Andrei Sabac

Evaluation of micromechanical properties of buckled SiOxNy-loaded membranes by combining the Twyman-Green interferometry with nanoindentation and point-wise deflection technique

Abstract In view of applications in MOEMS technology, an interferometric technique has been developed for determination of micromechanical properties of PECVD-deposited SiO x N y thin films. By combining the Twyman–Green interferometry with nanoindentation technique, an original “point-wise” deflection method is proposed, particularly appropriated to measure the residual stress in the case of silicon membranes compressively prestressed by SiO x N y deposition. For such SiO x N y membranes, operating at the first mode of buckling, the nonindentation permits the extraction of Youngs modulus and interferometry measures the out-of-plan displacements, respectively. The proposed point-wise deflection technique combines both the interferometric and Youngs modulus data, giving the access to the distribution of residual stress versus the optical quality of SiO x N y films. The residual stress is monitored as a function of the refractive index of SiO x N y , establishing the relationship between the optical and micromechanical properties of deposited thin films. High measuring accuracy and resolution have been demonstrated, allowing measurements to be used to enhance PECVD process control

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.

Interferometry system for the mechanical characterization of membranes with silicon oxynitride thin films fabricated by PECVD

In this paper, we present a method for the internal stress characterization of silicon membranes with silicon oxynitride thin films (SiOxNy) deposited by PECVD (plasma enhanced chemical vacuum deposition). Connecting the interferometric measurements (Twyman-Green interferometer) of out-of-plan displacements of SiOxNy-loaded membranes with evaluation of micromechanical parameters (Youngs modulus, Poisson ratio) obtained by nanoindentation we evaluated the residual stress of SiOxNy thin films via point-wise deflection technique. The magnitude of stress is monitored as a function of the refractive index of SiOxNy establishing the relationship between the optical and micromechanical properties of deposited films.

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.

Microsystem based optical measurement systems: case of opto-mechanical sensors

This paper describe design and investigations of a MOEMS family measurement devices based on light propagation via SiOxNy waveguide structures. Planar optical waveguide integration with micromachined structures and inclusion of micro-optic elements within MEMS environment offers significant promises for achieving advanced functionality of optomechanical architectures. The required optical sources and detectors can be outside the opto-mechanical system, then requiring light transport by fibers. As example, the resonant pressure sensor based on micromembrane with optical interrogation were designed, fabricated and tested. It works on the principle of resonance frequency shift, caused by the change of internal stress due to change of external physical environment. The introduced pressure sensor combine the advantages of the resonant operational mode with a MEMS fabrication process and optical signal detection, providing high sensitivity and maintaining stable performance. The technology has to be optimised in order to decreasing of initial stress state, what influence on sensitivity of sensor. The results indicated, that the sensor do not require vacuum encapsulation, but low-cost packaging is sufficient.

Development and investigation of high resolution resonant pressure sensor with optical interrogation

In this paper we present a new family of MOEMS device, which can be used as high resolution optical resonant pressure sensor. The architecture contains a membrane loaded with an optical branch of a Mach-Zehnder interferometer (MZI), monolithically integrated on top of a Si substrate. The measuring arm of MZI is crossing the MEMS actuator based on a piezoelectric thin-film PZT transducer integrated on SOI membrane. The PZT transducer is excited by applying a sinusoidal voltage from a waveform generator. The working principle of MZI read-out is based on the change of effective refractive index of guided waves of MZI, induced by displacements of the deformable structure via the elastooptic effect and waveguide elongation. When the membrane operating at resonance frequency, the application of a pressure on the membrane produces a significant shift of resonance frequency corresponding to a loaded pressure. For the characterisation of dynamic characteristic study of microdevices, the advanced testing methods are necessary. The point-wise measurement system was combined with the multifunctional interferometric platform based on Twyman-Green microinterferometer, working in stroboscopic mode. The prototype of the pressure sensor was evaluated and measurement results are presented.

Active membrane in situ read-out by monolithic integration of silicon-based Mach-Zehnder interferometer

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 and electrostatically actuated torsional micromirror. The application of membrane-type structure is in the area of pressure sensors. The monolithically integrated MZI on movable torsional mirror seems to be an easy solution for monitoring mechanical performances during lifetime of micromirror.

Evaluation of the mechanical properties of square membranes prestressed by PECVD silicon oxynitride thin films

A very broad field of relevant technologies and testing methods for silicon micromechanical elements had to be limited to specific elements and adapted methodologies including experimental and numerical methods. In particular, the bimorph micromembranes under buckling are key elements for investigations of their mechanical properties. Due to optical quality of silicon-based layers deposited on micromechanical devices under consideration, the two-beam interferometry with computer interferogram processing is well adapted for shape and deformation measuring, while the nanoindentation is able to extract the hardness as well as the Young’s modulus. In this contribution, we investigate the silicon square membranes prestressed by deposition of silicon oxinitride (SiOxNy) films fabricated by PECVD. The combination of experimental techniques with fine elements method (FEM) proposed here offers a powerful tools for investigation of residual stress of SiOxNy layers. The distribution of residual stress is monitored as a function of the refractive index of SiOxNy films, establishing the correlation between the optical and micromechanical properties of deposited thin films.

Interferometry system for out-of-plane displacements and microshape measurements of silicon membranes

In this paper we introduce a conventional interferometry system for MEMS/MOEMS characterization. We investigate the opto-mechanical properties of silicon micromembranes, where a thin film of PECVD SiOxNy was deposited. Monitoring the quality of this deposition we demonstrated the fabrication of technology for optical channel waveguides, compatible with the integration on micromechanical structures: based on low-stress and low-loss SiOxNy thin layers.