Investigation Of Single Event Effects And Synergistic Effects With Total Ionizing Dose In Flash Memory

Flash memories have been widely used in many space systems due to their high density,low power consumption and non-volatility.However,they are not immune to space radiation effects.The space radiation environment is characterized by a mixture of the particles which include protons,electrons,and heavy ions.The single event effects and total ionizing dose effects caused by the radiation environment can threaten the operation and service life of Flash memories in space.Using the heavy ions provided by the Heavy Ion Research Facility in Lanzhou(HIRFL)and ? rays provided by the 60 Co source at Xin Jiang Technical Institute of Physics and Chemistry,Chinese Academy of Science,we studied the single event upset(SEE)of floating gate(FG)cells,annealing of FG errors and retention errors in FG cells induced by heavy ion irradiation on 34 nm and 25 nm commercial Single Level Cell(SLC)NAND Flash memories and the influence of total ionizing dose(TID)on these three aspects.The main results are as follows:(1)The cross section of FG SEU in Flash memories is lower than that in SRAM.The errors only appear in the FG cells with binary ‘0'.The quantity of charge in FG cell decreases with the scale of technology,so the sensitivity of SEU in FG cells increases as the feature size decreases.For 34 nm and 25 nm Flash memories,the saturation cross section of SEU is 3.5×10-10 and 8.1×10-10 cm2/bit respectively,the LET threshold of SEU is 2.0 and 0.7 MeV·cm2/mg respectively.(2)After heavy ion irradiation,the charge trapped in the tunnle oxide could be recovered over time,annealing of FG errors is observed.The percentage of annealed FG errors in 34 nm Flash memories does not change significantly with the ion LET,remaining below 10%,when the ion LET is greater than 60 Me V·cm2/mg,the percentage of annealed errors in 25 nm Flash memories increases up to about 50% at LET 99.8 Me V·cm2/mg.The threshold voltage distribution of FG cells after heavy ion irradiation determines the percentage of annealed errors,which would be influenced by the feature size of device,ion LET and MCU.MCUs with higher multiplicity account for the majority of the errors in 25 nm Flash memories under high LET heavy ion irradiation,which leads to the large percentage of annealed errors.(3)Some defects are created in the tunnel oxide by the recombination of the carriers generated by heavy ions.These defects are randomly distributed in the ion track area to build a multi-Trap Assisted Tunneling(m-TAT)conductive path in the tunnel oxide,which would discharge the FG cell and cause a retention error in reprogramed FG cell.The LET threshold of retention errors for 34 nm memories is between 37.6 and 67.1 Me V·cm2/mg.The LET threshold of retention errors for 25 nm Flash memories is between 20.7 and 29.1 Me V·cm2/mg,which is smaller than that in 34 nm Flash memories.Both of them are an order of magnitude larger than the LET threshold of SEU.The cross section of retention errors increases with the increasing of ion LET,which is about several orders of magnitude lower than the cross section of SEU.(4)The total dose irradiation would affect the SEE sensitivity of FG cell in Flash memories.The increase in the SEU cross section of FG cells previously exposed to TID is observed,which is caused by the threshold voltage shift induced by the combination of ? ray and heavy ion irradiation.When the ion LET is 29.05 Me V·cm2/mg,the SEU cross section of FG cells in 34 nm Flash memories increases from 4.7×10-11 cm2/bit to 1.5×10-10 cm2/bit.The increase of SEU cross section reachs the maximum,which is about 220%.The total dose irradiation would increase of the number of trapped charge and FG errors and change the threshold voltage distribution of FG cells,which affects the percentage of annealed errors in FG cells.The defects also can be generated by trapped holes in the tunnel oxide after the total dose irradiation,which makes it easier to build the m-TAT path after heavy ion irradiation to discharge the FG cell,The increase in the retention error cross section of FG cells previously exposed to TID is observed and it becomes smaller and smaller with the increasing of ion LET.