Electron Sources: Types of Electron Sources

Added: 2026-05-06

[00:00:13]In the previous lectures, we've seen a general introduction to this course, the layout of microscope, layout of a microscopy lab and briefly touched upon various components and saw the ray diagram of the microscope. Now let us understand each of the components one after the other. Today's lecture I'll be focusing on electron sources.

[00:00:37]So when I talk about electron sources, it is what you have in this upper half or in the very top portion of the microscope. Uh here is the lanthanum hexaborite filament operated TM the jo000.

[00:00:54]And as you can see this portion which is enlarged and shown in this red border is where your source is. So this is called as actually the electron gun. And there are several components which form the electron gun. When I talk about the different kinds of electron sources in transmission electron microscopy or in scanning electron microscopes, we commonly have two kinds of emitters. Let us have a look at what these emitters are in the next slide.

[00:01:29]So as I was mentioning the two common sources that we have for electron emission are the thermionic sources or the field emission guns. Now when I talk about a thermionic source it is very much similar to something which we are very familiar with the incandescent bulb. So here in a thermmonic source the principle is you actually have a very fine filament typically a tungsten filament and then you really heat it to a very high temperature by passing an electrical current. So because of the resistive heating you not only have emission of light but at very high current when you overcome the work function barrier you also have the emission of electrons.

[00:02:17]These electrons are now converged to a point and you focus them. So if I want to converge a beam of electrons that are emanating from a filament, naturally the construction would require that I have some kind of a lens there. This lens should be able to control the electron beam. So the general construction of an electron source is you have a filament which is the cathode and you have something to converge. So you have something called as the venold grit and you also have an anode. So essentially having a cathode having a tungsten filament having some source similar to an incandescent bulb is not enough to emanate the electrons and converge them to a point. To have all this functionality you would want to have some kind of a lens which converges this to a point.

[00:03:12]So let us have a schematic layout and look at the components of a thermmonic source first and then we'll see how a field emission gun looks like and we'll draw a comparison between these kind of sources and we'll try to understand what effects this has on the quality of image and the resolution you can achieve in a transmission electron microscope. So let us first look at the layout of a thermionic source. Uh here's how the thermmonic source and the cathode part or the venold grid looks like. I'll show you a schematic from the microscope that I've shown you a demonstration that is the lantham exaboride filament 2100 tm.

[00:03:58]Here is the image taken from uh the user manual of the 2100 microscope. So the components you have are you have the base remember in mind to give a better understanding the layout in the microscope column is reversed here. So it is pointing so this is the way up in the TM. So just for convenience and to understand the components we've turned this upside down. So this base or what you see here is actually at the top of the microscope column. You have a small filament here and uh as you can see this portion acts as the cathode and uh you have a cap here. So you can unscrew these two components and remove it. It is like the way you place a small incandescent bulb in the filament holder or in the bulb holder. You similarly have a base and a filament holder here and uh you fix it inside a conicle-shaped venult cap or sometimes you call it as the venult grid and this goes into the top part and on the other side of this you get the anode.

[00:05:24]I'll show you another schematic very shortly. But let us see how a real filament looks like. Here is the filament that we have inside a lanthanumaborate TM. Uh this was in fact taken out from our own microscope here.

[00:05:40]So this is from I think a company called Dena. But what you have here is the posts you see here are placed here. And this is the reinium wire or a tungsten wire.

[00:05:59]And on top of this you have a tiny lanthnum hexaborate filament.

[00:06:06]If I look at a schematic of this here is how it actually looks like in the microscope. So I have the filament and this whole assembly which looks like this part.

[00:06:22]and I start applying some kind of a bias on the venold grid. The whole objective of this is from the cathode to have an emission. I have the oppositeely charged anode here and I also have a venel grid.

[00:06:36]It is typically a selfbiasing one in a lantham hexaboride gun. But uh in the other construction of a field emission gun, you would have two anodess. I'll not go there yet. But let us understand this one first. You apply a small bias or you do not have any bias on the anode. The anode is maintained at the earth potential or at the ground potential. So you're extracting electrons from the filament. So typically you start this from uh 0 to uh for example 700 volt or you go even up to 1500 volt or so. You might start seeing some kind of an emission. So the electrons are coming out in all the directions to converge them to a point and to have this along the trajectory of the electron uh beam path of the microscope or to have it along the optic axis of the microscope. You want these electrons to converge and that is why you have this venold grid and the anode here.

[00:07:40]Remember that once this emission happens beyond the gun, you're accelerating the electrons in the path of the electron microscope. Depending on the accelerating voltage you use, it can be a 60 KV, 100 KV, 120 KV, 200 or 300 kilovolts or even it could be a high voltage electron microscope. It can go even up to uh 1.3 million electron volts. But depending on how this goes into into which instrument the essential construction of a thermionic gun is the same. You have this kind of a venel tool or a venel grit and then uh the filament goes into this. you have an anode on the opposite side which is typically maintained at the uh ground potential and uh you start extracting electrons shortly and then you focus them or converse them to a point. So let me show this on the uh ray diagram of the microscope how this looks like and what exactly is happening when your emission happens in a thermionic filament. So in a thermionic emitter as you can see if I start extracting the electrons from the filament what happens here is uh you start from a current which is very low and then as the electrons start emitting here and the charge gets accumulated it gradually becomes slightly negatively charged and what happens is At this point you actually have to overcome a sort of a barrier or an electron cloud that is formed in front of the source to have a sustained emission. So that is where you have this optimum bias. And if I now look at the flux of the electrons emanating from such a filament, you can always write this with an equation which so if I say the flux emanating is J and if I have an exponential function like this.

[00:10:01]So this looks like an arhinous type of an equation. Uh this is called as the Richard Richardson's constant. Uh you have the flux emanating as a function of the temperature the work function and the temperature here again and this is the Boltzman constant. So typically you would expect this curve to look like an exponential function.

[00:10:30]But because I was just mentioning that as a small partial negative charge builds up here, you would have this appearing with a small kink where you have some critical current density beyond which you have a sustained emission. Now after you have this kind of an emission if you look at the ray diagram of the microscope let us see what happens here.

[00:11:00]So here is where your electron gun assembly is and I've shown that the path of the electrons are converging at a point here.

[00:11:10]This point is actually called as the first crossover.

[00:11:18]This is an extremely important thing here because this crossover point can act as the object for the subsequent condenser lenses that you have in the electron beam path. So just below the electron gun I have the condenser lenses which is also a part of the illumination system of the microscope. And this electron lenses are actually electromagnetic lenses. And by simply changing the strength of these and also changing the strength of the first condenser lens here, I can have a nice interplay between looking at where the crossover point is. Now what happens here is by making this first condenser to have the crossover point at the back focal plane of the second condenser. I can control whether my illumination or the electron beam is going to have a sort of a converged probe kind of a path or a parallel illumination. So if you understand the physics of this, if I have my second condenser lens here and my electron beam from the source is emanating to be at the back focal plane of the condenser lens. So if I have a point source here and this is coinciding with the focal plane of the second condenser, the electron beam that comes in would have a parallel path and just depending on whether it converges here or here I can always control whether I have a sort of a conical elimination or with the combination of these I can also form a probe and converge it to a very small point. So when you have a series of condenser lenses by converging the source to a point and allowing it to be at one of the focal planes of the second or the third condenser lenses by the combination of these condenser lenses, I can really get a very fine probe or I can get a parallel beam illumination. and the kind of the uh illumination that I use in a microscope would actually help me choose the different modes of operation I want.

[00:13:41]For example, if I'm using techniques that use parallel beam elimination, I might want to have a beam which is spread over. Now, why is this important and how does a thermmonic source matter?

[00:13:54]One of the important parameters is this crossover point for a thermionic emitter is not extremely fine.

[00:14:04]To have a better crossover and to have a very small probe, you would want to go to the field emitters. Let us now see how the thermionic emission functions.

[00:14:15]We'll have a look at how a thermionic emitter emits the uh electron. So electrons along the microscope column and then let us see how a schematic construction of a field emission gun is and how that looks like. So if I look at the thermic emission, I have to overcome the work function barrier. Work function is defined as the energy barrier that prevents electrons from escaping a material. Once I overcome this barrier, I have a sort of an emission which follows the uh equation that I just showed you the arheneous kind of an equation. The current density can be given by an equation like this.

[00:14:56]And then one important parameter is when the temperatures actually go very high, you would want to have a material which is sort of a refractory material and typically you use tungsten. But on tungsten you would not have a very low work function.

[00:15:16]You could still have pure tungsten emitters as well. But you use a material like lanthinum hexaboride or serium hexaboride. These crystals typically have a low work function. And it is established that uh the 110 uh plane the crystalallographic facet of 110 is a direction which has enhanced emission.

[00:15:39]So the construction that I showed you in the previous slide which is like this you have the lanthmum or a serium hexaborate crystal. Nowadays people are trying to replace this with carbon nanot tubes also for having some kind of an electron emission. That is an entire field of research which I'll not go into at this moment. But to give you an idea that on the tungsten filament you have a crystal which sits like this. Bear in mind even to the visible eye or when I take an optical image the crystal is still visible with a sharp facet which is along the 1 0 direction of the lanthanum hexaboride crystal and if you can see this particular size of the filament that you have plays an important role on one of the parameters which is called as coherency in particular spatial coherency which I'll define when I draw a comparison with a field emission gun. So let us have a look at how the thermionic emitter looks like. I'll play a small video here. So you can see that when I switch on the emission in a microscope column, I do not see anything. I'm trying to record the image I get on a fluorescent screen here and at some point I start seeing the whole area is eliminated. You just saw that this eliminated region there was a small jump here. Let me just play that little change once again for your reference. I see there is some bright image that I form here and then it slightly reduces and there is some kind of an adjustment that happens and then I have more bright sustained emission on the screen. If I now plot this current density as a function of time, I had the small jump and then I had the sustained emission. This was the graph that I showed you on the other screen as well some time ago when I defined this equation. I have to overcome this critical current density that is because of the kind of the shielding layer that is formed with all the electrons that are extracted and right in front of the tip and beyond this I have a sustained emission. So as you can see this is a very similar principle to an incandescent bulb. I just heat it up and by resistive heating uh I get emission of electrons. Bear in mind this green color you see here on the fluorescent screen is because the electron beams which are not visible to our naked eye which are not within the visible spectrum are striking a fluorescent screen or they go and strike a CCD camera and because of the uh phosphorusence from the material that is there on the screen you start seeing this color.

[00:18:49]In fact, uh when you have some kind of field emitters, you could have a different kind of a material and you might see this in a sort of a purple color. I'll try to see if I can get an example of emission from a cold field emitter.

[00:19:05]But please bear in mind that this constant bright current that you see and that is spread over this large area, you would actually have a sustained emission in a thermionic emitter.

[00:19:18]Please bear in mind that this illumination that I have spread across a large area, I can now converge this or I can spread this by controlling something called as an intensity knob on the uh microscope console.

[00:19:35]Having seen this, we look at the next kind of an emitter which is called as a uh field emission gun.

[00:19:43]If I look at a field emission gun like I was mentioning a while ago, you have the venold grid, you have the cathode and then you actually have a very fine tungsten wire tip and which is coated typically with uh uh a kind of a zirconium oxide and then uh the fine needle is positioned within the filament holder area and you have two anodess here. The function of these two anodess are you have an extraction anode where like in a field emission in a thermionic emitter you wanted to have some electrons emanating and then the accelerating anode which boosts these and you also want them to converge to a point. What happens here is your filament size is much finer than a lantham hexaboride crystal. If you remember I was mentioning to you that it is visible to the naked eye. If I look at a scanning electron microraph of how a filament tip in a field emission gun looks like.

[00:20:50]This is an image taken from the famous textbook from Williams and Carter. You have an extremely fine tip. Typically the tip radius here is less than 20 nanome or 50 nanometer.

[00:21:06]And this goes into an assembly which looks like this. Here is a schematic of a field emission gun. Uh this is typically used in a shortkey field emitter or even a cold field emitter has a similar construction. So essentially you have initially some extraction uh that you have in this anode and then uh the accelerating anode is in the microscope column where you're having this go along the accelerating voltage of the microscope but you have the first anode and the second anode and the crossover which is at a very fine point.

[00:21:49]So what you have here is combining the fields of both of these anodes you're actually having this behave like a lens and in comparison with a uh thermionic emitter the field emitter already before it is going into the condenser of the electron microscope it is focused to a very fine point. So in fact you can call that unlike a physical source from where you have the emission happening like an incandescent bulb in thermionic emitter here you sort of have a virtual source which is beyond the field emission gun.

[00:22:30]You have the electron beam uh focused to a very fine point uh which is typically coinciding close to the back focal plane of the second condenser lens.

[00:22:43]And uh with this kind of a combination of multiple anodes and addition of a magnetic field lens you have much better beam control. You can make the beam converge to a much finer point and uh you have increased brightness.

[00:23:02]So if I look at the energy diagram of how electrons are extracted and understand the principle of a field emitter in a bit more detail, you have the following.

[00:23:18]The idea is at the very fine tip I apply a very high field. Typically when I have even the field of the potential of the order of uh 3,500 volt or so or typically the extractions start from around uh 2.7 KV and you go up to 3.5 to 4.5 KV depending on the mode that you're operating in. And if my tip radius is less than or if it is in the order of tens of nanometers, you can see that my electric field is extremely high. It is of the order of a few uh 10,000s of volts per nanometer.

[00:24:04]And what happens here is when I have such a high field being applied and I also have the effect of the anode that is kind of lowering my work function barrier. This particular anode which is present in front of the fine tip. So if here is my cathode and I have series of anodes here. This anode is actually kind of helping me lower the work function barrier as well. This is called as actually the shortkey effect.

[00:24:46]So in a field emitter because of the shortkey effect by having the combination of two anodess I'm kind of lowering the work function barrier. So in a thermionic emitter if I had to extract electrons from within the uh filament into free space I had to overcome this work function barrier and with the addition of the anodess here I can kind of lower the work function barrier a little and I have a sustained emission at slightly lower potentials than in a thermionic emitter.

[00:25:26]So what this does is the anode not only focuses the emitted electrons to a confined path and to a very fine spot but I also have lowering of the work function barrier and I have a better control in a field emitter.

[00:25:46]So if I look at the advantages of this and in a little more detail when I have tungsten tips of a very fine radius I have a very high potential and this also overcomes the work function. You not only lower the work function barrier but in a few cases you would also allow the tunneling of the electrons. What do I mean by tunneling? It is a kind of a quantum mechanical effect where if I bring the anode extremely close and I apply such high fields. So if I bring this gap so within a venold grid in a thermionic emitter we've seen that the gap here was in the order of a few millmters or a few hundreds of microns. But in a field emitter if I bring it even closer to the order of half a micron or even lower let us say even a gap of 200 or 300 nanometers.

[00:26:51]What happens there is when such a high field is brought close to a tungsten filament which is very sharp the way you have in a scanning tunneling microscope.

[00:27:04]you have electrons just going out without overcoming the work function barrier and you can have a sustained emission. But bear in mind in order to have this happen you need to have an extremely uh good vacuum. So if in the ultra high vacuum systems let us say I have uh a vacuum of let us say of the order of 10 - 8 m you would want to have this even better than 10 -9 or even lower to have the tunneling happen.

[00:27:43]Also one of the consequences of such high fields and uh even within low vacuums of 10us 7 to 10us 8 m the filament tip is so fine that you cannot avoid the formation of some kind of an oxide layer on the tungsten filament. So you would naturally have some kind of effects of formation of an oxide layer and if you have an oxide layer on a conductive tungsten filament bear in mind that the oxide can also have some kind of a charging and flashing as well. So this will kind of reduce the brightness or the flux that is coming out from a field emitter. So to overcome these you would want to have a feedback loop. So I'm kind of mixing up the constructions or the principles of a thermonic or a shortkey FG and a cold field emitter. But you would want to have a pristine surface on the filament. If possible, you would want to have sort of a flashing of the filament and you can have tunneling of the electrons. Now within the field emitters also there are two kinds of emitters.

[00:29:00]You would have a thermal field emitter where the tip can be heated in a slightly poorer vacuum and in a very controlled fashion you kind of raise the potential on the anode such that you have extraction of the electrons. In the microscopy terms you can call it as a warm start or a cold start. So what do I mean by a cold status? If I want to achieve a potential of around 2.7 to 3 kilov volts to extract electrons from the tungsten filament and then form a crossover or form an image of the probe and converse them into a very fine point. I will do this at a very low rate that is I raise the uh potential at I don't know.1 uh volt per second or even lesser such that I reach this 2.7 to 3 kilovolt in a duration of about 2 and 1/2 to 3 hours.

[00:30:04]In the very old days on the older uh instruments like uh the Philips CM20 or CM30 series, you were manually turning the knobs with the digitization and having feedback controls. You can now do a warm start in a very gradual way. But if you're not having uh a good control on increasing the potential gradually, what happens is you might actually fracture the tip of the tungsten filament and you might actually lose it out. So in a thermal field emitter with a gradual increase in the potential by doing a warm start, you can have sustained emission at a potential slightly lower than the work function barrier.

[00:30:58]And uh because you do this gradually and you maintain the current or the emission current from the field emitter reasonably well, the stability of a thermal field emitter is not too bad in comparison with a thermionic gun.

[00:31:18]And if I look at the other kind of a filament uh or the cold field emitter, I was mentioning that you have to maintain an extremely high vacuum and you want to have tunneling of the electrons. You in fact bring the anode a little more closer to the uh tungsten filament to have sort of a electron tunneling effect and you do not go past the work function barrier. But the challenge here is when I'm doing this, the operating voltages in a field emitter would go slightly higher. You would go up to 7.1 or 7.5 kilovolts even.

[00:32:04]And you would have the same two anode construction here as well. And what you do is once you extract the electrons, you are also suppressing it by using the other anode.

[00:32:17]and you control the beam currents in a much controlled fashion with a feedback loop.

[00:32:25]Now the challenge here is the vacuums although are ultra high vacuum uh there is a chance that when you go to these high potentials if you do not have a good feedback control uh the filament tip can get fractured much more easier. And uh if you have this your stability of a cold field emitter was not extremely good in the mid90s to let us say the digitization that happened let us say around 2000 or 2005 from somewhere around 2000 onwards uh in fact I can say the manufacturers such as Hitachi and J have successfully mastered and very recently even thermofisher has come up with the uh field emitters cold field emitters thanks to the modernization and having a feedback loop you can do what is called as flashing of a filament. So essentially if I extract electrons in a cold field emitter over a duration of about 3 hours or so I might have a very good beam current but then this gra gradually drops down and uh I might end up with sort of an oxide layer formed and we do something called as flashing of the filament. What you do here is you go much beyond this 7 1/2 KV. You could go to a very high pulse over a very short span of 8 seconds or 10 seconds and you kind of sharpen the filament tip in C2 within the filament assembly and then or within the FG assembly and you you can redo the emission and imaging again within the microscope. So with this the lifespan of the filament is kind of uh naturally when such a technology is coming into the market people would have done all the homework in such a way that the overall lifespan of a cold field emitter where you're not doing a warm start. So the main difference between a thermonic a thermal fg and a cold uh field emitter is once you switch on a thermal field emission gun you always keep it on over a long duration unless there is a special maintenance or something where you are bound to switch off the uh field emission system whether it is within an SEM or within a TM you keep a thermal field emitter running it can run over a period of 1 year, 2 years or even more.

[00:35:04]And typically when a thermal field emitter is running that way, the lifespan of a thermal field emitter is typically over a duration of 2 to 3 years. But in the cold field emitter, like in a thermionic emitter, you're switching on, you're flashing the filament, and every time you flash, you're kind of sharpening the tip once again, and you have a feedback loop to have a sustained emission. your uh brightnesses you achieve can be quite high and also you would have a longer FEG uh lifetime. There have been cases where cold field emitters have run significantly beyond 5 years or even 7 years and it it it runs fine. That is because every time an oxide layer is formed and the vacuum is also maintained very well, you're kind of removing the top oxide layer by flashing. So that is about the lifetimes of field emitters and so on.