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

Imran Memon,Hartmut Hillmer,Yannan Shen,Abdullah Khan,Carsten Woidt

doi:10.1007/s13204-015-0468-9

Abstract

Miniaturized optical spectrometers can be implemented by an array of Fabry–Perot (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 FP-based 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.

Highlights

Low cost and miniaturized FP-based optical spectrometers are attractive for modern sensing systems e.g., for medical technologies, safety and security, process monitoring, etc
To achieve highly uniform residual layer (RL), 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
A Fabry–Perot (FP) filter comprises two highly reflecting dielectric mirrors implemented as Distributed Bragg Reflectors (DBRs) and a transparent polymer cavity with a defined thickness which determines the spectral position of the characteristic filter line of each single filter

Summary

Introduction

Low cost and miniaturized FP-based optical spectrometers are attractive for modern sensing systems e.g., for medical technologies, safety and security, process monitoring, etc. The optical nanospectrometer, a miniaturized spectrometer, is based on static FP filter arrays with 3D nanoimprinted cavities and a corresponding detector array where each filter transmits a distinct spectral filter line depending on its individual cavity thickness (Wang et al 2013). A Fabry–Perot (FP) filter comprises two highly reflecting dielectric mirrors implemented as Distributed Bragg Reflectors (DBRs) and a transparent polymer cavity with a defined thickness which determines the spectral position of the characteristic filter line of each single filter. Each DBR is based on multiple quarter wave thickness layers of alternating materials and produces a highly reflective band around center wavelength (kc), which is referred to as stopband. Introducing a cavity of halfwave thickness between two DBRs allows a narrow filter line inside the stopband to pass through the filter. The spectral position of the filter line depends on the height of a cavity. Fabrication of precise cavity heights with sub-nm spatial resolution is required to achieve high spectral resolution, low filter line width, and small spectral filter line separation

Methods

Results

Conclusion