Study Of Composite Hafnium High K Films Deposited By Dual-frequency Plasmas

The silicon based integrated circuits as the core of the microelectronics industry rapidly develop following the Moore’s law. After 2015 the feature size of semiconductor devices is below 22 nm and it continues to shrink, which is pointed out by the development plan of the international semiconductor technology. However the channel size of the device and the physical thickness of the gate dielectric can not be shrunk forever. In 65 nm technology, the thickness of Si O2 as the traditional gate dielectric material in MOSFET is close to 1nm, which has reached the physical limits. As a result, the leakage current caused by the direct tunneling mechanism between the gate electrode and Si channel region significantly increases. To overcome the problems, high k materials are considered to replace Si O2 because they have larger physical thickness to prevent the leakage current of tunneling effect and the diffusion of impurities at the condition of the same equivalent thickness. Hf O2 and the Hf-based high k film have been considered as one of the promising candidates due to its high k value, wide band gap and large conduction band offset(1.5 e V). However, there are still some deficiencies in the Hf-based high k material, such as oxygen vacancies as the intrinsic defects forming in the process of deposition, the formation of the interface layers between the Si substrates and the films and the Fermi energy level pinning effect. How to optimize the quality and improve the performance of the Hf-based high k materials has attracted many attentions. In our work, the composite Hf-based high k films with excellent properties are deposited by optimizing the preparation conditions. The quality of the films is improved by the thermal annealing and the post-deposition plasma treatments.(1) The Hf O2 films are deposited by the magnetron sputtering technique driven by the radio frequency(RF) source with three different frequencies(2 MHz, 13.56 MHz and 27.12 MHz). The structures, the optical properties and the wettability on the surface are observed. Results show that the magnetron sputtering technology driven by the RF source of 13.56 MHz is more suitable for the preparation of high k materials, because the films deposited by this RF source have higher deposition rate and more smooth surfaces. The wettability of the Hf O2 films can be optimized by the sputtering frequency and the sputtering power. The Hf O2 films deposited by the magnetron sputtering technology driven by the RF source of 27.12 MHz at the power of 180 W have hydrophobic surfaces with water contact angle of 91.5 o. The optical properties of the Hf O2 films are influenced by the ion energy.(2) The as-deposited Hf O2 films by sputtering show a large electric hysteresis loop in the process of quasi static C-V measurements at high testing frequency. The C-V curves appear serious dispersion effect with different testing frequencies. These results suggest that the electrical properties of the Hf O2 films need to be improved. The plasma density and the ion energy of the dual-frequency capacitively coupled plasmas(DF-CCP) can be controlled by the power of the high frequency source and the low frequency source, respectively. To improve the electrical properties of the Hf O2 films, plasma treatment on Hf O2 films by Octafluorocyclobutane(C4F8) DF-CCP is investigated. The fluorine atoms are incorporated into the Hf O2 films in the process of plasma treatment. The tetragonal structures are stabilized by both the plasma treatment through optimizing the power of the high frequency source and the process of post deposition annealing. The formation of the CF layers on the surface of Hf O2 is suppressed by optimizing the power of the low frequency source. The traps in the Hf O2 are reduced by the F incorporation and the interface states density at the Hf O2/Si interface are passivated by forming Hf-F bonds. As a result, the electrical characteristics of the Hf O2 films are greatly improved. The O2 gas are added in the discharge gases to suppress the formation of CF layers, which results in higher k value and further improved electrical properties.(3) The N2 DF-CCP treatment on the Si substrates before the deposition process is carried out in order to prepare high k films with high qualities on the Si substrates. Results show that the diffusion of Si atoms during the annealing process is effectively reduced and the formation of the interface layers of Hf Si O is suppressed by the plasma treatment on the Si substrates. As a result, the interface qualities are improved. Rare-earth element Gd is doped into Hf O2 to deposit the composite hafnium high k films. Compared with Hf O2, the Hf Gd O films have higher k value, which is ascribe that the cubic structures are stabilized after annealing by introducing the Gd atoms in the films. The highest k value, the smallest hysteresis, and excellent electrical performance are obtained in the Hf Gd O films deposited on the treated Si.(4) In order to realize the precise control of the content of the doping elements in the composite film, a simultaneous dual frequency magnetron sputtering technique is used to deposit the Hf Er O films. Results show that the thickness and the doped Er content of the Hf Er O films deposited by the 13.56 MHz / 27.12 MHz DF sputtering linearly increase with the power of the 27.12 MHz source. The high refractive index and the wide bandgap of the Hf Er O film are obtained through optimizing the sputtering power. While using the 13.56 MHz / 2 MHz DF sputtering, due to the formation of the ions with high energy in the plasmas, the growth of the film is suppressed as a result of the ion bombardment. Therefore, the frequencies of the sources need a comprehensive consideration,when the DF sputtering technique was applied to prepare compound high k material.(5) Our experimental results show that the oxygen vacancy is filled by the F atom incorporation resulting in the improvement of electrical properties. But the passivation mechanism needs to be further studied. At the application of CASTEP module and with in the framework of the first principles calculation, we investigate the influence on the density of states of the Hf O2 with monoclinic phase when the oxygen vacancy exists in Hf O2 and the oxygen vacancy is filled by F atom. Results show that oxygen vacancy introduces gap states. When F atom fills the oxygen vacancy, the hybridization between the 2p orbital of F atom and the 5d orbital of Hf atom results in pushing the band gap state above the conduction band of Hf O2, thus completely passivating the oxygen vacancy.