Abstract:
Apparatus for producing a flux of charge carriers that may be used in many applications including imaging and lithography comprises an electron source which includes an emitter with a tip radius of about one nanometer and a closely configured extractor, together with a specimen for receiving an electron beam from the source. The apparatus may operate in air under atmospheric conditions and at a much reduced operating voltage.

Description:
FIELD OF THE INVENTION 
     The present invention relates to apparatus for producing a flux of charge carriers. 
     BACKGROUND OF THE INVENTION 
     Apparatus for producing a flux of charge carriers have a variety of uses in many different applications. They may be put to use in displays for consumer electronics, in imaging and diagnostic systems used in research and development applications, in switching and amplifying circuits in sensors and in lithography systems for manufacturing. 
     In the field of imaging and lithography, apparatus for producing a flux of charge carriers may be used as proximity probes as well as to generate electron beams. 
     One type of proximal probe is a scanning tunnelling microscope (STM). 
     A STM comprises a tip that is scanned over the surface of a specimen. Electrons are emitted from the tip and tunnel into the specimen. The rate of tunnelling is highly sensitive to the separation of the tip and specimen and the measured current, together with the position of the tip, may be used to build an image of the specimen. 
     Although STM imaging has very high resolution, it has several disadvantages. For example, the tip is mechanically, rather than electrically, scanned across the specimen, thus reducing its raster speed compared with electron beam systems. Furthermore, image information may only be obtained from primary electrons, in this case, electrons tunnelling between the tip and the specimen. This may be compared with other electron imaging systems in which image information may also be obtained from secondary electrons that are generated when sufficiently energetic primary electrons strike the specimen. Another disadvantage is that the specimen must be arranged a few nanometers from the tip of the STM for electron tunnelling to occur. This is difficult to achieve and attempts to position the tip often result in the tip crashing into the specimen. 
     An example of using an STM in lithography is given in “Hybrid atomic force/scanning tunnelling lithography” by K. Wilder, H. T. Soh, A. Atalar and C. F. Quate, Journal of Vacuum Science and Technology, volume B15, pp 1811-1817 (1997). 
     Another type of apparatus for producing a flux of charge carriers and which may be used for imaging and lithography is microcolumn electron beam system. A microcolumn is a miniaturised version of a conventional electron beam system and an example of a microcolumn is given in “Experimental evaluation of a 20×20 mm footprint microcolumn” by E. Kratschmer, H. S. Kim, M. G. R. Thomson, K. Y. Lee, S. A. Rishton, M. L. Yu, S. Zolgharnain, B. W. Hussey and T. H. P. Chang, Journal of Vacuum Science and Technology, volume B14, pp 3792-3796 (1996). 
     A microcolumn comprises a field emitter, a beam-forming source lens, scanning electrodes and a beam focussing objective lens. The electrode and lenses are arranged around the axis of the column only a few millimeters long. The microcolumn operates with beam voltages of the order of 100-1000 volts. 
     One advantage of a microcolumn over larger electron beam systems is that lens aberration is reduced. Furthermore, an array of microcolumns separated from one another by a few millimeters may be used in parallel to expose the surface of a wafer. However, microcolumns have several disadvantages. For example, a microcolumn operates under high vacuum conditions, requiring the sample to be kept in a vacuum. This prohibits imaging of a sample in air, which would be advantageous in biological applications. Furthermore, microcolumns are generally complex and expensive to manufacture. In addition, the minimum beam voltage that can be used is limited by space charge, where electrons within the beam repel each other and cause the beam to broaden. A broader beam results in loss of resolution for lithography and imaging. 
     Electron beams can also be used to determine the composition of sample. For example, conventional scanning electron microscopes (SEMs) may be used to perform energy dispersive X-ray (EDX) spectroscopy and wave dispersive spectroscopy (WDS). However, the instruments used for these types of spectroscopy are cumbersome and require keeping in a vacuum. This prevents analysis of large samples or specimens that can only be analysed in air. 
     Apparatus for producing a flux of charge carriers may be used in flat panel displays. One type of device is a field emitting display device used in flat panel displays. An example of such a device may be found in U.S. Pat. No. 5,955,850 and comprises an array of field emitters, each comprising a conical cathode and an extractor electrode, and a common anode. The device may be fabricated using standard microelectronic processing techniques on a common substrate. However, the device has several disadvantages. The space between the cathode and the anode must be evacuated and the operative voltage is in the region of 1000&#39;s of volts. 
     Recently, a field emission device has been developed that has a much reduced operating voltage. The device is described in “Nanoscale field emission structures for ultra-low operation at atmospheric pressure” by A. A. G. Driskill-Smith, D. G. Hasko and H. Ahmed, Applied Physics Letters, volume 71, pp 3159-3161 (1997). The device comprises an emitter having a tip radius of about one nanometer and a closely configured extractor electrode. This laboratory experimental device allows field emission to occur at very low voltages and the emitted electrons to travel ballistically from the emitter tip to the extractor electrode even under atmospheric conditions. 
     This device has been modified to produce a nanoscale electron tube and is described in “The “nanotriode”: A nanoscale field-emission tube” by A. A. G. Driskill-Smith, D. G. Hasko and H. Ahmed, Applied Physics Letters, volume 75, pp 2845-2847 (1999). The device comprises a nanometer-scale chamber comprising an emitter and a closely configured gate electrode, sealed under vacuum by an integrated anode. One of the advantages of the triode is that it has a low operating voltage, while the gate electrode may be used to control the anode current. 
     However, such a triode is not suitable for generating an electron beam outside of the device because the structure is sealed. 
     The present invention seeks to provide an improved apparatus for producing a flux of charge carriers. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided apparatus for producing a flux of charge carriers comprising a source which comprises an emitter having a nanometer scale tip radius on a common substrate with an extractor arranged no more than 50 nm from the emitter to extract charge carriers therefrom and a specimen adjacent the source, to receive a flux of charge carriers from the source. 
     The emitter may have a tip radius less than 2 nm or less than 1 nm. 
     According to the present invention there is also provided apparatus for producing a flux of charge carriers comprising a source which comprises an emitter and an extractor to extract charge carriers from the emitter, wherein the emitter and the extractor are configured on a common substrate and a specimen, wherein the emitter and the specimen are arranged in a near-field configuration. 
     In the near-field configuration phase coherence of the charge carriers may be substantially maintained. 
     The near-field configuration may comprise an arrangement whereby the emitter and the specimen are disposed less than 200 nm from each other. 
     The extractor may be arranged no more than 50 nm from the emitter. 
     The extractor may be arranged no more than 30 nm from the emitter. 
     According to the present invention there is still further provided apparatus for producing a flux of charge carriers comprising a source which comprises an emitter and an extractor to extract charge carriers from the emitter, wherein the emitter and the extractor are configured so as to allow extraction of charge carriers under a gaseous atmosphere without ionisation of the gas and wherein a specimen adjacent the source, to receive a flux of charge carriers from the source. 
     The emitter and extractor may be configured such that said charge carriers are extracted while a bias is applied to the extractor relative to the emitter. 
     The relative applied bias may be positive and between 7 to 20 V. 
     According to the present invention there is further provided apparatus for producing a flux of charge carriers comprising a source which comprises an emitter and an extractor to extract charge carriers from the emitter and configured to extract charge carriers while a turn-on bias of less than 100V is applied to the extractor relative to the emitter and a specimen adjacent the source, to receive a flux of charge carriers from the source. 
     The turn-on bias may be less than 10V. 
     The apparatus may include a bias source to apply a bias to the specimen relative to the emitter. 
     The bias applied by the source to the specimen may be positive relative to the emitter and between 14 to 40 V relative to the emitter. 
     The emitter may comprise a metal, for example tungsten. 
     The emitter may comprise a tip member, for example comprising an alloy of gold and palladium, with a radius less than 2 nm. 
     The extractor may comprise tungsten and may comprise a sheet having an aperture. 
     The diameter of the aperture may be less than 100 nm or 50 nm. 
     The source may further comprise means for collecting charge carriers. 
     The source may further comprise means for deflecting flux of charge carriers. 
     The source may further comprise means for focussing the flux of charge carriers. 
     The flux of charge carriers may be a charge carrier beam. 
     The apparatus may be configured to operate in air at atmospheric pressure. 
     The charge carriers may be electrons. 
     The emitter and the specimen may be disposed less than 200 nm from each other. 
     According to the present invention there is also provided a method of producing a flux of charge carriers, the method comprising providing a source comprising configuring an emitter having a nanometer scale tip radius on a substrate with an extractor arranged no more than 50 nm from the emitter to extract charge carriers therefrom and wherein providing a specimen adjacent the source, to receive a flux of charge carriers from the source. 
     According to the present invention there is also provided a method of fabricating a source, the method comprising depositing a thin film and allowing said thin film to coalesce into individual particles. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the present invention will now be described by way of example with reference to the following drawings in which: 
     FIG. 1 is an exploded perspective view of a first embodiment of an electron source; 
     FIG. 2 is cross-sectional view of the first embodiment of an electron source taken along the line A-A′ as shown in FIG. 1; 
     FIG. 3 shows a first embodiment of the present invention; 
     FIG. 4 shows the fabrication sequence of the first embodiment of an electron source; 
     FIG. 5 a  is a cross-section of a second embodiment of an electron source taken along the line B-B′ as shown in FIG. 6; 
     FIG. 5 b  is a cross-section of a second embodiment of an electron source taken along the line C-C′, as shown in FIG. 6; 
     FIG. 6 is a plan view of the second embodiment of an electron source; 
     FIG. 7 shows a second embodiment of the present invention; 
     FIG. 8 is a cross-sectional view of a third embodiment of an electron source and 
     FIG. 9 shows a third embodiment of the present invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     First Embodiment 
     Electron Source Structure 
     Referring to FIGS. 1 and 2, a first embodiment of an electron source  1  is shown in an exploded perspective view and in cross-section respectively. 
     The source  1  comprises an insulating substrate  2  on which is disposed a cathode layer  3 . The insulating substrate comprises silicon dioxide grown on single crystal silicon. The cathode layer  3  comprises tungsten and has a thickness of 100 nm. A plurality of conductive posts  4  are upstanding from a shallow recess  5 , which is 15 nm deep and is substantially circular in plan view, in the cathode layer  3  and have particles  6  sitting thereon so as to form a plurality of pillars  7 . In this example, an individual particle  6  sits on each post  4 . The posts  4  comprise tungsten and have an aspect ratio of approximately 10 to 1. The conductive particles  6  comprise an alloy of gold and palladium and have a diameter of less than 1 nm, although conductive particles  6  having diameters in a range of less than 50 nm may be used. Preferably, the conductive particles  6  have diameters less than 10 nm. More preferably, the conductive particles  6  have diameters less than 2 nm. 
     One of the pillars  7  forms an emitter  8  by a self-selecting process, the details of which will be described later. The radius of the tip of the emitter  8  is defined by the size of the conductive particle  6 . 
     An insulating layer  9  overlies the cathode layer  3  and separates an extractor electrode  10  from the cathode layer  3 . The insulating layer  9  comprises silicon dioxide and has a thickness of 50 nm. The extractor electrode layer  10  comprises tungsten and has a thickness of 20 nm. Both the insulating layer  9  and the extractor electrode  10  have apertures, the centres of which are substantially coaxial with the centre of the recess  5 . The extractor electrode  10  causes cold emission of electrons from the tip of the emitter  8 , when positively biased relative to the cathode layer  3 . 
     A dielectric layer  11 , for example comprising silicon dioxide having a thickness of 50 nm, overlies the extractor electrode  10  and separates a collector  12  from the extractor electrode  10 . The collector  12  comprises a collector layer  13  and a capping layer  14 . In this example, the collector layer  13  comprises tungsten and has a thickness of 30 nm, while the capping layer  14  comprises aluminium and has a thickness of 10 nm. The dielectric layer  11 , the collector layer  13  and the capping layer  14  have apertures, the centres of which are substantially coaxial with the centre of the recess  5 . A plurality of conductive posts  15  are upstanding from the surface of the capping layer  14  furthermost from the substrate  2  and have conductive particles  6  sitting thereon so as to form a plurality of pillars. In this example an individual particle  6  sits on each post  15 . 
     The apertures are substantially circular in plan view and have diameters of approximately 50 nm, although apertures having diameters in the range of 10-100 nm may be used. Preferably, the apertures have diameters in the range of 10-50 nm. The apertures may be elliptical or regularly or irregularly polygonal. The edges of the apertures may be rough. 
     An advantage of using an emitter with such a small radius is that the energy spread of the emitted electrons is reduced. Furthermore, the beam brightness is increased. This is because the conductive particle  6  comprises a cluster of just a few atoms. The small size of the cluster results in quantisation of electronic states, which filters energy of electrons tunnelling from the conductive post into the cluster, thus reducing energy spread. The geometry of the tip causes concentration of the local electric field, resulting in a higher rate of emission of electrons. 
     Configuration of the Electron Source and Specimen 
     Referring to FIG. 3, a first embodiment of the present invention is a scanning electron microscope comprising the electron source  1  as described with reference to FIGS. 1 and 2 and a specimen  16 . In this example, the specimen  16  is a semiconductor chip. 
     The specimen  16  is arranged substantially parallel to the layers comprising the source  1  on a moveable x-y stage  17  driven by stepper motors (not shown). The position of the stage  17  is accurately determined using well-known laser interferometric techniques. The electron source  1  is arranged on a piezoelectric translator  18 , similar to those used for scanning probe microscopes. The separation of the tip of the emitter  8  from the facing surface of the specimen  16  is about 200 nm. 
     Unlike conventional electron beam systems, including microcolumns, the source  1  and the specimen  16  sit in air at room temperature and atmospheric pressure. The reason for this is that the specimen  16  is arranged in a near-field configuration relative to the tip of the emitter  8  and this will be discussed later in more detail. 
     To produce an electron beam P, the specimen  16  is grounded and first and second biases V 1 =−8V and V 2 =−15V are applied to the extractor electrode  10  and the cathode layer  3  respectively. However, a first bias in the range −1≧V 1 ≧−20V and a second bias in the range −1≧V 2 ≧−20V could be used. A person skilled in the art will be able to determine what biases may be applied to the extractor electrode  10 , cathode layer  3  and the specimen  16  in order to produce an electron beam P and to prevent insulating layer and dielectric layer breakdown. This may include application of positive biases. 
     The theory of operation of the source is not fully understood and the following discussion is presented by way of non-limiting explanation. The pillars  7  have slightly different shapes, sizes and other characteristics and it is believed that the pillar  7  having the most favourable conditions for field emission adopts the role of the emitter  8 . Field emission becomes possible when the local electric field at the surface of the emitter  8  exceeds approximately 10 9 Vm −1 . A number of factors determine which pillar  7  has the most favourable conditions for field emission, including aspect ratio. A high aspect ratio is advantageous. A higher aspect ratio is achieved by making the pillar taller or having a smaller tip radius. The degree of shielding from surrounding pillars  7  may also determine which pillar becomes the emitter  8 . If will be appreciated that conditions at any one pillar  7  may change, thus affecting whether the pillar becomes the most favourable pillar. For example, the emitter  8  may “burn” itself out and may be replaced by the next most favourable pillar. Alternatively, the conditions may change randomly or due to slight changes in applied voltages. 
     The emitter  8  and specimen  16  form a cathode and an anode respectively. A flux of electrons is directed in a beam P towards the specimen  16 . A proportion of the electrons are collected by the extractor electrode  10  and the collector  12  before they reach the specimen  16 . 
     The electron beam P reaches the specimen  16  having an electron beam energy of the order of 10 eV, in this example about 10 eV since the potential difference between the specimen  16  and the cathode layer  3  is 15V and the work function of tungsten is about 5 eV. This is at least an order of magnitude less than conventional electron beam systems, even microcolumns, although such low beam energies have been possible using complicated retarding energy stages. Furthermore, the electron beam energy is at least a factor of three larger than in scanning probe systems. The electron beam energy is a few times that of the work function of the surface of the specimen  16  and thus the electron beam P may be used to probe the surface electronic structure of the specimen  16 . 
     The electron beam P will reach the specimen  16  having a beam diameter of approximately  60  nm or smaller. Preferably, the electron beam P will reach the specimen  16  having a beam diameter less than 10 nm. The beam diameter may be smaller than 60 nm due to a narrow emission angle from the emitter  8  and focussing by the extractor electrode  10  and the collector  12 . 
     In this example, the beam current is of the order of a few nanoamperes. 
     The electron beam P strikes the specimen  16  and generates secondary electrons S having a lower energy than electrons of the electron beam P (so-called primary electrons), the secondary electrons S are collected by the collector  12 . The secondary electrons S may generate further secondary electrons S′ with still lower energies. The rate of secondary electron emission is dependent upon surface composition and topography. The greater the rate of secondary electron emission, the higher the current detected by the collector  12 . 
     A personal computer (not shown) runs an application, which controls movement of the x-y stage  17 , the piezo translator  18  and measures changes in the collected current A. Thus, an image of the specimen  16  may be obtained and displayed by the computer. It will be appreciated that other, customised systems may be used instead of a personal computer. 
     The advantage of this scanning electron microscope is that it need not operate under a vacuum. There are two reasons for this. Firstly, little or no scattering of the electron beam P takes place because the separation of the tip of the emitter  8  and the specimen is less than the electron mean free path in air, which is about 200 nm at low electron energies and at atmospheric pressure, i.e. 760 Torr. Secondly, there is no significant degradation of the emitter to impact ionisation because the operating voltage may be lowered below the first ionisation potential of molecules present in the air, namely 12.7 and 15.6 eV for nitrogen and water respectively. 
     If however, the ambient pressure is reduced below atmospheric pressure, the emitter  8  to specimen  16  separation may be increased. For example, at a pressure of 76 Torr, the electron mean free path is about 2 μm. Under high-vacuum conditions, the mean-free path limitation is relaxed and near-field condition is governed by geometric considerations. 
     The near-field configuration may be defined in terms of the behaviour of electrons within the beam P. In the near-field configuration, the phase coherence of the electron beam P is maintained to a high degree. This may be compared with the situation in the far-field configuration, where there is little or no phase coherence. Phase coherence is lost through interactions of electrons with other particles and electromagnetic fields. As the separation between the emitter  8  and the specimen  16  increases, electrons are subject to more interactions. In the case of in-air operation, the most significant interaction is with air molecules. As the mean free path of electrons in air is about 200 nm this is about the limit of separation between the emitter  8  and the specimen  16  for near-field operation. In the case of vacuum operation, the separation may be increased. 
     Those skilled in the art are able to test the degree of coherence in the electron beam P by routine experiment. For example, the image of a specimen  16  varies according to the degree of coherence of the electron beam P. A simulated image, dependent on the characteristics of the specimen  16  and the degree of coherence of the beam may be calculated. The specimen  16  is characterised using transmission electron microscopy, scanning tunnelling microscopy or atomic force microscopy. This characterisation is used to determine simulated images of the specimen  16  under near- and far-field configurations. The specimen  16  is then imaged at different distances from the source and at different ambient pressures, so as to change the field configuration. These images are compared with each other and with the simulated images to establish the limits of near-field configuration. 
     It will be appreciated that the electron beam P may be magnetically scanned placing external scan coils (not shown) around the source  1  and the specimen  16 . 
     Electron Source Fabrication 
     Referring to FIGS. 4 a  to  4   d , a method of fabricating the electron source  1  will now be described. 
     Using a p-type silicon wafer  2 ′, a silicon dioxide substrate  2  is grown by wet oxidation at 1000° C. The thickness of the silicon dioxide substrate  2  is 200 nm. A plurality of layers  3 ′,  9 ′,  10 ′,  11 ′,  13 ′,  14 ′ are radio frequency sputter deposited under an argon atmosphere in a cold-walled sputter chamber having a base pressure of less than 1×10 −6  Torr in a manner well known per se. 
     A first layer of tungsten  3 ′ of thickness 100 nm is sputtered and first layer of silicon dioxide  9 ′ of thickness 50 nm is deposited thereon. Then, a second layer of tungsten  10 ′ of thickness 20 nm is deposited. A second layer of silicon dioxide  11 ′ of thickness 50 nm is sputtered followed by a third layer of tungsten  13 ′ of thickness 30 nm. Finally, an aluminium layer  14 ′ of thickness 15 nm is deposited. 
     Polymethylmethacrylate electron beam resist is applied to the aluminium layer  14 ′, patterned using an 80 kV electron beam of spot size 10 nm. A circular window  19  of diameter 50 nm is left in the electron beam resist  20 . The corresponding structure is shown in FIG. 4 a.    
     The remaining electron beam resist  20  serves as a mask to dry and wet etches. 
     Using a 10:1 mixture of CF 4  and O 2  feed gases, a reactive ion etch (RIE) removes the unmasked portion of the aluminium layer  14 ′ and portions of layers underlying to give the configuration as shown in FIG. 4 b . The lateral extent of etching in each layer varies due to variation in exposure to the etch and due to different etch rates. 
     A granular thin film of gold-palladium is deposited by thermal evaporation at a pressure less than 1×10 −6  Torr. The film coalesces to form individual particles  6 ′ of gold-palladium across the surface of the structure as shown in FIG. 4 c . The gold-palladium particles  6 ′ have a diameter of approximately 2-3 nm. 
     Using CF 4 /O 2  RIE, the first tungsten layer  3  and the aluminium layer  14  are anisotropically etched with the gold-palladium particles  6 ′ serving as self-aligned masks to form tungsten and aluminium posts  4 ,  15  respectively. During etching the gold-palladium particles  6 ′ are reduced in size less than 1 nm to form smaller particles  6 . The resulting structure is shown in FIG. 4 d , which corresponds to the source  1  shown in FIGS. 1 to  3 . 
     Second Embodiment 
     Electron Source Structure 
     Referring to FIGS. 5 and 6, a second embodiment of an electron beam source  21  is shown in cross-section and plan view respectively. The second embodiment includes integral beam scanning electrodes. FIG. 5 a  is a cross-section of the device taken along the line B-B′ as shown in FIG.  6  and FIG. 5 b  is a cross-section of the device taken along the line C-C′, as also shown in FIG.  6 . 
     The source  21  comprises an insulating substrate  22  on which is disposed a cathode layer  23 . The insulating substrate  22  comprises silicon dioxide grown on single crystal silicon. The cathode layer  23  comprises tungsten and has a thickness of 100 nm. A plurality of conductive posts  24  are upstanding from a shallow recess  25 , which is 15 nm deep and is substantially circular in plan view, in the cathode layer  23  and have particles  26  sitting thereon so as to form a plurality of pillars  27 . In this example, one particle  26  sits on every post  24 . The posts  24  comprise tungsten and have an aspect ratio of approximately 10 to 1. The conductive particles  26  comprise an alloy of gold and palladium and have a diameter less than 1 nm, although conductive particles  26  having diameters less than 50 nm may be used. Preferably, the conductive particles  26  have diameters less than 10 nm. More preferably, the conductive particles  26  have diameters less than 2 nm. 
     One of the pillars  27  forms an emitter  28  by a self-selecting process as described hereinbefore. The radius of the tip of the emitter  28  is defined by the size of the conductive particle  26 . 
     An insulating layer  29  overlies the cathode layer  23  and separates an extractor electrode  29  from the cathode layer  23 . The insulating layer  29  comprises silicon dioxide and has a thickness of 50 nm. The extractor electrode layer  30  comprises tungsten and has a thickness of 20 nm. Both the insulating layer  29  and the extractor electrode  30  have apertures, the centres of which are substantially coaxial with the centre of the recess  25 . The extractor electrode  30  causes cold emission of electrons from the tip of the emitter  28 , when positively biased relative to the cathode layer  23 . 
     A dielectric layer  31 , for example comprising silicon dioxide having a thickness of 50 nm, overlies the extractor electrode  30  and separates scanning electrodes  32   a ,  32   b ,  32   c ,  32   d  from the extractor electrode  30 . The scanning electrodes  32   a ,  32   b ,  32   c ,  32   d  comprise a segmented scanning electrode layer  33  and a segmented capping layer  34 . In this example, the scanning electrode layer  33  comprises tungsten and has a thickness of 30 nm, while the capping layer  34  comprises aluminium and has a thickness of 10 nm. The dielectric layer  31  has an aperture. The scanning electrodes  32   a ,  32   b ,  32   c ,  32   d  are arranged about an aperture, the centre of which is substantially coaxial with the centre of the recess  25 . A plurality of conductive posts  35  are upstanding from the surface of the capping layer  34  furthermost from the substrate  22  and have conductive particles  26  sitting thereon so as to form a plurality of pillars. In this example, an individual particle  26  sits on every post  35 . A plurality of insulating posts are upstanding from the exposed surface of the dielectric layer  31  and have conductive particles sitting thereon so as to form a plurality of pillars. 
     The apertures are substantially circular in plan view and have diameters of approximately 50 nm. 
     Configuration of the Electron on Source and Specimen 
     Referring to FIG. 7, a second embodiment of the present invention is a scanning electron microscope comprising the electron source  21  as described with reference to FIGS. 5 and 6 and a specimen  37 . In this example, the specimen  37  is a semiconductor chip. 
     The specimen  37  is arranged substantially parallel to the layers comprising the source  21  on a base plate  38 . The electron beam source  21  is arranged on a moveable stage  39  attached to the inside of chamber cover  40 . The stage  39  is moved by means of x, y, and z screw gauge micrometers  41   a ,  41   b ,  41   c . It will be appreciated that the stage may also be moved by means of stepper motors. A chamber  42  is formed when the chamber cover  40  sits on the base plate  38  and is sealed by a rubber ‘O’-ring  43 . The chamber  42  is evacuated through a vacuum line  44  connected to a vacuum system (not shown) comprising a roughing pump and a turbo molecular pump. The chamber  42  may be evacuated to a pressure lower than 1×10 2  Torr. Preferably, the chamber  42  is evacuated to a pressure lower than 1×10 −2  Torr. 
     The separation of the tip of the emitter  28  from the facing surface of the specimen  37  is about 1 μm. 
     To produce an electron beam P, the specimen  37  is grounded and first and second biases V 1 =−8V and V 2 =−15V are applied to the extractor electrode  30  and the cathode layer  23  respectively. However, a first bias in the range −1≦V 1 ≦−10V and a second bias in the range −10≦V 2 ≦−20V could be used. A person skilled in the art would be able to determine what biases may be applied to the extractor electrode  30 , cathode layer  23  and the specimen  37  in order to produce an electron beam P and to prevent insulating layer and dielectric layer breakdown. This may include application of positive biases. This may also include application of biases of a greater magnitude. 
     The emitter  28  is the pillar  27  having the most favourable conditions for field emission as explained earlier with respect to the first embodiment. 
     The emitter  28  and specimen  37  form a cathode and an anode respectively. Electrons are directed in a beam P towards the specimen. A proportion of the electrons are collected by the extractor electrode  30 . 
     The electron beam P reaches the specimen  37  having a beam diameter of approximately 200 nm or smaller. Preferably, the electron beam P will reach the specimen  37  having a beam diameter less than 40 nm. The beam diameter may be smaller than 200 nm due to a narrow emission angle from the emitter  28  and focussing by the extractor electrode  30  and the scanning electrodes  32 . 
     In this example, the beam current is of the order of a few nanoamperes. 
     The electron beam P strikes the specimen  37  and generates secondary electrons S having a lower energy that the primary electrons, which are collected by a detector  45  mounted on the inside of the chamber cover  40 . The detector  45  comprises a sheet metal electrode held at substantially the same potential as the sample. The electron beam P strikes the specimen  37  and generates secondary electrons S. As explained earlier, the rate of secondary electron emission is dependent upon surface composition and topography. The greater the rate of secondary electron emission, the higher the current A detected by the collector  45 . 
     The electron beam P may be scanned by applying negative biases to the scanning electrodes  32   a ,  32   b ,  32   c ,  32   d . The negative applied biases are of the order of 10V, although the exact values are found by means of calibration by applying test biases and measuring electron beam P deflection. Scanning of the beam P is controlled using a personal computer  46 . The personal computer  46  also receives the value of current A detected by the collector  45  so is able to assemble an image of the surface of the specimen  37 . 
     It will be appreciated that the scanning electrodes may be used for focussing. It will be appreciated that an additional dielectric and segmented electrode layers may be added to form separate focussing electrodes. 
     Device Fabrication 
     The second embodiment of an electron source  21  is fabricated using the same process steps as the first embodiment of an electron source  1 . However, the polymethylmethacrylate electron beam resist  20  is patterned slightly differently. 
     A disc is exposed as before, to produce a circular window  19  in the electron beam resist  20 . However, a cross pattern, centred on the disc, is also exposed. This cross is underexposed with the effect that the resist is thinned during development. During dry etching the thinned resist is eventually consumed, exposing the surface of the layer structure and delaying the onset of etching. The cross pattern corresponds to the cross pattern as shown in FIG.  6 . 
     Third Embodiment 
     Electron Source Structure 
     A third embodiment of an electron beam source  47  is shown in cross-section in FIG.  8  and comprises a two-electrode structure. A plurality of sources are integrated in an array on a common substrate as shown in FIG.  9 . 
     Referring to FIG. 8, the source  47  comprises an insulating substrate  48  on which is disposed a cathode layer  49 . The insulating substrate  48  comprises silicon dioxide grown on single crystal silicon. The cathode layer  49  comprises tungsten and has a thickness of 100 nm. A plurality of conductive posts  50  are upstanding from a shallow recess  51 , which is 15 nm deep and is substantially circular in plan view, in the cathode layer  49  and have particles  52  sitting thereon so as to form a plurality of pillars  53 . In this example, an individual particle  52  sits on each post  50  respectively. The posts  50  comprise tungsten and have an aspect ratio of approximately 10 to 1. The conductive particles  52  comprise an alloy of gold and palladium and have a diameter less than 1 nm, although the conductive particles  52  may have diameters less than 50 nm may be used. Preferably, the conductive particles  52  have diameters less than 10 nm. More preferably, the conductive particles  52  have diameters less than 2 nm. 
     One of the pillars  53  forms an emitter  54  by a self-selecting process as described hereinbefore. The radius of the tip of the emitter  54  is defined by the size of the conductive particle  52 . 
     An insulating layer  55  overlies the cathode layer  49  and separates an extractor electrode  56  from the cathode layer  49 . The insulating layer  55  comprises silicon dioxide and has a thickness of 50 nm. The extractor electrode layer  56  comprises tungsten and has a thickness of 20 nm. Both the insulating layer  55  and the extractor electrode  56  have apertures, the centres of which are substantially coaxial with the centre of the recess  51 . The extractor electrode  56  causes cold emission of electrons from the tip of the emitter  54 , when positively biased relative to the cathode layer  49 . 
     A capping layer  57  overlies the extractor electrode  56 . It comprises aluminium and has a thickness of 10 nm. A plurality of conductive posts  58  are upstanding from the surface of the capping layer  57  furthermost from the substrate  48  and have conductive particles  52  sitting thereon so as to form a plurality of pillars. In this example one particle  52  sits on every post  50 . 
     The apertures are substantially circular in plan view and have diameters of approximately 50 nm. 
     Configuration of the Electron Source and Specimen 
     Referring to FIG. 9, an array of the electron beam sources  47  is used in parallel electron beam lithography system to expose a wafer  59  coated with electron beam resist  60 . 
     The electron beam sources  47   a ,  47   b ,  47   c  share the same insulating substrate  48  and are electrically isolated from each other by laterally disposed isolation layers  61   a,    61   b ,  61   c ,  61   d . In this example, the electron beam sources  47   a ,  47   b ,  47   c  are separated from one another by 5 mm and are arranged according to a rectangular array over a circular area of about 110 square inches. It will be appreciated that the electron sources  47   a ,  47   b ,  47   c  may be separated by only a few 100&#39;s nm. 
     The array of electron beam sources  47  and the wafer are held under vacuum and the emitter  54  of each source  47  is disposed from the surface of the resist  60 , by 200 to 400 nm. The emitters  54  and the resist  60  are arranged in the near-field configuration and thus there is no need to use focussing lenses. 
     To produce an array of electron beams Pa, Pb, Pc, the wafer is grounded and first and second pairs of biases V 1a , V 2a , V 1b , V 2b , V 1c , V 2c  are applied to the extractor electrode  56  and cathode layers  49  of each respective electron beam sources  47   a ,  47   b ,  47   c . The first and second pairs of biases V 1a , V 2a , V 1b , V 2b , V 1c , V 2c  of each respective source  47   a ,  47   b ,  47   c  are determined during a calibration process. The process includes moving a Faraday cup from one source  47   a ,  47   b ,  47   c  to another, measuring the beam current and changing the first and second biases V 1a , V 2a  until a desired beam current is obtained. The first and second biases V 1a , V 2a  have values of the order of 1-10V. 
     To expose the resist  60 , the array of sources  47  are mechanically scanned by means of an x-y stage (not shown), driven by stepper motors and whose position is determined by well-known laser interferometric techniques. The stepper motors are controlled by a personal computer (not shown). 
     It will be appreciated that each source  47  may be mounted on a micromachined combdrive actuated membrane disposed on a common substrate. This would allow each source to be moved independently of one another. Principles of how micromachined combdrive actuated membrane are fabricated may be found in “Integrated Polysilicon and DRIE Bulk Silicon Micromachining for an Electrostatic Torsional Actuator” by J-L. A. Yeh, H. Jiang and N. C. Tien, Journal of Micromechanical Systems, volume 8, pp 456-465 (1999). As an alternative to keeping the array of electron beam sources  47  and the wafer under vacuum using conventional pumping systems, it will be appreciated that the region about each source  47  may be evacuated using an integrated micromachined evacuation system. 
     Device Fabrication 
     The third embodiment of an electron source  47  is fabricated using similar process steps as the first embodiment of an electron source  1 , but without the process steps required for the dielectric layer  11  and the capping layer  13 . 
     It will be appreciated that many modifications may be made to the embodiments described above. 
     A non-insulating substrate may be used instead of an insulating substrate. 
     The cathode layer may be as thin as 10 nm and thicker than 100 nm. 
     The apertures in the electron source may have diameters of less than 100 nm. 
     The insulating layer or dielectric layer material may have different thicknesses in the range 10 to 50 nm and may comprise silicon nitride, tantalum oxide, titanium pentoxide or multilayer dielectric layers. 
     The extractor electrode material may comprise other metals, for instance gold or aluminium. 
     The alloy of gold and palladium may be deposited using ion-assisted deposition process with a landing energy of 100-300 eV. 
     Other conductive particle materials may be used other than an alloy of gold and palladium. The material should allow selectivity over the cathode layer material during dry etching. The material may have a lower work function than gold-palladium, for example alloys of caesium and barium. 
     The emitter may be formed out of a different material from the cathode material. The pillars may be formed by application of self-righting posts. The emitter may comprise a carbon nanotube. 
     The source may be used in other applications including metrology and information storage. 
     The secondary electron currents detected by a detector may be measured by an electrometer.