Systems for imaging require to employ high quality optical components in order to dispose of optical aberrations and thus reach sufficient resolution. However, well-known methods to get rid of optical aberrations, such as aspherical profiles or diffractive corrections are not easy to apply to micro-optics. In particular, some of these methods rely on polymers which cannot be associated when such lenses are to be used in integrated devices requiring high temperature process for their further assembly and separation. Among the different approaches, the most common is the lens splitting that consists in dividing the focusing power between two or more optical components. In here, we propose to take advantage of a wafer-level technique, devoted to the generation of glass lenses, which involves thermal reflow in silicon cavities to generate lens doublets. After the convex lens sides are generated, grinding and polishing of both stack sides allow, on the first hand, to form the planar lens backside and, on the other hand, to open the silicon cavity. Nevertheless, silicon frames are then kept and thinned down to form well-controlled and auto-aligned spacers between the lenses. Subsequent accurate vertical assembly of the glass lens arrays is performed by anodic bonding. The latter ensures a high level of alignment both laterally and axially since no additional material is required. Thanks to polishing, the generated lens doublets are then as thin as several hundreds of microns and compatible with micro-opto-electro-systems (MOEMS) technologies since they are only made of glass and silicon. The generated optical module is then robust and provide improved optical performances. Indeed, theoretically, two stacked lenses with similar features and spherical profiles can be almost diffraction limited whereas a single lens characterized by the same numerical aperture than the doublet presents five times higher wavefront error. To demonstrate such assumption, we fabricated glass lens doublets and compared them to single lenses of equivalent focusing power. For similar illumination, the optical aberrations are significantly reduced.
In this paper, we present construction, fabrication and characterization of an electrostatic MOEMS vertical microscanner for generation of an optical phase shift in array-type interferometric microsystems. The microscanner employs asymmetric comb-drives for a vertical displacement of a large 4x4 array of reference micromirrors and for in-situ position sensing. The device is designed to be fully compatible with Mirau configuration and with vertical integration strategy. This enables further integration of the device within an "active" multi-channel Mirau micro-interferometer and implementation of the phase shifting interferometry (PSI) technique for imaging applications. The combination of micro-interferometer and PSI is particularly interesting in the swept-source optical coherence tomography, since it allows not only strong size reduction of a system but also improvement of its performance (sensitivity, removal of the image artefacts). The technology of device is based on double-side DRIE of SOI wafer and vapor HF releasing of the suspended platform. In the static mode, the device provides vertical displacement of micromirrors up to 2.8μm (0 - 40V), whereas at resonance (fo=500 Hz), it reaches 0.7 μm for only 1VDC+1VAC. In both operation modes, the measured displacement is much more than required for PSI implementation (352nm peak-to-peak). The presented device is a key component of array-type Mirau micro-interferometer that enables the construction of portable, low-cost interferometric systems, e.g. for in vivo medical diagnostics.
In this work, vertical integration of miniaturized array-type Mirau interferometers at wafer level by using multi-stack anodic bonding is presented. Mirau interferometer is suitable for MEMS metrology and for medical imaging according to its vertical-, lateral- resolutions and working distances. Miniaturized Mirau interferometer can be a promising candidate as a key component of an optical coherence tomography (OCT) system. The miniaturized array-type interferometer consists of a microlens doublet, a Si-based MEMS Z scanner, a spacer for focus-adjustment and a beam splitter. Therefore, bonding technologies which are suitable for heterogeneous substrates are of high interest and necessary for the integration of MEMS/MOEMS devices. Multi-stack anodic bonding, which meets the optical and mechanical requirements of the MOEMS device, is adopted to integrate the array-type interferometers. First, the spacer and the beam splitter are bonded, followed by bonding of the MEMS Z scanner. In the meanwhile, two microlenses, which are composed of Si and glass wafers, are anodically bonded to form a microlens doublet. Then, the microlens doublet is aligned and bonded with the scanner/spacer/beam splitter stack. The bonded array-type interferometer is a 7- wafer stack and the thickness is approximately 5mm. To separate such a thick wafer stack with various substrates, 2-step laser cutting is used to dice the bonded stack into Mirau chips. To simplify fabrication process of each component, electrical connections are created at the last step by mounting a Mirau chip onto a flip chip PCB instead of through wafer vias. Stability of Au/Ti films on the MEMS Z scanner after anodic bonding, laser cutting and flip chip bonding are discussed as well.
Some of the critical limitations for widespread use in medical applications of optical devices, such as confocal or optical coherence tomography (OCT) systems, are related to their cost and large size. Indeed, although quite efficient systems are available on the market, e.g. in dermatology, they equip only a few hospitals and hence, are far from being used as an early detection tool, for instance in screening of patients for early detection of cancers. In this framework, the VIAMOS project aims at proposing a concept of miniaturized, batch-fabricated and lower-cost, OCT system dedicated to non-invasive skin inspection. In order to image a large skin area, the system is based on a full-field approach. Moreover, since it relies on micro-fabricated devices whose fields of view are limited, 16 small interferometers are arranged in a dense array to perform multi-channel simultaneous imaging. Gaps between each channel are then filled by scanning of the system followed by stitching. This approach allows imaging a large area without the need of large optics. It also avoids the use of very fast and often expensive laser sources, since instead of a single point detector, almost 250 thousands pixels are used simultaneously. The architecture is then based on an array of Mirau interferometers which are interesting for their vertical arrangement compatible with vertical assembly at the wafer-level. Each array is consequently a local part of a stack of seven wafers. This stack includes a glass lens doublet, an out-of-plane actuated micro-mirror for phase shifting, a spacer and a planar beam-splitter. Consequently, different materials, such as silicon and glass, are bonded together and well-aligned thanks to lithographic-based fabrication processes.
Scientific articles focusing on fabrication of micro-components often evaluate their optical performances by techniques such as scanning electron microscopy or surface topography only. However, deriving the optical characteristics from the shape of the optical element requires using propagation algorithms. In this paper, we present a simple and intuitive method, based on the measurement of the intensity point spread function generated by the micro-component. The setup is less expensive than common systems and does not require heavy equipments, since it requires only a microscope objective, a CMOS camera and a displacement stage. This direct characterization method consists in scanning axially and recording sequentially the focal volume. Our system, in transmissive configuration, consists in the investigation of the focus generated by the microlens, allowing measuring the axial and lateral resolutions, estimating the Strehl ratio and calculating the numerical aperture of the microlens. The optical system can also be used in reflective configuration in order to characterize micro-reflective components such as molds. The fixed imaging configuration allows rapid estimation of quality and repeatability of fabricated micro-optical elements.
In this paper, we adapt a technique employed for glass microlenses fabrication in order to obtain matrices of millimeter size lenses for inspection applications. The use of microfabrication processes and Micro-Electro-Mechanical Systems (MEMS) compatible materials allow the integration of lenses larger than usual in microsystems. Since the presented lenses can have 2 mm in diameter or more, some aspects apparently irrelevant when diameters are lower than 500 μm must be reviewed and taken into account. Indeed, when the lenses are in the millimeter range, problems such as size nonuniformities within a matrix and asymmetric shapes of each lens are dependent on parameters as mask design, depth of the silicon cavities and enclosed vacuum control after anodic bonding, glass reflow temperature and even the position of the lenses on the substrate. Issues related to the fabrication flow-chart are addressed in this paper and solutions are proposed. First results are shown to prove the pertinence of this technique to fabricate MEMS-compatible millimetersized lenses to be integrated in miniature inspection systems. We also discuss some of the paths to follow that could help improving the performances.
KEYWORDS: 3D metrology, Optical components, Objectives, Imaging systems, Point spread functions, Microscopes, Cameras, Microlens, CMOS cameras, High dynamic range imaging
High-resolution miniature imaging systems require high quality micro-optical elements. Therefore, it is essential to characterize their optical performances in order to optimize their fabrication. Usually, basic evaluation of micro-optical elements quality is based on the measurement of their topography since their optical properties are largely defined by their shape. However, optical characteristics have to be derived from the measured geometry. An alternative method is the direct measurement of their optical properties. Unlike topography measurement, it allows characterization of high numerical aperture components. Moreover, it can be applied to single elements but also to optical systems composed of several micro-optical components. In this work, we propose a simple method based on the measurement of the 3D intensity point spread function (IPSF). IPSF is defined by the 3D shape of the focal spot generated by the micro-element. The direct characterization of focusing response through the measurement of IPSF allows very precise estimation of micro-structures quality. The considered method consists in imaging different slices of the focal volume generated by the focusing component. It allows, depending on the configuration, characterizing both transmissive and reflective micro-optical components.
The presented paper shows the concept and optical design of an array-type Mirau-based OCT system for early diagnosis of skin cancer. The basic concept of the sensor is a full-field, full-range optical coherence tomography (OCT) sensor. The micro-optical interferometer array in Mirau configuration is a key element of the system allowing parallel imaging of multiple field of views (FOV). The optical design focuses on the imaging performance of a single channel of the interferometer array and the illumination design of the array. In addition a straylight analysis of this array sensor is given.
We present a simple method to check the exact realization of a radially polarized light beam by means of a vortex-sensing diffraction grating. The use of this grating easily allows the determination of the topological charges included in each of the two circular polarization components of the incident beam. Therefore a pure radially polarized beam can be easily distinguished from a pseudo-radially polarized beam. Experimental results are presented with radial beams generated with two different devices: a patterned radial polarizer and a specially designed liquid crystal device.
We report on diffraction efficiency considerations and experimental implementation of diffraction gratings by means
of a phase-only spatial light modulator, parallel aligned (PAL) liquid crystal on silicon (LCoS) display. We present
results of the implementation of continuous phase profiles for optimal efficiency, and their application for blazed
gratings and diffractive lenses onto displays with reduced phase modulation range and also onto displays with large
phase modulation range.
We used patterned radial polarizers to easily generate and detect radially polarized beams. We then showed how to spatially manipulate the two-dimensional polarization mapping provided by a radial polarizer using different waveplate systems to obtain new two-dimensional polarization states. One system is particularly useful, since it converts the radial polarized beam into the azimuthally polarized beam. The transformed beams were analyzed using linear, circular, and radial polarizers. The Jones matrix formalism was applied for the theoretical analysis.
We present the development of an array-type 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 microoptical technologies and silicon micromachining, based on the vertical assembly of two glass wafers.
The probing wafer is carrying 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 5x5 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.
This paper reports a batch-fabrication technique based on micromachining of silicon molds to create, after replication,
arrays of microlenses characterized by high fill factors. The technique for single microlens generation (compatible with
various types of replication or integration so that microlenses made of plastics or glass can be generated) was reported
previously and showed its potential in terms of range of shapes and cost. However, subtleties of chemical etching makes
more difficult the generation of high fill factor matrices when microlenses size overcomes several tenth of microns.
Thus, in this paper, we describe the analysis of the chemical etching process and the corresponding adaptation of the
mask design to achieve 100% fill factors arrays of microlenses. The process to fabricate arrays of microlens, with
hexagonal footprints and element sizing from 30 to 270 microns and having NA from 0.2 to 0.4, is described. The
hexagonal footprint shape of the elements in the arrays leads to 100% geometrical fill factor of fabricated structures.
Isotropic etching used for the molds fabrication preserves the spherical profile of the resulting microlenses.
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.
The article describes application of Level Set method for two different microfabrication processes. First is shape
evolution of during reflow of the glass structure. Investigated problem were approximated by viscous flow of
material thus kinetics of the process were known from physical model. Second problem is isotropic wet etching
of silicon. Which is much more complicated because dynamics of the shape evolution is strongly coupled with
time and geometry shapes history. In etching simulations Level Set method is coupled with Finite Element
Method (FEM) that is used for calculation of etching acid concentration that determine geometry evolution of
the structure. The problem arising from working with FEM with time varying boundaries was solved with the use
of the dynamic mesh technique employing the Level Set formalism of higher dimensional function for geometry
description. Isotropic etching was investigated in context of mico-lenses fabrication. Model was compared with
experimental data obtained in etching of the silicon moulds used for micro-lenses fabrication.
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 concept of the fabrication process of glass microlenses integrated with silicon and polymer replicas is presented.
These kinds of microlenses are formed using a silicon master which is wet etched in alkaline solutions (anisotropic
etching) and/or in acid solutions (isotropic etching). The control of the times and the selection of the solutions, joined
with the designs of the mask for conventional photolithography and the quality of the silicon wafers are the key for
obtaining the desired shapes and sizes. The fabricated moulds are used to replicate microlenses in polymer by the
standard well known replication technologies and also to fabricate glass microlenses integrated on a silicon frame.
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