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Radiation Damage Physics

3.1 General overview Radiation-induced damage on electronics takes place in a number of ways. In order to get a simple idea of what is happening we should consider that in general electronic devices are based upon charge carriers flow and its control by mean of electric potentials. Now radiation acts in the direction of changing carrier concentration or device potentials. For instance protons passing through a semiconductor can make it more conductive by simple ionization. There exist several ways of classifying radiation effects on electronics. For instance we could be interested in the final effect induced by exposition, without caring about the microscopic modifications inside the device. At this level we could distinguish between permanent and transient effects. Permanent effects are concerned with structural modifications in materials. The name refers to the fact that once we get the damage, the device will not recover. On the other side transient effects are concerned with an instantaneous failure of a device. This failure is often related to a small current discharge modifying normal operating conditions. For instance a current peak in a bistable device could switch it from one logic state to another. Similar failures are known as Single Event Latch-up (SEL). Another common kind of transient effect is the Single Event Upset (SEU), in which a more severe failure happens, leading the device outside its operating range and thus damaging it. In the following we will focus our attention on permanent effects. The common way of classifying radiation effects from a physical point of view is based on the distinction between displacement damage and ionization damage. Actually displacement is due to neutrons (heavy and neutral) and charged particles when very energetic, while ionization is mainly due to protons, electrons and photons. The main difference between neutrons and charged particles is that neutron damage takes place by mean of nuclear interactions, while ionization damage is related to atomic interactions, i.e. electromagnetic ones. So neutrons are more penetrating than charged particles, and act mainly on the bulk of the device (i.e. on Silicon). On the other side ionizing radiation often cannot overcome the Silicon Dioxide layer, so the effect is referred also as surface damage.