Silicon dioxide, SiO2, is an amorphous material used in microsystems as a dielectric in capacitors and transistors, insulator to isolate various electronic elements, and structural or sacrificial layer in many micromachining processes. Thin oxide films are quickly grown or deposited on silicon wafers using different techniques reviewed in Section 2.3. High-quality oxide films provide excellent electrical insulation with resistivity values as high as 1010 Ω-m. Oxide films are also good thermal insulators, with a low thermal conductivity of around 1.4 W/m-K. Phosphorous-doped and boron-doped oxides, known as phosphosilicate glass (PSG) and borosilicate glass (BSG), respectively, will flow at temperatures above approximately 900°C. Reflow of PSG and BSG films is commonly employed for planarization purposes. Heavily doped PSG films are used as a sacrificial material in many surface-micromachining processes because of the rapid etch rate in hydrofluoric acid. As grown or deposited on a substrate, the residual stress in oxide films is always compressive (on the order of 1 GPa).
Therefore, micromechanical structures made from oxide are constrained in design and size to avoid mechanical buckling upon release from the substrate. One of the crystalline forms of SiO2 quartz is helpful as a microsystem substrate. Quartz sensors are generally bulk micromachined devices and exploit the piezoelectric properties to convert mechanical displacement into electrical signals. Various ultrasonic transducers make use of quartz. Quartz has extremely low structural damping, making it an ideal material for mechanically resonant structures, such as low-noise oscillators. Borosilicate glass substrates are used in many bulk micromachined processes. Specifically, electrostatic (or anodic) bonding of silicon to glass is almost always accomplished with Coming #7740 glass because of its close match of thermal coefficient expansion to silicon. Channels, depressions, and holes in glass substrates can be etched chemically or machined conventionally. If you are looking for high quality, high purity and cost-effective silicon dioxide, or if you require the latest silicon dioxide price, please email contact mis-asia.
