New photoplastic fabrication techniques and devices based on high aspect ratio photoresist

This thesis deals with the development of microfabrication technologies based on photoplastic structuring by lithographic and molding techniques. These technologies, combined with an original releasing method, allow for the simple fabrication of pseudo three-dimensional, soft, photoplastic microstructures with features and shapes that are difficult to obtain with standard micro-machining. The photoplastic material used is the SU-8 photoresist, a material that is used increasingly in the growing field of micro-electro-mechanical systems (MEMS). Photoplastic SU-8 microstructure fabrication is based on a combination of multi-layer spin-coating, molding and photolithographic processing of the resist on a prestructured silicon substrate. The final product is obtained by release of the structure from the substrate. This thesis describes several SU-8 microstructures which have been developed with an emphasis on atomic force microscopy (AFM) and scanning-near field optical microscopy (SNOM) probes. Scanning probe microscopy is a well-established technique for surface analysis, but batch-fabricated, low-cost probes still remain a challenging issue. Using a polymer for the cantilever facilitates the realization of mechanical properties that are difficult to achieve with classical silicon technology. The design, fabrication and testing of single lever and cassettes of multiple single-lever probes are presented and demonstrates the potential for fabrication structures with complicated shape and features. The fabrication process for SU-8 AFM probes is a simple batch process in which the integrated tips and the levers are defined in one photolithography step. Tip radii of curvature smaller than 15 nm have been obtained. The cantilever thickness depends on the spin-coating parameters, hence it can be very well controlled over a full wafer. Photoplastic cantilevers with thicknesses ranging from 1 to 6 μm have been produced. Imaging soft, condensed matter with photo-plastic levers, which uses laser beam deflection sensing, exhibits a resolution that compares well with that of commercially available silicon cantilevers. Lateral resolution of 5–6 nm has been estimated from imaging DNA-plasmid molecules. A vertical resolution of the order of 0.1 nm has been found. A similar fabrication technique was also developed to fabricate photo-plastic tips for SNOM that are to be attached to optical fibers. This technique allows optical apertures to be integrated at the end of the well-defined tip directly by probe fabrication, without the need for any post-processing for the aperture formation. Sub-100 nm aperture have been fabricated using this technique. Simple fabrication, as well as topographical and optical imaging demonstrate the potential of photoplastic-based probes for both AFM and SNOM applications, as well as for future combined probes development. In addition, the fabrication of functional microstructures by using SU-8 processing needs to be combined with other microfabrication techniques. Simple releasing of the molded structures from the substrate is especially of great importance. A sacrificial layer technique based on electrochemical etching enhancement has been developed and combined with the fabrication of the different photoplastic SU-8 probes presented. This technique allows the fast releasing of large microstructures and has been demonstrated by releasing other SU-8 photoplastic microfabricated devices.

A new versatile hybrid MEMS technology for high sensitivity, fluid proof sensing applications.

Matthias Neuenschwander

The field of micro electromechanical systems (MEMS) evolved from the microelectronic industry and the technologies developed to fabricate integrated circuits. As a result, MEMS are commonly fabricated on silicon wafers. The development of MEMS has been driven by three main merits: miniaturisation, microelectronics integration, and parallel fabrication with high precision. Many operational properties scale well with smaller sizes. In addition, integrated electronic circuitry allows embedding MEMS with computing or networking capabilities, while parallel manufacturing enables the fabrication of many identical devices on a single wafer, reducing the unit cost. The main MEMS application is transducers which transform signals from one form of energy to another, and can be used for perception (sensors) or to produce actions (actuators). Silicon and other microelectronic materials like metals have enabled very high-performance MEMS transducers because these materials can be used for a range of very effective actuating and sensing principles.More recently, polymers have started to be used in MEMS because of their unique electrical, physical and chemical properties which include biocompatibility, viscoelasticity and mechanical shock tolerance. They are also much softer than silicon or metals, and they can be processed using many techniques that allow unique low-cost, batch-style fabrication and packaging. However, polymers have relatively low glass-transition and melting temperatures. As a result, the advantages of polymers cannot easily be combined with high-performance actuating and sensing elements as these are based on silicon technology and require high-temperature fabrication steps. Here, a hybrid-MEMS fabrication process is used to address this issue. The developed devices are based on a trilayer structure, where a thick polymer core is sandwiched between two hard thin films, while electronic layers are embedded within in a fluid-compatible way.The objective of this thesis is to turn hybrid-MEMS into a benchmark MEMS technology. This involves many different aspects. First, sensing and actuation elements are integrated into trilayer devices, and their performance is analysed. Then, some more unique possibilities offered by hybrid-MEMS are exploited. A way to fabricate electronics on multiple layers is developed, which allows different electronic features to be integrated in parallel. In addition, three-dimensional bulk features fabricated at several levels are demonstrated. This is possible because the hybrid-MEMS process is based on the bonding of multiple wafers.All these new fabrication features are demonstrated in atomic force microscopy (AFM) applications. Self-actuated trilayer AFM cantilevers with sharp silicon tips are used to boost imaging speeds in the off-resonance tapping (ORT) mode, thanks to an order of magnitude faster surface probing rates. Furthermore, fully integrated ORT imaging is demonstrated with self-actuated piezoresistive cantilevers. Next, trilayer cantilevers with shielded conductive tips are introduced for electrical AFM modes. Finally, ongoing research projects are touched upon, including approaches to fabricate hybrid-MEMS with single crystal silicon piezoresistors for improved sensitivity, and an analysis of reproducibility of fabricated trilayer cantilevers.

EPFL2022

Scanning near-field infrared microscopy and spectromicroscopy applied to nano-systems and cells

Dusan Vobornik

This thesis further explores the possibilities of scanning near-field optical microscopy (SNOM) in both materials and life sciences. Two experimental SNOM setups were developed: one designed for infrared spectroscopy applications and the other for the imaging of fluorescently labeled samples. The results of the experiments that were conducted with both setups are presented and analyzed in this thesis. Diffraction limits the resolution of lens-based microscopes to a value close to that of the wavelength of the light that is used to illuminate the samples. SNOM instruments overcome this limit by probing the near-field light — the light that remains within close vicinity of the sample and decays exponentially away from it. The SNOM concept and instrumental design are described. A comparison of SNOM with other novel microscopies (electron, atomic force, and scanning tunneling microscopies) outlines the main advantages of SNOM — the sole optical microscopy technique with a nanometer resolution. Aperture-based infrared SNOM (IR-SNOM), performed in the spectroscopic mode using the Vanderbilt University free electron laser, recently started delivering spatially resolved information on the distribution of chemical species and on other laterally fluctuating properties. The IR-SNOM combines the IR spectroscopy's chemical specificity and the SNOM's high optical and topographical resolutions. The IR-SNOM experimental setup is described in detail and results from the study of cells, boron nitride and lithium fluoride thin films are presented and analyzed. They demonstrate the great potential of this new technique. An overview of the development of IR-SNOM, as well as a consideration of its future possibilities are presented. The newly built SNOM instrument at EPFL was used for experiments that are complementary to the aforementioned IR-SNOM experiments. Specifically, the new SNOM enabled the study of fluorescently marked samples. Images of fluorescently labeled cells and carbon nanotubes are presented and analyzed. The results further demonstrate the exciting possibilities of SNOM.

EPFL2005

Micro and nano tools for magnetic field imaging

Schahrazede-Lila Mouaziz

Scanning probe microscopy has become, nowadays, a branch of microscopy widely used to image surfaces of different specimens. Among the various applications, the magnetic imaging is one of the most exciting scanning probe microscopy techniques. It allows the study of topography of samples at the nanometer scale. The magnetic force microscopy is the best known and the most employed magnetic imaging technique. It has been successfully used for the imaging of various samples but with the major inescapable limitations that it is confined to the surface, non-quantitative and invasive. Techniques such as the scanning Hall probe microscopy (SHPM) and the magnetic resonance force microscopy (MRFM) have emerged, several years later, as promising approaches to overcome these limitations. In this thesis, we present the design, the fabrication and the characterization of, first, micro-Hall sensors on SU-8 cantilevers used for SHPM and, second, of ultrasensitive Si cantilevers used for MRFM experiments. A microfabrication technique has been developed to produce hybrid devices consisting of micro-nano Hall sensors integrated on SU-8 cantilevers. Two materials have been used to produce the Hall sensors. First, bismuth (Bi) was chosen for its low carrier charge density and its negligible surface charge depletion effects so that it should provide sensitive sensors. The second material tested was nickel (Ni), chosen for the dominant effect called the extraordinary Hall effect. The active areas of 5×5, 2×2 and 1×1 µm2 were realized and reproduced with the standard photolithography. Producing submicron Hall sensors required the use of the focused ion beam (FIB) that allowed a decrease of the size of the sensors down to 80×80 nm2. The delicate transferring of the nano sensors on the polymer cantilevers was successful. Bi micro-Hall sensors have shown a fragility making them delicate to handle. However, the Ni micro-Hall sensors had an excellent functionality. Their characterization has shown sensors with a low sensitivity compensated by the high applied current providing a high magnetic field resolution. The magnetic field images were carried out using the Ni micro-Hall sensors. They had a high stability during the imaging experiments. The spatial resolution was set by the sensors active area limited to the micro-meter scale. However, the magnetic field resolution is better than 1 T/√Hz a result comparable to the high resolution offered by the semiconductors. Single crystal silicon cantilevers have been fabricated for use in MRFM experiments. Also, cantilevers with integrated micro-magnets have been successfully produced. The developed microfabrication technique has allowed a high yield production (up to 70%). The softest fabricated cantilevers are 0.34 µm thick, 500 µm long and 10 µm wide. These cantilevers have been characterized under vacuum conditions (1×10-6 mbar). The resonance frequencies and the quality factors were measured using the frequency sweep or noise methods and both provided the same results. The Rayleigh method that we have adapted to the cantilevers geometry, made it possible to estimate the theoretical resonance frequencies with a good approximation and the calculated values fit well with the measured ones. All the cantilevers have a quality factor above 20000 and the force sensitivity achieved was between 10-16 and 10-17 N/√Hz A mathematical model has been developed based on the motion equation of a harmonic oscillator under the influence of a magnetic field. The model was studied to find the expression of the resonance frequency of cantilevers with integrated magnetic tips in the presence of a parallel or a perpendicular magnetic field. The shift in frequency due to the presence of the applied magnetic field depended on the magnetic tip magnetization. The measurement of the frequency shift should permit the calculation of the magnetization of these micro-magnets directly integrated on the cantilevers. The probes that we have realized represent micro and nano tools suitable for the magnetic field imaging that can be done with the SHPM and MRFM techniques.

EPFL2007