Akiyoshi Muto

Effect of Hf sources, oxidizing agents, and NH3/Ar plasma on the properties of HfAlOx films prepared by atomic layer deposition

The effect of Hf sources, oxidizing agents, and NH3/Ar plasma on the properties of HfO2 and HfAlOx films prepared by atomic layer deposition (ALD) have been investigated. The use of tetrakis(ethylmethylamino)hafnium: Hf(NEtMe)4 (TEMAHf) as a source of hafnium has avoided the problems associated with hafnium tetrachloride (HfCl4), such as poor throughput and the generation of a large number of particles. This source has also been effective in reducing the level of residual chlorine (Cl). However, the level of residual carbon (C) increased. Hydrolysis was found to be more effective than oxidation using O3 in removing ligands from the TEMAHf and/or trimethylaluminum (TMA); the film formed using O3 contained a higher level of C than that using H2O, resulting in the deterioration in field-effect transistor (FET) properties, such as the subthreshold swings and the mobilities. The total levels of residual impurities (C+Cl) were successfully reduced through the combination of TEMAHf and NH3/Ar plasma, leading to improvements in FET properties, particularly in the reduction of the gate leakage current.

Selective Dry Etching of HfO2 in CF4 and Cl2/HBr-Based Chemistries

We investigated HfO2 etching characteristics in conventional Si gate etching chemistries, namely, CF4 and Cl2/HBr/O2-based chemistries. We obtained an adequate etch rate of 2.0 nm/min for both chemistries and a selectivity of 1.9 over SiO2 for Cl2/HBr/O2-based chemistry. We examined the etch rate dependence on source power, bias power, O2 flow rate, and Cl2 flow rate in the Cl2/HBr/O2 chemistry. It was clarified that a physical component is dominant in HfO2 etching in this chemistry. The possibilities of achieving a higher HfO2/SiO2 selectivity and of controlling the anisotropic/isotropic component in HfO2 patterning were also discussed. Moreover, it was clarified that the surface portion of the damaged layer created by the dry-etching step can be removed by a subsequent wet etching. Based on these results, the sub-100 nm patterning of poly-Si/HfO2 gate stacks was successfully demonstrated.

Nucleation and growth of Ge on Si(111) in solid phase epitaxy

The nucleation and growth of Ge on Si(111) surfaces using solid phase epitaxy (SPE) have been investigated by scanning tunneling microscopy (STM). In order to clarify the growth mechanism of two-dimensional (2D) and three-dimensional (3D) Ge islands in SPE, 0.25-bilayer (BL)-thick and 3-BL-thick Ge films on Si surfaces were grown. In the case of the 2D-islanding process, initial amorphous Ge clusters formed into 2D-islands on Si surface. We observed that disordered structures on the island surface and the Si surface transformed into well-ordered surface reconstructed structures on both surfaces as annealing time increased. From these results and the dependence of the 2D-island density on the annealing temperature, we found that growth process of 2D-islands in SPE is similar to that in conventional molecular beam epitaxy (MBE). On the other hand, in case of the 3D-islanding process, crystalline 3D-islands and islands with irregular shapes were initially formed. The 3D-islands preferentially grow with the expense of the irregular island, which corresponds to the ripening process. Considering the results of STM observation, we discuss the origin of the activation energy which was obtained by annealing temperature dependence of the 3D-island density.

Hydrogen Effects on Heteroepitaxial Growth of Ge Films on Si(111) Surfaces by Solid Phase Epitaxy

The influences of hydrogen atoms on Ge heteroepitaxial growth on Si(111) surfaces using solid phase epitaxy (SPE) have been investigated by scanning tunneling microscopy (STM). In the SPE growth of Ge films on H-terminated Si(111) surfaces, the formation of 3-dimensional (3D) Ge islands are suppressed. With the desorption of H atoms, the 3D islands appear on the Ge surface, which are considered to be formed by the agglomeration of Ge atoms. The density of the islands is decreased and the size of the islands is increased by the existence of Si-H bonding at the Ge/Si interface. These phenomena are considered to be caused by the release of the stress in the Ge film and the weakening of the interaction between the Ge film and the Si substrate by Si–H bonding. In addition, the activation energy of the migration of Ge atoms during the agglomeration is determined to be 0.3 eV.

Thermal Instability of Poly-Si Gate Al2O3 MOSFETs

The thermal stability of a SiO2/Al2O3 dielectric stack under high-temperature annealing after poly-Si gate deposition was examined. An interface reaction occurs when the activation temperature is higher than the post deposition annealing temperature, which results in the reduction of the interfacial oxide layer and hence silicate formation. As a result, the characteristics of metal oxide semiconductor field effect transistors (MOSFETs), such as mobility, reliability, and the distribution of capacitance are severely degraded. The formation of a SiON interfacial layer is found to be effective in suppressing the interface reaction.

Effects of H-termination on Ge film growth on Si(111) surfaces by solid phase epitaxy

We have investigated solid phase epitaxy (SPE) of Ge films on H-terminated Si(111) surfaces using scanning tunneling microscopy (STM). In order to clarify the influence of adsorbed H atoms on the initial growth of Ge SPE on Si(111) surfaces, sub-bilayer (BL)-thick Ge films on the H-terminated Si surfaces were grown. It has been found that Ge clusters are formed at room temperature and the cluster shape changes by the existence of H atoms at the Ge/Si interface due to the reduction of interaction between Ge and Si atoms. Moreover, H atoms at the Ge/Si interface suppress the progress of surface reconstruction of Ge films and the crystallization of Ge films occurs with the desorption of H atoms from the Ge/Si interface. Two-dimensional (2D) islands formed by SPE at 500°C contain twinned domains and have irregular shapes while triangular-shaped 2D-islands are formed in Ge epitaxial growth by molecular beam epitaxy (MBE). The irregular shapes observed in the SPE growth are considered to result from the small surface migration of Ge atoms during annealing.

Physical and electrical properties of HfAlOx films prepared by atomic layer deposition using NH3/Ar plasma

The effects of introducing NH3/Ar plasma pulses during the atomic layer deposition (ALD) of HfAlOx films and during the in situ nitridation of an interfacial layer (IL) on the properties of HfAlOx-FETs were investigated. The interaction between HfAlOx and IL and the dopant penetration were suppressed by the introduction of a NH3/Ar plasma pulse during ALD, because it increases the nitrogen content at the interface of HfAlOx/IL. Moreover, the nitrogen content in the IL could be controlled by adjusting the number of NH3/Ar plasma pulses used for in situ nitridation. No threshold voltage shift due to the nitridation of the Si substrate was observed, indicating that the nitrogen profile was well controlled when using a NH3/Ar plasma pulse for in-situ nitridation. It was found that in-situ nitridation with 5 cycles of NH3/Ar plasma pulses was optimum for reducing C–V hysteresis and improving carrier mobility.

Ni-Salicided Poly-Si/poly-SiGe-Layered Gate Technology for 65-nm-node CMOSFETs

A poly-Si/poly-SiGe-layered gate electrode with a high Ge content that is suitable for Ni silicidation has been examined. The optimum Ge content for suppressing gate depletion was found to be 30%. It was found that the in situ deposition of a thin Si layer after SiGe deposition was effective in suppressing void formation. A smooth Si0.7Ge0.3 film without voids was obtained as either polycrystalline or amorphous. The enhancement of boron activation in poly-Si0.7Ge0.3 was confirmed while gate depletion became large for α-Si0.7Ge0.3 because of its lower boron diffusivity. Stable Ni silicidation over the 300 mm wafers was achieved by using a poly-Si/poly-Si0.7Ge0.3-layered gate electrode.

Improved performance of FETs with HfAlO x gate dielectrics using optimized poly-SiGe gate electrodes

In this study, we investigated the growth manner of poly-Si/poly-SiGe layered films on HfAlO/sub x/ films with various Hf contents. The effect of the poly-SiGe gate electrode on FET performances was also evaluated.