Hello and welcome to the Methods of Surface Analysis video course. And today it's the first actual method we're going to talk about. And this is secondary ion mass-spectroscopy. And the abbreviation is SIMS. The method is quite obvious. I think it's the most straightforward actually. Why don't you just take some atoms of the surface and just separate them by their mass. And there you’ll have the composition of the sample. It's quite simple. How do you get atoms out of the surface? You all know it’s the process called sputtering. If you irradiate the sample with the particles, obviously ions, heavy ions, you have this surface sputtered. Some atoms fly off the surface. And it's a tough task to analyze neutral atoms, but thankfully some of them are ionized upon ejection of the surface, these are called secondary ions. You get the atoms out the surface, you analyze them in some kind of electromagnetic analyzer and that's how we get information about the surface composition. That's secondary ion mass-spectroscopy. A brief history look through. The discovery of sputtering was made in 1853. And then in 1910 the secondary ion emission was discovered. And in 1949 first SIMS device prototype was built. And then in 1970’s various SIMS devices were made and they are still made up to this time. SIMS is in constant evolution. New variations of the method are constantly being made. Parameters are upgraded, sensitivity rises and so on. So what do you need for a basic SIMS device? What are the basic components? Well obviously you have some kind of a sample which you are going to analyze the surface. You need to sputter it. Right? So you need some kind of ion gun. Let's put it right here, the ion gun. Ok, it sputters the sample. And the sputtered particles fly in any direction, some of them are already ionized, these are secondary ions. So we need to catch them and divert them to some kind of analyzing system. Right? And that’s it. These are the basic components of the device. You need some sample stage, you need the ion gun and you need an analyzing system. These are the most basic components. Of course, there are some secondary systems like extraction systems, acceleration systems, vacuum systems obviously. It could… I think you realize that particles can fly only in vacuum chamber. Right? So what do you favor as a primary beam for this ion gun? To get high sputtering rates you need heavy particles, right? And you won't need any chemical interaction of the primary beam and the sample. So this limits us to the use of mainly noble gases. You all know them. The first noble gas is helium. But it's too light to achieve good sputtering. Neon, argon, xenon are most widely used. Mostly argon and xenon, I think. Argon is quite heavy. So good sputtering may be reached with argon. And what are the energies primary used in these ion sources? Well, you all know that sputtering depends on primary beam energy. The dependence is the same for any pair of primary particle and target particle. The dependence… it usually goes up that it has a maximum somewhere and then it decreases. And, for example, this is the dependence of sputtering yield coefficient for the pair of argon ion and iron. Just as an example. And we see that the maximum on this dependence is somewhere around about 10 keV. And that's quite typical energy for this kind of setup. So usually the energies of the primary beam are on the order of several keVs or may be somewhat higher than 10 keV, but around that. So that’s it. We are defined with our primary beam. It's a noble gas ion gun with the energies of several keVs. So when we get secondary ions, we somehow must separate them, and then we get the raw data output, which is… just see the spectrum on which on the x-axis it has the m versus z and some kind of the signal on the y-axis, just some kind of units. And that's how it could look. And one might say that just by looking at this spectrum you could tell the composition of the sample. Well that's partially right. Of course, if you see a peak of some kind of m and q, you could tell that these substances present in the sample. But not always. It could be quite tricky. And we will talk about that in the second lecture about SIMS. But anyway qualitative elemental analysis can be performed almost straight away, almost, not at every time. So then we talk about quantitative analysis and this could be even more tricky. Why? Well, because as you know sputtering coefficients, they depend on a pair of irradiating particle and substrate particle. They are different. So if you have the compound material which consists of several types of particles, they are all sputtered in different ways. Their sputtering yields differ. So the quality of… the quantity of atoms you get in the sputtered beam is not always corresponding to their composition. Moreover, we are not analyzing only sputtered particles, we are analyzing secondary ions, remember. So anything worse. A secondary ion emission… it depends on surface conditions and on its cleanness, on the state of the surface, mostly on its… whether it is oxidized or not. Secondary ion emission heavily depends on the oxide state of the surface. If the surface is oxidized secondary emission usually arises. And it may arising orders. So that's what you want to look about, if the surface is oxidized or not. So quantitative analysis is tricky. But it could be done, it could be done. If you have some reference samples, it's even better.
