Anna Rissanen

MOEMS miniature spectrometers using tuneable Fabry-Perot interferometers

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.

MEMS and piezo actuator-based Fabry-Perot interferometer technologies and applications at VTT

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.

Advances in miniature spectrometer and sensor development

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 chemometrics modeling. 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.

Spectral imaging device based on a tuneable MEMS Fabry-Perot interferometer

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.

Low-temperature processes for MEMS device fabrication

The high temperatures typical in semiconductor and conventional MEMS fabrication limit the material choices in MEMS structures. This paper reviews some of the low-temperature processes and techniques available for MEMS fabrication and describes some characteristics of these techniques and practical process examples. The techniques described are plasma-enhanced chemical vapour deposition, atomic layer deposition, reactive sputtering, vapour phase hydrofluoric acid etching of low-temperature oxides, and low-temperature wafer bonding. As a practical example of the use of these techniques, the basic characteristics of a MEMS switch and other devices fabricated at VTT are presented.

Bragg reflectors for large optical aperture MEMS Fabry-Perot interferometers

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.

Use of ALD thin film Bragg mirror stacks in tuneable visible light MEMS Fabry-Perot interferometers

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 defined.

Compact large-aperture Fabry-Perot interferometer modules for gas spectroscopy at mid-IR

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.

Vapor-phase self-assembled monolayers for improved MEMS reliability

This paper presents the application of vapor-phase DDMS (dichlorodimethylsilane) self-assembled monolayer (SAM) coating which significantly reduced stiction behavior in optical MEMS components exposed to humidity. Previously SAMs have been deposited in liquid form, making them unsuitable for application in high aspect ratio MEMS/NEMS structures; now vapor-phase SAM deposition is a novel option for improving MEMS in-use reliability. DDMS and ODS (n-octadecyltrimethoxysilane) SAM coatings were tested on surfaces with different pre-treatments and the quality of coatings was assessed through static water contact angle measurements and humidity exposure tests for both test membrane structures (100% stiction on uncoated structures vs. 17% stiction of DDMS SAM coated structures) and optical MEMS FPI components (0% stiction of DDMS SAM coated components). The obtained contact angle of the DDMS SAM coating was ∼ 104°. Both long term stability and thermal stability of the DDMS SAM coatings were found to be good.

Gas detection with microelectromechanical Fabry-Perot interferometer technology in cell phone

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.