Abstract:
A radiation assisted electron emission device uses semiconductor or semi-insulator material formed with an emission mechanism such as a field emission tip, a thermionic emission device, or a negative electron affinity emitter as a source of electrons. The material is irradiated with a source of radiation, such as electromagnetic radiation, neutron radiation, or charged particle radiation, which excites electron-hole pairs in the material to generate a population of free conducting electrons. The electrons are driven to the emission mechanism by a suitable transport process, such as diffusion or drift. The electron emission device has applicability to a broad range of technologies where an electron beam or current is used.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to electron sources, and more particularly to an electron emission source that is driven into an electron emissive state with the assistance of radiation, such as photon emission from a radiation source. 
     2. Background and Prior Art 
     Electron emission sources are generally known in the art, and have applicability in many areas of technology which use electron beams, such as vacuum microelectronics applications, semiconductor device manufacturing, electron beam exposure apparatus, electron microscopes, flat panel display devices, radiation spectrometers and imagers, etc. 
     Various techniques of generating a source of electron beam emission are known in the art, including thermionic emission, field emission and negative electron affinity emission. The surface of any. conducting solid material presents an energy barrier that binds to the host material. Electrons incident upon the energy barrier at energy levels capable of surmounting the barrier may leave the solid, and thus give rise to an emission current. 
     Thermionic emission is generally achieved by heating a filament to high temperatures, by which electrons gain enough energy to surmount the energy barrier and leave the heated surface. The emission current density of a thermionic emission electron source is given by 
     
       
           J=AT   2  exp(− w/kT )  (1) 
       
     
     where A is a constant, T is the temperature of the source material in degrees Kelvin, k is the Boltzmann constant, and w is the work function of the source material. 
     Another method of generating electron emission is by altering the energy barrier to make it very thin, whereby electrons can tunnel through it, or to make it very small whereby electrons do not encounter any appreciable barrier to hinder their emission from the surface of the material. 
     Field emission cathodes present a different mode of emission from a surface than thermionic emission. A field emission source has a tip fashioned from a conducting material such that the energy barrier is very thin. Because contamination of the field emission tip can inhibit emission, field emission electron sources work best when the tip is exposed only to a vacuum. 
     Another mode of emission is provided by negative electron affinity devices. In such devices a coating is applied to a surface that produces “bends” in the energy bands such that there is no barrier present to prevent the emission of electrons into a vacuum. Hence negative electron affinity emission also generally is carried out where the treated surface is exposed only to a vacuum. 
     Regardless of the material, electron emission is best realized where the material has a large density of freely conducting electrons. For this reason, degenerately doped semiconductor materials are often used for both field emission and negative electron affinity devices, because of the high density of freely conducting electrons present in the material. Emission achieved primarily by increasing the voltage drop across the emission region of the device. 
     There exists a need in the art for improvement in such electron emission devices, and specifically to provide electron emission devices that can use semiconducting material that does not have to be degeneratively doped. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the disadvantages discussed above by providing an electron emission device that can be manufactured from a semiconductor material that is very pure (i.e., with a very low doping concentration), or from a semi-insulating material that has a very low doping concentration. 
     The device according to the present invention uses the change in conductivity caused by interactions of ionizing radiation within the device to increase the population of free electrons near the surface of the device. The ionizing radiation can be provided by any suitable mode of radiation, such as electromagnetic radiation, charged particle radiation, or neutron radiation. The free electrons excited by the ionizing radiation either can drift or diffuse to the emission region, whereby the conductivity in the emission region increases, allowing for increased electron emission from the emission region. 
     In particular, the present invention provides a radiation assisted electron source, including semiconducting or semi-insulating material capable of producing electron-hole pairs, a source of radiation for providing incident radiation to the material, the incident radiation exciting electron-hole pairs within the material, an emission mechanism formed in the material, and a transport mechanism for driving electrons of the electron-hole pairs to a local region of the emission mechanism, where the electrons are released from the emission mechanism to provide an electron beam. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and which are not limitative of the present invention, and wherein: 
     FIG. 1 is a diagram of a radiation assisted electron emission device according to a general embodiment of the present invention; 
     FIG. 2 is a diagram of a radiation assisted electron emission device according to a second embodiment of the present invention; 
     FIG. 3 is a diagram of a first possible application of a radiation assisted electron emission device according to the general embodiment of the present invention; 
     FIG. 4 is a diagram of the first possible application of a radiation assisted electron emission device according to the second embodiment of the present invention; 
     FIG. 5 is a diagram of a second possible application of a radiation assisted electron emission device according to the general embodiment of the present invention; 
     FIG. 6 is a diagram of the second possible application of a radiation assisted electron emission device according to the second embodiment of the present invention; 
     FIG. 7 is a diagram of an array of radiation assisted electron emission sources according to a third embodiment of the present invention; 
     FIG. 8 is a diagram of a radiation assisted electron emission device according to a fourth embodiment of the present invention; and 
     FIG. 9 is a diagram of a radiation assisted electron emission device according to a fifth embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention provides an electron emission source wherein the device is made from semiconducting or semi-insulating material, and wherein the electron emission may be from a thermionic emission tip, a field emission tip, a negative electron affinity region, or any other emission mechanism. Ionizing radiation is allowed to interact within the emission region of the emission mechanism, or in a region proximate the emission region made from the same material as the emission region. Freely conductive electrons excited by the ionizing radiation either are diffused to the emission region of the emission mechanism, drifted to the emission region by external or internal voltage potential, or migrate to the emission region by a combination of diffusion and drifting. 
     Upon reaching the emission region, the electrons provide a high density of electrical charge, which changes the conductivity in the local region of the emission mechanism to allow electron emission to occur. Electron emission is controlled by controlling the ionizing radiation applied to the device to cause electron emission from the emission mechanism. Emission may be controlled to be either continuous or in discrete packets. In the latter case, once the packet of electrons leaves the emission region and exits the device, emission stops until another packet of electrons excited by the ionizing radiation arrives in the local region of the emission mechanism. 
     Referring now to FIG. 1, a cross section of a basic form of radiation assisted electron emission device (RAEED)  10  is shown. Ionizing radiation  1 , such as photons (elg., visible, X-ray, gamma-ray, etc.), neutrons, or charged particles (e.g., beta, alpha, heavy ions, electrons, etc.) strike or impinge upon a semiconducting or semi-insulating material  2  at location proximate the local region of emission mechanism  3 . Mechanism  3  is any mechanism through which electrons may be emitted from a material, such as a thermionic mechanism, a field emission mechanism, or a negative electron affinity mechanism. 
     The absorbed radiation causes ionization in the material  2 , thereby generating electron-hole pairs  8 . Electrons  12  are driven to the emission region of the mechanism  3  by a transport process such as diffusion, drift or a combination of the two, and electrons  4  are emitted from the surface of the mechanism  3 . Voltage potential is applied to extractor grids  19  to assist in the extraction of electrons  12  from the material and/or to focus the emitted electrons  4  that have left the material into a directed beam. 
     FIG. 2 shows an example of an RAEED  20  of the invention wherein the emission mechanism is a field emission tip  5 . Electrons excited by the ionizing radiation  1 ′ are driven to the local region of the field emission tip  5 , wherein the energy barrier is very thin, allowing the excited electrons to exit the material to form a beam of electrons  4 ′. Optional extractor grids  19 ′ can be provided to assist in electron extraction or to focus the emitted electrons into a directed beam. 
     FIG. 3 illustrates an application of the RAEED of the present invention as a radiation detector. Photons  1 ″, such as gamma-rays, X-rays or other photon radiation, enter into a scintillating material  9  (such as a scintillation crystal) wherein the absorbed photons interact with the scintillating material to cause it to emit photons  7  of different electromagnetic energy or wavelength (such as visible light photons). 
     The photons  7  are reflected by a reflector  6  into a semiconducting or semi-insulating material  2 ″, where they excite electron-hole pairs  8 . Electrical contacts  10  and  11  (such as conductive metals or p-type or n-type semiconductor material) are provided on the opposite surfaces of the material  2 ″, whereby the voltage potential across the material  2 ″, from one side to the other may be varied (e.g., made negative, positive, or grounded). Electrons  12  from the excited electron-hole pairs are driven to the emission region of mechanism  3  either by drift or by diffusion from an appropriate applied voltage potential to contacts  10  and  11 , or a combination of drift and diffusion, where they are emitted as electrons  4 ″. The emitted electrons then can be fed to an amplifier such as a photomultiplier dynode bank, avalanche photodiode, or other electron amplifier, to produce a signal proportional to the amount of radiation  1 ″ impinging on the device. FIG. 4 shows an example of a radiation detector using the RAEED of the present invention wherein the emission mechanism  3  is a field emission tip  5 ′. 
     FIG. 5 shows an example of an embodiment of the present invention wherein a light source  14  is used as the source of ionizing radiation. The light source  14  may be implemented by any appropriate source of light photons, including a laser, light emitting diode, or incandescent light bulb. The light may be focused into the material  2 ″″ by a lens  13 . Photons absorbed into the semiconducting or semi-insulating material  2 ″″ excite electron-hole pairs  15 , from which electrons  12  are driven to the emission region of mechanism  3  as in the earlier examples disclosed above. The remaining reference numerals describe like elements as disclosed above. FIG. 6 shows an embodiment of the invention wherein the emission mechanism is field emission tip 
     FIG. 7 shows an alternate embodiment of the present invention wherein the RAEED is provided as an array of electron emission devices. An array of light sources  18 , such as light emitting or laser diodes, are provided on a substrate material  70  behind an array of field emission tips  5 ″′ formed in semiconducting or semi-insulating material  2 ″″″. The diodes  18  alternately ma be individual devices, or may be fabricated on the same material  2 ″″″ as the field emission tips  5 ″′. A coupling compound  17  may be inserted between the diodes  18  and the material  2 ″″″ in order to improve light coupling into the material. Wells  16  may be etched directly behind, the field emission tips to improve diffusion and drift of electrons to the field emission tips. The diodes  18  may be turned on and off individually according to the field emission tips that are desired to emit electrons. 
     FIG. 8 shows an embodiment of the invention as a depletion mode-operated RAEED. The device operates as follows. A pulse of radiation is incident on initial polarizer  81 . Dependent upon the bias applied to birefringent crystal  83  by voltage source  84 , the polarization of the incident radiation will change, which determines whether or not the radiation will pass through the second polarizer  82  and into the surface of fully depleted silicon material  89 . If the polarization is correct, the radiation creates excited electron-hole pairs in the depleted bulk silicon material. Electrons from the electron-hole pairs are driven to a field emitter  88  by an internal electric field  86  created by a voltage potential applied to P++ doped regions  85  and  87 . P++ doped regions  87  serve to focus the electrons to the field emission tip  88 . With no electrons in the vicinity of the tip, a net positive charge density exists due to the ionized N D . The extractor also exhibits a net positive charge density. When a charge packet approaches the tip, the electric field will become correspondingly larger, thereby allowing field emission to occur. As the full charge packet is being released from the tip, field emission will become correspondingly smaller, to the point of termination. Thus, as modulated by a birefringent crystal, the radiation source produces a charge packet that creates its own field emission. This embodiment of the invention can be used an electron source to dose electron beam sensitive resists. 
     FIG. 9 shows an embodiment of the invention as a single element radiation activated field emitter device. Photons/phonons  91  or charged particles  92  interact with mechanism  93  that controls the flux of incident radiation into bulk semiconductor material  94 . Material  94  is operated at a predefined temperature T and is depleted of charge carriers with proper biasing and doping. Doped areas  95  (opposite of the type of the bulk material  94 ) are used to deplete the bulk material and to shape the internal electric field  96 . The primary function of the electric field  96  is to drive the free electrons created by radiation interaction in the bulk semiconductor material to field emitter region  97 . The internal electric field also may be used to control mean drift times, generation/recombination rates, avalanche creation, and signal-to-noise ratio of the emission current. The field emitter region  97  may be a single tip or an array of tips. The emission mechanism may be thermionic, Schottky emission, cold field emission, or any combination of these. It may also include a negative electron affinity emission process. Extractor plates  98  apply an additional field to the emitter area, and function to control the emission beam. Depending on the application, the electron beam  99  may travel toward or away from the field emitter area  97 . 
     Examples of Particular Applications 
     (1) Electron Source for E-Beam Writing 
     The RAEED can be implemented as a single or multiple electron beam source for e-beam writing applications. The radiation dose can be modulated by adjusting the intensity of the incident radiations. (such as photons). For such applications, the device size would be approximately 100 μm by 100 μm, allowing approximately 250 active devices per linear inch, operating at 250 MHZ. 1524 devices could be implemented for six inch mask writing. At 250 MHZ, write times can be decreased by 3 orders of magnitude. Such devices will be very stable and reliable, because a high electric field exists at the tip only during electron emission. 
     (2) Electron Source for E-Beam Metrology 
     The RAEED can be implemented as a single or multiple electron beam source for metrology applications. Depletion mode backscatter detectors can be fabricated on the same side of the device(s) as the field emitter. The closer the electron source is placed to the surface under investigation, the greater the solid angle, thereby improving the detection efficiency. 
     (3) Radiation Spectrometer/Imager 
     In this application, a scintillator is used as the interacting medium. The scintillator photon emission is captured at the topside of the RAEED, and the electron signal of the RAEED is amplified with a high S/N ratio dynode bank or avalanche diode. The signal will be proportional to the energy of the incident photons. If implemented as an array, the X-Y coordinate position of the radiation interaction can be determined. 
     (4) High Gain/Speed Optical Switch/Router 
     Assuming single wavelength incident photons, the signal created by the incident photons can be turned on and off by proper biasing of a birefringent crystal. Estimated switching speeds are on the order of 250 MHZ. Amplification of the electron signal can be achieved by increasing extractor voltage to increase the energy of the emitted electrons. When incident on a detecting substrate (such as a p-i-n diode), the signal will be increased in proportion to the increase in extractor voltage. This signal can be used to drive other components in an optical switch/router. 
     (5) High Reliability Flat Panel Display 
     The RAEED can be implemented as a high reliability flat panel display, as high electric field is present at the emitter tip only during electron emission. The electron emission can be modulated so that ions do not experience enough drift to interact with the emitter, thereby eliminating the failure mechanism caused by ion attraction to the high electric field emitter, which causes a change in work function when ions interact with the emitter tip. 
     (6) Modulated Electron Source 
     The RAEED can be modulated by modulating the intensity of the light source that stimulates or assists in the electron emission. For example, a laser diode as shown in FIG. 7 may be modulated so that the emission of electrons changes in intensity in real time as the light intensity of the laser diode changes. As such, the device does not need to be driven by changing the operating voltages on the device. The operating voltages can remain constant while the electron emission intensity changes as a result of the differences in conductivity of the semiconducting or semi-insulating material, which is controlled by the photoelectric effect from the modulating light source. 
     The invention having been thus described, it will be apparent to those skilled in the art that the same may be varied in many ways without departing from the spirit and scope of the invention. Any and all such modifications are intended to be included within the scope of the following claims.