Space Radiation Effects on Electronics-TID Effects
In our previous post we described where space radiation comes from. Now that we understand that space contains electrons, protons and heavy ions which are produced by energetic solar events, galactic cosmic rays and trapped particles, we will dive into how space radiation affects spacecraft electronics. This will be divided into several parts where we will explain thoroughly the types of radiation effects and their implications to the electronics.
Why is radiation testing important?
As we mentioned in our previous post, there are three sources of space radiation that an electronic device has to deal with when placed in space. Once a satellite is launched into the space radiation environment, functional drifts and performance deterioration are noticed in the subsystems. Repairing damages in space is very difficult and replacing devices is not a cheap option. For this reason, it is crucial to conduct radiation testing on Earth prior to launching any device into space. By doing so, we are able to predict the components’ responses to ionising radiation during flight and set-up an appropriate mitigation strategy.
They are multiple ways that radiation can affect electronics function. Those effects can be regrouped into two categories: cumulative effects and single event effects (SEE).
Cumulative effects take place through continuous radiation exposure that is happening during the active lifetime of the electronics, causing permanent damages and thus, making them out of specification. In cases like this, the effect is permanent and a power reset does not solve the problem.
Cumulative effects can be subcategorized into Total Ionizing Dose (TID) and Displacement Damage Dose (DDD).
Total Ionizing Dose
The main issue with TID is the gradual performance deterioration of the circuits that potentially leads to system failure. This degradation in integrated circuits is caused by all ionizing radiation sources such as trapped electrons and protons, gamma and X-rays, solar flares, cosmic particles. Total Ionising Dose (TID) is the measure of the total energy absorbed by matter. The most common unit used is rad (radiation absorbed dose) or the International System Unit, gray (GY), where 1 Gy= 100 rad= 1 J/kg.
When it comes to testing a device for TID, it needs to be highlighted that an accelerated-life test approach is usually adopted. Since a mission is multiple years long, it is not viable to perform an equally long earth-based radiation exposure.
In order to reach reasonable running times of the experiments, accelerated testing is done at substantially higher dose rates than in space. This is called High dose rate (HDR) TID testing and is the most common TID test performed on electronics intended for space use. Typical HDR dose rates vary between 50 rad/s and 300 rad/s.
As an example, for a hypothetical polar LEO mission such as PROBA-II with 6mm of Al shielding, an average TID of 10 krad (SiO2) per decade is expected. In an HDR TID testing scenario, this dose can be reached within 3 minutes to 28 hours of irradiation. The device can reach 100krad in less than six minutes, while in space it can take up to 10 years to reach 100 krad.
Nevertheless, the assumption that the End-of-Life state of the component depends solely on the totally received dose has been proved wrong for a specific family of devices.
More specifically, it is known that for bipolar devices the lower the dose rate the more detrimental the effects. This phenomenon is coined Enhanced Low Dose Rate Sensitivity (ELDRS) and is a serious risk to consider when designing power distribution systems and their shielding. Usually, when talking about low dose rate (LDR), the radiation rate is considered to be less than or equal to 10mrad/s. Compared to HDR, LDR testing at 10krad takes up to 28 hours.
To illustrate the difference between different levels of TID immunity, commercial unhardened devices (COTS) are typically rated below 10 krad (SiO2), radiation-tolerant devices (Rad-Tol) below 100 krad (SiO2) whereas radiation-hardened devices (Rad-Hard) above 1 Mrad (SiO2).
But what are the TID effects in devices?
In microelectronics, ionising dose defects relate to the accumulation of trapped charge in the field oxides of the circuit. As ionization in silicon dioxide occurs the electron-hole pairs formed in this material do not entirely recombine but move under the drift of the present electric field. Since electrons have a much higher mobility, they can easily exit the oxide, leaving trapped holes in defect centers in the oxide volume.
Moreover, this process can activate defects at the silicon-oxide interface. The main reason for device degradation caused by TID is because of the charge buildup and the creation of defects.
Here are a few examples of TID Effects on electronic devices:
- Threshold Voltage Shifts
The trapping of holes in the oxide is what causes charge buildup and it occurs in the bulk of the oxide. These charges will screen or increase the gate oxide of MOS transistors electric fields, leading to a change in its I-V characteristic. The most prominent change is the shift of the power-ON (threshold) voltage which is negative for NMOS and positive for PMOS. As a result, a device might become unresponsive to some commands as it might be “stuck” on a specific state.
- Increased Leakage Current
In the Field-Isolation oxides such as Shallow Trench Isolation oxide, charges might draw an image charge in the semiconductor which can reverse the interface and free leakage paths. This can only occur in NMOS transistors. These parasitic leakage currents have degraded timings and increased power consumption.
- Amplifier Gain Degradation
TID-induced damage in bipolar transistors usually manifests as a reduction in bipolar gain (hFE) with increasing total dose exposure. To compensate for it, more power needs to be supplied to the device.
- Dark Signal in Camera Sensors
As a direct effect of the charging of gate oxides, the electrostatic potential in pixels shifts and gets “un-pinned” with the result of spurious thermally generated charges not being suppressed anymore. This is manifested as an increased noise background and is observed in both CCD and CMOS technologies. As a consequence, the dynamical range of the imager is compromised. This is a major problem with Star Trackers that could fail to locate reference stars.
In our next post, we will explore the Displacement Damage Dose (DDD) and the effects on the electronics. If you want to learn more about TID, you can check out our sources below and don’t forget to subscribe to our newsletter!