Patent Publication Number: US-2007114434-A1

Title: Multi-pixel electron microbeam irradiator systems and methods for selectively irradiating predetermined locations

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
      This non-provisional patent application claims the benefit of U.S. Provisional Application No. 60/639,958, filed Dec. 29, 2004, the disclosure of which is incorporated by reference herein in its entirety. 
    
    
     TECHNICAL FIELD  
      The subject matter disclosed herein relates to electron irradiation devices and methods using electron field emission technology. More particularly, the subject matter disclosed herein relates to multi-pixel cell irradiator systems and methods for selectively irradiating cells at predetermined locations.  
     BACKGROUND ART  
      Microbeam irradiators can be utilized for studying the microscopic biological effects of radiation in the cellular and sub-cellular level. Such devices can be utilized to bombard one or more cells with charged particles or photons for predetermined radiation dose and dose rate. An understanding of radiobiology at the cellular and sub-cellular level is important for improvements of cancer treatment and for an understanding of low dose radiation risk. Cellular micro-radiation is recognized as a powerful technique for this endeavor. One of the most active areas of research using a microbeam irradiator is the “bystander effect”, which involves the response of “unhit” cells to the radiation deposited in their neighboring cells. “Bystander effect” studies using microbeam devices have revealed the complicated intra-cellular and inter-cellular response to radiation that may have significant impact on the policy making of radiation protection for general public and for the space program.  
      Cell irradiation can include exposing cells to alpha particles, electrons, or X-rays. Recently, there has been interest in low-linear energy transfer (LET) microbeams using electrons and ultrasoft X-rays. Thermionic emission and field emission are two mechanisms for generating electrons. Although field emission is a sometimes preferred mechanism to extract electrons, many currently available vacuum electronic devices utilize thermionic electron sources. The wide use of thermionic electron sources may be partly due to the lack of effective electron field emitters. Recent studies have shown that carbon nanotubes have promising electron emission properties with high emission current, low turn-on field, and lifetime that can be required for device applications. A typical field emission device can include a cathode having a plurality of electron field emitters (e.g., carbon nanotubes) and an anode spaced from the cathode. A voltage applied between the anode and the cathode can induce the emission of electrons from the electron field emitters towards the anode.  
      A few large research institutions in the world are capable of delivering microbeam irradiation at the cellular level. These microbeam devices require major resources to develop and maintain. Further, in currently available microbeam irradiators, there is only a single microbeam port. One exemplary single port electron microbeam having a thermionic electron source includes an electron gun consisting of a heated filament and electron optics for accelerating and collimating a broad electron beam. The electron optics includes a suppression aperture that allows pulsing of the beam with sub-microsecond time resolution. The final spatial resolution of an electron microbeam can be achieved either by focusing or collimation. The electron microbeam can be used to target individual cells without significant scattering to neighboring cells. Target cells or sub-cellular regions designated for irradiation in a population must be irradiated one at a time by physically aligning the microbeam with each of the target cells. Some research experiments require the irradiation of a large number of individually selected cells, as many as 10,000, to obtain statistically significant results. This requirement makes it difficult to utilize single port microbeam devices for experiments requiring irradiation of a large number of cells and, especially, if real-time observation is also required.  
      Most cell irradiations in research labs today use irradiators that cannot deliver radiation beams much smaller than a centimeter. Therefore, researchers often have to deduce conclusions about microscale activities of a cell from macroscopic studies of irradiating a group of cells. The lack of microscale radiation manipulation ability can be especially detrimental when studying a minority cell type in a large cell population because of the low signal to noise ratio in the experimental data.  
      Accordingly, in light of desired improvements associated with microbeam cellular irradiators, there exists a need for improved microbeam irradiator functionality and availability and related methods.  
     SUMMARY  
      In accordance with this disclosure, novel multi-pixel electron microbeam irradiator systems and methods for selectively irradiating predetermined locations are provided.  
      It is an object of the present disclosure therefore to provide novel microbeam irradiator systems and methods for selectively irradiating predetermined locations. This and other objects as may become apparent from the present disclosure are achieved, at least in whole or in part, by the subject matter described herein. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Preferred embodiments of the subject matter described herein will now be described with reference to the accompanying drawings, of which:  
       FIG. 1A  is a schematic, cross-sectional side view of a multi-pixel electron microbeam irradiator system and a Petri dish in a cell irradiation setup according to one embodiment of the subject matter described herein;  
       FIG. 1B  is a schematic top view of predetermined locations that can be individually irradiated by the electron field emitters of the multi-pixel electron microbeam irradiator system shown in  FIG. 1A  in accordance with one embodiment of the subject matter described herein;  
       FIG. 2  is a schematic, cross-sectional side view of another multi-pixel electron microbeam irradiator system including a plurality of individually addressable electron field emitters for selectively irradiating predetermined locations according to one embodiment of the subject matter described herein;  
       FIGS. 3A-3F  are steps of a method for fabricating multiple electron field emitters, a gate electrode, and a focusing electrode according to one embodiment of the subject matter described herein;  
       FIGS. 4A-4E  are steps of a method for fabricating a gate electrode according to one embodiment of the subject matter described herein;  
       FIGS. 5A-5D  are steps of a method for fabricating an anode including a plurality of electron transparent windows according to one embodiment of the subject matter described herein;  
       FIG. 6  is an optical image of an individually addressable electron field emitter structure; and  
       FIG. 7  is an optical image of a top view of three electron field emitters after carbon nanotube deposition and removal of a photoresist. 
    
    
     DETAILED DESCRIPTION  
      In accordance with the present disclosure, multi-pixel, electron microbeam irradiator systems and methods are provided. The systems and methods described herein can have particular application for use in selectively irradiating predetermined locations such as cells or cellular locations as described herein. An electron microbeam irradiator system according to the present disclosure can include a plurality of individually addressable electron field emitters sealed in a vacuum. Further, an electron microbeam irradiator system can include an anode comprising one or more electron permeable portions corresponding to the plurality of electron field emitters. A controller can be operable to individually control electron extraction from each of the electron field emitters for selectively irradiating predetermined locations. The electron emitters can be individually turned on and off for individually irradiating the predetermined locations for a predetermined time duration and radiation dosage using a predetermined sequence.  
      The electron field emitters can be any suitable conductive structure and can have a sharp tip or protrusion for electron emission under an electrical field. Exemplary electron field emitters can include “Spindt” tips and other suitable nanostructures. “Spindt” tips and related processes are described in the publication “Vacuum Microelectronics,” I. Brodie and C. A. Spindt,  Advances in Electronics and Electron Physics,  83: 1-106 (1992), the disclosure of which is incorporated by reference herein. Exemplary materials of electron field emitter tips can include molybdenum (Mo), silicon (Si), diamond (e.g., defective CVD diamond, amorphic diamond, cesium-coated diamond, a nano-diamond), and graphite powders.  
      Nanostructures suitable for electron emission can include nanotube and nanowires/nanorods composed of either single or multiple elements, such as carbon nanotubes. A single carbon nanotube can have a diameter in the range of about 0.5-500 nm and a length on the order of about 0.1-100 microns.  
      Carbon nanotubes readily emit large fluxes of electrons with small angular divergence. A carbon nanotube can include a single graphene shell, which is termed a single-wall carbon nanotube, or multiple concentric graphene shells, which is termed a multi-wall carbon nanotube. Carbon nanotubes, nanowires and nanorods can be fabricated by techniques such as laser ablation, arc discharge, and chemical vapor deposition (CVD) methods. Further, carbon nanotubes can be made via solution or electrochemical synthesis. An exemplary process for fabricating carbon nanotubes is described in the publication “Materials Science of Carbon Nanotubes: Fabrication, Integration, and Properties of Macroscopic Structures of Carbon Nanotubes,” Zhou et al.,  Acc. Chem. Res.,  35: 1045-1053 (2002), the disclosure of which is incorporated herein by reference. A single carbon nanotube or a nanotube bundle can produce a current of about 0.1-10 μA.  
      Table 1 below summarizes the threshold field required to obtain a current density of 10 mA/cm 2  for several electron field emitter/cathode materials.  
               TABLE 1                          Emission Threshold Field of Different Field Emitter/Cathode Material                             Electron Field Emitter/Cathode   Threshold Field (V/μm) for           Material   10 mA/cm 2                         Mo Tips   50-100           Si Tips   50-100           p-type Diamond   160           Defective CVD Diamond   30-120           Amorphic Diamond   20-40            Cesium-coated Diamond   20-30            Graphite Powders   10-20            Nano-Diamond   3-5 (unstable &gt;30 mA/cm 2 )           Assorted Carbon Nanotubes   1-2 (stable &gt;4000 mA/cm 2 )                      
 
      In one embodiment, a multi-pixel electron microbeam irradiator system in accordance with the subject matter disclosed herein can include electron field emitters for irradiation of an area, such as, for example, 10 3 -10 6  (per square centimeter of area) individually selected cells or sub-cellular regions in a Petri dish. This radiation can be simultaneous, sequential, or of any predetermined temporal pattern. The radiation to each selected region can be controlled (i.e., individually turned on or off). The radiation can be controlled with high spatial (micrometer scale) and/or temporal (microsecond scale) resolution. Further, the radiation dose rate can be controlled and it can produce low to ultra-high dose rate (about 10 3  Gy/sec 3 ). The physical size of a multi-pixel electron microbeam irradiator system according to the present disclosure can be small and portable. It is envisioned that the device can be placed directly under an optical microscope for in-situ observation during the irradiation process.  
       FIGS. 1A and 1B  illustrate views of a multi-pixel electron microbeam irradiator system having a plurality of locations that can be selectively irradiated by the multi-pixel electron microbeam irradiator system according to one embodiment of the subject matter described herein.  FIG. 1A  is a schematic, cross-sectional side view of a multi-pixel electron microbeam irradiator system generally designated  100  and a Petri dish PD in a cellular irradiation setup. System  100  can include a plurality of individually addressable and controllable electron field emitters FE (also referred to herein as “pixels”) for selectively irradiating predetermined locations L in Petri dish PD.  FIG. 1B  is a schematic top view showing predetermined locations L that can be individually and selectively irradiated by electron field emitters FE of system  100 . Referring now to  FIG. 1A , each electron field emitter FE can comprise one or more carbon nanotubes and/or other suitable electron field emission materials. Electron field emitters FE can be attached to a surface of a cathode, conductive or contact line, or other suitable conductive material. The cathodes can be attached to a suitable non-conductive substrate such that the electron field emitters are electrically isolated.  
      Electron field emitters FE can be individually controlled (i.e., turned on and off) to emit electrons for selectively irradiating predetermined locations L (shown in  FIG. 1B ). In one embodiment, a controller CTR can control a voltage source VS to individually apply voltages between each electron field emitter FE and a gate electrode (not shown) to generate electric fields for extracting electrons from electron field emitters FE. Controller CTR can include hardware, software, and/or firmware, such as memory (e.g., RAM, ROM, and computer-readable disks), transistors, capacitors, resistors, inductors, logic circuitry, and other components suitable for individually controlling electron emission from electron field emitters FE. Controller CTR can also control the intensity, timing, and duration of electron emission for each of the electron field emitters FE.  
      Controller CTR can execute instructions for performing a sequence by which locations L are irradiated with electron beams generated by electron field emitter FE. The executable instructions can be implemented as a computer program product embodied in a computer readable medium. Exemplary computer readable media can include disk memory devices, chip memory devices, application specific integrated circuits, programmable logic devices, downloadable electrical signals, and/or any other suitable computer readable media.  
      Electron field emitters FE can be oriented such that extracted electrons are directed towards respective predetermined locations L. Further, a voltage can be applied between the gate electrode and an anode A for accelerating the extracted electrodes towards anode A. The voltage for accelerating the extracted electrodes can be between about 20-60 kV and adjusted such that electrons reach a desired energy for delivering radiation to predetermined locations L. The energy of a beam of electrons at anode A can be about 10 KV or greater. System  100  can also include a collimator and/or focusing electrode for collimating and focusing electrons extracted from electron field emitters FE and thus reducing the size of electron microbeams EM.  
      A vacuum chamber VC can include a sealed interior for containing electron field emitters FE and the gate electrode. The interior of vacuum chamber VC can be evacuated to achieve a desired interior pressure. An exemplary interior pressure of vacuum chamber VC can be about 10 −7  Torr. Predetermined locations L can be positioned on an exterior of vacuum chamber VC. Extracted electrons can travel from the interior of vacuum chamber VC to its exterior through electron permeable portions, such as one or more thin Si 4 N 3  or polymer exit windows. The electron permeable portions can be made of any suitable material having sufficient thickness for permitting electrons to pass and having sufficient robustness for withstanding a pressure difference between the interior and exterior of vacuum chamber VC.  
      In one embodiment, Petri dish PD can be positioned to receive electron microbeams EM emitted from microbeam irradiator system  100 . Petri dish PD can include a bottom component comprising a polyester film such as a Mylar® brand film F for allowing minimal attenuation of electron microbeams EM before the beams reach locations L. Referring now to  FIG. 1B , Petri dish PD can include a monolayer of cells C that can be selectively irradiated by system  100  (shown in  FIG. 1A ). In one embodiment, a reflective microscope (not shown) can be used to image the cell population and identify the target cells or regions and their locations, as well as for real-time observation of cells C during and after radiation. Using available cellular imaging and image analysis packages, an operator can quickly identify target cells TC for irradiation. Automated cellular microscope image analysis software as utilized are understood by those of skill in the art.  
      In one exemplary process for irradiating target cells TC using microbeam irradiator system  100 , all of electron field emitters FE can be turned on for identifying all positions P that can be irradiated by system  100 . The position identification process can occur without Petri dish PD in position to receive radiation. Next, while all of electron field emitters FE are turned on, an image capture device having control and imaging equipment suitable for capturing microscopic images and relative positions can be used to capture an image of the positions irradiated by the emitted electrons. Next, the imaging equipment can determine the relative coordinates of the positions. When system  100  is placed in a position for irradiating cells C in dish PD, two or more “calibration” electron field emitters FE can be turned on and imaged in the same coordinate system where target cells TC are identified.  
      Subsequent to a determination of the coordinates of the “calibration” electron field emitters FE, the coordinates of other “non-calibration” electron field emitters FE can be determined. Next, electron field emitters FE can be selected that correspond to locations of respective target cells TC. The electron field emitters FE corresponding to respective target cells TC can be activated and controlled to deliver a predetermined radiation dosage to target cells TC. Controller CTR can be programmed to control the intensity, timing, and duration of electron emission by electron field emitters FE to target cells TC. Further, one or more images of the cells in dish PD can be captured during and subsequent to exposure for obtaining information for radiation biology research.  
       FIG. 2  illustrates a schematic, cross-sectional side view of a multi-pixel electron microbeam irradiator system  200  including a plurality of individually addressable electron field emitters FE for selectively irradiating predetermined locations L according to one embodiment of the subject matter described herein. Referring to  FIG. 2 , system  200  can include a gate electrode GE operable to extract electrons from field emitters FE on generation of an electric field between gate electrode GE and field emitters FE. After extraction from field emitters FE, the electrons can be formed as electron microbeams and directed by generated electrical fields to travel along pathways P. Field emitters FE may be spaced apart in an array or grid such that different locations may be irradiated by the emitted electrons. A controller CTR may be operable to individually control electron emission from each field emitter FE for selectively irradiating locations L along pathways P.  
      In one embodiment, controller CTR can individually operate a plurality of metal-oxide-semiconductor field-effect transistors (MOSFETs) T for individually addressing field emitters FE to emit electrons. In one embodiment, controller CTR can be a digital I/O board. Controller CTR can include a field emitter addressing function AF that can individually switch on and off transistors T. The drains of field emitters FE can be connected to a corresponding one of a plurality cathodes C. Each cathode C can be connected to a respective field emitter FE via a resistor such as a 100 kilo ohm protection resistor R. Function AF can control power supply PS to individually turn on and off power to the sources of transistors T for individually turning transistors T on and off. Transistors T can be turned on and off by the individual application of a high signal (e.g., 5 V) and a low signal (e.g., 0 V), respectively, to the gates of the transistors T. When a high signal is applied to the gate of a transistor, a drain-to-source channel of the transistor is turned on to apply a voltage difference between a respective cathode C and gate electrode GE. A voltage difference exceeding a threshold can generate an electric field between cathode C and gate electrode GE such that electrons are extracted from respective electron field emitters FE. Conversely, when a low voltage (e.g. 0 V) is applied to the gate of a transistor, a corresponding drain-to-source channel is turned off such that the voltage at electron field emitter FE is electrically floating and the voltage difference between a respective cathode C and gate electrode GE cannot generate an electric field of sufficient strength to extract electrons from the respective electron field emitter. Function F is operable to individually control the voltages applied to the gates of transistors T. Thus, function F can individually address and control the extraction of electrons from field emitters FE.  
      Cathodes C can be attached to a substrate S in an array or grid-like spacing. Substrate S can be made of silicon or any other suitable non-conductive substrate material for electrically isolating cathodes C. A spacer S 1  can be disposed between substrate S and gate electrode G for suitably spacing electron field emitters FE and gate electrode G. Spacer S 1  can be made of an insulation material for electrically insulating substrate S and gate electrode GE.  
      A vacuum chamber VC can include a sealed and evacuated interior between anode A and electron field emitters FE. The interior of vacuum chamber VC can have a pressure differential with its exterior.  
      Further, microbeam array system  200  can include an anode A having a plurality of electron permeable portions EPP through which emitted electrons can pass. Each electron permeable portion EPP can be positioned for passing electrons from a respective electron field emitter FE. Further, electron permeable portions EPP can be made of silicon nitride (Si 3 N 4 ) or any other suitable material that is permeable to electrons. Silicon nitride has a high permeability to electrons and is mechanically robust for supporting a pressure differential of the interior and exterior of vacuum chamber VC, which can be one or more atmospheres.  
      An voltage difference can be applied between anode A and gate electrode GE such that respective fields are generated for accelerating elections emitted by respective electron field emitters FE towards respective electron permeable portions EPP. In one embodiment, anode A and electron permeable portions EPP can be electrically connected to a ground G. Further, the energy of the electrons can be adjusted by adjusting the electrical field applied between gate electrode GE and cathode C. The voltage between gate electrode GE and cathode C can be adjusted to change the electrical field generated therebetween.  
      Microbeam array system  200  can include a plurality of focusing electrodes FEL for at least partially focusing electrons emitted by respective electron field emitters FE. The electrons can be focused in a direction substantially towards respective locations L. A focusing electron voltage (V f ) can be applied to focusing electrode FEL by power supply PS. Focusing electrode FEL can be made of any suitable conductive material, such as Al, Fe, Cu, and Mo. Focusing electrode FEL can be spaced from gate electrode GE by a spacer S 2  made of a suitable material for electrically isolating focusing electrode FEL and gate electrode GE.  
      A plurality of collimators CL can be positioned between respective electron permeable portions EPP and electron field emitters FE. Collimators CL can collimate and tune the beam profile of the emitted electrons before the electrons pass through electron permeable portions EPP. Collimators CL can be electrically isolated from electron permeable portions EPP and anode A by a spacer S 3  made of insulation material. Further, collimators CL can be spaced from focusing electrode FEL by a spacer S 4  made of insulation material.  
      Collimators CL, focusing electrodes FEL, and gate electrodes GE can be each formed of a single layer of conductive material. For example, collimators CL can be formed of a single layer of metal. Further, collimators CL, focusing electrodes FEL, and gate electrodes GE can each include apertures for providing a pathway for emitted electrons to travel from respective electron field emitters FE.  
      Gate electrodes, focusing electrodes, collimators, anodes having electron permeable portions, spacers and other components of the multi-pixel electron microbeam irradiator systems described herein can be fabricated by either bulk or surface micromachining techniques. Bulk micromachining generally involves sculpting one or more sides of a substrate to form desired three dimensional structures in the same substrate material. The substrate can be made of a material that is readily available in bulk form, such as silicon or glass. Wet and/or dry etching techniques can be employed in association with etch masks and etch stops to form microstructures and apertures within the material. Etching can be performed through the backside of the substrate. The etching technique can be either isotropic or anisotropic in nature. Etch masks and etch stops are used to prevent predetermined regions of the substrate from being etched.  
      Conventional lithographic techniques can be employed in accordance with micromachining of the gate electrodes, focusing electrodes, collimators, anodes having electron permeable portions, spacers and other components of the multi-pixel electron microbeam irradiator systems described herein. Accordingly, basic lithographic process steps such as photoresist application, optical exposure, and the use of developers are not described in detail herein.  
      Similarly, generally known-etching processes can be employed to selectively remove material or regions of material. An imaged photoresist layer is ordinarily used as a masking template. A pattern can be etched directly into the bulk of a substrate, or into a thin film or layer that is then used as a mask for subsequent etching steps.  
      The type of etching process employed in a particular fabrication step (e.g., wet, dry, isotropic, anisotropic, anisotropic-orientation dependent), the etch rate, and the type of etchant used will depend on the composition of material to be removed, the composition of any masking or etch-stop layer to be used, and the profile of the etched region to be formed. As examples, poly-etch (HF:HNO 3 :CH 3 COOH) can generally be used for isotropic wet etching. Hydroxides of alkali metals (e.g., KOH), simple ammonium hydroxide (NH 4 OH), quaternary (tetramethl) ammonium hydroxide ((CH 3 ) 4  NOH, also known commercially as TMAH), and ethylenediamine mixed with pyrochatechol in water (EDP) can be used for anisotropic wet etching to fabricate V-shaped or tapered grooves, trenches or cavities. Silicon nitride is typically used as the masking material against etching by KOH, and thus can be used in conjunction with the selective etching of silicon. Silicon dioxide is slowly etched by KOH, and thus can be used as a masking layer if the etch time is short. While KOH will etch undoped silicon, heavily doped (p++) silicon can be used as an etch-stop against KOH as well as the alkaline etchants and EDP. A metal that can be used to form contacts and interconnects is gold, which is resistant to EDP. The adhesion layer applied in connection with forming a gold component (e.g., chromium) is also resistant to EDP.  
      It will be appreciated that electrochemical etching in hydroxide solution can be performed instead of timed wet etching. For example, if a p-type silicon wafer is used as a substrate, an etch-stop can be created by epitaxially growing an n-type silicon end layer to form a p-n junction diode. A voltage is applied between the n-type layer and an electrode disposed in the solution to reverse-bias the p-n junction. As a result, the bulk p-type silicon is etched through a mask down to the p-n junction, stopping at the n-type layer. Furthermore, photovoltaic and galvanic etch-stop techniques are also suitable.  
      Dry etching techniques such as plasma-phase etching and reactive ion etching (RIE) can also be used to remove silicon and its oxides and nitrides, as well as various metals. Deep reactive ion etching (DRIE) can be used to anisotropically etch deep, vertical trenches in bulk layers. Silicon dioxide can be used as an etch-stop against DRIE, and thus structures containing a buried silicon dioxide layer, such as silicon-on-insulator (SOI) wafers, can be used as starting substrates for the fabrication of microstructures.  
      An alternative patterning process to etching is the lift-off process. In this case, the conventional photolithography techniques are used for the negative image of the desired pattern. This process can be used to pattern metals, which are deposited as a continuous film or films when adhesion layers and diffusion barriers are needed. The metal can be deposited on the regions where it is to be patterned and on top of the photoresist mask (negative image). The photoresist and metal on top can be removed to leave behind the desired pattern of metal, such as the patterning of contact lines.  
      Suitable electron field emitters, such as carbon nanotubes, can be formed on conductive or semiconductive surfaces, such as contact lines and cathodes, described herein by electrophoretic deposition techniques and any other suitable techniques known to those of skill in the art, such as screen printing, chemical vapor deposition, and spraying. Generally, for example, carbon nanotubes can be electrophoretically deposited on a surface by a combination of some or all of the following steps: (1) forming a solution or suspension containing the carbon nanotubes; (2) selectively adding “chargers” to the solution; (3) immersing electrodes in the solution, with the surface upon which the carbon nanotubes are to be deposited acting as one of the electrodes; (4) applying a direct and/or alternating current for creating an electrical field between the electrodes for a predetermined period of time to thereby cause the carbon nanotubes in the solution to migrate toward and attach themselves to the conductive or semiconductive surface; and (5) optional subsequent processing of the coated surface.  
      The components of multi-pixel electron microbeam irradiator systems described herein can be assembled by a variety of methods. Generally, for example, electron field emitters, a gate electrode, focusing electrodes, collimators, an anode having electron permeable portions, spacers and other components of a multi-pixel electron microbeam irradiator system can be positioned together as described herein to form a vacuum chamber. The electron field emitters can be positioned within the interior of the chamber along with other components, such as a gate electrode and anode, for generating electric fields for extracting electrons from the electron field emitters and accelerating the electrons through electron permeable portions of the anode. The vacuum chamber can be subsequently sealed and evacuated to a predetermined minimum pressure, or back-filled with an inert atmosphere, for preparation of generating electron beams.  
      According to one embodiment, an array of electron field emitters, a gate electrode, and a focusing electrode can be fabricated on a substrate by combination of electrophoresis and photolithography processes. Referring to  FIGS. 3A-3F , a method for fabricating multiple electron field emitters, a gate electrode, and a focusing electrode according to one embodiment of the subject matter described herein is illustrated. Referring to  FIG. 3A , a substrate  300  can be provided, which can be a 3″ silicon wafer, glass, or other suitable substrate. A thermal oxide layer  302  can be disposed on a top surface of substrate  300 . Further, electrically-isolated conductive contact lines  304  can be disposed on a top surface of layer  302 . Contact lines  304  can be parallel electrode lines made of silver (Ag). Further, in one embodiment, contact lines  304  can be 150 μm in width and spaced by 100 μm.  
      Referring to  FIG. 3B , electron field emitters FE can be deposited on a top surface of lines  304  by a photolithography/electrophoresis process. In one embodiment, a release/photoresist layer  306  can be deposited on portions of layer  302  that are not to be covered by electron field emitters FE. Further, in one embodiment, electron field emitters FE can be carbon nanotubes. Next, Referring to  FIG. 3C , layer  306  can be removed without disturbing electron field emitters FE. An exemplary lithography and electrophoresis process for patterning carbon nanotubes is described in the publication “Liquid-Phase Fabrication of Patterned Carbon Nanotube Field Emission Cathodes,” Oh et al.,  Appl. Phys. Lett.,  87(19): 3738 (2004), the disclosure of which is incorporated herein by reference in its entirety.  
      Referring to  FIG. 3D , a dielectric insulation layer or spacer  308  comprising a plurality of apertures corresponding to electron field emitters FE can be disposed on the top surface of layer  302  between electron field emitters FE. Insulation layer  308  can be aligned using a mask aligner. In one embodiment, insulation layer  308  can be made of glass or any other suitable insulation material. Insulation layer  308  can be about 15 μm in thickness and patterned with an opening for exposing contact lines  304 .  
      Referring to  FIG. 3E , a gate electrode GE comprising a plurality of apertures corresponding to electron field emitters FE can be disposed on insulation layer  308 . Next, referring to  FIG. 3F , another insulation layer  310  and a focusing electrode FEL can be disposed on gate electrode GE. Insulation layer  310  and focusing electrode FEL can include a plurality of apertures corresponding to electron field emitters FE. An anode including a plurality of electron transparent windows can be fabricated. Each of the windows can correspond to one of electron field emitters FE. Alignment can be accomplished using alignment marks under a mask aligner and bonded together using a suitable wafer bonding technique.  
      According to one embodiment, electron field emitters FE can be about 50-100 μm in diameter. Further, electron field emitters FE can be electrically insulated from one another so that they can be individually addressed or controlled. Spacing between gate electrode GE and electron field emitters FE can be about 100 μm or any suitable distance such that a desired current can be reached with 1-2 kV driving voltage on gate electrode GE. Focusing electrode FEL can focus the field-emitted electrons on the electron transparent windows of the anode.  
      In an alternate embodiment, insulation layer or spacer  308  and gate electrode GE can be deposited on the top surface of layer  302  prior to the deposition of electron field emitters FE. Next, insulation layer  308  and gate electrode GE can be covered by layers of resist and release materials. For example, contact lines  304 , insulation layer  308 , and gate electrode GE can be spin-coated with a uniform layer of OMNICOAT™ release (available from MicroChem, Inc. of Newton, Me.). Next, contact lines  304 , insulation layer  308 , and gate electrode GE can be spin-coated with a uniform layer of epoxy-based SU-8 negative photoresist product (available from MicroChem, Inc.) of about 10-20 mm in thickness. Depending on the desired SU-8 thickness, spin speed and viscosity of SU-8 can be controlled. The photoresist can be insoluble in alcohol. Next, the photoresist can be patterned by contact-mode UV photolithography and developed such that the area contact lines  304  to be deposited with electron field emitters FE is removed while the other surfaces are covered with cross-linked SU-8. Subsequently, the exposed OMNICOAT™ release can be chemically removed to reveal contact lines  304 . Next, electron field emitters FE (in this example, carbon nanotubes) can be electrophoretically deposited onto contact lines  304  by applying a DC voltage between contact lines  304  and a counter-electrode submerged in alcohol containing carbon nanotubes. Further, MgCl 2  “chargers” can be added to the solution and a voltage applied between contact lines  304  and counter-electrode to cause the carbon nanotubes to deposit on contact lines  304 . After deposition of the carbon nanotubes, the photoresist can be stripped using a release such as an OMNICOAT™ release.  
      According to one embodiment, a gate electrode can be fabricated by combination of photolithography and deep reactive ion etch (DRIE) processes. Referring to  FIGS. 4A-4E , a method for fabricating a gate electrode according to one embodiment of the subject matter described herein is illustrated. Referring to  FIG. 4A , a substrate  400  can be provided, which can be a silicon wafer or other suitable substrate. Next, substrate  400  can patterned with an etch mask  402  for forming a metal mesh structure. Referring to  FIG. 4B , a mesh structure MS can be formed by performing a deep reactive ion etch. The etch can be about 50 μm deep. Referring to  FIG. 4C , mesh structure MS can be covered with a photoresist  404  and an opposing side of substrate  400  covered with an etch mask  406 . Next, referring to  FIG. 4D , a deep reactive ion etch can be performed from the opposing side until an aperture is formed to mesh structure MS. Referring to  4 E, photoresist  404  and etch mask  406  can be removed for completing the fabrication of gate electrode GE.  
      According to one embodiment, an anode including a plurality of electron transparent windows can be fabricated by combination of lithography, anisotropic silicon etching, and DRIE processes. Referring to  FIGS. 5A-5D , a method for fabricating an anode including a plurality of electron transparent windows according to one embodiment of the subject matter described herein is illustrated. Referring to  FIG. 5A , a substrate  500  can be provided, which can be a silicon wafer, ITO-coated glass, or other suitable substrate. Electron permeable layers  502  and  504  can be deposited on opposing sides of substrate  500 . Layers  502  can be made of silicon nitride (Si 3 N 4 ) or any other suitable material that is permeable to electrons. Silicon nitride has a high permeability to electrons and is mechanically robust for supporting a pressure differential of at least one atmosphere. In one embodiment, layer  502  may be a layer of silicon nitride having a thickness between about 500-1000 A, which can function as an etch mask for an etching process. Layer  504  can be a 2500 A layer of silicon nitride, a portion of which can function as electron transparent windows.  
      Referring to  FIG. 5B , areas on layer  502  can be defined for etching. For example, layer  502  can be patterned with a photoresist  506 . Referring to  FIG. 5C , a reactive ion etching process can be applied to remove the portions of layer  502  left uncovered by photoresist  506 . Next, photoresist  506  can be removed. Referring to  FIG. 5D , exposed portions of substrate  500  are removed by an anisotropic hot KOH etch. Trenches T can be formed by etching through substrate  500  to form electron transparent windows TW. In one example, transparent windows TW can be about 20 μm×20 μm. Transparent windows TW can be positioned for intercepting electron beams emitted from electron field emitters (such as electron field emitters FE shown in  FIGS. 1 and 2 ). Alignment marks can be patterned on the wafer to assist subsequent assembly with the electron field emitters. The resulting structure can function as a collimator of the electron beams when substrate  500  is of a material and thickness for preventing the penetration of electrons therethrough. For example, a silicon wafer having a thickness of 400 μm can prevent electrons from penetrating through.  
      In one embodiment, the nitride can be unaffected by the etch process. After completion of the KOH anisotropic etching, a metal layer can be deposited onto the resulting wafer. The metal can be deposited onto the areas of the wafer not covered by a photoresist. The photoresist can be lifted off in a solvent. The metal layer can serve as an anode component in a multi-pixel electron microbeam irradiator system according to one embodiment of the subject matter described herein.  
       FIGS. 6 and 7  are representations of optical images of actual electron field emitter structures. In particular,  FIG. 6  is an optical image of an individually addressable electron field emitter structure. The image shows a cathode electrode  600 , a gate electrode  602 , and an active area  604 .  
       FIG. 7  is an optical image of a top view of three electron field emitters after carbon nanotube deposition (dark areas) and removal of a photoresist. The image shows carbon nanotubes  700  on cathode lines and a gate electrode  702 .  
      A multi-pixel electron microbeam irradiator system according to the subject matter described herein can be used for individually irradiating a selected single cell or a subcellular region in a cell population in vitro, without depositing radiation doses to adjacent cells and regions. Further, multi-pixel electron microbeam cell irradiator systems according to the subject matter described herein can be used by researchers for studying microscopic processes activated by particular cellular or sub-cellular components of interest following radiation.  
      The above-described components and techniques can be applied to devices and systems having electron field emitters and related methods. In addition to uses described above, exemplary devices and systems that can be used with the subject matter described herein can include flat-panel displays, microwave vacuum tubes, portable X-ray devices, and gas discharge tubes.  
      It will be understood that various details of the subject matter described herein may be changed without departing from the scope of the subject matter described herein. Furthermore, the foregoing description is for the purpose of illustration only, and not for the purpose of limitation, as the subject matter described herein is defined by the claims as set forth hereinafter.