The Fabry-Perot interferometers (FPI) are essential components of many hyperspectral imagers (HSI). While the Piezo-FPI (PFPI) are still very relevant in low volume, high performance applications, the tunable MOEMS FPI (MFPI) technology enables volume-scalable manufacturing, thus having potential to be a major game changer with the advantages of low costs and miniaturization. However, before a FPI can be utilized, it must be integrated with matching optical assembly, driving electronics and imaging sensor. Most importantly, the whole HSI system must be calibrated to account for wide variety of unwanted physical and environmental effects, that significantly influence quality of hyperspectral data. Another challenge of hyperspectral imaging is the applicability of produced raw data. Typically it is relatively low and an application specific software is necessary to turn data into meaningful information. A versatile analysis tools can help to breach the gap between raw hyperspectral data and the user application. This paper presents a novel HSI hardware platform that is compatible with both MFPI and PFPI technologies. With an MFPI installed, the new imager can have operating range of λ = 600 - 1000 nm with FWHM of 15 - 25 nm and tuning speed of < 2 ms. Similar to previous imager in Ref. 1, the new integrated HSI system is well suited for mobile and cloud based applications due to its small dimensions and connectivity options. In addition to new hardware platform, a new hyperspectral imaging analysis software was developed. The new software used in conjunction with the HSI provides a platform for spectral data acquisition and a versatile analysis tool for a processing raw data into more meaningful information.
Recently developed tunable MEMS Fabry- Perot interferometers based on Ag thin-film mirrors have enabled building highly miniaturized spectral imagers covering almost the complete VNIR wavelength range. The level of miniaturization required by modern smartphone industry has created extremely compact, high performance electronics and camera technologies and by utilizing these technologies together with the novel MEMS FPI’s, it is possible to create extremely compact spectral imagers while still achieving good performance. This paper presents a spectral imager design that can be fit inside an envelope of 1 cubic inch (25.4 × 25.4 × 25.4 mm3 ) and it will be capable of recording images at freely selectable wavelengths within the range of ca. 650 nm - 950 nm. The imager field of view is ca. 12.5° × 10° and the image size is 640 × 512 pixels. Nominally the imager will be focused from ca. 0.5 m to infinity, but with additional optics it is possible to use the imager as a microscope. The compact size of the imager allows the easy integration to almost any available platform, including small drones, nanosatellites or planetary rovers, where small size is essential. It is also possible to integrate the imager to handheld devices, so the potential field of applications will be extensive.
This paper presents a novel miniaturized hand-held hyperspectral imager for VNIR range of λ = 600 – 900 nm based on MEMS Fabry-Perot interferometer (MFPI) technology. In recent years, tunable MFPI optical filters have been utilized to demonstrate sensors for mobile applications, including CO2 smartphone sensor for mid infra-red region and hyperspectral iPhone for visible spectrum. This hand-held sensor module targets the VNIR range in order to enable food sensing, while utilizing low-cost camera technology to enable potential volume scalability for future sensing applications. The sensor module is wirelessly connected to a mobile device, which enables further application algorithms development and cloudbased solutions.
Small, tunable MEMS Fabry-Perot interferometer (FPIs) have recently been demonstrated to enable hyperspectral imaging in mobile devices, so far using Bragg reflector mirror technologies, which however limit the device tuning range. This paper presents the realization of a novel MEMS FPI structure based on Ag thin film mirrors (AgMFPI), which allows tuning the entire visible - very near infrared (VNIR) wavelength range (for example 450 - 900 nm) with one single component. The characterized transmission of the components is 12 - 45% with full-width-half-maximum values (FWHM) between 11 - 18 nm using 2 mm optical aperture, while using 3 mm optical aperture results in FWHM values of 12 - 20 nm. A very compact hyperspectral imager can thus be built to cover this spectra by combining this AgMFPI and a typical RGB type of image sensor.
VTT’s Fabry-Perot interferometers (FPI) technology enables creation of small and cost-efficient microspectrometers and hyperspectral imagers – these robust and light-weight sensors are currently finding their way into a variety of novel applications, including emerging medical products, automotive sensors, space instruments and mobile sensing devices. This presentation gives an overview of our core FPI technologies with current advances in generation of novel sensing applications including recent mobile technology demonstrators of a hyperspectral iPhone and a mobile phone CO2 sensor, which aim to advance mobile spectroscopic sensing.
This paper demonstrates a mobile phone- compatible hyperspectral imager based on a tunable MEMS Fabry-Perot interferometer. The realized iPhone 5s hyperspectral imager (HSI) demonstrator utilizes MEMS FPI tunable filter for visible-range, which consist of atomic layer deposited (ALD) Al2O3/TiO2-thin film Bragg reflectors. Characterization results for the mobile phone hyperspectral imager utilizing MEMS FPI chip optimized for 500 nm is presented; the operation range is λ = 450 – 550 nm with FWHM between 8 – 15 nm. Also a configuration of two cascaded FPIs (λ = 500 nm and λ = 650 nm) combined with an RGB colour camera is presented. With this tandem configuration, the overall wavelength tuning range of MEMS hyperspectral imagers can be extended to cover a larger range than with a single FPI chip. The potential applications of mobile hyperspectral imagers in the vis-NIR range include authentication, counterfeit detection and potential health/wellness and food sensing applications.
This paper presents near- and mid- infrared (NIR-MIR) wavelength range optical MEMS Fabry-Perot interferometers (FPIs) developed for automotive and multi-gas sensing applications. MEMS FPI platform for NIR-range consist of LPCVD (low-pressure chemical vapour) deposited polySi-SiN λ/4-thin film Bragg reflectors, with the air gap formed by sacrificial SiO2 etching in HF vapour. Characterization results for the NIR MFPI devices for λ = 1.5 – 2.0 μm show resolution of 15 nm at the optimization wavelength of 1750 nm. We also present a MIR-range MEMS FPI for λ = 2.5 – 3.5 μm, which utilizes silicon and air in within the Bragg reflector structure to provide a high contrast for improved resolution. Characterization results show a FWHM (Full Width Half Maximum) of 20 nm in comparison to the 50 nm resolution provided by earlier MEMS FPIs realized for hydrocarbon sensing with conventional CVD-thin film materials. The improved resolution and the extended operation region shows potential to enable simultaneous sensing of CO2 and multiple hydrocarbons.
VTT Technical Research Centre of Finland has developed a miniaturized optical sensor for gas detection in a cell phone. The sensor is based on a microelectromechanical (MEMS) Fabry-Perot interferometer, which is a structure with two highly reflective surfaces separated by a tunable air gap. The MEMS FPI is a monolithic device, i.e. it is made entirely on one substrate in a batch process, without assembling separate pieces together. The gap is adjusted by moving the upper mirror with electrostatic force, so there are no actual moving parts.
VTT has designed and manufactured a MEMS FPI based carbon dioxide sensor demonstrator which is integrated to a cell phone shield cover. The demonstrator contains light source, gas cell, MEMS FPI, detector, control electronics and two coin cell batteries as a power source. It is connected to the cell phone by Bluetooth. By adjusting the wavelength range and customizing the MEMS FPI structure, it is possible to selectively sense multiple gases.
VTT’s optical MEMS Fabry-Perot interferometers (FPIs) are tunable optical filters, which enable miniaturization of
spectral imagers into small, mass producible hand-held sensors with versatile optical measurement capabilities. FPI
technology has also created a basis for various hyperspectral imaging instruments, ranging from nanosatellites,
environmental sensing and precision agriculture with UAVs to instruments for skin cancer detection. Until now, these
application demonstrations have been mostly realized with piezo-actuated FPIs fabricated by non-monolithical assembly
method, suitable for achieving very large optical apertures and with capacity to small-to-medium volumes; however
large-volume production of MEMS manufacturing supports the potential for emerging spectral imaging applications also
in large-volume applications, such as in consumer/mobile products. Previously reported optical apertures of MEMS FPIs
in the visible range have been up to 2 mm in size; this paper presents the design, successful fabrication and
characterization of MEMS FPIs for central wavelengths of λ = 500 nm and λ = 650 nm with optical apertures up to 4
mm in diameter. The mirror membranes of the FPI structures consist of ALD (atomic layer deposited) TiO2-Al2O3 λ/4-
thin film Bragg reflectors, with the air gap formed by sacrificial polymer etching in O2 plasma. The entire fabrication
process is conducted below 150 °C, which makes it possible to monolithically integrate the filter structures on other ICdevices
such as detectors. The realized MEMS devices are aimed for nanosatellite space application as breadboard
hyperspectral imager demonstrators.
Miniaturization and cost reduction of spectrometer and sensor technologies has great potential to open up new
applications areas and business opportunities for analytical technology in hand held, mobile and on-line applications.
Advances in microfabrication have resulted in high-performance MEMS and MOEMS devices for spectrometer
applications. Many other enabling technologies are useful for miniature analytical solutions, such as silicon photonics,
nanoimprint lithography (NIL), system-on-chip, system-on-package techniques for integration of electronics and
photonics, 3D printing, powerful embedded computing platforms, networked solutions as well as advances in
This paper will summarize recent work on spectrometer and sensor miniaturization at VTT Technical Research Centre of
Finland. Fabry-Perot interferometer (FPI) tunable filter technology has been developed in two technical versions: Piezoactuated
FPIs have been applied in miniature hyperspectral imaging needs in light weight UAV and nanosatellite
applications, chemical imaging as well as medical applications. Microfabricated MOEMS FPIs have been developed as
cost-effective sensor platforms for visible, NIR and IR applications. Further examples of sensor miniaturization will be
discussed, including system-on-package sensor head for mid-IR gas analyzer, roll-to-roll printed Surface Enhanced
Raman Scattering (SERS) technology as well as UV imprinted waveguide sensor for formaldehyde detection.
VTT has developed Fabry-Pérot Interferometers (FPI) for visible and infrared wavelengths since 90’s. Here we present
two new platforms for mid-infrared gas spectroscopy having a large optical aperture to provide high optical throughput
but still enabling miniaturized instrument size. First platform is a tunable filter that replaces a traditional filter wheel,
which operates between wavelengths of 4-5 um. Second platform is for correlation spectroscopy where the
interferometer provides a comb-like transmission pattern mimicking absorption of diatomic molecules at the wavelength
range of 4.7-4.8 um. The Bragg mirrors have 2-4 thin layers of polysilicon and silicon oxide.
This paper presents a novel MOEMS Fabry-Perot interferometer (FPI) process platform for the range of 800 – 1050 nm. Simulation results including design and optimization of device properties in terms of transmission peak width, tuning range and electrical properties are discussed. Process flow for the device fabrication is presented, with overall process integration and backend dicing steps resulting in successful fabrication yield. The mirrors of the FPI consist of LPCVD (low-pressure chemical vapor) deposited polySi-SiN λ/4-thin film Bragg reflectors, with the air gap formed by sacrificial SiO2 etching in HF vapor. Silicon substrate below the optical aperture is removed by inductively coupled plasma (ICP) etching to ensure transmission in the visible – near infra-red (NIR), which is below silicon transmission range. The characterized optical properties of the chips are compared to the simulated values. Achieved optical aperture diameter size enables utilization of the chips in both imaging as well as single-point spectral sensors.
Tunable MOEMS Fabry-Perot interferometers (FPIs) are key elements in the miniaturization of spectroscopic instrumentation. Robustness and high reliability of the MOEMS structure are important factors especially for sensors utilized in challenging environments such as in space- and automotive applications. This paper presents reliability assessment of two types of MOEMS optical filters; a tunable ALD (atomic layer deposition) –based surface micromachined FPI for visible – near-infrared range, and a tunable FPI for mid- infrared applications based on LPCVD (low-pressure chemical vapor deposition) thin-film micromachining. High-G shock tests were performed on both MOEMS FPIs. The FPI structures can survive mechanical impact up to 18 000 G without any detectable changes in the capacitance, while detected failure mechanisms in this range arise from packaging and not from the MOEMS structures. The effect of DDMS SAM (dichlorodimethylsilane self-assembled monolayer) coating to prevent in-use stiction was evaluated in both humidity- and impact tests. In humidity tests, 20% stiction rate in non-coated devices vs. 0% stiction rate in DDMS-coated LPCVD FPIs under pull-in was observed. These results indicate good shock-impact robustness for both types of surface-micromachined structures, while DDMS SAM can be utilized to improve in-use reliability of MOEMS.
New tuneable MOEMS filters have been developed to cover the spectral range from 400 to 750 nm. Compared with previous MEMS based visible light filters, these Fabry-Perot Interferometers (FPIs) have increased transmission (90%), spectral resolution of ∼ 4 to 9 nm, and larger aperture diameter (2 mm), which allows them to be used in spectral imaging devices. We present the fabrication process and characterization of tuneable MOEMS FPIs for central wavelengths of λ = 420 nm and λ = 670 nm. Miniature imaging spectrometers have potential novel applications in diagnostics and health care, bioprocess, and environmental monitoring, process analytical instrumentation, and water-quality analysis.
The trend in the development of single-point spectrometric sensors is miniaturization, cost reduction and increase of
functionality and versatility. MEMS Fabry-Perot interferometers (FPI) have been proven to meet many of these
requirements in the form of miniaturized spectrometer modules and tuneable light sources. Recent development of
MEMS FPI devices based on ALD thin film structures potentially addresses all of these main trends. In this paper we
present a device and first measurement results of a small imaging spectrometer utilizing a 1.5 mm tuneable MEMS FPI
filter working in the visible range of 430-580 nm. The construction of the instrument and the properties of the tuneable
filter are explained especially from imaging requirements point of view.
This paper presents the fabrication of large-aperture low-pressure chemical-vapour deposited (LPCVD) Bragg reflectors
utilizing low-stress polysilicon (PolySi) and silicon-rich silicon nitride (SiN) λ/4-thin film stacks. These structures can
function as the upper mirror in a MEMS FPI device. High aspect-ratio mirror membranes were successfully released for
5 - 10 mm diameter range by sacrificial SiO2 etching in HF vapour. Optical simulations are presented for the Bragg
reflector test structures designed for FPIs operating in the NIR range and the properties such as release yield and
mechanical stability of the released LPCVD deposited polySi-SiN mirror membranes are compared with similar released
atomic layer deposited (ALD) Al2O3-TiO2 λ/4-thin film mirror stacks. The realization of these Bragg reflector
structures is the first step in the process integration of large-aperture MEMS FPI for miniature NIR imaging
spectrometers, which can be applied to a variety of applications ranging from medical imaging and diagnostics to spaceand
environmental monitoring instrumentation.
This paper discusses the use of ALD thin films as Bragg mirror structure materials in MEMS Fabry-Perot
interferometers in the visible spectral range. Utilizing polyimide sacrificial layer in the FPI fabrication process is also
presented as an alternative method to allow higher temperature (T= 300 °C) ALD FPI processing. ALD Al2O3 and TiO2
thin films grown at T= 110 °C are optically characterized to determine their performance in the UV - visible range
(λ>200nm) and effects of the ALD temperature on the thin film stacks and the FPI process is discussed. Optically
simulated 5-layer Bragg mirror stacks consisting of ALD Al2O3 and TiO2 for wavelengths between 420 nm and 1000 nm
are presented and corresponding MEMS mirror membrane structures are fabricated at T= 110 °C and tested for their
release yield properties. As a result, the applicable wavelength range of the low-temperature ALD FPI technology can be
Miniaturized spectrometers covering spectral regions from UV to thermal IR are of interest for several applications. For
these purposes VTT has for many years been developing tuneable MEMS-based and more recently piezo-actuated
Fabry-Perot Interferometers (FPIs). Lately several inventions have been made to enter new wavelengths in the VIS range
and enlarge apertures of MEMS devices and also extending the wavelength range of piezo-actuated FPIs. In this paper
the background and the latest FPI technologies at VTT are reviewed and new results on components and system level
demonstrators are presented. The two FPI technologies are compared from performance and application point of view.
Finally insight is given to the further development of next generation devices.