What is MEMS micromachining ? Explain in details fabrication process flow of LIGA. Why electroplating is necessary in LIGA process.
MEMS micromachining refers to fashioning microscopic mechanical parts out of a silicon substrate or on a silicon substrate. The overall process of obtaining the small structure is called micromachining.
The term LIGA is an acrorym derived from the German words lithography (lithographie) electroforming (galvan forming) and molding (abforming). This technique was developed at karlsruhe Nuclear Research center in karlsruhe, Germany The LIGA process is completely different from the bulk and surface micromachining processes for fabricating microstructers. It is a versatile process and can be used to incorporate materials other than silicon in the manufacturing sequence. Fig(a) shows major steps involved in the LIGA process. Patterns and created in Photo resist wing deep X-ray lithography technique. Since one of the major attraction of LIGA process is the ability to fabricate thick microstructure, a thick photoresist is used in the photolithography step X-ray lithography is chosen over conventionaI lithography due to the short wavelengths that are required to penetrate and develop thick photoresist material. This short wavelength leads to high resolution and ultimately results in the capability to fabricate structures with high aspect ratios. Typically line width of 0.2μm with aspect ratios greater than 100:1 are achievable. X-rays are provided by a synchrotron radiation source. Due to the high flux of collimated way from a synchrotron source the exposure time can be considerably shortened inspite of the use of thick photo resists. A popular photoresist material is polymethimethacrylate. (PMNA). X-ray lithography requires appropriate masking moterial. Thin layer of Silicon Nitride with gold layer on top good masking combination.
- The substrate in LIGA process is called the Base plate. Electroplating process is carried out for refilling the trenches in the photoresists for electroplated metal refilling process, the substrate or the base plate should either be a conductor or should have a suitable coating of a conductor.