Microscopic homodyne interferometry with monochromatic or white light illumination is up to now the most widely used technique for micromechanical devices and MEMS surface profiling. In the last five years its capabilities have been largely extended and this technique can now be applied to out-of-plane or in-plane vibration measurements, to micromechanical testing, to transparent film thickness mapping and to surface spectral reflectivity mapping. In this paper we will review the performances and limits of this technique and its various applications in the MEMS field from technology assessment up to final device characterization. Some guidelines are provided to achieve high frequency vibration measurements, transient response measurements as well as on wafer or in vacuum measurements. Finally, future developments needed are discussed.

In the last decades, electronic speckle pattern interferometry (ESPI) was successfully used for detecting deformation, vibration, and strain on specimens and components with dimensions on the macroscopic level. The outstanding features of ESPI are it's ability for fully automatic operation combined with the excellent sensitivity controlled by the wavelength of laser light. In the field of MEMS' (MicroElectroMechanicalSystems) testing, however, where the object size scales down below one millimeter, a number of problems arises when speckle techniques are to be applied. On the other hand, speckle solutions sound really promising to satisfy certain demands in MEMS technology. In this situation, some recent research concentrated on the further development of speckle interferometry to serve best for the specifics of MEMS characterization and quality assurance. The paper explains the benefits and the application limits of micro speckle interferometry (MSI) and it shows the potential for improvements when a deep UV laser source is used. For the experiments, a new deep ultraviolet micro speckle interferometer (DUV-MSI) was designed operating at 266 nm of wavelength. The implemented optics enables for the measurement of both, in-plane and out-of-plane movements on the microparts. In this way, a complete motion analysis can be performed with nanometer accuracy.

Increasing technological capabilities to produce active microelements (incl. microbeams, micromembranes and micromirrors) and their expanding areas of application introduce unprecedented requirements concerning their design and testing. The paper presents a concept of an optical measurement system and methodology for out-of-plane displacement testing of such active microelements. The system is based on Twyman-Green microinterferometer. It gives the possibility to combine the capabilities of time average and quasi-stroboscopic interferometry methods to find dynamic behavior of active microelements (e.g., resonance frequencies and amplitude distributions in vibration modes). For mapping the zero-order Bessel function modulating the contrast of two-beam interference fringes the four-frame technique is applied. The calibration of the contrast variation in time-averaged interferograms enables quantitative evaluation of the vibration amplitude encoded in the argument of the Bessel function. For qualitative estimation of the vibration amplitude sign a simple quasi-stroboscopic technique is proposed. In this technique, laser pulses have the same frequency as the signal activating the microelement under test. This self-synchronous system enables to visualize the shape of the tested element at maximum deflection. Exemplary results of measurements performed with active micromembranes are presented.

Microscopic interferometry with monochromatic or white light stroboscopic illumination allows time-resolved measurements of out-of-plane MEMS vibration mode shapes with (sub)micron lateral resolution and a vertical resolution in the (sub)nanometer range. In this method, light pulses are synchronized with the sinusoidal excitation voltage to "freeze" microdevice vibrations, and automatic interferogram analysis techniques are used to get a 3D surface profile of the vibrating device. To obtain quantitative measurements of the vibration amplitudes, it is necessary to know in each point the phase delay between the light pulses and the microdevice vibrations. One way is first, to search manually for the phase delay t_max corresponding to an extremum of the vibration cycle, and then to perform two measurements at t_max and t_max+180°. The difference and the sum of these two measurements provide respectively the map of twice the vibration amplitudes and the map of twice the static deformations. Another way is to perform measurements while the phase delay is scanned in order to reconstitute the whole vibration cycle. We investigate herein an alternative method which enables, from only 3 measurements with different phase delays, a fully automatic mapping of the vibration phase, of the vibration amplitudes and of the static deformations. This method is illustrated by monochromatic and white light stroboscopic measurements on micromechanical devices. Factors affecting its accuracy such as the light pulse delay, the light pulse duty cycle and sample drift between acquisitions are analyzed from simulations and measurements.

The material properties of silicon, as well as the planar and monolithic nature of the microstructures make electrostatic field energy conversion the most suitable driving principle on the micrometer scale. Moreover, compared with most other actuation principles, the scaling of electrostatic forces is particularly suitable for actuator downsizing. In spite of the advantages, it is still difficult to obtain appropriate driving characteristics because of silicon based actuator limitations such as small structural height, micrometer gap requirements and material limitations in the shaping process. Actuators require specific tools to verify that their mechanical properties and motions obey the designer's intent. In this paper capabilities of future direct-drive electrostatic actuators SDA (Scratch Drive Actuators) are investigated through the characterisation of their out-of-plane displacements by interferometry. The actuation involves contact interactions by using flexible polysilicon elementary actuator plate. The region of the physical contact is measured using Twyman-Green interferometer incorporated within a metallurgical microscope. The shapes and out-of-plane displacements of microstructures are extracted from interferograms by temporal phase shift method (TPS). Additionally, the results from interferometric method are compared with numerical simulations given by finite elements software - ANSYS.

Thin membranes are part of numerous microelectromechanical systems (MEMS) like sensors and bulk acoustic wave filters for example. In most applications the material properties of the membranes are key parameters for the correct working of the MEMS devices. Measuring bulk acoustic waves excited in MEMS-structures with ultra-short laser pulses is a powerful method for accurate and non-destructive evaluation as well as for the characterization of material properties. The laser based acoustic method generates acoustic bulk waves in a thermo-elastic way by absorbing the pump laser pulses at the surface of the MEMS-structure. The propagating acoustic pulses are partly reflected at any discontinuity of the acoustic impedance. Back at the surface the partly reflected acoustic pulses cause changes of the optical reflection coefficient, which are measured with the probe laser pulses. This technique is used for measuring the bulk wave propagation in very thin membranes. The bulk acoustic wave propagation in freestanding aluminium-silicon nitride multi-layer membranes with total thickness in the order 500 nanometers is measured and discussed. Furthermore comparisons of measurements on freestanding and supported membranes and of thermo-elastic models are presented. The measured results are used for the estimation of the Moduli of the aluminium-silicon nitride multi-layer. The technique presented in this work can also be applied for the characterization of material or geometrical properties of other components of MEMS like ultrasonic reflection layers and cantilevers. The advantage of the method lies in its non-destructive and non-contact approach, which is crucial for very thin and brittle structures.

A digital control system implemented to operate the Microelectromechanical system (MEMS) optical switch is described. This MEMS switch is based on a voltage-controlled moving-mirror structure to control the optical path connections and optical power levels. The tiny MEMS devices may replace bulk optical-mechanical devices in lightwave equipment, and enable new functions not available from conventional devises. Here, by use of the proportional integral (PI) control system, each switch module functions either as a reflective optical switch or a variable attenuator. Moreover, this control system promises to improve the performance of the device, as well as make possible the monolithic integration packaging of MEMS with driving, controlling and signal processing electronics. One more attraction of this control system is its programmability. The rest of the paper describes the control system and experimental results for this MEMS optical switch.

Based on the experiences made with NanoMES the interferometric in situ measuring devices in real-time and during plasma etching of micro- and opto-electronic devices GFM designed a new optical system for interference and imaging. Also the measuring program was upgraded from a 1 dimensional analysis of interference stripes to a 2 dimensional evaluation. GFM and FBH have a Patent that makes it possible to measure distances that are much shorter than the wavelength of the used laser for the interferometer. The optical positioning of the wafers in the etching chambers poses a problem for standard optical imaging, since the objects are very far from the camera lens. Therefore it is impossible to use normal long distance microscope lenses. The new modular design allows special adaptations for special problems and variable magnifications. If required a zoom lens module could be integrated. Another problem for the interferometer are vibrations and shocks. The previous NanoMES used a pulsed laser diode for stroboscopic imaging of the interference stripes. The new measuring system is able to work with a continuous mode (cw) laser. That opens the view for new possible applications. The new NanoMES is developed and tested at the FBH.

The notching and stiction problem, which widely exists in silicon on insulator (SOI) microstructure fabrication, were resolved in this study. In this paper, a new plasma trench technique that is based on the deep reactive ion etching (DRIE) process is proposed. In this modified process, the deep reactive ion etching (RIE) was divided into several steps, where conformal plasma enhanced chemical vapor deposition (PECVD) oxide coating, and directional oxide etch back were employed to prevent the notching effect and the reactive ion etching (RIE) lag effect is also improved. Therefore, the microstructures regardless of the feature sizes could be realized. The stiction problem is eliminated by using dry chemical release replacing wet release in this approach, where the notching effect is used. The notching or footing effect was exploited for attaining the lateral etch following the deployment of the anisotropic plasma etching of the inductively coupled plasma (ICP). This method was proven useful for both the uniform and non-uniform feature designs. With this novel process, the high aspect ratio beams can be obtained. The thickness of the silicon layer is 75 μm, while the depth of the beams is 70 μum where the 5 μm silicon was etched to suspend the movable beams. The aspect ratio is as high as 35. Trenches with very different widths of 2.5 μm and 35 μm are also achieved at the same time.

In this paper we present Deep Lithography with Protons (DLP) as a promising technology for the fabrication of mechanical fiber alignment structures accurately ordered in massive 2D arrays. The fabrication process consists of irradiating PMMA-resist layers with high-energetic proton beams through a lithographic mask with a well-defined circular shape, followed by a selective development of these irradiated zones. To increase the coupling efficiency, we can additionally integrate uniform spherical micro-lenses created by swelling the proton-bombarded zones in a monomer vapor. We highlight the influence of the etching time, the proton beam intensity and the absorbed doses in the PMMA layers on the diameters of the finally developed alignment holes. While selecting the correct process parameters, we prove DLP to be a suitable technology for the fabrication of circular micro-holes with diameters of 125&mum and 155&mum at the front and the back side of a 500&mum thick PMMA plate respectively. We finally illustrate the potentialities of these type of fiber holding plates to realize a user-friendly and accurate 2D fibre positioning component.

This paper reviews and introduces the fundamentals of photonic bandgap crystal, nano-fabrication technologies and the potential applications of new generation optical devices for telecommunication applications. The integration of MEMS with photonic bandgap crystal promises greater flexibility because of the greater control over the properties of photonic crystals than over the electronic properties of semiconductors. The photonic bandgap crystals technology is expected to an even greater role in the 21st century, particularly in the optical-communications industry where photonic crystals could address many problems that currently constrain the integration technology of optical device and system.

Microinterferometric tomography method for determination of 3D refractive index distribution in phase elements is described. Applications of this method to measurement of gradient index fibers, fiber splices and single mode fiber are presented. Initial results of holey fiber testing are given and future trends in development of this method (applications to photonic structures) are discussed.

In high speed grinding research it is required to measure temperature within the workpiece. Present techniques are thermocouple based, and often suffer from excessive electrical noise on the signal. This paper presents a number of novel fiber optic sensing devices that overcome this limitation and also offer greater performance. Optical fiber devices are simpler to place in situ prior to the machining tests and they offer faster response and greater sensitivity than was previously possible. Results are presented from machining tests and the new devices are compared with each other and previous techniques.

A classical cracked metallic structure repaired with a "smart" bonded composite patch has been studied, using finite element analysis, in order to determine the debonding detection capabilities of the optical network and to select the appropriate optical fibers paths and Bragg Grating sensors locations. The patch is bonded over a cracked aluminum plate, by means of a thin adhesive layer, while the primary loading axis of the metal is assumed to be parallel to the direction of the optical fibers. Different optical fiber paths and sensor positions were considered and their ability to measure the variation of the developed strain field due to the patch debonding propagation around the crack tip was studied. It was concluded that a fiber optics network is capable of evaluating the increasing debonding area around the crack tip and can provide adequate information concerning the critical parameters required for the monitoring of the structural integrity of composite patch reinforced structures (i.e. strains developed at the patch debonding boundary and position of the crack tip). At least two Bragg Grating sensors should be used at each side of the crack per optical fiber, to enable adequate monitoring of the adhesive debonding and crack propagation.

In this paper we describe a highly miniaturized sensor head based on digital holography for endoscopic measurements. The system was developed to measure the shape and the 3D-deformation of objects which are located at places that cannot be accessed by common measurement systems. Therefore, a miniaturized optical sensor including a complete digital-holographic interferometer with a CCD-camera is placed at the end of a flexible endoscope. The diameter of the head is less than 10 mm. The system enables interferometric measurements with a speed of three reconstructions per second and it can be used outside of the laboratory in a usual environment. Shape measurements are performed with two wavelength contouring, while the deformation is measured with digital holographic interferometry. To obtain full 3D-data in displacement measurements the object can be illuminated sequentially from three different illumination directions. To increase the lateral resolution temporal phase shifting is used.

In this paper we study silicon MEMS (Microelectromechanical systems) structures subjected to thermal loading. Digital holography has been investigated as inspection tool to evaluate the deformation induced by the thermal loading. Application of DH on structures with several different geometries and shapes, like cantilever beams, bridges and membranes is reported and result will be discussed. Dimensions of the inspected microstructures, varies in the range 1÷50μm. The experimental results shown that a "bimorph-effect" induces a deformation in MEMS structures. The difficulties encountered in performing the deformation analysis by digital holography in real-time will be afforded and discussed. A method with automatic focus tracking in Digital Holography is proposed allowing inspection of MEMS, under thermal loading, in real-time.

Two novel micro/nano moire method, SEM scanning moiré and AFM scanning moire techniques are discussed in this paper. The principle and applications of two scanning moire methods are described in detail. The residual deformation in a polysilicon MEMS cantilever structure with a 5000 lines/mm grating after removing the SiO₂ sacrificial layer is accurately measured by SEM scanning moire method. While AFM scanning moire method is used to detect thermal deformation of electronic package components, and formation of nano-moire on a freshly cleaved mica crystal. Experimental results demonstrate the feasibility of these two moire methods, and also show they are effective methods to measure the deformation from micron to nano-scales.

Optical probing is a non invasive tool useful to characterize the vibrations of the small moving components of microsystems (MEMS/MOEMS). This paper presents two complementary methods that can sense in-plane components of the vibration. The first one is a heterodyne interferometer, commonly used for out-of-plane component detection. The edge of the sample partially occults the laser beam, and, consequently, the intensity is amplitude modulated when the sample vibrates. The electronics has been modified so that both phase and amplitude of the output signal are extracted. Actual sensitivity is about 10^-11 m/√Hz. In the second gradient method, a parallel acquisition of synchronous images is performed with a camera and a microcomputer, which stores the successive images for subsequent processing. Before digital lock-in processing, the images sequence is inter-correlated and interpolated to increase the accuracy of the method. This simple processing technique allows nanometer sensitivity. Both techniques are presented, analyzed and compared from theoretical and experimental point of view.

Several optical methods have been developed for the measurement of in-plane vibration of microscopic objects. However most of them need a scattering surface or a specific surface structuring. A low cost method which has not these limitations is optical stroboscopic microscopy combined with image processing by optical flow techniques. Previous works have shown that a nanometric sensitivity can be obtained. In this paper, we investigated several subpixel image processing methods for in-plane vibration measurements of MEMS by this technique. Emphasis was put on whole displacement field measurements and on fast algorithms able to process a large sequence of images without the need of a multi-resolution approach to get local vibration amplitudes. It is notably shown that use of spatiotemporal regularity between images is an efficient way to reduce noise and that a resolution in the 0.01 - 0.03 pixel range can be achieved. Results are applied to in-plane vibration local measurements in two perpendicular directions at video rate as well as to full-field mapping of in-plane vibration mode of electrostatically actuated MEMS devices in SOI technology.

Surfaces as well as interfaces between two neighboring materials are often subjected to various diffusion processes. Such diffusion processes like oxidation or migration of atoms of neighboring materials can cause layers having gradually varying mechanical properties -- like density, Young’s modulus, or shear modulus -- perpendicular to the surface or interface. The growing miniaturization of MEMS devices enlarges the relative size of these layers and thus enhances the importance of phenomena occurring at such material or phase interfaces thus demanding a detailed quantification of its mechanical properties. In this investigation particular interest is drawn on the question how the propagation characteristics of bulk acoustic waves are affected by diffusion layers. The reflection and transmission behavior of bulk acoustic waves encountering a continuum having a spatially dependent sound velocity is discussed based on numerical simulations as well as on experimental verifications. In contrast to previous work done in this field in which diffusion effects are generally considered as undesirable phenomena, the deliberate realization of microstructures having well defined gradually varying material properties in one or more dimensions represents a goal of this investigation. For metallic thin film multi layers thermally induced diffusion processes have shown to be an easy and reliable technique for the realization of layered structures having continuously varying mechanical properties within several 10 nanometers. Among the experimental methods suitable for the in-depth profiling of submicron metallic thin films providing resolutions of several nanometers, are short pulse laser acoustic methods, Rutherford Backscattering Spectroscopy (RBS), and Glow Discharge Optical Emission Spectroscopy (GDOES). Short pulse laser acoustic methods and Rutherford Backscattering Spectroscopy (RBS) have the advantage to be nondestructive. The short pulse laser acoustic method is described in detail and RBS measurements are presented for verification purposes. Finally potential engineering applications like micro-machined spectrum analyzers, acoustic isolation layers, and band pass filters, operating at very high frequencies are presented.

While testing electrical properties in microsystems is a well-developed art, the testing of mechanical properties of MEMS devices is not. There is a great need for techniques that will permit the evaluation of MEMS devices, in all stages of manufacturing, with respect to material and micromechanical properties. In this contribution we propose a new approach, based on the integrated optical read-out using a Mach-Zehnder interferometer, monolithically integrated into the PZT actuated membrane.

In this paper, the application of fiber interferometry in the nano-scale displacement measurement of microelectromechanical system (MEMS) device is being presented. Fiber optic interferometry combines the benefits of the optical fiber such as lightweight, small size and wide bandwidth with the high resolution, high sensitivity capability of the interferometry. It also provides easier setup and offers lower energy loss than the conventional free-spaced interferometry. The fiber optic interferometric system comprises a laser source and a 2 X 2 fiber coupler. The reference arm and the sensing arm of the interferometer are formed within a single output of the coupler. The resultant interference intensity is measured at one of the fiber coupler input. The fiber optic interferometry could be used for the MEMS with moving structure. A case study is being carried out to investigate the displacement of the micromirror in the MEMS Fabry-Perot Filter. The mirror is being driven by the comb drive actuator under the effect of applied voltage. It selectively reflects certain wavelengths while allows others to pass through determined by the air cavity length. The displacement under different applied voltages will be measured using the fiber optic interferometry. The experiment and the result will be demonstrated.

Fizeau interferometers with an additional diffractive optical element are frequently used for measuring spherical and aspherical surfaces. We present a new design, were the Fizeau principle is now perfectly fulfilled, by generating reference and measuring wavefront on the last optical surface, carrying a diffractive optical element. Several advantages of this design are discussed and proved experimentally.