Tham khảo tài liệu 'an introduction to mems engineering - nadim maluf and kirt williams part 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 60 Processes for Micromachining The process may be stopped at this point with a metal microstructure suitable for some purposes. Alternatively the metal can be used as a mold for plastic parts the A in LIGA . Precision gears and other microstructures have been fabricated using LIGA but the method is considered expensive because of the requirement to use collimated x-ray irradiation available only from synchrotrons. Mold formation using optical lithography is often called poor man s LIGA. Guckel 23 provides additional details on the molding of high aspect ratio structures fabricated with x-ray lithography. In a variation known as electroforming the plated metal is peeled off of the substrate and is the useful structure. Examples of electroformed products are electric shaver screens and some ink-jet heads. Supercritical Drying The final step of many micromachining processes is the removal of a sacrificial layer . using hydrofluoric acid to etch 1 um of silicon dioxide from under a polysilicon beam . After rinsing the water must be dried from the wafer. If a freestanding structure overhangs the substrate surface tension forms a meniscus of water between the two see Figure . As the water dries its volume and hence thickness decreases. If the structure is compliant as is usually the case in surface micromachining it is pulled down contacting the substrate. If a sufficiently large smooth area of the structure makes contact it can stick which is known as stiction in the micromachining community. Such stuck structures can often be freed by pushing with a probe tip but this is hardly suitable for production. A solution to avoid stiction after release is supercritical drying also known as critical-point drying 24 . In this process the wafer is moved without drying into methanol which is miscible with the small amount of water left on the wafer during transfer. The wafer is then placed in a pressure chamber covered by methanol. Liquid carbon dioxide which is miscible
