Silicon carbide (SiC) is the only wide band-gap semiconductor that can be thermally oxidised to form a native oxide insulator, which has the same properties as the SiO2 on
Si. This oxide can form a chemically and thermally stable insulation layer as part of the SiC MOSFET structure. It can also be used as a passive oxide film for passivation and edge termination. The thermal oxidation kinetics and the SiO2/SiC interface is inferior to
that of SiO2/Si interface, which the interface traps density for SiC is at least an order of
magnitude higher than the Si [74]. This Section discussed different oxidation techniques that used in this work for fabricating reliable power devices such as diodes and MOSFETs.
2.5.1
Thermal Oxidation Process
The growth process of SiO2 layer on silicon carbide is similar to the growth on silicon,
except that the growth rate of silicon carbide is much slower and requires higher tem- peratures. Thermal oxidation of silicon is usually performed at a temperature below 1200◦C, and dry oxidation temperature for silicon carbide is typically between 1000 and 1300◦C [75]. The oxidation process can be carried out in a dry or wet atmosphere. The difference between these two processes are the oxide growth rate and the oxide dielectric breakdown strength. Dry oxidation has higher quality and hence the dielectric breakdown strength is also high, but growth rate is slower. In contrast, the wet oxidation provide a bad interface and oxide quality but faster growth rate [75]. Since the interface and oxide quality is crucial for the performance of SiC power MOSFET, dry oxidation is preferred oxidation process for our work.
1200◦C) is achieved by supplying an oxygen gas to the high temperature dry environment (>1000◦C). Dry oxidation results in a very good interface quality (1011−1012 cm−2eV−1
for 4H-SiC) and high dielectric breakdown (11-12 MV/cm) [75]. Depending upon the oxygen concentration, high temperature oxidation of silicon carbide may be either active or passive. When the oxygen pressure is less than one bar, it is regarded as active oxidation according to the following equation:
SiC(s) + O2(g)→SiO(g) + CO(g) (2.38)
SiO formed gets vaporised after its formation leading to loss of mass according to the oxidation model [76]. There are three chemical reactions in the case of active SiC oxidation, which are SiC with SiO2, SiC with O2 and SiC with H2O for the case of wet
oxidation. All of them result in the formation of SiO and CO gases. These gases react with oxygen to produce CO2 (gas) and SiO2 (solid) which is deposited on the oxide interface.
In the case of passive oxidation, SiO2 is directly produced from the first chemical
reaction with O2 in the oxidation environment. There are no intermediate reactions or
product such as SiO. Passive oxidation occur by the following reaction
2SiC(s) + 3O2(g) →2SiO2(s) + 2CO(g) (2.39)
1300◦C, 1400◦C and 1500◦C. The details of oxidation procedure is discussed in Chapter 6.
2.5.2
Novel High Temperature Oxidation
A typical oxidation furnace which designed for silicon has maximum temperature of about 1200◦C. However, a higher oxidation temperature is usually required for SiC because the oxidation rate is much slower for SiC compare with for Si [51, 52, 77, 78]. It has been reported in some literature that oxidation at 1200◦C results in the formation of high defect density near the SiO2/SiC interface. Although the origin of these interface trap
charges which lead to high interface trap density and degrade the channel mobility is not well understood, it is believed that the carbon dimer [79] in the SiC and intrinsic oxide defects [80, 81] are responsible. Many researchers have mainly focused on passivating the traps after oxide growth with nitrogen or phosphorus to reduce the Dit and also increase
the field effect mobility [16,31,51,52]. Dry oxidation of SiC at high temperature (≥1200◦C) was also reported in the literature to reduce the Dit [82, 83], although [83] indicates that
the corresponding MOSFETs have a mobility of about 2 cm2/V.s. Recent work by our research group in the University of Warwick has shown that Dit can be reduced to about
2 cm2/V.s by oxidation at higher temperature (1500◦C) under a low oxygen flow rate [32].
MOSFET fabricated under 1500◦C in an ambient of 7% oxygen and 93% argon results the peak field effect mobility of approximately 40 cm2/V.s [33].
2.5.3
Post Oxidation Annealing
Post oxidation annealing after the oxide grown can effectively passivate the interface traps between the SiC/SiO2interface. Post oxidation annealing in many different ambients have
been reported in literature such as argon (Ar), hydrogen (H), helium (He), nitric oxide (NO), nitrous oxide (N2O) and phosphorous [16, 46, 53]. Nitrous oxide and phosphorous
passivation on 4H-SiC MOS capacitor and MOSFET are the main focus of this work. The theory of N2O passivation is that the Si ≡ N bonds that are created at the
SiO2/SiC interface can (a) passivate the interface traps due to dangling and strained
bonds and (b) act as a barrier for removal of carbon-oxides and other complex silicon oxides compounds. The incorporation of nitrogen during post oxidation annealing can also reduce stress between SiC and oxide because of the large mismatch between 4H-SiC and SiO2 as discussed in as discussed in [84].
Phosphorous passivation in a P2O5 ambient which converts the SiO2 layer to PSG
(phosphosilicate glass) [53] has shown to have best result so far in this work with peak mobility up to approximately 80 cm2/V.s on 4H-SiC MOSFET(0001) face with Al+ im-
planted P-body. Details of fabrication and characterisation results on 4H-SiC MOS ca- pacitors and MOSFETs are shown in Chapter 6 and Chapter 7.
2.5.4
N
2O Grown Oxide
Direct growth oxide under N2O environment has also been investigated and compared with
other post oxidation annealing techniques. Some authors [85] have reported that oxides grown in N2O have better reliability under high-field stress compared to those annealed
in N2O on 6H-SiC. And [16] also support this finding that the direct grown oxides in a
nitrogen rich ambient exhibit better electrical properties compared to those annealed in N2O. However, [46] indicates that results on oxide grown on 4H-SiC in 100% pure N2O
on SiC were not encouraging, as Dit and the near-interface trap density were increased,
whereas the oxide grown in diluted N2O results in improvements. This suggests that the
rates of carbon accumulation and carbon removal are closer in diluted N2O. Diluted N2O
(20% N2O and 80% Ar) was used in this work because of the flow rate limitation of N2O
gas (maximum 1 L/min) in our clean room. To prevent O2 in the air entering into the
furnace, the N2O oxidation process is mixed with Ar gas. Oxide grown under diluted N2O
environment in Ar for 4H-SiC MOS capacitors and MOSFETs have been fabricated and electrically characterised. Results are shown in Chapter 6 and Chapter 7.