Advanced ALD Processes and Interfacial Treatments for Next Generation Complementary Metal-Oxide-Semiconductor Field Effect Transistor (CMOSFETs) Technology
- Advanced ALD Processes and Interfacial Treatments for Next Generation Complementary Metal-Oxide-Semiconductor Field Effect Transistor (CMOSFETs) Technology
- Hyun Soo Jin
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- Surface sulfur passivation on InP substrate was performed using a dry process rapid thermal annealing under H2S atmosphere for III–V compound semiconductor based devices. The electrical properties of metal-oxide-semiconductor (MOS) capacitor fabricated with atomic-layer-deposited HfO2 film as a gate insulator were examined, and were compared with the similar devices with S passivation using a wet process (NH4)2S solution treatment. The hydrogen sulfide annealing gave a solid sulfur passivation with the strong resistance against oxidation compared with the ammonium sulfide solution treatment, although the sulfur profiles at the interface of HfO2/InP were similar. The electrical thickness reduction of the gate insulator by S passivation was similar in both treatments. However, the H2S annealing was more effective to suppress interface state density near the valence band edge, because thermal energy during the annealing resulted in stronger S bonding and InP surface reconstruction. Moreover, the flatband voltage shift by constant voltage stress was lower for the device with H2S annealing.
ALD Al2O3 films were grown on ultrathin-body In0.53Ga0.47As substrates for III-V compound semiconductor based devices. Interface S passivation was performed with wet-process using ammonium sulfide solution and dry-process using post-deposition annealing (Post-DA) under H2S atmosphere. Post-DA under H2S atmosphere results in lower S concentration at the interface and a thicker interfacial layer than the case with ammonium sulfide pre-treatment. Therefore, the electrical properties of the device including interface property estimated by frequency dispersion in capacitance were better for ammonium sulfide pre-treatment than post-DAunder H2S atmosphere. However, they might be improved by optimizing process conditions of post-DA. The post-DA under H2S atmosphere following ammonium sulfide pre-treatment resulted in an increased S concentration at the interface which improved the electrical properties of the devices.
We examined the S distribution and improvement of the electrical properties at the atomic-layer-deposited HfO2/InP interface according to H2S pre- and post-DA. As a result of X-ray adsorption spectroscopy analysis, the S ion through the H2S pre-DA dominantly formed a sulfide with -2 valence electron due to its direct chemical reaction with the InP substrate material. In addition, physical S passivation on the InP surface effectively inhibited interfacial layer growth during ALD process. However, in the case of H2S post-DA, the S ion was dominantly formed a sulfates (SO42-) with +6 valence electron because it is oxidized by abundant oxygen in HfO2 film during diffusion to the HfO2/InP interface. In conclusion, the S ion using H2S pre-DA was mainly exist at the HfO2/InP interface, and in case of H2S post-DA, the S ion showed broad distribution at the interface and bulk near interface due to the nature of process sequence.
The high concentration of S through H2S pre-DA showed excellent electrical characteristics by effectively removing the electrical defects at the interface, and finally showed that a semiconductor device having an electrically solid interface can be fabricated. The H2S post-DA showed higher electrical interface defects due to low S concentration at the interface compared to H2S pre-DA, but showed improved charge trapping characteristics as the thin film was healed by applied thermal energy and S during the annealing.
We systematically examined the influence of two oxygen source types, H2O and O3, on residual C-related impurities in atomic-layer-deposited Al2O3 film. ALD Al2O3 film grown using H2O contains negligible C-related impurities irrespective of growth temperature. However, the C-related impurity in film grown using O3 exhibited strong dependence on growth temperature
only Al carbonate (Al-CO3) was present in film grown at 300 ℃, but C-related impurities with lower oxidation states, such as Al-COOH and Al-CHO, appeared as the temperature decreased to 150 ℃. This suggests that the reactivity of O3 and H2O in the ALD process has a different temperature dependence
from a residual impurity perspective, compared to O3, H2O is a beneficial oxygen source for low temperature processes. Electrical properties, such as charge trapping and gate leakage current, were also examined. For a growth temperature of 300 ℃, the film grown using O3 was slightly superior to the film grown using H2O due to its high film density. However, the film grown using H2O demonstrated better electrical characteristics at low growth temperature, 150 ℃.
Typical Ru ALD process has island growth characteristics due to low nucleation density at the beginning of thin film deposition, these islands must grow and connect together to form a continuous film. The resulting thin film has high roughness after deposition and low electrical properties due to low film density. We have experimented on the formation of continuous high quality Ru thin films at thin thickness suitable for next-generation semiconductor device structure using discrete feeding method (DFM). The DFM is an advance ALD process which can improve the initial nucleation density by sub-dividing the metal precursor injection and purge. As the DFM process was subdivided, Ru nucleus formation density at the initial stage of deposition could be obtained due to the efficient removal of physically adsorbed Ru precursor molecules. In the case of DFM, the Ru island was connected to each other at a relatively low ALD cycle, and a continuous Ru film was obtained at a thinner thickness than the typical ALD Ru. In addition, it is confirmed that a Ru thin film with a high density is formed after the deposition because the most reaction sites exist on the surface are filled in the DFM Ru ALD process, therefore, chemically more metallic Ru thin film is formed. Due to the improvement of the physicochemical thin film properties, the DFM Ru thin film exhibited a very low electrical resistivity. The DFM process involves the injection of small amounts of precursor molecules several times, resulting in efficient precursor diffusion even in complex structures. Therefore, perfect Ru thin film with a high step coverage can be obtained in a complicated structure.
As another attempt to solve the problem of the conventional Ru ALD process, we introduced EA-ALD which is advanced ALD technology. The EA-ALD, which can form a +30V or -30V electric field and forcibly adsorb the precursor molecules on the surface, was applied to the EBECHRu/O2 ALD process. In the case of EA-ALD, the EBECHRu precursor molecule was effectively attracted to the SiO2 surface regardless of the polarity, but there was a difference in the quality of the Ru film formed according to the polarity. At Ru ALD process with +30V, more Ru nuclei were formed at the initial stage of Ru ALD process because the positive electric field assisted in the reduction of RuO2. Due to the increased nucleation density, a very uniform continuous Ru film was obtained. In addition, due to the increase of impingement flux and the efficient reduction of RuO2, the formed thin films showed high density and chemically more metallic Ru characteristics. In the Ru ALD process with -30V, the polarity of the electric field interferes with the reduction of RuO2, so that even if the impingement flux is increased, the effect of improving nucleation in the initial stage of deposition is insignificant. In addition, despite the high Ru density due to the increase of impingement flux, the deposited thin films exhibited low electrical characteristics due to the presence of most Ru in the oxidized state. Therefore, it has been confirmed that it is advantageous to use an positive electric field to form a high-quality continuous Ru thin film in the EBECHRu/O2 ALD process.
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