First-Principle Calculation of Novel Channel and Ferroelectric Materials with additional BEOL TDDB Reliability Modeling

• Main Authors: Pin-Shiang Chen, 陳品翔
• Other Authors: CheeWee Liu
• Format: Others
• Language: en_US
• Published: 2019
• Online Access: http://ndltd.ncl.edu.tw/handle/4395f4

博士 === 國立臺灣大學 === 光電工程學研究所 === 107 === In this dissertation, the characteristic of lonsdaleite germanium as the novel channel material and hafnia-based ferroelectric material are investigated. Besides, the failure model of the low-k dielectrics in the back-end-of-line is also discussed. As the device keeps scaling down, the high mobility channels are proposed to enhance drive current and reduce power consumption. To pursue high performance channels, new material such as III-V semiconductor is one way to explore. Alternatively, the same material with an optimum crystalline structure is more cost-effective to obtain high mobility. In the first part of this dissertation, lonsdaleite Ge, the allotrope of diamond structure Ge is discussed including the bandstructure, effective mass, ballistic current and strain response. The stable lonsdaleite Ge has the potential to enhance the performance of n-channel FETs without introducing new materials. The direct-bandgap characteristic also makes lonsdaleite Ge potentially useful for photonic applications. When the performance of devices enhances, it is also important to reduce the static power consumption. Transistor operating frequency and capacitance cannot be lowered in pursuit of higher speed and on current. Therefore, lowering VDD is the solution to reduce the dynamic switching power as technology node progress. In the transistor design, IOFF should remain the same or become even lower to maintain the low static power (IOFFVDD), and ION should become larger for decreasing VDD. As a result, devices with steep subthreshold slope (SS) are desired. However, SS of the traditional transistor is limited at 60 mV/decade at room temperature due to the thermionic emission transport mechanism. Negative capacitance FET (NCFET) is a method to overcome the limited SS. Ferroelectric material can be used to amplify the gate voltage by the internal polarization of it. A capacitor made with such a ferroelectric material can exhibit “negative capacitance,” where the stored charge in a stable state is negative with respect to the applied voltage. Such negative capacitance can be exploited for “voltage amplification”. This voltage amplification can be exploited in the gate-stack of a transistor in order to achieve SS < 60 mV/decade without changing the transport physics of the FET. However, it remains challenging whether the operation speed of NCFETs is applicable for high-speed circuit. Therefore, the research of the transient behavior of ferroelectric is important for the development of NCFET. In the second part of this dissertation, the characteristic of the HfO2 ferroelectric material with different Zr content is calculated by first principle. And the dynamic polarization with time is calculated based on density functional theory (DFT) and molecular dynamic model. The metastable life time of HfZrO2 is smaller than 0.2 ps, which has potential of high-speed operation for device applications. In addition, the strain response of HfO2 is also discussed. Finally, the breakdown mechanism of the back-end-of-line (BEOL) dielectric is investigated since it has become an important failure mechanism with the reduction of device dimensions. Several models of the breakdown process have been proposed, with the mechanisms being broadly classified as being dependent on the dielectric electric field, or the leakage current through the dielectric under a voltage bias. The most pressing issue to be resolved for accurate reliability estimation of BEOL dielectrics is the field dependence of the breakdown mechanism. By combining modeling of the leakage current of BEOL capacitors with time dependent dielectric breakdown (TDDB) data, we show that the hard breakdown of capacitors during electrical stress is related to the leakage current flowing through the dielectric. Moreover, we find the breakdown occurs after a critical energy density has been dissipated in the dielectric.