Abstract:
A microcavity discharge device generates radiation with wavelengths in the range of from 11 to 14 nanometers. The device has a semiconductor plug, a dielectric layer, and an anode layer. A microcavity extends completely through the anode and dielectric layers and partially into the semiconductor plug. According to one aspect of the invention, a substrate layer has an aperture aligned with the microcavity. The microcavity is filled with a discharge gas under pressure which is excited by a combination of constant DC current and a pulsed current to produce radiation of the desired wavelength. The radiation is emitted through the base of the microcavity. A second embodiment has a metal layer which transmits radiation with wavelengths in the range of from 11 to 12 nanometers, and which excludes longer wavelengths from the emitted beam.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The invention relates to microcavity devices and more particularly to a method and device for producing radiation useful in lithography systems.  
         DESCRIPTION OF RELATED ART  
         [0002]    Integrated circuits are fabricated using lithography systems with a variety of radiation sources, such as for example mid-ultraviolet lithography. These sources produce ultraviolet radiation with wavelengths in the range of 100 to 500 nanometers. The ultraviolet radiation is used to expose photoresist during integrated circuit fabrication. Radiation emissions with wavelengths of 253-254 nanometers are produced by known microdischarge lamps using a discharge gas.  
           [0003]    A known microdischarge lamp has a substrate, a cathode plug, a dielectric layer, and an anode layer. The lamp has a microcavity etched in the shape of a cylinder. The microcavity has an open end and a closed end. The microcavity extends through the anode and dielectric layers. The microcavity extends into the cathode layer to form a microcavity base. The diameter of the microcavity is in the range of 1 to 400 microns. The microcavity acts as a container for a discharge gas of mercury or xenon iodine. The discharge gas is supplied to the microcavity under pressure. The substrate layer and anode layer are formed of conductive materials. The cathode layer is formed of a doped silicon and the dielectric layer is formed of silicon dioxide. The cathode layer is secured to the substrate layer by an epoxy layer.  
           [0004]    By using a semiconductor material for the cathode layer, uniform voltages can be formed along the length of the microcavity. A discharge gas that is maintained in the microcavity under pressure and subjected to electric current emits radiation through the open end of the microcavity. High energy electrons are released by the discharge gas which allows access to higher energy or ion states of gaseous atoms or molecules.  
           [0005]    It has been suggested to operate a lamp by supplying a discharge gas to a microcavity and applying a constant electrical current of 4 milliamps between the anode and substrate layers. The discharge gas is supplied to the microcavity at a pressure of up to 200 torr. The lamp emits radiation with wavelengths in the 253 to 254 nanometer range. The lamp can be used in a lithography system. Radiation emitted from the lamp may be reflected off mirrors and through masks or reticles and onto the semiconductor wafer surface.  
           [0006]    Ideal reflective surfaces for mirrors used in lithography systems include surfaces formed from molybdenum silicon (MoSi) and molybdenum beryllium (MoBe) compounds. These compounds attain their highest reflectivities, approximately 70%, when reflecting radiation with wavelengths in the 11 to 14 nanometers range. Therefore, what is needed is a microcavity discharge device which produces radiation emissions with wavelengths of less than 253 nanometers, and more particularly wavelengths in the range of from about 11 to about 14 nanometers.  
         SUMMARY OF THE INVENTION  
         [0007]    The invention relates to a microcavity device which produces radiation with wavelengths in the extreme ultraviolet region. In accordance with one embodiment, the device has a semiconductor plug, a dielectric layer, and an anode layer. The dielectric layer electrically separates the semiconductor layer from the anode layer. A microcavity with an open end is formed in the anode layer. The microcavity extends through the dielectric layer and has a base in the semiconductor plug. Optionally, a substrate layer having an aperture aligned with the microcavity can be formed on the bottom surface of the semiconductor plug.  
           [0008]    The microcavity is filled with a pressurized discharge gas, and the anode and substrate layers are supplied with a combination of constant and pulsed currents. The electrical pulses produce radiation from the discharge gas which are emitted from the microcavity through the bottom of the semiconductor layer and the aperture of the substrate layer. The radiation can be directed as a beam onto mirrors in an optical system. The mirrors be formed with highly reflective surfaces. When the discharge gas is xenon, the radiation has wavelength peaks in the range of from about 11 to about 14 nanometers.  
           [0009]    In accordance with another aspect of the invention, a thin metal layer is located between the semiconductor plug and the substrate layer. When the metal layer is beryllium, the emitted radiation has wavelengths between 11 and 12 nanometers (wavelengths greater than about 12 nanometers are absorbed by the beryllium layer). 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    These and other advantages and features of the invention will be more readily understood from the following detailed description of the invention which is provided in connection with the accompanying drawings.  
         [0011]    [0011]FIG. 1 is a top view of a microcavity discharge device fabricated in accordance with a first embodiment of the invention;  
         [0012]    [0012]FIG. 2 illustrates a discharge system fabricated in accordance with the first embodiment of the invention, and includes a cross sectional view of the device of FIG. 1, taken along line II-II;  
         [0013]    [0013]FIG. 3 is a top view of a microcavity discharge device fabricated in accordance with a second embodiment of the invention;  
         [0014]    [0014]FIG. 4 illustrates a discharge system fabricated in accordance with the second embodiment of the invention, and includes a cross sectional view of the device of FIG. 3, taken along line IV-IV;  
         [0015]    [0015]FIG. 5 is a top view of a microcavity discharge device fabricated in accordance with a third embodiment of the invention;  
         [0016]    [0016]FIG. 6 illustrates a discharge system fabricated in accordance with the third embodiment of the invention, and includes a cross sectional view of the device of FIG. 5, taken along line VI-VI;  
         [0017]    [0017]FIG. 7 is a top view of a microcavity discharge device fabricated in accordance with a fourth embodiment of the invention;  
         [0018]    [0018]FIG. 8 illustrates a discharge system fabricated in accordance with the fourth embodiment of the invention, and includes a cross sectional view of the device of FIG. 7, taken along line VIII-VIII;  
         [0019]    [0019]FIG. 9 shows the time intervals and amounts of current supplied to the discharge devices of FIGS. 1 through 8.  
         [0020]    [0020]FIG. 10 illustrates a lithography system fabricated in accordance with one aspect of the invention.  
         [0021]    [0021]FIG. 11 illustrates a second lithography system fabricated in accordance with the invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0022]    The present invention will be described as set forth in the preferred embodiments illustrated in FIGS.  1 - 11 . Other embodiments may be utilized and structural and functional changes may be made without departing from the spirit or scope of the present invention. Like items are referred to by like reference numbers.  
         [0023]    [0023]FIG. 1 shows a microcavity device  100  with a microcavity  112  fabricated in accordance with a first embodiment of the invention. The microcavfty  112  has a diameter D with an open end in the anode layer  120  and a closed end or bottom surface  117 . The anode layer  120  is connected to a power supply via electrical connection  152 .  
         [0024]    As shown in FIG. 2, the device  100  has a semiconductor plug  114  and a dielectric layer  118 . The dielectric layer  118  is located between the plug and the anode layer  120 . Thus, the dielectric layer  118  separates or electrically isolates the semiconductor plug  114  from the anode layer  120 . The device  100  can be fabricated by depositing the dielectric layer  118  on the semiconductor plug  114  and depositing the anode layer  120  on top of the dielectric layer  118 .  
         [0025]    The microcavity  112  is formed in the device  100  with the bottom surface  117  formed in the semiconductor plug  114 . The microcavity  112  is preferably cylindrical. The microcavity diameter D is preferably less than 120 microns, and more preferably in a range between 10 and 120 micrometers. Other microcavity shapes are also possible. The illustrated microcavity  112  is etched or drilled through the anode layer  120  and the dielectric layer  118  to a predetermined distance or depth L from the semiconductor layer top surface  119 . The depth L of the microcavity  112  is preferably such that the distance between the microcavity bottom surface  117  and the semiconductor bottom surface  115  is in the range of from about 0.2 to about 0.8 microns. The hole depth L may be in the range of from about 20 to about 100 microns. The transmissivity of the closed end (through the bottom surface  117 ) may be about 50% for light at a wavelength of 13.5 nanometers.  
         [0026]    The microcavity  112  acts as a container for a discharge gas  116 . The gas  116  may include, for example, xenon. The discharge gas  116  is supplied through the open end of the microcavity  112 . In FIG. 2, the device  100  is shown inside a pressure system  30  which supplies the discharge gas  116  to the microcavity  112 . Formation of the microcavity  112  in a single piece integral semiconductor plug  114  allows the microcavity  112  to operate under higher pressure. The discharge gas  116  may be supplied to the microcavity  112  at a pressure that is greater than or equal to 200 torr. The pressure of the gas  116  may be in the range of from about 200 to about 600 torr.  
         [0027]    The semiconductor plug  114  is preferably a highly conductive doped crystalline silicon or polysilicon cathode material with a thickness in the range of 20 to 100 microns. A silicon-based material is preferred because of its resistance to ion sputtering. The dielectric layer  118  is preferably a silicon dioxide or aluminum oxide with a dielectric strength range of 5 to 10 megavolts per centimeter and a thickness range of 4 to 10 microns. The anode layer  120  is preferably a high conductivity metal or a doped polysilicon. The anode layer  120  should have a resistivity of less than 1×10 −7  Ohms-meter and a thickness of 4 to 20 microns. The anode layer  120  may be formed of copper, gold, tungsten, aluminum, silver, doped silicon, nickel chromium, or the like.  
         [0028]    The plug  114  and the anode layer  120  are connected to an electrical source  150  by respective electrical connections  151 ,  152 . The electrical charge supplied by the source  150  consists of a small constant DC current and a short interval larger pulsed current, as described in more detail below. The DC current establishes a virtual anode plasma in the discharge gas  116 . When the discharge gas  116  is subjected to the pulsed current, radiation  170  is emitted from the discharge gas  116  and exits through the microcavity bottom surface  117 . The discharge gas  116  produces high energy electrons when subjected to electrical currents in the amount and size as described below.  
         [0029]    The pulsed current further allows access to higher energy states of the gaseous atoms or molecules therein, such as for example Xe +10  and Xe +11 , such that the wavelength of the radiation  170  is less than or equal to 253 nanometers, and may be in the range of approximately 11 to 14 nanometers. In a preferred embodiment of the invention, the radiation has wavelength peaks at 11.3 and 13.5 nanometers. These wavelengths may be used in extreme ultraviolet lithography systems.  
         [0030]    [0030]FIG. 3 shows a microcavity discharge device  101  with microcavity  122  fabricated in accordance with a second embodiment of the invention. The microcavity  122  has a diameter D and an open end in the anode layer  120  and a closed end or bottom surface  127 . The anode layer  120  is connected to a power source  150  via electrical connection  152 . The lower end of the semiconductor plug  114 ′ is connected to the source  150  by another suitable electrical connection  151 .  
         [0031]    Device  101  differs from device  100  of FIGS. 1 and 2 by the presence of metal layer  126  located on the bottom surface  115  of the semiconductor plug  114 ′. The metal layer  126  may be formed on the bottom surface  115  of the semiconductor plug  114 ′ by a known film growth process. The metal layer  126  has a thickness in the range of 0.2 to 0.8 microns, preferably about 0.6 microns. The metal layer  126  is preferably formed from beryllium. After the metal layer  126  is secured to the semiconductor plug bottom surface  115 , the microcavity  122  is then etched completely through the semiconductor plug  114 ′. Thus, the base of the microcavity  122  is the top surface  127  of the metal layer  126 . The device  101  is otherwise operated as described above in connection with FIG. 2. The beryllium layer  126  filters out (excludes) radiation that is outside the 11 to 12 nanometer range. Consequently, when the radiation  171  is emitted through the metal layer  126  the wavelengths are in the range of from about 11 to about 12 nanometers. The transmissivity of the closed end (through the metal layer  126 ) may be about 50% for light at a wavelength of 11.3 nanometers.  
         [0032]    [0032]FIG. 5 is a top view of a microcavity discharge device  102  with a microcavity  132  fabricated in accordance with a third embodiment of the invention. The device  102  has a substrate  124 . The microcavity  132  has a diameter D with an open end in the anode layer  120  and a closed end or bottom surface  137 . The anode layer  120  is connected to a power source  150  via electrical connection  152 .  
         [0033]    As shown in FIG. 6, device  102  varies from FIGS. 1 and 2 by the presence of a conductive substrate layer  124  located on the bottom surface  135  on the semiconductor plug  114 . The substrate layer  124  can be secured or formed as a thin film by chemical or physical vapor deposition or as a metallic layer secured by epoxy or other techniques. An aperture  125  is formed in the substrate layer  124  and is aligned with the microcavity  132 . The substrate layer  124  is connected to the source  150  by a suitable electrical connection  151 . The substrate layer  124  is preferably a conductive material with a resistivity of less than 1×10 −7  ohms-meter and a thickness of 4 to 20 microns. The substrate layer  124  may include copper, gold, tungsten, aluminum, silver, doped silicon, nickel chromium, or the like.  
         [0034]    The aperture  125  has sloped sides  121 . The sloped sides  121  form a cone having an angle  123  with respect to vertical which is preferably in a range between 10 and 30 degrees. The angle  123  is preferably 10 degrees. The diameter of the truncated top of the cone  121  may be greater than or equal to the diameter D of the microcavity  132 . The radiation  172  is emitted through the aperture  125  and are directed by the sloped sides  121 . The device  102  is otherwise operated like the devices  100 ,  101 , shown in FIGS.  1 - 4 .  
         [0035]    [0035]FIG. 7 is a top view of a microcavity discharge device  103  with a microcavity  142  fabricated in accordance with a fourth embodiment of the invention. The microcavity  142  has a diameter D and an open end in the anode layer  120  and a closed end or bottom surface  147 . The device  103  has a substrate  124 . The anode layer  120  is connected to a power source  150  via electrical connection  152 . The source  150  is also connected to the substrate  124  by a suitable electrical connection  151 .  
         [0036]    Device  103  varies from FIGS. 5 and 6 by the presence of metal layer  126  located on the bottom surface of the semiconductor plug  114 ′. The metal layer  126  has a thickness in the range of about 0.2 to about 0.8 microns, preferably 0.6 microns. The metal layer  126  may include beryllium with a thickness of 0.6 microns. After the metal layer  126  is secured to the bottom of the semiconductor is plug  114 ′, the microcavity  142  is then etched completely through the center of the semiconductor plug  114 ′. The base  147  of the microcavity  142  is the top surface of the metal layer  126 . The device  103  is operated as described above in connection with FIGS. 1 through 6. The metal layer  126  can filter out undesired wavelengths. When the metal layer  126  is beryllium, radiation  173  having a wavelength in the range of about 11 to about 12 nanometers will be emitted through the closed end  147  of the microcavity  142 .  
         [0037]    [0037]FIG. 9 is a plot of the amount of current versus duration that may be applied to the discharge devices  100  through  103  of FIGS. 1 through 8 to produce radiation with the desired wavelengths. The discharge devices operate at lower pulsed currents and are more compact than known devices. The x-axis represents time in microseconds, while the y-axis represents the current in amps supplied by the external power source  150 . The DC current can vary between approximately 1 and 3 milliamps and the pulsed current  62  can vary between approximately 60 and 100 amps at a voltage of approximately 220 volts.  
         [0038]    As shown in FIG. 9, a constant 1 milliamp DC current is supplied to the devices of FIGS. 1 through 8 (line  61 ) and a pulsed current of approximately 60 amps is supplied to the devices  100  through  103  (line  62 ). The pulsed current  62  may be supplied to the devices at a repetition rate of up to approximately 1×10 3  pulses per second to prevent adverse heating. The time between successive pulses should be approximately 1×10 −3  seconds or greater. The present invention should not be limited to the preferred embodiments described in detail herein. The duration of the pulsed current  62  may be about 1 microsecond or 1×10 −6  seconds or less.  
         [0039]    [0039]FIG. 10 illustrates a lithography system  50  constructed with a pressure system  30  containing a discharge device  100 . The pressure system  30  supplies pressurized gas to the microcavity. Radiation  170  exits through the bottom of device  100  and strikes a first optical mirror  56 . The embodiment is shown with only a second optical mirror  59  although additional mirrors could be added. The second mirror  59  reflects the radiation  170  through a mask or reticle  58  to the wafer  54 . The wafer  54  is shown on a wafer station  53 . The mirrors  56 ,  59  have reflective surfaces formed from molybdenum silicon (MoSi) or molybdenum beryllium (MoBe) compounds. Such compounds have a peak normal incidence reflectiveness of approximately 70% within a reflectivity bandwidth of approximately 1 nanometer. These compounds have high reflectivity in the 11 to 14 nanometer spectral wavelength region. In particular, multi-layer MoSi reflecting surfaces have their highest reflectivity in the 13 to 14 wavelength region and MoBe reflecting surfaces have their highest reflectivity in the 11 to 12 wavelength region  
         [0040]    [0040]FIG. 11 illustrates a second lithography system  350 . The system  350  differs from FIG. 10 in that the wafer  54  is located outside the lithography system  350 . The wafer  54  is conveyed on a wafer transport  360 . The radiation  170  is transmitted across the system perimeter  355 . The system  350  may be suitably arranged to prevent atmosphere contamination and refraction of the radiation  170 .  
         [0041]    Having thus described in detail the preferred embodiments of the invention, it is to be understood that the invention defined by the appended claims is not to be limited by particular details set forth in the above description as many apparent variations thereof are possible without departing from the spirit or scope of the invention. Accordingly, the above description and accompanying drawings are only illustrative of preferred embodiments which can achieve the features and advantages of the present invention. It is not intended that the invention be limited to the embodiments shown and described in detail herein. The invention is only limited by the scope of the following claims.