Abstract:
A method of generating light comprising the step of applying an electric field to an excimer-forming gas such as a gas mixture containing noble gases and hydrogen or halogen, and providing free electrons in the gas. The electric field is configured to accelerate electrons to at least the energy required to form excimers, but in at least one region of the electric field, the field does not substantially ionize the gas, so that the field does not induce arcing through the gas. For example, electrons can be injected from one or more field emission electrodes ( 18 ) such as one or more a metal needle tip conductors, whereas the electric field can be a field between the field emission electrodes and a counterelectrode ( 13 ).

Description:
The present application claims the benefit of U.S. Provisional Patent Applications No. 60/147,912, filed Aug. 9, 1999, and No. 60/157,813, filed Oct. 4, 1999, the disclosures of which are hereby incorporated by reference herein. The disclosure of U.S. patent application Ser. No. 08/873,957 filed Jun. 12, 1997, now U.S. Pat. No. 6,052,401, is also incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to methods and apparatus for generating light such as ultraviolet light from excimer-forming gases. 
     BACKGROUND OF THE INVENTION 
     There has been a need for improved light sources capable of generating ultraviolet light in the spectral region of between about 50 and 200 nanometers wavelength, commonly referred to as the “vacuum ultraviolet” or “VUV” region. VUV photons have energies on the order of 10 electron volts (10 eV) and are capable of breaking chemical bonds of many compounds. Thus VUV light can be used to accelerate chemical reactions as in chemical vapor deposition, curing of photosensitive material, production of ozone, and cracking gaseous waste products. Moreover, the minimum feature size that can be imaged with light is directly proportional with the wavelength of the light. VUV light has the shortest wavelength of any light that can be focused and reflected with conventional optical elements. Therefore, photographic processes employing VUV lights can image smaller feature sizes than those imaged with other light wavelengths. This is of particular importance in photographic processes used to fabricate semiconductors. In addition, such microimaging of features requires high brightness of light sources with such short wavelengths. 
     Additional needs exist for broadband VUv light sources, i.e., light sources which emit the VUV light over a continuum of wavelengths within the VUV range. A broadband source can be used for absorption spectroscopy in the VUV range. Because gases such as hydrogen and oxygen have resonance lines in this range, VUV absorption spectroscopy can be used for sensitive analytical measurements. A light source for use in spectroscopy desirably can operate continuously, with stable emission characteristics over time. A stable, continuously operable broadband VUV source is also needed for use as a calibration standard, for measuring the sensitivity of VUV light detection systems in laboratory apparatus. 
     Deuterium arc lamps have been used as VUV light sources. However, such lamps emit a relatively weak continuum or broadband radiation in the VUV range together with intense line radiation at particular wavelengths. This spectral characteristic requires that the detector system used to measure the light have a very high dynamic range, i.e., the capability of measuring a weak light at some wavelengths and very intense light at others. 
     Some consideration has been given to the use of excimer radiation as a source of UV light. Excimers are temporary chemical compounds composed of atoms that normally do not combine with one another. One or more of the atoms constituting an excimer is an excited state, i.e., a state in which the [electrons have] atom has been momentarily promoted to a higher energy state as, for example, by promoting one or more electrons to higher-energy orbitals. The excimer molecule as a whole is also in an excited state, and will ultimately decay to yield the constituent atoms. For example, elements commonly referred to as inert gases, helium, neon, argon, krypton and xenon, which normally exist only as isolated atoms can form excimer molecules when in the excited state. Diatomic inert gas excimers such as Ar 2 *, Kr 2*,  and Xe 2*  emit relatively broadband continuum radiation in the VUV range. However, to form these excimers in appreciable quantities, it is necessary to provide excitation energies on the order of 10 to 40 electron volts per atom. Moreover, this excitation energy must be provided while the gas is maintained at relatively low temperatures, typically below 200° C. and most typically about room temperature. The gas also should be maintained under appreciable absolute pressure, desirably at least about 100 millibars (mbar) and most preferably about 0.5 bar or more, i.e., most preferably at about atmospheric pressure or more. Such substantial gas pressure is needed to provide a dense gas, which facilitates the excimer forming reactions. A simple direct current electrical arc discharge is ineffective to form excimers under these conditions. Other specialized arc discharge arrangements such as surface barrier discharges or arcs applied in short pulses can produce excimer light, but these devices operate only intermittently and do not provide stable, continuous emission. 
     Co-pending commonly assigned application Ser. No. 08/873,957, now U.S. Pat. No. 6,052,401, addresses the use of applying electron beams to gases to produce excimers to produce such broadband VUV light. However, all such electron beam approaches require the separate creation of an electron beam. It would be desirable to avoid the space and expense of producing an electron beam when creating the VUV light. Furthermore, the window for electron beam entry into the emission chamber still places limitations on the light source. Therefore, there is a need for broadband VUV light sources and monochromatic VUV light sources which can be produced at less expense and take up less space. 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention provides methods of forming excimers in a gas. A method according to this aspect of the invention desirably includes the steps of providing free electrons in the gas disposed within a volume and imposing an electric field within such volume so as to accelerate the free electrons within the gas. The electric field is configured so that within a first region of said volume, said free electrons have mean energy equal to or greater than the excitation energy required for excimer formation. However, the field is configured so that within at least one region of the volume, the free electrons have mean energy less than the ionization energy of the gas. Stated another way, the field is configured so that any path through the electric field from negative potential to positive potential passes through at least one region of the volume in which the field is less than the field required to substantially ionize the gas. Thus, the free electrons excite the gas and form excimers without causing breakdown and arcing within the volume. 
     For example, the step of providing an electric field may include providing a point electrode within the volume and providing a counterelectrode remote from the point electrode, and imposing the electric field between the point electrode and the counterelectrode. As further discussed below, the field is very high in the immediate vicinity of the point electrode, but declines rapidly with distance from the point electrode. In this arrangement, free electrons may be provided in the gas by emission from the point electrode. The field immediately adjacent to the point electrode typically is so large that the mean energy of the electrons is far above the ionization potential of the gas, and the gas is substantially ionized and heated. The high temperature prevailing in this innermost region inhibits excimer formation in this region. In another region of the gas, immediately surrounding the innermost region, mean energy of the electrons is lower, and the temperature of the gas is lower. In this region, the field is such that the mean energy ranges from slightly above the ionization energy of the gas to below the ionization energy but above the excitation energy required for formation of the excimers. Substantial excimer formation occurs in this region. In yet another region, extending from the outside of the excimer-forming region to the counterelectrode, the field is below the excitation energy of the excimers and hence far below the ionization energy of the gas. 
     In another embodiment, an additional electrode may be provided. For example, the additional electrode and the counterelectrode may be provided as a pair of parallel plate electrodes. A substantially uniform field is maintained between the additional electrodes and the counterelectrode, with the counterelectrode being at a positive potential with respect to the additional electrode. This field has a substantially constant magnitude such that within this field, free electrons have mean energy equal to or greater than the excitation energy required to form the excimer but less than the ionization energy of the gas. Here again, a negative potential is applied to the point electrode and there is a high field in the immediate vicinity of the point electrode. There may be substantial ionization of the gas in the immediate vicinity of the point electrode. The point electrode and the surrounding ionized gas serve as a source of free electrons. Here, however, substantial excimer formation occurs within a region which extends from the outside of the innermost, hot region immediately surrounding the point electrode all the way to the counterelectrode. 
     The excimers decay and emit light, typically ultraviolet light, specifically in the vacuum ultraviolet region of the spectrum. Thus, a further aspect of the present invention provides methods of generating light such as VUV light. Typically, the gas is contained within a chamber, and the light is directed outside of the chamber through a window transparent to the light generated by the excimers. 
     Because the gas remains substantially non-ionized in at least some regions within the volume, there is no arcing or uncontrolled dielectric breakdown of the gas. The electrical discharge or electron flow from the point electrode has properties resembling those of a resistor. That is, the electron current increases with the applied voltage between the point electrode and the counterelectrode. The discharge is stable and can operate at steady state to provide a continuous source of excimers and continuous illumination. Moreover, multiple point electrodes can be operated in parallel to increase the amount of excimer formation and to increase the amount of light emission. 
     Preferred methods according to this aspect of the invention provide a new VUV light source which can operate at high gas pressures within the chamber or gas volume, because no window is needed for electron beam excitation. As high gas pressures contribute to excimer formation and discourage the formation of plasma conduits which lead to arc discharge, this allows a substantially brighter and smaller source of VUV light that has been previously available. 
     In further variants, multiple point electrodes connected in parallel can be replaced by an elongated electrode such as a wire or blade. The term “field emission electrode” is used herein to denote an electrode which can emit electrons into the gas. Typically, such an electrode has a relatively small radius in one dimension (such as a blade or wire) or in two dimensions (such as the point of a needle) and provides a high local electric field in the vicinity of the electrode. Here again, a high field magnitude is provided in the vicinity of the elongated electrode. In still other embodiments, the source of free electrons does not include an electrode which provides a locally high field magnitude. For example, electron sources such as surface plasmas in a localized region of the gas, referred to as a “electron source region”, can be employed. Such a plasma can be created by laser excitation. Electron beams can be employed, although the use of electron beams typically requires a beam window and hence limits the gas pressures which can be employed. 
     Another aspect of the present invention provides an apparatus for generating VUV light. Apparatus in accordance with this embodiment of the invention desirably includes, a chamber for holding a gas, means for providing free electrons in said gas; and means for imposing an electric field within a region of said gas so as to accelerate said free electrons, said electric field being configured as discussed above in connection with the methods. 
     In one exemplary apparatus, application of voltages of the order of magnitude of 10 kV to a metal needle tip electrode with a radius of curvature of approximately 20 μm in an excimer-forming gas with a pressure on the order of 1-5 bars will produce VUV light. Although the present invention is not limited by any theory of operation, it is believed that the high negative voltage (on the order of 5 kV to 20 kV) at the needle tip of creates a high electric field around the metal needle tip electrode. This high intensity electric field is believed to generate free electrons through field emission from the metal needle tip electrode. The electric field accelerates these free electrons away from the metal needle tip electrode to an energy needed to cause excimer formation in a gas. These accelerated free electrons will collide with gas atoms or molecules and cause the excitation necessary to generate the VUV emitting excimers in the gas. This embodiment thus uses the phenomenon of point corona discharge (also known as St. Elmo&#39;s Fire). It is important in harnessing the phenomena that no arc discharges occur. 
     These and other objects, features and advantages of the present invention will be more readily apparent from a detailed description of the preferred embodiments set forth below taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic sectional view depicting apparatus according to one embodiment of the invention. 
     FIG. 2 is a diagrammatic sectional view depicting the point source and surrounding area the embodiment of FIG.  1 . 
     FIG. 3 is a graph showing the spectra of the light emitted from a specific example of the present invention. 
     FIG. 4 is a graph showing the spectra of light emitted from a specific example of the present invention. 
     FIG. 5 is a circuit diagram of the effective circuit of a particular embodiment of the present invention. 
     FIG. 6 is a graph showing the relationship between current and gas pressure for a specific example of the present invention. 
     FIG. 7 is a graph showing the relationship between current and gas pressure for a specific example of the present invention. 
     FIG. 8 is a graph showing the relationship between current and gas pressure for a specific example of the present invention. 
     FIG. 9 is a graph showing the relationship between current and applied voltage for a specific example of the present invention. 
     FIG. 10 is a graph showing the relationship between current and applied voltage for a specific example of the present invention. 
     FIG. 11 is a diagram of another embodiment of the present invention, this embodiment having three electrodes. 
     FIG. 12 is a diagram of another embodiment of the present invention, this embodiment having two electrodes and one virtual electrode. 
     FIG. 13 is a diagram of another embodiment of the present invention, this embodiment having an electron gun. 
     FIG. 14 is a diagram of another embodiment of the present invention, this embodiment having a laser-generated plasma. 
     FIG. 15 is a diagram of another embodiment of the present invention, this embodiment having a blade electrode. 
     FIG. 16 is a diagram of another embodiment of the present invention, this embodiment having an AC power source. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     One embodiment of the present invention is an apparatus as depicted in FIG.  1 . An apparatus includes an emission chamber  10  having insulating walls  12  and conducting walls  13 . The insulating walls  12  may be composed of glass or Plexiglas. The conducting walls  13  may be composed of metals, as, for example, copper. The emission chamber  10  also ha s an optical window  14 . The optical window  14  may be composed of MgF 2 . The window is transparent to the VUV radiation which will be emitted by the gas. A field emission electrode in the form of a metal needle tip  16  is provided which pierces the insulating walls  12 , so that the tip  18  is disposed within the internal volume of the emission chamber. The metal needle tip electrode  16  has a sleeve  50  where it pierces the insulating wall  12 . This sleeve  50  allows the emission chamber  10  to remain airtight. The tip  18  of the metal tip needle electrode  16  has a radius of curvature r tip , and is disposed within the emission chamber  10  a distance from the insulating walls  12  and from conductive walls  13 . The metal needle tip electrode  16  is attached to a resistor  20  outside the emission chamber  10 . The resistor  20  is connected to a voltage source  22 , again outside the emission chamber  10 . A circuit-completing ground  24  is attached to conductive walls  13 . Thus conductive walls  13  act as a counter electrode. 
     A conducting layer  42  such as a copper foil is disposed outside of the chamber along one insulating wall  12  and is electrically connected to the conducting walls  13 , so that the conducting layer serves as part of the counterelectrode. The counterelectrode, including conducting layer  42  and conductive walls  13 , is electrically connected to ground  48 . For monitoring purposes, the connection between the counterelectrode and ground passes through an ammeter  43 , including a resistor  44  and a voltmeter  46  in parallel with each other. 
     The voltage source  22  provides a voltage from 0 to −20 kV. The resistor  20  provides 100 MΩ of resistance. The chamber size is on the order of one inch (2.5 cm) to a side. The distance d is ˜1 mm. The radius r tip  is ˜20 μm. 
     Excimer-forming gas  26  is provided disposed within the emission chamber  10 . Emission chamber  10  will of course be as air tight as necessary to contain the excimer-forming gas  26  at high pressure. In general, the gas  26  within the emission chamber  10  can be composed of one or more gases selected from the group of helium, neon, argon, krypton, and xenon. In addition, the gas  26  can also be composed of one ore more gases from the aforementioned group and a second gas different from the first gas. Such a second gas is preferably a halogen or halogen compound. Such second gas is more preferably fluorine or fluorine compound, as noble-gas and fluorine mixtures. For example, mixtures of two or more noble gases and a halogen can be used to form noble gas-halogen excimers. Thus a mixture of argon and helium with fluorine can be excited to from ArF* excimers. Also, neon and hydrogen mixtures can be excited to form NeH* excimers. The gas  26  within the emission chamber  10  is generally at about 2-6 bars of pressure. 
     FIG. 3 shows the spectra of the light emitted from the apparatus of FIGS. 1 and 2 with a −10 kV voltage applied to field emission electrode  16 , using a gas  26  within the emission chamber  10  containing 100 mbar partial pressure Ar, 5 mbar partial pressure F 2 , and 4 bar partial pressure He. Under these conditions a 13 μA current is detected at ammeter  43 . The spectrum clearly shows a peak at 193 nm, consistent with the spectrum line expected from the emission from a gas with ArF* excimers. 
     Similarly, FIG. 4 shows the spectra of light emitted from the same apparatus using a gas containing, 3.6 bar Ar, and 3 bar He. The voltage 13 kV was applied with a current 30 μA recorded. The spectrum clearly shows a peak at 128 nm, which is the expected peak from an emission from Ar 2 * excimers. 
     Spectra showing peaks at 248 nm (Kr 2 *), 172 nm (Xe 2 *) and 248 nm (KrF*) have been observed. Essentially any excimer VUV radiation can be generated by the present invention by using the appropriate gas mixture. 
     In operation, the voltage can be applied continuously, and a continuous emission of light is seen. Over a larger time scale, the current may begin to drop gradually. Although the present invention is not limited by any theory of operation, it is believed that this gradual decline is related is related to impurities in the gas. This decline can be suppressed by using highly purified gases and by continually replacing the gases with a relatively slow flow of fresh gases through the chamber. Where the gas mixture consists essentially of noble gases, this decline can be suppressed by incorporating a “getter” such as a heated tungsten filament which reacts with impurities within the chamber. With such measures, the emission remains substantially constant for many hours. 
     While the present invention is not limited by any theory of operation, the demonstrated results below may be interpreted in the following way. The high negative voltage applied to the tip  18  of the metal needle tip electrode  16  creates a high intensity electric field around the tip. This electric field is enhanced by the geometry of the tip  18 . In essence, tip  18  will act as a point source for a field. The tip  18  will display the phenomena of field emission, and emit free electrons. The electric field will then accelerate the free electrons. At certain electric field strength, the free electrons will be accelerated enough to have enough energy to cause excimer formation. This is the “ignition”, when the apparatus will begin to emit VUV light from the decay of excimers. As excimers begin to emit VUV light, they will decay to their constituent parts, other excimers will form until a steady state of emission and creation is achieved. 
     However, it is believed that in this point source geometry that the electric field strength closest to tip  18  will be strong enough to cause ionization, resulting in a double sphere structure. FIG. 2 shows this structure. The arrows in FIG. 2 represent the magnitude of the electric field at various points. The field strength is highest immediately adjacent the tip, and decreases with the square of the distance r from the tip. An inner sphere  60  has a range from the tip to a given radius r inner . Within this region, free electrons are accelerated to a mean energy substantially higher than the ionization energy of the gas. Stated another way, the work done by the field on an electron moving along a path length equal to the mean free path of an electron between collisions with gas molecules is substantially greater than the ionization energy of the gas. Within this region, the gas is at a high temperature and excimer formation is limited. In the outer sphere  62 , between r inner  and r outer , the free electrons are accelerated to a mean energy slightly above the ionization energy of the gas, and above the excimer formation energy of the gas. In the region outside the outer sphere  66 , the electric field strength is insufficient to give the free electrons the mean energy to cause excimer formation. Therefore, excimer formation will occur principally within outer spherical shell  62 , and this shell will show emission of VUV light. The region  64  will be the coolest, and show little excimer formation and ionization. If r outer  should be increased to the point where is touches the counterelectrode, arc discharge would occur due to break down of the gas  26 . It should be appreciated that the inner sphere  60  and outer shell  62  are not sharply bounded; at r inner  and r outer ; these regions merge gradually into one another. Also, there is some ionization immediately outside of r outer . Therefore, to reliably prevent arcing, r outer  should be considerably smaller than the distance to the counterelectrode. 
     The limit of the outer shell  62  of excimer formation (the so called the so-called “lamp zone”) may be modeled by the equation:          r   outer     =           U   ·   e       E   i       ·     r   tip     ·     λ   F                                
     where r outer  is the radius from the point source at which the mean free energy of electrons is insufficient to cause excimer formation, U is the voltage at the tip, e is the elementary charge, r tip  is the radius of the tip, E i  is the ionization energy of the gas atoms or molecules, and λ F  is the mean free path of the electron. It is believed that some excimer formation occurs outside of r outer  as modeled by this equation. That is, within a thin shell just outside of r outer , the mean energy of the electrons is below the ionization energy E i  but still above the excitation energy E e  required to form excimers. 
     As corona discharge (ignition) is achieved, a weak current will be established through the conducting wall  13 . This allows the completion of a circuit, and the maintenance of the excitation of the gas over a relatively continuous period. The functional circuit created is shown in FIG.  5 . This circuit shows that the applied voltage, U a , does not equal the voltage applied at the tip, U tip . The circuit shown in FIG. 6 allows us to calculate this U tip . In one example in the even more preferred embodiment, application, of −12 kV at the voltage source  22  generates 2 V as detected on voltmeter  46 . This indicates 20 μA of current. Therefore, the resistance of the entire circuit, R cir , equals 600 MΩ. As the resistor 20 is 100 MΩ, we determine that the resistance of the emission chamber is ˜500 MΩ. Therefore, the voltage at the tip  18 , U tip , equals ˜10 kV. Also, we see that the Utip will be some constant fraction of the U a  for a given experimental set up. Thus the relationship will be linear. 
     As the applied voltage is increased, the size of the lamp zone and the brilliance of the resultant VUV light source will increase. However, a maximum applied voltage exists, the breakdown voltage. At this voltage, the gas under goes dielectric breakdown, the emission ceases and the current ceases. The composition of gas  26 , the pressure of gas  26 , and the distance d determine the break down voltage. By way of example, in the more detailed embodiment, Xe gas at 5 bars of pressure demonstrates a breakdown voltage at about 20 kV. 
     FIGS. 6-8 demonstrate the dependence between the detected current (in μA) and the gas pressure (in bars) in the apparatus of FIG. 1 for a given gas and applied voltage. FIG. 6 shows the relationship for Kr at 7 kV. FIG. 7 shows the relationship for Ar at 4 kV. FIG. 8 shows the relationship for Xe at 6 kV. In each case, the curves reveal a decrease in detected current as gas pressure is increased. 
     FIG.  9  and FIG. 10 demonstrate the relationship between the detected current and the applied voltage for a given gas and gas pressure. FIG. 9 shows the relationship for Ar at 3.6 bar pressure. FIG. 10 shows Xe at 5 bar pressure. In each case, an increase of applied voltage yields an increase in detected current. 
     The above relationships between current, voltage and pressure can be summarized in the following approximate scaling law:        I   ∝     (       r   outer   3     -     r   tip   3       )     ∝       (     U   -     U   ign       )       3   2       ∝     p     -     3   2                                
     where I is the current, r outer  is the distance from the point source, r tip  is the radius of curvature of the tip  18 , U is the voltage at the tip, U ign  is the voltage at the tip at which corona point discharge starts, and p is the pressure. 
     In another embodiment is shown in FIG.  11 . In this embodiment both a metal needle tip electrode or field emission electrode  100  and a planar additional electrode  102  are held at a negative potential. The planar additional electrode  102  is held a constant distance from the planar counterelectrode  104 . The metal needle tip electrode  100  and the planar additional electrode  102  are provided with a negative potential by voltage source  106 . The circuit is completed by a connection to ground  108 . An optically transparent window  110 , such as a window of MgF 2 , is provided to allow VUV light to escape. Planar counter electrode  104  may be a wire mesh as shown in order to allow UV light through. Only some walls  112  of the chamber  113  are illustrated in FIG. 11; in practice, the walls extend entirely around the chamber, and enclose the gas  114  in an airtight manner. In operation, the metal needle tip electrode  100  emits free electrons through field emission. The planar additional electrode  102  may be wire mesh as shown, which allows the free electrons emitted to pass through. The additional electrode  102  and counter electrode  104  create an approximately constant electric field between them. At the correct applied voltage, the electric field produced will be less than the ionization energy of the gas inside the chamber but more than the energy necessary for excimer formation. This avoids arc discharges while allowing excimer formation. In a specific example, 2.5 bar Xe is provided. The planar additional electrode  102  and the planar counterelectrode  104  are kept 5 mm apart. A potential of −9 kV is applied to the additional electrode  102  and to field emission electrode  100 , and 17 μA is read as the resulting current. 172 nm excimer radiation is detected in this example, in accord with radiation expected from Xe 2 *. 
     Thus, in this embodiment, the region  121  immediately surrounding the tip of the field emission electrode serves principally as a source of free electrons. Excimer formation occurs principally within region  120 , remote from the tip, under the influence of the field applied between additional electrode  102  and counterelectrode  104 . 
     In yet another embodiment a plurality of field emission electrodes, such as a plurality of metal needle tip point electrodes, are run simultaneously in parallel to increase total output intensity. The plurality of field electrodes may be arrayed in a line or in a planar grid. FIG. 12 shows this embodiment. An emission chamber  130  has walls  132  which contain the gas  136 . Only some walls  132  of the chamber are illustrated in FIG. 11; in practice, the walls extend entirely around the chamber, and enclose the gas  136  in an airtight manner. A transparent window  138  allows VUV radiation to pass though. A plurality of metal needle tip point electrodes  140  are arrayed in a planar grid. They are attached to a voltage source  142 . The planar counterelectrode  144  is attached to a ground  146  to complete the circuit. Again, the planar counterelectrode  144  may be wire mesh to allow through the emitted VUV light. 
     The ends of the plurality of metal needle tip electrodes  140  define a surface  147 . A small distance from this surface a substantially equipotential plane  148  will exist. The equipotential plane  148  acts as a virtual planar electrode, giving results similar to the additional electrode  102  in the embodiment of FIG.  11 . Thus, the region immediately surrounding each field emission electrode serves as a source of free electrons, whereas the array of field emission electrodes and counterelectrode  144  apply a field so that in a region  149 , the field produces excimer formation without substantial ionization. 
     An arrangement of electrode and counterelectrode can be used with other sources of free electrons as, for example, with an electron gun for injecting free electrons into the space between the electrodes. FIG. 13 shows an electron gun  150  passing a bream of electrons  152  through an electron window  154  of an emission chamber  156 . The electrode  158  is connected to a voltage source  162 . The counter electrode  160  is connected to a ground  164  to complete the circuit. The emission chamber  156  contains gas  166  in an airtight manner, and has a VUV transparent window  168 . In operation, the voltage source  162  applies a voltage necessary to create an approximately constant electric field between electrode  158  and counterelectrode  160 . The electron beam  152  provides free electrons. The free electrons are excited by the constant electric field sufficiently to form excimers, and thus generate VUV light. 
     In another alternative embodiment as shown in FIG. 14, a laser  180  is used to create a laser beam  182 . The emission chamber  188  contains a target  184  and gas  190  in an airtight manner. It has a laser window  183  and a VUV transparent window  192 . The electrode  194  is connected to a voltage source  196 , while the counterelectrode  198  is connected to a ground  200 , completing the circuit. In operation, a voltage is applied by the voltage source  196  to create an approximately constant electric field between electrode  194  and counterelectrode  198 . The laser beam  182  passes though the laser window  183  and impinges the target  184 , and generates plasma  186 . The plasma  186  is a source of free electrons. The free electrons are excited by the constant electric field sufficiently to form excimers, and thus generate VUV light. The present invention can by implemented with essentially any source of free electrons which can be employed. 
     In yet another embodiment, the multiple tips described above can be arrayed in a line. In yet another alternative embodiment, blade or wire is used rather than a needle tip, allowing the production of a cylindrical light source. FIG. 15 shows such a blade electrode  210  in a chamber  212 . The chamber holds a gas  213  in an airtight manner. The blade electrode  210  is attached to a voltage source  214 . A counterelectrode  216  is electrically connected to a ground  218  to complete the circuit. An optically transparent window  220  allows the transmission of VUV light. In operation, the blade electrode  210  will generate a local, high-magnitude electric field in the vicinity of the blade edge. In other respects, this embodiment is similar to the embodiment of FIG.  1 . 
     In the arrangements discussed above, constant DC potentials are applied. Pulsating or varying DC potentials can be employed to provide a pulsating or modulated light intensity. Also, alternating potentials can be applied. Where an alternating potential is applied in an embodiment which uses field emission from an electrode, both electrodes should be capable of acting as field emission electrodes. For example, as shown in FIG. 16, an alternating potential can be applied between a first electrode  301  and a second electrode  302 . When the first electrode  301  is negative with respect to the second electrode  302 , the first electrode serves as the field emission electrode and the second electrode acts as the counterelectrode. When the potential is reversed, electrode  302  acts as the field emission electrode and electrode  301  acts as the counterelectrode. The AC potential may be at a high radio frequency. Use of a radio frequency potential minimizes sputtering and minimizes damage to the electrodes. 
     In the arrangements discussed above, a window is used to allow the VUV light to escape from the chamber, so that the light can interact with objects outside of the chamber. In other embodiments, the VUV light can be utilized within the chamber as, for example, to treat an object disposed within the chamber. For example, Ne 2 * can be used to emit 80 nm radiation, which can not be transmitted currently any material, for such an application. In still other embodiments, the excimers created as discussed above can be employed without using light emitted by decay of the excimers. 
     Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.