Simulation Research And Design Of NMOS Devices For TID Tolerance

When electronic devices are continuously exposed to high energy rays,like X ray and ? ray,a time-accumulation-related effect called TID(total ionizing dose)would occur,causing degradation or even failure to transistors.Generally speaking,the electronic devices working in space are not easy to change frequently,so it is very necessary to develop radiation-hardened transistors.Since the MOSFET based on bulk silicon is still the most wildly used,this thesis mainly focus on developing radiation-hardened layout based on bulk silicon MOSFET.In the introduction of the theory of TID effect,the mechanism of total ionizing dose effect on MOSFETs is analyzed,and two main radiation-induced charge,oxide-trapped charge and interface traps are introduced.Due to the dominant impact of oxide-trapped charge on MOSFETs,interface traps are ignored in this paper.In the MOSFETs,it's the oxide that sensitive to total ionizing dose effect.The oxide in MOSFETs are gate oxides and isolation oxides.The gate oxide is very thin,and the thickness is usually several nanometers to tens of nanometers,while the isolation oxide is usually 100 nm to several hundred nanometers.It has been shown that when the oxide thickness is less than 12 nm,the thin oxide can be seen as radiation-hardened.Therefore,the effect of oxide-trapped charge in isolation oxide is mainly studied in this paper.Based on the study of TID theory and the existing TID-hardened transistors,three new kinds of TID-hardened transistors are developed.The TID-hardened characteristic of the three new transistors are verified that the level of radiation-hardness can reach 500krad(Si)dose through Sentaurus TCAD in 3D,and this level meets the nowadays Frequirements.The novel layouts are designed for NMOSFET because NMOSFET are more sensitive to PMOSFET which will be introduced in chapter 2.The first novel layout is a NMOSFET with semi-p+ guard rings(SPGR-NMOSFET).The design isolates STI from active by introducing p+ doping area,resulting in eliminating parasitic channel and thus the effect of TID.Differing form the existing radiation-hardened layout,H gate layout and enclosed gate layout,SPGR-NMOSFET isn't introduced extra gate area and thus will not increase transistor area and achieve radiation-hardness at the same time.The second novel layout is NMOSFET with Z gate,the name comes from the similarity of gate with the English letter Z.The ultra-thin gate is radiation-hardened because the TID-induced charge occur only little in it,and thus the effect on ultra-thin gate can be ignored.The radiation-hardened design of Z gate is to isolate STI form active by introducing extra gate in one side of STI/active interface.Z gate layout complements many disadvantages of enclosed gate layout—flexibility of W/L design,smaller transistor area,solving transistors applyed for analog circuit with small W/L design difficulties and digital circuit with speed limitation problem.The third novel layout is multi-finger Z gate NMOSFET.The multi-finger design of a single transistor with large W/L would improve the flexibility of IC design,decrease gate resistance and gate parasitic effect.However,due to the increase number of fingers,the number of STI/active interface increase,leading to more sever response of TID effect.And this is bad for transistors applying for working in space.The idea of radiation-hardened design of multi-finger Z gate NMOSFET is similar to Z gate NMOSFET,and the novel layout is able to be radiation tolerant and independent of transistor fingers.The design of multi-finger Z gate NMOSFET,realizing the single gate transistors with large W/L applied for space by splitting long gate to several small gates.In this thesis,besides radiation tolerant characteristic,the three novel radiation-hardened layouts has a smaller transistor footprint,no limitation of W/L design and smaller gate capacitance compared with enclosed gate layout.Through the simulation and analysis of these layouts,proving some references for developing radiation-hardened layouts working in space in the future.