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96 MEMS and Microstructures in Aerospace Applications

In contrast, ions passing through insulators and semiconductors are capable of
generating sufficient charge to cause noticeable radiation effects in devices such
as transistors and diodes.
Charge generated in insulators may become trapped at sites where they can reside
foralongtime.Theirpresencedistortsthelocalelectricfieldsandcanaffectthedensity
of carriers in the semiconductor near the interface. For instance, positive charge
trapped in the oxides used in the construction of a transistor will attract electrons in
the semiconductor to the interface. The increased concentration of electrons at the
field-oxide or semiconductor interface may lead to increased leakage currents in the
transistor, whereas positive charge trapped in the transistor’s gate oxide may prevent
the transistor from switching on and off, thereby causing functional failure.
The amount of trapped charge is a function of the TID, which increases with
exposure. Therefore, in space where devices are continuously exposed to radiation,
there is a steady increase in the amount of trapped charge that is first observed as an
increase in the leakage current and eventually a failure to operate.
TID effects in MEMS can originate in either the electronic or mechanical parts
of the device, or both. Whatever the origin, the essential requirement is that charge
be trapped in an insulator and that the trapped charge distort the existing electric
field to such an extent that the operation of the device is affected.
Electrons and holes generated by energetic ions passing near or through a
semiconductor metallurgical (n/p) junction will be separated by the associated
electric field. Charge separation disturbs the electrical potential across the junction,
and that voltage disturbance may propagate through the circuit to other nodes.
When the voltage disturbance occurs in a latch or a memory, the information stored
there may be nondestructively altered. The change in the state of the latch is known
as a single-event upset (SEU). It is called a SEU because a single particle interact-
ing with the material liberates sufficient charge to cause the effect. Of the many
different kinds of single event effects, those that occur when charge is deposited in
the semiconductor part of a device include single-event upset, single-event latchup,
single-event snapback, single-event transient, and single-event burnout. In some
cases, charge deposited in the gate oxide of a power MOSFET will lead to single-
event burnout. These types of effects are expected to occur in the electronic circuits
of MEMS but are unlikely to occur in the mechanical parts.

5.2.2 SPACE RADIATION INTERACTION WITH MATERIALS AND DEVICES
(DISPLACEMENT DAMAGE)

Particle radiation may also interact with the atomic nuclei of the materials through
which they pass. Those interactions consist of either elastic or inelastic nuclear
scattering events. In either case, the atomic nuclei of the constituent atoms recoil
and move away from their normal lattice sites, thereby disrupting the regular crystal
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lattice, and producing vacancies and interstitials. Vacancies in semiconductors are
usually electrically active whereas interstitials are not. Electrically active sites act
as either short-lived traps or recombination centers for free carriers. Such traps
reduce minority carrier lifetimes and doping levels, causing certain devices, such as

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