 Hello everyone, welcome to this material characterization course. In the last class we just started with the introduction of scanning electron microscopy and we have just reviewed what all the information one can get out of this scanning electron microscopic techniques and what all the salient features that you can obtain related to microstructural information. And then we started looking at the instrumentation details. So we will continue in that session. So this is what I was just showing yesterday. The schematic shows the cross sectional view of ACM and I just started describing this each parts. So you have this electron then and then you have series of electron lenses and then something called scanning coils and then you have this magnification control and scan generator and then you have final lens aperture and this is where your specimen is kept in the specimen chamber which is maintained at a vacuum of 10 to the power minus 4 Pascal. I just mentioned yesterday and then you have this detector system and then you have the control console. So what we have to understand from this schematic, the electron then just generates the electron and accelerates to 0.1230 kilo electron volt and then the electron are passing through this electron lenses or scan coils and then the primary function of this section is to demagnify this probe diameter because typically if you take a tungsten harpin the probe diameter is not sharp enough to obtain the microstructural details. However this scan coils or electron lenses demagnify that probe to a very small size in the order of 10 nanometers when it finally reaches on this specimen surface. So this is the primary function of this coils and electron lenses and if you look at the right hand side schematic which shows the specific action of this scan coils what you see here is the high energy electron beam which is accelerated by the electron gun comes through this scan coils and you see that the electron beam is deflected of the optic axis in a discrete locations in a line. You can see that and then finally the second coil is also deflects again on a discrete location in a second line. So like that it will go on depending upon the number of lens coils you have in the column and what you have to understand is before it reaches the beam reaches the final aperture at a pivot point it has been deflected of the optic axis by the first coil and then the beam is brought back to this optic axis by the second coil and it crosses the final aperture and this action this deflecting of and on the optic axis of the electron beam occurs till it makes the rectangular raster on the specimen surface that is a scanning action here. So this happens. So finally the magnification of the image is the ratio between the specimen region on which the electron beam is probing and the in the CRT screen where the raster is going from the one end to the other. So we will look at that ratio and understand the magnification just since we are talking about the rastering here I just want to mention the magnification related to the specimen region where the probe is scanning this area and the CRT screen what you are looking at the area of the CRT screen and what you are seeing is also a W is a working distance we will discuss about it it is again a very important one of the parameters to for the operator control in the scanning electron microscopy. So now we look at the other schematic where it also describes the what kind of detecting system is employed in this scanning electron microscopy and you see here this electron beam strikes the specimen and you have some interaction volume shown here and from there you get typical signals secondary electrons as well as backscattered electrons like we discussed yesterday and you see that these signals are collected by the detectors. The image is formed by I mean the electronic system converts the signal point by point and form an image. So you see that the signals are collected by these detectors we will look at the detector details little later just to give a kind of an idea to understand how a CM works. Let us assume that this is an detector which can detect these 2 signals and you see the details if you have the positive potential or positive voltage then it can accept secondary electrons as well as backscattered electrons but when you apply a negative voltage it can only accept backscattered electrons and not the secondary electrons this is because your secondary electrons have a lower energy which will get repelled by this field then the signals are collected by the scintillator and the photo multiplying multiplier tube and which is getting further amplified with an amplifier and finally it reaches the CRT where you see the image of your specimen of interest. So this gives you a kind of overall a function of how the CM works I hope you got some rough idea by looking at all these 3 schematics. What now we will do is we will summarize whatever we have just discussed in the form of few sentences so that you can just verify this again and again a source of electron is focused in a vacuum into a fine probe that is rastered over the surface of the specimen as the electron penetrates the surface a number of interactions occurs that can result in the emission of electrons or photons from the surface a reasonable fraction of the electrons emitted can be collected by the appropriate detectors and the output can be used to modulate the brightness of a cathode rate tube whose X and Y inputs are driven in synchronizing with the XY voltage rastering the electron beam. So if you look at the description of the magnetic lenses the beam is defocused by series of magnetic lenses or the each lens has an associated defining aperture that limits the divergence of the electron beam. So what I just want to go back and show these are the apertures we talk about each lens has got some kind of aperture which decides the divergence of the electron beam by increasing the current through the condenser lenses the focal length is decreased and the divergence increases the lens therefore passes a less beam current onto the next lens in the chain. Remember the smaller the spot sizes often given higher dial numbers to the numbers to correspond with the higher lens currents required for the better resolution or attained with the smaller signal to noise ratio. This is very common practice in an SEM as well as TEM probably I will show you when we go to that appropriate lab and look at the actual equipment and the controls you can see that all small smaller spot size often given higher dial numbers. The beam next arrives at the final lens aperture combination the final lens does the ultimate focusing of the beam onto the surface of the sample. The sample is attached to a specimen stage that provides X and Y motion as well as the tilt with respect to the beam axis and rotation about an axis normal to the specimen surface. A final Z motion that is vertical motion allows for the adjustment of the distance between the final lens and the sample surface this distance is called the working distance I just mentioned in the schematic. So the working distance is the distance between the final lens and the sample surface. Now let us look at the some more description of this parameters. The working distance and the limiting aperture size determine the convergence angle shown in the figure. Typically the convergence angle is a few radians and it can be decreased by using a smaller final aperture or by increasing the working distance. The smaller the convergent angle the more variation in the Z direction topography that can be tolerated while still remaining in focus to some prescribed degree. This large depth of focus contributes to the ease of observation of topographical effect. You see we also discussed the this phenomenon yesterday in the introduction of an SEM. One of the important feature of this equipment is this can achieve very large depth of focus and also I said that you get a feel of 3D like image and this is convergent angle is one of the parameters which contributes to this large depth of focus. So an image is produced on CRT every point that the beam strikes on the sample is mapped directly on to the corresponding point on the screen. If the amplitude of voltage applied to the deflection amplifiers in the SEM is reduced by some factor while the CRT voltage is kept fixed at the level necessary to produce a full screen display the magnification as viewed on the screen will be increased by the same factor. See this is just in terms of operator control the magnification is explained. I also talked about the ratio between the region on the sample as well as the CRT screen area and this particular explanation is in terms of what you actually control on the specimen that is so you have the deflection amplifiers which is been controlled by some factor whether you reduce or increase the same effect you see it on the CRT screen on the magnification of the appropriate specimen region. So now what kind of samples can be examined the sample requirements are more stringent they must be vacuum compatible they must be either conducting or coated with a thin conducting layer and we will look at the details of the sample preparation and its requirements little later we will see it but just give you an kind of introductory remark you should realize that the material should be vacuum compatible and it should be either conducting or we have to coat a thin conducting layer on the specimen a variety of contrast mechanisms exist in addition to the topological enabling the production of maps distinguishing high and low atomic number elements defects magnetic domains and then even electrically charged regions in semiconductors this also we discussed yesterday different kinds of mechanisms are possible when a high energy primary electron interacts with an atom it undergoes either in elastic scattering with atomic electrons or elastic scattering with the atomic nucleus in an inelastic collision with an electron some amount of energy is transferred to the other electron if the energy transfer is very small the emitted electron will probably not have enough energy to exit the surface. So we are now getting to the details of electron beam and and interaction with the specimen we will see when the energy of the emitted electron is less than about 50 electron volt by convention it is referred to as a secondary electron yesterday I just mentioned that the classification of this signals something like secondary electron and backscattered electrons is based upon its varying energies so you have the effects number here when the emitted electron is less than about 50 electron volt it is referred as secondary electron most of the emitted secondaries are produced within the first few nanometers of the surface backscattered electrons are considered to be the electrons that exit the specimen with an energy greater than 50 electron volt including OJ electrons the higher the atomic number of the material the more likely it is that back scattering will occur thus a beam as a beam passes from a low atomic number to a high atomic number area the signal due to back scattering and the consequently the image brightness will increase there is a built-in contrast caused by the elemental differences. So you have to understand that the atomic number of the element increases the scattering event also increases and eventually you get image brightness as well we will look at this kind of electron beam and its interaction and its volume everything we will look at them in later and this is just and I am introducing how these signals are classified and what kind of interaction they will make with the specimen. One further breaks down the secondary electron contribution into three groups secondary electron one secondary electron two and second electron three secondary electron ones result from the interaction of the incident beam with the sample at the point of entry secondary electron twos are produced by backscattered electrons on exiting the sample a CM's are produced by backscattered electrons which have exit to the surface of the sample and further interact with the components of the interior of this SEM usually not related to the sample and SEMs and secondary electron threes come from the region far outside that defined by the incident probe and can cause serious degradation of the resolution of the image. You see these classification the further classification of the secondary electrons again based upon the energy variation however these energy variation happens at particular location and the event and that is how it is been described in the last two slides this is just for a clarity we will just show the effect of these one two and three secondary electrons when we discuss the contrast mechanisms in detail. Now we will just go to the scanning electron microscopy imaging modes what are the kinds of imaging mode we employ while we carry out the microstructure investigation using scanning electron microscopy you see this schematic this is the electron beam you have some notation called dp, ip, alpha p and v naught four major electron beam parameters are defined where the electron probe impinges on this specimen what are those four parameters electron probe diameter that is dp beam beam size electron probe current ip electron probe convergence alpha p there is a typo here alpha p and electron beam accelerating voltage v naught. Please remember all these beam parameters will have a specific effect on the image quality and the information which you get as well as on the resolution that is why we specifically talk about these parameters we will look at the effect of each one of these beam parameters on the microstructural details which you obtain as well as on the resolution we will see one by one the effect of probe size and the probe current on the resolution and the and the high current mode. So you have a three micrographs of some surface the voltage employed is 20 kilo volt the magnification for these two would sorry this is not magnification there is something wrong here the magnification is about 10000 X for all these three you have dp of 15 nanometers ip of 1 picoamperes and you see that schematic I mean so this micrograph b is obtained with the dp in the order of 20 nanometers and an ip in the order of 5 picoamperes and the third micrograph is obtained with the dp of 130 nanometers and ip of 320 PA. So what do you see it is not that you have a specific combination of all these parameters is well defined you see that as the probe diameter increases you are not seeing the clear resolution here resolution is not improved at the same time if you increase the probe current also the resolution is not improved but at this particular combination of dp and ip you have a better result compared to this first one and third one. So you see that you have a combination of a probe diameter and the probe current gives a better resolution and next we look at the effect of convergent angle on the depth of focus you see the image taken in the same region here the alpha p that is convergent angle is 15 milli radians and here it is 1 milli radian the voltage is 20 kilo volts and this marker is 11.6 microns and you have the varying aperture size you see that a loss of background features occurs as the convergent angle increases. So if you want if you look at this image the background is not details are not clear however you can see that much more details are seen which is lying behind this region. So you have the convergent angle effect as well on the resolution of the micrograph. Now if you look at the effect of accelerating voltage in a low voltage mode the micrographs taken here with 5 kilo KVA and 15 KV and 30 KV and you see that the surface detail of a surface oxide growth on a copper is seen with the different voltage you would clearly appreciate that the increase in the acceleration voltage not necessarily help the resolution you see only at the lower accelerating voltage you are able to look at the details of you are able to see the details of the oxide layer on the specimen. The low KV image shows greater surface detail the high KV image shows loss of information about the surface oxide due to the beam penetration. So now we will see what are all the operator control in ACM to obtain a better resolution or a control. We will see that the animation which is showing the kind of aperture size effect you will see and let me first describe this schematic and then we will see what is that we try to understand from this schematic. So this is an electron gun then again the beam comes through the condenser lenses and then final aperture and then again goes to the next stage at the objective lens and then you see finally it reaches the sample. What we are trying to say here is when you obtain optimal aperture angle that minimizes the aberrations on the final probe size that means we need to understand what is this optimum aperture angle by looking at the image quality where relatively it is free from the aberrations you judge this the final convergent angle controls the image depth of focus. The aperture determines the current in the final probe because only a fraction of the current sprayed out to the angle alpha 1 passes in the aperture angle alpha finally. So what we are trying to say here is you see that the beam is spreading to the angle of alpha 1 it is quite large and only fraction of this is going to enter the final aperture and which has the aperture angle controlled by this objective lenses or aperture the aperture belongs to objective lens controls the final angle from the large sprayed out angle in the previous lenses. So that is what we are trying to understand here. So that aperture an operator can control and then decide whether this particular settings will be useful in obtaining the information with minimal aberrations. Now we will look at the effect of working distance again you see these two animations of the ray diagram and you see that the distance between the final aperture and the sample surface is working distance like we defined in the previous one. So you have this two schematic displaying the ray diagram with two different working distance this is W1 and this is W2 and then the schematic nicely displays increasing the working distance how your converging the ray converging positions are I mean displayed here or how they are different with the adjustment of the working distance. The increase in working distance produces a large spot size at the specimen. So you can see that here it is small spot and here it is a large spot and the degradation of the image resolution obviously that is going to cause some resolution decrease in the resolution converging angle decreases and improved depth of focus and the converging angle which we have already discussed and smaller the converging angle we improved the depth of focus that we have seen already weakening the objective to focus aberration to the lens. So if you increase the working distance this also will happen and which also increases the scan length and reduces the magnification. So working distance if you play around these are all the points which have to keep in mind and the operator should again judge by looking at the working distance and the image quality he obtains and then decide what gives him the best. Now we will look at the effect of condenser length strength here is the schematic again you can see that the effect of condenser length strength on the final probe diameter the increase in the condenser length strength increase the demagnification of the each lens and reduces the probe size. The final probe size can only be reduced at the expense of decreasing the probe current and a conscious choice between minimizing the probe size or maximizing the probe current. So either you if you want to reduce the final probe size either you play with the minimum probe size or maximum probe current that you have to take a call by looking at the again the kind of information you are interested in and also kind of resolution you want to obtain at particular magnification. So you can clearly see that from the schematic depending upon the condenser length current so you see that how the final probe diameter which is falling on the sample is reduced to a very small probe here. So having talked about these probe diameter we will go through some of the important aspects to be noted. We are always interested in minimum probe size if we in order to resolve very smaller details and if you recall in the beginning of this second part of the course where we talked about fundamentals of electron optics we also discussed about quite a bit of information on the lens aberrations in the case of electromagnetic lenses and its optical systems. Where we discussed that all the aberrations are going to contribute little bit to the final probe diameter or the electron beam. So this is what we are now summarizing here the calculations of the probe size assume that Dp is quadrature sum of the diameters of Gaussian and other aberration discs. So the final diameter is Dp is equal to Dg square plus dc square plus dd square plus dc square to the power half. At normal voltages of 10 to 30 kv the relationship between probe size and probe current can be calculated at alpha optimum d minimum is equal to kcs to the power 1 by 4 times lambda to the power 3 by 4 into ip by beta lambda square plus 1 volt to the power 3 by 8. And the maximum probe current at 10 to 30 kilo volt has got an expression similar to this I max equal to 3 pi square by 16 into beta into dp to the power 8 by 3 divided by cs to the power 2 by 3. And we also look at what is this Gaussian probe diameter to fully understand how probe size varies with the probe current we need to calculate the minimum probe size and the maximum probe current. The aberration free Gaussian probe diameter Dg which is the full width at half maximum height of the intensity distribution of Dg where Dg equal to square root of 4ip divided by beta pi square alpha p square. The current in the final probe can be estimated as ip is equal to square root of beta pi square alpha p square Dg square by 4. So all these expressions will give you a kind of an idea the important four parameters which we talked about how they are related basically with respect to the probe diameter. Please understand there is you should not confuse this probe diameter with the electron beam size. So electron beam along the column is not called probe diameter the probe diameter is a final probe electron beam which exit from the the final aperture and next to immediately to the specimen surface. So that is called probe diameter. So do not confuse this parameter with the electron beam size along the rest of the column. And then you see that that probe diameter has got the dependence on all the other four parameters and that is what this mathematical expressions relate that is all I want you to appreciate. If there were no aberrations in the system it would only be necessary to increase the convergent angle to increase the probe current at the constant probe diameter. So I would like to stop this lecture here and then we will continue on the various aspects of the SEM operations and little bit of theory of contrast mechanisms and how this equipment can be exploited can be exploited in order to obtain more microstructural details. We will continue in the next class. Thank you.