Radiation Hardening of Electronics and Instruments for Fusion 1
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Radiation Hardening of Electronics and Instruments for Fusion K. Vetter Oak Ridge National Laboratory, Oak Ridge, TN 37831 Email: [email protected] 1. Technology to be assessed This white paper is submitted to the U.S. FESAC Transformative Enabling Capabilities (TEC) subcommittee seeking to raise awareness of the specific R&D needs to ensure robust reliable operation of electronics based instrumentation, controls and diagnostics in harsh radiation environmental conditions. The D-T environment in fusion reactors such as ITER poses significant challenges in deploying robust systems in high radiation environments that may also be coupled with high magnetic fields. Embedded electronics are pervasive in a multitude of modern systems and components that span instrumentation sensors and actuators, detectors, analog and digital signal processing elements, and power devices. Many of these elements may be required for critical safety, diagnostics, power systems, and remote handling. The associated transmission mediums for signals and power distribution must also be considered. In addition to the various domains embedded electronics are associated with, there are many semiconductor device technologies that exhibit different dependencies on nuclear radiation. For instance, newer deep submicron (<100nm) CMOS devices have reduced sensitivity to long-term cumulative radiation as a result of reduced gate oxide thickness, but are more sensitive to instantaneous bursts of ionizing radiation. Conversely, bipolar devices are typically less sensitive to short bursts of ionizing radiation, but are sensitive to cumulative radiation effects that typically reduce device gain and increase leakage current. However, bipolar devices may be prone to Enhanced Low Dose Rate Sensitivity (ELDRS) which is manifested as device degradation with decreasing dose rate. It is essential to employ significant computational nuclear radiation transport analysis tools such as Monte Carlo N-Particle (MCNP) with sophisticated hybrid variance reduction techniques to quantify the operational radiation environment (e.g. neutron flux and fluence, gamma dose) with deterministic confidence intervals. The ability to quantify the radiation environment will allow for setting realistic radiation hardness design requirements and margins. Nuclear radiation analysis using MCNP and ADVANTG for US ITER Instrumentation & Control (I&C) has implemented tallied responses on a uniform mesh size of 5 cm over the region of interest relating to electronic devices, with 100 cm uniform mesh grids for global analysis [1]. Given the enormous scale of ITER and fusion facilities beyond ITER, a strategic development employing large-scale MCNP analysis coupled with local high-resolution capabilities, targeted at exascale computing, would likely realize a transformative enabling capability for the U.S. to develop and deploy electronic based systems in future fusion energy facilities. 2. Application of the Technology for Tokamaks and Stellerators Fusion reactors, such as ITER, require radiation hardened electronics for a very wide range of applications within close proximity to the tokamak core (e.g. port-cells and gallery) to operate in a reliable robust manner. Future fusion facilities may impose requirements for 1 diagnostics to operate to 10#$ neutrons/cm0. Applications of electronics in a tokamak fusion reactor include: • Instruments with embedded electronics (e.g. flow, pressure, temperature) • Vacuum gauges with embedded electronics (e.g. Peizo, Pirani, Penning) • Diagnostics (sensors, signal processing, opto-electronics) • Signal conditioning (e.g. amplifiers, optical transceivers) • Remote handling Reliability of electronics is a very important factor as space allocations in port-cells and gallery locations are very limited. Access to electronic devices in these areas is also very limited due to material activation and significant time and effort to access these areas. It is likely that a planned service access may be required to wait 3 days or more to ensure a radworker does not exceed defined occupational dose limits. It is important to note that a number of standard modern instruments qualified for nuclear fission reactors with embedded electronics are only qualified for gamma radiation and not neutrons. The fusion reactor environment introduces a mixed radiation spectrum of gamma and neutrons that requires development of embedded radiation hard electronic solutions that are hardened and qualified for both neutron and gamma irradiation. 3. Critical Variables Radiation effects induced in electronics fall into the two broad categories of long-term effects and transient effects, sometimes called single-particle effects. Transient effects are called “single event effects” (SEE) and can be further decomposed into soft or hard errors. A single event upset (SEU) results in a stored bit changing its value. An SEU is caused by an incident energetic particle liberating charge in the semiconductor which in turn migrates to the reversed biased junction inducing a current pulse. A transition of the bit will occur if the injected charge exceeds the critical charge to of the device. The migration of semiconductor devices to smaller geometry results in a reduced critical charge, and therefore increased sensitivity to SEUs. As an example, the stored charge for a 28nm CMOS device is approximately 101#2 coulumbs [2]. Since an atmospheric neutron can deposit approximately 150fC, SEU events are becoming possible for atmospheric neutrons at sea level [3]. Transient events are typically considered relevant to only digital devices. Radiation induced noise may be considered a transient event that can be manifested in analog devices. SEEs are prompt events with a probability of occurring (cross-section) for each particle interaction. Hard SEEs may result in device failure. CMOS devices contain parasitic NPN and PNP structures that are normally off, but can be transitioned to an on state by a pulse of ionizing radiation, forcing excess minority carriers into the base region. The result is a low-impedance condition between the power supply and ground. If left in this state the device will eventually fail as a result of localized heating [4]. Long-term effects are considered to be cumulative in that device damage increases over time. Long-term effects are described in terms of Total Ionizing Dose (TID) and Displacement Damage (DD), which is a non-ionizing effect. TID effects are induced by energetic particle irradiation over time. Damage is typically gradual and is manifested in devices as threshold shifts, increased leakage currents, timing changes and device failures. 2 TID is quantified as accumulated dose in silicon in terms of Grays [Gy], where 1Gy = 100 rad = 1 J/kg. CMOS devices typically experience a biasing of the gate threshold voltage as the result of trapped holes in insulating oxides. Displacement damage is induced by energetic neutrons displacing semiconductor lattice atoms, generating holes and electrons (i.e. Frenkel pairs). The holes tend to get trapped in defects thereby reducing minority carrier lifetime. Bipolar and BiCMOS technologies are more sensitive than CMOS to displacement damage. Bipolar devices exhibit parametric degradation at approximately 10#9 neutrons/ cm0, while CMOS device degradation begins at approximately 10#2 neutrons/cm0. It is important to note that optical devices and CCDs are very sensitive to displacement damage induced by neutrons. For sensors such as CCDs, displacement damage introduces defects in the CCD bulk material. Although the cross-section of Si for a 1MeV incident neutron is only 2.4 ∙ 1010<cm0, the energy transfer to the silicon recoil atom is ~70 keV, on average. This energy transfer corresponds to approximately 1500 displaced atoms. Bulk defects include increased dark current and dark current spikes, degradation of output amplifier performance, and a reduction in charge transfer efficiency (CTE) [5]. 4. Design variables The design of radiation hard electronics should be considered a system optimization problem. Since radiation hard device development is expensive and time-consuming it is essential to first know the radiation environment with minimal uncertainty. A comprehensive radiation transport analysis that is highly correlated to the facility physical structure is essential to quantify the radiation environment, ideally iterating with the design of the facility with the objective of minimizing electronic device radiation exposure, either through physical facility changes and/or shielding. The uncertainty of the rad-hard electronics development is a direct result of the quality of the radiation analysis. Minimization of active electronics exposed to high radiation, carefully considering the device technology and level of reliability required (e.g. critical safety) should follow. There are typically two approaches to radiation hardening of electronic devices; radiation hard by design, and radiation hard by process. Radiation hard by design utilizes proven hardened design methodologies implemented on a radiation hardened transistor library. Radiation hard by process utilizes enhanced silicon fabrication processes for improved radiation performance. In order to mitigate SEEs in digital devices, redundancies are introduced into the signal processing path, as well as error detection and correction codes. Triple modular redundancy is a popular technique utilized in programmable devices. Error correction codes are typically employed in communication data links.