Abstract:
A cold field emission (CFE) electron source for a focused electron beam system such as a transmission electron microscope (TEM), scanning transmission electron microscope (STEM), or scanning electron microscope (SEM) is disclosed. The source employs an emitter enclosure electrode behind the CFE tip which, in conjunction with the extractor electrode, defines a closed volume that can be thoroughly cleaned by electron impact desorption (EID) and radiative heating from a heated filament located between the emitter enclosure electrode and extractor electrode. The extractor electrode may have a counterbore which restricts backscattered electrons generated at the extractor from reaching portions of the source and gun which have not been cleaned by EID. Pre-cleaning of the emitter enclosure electrode and extractor electrode prior to cold field emission substantially improves both source emission stability and frequency noise characteristics, enabling source operation over time intervals adequate for application to TEMs, STEMs, and SEMs.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates generally to electron sources, and more particularly to cold field emission electron sources for application in focused electron beam systems. 
     BACKGROUND OF THE INVENTION 
     In focused electron beam systems a column is typically used to focus an electron beam onto the surface of a target to be imaged and (optionally) processed using the beam. In these columns, an electron source generates the initial beam of electrons, which then passes into an electron “gun”, which typically focuses the charged particles into a roughly parallel beam which enters the main body of the column. Various types of electron sources have been used in focused electron beam systems, including thermionic cathodes, Schottky emitters, and cold field emitters (CFEs). Of these, CFEs are characterized by the highest brightnesses and smallest energy spreads, potentially enabling the smallest beam sizes at the target, with the highest current densities, thus enabling improved imaging resolution. Unfortunately, CFE sources have also shown a tendency for very fast (˜0.5 to 1.5 hours) quenching of the emission current even in excellent UHV (˜10 −10  Torr) conditions. In an attempt to solve or ameliorate this problem, FEI Company, Hillsboro, Oreg., developed and patented (U.S. Pat. No. 7,888,654, to Tessner II et al. for “Cold Field Emitter”) an oxidized W(111) CFE that demonstrates much slower quenching than previous (unoxidized tip) CFE sources. These improved CFE sources, however, still demonstrate noise in the emission current after a short period of source operation. Thus, there is a need for a CFE source with improved emission stability while demonstrating reduced noise. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a cold field emitter (CFE) electron source having improved emission stability and reduced noise. 
     In some embodiments of the invention, a filament positioned between an emitter enclosure electrode and an extractor electrode is used to clean surfaces near the emitter tip. In some embodiments, the gap between the emitter enclosure electrode and the extractor electrode is configured to limit the paths of backscattered electrons and/or to reduce the influx of gas molecules into the region of the tip. Embodiments of the invention have been shown to significantly improve stability and reduce the noise of CFEs. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter. It should be appreciated by those skilled in the art that the conception and specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more thorough understanding of the present invention, and advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a schematic diagram of a prior art cold field emitter electron source in a test set-up for measuring the on-axis emission current stability. 
         FIG. 2  is a graph of experimental results for a prior art cold field emitter source, demonstrating the quenching of emission. 
         FIG. 3  is a graph of experimental results for a prior art cold field emitter source, demonstrating undesirable emission instability. 
         FIG. 4  is a schematic diagram of a prior art cold field emitter electron source with an outgassing filament in the source base. 
         FIG. 5  is a schematic diagram of a cold field emitter electron source of the present invention, operating in the degassing mode. 
         FIG. 6  is a schematic diagram of a cold field emitter electron source of the present invention, operating in the cold field emission mode in a test set-up for measuring the on-axis emission current stability. 
         FIG. 7  is a schematic diagram of a portion of a cold field emitter electron source of the present invention, illustrating a first embodiment of the source tip region. 
         FIG. 8  is a schematic diagram of a portion of a cold field emitter electron source of the present invention, illustrating a second embodiment of the source tip region. 
         FIG. 9  is a schematic diagram of a portion of a cold field emitter electron source of the present invention, illustrating a third embodiment of the source tip region. 
         FIG. 10  is a graph of experimental results for the cold field emitter electron source of the present invention. 
         FIG. 11  is a flowchart illustrating a method for making a cold field emitter electron source of the present invention. 
         FIG. 12  is a flowchart illustrating the typical operation of an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Embodiments of the present invention provide a cold field emitter (CFE) electron source having improved emission stability and reduced noise. Embodiments provide source structures that are thought to greatly reduce levels of ion and neutral molecule bombardment of the emitter tip during normal cold field emission operation of the CFE source, while also reducing the rate of accumulation of adsorbates on surfaces impacted by the electron beam. In some embodiments, this is accomplished by enclosing the emitter tip between an emitter enclosure electrode and the extractor, and then thoroughly cleaning both inner surfaces by a combination of electron bombardment and radiative heating. This cleaning process utilizes a thermionic filament capable of electron bombarding the inner surfaces of the emitter enclosure electrode and extractor, and also radiantly heating these surfaces—both the electron bombardment and heating serve to remove essentially all adsorbates from these surfaces prior to the initiation of CFE emission to generate an electron beam. The thermionic filament may be annular and centered on the emission axis, or may be of any shape and in any position or orientation such that the emissions and/or radiation from the filament are sufficiently distributed onto the surfaces of the emitter enclosure electrode and extractor to adequately clean the surfaces. 
     In some embodiments, a confining structure, such as a counterbore or countersink structure in the side of the extractor facing the emitter tip, serves to confine backscattered electrons (BSEs) generated by impact of the CFE electrons with the emitter, preventing these BSEs from striking surfaces within the electron gun which may have desorbable gas layers. Studies have shown that the causes of tip emission instabilities are variations in the local work function and electric field due to contamination by adsorbates and by geometrical modifications of the tip shape due to ion bombardment. The discussion below considers the origins of these effects. 
     First, we discuss prior art measurements of CFE emission stability and noise, including an analysis of the origins of the observed emission instability. Quantitative measurements of noise and emission quenching are presented in order to put the experimentally demonstrated benefits of the present invention in context. Prior art attempts at reducing noise and improving stability are then discussed, including the causes for the failure of these attempts to fully solve the noise and stability problems with CFE sources. Finally, the source structure of the present invention is presented and its operation in both the degassing and CFE operating modes described. 
     Measurements of Emission Current Stability for Prior Art CFE Sources 
       FIG. 1  is a schematic diagram  100  of a cold field emitter (CFE) electron source in a test set-up for measuring the on-axis emission current stability. A number of physical effects occur at, and in the proximity of, the CFE emitter tip  103  which may have a deleterious influence on the stability and noise levels of the source in a focused electron beam system such as a scanning electron microscope (SEM), transmission electron microscope (TEM), or scanning transmission electron microscope (STEM). In a typical CFE source, electrons are emitted from the sharp end  103  of an oriented wire  102  which is welded to a support filament  104 . A bias voltage is applied between the emitter tip  103  and an extractor electrode  108 , which creates a very high electric field at the surface of the tip  103 , thereby inducing tunneling of electrons out of the tip  103  into the vacuum in front of the tip. These emitted electrons form a beam  106  which is directed toward the right of  FIG. 1 . The great majority of the electrons in beam  106  strike the extractor  108  at area  110 , while a small portion of the center of the emission distribution passes through hole  113  in extractor  108  to form beam  112 . The test set-up for measuring the emission stability comprises a shield plate  114 , and a Faraday cup  120  electrically connected to an electrometer  122 . The majority of beam  112  strikes the shield plate  114  at area  116 , while a small portion  118  from the center of the emission distribution passes through the hole  130  in shield plate  114  and is collected by the Faraday cup  120 . This collected current is measured by electrometer  122  and then passes to the system ground  124 . Since, in a typical focused electron beam system, only a very small portion of the center of the emission distribution from the tip  103  contributes to the final beam current at the sample, the test system illustrated here is configured to measure that center portion, while excluding other portions of the emission distribution which would be apertured away (i.e., blocked from passing to the sample) in an actual application. 
     The impact of beam  106  with the extractor  108  at area  110  induces the emission of secondary electrons (SEs)  160 . Although the voltage difference between the tip  103  and the extractor  108  mostly creates an electric field in the proximity of the tip  103 , enough residual electric field remains near area  110  that secondary electrons  160  (which have low energy) are attracted back to the extractor  108 , as shown by the curved trajectories in  FIG. 1 . The impact of beam  106  with the extractor at area  110  also induces backscattered electron (BSE) emission  130 —the majority of the BSEs have nearly the same energy as the impacting electrons from beam  106 , thus the small electric field at area  110  cannot prevent the escape of BSEs  130  into the overall volume of the electron gun, as shown. Some BSEs  130  travel to regions of the gun far enough away from tip  103  to cause no obvious effects on source operation. BSE  132  can be seen striking surface  136 , desorbing a gas molecule  134  which may travel towards the emitter tip  103 , potentially inducing sputter damage to the structure of tip  103  or adsorbing onto tip  103 . On the other side of tip  103 , BSE  142  desorbs and ionizes molecule  144  from surface  138  to travel towards the tip  103 —due to the negative bias voltage on tip  103 , ion  144  will be accelerated towards tip  103 , potentially causing sputter damage to the local tip structure near the point of impact of ion  144  with tip  103 . Note that surfaces  136  and  138  represent any surfaces within the electron gun which are not cleaned (i.e., from which adsorbed molecules have not been desorbed) prior to initiation of cold field emission from tip  103 —surfaces  136  and  138  are shown closer to tip  103  than would generally be the case in an actual source. In the prior art, it has been found to be difficult to find means for completely cleaning all surfaces within electron guns which may be exposed to BSEs. Polarizable gas molecules  150  may be attracted to, and adsorbed onto, the tip  103  by the local electric field gradient, changing the work function and thus causing variations in the emission currents. The impact of beam  106  with the extractor  108  can also desorb gas molecules  182  from area  110  of extractor  108 —these molecules  182  may subsequently adsorb on the emitter tip  103 , usually increasing the local work function and thus reducing the cold field emission current. Gas molecule  184  is ionized by an electron  186  from beam  106  and is then attracted to the negatively-biased tip  103 , as was the case for ion  144 . In some cases, sputtering damage to the tip  103  may create locally very sharp regions which will demonstrate increased field emission—this effect may cause a catastrophic momentary increase in the emission current, possibly leading to arcing and subsequent destruction of the emitter tip  103 . 
       FIG. 2  is a graph  200  of experimental results for a prior art cold field emitter source, demonstrating the quenching of emission as observed with the CFE source and test set up illustrated in  FIG. 1  as a function of time  202 . The normalized current plotted along axis  204  represents the output from electrometer  122  in FIG.  1 —note that for curves  206 ,  208  and  210 , the value “1.0” on axis  204  represents the initial collected current, which may differ in absolute value between the curves  206 ,  208  and  210 , respectively. Three different experiments are compared here: 1) curve  206  represents a series of Faraday cup current readings taken over a period of nearly 1.5 hours, where the source was off between measurements (indicated by the circles) and the total initial emission current was 100 μA, 2) curve  208  represents continuous operation of a source with an initial total emission current of 80 μA, while 3) curve  210  represents continuous operation of a source with an initial total emission current of 100 μA. Several significant conclusions can be drawn from comparison of curves  206 ,  208  and  210 :
         1) In all three cases, the normalized Faraday cup current drops significantly over less than 1.5 hours—this time-frame is too short for the CFE source to have practical application in a typical focused electron beam system.   2) For the OFF mode case (curve  206 ), the rate of drop is less—this indicates that some of the current drop-off must be beam-induced, such as the bombardment of tip  103  by ions  144  and  184  illustrated in  FIG. 1 .   3) For both ON modes (curves  208  and  210 ), the rate of drop-off is higher than for the OFF mode curve  206 .   4) For higher total initial tip emission (i.e., 100 μA in curve  210  compared with 80 μA for curve  208 ), the collected current drop-off is faster since for larger total emission currents the rates of local gas desorption and ion bombardment are increased—confirming that beam induced processes contribute to the current drop off. Especially for the 100 μA curve  210 , the rate of drop-off is catastrophic—within 0.25 hours (15 minutes), the normalized Faraday cup current has dropped by roughly 80%.       

       FIG. 3  is a graph  300  of experimental results for a prior art cold field emitter (CFE) source such as in  FIG. 1 , demonstrating unacceptable levels of noise. The beam current (in nA)  304  is plotted as a function of time  302  for nearly a 10 hour span. Even near the beginning of the run, the source demonstrates noise  306  which begins abruptly at around 0.75 hours and starts decreasing near 1.5 hours, tailing off to around 3.0 hours into the run—this is characteristic of prior art emitters and is believed to arise at least partially from the fact that the physical area on the emitter tip  103  which contributes to the final beam current (i.e., the current  118  into Faraday cup  120  in  FIG. 1 ) is near atomic dimensions in size, thus even a single adsorbate molecule may have a significant effect on the local work function leading to rapid, oscillatory current fluctuations as seen here which are thought to arise from rapid motions of these adsorbates into, and out of, the relevant region on the tip  103 . Rapid motions of atoms in the tip itself are also believed to contribute to these fluctuations. The burst of noise  308  near 7.0 hours also demonstrates this abrupt turn-on and turn-off behavior. 
     Noise is also apparent during time-period  320 , from 4.2-5.3 hours, and again during time-period  322 , at around 6.25 hours—although at lower frequency than for time-periods  306  and  308 , this lower-f noise is also unacceptable in a focused electron beam system. Finally, at the far right, time frame  330  after around 7.9 hours demonstrates even wilder emission current fluctuations, which may precede the failure of the emitter tip due to vacuum high voltage breakdown (arcing). 
     Prior Art Attempts to Improve the Emission Current Stability 
       FIG. 4  is a schematic diagram  400  of a prior art cold field emitter electron source with an outgassing filament in the source base. The CFE emitter tip  403  comprises the sharpened end of an oriented wire  402 , typically spot-welded to a filament  404  which is, in-turn, welded to two mounting posts  490  and  492  which are attached to an insulating structure such as disk  480 , shown in cutaway. A filament  498  is mounted between posts  494  and  496  and a voltage is applied between posts  494  and  496  to induce ohmic heating of filament  498 . A bias voltage is applied between filament  498  and the extractor  408 —this bias voltage serves to attract the electrons  420  which are emitted thermionically from filament  498  toward the extractor  408 . Impact of electrons  420  with adsorbed molecules  422  on extractor  408  causes electron induced desorption (EID). With sufficient numbers of electrons  420 , the great majority of the adsorbed molecules  422  can be removed. Unfortunately, in the prior art, other surfaces  436  and  438  (shown closer to tip  403  than would generally be the case in an actual source) within typical gun structures were found to receive an inadequate flux of bombarding electrons  420  and thus retained a portion of their original coating of adsorbates, such as  434  and  444 , respectively. The deleterious effects of these adsorbates  434  and  444  on normal source operation were discussed in  FIG. 1 . Thus, a fundamental problem with prior art methods for in situ cleaning of the electron source and gun was an inability to adequately remove adsorbates from surfaces within the gun which are subsequently impacted by backscattered electrons emitted from the extractor due to impact of the primary electron beam from the emitter tip. In prior art CFE source and gun designs, the pumping speed between the region surrounding the emitter tip and the overall gun structure was made as large as possible to allow ambient gas to flow away from the tip—however this clearly also allows gas to flow towards the tip. 
     CFE Electron Source Design of an Embodiment of the Invention 
       FIG. 5  is a schematic diagram  500  of an embodiment of a cold field emitter electron source of the present invention, operating in the degassing mode. The CFE emitter tip  503  is the sharpened end of the tip wire  502 , typically an oriented wire of a refractory metal such as tungsten. The tip wire  502  may be spot-welded to a filament  504 , supported by posts  590  and  592  mounted in an insulating disk  580 . To clean just the emitter wire  502  and tip  503 , a current can be “flashed” through filament  504  by applying a voltage between posts  590  and  592 , thereby momentarily ohmically heating filament  504 , wire  502 , and tip  503  hot enough to remove adsorbates, restoring the initial clean tip structure characterized by the work function of the base metal of the oriented tip wire  502 , or of the oxidized W(111) surface of the wire  502  and tip  503  as described in U.S. Pat. No. 7,888,654, issued 15 Feb. 2011. Although this tip flashing process is effective in cleaning the wire  502  and tip  503 , it does not remove adsorbates from the extractor  508  or other surfaces within the gun which may be impacted by BSEs from the extractor, as shown in  FIG. 1 . Thus, a more thorough source and gun cleaning method is preferred, capable of more complete removal of adsorbates than the prior art illustrated in  FIG. 4 . In the present invention, an emitter enclosure electrode  552  with a hole  554  through which the tip wire  502  protrudes (typically by ˜1.5 mm) is mounted as shown. The distance from the tip  503  to the extractor  508  is typically ˜0.75 mm. In addition, a circular filament  530  is located radially outwards from the tip wire  502  and between the emitter enclosure electrode  552  and the extractor  508 . Three different cleaning modes using filament  530  are possible:
         1) Pure EID desorption—in this mode, the inner surfaces of the emitter enclosure electrode  552  and extractor  508  are cleaned using bombardment by electrons  520  and  524  emitted from the filament  530  due to a voltage applied between the filament  530  (which is heated by an electric current) and the emitter enclosure electrode  552  and extractor  508 —this induces thermionic emission of electrons  520 . Adsorbate molecules  522  and  526  are then desorbed and pumped-away. Typical gaps between the facing surfaces of the emitter enclosure electrode  552  and the extractor  508  may range from 1.8 to 2.2 mm—this allows adequate radial pumping speeds for removal of the desorbed gas molecules.   2) Pure Thermal Desorption—in this mode, the filament  530  is still heated, but a bias voltage need not be applied between the filament  530  and either the emitter enclosure electrode  552  or the extractor  508 —adsorbates  522  and  526  are then removed by thermal excitation of the surfaces and molecules. Pumping of desorbates is the same as for the first mode, above. Within a short time, temperatures exceeding 400° C. may be reached on the emitter enclosure electrode  552  and extractor  508 , effectively removing adsorbed molecules.   3) Combined EID and Thermal Desorption—in this mode, adsorbates  522  and  526  are removed both by impact of thermionic electrons  520  and  524  from the filament  530  and by heating due to radiation from the heated filament  530 .       

     A gap  560  is formed between emitter enclosure electrode  552  and extractor  508 . In some embodiments, the gap  562  is wider near the hole  550  in extractor  508  than farther off-axis from the hole  550 , forming a partial enclosure or shield in the region around the emitter tip. The confining space may have a concave shape, that is, thicker near the center than at the edge. For example,  FIG. 5  shows the surface of extractor  508  that faces the tip  503  has a shallow depression (˜0.45 mm deep), such as a counterbore which, coupled with the narrow gap  560  between the emitter enclosure electrode  552  and the extractor  508  away from the depression, forms a partly enclosed volume around tip  503 . During source operation, this partly enclosed volume serves to confine the production of ionized and neutral molecules to the surfaces of the emitter enclosure electrode  552  and extractor  508 , while preventing BSEs generated at the bottom of the depression from reaching other surfaces within the gun (such as surfaces  136  and  138  in  FIG. 1 ). Other shapes for emitter enclosure electrode  552  and extractor  508  can accomplish the same purpose. For example, a countersink, counterbore, or curved surface can be incorporated into either the emitter enclosure electrode  552 , the extractor  508 , or both, to produce a partly enclosed space around the emitter. 
       FIG. 6  is a schematic diagram  600  of the CFE electron source of  FIG. 5 , operating in the cold field emission mode in the test set-up for measuring the on-axis emission current stability illustrated in  FIG. 1 . Comparison of  FIG. 6  to  FIG. 1  illustrates the advantages of some embodiments of the present invention over prior art CFE sources. The cleaning process shown in  FIG. 5  has thoroughly removed adsorbed molecules from all surfaces which are impacted by the beam  602  emitted from tip  503  due to the electric field induced by the voltage applied between the tip  503  and the extractor  508 . Thus, the inner surface  604  of extractor  508  is relatively free of adsorbates. BSE emission  630  from surface  604  is confined by the combination of the emitter enclosure electrode  552  and the counterbore in the extractor  508 , thus gas desorption from surfaces  436  and  438  (not shown here—see  FIG. 1 ) cannot occur. The current measurement set-up is the same as in FIG.  1 —a small portion  606  of beam  602  passes through hole  550  in extractor  508 . The majority of the resultant beam  606  strikes the shield plate  114  at area  610 , while a small center portion  612  passes through hole  130  to enter the Faraday cup  120 . Current collected by the Faraday cup  120  is measured by electrometer  122  and then passes to the system ground  124 . 
     First Embodiment of the Emitter Tip Region 
       FIG. 7  is a schematic diagram of a portion of a cold field emitter electron source, illustrating a first embodiment  700  of the emitter tip region. The emitter wire  702  has a sharpened end  703  which emits electrons  710  under the influence of a high electric field induced at tip  703  by a high voltage applied between the emitter tip  703  and extractor  708 . A volume enclosing tip  703  is formed between the inner surfaces of emitter enclosure electrode (EEE)  752  and extractor  708 . A cleaning filament  730  is shown between the EEE  752  and the extractor  708 . An important consideration in the design of the source region for this first embodiment is the aspect ratio between the outer radii of EEE  752  and extractor  708 , and the gap separating the inner surfaces of EEE  752  and extractor  708 . The larger this aspect ratio, the more backscattered electrons generated from the inner surface of extractor  708  are prevented from striking other (possibly unclean) surfaces within the gun, such as surfaces  136  and  138  in  FIG. 1 . For this first embodiment, the inner surfaces of EEE  752  and extractor  708  are illustrated as flat surfaces near their outer radii—thus a small number of backscattered electrons  771  emitted at large angles from area  704  on extractor  708  may escape from the source tip region. It is also possible for a small number of backscattered electrons  772  reflected at large angles off EEE  752  to also escape from the source tip region. 
     Second Embodiment of the Emitter Tip Region 
       FIG. 8  is a schematic diagram of a portion of a cold field emitter electron source of the present invention, illustrating a second embodiment  800  of the emitter tip region. The emitter wire  802  has a sharpened end  803  which emits electrons  810  under the influence of a high electric field induced at tip  803  by a high voltage applied between the emitter tip  803  and extractor  808 . A cleaning filament  830  is shown between the EEE  852  and the extractor  808 . A volume enclosing tip  803  is formed between the inner surfaces of emitter enclosure electrode (EEE)  852  and extractor  808 . For this embodiment, extractor  808  has an outer shield ring  890 , which prevents the escape of backscattered electrons  871  emitted from area  804  on extractor  808 , and backscattered electrons  872  reflected off EEE  852 , as shown. The benefits of improved BSE containment in this second embodiment must be balanced against a slightly reduced pumping speed from the source tip region. An additional benefit of this second embodiment is that the outer radii of EEE  852  and extractor  808  may be smaller since the aspect ratio considerations for the first embodiment  700  are less important here due to the outer shield ring  890 . 
     Third Embodiment of the Emitter Tip Region 
       FIG. 9  is a schematic diagram of a portion of a cold field emitter electron source of the present invention, illustrating a third embodiment  900  of the emitter tip region. The emitter wire  902  has a sharpened end  903  which emits electrons  910  under the influence of a high electric field induced at tip  903  by a high voltage applied between the emitter tip  903  and extractor  908 . A cleaning filament  930  is shown between the EEE  952  and the extractor  908 . A volume enclosing tip  903  is formed between the inner surfaces of emitter enclosure electrode (EEE)  952  and extractor  908 . For this embodiment, EEE  952  has an outer shield ring  990 , which prevents the escape of backscattered electrons  971  emitted from area  904  on extractor  908 , and backscattered electrons  972  reflected off EEE  952 , as shown. The benefits of improved BSE containment in this third embodiment must be balanced against a slightly reduced pumping speed from the source tip region. An additional benefit of this third embodiment is that the outer radii of EEE  952  and extractor  908  may be smaller since the aspect ratio considerations for the first embodiment  700  are less important here due to the outer shield ring  990 . 
     Experimental Results for Operation of the CFE Electron Source of the Invention 
       FIG. 10  is a graph  1000  of experimental results for the cold field emitter electron source embodying aspects of the present invention. The beam current collected on the Faraday cup  120  (see  FIG. 1 ) is plotted on axis  1004  as a function of time (in hours)  1002 , extending to slightly past 9 hours of source operation. Curve  1006  can be compared with the data in FIG.  3 —a substantial reduction in noise is apparent, especially for the first 5 hours of operation. Box  1008  is expanded as inset  1010  showing curve  1012  as a portion of curve  1006  to highlight this reduction over an expanded time scale from 4 to 6 hours. Various embodiments of the present invention thus demonstrates the following advantages:
         1) Removal of adsorbates from surfaces which will be bombarded by the emission current from the emitter tip, including both the surface of the extractor facing the tip, and the surface of the emitter enclosure electrode facing the extractor.   2) Shielding of the tip volume from in-flowing gas by the emitter enclosure electrode.   3) Shielding of internal gun surfaces from BSEs emitted from the extractor.   4) The heating/bombarding filament is inside the small source volume, allowing inside surfaces to be cleaned sufficiently in a relatively short time.   5) The source is based on standard source mounting structures and can be compatible with commercial electron microscopes, such as those sold by FEI Company, Hillsboro, Oreg.       

       FIG. 11  describes a method  1100  of making a cold field emitter electron source. In step  1102 , an emitter having an emitter tip and an emitter axis is provided. A method of making an emitter is described, for example, in U.S. Pat. No. 7,888,654, which is hereby incorporated by reference. In step  1104 , an emitter enclosure electrode having a hole centered on the emitter axis through which the emitter extends is provided. In step  1106 , an extractor electrode having a hole along the emitter axis for passage of an electron beam is provided. In optional step  1108 , a filament electrode is provided between the extractor electrode and the emitter enclosure electrode, preferably the filament having an annular shape and being centered on emission axis. Step  1110  then comprises the steps necessary for assembling the cold field emitter source using the sub-assemblies and parts provided in steps  1102 ,  1104 ,  1106 , and  1108 . In the source assembled in step  1110 , the emitter enclosure electrode and extractor electrode are optionally configured to produce a confinement space containing the emitter tip, the confinement space limiting the paths of electrons backscattered from the extraction electrode and/or reducing the flow of gas into the confinement space. This confinement space may be formed by providing an extractor electrode having a depression facing the emitter enclosure electrode. 
       FIG. 12  shows a flow chart  1200  of a typical operation of an embodiment of the invention. In step  1202 , the emitter tip is first flashed, and then a voltage is applied across the cleaning filament to induce a heating current through the cleaning filament to ohmically heat the cleaning filament to an elevated temperature—heating currents may typically range from 1.5 to 5.0 A. In optional step  1204 , a bias voltage is applied between the cleaning filament and both the emitter enclosure electrode and the extractor electrode—typical bias voltages may range from a few V to a few kV, with the voltage on the filament being negative relative to the emitter enclosure electrode and the extractor. If step  1204  is omitted, cleaning of the electrodes in step  1206  will be solely thermal (mode  2  in  FIG. 5 ). If step  1204  is not omitted, the combination of heating and bias voltage on the filament induces thermionic emission of electrons from the cleaning filament in step  1206  to bombard the surfaces of the emitter enclosure electrode and extractor electrode facing the emitter wire, the electron bombardment stimulating electron impact desorption of molecules from the emitter enclosure electrode and extractor electrode—this corresponds to cleaning mode  1  or  3  in  FIG. 5 . Note that in the case of electron impact desorption (EID) of gas off the surfaces of the emitter enclosure electrode and the extractor, a certain minimum electron bombardment energy may be necessary, typically in the range from a few eV up to a few keV. Thus, even with a certain power into the electrodes to be cleaned (calculated as the product of the filament bias voltage times the electron current), there may be inadequate desorption of gas if the bombarding electrons have insufficient energy to desorb individual gas molecules. Conversely, for thermal gas desorption, the total power imparted to the electrodes is the prime consideration for cleaning rates. Cleaning step  1206  typically lasts from a few minutes to 1-2 hours. After cleaning step  1206 , the heating current and bias voltage are turned off in step  1208 . In step,  1210 , the emitter tip is flashed and then an extraction voltage is applied between the extractor and emitter (positive voltage on emitter) to induce cold field emission of electrons from the emitter tip towards the extraction electrode. The extraction voltage is typically between 100 V and 4000 V, more typically between 1000 V and 3000 V. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made to the embodiments described herein without departing from the spirit and scope of the invention as defined by the appended claims. Multiple aspects of the invention are novel and not every embodiment will require the use of every aspect. For example, the use of an emitter enclosure electrode behind the emitter tip, the use of a electrode filament between the tip and the extractor electrode, and the use of a confining space around the emitter tip and all inventive. Aspects of the invention can be applied to other types of emitters, such as Schottky emitters. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.