Front-to-back alignment techniques in microelectronics/MEMS fabrication: A review

The front-to-back alignment technique in photolithography process is often required for registering mutually aligned patterns on both the sides of the wafer in the fabrication process of power devices such as power transistors and various kinds of microelectromechanical systems (MEMS) based devices such as RF MEMS components, mechanical sensors, bio/chemical sensors, microcalorimeters, and microfluidic devices etc. This paper reviews the progress made in the front-to-back alignment techniques in semiconductor processing for the realization of microstructures for MEMS and other microelectronics devices. Various techniques, which are used in commercial front-to-back mask aligners, are discussed in detail. Some other alternative methods, which use the conventional contact/proximity single sided mask aligner for defining the mutually aligned patterns on both the sides of the wafers, are also covered in this review. The principle and approach of various front-to-back alignment techniques have been presented. The alignment accuracy and the source of errors have been discussed. A list of the references which incorporate front-to-back alignment methods for the development of MEMS/microelectronics devices such as, mechanical sensors, bio/chemical sensors, microcalorimeters, RFMEMS components, microfluidic devices, power transistors is also included.