Highly uniform residual layers for arrays of 3D nanoimprinted cavities in Fabry–Pérot-filter-array-based nanospectrometers

Abstract

Miniaturized optical spectrometers can be implemented by an array of Fabry-Pérot (FP) filters. FP filters are composed of two highly reflecting parallel mirrors and a resonance cavity. Each filter transmits a small spectral band (filter line) depending on its individual cavity height. The optical nanospectrometer, a miniaturized FPbased spectrometer, implements 3D NanoImprint technology for the fabrication of multiple FP filter cavities in a single process step. However, it is challenging to avoid the dependency of residual layer (RL) thickness on the shape of the printed patterns in NanoImprint. Since in a nanospectrometer the filter cavities vary in height between neighboring FP filters and, thus, the volume of each cavity varies causing that the RL varies slightly or noticeably between different filters. This is one of the few disadvantages of NanoImprint using soft templates such as substrate conformal imprint lithography which is used in this paper. The advantages of large area soft templates can be revealed substantially if the problem of laterally inhomogeneous RLs can be avoided or reduced considerably. In the case of the nanospectrometer, non-uniform RLs lead to random variations in the designed cavity heights resulting in the shift of desired filter lines. To achieve highly uniform RLs, we report a volume-equalized template design with the lateral distribution of 64 different cavity heights into several units with each unit comprising four cavity heights. The average volume of each unit is kept constant to obtain uniform filling of imprint material per unit area. The imprint results, based on the volume-equalized template, demonstrate highly uniform RLs of 110 nm thickness.

Role of Nanoimprint Lithography for Strongly Miniaturized Optical Spectrometers

“…These cavity heights do not include the residual layer thickness (of about 100 nm). The arrangement of these cavities follows the volume-equalized design [ 80 ] to ensure the lateral homogeneity of residual layer as well as possible.…”

“…Thus, extremely high vertical accuracy is obtainable at the expense of residual layers and requires complicated reservoir technologies. Even if our zero-residual layer technology [ 85 ] is applied, these reservoir technologies and volume equalizing technologies [ 80 ] must be applied. Figure 13 depicts a schematic for FP array structures, including two different mesa heights, to visualize the vertical resolution limit in 3D nanoimprint lithography.…”

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

“…This value has been obtained from the previous investigation series on this topic. In addition, the arrangement of these cavities follows the volumeequalized design (Memon et al 2015) to preserve the lateral homogeneity of residual layer as well as possible. Each filter array contains 64 different cavity heights, which is able to transmit 64 distinct transmission lines.…”

Miniaturized Interferometric Sensors with Spectral Tunability for Optical Fiber Technology—A Comparison of Size Requirements, Performance, and New Concepts

“…To keep the residual layer constant in lateral directions despite the different mesa volumes, four adjoining mesa heights are laterally grouped in a 2 × 2 submatrix consistently, where all sub-matrices of four mesas have the same combined volume. Thus, the lateral positioning of these cavities is done applying the volumeequalized design methodology [86] to ensure that the residual layer thickness is as constant as possible.…”

“…Furthermore, very high vertical resolution can be obtained if larger residual layers are acceptable or reservoir for excess resist are included in the design. Even if our technology which enables zero-residual layers is used [92], these reservoir technologies and volume equalizing technologies [86] have to be applied.…”