Optoelectronic components must be protected from the radiation they are exposed to in military, space and nuclear environments to prevent malfunction and damage. Radiation hardening electronics makes them resistant to this damage by letting them retain characteristics, and keeps imaging and electrical performance consistent with pre-radiation values. This is especially critical for components used in applications like a satellite or a nuclear reactor that cannot be accessed and must work properly for many years.
Radiation Effects on Optoelectronics
Radiation has the potential to pose many problems for electronics including de grading performance. Ionizing sources, gamma rays, protons and heavy ions create a path when hitting a semiconductor that charge up oxides and create traps or interface states, resulting in a number of changes to the sensor. Ionizing radiation is known to increase dark currents and noise and, because of this, dynamic range is lost. The trapped charges in oxides can cause a drop in threshold voltage, which will then minimize amplifier liearity and charge transfer efficiency. These are all important sensor specifications that customers want to maintain, even after radiation exposure.
Other radiation effects include displacement from a non-ionizing dose of neutrons or protons that can result in lasting damage to the lattice structure of the sensor. Plus, as we move from analog to digital components like CMOS, there is an additional set of effects that we have to be concerned about. In the digital realm, live events are happening as the camera is operating. These single-event effects (SEEs) occur when charged particles upset stored digital information and cause glitches in the image or even catastrophic effects like a latchup. A latchup occurs when an energetic particle creates an ionizing impact or short that results in a transistor being “stuck” in a high current and, possibly, burning out that location on the sensor. A short is self-propagating and can only be removed by powering the device down.
Different Radiation Hardening Methods
Radiation-hardening an electronic component touches upon all facets of its design, process and testing. Well known, published methods of radiation hardening by design do exist. One such method looks to reduce the impact of ionizing damage by isolating sensitive regions of pixels and amplifiers away from charge traps or interface states. Another example of rad-hard by design is to develop digital components with redundant circuity to mitigate single-event effects (SEEs) like upsets in digital information. Unlike rad-hardening by design methods, which are well documented, radiation- hardening by process employs proprietary methods that are closely guarded by the device manufacturers.
Gamma ray testing has been the standard space qualification for many years as ionizing radiation has been the dominant factor in developing radiation-tolerant sensors. With CMOS radiation sensors continuing to evolve, this “traditional” testing method, often based on MIL standards, is being supplemented by more complicated and involved testing like proton radiation and heavy ions.
Factors to Consider When Radiation-Testing
The biggest challenge of radiation testing optoelectronic products is to replicate a lifetime of radiation in a short test. You have to determine the dose, and the types of radiation and their corresponding energy ranges – protons, for example, have a broad range from 5 MeV to 100s of MeV. In addition, radiation effects vary according to extremes in temperature, like those experienced in a space environment. These variances are significant since occurrences like latchups are more sensitive to higher temperatures. Also, if you look at displacement damage from a proton or a neutron, which impacts the actual lattice structure, an annealing effect can happen at higher temperatures that doesn’t occur at lower temperatures.
When testing CMOS sensors, not only should you be concerned about the total lifetime effects of radiation, you must also be aware of short term effects. Because your test suite must be sensitive to these single effects, you’ll have to perform live imaging and monitoring of the sensor while you expose. In addition to an extreme effect like a latchup, you are also looking to predict the occurrence of “softer” errors like single-event upsets (SEUs), which happen when a bit is flipped by a proton or heavy ion interaction. A SEU also has the potential to seriously impact a digital device, resulting in a timing error that could cause the device to enter into an invalid operating mode or, more critically, a lock-up.