1. Introduction
Although silicon carbide (SiC) power devices have been proven to be stable and reliable for high temperature operation, the present control and drive circuits are based on silicon material, which restricts the maximum operating temperature of the power electronics system[1]. Developing SiC integrated circuits (ICs) is beneficial for realizing the full potential of SiC material in high temperature applications[2-6]. Compared with other technology, the complementary-metal-oxide-semiconductor (CMOS) technology could achieve full rail-to-rail output voltage switching, low power losses and temperature-independent logic levels[7, 8]. Unfortunately, the reported SiC CMOS ICs in the literature are fabricated using specially developed technology, which is not compatible with the mainstream 4H-SiC power device processing technology.
Motivated by developing 4H-SiC based power ICs which integrates the control circuits and power device in a single chip, this article reports the implementation of fundamental CMOS-based digital gates based on 6-inch SiC wafer processing technology, in which the digital gates and power devices are fabricated simultaneously. The fundamental characteristics of the inverter and NAND gates are characterized and analyzed. The experimental results that are obtained by this work will serve as useful guides for driving the development of SiC-based power ICs.
2. Device fabrication
The CMOS circuit requires both N-channel MOS (NMOS) and P-channel MOS (PMOS) devices. To implement the SiC NMOS and PMOS while ensuring compatibility with the SiC power device processing technology, the CMOS configuration in Fig. 1 is adopted [7, 8]. The PMOS is formed directly in the drift layer, while the NMOS is fabricated in a P-well. The P-well also functions as a base layer for the SiC power MOSFET.
In fabricating the SiC power MOSFET, only one thick metal is used for the cell interconnect [9]. In this work, a metal interconnect strategy is proposed to implement the CMOS circuits. The proposed metal interconnect strategy is shown in Fig. 2. The polysilicon acts as the gate (G) and the ohmic metal is extended to the other side to form the source (S), as shown in Figs. 2(a) and 2(b), respectively. After depositing the interlayer dielectric, contact holes are etched, as shown in Fig. 2(c). After sputtering the thick Aluminum metal, the wet etching technique is used to form the three electrodes, as shown in Fig. 2(d). The fabrications of SiC NMOS and PMOS are thus finished. The channel length for both NMOS and PMOS is 2 μm. The gate oxide thickness is 50 nm.
High temperature and high energy aluminum implantations and nitrogen implantations are used to form the P-wells, P+ regions and N+ regions, respectively. The process conditions for the implantations are summarized in Table 1. The obtained doping profiles for these regions are shown in Fig. 3. The depth of P-well should be larger than that of N+ region because the NMOS is located in the P-well.
| Item | Temperature (°C) | Energy (keV) |
| P-well | 500 | 550 |
| 500 | 360 | |
| 500 | 200 | |
| P+ | 500 | 320 |
| 500 | 200 | |
| 500 | 40 | |
| N+ | 500 | 160 |
| 500 | 130 | |
| 500 | 40 |
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3. Results and discussions
Based on the mainstream 6-inch SiC wafer processing technology, we realized the simultaneous fabrication of SiC digital gates and a SiC power MOSFET. Fig. 4 gives the photos of the fabricated SiC NMOS, PMOS and the CMOS gates. As shown in Figs. 4(a) and 4(b), the drain (D), gate (G) and source (S) of the NMOS and PMOS are clearly distinguished. The Inverter gate shown in Fig. 4(c) contains both PMOS and NMOS device, and the NAND gate in Fig. 4(d) integrates four devices. The measured transfer characteristics of the PMOS and NMOS are shown in Fig. 5. The typical threshold voltage Vth of the PMOS is 8.2 V while that of the NMOS is only 2.1 V. The asymmetry of Vth between PMOS and NMOS should be attributed to the unavoidable fixed charges located at the interface of SiC and SiO2[10].
After bonding the fabricated inverter and NAND gates, the functions of the two gates are evaluated. The measured voltage transfer curves of the inverter gate at various VDDs are shown in Fig. 6(a). The inverter gate functions are as expected. We also extract the inverter gain from the voltage transfer curves and the results are shown in Fig. 6(b). It is found that the inverter gain increases with VDD. At a VDD of 15 V, the inverter gain is larger than 50, which is beneficial for fabricating more complex integrated circuits, such as a multi-stage ring oscillator.
Fig. 7 give the typical measured input-output voltage curves of the inverter gate. The obtained waveform demonstrates that the inverter gate functions well. In addition, from the voltage curves, the calculated low-to-high delay tpLH (low-to-high output transition) and high-to-low delay tpHL (high-to-low output transition) are 49.9 and 90 ns, respectively. The measured dynamic switching characteristic of the NAND gate is shown in Fig. 8. Obviously, the obtained results verify the normal operation of the NAND gate.
The fabricated SiC power MOSFET is of a trench structure. The related experimental results have been reported in Ref. [11]. Based on the proposed metal interconnect strategy, the SiC IC gates and trench power MOSFET are fabricated on the same wafer. It is expected that SiC power ICs which integrate the SiC ICs and a power MOSFET will be realized in future work.
4. Conclusion
In this article, 4H-SiC CMOS digital gates are successfully fabricated based on the mainstream SiC processing technology. The functions of the inverter and NAND gate are verified through experimental measurements. Based on these fundamental gates, more complex SiC CMOS based integrated circuits can be fabricated. Moreover, the simultaneous fabrication of the SiC CMOS circuits and power MOSFET can be realized. Therefore, the results of this research can provide valuable guides for the future fabrication of power integrated circuits to realize the full potential of SiC power devices in high temperature applications.
Acknowledgements
The authors would like to thank State Key Laboratory of Advanced Power Transmission Technology, Global Energy Interconnection Research Institute Co. Ltd. for the fabrication of the SiC trench MOSFET and Datang Microelectronics Technology limited Company for wafer dicing.
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