Stéphane Perrin

A simple method for quality evaluation of micro-optical components based on 3D IPSF measurement.

This paper presents a simple method based on the measurement of the 3D intensity point spread function for the quality evaluation of high numerical aperture micro-optical components. The different slices of the focal volume are imaged thanks to a microscope objective and a standard camera. Depending on the optical architecture, it allows characterizing both transmissive and reflective components, for which either the imaging part or the component itself are moved along the optical axis, respectively. This method can be used to measure focal length, Strehl ratio, resolution and overall wavefront RMS and to estimate optical aberrations. The measurement setup and its implementation are detailed and its advantages are demonstrated with micro-ball lenses and micro-mirrors. This intuitive method is adapted for optimization of micro-optical components fabrication processes, especially because heavy equipments and/or data analysis are not required.

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.

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.

Monolithic integration of a glass membrane on silicon micro-actuator for micro-interferometry

We report the monolithic integration of glass membranes on a highly structured SOI wafer of electrostatic comb-drive actuator. The integration method combines glass micromachining techniques and non-standard processing of SOI, based on spray coating and projection photolithography. Proposed solution overcomes several basic difficulties of monolithic glass integration. We demonstrate its potential by fabrication of 4×4 matrix of reference mirrors on very thin (25 μm) and large (φ=2 mm) light-transparent glass membranes, which is of great importance for MOEMS-based phase-shift micro-interferometry systems.

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.

Wafer-level fabrication of arrays of glass lens doublets

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.

Sub-diffraction surface topography measurement using a microsphere-assisted Linnik interferometer

Microscopic surface topography measurement is an important aspect of industrial inspection. Optical and near field scanning techniques are increasingly replacing the use of the traditional mechanical stylus since they provide better lateral resolutions and higher measurement speeds. The main far field optical techniques used are interference microscopy and confocal microscopy, with the advantages of having larger fields of view and higher measurement speeds. Interference microscopy is now widely used, mainly because of its nanometric axial measurement sensitivity and its ease of use but suffers from a limitation in lateral resolution of about /2 due to diffraction. A new technique for high resolution 2D imaging using a microsphere placed on the sample has been recently combined with interferometry by several groups to greatly improve the lateral resolution. In this paper we present some of our own first results using glass microspheres with a white light Linnik interferometer and demonstrate a lateral resolution of /4 and an axial measurement sensitivity of several nm. Results are shown on calibrated square profile gratings with periods down to 400 nm, with a minimum feature size of 200 nm and a height of 148 nm and a field of view of several μm. While these features are not visible directly with the microscope objective, they become observable and measurable through the microsphere. An analysis using rigorous electromagnetic simulations is also given to help better understand the imaging properties of the technique. These first experimental and simulation results clearly indicate that this is an important new technique that opens new possibilities for surface metrology with a lateral resolution well beyond the diffraction limit.

Aberration retrieval for the characterization of micro-optical components

This paper proposes a method for the characterization of focusing micro-optical components such as microlens. Based on the measurement of the focal volume generated by the micro-element, the wavefront map reconstruction as well as the optical aberrations can be estimated. To record the slices of the focal volume, this technique requires a simple optical arrangement which consists of a microscope objective and a camera. Then, an iterative phase retrieval algorithm is applied on each recorded intensity slice. This approach is less sensitive to the environmental variations than interferometry and is less expensive than wavefront sampling sensors although it leads to similar results than interferometry. As an example, ball microlens with 596μm diameter and 0.56 numerical aperture, has been characterized and comparison with more conventional technique demonstrates the good performances of the proposed phase retrieval method.

Array-type miniature interferometer as the core optical microsystem of an optical coherence tomography device for tissue inspection

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.