4-1 Conclusions
In this thesis, the Dit of MOS capacitors and of MOSFETs are studied. The high Dit of sample WO is because that carbon clusters at interface after thermal oxidation process. We also fabricate gate dielectric by depositing a PE-oxide (sample PE) and N2O oxidation at 1100 ℃ (sample N4 and N0.25). We expect these low thermal budget methods would reduce Dit. But the Dit at the energy level of EC-E=0.2 eV of sample PE, N4 and N0.25 are at the same order of magnitude as sample WO.
The high Dit of sample PE may be attributed to the poor atomic bonding between PE-oxide and 4H-SiC. Sample N4 has high Dit since the temperature of the N2O oxidation is not high enough so that nitrogen atom wouldn’t passivate the interface effectively. The reason of sample N0.25 has high Dit might be the N2O oxidation time is still too long so that SiC/SiO2 interface has already formed carbon cluster and the oxidation temperature is not high enough so that nitrogen passivation does not occur.
We want to understand the thermal stability of the hydrogen passivation effect after additional high temperature processes. Ammonia plasma treatment can reduce the Dit, the passivated interface can’t sustain processes with temperatures higher than 500 C. Therefore, NH3 plasma treatment after device fabrication should be evaluated.
We fabricated the MOS capacitor which has similar gate structure of MOSFETs.
We wanna evaluate if the NH3 plasma treatment after gate electrode formation still could improve Dit or not. Samples have the structure of 150 nm-thick-SiO2/150 nm-thick-poly-Si/30 nm-thick SiO2 which than exposed ammonia plasma treatment
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by different time. The result shows that post-gate-patterning NH3 plasma treatment still can improve Dit if only if the exposure time is long enough. Hence we could use this method in the MOSFETs fabrication process.
MOSFETs were fabricated by coating different PRs which as the high temperature graphite capping layer. The surface roughness of samples were coated by FH-6400 PR is smoother than that coated by S-1813 PR. This is because the graphite capping layer formed by coating FH-6400 is thicker than S-1813 which leads to better protective effect. Hence, sample FP has higher channel mobility than sample SP.
However, our samples have very low channel mobility and only slightly improvement channel mobility after ammonia treatment. These results have three reasons. The first reason is the ability of using ammonia plasma treatment to reduce Dit is not significant.
The second reason is the surface roughness caused by high temperature dopant activation. The third reason is near interface oxide traps may result in coulomb scattering. We expect better surface roughness by slightly chemical etching SiC surface before depositing field oxide. However, the surface becomes rougher after growing dielectric layer compared with those samples without etching process. The higher surface roughness of sample FEP and FE leads to worse channel mobility.
Ammonia plasma treatment can passivate the interface defects so that the VT is reduced and the SS is improved for sample FEP.
S/D series resistance is 107 ohm. This is because that the heavily doped region has almost been exhausted after sacrificial oxidation and gate oxidation processes.
This problem leads to the abnormally high series resistance. Despite of the series resistance is 107 ohm, the VDS almost drops on the channel region. That is, the inversion channel region has higher resistance than that of S/D region due to the low mobility.
The result of high temperature measurement shows that the channel mobility will
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enhance with higher temperature which is contradiction to Si. This is because the mobility is dominated by the coulomb scattering mobility, which has positive temperature dependence. VT becomes smaller with higher temperature. It’s because the reduction of trapped negative charges.
Body effect measurement shows the channel mobility will decrease with the more negative VB bias. This is because as VBS becomes more negative, a stronger would exert on the electrons in the inverted channel. Hence, electrons will suffer stronger surface scattering and coulomb scattering which lead to mobility degradation.
This is the first time for our group to fabricate 4H-SiC MOSFETs. Our 4H-SiC MOSFETs are needed to be improved in the future. From the results we realized the mechanism of leading mobility degradation. Coulomb scattering caused by high Dit
and surface roughness are the most critical factors to mobility degradation. Sample FP has highest mobility, because it has smoothest surface roughness and lowest Dit. We expect better performance of our 4H-SiC MOSFETs by solving these problems in the future.
4-2 Future works
In this study, important results have been summarized in previous section.
However, many works are worthy for further investigation. They are listed here.
1. The source of the negative charges of sample H800 and H1000 are not clear at this moment. Further material analysis will be performed in the future.
2. The heavily doped region has almost been exhausted after sacrificial oxidation and gate oxidation processes. We will adopt different ion implantation conditions, such as different dosage, energy and the ion source. Dopant activation will adopt different annealing temperature and time in the future.
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3. Why sample FEP and FE have high surface roughness is still a problem. Whether it’s caused from wet oxidation after implantation process would be clarified in the future.
4. Channel mobility is suffered from coulomb scattering which is caused by high Dit. The high Dit extracted from DLTS signal should be reduced in the future, or the channel mobility wouldn’t enhance.
5. The method of NH3 plasma treatment to reduce Dit is not strong enough. More effective methods to reduce Dit would be investigated in the future.
6. Samples through high temperature annealing were capped with graphite, but the surface roughness is about 2 nm after de-capping graphite. To reduce surface roughness, we might adopt other material such as AlN as high temperature capping layer in the future [48] .
7. The reason of SS becomes smaller with higher temperature is not clear at this moment. Further investigation should be studied in the future.
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