In this fourth and final lecture, the Professor will discuss radiation impacts on the entire crystal oscillator assembly..
Up until now we have not discussed the effects of radiation on oscillator components other than the quartz crystals. Radiation effects on other electronic components is a more complicated subject, because all active devices (semiconductors, transistors, digital electronic devices, etc.) are subject to degradation due to several types of radiation. Some of the major types of radiation of interest are:
TID Total Ionizing Dose (TID) is the cumulative absorbed dose in a given material resulting from the energy of ionizing radiation at a dose rate between 50-300rad(Si)/s. For electronic components, TID is a possible long term failure mechanism. Typically, space applications have required components to be certified reliable for at least 100 kRads. For use in LEO New Space applications, 30 kRad or 50kRad compliance is commonly accepted. On the other hand, in some applications, 300kRad or even 1MRad are required. In order to be so certified, components must come from a lot from which a representative sample have survived at least twice the TID level in question, or 200 kRads in order to certify 100 kRad compliance of the lot. This testing is reported in what’s called an RLAT (Radiation Lot Acceptance Test report).
Enhanced Low Dose Rate Sensitivity (ELDRS) is similar to TID but the total radiation required, for example, 100 kRads, is administered at a much lower dose rate, usually 0.01rad(Si)/s to 0.1rad(Si)/s, thus requiring the radiation testing to take a much longer period of time up to 120 days of test. This is because, paradoxically, some components are more affected by slower rates of radiation than by faster rates. Fortunately, the primary components susceptible to ELDRS are bi-polar semiconductors, and if they are not used, it is not necessary to test ELDRS.
A very important kind of radiation is Single Event Effects (SEE). Single events are caused by any singular impact of either a particle (usually a heavy ion), and the magnitude of the event is measured in MeV (million electron volts). SEE is broken up into at least three major kinds, in increasing order of severity:
- Single Event Transients (SET)
- Single Event Upsets (SEU)
- Single Event Latchups (SEL).
A key difference between TID effects and SEE is that TID is a cumulative effect that builds up over time, from all types of ambient radiation, while SEE damage comes almost instantaneously from a highly energetic particle impacting a semiconductor device. Because the line spacing of integrated circuits keeps getting closer and closer together, an impact by an actual particle can cause a short between two lines, or other catastrophic damage, potentially resulting in a dead device.
An SET happens when the charge collected from an ionization event discharges in the form of a spurious signal traveling through the circuit. This is de facto the effect of an electrostatic discharge. It is a soft error and is reversible. Many modern components have some form of SET, and it is important to carefully characterize the SET. If it is a very short distortion of a wave cycle, perhaps of only a few nanoseconds, it may be perfectly acceptable in some applications. SET events that completely self-recover are the least disastrous of SEE events, but they still are important and will certainly sometimes disqualify a part from being used in its intended application.
The next more serious SEE is an SEU. SEUs are state changes of memory or register bits caused by a single ion interacting with the chip. They do not cause lasting damage to the device but may cause lasting problems to a system which cannot recover from such an error. These are soft errors and are reversible. In very sensitive devices, a single ion can cause a multiple-bit upset (MBU) in several adjacent memory cells. SEUs can become Single-event functional interrupts (SEFI) when they upset control circuits, such as state machines, placing the device into an undefined state, a test mode, or a halt, which would then need a reset or a power cycle to recover.
The worst SEE of all is an SEL. SEL events result in a semiconductor “latching up” or dying. It will not self-recover at all. It is never acceptable for any space application. A heavy ion or a high-energy proton passing through a semiconductor can “short” (an effect known as latch-up) at least until the device is power-cycled back on. As the effect can happen between the power source and substrate, destructively high current can be involved and can cause other problems in the circuit. It is a hard error and is irreversible. Bulk CMOS devices are most susceptible.
Prompt Dose is most likely to occur due to the detonation of a nuclear bomb that releases a large amount of radiation at a rate of between 1E8 to 1E13 rads per second. Quite frankly, in this kind of scenario, all people in the vicinity will be dead, but the electronics needs to continue to fight on. The radiation from a nuclear weapon consists primarily of photons (x-rays and gamma rays) and neutrons. The prompt dose radiation which is emitted produces a transient ionization pulse known as a dose rate pulse. For these kinds of applications, a specific testing program on prompt dose latchup and upset at both the component and the oscillator level must be implemented. Dose rate testing is usually performed on an electron linear accelerator (LINAC) or a flash x-ray machine which is emitted with a pulse width between 20ns and 100ns of radiation to determine the threshold dose rate for upset. Prompt Dose applications require the most rad hard active devises of all applications. But even these components may experience an outage. Interestingly, properly designed high reliability SC Cut crystal resonators made from swept quartz can not only survive this most extremely energetic and violent of radiation events but will even continue to mechanically vibrate while even the best electronic components experience a short outage of a few to 20 nanoseconds, and the quartz crystal can maintain both phase and frequency to pull the oscillator back into shape when the other active devices recover. This important maintenance of phase and frequency by the mechanical vibration a doubly rotated quartz crystal in an OCXO is called the flywheel effect.
There are many other kinds of radiation, and each kind of radiation discussed above has many layers of important details and nuances not discussed here. My hope is this article gives the reader a brief overview and some level of intuitive understanding of the scope of what is involved.
