n
+poly
1.12eV 3.1eV
4.8eV 6.9eV
1.88eV 0.24eV
SiON Si substrate HfSiON
n
+poly
Fig.4.21 The band diagram during negative stress.
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Chapter 5 Conclusion
Alternative gate oxide material is a trend for the scaling of oxide thickness in CMOS device technology. Hafnium based dielectrics are among the most promise candidates as a replacement for SiO2 in conventional CMOS process. Interfacial layer and halo implant are mandatory for high-k and sub-65nm CMOS devices.
.In this dissertation, the basic principle of the CP measurement has been introduced briefly, and the trap density at different depths could be measured by different frequencies. Subsequently, the variation of the traps is investigated using this method after the device is stressed by positive and negative voltage. It has been shown that the traps created by the positive stress are mainly generated by the capture of electrons into the traps in the high-k dielectric and thus produces the defects.
Nevertheless, the mechanism of trap generation under the negative stress is more complicated. The device is in its accumulation state under a negative voltage, and the holes in the channel would be captured by the defects located at the Si/IL interface, which may damage the device. The gate poly-Si, which is n-type, would cause the electrons to be trapped into the dielectric due to the negative bias at the gate side, and more traps are created inside the device. For the electrons are captured in the dielectric under the positive stress, the electrical properties changed is essentially about the variation of the threshold voltage. However, in the situation of the negative stress, the electrons would be trapped in the dielectric, and besides the holes would be captured at the Si/IL interface. Because where the holes are trapped is closer to the channel, the variation of electrical properties would be much more obvious, and this could be seen by the degradation of the subthreshold swing under negative stress and the change of Gm,max during positive and negative stress. The result is not found in related researches so far,
72
and it is the primary contribution of this thesis.
73
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