Fabrication and characterization of multi-stopband Fabry–Pérot filter array for nanospectrometers in the VIS range using SCIL nanoimprint technology

Abstract

Miniaturization of optical spectrometers can be achieved by Fabry-Pérot (FP) filter arrays. Each FP filter consists of two parallel highly reflecting mirrors and a resonance cavity in between. Originating from different individual cavity heights, each filter transmits a narrow spectral band (transmission line) with different wavelengths. Considering the fabrication efficiency, plasma enhanced chemical vapor deposition (PECVD) technology is applied to implement the high-optical-quality distributed Bragg reflectors (DBRs), while substrate conformal imprint lithography (one type of nanoimprint technology) is utilized to achieve the multiple cavities in just a single step. The FP filter array fabricated by nanoimprint combined with corresponding detector array builds a so-called "nanospectrometer". However, the silicon nitride and silicon dioxide stacks deposited by PECVD result in a limited stopband width of DBR (i.e., < 100 nm), which then limits the sensing range of filter arrays. However, an extension of the spectral range of filter arrays is desired and the topic of this investigation. In this work, multiple DBRs with different central wavelengths (λ c ) are structured, deposited, and combined on a single substrate to enlarge the entire stopband. Cavity arrays are successfully aligned and imprinted over such terrace like surface in a single step. With this method, small chip size of filter arrays can be preserved, and the fabrication procedure of multiple resonance cavities is kept efficient as well. The detecting range of filter arrays is increased from roughly 50 nm with single DBR to ~ 163 nm with three different DBRs.

Role of Nanoimprint Lithography for Strongly Miniaturized Optical Spectrometers

“…Nanoimprint technology is able to mold the whole cavity structure of millions of pixels, and the mold/template can be replicated and reused several times. Theoretically, there is no limit to the number of pixels that can be fabricated in a single cavity layer imprint step [ 32 , 33 , 34 , 35 , 36 ], in contrast to alternative technologies [ 27 , 28 , 29 , 30 ]. In a batch process many FP filter arrays can be imprinted at the same time, potentially resulting in substantial cost reduction.…”

“…Next, the 3D cavity layer is defined by a single 3D nanoimprint step, followed by deposition of a specific Si 3 N 4 protection layer that also acts as the first quarter-wave layer of the top DBRs. The tricky technology involved for this protection layer is reported in [ 36 ]. Then, negative photoresist is used again to cover distinct parts of the samples (including three bottom DBRs, the 3D cavity layer, and the protection layer) and precisely structure the blank deposition areas before depositing top DBR 1.…”

“… Optical microscope image of three FP filter arrays with corresponding optical transmission spectra below. Reproduced with permission from [ 36 ] © Springer Applied Nanoscience, 2018. …”

“…The third interferometric spectrometer is based on the Fabry–Pérot (FP) principle [ 4 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 , 53 , 54 , 55 , 56 , 57 ]. Although only two parallel transparent dielectric mirrors are involved, the FP interferometer is also highly complex and involves multipath interference.…”

“…As a selection of alternatives to these mini grating spectrometers, we compare in this review paper four strongly miniaturized optical spectrometers ( Table 1 ): AWG devices [ 20 , 21 , 22 , 23 , 24 , 25 , 26 ], static FP filter arrays on a charge-coupled device (CCD) or complementary metal oxide semiconductor (CMOS) sensor arrays [ 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 ], tunable MEMS FP filter arrays [ 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 , 53 , 54 , 55 , 56 ] on photodetector (PD) arrays, and tunable MEMS photonic crystal (PC) filters [ 61 , 70 , 71 , 72 , 73 , 74 ]. At first glance, we identify that all these alternatives are smaller in size and reveal higher resolutions, as well as smaller FWHM than the grating spectrometers.…”