MEMS技术为实时时钟提供了实实在在的好处

　　The MEMS resonator technology discussed here was developed in standard complementary metal-oxide semiconductor （CMOS） fabs. CMOS fabrication is especially advantageous for meeting targeted frequency responses based on the shapes and sizes of device elements established at the photolithographic stage of development. Since MEMS is a silicon technology， the benefits of repeatability and sustainability apply to the manufacturing of MEMS wafers. The manufacturing temperatures reached while processing MEMS wafers can exceed +700°C. Subsequently， during processing， the MEMS resonator can be subjected to multiple reflow temperatures of +260°C without any degradation of performance. （We will talk about this in more detail below.） This durability can be attributed to its material makeup， design， and wafer processing flow.

　　In contrast （and well understood）， crystal assembly is a less robust process and prone to sizeable variations in product-to-product output. Frequency tuning and trimming generally require the deposition or removal of material from the crystal electrode to achieve the desired frequency. Additionally， a vacuum must be established in the cylindrical carrier for the crystal resonator to vibrate once voltage is applied to the device. Consequently， to produce high-quality devices， special materials are required for attachment of the crystal to its leads. These materials help the crystal survive high-temperature （approximately 260°C） reflow operations. There is a caveat， nonetheless. Care must be taken when subjecting crystals to multiple high temperature reflow cycles. Frequency shifts can be attributed to aging of “crystal-attach” material， quality of the vacuum， and/or imperfections in the crystal blank.