LIL4NOEMS

Optical modulators are essential components in telecommunication, spectroscopy and microfabrication. External modulators act on the laser beam after its generation, within waveguides, optical fibers or in free space, and usually rely on light propagation through non-linear optical materials to alter the local refractive index or absorption of the material, enabling amplitude, phase, polarization or direction modulation of a continuous or pulsed beam. Although these modulators provide a high frequency response (ranging up to MHz and GHz), they require high modulation voltages, have narrow spectral bandwidth, and have a strong wavelength-dependent response. Free-space mechanical modulators, such as choppers or MEMS mirrors or gratings, provide a wavelength-agnostic intensity modulation with high extinction ratios up to a few tens of kHz, and small signal modulation up to a few 100 kHz. Recently, INL demonstrated a grating-based MEMS optical modulator with an aperture of 100×100 µm2 at 290 kHz operation presenting around 35% shutter coverage, limited by the actuation travel and the 2 µm grating pitch. To reach 1 MHz operation at high extinction ratios (full shutter coverage), a sub-µm periodic grating pitch resolution must be achieved,

Nano-opto-electro-mechanical systems (NOEMS) combine both nanophotonics and optomechanics to confine the electromagnetic and displacement fields at the nanoscale. The fabrication of these devices rely on the advances of nanofabrication methods spearheaded by CMOS and foundry-level technologies. The features and alignment requirements of NOEMS devices range between the nano (optical) and hundreds of microns (electromechanical) scales, and rely on electron-beam or optical stepper lithography for production. E-beam lithography is a maskless technique that allows resolutions down to 10 nm but requires scanning a focused beam on the resist to define the structures, limiting its throughput. Stepper photolithography uses deep (λ~248-193 nm) or extreme (λ~13 nm) ultraviolet radiation and a costly prefabricated mask to expose nanostructures onto the photoresist. Advances in mask magnification and multiple patterning have allowed stepper features to reach down to 8 nm.

Nanophotonic devices largely employ periodic nanostructures to achieve their optical functions. The combination of NOEMS with periodic gratings and subwavelength metamaterials have enabled the tunability of some of these optical properties. Laser interference lithography (LIL) is a powerful flexible technique to produce high-resolution periodic nanostructures over large areas without the use of a mask. LIL exploits the interference in the overlapping region between multiple coherent laser beams, where the periodicity (Λ) is dependent on the wavelength (λ) and angle between the beam and the substrate. The intensity of the interference pattern and the photoresist contrast curves define the duty cycle and sharpness of the nanostructured features, allowing for asymmetric line-spaces, where features can be smaller than half the interference period.

LIL typically requires exposing areas as large as the overlapping multiple beam spots, resulting in distortions at the edges, which must typically be removed in subsequent process steps to define the active optical region or aperture of the device. However, LIL can also be combined with other lithography methods on the same optical photoresist for additional flexibility in feature and region definition. Recently, a combined multiple exposure of LIL and grayscale secondary lithography has been demonstrated for producing nanogratings with same periodicity but varying linewidths (linewidths between 110-250 nm, Λ=600 nm, λ=405 nm). In a similar fashion, an additional flood exposure step using an aperture mask may be employed to eliminate the periodic nanostructures outside the region of interest.

The proposed LIL4NOEMS project compounds on the previous experience of the INL team in lithography and microfabrication process development, in optical MEMS and NEMS development, and in non-linear optics and optical system design. The objective of the LIL4NOEMS project is to develop and explore a novel method to define periodic sub-micron photonic features and metasurfaces on the active optical area of a NOEMS device together with microscale electromechanical features, without relying on e-beam or stepper lithography. In this concept, a multibeam LIL setup will be developed and combined with secondary lithography methods to enable the definition of both microscale and periodic nanoscale features down to 200 nm, targeting the application of a NOEMS shuttering optical modulator with high extinction ratio up to 1 MHz.