Kay Gastinger

Micromachined array-type Mirau interferometer for parallel inspection of MEMS

We present the development of an array type of micromachined Mirau interferometers, operating in the regime of low coherence interferometry (LCI) and adapted for massively parallel inspection of MEMS. The system is a combination of free-space micro-optical technologies and silicon micromachining, based on the vertical assembly of two glass wafers. The probing wafer carries an array of refractive microlenses, diffractive gratings to correct chromatic and spherical aberrations and reference micro-mirrors. The semitransparent beam splitter plate is based on the deposition of a dielectric multilayer, sandwiched between two glass wafers. The interferometer matrix is the key element of a novel inspection system aimed to perform parallel inspection of MEMS. The fabricated demonstrator, including 5 × 5 channels, allows consequently decreasing the measurement time by a factor of 25. In the following, the details of fabrication processes of the micro-optical components and their assembly are described. The feasibility of the LCI is demonstrated for the measurement of a wafer of MEMS sensors.

Optical, mechanical and electro-optical design of an interferometric test station for massive parallel inspection of MEMS and MOEMS

The paper presents the optical, mechanical, and electro-optical design of an interferometric inspection system for massive parallel inspection of MicroElectroMechanicalSystems (MEMS) and MicroOptoElectroMechanicalSystems (MOEMS). The basic idea is to adapt a micro-optical probing wafer to the M(O)EMS wafer under test. The probing wafer is exchangeable and contains a micro-optical interferometer array. A low coherent and a laser interferometer array are developed. Two preliminary interferometer designs are presented; a low coherent interferometer array based on a Mirau configuration and a laser interferometer array based on a Twyman-Green configuration. The optical design focuses on the illumination and imaging concept for the interferometer array. The mechanical design concentrates on the scanning system and the integration in a standard test station for micro-fabrication. Models of single channel low coherence and laser interferometers and preliminary measurement results are presented. The smart-pixel approach for massive parallel electro-optical detection and data reduction is discussed.

Low-coherence speckle interferometer (LCSI) for characterization of adhesion in adhesive-bonded joints

Low Coherence Speckle Interferometry (LCSI) combines the depth-resolved measurement of Low Coherence Interferometry (LCI) with the high-accuracy deformation measurement of Electronic Speckle Pattern Interferometry (ESPI). Depth-resolved deformation measurement enables the characterization of the behavior of interfaces in multi-layer materials or structures while changing the ambient conditions. In this paper LCSI is introduced as a new tool for characterization of adhesion. The experimental set-up and the principle of work are described. A FEM-model of an adhesive bonded aluminum joint is developed to analyze the behavior of the Al-adhesive interface during mechanical testing. Some recent results are shown. This application demand measurements on a microscopic scale (camera field of view down to 500 x 500 μm).

Next-generation test equipment for micro-production

The paper introduces different approaches to overcome the large ratio between wafer size and feature size in the testing step of micro production. For the inspection of Micro(Opto)ElectroMechanicalSystems (M(O)EMS) a priori information are available to optimise the inspection process. The EU-project SMARTIEHS develops a new concept for high volume M(O)EMS testing. The design of the test station and the fabrication of the first components are presented and the advancements compared to the state of the art are introduced within the following fields: micro-optical interferometer design, micro-optical production, smart-pixel camera and mechanical design. Furthermore the first demonstrators are introduced and experimental results are presented.

Determination of the phase- and polarization-changing properties of reflective spatial light modulators in one set-up

The use of liquid crystal spatial light modulators in applications, require good characterization of phase, polarization and amplitude shifting properties. This report presents a new approach for simultaneously characterizing the depolarization and controlling the polarization properties of a reflective twisted nematic liquid crystal spatial light modulator (LC SLM). The SLM was set up as a part of a Michelson interferometer. The phase response was determined by using a piezo-electric actuator for phase stepping in the reference arm. During the polarization measurement the reference beam was removed and the polarization state of the input and output was determined by a polarization state generator (PSG) and a polarization state analyzer (PSA), each consisting of a polarizer and a quarter-wave plate. Hereby, both phase response and polarization control properties could be determined independently in the same measurement configuration simply by changing static polarization components. The systematic rotation of the quarter wave plates of the PSG and the PSA using stepper motors gives out-put data whose Fourier transform in terms of angular frequency components can be used to determine all the elements of the Mueller matrix. The Mueller matrix of a commercial SLM (Holoeye LC-2500) was determined for 17 evenly spaced voltage levels addressed to the SLM.

Design of a micro-optical low coherent interferometer array for the characterisation of MEMS and MOEMS

The EU-project SMARTIEHS (SMART InspEction system for High Speed and multi-functional testing of MEMS and MOEMS) develops a new inspection concept for MEMS and MOEMS (M(O)EMS) testing at the wafer level [1]. The inspection systems on the market today are based on a serial approach inspecting one M(O)EMS structure at the time. In SMARTIEHS a parallel wafer-to-wafer inspection concept is adopted from the electronic probing cards in the micro electronics industry. A microoptical probing wafer is aligned with the M(O)EMS wafer under test. The 4-inch probing wafer contains a 5x5 array of micro-optical low coherent interferometers, inspecting shape and deformations of 25 M(O)EMS structures within one measurement cycle. The measurement time can thus be reduced by a factor of 25, scaling with the no. of interferometers on the probing wafer. A 5x5 channel smart pixel camera array detects and demodulates the interference signals [2]. The design of the micro-optical low coherent interferometer array is presented. The configuration of the array elements is based on a Mirau interferometer. The main challenge is to use standard micro-fabrication processes to produce the micro optical interferometer array.

Low coherence speckle interferometry (LCSI): when speckle interferometry goes sub-surface

In this paper the theoretical background and basic principles of Low Coherence Speckle Interferometry (LCSI) are described. Furthermore the main parts of our research work are briefly introduced; (1) the development of a dual wavelength, open-path LCSI setup (2) the introduction of a new method for the detection of zero path length difference for temporal phase shifting (3) the optimisation of the optical parameters of LCSI to increase the probing depth (beam ratio, position of the coherence layer and imaging parameters) (4) the fundamental understanding of the measurement effect and quantification of the measurements using a one-dimensional transmission line model. Finally, LCSI is introduced as a tool for characterisation of adhesion. In this work fundamental studies on interfacial instabilities in adhesive bonded aluminium joints are carried out. The basic hypothesis is that low adhesion is due to the existence of microscopic delaminations at the interface between the substrate and the adhesive. These delaminations can be caused by imperfect pre-treatment, surface topography or other surface phenomena (e.g. corrosion, inter-metallic particles, etc.).

Multi-technique platform for dynamic and static MEMS characterisation

MEMS characterisation is an important application area for interferometry. In this paper a Mach-Zehnder interferometer configuration is presented that combines both coherent and low coherent techniques in one setup. It incorporates the application of classical Laser Interferometry (LI) and Electronic Speckle Pattern Interferometry as well as classical Low Coherence Interferometry (LCI), full-field Optical Coherence Tomography and Low Coherence Speckle Interferometry. Digital Holography can be applied by minor modifications of the setup. The setup, working principle, and applications of the interferometer will be described. Measurements on a MEMS-based pressure sensor are presented. The sensor consists of a glass wafer attached to a silicon membrane. A cavity is etched into the glass wafer. The wafers are bonded and form a vacuum cavity. Membrane deformations are measured through the window using LI and LCI. LCI provides information about the shape of the glass window. Results from speckle techniques are compared with similar results from plane wave techniques. The influence of the glass window and the illumination of the object are investigated.

Low-coherence speckle interferometry (LCSI) for detection of interfacial instabilities in adhesive-bonded joints

In this paper the basic principles of Low Coherence Speckle Interferometry (LCSI) are described. Theoretical background and experimental results for the systematic investigation of LCSI are presented. To understand and quantify the measurement results of adhesive bonded joints a modelling of the interference signal is required. For this purpose, a one-dimensional transmission line model is developed, including changes in the refractive index in a stressed adhesive layer and delamination of the glued interface. A new method for the detection of zero path length difference is introduced. Investigations of the probing depth in semi-transparent adhesive and recent experimental results of the characterisation of adhesive-bonded aluminium joints are presented.

Design, technology and signal processing for DOE-based microinterferometer array applied in new generation M(O)EMS test equipment

The paper introduces different approaches to overcome the large ratio between wafer size and feature size in micro production. The EU-project SMARTIEHS develops a new concept for high volume M(O)EMS testing. The design of the test station is presented and the advancements compared to the state of the art are introduced within the following fields: micro-optical laser interferometer (LI) design, DOE-based microinterferometer production, smart-pixel camera and signal processing for resonance frequency and vibration amplitude distribution determination. The first experiments performed at LI demonstrator are also reported.