Geometrical properties of multilayer nano-imprint-lithography molds for optical applications
Graphical abstract
Highlights
► Multi-layer masters with μm and nm patterns in one mold. ► Double polymer mold casting for tone inversion. ► Residual layer free reverse imprint.
Introduction
Integrated optical sensors, planar optical devices and circuits as well as large area functional optical patterns make new device functionalities possible or can help to increase the performance of a certain application. With three-dimensional features, the efficiency of optical coatings like anti-reflective patterns [1] can be increased or totally new device functionalities using gratings [2] or photonic crystals [3] become possible in planar optics. Patterning technologies like grayscale lithography [4] are not able to produce the patterns needed for optical applications with the necessary accuracy. Alternatives like three-dimensional laser direct-write [5] or focused ion-beam patterning [6] are not scalable to large area production.
Nano-imprint-lithography (NIL) is a diffraction unlimited replication technology [7] which offers large area patterning with high throughput [8] and moderate cost. Moreover, the direct patterning of high aspect-ratio structures and the use of functional resist are possible [9]. With NIL, one-dimensional functional elements of optical [10], micro-fluidic [11] or micro-mechanical systems [12] have been produced.
If a dedicated master is available, NIL is able to reproduce multilayered or even three-dimensional structures without increasing the fabrication effort. For the fabrication of those masters, various processes using different patterning technologies have been proposed [13], [14], [15], [16]. To be able to obtain wafer-scale master structures with full pattern diversity in silicon, previously a process has been shown using contact lithography as pattering technology [17].
This paper reports on the production of such silicon NIL molds using projection lithography to extend the resolution limit while maintaining the other parameters like etch depth and sidewall angle. The in-plane and out-of-plane resolution capability has been investigated concerning optical applications. Further, the replication of the silicon master in a UV-transparent polymer daughter mold is shown. Subsequently a tone inversion by casting another sub-master from the daughter mold and a residual layer free replication of the structures in a UV-curing polymer on a silicon substrate is shown.
Section snippets
Multilayer nano-imprint molds
The basic process flow for the production of multilayer NIL molds can be obtained from Fig. 1. Silicon is used as mold material due to its excellent mechanical properties and the availability of structuring methods as it is the mature material of the semiconductor industry. In the multilayer mold production process, a previously patterned etch mask is transferred into the silicon using reactive ion etching (RIE). Afterwards, the structures are entirely covered with silicon dioxide by plasma
Polymer mold casting, tone inversion and feature replication
The fabricated silicon molds cannot be used for standard through mold UV–NIL exposure as they are not transparent for UV radiation. Unless they can be used in a through substrate exposure system or using NT–UV–NIL [19], the use of polymer working stamps allows a through mold exposure. Polymer working stamps can be cast from master structures with lower effort compared to an imprint step used for replication and can moreover be reused for several imprints. For the replication 15 × 15 mm2 large
Conclusion
The fabrication of silicon master structures with two height levels using standard microelectronics processes and projection lithography has been shown. The achieved in-plane and out-of-plane resolution as well as the overlay accuracy of the two pattern layers are sufficient to gain planar optical waveguides with functional elements like gratings. For the feature replication, polymer workings stamps have been used, which enable a double tone inversion as well as the application of UV-curing
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