Đang chuẩn bị liên kết để tải về tài liệu:
An Introduction to MEMs Engineering - Nadim Maluf and Kirt Williams Part 4

Tham khảo tài liệu 'an introduction to mems engineering - nadim maluf and kirt williams part 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 40 Processes for Micromachining For deposition below 400 C nonstoichiometric silicon nitride SixNy is obtained by reacting silane with ammonia or nitrogen in a PECVD chamber. Hydrogen is also a byproduct of this reaction and is incorporated in elevated concentrations 20 -25 in the film. The refractive index is an indirect measure of the stoichiometry of the silicon nitride film. The refractive index for stoichiometric LPCVD silicon nitride is 2.01 and ranges between 1.8 and 2.5 for PECVD films. A high value in the range is indicative of excess silicon and a low value generally represents an excess of nitrogen. One of the key advantages of PECVD nitride is the ability to control stress during deposition. Silicon nitride deposited at a plasma excitation frequency of 13.56 MHz exhibits tensile stress of about 400 MPa whereas a film deposited at a frequency of 50 kHz has a compressive stress of 200 MPa. By alternating frequencies during deposition one may obtain lower-stress films. Spin-On Methods Spin-on is a process to put down layers of dielectric insulators and organic materials. Unlike the methods described earlier the equipment is simple requiring a variable-speed spinning table with appropriate safety screens. A nozzle dispenses the material as a liquid solution in the center of the wafer. Spinning the substrate at speeds of 500 to 5 000 rpm for 30 to 60 seconds spreads the material to a uniform thickness. Photoresists and polyimides are common organic materials that can be spun on a wafer with thicknesses typically between 0.5 and 20 um though some specialpurpose resists such as epoxy-based SU-8 can exceed 200 um. The organic polymer is normally in suspension in a solvent solution subsequent baking causes the solvent to evaporate forming a firm film. Thick 5-100 um spin-on glass SOG has the ability to uniformly coat surfaces and smooth out underlying topographical variations effectively planarizing surface features. Thin 0.1-0.5 um SOG was heavily investigated