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Zusammenfassung Mikroelektromechanische Systeme (MEMS) werden heute in einer Vielzahl von Anwendungen eingesetzt. Zur Herstellung von MEMS werden üblicherweise fotolithografische Verfahren eingesetzt. Insbesondere bei sicherheitsrelevanten MEMS, wie z. B. Airbag- oder ESP-Sensoren, ist eine 100 % Inspektion erforderlich. Nach einer optischen Inspektion der MEMS-Strukturen werden diese durch das Bonden eines Silizium-Kappenwafers geschützt. Dieses Bonding oder das anschließende Packaging kann zu einer zusätzlichen Spannung im Waferstapel führen, die die MEMS-Funktion negativ beeinflussen kann. Im Falle eines fehlgeschlagenen elektronischen Tests kann das Problem nicht lokalisiert werden, da der Kappenwafer für die gängigen optischen Oberflächensensoren undurchsichtig ist. In dieser Publikation wird die Herausforderung der optischen Topografiemessung von verkapselten MEMS-Strukturen diskutiert. Dabei werden Defekte wie Verbiegung oder Festklemmen einzelner MEMS-Finger betrachtet. Für diese Messaufgabe wird ein kurzkohärentes Interferenzmikroskop implementiert. Die Wellenlänge liegt im Short-Wave Infrarot (SWIR), da Silizium im Infraroten transparent wird. Die interferometrische Oberflächenmessung der MEMS-Strukturen wird durch den Kappenwafer stark beeinflusst. Hierfür wurden Simulationen durchgeführt, die die systematischen Messabweichungen aufgrund der Kappe zeigen. Eine Möglichkeit zur Korrektur und Reduzierung der systematischen Abweichungen wird ebenfalls beschrieben.

This chapter reviews the confocal microscopy, a description of the depth-scanning fringe projection system, and an overview about low-coherence interferometric microscopy for the optical profiling of encapsulated Microelectromechanical Systems. The strong depth discrimination of the confocal sensor leads to a good depth resolution. When a depth scan is performed, the peak intensity is measured if the measurement spot falls onto the object surface. The measured confocal intensity signal has to be analyzed relative to a common zero point. The spectrally encoded confocal intensity signal can be used to calculate the topography. The reflected light is focused onto the pinholes of the Nipkow disk, which are also the confocal detection pinholes in this setup. As the microlenses alone cannot do the confocal discrimination, a central pinhole is placed in front of the camera, which can be used for all microlenses in parallel. Confocal microscopes translate the defocus in each position into an intensity signal.

Micro-electro-mechanical systems (MEMS) are used today in a multitude of applications. The production of MEMS is a fully automated wafer process consisting of deposition, patterning and etching. Especially for safetyrelevant MEMS devices such as airbag or ESP sensors, a 100% inspection is required. After an optical inspection of the MEMS structures, they are sealed and protected by bonding a silicon cap wafer on top. This bonding can lead to an additional stress in the wafer stack that interferes with the MEMS function, which is tested by an electrical test. This stress can cause individual elements to stick together or to be bent. In the case of a failed test, the problem cannot be located on the MEMS structures because they are opaque to visible light. The defects are to be detected by a topography measurement. This publication discusses the challenge of topography measurement using an low-coherence interference microscope, which is necessary to achieve a high lateral and axial resolution. It turns out that dispersion does not have such a large influence on the measurement signal as the uneven thickness of the cap. The influence of thickness variation could be reduced by measuring both cap surfaces and subtracting them from the MEMS topography. A comparative measurement of the MEMS without a cap shows that the residual deformation is of a similar magnitude. Removing the cap, however, will cause any stress that may be present to dissolve.

Micro-electro-mechanical system's (MEMS) applications have greatly expanded over the recent years, and the MEMS industry has grown almost exponentially. One of the strongest drivers are the automotive and consumer markets. A 100% test is necessary especially in the production of automotive MEMS sensors since they are subject to safety relevant functions. This inspection should be carried out before dicing and packaging since more than 90% of the production costs are incurred during these steps. An electrical test is currently being carried out with each MEMS component. In the case of a malfunction, the defect can not be located on the wafer because the MEMS are no longer optically accessible due to the encapsulation. This paper presents a low coherence interferometer for the topography measurement of MEMS structures located within the wafer stack. Here, a high axial and lateral resolution is necessary to identify defects such as stuck or bent MEMS fingers. First, the boundary conditions for an optical inspection system will be discussed. The setup is then shown with some exemplary measurements.

International audience; OCT instruments permit fast and non-invasive 3D optical biopsies of biological tissues. However, they are bulky and expensive, making them only affordable at the hospital and thus, not sufficiently used as an early diagnostic tool. Significant reduction of system cost and size is achieved by implementation of MOEMS technologies. We propose an active array of 4x4 Mirau microinterferometers where the reference micro-mirrors are carried by a vertical comb-drive microactuator, enabling the implementation of the phase-shifting technique that improves the sensitivity and eliminates unwanted interferometric terms. We focus on the design of the imaging system, the microfabrication and the assembly of the Mirau microinterferometer, and the swept-source OCT imaging.

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.

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.

International audience; In recent years, optical coherence tomography (OCT) became gained importance in medical disciplines like ophthalmology, due to its noninvasive optical imaging technique with micrometer resolution and short measurement time. It enables e. g. the measurement and visualization of the depth structure of the retina. In other medical disciplines like dermatology, histopathological analysis is still the gold standard for skin cancer diagnosis. The EU-funded project VIAMOS (Vertically Integrated Array-type Mirau-based OCT System) proposes a new type of OCT system combined with micro-technologies to provide a hand-held, low-cost and miniaturized OCT system. The concept is a combination of full-field and full-range swept-source OCT (SS-OCT) detection in a multi-channel sensor based on a micro-optical Mirau-interferometer array, which is fabricated by means of wafer fabrication. This paper presents the study of an experimental proof-of-concept OCT system as a one-channel sensor with bulk optics. This sensor is a Linnik-interferometer type with similar optical parameters as the Mirau-interferometer array. A commercial wavelength tunable light source with a center wavelength at 845nm and 50nm spectral bandwidth is used with a camera for parallel OCT A-Scan detection. In addition, the reference microscope objective lens of the Linnik-interferometer is mounted on a piezo-actuated phase-shifter. Phase-shifting interferometry (PSI) techniques are applied for resolving the conjugate complex artifact and consequently contribute to an increase of image quality and depth range. A suppression ratio of the complex conjugate term of 36 dB is shown and a system sensitivity greater than 96 dB could be measured.

International audience; The EU-funded project VIAMOS1 proposes an optical coherence tomography system (OCT) for skin cancer detection, which combines full-field and full-range swept-source OCT in a multi-channel sensor for parallel detection. One of the project objectives is the development of new fabrication technologies for micro-optics, which makes it compatible to Micro-Opto-Electromechanical System technology (MOEMS). The basic system concept is a wafer-based Mirau interferometer array with an actuated reference mirror, which enables phase shifted interferogram detection and therefore reconstruction of the complex phase information, resulting in a higher measurement range with reduced image artifacts. This paper presents an experimental one-channel on-bench OCT system with bulk optics, which serves as a proof-of-concept setup for the final VIAMOS micro-system. It is based on a Linnik interferometer with a wavelength tuning light source and a camera for parallel A-Scan detection. Phase shifting interferometry techniques (PSI) are used for the suppression of the complex conjugate artifact, whose suppression reaches 36 dB. The sensitivity of the system is constant over the full-field with a mean value of 97 dB. OCT images are presented of a thin membrane microlens and a biological tissue (onion) as a preliminary demonstration. © (2015) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Skin cancer is the most diagnosed type of cancer. While an early treatment usually offers good therapy prospects, effective diagnosis techniques are required. Today traditional biopsy is still the reference diagnostic technique, which has the drawback of a long diagnosis time and invasiveness. In recent years, new optical system based on optical coherence tomography were developed for early diagnostics of skin cancer. But as these systems are still bulky and expensive, they're still available at specialized hospitals only. Therefore we present the optical design of a Vertically Integrated Array-type Mirau-based OCT System (VIAMOS), which will allow non-invasive 3D optical biopsies of skin for early diagnostics of skin cancer. The VIAMOS system constitutes a miniature, low cost, full-field optical coherence tomography system combining swept-source OCT, phase-shifting interferometry and MOEMS technologies. It will provide 3D tomograms of human skin from a measurement volume of 8 x 8 x 0.5 mm³ with an axial and transverse resolution of about 6 µm. The swept-source will be realized as a miniaturized, MEMS-based wavelength-tuning Fabry-Perot Interferometer and a near-infrared superluminescent diode resulting in a linewidth of approximately 1 nm and a scanning range of 50 nm. The wafer stacked measurement device consists of 4 x 4 Mirau-Interferometer channels with movable reference mirrors to realize heterodyne detection and will be fabricated by means of MOEMS technologies. The signal detection will be realized using a fast new developed smart camera chip, which features 4 x 4 multiple regions of interest and on-chip signal processing. In this contribution, we present the optical design of the sensor and provide results of an extensive stray light analysis of the system.

Low-coherence-interferometry (LCI) is a well-known optical metrology technique which offers high resolution and a low measurement uncertainty. For the inspection of dynamic and fast processes, single-shot point sensors can show their full capability. Therefore, a new approach of an interferometer is presented, allowing an single-shot acquisition of a LCI signal. The presented concept is based on tilted wavefronts to generate a spatial modulated interferogram. Using a three faceted mirror (TFM) in the reference path, a virtual field point is generated leading to a tilted planar reference wavefront in the pupil plane of the telecentric objective lens. Three mirror surfaces, which are diamond milled out of a monolithic copper cube, are applied to guarantee a stable TFM geometry, resulting in an invariant reference field point position.

International audience; This Letter reports on the fabrication of glass lens doublets arranged in arrays and realized at wafer level by means of micro-fabrication. The technique is based on the accurate vertical assembly of separately fabricated glass lens arrays. Since each one of these arrays is obtained by glass melting in silicon cavities, silicon is employed as a spacer in order to build a well-aligned and robust optical module. It is shown that optical performance achieved by the lens doublet is better than for a single lens of equivalent numerical aperture, thanks to lower optical aberrations. The technique has good potential to match the optical requirements of miniature imaging systems.

This paper presents the study of a fabrication technique of lenses arrays based on the reflow of glass inside cylindrical silicon cavities. Lenses whose sizes are out of the microfabrication standards are considered. In particular, the case of high fill factor arrays is discussed in detail since the proximity between lenses generates undesired effects. These effects, not experienced when lenses are sufficiently separated so that they can be considered as single items, are corrected by properly designing the silicon cavities. Complete topographic as well as optical characterizations are reported. The compatibility of materials with Micro-Opto-Electromechanical Systems (MOEMS) integration processes makes this technology attractive for the miniaturization of inspection systems, especially those devoted to imaging.