Patent Publication Number: US-6664722-B1

Title: Field emission material

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application is a continuation-in-part of U.S. patent application Ser. No. 09/453,304, filed Dec. 2, 1999 now U.S. Pat. No. 6,479,939. 
    
    
     TECHNICAL FIELD 
     The present invention relates in general to field emission devices, and in particular, to a cold cathode for use as a field emitter. 
     BACKGROUND INFORMATION 
     Cold cathodes are materials or structures that emit electrons with the application of electric fields without heating the emitter significantly above room temperature. Examples of cold cathodes are small metal tips with sharp points that are fabricated together with a grid structure around the tips such that an appropriate bias placed between the grid structure and the tips will extract electrons from the tips when operated in a suitable vacuum environment (Spindt emitters). 
     Diamond, diamond-like carbon (DLC) and other forms of carbon films have also been investigated for use as cold cathode electron emitters for many applications, such as flat panel displays, microwave device applications, backlights for liquid crystal displays (LCDs), etc. Many different techniques for growing the carbon films were tried resulting in a wide variety of carbon films. The mechanism for electron emission from these carbon films is not clear and is the subject of much investigation. What has been found consistently is that electrons are not emitted uniformly from the carbon cold cathodes, but are instead emitted from specific areas or sites of the carbon film. These areas are the emission sites (ES). The density of these sites in a unit area is referred to as the emission site density (ESD). 
     Researchers recognized early on that the negative electron affinity of the hydrogen terminated &lt;111&gt; and &lt;100&gt; faces of diamond may be important. A material having negative electron affinity (NEA) means that if an electron is in the conduction bands of the material, this electron has no barrier to prevent it from leaving the material if the electron diffuses to the surface having the NEA property. 
     The question for diamond has always been how to get an electron into the conduction band of diamond. This is not an easy question since diamond is an insulator with a very wide energy gap (5.5 eV) between the conduction band and the valence band. For an insulator at room temperature with this large a band gap, the population of electrons in the conduction band is too small to support any substantial emission current. Researchers have speculated that the electrons are injected into the diamond from a back side contact. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 illustrates an apparatus for measuring the dielectric constant of a material; 
     FIG. 2 illustrates results of one area of a carbon film showing the dielectric properties and the field emission properties; 
     FIG. 3 illustrates a field emitter device configured in accordance with the present invention; 
     FIG. 4 illustrates a digital image of a single site emission current image; 
     FIG. 5 illustrates a digital image of the topography of a single emission site taken simultaneously with the emission current image of the same site as illustrated in FIG. 4; 
     FIG. 6 illustrates a digital image of a non-contact topography image of an emission site showing “grainy” distribution of physical parameters; 
     FIG. 7 illustrates a digital image of a contact topography of the single site illustrated in FIG. 6 showing a structure of “bumps”; 
     FIG. 8 illustrates a cross-section of emission current data from a single emission site; 
     FIG. 9 illustrates a cross-section of fluctuations of a topographic image of a single emission site whose current image is illustrated in FIG. 8; 
     FIGS. 10-12 illustrate graphs of wide variations in conductivity of non-emitting regions of a carbon film; 
     FIG. 13 illustrates a topographic image cross-section of an emitting carbon film; 
     FIG. 14 illustrates a digital image of a portion of the emitting film as graphed in FIG. 13; 
     FIG. 15 illustrates a graph of semiconductor-type behavior of an emission site of a film in accordance with the present invention; 
     FIG. 16 illustrates a data processing system configured in accordance with the present invention; 
     FIG. 17 illustrates a graph of emission current versus time; and 
     FIG. 18 illustrates an emission site. 
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are set forth such as specific emitter types, etc. to provide a thorough understanding of the present invention. However, it will be obvious to those skilled in the art that the present invention may be practiced without such specific details. In other instances, well-known circuits have been shown in block diagram form in order not to obscure the present invention in unnecessary detail. 
     Refer now to the drawings wherein depicted elements are not necessarily shown to scale and wherein like or similar elements are designated by the same reference numeral through the several views. 
     The inventor produced films for testing to characterize emission sites at better than 100 nm spatial resolution using a modified atomic force microscope (AFM). 
     Referring to FIG. 1, a modified scanning microscope was operated in two modes. In the first mode the tip  104  was placed touching the surface of the sample  101  (mounted on sample holder  102 ) and scanned across the surface  101  to measure the physical topography (AFM mode). The height of the tip  104  was detected by bouncing a light beam from light source  105  off of the end of the tip  104  and reflected into a position detector  106 . The position of the light hitting the detector  106  is dependent on the height of the needle tip  104 . In another mode (scanning polarization force microscopy (SPFM) mode), the tip  104  was placed about 100 nm away from the surface as shown in FIG.  1 . SPFM is a non-contact form of AFM in which the electrostatic forces between a biased, conductive AFM tip and a sample deflect the AFM cantilever. This deflection is used as the control signal to image both conducting and insulation substrates. A voltage bias from source  103  was placed on the needle tip  104  while the tip  104  was scanned across the surface. By biasing the tip  104 , an electric charge was placed on the tip  104  relative to the surface. The material reacted to the charge on the tip  104  by placing charges on the surface in such a way as to form what appears to be an image charge inside the material  101 . The strength of the image charge is dependent on the dielectric constant (ε) of the material  101  as given by the equation: 
     
       
         − q′=q *(ε−1)/(ε+1) 
       
     
     Here q′ is the magnitude of the image charge and q is the charge placed on the tip  104 . Since the image charge is of opposite polarity to the charge on the tip  104 , an attractive force develops between the needle tip  104  and the surface of the substrate  101 . This force deflects the needle  104 . The magnitude of the force is detected by the position of the light hitting the detector  106 . By scanning the needle tip  104  across the surface, a mapping of the relative dielectric constant across the surface is obtained. Simultaneously, if the bias on the tip  104  is high enough, electrons from the carbon film  101  can be field emitted from the surface of the sample  101  to the tip  104 . By monitoring the current to the tip  104 , the emission sites of the carbon film  101  can be located. Thus this instrument can map simultaneously the spatial emission properties of the sample  101  and the dielectric properties of the material  101 , allowing a correlation of the results. 
     FIG. 2 shows the results of one area of the carbon film  101  showing both the dielectric properties (left side image) and the field emission properties (right side image). What was discovered was that the field emission sites are correlated with specific dielectric properties of the sample  101 . The features that are correlated to the emission sites are characterized by a dark area surrounded by a ring of bright area. They look like small volcanoes. Examples of these are features labeled A, B, and C. The emission sites are actually centered on the dark part of the volcano features. This corresponds to an area of material having a relatively low dielectric constant surrounded by a ring of material  101  having a relatively higher dielectric constant. Classically, the dielectric constant is related to the conductivity of the material. We correlate the areas of relatively high dielectric constant to material that is more conductive. We correlate the areas of relatively low dielectric strength to material that is more insulating. Since the film was grown by a diamond CVD process, we concluded that the insulating material was diamond and that the conductive ring was amorphous or graphitic carbon. 
     We also noted that the dielectric distribution going towards the center of these volcanoes was not abrupt but instead was gradual until it reached the dark center of the volcano. This suggests that the dielectric constant of the material  101  varies gradually towards the center of the volcano feature. In other words, the material surrounding the diamond has a graded dielectric constant, the interfaces are not abrupt, but gradual. One of the emission sites (site A) has the volcano feature as well as a well defined area of high dielectric constant next to it. 
     We also noted that there are other features in the dielectric map that do not correspond to emission sites. Two features marked E and F do not have the dark centers of the volcano features. Another volcano-like feature (labeled G) is not correlated with an emission site. Note that this feature also is not surrounded by a significant conducting ring as the other features A, B, C. 
     Finally we noted that the intensity of emission from different sites was not uniform. The site marked D has the smallest emission intensity of the sites that emit. Its volcano features are hardly discernible. 
     Thus we discovered that a certain structure promotes electron field emission from the diamond films. These structures consist of a small diamond particle (less than 2000 Å in diameter) surrounded by a material that has a dielectric constant that changes gradually in a volcano-like structure. It is believed this structure is necessary to promote injection of electrons into the low dielectric material which is presumably diamond. Once in the diamond conduction band, these electrons have little or no barrier for emission because of the low or negative electron affinity of the diamond surfaces. 
     The inventors have since performed additional tests on samples and have arrived at new discoveries. FIG. 4 illustrates an image of emission current of a single emission site measured using the SPFM mode with a tip bias of +9.33 volts and a separation of 100 nm. FIG. 5 illustrates the same single emission site illustrating a distinct SPFM topography image matching the same geometrical area of the emission current image in FIG.  4 . The correlation is similar to what was explained for FIG. 2, emission properties and dielectric properties are both imaged together over the same area of the sample. 
     FIG. 7 illustrates a digital image showing the topography of a single emission site using the AFM mode, which illustrates a “grainy” structure of “bumps” of approximately 50-100 nm. FIG. 6 illustrates a topography image using the SPFM mode of the same emission site, which also shows a “grainy” distribution of physical parameters, which correlate to the “grainy bumps” from the AFM image in FIG.  7 . The noise spikes within the FIG. 6 image are to be ignored. 
     When searching for these emission sites, an area of the sample measuring approximately 6 micrometers ×6 micrometers was searched. In general, it took about three such general scans to locate an emission site. A conclusion from the foregoing is that the emission site density is equivalent to one site per one hundred square micrometers or one million emission sites per square centimeter. 
     Referring to FIG. 8, it has also been discovered by viewing the cross-section of the emission current data from a single emission site that the emission is time-dependent with a lateral resolution of 50 nm. The cross-section of the image data in FIG. 8 was taken using the SPFM mode. There are two items to note in this scan: (a) the image does not repeat itself on consecutive scans, and (b) the feature sizes along the length scale are sharper than what is expected given the resolution of the measurement in SPFM mode. FIG. 9 illustrates the simultaneous topographic image in SPFM mode of such a single site. The features in this scan also do not repeat in consecutive scans and are also sharper than expected given the resolution of the instrument in this mode. The area of sharp features in FIG. 8 is correlated with the area of sharp features in FIG.  9 . These two figures show that the emission current is changing with time and the surface potential due to surface charging is also changing with time. This is further illustrated in FIG. 17, which illustrates a graph of emission currents versus time when the AFM tip is located at a single position within the emission site illustrated in FIGS. 4 and 5. This quite clearly shows that emission current from a single point in an emission site varies as a function of time. Therefore, it can be concluded that within an emission site such as illustrated in FIGS. 4 and 5, emission of electrons is dependent upon time, and the emission of electrons across the entire emission site also varies, so that emission from an emission site is a function of distance also. Referring to FIG. 18, an emission area such as the one imaged in FIGS. 4 and 5 is illustrated. As discussed herein, it is believed that such an emission area will be comprised of grainy bumps represented by the smaller circles within the emission area  1800 . The cross-hatched bump  1801  will at one point in time emit electrons, essentially making it a wide band gap material, while bumps  1802  surrounding emitting bump  1801  are at that point in time not emitting, which essentially makes such grainy bumps formed of insulators. 
     FIGS. 10-12 show that non-emitting regions of a sample do not show semiconductor interface behavior, but instead a wide variation in conductivity from a perfect insulator to nearly ohmic behavior. FIG. 15, however, illustrates a contact I-V spectra of an emission site, which shows a semiconductor type behavior with a relatively large band gap. 
     FIGS. 13 and 14 illustrate that there is typically no geometric enhancement at the vacuum/film interface. The image in FIG.  14  and the graph in FIG. 13 were taken using a contact AFM mode with a +6 volt biased tip. 
     Some conclusions can now be made. The emission sites are formed of geometric grainy bumps. However, such emission sites do not have microtips, but are very relatively flat. The sharpest features have a rise of ˜±20 nm over a distance of ˜1.0 nm (˜1000 nm). This corresponds to an enhancement of 2% or less, which is very flat compared to microtip cathodes. A location that emits exhibits a semiconductor behavior with a wide band gap. Furthermore, the gradient portion described previously with respect to FIG. 2 is time-dependent so that nonactive sites, such as sites E and F become active at a later time, and the active sites, A, B, and C, will become inactive. Furthermore, such periods of activity and inactivity may be coupled and may oscillate, as the transitional intermittent interfaces between the grainy bumps behave as semiconductors whereby a charge builds up and is then emitted as electrons, resulting in an emission site. Subsequently the interface between the bumps loses the charge and must again charge up to a threshold limit. During the charge up period, the emission site is inactive. For further discussion, refer to J. Robertson; “Mechanism of Electron Field Emission From Diamond and Diamond-Like Carbon,” IVMC 98, pp. 162-163, which is hereby incorporated by reference herein. 
     The areas of grainy bumps may be areas of carbon growth on the surface of the substrate. It is known by scanning electron microscope images that the carbon film is not continuous across the surface of the sample. The grains within the bump may be grains of diamond plus grains of graphite in an amorphous carbon matrix. 
     Please note that the carbon emitter of the present invention may comprise any known carbon-based field emission device, including carbon films, microtip structures, and carbon nanotubes. 
     Referring next to FIG. 3, there is illustrated field emitter device  80  configured with a film produced in accordance with the invention discovered above. Device  80  could be utilized as a pixel within a display device, such as within display  938  described below with respect to FIG.  16 . 
     Device  80  also includes anode  84 , which may comprise any well-known structure. Illustrated is anode  84  having a substrate  805 , with a conductive strip  806  deposited thereon. Then, phosphor layer  807  is placed upon conductive film  806 . An electrical potential V+ is applied between anode  84  and cathode  82  as shown to produce an electric field, which will cause electrons to emit from film  501  towards phosphor layer  807 , which will result in the production of photons through glass substrate  805 . Note that an alternative embodiment might include a conductive layer deposited between film  501  and substrate  101 . A further alternative embodiment may include one or more gate electrodes (not shown). 
     As noted above, field emitter device  80  may be utilized within field emission display  938  illustrated in FIG. 16. A representative hardware environment for practicing the present invention is depicted in FIG. 16, which illustrates a typical hardware configuration of workstation  913  in accordance with the subject invention having central processing unit (CPU)  910 , such as a conventional microprocessor, and a number of other units interconnected via system bus  912 . Workstation  913  includes random access memory (RAM)  914 , read only memory (ROM)  916 , and input/output (I/O) adapter  918  for connecting peripheral devices such as disk units  920  and tape drives  940  to bus  912 , user interface adapter  922  for connecting keyboard  924 , mouse  926 , speaker  928 , microphone  932 , and/or other user interface devices such as a touch screen device (not shown) to bus  912 , communication adapter  934  for connecting workstation  913  to a data processing network, and display adapter  936  for connecting bus  912  to display device  938 . CPU  910  may include other circuitry not shown herein, which will include circuitry commonly found within a microprocessor, e.g., execution unit, bus interface unit, arithmetic logic unit, etc. CPU  910  may also reside on a single integrated circuit. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.