Research On Equivalent Substrate Model And Nonfull Depletion Mode Of The Super Junction Power Device

Over 75% of the electric energy used by human is converted by the power semiconductor devices.The power MOSFET is the main force in the area of the power semiconductor.The characteristics with both the high breakdown voltage V_B and low specific on-resistance R_on are the hot research topic of the power MOSFET.This thesis discusses the up rising star super junction（SJ）.The junction-type voltage sustaining layer with the periodic N and P-type dopings of the SJ device is used to replace the single conduction resistance-type voltage sustaining layer of the conventional power MOS,which is a qualitative change of the voltage sustaining layer.The equal positive and negative charges are introduced into the voltage sustaining layer of the SJ,leading to the the high surface electric field turns to the bulk and realizing the optimization from the surface to the bulk.From the researches of numerous scholars,the doping concentration N of the SJ is almost independent of V_B in a certain range.For a given V_B,the SJ reduces R_on efficiently and then the important R_on-V_B relationship of the power MOS is reduced from the R_on ∝ V_B^2.5 to V_B^1.32.Therefore,SJ is highly praised as the “milestone in high voltage power MOS device”.In recent ten years,the development of the SJ is in two aspects: Firstly,the processes obtained the SJ voltage sustaining layer with the narrower cell pitch and more stringent change balance;Secondly,the device physics focused on the analyses of the1-D and 2-D electric fields by using the field criterions.The local values of R_on were obtained with the exactly depletion condition,and so on.It is concerned by researchers whether the minimum specific on-resistance R_on,min can be searched by the full-region optimization,which has not been solved yet.The theoretical minimum R_on of the SJ gives a standard of comparison to show how much could be expected by using the SJ voltage sustaining layer and give the clear improvement direction for a existing result.By following the spirits of “to exhaust the mechanism”,this thesis focuses on the basic R_on question of the SJ device.The concept of the charge electric field is developed and its modulation mechanism is revealed.The R_on optimization theory is proposed and realize the most optimized relationship so far of R_on ∝ V_B^1.03.The theory includes 2 model,1 mode,and 1 method,i.e.,the specific on-resistance well（R-well）model and the the equivalent substrate（ES）model;the non-full depletion（NFD）mode;the minimum specific on-resistance R_on,min optimization method.The above theory gives the full region R_on optimization to search the R_on,min.The leading innovative points are:1.The new non-full depletion（NFD）mode is proposed.It’s found by introducing the concept of the charge electric field that its vertical peak value is uniquely determined by the doping dose.The normalized charge electric field analysis method is proposed and introduces the normalized V_B factor.The SJ can work in the NFD mode with the strong modulation effect to provide the lower R_on.The new NFD mode is thus proposed and shows a theoretical R_on ∝ V_B^1.03 relationship with the analytical R_on,min condition.As an example,if V_B of the device increases from 200 V to 800 V,R_on form the conventioal R_on ∝ V_B^2.5 relationship is increased to 32 times,however,the new relationship leads to the multiple reducing to 1/8.2.The R-well mode is developed for the full region optimization.The two basic restrictions of the SJ is the maximum doping concentration N_max by the cell size and the minimum voltage sustaining layer length L_d,min by V_B,which together determine the U shape distribution of R_on as a function of N,called the R-well.The R-well model covers all the possible optimized points in both the NFD and the FD modes under the breakdown states with the superposition of charge and potential electric fields.The R-well model provides the optimal path of the R_on optimization and thus is universal for all the existing optimizations with the respective selected points at the R-well.3.The R_on,min optimization methodology is developed.On the basis of the R-well model,the methodology is with three steps: firstly,the different R-wells in the application ranges of SJ are obtained by varying the parameter values;secondly,the unique R_on,min of each R-well is searched by the golden section search method as the optimal design;thirdly,the design formulas and accurate R_on-V_B are obtained by the least square fitting method.From the R_on,min optimization,a 900 V / 10 A vertical SJ devic is fabricated with the trench etch and filling process.For a 950 V device,R_on is reduces by 77% of the conventional R_on ∝ V_B^2.5 relationship to 53 mΩ·cm2.4.The ES model is developed for the lateral SJ device.The substrate voltage sustaining layer containing the charge compensation layer is treated as a whole ES layer of the lateral SJ device to analyze its modulation to the surface SJ layer.From the ES model,the essence of the substrate-assisted depletion effect is the charge balance of the SJ layer is interrupted due to the ionized negative charges of the P-sub,which leads to the non-full depletion of the P-pillars and a decreased V_B.Then the optimized ES conditions of the electrical neutrality and the uniform ES surface electric field are produced to maximize V_B similar with that of the vertical SJ devices.The R_on,mim of both the lateral SJ devices with both the optimized and non-optimized ESs are further obtained to give the design formulas and a lateral single cell SJ device with the interface thin oxide layer is proposed.According to the R_on optimization of the lateral SJ,two lateral SJ devices are experimentally obtained: 1)The partial SOI SJ device.The silicon layer with thicknesses of 1 and 0.15 μm near the source and the drain are designed to contain the SJ layer in the thick silicon region by three times implantations.The new device combines the advantages of the low R_on of the SJ and high V_B of the thin silicon layer and thus realizes a high V_B of 977 V with a R_on reduced by 34.8% compared with the conventional thin layer SOI device.The critical electric field Ec of the silicon of the new device is up to 106.7 V / μm,which is predicted by the proposed Ec expression.2)The surface single cell SJ device on the bulk silicon substrate.The vertical single cell SJ layer is introduced at the surface of the conventional devcie,forming the full surface low-resistance path connected with the source and the drain.The test result shows that the single cell SJ device realizes a V_B of 805 V and the lowest R_on of 86.49 mΩ·cm2 among the lateral SJ devices formed by two time surface N and P-type dopings.