Bulk Micromachining of Silicon

Anisotropic silicon micromachining Bulk micromachining of silicon... 

Kovacs GTA, Maluf NI, Petersen KE (1998) Bulk micromachining of silicon. Proc IEEE 86(8) 1536-1551... 

Bulk micromachining of silicon Anisotropic silicon micromachining... 

Micromolding, Figure 1 Bulk micromachining of silicon masters... 

D Processing Technology 200 Sensors, Actuators and Passive Components 201 Bulk Micromachining Technology 201 Surface Micromachining of Silicon 205 Summary 205... 

The SOIMUMPS process is based on bulk micromachining of a silicon on insulator (SOI) wafer using four mask levels. It was originally developed for the fabrication of MEMS variable optical attenuators (VOAs) based on the use of a thermal actuator to control an optical shutter [18]. A cross-sectional diagram of an SOI wafer is shown in Figure 1.12. 

Fabry-Perot interferometer is an optical resonator consisting of two parallel mirrors. Fabry-Perot interferometers can be made by silicon bulk microma-chining." " Silicon surface micromachining is also a suitable technique for making interferometers for infrared wavelengths. 

Tu, J. K., Huen, T., Szema, R., Ferrari, M., Filtration of sub-100 nm particles using a bulk-micromachined, direct-bonded silicon filter, J. Biomed. Microdevices 1 (1999) 113-119. 

Electrochemical etching is one way of controlling the etch rate and determine a clear etch stop layer when bulk micromachining Silicon. In this case, the wafer is used as anode in an HF-Electrolyte. Sufficiently high currents lead to oxidation of the silicon. The resulting oxide which is dissolved by the HF-solution. Since lowly doped silicon material is not exhibiting a notable etch rate, it can be used as an etch stop. 

Bulk micromachining Microfabrication of three-dimensional features such as membranes, cavities, and so on by anisotropic dry or wet etching into the bulk of substrate materials like silicon, quartz, or others. 

Substrates The substrates in microelectronics are mainly Si wafers. For mobile applications, silicon-on-insulator (SOI) wafers increasingly replace bulk Si wafers and for very specific high-frequency applications, III-V compound semiconductors (e.g., GaAs) are used. The majority of substrates in microfabrication are Si wafers, but metal, glass, and ceramic substrates are also common. Particularly when using glass, quartz, and ceramic wafers in CMP processes, it has to be taken into account that they are brittle and easy to break. The situation is worse when the material is also under stress induced by deposited layers. For applications where the backside of the wafer has to be structured (e.g., in bulk micromachining), double-side polished substrates are employed. 

The mechanical properties of thin films used as membranes or moving structures and produced either by bulk or surface silicon micromachining are of crucial importance. In either type of application, the mechanical stress and the fracture properties of the applied thin films determine the behavior and long-term reliability of the device. In automotive applications, the environmental constraints (e.g., high ambient temperatures and temperature variations) and the required long life expectancy of the sensors require the use of high-quality thin films with well-controlled properties. 

Bulk-micromachined membranes are usually formed from dielectric materials like silicon oxide or silicon nitride combined with additional materials for example, in pressure sensors, silicon is used to increase the membrane thickness to the required values and in thermal sensors, platinum or other metals are needed for the sensing elements. The overall stress state of the membranes has to be controlled well to prevent buckhng (under high compressive stress) or fracture (under high tensile stress). With proper processing control, silicon oxide and silicon nitride thin films meet this requirement, making them ideal candidates for membrane-type devices. 

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