Abstract:
A multipixel electron emission source is generated by separating a point electron source from a plasma region. The point electron source produces an electron beam that is passed through the plasma region. The plasma region diffuses the electron beam thereby producing electrons with uniform energy. Moreover, the maximum current of the device is advantageously controlled by the maximum electron current produced by the point electron source and not the characteristics of the plasma and wall interactions as found in conventional devices. The electrons are then pulled out of the plasma region by an aperture grid that is also used as a blanking array. A focusing chamber is positioned down stream of the plasma region and aperture grid. The aperture grid includes a base electrode and a blanker electrode, which is isolated from the base electrode. The base electrode is held at a potential. In the off state, the blanker electrode is floating permitting the blanker electrode to become negatively charged from the electron stream. Once negatively charged, the blanker electrode pinches off the electron stream. In the on state, the blanker electrode is switchably coupled to the base electrode which drains the negative charge and permits the electron stream to pass. The aperture grid may be an integrated blanking and switching device.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an electron source, and in particular to an electron source that uses a glow discharge and multiple individually extinguishable electron-emitting apertures to create multiple electron beamlets. 
     BACKGROUND 
     Electron lithographic or detection systems typically use a single electron beam to expose or image a substrate. A single beam approach, however, poses severe limitations to the maximum achievable pixel rate, whether used for pixel exposure or pixel detection. In order to satisfy the throughput requirements of present day manufacturing environments several techniques are being developed to increase this pixel rate. For example, one technique used to increase throughput is to increase the number of electron beams that are used for exposure or detection. 
     In order for such a multiple electron beam system to function properly numerous requirements must be met for each single beam, i.e., each beamlet, as well as the collection of beamlets, i.e., the array. Crucial parameters for each beamlet include, e.g., spot size, brightness, beam uniformity and energy spread, while parameters for the array include uniformity, reliability and manufacturability standards. 
     In a multi-beam system it is typically desirable to have the ability to individually extinguish each beamlet independently, i.e., blanking a beam. Conventionally, a beam is blanked by shifting the direction of the beam away from a transmission aperture thereby stopping the flow of electrons through the aperture. Electron beamlets, however, propagate in close proximity to each other and, thus, such an approach might be undesirable. The stray electrons created by this type of blanking action could very well disturb the propagation of the neighboring beamlets. It is therefore preferred to extinguish the beamlets at the source thereby preventing any unneeded electrons from entering the optical system. 
     SUMMARY 
     A multipixel electron emission device in accordance with the present invention separates a source of electrons from a plasma region. The electron source is contained in an electron source chamber and produces an electron beam that is passed through a wall separating the electron source chamber and the plasma region, e.g., through an entrance aperture. The plasma region, for example, may contain a heavy noble gas, such as Xenon, at low pressure and is surrounded by a high frequency helical coil to produce a plasma. The electron beam enters the plasma region and is diffused in the plasma, which advantageously provides a more uniform energy to the electrons in the electron beam. Moreover, the current of the electron emission device is advantageously controlled by the electron current produced by the electron source and is not limited by the characteristics of the plasma and wall interactions as found in conventional devices. An aperture grid coupled to the plasma region pulls electrons out of the plasma region over a large area thereby producing a broad area electron emission. A focusing chamber is positioned down stream of the plasma region and aperture grid and includes, for example, an multi-beam optical system with beam acceleration grids and deflection devices. 
     In accordance with an aspect of the present invention, an aperture grid is used as a blanking array. The aperture grid may be used in the above described electron source or may be used in other suitable electron sources. The aperture grid includes a base electrode, which is at a certain potential, and has at least one aperture. A dielectric layer fully or partially overlays the base electrode and surrounds the aperture. A blanker electrode overlays the dielectric layer and also surrounds the aperture. The dielectric layer isolates the blanker electrode from the base electrode. The blanker electrode and base electrode are switchably coupled. 
     In an “off” state, the blanker electrode is floating, i.e., not coupled to the base electrode, which permits the blanker electrode to become negatively charged from the electron stream that contacts the blanker electrode. Once the blanker electrode is negatively charged, the blanker electrode pinches off the electron stream through the aperture. In an “on” state, the blanker electrode is switchably coupled to the base electrode which drains the negative charge. Thus, the blanker electrode is at the same potential as the base electrode and the electron stream is permitted to pass through the aperture. 
     The aperture grid may be an integrated blanking and switching device which is manufactured using conventional thin film deposition and patterning techniques. A method of fabricating the integrated blanking and switching device includes providing a conductive substrate, such as a silicon substrate; forming, e.g., a pnp type transistor on the bottom side of the substrate, i.e., on the side that will not be exposed to the plasma region; etching at least one aperture through the substrate so that it extends through the collector of the transistor; depositing and patterning an insulating layer over the substrate so that it surrounds the aperture and covers the sidewalls of the aperture; and depositing and patterning a conductive layer to form a blanker electrode that surrounds the aperture and covers the sidewalls of the aperture. In addition, an inert conductor may be deposited on the top side of the substrate, i.e., the side that will be exposed to the plasma region. The inert conductor can serve as a base electrode or as merely a protective layer that protects the substrate from the glow discharge in the plasma region or electron bombardment from the electron sources. The blanker electrode is coupled to the collector of the pnp type transistor, the emitter is coupled to the conductive substrate and the base is coupled to an external lead and is used to turn on and off the transistor. If desired, additional embedded logic may be included on the aperture grid. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a multipixel electron emission device in accordance with the present invention. 
     FIG. 2 shows a block diagram view of the electron emission device in accordance with the present invention. 
     FIG. 3 shows a top view of an aperture grid in accordance with an embodiment of the present invention. 
     FIG. 4 shows a cross-sectional view of a blanking device on the aperture grid shown in FIG.  3 . 
     FIGS. 5A and 5B show a qualitative depiction of the equipotential lines in the blanked and unblanked conditions, respectively, of a blanking device in accordance with an embodiment of the present invention. 
     FIG. 6 shows a cross sectional view of one embodiment of an integrated blanking and switching device. 
     FIGS. 7-13 show cross-sectional views of an integrated blanking and switching device in various states of fabrication in accordance with an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     In accordance with the present invention, a multipixel electron source uses a glow discharge and an accompanying blanking apparatus which advantageously relaxes the vacuum and cleanliness requirements compared to conventional electron sources. The multipixel electron source draws electrons from a conventional electron source and injects the electrons into a glow discharge region, e.g., a RF glow discharge of a heavy noble gas. The blanking apparatus utilizes the difference in mobility between ions and electrons to achieve the potentials needed to stop the electron flow. These potentials are modulated to achieve control over the electron flow through each aperture of the blanking apparatus, thereby allowing individual control over each beamlet. The blanking apparatus uses low voltage signals to open and close apertures and therefore allows high frequency operation without the need for elaborate electronic circuitry. 
     FIG. 1 shows a multipixel electron emission device  100  in accordance with the present invention. As shown in FIG. 1, the multipixel electron emission device  100  is constructed from three distinct sections with vacuum walls having appropriate apertures separating the sections. The first section is the electron source region  102 , which is a high vacuum volume that holds a conventional high current electron source. The second section is the glow discharge region  104  of the device, which operates at a higher pressure than the first section. The third section is the focusing region  106 , which includes a chamber containing an appropriate multi-beam optical system with, for example, beam acceleration grids and deflection devices. 
     Electron source region  102  is a chamber that may be formed, e.g., of stainless steel, and maintained at a high vacuum, e.g., 10 −8  Torr, by appropriate pumping via outlet  114 , as indicated by arrow  115 . Electron source region  102  includes a high current electron source  112 , which may be, for example, a tungsten or LaB 6  thermionic emitter, that produces an electron beam  113 . The requirements for the electron source  112 , however, are minimal, and thus, electron source  112  may be any conventional electron source that can deliver high current. The vacuum in electron source region  102  prevents sputtering and extends the life of electron source  112 . Electron beam  113  is directed towards an entrance aperture  116  between electron source region  102  and glow discharge region  104  by conventionally deflecting and/or focusing the electron beam  113 , e.g., by an appropriately charged plate  118 . Electron source  112  produces an electron beam  113  that may have a large energy distribution. For example, an energy distribution of approximately 5 eV is common. The energy distribution of the electrons advantageously will be minimized in the glow discharge region  104 . 
     The entrance aperture  116  between the electron source region  102  and the glow discharge region  104  is sized to create a restriction in the flow of gas from the glow discharge region  104  to the electron source region  102  such that the desired pressures may be maintained in each region without blocking the electron beam  113 . By way of example, entrance aperture  116  may be approximately 100 μm to 1000 μm in diameter. Differential pumping or a blocked flow condition in the entrance aperture  116  may be used to maintain the appropriate pressures in the electron source region  102  and the glow discharge region  104 . 
     The glow discharge region  104  is a chamber  105  containing an inductively coupled plasma  120 . The glow discharge region  104  chamber is filled with a noble gas, e.g., a heavy noble gas such as Xenon, which is maintained at a low pressure. A desired pressure, e.g., 10 −3  to 10 −2  Torr, is controlled by regulating a low flow of gas into the chamber  105  via aperture  121  and pumping the gas out of the chamber at outlet  123 . The flow of gas through the chamber is indicated by arrows  122  and  124 . 
     Chamber  105  includes quartz walls  127  that are surrounded by a helical coil  126 . High frequency power, e.g., radio frequency (RF)—MHz to GHz, is applied to helical coil  126  via plasma power supply  128  and a matching network system. The uniformity of the electric field inside the coil  126  will produce a radially uniform inductively coupled plasma  120  and, consequently, a uniform electron density in areas removed from walls  127 . 
     Electron beam  113  enters chamber  120  by way of entrance aperture  116  and passes through the plasma  120 . Plasma  120  acts as a conductor. Thus, when electrons within electron beam  113  pass through plasma  120 , the electrons will be uniformly distributed. In addition, because plasma  120  is a good conductor, once the electrons enter plasma  120 , the electrons will pick up and lose energy thereby greatly reducing the energy distribution in electron beam  113 . Thus, the plasma  120  acts as an electron diffuser. 
     In a conventional system, such as that described in U.S. Pat. No. 4,684,848, the plasma itself is used directly or indirectly to generate the electrons. Consequently, the maximum current that may be delivered from a conventional glow discharge based electron source is limited by the characteristics of the discharge. Multipixel electron emission device  100  advantageously separates the electron source region  102  from the glow discharge region  104 . Thus, the maximum current of the electron emission device  100  is independent of the characteristics of plasma  120 , and is determined by the maximum electron current of electron source  112 . The inductively coupled plasma  120  is used to diffuse the electrons to reduce the energy range of the electrons and to uniformly distribute the electrons over a large area. Thus, electron source  112 , which acts as a point source with a poor energy distribution, is converted to a broad area electron source with superior energy distribution. 
     Once the electrons from electron beam  113  are diffused in plasma  120 , the electrons are pulled out by an aperture grid  150 . Aperture grid  150  also separates the glow discharge region  104  from the focusing region  106 . Aperture grid  150  is an array of closely spaced, small apertures that is used to pull the electrons out of plasma  120 . Aperture grid  150  can be used to accelerate, decelerate or blank the electrons exiting the apertures by altering the bias of the aperture grid  150 . A blocked flow condition is maintained in each aperture in the aperture grid  150  to maintain the desired pressures in the glow discharge region  104  and the focusing region  106 . Each aperture in aperture grid  150  may be, e.g., approximately 10 to 100 μm in diameter and there may be many apertures in aperture grid  150 . In one embodiment, there may 1000 apertures or more per square inch. The size and number of apertures is dependent on the trajectory of the electrons as they are pulled out of plasma  120 . The operation of aperture grid  150  along with an embodiment of the manufacture of aperture grid  150  is described in further detail below. 
     Focusing region  106  contains the remainder of the optical system, such as a conventional multi-beam optics system using beam accelerators and deflection devices. The focusing region  106  is kept under a high vacuum by pumping aperture  144 , as indicated by arrow  145 . Any conventional accelerating and imaging optics capable of focusing multiple pixel electron beams may be used. 
     FIG. 2 shows a block diagram view of the electron emission device  100  in accordance with the present invention. As shown in FIG. 2, the electron source  112  is coupled to a cathode supply  152  and an anode supply  154 , both of which may be a current or voltage supply. As discussed above, electron source  112  may be a conventional electron source with a filament  112   a  that is coupled to the cathode supply  152 , an extraction electrode  112   b  coupled to ground, and an anode electrode  112   c  coupled to the anode supply  154 . The glow discharge region  104  chamber  105  includes a top plate  156  and the aperture grid  150  that includes a base electrode  158 . The top plate  156  and the base electrode  158  are coupled to a negative and positive terminal, respectively, of a bias supply  160 , which supplies a current or voltage. As shown in FIG. 2, in one embodiment, the positive terminal of the anode supply  154  and the negative terminal of the bias supply  160  are coupled together to create a field free region between the anode electrode  112   c  and the top plate  156 . 
     FIG. 2 shows helical coil  126  around chamber  105 . Helical coil  126  is coupled to plasma supply  128 , which supplies radio frequency (RF) power via conventional matching network  129 . 
     The bias supply  160  is also coupled to the aperture grid  150  through a high frequency beamlet switch system  162 , which controls the individual switching devices of each aperture in aperture grid  150  to open and close each aperture as an individual or as a group of apertures. The opening and closing of each aperture may be controlled, for example, by a microprocessor, digital signal processor or other appropriate device. 
     If desired, electron emission device  100  may also include a biased screen (not shown) above aperture grid  150 . A biased screen may filter out plasma ions before contacting aperture grid  150  while permitting electrons to pass through. 
     In accordance with an embodiment of the invention, the aperture grid  150  can be used to accelerate, decelerate or blank the electrons exiting the apertures by altering the bias of the aperture grid  150  and by operating individual blanking devices contained in the aperture grid  150  structure. FIG. 3 shows a top view of aperture grid  150  in accordance with an embodiment of the present invention. Aperture grid  150  includes a floating shield  170  and a base electrode  172  separated by insulation  174 . Arranged on base electrode  172  is an array of blanking devices  176  that include blanker electrodes  178  having apertures  180  though which individual electron beamlets may travel. The blanker electrode  178  surrounds the aperture  180  and is insulated from the base electrode  172  by a dielectric layer. The base electrode  172  can be biased with different voltages and each blanker electrode  178  can be individually connected through a switching device to the base electrode  172 . 
     FIG. 4 shows a cross-sectional view of a blanking device  176  on aperture grid  150  in accordance with an embodiment of the present invention. Blanking device  176  enables the rapid and independent blanking of exit aperture  180 , based on the self-bias generated by the incoming electrons. For a plasma source, the blanking device  176  utilizes the difference in mobility between ions and electrons to achieve the potentials needed to stop the electron flow. These potentials are modulated to achieve control over the electron flow through each aperture of the blanking apparatus, thereby allowing individual control over each beamlet. The blanking device  176  may use low voltage signals to open and close apertures. Thus, blanking device  176  allows high frequency operation without the need for elaborate electronic circuitry. The opening or closing of aperture  180  is achieved by grounding (or increasing the potential) and floating blanker electrode  178 , respectively. Advantageously, the potential to drive the blanking of an aperture in aperture grid  150  is independent of the electron energy and is determined only by a switching device used to operate the blanker. 
     As shown in FIG. 4, blanking device  176  includes base electrode  172  that is coupled to a voltage source  182 , which may be, for example, bias supply  160  shown in FIG.  2 . Voltage source  182  either grounds or positively biases base electrode  172  to prevent sputtering and to increase electron potential so as to pull electrons (illustrated by arrows  184 ) out of plasma  120  (shown in FIG.  1 ). The actual blanking action of blanking device  176  is performed by blanker electrode  178 . It should be understood that each aperture in aperture grid  150  includes a separate and independent blanker electrode  178  but shares the same base electrode  172 . The blanker electrode  178  is insulated from base electrode  172  by a dielectric  186  but is switchably connected to base electrode  172  by a switch  188 . 
     As illustrated in FIG. 4, blanker electrode  178  overlaps the base electrode  172  with a surface area (illustrated by dimension a), which is a fraction of the total surface area of blanker electrode  178  (illustrated by dimension A). In addition, blanker electrode  178  is separated from base electrode  172  via dielectric  186  by a distance D. The dimensions a, A, and D are parameters that are governed by the speed requirements of the blanker device  176 . Dielectric  186  and blanker electrode  178  surround aperture  180 . 
     In the “off” state, the blanker electrode  178  is floating, i.e., is not coupled to base electrode  172 . In a floating state, the self bias of the electron source will charge the blanker electrode  178 , e.g., by electrons contacting the blanker electrode  178 , with a negative potential. Once the blanker electrode  178  is fully charged, the negative potential will prevent electrons from reaching the blanker electrode  178 . The presence of the negative potential on blanker electrode  178 , which surrounds aperture  180 , will pinch off the electron flow through aperture  180 . Consequently, aperture  180  is blanked. 
     In an “on” state, the blanker electrode  178  is electrically coupled to the base electrode  172  by closing switch  188 , which will drain the accumulated charge from blanker electrode  178 . Thus, the blanker electrode  178  is discharged until it has the same potential as the base electrode  172 . Consequently, the pinching field around aperture  180  is removed thereby permitting electrons to pass through aperture  180 . 
     The dimensions (a and A) of blanker electrode  178  may be altered to optimize the characteristic charging and discharging times, i.e., the times to turn “off” and “on”. Increasing the total area (A) of blanker electrode  178  will expose a larger area to plasma  120  thereby decreasing the time to charge blanker electrode  178 . Increasing the thickness (D) of dielectric  158  and decreasing the non-overlap area (a) will decrease the capacitance of blanker electrode  178 . Consequently, the time to reach the “pinch-off” potential will be decreased. The unblanking time is related to the amount of charge on blanker electrode  178  because the “pinch-off” potential must be removed from blanker electrode  178  to unblank aperture  180 . Thus, the unblanking time is decreased by storing only a small amount of charge on blanker electrode  178 . 
     FIGS. 5A and 5B show a qualitative depiction of the equipotential lines in the blanked and unblanked conditions, respectively, of a blanking device  176 . As shown in FIG. 5A, in the blanked condition, the blanker electrode  178  is insulated from the base electrode  172  by insulator  186 , which permits negative charge to accumulate on blanker electrode  178 , i.e., blanker electrode  178  has a voltage V=−v while base electrode has a voltage V=+v. Consequently, a negative field is created around blanker electrode  178 . Because electrons are negatively charged, electrons will not pass through the negative field that is created and, thus, aperture  180  is blanked. 
     As shown in FIG. 5B, when blanking device  176  is unblanked, by electrically coupling blanker electrode  178  to base electrode  172 , both blanker electrode  178  and base electrode  172  will have the same voltage V=+v. Thus, electrons will be able to pass through the positive electric field and pass through aperture  180 . 
     The switch  188  that couples blanker electrode  178  to base electrode  172  may be integrated in accordance with an embodiment of the present invention. FIG. 6 shows a cross sectional view of one embodiment of an integrated blanking and switching device  200 . Device  200  is manufactured from a silicon substrate that is overlaid with thin films and patterned to produce the desired switch using conventional thin film deposition and patterning techniques. As shown in FIG. 6, device  200  includes a conductive substrate  202  with an inert conductor  203  (in an embodiment of a non-floating top electrode) and a blanker electrode  204  that surrounds an aperture  206  and that is insulated from the conductive substrate  202  by an insulator  208 . The more sensitive components, e.g., the transistor  210  and the switch line  212 , are embedded on the back side of the device so as to avoid exposure to the glow discharge. In another embodiment, inert conductor  203  may be replaced with a dielectric material, e.g., insulator  208  may extend over the entire top surface of substrate  202 , to provide a floating top electrode. This embodiment may be particularly useful where blanking device  176  is used in an electron beam source that does not use plasma or where the plasma ions are prevented from contacting blanking device  176 , e.g., where a biased screen separates plasma ions from an aperture grid  150  that includes blanking devices  176 . 
     FIGS. 7-13 show cross-sectional views of an integrated blanking and switching device in various states of fabrication in accordance with an embodiment of the present invention. The manufacturing of the device may be based on conventional pnp-type transistor fabrication. 
     As shown in FIG. 7, a double sided polished n-doped silicon substrate  202  is provided. As shown in FIG. 8, one side  214  of the substrate  202  is coated with a metal or metal compound layer  203  with low sputtering yield to minimize deterioration that will result from exposure to a plasma because side  214  will later be the side that is exposed to the glow discharge. Layer  203  should be resistant to oxidation, and may be, for example, Titanium Nitride (TiN) that is deposited using plasma vapor deposition (PVD). Of course, other suitable materials may be used if desired. 
     A pnp transistor  210  is produced on the other side  215  of silicon substrate  202  as shown in FIG.  9 . Transistor  210  may be conventionally fabricated, as will be well understood by those of ordinary skill in the art, with an emitter  218 , a base  219  and a collector  220  that should be large enough to extend around or at least contact the aperture that will be formed. For example, in one embodiment collector  220  may not completely surround the aperture but merely abut the aperture in a limited area. 
     The aperture pattern is etched in the TiN layer  203  so that it lines up with the extended collector  220  of the transistor  210  as shown in FIG.  10 . The aperture  206  is then produced in silicon substrate  202  by a deep etch process. The thickness of the aperture  206  may be adjusted by etching silicon substrate  202  around aperture  206  using TiN layer  203  as an etch mask. Thus, for example, a silicon layer  222  may be etched away to decrease the thickness of aperture  206 . As shown in FIG. 10, collector  220  surrounds aperture  206 . 
     An oxide layer  224 , which serves as insulator  208 , shown in FIG. 6, is then deposited using, e.g., a wet silicon oxidation process, to a thickness that meets the insulation and capacitance requirements. The oxide layer  224  should be formed such that it covers the sidewalls of aperture  206 . Oxide layer  224  does not form over layer  203 , because layer  203  is formed from a material, such as TiN, that is resistant to oxidation. The resulting structure is shown in FIG.  11 . As described above, if a floating top electrode is desired, oxide layer  224  may be used to completely cover side  114  rather than layer  203 . 
     As shown in FIG. 12, the openings for the contacts for transistor  210  are then created, for example, using a dry etching technique so oxide layer  224  on the sidewalls of aperture  206  is maintained. 
     Both sides  214  and  215  of substrate  202  are then covered with a conductive layer  226  that has a low sputtering yield to avoid future deterioration caused by exposure to a plasma. Layer  226 , for example, may be a Tungsten layer which is deposited using chemical vapor deposition. The conductive layer  226  forms the blanker electrode  204  (shown in FIG.  6 ). Conductive layer  226  may cover the sidewalls of aperture  206 . 
     The conductive layer  226  is then patterned and etched to separate the base, emitter and collector regions of transistor  210  and to form switch line  212  resulting in the structure shown in FIG.  6 . The emitter  218  of transistor  210  is coupled to the conductive substrate  202  via a lead  221  formed from conductive layer  226 . The collector  220  is coupled to blanker electrode  204 . The base  219  contacts switch line  212  which extends to the side of the substrate to allow external control of transistor  210 , e.g., via beamlet switch system  162  (shown in FIG.  2 ). As shown in FIG. 6, conductive layer  226  is also patterned to form blanker electrode  204  which is insulated from the inert conductor  203 . 
     It should be understood that FIG. 6 shows aperture  206  having the same thickness as substrate  202 , but if desired, silicon layer  222 , shown in FIG. 10, may be etched away to decrease the thickness of aperture  206 . 
     Thus, in accordance with an embodiment of the present invention, aperture grid  150  has a Titanium-Nitride surface, which is exposed to the glow discharge  120 , with small apertures which are surrounded by a Tungsten coating. The non-exposed side of aperture grid  150  includes the more sensitive components of the device, include the switching device, i.e., transistor  210  and conductive switching lines. In addition, the aperture grid  150  may also include, for example, on the non-exposed side, embedded logic circuits which may control the aperture grid. 
     It should be understood that aperture grid  150  is one example of an aperture grid that may be used with electron source device  100 . If desired, other aperture grids, including blanking aperture arrays, may be used. Moreover, the use of aperture grid  150  need not be limited only to electron source device  100 . Aperture grid  150  may be used with any appropriate charged particle device, for example, a device that includes an electron emitting material, a glow discharge or plasma that can self-bias the blanker electrode  178 . 
     While the present invention has been described in connection with specific embodiments, one of ordinary skill in the art will recognize that various substitutions, modifications and combinations of the embodiments may be made after having reviewed the present disclosure. The specific embodiments described above are illustrative only. Various adaptations and modifications may be made without departing from the scope of the invention. Thus, the spirit and scope of the appended claims should not be limited to the foregoing description.