Single Event Effects Testing Primer

Basics of Testing

Radiation in the space environment consists of a variety of charged particles that greatly differ in their ionizing ability, and, therefore, in the energy that can be deposited into electronic instruments. A particle's ionization loss-rate is called linear energy transfer (LET) and is measured in MeV cm^2 / mg.

The constituents in the space-radiation environment primarily responsible for Single Event Effects are energetic atoms ranging in atomic number from 1 (hydrogen) to 26 (iron) and beyond. A particle passing through matter (for example, silicon) transfers its energy to the medium primarily by ionizing atoms along its path. The amount of energy lost by the particle per unit path length is called linear energy transfer (LET) and varies directly as the square of the atomic number of the particle and inversely as its energy. Thus, the amount of energy deposited (and therefore, charge created) in a vulnerable region of a circuit component is proportional to LET x path length in the region. By convention, "path length" is measured in units of mass per unit area (mg / cm^2) and energy in MeV. Thus, LET has the units of MeV - cm^2 / mg. Note that in a vulnerable region shaped like a thin slab, particles incident at an angle theta have a path which is 1 / cos theta longer than a path at normal incidence and hence, produce more ionization charge than normally incident particles do.

The figure above illustrates a simple Single Event Effect test technique based on the concepts outlined above. The device under test is monitored while it is being irradiated with energetic particles. By counting the number of Single Event Effects and knowing how many particles passed through the part, we can calculate the likelihood of a particular particle causing a Single Event Effect. This resultant number, which is the number of upsets divided by the number of particles per cm^2 causing the upsets, is called the cross-section of the part and is measured in units of cm^2 / device.

The goal of Single Event Effects testing is to determine the cross section vs. LET curve by irradiating the device being tested with different species of particles, at various angles, to render a range of effective LETs.

The Effects of Varying LET

The following chart shows a sample cross-section curve for a microelectronic device and the ions that were used to test it.

With most devices, there is some minimum LET that is required to affect the part. We call this minimum LET the threshold LET. Above the threshold LET, there is a range where, as the LET increases, the cross-section also increases. As LET gets larger, eventually a point is reached, called the knee, where an increase in LET no longer affects upset rate. All particles with LET higher than the knee value affect the part equally.

The following graph shows the LET of the ions that are encountered in an Earth orbit.

Notice the selection of ions used to cover the range encountered in space. Fortunately, for satellites, the low LET ions are much more common than ones with high LET.