**2.4 Polyimide deposition and process techniques**

There are several routes to shape polyimides depending on the targeted application. Among them, one can cite the biaxial stretching for film production, the spincoating and lithography process for wafer-level deposition and the vapor-deposition process. The following sections present all these.

### *2.4.1 Biaxial stretching for film production*

Compared with the lab-scale preparation, the greatest difference for industrial manufacturing of polyimide films is the stretching process [9, 10]. Stretching process, either uniaxial or biaxial stretching of the gel-like PAA films, will result in the full orientation and extension for the polyimide molecular chains.

**Figure 5** shows the main steps of the industrial scale production line of polyimide film by biaxial stretching from PAA precursors. In this procedure, the precursor monomers are firstly introduced into the polymerization reactor containing the solvent. After PAA synthesis, the obtained solution is deaerated and cast in the form of a continuous film onto the surface of a heated rotating stainless steel drum. The solvent is partially evaporated and a part of the imidization reaction takes place simultaneously. Thus, a self-supported PAA film is formed. The gel-like PAA film is peeled from the metal drum and first stretched in the machine direction (MD) while controlling the stretching rate. The PAA film is then stretched in the transverse direction (TD). The solvent is removed by evaporation, and the film is heat treated by means of hot air or radiant heat from an
