ABSTRACT
Various matured fabrication technologies and processes for distinct mechatronic systems and their components are available. To fabricate conventional, mini-, and microscale electromechanical motion devices and ICs, distinct technologies, processes, and materials are used. Affordable, high-yield, and effective conventional technologies have been used for many decades to fabricate various high-performance electromechanical motion devices, while complementary metal oxide semiconductor (CMOS) technology ensured high-yield for power electronics and ICs [1-7]. Power electronics, ICs, actuators, and sensors are profoundly different. However, the majority of microelectromechanical systems (MEMS) can be fabricated enhancing CMOS technologies [8-10]. MEMS imply not simply integration of electromechanical and electronic components, but utilization of enhanced microelectronics technologies and application of enabling materials to guarantee high-yield mass-produced cost-efficient system fabrications and packaging. This chapter is aimed to introduce the reader to the basics of MEMS fabrication. Different techniques should be utilized taking into the account various system components and specification. We will center on the fabrication basics for microelectromechanical devices which can be integrated with ICs forming MEMS [11]. Core micromachining and microelectronics processes will be introduced. Micromachining, as a core fabrication technology, can be used to fabricate complex
structures and devices which could be within micrometers in size. Integrated MEMS are composed from functional actuators, sensors, radiating energy devices, energy sources, ICs, etc. These MEMS can be fabricated utilizing different microfabrication technologies, e.g., CMOS, micromachining (bulk and surface), or high-aspect ratio processes. In contrast, the microelectronic components are fabricated using CMOS and biCMOS processes. The micromachining processes were developed as a well-defined sequence of steps to selectively etch away parts of the silicon wafer or other materials and deposit various structural and sacrificial layers of different materials forming mechanical, electromechanical, electro-opto-mechanical, electromagnetic, electrostatic, piezoelectric, and other devices and their components. For integrated MEMS, the main goal is to integrate microelectronics with micromachined
electromechanical devices in order to design high-performance systems. To guarantee highperformance, affordability, reliability, andmanufacturability, well-developed CMOS-based batch-fabrication processes have been modified and enhanced. MEMS are packaged, and usually sealed, to protect them from harsh environments, prevent mechanical damage,
Analysis, and Design with
minimize stresses, vibrations, contamination, and electromagnetic interference. It is impossible to specify generic MEMS, fabrication, and packaging solutions due to distinct device physics and unlimited number of possible application-dependent solutions. Various fabrication techniques, processes, and materials must be applied to attain the
desired performance, reliability, and cost. Bulk and surface micromachining, as well as high-aspect-ratio technologies such as Lithography-Galvanoforming-Molding (abbreviated as LIGA because it stands for Lithografie-Galvanik-Abformung in German), are the most developed fabrication methods. Silicon is the primary material which is used by the microelectronic industry. A single crystal ingot (solid cylinder hundreds millimeters in diameter and length) of very high purity silicon is grown, sawed to the desired thickness, and polished using chemical and mechanical polishing techniques. Electromagnetic and mechanical wafer properties depend on the orientation of the crystal growth, concentration and type of doped impurities. The major steps for ICs fabrication are diffusion, oxidation, photolithography, masking, deposition, etching, metallization, doping, planarization, packaging, and bonding. Extending conventional CMOS processing, additional processes and enablingmaterials are used to fabricateMEMS. There are a number of micromachining techniques which can be used in order to pattern and deposit thin films, as well as to shape silicon and other materials forming the needed microstructures and devices. Photolithography (lithography) is the process used to transfer the mask pattern (desired
pattern, surface topography, and geometry) to a layer of radiation-or light-sensitive material (photoresist). Then, the pattern is transferred to the films or substrates through development, etching, and other processes. The major steps in lithography are the masks production (pattern=topography generation) and transfer of the pattern. Important photoresist characteristics are resolution, sensitivity, etch resistance, thermal stability, adhesion, viscosity, flash point, and toxicity. The photoresist processing includes dehydration baking and priming, coating, soft baking, exposure, development, inspection, postbake (UV hardening), etc. The specified pattern is transferred to the wafer through etching, after which the photoresist is stripped by strong acid solutions (e.g., H2SO4), acid-oxidant solutions (H2SO4þCr2O3), organic solvents, alkaline strippers, oxygen plasma, or gaseous chemical reactants. Using wet and dry stripping, the photoresist must be removed without damaging silicon structures. In surface micromachining, an alternative solution to etching (in order to transfer the patterns from photoresist to thin films) is the lift-off. In lift-off, the photoresist is first patterned, the thin film to be patterned is deposited, and then the photoresist is dissolved. The photoresist acts as a sacrificial material under thin film regions to be removed. High-resolution photolithography defines two-dimensional (planar) shapes, and, indir-
ectly leads to three-dimensional features. Hence, microdevice and their structures (plates, stator, rotor, bearing, coils, and cavities.) geometry are defined photographically. First, a mask is produced on a glass plate. The silicon wafer is then coated with a polymer which is sensitive to ultraviolet light. This photoresistive layer is called photoresist. Ultraviolet light is shone through the mask onto the photoresist. The positive photoresist becomes softened, and the exposed layer can be removed. There are two types of photoresist, e.g., positive and negative. Where the ultraviolet light strikes the positive photoresist, it weakens the polymer. As the image is developed, the photoresist is rinsed where the light struck it. In contrast, if the ultraviolet light strikes negative photoresist, it strengthens the polymer. Therefore, a negative image of the mask results. When the photoresist is removed, the patterned oxide appears. Different chemical processes are involved to remove the oxide and other materials where they are exposed through the openings in the photoresist.
Alternatively, electron beam lithography can be used. Various computer aided design (CAD) tools are applied to support photolithography. The photolithography process and a photolithography system are illustrated in Figure 13.1. A high-resolution positive image is needed, and different photolithography processes
are developed. Deep UV lithography processes were developed to ensure the feature sizes of microstructures to be 0.1 mm. Different exposure wavelengths l are used (e.g., 435, 365, 248, 193, and 157 nm) which are mainly predefined by the photoresist chemistry. Using the Rayleigh model for image resolution, one finds the expressions for image
