cell  102  of a less pure grade of sapphire. 
         [0042]      FIG. 8  depicts an embodiment where a single high-powered laser diode  302  focuses a beam  110  directly into the gas  118  to create the plasma  114 . The laser diode  302  in one embodiment emits between about twenty watts and about two-hundred and fifty watts of infrared or visible radiation. The laser-diode light source  302  can be used in combination with any of the other features and embodiments described herein including, but not limited to, different configurations of lenses and minors for the focusing optics  306 , anti-reflection coatings  308 , and the cylinder  102 , with or without flat windows  104  and  106 . 
         [0043]      FIG. 9  depicts another embodiment of an ellipsometer  200 . Again it should be understood that some of the optical elements depicted in  FIGS. 9 and 10  may lie above or below the plane of the page and so are shown in projection. For example the light  116  passes in front of minor  908  and aperture  910  (both of which lie below the plane of the page in these figures) and is reflected off mirror  906 . Mirror  906  is tilted at a slight angle so that the reflected light from  906  passes through aperture  910  and strikes minor  908 . The light is reflected from  908  to polarizer  218 . Other optical components may similarly lie above or below the plane of the page. This ellipsometer  200  includes a monitor  902  that monitors the intensity of the light  116  that is emitted from the light source  100 . In this embodiment, the minor  906  reflects most of the light  116  that is incident upon it, but a small fraction  904  of the incident light  116  is transmitted to the monitor  902 . The monitor  902  in some embodiments is a photodiode that monitors the intensity of the light source  100  over a wide range of wavelengths. In other embodiments it is a spectrometer that individually monitors the intensity of many wavelengths. 
         [0044]    The signal from the monitor  902  in some embodiments is used to adjust the laser  302  within the light source  100  by, for example, controlling the current through a laser diode  302 , to compensate for intensity fluctuations in the light source  100 . In some embodiments the monitor  902  is part of a control loop that controls the light output  116  to be more stable than would be possible without the monitor  902 . In an alternate embodiment, the signal from the monitor  902  is used to normalize the data collected by the spectrograph  224 , and thereby to correct for fluctuations in the light source  100 . This can be done wavelength by wavelength as described in U.S. Pat. No. 5,747,813, or by a single global correction value that is applied to all wavelengths. 
         [0045]    In various embodiments the monitor  902  is placed in different positions. For example, instead of mirror  906  transmitting a small fraction  904  of the incident radiation  116 , the mirror  906  is opaque and the mirror  908  is partially transmissive, with the monitor  902  located behind the mirror  908 . In yet another embodiment, aperture  910  is inclined at a slight angle to the main propagation direction of the light  116 , and the monitor  902  is positioned so as to capture the light  904  that is reflected from the aperture  910  that is not transmitted through the aperture in  910 , as depicted in  FIG. 10 . 
         [0046]    In some embodiments, the polarizer  218  of the ellipsometer  200  incorporating the light source  100  is not rotated during data collection, but instead the analyzer  222  is rotated. In various embodiments, either or both of the analyzer  222  or polarizer  218  includes not just a polarizing element, but also a compensator (also known as a waveplate or retarder). In some embodiments, the compensator can be rotated instead of the polarizing element. In other embodiments, both the analyzer  222  and the polarizer  218  (or the compensators within those functions) are rotated, in some embodiments at different rotation speeds and in some embodiments in opposite directions. 
         [0047]    The foregoing description of embodiments for this invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiments are chosen and described in an effort to provide illustrations of the principles of the invention and its practical application, and to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.