Abstract:
A source of photons includes a discharge chamber, first and second groups of ion beam sources in the discharge chamber and a neutralizing mechanism. Each of the ion beam sources electrostatically accelerates a beam of ions of a working gas toward a plasma discharge region. The first group of ion beam sources acts as a cathode and the second group of ion beam sources acts as an anode for delivering a heating current to the plasma discharge region. The neutralizing mechanism at least partially neutralizes the ion beams before they enter the plasma discharge region. The neutralized beams and the heating current form a hot plasma that radiates photons. The photons may be in the soft x-ray or extreme ultraviolet wavelength range and, in one embodiment, have wavelengths in a range of about 10-15 nanometers.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to plasma sources and, more particularly, to sources of soft X-ray or extreme ultraviolet photons, wherein high power production of photons is achieved by electrostatic acceleration of ions toward a plasma discharge region, neutralization of the ions to avoid space charge repulsion as the discharge region is approached and the application of a heating current through the central plasma in order to raise its temperature and density.  
         BACKGROUND OF THE INVENTION  
         [0002]    Soft X-ray and extreme ultraviolet photons can be generated in a hot plasma. The wavelength of the photons is determined by the mixture of ionization states present, with generally shorter wavelength photons being produced by the radiation of higher ionization states within the plasma. An example relevant to lithography is the xenon plasma that contains states Xe 10+ , Xe 11+  and Xe 12+  and radiates strongly in the 10-15 nanometer (nm) band of the spectrum. Within this band, the 13.5 nanometer wavelength is considered to be the optimum for lithography because it can be reflected with up to 70% efficiency by molybdenum-silicon multilayer mirrors in a combination that re-images the pattern of a semiconductor circuit from a mask onto a silicon wafer.  
           [0003]    Several approaches to the generation of these energetic photons have been researched in recent years. The plasma has been heated by laser pulses in the so-called laser-produced-plasma (LPP) method. Also, the plasma has been heated directly by the passage of a pulsed electric current in a variety of discharge-produced plasma (DPP) photon sources. These include the capillary discharge, the dense plasma focus and the Z-pinch. It is believed that a viable 13.5 nm source for commercial, high throughput lithography will be required to emit approximately 100 watts of photon power into 2 steradians in a 2% fractional band at 13.5 μm, from a roughly spherical source of diameter less than 1.5 millimeters. In xenon, which is the most efficient 13.5 nm radiator (of room temperature gaseous elements), the 2% fractional band is produced at an electrical efficiency of approximately 0.5% into 2π steradians in DPP sources and up to 1% into 2π steradians in LPP sources relative to laser power absorbed. For the lithography source, a plasma power of 30-60 kilowatts (kW) is therefore required. Other requirements are for precise plasma positioning, to provide uniform illumination, and a repetition frequency greater than 6 kHz.  
           [0004]    In the prior art, the plasma has been positioned, in the case of a laser produced plasma, by the intersection of a stabilized beam of liquid xenon with a focused laser beam. The size and positional stability of the resulting plasma are compatible with the application, but with laser efficiencies of only 4% for the pulsed lasers of interest, an electrical input power of 750 kW to 1.5 megawatts is likely to be needed in order to generate 100 watts of 13.5 nm photons, making the economics of the LPP source very unfavorable.  
           [0005]    By supplying electrical energy directly to the plasma, the DPP source can, in principle, have a power input not much greater than the 30-60 kW plasma power. However, in prior art discharges, the plasma has, with the exception of the dense plasma focus, been too large in at least one dimension, and the dense plasma focus itself depends on a closely positioned electrode, only a few millimeters distant from the plasma, to create a small, positionally stable plasma focus. There are limits to the plasma power that can be generated in such close proximity to a solid electrode, presenting a difficult scaling challenge for the dense plasma focus source.  
           [0006]    Pending application Ser. No. 09/815,633 filed Mar. 23, 2001 discloses a new photon source, referred to herein as the astron source, wherein energy and material are fed into a plasma at a central location via numerous energetic neutral beams. In this source, a relatively large separation has been achieved between the plasma and the nearest solid surface. The astron source also has a distributed electrode which exhibits low current density and anticipated longer life. Although this approach has enabled the generation of a hot plasma that emits extreme ultraviolet photons and is capable in principle of being scaled to 30-60 kW plasma power, it depends on a high acceleration efficiency for the neutral beam particles. To date, only 20% efficiency has been measured, and improvements in acceleration efficiency are required in order to give this photon source a good electrical efficiency.  
           [0007]    Accordingly, there is a need for improved methods and apparatus for generating soft X-ray or extreme ultraviolet photons.  
         SUMMARY OF THE INVENTION  
         [0008]    According to a first aspect of the invention, a source of photons comprises a discharge chamber, first and second groups of ion beam sources in the discharge chamber, and a neutralizing mechanism. Each of the ion beam sources electrostatically accelerates a beam of ions of a working gas toward a plasma discharge region. The first group of ion beam sources acts as a cathode, and the second group of ion beam sources acts as an anode for delivering a heating current to the plasma discharge region. The neutralizing mechanism at least partially neutralizes the ion beams before they enter the plasma discharge region. The neutralized beams and the heating current form a hot plasma in the discharge region that radiates photons.  
           [0009]    The first and second groups of ion beam sources may comprise concentric electrode shells having sets of apertures aligned along axes which pass through the plasma discharge region. An inner electrode shell is divided into two half shells. The inner electrode half shells receive a voltage pulse that causes the heating current to be delivered to the plasma discharge region through channels formed by the ion beams. Electrical energy is delivered efficiently to the central plasma, causing heating and radiation from the ionic species of interest.  
           [0010]    In one embodiment, the source includes a transformer having a secondary with one terminal coupled to the first group of ion beam sources and a second terminal coupled to the second group of ion beam sources. The primary of the transformer is coupled to a pulsed voltage source.  
           [0011]    According to another aspect of the invention, a system is provided for generating photons. The system comprises a housing defining a discharge chamber, first and second groups of ion beam sources in the discharge chamber, a first voltage source for applying an accelerating voltage to the first and second groups of ion beam sources, a second voltage source for supplying a heating current through a plasma discharge region between the first and second groups of ion beam sources, a gas source for supplying a working gas to the discharge chamber, a neutralizing mechanism, and a vacuum system for controlling the pressure of the working gas in the discharge chamber. The ion beam sources each electrostatically accelerate a beam of ions of a working gas toward the plasma discharge region. The first group of ion beam sources acts as a cathode and the second group of ion beam sources acts as an anode for supplying the heating current through the plasma discharge region. The neutralizing mechanism at least partially neutralizes the ion beams before they enter the plasma discharge region, wherein the neutralized beams enter the plasma discharge region and form a hot plasma that radiates photons. The heating current heats and compresses the plasma, raising its temperature and density.  
           [0012]    According to a further aspect of the invention, a method is provided for generating photons. The method comprises the steps of electrostatically accelerating a plurality of beams of ions of a working gas toward a plasma discharge region, at least partially neutralizing the ions beams before they enter the plasma discharge region and supplying a heating current through the plasma discharge region, wherein the neutralized beams and the heating current form a hot plasma that radiates photons.  
           [0013]    According to a further aspect of the invention, a source of photons comprises a discharge chamber, a plurality of ion beam sources in the discharge chamber, a neutralizing mechanism and an external electrode. The ion beam sources each electrostatically accelerate a beam of ions of a working gas toward a plasma discharge region. The neutralizing mechanism at least partially neutralizes the ion beams before they enter the plasma discharge region. The external electrode is spaced from the plasma discharge region and delivers a heating current to the plasma discharge region, wherein the neutralized beams and the heating current form a hot plasma that radiates photons. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    For a better understanding of the present invention, reference is made to the accompanying drawings, which are incorporated herein by reference and in which:  
         [0015]    [0015]FIG. 1A is a cross-sectional side view of an embodiment of an extreme ultraviolet source based on the acceleration of multiple ion beams to a central plasma discharge region;  
         [0016]    [0016]FIG. 1B is a cross-sectional top view of the extreme ultraviolet source shown in FIG. 1A;  
         [0017]    [0017]FIG. 2A is a cross-sectional side view of a first embodiment of a photon source in accordance with the invention;  
         [0018]    [0018]FIG. 2B is a cross-sectional top view of the photon source shown in FIG. 2A;  
         [0019]    [0019]FIG. 3 is a cross-sectional side view of a second embodiment of a photon source in accordance with the invention;  
         [0020]    [0020]FIG. 4 is a cross-sectional top view of the photon source shown in FIG. 3;  
         [0021]    [0021]FIG. 5 is a schematic representation of an embodiment of a system for generating photons in accordance with the invention;  
         [0022]    [0022]FIG. 6A is a cross-sectional side view of a third embodiment of a photon source in accordance with the invention; and  
         [0023]    [0023]FIG. 6B is a cross-sectional top view of the photon source shown in FIG. 6A. 
     
    
     DETAILED DESCRIPTION  
       [0024]    A photon source in accordance with a feature of the invention operates in two stages to produce X-ray or extreme ultraviolet radiation. In a first stage, a central plasma is formed using multiple ion beams directed at a central plasma discharge region as described below. In a second stage, a heating current pulse is passed through the central plasma in order to heat and compress the plasma, raising its temperature and density.  
         [0025]    The astron source is a source of photons comprising a discharge chamber, a plurality of ion beam sources in the discharge chamber, each electrostatically accelerating a beam of ions of a working gas toward a plasma discharge region, and a neutralizing mechanism for at least partially neutralizing the ion beams before they enter the plasma discharge region. The neutralized beams enter the plasma discharge region and form a hot plasma that radiates photons.  
         [0026]    The astron principle that operates in the first stage of the photon source described above is illustrated in FIGS. 1A and 1B. The embodiment of the source shown in FIGS. 1A and 1B has a two gap ion acceleration structure  100 . Acceleration structure  100  includes concentric spherical electrode shells  112 ,  113  and  114 . The electrode shells  112 ,  113  and  114  have a plurality of sets of holes aligned along axes which pass through a central plasma discharge region  120 . Thus, for example, holes  122 ,  123  and  124  in electrode shells  112 ,  113  and  114 , respectively, are aligned along an axis  126  that passes through plasma discharge region  120 . Each set of holes, such as holes  122 ,  123  and  124 , defines an acceleration column  128 . The spaces between electrode shells  112 ,  113  and  114  constitute acceleration gaps for electrostatic acceleration of ion beams. Thus, each acceleration column has two gaps in the embodiment of FIGS. 1A and 1B. The embodiment of FIGS. 1A and 1B includes  36  acceleration columns  128 , arrayed in three sets of 12. Thus, the acceleration structure directs  36  ion beams toward plasma discharge region  120 . However, different numbers of ion beams may be utilized within the scope of the invention.  
         [0027]    The electrode shells  112 ,  113  and  114  may be supported by insulating spacers  130 . A plenum  132  having ports  134  encloses acceleration structure  100 .  
         [0028]    A working gas is introduced, either in a pulsed mode or continuously, through ports  134  into a space  144  behind the outermost electrode shell  114 . Some of the working gas flows down the acceleration columns  128 . When the appropriate gas density is present in the acceleration columns, a pulsed voltage may be applied between electrode shells  112  and  114 , with the polarity of electrode shell  114  being positive with respect to electrode shell  112 . In the configuration of FIGS. 1A and 1B, provided the appropriate gas density is present and provided that sufficient voltage is applied, a pseudospark discharge develops simultaneously in each of the acceleration columns  128 . The pseudospark discharge is characterized by the development of oppositely directed electron and ion beams that can have extremely high intensity. The ion beam exits from the negative polarity end of the acceleration column  128  at electrode shell  112  and progresses toward the central plasma discharge region  120 .  
         [0029]    By correct adjustment of the working gas density at an exit region  146  of each of acceleration columns  128 , most of the ions can be neutralized by resonant charge exchange, so as to form a neutral beam that propagates without deflection to the plasma in plasma discharge region  120 . Those ions that are not neutralized contribute excess positive charge to each of the ion beams, causing electrons to be attracted from the nearby surface of electrode shell  112 , which is already primed as a cathode due to the breakdown into a pseudospark discharge. Thus, the neutral atoms are accompanied by a nearly charge-balanced beam plasma, including the remaining unneutralized ions and electrons. The slow ions resulting from resonant charge exchange define tracks that are favored for a conduction of a high current heating pulse in the second stage of device operation, as described below. Additional details and embodiments of the astron photon source are described in the aforementioned application Ser. No. 09/815,633, which is hereby incorporated by reference.  
         [0030]    In the second stage of device operation, the newly formed plasma is heated and compressed, or pinched, by passage through the plasma of a pulse of electric current. A first embodiment of a photon source which incorporates both the first stage of operation, wherein beams of ions are electrostatically accelerated toward a plasma discharge region and are at least partially neutralized, and the second stage, wherein an electric current is passed through the plasma discharge region, is shown in FIGS. 2A and 2B. FIG. 2A is a simplified cross-sectional side view of the photon source, and FIG. 2B is a cross-section defined by revolution of line A-A in FIG. 2A around axis  200 . In FIGS. 2A and 2B, a central cathode shell, corresponding to electrode shell  112  in FIG. 1A, is divided into two half shells  202  and  204  that are electrically connected to a pulse voltage source  205 . The anode shell of the photon source is divided into two half shells  212  and  214  which are electrically connected by a conductor  216 . The working gas is introduced at low pressure through ports  218  and flows through passages  220  to enter hollow anode volumes  222  within anode half shells  212  and  214 . Cathode half shells  202  and  204  are electrically isolated by insulator  225 . The cathode half shells are electrically isolated from the respective anode half shells by insulators  227  and  229 . A pulse voltage source  215  is connected between cathode half shells  202  and  204  and anode half shells  212  and  214 .  
         [0031]    During the first phase of operation of the photon source shown in FIGS. 2A and 2B, a pulsed voltage V 1  from pulse voltage source  215  is applied between anode half shells  212  and  214  and cathode half shells  202  and  204 . In the absence of any applied voltage V 2  from pulse source  205 , the potential difference between cathode half shells  202  and  204  remains at zero. The combined cathode half shells are therefore pulsed negatively by voltage V 1  relative to the combined anode half shells, and a discharge develops as described above in connection with FIGS. 1A and 1B. Neutralized beams from this discharge pass through a plasma discharge region  224  to form a small spherical plasma. At the same time, the passage of ions and energetic neutral atoms forms ionized tracks  230  between cathode half shell  202  and plasma discharge region  224 , and ionized tracks  232  between cathode half shell  204  and plasma discharge region  224 . The ionized tracks  230  and  232  lie on the surfaces of two cones that have their vertices located at plasma discharge region  224  and provide conducting paths between cathode half shells  202  and  204 .  
         [0032]    During the second phase of operation, a pulsed voltage V 2  from pulse voltage source  205  is applied between cathode half shells  202  and  204 . The circuit is completed by conduction through the conical configuration of ionized tracks connecting cathode half shells  202  and  204 . The current flows through the plasma in plasma discharge region  224 , heating and compressing it via the magnetic pinch effect. The plasma temperature and density rise to the point where the desired X-ray or extreme ultraviolet radiation is emitted copiously. The radiation escapes the photon source in conical beams  234  that are relayed to the point of use by collecting optical surfaces (not shown in FIGS. 2A and 2B).  
         [0033]    The working gas pressure in the central part of the acceleration structure is preferably maintained in a range of about 1.0 to 100 millitorr. As noted above, one suitable working gas is xenon. Other suitable working gases include, but are not limited to, hydrogen, lithium, helium, nitrogen, oxygen, neon, argon and krypton.  
         [0034]    The ion beams may be pulsed or continuous, and the ion acceleration voltage V 1  may be from 2 kV to 20 kV, but is not limited to this range. Voltage V 1  may have a typical pulse duration of 100 nanoseconds to 10 microseconds, but may also be applied continuously. The heating voltage V 2  is applied typically within 100 nanoseconds to 10 microseconds of the initial application of voltage V 1  . The amplitude of voltage V 2  is typically in the range of 100 volts to 10 kV, and the width of this pulse typically ranges from 10 nanoseconds to 1 microsecond.  
         [0035]    A second embodiment of a photon source in accordance with the invention is shown in FIGS. 3 and 4. FIG. 3 is a simplified cross-sectional side view of the photon source, and FIG. 4 is a cross-section defined by revolution of line A-A in FIG. 3 around axis  200 . Like elements in FIGS. 2A, 2B,  3  and  4  have the same reference numerals. The embodiment of FIGS. 3 and 4 differs from the embodiment of FIGS. 2A and 2B by the addition of a transformer  211  for coupling pulsed electrical current from pulse voltage source  205  to cathode half shells  202  and  204 . Transformer  211  includes multiple primary windings  208 , a toroidal core  210 , which may be of non-magnetic or magnetic material, and an armature or secondary  206 , also having a toroidal configuration. Primary windings  208  are connected to pulse voltage source  205 , and secondary  206 , which may have a single turn, is connected between cathode half shells  202  and  204 .  
         [0036]    During the first phase of operation, a pulsed voltage V 1  is applied between the anode half shells  212  and  214  and secondary winding  206  that is connected to cathode half shells  202  and  204 . In the absence of any applied voltage V 2  from pulse voltage source  205  to primary windings  208 , the electric potential between the cathode half shells  202  and  204  remains at zero. The combined cathode half shells are therefore pulsed negatively by voltage V 1  relative to the combined anode half shells  212  and  214 , and a discharge develops as described above. The neutralized beams of this discharge pass through plasma discharge region  224  to form a small spherical plasma. At the same time, the passage of ions and energetic neutral atoms forms ionized tracks  230  and  232  as described above.  
         [0037]    During the second phase of operation, a pulsed voltage V 2  is applied simultaneously and in parallel across all the primary windings  208 , with the result that a voltage is induced between cathode half shells  202  and  204  that are connected to opposite ends of transformer secondary  206 . The transformer secondary circuit is completed by a conduction through the ionized tracks  230  and  232  connecting cathode half shells  202  and  204 . The secondary current flows through the plasma in plasma discharge region  224 , heating and compressing it via the magnetic pinch effect. As described above, the plasma temperature and density rise to the point where the desired X-ray or extreme ultraviolet radiation is emitted.  
         [0038]    An embodiment of a system for generating photons in accordance with the invention is shown schematically in FIG. 5. An acceleration structure  500  may correspond to the acceleration structure shown in FIGS. 2A and 2B, the acceleration structure shown in FIGS. 3 and 4, or in any other acceleration structure within the scope of the present invention. In the system of FIG. 5, acceleration structure  500  is a modification of acceleration structure  100  shown in FIGS. 1A and 1B and described above. Like elements in FIGS. 1A, 1B and  5  have the same reference numerals.  
         [0039]    Acceleration structure  500  includes concentric spherical electrode shells  112 ,  113  and  114 , each of which is divided by an insulator  503  into electrode half shells. A pulse voltage source  540  is connected between inner electrode half shells  112   a  and  112   b . A pulse voltage source  530  is connected between outer electrode half shells  114   a  and  114   b  and inner electrode half shells  112   a  and  112   b.    
         [0040]    Acceleration structure  500  is enclosed within a housing  502  that defines a discharge chamber  504 . A top aperture  140  of acceleration structure  500  is coupled through a screen  510  to a collection region  514  that is defined by an enclosure  516 . Enclosure  516  contains collection optics  518  for relaying a photon beam  150  to a remote point of use. Screen  510  allows propagation of photons from discharge chamber  504  to collection region  514  but impedes flow of gas from discharge chamber  504  to collection region  514 . A gas source  520  coupled to housing  502  supplies a working gas through inlets  522  and ports  134  in plenum  132  to acceleration structure  500 . A bottom aperture  142  of acceleration structure  500  is coupled to a vacuum pump  524 . An outlet  526  of vacuum pump  524  is connected to gas source  520  to form a gas recirculation system. The gas source  520  and the vacuum pump  524  are connected to housing  502  in a closed loop configuration that permits recirculation of the working gas through discharge chamber  504 . Gas source  520  may include elements for removing impurities and particulates from the working gas. The system may include a detector  550  located in collection region  514 , a control circuit  552  and a flow controller  554  for a feedback control of the rate of flow of the working gas into the discharge chamber  504  in response to a measured spectrum of the radiated photons.  
         [0041]    The system of FIG. 5 operates with first and second phases as described above in connection with FIGS. 2A and 2B. In particular, pulse source  530  applies a pulsed voltage between inner electrode half shells  112   a  and  112   b  and outer electrode half shells  114   a  and  114   b , causing neutralized beams to be directed toward plasma discharge region  120 . In the second stage, the plasma in discharge region  120  is heated and compressed by passage of a pulse of electric current. The neutralized beams form ionized tracks between cathode half shells  112   a  and  112   b  and plasma discharge region  120 . Application of a pulse to cathode half shells  112   a  and  112   b  by pulse source  540  causes electrical current to flow along the ionized tracks through plasma discharge region  120 , heating and compressing the central plasma. The plasma temperature and density rise to the point where the desired x-ray or extreme ultraviolet radiation is emitted. The radiation escapes the acceleration structure  500  as conical photon beam  150 .  
         [0042]    A third embodiment of a photon source in accordance with the invention is shown in FIGS. 6A and 6B. Like elements in FIGS. 1A, 1B,  6 A and  6 B have the same reference numerals. The embodiment of FIGS. 6A and 6B differs from the structure of FIGS. 1A and 1B by the addition of an external electrode  600  for supplying an electrical heating current to plasma discharge region  120 . A pulse voltage source  601  is connected between inner electrode shell  112  and outer electrode shell  114 . A pulse voltage source  602  is connected between inner electrode shell  112  and external electrode  600 . Insulators  603  and  604  electrically isolate the connections to electrode shells  112  and  114 , respectively. External electrode  600  may have a cylindrical configuration and may be positioned in the bottom aperture of the acceleration structure in spaced relationship to plasma discharge region  120 .  
         [0043]    During the first phase of operation, a pulse voltage V 3  from voltage source  601  is applied between electrode shells  112  and  114 . A discharge develops as described above, and the neutralized beams of the discharge pass through plasma discharge region  120  to form a plasma. At the same time, the passage of ions and energetic neutral atoms forms ionized tracks as described above.  
         [0044]    During the second phase of operation, a pulse voltage V 4  from voltage source  602  is applied between external electrode  600  and electrode shell  112 . The circuit is completed by conduction through the ionized tracks connecting electrode shell  112  and plasma discharge region  120  and through a glow region  605  between plasma discharge region  120  and external electrode  600 . The current flows through the plasma in plasma discharge region  120 , heating and compressing it. The plasma temperature and density rise to the point where the desired x-ray or extreme ultraviolet radiation is emitted. The plasma tends to be elongated in the direction of external electrode  600 . The working gas extends to electrode  600  at approximately the same pressure as inside electrode shell  112 .  
         [0045]    While there have been shown and described what are at present considered the preferred embodiments of the present invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.