ALD/ALE 2021 Session EM2: Organic-Inorganic Hybrid Materials

Foldable phones are now a commercially available product, with some manufacturers already several generations into the product cycle. Use of thin film encapsulation solutions (TFEs) in organic electronic manufacturing (OEM) has become more common as they are lighter and enable folding and stretching of a device, unlike conventional encapsulation methods. The commercial demand has created a large driving force to develop the quality, reliability, and process flow integration of TFEs to enable future product iterations. Inorganic films, such as those generally associated with atomic layer deposition (ALD) are vital building blocks for electronic devices due to the variety of properties they offer to a fabrication scheme. One of the main reliability issues TFEs face is that inorganic films are susceptible to crack under mechanical stress, compromising the barrier performance and cutting the lifetime of the device short. [1]

One approach to address this issue is the nanolamination of dense inorganic layers deposited with ALD for enhanced barrier performance, with organic or hybrid molecular layer deposition (MLD) layers to offer enhanced flexibility [2, 3]. To be an industrially viable solution both components of this process need to be scalable to large substrate sizes, have reasonable deposition time and show good stability in ambient conditions. The more studied MLD processes have been shown to be prone to degradation in ambient atmosphere and quickly lose their beneficial properties [4].

We present process development results from a MLD hybrid with several different inorganic linkers deposited in Picosun R-200 advanced ALD reactor on 200 mm Si substrates. The hybrid films analysed with spectroscopic ellipsometer and Drop Shape Analyzer show excellent large area uniformity at OEM friendly temperature of 90 °C. Long term stability was studied with XPS and FTIR in addition to the previously mentioned methods and demonstrates excellent film stability after months of ambient ageing.

Industrially viable MLD processes can be a valuable tool to have in the fields of OEM and flexible electronics. In this work we show that industrial-scale ALD reactors are capable of producing MLD and ALD layers in the same chamber with low thermal budget and that ALD/MLD nanolaminates (Fig 1) can be easily deposited in a single process step on a large-scale substrate, a process that has a potential as a TFE solution.

[1] Steinmann et al. (2018), doi:10.1557/jmr.2018.194

[2] Yoon et al. (2017), doi:10.1021/acsami.6b15404

[3] Jeong et al. (2020), doi:full/10.1080/15980316.2019.1688694

[4] Ruoho et al. (2018), doi:10.1016/j.mtchem.2018.09.004

The little-known iron(III)oxide polymorph ε-Fe₂O₃ has risen intrigue for its ferrimagnetic character and high coercive field, that make it an interesting material for magnetic storage applications.¹ To enhance the appeal of ε-Fe₂O₃ thin-film magnets for flexible electronics, tailoring its mechanically rigid character is however desired. Here we demonstrate tailoring of the mechanical properties of ε-Fe₂O₃ thin films through insertion of Fe-terephtalate interlayers by using the combined atomic/molecular layer (ALD/MLD) deposition technique.^2,3

We deposited ε-Fe₂O₃ oxide films, Fe-terephtalate (Fe-TP) hybrid films and ε-Fe₂O₃/Fe-TP superlattice thin films using FeCl₃ as the Fe metal precursor, H₂O as the oxygen source, and terephtalatic acid as the organic precursor.^2,3,4Nanoindentation experiments indicate that the insertion of the Fe-TP interlayers in the ε-Fe₂O₃ matrix enables a factor-of-two decrease in elastic modulus for the superlattice films down to 70±20 GPa. In-situ tensile fragmentation testing indicates that the crack onset strain and critical bending radius can be tuned by a factor of three.Modeling of the tensile fragmentation patterns through Weibull statistics shows that cohesive and interfacial shear strain, following the trend for crack onset strain, increase with increasing organic content, while gains for the respective strengths are limited by the simultaneous reduction in elastic modulus. Importantly, we demonstrate through magnetization vs. magnetic field (M-H) measurements that the unique magnetic characteristics of ε-Fe₂O₃ are not sacrificed in the superlattices with enhanced mechanical flexibility. Magnetic characterization of strained films indicates absence of catastrophic failure upon fragmentation and modest debonding of the films, while the improved mechanical characteristics for the superlattices translate to improved preservation of the magnetic characteristics.

The present results are interesting for magnetic storage applications in flexible electronics and highlight the potential of the combinatorial ALD/MLD technique for enhancing mechanical properties of—not only inorganic-organic alloys as known from the past⁵—but also of multilayer structures with inorganic layers thick enough to generate usable functionalities.

(1) S. Ohkoshi, Angew. Chem. Int. Ed. 2016, 55, 11403-11406. (2) A. Philip, J.-P. Niemelä, et al., ACS AMI 2020, 12(19), 21912.(3) J.-P. Niemelä et al., ACS ANM 2021, 4, 2, 1692. (4) A. Tanskanen et. al, Phys. Status Solidi RRL 2018, 12, 1800390. (5) S. Jen et al., Appl. Phys. Lett. 2012, 101, 234103.

Due to the limitation of resolution with scaling down of feature size, extreme ultraviolet (EUV) was has drawn great attention as the light source for the next-generation lithography process. Unlike the photoresists for UV irradiation, EUV photoresist interacts with secondary electrons. However, the current polymer-based resist has confronted a number of challenges, such as pattern collapsing due to resist thickness, poor EUV adsorption, and etch-selectivity, etc. To circumvent the limitation, research on new materials, particularly metal incorporation with organic resists have been recently reported. With introduction of metal, the hybrid resist can increase the EUV absorption as well as the mechanical stability during the pattern transferring.^2,3 Among the various synthesis of inorganic-organic hybrid EUV resists, vapor-phase infiltration of metal source into existing organic resist using ALD process can be employed. This ex-situ process can avoid the complex chemistry and employ the standard fabrication process.

In this work, we have demonstrated the infiltration of Hf into PMMA e-beam resist using ALD process. The PMMA resist is coated on the Si substrate using a spin-coating technique, followed by exposure of TDMA-HF precursor in the ALD chamber at 85 ^oC. In XPS gas cluster ion beam depth profile, Hf concentration showed 5 to 10 at. % in PMMA film, implying that Hf is distributed in the PMMA resist. Specifically, we investigated the effect of metal-infiltrated resists on electron-induced solubility by understanding the change of chemical bonds using an in-situ Fourier transform infrared spectroscopy (FTIR) equipped with electron gun capability. Each sample was exposed with electrons with 100 eV, which was similar energy of incident photons from EUV, and characterized the difference of chemical bonds before and after exposure. Based on these theoretical results, pure PMMA and metal infiltrated PMMA was exposed with a variable dose of electrons, followed the developing test with various solvent to investigate the effect of metal infiltration.

This work is funded by Brain Pool Program through National Research Foundation by the Ministry of Science and ICT in Korea (No. 2019H1D3A2A01101691). This work was also partially supported by SRC-NMP program (task# 3035.001).

1. Mattson, E. C. et al., Chem. Mater.30, 6192 (2018).

2. Fallica, R. et al., J. Micro/Nanolithography, MEMS, MOEMS17, 1 (2018).

3. N. Tiwale et al., J. Mater. Chem. C, 7, 8803 (2019).

The deposition rate and properties of MLD films are for a large part determined by what happens during the precursor exposure. In some cases, however, the purge step is of equal importance, for example in the MLD of alucone films using trimethylaluminum (TMA) and ethylene glycol (EG). These alucone films tend to be porous in nature due to which the reactants during their exposure step not only react at the film surface but also tend to infiltrate into the film. The outgassing of the infiltrated reactant can take relatively very long thereby becoming the deposition rate limiting step. If enough purge time is not provided for the reactant to outgas, it will lead to an undesirable CVD component alongside MLD in the overall growth. We have investigated the MLD of alucone focusing on the effect of purge time of TMA on the growth kinetics. To avoid any negative impact of the CVD component on the deposition rate and the film’s properties, we have also developed a kinetic model to correlate parameters like exposure times, partial pressures, purge times and deposition temperature to the CVD component in the growth. With an intention to improve the outgassing efficiency of TMA, the influence of purge gas flow on the CVD growth component has also been briefly investigated. Finally, we show that using a bulkier precursor like DMAI instead of TMA can overcome the problem of precursor infiltration altogether and increase the deposition rate of alucone processes by at least an order of magnitude.

Gas permeation barriers are widely employed in many technological applications. From medium barrier properties (Oxygen transmission rate, OTR ≈ 1 cc m^-2 day^-1 bar^-1; Water vapour transmission rate, WVTR ≈ 1 g m^-2day^-1) as in the food packaging industry. To very high barrier properties (OTR ≈ 10^-5 cc m^-2 day^-1 bar^-1 ; WVTR ≈ 1 g m^-2day^-1) such as in flexible organic opto-electronics. Here, it is even more critical to avoid the presence of water and oxygen in the active layer.

Atomic layer deposition (ALD) of amorphous alumina show very promising properties for passivation or encapsulation layer, for gas barrier layers. However, the deposition conditions usually require temperatures above 100 °C or highly oxidative conditions (ozone or oxygen plasma) to obtained reasonable deposition rates and high-density material. And due to its brittle nature, it limits the implementation on rigid or low flexibility substrates. A process for temperature sensitive substrates used in food packaging would be highly welcomed.

In this work, we combined amorphous alumina with alucone, in multi-stack structures on flexible PET substrate for food packaging. The inorganic alumina layer serves as gas barrier layer and the organic alucone layer decouples the adjacent alumina layers, thus avoiding direct passage of the permeant gas through pinholes.

By developing an in-house He permeation tester, we have investigated the physical diffusion of Hellium on these structures; and a method of extracting diffusivity and solubility from different substrate thickness is proposed. To further understand the morphology of these layers, we have simulated those structures using Finite Element Method (FEM).

The development of hybrid inorganic-organic films with well-controlled properties is vital for the future of nanotechnology for many applications. Molecular layer deposition (MLD) allows the deposition of hybrid films using sequential, self-limiting reactions, similar to ALD. In this study, we investigate the growth mechanism of different hybrid organic−inorganic thin films by using first principles density functional theory (DFT).

The prototype for MLD hybrid films are composed of thin films of aluminium alkoxides, known as ”alucones”. We investigated the MLD reaction productsbetween the post-TMA Monomethyl-Al₂O₃ (Al-CH₃-Al₂O₃) and Dimethyl-Al₂O₃ (Al(CH₃)₂-Al₂O₃) surface and diol organic precursors, including ethylene glycol (EG) as well as glycerol (GL). The results show that Al–O formation with release of methane is favorable for all diols. Interaction energies show that longer diols will not maintain an upright configuration compared to lying flat and participating in the “double reaction” where the two terminal –OH groups bind to surface TMA. A detailed comparison of ethylene glycol (EG) and glycerol (GL) precursors is presented to assist the interpretation of experimental findings regarding the differences in the hybrid films grown by EG and GL. EG and GL can lie flat and create double reactions through the two terminal hydroxyl groups. This phenomenon removes active hydroxyl sites for EG. For GL the third hydroxyl group is available and growth can proceed. This analysis shows the origin of differences in thickness of alucones found for EG and GL.

We also investigate the growth mechanism of magnesium containing hybrid films known as “magnesicone”from the reaction of EG and GL at MgCp-terminated MgO as a model system. Interaction energies show that while the ligand elimination process is favorable for both precursors, GL species prefers to lie in an upright position and EG prefers to orient in a flat configuration and interacts at the MgO surface, resulting in a thicker GL-based magnesicone compared to the EG-based magnesicone. This is consistent with the experimental findings for magnesicone growth using these precursors.

Finally we study the growth of titanium-containing hybrid organic−inorganic films known as “titanicones”. using titanium tetrachloride (TiCl₄) or tetrakisdimethylamido (Ti(TDMA)₄) as metal source and ethylene glycol (EG) or glycerol (GL) as organic reactants. This investigation contributes to the understanding of growth process of EG and GL based titanicones at different surfaces and is valuable in supporting experimental data on titanicone film growth.