REFERENCE TO CO-PENDING APPLICATION

Applicant hereby claims priority of U. S. Provisional Patent Application Serial No. 61/162,339, filed on March 23, 2009 entitled "Carbon Nanotube Modules and Methods for Fabrication Thereof." which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates generally to the field of electron emission based systems such as ion microscopy, Electron Microscopy (EM), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), and Electron Beam Lithography (EBL), and more specifically to electron nanotube-based electron emission devices and methods for fabrication thereof.

BACKGROUND OF THE INVENTION

Electron emission based systems are commonly used for the semi-conductor industry as well as for Nanotechnology research and general science. One of the core components of such a system is the electron source. Such an electron source may be an electron emitting fiber, typically a nanotube, such as a Carbon Nanotube (CNT), incorporated in a nanotube-based electron emission device. GENERAL DESCRIPTION

There is a need in the art to provide a nanotube-based electron emission device designed to provide maximal electron emission therefrom. This may be achieved, inter alia, by ensuring minimal current leakage within the electron emission device, namely, minimal absorption of electrons, emitted from the electron emitting fiber, by the electron emission device.

It is noted that the term "nanotube" has a broad meaning within the present application, and includes nanowires, nanofibers and all other types of nanofilaments.

There is thus provided according to an embodiment of the present invention an electron emission device including a substrate, an intermediate layer overlaying the substrate and including a crater having an annular wall extending from an upper surface of the substrate along a central longitudinal axis of the crater, a catalyst film having a predetermined catalyst film diameter and disposed within the crater on the upper surface such that a catalyst central region of the catalyst film is substantially concentric to a central region of the crater along the central longitudinal axis, at least one nanotube is attached to the catalyst film at the catalyst central region and extending therefrom along the central longitudinal axis, and a gate layer overlaying the intermediate layer and including a central annular aperture having a predetermined gate layer aperture diameter formed over the crater for controlling the application of electric fields on the nanotube to control emission of electrons from the nanotube, wherein a ratio between the catalyst film diameter and the gate layer aperture diameter is in the range of 1/14 to 1/6.

According to an embodiment of the present invention the intermediate layer is made of an insulating material selected from silicon oxide and silicon nitride. Additionally, the gate layer is made of a flexible and low stress material. Moreover, the flexible and low stress material is selected from doped poly-silicon and doped metal.

According to another embodiment of the present invention the nanotube is a carbon nanotube. Additionally, the nanotube has an average height to width ratio greater than about 2.

There is thus provided according to another embodiment of the present invention an electron emission device fabricated in a wafer-scale fashion, thereby having the substrate as a common substrate and the intermediate layer as a common intermediate layer, and further including a plurality of catalyst films disposed within the craters on the upper surface such that a central region of each catalyst film is substantially concentric to a central region of the corresponding crater along the corresponding central longitudinal axis, each catalyst film having a predetermined catalyst film diameter, a plurality of nanotubes, wherein at least one nanotube is attached to the corresponding catalyst film at the catalyst central region of the corresponding crater, the plurality of nanotubes extending from the catalyst film along the corresponding central longitudinal axis, and a plurality of gate layers overlaying the common intermediate layer, each gate layer having a central annular aperture formed over the corresponding crater for controlling the application of electric fields on the corresponding nanotubes, each gate layer having a predetermined gate layer aperture diameter, wherein for each crater a ratio between the catalyst film diameter and the gate layer aperture diameter is in the range of 1/14 to 1/6.

There is thus provided according to yet another embodiment of the present invention an electron emission device including a common substrate, a common intermediate layer overlaying the substrate and including a plurality of craters having annular walls extending from an upper surface of the substrate along a central longitudinal axis of the corresponding crater, a plurality of catalyst films disposed within the craters on the upper surface such that a catalyst central region of each catalyst film is substantially concentric to a central region of the corresponding crater along the corresponding central longitudinal axis, each catalyst film having a predetermined catalyst film diameter, a plurality of nanotubes, wherein at least one nanotube is attached to the corresponding catalyst film at the catalyst central region of the corresponding crater, the plurality of nanotubes extending from the catalyst film along the corresponding central longitudinal axis, and a plurality of gate layers overlaying the common intermediate layer, each gate layer having a central annular aperture formed over the corresponding crater for controlling the application of electric fields on the corresponding nanotubes, each gate layer having a predetermined gate layer aperture diameter, wherein for each crater a ratio between the catalyst film diameter and the gate layer aperture diameter is in the range of 1/14 to 1/6.

According to an embodiment of ¹ the present invention the electron emission device includes conductive electrical wiring disposed on a contact pad formed on the gate layer to provide coupling of the electron emission device to auxiliary electrical - A -

components, thereby providing individual electrical addressability and control of each gate layer of the plurality of gate layers.

There is thus provided according to still another embodiment of the present invention a method of fabrication of an electron emission device, including providing a substrate, disposing an intermediate layer over the substrate, disposing a gate layer over the intermediate layer, forming a central annular aperture in the gate layer having a predetermined gate layer aperture diameter, forming a crater under the gate layer having an annular wall along a central longitudinal axis of the crater, the annular wall extending from an upper surface of the substrate across the intermediate layer, disposing a catalyst film having a predetermined catalyst film diameter within the crater on the upper surface of the substrate such that a catalyst central region of catalyst film is substantially concentric to a central region of the crater along the central longitudinal axis, wherein a ratio between the catalyst film diameter and the gate layer aperture diameter is in the range of 1/14 to 1/6, and growing at least one nanotube on the catalyst film at the central region along the central longitudinal axis.

According to an embodiment of the present invention forming the central annular aperture in the gate layer includes coating a first photo-resist layer over the gate layer, placing a first photo-mask layer having a mask aperture above the first photoresist layer, forming an aperture within the first photo-resist layer having a diameter equal to the predetermined gate layer aperture diameter, and etching the central annular aperture, having the predetermined gate layer aperture diameter, to extend from the aperture of the photo-resist layer to an upper surface of the insulating layer. Additionally, forming the crater includes etching the intermediate layer. Moreover, forming the crater includes the following steps carried out after forming the central annular aperture in the gate layer: removing the first photo-resist layer, coating a second photo-resist layer over the gate layer, placing a second photo-mask layer having a second mask aperture above the second photo-resist layer, forming an aperture within the second photo-resist layer having a diameter equal to the predetermined catalyst film diameter such that the central region of the aperture being substantially concentric to the central region along the longitudinal axis, and etching the intermediate layer to create the crater.

According to another embodiment of the present invention the method is repeated wafer-scale, thereby providing a plurality of the electron emission devices having the substrate as a common substrate and the intermediate layer as a common intermediate layer. Additionally, the method further includes disposing conductive electrical wiring on a contact pad formed on the gate layer to provide coupling of the plurality of the electron emission devices to auxiliary electrical components, thereby providing individual electrical addressability and control of each gate layer of the plurality of gate layers.

According to another embodiment of the present invention the electron emission device is utilized as an electron-emitter or field-ionizer in at least one system selected from an ion microscope, electron microscope, electron beam lithography system, inspection system, mass spectrometer, flat panel display or ionizer.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

Figs. IA and IB are a simplified cross-sectional view of a nanotube-based electron emission device constructed and operative in accordance with an embodiment of the present invention and a top view of Fig. IA, respectively; Fig. 2 is a simplified schematic illustration of a trajectory of electrons transmitted by the nanotube-based electron emission device of Figs. IA and IB;

Figs. 3A and 3B are a simplified cross-sectional view of a nanotube-based electron emission device constructed and operative in accordance with another embodiment of the present invention and a top view of Fig.3 A, respectively; Fig. 4 is a simplified schematic illustration of a trajectory of electrons transmitted by the nanotube-based electron emission device of Figs. 3A and 3B;

Fig. 5 is a simplified cross-sectional view of a nanotube-based electron emission device constructed and operative in accordance with yet another embodiment of the present invention; Fig. 6 is a simplified cross-sectional view of a nanotube-based electron emission device constructed and operative in accordance with still another embodiment of the present invention; Fig. 7 is an exemplary graph illustrating a dependence of the percentage of electrons transmitted from the nanotube-based electron emission device of Figs. 1A-6 from the total number of electrons emitted from a nanotube on a ratio between a catalyst film diameter (D2) and a gate layer aperture diameter (Dl); Fig. 8 is a simplified cross-sectional view of the nanotube-based electron emission device of Fig. 5 shown with individual electrical addressability functionality;

Fig. 9 is a simplified cross-sectional view of the nanotube-based electron emission device of Fig. 6 shown with individual electrical addressability functionality;

Figs. lOA-lOG are simplified sectional illustrations of steps in the fabrication of the nanotube-based electron emission devices of Figs. 1A-6 constructed and operative in accordance with an embodiment of the present invention; and

Figs. 1 IA-I II are simplified sectional illustrations of steps in the fabrication of the nanotube-based electron emission devices of Figs. 1 A-6 constructed and operative in accordance with another embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

As indicated above, the present invention is intended to be used with an electron emission based systems and the embodiments will therefore be described below with reference to this application. The principles of the electron emission based systems according to the present invention may be better understood with reference to the drawings and the accompanying description, wherein similar reference numerals have been used throughout to designate identical elements. It should be understood that these drawings, which are not necessarily to scale, are given for illustrative purposes only and are not intended to limit the scope of the invention. Examples of constructions, materials, dimensions, and fabrication processes are provided for selected elements. Those versed in the art should appreciate that many of the examples provided have suitable alternatives which may be utilized.

For purposes of explanation, specific configurations and details are set forth in order to provide a thorough understanding of the present invention. However, it will also be apparent to one skilled in the art that the present invention may be practiced without the specific details presented herein. Furthermore, well known features may be omitted or simplified in order not to obscure the present invention. Referring to Figs. IA, IB and 2, there is illustrated a nanotube-based electron emission device 100 according to an embodiment of the present invention. As seen in Fig. IA, the nanotube-based electron emission device 100 comprises a substrate 102 formed of any suitable material, such as any suitable conductive material, typically silicon, a transparent material or a combination thereof, for example. The substrate is a structure that provides the mechanical support for the emission device 100 and a growth surface for a source of electrons, as will be described hereinbelow. The substrate 102 can be formed of any suitable material, such as a single crystal, polycrystalline, glassy or amorphous material. The substrate can comprise one or more sublayers that may be structured to form an electronic architecture.

Overlaying the substrate 102 is an intermediate layer 104. Intermediate layer 104 may be an insulating layer formed of any suitable insulating material, such as silicon oxide or silicon nitride, for example. Overlaying insulating layer 104 is a conductive gate layer 106 operative as an anode-electrode. Gate layer 106 is preferably formed of a flexible, low stress material, such as a low stress, doped poly-silicon or a low stress, doped metal, for example. Insulating layer 104 is formed with a generally central crater 110 formed with an annular wall 114 extending from an upper surface 120 of the substrate 102 to a generally central annular aperture 126 formed in gate layer 106. Aperture 126 is formed with a diameter designated by Dl. The diameter of central crater 110, at any point along wall 114, may be any suitable size, such as equal to Dl or larger than Dl so as to prevent charging of the insulating layer 104 by electrons emitted from a nanotube 128, as will be described hereinbelow.

On upper surface 120 is configured a nanotube growth catalyst film 130 provided for growth of the nanotube 128 therefrom. The catalyst film 130 may be formed of any suitable material, typically a metal, and the nanotube 128 may be any suitable nanotube, such as a Carbon Nanotube (CNT).

A central region 150 is defined on upper surface 120 of the substrate 102. Central region 150 is a location wherein a vertical central axis 160 of the electron emission device 100 crosses the horizontal central axis 164 of the electron emission device 100 (Fig. IB). Catalyst film 130 is formed with a diameter designated by D2 and is preferably located on upper surface 120 such that a catalyst central region 170 of catalyst film 130 is sustainably concentric to central region 150 along a central longitudinal axis 180 of the electron emission device 100. The nanotube 128 is preferably attached to the substrate 102 at the catalyst central region 170, thereby situating the nanotube 128 at the central region 150. The carbon-containing fiber has an average height to width ratio greater than about 2. Gate layer 106 is operative to perform as an anode-electrode for selectively urging electrons emitted from a tip 184 of the nanotube 128 to be attracted to the gate layer 106 and thus to propagate longitudinally along longitudinal axis 180 so as to facilitate egress and transmission of the electrons from the electron emission device 100. Wherein a suitable voltage is supplied to the gate layer 106 by an appropriate voltage supply utility (not shown), an electrical field is induced on the nanaotube 128 thereby urging electrons to emit from the tip 184 of the nanotube 128. A suitable supplied voltage may be a relatively low voltage, such as in the range of about 50V- 200V, for example. Typically the supplied voltage is in the range of about 100V- 150V, for example. It is a particular feature of the present invention to design the electron emission device 100 such that the emitted electrons will be attracted to gate layer 106 so as to urge their propagation in a longitudinal orientation along longitudinal axis 180 while minimizing currant leakage of the emitted electron beam, namely, absorption of the emitted electrons within the gate layer 106, as shown in Fig. 2. It was found by the inventors of the present application that the dispersion of the electrons in the beam emitted from the nanotube 128 essentially depends on a ratio between the catalyst film diameter D2 and the gate layer aperture diameter Dl . Thus, in accordance with an embodiment of the present invention the electron emission device 100 is constructed such that the ratio between the catalyst diameter D2 and the gate layer aperture diameter Dl is predetermined so as to minimize the dispersion of the electrons in the beam, and thereby to minimize the current leakage.

In accordance with an embodiment of the present invention, the predetermined ratio (D2/D1) between the catalyst film diameter and the gate layer aperture diameter is in the range of 1/14 to 1/6. For example, D2/D1 can be at least 1/8. In accordance with another embodiment of the present invention the predetermined ratio D2/D1 may be at least approximately 1/12. In accordance with yet another embodiment of the present invention the predetermined ratio D2/D1 may be greater than 1/8, namely, Dl may be larger than 8 multiplied by D2. In a non-limiting example D2 may be approximately 0.5 microns and Dl may be approximately 5 microns.

A dependence of the percentage of electrons transmitted from the nanotube- based electron emission device 100 from the total number of the electrons emitted from the nanotube 128 on the D2/D1 ratio (when the gate voltage supply is 100V) is illustrated in Fig. 7. As can be seen in Fig. 7, when the D2/D1 ratio is less than about 1/14 (i.e., wherein Dl is equal to or greater than 14 times D2), the percentage of electrons emitted by the nanotube-based electron emission device 100 is relatively small, since the effect of the electronic field induced by gate layer 106 is negligible and a relatively small amount of electrons are emitted from nanotube 128. Likewise, when the D2/D1 ratio is in the range of about 1/6 to 1, the percentage of electrons transmitted from the nanotube-based electron emission device is also relatively small, due to the relatively high degree of current leakage. In other words, when D2/D1< 1/14 or l/6< D2/D<1, the transmission percentage is relatively small, since most of the electrons are absorbed by the gate layer 106. On the other hand, when the D2/D1 ratio is in the range of about 1/13 to 1/6, the percentage of electrons transmitted from the nanotube-based electron emission device 100 is relatively high, due to the relatively low degree of current leakage.

Additionally, in accordance with an embodiment of the present invention the nanotube 128 is grown on catalyst film 130 employing a method designated to ensure convergence of a central longitudinal axis 190 of nanotube 128 with longitudinal axis 180, namely, minimal angular deviation of the nanotube 128 from longitudinal axis 180, thus minimi/ing current leakage due to excessive proximity of nanotube tip 184 to gate layer 106 in the orientation of horizontal axis 164. Turning to Fig. 2, a trajectory of a beam transmitted by the nanotube-based electron emission device 100 is schematically illustrated. It can be seen that the electrons emitted from nanotube 128 propagate in a longitudinal orientation along longitudinal axis 180 without being absorbed within the gate layer 106 and are thus transmitted from the nanotube-based electron emission device 100. Referring to Figs. 3A, 3B and 4, there is illustrated a nanotube-based electron emission device 200 according to another embodiment of the present invention.

Electron emission device 200 is formed similarly to electron emission device 100 of Figs. 1A-2 and is provided with a cluster 210 of nanotubes 128. In accordance with an embodiment of the present invention each nanotube 128 of the nanotube cluster 210 is grown on catalyst film 130 employing a method designated to ensure that a central longitudinal axis 220 of each nanotube 128 is coplanar with longitudinal axis 180, namely, minimal angular deviation of each of the nanotubes 128 within the nanotube cluster 210 from longitudinal axis 180, thus minimizing current leakage due to excessive proximity of a nanotube tip 184 to gate layer 106 in the orientation of horizontal axis 164.

Turning to Fig. 4, a trajectory of a beam transmitted by the nanotube-based electron emission device 200 is schematically illustrated. It can be seen that the electrons emitted from nanotube cluster 210 propagate in a longitudinal orientation along longitudinal axis 180 without being absorbed within the gate layer 106 and are thus transmitted from the nanotube-based electron emission device 200.

Referring to Fig. 5, there is illustrated a nanotube-based electron emission device 300 according to yet another embodiment of the present invention. The electron emission device 300 may be formed wafer-scale comprising a plurality or an array of electron emission devices 100 of Figs. 1A-2, thereby having the substrate 102 as a common substrate and the intermediate layer 104 as a common intermediate layer (only two electron emission devices 100 are shown, but any other number of the devices is also contemplated). The nanotube-based electron emission device 300 may be fabricated by a suitable method. Examples of the fabrication methods include, but are not limited to the methods shown in Figs. 10A-11I modified to wafer-scale fabrication.

Referring to Fig. 6, there is illustrated a nanotube-based electron emission device 400 according to still another embodiment of the present invention. The electron emission device 400 may be formed wafer-scale comprising a plurality or an array of electron emission devices 200 of Figs. 3A-4 thereby having the substrate 102 as a common substrate and the intermediate layer 104 as a common intermediate layer. The nanotube-based electron emission device 400 may be fabricated by any suitable method, such as the methods described hereinbelow with reference to Figs. 10A-11I, modified to wafer-scale fabrication. Referring to Figs. 8 and 9, it is shown that each of the plurality of electron emission devices 100 or 200 within respective wafer-scale electron emission device 300 or 400 may be in direct electrical communication with auxiliary electrical components 420, such as a voltage supply, thereby providing individual electrical addressability functionality for each of the electron emission devices 100 or 200. This may be achieved in any suitable manner, such as by coupling each of the plurality of electron emission devices 100 or 200 to auxiliary electrical components 420 via conductive electrical wiring 430 disposed on a contact pad 440 formed on gate layer 106, thereby allowing direct electrical access and control of each electron emission devices 100 or 200. The wiring 430 may be disposed on contact pad in any suitable manner, such as by soldering, welding or brazing the wiring 430 thereon, etc.

Thus, for example, only predetermined electron emission devices 100 or 200 may selected to be operated while other electron emission devices 100 or 200 may be simultaneously selected to be inactive. Electron emission devices with individual addressability functionality may be utilized for a variety of applications and devices such as bio-chips, sensors, nano-pillars and field-emitters, for example.

It is further noted that electron emission devices 100, 200, 300 and 400 may be utilized as electron-emitters or field-ionizers, typically incorporated in electron emission based systems such as ion microscopes, electron microscopes, such as SEMs or TEMs, electron beam lithography, inspection systems, mass spectrometers, flat panel displays or ionizers, for example.

The fabrication of an electron emission device containing nanotube emitters according to the present invention combines semiconductor processing technology with catalytic carbon fiber growth technology. A non-limiting general procedure for fabricating an electron emission device involves the following process steps: fabricating the basic structure of the device using semiconductor processing techniques, depositing a catalyst film on the desired area of the device using semiconductor processing techniques, such as photolithographic techniques, and growing nanotube emitters. Referring to Figs. lOA-lOG, there are illustrated steps in the fabrication of the nanotube-based electron emission devices of Figs. 1A-6 constructed and operative in accordance with an embodiment of the present invention. As seen in Fig. 1OA, there is provided a stack comprising the substrate 102. The substrate 102 may, for example, have a thickness of approximately 300-500 microns, it being appreciated that the substrate thickness may be any suitable thickness. The insulating layer 104 overlying the substrate 102 may be formed in any suitable manner, such as by growing insulating layer 104 on substrate 102 by any suitable means or by depositing insulating layer 104 thereon. The insulating layer 104 may have a thickness of approximately 1-3 microns, it being appreciated that the insulating layer thickness may be any suitable thickness.

The gate layer 106 may, for example, be deposited over insulating layer 104 typically to a thickness of approximately 0.1-0.3 microns, it being appreciated that the gate layer thickness may be any suitable thickness.

A photo-resist layer 500 may, for example, be spin coated over gate layer 106 typically to a thickness of approximately 1-5 microns, for example. The photo-resist layer 500 may be formed of any suitable material, preferably a material durable to etching processes, as described hereinbelow. A photo-mask layer 508 is placed above photo-resist layer 500 and is provided to form an aperture 520 (Fig. 10B) within photoresist layer 500, via a corresponding mask aperture 522.

Aperture 520 may be formed by any suitable method, such as by employing conventional photolithography, such as contact alignment photolithography or by employing steppers so as to expose the photo-resist layer 500, thereby forming aperture 520 therein. A resultant annular wall 524 is formed within photo-resist layer 500 surrounding aperture 520 and extending to an upper surface 526 of gate layer 106. Aperture 520 is formed with the diameter D2, as seen in Fig. 1OB and a central region 530 thereof is spatially aligned to be substantially concentric to central region 150 along the longitudinal axis 180. In a non-limiting example D2 may be approximately 0.5 microns and Dl may be approximately 5 microns.

As seen in Fig. 1OC, central annular aperture 126, formed with diameter Dl, is etched within gate layer 106 and extends from aperture 520 of photo-resist layer 500 to an upper surface 534 of insulating layer 104. Aperture 126 is formed in any suitable manner, such as by wet isotropic etching of gate layer 106 or by dry isotropic etching of gate layer 106, for example. Turning to Fig. 10D, it is seen that the central crater 110 is formed within insulating layer 104. Crater 110 extends from aperture 126 to upper surface 120 of substrate 102. Crater 110 is formed in any suitable manner, such as by wet isotropic etching of insulating layer 104, for example. Catalyst film 130 is formed, such as by deposition, on upper surface 120, as seen in Fig. 1OE. The catalyst film 130 is formed via aperture 520 so as to configure the catalyst film 130 with a substantially identical diameter D2 and to align central region 170 of catalyst film 130 to be substantially concentric to central region 530 of aperture 520 along longitudinal axis 180. Following "lift-off ¹ of photo-resist layer 500 by any suitable method, such as by conventional stripping, for example, as seen in Fig. 1OF, the nanotube 128 is formed on catalyst film 130 as seen in Fig. 1OG. The nanotube 128 may be formed by any suitable formation methods. As described hereinabove in reference to Figs. IA, IB and 2, the nanotube 128 is grown on catalyst film 130 employing a method designated to ensure convergence of the central longitudinal axis 190 with longitudinal axis 180, namely, minimal angular deviation of the nanotube 128 from longitudinal axis 180. The method of growing the carbon nanotubes, can, for example, include heating the catalyst film in a gas environment containing hydrocarbons, carbon-containing compounds and/or carbon monoxide. In general, any transition metal that is a catalyst for the growth of carbon nanotubes is sufficient for the fabrication of carbon fiber emitters. Examples of the materials for the catalyst film 130 include Fe, Co, Ni, Cr, Mn, Mo, W, Re, Ru, Os, Rh, Ir, Pd, Pt, Cu, Zn, and compounds or alloys containing these elements. In a non-limiting example, the nanotube growth may be performed by Plasma

Enhanced Chemical Vapor Deposition (PECVD), which may be applied by use of commercial systems such as the BLACK-MAGIC™ system commercially available from the Aixtron Ltd. company of Buckingway Business Park, Anderson Road Swavesey, Cambridge CB244FQ, United Kingdom. It is appreciated that the order of the fabrication steps described hereinabove in reference to Figs. lOA-lOG may be alternated. Furthermore, some steps may be added or obviated.

It is noted that fabrication of electron emission devices 100, 200, 300 and 400 may be performed in any suitable manner. Referring to Figs. 1 IA-I II, there are illustrated steps in the fabrication of the nanotube-based electron emission devices of Figs. 1A-6 constructed and operative in accordance with another embodiment of the present invention. As seen in Fig. HA there is provided a stack comprising the substrate 102. The substrate 102 may have a thickness of approximately 300-500 microns, it being appreciated that the substrate thickness may be any suitable thickness. The insulating layer 104 overlying the substrate 102 may be formed in any suitable manner, such as by growing insulating layer 104 on substrate 102 by any suitable means or by depositing insulating layer 104 thereon. The insulating layer 104 may have a thickness of approximately 1-3 microns, it being appreciated that the insulating layer thickness may be any suitable thickness.

The gate layer 106 may be deposited over insulating layer 104 typically to a thickness of approximately 0.1-0.3 microns, it being appreciated that the gate layer thickness may be any suitable thickness.

A first photo-resist layer 600 may be spin coated over gate layer 106 typically to a thickness of approximately 1-5 microns, for example. The first photo-resist layer 600 may be formed of any suitable material, preferably a material durable to etching processes. A first photo-mask layer 608 is placed above first photo-resist layer 600 and is provided to form an aperture 620 (Fig. HB) within first photo-resist layer 600, via a corresponding mask aperture 622.

Aperture 620 may be formed by any suitable method, such as by employing conventional photolithography, such as contact alignment photolithography or by employing steppers so as to expose the first photo-resist layer 600, thereby forming aperture 620 with the diameter Dl. Central annular aperture 126 is etched within gate layer 106 and extends from aperture 620 of first photo-resist layer 600 to upper surface 534 of insulating layer 104. Aperture 126 is formed in any suitable manner, such as by wet isotropic etching of gate layer 106 or by dry isotropic etching of gate layer 106, for example. Aperture 126 is configured with the diameter Dl. Following "lift-off ¹ of the first photo-resist layer 600 by any suitable method, such as by conventional stripping, for example, as seen in Fig. HC, a second photoresist layer 640 may be spin coated within aperture 126 and further over gate layer 106 typically to a thickness of approximately 1-5 microns, for example, as seen in Fig. HD The second photo-resist layer 640 may be formed of any suitable material, preferably a material durable to etching processes. A second photo-mask layer 648 is placed above second photo-resist layer 640 and is provided to form an aperture 650 (Fig. HE) within second photo-resist layer 640, via a corresponding mask aperture 652.

Aperture 650 may be formed by any suitable method, such as by employing conventional photolithography, such as contact alignment photolithography or by employing steppers so as to expose the second photo-resist layer 640, thereby forming aperture 650 with the diameter D2. A central region 660 of aperture 650 is spatially aligned to be substantially concentric to central region 150 along the longitudinal axis 180. In a non-limiting example D2 may be approximately 0.5 microns and Dl may be approximately 5 microns.

Turning to Fig. HF, it is seen that the central crater 110 is formed within insulating layer 104. Crater 110 is formed in any suitable manner, such as by wet isotropic etching of insulating layer 104, for example. Catalyst film 130 is formed, such as by deposition, on upper surface 120, as seen in Fig. HG.

The catalyst film 130 is formed via aperture 650 so as to configure the catalyst film 130 with a substantially identical diameter D2 and to align central region 170 of catalyst film 130 to be substantially concentric to central region 660 of aperture 650 along longitudinal axis 180.

Following "lift-off' of photo-resist layer 640 by any suitable method, such as by conventional stripping, for example, as seen in Fig. HH, the nanotube 128 is formed on catalyst film 130 as seen in Fig. HI. The nanotube 128 may be formed by any suitable formation methods. As described hereinabove in reference to Figs. IA, IB and 2, the nanotube 128 is grown on catalyst film 130 employing a method designated to ensure convergence of the central longitudinal axis 190 with longitudinal axis 180, namely, minimal angular deviation of the nanotube 128 from longitudinal axis 180.

It is appreciated that the order of the fabrication steps described hereinabove in reference to Figs. HA-IlI may be alternated. Furthermore, some steps may be added or obviated.

It is noted that fabrication of electron emission devices 100, 200, 300 and 400 may be performed in any suitable manner.

Those skilled in the art to which the present invention pertains, can appreciate that while the present invention has been described in terms of preferred embodiments, the concept upon which this disclosure is based may readily be utilized as a basis for the designing of other structures and processes for carrying out the several purposes of the present invention. Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting. It should be noted that the word "comprising" as used throughout the appended claims is to be interpreted to mean "including but not limited to".

It is important, therefore, that the scope of the invention is not construed as being limited by the illustrative embodiments set forth herein. Other variations are possible within the scope of the present invention as defined in the appended claims. Other combinations and sub-combinations of features, functions, elements and/or properties may be claimed through amendment of the present claims or presentation of new claims in this or a related application. Such amended or new claims, whether they are directed to different combinations or directed to the same combinations, whether different, broader, narrower or equal in scope to the original claims, are also regarded as included within the subject matter of the present description.