Jorge Albero

Fabrication of spherical microlenses by a combination of isotropic wet etching of silicon and molding techniques

We report a novel process technology of hemispherical shaped microlenses, using isotropic wet etching of silicon in an acid solution to produce the microlenses molds. Governed by process parameters such as temperature and etchant concentration, the isotropic wet etching is controlled to minimize various defects that appear during the molding creation. From the molds, microlenses are fabricated in polymer by conventional replication techniques such as hot embossing and UV-molding. The characterization of molds and measurements of replicated microlenses demonstrate high smoothness of the surfaces, excellent repeatability of mold fabrication and good optical properties. Using the proposed method, a wide range of lens geometries and lens arrays can be achieved.

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 Design of a Vertically Integrated Array-type Mirau-based OCT system

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.

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.

Impact of mirror spider legs on imaging quality in Mirau micro-interferometry.

We report the impact on imaging quality of mirror suspensions, referred to as spider legs, used to support the reference mirror in a Mirau micro-interferometer that requires the vertical alignment of lens, mirror, and beamsplitter. Because the light goes from the microlens to the beamsplitter through the mirror plane, the spider legs are a source of diffraction. This impact is studied as a function of different parameters of the spider legs design. Imaging criteria, such as the resolution as well as the symmetry of the imaging system, are determined using the point spread function and the modulation transfer function of the pupil. These imaging criteria are used to determine the optimum radius of curvature, thickness, and number of legs of the spider structure. We show that 3 curved legs give performances, with specific radius of curvature and thickness, similar to a suspension-free mirror.

Simple setup for optical characterization of microlenses

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.

Arrays of millimeter-sized glass lenses for miniature inspection systems

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.

Fabrication of 100% fill factor arrays of microlenses from silicon molds

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.

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.

Micromachined array-type Mirau interferometer for MEMS metrology

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.