Organic-inorganic-hybrid-polymer microlens arrays with tailored optical characteristics and multi-focal properties

Plano-convex microlens arrays of organic-inorganic polymers with tailored optical properties are presented. The fine-tuning of each microlens within an array is achieved by confining inkjet printed drops of the polymeric ink onto pre-patterned substrates. The lens optical properties are thus freely specified, and high numerical apertures from 0.45 to 0.9 and focal lengths between 10 μm and 100 μm are demonstrated, confirming theoretical predictions. Combining nanoimprint lithography approaches and inkjet printing enables using the same material for the microlenses and their substrates, improving the optical performances. Microlens arrays with desired specifications are printed reaching yields up to 100% and high lens reproducibility with standard deviations of the apparent contact angle under 1° and of the numerical apertures and focal lengths under 6%. Microlens arrays involving lenses with different characteristics, e.g. multi focal length, and thus focal planes separated by only few microns are printed with the same reproducibility.

Inkjet printing and high aspect ratio structuring for polymer-based micro and nano systems

Vahid Fakhfouri

Inkjet printing of functional materials, photolithography and micromolding are 3 different techniques largely used in the field of micro-electromechanical systems (MEMS) with increasing interest for the fabrication of new devices and the development of new applications. This Ph.D. thesis provides new insights into each of these fields and, by combining them, aims to enable the access to new application fields. Complementary to photolithography, inkjet printing is increasingly considered a cost-effective and flexible microfabrication method to structure functional materials. The ease of mass fabrication and the inherent flexibility of inkjet technology make it a suitable method for the manufacturing of microsystems and components. The results presented in this dissertation summarize recent achievements in this relatively new technology for the development of miniaturized devices. First the piezo-acoustic inkjet printing technology is described and a theory to explain the drop generation process is developed and evaluated. The experimental work was focused on the drop-on-demand inkjet printing of viscous polymers, particularly SU-8, a negative tone photo-resist epoxy. It validates the theoretical predictions and shows the shortcomings of the existing theories. The camera-assisted high accuracy alignment (< 3 µm misalignment) technique developed in the frame of this work enables the precise positioning of microdrops on pre-structured substrates and accurate filling of high aspect ratio micromolds. Additionally, polymeric microlenses were also Inkjet-printed and characterized. The highly reproducible optical properties of such microlenses, i.e. the focal length in the range of 40-150 µm and the numerical aperture in the range of 0.25-0.75, can be adjusted for different applications in micro-optical systems. An innovative technique, based on micromolding, for the microfabrication of high aspect ratio ceramic structures using polymeric precursors is also presented. Polyureasilazane, a precursor for silicon-carbide-based ceramics was investigated. Pipettes and inkjet printers were used to fill high-aspect ratio SU-8 molds. The micromolded structures were released using a sacrificial layer. After a pyrolysis step which enables the polymer-to-ceramic transformation in a simple tube furnace, the final structures showed that this process is very reliable as the mold shapes were perfectly replicated and a smooth surface was obtained. This technique is an interesting candidate for the fabrication of ceramic devices for harsh environments and high temperatures. Incorporation of luminescent nanocrystals (NCs) in SU-8 is another contribution of this thesis. For this purpose, the selection of an appropriate solvent for both NCs and the epoxy-resist formulation is fundamental in directing the nanocomposite resist preparation. The NC-modified SU-8 was successfully patterned and high aspect ratio structures with micrometer resolution were fabricated, showing that the overall UV-structuring capability of the modified resist was not affected by the NC incorporation. The obtained microstructures were shown to retain the NC emission properties. Finally the possibility to combine inkjet printing, micromolding and photopolymerization for the fabrication of Hybrid AFM probes by a single-plane surface-micromachining technique was demonstrated. Hybrid AFM probes, with a tip made of a hybrid organic/inorganic polymer and a cantilever and a holder made of SU-8 were micro-structured, characterized and tested for AFM imaging. The results were compared with a commercial silicon probe and a SU-8 AFM cantilever. This technology can be used to convey new properties to scanning microscopy probes for new sensing applications or to improve the quality of the existing devices. Functional materials can be inkjet-printed to form the tips with a minimum material waste. This thesis is intended to contribute to the advancement of micromachining technologies in the area of micro-devices with enhanced properties. In particular, it shall contribute to the cost effective fabrication of hybrid components made of different functionalized materials.

EPFL2008

Microlenses with defined contour shapes

Jürgen Brugger, Victor Javier Cadarso Busto, Loïc Jacot-Descombes

Ink-jet printing of optical ink over SU-8 pillars is here proposed as a technology for obtaining microlenses with shape control. To demonstrate the flexibility of this method, microlenses with five different contour shapes (ranging from circular and elliptical to toric or more advanced geometries) have been fabricated. Furthermore, the optical properties of the different fabricated lenses have been experimentally investigated. Focal distance, numerical aperture (NA) and full-width at half maximum (FWHM) of the microlenses have been determined. Arrays of microlenses showed an identical behavior with a standard deviation in the total intensity of only 7%. Additionally, the focal plane of the fabricated symmetric microlenses and the Sturm interval of the non-symmetric ones have been obtained. The experimental results demonstrate the validity and flexibility of the proposed technology.

Optical Society of America2011

Inkjet printing of SU-8 for high-aspect ratio spherical polymeric microstructures

Jürgen Brugger, Victor Javier Cadarso Busto, Rosario Maurizio Gullo, Loïc Jacot-Descombes

SU-8 exhibits interesting mechanical and optical properties for various MEMS applications, such as the development of micro lens and ball-like lens arrays. One of the most common method to fabricate such structures, is thermal reflow. However, a promising alternative is inkjet printing. It is generally reported that inks with a dynamic viscosity below 40 mPa•s and a surface tension from 10-3 to 10-1 N•m-1 can be inkjetted. The ability of inkjet printing SU-8 based material and fabricate micro lens has already been demonstrated. Another area of interest for SU-8 is MEMS packaging. Once the MEMS active parts are fabricated, it is often important to protect them from environmental damages or perturbations affecting their functionality. In this work, SU-8 based material has been inkjet printed for the development of optical micro lenses with controlled contact angle and for MEMS like parts embedding. Chemical functionalization and dedicated topographical features allows self-alignment of the deposited drops, and using the fabrication method described in high drop confinement is also performed. It was possible to fabricate spherical shapes with different radius of curvature on practically 1 mm diameter round platforms in a controlled manner. The experimental standard deviation is below 1.5% from theoretical expectations. Furthermore a precise positioning of the inkjetted drop can be used to micromachine overhanging ball-like lens structures with edge angle up to 160°. This paper will present three case studies; a) the fabrication of micro lens arrays, with a controlled radius of curvature and thus controlled focal lens, b) the possibility to prototype ball like lens structures and c) the ability to precisely encapsulate MEMS parts.

2011

Inkjet printing and high aspect ratio structuring for polymer-based micro and nano systems

Vahid Fakhfouri

EPFL2008