Thursday, May 14, 2015

Some Key Considerations for Aerospace RF/Microwave Devices

Traditional stress testing versus Knowledge Based Approach for
Aerospace RF/micro devices.
13 Key Considerations for Aerospace RF/Microwave Devices | Active components content from Microwaves & RF


Aerospace platforms put their solid-state electronic components through an extreme range of temperatures and G-forces. Combine those factors with shock, vibration, and pressure changes, and the resulting environment makes it challenging for highly sophisticated RF/microwave devices to perform reliably. Future aerospace platforms will rely even more on RF/microwave telecommunications, geolocation, imaging, and electronic-warfare (EW) technologies. Fortunately, a wealth of ruggedization standards and qualifications have been developed since radios first took to the air. Now, new methods of employing analysis tools in conjunction with accelerated tests could take RF/microwave aerospace systems to the next level of reliability.

Breakdown Constraints

RF/microwave components in the aerospace channel must be able to withstand extreme conditions, of course, but beyond simply functioning in survival mode, these components must continue to satisfy stringent performance criteria. RF/microwave components tend to be mission-critical. It is therefore necessary to design these components with breakdown constraints in mind.


Space Dangers and How API Mitigates Them

Space-Breakdown Mechanisms. 

For RF/microwave technology designers who are familiar with space applications, the enormous differences in temperature and pressure are a given.
  1. Temperature operation ranges have been designed for, simulated, and predicted with a good level of accuracy. It is possible to account for temperature extremes. 
  2. Physical stressors, such as 
    1. vibration, 
    2. shock, and 
    3. pressure differentials. 
  3. unique material, radiation, and electrical phenomena, which can lead to component degradation and, later, system failure.
    1. Outgassing. NASA has developed a database of materials and their estimated outgassing behavior through batteries of experiments.
    2. Multipaction. Hazardous-free electrons can become trapped in metallic structures under high-vacuum conditions—especially when these devices are open to radiation and electromagnetic field emission from celestial bodies. If they develop a resonant cascade with the RF signal, these free electrons are particularly troublesome for RF/microwave electronics.
    3. Corona. Free electron emissions in space can also wreak havoc when interacting with the ionized atmospheric gas trapped with RF/microwave devices. If the energy of these electrons increases because of a spacecraft’s RF/microwave electronics, the electrons can excite the gaseous molecules within the devices.
    4. Solid-State-Transistor Breakdown Mechanisms. Each solid-state failure mechanism must therefore be identified, designed against, and verified under the strictest standards. Materials steadily allow liquids and gases to permeate when there is a pressure differential.
    5. Stress Migration. Hydrostatic stress gradients can drive electromigration, thus inducing voids in integrated-circuit (IC) metallization.
    6. Electromigration. Under high-current conditions, ions within a conductor can gradually travel in response to momentum transfer by electrons and diffusing metal atoms. Like stress migration, electromigration can induce voiding-related failures.
    7. Hot Carrier Injection. Hot carrier injection (HCI) is one of the most significant limiting factors affecting the reliability of a MOSFET device.
    8. Negative-Bias Temperature Instability (NBTI). Also affecting MOSFETs, NBTI can lead to transconductance degeneration of the transistor. It is believed that nitrogen—used to retard boron penetration—may be responsible for increased NBTI in both p- and n-type MOSFETs.
    9. Time-Dependent Dielectric Breakdown (TDDB). When a steady voltage is applied across them, many performance dielectric materials experience time-related degradation. TDDB is thought to occur when conductive paths are formed through the gate-oxide substrate as electrons tunnel through the material. Higher densities of electrons tend to accelerate TDDB—an inevitability with MOSFETs. Accelerated statistical testing is used to develop reliability plots to predict TDDB failures.
    10. Radiation Effects on Electronic Components in Low-Earth Orbit - To ensure dependable and reliable electronic circuit designs, the radiation environment for Total Ionizing Dose (TID) and Single Event Effects (SEE) encountered at a specific height and orbital orientation during the space-craft mission must be determined. Such data is available from NASA documentation such as SSP 30512, "Space Station Ionizing Radiation Design Environment" and SSP 30513, "Space Station Program Natural Environment Effects Test and Analysis Techniques", applicable to the International Space Station Alpha. Goddard Space Flight Center documentation also provides this information.
      All electronic devices/components will experience two radiation-related effects in space. The first, the TID effect is time dependent, and the second, SEE, depends on many factors and is independent of time. The two effects must be addressed separately in design, and as such, this guideline will define basic ground rules for selection of rad-hard devices (radiation tolerant up to a certain specified dose) which can tolerate the effects produced by space radiation, within specified safe limits.

Military & Aerospace Standards. 

Many standards and qualifying tests have been developed to ensure that only high-reliability parts are used in real systems. These qualifications cover virtually every component of an RF/microwave system with a battery of requirements.
  1. U.S. Department of Defense has developed a wide range of “mil standards” in order to prevent the inclusion of unqualified parts. 
  2. NASA has performed many qualifying tests in order to recommend parts that are known to have performed well in prior space applications. 
  3. many part failures still occur, prompting greater investigation into how to effectively analyze and predict breakdowns.

Simulating RF Breakdown.  

In the hunt for methods to accelerate the analysis and prediction of breakdown mechanisms, designers are increasingly relying on advanced electromagnetic, mechanical, and environmental simulators. These integrated simulation environments, such as CST Studio Suite and Ansys Multiphysics, have the potential to replicate real-world conditions. In some cases, the simulations can be more illuminating than real-world tests. The simulation environments can include models of different physical interactions to reveal their impact on highly detailed 3D structures. As a result, previously masked internal failure modes may become apparent. Such modes may be hard to detect/analyze in real tests and could reduce the number of tests needed. For example, a recent application note details the use of CST Studio Suite and Spark3D to predict multipaction and coronal discharge within a 2-pole L-Band filter for space applications. A difference of roughly 25% was determined between the tested and simulated breakdown power levels. As shown, simulation tools may have reached a level of sophistication necessary to reduce the need for costly and time-consuming qualification testing.

DARPA IRIS. 

Integrity and Reliability of Integrated Circuits
| Armed with Science
Even if all the right qualification steps are performed, the industry’s reliance on offshore foundries for IC manufacturing still leaves room for counterfeiting and device performance below expectations. In light of such scenarios, DARPA has begun its Integrated and Reliability of Integrated Circuits (IRIS) initiative. Rigorous qualification processes and testing can hopefully ensure that these complex devices operate reliably. Yet such steps are leading to significant cost and time requirements. In the meantime, counterfeiting and out-of-spec devices are contributing to assembly and system failures.

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