Radiation Hardening In Space
The Problem
Degradation
Radiation
Risk Assessment
Processing
Techniques for Space Devices
Summary
The space radiation environment poses a risk to all earth orbiting satellites and missions to other planets. Also, the need for radiation hardening of microelectronics has become even more crucial with the strategic goal of creating autonomous spacecraft which will rely on information processing on-board the vehicle
Satellites in space are depended upon by those of us on the earth, without much thought to their design. Satellites are becoming increasingly complex and versatile, and as such, contain a large amount of computing power and electronic equipment. Care must be taken to protect the equipment from temporary and permanent damage. Designing equipment for space travel requires that the designers know exactly the intended usage of the electronics, as radiation effects vary widely depending on where the satellite is used, its orbit, and which direction it is facing.
Much of the radiation in space near Earth comes from the sun, as fusion in the sun creates a constant stream of particles flowing through space. Around planets with magnetic fields, these particles can be trapped or deflected away from the planet. In space away from planets, the particles themselves constitute the entirety of the threat to the equipment.
Most satellites are deployed in orbit around the earth, and as such, the different types and levels of radiation around the earth must be understood. The lowest radiation doses occur in Low Earth Orbit, less than 500 km from the surface. Only a few heavy ions penetrate the magnetic fields to this level. In the polar regions, there are slightly increased levels due to the Van Allen belts which will allow more heavy ions to penetrate. At geosynchronous orbit, doses are somewhat higher, but still low compared to interplanetary space due to geomagnetic shielding. However, more heavy ions can penetrate to this level than occur in Low Earth Orbit.
Figure 5. A Linear Energy Transfer Spectrum for Spacecraft near Earth
As ions penetrate the skin of the space craft, they can emit X-rays. These X-rays will enter the semiconductor, and cause the silicon and silicon dioxide layers to ionize. This can be temporary, such as corrupting the contents of a memory cell, or permanent, when the ionization triggers latchup in the device. The charge injected into the device will collect at a circuit node and cause data in the device to change.
If radiation is exposed to a device continuously, over time they may also gradually become degraded. This amount of degradation depends on the total dose and on the dose rate of irradiation. Depending on the device’s susceptibility to radiation and the type of environment the electronic device will be exposed to, devices may only last a few days or many years.
Electrons and X-rays produce electron-hole pairs which are normally collected at the power supply nodes. However, the ionization can cause eventual shifts in MOS transistor thresholds, which causes changes in the characteristics of the device. When there is no bias on the transistor, almost all of the electron-hole pairs can immediately recombine. When there is a positive bias, however, the electrons are swept away while the holes migrate slowly to the negative channel, and become trapped. This causes N-channel enhancement transistors to become easier to turn on, while P-channel transistors are harder to turn on.
Figure 6. X-Rays and other high energy radiation create electron-hole pairs in the semiconductor, which are then swept away.
Radiation risk assessments must be done before objects are launched into space. "A radiation risk assessment for any electronic device includes the determination of total does damage and SEE susceptibility of the device caused by the projected radiation environment of the spacecraft." The total dose damage is caused mostly by high energy protons and electrons and by secondary radiation, such as Bremsstrahlung. Heavy ions are the main contribution to SEE. The South Atlantic Anomaly, a region of intense particle radiation in the South Atlantic atmosphere, can contribute in a major way to SEE. This is due to the very high influences of high-energy protons seen during heavy solar flares during the excursion of the satellite.
In order to assure system hardness for space devices, each team working on a project should perform an application analysis for all electronic parts which may be susceptible to SEE. The analysis should consist of knowing the number of SEE occurrences anticipated during the mission-lifetime, along with the worst-case SEE rates resulting from transient peaks in the radiation environment, such as high proton fluences from solar flares and the SAA.
In looking into the suitability of electronic devices in their intended application, it is very important to determine the radiation environment to which these parts will be subjected. The radiation environment varies significantly with the orbital parameters and solar activity level. One factor that effects the radiation environment experienced by a device inside the spacecraft is the shielding provided by the spacecraft walls and other materials interposed between the device and the outside environment. Therefore preliminary checks should be made (i.e. evaluating the radiation environment that the device will be exposed to) before assessing the total dose and SEE sensitivity of the devices selected for system design. Total dose testing is normally performed by exposing devices to gamma rays from a Cobalt-60 source. "For space applications, the recommended dose rates should be kept as low as possible, preferably 0.01 - 2 rads(Si)/sec. The dose rates used depend on the predicted device radiation sensitivity and the projected mission total dose." Some devices are more radiation sensitive than others and most device types and technologies show higher total does tolerance at low dose rates.
Figure 7. I-V Curves for MOS semiconductors subjected to a different total radiation doses.
Processing Techniques for Space Devices
The next step is to find out how to protect a device from harmful radiation. To do this, the active layer of the device is protected by an insulating layer of silicon dioxide. This process is known as Silicon-on Insulator isolation. This method of radiation hardening completely protects the devices in space from latchup, and produces very few data errors due to radiation (less than 7 x 10-19 errors / bit day).
[Process Flowchart for Space System Design]
The advances in radiation hardening are allowing satellite designers greater flexibility than ever before. In the past, it has been difficult to protect equipment with read / write RAM, and thus designers were locked into one program for their equipment early in the design stage. Now that SRAM can be radiation hardened, it allows designers to use field programmable gate arrays, which can be reprogrammed during design and in some cases even remotely in space. This has led to a simplification of circuitry and a reduction in costs, in addition to much enhanced flexibility in space-based usage.
References: See Bibliography
references 6 and 7.
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