Abstract:
A photocathode having a gate electrode so that modulation of the resulting electron beam is accomplished independently of the laser beam. The photocathode includes a transparent substrate, a photoemitter, and an electrically separate gate electrode surrounding an emission region of the photoemitter. The electron beam emission from the emission region is modulated by voltages supplied to the gate electrode. In addition, the gate electrode may have multiple segments that are capable of shaping the electron beam in response to voltages supplied individually to each of the multiple segments.

Description:
BACKGROUND  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to electron beam sources and, more particularly, to photocathodes for the generation of single or multiple electron beams.  
           [0003]    2. Prior Art  
           [0004]    High resolution electron beam sources are used in systems such as scanning electron microscopes, defect detection instruments, VLSI testing equipment, and electron beam (e-beam) lithography. In general, e-beam systems include an electron beam source and electron optics. The electrons are accelerated from the source and focused to define an image at a target. These systems typically utilize a physically small electron source having a high brightness.  
           [0005]    Improvements in optical lithography techniques in recent years have enabled a considerable decrease in the linewidths of circuit elements in integrated circuits. Optical methods, however, will soon reach their resolution limits. Production of smaller line-width circuit elements (i.e., those less than about 0.1 μm) will require new techniques such as X-ray or e-beam lithography.  
           [0006]    In e-beam lithography, a controllable source of electrons is desired. A photocathode used to produce an array of patterned e-beams is shown in FIG. 1. U.S. Pat. No. 5,684,360 to Baum et al., “Electron Sources Utilizing Negative Electron Affinity Photocathodes with Ultra-Small Emission Areas,” herein incorporated by reference in its entirety, describes a patterned photocathode system of this type.  
           [0007]    [0007]FIG. 1 shows a photocathode array  100  with three photocathodes  110  comprising a transparent substrate  101  and a photoemission layer  102 . The photocathode is back-illuminated with light beams  103  which are focused on photoemission layer  102  at irradiation region  105 . As a result of the back-illumination onto photoemission layer  102 , electron beams  104  are generated at an emission region  108  opposite each irradiation region  105 . Other systems have been designed where the photoemitter is front-illuminated, i.e. the light beam is incident on the same side of the photoemitter from which the electron beam is emitted.  
           [0008]    Often, light beams  103  or electron beams  104  are masked. In FIG. 1, light beams  103  are masked using mask  106  which allows light onto irradiation spots  108  but prevents light from being incident on other areas of photoemission layer  102 . FIG. 1 also shows mask  107  which allows electrons to exit photoemission layer  102  only at certain surface spots corresponding to emission regions  105 . A photocathode may also have a mask between transparent substrate  101  and photoemission layer  102  to block light beam  103  so that it is only incident at irradiation spots  105 . In general, photocathode  110  may include no masking layers or may have one or more masking layers.  
           [0009]    Each irradiation region  105  may be a single circular spot representing a pixel of a larger shape, the larger shape being formed by the conglomerate of a large number of photocathodes  110  in photocathode array  100 . In that case, irradiation region  105  may be as small as is possible given the wavelength of the light beam incident on photocathode  100 . Typically, a grouping of pixel irradiation regions has dimensions of 100-200 μm. Each pixel can have dimensions (i.e. diameter) as low as 0.1 μm. Alternatively, irradiation spot  105  and emission region  108  can be a larger shape. In either case, the image formed by emission region  108  will be transferred to e-beam  104  so long as the entirety of irradiation region  105  is illuminated by light beam  103 .  
           [0010]    Photoemission layer  102  is made from any material that emits electrons when irradiated with light. These materials include metallic films (gold, aluminum, etc.) and, in the case of negative affinity (NEA) photocathodes, semiconductor materials (especially III-V compounds such as gallium arsenide). Photoemission layers in negative electron affinity photocathodes are discussed in Baum (U.S. Pat. No. 5,684,360).  
           [0011]    When irradiated with photons having energy greater than the work function of the material, photoemission layer  102  emits electrons. Typically, photoemission layer  102  is grounded so that electrons are replenished. Photoemission layer  102  may also be shaped at emission region  108  in order to provide better irradiation control of the beam of electrons emitted from emission region  108 . Further control of the e-beam is provided in an evacuated column as shown in FIG. 2.  
           [0012]    Light beams  103  usually originate at a laser but may also originate at a lamp such as a UV lamp. The laser or lamp output is typically split into several beams in order to illuminate each of focal points  105 . A set of parallel light beams  103  can be created using a single laser and a beam splitter. The parallel light beams may also originate at a single UV source. Alternatively, the entire photoemission array  100  may be illuminated if the light source has sufficient intensity.  
           [0013]    Photons in light beam  103  have an energy of at least the work function of photoemission layer  102 . The intensity of light beam  103  relates to the number of electrons generated at focal point  105  and is therefore related to the number of electrons emitted from emission region  108 . Photoemission layer  102  is thin enough and the energy of the photons in light beam  103  is great enough that a significant number of electrons generated at irradiation region  103  migrate and are ultimately emitted from emission layer  108 .  
           [0014]    Transparent substrate  101  is transparent to the light beam and structurally sound enough to support the photocathode device within an electron beam column which may be a conventional column or a microcolumn. Transparent substrate  101  may also be shaped at the surface where light beams  103  are incident in order to provide focusing lenses for light beams  103 . Typically, transparent substrate  101  is a glass although other substrate materials such as sapphire or fused silica are also used.  
           [0015]    If mask  106  is present either on the surface of transparent substrate  101  or deposited between transparent substrate  101  and photoemission layer  102 , it is opaque to light beam  103 . If mask  107  is present, it absorbs electrons thereby preventing their release from emission region  108 . Mask  107  may further provide an electrical ground for photoemission layer  102  provided that mask  107  is conducting.  
           [0016]    Photocathode  100  may be incorporated within a conventional electron beam column or a microcolumn. Information relating to the workings of a microcolumn, in general, is given in the following articles and patents: “Experimental Evaluation of a 20×20 mm Footprint Microcolumn,” by E. Kratschmer et al., Journal of Vacuum Science Technology Bulletin 14(6), pp. 3792-96, November/December 1996; “Electron Beam Technology—SEM to Microcolumn,” by T. H. P. Chang et al., Microelectronic Engineering 32, pp. 113-130, 1996; “Electron Beam Microcolumn Technology And Applications,” by T. H. P. Chang et al.,  Electron-Beam Sources and Charged-Particle Optics , SPIE Vol. 2522, pp. 4-12, 1995; “Lens and Deflector Design for Microcolumns,” by M. G. R. Thomson and T. H. P. Chang, Journal of Vacuum Science Technology Bulletin 13(6), pp. 2445-49, November/December 1995; “Miniature Schottky Electron Source,” by H. S. Kim et al., Journal of Vacuum Science Technology Bulletin 13(6), pp. 2468-72, November/December 1995; U.S. Pat. No. 5,122,663 to Chang et al.; and U.S. Pat. No. 5,155,412 to Chang et al., all of which are incorporated herein by reference.  
           [0017]    [0017]FIG. 2 shows a typical electron beam column  200  using photocathode array  100  as an electron source. Column  200  is enclosed within an evacuated column chamber (not shown). Photocathode array  100  may be completely closed within the evacuated column chamber or transparent substrate  101  may form a window to the vacuum chamber through which light beams  103  gain access from outside the vacuum chamber. Electron beams  104  are emitted from emission region  108  into the evacuated column chamber and carry an image of emission region  108 . Electron beam  104  may be further shaped by other components of column  200 .  
           [0018]    Electron beams  104  are accelerated between photocathode array  100  and anode  201  by a voltage supplied between anode  201  and photoemission layer  102 . The voltage between photocathode array  100  and anode  201 , created by power supply  208  (housed outside of the vacuum chamber), is typically a few kilovolts to a few tens of kilovolts. The electron beam then passes through electron lens  204  that focuses the electron beam onto limiting aperture  202 . Limiting aperture  202  blocks those components of the electron beams that have a larger emission solid angle than desired. Electron lens  205  refocuses the electron beam. Electronic lenses  204  and  205  focus and demagnify the image carried by the electron beam onto target  207 . Deflector  203  causes the electron beam to laterally shift, allowing control over the location of the image carried by the electron beam on a target  207 .  
           [0019]    In 0.1 μm lithography systems, the size of a circular pixel incident on target  207  is on the order of 0.05 μm. Therefore, the image of emission area  108  needs to be reduced by roughly a factor of 2 to 10, depending on the size of emission region  108 . Target  207  may be a semiconductor wafer or a mask blank.  
           [0020]    Conventional variable shaped electron beam lithography columns shape the electron beam by deflecting the electron beam across one or more shaping apertures. The resulting image in the shaped electron beam is then transferred to target  207  with a large total linear column demagnification. The requirement of large total linear demagnification (supplied by electron lenses  204  and  205 ) results in large column lengths, increasing electron-electron interactions that ultimately limit the electron current density of the column. The low electron current density results in a low throughput when the column is used in lithography.  
           [0021]    Another major drawback in using known e-beam systems include the inability to modulate the electron beam without modulating the light source itself, usually a laser. Modulating a laser typically involves a large amount of control circuitry, requiring a large amount of space, and can be slow. In addition, in a patterned array of photocathodes, modulation of individual photocathodes in the array is extremely difficult. Finally, better resolution is required of lithography systems in order to meet future demands of semiconductor materials processing.  
         SUMMARY  
         [0022]    According to the present invention, a photocathode has a gate electrode that modulates and, in some embodiments, shapes the emission of an electron beam.  
           [0023]    A photocathode emits electrons upon irradiation by a photon beam if the photon energy is greater than the work function of the photocathode. By masking the photocathode selectively with an opaque material, the emission is confined to pre-defined regions. Providing an electrically isolated gate structure that encompasses an emission region of the photocathode allows the intensity of the electron beam to be modulated by application of a gate bias voltage to the gate structure. If the gate structure has multiple segments, the electron beam emitted from the photocathode can also be shaped.  
           [0024]    In a photocathode according to the present invention, an emission area is surrounded by a gate electrode that is offset from an electron emitting surface by an insulator. The gate electrode can be electrically controlled in order to turn the electron beam on or off or to vary the intensity of the electron beam. The electron beam is modulated in the region between the gate electrode and electron emitting surface rather than at a light beam source such as a laser, resulting in faster switching times and space savings in the electron beam system.  
           [0025]    Embodiments of this invention can be utilized to form an array of photoemission sources each having a precisely controlled emitting region and position. In embodiments where the gate structure of each of the photoemission sources in the array includes a single gate electrode, each of the single gate electrodes in the array may be individually controlled or controlled in groups. In embodiments where the gate structure of each of the photoemission sources in the array includes multiple gate electrodes, each of the multiple gate electrodes may be individually controlled or controlled in groups. In yet other embodiments, the array of photoemission sources may include a combination of photoemission sources having a single gate electrode and photoemission sources having multiple gate electrodes where each gate electrode is individually or group controlled.  
           [0026]    In general, emission regions can be of any size or shape that are within the limits of microfabrication technology. Some embodiments of the invention include self-biasing circuitry utilizing photoemission as the feed-back for stable emission intensity.  
           [0027]    A photocathode includes a transparent substrate and a photoemission layer. The transparent substrate is transparent to a light source. The light source generates an array of light beams which are focused on an array of irradiation regions directly above the emitting areas on the photoemission layer. In one embodiment, the light source is a laser and the array of light beams results from the laser beam being split into multiple light beams using a beam splitter. Alternatively, the light source may be a UV lamp.  
           [0028]    In some embodiments, each emitting area on the photoemission layer is a single pixel, a larger shape being formed by the aggregate of all of the pixels. Alternatively, the emitting area itself may represent any shape that is to be transferred to a target.  
           [0029]    In some embodiments, masks are formed on top of the substrate in order to form the light beams into the desired images before the light beams are incident on the irradiation region. Other embodiments place a mask on the emitting surface of the photoemission layer. Yet other embodiments place a mask between the photoemission layer and the substrate in order to form the image in the light beam. In some embodiments, a back surface of the substrate, where the light beams are incident and opposite the photoemission layer, is shaped to provide lenses. The lenses help to focus the light beams onto the irradiation region.  
           [0030]    According to the present invention, the emitting area is surrounded by an insulator. The emitting area itself is left uncovered by the insulator. In some embodiments, a single conductor is mounted on the insulator to form a gate electrode. In other embodiments, multiple electrically independent conductors are mounted around the emitting area on the surrounding insulator to form a gate electrode having multiple segments. Each segment of the gate electrode is independently controlled in order to turn on and off a corresponding portion of the electron beam that is initiated at the emitting area.  
           [0031]    A photocathode according to the present invention is suitable for use in an arrayed electron source for conventional electron beam columns. Other embodiments of the invention are suitable for use as a miniaturized arrayed electron source for electron beam microcolumns. Some embodiments are suitable for use as a single gated source for conventional electron beam columns and microcolumns.  
           [0032]    Photocathode arrays having gate electrodes with multiple segments allow variable shaping at the electron source in an electron beam lithography column without using shaping apertures or shaping optics. Use of these embodiments results in shorter column length because of the reduced need for further beam shaping and demagnification. The shorter column length results in less electron-electron interactions and ultimately a higher throughput in systems such as lithography systems because of the higher intensity electron beams.  
           [0033]    The invention and its various embodiments are further discussed along with the following figures and the accompanying text. 
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0034]    [0034]FIG. 1 shows a patterned photocathode according to the prior art.  
         [0035]    [0035]FIG. 2 shows a conventional electron beam column using the photocathode shown in FIG. 1.  
         [0036]    [0036]FIGS. 3A and 3B show a photocathode according to the present invention.  
         [0037]    [0037]FIG. 4 shows a portion of a photocathode array having two photocathodes according to the present invention.  
         [0038]    [0038]FIG. 5 shows a photocathode according to the present invention having a gate electrode with multiple segments.  
         [0039]    [0039]FIG. 6A shows a photocathode according to the present invention having multiple independent segments in the gate electrode.  
         [0040]    [0040]FIGS. 6B and 6C show sample patterned e-beams resulting from selectively turning on the segments shown in the gate electrode of FIG. 6A.  
         [0041]    [0041]FIG. 7 depicts the process of forming a photocathode according to the embodiment of the invention presented in FIG. 4.  
         [0042]    [0042]FIG. 8 shows a photocathode array according to the present invention.  
         [0043]    [0043]FIG. 9 shows a micro-column utilizing a photocathode according to the present invention.  
         [0044]    [0044]FIG. 10 shows a multiple segment gated photocathode used in an electron beam column where the beam shaping is accomplished at the photocathode.  
         [0045]    [0045]FIG. 11 shows a conventional variable shaped beam electron beam column having multiple shaping components.  
         [0046]    In the figures, components having the same or similar functions are identically labeled. 
     
    
     DETAILED DESCRIPTION  
       [0047]    [0047]FIGS. 3A and 3B show in a side view an embodiment of a photocathode  300  according to the present invention. (The conventional associated housing, electrical leads, etc. are not shown.) In FIG. 3A, a photoemitter  302  is deposited on a transparent substrate  301 . Transparent substrate  301  is usually glass, fused silica or sapphire, although other transparent materials having structural strength sufficient for support can be used.  
         [0048]    A light beam  303  is incident on transparent substrate  301 , passes through transparent substrate  301 , and is absorbed by photoemitter  302  at irradiation region  308 . Photoemitter  302  emits electrons from emission area  305 , located on the surface of photoemitter  302  opposite of irradiation region  308 , when light beam  303  is incident upon irradiation region  308 .  
         [0049]    Emission area  305  can, in general, be of any shape and any size where gate electrode  307  determines the electric field. Some useful shapes include a circle, a square, a rectangle, an octagon and a hexagon. Irradiation region  308  should at least cover emission area  305 .  
         [0050]    A gate insulator  306  is deposited on photoemitter  302  such that emission area  305  is surrounded, but not covered, by gate insulator  306 . Gate insulator  306  may be made from any electrically insulating material and is preferably made from SiO 2 . Gate electrode  307  is deposited on the side of gate insulator  306  away from emission region  305 . Gate electrode  307  can be made from any conducting material.  
         [0051]    Photoemitter  302  can be made from any material that emits electrons when illuminated. The most efficient photoemitting materials include gold, aluminum, and carbide materials. In addition, many III-V semiconductors, such as GaAs, are suitable photoemitter materials. Preferably, photoemitter  302  is made from gold and has a thickness of about 100 Å.  
         [0052]    Photoemitter  302  will have a work function that is determined by the actual photoemitter material. The work function is the minimum energy required to release an electron from the material. The photons in light beam  303  must have an energy at least as great as the work function in order that photoemitter  302  will emit electrons.  
         [0053]    Light beam  303  is absorbed by photoemitter  302  at, or nearly at, the surface of photoemitter  302  corresponding to irradiation region  308 . At that point, electrons will have a kinetic energy equal to the photon energy minus the work function. These electrons migrate from irradiation region  308  to emission area  305  and are emitted from the material at emission area  305  provided that the electrons have not lost too much energy to collisions within the photoemitter material. As such, the thickness of photoemitter  302  should be sufficient to absorb light beam  303  but not so thick as to reabsorb a significant number of the free electrons created.  
         [0054]    It is also desirable that, in embodiments of this invention, the kinetic energies of the emitted electrons not be too great, preferably less than 0.5 eV but can be as great as a few eV, so that the emitted electrons can be reflected by a voltage applied to gate electrode  307 . If photoemitter  302  is gold, then a light beam having a photon wavelength of 257 nm or less is needed to produce photons having an appropriate photon energy.  
         [0055]    Transparent substrate  301  must be transparent to light beam  303  so that the maximum amount of light possible is incident on irradiation region  308 . Transparent substrate  301  can be of any thickness but preferably is a few millimeters thick. In addition, light beam  303  may be focused to cover irradiation spot  308  in an area corresponding to emission region  305 .  
         [0056]    The intensity distribution of light beam  303  is generally gaussian in shape, therefore light beam  303  will be more intense at its center than at its edges. Light beam  303  is preferably focused in such a way that its intensity is nearly uniform across irradiation region  308  so that electron beam  304  has nearly uniform intensity. In general, however, light beam  303  can be as focused as is desired.  
         [0057]    Gate electrodes  307  are mounted to insulators  306  and can be constructed from any conducting material. The thickness of gate insulator  306  is preferably about 1000 Å and the thickness of gate electrode  307  is also preferably about 1000 Å. In one embodiment, photoemitter  302  is held at ground voltage and gate electrode  307  is biased at a voltage greater than ground, approximately +10 V, in order to accelerate the electrons that are emitted from photoemitter  302 . Gate electrode  307  is biased at voltages less than ground, approximately −10V, in order to reflect the emitted electrons back towards photoemitter  302 . Moreover, stable emission can be achieved by coupling a resistor  311  between photoemitter  302  and gate electrode  307  and using the emission-intensity for feed-back (i.e., a self-biasing system). For example, when electron emission increases the gate voltage decreases correspondingly which in turn lowers the emission intensity.  
         [0058]    Anode electrode  310  is held at a voltage of from a few kilovolts to several tens of kilovolts and accelerates the electrons out of photocathode  300  and into an evacuated electron beam column. Alternatively, photoemitter  302  is held at a high negative voltage, gate electrodes  307  are biased at ±10 V compared to photoemitter  302 , and anode electrode  310  is grounded.  
         [0059]    In FIG. 3A, gate electrode  307  is held at +10 V. This voltage is chosen so as to be consistent with the electric field which would be set up between anode electrode  310  and photoemitter  302  if insulator  306  and gate electrode  307  were absent. With the voltage of gate electrode at 10 V, electron beam  304 , which carries the image of emission region  305 , is accelerated out of emission region  305 . Insulators  306  and gate electrode  307  also act as a mask in order to better shape the image of emission region  305  contained in electron beam  304 .  
         [0060]    In FIG. 3B, gate electrode  307  is held at −10V. At this voltage, the electrons emitted by emission region  305  are accelerated back towards emission region  305  by the electric field created between gate electrode  307  and photoemitter  302 . No electron beam  304  is created because the electrons emitted from emission region  305  are reflected back into photoemitter  302  rather than being accelerated away from photoemitter  302 . Instead of an electron beam, electron cloud  309  is created where electrons are emitted out of photoemitter  302  and promptly accelerated back into photoemitter  302 .  
         [0061]    In some embodiments of the invention, the voltage at gate electrode  307  is varied in order to control the intensity of the electron beam. The higher the voltage difference between gate electrode  307  and photoemitter  302  the greater the number of electrons that leave photocathode  300 . The maximum number of electrons available, those that are emitted from emission region  305  as a result of light beam  303 , are extracted when the gate electrodes are set at full on (about 10V).  
         [0062]    Although the examples shown here have the gate biasing voltage at +10V for full-on operation and −10V for full-off operation, other parameters for gate voltages are possible. The full-on bias voltage and the voltage applied to anode electrode  310  determines the thickness of insulator  306  because the electric field created by gate electrode  307  at the full-on bias voltage should be consistent with that field which would exist in the absence of gate electrode  307  and gate insulator  306 . The full-off bias voltage limits the incident light beam photon energy because in full-off operation the electrons emitted from emission region  308  must be reflected back into photoemitter  302 . In addition, gate electrode  307  should be the dominant feature determining the electric fields near emission region  308 . The size of emission region  308  is therefore limited by the relative sizes and distances between gate electrodes  306  and anode electrode  310 .  
         [0063]    In embodiments where switching times are important, the RC time constant of gate electrode  307  should be relatively small. The spacing between gate electrodes, the spacing between the gate electrode and the photoemitter, and the thickness of the electrodes determine the RC time constant and therefore the maximum rate of switching.  
         [0064]    [0064]FIG. 4 shows an embodiment of a photocathode array  400 . In FIG. 4, two emission regions  402  of photocathode array  400  are shown and both emission regions  402  are illuminated by light beam  403  which simultaneously illuminates the entire portion of photocathode array  400  shown. Parallel light beams  403  could be used instead with each beam being focused on an individual emission region  402 . Photocathode array  400  comprises a transparent substrate  401 , a conductor  408 , gate insulator  406 , gate electrodes  407 , and photoemitters  402 .  
         [0065]    Conductor  408 , which can be made from any conducting material but is preferably aluminum, is deposited on transparent substrate  401  and has an opening within which photoemitter  402  is mounted. Photoemitter  402  may be any material which emits electrons when illuminated with photons, as was previously discussed. Again, photoemitter  402  has a work function and the photons in light beam  403  must have an energy at least as great as the work function in order that electrons are emitted from emission region  405 . Transparent substrate  401  is preferably glass but can be any material that is transparent to light beam  403  such as sapphire or fused silica. Conductor  408  is opaque to light beam  403  and does not emit electrons from its front when illuminated from the back by light beam  403 . Conductor  408 , therefore, acts as a mask and defines an irradiation region  408 . Emission region  405  lies directly opposite irradiation region  408  on photoemitter  402  and can be of any shape and any size where gate electrodes  407  dominate the electric field.  
         [0066]    A gate insulator  406  is mounted on conductor  408  and has an opening  410  such that photoemitters  402  are not covered by insulators  406 . Gate electrodes  407  are deposited on gate insulator  406 . In this embodiment, gate electrodes  407  overhang opening  410  by an amount sufficient to cause the electric fields created at emission area  405  to be not substantially distorted by gate insulator  406 . As in FIGS. 3A and 3B, gate electrode  407  has the ability to turn electron beam  404  on and off with a voltage applied to gate electrode  407 . The on and off voltage roughly correspond to +10V and −10V, respectively. In addition, ultimate electron beam intensity may be regulated by varying the gate electrode voltage. A self-biasing resistor  411  also may be connected between gate electrode  407  and conductor  408  in order to provide feedback for controlling the intensity of electron beam  404  by self-biasing.  
         [0067]    In the photocathodes shown in FIGS. 3A, 3B and  4 , the intensity of the electron beams may be controlled by controlling the actual voltage between the gate electrode and the photoemitter. The lower the voltage, the less intensity that the electron beam will have because fewer of the electrons will escape the electron cloud where the electrons have a statistical distribution of velocities in the direction of electron beam propagation. In addition, the gate electrodes may be used to regulate the intensity of the resulting electron beam. In some embodiments, a resistor is placed between the gate electrode and the photoemitter so that a self-biasing feedback is created, i.e., if emission increases, the gate voltage lowers correspondingly.  
         [0068]    In FIG. 4, gate electrode  407  is shown as being the same for each emission area  405 . However, in general each emission area  405  has a gate electrode  407  that is electrically isolated from the other gate electrodes. In addition, a gate electrode for a particular emission area may include several segments each of which are electrically isolated from all of the others.  
         [0069]    In some embodiments, the gate electrode surrounding the emission region has multiple segments. Multiple segments allow the ability to turn on parts of the emission region while turning off other parts of the emission region, shaping the image carried by electron beam  404 .  
         [0070]    [0070]FIG. 5 shows a photocathode as in FIG. 3 but with a right gate segment  510  and a left gate segment  511  instead of single segment gate electrode  307 . The result of this construction is that the electron beam can be selectively switched on. For example, in FIG. 5 right gate segment  510  is held full-on at a bias voltage of 10 V and left gate segment  511  is held full-off at a bias voltage of−10 V. The resulting electric field reflects electrons which are emitted by emission region  305  near left gate segment  511  while accelerating electrons are emitted out from emission region  305  near to right gate segment  510  of photocathode  500 . The resulting electron beam  504  is an image of, in this example, half the emission region  305 . The resulting electron beam  504  distribution is not uniform and is most intense near right gate segment  510  and is essentially off at a point midway between the two segments  510  and  511 .  
         [0071]    [0071]FIG. 6A shows in a plan view a four segment gate electrode configuration. The gate segments are segment A  601 , B  602 , C  603 , and D  604 . Emission region  305  in this example is a square. Emission region  305  can be of any shape but is preferably a square. Other useful shapes include a circle, a rectangle, an octagon and a hexagon.  
         [0072]    [0072]FIG. 6B shows in a plan view electron beam  504  that results when gate segments A  601 , C  603 , and D  604  are turned on (i.e., held at +10V) and gate segment B  602  is turned off (i.e., held at −10V). FIG. 6C shows in a plan view electron beam  504  that results when gate segments A  601  and D  604  are turned on while gate segments B  602  and C  603  are turned off. Other shaped electron beams can be formed by selectively controlling the voltages of the segments of the gate electrodes. This ability lends great versatility to constructing photocathode arrays that are useable for a variety of different tasks. FIG. 10 shows a photocathode having a segmented gate electrode used in an electron beam column for electron beam lithography.  
         [0073]    In general, any number of gate segments can be used. The more gate segments there are, the more control a user of the photocathode has over the electron beam created from a given emission area. This ability may be of great importance in efficiently writing features onto semiconductor substrates. In addition, resistors can be coupled between individual segments of the gate electrode and the photoemitter in order to provide self-biasing control over electron beam intensity as described above.  
         [0074]    The photocathodes described above are conducive to miniaturization and precise integration into multiple photocathode sources. A photocathode array can be constructed on a single substrate with precise positioning of photocathodes. In particular, FIGS.  7 A- 7 F illustrate a process of manufacturing the photocathode illustrated in FIG. 4 using conventional semiconductor processing steps. The illustrated process shows only a single photocathode of the photocathode array. However, one skilled in the art can produce a photocathode array having precisely placed photocathodes with various emission area shapes and gate structures from this illustration. In addition, one skilled in the art can modify this process in order to manufacture other photocathodes according to this invention or alter this process in ways that result in the same photocathode construction.  
         [0075]    [0075]FIG. 7A shows in a cross sectional view the first step in the process where an opaque layer of conducting film is deposited on a transparent substrate  401  such as glass, fused silica, or sapphire. Preferably, transparent substrate  401  is a glass substrate. As shown in FIG. 7B, the conducting film is masked and a window having an appropriate size and shape to form an emission area  410  is etched through the conducting film. A gate insulator  406  is then deposited on top of conducting film  408  and also fills the window of emission area  410 . Gate insulator  406  can be any electrical insulator but preferably is SiO 2 . A gate electrode layer  407  is then deposited on top of gate insulator  406  as shown in FIG. 7D.  
         [0076]    Gate insulator  406  is then masked and a hole  411  is etched through gate electrode layer  407  and insulating film  406  as is shown in FIG. 7E. Hole  411  is aligned with emission area  410  and is slightly larger than emission area  410 . In addition, all of insulating film  406  is removed from the window of emission area  410  by this etch.  
         [0077]    In FIG. 7F, a selective isotropic etch has created a recessed hole  412  in insulating film  406  so that gate electrode  407  now overhangs the opening created at hole  411  and recessed hole  412 . Finally, photocathode material  402  is deposited using a directional deposition technique such as thermal evaporation from a point source or ionized sputter deposition. This final deposition forms a photocathode  400  with a self-aligned gate aperture and is formed such that the photocathode is electrically connected to conducting layer  408  but maintains electrical isolation from gate electrodes  407 .  
         [0078]    In addition, in an array of photocathodes manufactured by this process, each gate electrode segment surrounding each of the photocathodes may be formed by appropriately masking the gate insulator  406  during deposition of gate electrode layer  407 . Alternatively, gate electrode layer  407  may be individually etched to form individual gate segments. Also, interconnect lines that connected gate electrode segments to pads can be formed along with the gate electrode segments or may be deposited at a later process step.  
         [0079]    As an alternative manufacturing method, the substrate could be coated with conducting layer  408 , gate insulator  406  and gate electrode  407  first. Window  411  is then etched through all films down to transparent substrate  401 . Using a selective isotropic etch, the opening in gate electrode  407  could be enlarged slightly with respect to the corresponding window  410  in conducting layer  408 . Also alternatively, multiple segments of gate electrodes are created around each of holes  411  by isotropically etching insulating breaks in gate electrode  407 .  
         [0080]    In some embodiments, the surface of substrate  401  may be shaped in order to focus the light beam onto an irradiation region corresponding to emission area  410  of photoemitter  402 . Also, in some embodiments, photoemitter  402  may itself be shaped so as to better focus the resulting electron beam that is emitted from the photocathode.  
         [0081]    [0081]FIG. 8 shows in a plan view a four by four array of patterned photocathodes. Emission areas  801  in this example are squares although any shape, including circles, rectangles, hexagons and octagons, can be fabricated. Gate electrode  804  fully surrounds each emission area  801 . Although only a single segment gate electrode is shown in FIG. 8, gate electrode  804  may in general be constructed of multiple electrode segments for further control of the electron emission from emission area  801 . Gate electrode  804  is connected to a bonding pad  803  by an interconnect line  802 . Both bonding pad  803  and interconnect  802  are preferably made from the same material as is gate electrode  804  but any conductor making electrical contact with gate electrode  804  can be used. In general, for lithography systems it is desirable that the physical separation between two adjacent emission regions be such that the array is a square. The minimum separation between emission regions is approximately four times the physical dimensions of the emission region. In FIG. 8, the dimension of the square emission region with current microfabrication technology can be as small as 0.1 μm. Preferably, the side dimension of the emission region is 0.1 μm. Therefore, the whole four by four array shown in FIG. 8 is constructable within a square 1.6 μm on a side, which is well within conventional microfabrication limits.  
         [0082]    [0082]FIG. 9 shows in a side view a photocathode array  910  according to this invention mounted within a microcolumn  900 . Microcolumn  900  is contained within an evacuated chamber (not shown). The substrate of photocathode array  910  may suffice as a vacuum window allowing a laser light source onto the irradiation regions of photocathode array  910  or alternatively photocathode array  910  may be fully enclosed in the vacuum chamber. Electron beams  911  are emitted from the emission regions of photocathode array  910  and, depending on the control inputs to gate electrodes  909  of photocathode array  910 , are accelerated through anode  901 . Anode  901  is held at a voltage of from one kilovolt to several tens of kilovolts over that of the photoemitters in photocathode  910 . Limiting aperture  902  blocks a portion of beams  911  which have a larger emission solid angle than desired. Deflector  903  allows the image of the emission regions contained in electron beams  911  to be laterally shifted. Einzel lens, having electrodes  904 ,  905 , and  906 , focuses and demagnifies the image onto target  907 . Target  907  may be either a semiconductor wafer or a mask blank for electron beam lithography.  
         [0083]    Photocathode array  910  can include any number of individual photocathodes. Each of the individual photocathodes can include a single segment gate or a multiple segment gate. The image formed in electron beam  911  is dependent upon the emission areas of each of the individual photocathodes and the states of the gate electrodes of each of the individual photocathodes. For example, a photocathode array  910  having one photocathode with a single segment gate can only produce an image of the emission area of the photocathode. With a photocathode array  910  having multiple photocathodes, each with an individually controlled single segment gate, various images can be formed by selectively turning on the individually controlled photocathodes to form conglomerates of the images of each of the emission areas of the “on” photocathodes. A photocathode array  910  where some of the photocathodes have multisegmented gate electrodes have the most versatility because images can be formed using portions of emission areas of the individual photocathodes.  
         [0084]    [0084]FIG. 10 shows an electron source  1001  having a single photocathode  1004 . Photocathode  1004  has an emission area  1002  and a four segment gate structure  1003 . The four segment gate structure is capable of selectively imaging emission area  1002 . In the example of FIG. 10, the four segment gate structure  1003  is used to shape an electron beam image equivalent to one half of emission area  1002 . The electron beam carrying the electron beam image is accelerated out of photocathode  1004  by extraction electrode  1005 . Demagnification lens  1006  demagnifies the electron beam image onto wafer or mask blank  1008  to form the final shaped beam image. The system shown in FIG. 10, having a minimal number of components, allows shaped electron beam columns to be constructed utilizing a minimum amount of space.  
         [0085]    [0085]FIG. 11 shows a conventional variable shaped electron beam column, in contrast to the electron beam column shown in FIG. 10. An electron beam is formed at electron source  1101 . Electron source  1101  may be a thermionic cathode such as lanthanum hexaboride, LaB 6 , or a single gated photocathode similar to that shown in FIG. 3. The electron beam is shaped by square aperture  1102  to form a shaped electron beam. The shaped electron beam is focused by electron lens  1103  into region  1110 . Spot shaping deflector  1104  deflects the electron beam at focus region  1110  so that the shaped electron beam is shifted. The shaped electron beam is then passed through square aperture  1105  to form an intermediate shaped electron beam. Square aperture  1105  passes that portion of the electron beam that overlaps with the aperture and blocks that portion of the electron beam outside the aperture so that only a portion of the image formed by square aperture  1102  is passed into the intermediate shaped beam image. Demagnification lens  1106  demagnifies the image and focuses the image onto a final shaped beam image  1108  on a wafer or mask blank  1109 .  
         [0086]    The above described examples are demonstrative only. Variations that are obvious to one skilled in the art fall within the scope of this invention. As such, this application is limited only by the following claims.