**2.5 PI film casting using glass and brass substrate**

Higher molecular weight polyamic acid solution dissolved in dimethylacetamidebased solvents is suitable for PI film casting using a variety of substrates, for example, teflon, alumina, glass, silicon wafers, brass, and copper. After coating the polyamic solution on the substrate, the precursor is thermally cured into an aromatic polyimide film. Both static and dynamic casting techniques can be used depending on the final product size and available tooling. In static casting, glass was used as a substrate to cast the PAA solution on it using a glass rod. Typically, this technique is easy, but it requires more material per substrate and difficult to control the thickness of PI films.

### **2.6 Casting PI films using a spin coating technique**

Spin coating is the most prevalent technique to deposit PAA solution onto substrates such as silicon wafers and brass. The dynamic deposition technique uses less material, but it requires precision to control the operation. While depositing, it is important to pour PAA solution in the center of the wafer. Generally, we can achieve the maximum speed up to 6000 rpm, if the vacuum chunk has enough suction to hold the substrate while depositing PAA solution onto the surface of substrates. The PAA solution was homogenously distributed over the silicon wafer substrates during spinning. The acceleration of speed was programmed to start with low speed, slowly rise to maximum speed, and finally decrease the speed slowly to allow the coating flow across the substrate edges homogenously. Several spin speed steps can be used to control the flow of solution to cover more than 80% of the substrate before achieving the final speed.

### **2.7 Casting PI and nanocomposite/PI multilayer films**

PI is obtained using the curing method in which a solvent is evaporated with the increase of temperature. During the PI film curing process from the polyamic acid (PAA) solution, there is a strong tendency of nanoparticles to get agglomerated. They may float on the surface or decant. Both effects can increase the chances of agglomeration and may influence the interface thickness and permittivity of samples and result in lower dielectric properties of PI films. Keeping this in mind, we prepared a multilayer (two and three layers) structure in which the top layer consists of a very thin PI/SiO2 nanocomposite (NPI) layer, and the bottom is composed of pure PI layer [4]. By doing this, we are giving less space for nanoparticles to get agglomerated. In order to cast the PI/nanocomposite multilayer films, two different spin speeds were used for a two-layer structure. After calibrating the right speed to get right thickness, we select 20 s at speed of 500 rpm to get around 60 μm thick first base layer of PI film and soft bake for 30 min at 60 and 100°C temperature, then we use 30 s of 1000 rpm to get second nanocomposite/PI 20 μm thick layer.
