An original new design for manufacturing a piezo-resistive type micro-accelerometer made by Si bulk micro-machining is proposed. The enhancements applied by the authors especially in design and also in processing and control techniques lead to a more precise device, having a superior reliability. Opposite to classical type, the new model have shorter cantilever, enabling to have an uniform stress and so permitting us a long piezo-resistor design. This in turn enables the use of low surface boron concentration of diffused resistors, resulting in a low temperature drift. After ANSIS simulation, showing the benefits of this new design, it follows the description of layout-based surface and volume control elements, and finally the processing enhancements applied to give a 1g piezo-resistive accelerometer
The "traditional" silicon planar technology, generating the field of micro-optics produces only a few shapes. To avoid that, many "non-conventional" technologies, based especially on photo-polymers, are investigating. This paper reports on some enhancements of such three technologies: LiGa, gray-tone lithography and embossing.
An interesting alternative to the former use of HEBS glass (suitable e-beam exposed to give a “true” variable transmission contact mask) is the “simulated” variable transmission gray-tone projection reticle. It is simply obtained from a film generated on a 4000 dpi laser writer and then 10x reduced on a Cr.-glass plate. This projection gray-tone reticle, having sub-resolution pixel elements, is able to modulate light intensity in a thick positive resist to give arbitrary 3-D shapes. A tremendous enhancement in resolution, both for horizontal and vertical direction, was obtained by applying an original combination of binary masking technique (N powered by M levels for M reticles having each N gray-tones) with a proper focusing depth of the projection objective (just at the middle of the exposed resist layer). So, it was possible to structure a thick resist in very complex smooth 3-D shapes, which are very useful for micro-optics, by using M exposing steps. Interesting applications for micro-optics are given.
Although, by using gray-tone lithography and common technologies in standard IC fabrication it's easy to obtain an arbitrarily 3-D shaping of positive thick resists, there are some limitations too. E-beam writing implies a maximum of only 200 gray-levels on the project retilce, and the limited focus depth of the projection objective gives a poor lateral resolution. That's why we applied a new approach to enhance the 3-D resolution of gray-tone lithography. By combining gray-tone lithography with binary masking technique, it was possible to obtain a high resolution (vertical and horizontal directions) into thick resist. Considering it as a primary mold, a metallic variable absorber mask for deep X-ray lithography may be processed. Previously, it's necessary to transform the resist surface into a conductive layer as follows: conditioning, nucleation and electroless plating, respectively. After that, a metallic deposit is obtained by electroplating at a desired thickness, resulting in a complementary shape of the mold resist. The original design and fabrication method for the gray-tone test reticle were supported by preliminary experiments showing the main advantage of this new technology: the 3-D structuring of thick resists in a single exposure step and also a very promising aspect ratio obtained of over 9:1.
With the conventional micro machining technologies: isotropic and anisotropic, dry and wet etching, a few shapes can be done. To overcome this limitation, both binary multi- tasking technique or direct EB writing were used, but an inexpensive one-step UV-lithographic method, using a so- called 'gray-tone reticle', seems to be the best choice to produce local intensity modulation during exposure process. Although, by using this method and common technologies in standard IC fabrication it is easy to obtain an arbitrarily 3D shaping of positive thick resists, there are some limitations, too. The maximum number of gray-levels, on projection reticle, achieved by e-beam writing, are only 200. Also, for very thick resists, the limited focus depth of the projection objective gives a poor lateral resolution. These are the reasons why the author prose da new approach to enhance the 3D resolution of gray-tone lithography applied for thick resist. By a high resolution, both for vertical direction, as well as for horizontal direction. Particular emphasis was put on the design, manufacturing and use of halftone transmission masks, required for UV- lithographic step in the fabrication process of mechanical, optical or electronics components. The original design and fabrication method for the gray-tone test reticle were supported by experiments showing the main advantage of this new technology: the 3D structuring of thick resist in a single exposure step and also a very promising aspect ratio obtained of over 9:1. Preliminary experimental results are presented for positive thick resists in SEM micrographs. A future optimization of the lithographic process opens interesting perspectives for application of this high 3D resolution structuring method in the fabrication process of different products, with imposed complex smooth profiles, such as: x-ray LiGA-masks, refractive optics and surface- relief DOEs.
A new fabrication method suitable for cheap assembling of MEMS and MOEMS is described. As a present, the very expensive methods for assembling and packaging, applied in the case of MEMS and MOEMS, make almost prohibit the final cost of a new prototype. For this reason it is so important to reduce the prize of the assembling techniques, and to find a way for batch mounting and packaging. The proposed method consists of generating a high aspect ratio structure, by means of soldering some different shaped metallic foils. The technological process consists of the following steps. In the first step, the desired 3D structure is divided into N different levels, while maintaining the thickness of each of them between 10-300 micrometers. Next, the pattern for each level is achieved by applying, to the copper metallic substrates, a double side lithography, commonly used in printed circuits fabrication, such as: solid negative resist lamination; UV exposure; alkaline spray developing, and acid spray etching. After a double side Sn-Pb electroplating of each metallic foil, all of them are properly stacked and aligned in a frame, by means of some special etched holes. The resulting stack is pressed and heated at soldering alloy's melting temperature, preferably by using a vacuum system. Finally, the temperature is ramped down and the pressure is released, resulting in the desired 3D arbitrary shaped sandwich type structure. This new technique opens wide perspectives to replace some expensive assembling methods, in a variety of prototypes containing MEMS and MOEMS, being also recommended for metal can packaging in special applications and microfluidic devices assembling too.
In building the movable elements of robots, peripheral devices and measuring apparata, increasing the resolution of the angular sensor systems, based on incremental rotary encoders, is essential, together with decreasing the complexity, dimensions and weight. Especially when the angular sensor is integrated in a measuring system, belonging to a programmed light airplane for surveillance, the key issue is to reduce both dimensions and weight. This can be done using a simplified design, which consists in the following solutions: replacement of the fragile Cr on glass substrate, 1.5 mm thick (normally used for the fabrication of incremental disks), with light Cr on polycarbonate substrate, with only 0.15 mm thick; the absence of collimating optics (based on microlenses, used in IR emitter-photocell receiver assembly), as a result of the good coupling efficiency (due to the possible approaching of these elements at minimum 0.45 mm); the shrinkage of the disk's diameters to only 14 mm; the use of surface mounting devices and the related surface mounting technology, enabling to reduce dimensions and weight. The maximum number of slits on a 14 mm diameter dividing disk, usually obtained in a Cr on polycarbonate version, being approx. 1000, no problem occurs in our case, for 360 slits. The requested angular resolution (only 0.5 degrees for the light airplane), using the whole classical '4x digital multiplication' is not necessary, but a lower one of only 2x, resulting in a simplified electronics. The proposed design permitted, that an original arrangement, for building a small size, lightweight, heavy-duty incremental transducer based angular sensor system, to be obtained, useful not only in avionics, but also in robotics, or other special applications. Besides, extending the number of fixed gratings (masks) allows, that many primary signals to be derived, and a further increase in resolution of even 6 angular minutes to be obtained from the initial 360 slits.
The original diffraction grating was fabricated using a well known Si anisotropic etching technique. By means of this micromachined Si stamper it is possible to transfer its surface-relief profile into a dry negative resist film. The transfer is performed during an embossing process which contains the following steps: In the first step, the dry photosensitive film having 1.5 mils thick, commonly used in PCB fabrication, is laminated onto a quartz UV transparent glass. After that, the protective cover sheet is removed from the laminated substrate, and the Si stamper is placed on the free surface of the photopolymer film. Next, the shimmed sandwich is loaded at the embossing temperature of 115 degree(s)C into a vacuum UV contact printing unit, while a moderate external pressure is applied, during aprox 5 second. The embossed information layer is then firmly bonded to the substrate, and the embossing is made permanent by ultraviolet radiation curing. This treatment consists of exposing the photopolymer, through the transparent substrate, at a UV broad band light source, with a hardening dose of at least 2500 mJ/cm<SUP>2</SUP>. Finally, the vacuum is stopped, the sandwich is removed from the exposing unit, and the assembly is separated by slightly flexing, obtaining the permanent complementary replica of the stamper. This fabrication method, tested only for diffraction grating replicas, has also great potential in batch production of many other low-cost integrated optical components.
This paper describes the fabrication of a new type of metal stencil mask. By using electrodeposited (ED) photoresist in an electrophoretic bath, conformal layer is deposited on both sides of a copper foil, making thus possible double side photolithography. After the exposure through two well aligned mask, containing the structure, and the developing of the photoresist, the copper foil is simultaneous selective electroless nickel deposited on both sides, allowing the stress in the sandwich structure nickel films to balance each other. Transmission holes are etched entirely through the copper membrane resulting in an inherent stress compensated metal stencil mask. The attainable 5 micrometers resolution and high reliability offered by stress compensation of this kind of metal stencil, open very exciting possibilities for low costs selective deposition of metals and insulators in microelectronics, micromechanics and optoelectronics.