Michal Jozwik

Active microelement testing by interferometry using time-average and quasi-stroboscopic techniques

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.

A micromachined silicon-based probe for a scanning near-field optical microscope on-chip

The novel concept of an integrated scanning near-field optical microscope on-chip (SOMOC) has been presented. SOMOC consists of a micromachined cantilever probe and an emission-detection device realized by the use of a vertical-cavity surface-emitting laser (VCSEL). The construction and the first technological results of the silicon probe with a light transparent SiO2 tip have been described. The basic mechanical properties of the probe have been determined by stroboscopic interferometry.

Design, testing, and calibration of an integrated Mach-Zehnder-based optical read-out architecture for MEMS characterization

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 allow 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 (MZI), monolithically integrated with the piezoelectric (PZT) actuated membrane.

New generation of fully integrated optical microscopes on-chip: application to confocal microscope

In the paper the new concept of fully integrated scanning confocal optical microscope on-chip is proposed. The operation of this microscope combines the 3-D transmissive scanning of VCSEL laser beam by use of two MOEMS scanners, and active signal detection, based on the optical feedback in the VCSEL laser cavity. The silicon-based electrostatically driven scanners provide controlled movement of two convex microlenses, working as an objective lens of microscope. Glass microlenses are monolithically integrated on movable silicon tables of scanners. The first results of technological investigation on the microscope components are presented.

Characterization of internal stress of silicon oxinitride thin films fabricated by plasma-enhanced chemical vapor deposition: applications in integrated optics

In view of applications of SiOxNythin films in MOEMS technology, a study of optomechanical characteristics of this material PECVD deposited are investigated. To optimize the quality of SiOxNy layers we established the relationship between the chemical properties, optical performances and micromechanical stress of deposited films. To use the SiOxNy thin film for the core layer of a channel waveguide, we need to obtain a structure with low optical attenuation, well-controlled refractive index, and low-internal stress. To study the stress characteristics of SiOxNy material we used an interferometric technique, and we fabricated for this purpose special membranes with deposition of variable quality SiOxNy thin films.

Optical microlenses for MOEMS

A new concept of the fabrication process for glass microlenses (external diameter ED<1 mm, focal length a few millimeters), based on the silicon master mask-less anisotropic wet etching in KOH, vacuum anodic bonding and re-flow of borosilicate glass, followed by the precise wafer-scale polishing and DRIE has been presented. A single spherical microlens as well as an array of spherical microlenses with focal length between 44.8 and 8.6 mm and external diameter 0.35 to 0.985 mm have been repeatable manufactured.

Silicon oxynitride waveguides developed for optomechanical sensing functions

In the last decade, the advances in the MEMS technology lead to the integration of optical structures with MEMS. This association between MEMS technologies and integrated optical structures may provide complex functionality such as sensing, modulation or switching. Optical MEMS integrated on silicon are very attractive in terms of potential for cheap mass production and compatibility with CMOS technologies. In this paper we present the technology of SiON waveguide fabrication including aspects of PECVD and micromachining. PECVD process optimisation in order to increase the waveguide performances is presented. Finally the integrated opto-mechanical sensing structures are discussed.

Optical interferometry investigation of internal stress and optomechanical characteristics of silicon-oxynitride thin films fabricated by PECVD

The full-field interferometry is very well suited for evaluation of micromechanical and material properties of microsystems. In this paper, we presented a Twyman-Green interferometer for MEMS/MOEMS testing. The measurements of out-of-plan displacements of special silicon membranes with thin film of SiOxNy deposited by PECVD enable the analysis of opto=mechanical properties.

LCOS as a reference element in Twyman-Green laser interferometer

Micro-Electro-Mechanical Systems are nowadays frequently used in many fields of industry. The number of their applications increases and their functions become more responsible, therefore precise knowledge about their properties is necessary. Due to its fragility and small sizes non-contact and high sensitive measurement method is required. Two-beam laser interferometry is one of the most popular testing methods of microelements. Such method implemented in Twyman-Green interferometer allows for full-field shape determination and out-of-plane displacement measurement. However the elements under test may bring additional challenges: their surfaces may have complicated shape or large shape gradients which prohibit their testing by means of interferometer with a flat reference mirror. To overcome such problems we propose to use LCOS (Liquid Crystal On Silicon) - phase, reflective SLM as an active reference element. LCOS serves as an adaptive reference mirror and phase shifter. The use of such element allows increasing measurement range of the interferometer and simplifies out-of-plane displacement measurement through object wavefront compensation. The applicability of the modified Twyman-Green interferometer will be shown at the examples of active micromembranes testing.

Dynamic measurements of actuators driven by AlN layers

Micro-Electro-Mechanical Systems are nowadays frequently used in many fields of industry. The number of their applications increase and their functions became more complex and demanding. Therefore precise knowledge about their static (shape, deformations, stresses) and dynamic (resonance frequencies, amplitude and phase of vibration) properties is necessary. Two beam laser interferometry is one of the most popular testing methods of micromechanical elements as a non-contact, high-accurate method allowing full-field measurement. First part of the paper present microbeam actuators designed for MEMS/MOEMS applications. The proposed structures are the straight silicon microbeams formed by KOH etching of Si wafer. Aluminium nitride (AlN) thin films are promising materials for many acoustic and optic applications in MEMS field. In the proposed architecture the actuation layer is sandwiched between two metal electrodes on the top of beam. In the second part we describe the methodology of the actuator characterization. These methods applied are: stroboscopic interferometry and active interferometry (LCOS SLM is used as a reference surface in Twyman-Green interferometer). Moreover some results of FEM analysis of the sample are shown and compared with experimental results. Dynamic measurements validate the design and simulations, and provide information for optimization of the actuator manufacturing process.