Optimization Of SOI Lateral Power Devices With 3D Variation Of Doping Profile

Many techniques have been proposed by the researchers to satisfy the increasing requirement of superior breakdown characteristic of lateral power devices.Among these techniques,Variation of Lateral Doping?VLD?technique is the most mature and commercialized technique for evening the surface electric field and maximizing the lateral breakdown voltage in the lateral power device.However,the power devices with VLD technique always encounter some practical problems caused by the process.For instance,the non-uniform vertical drift doping profile caused by the anneal,the Three-Diminsion?3D?curvature effect caused by the layout geometry and some other effects will make the electric field fail to be uniform.These unideal situations will degrade the breakdown characteristics of lateral power devices with VLD technique.Considering the difficulty of modeling power devices with unideal situations,it is hard for researchers to explore a clear breakdown mechanism and give an optimization guidance of the practical lateral power devices.Thus,in this thesis,the 2D/3D breakdown model and the optimal design of lateral power devices with variable drift doping profile are addressed.First,this thesis develops the breakdown model of lateral power with 2D doping profile.Then,the 3D curvature effect in pratical layout is considered into this established breakdown model,and the 2D withstand voltage theory is extended to a 3D withstand voltage theory.With the guidance of the built model,a novel 3D VLD technique is proposed to diminish the 3D curvature effect in the lateral power device with circular layout.By optimizing the drift doping profile,this technique can totally suppress the electric field crowding caused by the layout and obtain a better on/off-state performance.Finally,in terms of process,a new method to calculate the mask geometry of VLD region under a given process condition is proposed.Meanwhile,a feasible scheme for fabricating a VLD device is given.1.A new modeling method,the Effective Substrate Potential?ESP?method,is proposed,and the2D breakdown model of lateral power devices with arbitrary doping profile in x and y directions is derived.First,the impact of depleted charges in the drift region is replaced by the substrate potential by using the ESP method.This method reduces the difficulty of directly solving the 2D Poisson's equation and the analytical model of the device with arbitrary doping profile can be developed.According to the breakdown model,the RESURF criterion of this device is given.With the guidance of this criterion,the device with the optimal doping profile can achieve a maximum lateral breakdown voltage.Thus,this criterion can be applied into the optimal design of devices with various doping profiles.In conclusion,the analyzation of this breakdown model extends the withstand voltage theory from the device with 1D doping distribution to the device with 2D doping distribution.2.A new theory of 3D withstand voltage is proposed and the 3D breakdown model of lateral power devices with arbitrary doping profile in r and y directions is derived.By using the method of approximate superposition of electric field and ESP method,the analytical model for the SOI LDMOS with a variable doping profile in a circular layout is developed based on the 3D Poisson's equation.Analytical models and simulation results illustrate the influence of drift doping profiles and curvature radius on the breakdown performance of the lateral power device.The structural parameters of the turning point at which the breakdown voltage sharply decreases are provided and this gives a theoretical guidance for the optimal design of the devices.In addition,the impact of 3D curvature effect on specific on-resistance and BFOM?Baliga's Figure Of Merits?is investigated.The analyzation shows that by optimizing the drift doping profile,the BFOM of the drain-centered structure can reach the optimal value of the 2D case.Yet,the source-centered structure is difficult to achieve the BFOM of 2D case through the optimization.The study on this breakdown model extends the withstand voltage theory of the device with a variable doping profile from the 2D structure to the 3D structure.3.A new technique to gain a uniform 3D surface field is proposed.This technique is formulated by solving the 3-D Poisson equation based on the assumption of uniform surface electric field.The proposed doping profile is adaptive to the change of curvature radius.The analyzation reveals that the 3D VLD device can obtain a uniform surface electric field regardless of the size of curvature region.Compared to the 2D VLD device,the breakdown voltage of the 3D VLD device is increased by 18%at r_in=2?m.Moreover,the 3-D VLD device presents an excellent operation performance,including high on-states breakdown voltage,low specific on-resistance,increased large saturation current,large transconductance and suppressed quasi-saturation effect.The BFOM of the 3-D VLD device is about 2.3 times of BFOM of the 2-D VLD device at r_in=2?m.Therefore,3D VLD technique can weaken the influence of curvature effect and contributes to a better tradeoff performance between breakdown voltage and on-resistance.This technique could be expected a higher performance in analog applications.4.A new method to design the process parameters of fabricating VLD region is proposed,and the process of 3D VLD device is verified by the TCAD tools.First,the mask geometry of 3D VLD region is given based on the diffusion equation.The process simulation by using TCAD tool proves the feasibility of the designed process.Then,the software MATLAB is implemented to develop the Minivld tool based on least squares.The Minivld tool can provide mask geometry for fabricating VLD regions with the given process conditions/parameters,and it has a good human-computer interaction interface.The process simulation results reveal the accuracy and feasibility of this tool in designing process conditions/parameters of the VLD region.Compared with the traditional method,this tool takes the impact of process parameters into account,including impurity types,annealing time/temperature,and minimum lithographic accuracy.Thus,this tool can provide a more comprehensive and user-friendly design solutions.Finally,the impact of key process parameters on the on/off-state characteristics of VLD devices is studied.In order to improve the trade-off relationship between breakdown voltage and on-resistance,the annealing should be as long as possible or the dose of the drift region should be large enough.