Abstract
In order to meet the requirement of device scaling for ultra large scale integration (ULSI) technology, a high dielectric constant (high-k) gate insulator should be applied in MOS devices to further scale down the equivalent oxide thickness (EOT). However, regrowth of the interfacial layer may be induced by high-k dielectric processes, which render EOT scaling difficult. In addition, carrier mobility is degraded because of the poor quality of the high-k/Si interface. To overcome these challenges, studies of high-k gate dielectrics, SiGe and Ge channel have attracted much attention in the research fields of MOS field-effect-transistor (MOSFET) devices. The dielectric constant k of the traditional HfO2 dielectric (monoclinic phase), which is approximately 15 to 20, is not sufficiently high to reduce the EOT to below 0.9 nm for MOS devices. Thus, some exotic higher-k dielectrics deserve investigation. Because of k ≈ 60 for TiO2, HfO2 doped with Ti is studied. An EOT of 0.76 nm is obtained for a sample with a Ti/HfO2 higher-k dielectric after post-metallization annealing (PMA) at 600 °C. As the PMA is above 700 °C, the electrical characteristics of MOS device would be severely degraded. For reducing the EOT value, it is crucial to maintain low gate leakage current and the reliability of the MOS device. The tetragonal phase HfO2 (k > 30) is proposed to be a higher-k dielectric candidate. t-HfO2 can be formed by using a Cl2 plasma treatment at the HfO2/Si interface after post-deposition annealing at 650 °C. Using the results of cross-sectional transmission electron microscopy and capacitance–voltage measurement, we estimated the value of this t-HfO2 to be approximately 35. The optical bandgap value for t-HfO2 is similar to that of the monoclinic. Because Ge can offer twice the electron mobility and four times better hole mobility than Si, Ge is a promising candidate to resolve the issue of mobility degradation by high-k dielectrics. At first, a thin Si layer was grown upon SiGe channel layer as a capping layer. Different Ge contents from 7% to 32% in SiGe channel on electrical characteristics of MOS device were investigated. Based on gate leakage and reliability properties of MOS devices, the optimal Ge content in SiGe channel to achieve sufficient mobility enhancement is around 20%. Further, high-k gated MOS devices with a SiGe channel and nitridation treatment using plasma immersion ion implantation (PIII) were studied. Experimental results indicate that the electrical characteristics of high-k dielectric can be obviously improved by PIII nitridation. For instance, the values of stress-induced leakage are reduced by more than 50%, and the values of stress-induced-flat-band voltage shifts are reduced by more than 33%. Moreover, the EOT value of the MOS device with 30% Ge content in the SiGe channel and PIII nitridation can be reduced to 9.6 A. In order to achieve a high-quality interfacial layer between the high-k dielectric and the Si substrate as well as to enhance the carrier mobility degraded by an ultrathin high-k gate dielectric, a pMOSFET device with a novel super-lattice (SL) Si/Ge channels is proposed. Experimental results show that the electrical characteristics of the MOSFET can be obviously improved by using the SL virtual substrate. The peak hole mobility of the pMOSFET device with SL is enhanced by approximately 100% than that with Si. Moreover, the on-off ratio of the Id–Vg curve is more than eight orders, and the EOT value of the gate dielectric is ~1 nm. The source/drain activation temperature of 650 °C is particularly suitable to the high-k dielectric process. Finally, a Ge MOS device with an ultralow EOT of ~ 0.5 nm and acceptable leakage current of 0.5A/cm2 is presented in this work. The superior characteristics can be attributed to a tetragonal HfO2 with a higher k value (k~31) and comparable band gap. Besides, a Ge MOS device with t-HfO2 also shows a lower leakage current and better thermal stability. The IL with k~13 can be formed by in-situ H2O plasma treatment. Moreover, a Ge MOS device with the IL grown by H2O plasma shows smaller interface trap density and hysteresis effects due to a high composition of Ge+4.