Research Of 22nm Fully Depleted Silicon-On-Insulator (FDSOI) Transistor And Its High Flexibility

As the feature sizes of IC core devices decrease,the short channel effects of the conventional bulk MOSFET become increasingly uncontrollable.When the semiconductor process technology reaches the 28 nm node,FDSOI devices are demonstrated to be a promising candidate due to the superior gate control capability and low leakage current.At present,the gate-first process is widely used to prepare the FDSOI devices in the industry,but the process flow is difficult to precisely and effectively control the threshold voltage of the devices.Therefore,preparation of FDSOI devices by gate-last process has been proposed in this paper.In addition,FDSOI technology should have great flexibility for circuit designers.In order to improve the flexibility of FDSOI devices,different threshold characteristics can be achieved by body bias technology.Based on TCAD,this paper also analyzes the body bias technology from the physical mechanism in detail.The main research contents in this paper are as follows:1.DC and AC characteristics of FDSOI devices based on 22 nm technology node were simulated by TCAD tool.Based on the simulation results,the basic electrical parameters of FDSOI devices were analyzed,including gate capacitance,ON state current,OFF state current,subthreshold swing,threshold voltage,etc.2.The preparation flow and process optimization of FDSOI devices were analyzed.FDSOI devices have been fabricated by gate-last process,which is completely compatible with the planar bulk process.In terms of process optimization,the effective solutions to solve the problem of non-uniform RSD structure and aluminum void have been proposed.Finally,we tested the devices by wafer acceptance test,and the results showed that the threshold voltage of fabricated devices was around 300 m V,DIBL was in the range of 60 m V/V to 80 m V/V and SS was around 70 m V/dec.The ON state current/OFF state current ratio of n-type and p-type devices were 104 and 105 respectively.The electrical performance of the devices accorded with the industry standard and matched with simulation results.3.To improve the flexibility of FDSOI technology,we have researched the body bias technology.According to the types of backplane doping,FDSOI devices were divided into conventional well structure and reverse well structure.Based on the actual test results and TCAD simulation,the effect of back bias was analyzed from physical mechanism.The results showed that back bias significantly affected the threshold voltage of FDSOI devices with different sizes and different types of backplane doping.For example,positive back bias can reduce the saturation threshold voltage of small-size n-type devices with conventional well by 265.4m V,and it can reduce large-size n-type devices with conventional well by 320.3m V.Depending on the demands,the circuit designers can choose forward body bias mode or reverse body bias mode.Besides,we demonstrated another strategy to realize multi-Vt devices.Based on different back bias and types of backplane doping,FDSOI devices can realize LVT,SVT(i),SVT(ii)and HVT.The multi-Vt strategy greatly improves the flexibility of FDSOI technology.Finally,the circuit delay was described through the ring oscillator circuit based on devices of different threshold characteristics.The content of this paper has guiding significance for optimizing 22 nm FDSOI process and impoving the flexibility of the device design and the circuit design.