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Impact of Space Environmental Factors on Microtechnologies 77

that follow are: radiation, zero gravity, zero pressure, plasma, and atomic oxygen
(AO), along with long-life requirements. These influences are reviewed in relation
to their effects at the system and individual part levels.
A more in-depth discussion of the radiation environment is found in the chapter
on space environment; however, some discussion of device level concerns is
contained herein and would be applicable to device designer’s incorporation of
MOS components in their MEMS designs.
Commercial MEMS are designed to operate in our low radiation biosphere and
the CMOS portions of the electronics can tolerate total radiation doses of up to 1 to
10 kRads. Terrestrial radiation levels are only about 0.3 rad/year so radiation
damage is not normally an issue if you stay within the biosphere. 10
There are primarily two types of radiation environments in which a system may
be operated: a natural environment and a threat environment. Earth-orbiting satel-
lites and missions to other planets operate in a natural environment. The threat
environment is associated with nuclear explosions; this neutron radiation normally
is a concern of non-NASA military missions. Irradiating particles in the natural
environment consist primarily of high-energy electrons, protons, alpha particles,
and heavy ions (cosmic rays). Each particle contributes to the total radiation fluence
impinging on a spacecraft. The radiation effects of charged particles in the space
environment cause ionization. Energy deposited in a material by ionizing radiation
is expressed in ‘‘rads’’ (radiation absorbed dose), with 1 rad equal to 100 ergs/g of
the material specified. The energy loss per unit mass differs from one material to
another. Two types of radiation damage can be induced by charged particle ioniza-
tion in the natural space environment: total dose effects and single event phenom-
ena. In semiconductor devices, total dose effects can be time-dependent threshold
voltage shifts, adversely affecting current gain, increasing leakage current, and even
causing a loss of part functionality. A single-event phenomenon (SEP), which is
caused by a single high-energy ion passing through the part, can result in either soft
or hard errors. Soft errors (also referred to as single event upsets [SEUs]) occur
when a single high-energy ion or high-energy proton causes a change in logic state
in a flip-flop, register or memory cell of a microcircuit. Also, in low-power high-
density parts with small feature sizes, a single heavy ion may cause multiple soft
errors in adjacent nodes. Soft errors may not cause permanent damage. A hard error
is more permanent. An example of hard error is when a single high-energy ion
causes the four-layer parasitic silicon controlled rectifier (SCR) within a CMOS
part to latch-up, drawing excessive current and causing loss of control and func-
tionality. The part may burnout if the current is not limited. Single event latch-up
(SEL) in CMOS microcircuits, single-event snapback (SES) in NMOS parts and
single-event burnout (SEB) in power transistors are examples of hard errors that can
lead to catastrophic art failures. Major causes of SED and latch-up are heavy ions.
To valuate SED and latch-up susceptibility, the heavy-ion fluence is translated into
linear energy transfer (LET) spectra. While the total dose radiation on a part may
vary considerably with the amount of shielding between the part and the outside
environment, the LET spectra (and hence the SED susceptibility) do not change
significantly with shielding. SEU and latch-up problems are most critical for

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