Modeling And Hardening Techniques For Atmospheric Neutron-induced Single Event Effects In Microelectronics

With the popularization of civil electronics and avionics in our country,the stability of electronic chips working under atmospheric environment in long-term work is of great significance.The single event effect caused by high-energy particles in the space has been widely researched by the industry for a long time.However,it is shown by recent experiments and accident reports that chips working in the atmosphere could also be affected by single event effects,which are mainly caused by atmospheric neutrons.As technology scaling,the impact of single particle effect caused by atmospheric neutrons is showing serious impact.Radiation hardening has long been an important topic for electronics in the aerospace.Paying high expenses for robustness is not an issue for aerospace electronics,however,the chips working in the atmospheric environment have spreaded to every corner of human life,with a wide range of application scenarios.It is difficult for the chips to meet the market demand and stay valuable if the robustness is achieved with high expenses.Products will be more valuable if the same performance is achieved with lower overheads and development costs.Therefore,considering the market requirements,it is important to improve the robustness at lowest cost possible when protecting chips against the atmospheric neutron-induced single event effect.First of all,relevant models should be established to evaluate the robustness of the circuit in the design phase.The establishment of models can reduce the trial-and-error cost,and shorten the research and development cycle.Furthermore,models could be used to evaluate the most sensitive parts in the integrated circuit for selective hardening,so as to improve the overall robustness at the lowest possible cost.Secondly,hardened circuit modules should be developed with lower overheads,such as flip flops,comparators and other modules,so that the hardened modules can achieve higher robustness while introducing as little additional overhead as possible.In this dissertation,the modeling technology and hardening technology for atmospheric neutron-induced single event effects are studied,with following achievements:(1)The transient current caused by the nuclear reaction products is studied.Through TCAD simulation,two scenarios have been identified and studied: Firstly,it is verified that when the track of the secondary particle is far from the space charge zone of the sensitive drain,the waveform of transient current is mainly controlled by the diffusion-andcollection process of the deposited charge;Secondly,it is found that in nano scale CMOS process,when the ionization track is closer to the sensitive drain,even though the particle is not incident on the sensitive drain,the transient current waveform is composed of a narrow peak and a flat plateau.Two models are established to simulate the two scenarios:Firstly,the effectiveness of diffusion-collection model in nanoscale CMOS process is studied for the scenario where the ionizing track is sufficiently far from the sensitive drain.Two diffusion-collection models with different advantages have been implemented.It is found that recombination of non-equilibrium carriers cannot be ignored in nano CMOS process.It is proved that the parameters describing the carrier lifetime is very important for the accuracy of the diffusion-collection model.The variations of model parameters have been studied for particles with different positions,LET values and track lengths.The established models have been validated by TCAD simulations.Secondly,for the scenario where the ionizing track is close to the sensitive drain,an equivalent circuit model is established for the transient current waveform,which adopts the diffusion-collection model to describe the generation of charge.The influence on model parameters of the LET value is studied,as well as the influence of the distance between the ionizing track and the sensitive drain.In the end,some transient current waveforms are simulated with predicted model parameters as examples,and the results are compared with the TCAD simulation results to verify the effectiveness of the diffusion-collection model and the equivalent circuit model established in this dissertation.(2)According to the requirements of civil electronics and avionics,which are dominant in the atmosphere,the designing techniques of radiation hardening by design and pulsetriggered flip-flop are studied,and a hardened flip-flop with low overall overhead is proposed.The hardened flip-flop achieves robustness to single-event effect by redundancy,and the additional overhead is minimized by the advantage of pulse triggering designs.The proposed flip-flop shows lower overall overhead among hardened flip-flops,therefore it is very suitable for hardening scenarios requiring low power consumption or low overall overhead.Single event effect is simulated with both the classical model and the models proposed in this dissertation to verify the robustness of the proposed flip-flop design.A shift register chip for irradiation experiment is designed and fabricated to further verify the robustness of the proposed hardened flip-flop.(3)A hardened dynamic comparator is proposed.Dynamic comparator is an important module in integrated circuits which is sensitive to single event effect.Therefore,a classic double-tail dynamic comparator is studied through single event transient injection simulations.It is found that the dynamic comparator is insensitive to single event in the reset phase,and is sensitive during the latch phase.The characteristic of the hardened memory cell is studied,as well as the difference with the characteristic of dynamic comparator.Then the reset mechanism and the writing mechanism of the hardened memory cell are adjusted to work as the latch stage of the dynamic comparator.The hardened comparator has improved the robustness against single event effect with acceptable costs.Moreover,the comparator is controlled by a single-phase clock,which reduces the complexity of the clock tree.As a result,it is suitable for applications that requires robustness and is constrained by overall overheads.