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Optical interferometric sensors have acquired significant importance in metrology and information technology, especially in terms of their potential application in launching size, selectivity, sensitivity, resolution, spectral tuning ranges, efficiency, and cost. However, these demands are often contradictory and counteract one another, and are thus difficult to simultaneously fulfill during their interaction. This review focuses on a detailed comparison of seven different strongly miniaturized sensor concepts investigating the limits of these demands. For the visible and near-infrared spectral range, seven optical sensors were reviewed based on the following methodologies: classical optical transmission and reflection gratings, arrayed waveguide gratings, static Fabry–Pérot (FP) filter arrays, MEMS tunable FP interferometers, MEMS tunable photonic crystals, plasmonic filters, and fiber tip sensors. The comparison between the selected concepts concentrates on (i) the minimum space required for a particular spectral range, (ii) resolution, (iii) the integration in optical fiber technology, (iv) tunability to save space, (v) efficiency in using available light, (vi) multiplexing, (vii) miniaturization limits, and (viii) the potential of nanoimprint for cost reduction. Technologies for enhancing efficiency to obtain more available light and their applicability to the different methodologies were studied.

Miniaturization of optical spectrometers can be achieved by Fabry–Perot (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.

Ultraviolet Substrate Conformal Imprint Lithography (SCIL) is an economic nanolithography technique of imprinting high-fidelity patterns over large areas. It is an improvement to conventional UV-Nanoimprint lithography (NIL) with high resolution, flexibility and conformal imprinting. However the aspect-ratio of SCIL is limited due to its patterning by capillary forces and the PDMS material has very low Young’s modulus (

Optical measurement techniques are widely applied in high-resolution contour, topography and roughness measurement. In this context vertical scanning white-light interferometers and confocal microscopes have become mature instruments over the last decades. The accuracy of measurement results is highly related not only to the type and physical properties of the measuring instruments, but also to the measurement object itself. This contribution focuses on measurement effects occurring at edges and height steps using white-light interferometers of different numerical apertures. If the edge is perfectly perpendicular, batwing effects appear at height steps. These batwings show maximum height if the height-to-wavelength-ratio (HWR) is about one forth or three forth, and they disappear if the HWR value is about an integer multiple of one half. The wavelength that is relevant in this context is the effective wavelength, i.e. the center wavelength of the illuminating light multiplied by a correction factor known as the numerical aperture correction. However, in practice the edges are usually not perfectly perpendicular. In this case, the measurement results depend also on the derivative of the surface height function and they may differ from theory and the prediction according to the HWR value. Measurements of such steps show systematical effects depending on the lateral resolution of the instrument. In this context, a Linnik interferometer with a magnification of 100x and NA = 0.9 is used to characterize the three dimensional topography of more or less rectangular calibration specimens and quasi-perpendicular structures produced by the nanoimprint technology. The Linnik interferometer is equipped with LED light sources emitting at different wavelengths, so that the HWR value can be changed. This is possible since the high NA objective lenses show a rather limited depth of focus such that the temporal coherence gating may be replaced by focal gating in this particular instrument. In addition, measurement results are compared with those achieved by a Mirau interferometer of NA = 0.55. A commercial confocal microscope with NA of 0.95 serves as a reference instrument for further comparison. Numerical simulations considering diffraction effects are carried out in order to explain the experimental results obtained by the different white and colored light interferometers

Fabry–Perot (FP) filter arrays fabricated by high-resolution three dimensional (3D) NanoImprint technology are presented. A fabrication process to implement 3D templates with very high vertical resolution is developed. Filter arrays with 64 different cavity heights have been fabricated requiring only one single imprint step. Different optical methods are involved in this paper to characterize geometric and spectral properties. In order to investigate the transfer accuracy of the surface quality from the NanoImprint template to the filter, we use white light interferometry (WLI) measurements. Surface roughness and structure height accuracy of

In daily life networked sensing systems become more and more extensive, important and accurate. Often, actuators and sensors, are based on MEMS (micro-electromechanical-systems), such as Fabry-Perot resonators or micro mirrors. To open up new fields for electrostatic actuable micro mirror array modules, we are going to develop a specific system for light steering or visual protection in airplanes. Interaction of the sensors with the aircraft interior will be used to monitor the health of the passengers as well as the air conditions. This creates new possibilities of highly accurate interpolation of temperature and illumination control. Information about the outside conditions, e.g. sun position and cloudiness, will be gathered to optimize the lighting for pilot in the cockpit as well as energy efficiency of the aircraft cabin itself. Additionally we developed nano mirrors to be applied to implement interactive features, such as advertisements or multimedia and informational displays.

Producing miniaturized and low-cost components of high precision optical sensors has attracted a wide variety of fields. Fabry-Perot-based filters can be considerably miniaturized without decreasing high spectral resolution, compared to grating-based spectroscopic devices. The filters are assembled by using MEMS (micro-electro-mechanical-systems) technologies. Tunable Fabry-Perot filters enable miniaturized sensor components for spectrometers covering a wide area of applications in the visible and near infrared range. We demonstrate a concept of micromachined tunable Fabry-Perot filters, suitable for precisely selecting very narrow wavelength bands and, thus, by tuning for measuring spectra in the visible and infrared spectral range. These tunable filters integrated into optical devices act as sensors for process monitoring, color detection or medical applications. The design and the fabrication processes are compatible with nanoimprint requirements to assemble arrays of filters for increasing the spectroscopic range.

Nowadays, nanosensor arrays have a great potential in industrial process control, automatic identification or medical applications. Regarding the medical field, the possibility to detect characteristic biomarkers or environmental toxins in living spaces in a transcutaneous, non-invasive and user-friendly way will open new markets. Furthermore, by combination of an array of miniaturized spectroscopic sensors a fast and reliable overview-scan of the most important parameters will be enabled. In this paper, we describe the fabrication of a miniaturized spectrometer (nanospectrometer), the measurement setup used for its characterization and reveal measurement results. In this work, a microscope spectrometer is used to measure transmission spectra of nanosensor arrays in visible spectral range. This system can provide many advantages in terms of high accuracy, measurement speed and the ability of measuring microstructure sizes. We focus on the spectral resolution as key results of our optical characterization. We have obtained good agreement between theoretical model calculations and two different experimental set-ups: our fast and flexible microscope spectrometer and the Lambda 900 reference spectrometer which calibrates and verifies the flexible system. The agreement with the simulations provides a proof of concept of our novel nanospectrometer.

High vertical resolution 3D Nanoimprint technology for nanophotonic applications is presented. Opaque and transparent 3D templates with 64 different heights are fabricated. Surface roughness and structure height accuracy better than 1nm are achieved. The conservation of these critical parameters during the 3D Nanoimprint process is proved. A Fabry-Perot filter array is shown as a possible application with a filter transmittance of up to 90% and a linewidth (FWHM) of less than 3nm.

Miniaturized spectrometers can be implemented using Fabry–Perot (FP) filter arrays. Such filters are defined by two parallel mirrors with a resonance cavity in between. For high optical quality, ion beam sputtered distributed Bragg reflectors (DBRs), with alternating high and low refractive index material pairs, can be used as the FP mirrors; while 3D nanoimprint technology provides an efficient way of implementing multiple organic FP cavities of different heights in a single step. However, the high residual stress in ion beam sputtered films results in poor adhesion between the DBR films and the organic polymer cavities, causing debonding of the DBR. Therefore, the residual stress of the ion beam sputtered films forming the DBRs must be reduced. Niobium pentoxide (Nb2O5) and silicon dioxide (SiO2) are used as the DBR materials in this work due to their high index contrast, resulting in high reflectivity for only a few alternating pairs. Stress relaxation in ion beam sputtered Nb2O5 and SiO2 films is achieved in this work by deposition under simultaneous high energy ion bombardment (oxygen and argon gas mixture) from a second ion source. Using this technique, the film density and hence compressive film stress for both Nb2O5 and SiO2 films is reduced without introducing any additional optical absorption in the films. FP filter arrays fabricated with stress reduced Nb2O5 and SiO2 as DBR films exhibit high optical and mechanical performance, with good adhesion between the films and the polymer cavity.

We report on the fabrication of miniaturized NIR spectrometers based on arrays of multiple Fabry-Perot (FP) filters. The various cavities of different height are fabricated via a single patterning step using high resolution 3D nanoimprint technology. Today, low-cost patterning of extended cavity heights for NIR filters using the conventional spin-coated nanoimprint methodology is not available because of insufficient coating layers and low mobility of the resist materials to fill extended cavity structures. Our investigation focuses on reducing the technological effort for fabrication of homogeneous extended cavities. We study alternative cavity designs, including a new resist and apply large-area 3D nanoimprint based on hybrid mold and UV Substrate Conformal Imprint Lithography (UV-SCIL) to overcome these limitations. We compare three different solutions, i.e. (1) applying multiple spin coating of the resist to obtain thicker initial resist layers, (2) introducing a hybrid cavity (combination of a thin oxide layer and the organic cavity) to compensate the height differences, and (3) optimizing the imprint process with a novel resist material. The imprint results based on these methods demonstrate the implementation of NIR FP filters with high transmission intensity (best single filter transmission >90 %) and small line widths (

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.