Research On Design And Process Integration Technology Of The Core Configuration Cell For Radiation-Hardened FLASH-Based FPGA

With the development of electronic systems in key projects of aviation,aerospace and shipbuilding towards multi-function,high performance,high reliability,miniaturization and low power consumption in China,the demand for high-capacity,high reliability and radiation-hardened Flash-based FPGA is becoming more and more prominent,and the Flash switch is the basic core configuration cell for the radiation-hardened Flash-based FPGA.In this paper,the design andprocess integration technology of configuration cell for radiation-hardened Flash-based FPGA are studied.The scientific problems of"design structure of the high reliability and radiation-hardened Flash-based configuration cell","working mechanism and method of the Flash-based configuration cell","total dose radiation response and physical model of the floating gate Flash-based configuration cell"have been solved.The key technologies such as structure design,process integration,"ON/OFF"state characterization and reliability of radiation-hardened Flash-based configuration cell have been broken through.The radiation-hardened Sense-Switch p FLASH and Push-Pull p FLASH cells are successfully developed.The total dose radiation ionization damage mechanism of Flash switching cell is revealed.The equvalient physical model of ionization effect is established,and the programming/erasing mechanism and working method are mastered.In summury,the main research results of this paper are as follows:（1）The structure and working principle of radiation-hardened Sense-Switch p FLASH are proposed.The structure consists of two p MOS with common floating gate and control gate.The charge in the shared floating gate is changed by BBHE programming of T₁ and FN erasing of T₁/T₂,and then the working"ON/OFF"states of T₂ are controlled.When the floating gate is charged,the T₂is in the ON state.When the electrons of the floating gate are removed,the T₂is in the OFF state.The basic physical model of charge sharing is established,that is,the programmed or erased threshold voltage of T₁and T₂mainly depends on initial threshold voltage(V_TH0)and shared floating gate charge（ΔQ）.The difference between the programmed or erased threshold voltages of T₁and T₂ is equal to the difference between their initial threshold voltages.The threshold voltage window depends on the device structure and processing factors under the same programming/erasing conditions.（2）The structure and working principle of radiation-hardened Push-Pull p FLASH are proposed.The gate potential of the signal transmission transistor is applied by the programmable transistor through an indirect coupling methold,that is,the Push-Pull p FLASH cell composed of two2T-FLASHtransistors（T₁/T₂）controls the"ON/OFF"states of the signal transmission transistor（T₃）.Therefore,the signal transmission transistor and the programming/erasing transistors are isolated effectively.The anti-interference ability and reliability is enhanced.In order to balance the influence of programming and erasing stress,a mirror programming/erasing methode is proposed.（3）The damage mechanism of total inorizing dose radiation effect in the floating gate p FLASH cell is studied.The programming state is electron emission and hole injection,and the erasing state is electron injection and hole emission.The relationship among threshold voltage,subthreshold swing,maximum transconductance and cumulative dose of ⁶⁰Co in two states of Sense-Switch p FLASH（i.e.programming and erasing state）is studied.It is found that:1)the change of threshold voltage of T₁ and T₂ is mainly determined by the change of floating gate charge,trap charge in oxide layer around floating gate and interface trap of tunnel oxidation;2)The absolute value of threshold voltage in programming and erasing states of T₁ and T₂decreased linearly with the cumulative dose rate of total dose,and the change rates are about 22.5 m V/krad（Si）and 14.5 m V/krad（Si）,respectively;3)The charge modification on the floating gate can be compensated by programming and erasing,but the interface damage(interface state N_it)and trap charge(N_ot)in the oxide layer caused by the total dose radiation ionization effect increase with the increase of cumulative dose rate.At the same time,the relationship between the"ON/OFF"state characteristics and the total inorizing dose radiation effect of Push-Pull p FLASH switching cells in two states of"T₁＿ERS-T₂＿PGM"and"T₁＿PGM-T₂＿ERS"is studied.It is found that the"ON/OFF"state function of Push-Pull p FLASH switching cells can resist the total dose irradiation of more than 150 krad（Si）,and there is moreadvantage i nradiation hardness of the total dose than that of Sense-Switch p FLASH obviously.（4）The leakage mechanism of TID effect on CMOS devices is analyzed.The radiation-hardened method of"STI implant+PA implant+HTO+low temperature gate oxidation method"is adopted to effectively solve the problems of threshold drift and leakage of NMOS transistors.The effects of STI implant dose and energy on the total dose ionization effect of LVMOS and HVMOS are studied.The radiation-hardeness of 3.3 V/5 V NMOS devices is successfully achieved more than 100 krad（Si）,and the total dose radiation-hardened ability of MOS devices is improved.At the same time,the effects of X-ray（40 k V）and ⁶⁰Co（310 k V）irradiation sources on physical parameters such as threshold-voltage drift,off-state leakage,saturation current and subthreshold swing of 5V NMOS devices are compared and analyzed under the same irradiation dose rate.It is found that the parameter modification caused by X-ray is around 4-6 times more than that caused by ⁶⁰Co.It is speculated that it is mainly due to the ionized electron-hole pairs in the Si O₂ layer around MOS and the interface state of Si/Si O₂ caused by X-ray irradiation source are higher than that of ⁶⁰Co.（5）The threshold characteristics,"ON/OFF"state characteristics,durability and charge retention characteristics of radiation resistant Sense-Switch p FLASH are studied.The device has excellent"ON/OFF"state characteristics and consistency of electrical parameters.The average value and uniformity of T₁/T₂ threshold window are about 10 V and 3.3%respectively.When the working voltage is-1.5 V,the leakage current of T₂ in the"off"state is about-0.24 p A/μm,the average driving current of T₂ in the"ON"state is about-126.8μA/μm,and which of uniformity is about 6.31%,the endurance can reach more than 10,000 times,and the charge retention characteristic at 25℃can reach more than 10 years.At the same time,the threshold characteristics and switching characteristics of Sense-Switch p FLASH with"bar-gate"and"dumbbell-gate"structures are compared.The effects of channel length（L）and field PIP capacitance size(W_PIPCD)on the coupling coefficient of dumbbell-gate structure devices are studied.Based on dumbbell-gate structure devices with better performance,the programming and erasing working conditions of Sense-Switch p FLASH devices are optimized.It is found that 1)the coupling coefficient(K_FG)ofSense-Switch p FLASH with“bar-gate”structure does not depend on L,but only depends on the design rule size of the PIP capacitor in the field,the K_FG shows an increasing trendwith the increase of W_PIPCD;2)The K_FGofSense-Switch p FLASH with“dumbbell-gate”structure is significantly affected by L and W_PIPCD.The K_FG shows an increasing trend with the increase of W_PIPCD with the same channel length.Under the condition of same W_PIPCD,the K_FG decreases with the increase of L;3)The increase of field PIP capacitance area is an effective method to improve the K_FG of Sense-Switch p FLASH;4)By increasing K_FG,the effective charge of Sense-Switch p FLASH in programming and erasing state can be increased,especially the driving ability of"ON"state can be significantly improved.（6）The threshold characteristics,"ON/OFF"state characteristics,durability and charge retention characteristics of radiation resistant Push-Pull p FLASH are studied.The device has good electrical characteristics and parameter consistency.The"ON/OFF"state function of signal transmission transistor T₃ can be realized through two programming/erasing states of"T₁＿PGM-T₂＿ERS"and"T₁＿ERS-T₂＿PGM".The threshold window of the device structure is about 10.5 V（T₁ programming state and T₂erasing state or T₁erasing state and T₂ programming state）,and the driving capacity of"ON"state is about-0.92 m A(V_D1=V_D3=-1.2 V)the"OFF"state leakage is about-40 p A.In particular,the uniformity of parameters such as threshold window and"ON"state driving capacity is less than 5%,and theendurancecan reach over 10,000 times.Under the"ON/OFF"stress condition of 25℃,they can meet the service life requirements of more than 10 years.The above research conclusion can provide core IP technology and theoretical support for radiation-hardened Flash-based FPGA circuits,and can have good application value for the realization of reconfigurable programmable logic devices.