Total ionizing dose mitigation by means of reconfigurable FPGA computing

[摘要] There is increasing use of commercial components in space technology and it isimportant to recognize that the space radiation environment poses the risk of permanentmalfunction due to radiation. Therefore, the integrated circuits used for spacecraftelectronics must be resistant to radiation.The effect of using the MOSFET device in a radiation environment is that the gate oxidebecomes ionized by the dose it absorbs due to the radiation induced trapped charges inthe gate-oxide. The trapped charges in the gate-oxide generate additional space chargefields at the oxide-substrate interface. After a sufficient dose, a large positive chargebuilds up, having the same effect as if a positive voltage was applied to the gate terminal.Therefore, the transistor source to drain current can no longer be controlled by the gateterminal and the device remains on permanently resulting in device failure.There are four processes involved in the radiation response of MOS devices. First, theionizing radiation acts with the gate oxide layer to produce electron-hole pairs. Somefraction of the electron-hole pairs recombine depending on the type of incident particleand the applied gate to substrate voltage, i.e. the electric field. The mobility of theelectron is orders of magnitude larger than that of the holes in the gate oxide, and is sweptaway very quickly in the direction of the gate terminal. The time for the electrons to beswept away is on the order of 1ps. The holes that escape recombination remain near theirpoint of origin. The number of these surviving holes determines the initial response of thedevice after a short pulse of radiation. The cause of the first process, i.e. the presenceof the electric field, is the main motivation for design method described in thisdissertation.The second process is the slow transport of holes toward the oxide-silicon interface dueto the presence of the electric field. When the holes reach the interface, process 3, theybecome captured in long term trapping sites and this is the main cause of the permanent threshold voltage shift in MOS devices. The fourth process is the buildup of interfacestates in the substrate near the interfaceThe main contribution of this dissertation is the development of the novel SwitchedModular Redundancy (SMR) method for mitigating the effects of space radiation onsatellite electronics. The overall idea of the SMR method is as follows: A chargedparticle is accelerated in the presence of an electric field. However, in a solid, electronswill move around randomly in the absence of an applied electric field. Therefore if oneaverages the movement over time there will be no overall motion of charge carriers inany particular direction. On applying an electric field charge carriers will on averagemove in a direction aligned with the electric field, with positive charge carriers such asholes moving in the direction of field, and negative charge carriers moving in theopposite direction. As is the case with process one and two above.It is proposed in this dissertation that if we apply the flatband voltage (normaly a zerobias for the ideal NMOS transistor) to the gate terminal of a MOS transistor in thepresence of ionizing radiation, i.e. no electric field across the gate oxide, both the freeelectrons and holes will on average remain near their point of origin, and therefore have agreater probability of recombination. Thus, the threshold voltage shift in MOS deviceswill be less severe for the gate terminal in an unbiased condition. The flatband conditionsfor the real MOS transistor is discussed in appendix E.It was further proposed that by adding redundancy and applying a resting policy,one can significantly prolong the useful life of MOS components in space. The factthat the rate of the threshold voltage shift in MOS devices is dependant on the biasvoltage applied to the gate terminal is a very important phenomenon that can beexploited, since we have direct control and access to the voltage applied to the gateterminal. If for example, two identical gates were under the influence of radiation andthe gate voltage is alternated between the two, then the two gates should be able towithstand more total dose radiation than using only one gate. This redundancy could beused in a circuit to mitigate for total ionizing dose. The SMR methodology would be to duplicate each gate in a circuit, then selectively onlyactivating one gate at a time allowing the other to anneal during its off cycle. The SMRalgorithm was code in the 'C language. In the proposed design methodology, the designengineer need not be concerned about radiation effects when describing the hardwareimplementation in a hardware description language. Instead, the design engineer makesuse of conventional design techniques. When the design is complete, it is synthesized toobtain the gate level netlist in edif format. The edif netlist is converted to structuralVHDL code during synthesis. The structural VHDL netlist is fed into the SMR 'Calgorithm to obtain the identical redundant circuit components. The resultant file is also astructural VHDL netlist. The generated VHDL netlist or SMR circuit can then be mappedto a Field Programmable Gate Array (FPGA).Spacecraft electronic designers increasingly demand high performance microprocessorsand FPGAs, because of their high performance and flexibility. Because FPGAs arereprogrammable, they offer the additional benefits of allowing on-orbit design changes.Data can be sent after launch to correct errors or to improve system performance. Systemincluding FPGAs covers a wide range of space applications, and consequently, they arethe object of this study in order to implement and test the SMR algorithm.We apply the principles of reconfigurable computing to implement the Switched ModularRedundancy Algorithm in order to mitigate for Total Ionizing Dose (TID) effects inFPGA's. It is shown by means of experimentation that this new design techniqueprovides greatly improved TID tolerance for FPGAs.This study was necessary in order to make the cost of satellite manufacturing as low aspossible by making use of Commercial off-the-shelf (COTS) components. However,these COTS components are very susceptible to the hazards of the space environment.One could also make use of Radiation Hard components for the purpose of satellitemanufacturing, however, this will defeat the purpose of making the satellitemanufacturing cost as low as possible as the cost of the radiation hard electronic components are significantly higher than their commercial counterparts. Added to this isthe undesirable fact that the radiation hard components are a few generations behind asfar as speed and performance is concerned, thus providing even greater motivation formaking use of Commercial components.Radiation hardened components are obtained by making use of special processingmethods in order to improve the components radiation tolerance. Modifying the processsteps is one of the three ways to improve the radiation tolerance of an integrated circuit.The two other possibilities are to use special layout techniques or special circuit andsystem architectures.Another method, in which to make Complementary Metal Oxide Silicon (CMOS) circuitstolerant to ionizing radiation is to distribute the workload among redundant modules(called Switched Modular Redundancy above) in the circuit. This new method will bedescribed in detail in this thesis.

[发布机构] Stellenbosch University