Abstract:
Disclosed is an encapsulated micro-diode and a method for producing same. The method comprises forming a plurality columns in the substrate with a respective tip disposed at a first end of the column, the tip defining a cathode of the diode; disposing a sacrificial oxide layer on the substrate, plurality of columns and respective tips; forming respective trenches in the sacrificial oxide layer around the columns; forming an opening in the sacrificial oxide layer to expose a portion of the tips; depositing a conductive material in of the opening and on a surface of the substrate to form an anode of the diode; and removing the sacrificial oxide layer.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     This invention was developed under Contract DE-AC04-94AL85000 between Sandia Corporation and the U.S. Department of Energy. The U.S. Government has certain rights in this invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to field emission arrays. Specifically, the present invention relates to a cold cathode field emission vacuum diode. 
     BACKGROUND OF THE INVENTION 
     Conventionally, field emission arrays have been fabricated using thin film deposition techniques, also known as Spindt tips. Electrons emitted from the cathode (Spindt tip) are accelerated by the electric field between the cathode and the anode electrode. The cathode has an approximately conical shape, to which a predetermined electric field is applied so as to emit electrons. Moreover, when producing this Spindt type of electron emission device, a hole having a diameter of about 1 micrometer is formed and inside this hole, the emitter electrode is formed by way of deposition or the like. 
     However, in such a Spindt type of electron emission devices, it is difficult to form the aforementioned conical emitter electrode with a desired configuration, therefore resulting an a device that does not have stable electron emission characteristic. In particular, when producing an emission array, it is necessary to uniformly form the emitter electrodes over a large substrate. In other words, unless the emitter electrodes are formed uniformly, the field electron emission characteristic varies depending on a position within the array. 
     SUMMARY OF THE INVENTION 
     The inventors of the present invention have now discovered a novel cold cathode field emission vacuum diode and method of producing same. 
     According to one aspect of the invention, an exemplary method for producing an encapsulated micro diode in a substrate comprises forming a plurality columns in the substrate with a respective tip disposed at a first end of the column, the tip defining a cathode of the diode; disposing a sacrificial oxide layer on the substrate, plurality of columns and respective tips; forming respective trenches in the sacrificial oxide layer around the columns; depositing a dielectric material in the trenches and on top of the sacrificial oxide; forming an opening in the dielectric material, extending into the sacrificial oxide but not so far as to expose a portion of the tips; depositing a conductive material in of the opening and on a surface of the dielectric to form an anode of the diode; and removing the sacrificial oxide layer. 
     According to another exemplary embodiment, the method further comprises forming a respective vent at an upper portion of the substrate adjacent the opening in the sacrificial oxide prior to depositing the conductive material, wherein the step of removing the sacrificial oxide includes introducing an etchant in the vent. 
     According to yet another exemplary embodiment, the method further comprises applying a vacuum through the vent prior to depositing a second conductive material such that the diode is sealed under vacuum. 
     According to still another aspect of the invention, a method for producing an encapsulated micro diode in a substrate comprises forming a plurality columns in the substrate with a respective tip disposed at a first end of the column, the tip defining a cathode of the diode; disposing a first sacrificial oxide layer on the substrate, plurality of columns and respective tips; forming respective trenches in the first sacrificial oxide layer around the columns; forming an opening in the first sacrificial oxide layer to expose a portion of the tips; disposing a second sacrificial oxide layer in a portion of the opening in order to conform to the exposed tip and form a spacer; depositing a conductive material in a remaining portion of the opening and on a surface of the substrate to form an anode of the diode, the anode conforming to a shape of the tip; and removing the first and second sacrificial oxide layers. 
     According to a further exemplary embodiment, the method further comprises forming a respective vent at an upper portion of the substrate adjacent the opening prior to depositing the conductive material, wherein the step of depositing the conductive material includes depositing the conductive material in the vent. 
     According to yet another exemplary embodiment, the method further comprises applying a vacuum through the vent prior to depositing the conductive material such that the diode is formed and sealed under vacuum. 
     According to still another exemplary embodiment, the tip is clad with tungsten. 
     According to yet a further exemplary embodiment, the conductive material is tungsten. 
     According to yet another aspect of the invention, a exemplary method for producing an encapsulated micro diode in a substrate comprises forming a plurality of trenches in the substrate to form columnar portions therein; depositing a first sacrificial oxide in the trenches and on a surface of the substrate; polishing the first sacrificial oxide to remove the sacrificial oxide from the surface of the substrate; disposing a resist layer on a portion of the surface of the columns; applying an etchant to form a tip in the substrate at a first end of the column; removing the resist layer to expose the formed tip; depositing a second sacrificial oxide layer on the substrate and exposed tip; etching a trench in the sacrificial oxide layer to form a column of oxide within which a respective column of the substrate is encapsulated; disposing an insulator in the trench and on a surface of the sacrificial oxide; forming an orifice in the insulator above a respective one of the formed tips; forming an opening in the sacrificial oxide layer in line with the orifice to expose a portion of the tips; disposing a third sacrificial oxide layer in a portion of the opening in order to conform to the exposed tip and form a spacer; depositing a conductive material in a remaining portion of the opening and on a surface of the substrate to form an anode of the diode, the anode conforming to a shape of the tip; and removing the sacrificial oxide layers. 
     According to a further aspect of the invention, an exemplary encapsulated micro diode comprises a plurality of columnar portions formed from a substrate having a pyramidal tip at a first end forming a cathode of the diode; an insulation layer disposed between adjacent ones of the columnar portions, the insulation layer overlying the columnar portion and having an aperture therethrough in a region overlying respective ones of the tips; and an anode formed through the aperture in the insulation layer and disposed above the cathode. 
     According to another exemplary embodiment, the anode conforms to a shape of the pyramidal tip. 
     According to yet another exemplary embodiment, the tip is disposed within an envelope of the anode. 
     According to still another exemplary embodiment, the micro diode includes a second aperture in the insulation layer above the columnar portion and adapted to provide a vacuum to the diode during formation of the anode. 
     According to a further exemplary embodiment, the tip is clad with tungsten. 
     According to another exemplary embodiment, the anode is formed from tungsten. 
     Common Aspects of all Embodiments—Fabrication Up to Anode Formation:
         An exemplary method for producing an encapsulated micro diode in a substrate comprises forming a plurality of trenches in the substrate to form columnar portions therein; depositing a first sacrificial oxide in the trenches and on a surface of the substrate; polishing the first sacrificial oxide to remove the sacrificial oxide from the surface of the substrate; disposing a resist layer on a portion of the surface of the columns; applying an etchant to form a tip in the substrate at a first end of the column; removing the resist layer to expose the formed tip; depositing a second sacrificial oxide layer on the substrate and exposed tip; planarizing this second sacrificial oxide; etching a trench in the sacrificial oxide layer and silicon substrate material below to form a column of oxide within which a respective column of the substrate is encapsulated; disposing an insulator in the trench and on the surface of the sacrificial oxide; forming an orifice in the insulator above a respective one of the formed tips       

     Anode Formation—Conformal
         Forming an opening in the sacrificial oxide layer in line with the orifice in the above nitride to expose a portion of the tips using a combination of dry and wet etching processes to modify anode geometry; disposing a third sacrificial oxide layer in a portion of the opening in order to conform to the exposed tip and form a sacrificial spacer film; depositing a conductive material in a remaining portion of the opening and on a surface of the substrate to form an anode of the diode, the anode conforming to a shape of the tip.       

     Anode Formation—Non-Conformal
         Forming an opening in the sacrificial oxide layer in line with the orifice in the above nitride, using a combination of dry and wet etching processes to modify anode geometry, such as to approach the tips but not expose a portion of the tips; depositing a conductive material in a remaining portion of the opening and on a surface of the substrate to form an anode of the diode.       

     Common Aspects of all Embodiments:
         The conducting anode material is polished to remove excess material from above the dielectric film; form a vent in the dielectric material adjacent to the anode structure; dispose an etchant into the vent to remove sacrificial oxide;       

     Common Aspects of all Embodiments: Vacuum Encapsulation:
         Deposit conducting material in a non-conformal process at sufficient angle to minimize deposition below the nitride vent orifice;   According to a further exemplary embodiment, the conducting material is deposited at a thickness sufficient to seal the vent, forming a plurality of microcavities sealed at the vacuum level of deposition.   According to another exemplary embodiment, the conducting material is deposited at a thickness insufficient to seal the vent, forming a plurality of microcavities that are exposed to the environment.       

     These and other aspects of the invention will become evident in view of the detailed description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is best understood from the following detailed description when read in connection with the accompanying drawings, in which: 
         FIGS. 1   a - 1   f  are cross-sectional views illustrating steps for producing the tip portion of an encapsulated micro-diode in accordance with a first exemplary embodiment of the present invention; 
         FIG. 2  is an SEM micrograph of the tip produced in according with the process of  FIGS. 1   a - 1   f;    
         FIGS. 3   a - 3   c  are cross-sectional views illustrating further process steps for producing the encapsulated micro-diode in accordance with the first exemplary embodiment of the present invention; 
         FIG. 4  is an SEM micrograph of the encapsulated micro-diode of  FIGS. 3   a - 3 , after anode etch with the sacrificial oxide removed for clarity; 
         FIGS. 5   a - 5   h  are cross-sectional views illustrating still further process steps for producing the encapsulated micro-diode in accordance with the first exemplary embodiment of the present invention; 
         FIG. 6  is an SEM micrograph of the complete encapsulated micro-diode of in accordance with the first exemplary embodiment of the present invention; 
         FIGS. 7   a - 7   d  are SEM micrographs of additional views of the complete encapsulated micro-diode of  FIG. 6 ; 
         FIGS. 8   a - 8   b  are SEM micrographs of an encapsulated micro-diode in accordance with a second exemplary embodiment of the present invention; 
         FIG. 9  is a graph of the exemplary micro-diode illustrating current versus voltage in a forward bias condition; 
         FIG. 10  is a graph of the exemplary micro-diode illustrating current versus voltage in both forward and reverse bias conditions; 
         FIG. 11  is a graph of a simulation of the first exemplary embodiment of the present invention; 
         FIG. 12  is a graph of a simulation of the second exemplary embodiment of the present invention; and 
         FIGS. 13   a - 13   c  are a flow chart outlining a process according to an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Field emission arrays have traditionally been fabricated using thin film deposition techniques (known as Spindt tips). The inventors have discovered that the use of micro-electromechanical systems (MEMS) processing technology to fabricate a field emission vacuum diode has beneficial effects. 
     An exemplary device according to one aspect of the invention comprises an array of cold cathode field emitter tips, each associated with a blunt anode counter electrode. Both electrodes are in a vacuum cavity, created in-situ by physical vapor deposition of a metal film that seals the device at the deposition pressure, typically between 1E-03 and 1E-08 torr. An external vacuum chamber may also be incorporated to obtain sufficient vacuum levels. 
     When the exemplary device is forward biased, the field compression associated with the sharp tip of the cathode causes energy band bending that allows Fowler-Nordheim tunneling of electrons from the tip into vacuum, where they are attracted by the relative positive bias of the anode and collected. When the exemplary device is reverse biased, the rounded shape of the anode does not result in compression of the electric field lines. 
     While the field lines will still compress at the tip in reverse bias, the sign of the field is incorrect for electron emission. Without an intense field gradient at the anode surface, there is insufficient bending of the energy barrier for tunneling of electrons to occur. Current flow in reverse bias will be by field ionization rather than field emission. Field ionization occurs at local field gradients that are 3-10 times greater than field emission, thus producing asymmetrical current-voltage characteristics (diode behavior). 
     As would be understood by those skilled in the art the turn-on voltage of the exemplary device in forward bias operation is determined by the sharpness of the tip, the tip work function, the shape of the tip (half-angle of tip and shape of the shank), the gap between the cathode and the anode, and the vacuum level of the cavity. These same factors, except work function, will also affect reverse bias breakdown voltage. The onset of electron emission typically occurs at fields of 2-3 V/nm, while field ionization typically begins at about 10 V/nm or greater. 
     An exemplary fully integrated device is fabricated using MEMS processing technology. In one exemplary embodiment, the tip is fabricated from tungsten clad silicon. In an exemplary embodiment, the anode is comprised of tungsten, fabricated in a damascene process. 
     The damascene process uses both dry plasma and wet chemical etching to create a mold into which chemical vapor deposition (CVD) tungsten is deposited. These etches can be modified to manipulate the shape of the anode. This provides a smooth, rounded anode to minimize field compression. The structures can be comprised of a single cathode/anode, or an array of many cathodes/anodes. 
     Arrays are used to increase the current carrying capacity of the device. Variability in the gap spacing between the anode and cathode across an array of structures can have profound impact on device performance. Tips that are closer to the anode will turn on before tips that are farther away, such that some tips may not turn on at all, while others may be stressed with higher fields, currents and temperatures. To minimize the variability between the tips and anodes, in one exemplary embodiment, a sacrificial film is used as a spacer. The sacrificial film, an oxide deposited by CVD, for example, is highly uniform, thus creating a highly uniform gap between the tip and the anode. This type of anode is later referred to herein by the inventors as a conformal anode. 
     Furthermore, the inventors have conducted numerical simulation which indicates that the resulting conformal shape of the anode over the cathode enhances the electric field at the cathode, as compared to a simple, flat anode. The increased field reduces the turn-on voltage, and increases the tunneling current at a given operating voltage. 
     Referring to  FIGS. 1   a - 1   f  are cross-sectional views illustrating steps for producing the tip portion of an encapsulated micro-diode in accordance with a first exemplary embodiment of the present invention. Reference will also be made herein to the steps  1300 - 1330  outlined in  FIGS. 13   a - 13   c  as appropriate. 
     As shown in  FIG. 1   a  isolation trenches  102  are etched into substrate  100  (Step  1300 ), such as silicon or another semiconductor material, using known semiconductor processing techniques. As viewed from the top (see  FIG. 2  for example), trenches  102  are circumferential such that they form columns  108  in substrate  100 . Columns  108  will ultimately form the cathode of the exemplary embodiment and may thus also be referred to herein as cathode  108 . It is understood that although trenches  102  are shown as extending through substrate  100 , in fact there is substrate material below the trench area as illustrated in the micrographs shown in  FIGS. 4 ,  6 ,  7   d ,  8   a  and  8   b  for example. 
     As shown in  FIG. 1   b  a sacrificial oxide  104  is disposed in trenches  102  and on the surface of substrate  100  (Step  1302 ). This process is sometimes referred to as oxide overburden. Sacrificial oxide  104  will ultimately be etched away at the completion of processing to create a vacuum moat. 
     Next, as shown in  FIG. 1   c , oxide  104  is removed from the upper surface of substrate  100  by chemical mechanical planarization (CMP), as is well understood by those skilled in the art, resulting in oxide  104  remaining in trenches  102  (Step  1302 ). 
     Next, as shown in  FIG. 1   d , photo resist  106  is patterned on the upper surface of columns  108 . This is followed in  FIG. 1   e  by an isotropic etch of substrate  100  in the areas of columns  108  resulting in the formation of the cathode of the exemplary micro-diode (Step  1304 ). Other areas of substrate  100  that are not desired to be etched are protected using known means (not shown for simplicity). 
     Next, as shown in  FIG. 1   f , photo resist  106  is stripped from substrate  100  resulting in columns  108  revealing a pronounced tip  110  of the cathode having a pyramidal shape, for example. At this point, oxide  104  still surrounds columns  108 . This is best shown in the micrograph illustrated in  FIG. 2 . 
     Next, as shown in  FIG. 3   a , a second sacrificial oxide  112  is placed over the surface of substrate  100  and into the area of column  108  that was etched away in a previous step (Step  1306 ). Next, as shown in  FIG. 3   b , a trench  114  (circumferential in nature) is formed in substrate  100  so as to remove a portion of oxide  112  as well as the portion of substrate  100  that was disposed between columns  108  (Step  1308 ). Again, for simplicity of the figures, the portion of substrate  100  that exists below columns  108  and trench  114  is not show in this figure, but is readily understood to those skilled in the art to be present, especially when  FIGS. 4 ,  6 ,  7   d ,  8   a  and  8   b  are considered. It is contemplated that a trench  114  may extend beyond what is shown in  FIG. 3   b  and into the underlying substrate (not shown in this figure—refer to  FIG. 6  for example to illustrate this). 
     Next, as shown in  FIG. 3   c , an insulator  116 , such as low stress silicon nitride, for example, is disposed in trenches  114  and on the surface of oxide  114  (Step  1310 ). A portion of insulator is then removed using known techniques to form a place  118  (void) for the anode of the inventors&#39; micro-diode to be formed (Step  1312 ). Alternatively, a mask may be placed on the surface of oxide  112  in the positions corresponding to the anodes to be formed, followed by deposition of the insulator which is then followed by removal of the mask thus resulting in the formation of void  118 . 
       FIG. 4  is an SEM micrograph of the encapsulated micro-diode of  FIGS. 3   a - 3   c , after anode etch with the sacrificial oxide removed for clarity. 
       FIGS. 5   a - 5   c  are cross-sectional views illustrating further process steps for producing the encapsulated micro-diode in accordance with the first exemplary embodiment of the present invention. As shown in  FIG. 5   a , an conventional oxide etch is performed, through opening  118  in insulator  116 , on oxide  112  to form a void  120 , preferably having a substantially semi-circular cross section, for formation of the anode. It should be noted that in this exemplary embodiment, enough of oxide  112  is removed so as to expose cathode tip  110  (Step  1313 ). 
     Next, as shown in  FIG. 5   b , another sacrificial oxide layer  122  is disposed in void  120  to form a smaller void  123 . As shown, the oxide  122  is disposed such that it conforms with the walls of void  120  and cathode tip  110 . The thickness of oxide  122  is selected based on a desired distance between the cathode and the yet to be formed anode of the ultimate micro-diode (Step  1315 ). 
     Next, as shown in  FIG. 5   c , anode  124 , formed from tungsten for example, is disposed in void  123  (Step  1316 ) and desirably planarized (Step  1318 ) to conform with the upper surface of insulator  116 . Although tungsten is one preferable example, the invention is not so limited. Any conducting material that can be deposited by a conformal process (such as CVD, atomic layer deposition (ALD), or electroplating) are contemplated. This includes high work function materials such as gold, platinum, palladium, etc. 
     Next, as shown in  FIG. 5   d , an etch is performed to form a recess  125  in oxide  112 , followed by a second metal deposition, such as with tungsten for example, to form a cap  127  on anode  124 , as shown in  FIG. 5   e . The second metal deposition is also polished as necessary so that cap  127  is substantially level with the upper surface of insulator  116 . As a result, anode  124  will be supported after the oxides are removed in subsequent steps (described below). 
     Next, as shown in  FIG. 5   f , an orifice  126  (vent) is formed in insulator  116  (Step  1320 ). Next, as shown in  FIG. 5   g , the previously formed oxide layers are removed using known techniques revealing a well defined space  131  between anode  124  and cathode tip  110  (Step  1322 ). The inventors refer to this type of anode as a conformal anode. 
     Although cathode tip  110  is formed from the material comprising substrate  100 , the invention is not so limited. It is also contemplated that cathode tip  110  can be clad with a material such as tungsten (Step  1324 ). 
     Next, as shown in  FIG. 5   h , the substrate may be subjected to a vacuum and a conductive layer  128  formed on the surface of insulator  116  and disposed within vent  126  forming plug  130 , using angle physical vapor deposition, for example (Step  1326 ). Thus, the micro-diode may be formed in a vacuum state. Conductive layer  128  also provided electrical contact to anodes  124 . Examples of conductive layer  128  include aluminum and tungsten. Any conductive material that can be deposited by physical vapor deposition, at an appropriate angle to the substrate, are also contemplated. Included among these are gold, nickel, titanium, etc. 
     This last metal deposition are non-conformal to prevent excessive metal deposition below vent  126 . Preferably, this deposition is performed at an angle, so that the vent  126  can be choked off without deposition below the vent (which may cause electrical shorting). Consider, for example, a vent  126  with a 1 μm diameter, through a 1 μm thick nitride film, then an angle greater than 45 deg (from normal) is sufficient to seal the port without deposition below—as long as the deposition is line-of-sight. 
     Although the above embodiment illustrates that vent  126  is sealed with conductive layer  128 , the invention is not so limited. It is also contemplated that vent  126  may remain unsealed (Step  1328 ) and be exposed to the environment such that it is useful as a vacuum sensor, gas ionization detector, or other electron/ion source with closely paired anode/cathode. 
     Next, at Step  1330 , conductive layer  128  may be patterned and etched as desired. 
       FIG. 6  is an SEM micrograph of the complete encapsulated micro-diode in accordance with the first exemplary embodiment of the present invention, showing insulator  116 , cathode  108  and anode  124 . Tip  110  of cathode  108  is disposed within the envelope of anode  124  and thus not able to be seen in this figure. As mentioned above, insulator  116  extends beyond column  108  and into the underlying substrate. 
       FIGS. 7   a - 7   d  are SEM micrographs of additional views of the complete encapsulated micro-diode of the first exemplary embodiment.  FIG. 7(   a ) is an enlarged view of the cathode  108 , anode  124 , cathode tip  110  (partially disposed within the envelope of anode  124 ), vent  126  and conductive layer  128 . 
       FIG. 7   b  is a view from below illustrating the spatial relationship between cathode tip  110  and anode  124 . It is clear from this figure that cathode tip  110  is disposed within the envelope of anode  124 .  FIG. 7   c  is an enlarged view of vent  126  and plug  130 , and  FIG. 7   d  is a perspective view from above of a plurality of micro-diodes in accordance with the inventors&#39; first exemplary embodiment. 
       FIGS. 8   a - 8   b  are SEM micrographs of an encapsulated micro-diode in accordance with a second exemplary embodiment of the present invention. As can be readily seen in  FIGS. 8   a  and  8   b , cathode  108  and cathode tip  110  are formed in accordance with the first exemplary embodiment. The difference is in the formation of anode  125 . In this embodiment, void  120 , as discussed above with respect to  FIG. 5   a , is not formed so deep as to expose cathode tip  110 . Rather it is formed as a shallow void so that cathode tip  110  remains encapsulated within oxide  112 . The remaining process steps are the same, such that anode  125  is formed having a substantially smooth and curved surface disposed at a distance that is determined based on the desired biasing characteristics of the resultant micro-diode. The inventors refer to this type of anode as a non-conformal anode. The process steps for this embodiment are similar to those of the first exemplary embodiment except that in this embodiment, steps relating to the etch of the oxide to reveal the tip of the cathode (Step  1313 ) and deposition of the sacrificial oxide (Step  1315 ) are replaced with Step  1314  which is a Dry/wet etch into the oxide, where the tip of the cathode is not revealed. 
       FIG. 9  is a graph of an exemplary micro-diode illustrating current versus voltage in a forward bias condition of a conformal anode device. In  FIG. 9 , five different V/I sweeps are shown. The inventors observed that the first sweep demonstrated greater noise than follow-on sweeps because of a lower turn-on voltage. The remaining sweeps demonstrated almost identical characteristics to each other. The space charge and/or series resistance had the effect of limiting current at high applied voltage conditions. 
       FIG. 10  is a graph of the exemplary micro-diode illustrating current versus voltage in a both forward and reverse bias conditions. In  FIG. 10 , the sweeps were from −95V to +95V. Although a slight leakage current was observed, there was no breakdown in the diode under test. The noted dip in the V/I curve at about −60V is the crossover point superimposed with a positive displacement current. 
       FIG. 11  is a graph of a simulation of the first exemplary embodiment of the present invention, and  FIG. 12  is a graph of a simulation of the second exemplary embodiment of the present invention. These simulations suggest that the conformal anode embodiment produces an enhanced field at the tip of the cathode. 
     While preferred embodiments of the invention have been shown and described herein, it will be understood that such embodiments are provided by way of example only. Numerous variations, changes and substitutions will occur to those skilled in the art without departing from the spirit of the invention. Accordingly, it is intended that the appended claims cover all such variations as fall within the spirit and scope of the invention.