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Hi, I'm Rob Baumann, the Chief Technologist for High Reliability. If you want to know more about TI's space products after this video, please visit ti.com/space. Thanks for watching.

And in this lecture, we'll be talking about Total Ionizing Dose. So we focus on the physical manifestation of radiation. That is, how does radiation interact with matter, and what does it do to electronic devices?

So one of the primary ways in which radiation loses energy in matter is by the creation of charge. Now, this excess charge, if nothing is done, will recombine or move away, depending on electric fields in the device. So in general, the charge generation in semiconductors is transient. In other words, the radiation passes through the semiconductor, excess charge is generated, and then a lot of this charge, as I said, recombines, and some of it may be collected by junctions and nodes that are at different potentials.

Another way in which radiation can lose energy in material is to actually create structural damage. This is often referred to as NIEL, Non-Ionizing Energy Loss, because it is not a direct form of charge creation, but, instead, is a physical damage to the structure of the material by the radiation.

So for example, if we have a proton, a high energy proton incident on a silicon device, that proton can induce structural damage. That is, it can knock out silicon atoms or dopant atoms from their normal position, and, thereby, create electronic effects through structural damage, even though no charge is generated by that initial reaction. So basically, we have transient charge generation and structural damage.

There is also a third way in which radiation can basically lose energy in matter, and that is nuclear reactions. So this is, for example, a nuclear particle, like a proton or neutron, incident on a silicon wafer may actually react with the silicon nuclei and form nuclear reactions. And of course, these are like small explosions of the nuclei. And so you get secondary products.

And these secondary products are pieces of the nucleus. So they're highly-ionizing, so these secondary products can generate additional transient charge generation, as well as additional structural damage. So these are kind of the three mechanisms that are going on when radiation interacts with matter.

Now, we can divide the effects that we see in devices into two categories. The first is the so-called dose effects. And there are dose effects related to charge, which is what we're talking about in this section. And there are also dose effects related to structural damage, which we'll deal with in a separate section.

But on top of dose effects, which are basically chronic exposure to radiation-- So exposure to radiation over time, slowly shifts device parameters until the device doesn't work anymore. But the key point is it's a lot of radiation events integrated over a long period of time, typically. So if you think about a satellite in orbit, it's going around the Earth. Every hour it goes around the Earth. And it may be in orbit for 10 years or 20 years. It's being exposed to radiation all during its lifetime. And dose effects will start to take over, over time.

In sharp contrast, the other key mechanism is the single event effects. And again, it's not the topic of this talk. But I just mentioned it for completeness. This is where a single nuclear particle or radiation event causes a measurable upset or an actual circuit failure. So again, dose effects are chronic exposure over time. And single event effects are like lightning strikes. Basically, the event occurs, and then something bad happens to your electronics.

So what is Total Ionizing Dose, with that introduction? We think about it as an accumulated exposure to many radiation events. So it's a chronic exposure. Again, think of a satellite in orbit going around and around through the radiation belts. And it's accumulating the radiation dose over time. And the primary mechanism is that this exposure to radiation creates trapped charge and interface states in the gate oxides and isolation oxides. So basically, insulators, because of their electrical properties, basically trap this charge.

So I talked about the transient charge generation in semiconductors. In insulators, this transient charge can actually be permanent because it gets trapped where it can't move in the insulator. And finally, this trapped charges accumulating over time causes quasi-permanent device shifts that lead to functional failures.

I have a little cartoon here that can maybe help you think about what total dose is like. We've got these two little pigs, one that used suntan lotion and one that didn't. And one turned to bacon. Very much like a sunburn, total ionizing dose is something that, over time, you're accumulating a radiation exposure. Your oxides are trapping charge. And over time, those oxides and that trapped charge will build up and cause shifts in your devices.

So let's look in a little bit more detail of what's happening electrically to our semiconductor materials. What I show here as a [INAUDIBLE] diagram. And it's a way of showing energy in the vertical axis. And on the horizontal axis, we have actual physical distance. So this is actually a vertical cut through a transistor structure, a field-effect transistor. And I show the gate here. Then, of course, this big block of material is the gate oxide. And then on the right-hand side, I show the silicon substrate where all the magic happens, the device physics.

But the actual total dose effect takes place completely in the oxide. And effectively, what happens is, incoming radiation ionizes electrons from the valence band-- so down at the bottom here-- leaving a hole, and the electron goes up here. And this can happen throughout the bulk of the oxide. In this diagram, it shows it right at the edge. But imagine radiation streaming through this oxide, creating charge everywhere within this oxide block.

Now, you'll note there's a slant here. This is because we've applied a positive gate voltage. And we're pulling down the gate. And again, this is electrical, or electronic potential energy. So up on this diagram means the electron has more potential energy, down means that it has lost its potential energy. So again, electrons will travel downhill, attracted to the positive gate voltage. They'll be losing potential energy. So they've absorbed energy from the radiation. Now, they're moving fast. They get pulled down by the positive. They get attracted to the positive gate. And they lose that energy going into the gate.

Now, the key feature of oxides, and the reason why oxides are problematic from a total dose point of view, is that the mobility or how easy it is for the particles to move, the mobility of electrons is very high relative that of holes. So even though you've got a positive and a negative, they are not created equal. The electrons can very readily be removed from the conduction band, leaving the holes to move very slowly by a hopping mechanism.

And you'll note that they float or move in a hopping mechanism, as per these green arrows, towards the interface. Again, they're being repelled by the positive gate voltage. And they're moving by hopping in localized states of the gate oxide.

And the key feature here is that they move slowly. And when they get close enough to the interface, there's some deep hole traps. So these are actually locations that want to capture a hole. And when the holes get in close proximity here, they can basically populate these traps. And unfortunately for the holes and for us, these holes get quasi-permanently trapped in these locations. So you're left with a positive charge very close to your silicon.

And obviously, since this is a field-effect device in this case, that positive charge will affect the localized fields. And it affects the device characteristics, as we'll see in other lectures. But the key point is holes move up towards the interface and get trapped in the oxide deep hole traps. But what's also happening, and I note in the purple here, is proton release.

Now, every semiconductor process has hydrogen in it. Even if you do a good job of trying to keep the hydrogen out, you'll get some hydrogen in yours, particularly in your gate oxides. Now, this hydrogen occupies traps or localized states. And it's pretty much trapped in the oxide. However, when the hole displaces those localized states-- in other words, it creates a warp in the local field-- that hydrogen is free to escape. So not only do you get holes moving towards the interface, but because of the release, the proton release and then proton transport-- again, remember, protons are ionized hydrogen. So they're positive. They're going to be pushed in the same direction as the holes.

Now, the protons are slightly different than the holes. They actually go to the interface. And they actually wreak havoc in the interface states, creating defects right at the surface where the silicon meets with the oxide. And this is very problematic for bipolar devices. We'll see this in a future lecture. For MOSFETs, it's less important. Typically, the quality of the oxide is such that it doesn't do too much damage. But you need to be aware of this.

So these are the two mechanisms. So radiation is absorbed. It creates electron hole pairs. Electrons are more mobile than the holes. They get very quickly removed. So that leaves us with a positive charge, which then moves towards the interface. And it's this positive charge, as well as the interface state creation by protons, that causes all the problems related to total dose.

So what kinds of radiation can cause total dose? Basically, as we can see from this chart, just about any radiation will do. And what I show in this graph is electric field on the horizontal axis and fractional yield on the vertical axis. This is kind of a measure of how effective different radiations are at creating total dose problems-- in other words, trap charge in the oxide. And you'll note that basically highly energetic electrons and cobalt 60, this is gamma rays, are very efficient. What this means is, independent of electric field, you get close to one 100% fractional yield. In other words, of all the charge the generated during the irradiation, most of it stays in the oxide.

Now, you'll note, for different particles, particularly as you go to heavier ions like alpha particles versus protons, this efficiency goes way down. There's a lot of physical reasons for this. We don't have time to go into it. But you just need to be aware that different particles will produce different total dose effects. And you have to account for these when you're trying to do a measurement or understand data. Fortunately, the industry has pretty much focused on cobalt 60 exposures for total dose. And everything is calibrated in terms of the total dose with cobalt 60, sometimes X-rays. But generally, these particles are not used for total dose testing because of their low efficiency.

Now, one thing that you'll note is the reason electric field is important, you recall that I told you that electrons are more mobile than holes. Well, if you don't put an electric field on your gate oxide, in other words, if that [? band ?] diagram were flat, the electrons would remain in the oxide and could actually recombine with the holes. So recombination will reduce your fractional yield. So this is why all of these curves have that slope to the right. In other words, as you increase the electrical field, you basically can pull out more electrons, you reduce recombination, and your fractional yield of holes, basically, hole charge remains high or gets higher.

And just to calibrate this, again, electric field is in megavolts per centimeter. And this is showing technology nodes from two microns down to 90 nanometers. And you can see that the electric field in our gate oxides has actually been increasing pretty much with each technology node. And we're clearly in modern technologies. We're in the 5 to 6 MeV per centimeter-- or sorry, megavolts per centimeter. So fractional yield will be very high. You just need to be aware of that.

And again the fraction of holes remaining at 5 megavolts per centimeter with gamma rays, the so-called cobalt 60, 95% is remaining. With X-rays, we're at about 80%. If you look at protons, you're at 35%. And alpha particles is only about 12% of the charge that was actually generated is finally trapped in the oxide.

So that gives you an idea of what the total dose effect is. Well, what about the space environment? What should we be worried about in the space environment?

This is basically a diagram from NASA showing the Van Allen radiation belts. These are belts that form because of the earth's magnetic field. There's a trapping. So the magnetic field traps the charged particles in these sort of donut-shaped belts. And the outer belt is electrons. The inner belt is protons. Very energetic, 10 to 200 MeV. In the inner belt, they're less than energetic. That's why they're closer. But the key point is when we send a satellite into orbit, very frequently, it's low Earth orbit. So most of the communication satellites are in this green orbit. And there's a range of values. But it's somewhere inside the belts. The International Space Station, as well, is in so-called LEO, or low Earth orbit.

We do have some satellite applications out in geosynchronous orbit. These are typically GPS, Global Positioning Satellites, or some sort where basically, the satellite has to remain in a fixed position over a position on Earth, a location on Earth. So basically, the satellite tracks with the Earth exactly and is always in the same location above the Earth.

Now, the key point of these different satellite orbits is that if you travel within a belt, you will be exposed to a lot of total ionizing dose effects. If you travel inside the belts, if you stay inside the donut hole, if you will, you're actually protected. These belts give you an electromagnetic shield from external radiation sources. So if you think about it, a low Earth orbit will be better shielded than a geosynchronous orbit.

So again, I'll probably go into that in a lot more detail about environment in different orbits in a later lecture. But you just need to be aware that depending on the satellite mission, it may have a different orbit. And the radiation dose that it receives, and hence the total ionizing dose effect that you will see, is very dependent on orbit.

And this is kind of a summary. Most satellites are shielded in some way. In other words, the electronics are not stuck on the outside of the satellite. But they're typically in a metal box for electrical shielding, as well as radiation shielding. And that metal box is typically made of aluminum, and very typically somewhere between the 200 to 300 mills of aluminum is used.

So what this tells you, this chart, Bremsstrahlung Radiation, this is X-ray radiation from slowing particles. We have electrons here, again, from the electron belt. And then we have protons. What this is telling you is in a shielded system, typically, protons will dominate. They're the dominant form of dose that you'll see. Only in unshielded systems do electrons really become the dominant feature. And since most satellite electronics are shielded to some form, you're somewhere out here on the curve, protons are our primary concern from a total dose perspective. So as I said, in applications with any kind of shielding, total dose is dominated by the protons, at least for satellite orbits.

So to sum up, total ionizing dose affects electronic devices in the following ways. Radiation induces excess electron hole pairs in the insulators. The electrons are more mobile. And they exit, leaving hole charges. The hole charge is trapped in the oxide in deep level traps, and this includes gate oxide, as well as the isolation, and then moves very slowly by hopping from trap to trap. Hopping holes create additional damage and release trapped protons, or hydrogen, ionized hydrogen, that is also mobile.

Trapped holes, basically they get trapped in deep traps, as we discussed, very close to the interface, alter the surface carrier concentrations in silicon. And holes and protons that move to the interface create additional defects at the interface. And since most of our devices are surface devices, certainly MOSFETs, field-effect devices are very sensitive to the interface.

This clearly has very big effects in devices, as we'll see in a later lecture. This trapped charge and interface defect change the local carrier population and reduce carrier lifetime near the surface of the SiO2 silicon interface. And again, this effect has a direct impact on bipolar devices, as well as field-effect devices. Thank you.