Abstract:
The present disclosure relates to a data storage device, comprising a plurality of electron emitters adapted to emit electron beams, the electron emitters each having a planar emission surface, and a storage medium in proximity to the electron emitter, the storage medium having a plurality of storage areas that are capable of at least two distinct states that represent data, the state of the storage areas being changeable in response to bombardment by electron beams emitted by the electron emitters.

Description:
FIELD OF THE INVENTION 
     The present disclosure relates to a data storage device. More particularly, the disclosure relates to a data storage device incorporating ballistic or quasi-ballistic electron emitters. 
     BACKGROUND OF THE INVENTION 
     Researchers have continually attempted to increase the storage density and reduce the cost of data storage devices such as magnetic hard-drives, optical drives, and dynamic random access memory (DRAM). It has, however, become increasingly difficult to increase storage density due to fundamental limits such as the superparamagnetic limit, below which magnetic bits are unstable at room temperature. 
     Several approaches have been used to increase storage density of storage devices. One approach is based on scanned probe microscopy (SPM) technology. In such an approach, a probe is positioned extremely close to a storage medium. An example is atomic force microscopy (AFM) in which a probe is placed into physical contact with the storage medium. Another example is scanning tunneling microscopy (STM) in which the probe is placed within a few nanometers from the storage medium to ensure that the probe is within a tunneling range of the medium. Although limited success has been achieved through these approaches, it is difficult to inexpensively build a storage device having probes that contact or are in close proximity to the storage medium without eventually damaging the probe and/or the surface of the medium. Moreover, in STM, the spacing must be precisely controlled. As known by persons having ordinary skill in the art, such control is difficult to achieve. 
     In view of the difficulties associated with SPM, other researchers have developed methods that eliminate the need for extremely close proximity. One such technique is based on near-field scanning optical microscopy (NSOM). Although avoiding the proximity problem, this technique has limited lateral resolution and bandwidth and therefore is of limited applicability. Other techniques have been developed based on non-contact SFM, but these techniques typically suffer from poor resolution and poor signal to noise ratio. 
     Even where increased storage density can be achieved, hurdles to effective implementation exist. Once such hurdle is the time required to access data stored on the storage device the information. Specifically, the utility of the storage device is limited if a long time is required to retrieve the stored data. Therefore, in addition to high storage density, there must be a way to quickly access the data. 
     Recently, semiconductor-based electron sources have been developed that can be used in storage devices and which may avoid the difficulties noted above. An example of such a data storage device is described in U.S. Pat. No. 5,557,596. The device described in that patent includes multiple electron sources having electron emission surfaces that face a storage medium. During write operations, the electron sources bombard the storage medium with relatively high intensity electron beams. During read operations, the electron sources bombard the storage medium with relatively low intensity electron beams. Such a device provides advantageous results. For instance, the size of storage bits in such devices may be reduced by decreasing the electron beam diameter, thereby increasing storage density and capacity and decreasing storage cost. 
     One type of electron source described in the U.S. Pat. No. 5,557,596 is the “Spindt” emitter. As described in the patent, such an emitter has a cone shape that ends in a tip from which electron beams can be emitted. Typically, the tip is made as sharp as possible to reduce operating voltage and achieve a highly focused electron beam diameter. Unfortunately, utilization of Spindt emitters creates other problems. First, the fabrication of sharp emitter tips is difficult and expensive. In addition, focusing the electron beam from a Spindt tip in a temporally and spatially stable manner is difficult. Furthermore, the electron optics that provide the focusing can become complicated. Moreover, Spindt emitters do not operate well in poor vacuums. These problems become especially prominent as the electron beam diameter is reduced below 100 nanometers. 
     From the foregoing, it can be appreciated that it would be desirable to have a data storage device that employs electron emitters but that avoids one or more of the problems identified above. 
     SUMMARY OF THE INVENTION 
     The present disclosure relates to a data storage device, comprising a plurality of electron emitters adapted to emit electron beams, the electron emitters each having a planar emission surface, and a storage medium in proximity to the electron emitter, the storage medium having a plurality of storage areas that are capable of at least two distinct states that represent data, the state of the storage areas being changeable in response to bombardment by an electron beams emitted by the electron emitters, wherein data is written to the device by changing the state of the storage areas and data is read by the device by observing phenomena relevant to the storage areas. 
     In addition, the disclosure relates to a method for storing data, comprising the steps of emitting an electron beam from an electron emitter including a planar emission surface, directing the electron beam toward a storage medium comprising a plurality of storage areas, and bombarding one of the storage areas with electrons with the electron beam so as to change the state of a storage area. Typically, although not necessarily, the method further comprises the step of bombarding one of the storage areas with electrons with a lower current electron beam and observing its effect on the storage area. 
     The features and advantages of the invention will become apparent upon reading the following specification, when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. 
     FIG. 1 is a schematic side view of an example data storage device. 
     FIG. 2 is a schematic cross-sectional view of the data storage device of FIG. 1 taken along line  2 — 2 . 
     FIG. 3 is a schematic cross-sectional perspective view of the data storage device of FIGS. 1 and 2 taken along line  3 — 3 . 
     FIG. 4 is a partial schematic view of a storage medium of the data storage device shown in FIGS. 1-3. 
     FIG. 5 is a schematic side view of a first example reading arrangement for the data storage device of FIGS. 1-4. 
     FIG. 6 is a schematic side view of a second example reading arrangement for the data storage device of FIGS. 1-4. 
     FIG. 7 is a schematic side view of a first electron emitter suitable for use with the data storage device of FIGS. 1-4. 
     FIG. 8 is a detail view of a conductive layer of the first electron emitter shown in FIG.  7 . 
     FIG. 9 is a schematic side view of a second electron emitter suitable for use with the data storage device of FIGS. 1-4. 
     FIG. 10 is a schematic side view of a third electron emitter suitable for use with the data storage device of FIGS. 1-4. 
     FIG. 11 is a schematic side view of a fourth electron emitter suitable for use with the data storage device of FIGS. 1-4. 
     FIG. 12 is a schematic side view of a fifth electron emitter suitable for use with the data storage device of FIGS.  1 - 4 . 
    
    
     DETAILED DESCRIPTION 
     Referring now in more detail to the drawings, in which like numerals indicate corresponding parts throughout the several views, FIGS. 1-3 illustrate an example data storage device  100 . It is noted that this device  100  is similar in construction to that described in U.S. Pat. No. 5,557,596, which is hereby incorporated by reference into the present disclosure. 
     As indicated in FIGS. 1-3 the data storage device  100  generally includes an outer casing  102  that forms an interior space  104  therein. By way of example, the casing  102  can include a plurality of walls  106  that define the interior space  104 . Typically, the walls  106  of the casing  102  are sealed to each other such that a vacuum can be maintained within the interior space  104 . By way of example, the casing  102  maintains a vacuum of at least approximately 10 −3  torr within the interior space  104 . Although a particular configuration is shown for the casing  102 , it is to be understood that the casing can take many different forms that would be readily apparent to persons having ordinary skill in the art. 
     Within the interior space  104  is a plurality of electron emitters  108  that face a storage medium  110 . As described in relation to FIG. 4, the storage medium  110  comprises a plurality of storage areas (not visible in FIGS.  1 - 3 ). In a preferred embodiment, each storage area of the storage medium  110  is responsible for storing one or more bits of data. 
     The electron emitters  108  are configured to emit electron beam currents toward the storage areas of the storage medium  110  when a predetermined potential difference is applied to the electron emitters. Depending upon the distance between the emitters  108  and the storage medium  110 , the type of emitters, and the spot size (i.e., bit size) required, electron optics may be useful in focusing the electron beams. An example of such optics is provided below (FIG.  9 ). Voltage is also applied to the storage medium  110  to either accelerate or decelerate the emitted electrons and/or to aid in focusing the emitted electrons. 
     Each electron emitter  108  can serve many different storage areas to write data to and read data from the storage medium  110 . To facilitate alignment between each electron emitter  108  and an associated storage area, the electron emitters and storage medium can be moved relative to each other in the X and Y directions noted in FIG.  2 . To provide for this relative movement, the data storage device  100  can include a micromover  112  that scans the storage medium  110  with respect to the electron emitters  108 . As indicated in FIGS. 1 and 3, the micromover  112  can include a rotor  114  connected to the storage medium  110 , a stator  116  that faces the rotor, and one or more springs  118  that are positioned to the sides of the storage medium. As is known in the art, displacement of the rotor  114 , and thereby the storage medium  110 , can be effected by the application of appropriate potentials to electrodes  117  of the stator  116  so as to create a field that displaces the rotor  114  in a desired manner. 
     When the micromover  112  is displaced in this manner, the micromover scans the storage medium  110  to different locations within the X-Y plane such that each emitter  108  is positioned above a particular storage area. A preferred micromover  112  preferably has sufficient range and resolution to position the storage areas  110  under the electron emitters  108  with high accuracy. By way of example, the micromover  112  can be fabricated through semiconductor microfabrication processes. Although relative movement between the electron emitters  108  and the storage medium  110  has been described as being accomplished through displacement of the storage medium, it will be understood that such relative movement can alternatively be obtained by displacing the electron emitters or by displacing both the electron emitters and the storage medium. Moreover, although a particular micromover  112  is shown and described herein, it will be appreciated by persons having ordinary skill in the art that alternative moving means could be employed to obtain such relative movement. 
     Alignment of an emitted beam and storage area can be further facilitated with deflectors (not shown). By way of example, the electron beams can be rastered over the surface of the storage medium  110  by either electrostatically or electromagnetically deflecting them, as through use of electrostatic and/or electromagnetic deflectors positioned adjacent the emitters  108 . Many different approaches to deflect electron beams can be found in literature on scanning electron microscopy (SEM). 
     The electron emitters  108  are responsible for reading and writing information on the storage areas of the storage medium with the electron beams they produce. Therefore, the electron emitters  108  preferably produce electron beams that are narrow enough to achieve the desired bit density for the storage medium  110 , and that provide the different power densities needed for reading from and writing to the medium. Particular example embodiments for the electron emitters  108  are provided later in this disclosure. 
     As indicated in FIGS. 1 and 2, the data storage device  100  can further include one or more supports  120  that support the storage medium  110  in place within the interior space  104 . When provided, the supports  120  typically comprise thin-walled microfabricated beams that flex when the storage medium  110  is displaced in the X and/or Y directions. As is further indicated in FIGS. 1 and 2, the supports  120  can each be connected to the walls  106  of the casing  102 . 
     In a preferred embodiment, the electron emitters  108  are contained within a two-dimensional array comprising a plurality of emitters. By way of example, an array of 100×100 electron emitters  108  can be provided with an emitter pitch of approximately 5 to 100 micrometers in both the X and Y directions. As discussed above, each emitter  108  typically is used to access a plurality of storage areas of the storage medium  110 . FIG. 4 provides a schematic representation of this relationship. In particular, this figure illustrates a single electron emitter  108  positioned above a plurality of storage areas  400  of the storage medium  110 . As indicated in FIG. 4, the storage areas  400 , like the electron emitters  108 , are contained in a two-dimensional array. In particular, the storage areas  400  are arranged in separate rows  402  and columns  404  on the surface of the storage medium  110 . In a preferred an embodiment, each emitter  108  is only responsible for a portion of the entire length of predetermined numbers of rows  402 . Accordingly, each emitter  108  normally can access a matrix of storage areas  400  of particular rows  402  and columns  404 . Preferably, each row  402  that is accessed by a single electron emitter  108  is connected to a single external circuit. 
     To address a storage area  400 , the micromover  112  is activated to displace the storage medium  110  (and/or electron emitters  108 ) to align the storage area with a particular electron emitter. Typically, each emitter  108  can access tens of thousands to hundreds of millions of storage areas  400  in this manner. The storage medium  110  can have a periodicity of approximately 1 to 100 nanometers between any two storage areas  400 , and the range of the micromover  112  can be approximately 5-100 micrometers. As will be appreciated by persons having ordinary skill in the art, each of the electron emitters  108  can be addressed simultaneously or in a multiplexed manner. A parallel accessing scheme can be used to significantly increase the data rate of the storage device  100 . 
     Writing with the data storage device  100  is accomplished by temporarily increasing the power density of an electron beam produced by an electron emitter  108  to modify the surface state of a storage area  400  of the storage medium  110 . For instance, the modified state can represent a “1” bit, while the unmodified state can represent a “0” bit. Moreover, the storage areas can be modified to different degrees to represent more than two bits, if desired. In a preferred embodiment, the storage medium  110  is constructed of a material whose structural state can be changed from crystalline to amorphous by electron beams. An example material is germanium telluride (GeTe) and ternary alloys based on GeTe. To change from the amorphous to the crystalline state, the beam power density can be increased and then slowly decreased. This increase/decrease heats the amorphous area and then slowly cools it so that the area has time to anneal into its crystalline state. To change from the crystalline to amorphous state, the beam power density is increased to a high level and then rapidly reduced. Although temporary modification of the storage medium  110  is described herein, it will be understood that permanent modification is possible where write-once-read-many (WORM) functionality is desired. 
     Reading is accomplished by observing the effect of the electron beam on the storage area  400 , or the effect of the storage area on the electron beam. During reading, the power density of the electron beam is kept low enough so that no further writing occurs. In a first reading approach, reading is accomplished by collecting the secondary and/or backscattered electrons when an electron beam with a relatively low (i.e., lower than that needed to write) power density is applied to the storage medium  110 . In that the amorphous state has a different secondary electron emission coefficient (SEEC) and backscattered electron coefficient (BEC) than the crystalline state, a different number of secondary and backscattered electrons are emitted from a storage area  400  when bombarded with a read electron beam. By measuring the number of secondary and backscattered electrons, the state of the storage area  106  can be determined. 
     FIG. 5 illustrates example apparatus for reading according to the first reading approach. More particularly, FIG. 5 schematically illustrates electron emitters  108  reading from storage areas  500  and  502  of the storage medium  110 . In this figure, the state of storage area  500  has been modified, while the state of storage area  502  has not. When a beam  504  of electrons bombard the storage areas  500 ,  502  both the secondary electrons and backscattered electrons are collected by electron collectors  506 . As will be appreciated by persons having ordinary skill in the art, modified storage area  500  will produce a different number of secondary electrons and backscattered electrons as compared to unmodified storage area  502 . The number may be greater or lesser depending upon the type of material and the type of modification made. By monitoring the magnitude of the signal current collected by the electron collectors  506 , the state of and, in turn, the bit stored in the storage areas  500  and  502  can be identified. 
     In another reading approach, a diode structure is used to determine the state of the storage areas  400 . According to this approach, the storage medium  110  is configured as a diode which can, for example, comprise a p-n junction, a schottky barrier, or substantially any other type of electronic valve. FIG. 6 illustrates an example configuration of such a storage medium  110 . It will be understood that alternative diode arrangements (such as those shown in U.S. Pat. No. 5,557,596) are feasible. As indicated in this figure, the storage medium  110  is arranged as a diode having two layers  600  and  602 . By way of example, one of the layers is p type and the other is n type. The storage medium  110  is connected to an external circuit  604  that reverse-biases the storage medium. With this arrangement, bits are stored by locally modifying the storage medium  110  in such a way that collection efficiency for minority carriers generated by a modified region  608  is different from that of an unmodified region  606 . The collection efficiency for minority carriers can be defined as the fraction of minority carriers generated by the instant electrons that are swept across a diode junction  610  of the storage medium  110  when the medium is biased by the external circuit  604  to cause a signal current  612  to flow through the external circuit. 
     In use, the electron emitters  108  emit narrow beams  614  of electrons onto the surface of the storage medium  110  that excite electron-hole pairs near the surface of the medium. Because the medium  110  is reverse-biased by the external circuit  604 , the minority carriers that are generated by the incident electrons are swept toward the diode junction  610 . Electrons that reach the junction  610  are then swept across the junction. Accordingly, minority carriers that do not recombine with majority carriers before reaching the junction  610  are swept across the junction, causing a current flow in the external circuit  604 . 
     As described above, writing is accomplished by increasing the power density of electron beams enough to locally alter the physical properties of the storage medium  110 . Where the medium  110  is configured as that shown in FIG. 6, this alteration affects the number of minority carriers swept across the junction  610  when the same area is radiated with a lower power density read electron beam. For instance, the recombination rate in a written (i.e., modified) area  608  could be increased relative to an unwritten (i.e., unmodified) area  606  so that the minority carriers generated in the written area have an increased probability of recombining with minority carriers before they have a chance to reach and cross the junction  610 . Hence, a smaller current flows in the external circuit  604  when the read electron beam is incident upon a written area  608  than when it is incident upon an unwritten area  606 . Conversely, it is also possible to start with a diode structure having a high recombination rate and to write bits by locally reducing the recombination rate. The magnitude of the current resulting from the minority carriers depends upon the state of particular storage area, and the current continues the output signal  612  to indicate the bit stored. 
     As identified above, various hurdles exist to the use of Spindt (i.e., tip) electron emitters. Accordingly, alternative emitter configurations are contemplated. Generally speaking, these alternative electron emitters comprise ballistic or quasi-ballistic electron emitters. More particularly, the electron emitters are configured as flat emitters. FIG. 7 illustrates a first example flat electron emitter  700  that can be used in the data storage device  100  to bombard a target  702  (e.g., storage medium  110 ). As indicated in this figure, the emitter  700  includes an n++ semiconductor substrate  704  that, for example, can be made of silicon. Typically, the thickness of the substrate depends upon the size of the wafer used to form the substrate. By way of example, the substrate  704  can be approximately 400 to 1000 micrometers thick. The substrate  704  is fabricated such that it includes a volcano-like, funnel-like, or nozzle-like active region  706 . Stated in other words, the active region  706  generally has a wide base that quickly narrows into a neck  708 . 
     The active region  706  is surrounded by an isolation region  710  that limits the geometry of the active region  706  to limit the area from which the active region can emit electrons. By way of example, the isolation region  710  comprises silicon dioxide that is formed through an oxidation process (e.g., wet or dry oxidation). In addition to limiting the geometry of the active region  706 , the isolation region  710  isolates the active region  706  from neighboring active regions (not shown). However, it will be understood that bases of the active regions  706  of contiguous electron emitters  700  can be connected together. 
     Formed on the substrate  704  is a semiconductor layer  712 . By way of example, the semiconductor layer  712  is made of polysilicon or silicon carbide (SiC) and has a thickness of approximately 0.01 to 2 micrometers. In a preferred arrangement, the semiconductor layer  712  includes a planar outer surface  714  and a porous region  716 . As indicated in FIG. 7, the porous region  716  is limited in extent such that it is aligned with the neck  708  of the active region  706 . Limiting the porous region in this manner allows for higher current densities due to increased thermal energy dissipation. The porous region  716  terminates at the outer surface  714  to define an emission surface  718 . In that the surface  714  preferably is planar, the emission surface  718  likewise preferably is planar. This configuration permits better focusing of electron beams emitted from the emitter  700 . By way of example, the area of the emission surface  718  can be limited to less than approximately 10% of the total area of the outer surface  714  of the semiconductor layer  712 . Most preferably, the area of the emission surface  718  is limited to less than approximately 1% of the total area of the surface  714 . 
     The electron emitter  700  further includes an emission electrode  720  formed on the semiconductor layer  712  that is used to supply voltage to the semiconductor layer  712 . The emission electrode  720  typically is composed of a highly electrically conductive material such as chromium and can have a thickness of approximately 0.1 to 1 micrometer. In addition to the emission electrode  720 , the emitter  700  includes a conductive layer  722  that covers the emission electrode  720  and a portion of the outer surface  714  of the semiconductor layer  712 , including the emission surface  718 . This layer  722  is preferably thin and can, for instance, have a thickness of approximately 10 nanometers or less. The conductive layer  722  provides an electrical contact over the emission surface  718  and allows an electric field to be applied over the emission surface  718 . Preferably, the conductive layer  722  comprises an alloy that does not form an insulating oxide or nitride on its surface to avoid the creation of tunnel barriers that would negatively effect the efficiency of the electron emitter  700 . 
     By way of example, the conductive layer  722  can be made of a thin metal or conductive material such as gold, carbon (e.g., graphite, electrically conductive diamond, or combinations thereof), platinum, iridium, rhodium, conductive boron nitride, or other conductors or semiconductors. Generally speaking, materials having atomic numbers substantially below that of gold may also be used for the conductive layer  722  in that such materials do not scatter electrons (which lowers emission efficiency) to the extent that materials having higher atomic numbers do. As a low atomic number element, carbon exhibits very low electron scattering probability. The conductive layer  722  can be porous or semi-dense such that all conductive areas are electrically connected. For example, the conductive layer  722  can include electrically interconnected conductive islands, a mesh of interconnected filaments, or a combinations thereof. In an alternative embodiment, the conductive layer  722  can comprise multiple thin layers  800  of metal, as shown in the detail view of FIG.  8 . 
     The electron emitter  700  can further include a back contact  724  that is formed on the substrate  704  on a side opposite that on which the semiconductor layer  712  is formed. When provided, the back contact  724  establishes an equipotential surface for internal fields in the semiconductor substrate  704  and the porous region  716 . It is to be understood that the back contact  724  can be eliminated if the substrate  704  is highly doped, in which case a contact can be made to the substrate via a front contact through known means. 
     During operation, different potentials are applied (e.g., with on or off-chip drivers) to the substrate  704 , the emission electrode  712 , and the back contact  724 . The resulting emission electrode voltage causes electrons to be injected from the active region  706  of the substrate  704  into the porous region  716  of the semiconductor layer  712  and be emitted from the emission surface  718  and through the conductive layer  722 . This emission results in an electron beam  726  that impinges the target  702 . 
     As will be appreciated by persons having ordinary skill in the art, focusing means may be needed to focus the beam  726  on the target  702 . One example of such focusing means are illustrated in FIG. 9 which illustrates a second example flat electron emitter  900 . As indicated in this figure, the emitter  900  is similar in several ways to the emitter  700  shown in FIG.  7 . Accordingly, the emitter  910  comprises a substrate  704  including an active region  706  and an isolation region  710 , a semiconductor layer  712  including a porous region  716 , an emission electrode  720 , a conductive layer  722 , and a back contact  724 . In addition, the electron emitter  900  includes a focusing structure  902  that is used to focus the electron beams emitted from the emitter  900 . 
     As shown in FIG. 9, the focusing structure  902  comprises an insulating layer  904 , a lens electrode  906 , and a second conductive layer  908 . The insulating layer  904  isolates the emission electrode  720  from the lens electrode  906 . Like the conductive layer  722 , the conductive layer  908  provides a contact over the lens electrode  906  such that an electric field can be applied thereto. As indicated in FIG. 9, the lens electrode  906  and the conductive layer  908  are formed so as to define an aperture  910  through which electron beams can pass. In use, a potential is applied to the lens electrode  906 . The electric field resulting from the lens electrode voltage at the aperture  910  causes the emitted electrons to be focused. Typically, this focus can be adjusted by varying the potential applied to the lens electrode  906 . The electron beam can be focused to a very small spot size, e.g., less than 1 nanometer in diameter, on the target (not shown). Although a particular focusing arrangement has been shown and described, it will be appreciated by persons having ordinary skill in the art that many different focusing arrangements are possible and that others may even be more preferable. 
     FIG. 10 illustrates a third example flat electron emitter  1000  that can be used in the data storage device  100 . The electron emitter  1000  includes a n++ semiconductor substrate  1002  and a semiconductor layer  1004  that is formed on the substrate. By way of example, the substrate  1002  can comprise silicon and the layer can comprise polysilicon. In addition, the emitter  1000  includes an insulating layer  1006 , a patterning mask  1008 , and a conductive layer  1010 . The patterning mask  1008  is deposited on the semiconductor layer  1004  and the insulating layer  1006 . In similar manner, the conductive layer  1010  is deposited on the patterning mask  1008  and the semiconductor layer  1004 . The semiconductor layer  1004  includes a porous region  1012 . An opening  1014  in the patterning mask  1008  defines an emission area  1016  of the emitter  1000 . 
     Electron emission can be achieved with emitter structures distinct from those described above. For example, the electron source may be adapted to emit electrons from the surfaces of metal-insulator-metal (MINI) and metal-insulator-oxide (MIS) structures at or below room temperature. This type of electron emission is described in Wade &amp; J Briggs, “Low noise Beams from Tunnel Cathodes,”  Journal of Applied Physics  33, No. 3, pp. 836-840, 1962; Julius Cohen, “Tunnel Emission into Vacuum,” Applied  Physics Letters  1, No 3, pp. 61-62, 1962; and Yokoo, et al., “Emission characteristics of metaloxide-semiconductor electron tunneling cathode,”  Journal of Vacuum Science and Technology,  pp. 429-432, 1993. Electrons from MIM and MIS structures are emitted into the vacuum with small divergence angles as described in R. Hrach,  Thin Solid Films  15, p. 15, 1973. Small divergence angles allow the emitted electrons to be focused into small diameter electron beams. 
     FIG. 11 shows a flat electron emitter  1100  that includes a MIM-based electron emission structure. As indicated in this figure, the emitter  1100  includes a substrate  1102  including an active region  1104  and an isolation region  1106 , an insulator layer  1108 , an emission electrode  1110 , a conductive layer  1112 , and a back contact  1114 . Included in the active region  1104  of the substrate  1102  adjacent the insulator layer  1108  is a thin metal layer  1116 . Therefore, a metal-insulator-metal arrangement is obtained by the conductive layer  1112 , the insulator layer  1108 , and the metal layer  1116 . Although a particular MIM arrangement is shown and described, it will be appreciated by persons having ordinary skill in the art that alternative arrangements are feasible. 
     FIG. 12 shows a flat electron emitter  1200  that includes a MIS-based electron emission structure. As indicated in this figure, the emitter  1200  includes a silicon substrate  1202  including an active region  1204  and an isolation region  1206 , an insulator layer  1208 , an emission electrode  1210 , a conductive layer  1212 , and a back contact  1214 . The metal-insulator-silicon arrangement is obtained by the conductive layer  1212 , the insulator layer  1208 , and the substrate  1202 . Although a particular MIS arrangement is shown and described, it will be appreciated by persons having ordinary skill in the art that alternative arrangements are feasible. 
     While particular embodiments of the invention have been disclosed in detail in the foregoing description and drawings for purposes of example, it will be understood by those skilled in the art that variations and modifications thereof can be made without departing from the scope of the invention as set forth in the following claims.