Improving Breakdown Voltage for a Novel SOI LDMOS with a Lateral Variable Doping Profile on the Top Interface of the Buried Oxide Layer

In order to achieve a high breakdown voltage (BV) for the SOI (Silicon-On-Insulator) power device in high voltage ICs, a novel high voltage n-channel lateral double-diffused MOS (LDMOS) with a lateral variable interface doping profile (LVID) placed at the interface between the SOI layer and the buried-oxide (BOX) layer (LVID SOI) is researched. Its breakdown mechanism is investigated theoretically, and its structure parameters are optimized and analyzed by 2D simulation software MEDICI. In the high voltage blocking state, the high concentration ionized donors in the depleted LVIDmake the surface electric field of SOI layer (E S ) more uniform and enhance the electric field of BOX layer (E I ), which can prevent the lateral premature breakdown and result in a higher BV. Compared with the conventional uniformly doped (UD) SOI LDMOS, E I of the optimized LVID SOI LDMOS is enhanced by 79% from 119V/μm to 213V/μm, and BV is increased by 33.4% from 169V to 227V. Simulations indicate that the method of LVID profile can significantly improve breakdown voltage for the SOI LDMOS.


Introduction
SOI technology has been widely used in high voltage ICs due to its advantages such as superior isolation, high speed, and low loss [1].However, SOI power devices suffer from a low vertical BV.A feasible way to increase the BV is to enhance the dielectric layer field (ENDIF), and several new structures have been proposed by ENDIF, in which introducing interface charges is effective and attractive [2].The variable doping profile is a method of introducing interface charges which was firstly proposed to avoid high voltage breakdown for planar junctions in 1985 [3].The approach of designing and implementing the linear doping profile on thin SOI for lateral high voltage devices was presented in 1995 [4].A submicron thin film SOI LDMOS with the variable doping profile and numerical modeling of linear doping profiles were proposed in 1996 and 1999, respectively [5,6].After 2000, several new SOI devices structures with linear doping profile were proposed [7][8][9][10][11].
In this paper, a novel SOI device with a LVID profile (LVID SOI LDMOS) is designed.The doping profile of drift region is a combination of linear variable doping and uniform doping in vertical dimension, which is different from the above-mentioned structures.The influence of the parameters on BV and the specific on-resistance ( on,sp ) is presented.The simulated results indicate that the method of LVID profile can significantly improve BV compared with UD profile.

Structure and Mechanism
The cross-section of LVID SOI LDMOS is illustrated in Figure 1(a).The LVID profile is placed at the interface between the SOI layer and the BOX layer, the thickness of which is expressed as  lvid .The lateral doping concentration  of the LVID profile is shown in Figure 1(b), which can be expressed as where  is the distance away from source,  is the linear graded coefficient which determines the slope of the profile, () is the doping concentration of LVID profile, and  0 is the doping concentration at  = 0,  =   .Here,   is the thickness of the SOI layer and   is the doping concentration of uniform doping section.
According to Figure 1(b) and (1), where    is the doping concentration at  =   ,  =   .Since  0 is much less than    , it is ignored in the above expression.
is proportional to    .
The device mechanism at breakdown can be explained as follows.When a high positive voltage   is applied to the drain while the source, gate, and substrate are grounded, the LVID profile will be depleted and high concentration ionized donors are accumulated at the top interface of BOX layer just as shown in Figure 2, and due to the LVID profile, the concentration of ionized donors in LVID profile will linearly increase from source to drain.According to the theory of ENDIF, the interface charges can not only increase the electric field in BOX layer but modulate the surface electric field of drift region [2].The modulation effect is that the surface electric field peaks at source and drain are decreased and the surface electric field in the middle drift region is improved.Thus LVID profile should prevent the lateral premature breakdown.At the same time, because the vertical breakdown voltage is sustained by the BOX layer, the improved   can significantly enhance the vertical BV.The influences of the LVID profile on the breakdown performance of SOI LDMOS transistor will be analyzed and compared with UD SOI LDMOS in Section 3. The parameters used in the simulations are detailed in Table 1.Concentration of uniform doping in drift region,   (cm −3 ) Optimized The slope of the LVID profile,  (cm −4 ) Optimized    The vertical electric fields and potentials distributions for the UD and LVID SOI LDMOS at breakdown along line MN (shown in Figure 1(a)) are shown in Figure 4.It is clear that, for the LVID SOI LDMOS,   is improved by 79% from 119 V/m to 213 V/m, and BV is improved by 34.3% from 169 V to 227 V, which are both the results of the introduced interface charges in the LVID profile.

Results and Discussion
Figure 5 shows the dependences of BV on the  for different   .Simulation results indicate that there is always an optimum concentration slope for different   to make BV reach the maximum value and the optimum  decreases with the increase of   .The reason is that the total doping concentration of drift region needed for the device breakdown at the maximum BV is certain by RESURF (REduced SURface Field) theory [12].When   increases, the doping concentration of LVID profile needed for the maximum BV will decrease, which is proportional to  as defined in (2).So  is inversely proportional to   .Figure 6 shows the dependences of BV on   for different .Results indicate that there is also always an optimum   for different  to make BV reach the maximum value and the optimum   decreases with the increase of .The results accord with the inverse relationship of  and   .
Figure 7 shows the influence of  lvid on BV.It can be seen that  is inversely proportional to  lvid , which means the optimum  decreases with the increase of  lvid .This is also because the total doping concentration needed for the maximum BV is certain.Simulations show that when  lvid is less than 0.5 m, BV can reach maximum values above 210 V for different  lvid and attains 227 V at  lvid = 0.1 m.When  lvid is greater than 0.6 m, BV falls quickly and drops to 184 V at  lvid = 1 m.BV remains in about 180 V within 1 m to 2 m.It can be concluded that  lvid has a great influence on BV within 0.5 m and has a little influence on BV when  lvid is greater than 1 m.Figure 8 shows the influences of  lvid on BV and  on,sp .The results are obtained in the case of the optimized devices for the highest BV for different  lvid .It can be seen that BV and  on,sp both decrease with the increase of  lvid .Figure of Merit (FOM) is a value calculated by BV 2 / on,sp which balances the trade-off between BV and  on,sp [13].The optimized FOM is 6.22 MW/cm 2 at  lvid = 0.1 m when gate voltage is 15 V.  on,sp is calculated by   ×   /  at the linear region of - curve and is 8.28 mΩ⋅cm 2 at  lvid = 0.1 m.
Figure 9 shows the major fabrication steps for an LVID SOI wafer: (a) implanting arsenic to form the LVID profile region on the bottom of the SOI layer; (b) thermally growing to form the buried oxide layer on the p-type substrate; (c) bonding the SOI layer and the substrate layer; (d) annealing the wafer and then thinning and polishing the wafer.The LVID profile can be achieved by using the lateral variation doping technique [4,9].It can be seen that only an additional process of arsenic implanting before bonding is needed to form LVID SOI wafer.Other fabrication processes are fully compatible with conventional CMOS/SOI technology.

Conclusion
The LVID SOI high voltage device structure is proposed.The proposed structure increases greatly the BV because of the modulation effect of interface charges on the electric fields in the BOX layer and drift region.Compared with UD SOI LDMOS,   and BV of LVID SOI LDMOS are improved by 79% and 33.4%, respectively, which proves that the LVID profile is an effective method to improve the BV for SOI high voltage devices.

Figures 3 (
Figures 3(a) and 3(b) are the equipotential contours distributions at breakdown for the two devices.It can be seen that, compared with the UD SOI LDMOS shown in Figure 3(a), the equipotential contours of the LVID SOI LDMOS shown in Figure 3(b) are more uniform and denser.The surface electric field distributions at breakdown for the two devices are shown in Figure 3(c).It is clear that the   of LVID SOI LDMOS is optimized and more uniform, and the   at  =   /2 is improved by 111% compared with UD SOI LDMOS which means an increase of lateral BV, and the shadow part is just the improvement value of lateral BV.The optimization of   and improvement of lateral BV are the results of the high concentration ionized donors distributed in the LVID profile as shown in Figure 3(d), the inset of which is the lower ionized donors concentration distribution of UD profile.The vertical electric fields and potentials distributions for the UD and LVID SOI LDMOS at breakdown along line MN (shown in Figure 1(a)) are shown in Figure 4.It is clear that,

Figure 8 :Figure 9 :
Figure 8: The influences of  lvid on BV and  on,sp .

Table 1 :
Device parameters used in the simulations.