Infoscience

Thesis

Wafer-Scale Fabrication of Thin SiN Membranes and Au Films and Membranes with Arrays of Sub-um Holes Using Nanosphere Lithography

In this thesis, the wafer-scale fabrication of SiN membranes, Au films and Au membranes with arrays of sub-µm holes is described. Two conceptually different processes (1) and (2) were developed, both of which are based on nanosphere lithography (NSL) with self-assembled close-packed monolayers of polystyrene (PS) beads. PS beads with a diameter D in the range of 420 nm to 530 nm were used. The hole array periodicity p was thus determined by D. Different bead deposition methods were tested; by spin-coating, a monolayer wafer coverage > 90% could be obtained. By O2-RIE, D could be reduced in a controlled way down to 0.3D. In process (1), holes were etched into the device layer using a hole etch mask made by NSL. The hole size ⌀ was determined by the reduced D. In this way, 100 nm thick SiN membranes with a maximum size of 2400×2400 µm2 and a hole density on the order of 108 holes/cm2 were fabricated by etching holes into the SiN. The membrane release was done in a combined dry-/wet-etch procedure. Similarly, hole arrays were fabricated on 2" glass wafers by sputter-etching into 200 nm thick Au films. With this process, different ⌀, e.g. from 60 nm to 180 nm, could be obtained for initially identical mask holes by tuning the sputter-etch parameters. In process (2), NSL was used to realize high aspect ratio Si and oxidized Si pillars that were subsequently used as a lift-off template. Free-standing, 200 nm thick, 1200◊1200 µm2 large Au membranes with ⌀ from 100 nm to 300 nm were successfully fabricated using Si pillars as KOH lift-off template and a Si-DRIE release procedure. Using oxidized Si pillars for lift-off in HF instead, such hole arrays in Au films could be transferred to flexible, transparent parylene films. The fabricated devices were characterized and successfully tested for stencil and refractive index sensing applications: Holey SiN membranes were bulge-tested and withstood pressures up to 5 bar, showing their suitability for stencil and filtration applications. Stenciling was successfully done for (a) the deposition of arrays of 230 nm in diameter Au and Ag dots onto Si substrates and (b) the etching of holes into a 500 nm thick SiN membrane. The light transmission characteristics of wafer-scale hole arrays in Au films were reproducible for different measurement locations. Similar hole arrays were used to detect refractive index changes in water of varying glycerine concentrations. The transmission through Cr/Au membranes with sub-wavelength hole arrays was measured. Upon Cr removal, the transmission was enhanced by a factor 2.4.

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