Liquid transfer nanoimprint replication on non-flat surfaces for optical applications
Graphical abstract
Highlights
► Demonstrations of the liquid transfer nanoimprint technique on non-flat substrates. ► Creation of hybrid micro optical elements with combined diffractive and refractive structures. ► Nanostructured moth-eye structure imprinted on a spherical micro lens array. ► Anti-reflection texture imprinted on a rough multi crystalline silicon solar wafer.
Introduction
Nanoimprint plays a growing role for nano patterning of three dimensional structures. Therefore nanoimprint is the method of choice for wafer level micro lens fabrication used e.g. in wafer level camera system. But modern optics is evolving from flat towards spherical surfaces. The boosted use of micro optics in consumer electronic products demands low cost fabrication. To meet the demand, a new solution for the enhanced optical system with reduced size and cost is required. This can be realized e.g. by combining refractive and diffractive elements in one device, which is called hybrid or multi-function optics. Such a hybrid optic, however, requires patterns on curved substrates. Methods for patterning technique on curved substrates have been demonstrated by using a flexible PMDS template in a hot embossing process on spherical polymer substrates manually [1], without the need for resist coating. UV nanoimprint processes were applied on sparsely curved substrate either using a gasbag pressure system [2] of capillary force nanoimprint [3]. In both cases the substrates having tens of millimeter radiuses in curvature were used, allowing homogeneous resist layers by spin coating.
But there are very demanding applications where spin coating will not be applicable. The simplest example is sub wavelength anti-reflection patterns on a micro lens. The tools to replicate these complicated structures cannot be realized by simple reflow processes or diamond turning machines, because those do not offer the fine line patterning capabilities. Grey scale lithography combining reflow and optical lithography or e-beam and nanoimprint lithography offer a new path, but are usually limited to rather low aspect or flat geometries [4], [5]. Another important application for patterned nano structures on curved surface are photovoltaic devices. In photovoltaics the use of moth eye patterns to increase cell efficiency by reducing the reflection losses at the surface are of growing interest [6], [7]. Sub wavelength patterns offer the highest performance but cannot be patterned easily on the non-flat surface of a multi crystalline wafer, which is the dominating substrate for the photovoltaic market.
One of the main problems that have to be faced when performing lithography on non-flat or pre-patterned substrates is the deposition of a conformal and homogenous resist layer all over a complex geometry. Here, standard coating methods based on spin-coating are bound to fail. We propose here to prepare both micron and nano sized structures with the best method available and then combine them onto each other by liquid transfer imprint lithography using flexible elastomeric stamps [8], resolving the coating problem on non-flat substrates.
Section snippets
Experimental
For the micro lens experiments a replication of a standard micro lens array for back lighting application was chosen. The master was provided as a nickel shim from Temicon GmbH, Germany, consisting of dense and sparse arrays of half sphere micro lenses with a diameter of 100 μm. The dense areas had a pitch of 200 μm in both directions. The master was treated with a 10 nm anti-adhesion layer formed in plasma in order to reduce adhesion forces during preparation of the stamp. The flexible stamp was
Results and discussion
Fig. 1 shows schematically the template in contact with the micro lens surface at different pressures and the corresponding SEM image after curing and detachment. Depending on the pressure used the lenses are covered more or less with the nano pillar grating. In Fig. 1(a) an imprint pressure of 100 mbar was used. Only at the top area of the lens a nano grating was printed. The template did not reach the space between the lenses due to the low imprint force. A gap of 40 μm between bottom and lower
Summary
We have applied “Liquid Transfer Nanoimprint Lithography” to realize hybrid optic elements like sub wavelength moth eye patterns on micron scale lens array and sub wavelength anti-reflection texturing on multi crystalline wafer for solar cell. In both cases the ability of this process to transfer high resolution patterns on extreme non-flat surfaces was utilized to realize a new combination of optical function on one hand and patterning on severely topographic substrates in general. We showed
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