Direct writing technology—Advances and developments

Direct writing (DW), also known as digital writing or digital printing, is a family of flexible multi-length scale processes for the deposition of functional materials to form simple linear or complex conformal structures on a substrate. This paper provides an overview of key DW technologies and their process characteristics under a unified classification system. In DW, a variety of mechanisms and energy modes such as inkjet, laser, mechanical pressure and tips are used to create material transfer to produce features from the nm to the mm range. This new group of additive on-demand processes complements existing manufacturing methods especially in product miniaturization and geometrical footprint reduction due to its conformal writing capability. The range of materials is exceptionally wide, ranging from metallics, ceramics, dielectrics and polymers to biomaterials. The thickness of the layer ranges from a monolayer of molecules to hundreds of micrometres. As DW is a scalable process, it is capable of high-throughput volume production, especially in microelectronics. Industrial applications have been expanding and numerous niche examples are given to illustrate meso-, micro- and nano-scale applications. Finally, challenges for its future development are also discussed.