The paper presents the novel approach to an interferometric, quantitative, massive parallel inspection of
MicroElectroMechanicalSystems (MEMS), MicroOptoElectroMechanical Systems (MOEMS) and
microoptics arrays. 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 one of the micro-optical interferometer arrays based on:
(1) a low coherent interferometer array based on a Mirau configuration or (2) a laser interferometer array
based on a Twyman-Green configuration. The optical, mechanical, and electro-optical design of the system
and data analysis concept based on this approach is presented. The interferometer arrays are developed and
integrated at a standard test station for micro-fabrication together with the illumination and imaging modules
and special mechanics which includes scanning and electrostatic excitation systems. The smart-pixel approach
is applied for massive parallel electro-optical detection and data reduction. The first results of functional tests
of the system are presented. The concept is discussed in reference to the future M(O)EMS and microoptics
manufacturers needs and requirements.
Multi-wire sawing of silicon wafers is a tribological process. Slurry consisting of small silicon carbide particles
embedded in polyethyleneglycol carries out the abrasive material removal process. During this process small silicon
chips are removed from the bulk material. Low coherence interferometry (LCI) is widely used for high accuracy surface
topography measurements of materials. This paper presents an application of LCI where the surface of a material
(silicon) is inspected from the inside. Light in the near infrared (NIR) wavelength region is used. High spatial resolution
is necessary to be able to observe the processes on the micro scale. Therefore a modified solid immersion approach is
suggested. That makes it possible to reach a spatial resolution in the range of the illumination wavelength. The
topography changes produced by the chippings are in the range of some micrometers. To be able to estimate the
volumes of the Si chippings interferometric phase measurements are applied.
A setup for an NIR-LCSI instrument is introduced. It is based on a Superluminescence diode (SLD) (wavelength =
1280nm, FWHM = 50nm) and an InGaAs camera with VGA resolution. The paper presents a work in progress. The
aim of the research work is to measure the chipping process in a Si wafer saw with high spatial resolution. Hereby the
sawing channel inside the silicon block is investigated. The chipping generates a change of the interface topography of
the sawing channel. NIR-LCSI will be applied to measure this topography change and contributes thus to determine the
size and volume of the silicon chips and thus the cutting rate of the sawing process. This paper presents a novel concept
to increase the spatial resolution of the imaging system. With the use of a new type of "immersion" optics the numerical
aperture can be significantly increased and a spatial resolution close to, or even below, the nominal illumination
wavelength can be obtained. The speckle size and the resulting spatial and depth resolution of the LCSI measurements
are investigated.
KEYWORDS: Cameras, Demodulation, Interferometers, Interferometry, Imaging systems, Inspection, Signal detection, Signal processing, Data processing, Modulation
The paper presents the electro-optical design of an interferometric inspection system for massive parallel inspection of
Micro(Opto)ElectroMechanicalSystems (M(O)EMS). 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 interferometer (LCI) array based on a Mirau configuration and a laser interferometer (LI) array based on a
Twyman-Green configuration. The interference signals are generated in the micro-optical interferometers and are applied
for M(O)EMS shape and deformation measurements by means of LCI and for M(O)EMS vibration analysis (the
resonance frequency and spatial mode distribution) by means of LI. Distributed array of 5×5 smart pixel imagers detects
the interferometric signals. The signal processing is based on the "on pixel" processing capacity of the smart pixel
camera array, which can be utilised for phase shifting, signal demodulation or envelope maximum determination. Each
micro-interferometer image is detected by the 140 × 146 pixels sub-array distributed in the imaging plane. In the paper
the architecture of cameras with smart-pixel approach are described and their application for massive parallel electrooptical
detection and data reduction is discussed. The full data processing paths for laser interferometer and low coherent
interferometer are presented.
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.
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.
We present the application of glass microlenses for the fabrication of inspection systems based on interferometric
measurements. The microlenses are molded from wet etched silicon by using microfabrication techniques. The
concerned system requires lenses to be used in a Mirau interferometer configuration. The principle of the system is
presented, as well as different choices of lenses to be integrated. The use of glass microlenses monolithically molded on
a substrate is proven as the proper technology to be used in the system.
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.
Interferometry is a powerful and versatile tool for active MEMS characterisation. The high accuracy measurement of
deformations and vibrations of MEMS structures is an important application and well described by classical
interferometry. Deformation measurements in multi-layered structures requires a more sophisticated approach. All
phase changes along the optical path of the object light influence the measurements. Thus the shape and the
displacement of obstacles (like glass cover plates) must be included to quantify the measurement results.
The paper presents numerical simulations of the light path in an interferometric deformation measurement. A ray
tracing program is developed that keeps track of the optical path length and can thus be used to analyse disturbances
along the optical path. The simulations show how the deformation of more than one interface influences the phase
measurement. The phase errors are quantified and the reliability of the deformation measurements is evaluated.
Different interface geometries are examined. The simulations are compared to measurements on a MEMS pressure
sensor.
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.).
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.
In this paper the basic principles of LCSI are briefly described. Furthermore, we investigate the interference signal coming from a semitransparent, multilayer object theoretically and experimentally. The interference signal is modelled using a one-dimensional transmission line approach. The theory for this model is well known from the exact matrix theory of multilayered systems. We have extended the model for phase investigations and simulated the effect of a delamination when measuring through a semitransparent layer. The model is verified by experimental results from LCSI measurements.
In this paper the optical parameters of Low Coherence Speckle Interferometry (LCSI) are analysed in order to optimise the technique and increase the probing depth. The contrast of the interference signal depends on the configuration of the optical setup and the optical properties of the material. Theoretical investigations and measurements for optimising the beam ratio, the coherence function and imaging parameters are presented. By optimising the interference signal the probing depth of the technique can be extended. This is demonstrated for a semi-transparent polymer material. Finally, measurements of the deformation of interfaces in a multi-layered material are presented.
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.
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.
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).
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