Analysis of Breakdown Voltage and On Resistane of Super-juntion Power MOSFET CoolMOSTM Using Theory of Novel Voltage Sustaining Layer P. N. Kondekar, Student Member ZEEE, C.D. Parikh, Member ZEEE and M. B. Patil, Member ZEEE Miroeletronis Group, Department of Eletrial Engineering, ndian nstitute of Tehnology, Bombay Abstmt-Conventional VDMOS (Vertially Double diffused Metal Oxide Semiondutor) Tehnology for power devies was onstrained by the Silion Limit. This is now improved to have linear relation between On Resistane (L) and Breakdown voltage (BV) instead of the quadrati relation. Theory of novel voltage sustaining layers (SJ- Theory) reently published analytially models the super juntion drift layers (SJ-drift layer). We have designed SJlayers based on this theory and used to onstrut the SJ- MOSFET: CoolMOS struture. The laim of the theory that the doping level in the drift layer an now be inreased by at least one order of magnitude without lowering BV is analyzed in detail. With the new silion l it we now an inrease BV of a power devie, just by inreasing thikness of the SJ-drift layer. L and BV relationship as the thikness of the devie varies is analyzed with the help of simulation. The limitations and onstraints of applying SJ-theory for the CoolMOS struture are disussed. The SJ- theory does not model the behavior of R, and BV for a fixed geometry as doping level hanges. We observed that for a fixed geometry the rate of redution of the BV depends on the ell pith. This rate is large for the higher ell pith. The effet of harge imbalane reated due the hannel region in CoolMOS is also investigated. ndex &"-Super-juntion devies, CoolMOS, Geometry fator, On resistane, Breakdown voltage, Charge imbalane. NTRODUCTON Super-juntion (SJ) MOSFET (also alled as CoolMOSq is attrating lot of attention. This onept pushed the power MOSFETs utility in higher Breakdown Voltage (BV) and high wattage (Lower on state resistane R.) appliations by breaking the so-alled Silion Limit [ R.= BV *" [l] relationship and have SJ-MOSFET with at least 5 to 100 times lower R,,,, as ompared with VDMOST. Soure Gate Soure P- P+ / n- epi n+ Drain \L...J n+ Fig. 1 Cross-setion of the SJ-MOSFET (CoolMOS) t. VDMOS DRFT LAYER VS SJ -LAYER The drift layer of onventional VDMOS (Vertially Double-diffused Metal Oxide Semiondutor) power devie an be modified to SJ-drift layer keeping its geometry and doping levels same to obtain almost double breakdown voltage (BV). Further inrease in the BV an be ahieved by simply inreasing the thikness of the drift layer fepi. This inreases on resistane (&), sine the drift layer is divided in p and n pillar as shown in the Fig.2!- i Y 83lB P' E3 18 Now a large P-N juntion (Vertial) builds the eletrial field in both horizontal and vertial diretion as the drain bias is provided. P-pillar does not ontribute toward onstate ondution but essential for ahieving higher BV despite higher doping of N and P regions.?si4-7314 P?U14 w n the off state the N and P pillars will be ompletely depleted well before BV, reduing the harge inside the depletion region. The eletri field profile beomes flat, instead of triangular as in onventional VDMOST. [l] [2] This makes it possible to have [R. = BV] linear L Fig.2 VDM0S-M layer (a) Modified to SJ& layer (b) 0-7803-7262-X/02/$10.000 2002 EEE. 1769
With doping level of SJ-drift layer inreased by one order of magnitude, the simulation result shows that at least 5 times improvement in %,. without losing muh advantage of inreased BV. This is justified analytially with help of theory of novel voltage sustaining layer [l] or Super-Juntion Theory (SJ-Theory). Design and simulation of the onventional drift layer is given in Table for BV 390V, 200 W, 10 A VDMOS power devie. This an be shown as p+ n- n+ diode for off state of the devie, as in Fig.2 (a). Maximum doping required in onventional drift layer is given by [2] [3] E 390 DFSGN ( TABLE VDMOS E, Vlm DU T LAYER &xa Simul. 2.8~10~ 0.0196 375 The %, will be approximately 2 SZ for a 10 m width of the drift layer sine the ross setional area will be lo-* m'. The major ontribution (>95%) to %, of high BV devies omes from n-drift layer [2]. %, is defined at low drain voltage V,,=lV and high gate voltage V,=lOV, so that JFET effet resistane and hannel resistane will be minimum. The eletri field along the drift layer is shown in Fig. 3. Conventional triangular eletri field profile was the main reason for the so-alled silion limit (%a = BV2.5) [4] [5]. Thus high BV power MOSFETs were not suitable to use in atual appliations due to high %,. Here a P-pillar is inserted in the drift layer forming a vertial p-n juntion, whih builds the eletri field in both vertial as well as horizontal diretion. The P-pillar does not take part in ondution when the devie is ON, but is neessary in order to inrease BV of the devie when devie is OFF. Due to full depletion of the SJ-layer at muh lower drain voltage than the BV, it will behave similar to a very lightly doped drift layer. Simulation result of SJ-drift layer with same geometry and doping profile shows that BV of SJ-MOSFET will be 560V as against 390V for VDMOS power devie. Here R,,, almost doubled due to SJ-struture. Almost 3f 2 times inrease in the doping level without reduing the BV of the devie as ompared to onventional drift layer is possible using onept of SJ-layer. Here f is Geometry fator as defined in [ 13. One order inrease of magnitude will result in almost 5 times improvement in the %, as shown analytially. The simulation of SJ-drift layer with same geometry but inreased doping by one order shows that BV remains almost same as 560V. %, will be only 0.4 52 (using equation 6 in next setion) as against 2 SZ in this ase. The eletri field profile and potential ontours at high voltage near breakdown are shown in Fig. 4 and 5 respetively. 0 3.5&, 2e5 Eletrial Field in the VDMOS Drift Layer for 379-25.5 ' o 5 io ij 3 25 30 Vertial Distqne in the Drifr-layr in mirons Fig.4 Eletri field profile in SJ-Drift layer along the line shown in the fig.2 (b) Conept of SJ-layer. Fig. 3 Eletri field along the line shown in fig.2 (a) Keeping geometry and doping profile same the n-drift layer of VDMOS is now modified to SJ-layer as shown in Fig. 2 (b) We an see that the eletri field profile beomes flat one the harge inside the drift layer redues due to depletion region aross vertial p-n juntion. Sine the width of p and n pillars are very small as ompared with height, the horizontal depletion takes plae at lower drain voltage. One the drift layer is fully depleted, the field inreases vertially predominantly as drain voltage inreases till it reahes E, (Critial Eletri Field) where impat ionization is triggered. The potential ontours and uniformly distributed throughout the drift layer suggesting flat eletrial field profile. The intensity of ontours redues as the drain bias inreases in Fig. 5 1770
Theoretially, one an predit 50% improvement in BV assuming ideally flat eletri field profile. Simulation result shows 45% inrease in BV. Unfortunately, sine the drift layer is now divided equally in P and N pillars, the & will be doubled as ompared to VDMOS. The simulation of SJ-drift layer with same geometry but inreased doping by oneorder shows that BV remains almost same as 560 V, thereby reduing the k, by at least a fator of 5. pillar (kpi) one an hange BV of the devie. This is also observed with simulation. From the boundary ondition used we an find the thikness of the drift layer required as Then from the initially assumed value off we an find out the ell pith from equation 2. The maximum doping level required for this is given by Here, for same bpi in VDMOS and SJ drift layers, to obtain the same E,, we an find ratio Ns, N,, =3f 12. Thus the doping in SJ-layer is higher than the onventional and depends on the geometry of the devie. Thus for f =O. 1, N,, = 15 NCO, for f = 0.2, Nsj = 7.5 N, and for f =0.3, we get N, = 5 Nm. Pratially the value of E, itself is very high in SJ-layer as ompared to onventional [4]. The value of Area Speifi On Resistane &A in terms of the geometry and BV is given by Rod= 2.6~10-~C, BVam' (6) Fig. 5 Uniformly disbtibuted Potential Contours in the SJ-Driftlayer near breakdown We have tabulated the design for different BV using three geometry fators as follows. TABLE H. DESGN AND SMULATON OF SJ-DRFT LAYER Theory of a novel voltage-sustaining layer [l] gives analytial modeling of the SJ-drift layer. A simple design methodology an be formulated on the basis of this theory. Here the SJ-drift layer having P and N pillars with uniform doping is modeled as unitary funtion. Two-dimensional Poisson's equation with boundary onditions for the full depletion of Slayer is solved. 700 5.54~10'~ 3.4~10~ 40.75 5.49 67.16 900 3.96~10'~ 3.2~10~ 54.63 7.36 122.77 TABLE The y-omponent of the eletri field is important from design point of view. Geometry Fator ( f ) is defined as ratio of ell pith to the thikness of drift layer and an be approximated as in equation 2 if hpil 2% >1 Volt 400 500 700 900 tm3 v/m pm pm dm 2 4.91~10'~ 3.4~10~ 23.14 6.23 45.51 3.65~10'~ 3.3~10~ 30.02 8.09 77.75 2.33~10'~ 3.1~10~ 44.45 11.9 174.3 1.66~10'~ 3.0~10~ 59.59 16.1 318.66 The ritial eletri field in the layer is given by E=1.24x106BV" (1 +21.7f)-1/6 (3) Thus we an alulate E, for required BV by assuming $ t an be observed that ritial field E, for the devie depends on the BV and geometry. f Wn=Wp is kept fixed i.e. if ell pith is onstant then by hanging the height of TABLE.V Forfi0.3 BV N E, Volt /m3 Vm 24.49 9.90 80.95 31.77 12.8 138.30 1771
Simulation These drift layers were simulated using SE-TCAD [7] and ompared with analytial plot as shown in Fig. 6. Sine SJ-theory gives different maximum N for different dimensions, a first order omparison is possible, we an see that for the 60.1 the simulated BV are quite lose to analytial but for as f inreases the simulated BVs are smaller than the analytial. This is beause if t.pi 2% approah unity, the ritial eletrial field in the devie given by equation 3 will be smaller. Physially this means that the harges from the top and bottom omers near juntions ontribute to vertial field before omplete depletion of the drift layer forming P+N-N+ type of field. This redues the BV of the devie. Thus we see larger departure of BV from analytial as the ross setional area of the SJ-drift layer inreases. For smaller f the doping level are higher for same BV, suggesting lower R,,,. l i P.PULAB DBUT LAYER l&b DEAN CQNlAC Fig.7 The ross-setion of the CoolMOS used for simulation. AU dimensions are in mirons 600 9 f-i 400. 1,. - 2.O~lo'~ 4.0~10'~ 6.0~10'~ 8.0~10'~ 1.0~10'~ Doping Conentration /m3 Fig. 6 Analytial and Simulated BV of SElayers V. SJ-LAYER USED FOR SJ-MOSFET The SJ-layer designed is used to simulate the CoolMOS struture as shown in Fig.7 using SE- TCAD. We have simulated the struture with C, = 5pm. n the figure, it is shown that keeping the ell-pith onstant, we vary the thikness of the drift layer in order to study the BV and Ron. The operation of the CooMOS is same as VDMOS. When the devie is ON the onduting hannel is farmed in the P-body and eletrons flow toward drain vertially down toward the drain. The threshold voltage alulation is same as the VDMOS [3] and is about 3.5 V. The simulated &.V, harateristis show it as approximately 3.3 V. The BV of this devie as LPi inreases (keeping doping level fixed and ell pith onstant) is simulated and studied here. The analytial alulation based on SJ-theory for the same are shown in Table V. bpi lun 20 30 40 50 60 f BVVol. R,,, mszm2 N A S A S /m3 0.18 356 330 3.09 4.29 6.31~10'~ 0.12 527 475 4.53 6.25 6.22~10'~ 0.09 693 610 5.95 8.35 6.15~10'~ 0.07 857 740 7.36 10.22 6.08~10'~ 0.06 1017 880 8.78 12.15 6.01~10'~ From the plots shown in Fig. 8 we an see that the eletri field profile gets strethed as bpi inreases (Keeping doping level same), indiating proportional inrease in BV Thus we an onlude BV = tepi along the vertial left. edge of the devie 0 10 20 30 40 50 60 SJ-layer height in ym Fig 8 Eletrial field profiles as thikness of drift layer inreases 1772
From the plot as shown in Fig 9 simulated %, is quite high as ompared to analytial predition by SJ-Theory. This is expeted beause SJ-Theory neglets the depletion width at drain voltage of 1V and hannel resistane and JEFET resistane. %, is defined as the slope of the d-vd urve in the linear region i.e. when Vds = 1V and V, =10 V. We an learly observe the new silion limit here i.e. %, = BV instead of onventional %, a BV2.5. N-drift layer simultaneously. Here, we have defined Nno-d as the same level of doping i.e. N=P=Nno~~. We have simulated CoolMOS strutures for three different ell pith and the eletrial field profile along the vertial left edge of SJ-MOSFET as doping level (Nno-d ) inreases are studied. Simulation Results g 0.20.- 0 E 0.18.. --Analytial simulated for N=6e15 For a fixed ell-pith q=5 pm and t.pi =40 pm, is observed that, as the doping Nu- is inreased from l~lo'~t0 6 ~lo'~,%,drops rapidlyas shown in Fig.11. 400 600 800 Breakdown Voltage (Volts) Fig. 9 & resistane Vs Breakdown Voltage &-V, harateristis in the linear region are shown in Fig 10. The 4-V, urve shows that the threshold voltage is as alulated 3.2V and does not get affeted by inreasing the thikness of the SJ-layer. The saturation drain urrent is high in ase of the 20pm devie, sine %,,is very low. 1.OX1 0-5- 2 Vds=l Volt 1.- 2 3 34.0~10 4.- 5 m 3 2.0x10' 0.0 L 0 1 2 3 4 5 Gate Voltage Vds in Volts Fig. 10 av, in the linear region as t i inreases V. DOPNG LEVEL VARATON FOR A FXED GEOMETRY The theory of novel voltage sustaining layer [ 11 does not provide insight into the variation of Breakdown Voltage and L, if the geometry of the devie is fixed and the doping level is varied in the SJ-drift layer i.e. P-pillar and Doping Conentration in m' Fig. 11 The plot of N vs. BV and &for a fixed geometry SJ-MOSFET Sine the depletion width of the juntion is proportional to N-"', it will be more at lower doping level; this redues the ross-setional area of the onduting hannel inreasing &. As the doping inreases, depletion width of the juntion redues, ausing available ross setion area to be more. %, rapidly drops to lowest possible value at highest doping level. We an see BV redues almost linearly for this ase. We have observed that area under the eletrial field urve (BV) redues at faster rate in higher ell pith. We have given these plots of eletrial field along the left vertial edge of the devie, here in Fig 12 (a), (b) and (), Where we see that as the ell pith beomes smaller the rate of redution of the area under the urve i.e. BV redution beomes negligible with inreasing Nno-d. This observation also helps in designing the devie geometry for optimum breakdown voltage. The rate of redution of BV with Nno-d depends on ellpith. For larger ell-pith with higher doping the amount of harge from the drift layer ontributing to vertial field 1773
omponent will be large and this auses to trigger impat ionization proess at lower drain voltage. Thus, we have to use lower doping in order to get higher BV for large ellpith; this is not suitable to obtain lowest possible &,. Only Cp=5 pm and Cp=lO pm wil be suitable for devie design. Simulation results for the rate of redution of BV as Nn-d inreases by one order are shown in Fig. 13 L 1 3.0~101 h E 2.5x10' Y - 2.0X10'- a E 1.5~10'- m 0.- z 1.0X10'- 0 a, iij 5.0~10~- 0.0 t 0 10 20 30 40 Vertial Distane in SJ-layer km Fig. 13 Simulated BV as the doping level hanges by one order for different C, V. STATC CHARGE 50x10 N=le14 5 0.0 0 10 20 30 40 = E P 2.0x10.- 1.5~10 5.0~10 Vertial Distane in SJ-layer in pm C,=5 pm tea=40km Fig. 12 (b) 0.0 J 1 0 10 20 30 40 Vertial Distane in SJ-layer in pm (C) Fig. 12 a, b, Eletrial field profile along the left edge of SJ-MOSFET for various ell pith The SJ-theory assumes perfet harge balane [6] between the P-pillar and N-drift layer as in perfet symmetrial SJ-drift layer in Fig.2 (b) for obtaining maximum BV. The nek region (top of the N-drift layer) of CooMOS reates harge imbalane due to whih, N-drift layer harge is larger than the P-pillar. This affets the net obtainable maximum BV at a speified doping N. The harge imbalane due to top nek region an be estimated for the CoolMOS. One of the reasons for lower BV in simulation is the harge imbalane. To improve BV there are two ways, one is to redue the doping level in both P- pillar and N-drift layer, whih wil redue BV sensitivity and thereby inrease BV. This will inrease the &, whih is not desirable. Another way is to redue harge imbalane by inreasing P-pillar doping only to get maximum BV. The ideal SJ-drift layer as shown in Fig 2(a) gets modified in the atual CoolMOS struture as shown in Fig. 7. Charge imbalane is reated due to hannel region. The ideal symmetrial SJ-drift layer of 40pm thikness and Cp=5 pm is simulated and a CoolMOS struture using this drift layer is also simulated with hannel length of 1.8 pm and juntion depth of p-body 3 pm. The ompensation degree is defined in this ase as CD % = (N-P)/ N,,-d*lOO. The doping level of the P-pillar and N-pillar is varied and off-state breakdown voltage is observed as shown in Fig. 14. 1774
An inrease in the P-pillar doping orresponds to a negative value of CD and that in the N-pillar doping to a positive value of CD. The SJ-drift layer due its perfet symmetry as shown in Fig.2 @) gives maximum obtainable BV at N=P i.e. CD = 0%. nrease in doping level on either side will redue the BV, sine the net harge imbalane inreases the y-omponent of the field ausing impat ionization proess to trigger at lower drain voltage. The most important observation here is that, for CoolMOS struture the harge imbalane urve shifts left as ompared to ideal SJ-layer. The maximum obtainable BV from CoolMOS is not at N=P but at P>N by approximately 3.2%. This learly indiates that due to the nek region or hannel region the amount of harge in N- pillar is slightly more than P-pillar. n order to balane this surplus harge laterally, we have to inreases the P- pillar doping by 3.2%. The maximum obtainable BV in ase of CoolMOS is slightly less than the SJ-layer beause the net P-pillar height is redued due to the P-body diffusion. REFERENCES X. B. Chen and P.A. Mawby, Theory of a novel voltagesustaining layers for power devies, Miroeletroni Journal, 1998, ~01.29, pp.1005-1011. D. A. Grant, Power MOSFETs Theory and Appliations, John Wiley & Sons. 1989. B. J. Baliga, Modem Power Devies, John & Wiley n.,1987. L. Lorenz, M. Mar, J.P Stenil and A. Bahofner, Drasti Redution of On-Resistane with CoolMOS, PCM Europe, vo1.5, 1998. G. Deboy et. al., A new generation of high voltage MOSFETs breaks the limit of silion, Pro. EDM, p.683, 1998. [6] P. Shenoy, A. Bhalla and G. Dolny, Analysis of the effet of harge imbalane on the stati and dynami harateristi of super juntion MOSFET, Pro. SPSD 98, pp.99-102, 1999. [7] ntegrated System Engineering, SE-TCAD Manuals, AG, Zurih, Switzerland 1999 The detailed stati harge imbalane and its simulation and results will be published elsewhere. - 1 1-10 -5 0 5 10 GmpematimDegeea)%=(NP)/N* 100 Fig.14 The off-state harge imbalane urve ACKNOWLEDGMENT We would like to express sinere thanks to General Eletri Company, Shenetady USA for their finanial support. 1775