 Hello everyone welcome to this material characterization course. In the last class we looked at the details of the image contrast in the scanning electron microscopy and then and we also looked at some of the parameters which significantly influence the image contrast and typically the topographic and atomic number contrast is primarily controlled by this secondary electron and backscatter electron signals. And I also mentioned that there are some some more special contrast mechanisms possible in an SEM and today we will look at some of them very briefly if not detailed for the sake of completion of all the contrast mechanism possible under SEM. So what I will do is I will just write it on the blackboard so when we say that it is a special contrast mechanisms this can be observed only with a special class of materials or solids and structures. And typically these mechanisms carry the information on the specimen electric and magnetic fields and crystal structures and this can be imaged in any of the scanning electron microscopy. So first we will look through the electric fields how it can be imaged and what is the idea behind it and then we will move on to the magnetic fields. The first is electric field so we will first look at the how to image this electric fields there are two types of what we are now going to look at it is type one where you have the insulating material and engineering structures such as integrated circuits can develop electric field as a result of applied potential or in response to the charge injected by the primary electron beam. So what is that that causes the image that we will see now you see when this electron beam interacts with this any insulating material or the IC circuits the variation in the local surface potential on such specimens can be detected as the image contrast through the effects of that potential on collection of AC's collection of secondary electrons through the ET detector like we have seen in the other mechanisms. See this potential aids in collecting the secondary electrons which is coming out of this structures it could be IC circuits or any insulating material because of the variation in the local surface potential which can be imaged as a voltage contrast. So this is one type we look at the other type you see this is another very important aspect of ACM operation itself and some of you would have seen or you may you may see that sometimes your specimen is getting charged whether your operator tells this or if you are operating the machine you yourself will observe it that is a voltage contrast due to localized specimen charging under the beam of insulating specimen or conducting specimens with insulating inclusions. So non conducting inclusions will also cause this charging effect and that is because of so what happens is in the insulation insulation material the beam can inject sufficient number of electrons into the target to develop a surface potential locally equal to the beam potential. So with the result the specimen becomes a mirror actually you are just looking at the beam we can see that we can say that so that that is why you get the images of a charging effect you can see that in a gray micrograph some parts or some portions wherever you have this insulation material or insulation insulating inclusions you will see that very wide bright spots which may not be the actual feature of your material or part of microstructures. So this is the another type of imaging using this voltage contrast you know ACM. Now we will look at the magnetic fields. So in the case of magnetic fields imaging where you have the natural and synthetic magnetic materials fall into two distinct classes based upon the behavior of the magnetic field at the surface of the material. There are two types first we will see type one imaging so that is based on so the first type of the magnetic field imaging fall into the this category where this contrast arises because from the interaction of secondary electrons after they have excited the specimen surface with the leakage magnetic field of the magnetic specimens. So what is leakage magnetic field? So you see in a magnetic material you have all the spins oriented in same direction called the magnetic domain and then when the magnetic domain reaches the surface as free space there it is called magnetic leakage you can write in the bracket magnetic domain a magnetic domain passes through free space where it is called magnetic field leakage. So the interaction between that field with secondary electrons give the contrast. So I will just need to draw a schematic for that. So you see the schematic which I have drawn here is this is ET detector and this is the specimen we talk about and there are two distinct events happen here because of this the electric charge moving through the magnetic field experiences a vector force F is equal to minus EV cross B where V is the velocity of vector of secondary electron V is the magnetic field E is the electron charge. So what happens is and this field the job of this field is to accelerate the secondary electron it is not just the acceleration of the secondary electron but also the making directional component of the secondary electrons directionality is also controlled by this force. So in that process what happens is you have this a spin of particular orientation where the magnetic deflection of AC towards ET detector and where you have the opposite domain where the magnetic deflection of AC is away from the ET detector. So you know now very well when you do this when you are making a secondary electron yield very high and when it is moving away from the secondary electron the yield is reduced. So in that sense you have you develop a dark and a bright bands in the specimen image. So that is how the image contrast is produced we will write it down so that we would miss the point the effect of force that is F is to so whatever is shown in the schematic we have put them in the form of couple of sentences. So the effect of this force this F is to accelerate the secondary electron changing its velocity especially the directional component of the velocity. So when you have the control on direction then you have both options. If the acceleration imported by the secondary electron carries it away from ET detector the signal collected is reduced and the domain appear darker than the average. So we are talking about suppose if you have the this is the image so we are talking about this darker domain. Suppose if you suppose if this is the case then about domains of opposite magnetization the secondary electron will be deflected toward the ET detector and that domain will appear brighter than the average. So you will have a bands of a dark bright dark bright dark bright kind of an image which actually you are imaging the magnetic domains of particular orientations. So this is about type 1 of magnetic field imaging then we will look at the type 2. So this type 2 contrast or magnetic contrast arises from the effect of internal magnetic field upon the beam electrons as they initially scattered within the specimen. So it is the interaction between the internal magnetic field and the beam of electrons after they initially scattered within the specimen. I need to draw one schematic again based we will use the same equation here also F is equal to minus E cross V. So you have see what we have drawn here as a schematic is again a magnetic specimen and this is a tilt axis of this. So it is kept in this one direction and then after it tilts and it is in the other direction. So what is shown in this schematic is for certain orientation of the specimen magnetic field relative to the beam velocity the effect of magnetic field that is force with the domain of one polarity to deflect the beam of electrons towards the surface that is this case for this polarity the magnetic deflection pushes the beam of electrons towards the surface that means I get more backscattered electron signal here because of this force then that increase the backscattered electron yield whereas the domain of opposite polarity for example this is an opposite polarity the deflection tends to drive the beam electrons much more deeper inside the specimen where it reduces the BSE yield low backscattered electron yield in this case in this case it is a more backscattered electron yield. So that produces again a similar contrast of a bright and a dark bands in the image of the respective specimens. So this is a type 2 magnetic contrast which one can appreciate in the scanning electron microscopy and two more contrast mechanisms I would like to discuss namely the electron channeling and as well as electron backscattered diffraction EBSD these two I will briefly discuss in the next class and I will also show some of the laboratory demonstrations how we are going to acquire this EBSD maps using SEM. Thank you.