Solution processing spin-coating

Poly(amic acid) solution is spin-coated on a substrate such as Si, glass, and metals. This coated solution is then converted to polyimide film by heating along with ordering. Furthermore, film stress arises in the final process from high temperature to room temperature with the CTE difference between the polyimide and the substrate, and Ani is caused by this orientation and stress. 

Figure 15.24 shows the fabrication process of the optical filter on a fluorinated polyimide substrate. First, the low-thermal-expansion-coefficient PMDA/TFDB poly(amic acid) solution was spin-coated onto a Si substrate and baked. Then alternate TiO2 and SiO2 layers were formed on the polyimide film by ion-assisted deposition. The multilayered polyimide film was diced and peeled off from the Si substrate. In this way, thin optical filters on a fluorinated polyimide substrate are easily fabricated. 

Polyimide Deposition. The most accurate processes for depositing thin, uniform coatings of PAA or PI solutions are spin coating, spray coating, and a recently reported screening process (12). 

A variation of MIMIC for patterning polymer semicondnctor TFT arrays was recently reported by Salleo and coworkers [105]. This techniqne exploits capillary forces to pattern a solution-processable polymer semicondnctor. In this method, the polymer semiconductor solution is spin-coated onto a snbstrate. A chemically treated PDMS stamp is then placed directly on top of the substrate coated with the polymer semiconductor solution. In the regions of contact, the PDMS stamp absorbs the solvent, leaving behind a solid polymer semiconductor film between the stamp and the substrate. In the recessed regions of the PDMS stamp, the polymer semiconductor solution wicks into the stamp due to capillary forces, effectively leaving behind a clean snrface in the noncontact regions. 

Layers of in -PEDOT are deposited by mixing EDOT, an oxidant, e.g. Fe(III) tosylate, and optionally imidazole first [52, 89, 90]. The butanolic solution is spin-coated on to a substrate where EIX)T polymerizes to doped PEDOT while drying. The remaining oxidant is removed by purging the films in deionized water followed by a second drying sequence. Several successive processing steps are needed to obtain finished Alms. 

Polymer solutions of 10 mg polymer in 1 ml toluene were prepared. For the polymer blends, 75 % PMMA solution and 25 % PS with different molecular weights were mixed. The solutions were spin-coated on silicon substrates, freshly cleaned in a solution of 50 % H2SO4 and 50 % H2O2. Using a speed of 2000 rpm for the spin coating process, homogeneous films with thickness of about 150 nm were produced. 

Carbon membranes were fabricated by pyrolysis of a mixture of polyffurfuiyl alcohol) (PFA) and poly(ethylene glycol) (PEG) that was spin-coated on a mac-roporous stainless steel support. The stainless steel support was first modified by physical deposition of sub-micron-sized silica particles within the micropoies and then PFA/PEG/acetone solution was spin-coated onto the silica-modified support. Coated support was heated in a stream of Ar to 600°C for 4 h for pyrolysis. The process was repeated. Hydrauhc permeability was obtained under 40-50 psig. UF experiments with blue dextran as the solute was also condueted. Table 6.1 shows the hydraulic permeability and blue dextran sieving data. 

The use of spin-coated polymers was limited to the anti-reflection field, due to the lack of materials having comparatively lower refractive indices. Walheim et al. [48] demonstrated a superior anti-reflection nano-porous polymer-based film. Two immiscible polymers (PMMA and PS) were mixed in tetra-hydrofuron and the solution was spin coated on a substrate made of glass. This process resulted in the formation of the individual polymer domains and it was observed that the PS was selectively dissolved to form a nano-porous film. It was also found that increasing the volume ratio of the pores would result in minimising the refractive index of the material, a phenomenon which is highly desired in the anti-reflection coatings of broadband. 

Solution processes are used mainly for coating, film casting and fibre spinning, i.e. in processes where the distance the solvent has to diffuse out of the solution once coated, cast or spun is short. Regulation of this diffusion process is important if products of even quality are to be obtained. 

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