Polyimide deposition, processes

A number of dielectric films are deposited by the spin-on technique. In this case the film s constituent molecules are dissolved in a solvent to form a hquid. After spinning the Hquid over a semiconductor surface the solvent is driven off with a baking step, leaving behind the thin dielectric film. Common films include polyimide and benzocyclobutene (BCB). The deposition process for these films is simple, making it attractive for a manufacturing process. 

Chemical vapor deposition (CVD) is a process whereby a thin solid film is synthesized from the gaseous phase by a chemical reaction. It is this reactive process that distinguishes CVD from physical deposition processes, such as evaporation, sputtering, and sublimation.8 This process is well known and is used to generate inorganic thin films of high purity and quality as well as form polyimides by a step-polymerization process.9-11 Vapor deposition polymerization (VDP) is the method in which the chemical reaction in question is the polymerization of a reactive species generated in the gas phase by thermal (or radiative) activation. 

Attaching thin adherent titania films to polyimide polymers demonstrates the promise of such coatings as barrier layers. Potential applications must examine the effect of the deposition process on the mechanical and thermal properties of the polymer and the requisite thickness of the ceramic layer. Nevertheless, the low-cost and convenience of LPD coatings, their mild conditions and their lack of line-of-site limitations, recommend their use. 

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). 

The post deposition processing parameters (curing temperature, time) have a profound impact on the thermal properties of polyimide thin films. It has been observed that, films cured at around 300°C for 1 hour exhibit markedly superior thermal stability as compared to those that were not cured." ° Cured PMDA-ODA... 

Chemical vapor deposition of polynapthalene differs from the parylene and polyimide systems in terms of the deposition process. As mentioned in earlier sections deposition of parylenes and polyimides occurs on cold surfaces, and the deposition rate decreases with increasing substrate temperature. In other words, deposition is done in a "hot wall" reactor. In contrast, CVD of polynapthalenes is performed in a "cold wall" reactor, meaning that the substrate is maintained at a high temperature ( 350°C) while the surrounding wall temperature is kept at near room temperature. A schematic of the CVD reactor setup employed by Lang et al. can be found in Ref. 28. 

Polyimides Excellent electrical properties. Excellent barrier properties, eg., H2O High thermal Stability Two step deposition process CVD films not completely characterized... 

Figure 12.12 Image of an organo-metallic palladium precursor solution that was deposited by the author on a flexible polyimide substrate via spin coating and thermally processed at 200 °C for 30 seconds.

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. 

In the lift-off process, a blanket metal coating is deposited, usually by evaporation, over the photoresist, which is then dissolved to lift off the unwanted metal and leave the desired pattern. The lift-off process may be assisted by depositing and patterning a dielectric layer, a release layer, or both beneath the photoresist (131, 132). In both additive approaches, via posts are patterned in a step separate from that used to pattern the conductor lines. The polyimide is then coated over the lines and via posts, and shallow etching or mechanical polishing is done to expose the top of the via posts. The process sequence is then repeated to pattern additional layers. 

A recently reported alternative to spin or spray coating is screen printing of polyimide solutions (82, 85, 90). Screen printing is a low-cost, high-throughput process capable of directly patterning the polyimide films as they are deposited. Another alternative is the vapor deposition of polyimides, which was reported by researchers who co-evaporated the diamine and dianhydride monomers at stoichiometric rates (140). The evaporated films had better adhesion, a lower dielectric constant, and a lower dissipation factor compared with spin-coated polyimides. With this process, uniform, defect-free, conformal films can be cured in situ during deposition. 

We report herein LPD titania films deposited on three kinds of polyimides two high-temperature polyimide resins (PMR-15 and Kapton ) and a bismaleimide glass fiber composite (BMI). We have examined different solution conditions and different surface priming strategies. We have also learned to minimize film cracking by carefully controlling the drying process. 

Materials. Biaxially oriented polypropylene (PP) films of 50 um thickness were obtained from 3M and have been described (2). PMDA-ODA (PI) was Kapton H polyimide from Dupont. Copper-plated PTFE films were obtained from Spire Corporation (Bedford, MA). They were prepared using the Ion Beam Enhanced Deposition (IBED) process in which a 100 nm thick Cu film was vapor-deposited onto a PTFE substrate in the presence of a beam of 400 eV Ar+ ions of 25 uA/cm2 (IQ). Shortly before SIMS analysis, the Cu film was removed slowly by peeling at 90° in ambient conditions. Metal-coated PI films were prepared by sputtering 50 nm Cr and 1 um Cu onto a 50 um thick Kapton film on both sides. Thermal annealing was performed in a vacuum chamber at 2xl0 6 torr using a quartz lamp as the heating source. The samples were held for 15 min at the desired temperature and then cooled down to ambient temperature inside the chamber for about 2 hours. Just prior to SIMS analysis, the metal films were peeled slowly at 90° and then immediately introduced into the vacuum chamber of the instrument. 

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