Ion beam irradiated polyimide

Fig. 6.12. Auger (left) and total electron yield (right) NEXAFS spectra of rubbed polyimide (top), ion beam irradiated polyimide (middle), and amorphous carbon (bottom). The comparison reveals the presence of a layer of amorphous carbon at the surface of the ion beam irradiated polyimide film.

Fig. 6.10. (A) Liquid crystals align on rubbed and ion beam irradiated polyimide surfaces along the treatment direction, but with opposite pretilt angles. (B) The respective polarization dependences possess the same overall orientation, but opposite shifts with respect to a = 0° within the plane parallel to the rubbing direction (solid squares). This is in agreement with the presented alignment model, as the derived molecular distribution factors illustrate (C).

Our study of ion beam irradiated polyimide films revealed that in this case the aligning surface consists of an amorphous carbon layer, which is caused by degradation of the molecular polymer structure in the ion beam. This observation led to the idea to replace the polyimide film with an amorphous carbon film. Indeed, an ion beam irradiated amorphous carbon layer appears to align liquid crystals as well as an ion beam irradiated polymer film. These results convinced IBM to develop a new manufacturing technology for liquid crystal displays, which successfully produced displays of highest quality. 

We note that the tilt direction of the molecular distribution can directly be derived from the ratio of the 45° intensities, which are emphasized by the enlarged symbols in Fig. 6.10. The -45° intensity is larger than the 45° intensity for the rubbed polyimide film, which indicates an upwards tilt of the molecular distribution with respect to the rubbing direction. For the ion beam irradiated film, on the other hand, one notices the opposite intensity ratio, which corresponds to a downward tilt of the molecular distribution with respect to the in-plane direction of the ion beam irradiation. One can therefore characterize the anisotropy of the molecular distribution by two ratios the molecular tilt angle is determined by the ratio of the tt intensities observed for a = 45° incidence angle within the plane parallel to the rubbing or ion beam direction. And the ratio of the two normal incidence spectra with the electric field vector in the plane either parallel (sohd squares at a = 90°) or perpendicular to the treatment direction (open circles at a = 90°) reveals the in-plane anisotropy. We will make use of this later. 

In Fig. 6.12 we show the tt region of the more surface sensitive AEY (left column) and the deeper into the film sampling TEY spectra (right column) of a rubbed (top row) and an ion beam irradiated (middle row) polyimide fihn. Comparing the spectra one clearly notices that the ion beam irradiation leads to significant ring breaking at the fihn surface. While the characteristic fine structure of polyimide is still prominent in the deeper below the film... 

The observation of a layer of amorphous carbon at the film surface, in combination with the observation of liquid crystal alignment on this surface, suggested the breakthrough idea to replace the polyimide polymer film with an amorphous carbon layer [34]. The essential requirement for liquid crystal alignment (as stated by our model), namely the presence of an anisotropic distribution of directional bonds, can be fulfilled by an ion beam irradiated amorphous carbon layer. This is demonstrated by the presence of the resonance associated with tt orbitals at 285 eV in the absorption spectrum of amorphous carbon (bottom of Fig, 6.12). Its presence indicates that amorphous carbon contains unsaturated sp2 and sp hybridized carbon atoms. While sps hybridization does not lead to any anisotropy, the directional nature of carbon double and triple bonds formed by sp2 and sp hybridized carbon atoms can lead to a breaking of the isotropy of the molecular distribution. It therefore mainly remains the question whether a statistically significant anisotropy in these carbon bonds can be achieved by ion beam irradiation of an amorphous carbon layer. 

Comparing the spectra in the right panel, which characterize the molecular tilt angle, one finds the same polarization dependence for the two ion beam irradiated materials, which is opposite to the one of the rubbed polyimide film. Hence, a downwards liquid crystal pretilt angle is expected for both ion beam treated surfaces. Again, since the overall shape and the tt intensities and their dichroism is comparable for the two ion beam irradiated films, liquid crystals ai e expected to exhibit a technologically sufficient pretilt angle on an ion beam irradiated amorphous carbon layer. 

In this study, plasma fluorinated polyimide films of various thicknesses were exposed to a 2 MeV He2+ ion beam at different doses to determine beam effects. A thick PTFE film, irradiated under similar conditions, was also examined surface changes and fluorine loss during RBS analysis were compared. 

Swift heavy ion beams with extremely high LETs are used to degrade strongly polymer chains within tracks over a range of a few tens of micrometers. After irradiation, the top surface of bulk thermoplastics such as polyethyleneterephtalate, polycarbonate, polyvinylidene fluoride or polyimide materials as well as thin films made of the same material are subsequently etched by a wet chemical treatment that reveals pores of which the shape, surface density and dimensions can be controlled by choosing appropriate conditions. High aspect ratio cylindrical or conical traces of diameter ranging from a few nanometers to some... 

Dlubek, G., B5rner, F., Buchhold, R., Sahre, K., Krause-Rehberg, R., and Eichhom, K.-J., Damage-depth profiling of ion-irradiated polyimide films with a variable-energy positron beam, J. Polym. Sci. B, 38, 3062-3069 (2000b). 

Xu and Coleman studied 6FDA (2,2 -bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride)-pMDA (pyromellitic dianhydride) polyimide films irradiated by an ion beam [84]. A beam of 140 keV N ions with a low-current density was used, and three irradiation fluences (2 x 10 cm , 1 x 10 cm , and 5 x 10 cm ) were chosen. It was reported that even a small dose altered the microstructure of the surface layer. The AFM analysis of those films showed that low-fluence irradiation induced microvoids in the surface layer of the polymer, and high-fluence irradiation resulted... 

Chemical modifications of thermoset resins have been documented, using afm as an analytical tool. Ion beam modification of polyimide surfaces were imaged in contact mode, which showed a reduction in surface roughness with increasing irradiation, and generation of a graphitic structure in the degraded polymer (83). 

Xu and Coleman [76] modified the 6FDA-pMDA (polyimide) films by irradiating ion beam and studied the structure and morphology by AFM. The AFM images data indicated that free-standing polyimide films had deep surface valleys which could extend to a depth of several micrometers. 

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