Elsevier

Applied Surface Science

Volume 510, 30 April 2020, 145422
Applied Surface Science

Full Length Article
Facile fabrication of microlenses with controlled geometrical characteristics by inkjet printing on nanostructured surfaces prepared by combustion chemical vapour deposition

https://doi.org/10.1016/j.apsusc.2020.145422Get rights and content

Highlights

  • CCVD of a silane followed by CVD of a fluorosilane results in surfaces with a large contact angle.

  • Solvent-free photocurable hybrid ink enables direct curing of microlenses just after inkjet printing.

  • Microlenses with controlled geometrical characteristics have been prepared using this industrially viable method.

Abstract

Precise positioning of microlenses with well-defined optical characteristics is key in the further development of CCD cameras, biosensors or optical fiber interconnects. Inkjet printing enables accurate microfabrication of microlenses however inks generally employed for this purpose contain solvents that need to be evaporated before the lens solidification process. Besides, the receiving substrate needs to be conditioned, sometimes using complex photolithographic steps to lead to large contact angles of the deposited ink drop that are needed to attain large numerical aperture microlenses. This paper describes the fabrication of microlenses with controlled geometrical characteristics by inkjet printing a solvent-free photocurable formulation. The employed photoacid catalyzed organic–inorganic hybrid ink can be cured just after deposition, without any intermediate evaporation or annealing step, enormously simplifying microlens fabrication process. Besides, a simple combustion chemical vapour deposition process, leading to a porous layer with nano-roughness, followed by a silanization step using a fluorosilane enables the generation of a surface that provides access to a large range of contact angles for the ink drops that are printed on this surface. Single droplet microball lenses with contact angles up to 115°, beyond the hemispherical microlenses, are demonstrated with this industrially viable, cost-effective and high-throughput method.

Introduction

Microlenses and microlens arrays are used to redirect light and improve collection efficiency in sensing devices, light-coupling in optical fiber communication systems, light extraction from light emitting diodes (LEDs) or optical performance in displays [1], [2], [3], [4], [5], [6], [7], [8], [9]. In many of these applications, the precise positioning of microlenses with short focal lengths and high numerical aperture (NA) is highly demanded. For example, microball lenses beyond the hemispherical geometry, adequately integrated and precisely positioned, are useful elements to efficiently focus light from a laser diode into single-mode fibers for communication systems. The development of photolithographic techniques, that enabled the extraordinary progress of the semiconductor industry, also facilitated the first practical miniaturization of lenses into microlenses [10]. These were prepared starting with circular microposts created by using a photoresist. The posts were later thermally treated to melt the resist leading, through a thermal reflow process, to a lens with a spherical profile dictated by surface tension. Curing of the photoresist ultimately stabilizes the final microlens shape [11], [12]. Other photolithographic approaches such as gray-scale microlens projection as well as other chemical or mechanical methods, for example hot embossing, have also been used in the production of microlens arrays with well-defined profiles redirecting light in a controlled fashion [13], [14], [15], [16]. The preparation of microlenses has also been undertaken using inkjet printing technology. Compared to photolithographic or embossing based methods, inkjet printing digitally positions droplets of ink leading to microlenses at well-defined locations on virtually any type of substrate, with no contact and minimal post-processing [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30].

Typically a polymerizable material solution is used as a base material for the microlens fabrication. The final refractive index of the microlens, key in its optical performance, can be tailored to a large extent through the ink formulation. A solvent, not present in the final solid lens, is usually employed to adjust the ink properties so jetting is possible without clogging of the printhead nozzle [19], [23], [24], [27]. The prepolymer solution is then jetted and deposited on top of a substrate. After all the solvents are evaporated, the so generated microlens, can be fixed by polymerization [18], [19]. Disadvantageously, this solvent evaporation post-printing step complicates the manufacturing process and it can additionally result in undesired changes in the geometry and the optical properties of the final solid microlenses.

Besides the intrinsic properties of the ink, the interaction of this ink with the target substrate is also of key importance for the resultant microlens geometry and therefore for its optical characteristics. For the typical volumes of single droplets deposited by inkjet printing, their final shape on the substrate is not influenced by gravity, generally reaching a spherical cap geometry [31]. For a flat surface, the solid angle of the spherical cap is dictated by the contact angle and their volume determined by the amount of deposited ink [32]. For example, in order to generate microlenses with high NA, modified substrates leading to sessile ink droplets with large contact angles have been typically targeted. Simple chemical modification of conventional glass with fluorosilanes has been used for this purpose however the achieved ink contact angles are limited and therefore the lens NA is limited too (NA values around 0.4 are typically achieved) [18]. Besides, photolithographically structured substrates with pillars have been generated to create, on top of them, high NA microlenses by inkjet printing, however the preparation process is complex and involves sophisticated and expensive equipment [20], [21], [22], [33], [34]. Interestingly, Luo et al [27] prepared nanostructured layers consisting on fluorinated nanopillars to create high NA microlenses on top of these layers. The method to prepare them comprises the generation of a thin ZnO seed layer (30 nm thick) onto a glass substrate using a radio frequency (RF) magnetron sputter-deposition technique. Afterwards, ZnO nanopillars are synthesized onto the substrate in an aqueous solution of zinc nitrate hexahydrate and methenamine in deionized water. Finally a C4F8 coating is done by inductively coupled plasma chemical vapour deposition process, using C4F8 and CHF3 gases. The so prepared surfaces can be used to generate microlenses beyond the hemisphere (contact angle of 115°). Overall, despite all these advances and the intense efforts, the developed techniques involve complicated steps, difficult to implement in an industrial environment and in a cost-effective way.

Here we report a facile, robust, cost-effective and high-throughput method for the preparation of microlenses with well-defined optical characteristics on flat substrates by inkjet printing technology. A solvent-free photoacid catalyzed hybrid organic–inorganic polymerizable formulation has been used as an ink for microlens preparation. The deposited ink is directly cured just after deposition without need of any evaporation post-printing step. As a result, the lens geometry is immediately fixed after deposition, with no post-processing, facilitating the control of the lens optical properties. In order to tune the contact angle of the prepolymer droplets, a simple two-step method to modify the receiving substrate has been developed. Firstly, the combustion chemical vapour deposition (CCVD) of a silane leads to a nanostructured layer on top of the substrate. Secondly, this layer is chemically coated with a fluorosilane by conventional chemical vapour deposition (CVD) [35]. The introduction of the nanoroughness on the surface by CCVD and its subsequent fluorination by CVD leads to larger contact angles of the photopolymerizable ink droplets and microball lenses beyond the hemisphere can be attained. The drops can be immediately fixed by ultraviolet (UV) photopolymerization. The preparation of microlenses with controlled geometrical characteristics using this simple method, compatible with a continuous industrial production process, together with the lens morphological and optical characterization are described in this paper.

Section snippets

Ink material

3-glycidoxypropyltrimethoxysilane (GPTMS), a monomer with an epoxy and a trialkoxysilane group, was purchased from Alfa Aesar (Haverhill, MA, USA). The epoxy resin Epikote 157, a monomer with eight epoxide groups, was obtained from Momentive (Waterford, NY, USA). Dimethoxydiphenylsilane (dPDMS), a disilane monomer bearing two aromatic rings was acquired from Aldrich (Madrid, Spain). Triarylsulfonium hexafluorophosphate salts (50% in propylene carbonate) purchased from Aldrich (Madrid, Spain)

Microlens ink material

To generate the microlenses by inkjet printing we use as ink a solvent-free photoacid catalyzed organic–inorganic hybrid formulation previously developed in our laboratory, named HRI ink [37]. The ink consists mainly of three different monomers: an organic–inorganic hybrid molecule named GPTMS (50 wt%), bearing an epoxy and a triethoxysilane group, the multifunctional epoxide Epikote 157 (25 wt%) and dPDMS (25 wt%) as disilane. These two last monomers, are expected to polymerize with the epoxy

Conclusions

In this work we have presented a facile and robust methodology for the preparation of microlenses with controlled geometrical characteristics on flat substrates by using inkjet printing technology. A solvent-free photocurable organic–inorganic hybrid formulation has been used as an ink for microlens fabrication. This ink uses no solvent and therefore can be cured immediately after printing, without any evaporation or annealing post-printing step, notably simplifying the microlens preparation

CRediT authorship contribution statement

Jorge Alamán: Conceptualization, Methodology, Investigation, Writing - original draft. Ana María López-Villuendas: Investigation. María López-Valdeolivas: Investigation. María Pilar Arroyo: Methodology, Investigation. Nieves Andrés: Methodology, Investigation. Carlos Sánchez-Somolinos: Conceptualization, Methodology, Writing - original draft.

Declaration of Competing Interest

The authors declared that there is no conflict of interest.

Acknowledgements

Carlos Sánchez Somolinos acknowledges funding from the Spanish Ministry project BIO2017-84246-C2-1-R, Gobierno de Aragón project LMP150_18 and FEDER (EU). Authors would like to acknowledge the use of Servicio General de Apoyo a la Investigación-SAI, Universidad de Zaragoza.

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