ALD2019 Session AA1-WeA: Emerging Applications II

Polymer interfaces play a critical role in a variety of applications, including consumer products, structural components, biomedical devices, and flexible electronics. In many cases, polymers need to be bonded with adhesives to create structural joints or multi-layer structures. For adhesives to efficiently wet and bond to a substrate, the surface free energy of the substrate must be equal to or higher than the surface free energy of the adhesive. However, the surface energy of most polymers is low, which makes adhesion difficult. Thus, there often is a need to increase the surface energy of a polymer without changing the bulk mechanical and chemical properties.

In this work, we demonstrate that atomic layer deposition (ALD) can be applied on poly(methyl methacrylate) (PMMA) and fluorinated ethylene propylene (FEP) to increase their surface energies and, hence, to increase the interfacial fracture toughness when bonded to an epoxy adhesive.

ALD alumina films were deposited on each type of polymer to modify the surfaces towards high energy surfaces of metal oxide. Transmission-electron microscopy (TEM) and atomic-force microscopy (AFM) were used to study the film morphology on the polymers. These indicated that the ALD treatment increased the surface roughness and changed the sub-surface chemistry by vapor-phase infiltration (VPI). The increase in surface energy after ALD was measured by the sessile-drop test with water, ethylene glycol and glycerol.

The interfacial fracture toughness of each polymer-epoxy interface was measured using a customized motor-controlled wedge tester. After ALD film growth, the interfacial fracture toughness of the PMMA-epoxy and FEP-epoxy interfaces increased by factors of up to 7 and 60, respectively. The two ALD samples and two control samples were tested at the same level of humidity. Furthermore, we observed stress-corrosion cracking of the ALD-polymer interfaces. By conducting wedge tests in different levels of humidity, we found that although ALD increased equilibrium interfacial fracture toughness at all humidity, the effect decreased as humidity increased. Scanning-electron microscopy (SEM) of samples after testing provided additional evidence for stress-corrosion cracking of the ALD-polymer interface. These results suggest a new application of ALD for engineering the mechanical properties of chemically inert surfaces.

The metal oxide-metal heterostructure is reported to be effective for improving the performance of photocatalysts. Zinc oxide (ZnO) is widely used as a photocatalyst due to its proper band gap (3.3 eV) that enables outstanding semiconducting characteristics. Also, ZnO has been fabricated in variety of forms including thin film, particles or nanowires, that are useful for catalysis with high surface area. Palladium (Pd) is considered as a well-matching metal with ZnO to synergistically improve photocatalytic performance with an appropriate Fermi level. Recombination of generated charges are prohibited since Pd catches excited charges from ZnO leading to highly improved photocatalytic efficiency.

Accelerating light beam (ALB) or bended light along an arbitrary curvature enables many intriguing applications such as particle manipulation, optical illusion and cloaking¹. To date, one common method to generate ALBs is based on spatial light modulators (SLMs)², which are large and lack spatial resolution due to the large pixel size of the SLMs. Acceleration control of Airy beams (one representative form of ALB) in a photorefractive crystal by applying ultrahigh voltages have been recently reported³. However, the scheme usually only operates at a specific wavelength, or imposes a stringent requirement on the ALB generation process. Furthermore, the ALB is generated inside the crystal, not in free-space.

In this presentation we successfully demonstrated a novel scheme to generate ALBs through an ultrathin all- dielectric optical metasurface consisting of nano-posts with cylindrical cross-sections. By properly configuring the lateral dimensions of the nano-post (major axis length and short axis length), as well as its orientation angle, arbitrary phase modulations of an incident beam can be created, thus providing an efficient approach to generate ALBs. The dielectric metasurfaces are fabricated by first creating the reverse patterns in an electron beam (E-beam) resist, followed by low-temperature atomic layer deposition (ALD) of TiO₂, which fills the openings in the exposure E-beam resist in a conformal manner without causing any degradation of the resist (enabled by low temperature ALD).

The proof-of-concept devices demonstrated include: 1) Generation and switch between two arbitrary ALBs that follow different caustic trajectories in free-space (schematically represented in the following figure); 2) simultaneously achieving efficient and broadband generation and dynamic control of ALBs across the visible region. Our study opens up the possibility of creating ultra-compact, fine-spatial-resolution, and flat-profile nanophotonic platforms for efficient generation and dynamical control of structured light beams.

Literature:

1. Cai, W.; Chettiar, U. K.; Kildishev, A. V.; Shalaev, V. M. Nat. Photonics 2007, 1, 224−227; Valentine, J.; Li, J.; Zentgraf, T.; Bartal, G.; Zhang, X. Nat. Mater. 2009, 8, 568−571.

2. Siviloglou, G. A.; Broky, J.; Dogariu, A.; Christodoulides, D. N. Phys. Rev. Lett. 2007, 99, 213901.

3. Ye, Z.; Liu, S.; Lou, C.; Zhang, P.; Hu, Y.; Song, D.; Zhao, J.; Chen, Z. Opt. Lett. 2011, 36, 3230−3232.

Decorative colouring of pure silver and gold surfaces is an important application for jewelry and coinage, particularly collector’s coins. In order to preserve the purity of these substrates, colours created by adding materials like inks or patinas should be avoided. A novel colouring method that uses careful laser treatment to create a surface with nanoscale, plasmonic features has been developed but the bare surface features undergo dynamic coalescence that dulls and shifts the available colour palette. This work demonstrates that colours generated by nanostructured plasmonic silver surfaces can be preserved and tuned by overcoating with alumina films by ALD. These colours were observed to shift with increasing alumina film thickness.

Two types of laser treatment were used to create surface features on silver that gave rise to colours in the visible spectrum: burst and nonburst. For burst surfaces, the colours first degrade with increasing alumina film thickness but recover at larger thicknesses with an expanded colour range. For nonburst surfaces the colours degraded with increasing thickness without any recovery. Underlying periodic structures specific to the burst method are responsible for this behavior. FDTD modeling of representative surfaces, including the conformal alumina layer, helps explain these colour changes. For alumina thicknesses smaller than the nanoparticle gaps, the changes in the perceived colours are due to the perturbation of plasmonic resonances. For alumina thicknesses larger than the nanoparticle gap, the change in colours originates primarily from the complex reflectance response of the alumina coated structures and modification of the refractive index of the resulting complex surface.

In this paper we introduce a time space divided (TSD) plasma assisted deposition equipment available to in-situ atomic layer deposition (ALD) and chemical vapor deposition (CVD) process and a thin film encapsulation (TFE) of organic light emitting diodes (OLEDs) deposited by the above mentioned equipment, TSD hybrid system. Figure 1 represents a structure of the TSD hybrid system. The 1^st electrode, included protruding rod-like metal bars, is composed of two types of gas injection systems and the protruded parts are inserted into holes of the 2^nd electrode. Each electrode is rigorously designed considering the hollow cathode effect (HCE), the plasma sheaths and the surface area for optimizing RF power efficiency. The remote plasma source cleaning (RPSC) system enables an in-situ cleaning to etch away the film residue in the chamber.

Figure 2 show cross-sectional views of two kinds of TFEs in particle environments. Figure 2-1 represents seam defect of general SiO film, deposited by using the hexamethyldisiloxane (HMDSO), and figure 2-2 represents the cross-sectional view of the coated particle by flowable SiO, named pp-HMDSO. Because deposited films generally growth to the direction from which the gas is flowing, almost films which are deposited by the general CVD process cannot avoid to growth of the defect from the blind spot. So, we, by controlling process conditions, fabricated the TFE which includes flowable SiO film in order to fill the blind spot and hardened the film to reinforce the adhesion to neighbored layers.

Figure 3 show measured characteristics of the TFE structure of the figure 2-2, deposited by the TSD hybrid system at low substrate temperature (<100℃ ). In order to suppress film defects, induced by particles, first, we coated particles with the pp-HMDSO and then, the ALD SiO layer is deposited as a second barrier and a buffer layer between the SiO and the SiN. Even if the film coats conformally without any defect, the permeation of water and oxygen through the film bulk direction would be occurred. So, finally, we coated with the CVD SiN layer for an ultra-low water vapor transmission rate (WVTR). As shown in figure 3, the WVTR of <5x10^-5g/m²day at 40 ℃ /100%RH conditions is measured by MOCON Aquatran2 and also we get the optical transmittance (>95%) and the low film stress (<100MPa) data.

The TSD hybrid system can make various kinds of thin films, such as CVD SiN, SiON, SiO, ALD SiO, SiN, etc, by in-situ ALD or CVD process and also can control characteristics of thin films widely by changing the process conditions. By TSD hybrid system, manufactured by JUSUNG engineering, we expecting to contribute the OLED industry development.

In recent years, HfO₂/ZrO₂-based ferroelectric thin films have been recognized as promising candidates for memory devices and negative-capacitance field-effect-transistors to achieve a further improvement of device performance. The ferroelectric(FE) and antiferroelectric(AFE) properties of these CMOS-compatible oxides have been confirmed to originate from the polar orthorhombic phase and non-polar tetragonal phase, respectively. In this work, we report the significant impact of ALD-deposited interfacial layers on the microstructures and FE/AFE properties of ZrO₂ thin films. Sub-nanometer interfacial layers deposited by plasma-enhanced atomic layer deposition are intentionally introduced between the ZrO₂ thin film and the electrodes of metal-insulator-metal structures to tailor the crystalline phase and ferroelectricity of the ZrO₂. The interfacial layers boost the formation of orthorhombic ZrO₂, leading to significant enhancement of the ferroelectricity with a significant increment of the remanent polarization. On the other hand, another interfacial layers contribute to the formation of tetragonal ZrO₂, giving rise to the dramatic transformation of ZrO₂ from ferroelectricity to antiferroelectricity. The findings indicate that interface engineering by ALD is an effective and advantageous approach to tailor the FE/AFE characteristics of materials.

[1] Schlitz et al., Appl. Phys. Lett. 112, 242403 (2018)