Abstract:
The present disclosure relates to a data storage device. The data storage device comprises a closed interior space containing a noble gas, a plurality of electron emitters having emission surfaces exposed within the interior space, the electron emitters adapted to emit electron beams, and a storage medium contained within the interior space in proximity to the electron emitters, 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  
         [0001]    The present disclosure relates to a data storage device. More particularly, the disclosure relates to a data storage device including a mechanism for removing contaminants from emission surfaces of electron emitters within the device.  
         BACKGROUND OF THE INVENTION  
         [0002]    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). 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 emitters 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.  
           [0003]    During fabrication, various contaminants from the ambient air can form on the electron emission surfaces of the data storage device. Such contaminants include various materials containing oxygen, nitrogen, and/or carbon. Perhaps most problematic of these is carbonaceous materials such as hydrocarbons. The formation of contaminants is disadvantageous in that their presence adversely affects operation of the electron emitters. For instance, the presence of contaminants increases electron scattering. In addition, where the electron emitters comprise field (i.e., tip) emitters, the work function of the emitters can be decreased, lowering the potential needed to emit electron beams from the emitters and thereby raising the currents substantially. This phenomenon makes it more difficult to control operation of the emitters in that the magnitude of the electron beams emitted from the emitters may be greater than desired, therefore increasing the opportunity for misreading and/or miswriting to a storage medium of the device. Removing these contaminants from the atmosphere to prevent their deposition on the electron emission surfaces during fabrication is difficult, if not impossible.  
           [0004]    In addition to contaminants present in the ambient air during fabrication, further contaminants can be deposited on the emission surfaces of the electron emitters. For example, if the storage medium of the device is partially decomposed, or if contaminants on the surface are desorbed, during read and/or write operations, volatile components can be released that will settle on the electron emission surfaces. Like the airborne contaminants referenced above, these contaminants can similarly result in electron scattering and may significantly change emitter operational characteristics.  
           [0005]    From the foregoing, it can be appreciated that it would be desirable to have a data storage device that employs a mechanism to remove contaminants from the emission surfaces of the electron emitters contained within the device.  
         SUMMARY OF THE INVENTION  
         [0006]    The present disclosure relates to a data storage device. The data storage device comprises a closed interior space containing a noble gas, a plurality of electron emitters having emission surfaces exposed within the interior space, the electron emitters adapted to emit electron beams, and a storage medium contained within the interior space in proximity to the electron emitters, 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.  
           [0007]    In addition, the present disclosure relates to a method for removing contaminants from an electron emission surface of an electron emitter of a data storage device. The method comprises the steps of providing a noble gas within an interior space of the data storage device to which the electron emission surface is exposed, exciting atoms within the gas by impacting them with an electron beam emitted by the electron emitter, wherein the atoms of the gas are ionized by impact with the electron beam and accelerated toward the emission surface to sputter remove the contaminants from the emission surface.  
           [0008]    In preferred arrangements, the noble gas used in the device and method comprises neon gas.  
           [0009]    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  
       [0010]    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.  
         [0011]    [0011]FIG. 1 is a schematic side view of an example data storage device.  
         [0012]    [0012]FIG. 2 is a schematic cross-sectional view of the data storage device of FIG. 1 taken along line  2 - 2 .  
         [0013]    [0013]FIG. 3 is a schematic cross-sectional perspective view of the data storage device of FIGS. 1 and 2 taken along line  3 - 3 .  
         [0014]    [0014]FIG. 4 is a partial schematic view of a storage medium of the data storage device shown in FIGS.  1 - 3 .  
         [0015]    [0015]FIG. 5 is a schematic side view of a first example reading arrangement for the data storage device of FIGS.  1 - 4 .  
         [0016]    [0016]FIG. 6 is a schematic side view of a second example reading arrangement for the data storage device of FIGS.  1 - 4 . 
     
    
     DETAILED DESCRIPTION  
       [0017]    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.  
         [0018]    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 . As is described in greater detail below, the interior space preferably contains a noble gas, such as neon gas, which removes contaminants from emission surfaces within the device  100 . 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.  
         [0019]    Within the interior space  104  is a plurality of electron emitters  108  that face a storage medium  110 . These electron emitters can, for example, comprise field (i.e., tip) emitters as described in U.S. Pat. No. 5,557,596 identified above. Alternatively, the electron emitters  108  can comprise flat emitters such as those described in U.S. patent application Ser. No. 09/836,124 (HP Docket No. 10006168-1), filed Apr. 16, 2001, which is hereby incorporated by reference into the present disclosure. 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 bits more 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. Voltage is also applied to the storage medium  110  to accelerate the emitted electrons to aid in focusing the emitted electrons.  
         [0020]    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.  
         [0021]    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.  
         [0022]    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).  
         [0023]    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.  
         [0024]    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  or alternatively to the stator  116 .  
         [0025]    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.  
         [0026]    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 5 to 100 nanometers between any two storage areas  400 , and the range of the micromover  112  can be approximately 15 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 .  
         [0027]    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.  
         [0028]    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 (ie., 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.  
         [0029]    [0029]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.  
         [0030]    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.  
         [0031]    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 .  
         [0032]    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.  
         [0033]    As identified above, various contaminants can form on the electron emission surfaces of the electron emitters  108  during fabrication of the data storage device  100  and/or thereafter. The interior space  104  of the data storage device  100  therefore preferably contains a noble gas that removes the contaminants from the emitter emission surfaces during use of the emitters. In a preferred arrangement, this noble gas comprises neon gas. Neon gas is preferable because it is massive enough to remove the contaminants, yet not so massive as to damage the electron emitters  108 .  
         [0034]    The mechanism with which the selected gas removes the contaminants from the electron emitters pertains to ionization of the gas during data storage device use. Specifically, when an electron beam is emitted from an electron emitter  108 , the beam impacts atoms of the gas, exiting them to the point at which the atoms lose an electron and therefore ionize. Due to their positive charge, the generated ions are attracted to the negative charge of the electron emitters  108 , and are accelerated toward the electron emitters such that they ultimately bombard the emission surfaces of the electron emitters. In that the ions have a mass similar to that of the contaminants residing on the emission surface, the contaminants are displaced (i.e., sputter removed) from the surface by the ions, thereby cleaning the emission surfaces and ensunng proper operation of the electron emitters  108 .  
         [0035]    The strength of the vacuum maintained within the interior space may depend upon the type of electron emitters  108  used in the fabrication of the data storage device  100 . Due to the use of the gas within the interior space  104 , the vacuum need not be as strong as when air remains within the space. By way of example, where field emitters are used, the vacuum preferably is approximately 10 −4  to 10 −6  Torr, with 10 −3  Torr possible. For flat emitters, the vacuum preferably is approximately 10 −3  to 10 −6  Torr. In any case, however, the strength of the vacuum is maintained such that plasma generation within the interior space  104  is avoided.  
         [0036]    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.