Tham khảo tài liệu 'mems mechanical sensors - stephen beeby part 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 7ữ Mechanical Sensor Packaging liquid water forming at the interface 27 . These polymers however offer poor levels of protection against alkaline solutions. Protective silicon oxide and silicon nitride films possess a much greater resistance to the diffusion of water molecules. These films can be applied both at wafer level and on mounted chips using CVD processes. They must be free from cracks and pinholes and in the case of mounted chips the films must be deposited on all the exposed surfaces including wirebonds and contact pads. The chemical resistance of these films is fundamentally important since they will only be deposited in thicknesses of a few microns. Even very low corrosion rates 27 angstroms day will remove a 1-micron-thick protective film after 1 year. Silicon carbide thin-films have been found to offer the most promising levels of chemical resistance 28 . A further consideration is the effects of thermal cycling which can cause delamination of these films due to TEC mismatches. If the second order package is required to protect the device the sealing processes developed by the IC industry and described in Section can be used. In the case of MEMS packaging second order capping can be further complicated by the functionality of the device. The most common example of this is in pressure sensors where a stainless steel diaphragm in the second order package is used to provide media isolation 29 . Stainless steel offers excellent levels of chemical resistance and possesses good mechanical properties making it an ideal material for such a barrier diaphragm. This diaphragm must not only protect the sensor but transmit the media pressure to it. This is typically achieved by placing the sensing die in an oil-filled chamber behind the stainless steel diaphragm see Figure . The pressure exerted on the stainless steel diaphragm is transmitted through the hydraulic oil to the sensor diaphragm. Both the stainless steel diaphragm and the oil used to fill .
