ICMCTF 2023 Session C1-1-WeM: Optical Materials and Thin Films I

Development of a scalable nanofabrication technique is essential for the practical realization of nanostructured materials in functional transparent films. META® has developed discrete platform-specific proprietary technologies for large-scale lithography, allowing the manufacture of nanostructures to be carried out with high uniformity and low cost. Using these manufacturing techniques at META®, we design and mass-produce nano-technology-based films with applications in a wide variety of products and markets.

In this talk, I will present large-scale manufacturing techniques at META® including nanoimprinting, roll-to-roll processing, and holographic lithography. I will also discuss various industrial applications in automotive, authentication, 5G communication and consumer electronics. Our customizable patented nano-optics include visual authentication technology, KolourOptik® Stripe, with a unique combination of multiple colors, 3D depth and omni-directional movement. I will describe NANOWEB®, META®’s Roll to roll lithographically fabricated conductor with superior electrical and optical properties. This functional film is configured as a nanostructured conformable metal wire mesh, that is transparent to the naked eye. I will explain how we utilize NANOWEB® to enable smart solutions for distinct functions including electro-optical dimming, transparent antennas, anti-fog heaters and EMI Shielding.

In addition, we are also using a first of its kind Plasma Fusion® deposition technology, which enables high speed coating of any alloy on any type of thin flexible plastic substrate. Large scale and efficient metallization coating of ultra-thin substrate with zero defect, is a critical step for example toward volume production of safe reliable lightweight battery materials, requiring hundreds of millions of square meters per year. We use this metallization step in our roll-to-roll production process for functional films, which for solid state NCORE^TM battery current collectors will reduce battery weight, increase safety, significantly accelerate line speed, and hence reduce cost.

Calcium fluoride (CaF₂) is a ceramic material that exhibits versatile properties such as broadband transparency, lubrication behavior at high temperatures (up to 900 °C), high mechanical stability, and hydrophobicity. CaF₂ can be utilized as an antireflective (AR) coating for different optical needs and applications. Due to its enhanced transmission, mainly in the UV and IR regime, a broader transmission range can be achieved, greater than the common oxide-based coatings (SiO₂, TiO₂, or their combination). Moreover, CaF₂ as a constituent in composite-based coatings can effectively reduce wear and friction in mechanical devices and working tools that are subjected to heavy loads and high temperatures. One of the challenges in CaF₂ sputtering concerns with the low stability of the target under high sputtering power (above ~ 2 W/cm²). Therefore, CaF₂ deposition is usually characterized by low deposition rates. In the current research, CaF₂ coatings were deposited on silicon (Si) substrates by means of a radio frequency (RF, 13.56 MHz) magnetron sputtering technique in an argon atmosphere, and fixed power of 4.4 W/cm², whereas the substrate bias voltage (V_b) was varied from 0 to - 150 V. The role of V_b is to enhance the adatoms' mobility and to increase their probability of residing in permanent binding sites. This lowers the roughness and increases the layer density. The coatings were characterized using SEM/EDS, XRD, and XPS. A spectrophotometer was utilized to study the optical properties of the deposited films. The residual stress was studied using the Stoney formula, while the adhesion was studied by a scotch-tape method. In this research, we report a high deposition rate of 8-25 nm/min, depending on the V_b value, with a maximum deposition rate at the low voltages applied, 0 and - 30 V, as expected. To the best of our knowledge, this high deposition rate of CaF₂ coatings, using RF magnetron sputtering technique, is reported for the first time. The morphology of the surface was changed with increasing V_b, from a rough surface obtained at the low V_b regime to smoother at - 50 to - 75 V, which finally became rougher again at high V_b of - 150 V, because of pits and holes that were generated by the excessive ion bombardment. Moreover, at - 150 V, coatings were delaminated from the Si substrate immediately after deposition. Due to preferential sputtering of the light fluorine, the composition of the CaF₂ coatings turned fluorine deficient with increasing V_b.

A group of metallic materials characterized by low reflectivity is known as black metals. The transition of the metal from being highly reflective to nearly black can be achieved by its preparation in a way providing a highly porous structure . This phenomenon has been widely applied to the synthesis of macroscopic black metal materials based on Ag [1], Au [2], Al [3], Cr [4], Ni [5], Mo [6], P [7], Zn [8] etc. The dominant applications of black metals are in solar cell collectors [9], optical sensing10, electrochemical sensing11 and decorative coatings12. On the other hand, white coatings can find applications where a high degree of specular light reflection is required – e.g. in cooling coatings [13] or decorative coatings [14]. Depending on the function, several properties must be considered, such as absorbance of electromagnetic radiation in specific wavelengths, thermal emittance, thermal stability and wear resistance.

Our study presents a way to make thin porous zinc and zinc oxide nanosystems obtained by reactive sputter deposition with additional postdeposition annealing as functional black coatings. We show that a wide array of morphologies from highly porous nanocoral and branched structures to densely-porous ones can be obtained by addition of oxygen to inert working gas [15,16]. The purely Zn films are characterized by a wide degree of black or off-black colours. They can be oxidized at an elevated temperature to form matt white ZnO nanoporous films. We show the strategy to increase the mechanical stability and the optical quality of the films, by applying additional protective overcoats. We show that the doping with Al increases the thermal stability of the Zn films for harsh applications. We investigate creating patterns in the black and white films. The influence of morphology, crystal structure and chemical composition on the optical and mechanical properties of the films is discussed.

The comprehensive approach allowed us to achieve highly functional black and white coatings based on zinc with the use of a solvent-free deposition technique widely spread in large-scale thin film industries.

References:

[1] 10.1143/JJAP.39.551

[2] 10.3762/bjnano.8.46

[3] 10.1039/D0RA00866D

[4] 10.1016/j.proeng.2013.09.123

[5] 10.3103/S1068375515040055

[6] 10.1016/0927-0248(95)00021-6

[7] 10.1016/S1003-6326(06)60099-X

[8] 10.1016/0040-6090(84)90387-0

[9] 10.1016/j.solener.2022.01.042

[10] 10.3847/1538-4365/ab7af8

[11] 10.1016/j.bios.2013.12.002

[12] 10.1016/j.solmat.2010.05.019

[13] 10.1038/s41467-022-32409-7

[14] 10.1016/S0257-8972(99)00076-6

[15] 10.1002/pssa.201228041

[16] 10.1016/j.apsusc.2016.07.098