Nano-finishing of MEMS-based platforms for optimum optical sensing

Abstract

The surface finish of the microelectromechanical systems substrate, particularly the ones that are deployed in chip-based optofluidic systems, is of utmost importance, and the overall surface finish helps in preventing light scattering and associated losses. The proposed system is made up of a microchannel with a coating on its interior which acts as a cladding layer and possesses an air-like refractive index. The water-based solutions with refractive indices higher than the coating, when confined within such channels, act as waveguides with a refractive index difference which would allow grazing incidence at the solution film interface. The microchannel is fabricated over a piece of Si along <100> direction with the help of lithography and wet etching technique. After wet chemical etching of silicon, multiple pyramidal hillocks with overall large surface roughness is observed which are not appropriate for loss-free light transmission and cause various optical losses. So the overall optimization of surface roughness created by the etching processes is critical from an optical standpoint. Roughness in the microchannel surface mainly arises due to wet etching through tetra methyl alcohol, potassium hydroxide, potassium ferricyanide and isopropyl alcohol. In this work, we have obtained surface finish upto ~1.33 nm at an etch rate of 141 nm/min which is obtained by tweaking the composition of the participating reagents in the etchants and also the etching temperature. The surface roughness obtained is quite small in comparison to the wavelength range of the visible spectrum and thus losses were greatly avoided. The low refractive index films over silicon substrate were characterized by field emission scanning electron microscopy, energy dispersive analysis of X-ray, atomic force microscopy, 3D optical profilometry and ellipsometry. The transmission results show that transmission loss was reduced by 27.42% for the coated samples with 33 nm surface roughness as compared to surface with 250 nm roughness.

Sensing performance of room temperature operated MEMS gas sensor for ppb level detection of hydrogen sulfide: a review

“…Microfabrication techniques were successfully used to miniaturize the device size by fabricating a gas sensor. These techniques can be used for either bulk or surface micromachining of silicon in order to integrate multiple components on a single wafer [18]. The fabrication of sensors over silicon substrates typically involves the repetitive uses of the following techniques; such as deposition, photolithography, and etching [19,20].…”

Si-based MEMS resonant sensor: A review from microfabrication perspective

“…Hara et al deposited an AlN film using RF magnetron reactive sputtering technique that gives lower process temperature than CVD [94]. Gupta et al reported an optical sensing work with a surface coating of porous polymer-film; a reduction in surface-roughness up to 33 nm is obtained in contrast to the 235 nm surface-roughness of uncoated surfaces, which in turns reduces 27.42% relative transmission losses [95]. Prior to the deposition step, a high temperature baking step is performed to breaks down the SiO2 layer and allows the Si to reflow and smooth the surface features and finally leave the Si with a low-surface roughness [96].…”

Development of spiral square coils for magnetic labelling detection in microfluidic systems

“…The development of the microfluidic systems components is associated with the microtechnology based on silicon materials. They were developed after the discovery of deep etching and bonding methods [1], [2], which allow to realize microchannels in silicon [3], [4], glass [5], [6], polymer material polydimethylsiloxane (PDMS) [7], [8] and SU-8 [9].…”