Cathode
The Cathode is one of the fundemental parts of an Electron Microscope. It is the material from which the electrons that are accelerated in the Electron Gun originate from. Many types of cathode have been invented over the years, many tracing their liniage to the humble thermionic valve / Vacuum tube. Not all cathode types usable in a sealed thermionic valve can be used within the demountable vacuum system of an electron microscope.
The cathode can be ordered by their fundamental opperating principle. All of these serve the same purpoes, that being to transfer electrons from the wires of the electrical system, into the free space surounding the cathode surface. The basic categories are thus, Thermionic, Field Emission, Plasma cathodes, and Photoelectric Cathodes. It is interesting to note, that the first electron microscopes created by Ernst Ruska employed a Plasma cathode.
Thermionic Cathodes
These are the most common type of cathode encountered in the electron microscope field, though the sub types may be rare all the way to extinct. The Thermionic cathode works by overcoming the [| work function] of the bulk material by means of heating it. In sohrt, the work function is the energy required for an electron to exit the bulk material, and is thus also material dependent. Tungsten for example, having a work function of 4.32 – 4.55 eV. The most common of these cathodes encountered in electron microscopes, is the tungsten hairpin and the LaB6 (Lanthanum Hexa Boride) cathode.
Tungsten Hairpin
The tungsten hairpin, much like its name might suggest, is a usually V shaped tungsten wire of small crossection welded to a support structure, typically metal pins bonded to a ceramic carrier. The common wire diameters encountered are in the range of 0.1 to 0.2 mm. The very tip of the V shaped hairpin is heated by electic current to a temperature in the range of 2700K to 3300K in order to give it sufficiant energy for the electrons to overcome the barrier emposed by the work function. This heating current is either in the form of AC or DC current, with DC being the norm these days. The current is supplied either by means of a Battery floating at the accelerating voltage, or my means of isolation transformers.
The Emission area used for electron gun of microscopes, is the very tip of the V shaped hairpin, and is typically (dependen on many factors such as gun geometry and Wehnelt voltage) in an oval area. Thus the emission area is rather large in comparison to other cathode types, thus limmiting the useful resolution attained by this cathode type.
The lifespan of a tungsten hairpin is, on average 100 Hours, but can be as short as a few minutes if overheated or in some rare instances, such as the AEG Zeiss EM8, where the original cathode was rated at 10 hours of useful lifespan, owing to its very high working temperature required for the Steigerwald Kathode used therein. The Lifespan is somewhat dependent on the vacuum present in the emission chamber of the electron gun, the higher the vacuum the longer the lifespan.
The factors limiting the lifespan of the tungsten hairpin are simple to understand, these being the evaporation of the tungsten, and thus constriction of the wire to the point of failure. As well as the ion current bombarding the cathode, sputtering away the tungsten over time.
Tungsten Lancet
The lancet cathode is a subtype of tungsten hairpin, where the very tip of the hairpin is ground into a needle shape via (typically) 4 grinds. It has the advantage of smaller emission aria and thus a brighter electron gun. The downside of grinding the wire to this shape are obvious, in that the amount of metal avalible is far less, thus the aging process described above for the Hairpin, is faster.
Typically, such cathodes where employed when higher resolution was needed then the instrument could otherwise achieve, or a brightre image at higher magnifications was sought, owing to the higher current density this cathode is capable of producing.
Thoriated Tungsten
A method used in the manufacture of thermionic valves is the use of thoriated tungsten to depricate the work function and overall extend the service life of the cathode. This is accomplished by allowing up to 4% thorium into the tungsten wire. This cathode type was only experimentally uesd in the electron microscope, where it showed no aperciable advantage over the pure tungsten hairpin. Its use was thus religated to thermionic valves.
Oxide Cathode
Another type of cathode from the manufacture of thermionic valves is the oxide cathode. This cathode employs a carrier, typically in the form of a nickle tube, coated with Strontium and Barium oxide. These oxides have significantly lower work functions then normal refractory metals, and can thus run at a red to yellow heat. (exact temperatures elude the authers memory at time of writing). Other methodes of their use are the Platinum, molybdenum or tungsten wire coated in metal oxides, such as used in the 45T valves developed by Lee De Forest. Since the working temperature is lower, the cathode has a longer working life since less material evaporates over time.
The use of oxide cathodes was briefly explored and implemented by a Japanese Manufacturer (add name later) who used a hallow cylinder of platinum, filled with oxide, and heated by electric current flow, similar to the tungsten hairpin. It was however found, that the pump oil backstreaming from the diffusion pumps and the over all poor vacuum when compared to sealed thermionic valves, lead to quick poisoning of the oxide layer. Thus the lifespan of these cathodes in the electorn microscope was on the order of a few hours, to at most tens of hours. (citation needed).
Oxide cathodes, are susceptible to ion poisoning and are quite sensetive to contamination. If carbon from the air or oil gets into the oxide crystals, their work function quickly rises to a point where their use is no longer possible at the normally low temperatures.
LaB6 Cathodes
The second most common type of Cathode found in modern electron microscopes is the Lanthanum HexaBoride cathode. This cathode uses the low work function and high temperature resistance and mechanical hardness of the salt LaB6. This being a purple colored crystline solid. Early cathodes used Sintered powder in the form of long square rods with a pyramidal tip as the emmitter. Later and modern ones use single crystals of LaB6. It has the advantage of lower operating temperature and higher current density at that lower temperature, when compared to Tungsten Hairpins. The emission area is typically round in shape and of the order of 5 to 10µm.
The downside to LaB6 cathodes is their proclivity to be poisend by carbon, thus requiring significantly higher vacuum then a tungsten hairpin to opperate. This is typically affected by the use of a differental pumping system, i.e. a Aperture and second pump, and is of the order >1 e-7 mBar. The pumping power to achieve such pressures is supplied by Ion pumps usually connected directly to the emission chamber.
Sintered LaB6
These early types as stated previously are long square rods with a pyramid tip. The heating of these cathodes was typically done by either bombarding them with high current electron beams, typically radially, or by thermal absorbtion of a white hot tungsten filament in close proximity. The cathode would normally be assembled into a oven assembly which also acts as the whenelt cup, and mounted into the micoscope. Examples of an early use of this type of cathode is the Cambridge Stereoscan S4-10, which if ordered or upgraded to use LaB6 cathodes, had the ability to also be used with Tungsten Hairpin cathodes by simply replacing the LaB6 oven with a Tungsten Hairpin Wehnelt Assembly (as well as changing some settings).
Due to the complexity and requirement for a 4th conductor for the gun, sufficient room for the oven assembly, and being overall hard to set up and use, this design of cathode is now no longer used.
Single Crystal LaB6
The more modern type of LaB6 cathode is the single crystal. Here a single crystal in the form of a round rod is connected to a heating system, this usually being ohmic in nature, and thus taking a similar form to the tungsten hairpin. This crystal in the orientation <insert later> is ground down to a tip size of 4 to 10 µm, this forming the emission surface.
Since this cathode type is much simpler to use then the sintered type, as well as being smaller, and offering a more even emission area current distribution, it has supplanted the older type mentioned above.
Field Emission Cathode
This type of cathode is marked by its extremely small emission area, being down to a few nm in diameter, and thus overwhelming current density. It is however one of the hardest to produce and work with, as well as requiring the most stringent of vacuuo to opperate. This type of cathode can be subdevided into two main types, the cold field emission cathode, and the Schottky Hot Field Emission cathode. The vacuum required for the cold type is higher then that of the shottkey, these being in the range of <1e-10 mBar for the former, and <1e-8 mBar for the latter. The cathode takes the shape of a tungsten single crystal needle, typically mounted on a tungsten hairpin, with the tip having a radius of <100nm. These tips are, overly simplidied, made by etching the crystal in a solution of Sodium Hydroxide, half being sumberged and half being above water. The resultent needle is of remarkably small tip radius.
The opperating principle of these cathodes, is the use of the field emission effect, where a very strong electric field gradiant quite literally ripps the electrons out of the bulk material. The fields required are on the order of 2 to 5 MV/m (citation needed).
Due to the high complexity of the gun, its vacuum requirements and difficulty of manufacture, the field emission cathode is only found in high end instruments but offered the smalles spot size of all cathode types.
Cold Field Emission
Invented by Crewe, the cold field emission gun works like above, a tungsten needle is placed into a strong field gradiant and its electrons get ripped out. It however sufferes from the disadvantage of having unstable emission current, as in, it fluctuates rather strongly over time, even from second to second. Thus at first, no slow scan electron microscope was constructable with this cathode. The first such instruments where the likes of the Kwick Scan which only offered TV speed scanning. The solution to this instability was found by Hitachi, in the form of beam monitors which modulate the video signal (presumably via a Gilbert Cell or similar multiplyer) with the measured current from the gun at the same time. The first instrument to use this type of arangement was the Hitachi MRS2 scanning electron microscope, fallowed later by the Hitachi S800.
This cathode type also needs to be periodically heated up, in order to drive off embedded ions from the imperfect vacuum of the gun, this heating can and will eventually burn out the cathode itsself, but restores it back to fully operating condition for a time. The cold field emission cathode thus also has the longest service life of any cathode type, this being in the range of 10000 hours or more, but strongly depends on the vacuum at which it is run. The better the vacuum the longer the service life is. The cold field emission cathode also has the smallest emission area of any cathode, and is thus used in the most high resolution electron microscopes.
Schottky Hot Field Emission
The hot field emission cathode, also known as the Shottky Field emission cathode, has the same general design to the cold type above, but additionally has a layer of zirconium oxide melted onto its tip. At elevated temperature, much lower then any thermionic cathode, electrons can tunnel through this barrier layer by the schottky effect, thus its name (citation needed). It has the advantage of offering a higher total current then the cold field emission cathode as well as a complete lack of the instability of the cold type. This comes at the cost of a larger emission area and lower current density when compared to the cold field emission cathode. This type has mostly supplanted the cold field emission type in the high end instrument sector.
It also requires a slightly less stringent vacuum, with values typically in the range of 1 e-9 mBar, but benefits form higher vacuuo, just like any other cathode, bar the plasma types.
Plasma Cathode
This cathode type is the oldest of the cathodes used for electron microscope, and was the first used by Ernst Ruska. It basically consists of a gasious plasma discharge from which is drawn a stream of electrons by a second accelerating potential. Its ease of construction and high fault tolerence made it atractive for early electron microscopes. It however has the worst amonut of current density of all cathodes and requires differential pumping between the plasma chamber and the acceleration chambre. The plasma discharge also takes a significant amount of power to sustain, thus leading to large amounts of heat as well as errosion of the discharge electrodes by sputtering.
Photoelectric Cathode
A rare type of cathode used in ultra short pulse electron microscopes, is the photoelectric cathode. This usually consists of a metal surface of low work function, such as Cesium, which is illuminated by a suitable light source. The ara exposed thus, will emmit electrons via the photoelectric effect, which can then further be accelerated to form the electron beam of an electron microscope.
A common useage of this cathode type is within the photomultiplier tube of the secondary electron detector of a scanning electron microscope as well as night vision image intensification tubes. In the past, such cathodes also formed the basis of Videocon image capture tubes, used in television cameras and the first digital cameras.