Study Of Electric Field Modulation And Novel Lateral High Voltage Devices

LDMOS (Lateral Double-diffused MOSFET) is a key device in HVIC (High Voltage Integrated Circuit) and PIC (Power Integrated Circuit). In order to compatible with low-voltage circuit, novel LDMOS with thin epitaxy layer is a developing trend. SOI (Silicon-On-Insulator), which is a mainstream of the 21st integrated technology, is applied to the new style HVIC and PIC owing to the improved isolation, reduced leakage current, high speed performance, low power dissipation and perfect irradiation hardness. It is a hotspot for many scholars to improve the trade-off between the breakdown voltage and specific on-resistance of LDMOS. The vertical breakdown voltage is low as applying thin epitaxy layer or SOI substrate in LDMOS. The application of SJ ( Super Junction) LDMOS is restricted due to the effect of substrate-assisted-depletion in the conventional SJ LDMOS when the super junction layer is adopted firsthand.The ideas of this paper are applying the effects of electric field modulation and charge shielding to optimize the surface and bulk electric field of LDMOS. A novel technology, which is called substrate terminal technology for the first time in this paper, is different from the conventional surface terminal technology in which the surface of device is processed. Its key is the modulation effect of the bulk electric field by rebuilding the substrate. In order to design an LDMOS with certain breakdown voltage in ultra-thin epitaxy layer, REBULF (REduced BULk Field) technology is put forward for the first time. ENDIF (ENhanced Dielectric layer Field) technology is proposed in this paper to break through the vertical breakdown voltage of SOI LDMOS due to the Gauss' theorem without interface charge.By applying substrate terminal technology, several kinds of devices are designed as following:(1) SBOSOI (Step Buried Oxide SOI): The surface electric field is optimized by applying the electric field modulation effect of step buried oxide, which is different from the case of the step or linear-drift-doping profiles. The results show that the breakdown voltage increases by 30-50% and the on-resistance decreases by 40-50% at the 0.2～0.8μm oxide thickness, and the breakdown voltage increases by 10～50% and the on-resistance decreases by 10～50% as the drift region thickness is less than 1μm.(2) D-SBOSOI (Double-Step Buried Oxide SOI): Based on ENDIF technology the structure is provided with the merits of the developed electric field modulation, electric field shielding by introducing interface charge and simple isolation with low-voltage circuit by step buried oxide. The results show the surface electric field in this structure reaches nearly uniform distribution due to additive electric field modulation by double-step buried oxide, which results in the electric field of 200V/μm in the buried oxide layer.(3) APSOI (Air Partial SOI): The surface electric field is optimised by applying the air gap with low permittivity. The results show only 1/4 thickness of buried layer is needed compared with normal PSOI structure at the same breakdown voltage. When the thickness of drift region and buried layer are both 2μm the breakdown voltage more than 600V can be obtained.(4) BPSOI (Buried Partial SOI): The breakdown voltage is improved resulting from the additive electric field modulation by the P-type buried layer charges. The specific on-resistance is decreased as a result of increased doping of the drift region. The results show that the breakdown voltage is increased by 52～58% and the on-resistance is decreased by 45～48% compared with the conventional PSOI in virtue of 2-D MEDICI simulation.(5) N⁺ -Floating LDMOS: The mechanism of improved breakdown characteristics of this structure is the high electric field around the drain is reduced by N⁺-Floating layer which causes the redistribution of the bulk electric field in the drift region. REBULF, which provides a novel method to improve the vertical breakdown voltage of LDMOS, is proposed after the technology of RESURF (Reduce Surface Electric Field). The critical condition of REBULF is the multiplication of the substrate's doping and the distance between N⁺ -Floating layer and the drift region, which is less than l×10¹²cm^-2. The breakdown voltage of REBULF LDMOS is increased by 60% compared with RESURF LDMOS.(6) N⁺ -Floating SJ-LDMOS: The effect of substrate-assisted-depletion is suppressed by N⁺ -Floating Layer, which results from charge imbalance between the N-type and P-type pillars. The high electric field around the drain is reduced by N⁺ -Floating layer thanks to REBULF effect which causes the redistribution of the bulk electric field in the drift region, thus the substrate supports more voltage.(7) LDMOS with folded silicon: In this kind of technology the silicon substrate surface is trenched to form folded shape from the channel to the drain electrode and the gate is extended to the drain. Two structures of SOI-FALDMOS and FALDMOS are designed by applying this technology. It indicates that the ultra-low specific on-resistance is obtained with the breakdown voltage at less than 40 volts in SOI-FLDMOS. The ideal silicon limit of the breakdown voltage and specific on-resistance has been broken in FALDMOS. The turn-off of the FALDMOS is very fast due to no minority-carrier storage in the drift region compared with LIGBT.