(Invited) Three-Section Adjusted Field Limited Rings Applicable for SiC 2200V Power MOSFETs
Authors: Li Xuan , Yang Wenchi , Li Lijun , Deng Xiaochuan , Zhang Bo 
Published: February 2020

Due to excellent intrinsic properties of silicon carbide (SiC), SiC power metal oxide semiconductor field effect transistors (MOSFETs) potentially become a desirable candidate featuring high blocking voltage with ultra-low conduction resistance for high efficiency power applications [1]. In order to achieve expected avalanche breakdown in SiC device, junction termination techniques are paramount to alleviate the crowded high electric field at the periphery of active regions [2]. Previous works related with various SiC junction termination techniques mainly focus on junction termination extension (JTE) and related variations, like hybrid, bevel-assist, multiple-shallow-trench, and counter-doped JTE [3-5]. However, little reports on field limited rings (FLRs) technique for SiC power device are released. Actually, the FLR technique features less tight process control and desirable reliability, benefiting SiC-based industrial power applications.

In this paper, three section adjusted field limited rings (TS-FLRs) are proposed and fabricated for 2200V rating for motor extraction applied in high-speed trains and hybrid/electric vehicles. The proposal not only avoids tight control of etching depth and ion implantation dose required by aforementioned multiple JTE structures, but also shows a desired process tolerance without complicated designs and extra process step for the TS-FLRs.

The schematic cross-section view of the proposed TS-FLRs is fabricated on an 18μm thick drift layer of with doping concentration of 5E15 cm-3 as shown in Fig. 1. The TS-FLRs are implanted with formation of P base region of active region in same process step, followed by an 1800 degree C high temperature activation. Then a thick passivation layer is deposited on the termination region to effectively avoid influences from following processes. The TS-FLRs is designed to three sections characterizing space of FLRs S1 and width of FLRs W1 in section I, S2 and W2 in section II, as well as S3 and W3in section III for substantially reducing design complex of FLRs.

Firstly, due to formation at same fabrication step, the total implantation dose is carefully considered for both BV and V
th. TS-FLRs have an evidently better stability based on different total doses than that of fixed space and same width ones (F-FLRs) as shown in Fig. 2. Moreover, the TS-FLRs evidently show stable breakdown performances dependent on various spaces while a weak tolerance of space definition of F-FLRs.

Furthermore, the influence of TS-FLRs with different widths of ring and with different max energies of implantation on breakdown voltage is evaluated, combining analysis of electric field distribution inside termination region by Sentaurus simulation.

After optimized design, TS-FLRs and F-FLRs are fabricated at same process level with SiC power MOSFETs as shown in Fig. 3. Fig. 4 shows that the breakdown voltage of the MOSFETs with TS-FLRs is as high as 2500V, a significant enhancement in comparison with F-FLRs.

Acknowledge: The authors thank China Railway Rolling Stock Corporation.

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Figure 1