ALD2019 Session AF4-MoP: Plasma Enhanced ALD Poster Session

We used a double-side polished p-type Si (100) with a size of 20 × 20 mm² as a sample. Y₂O₃ ALD was performed by introducing a precursor from the heated container at 140 °C and the oxidizer alternately into a cumber at 80 °C. We designed the process condition of ALD as follows, (BuCp)₃Y injection of 300 sec and plasma excited humidified argon of 600 sec. The evacuation times after the (BuCp)₃Y and plasma excited humidified argon are 30 sec and 90 sec, respectively. We measured chemical status of Y₂O₃ films by X-ray photoelectron spectroscopy (XPS) to confirm the presence of Y₂O₃. We extracted a growth per cycle of ALD by spectroscopic ellipsometry. In fig. 1, we show a wide scan XPS spectrum obtained from the 80 °C grown Y₂O₃ film. Fig. 2 shows Y₂O₃ thicknesses as a function of ALD growth cycles. We estimated the growth per cycle of 0.084 nm/cycle.

In this work, we succeeded in 80 °C ALD of Y₂O₃. It must be possible to form Y₂O₃ films on most of flexible substrates by this process, because the process temperature was limited below 80 °C in this ALD process. In the conference, we show the related results and discuss the reaction mechanism.

In this study, we demonstrated that a multi chloride ligand organosilane precursor with no oxygen content, designated as Precursor T to deposit SiN film at the identical deposition temperature for a possible low thermal budget and reliable device operating in industrial applications. Precursor T was used as a Si precursor, and NH3 and N2 were used as the reactants. Ar gas was used as a main and a carrier gas. Plasma enhanced ALD (PEALD) method was conducted to obtain SiN film at 300 oC wafer temperature. SiN PEALD deposition conditions with two different reactants are as follows; Precursor T/purge/NH3*/purge (process A,* denotes plasma use), Precursor T/purge/N2*/purge (process B), Precursor T/purge/NH3*/purge/N2*/purge (process C, applied additional N2 plasma after NH3 plasma) and Precursor T/purge/N2*/purge/NH3*/purge (process D, applied additional NH3 plasma after N2 plasma).

Deposited SiN film with process A and process C exhibited good stoichiometric film compositions at about Si : N ratio of 3 : 4 with low oxygen and carbon contents and excellent SC of more than 95 %. SiN film with process D also showed good stoichiometric characteristic but, SC was ~ 65 % which is not good property than the above two methods. SiN film with process B showed a higher level of impurities over 3 % carbon and chlorine and 20 % oxygen contents, and a poor SC less than 20 % was found. Consistent with the above results, the XRR results also differed significantly from the other three process in SiN film with process B. The film density from process A to D were 2.65, 2.44, 2.73, and 2.73 g/cm3, respectively, which show that the film density of process B are not perfect SiN film.

For the WER performance with 0.1% HF solution, SiN with process C shows the lowest WER of 10 A/min among the four conditions. (WERs of process A, process B and process D SiN films were 88, 187 and 15 A/min, respectively.) It is demonstrated that WER is improved with combinatorial deposition with NH3*/N2* compared to the deposition with NH3* or N2* alone.

In summary, we have deposited SiN film by using a multi chloride ligand organosilane precursor and ALD process design. SiN film was deposited with NH3 and N2 reactant gases by PEALD. Excellent SC and WER results were obtained for SiN film deposited with Precursor T/NH3*/N2* PEALD process. These results would be able to be applied to spacers or passivation layer, etc. by depositing SiN films with process engineering and would be expected to reduce thermal budget or to obtain reliable device operation.

Plasma enhanced atomic layer deposition (PEALD) is an emerging energy enhanced ALD technique that utilizes plasma radicals to drive surface reactions rather than thermal energy, as in traditional ALD. PEALD allows for lower impurities, increased growth rates, improved stoichiometry, and lower deposition temperatures, which are essential for the development of high reflectivity VUV structures and multilayer Fabry-Perot bandpass filters. The goal of this project is to develop an oxygen and nitrogen free ALD process to grow both aluminum and AlF₃ in the same chamber to enable UV optical filters that can be operated at <120 nm. The system is designed to deposit aluminum on a hydrogen terminated Si substrate, by sequential exposures of trimethylaluminum (TMA), an Ar purge, and a hydrogen plasma. The deposition of metal fluorides has been limited by difficulty in handling, and storing, anhydrous HF. The use of Pyridine-HF as the fluoride source reduces the risk for deposition of UV fluoride layers including AlF₃ and MgF₂. The process designed for the growth of AlF₃ layers proceeds by exposing the fresh PEALD Al to HF followed by an Ar purge. The process is designed for growth of Al and AlF₃ at 100-300° C. The PEALD system is connected to an X-ray photoelectron spectroscopy (XPS) system for elemental analysis and a UV spectrometer for far UV optical properties. Ex-situ characterization was conducted using atomic force microscopy (AFM), Rutherford backscattering (RBS), UV-VIS ellipsometry, and X-ray topography (XRT).

Research supported by NASA through grant NNX16AC30G.

Recently, plasma enhanced atomic layer deposition (PEALD) is expected to overcome the limitation of conventional LPCVD and PECVD processes and could be used to deposit conformal thin films in convoluted high aspect ratio structures with thickness scalability.¹ A number of studies on PEALD SiN_x process have been reported and mostly capacitively coupled plasma (CCP), inductively coupled plasma (ICP) or microwave (MW) plasma source was employed.² The characteristics of the plasmas, such as low plasma density in CCP, high plasma damage in ICP and high oxygen contamination in MW, etc., can be problematic for film properties, especially wet etch resistance. On the other hand, hollow cathode plasma (HCP) can be applied for high-quality silicon nitride films due to its lower oxygen contamination, higher plasma density and lower plasma damage. Inspired by the unique characteristics of HCP, we proposed to explore the feasibility of high-quality PEALD-SiN_x films by improving the films properties with HCP. Additionally, the relationship between plasma source and PEALD-SiN_x films properties has rarely been studied.

In this work, we not only clarified the effect of plasma source on the oxygen contamination of PEALD SiN_x films, but also achieved high-quality SiN_x films using hollow cathode PEALD. PEALD system with ICP or HCP source was employed to study SiN_x films comprehensively. Hexachlorodisilane (HCDS, Si₂Cl₆) was used as the silicon precursor due to its higher reactivity than SiCl₄ and SiH₂Cl₂ while exhibiting a distinct self-limiting growth behavior. We explored the effect of process temperature (310−570 °C), precursor exposure, and plasma gas composition (NH₃/N₂ or NH₃/Ar) on the film properties. Within a range of process temperature window (310−480 °C), the combination of Si₂Cl₆ and NH₃ plasma showed the distinct self-limiting growth behavior at a low precursor exposure (5×10² L). In plasma source comparison, the SiN_x films deposited with ICP source showed higher oxygen contamination (> 7.0 at%) than the HCP-PEALD SiN_x films (> 3.5 at%). Particularly, the HCP-PEALD SiN_x film deposited at 480 °C had the wet etch resistance in HF acid (> 0.4 nm/min, 200:1 HF) and film density (2.9 g/cm³) comparable to the LPCVD SiN_x films. These result could pave the way for achieving high-quality SiN_x films using the unique HCP source.

¹ H.S. Kim, X. Meng, S.J. Kim, A.T. Lucero, L. Cheng, Y.-C. Byun, J.S. Lee, S.M. Hwang, A.L.N. Kondusamy, R.M. Wallace, G. Goodman, A.S. Wan, M. Telgenhoff, B.K. Hwang and J. Kim, ACS Appl. Mater. Interfaces 10, 44825 (2018).

² X. Meng, Y.C. Byun, H.S. Kim, J.S. Lee, A.T. Lucero, L. Cheng, and J. Kim, Materials (Basel). 9, 1007 (2016).

Plasma enhanced technology has been widely used in the fabrication of thin films. In an atomic layer process, for example in a plasma enhanced atomic layer deposition (PEALD) process, chemically reactive plasma is expected to achieve high growth per cycle (GPC) at relatively low process temperature. A typical PEALD process may also be used in multiple patterning. In such patterning processes, much more precise resolution of film patterning is highly required for the nanoscale pattering. However, the film surfaces suffer the bombardment of energetic ion species, which are generated in the plasma. In such processes, film properties are affected. Therefore, the goal of this study is to clarify the effects of ions or radicals in the plasma enhanced atomic layer processes and gain a further understanding of deposition mechanisms.

In this study, we have estimated the sputtering yields of a SiO₂ film deposited by a PEALD process, by an oxygen ion (O⁺) beam as well as inert gas species with the use of a mass-selected ion beam system. We have also examined the effects of low-energy ion bombardment on a precursor-absorbed SiO₂ or Si surface. After precursor exposure, the surface was exposed to energetic argon ions (Ar⁺). The surface chemical compositions after ion bombardment were analyzed by in-situ X-ray Photoelectron Spectroscopy (XPS). From the XPS observations, it has been found that impurities, such as nitrogen (N) and carbon © originating from the organic precursor molecules, remain in the surface region even after Ar⁺ ion bombardment. We have also examined the effects of oxygen, which will be discussed in the presentation.

With the continued down scaling of devices and structure changed to 3-dimensional, new ALD processes are in great demand. Microwave surfatron plasma is considered new plasma source because it enables very low-temperature deposition and good film quality due to its low electron temperature and higher plasma density.

In this work, surfatron plasma source was incorporated to ALD reactor. We studied the characteristics of oxide films grown by microwave plasma enhanced ALD in the reactor. The films had high growth rate and low impurities, and they grew conformally on 6-inch Si wafer. The successful incorporation of surfatron plasma source to ALD reactor encourages the study of challenging ALD processes.

In this work, the epitaxial growth of single crystalline GaN films on sapphire substrate is realized by plasma-enhanced atomic layer deposition (PEALD) at a growth temperature of 350 ℃. The XRD patterns show that the GaN films are single-crystalline and display a single hexagonal (002) phase. The high-resolution transmission electron microscopy reveals a nanoscale single-crystal GaN heteroepitaxy and a sharp interface. The full width at half-maximum of the X-ray rocking curve of the GaN epilalyers is 709 arcsec. The hetero-epitaxial GaN thin films have broaden its application in photovoltaic area, such as improving the performance of the solar cells.

In our continuous evaluation of disilane compounds as silicon source precursors for plasma enhanced atomic layer deposition of silicon nitride films (PEALD SiN), a halodisilane is found to give a superior film-forming performance and film properties. The halodisilane having 99% purity was evaluated with ammonia plasma on a PEALD tool with a remote hollow cathode plasma source. SiN films were deposited at about 1 Å/cycle growth rate within the ALD window up to about 450 ^oC. This growth rate represents a 20% improvement over the growth rate of hexachlorodisilane (HCDS). The wet etch rate (WER) of the PEALD SiN films of the halodisilane is 50% lower than that of HCDS in 200:1 HF. The step coverage is comparable to that of HCDS in high aspect ratio trenches. The PEALD SiN film results of this precursor including saturation behavior, ALD window, composition, refractive index, uniformity, density, wet etch rate and step coverage are summarized and compared with those of HCDS and pentachlorodisilane (PCDS) in this presentation.

In this study, to improve the properties of thin films deposited by plasma enhanced atomic layer deposition (PEALD), two different very high frequency (VHF) plasmas (60MHz, 100MHz)were used for the deposition of cobalt film by PEALD and, the characteristics of plasmas and cobalt films deposited by different frequencies were investigated. For the characterization of VHF plasmas, ion density and electron temperature were measured using a Langmuir probe. It is found that the higher frequency showed a higher plasma density with a lower electron temperature at the same rf power. When the NH radicals, which are required to remove the ligands of the cobalt precursor during the plasma exposure step in the ALD cycle, w ere measured by OES, the intensity of NH peak at 100 MHz was higher than that at 60 MHz, indicating that more NH₃ plasma dissociation at the higher frequency. The composition and the RMS surface roughness of the deposited cobalt films were measured by XPS and AFM, respectively. AFM data showed the lower RMS surface roughness value at the higher frequency possibly indicating denser films due to more active surface reactions at the higher frequency. As a result, it is expected that the cobalt thin films deposited by the higher VHF PE-ALD will improve the characteristics of deposited thin films.

Keywords : Very high frequency (VHF) plasma, Atomic layer deposition (ALD), Cobalt, dissociation, radical