Film densification

The soft bake process can affect the solubility properties of some resists in the developing solvent. For instance, in DNQ/novolac resists, the solubility of the exposed resist as a function of prebake temperatures shows a maximum at around 120°C (Fig. 11.16). Four zones can be distinguished in this plot (i) a no-bake zone where residual solvent and dissolution rates are high, (ii) a low-temperature zone (up to 80°C) where the dissolution rate shows an appreciable decrease due to solvent removal, (iii) a mid-temperature zone (80-110°C) where the DNQ is thermally and preferentially converted to indene carboxylic acid, leading to an increase in dissolution rate, and (iv) a high-temperature zone (>120°C) where the film densification takes place, DNQ is thermally decomposed, the film is depleted of water, and the novolac resin is cross-linked, resulting in dramatic dissolution inhibition. ... 

Sheetz [10c] developed a model which tried to rationalize the observation that most latex dispersions diy from the peripheiy inward. He construded experiments using dispersion-saturated blotting paper whidi avoided die problem. In this system, he noted that latex dispersions containing vinylidene chloride as a comonomer dried more slowly, at later stages, than poly[(ethyl aciylate)-co-(methyl methacrlyate) (P(EA-co-MMA)) latex dispersions, and noted that the latter formed films more permeable to water vapour. He therefore proposed that protrusion of the meniscus below the tops of the latex at close packing was accompanied by skin formation that closed the pores to further evaporation of water. Water vapour then had to diffuse through the continuous polymer film, and the resulting (osmotic) pressure led to particle deformation and film densification. 

Imai H., Hirashima H. Novel modification method of sol-gel thin films densification of silica gel films with electronic excitation. J. Ceram. Soc. Jpn. 1994a 102 1094-1096 Imai H., Hirashima H., Awazu K., Onuki H. Structural change in sol-gel-derived Si02 films using ultraviolet irradiation. Proc. Soc. Photo-Optical Instrumentation Eng. 1994b 2288 71-76 Imai H., Yasumori M., Hirashima H., Awazu K., Onuki H. Significant densification of sol-gel derived amorphous siliea films by vacuum ultraviolet irradiation. J. Appl. Phys. 1996 79 8304-8309... 

Because of the role of precursor structure on film processing behavior (consolidation, densification, crystallization behavior), the reaction pathways are typically biased through the use of the catalyst, which is simply an acid or a base. This steers the reaction toward an electrophilic or nucleophilic attack of the M—OR bond.1,63 Hydrolysis sensitivity of singly or multiply hydrolyzed silicon alkoxides is also influenced by the catalyst, which contributes to the observed variations in oligomer length and structure. Figure 2.3b illustrates... 

STRUCTURAL EVOLUTION FILM FORMATION, DENSIFICATION, AND CRYSTALLIZATION... 

Precursor Structure Effects. The precursor structure can impact a broad range of properties, including crystallization temperature, the formation of intermediate phases during thermal treatment and film density, among other properties. Table 2.4 reports some of the key precursor properties that may affect densification and crystallization behavior, as well as the final film microstructure. 

Researchers who have focused more on understanding cause-effect relationships in solution processing have given attention to film drying and pyrolysis behavior, densification processes, and nucleation and growth into the desired crystalline state. Both thermodynamic and kinetic factors associated with the phase transformation from the amorphous state to the crystalline state have been considered.11 119 Control of these factors can lead to improvements in the ability to influence the microstructure. It is noted that in the previous sentence, influence has been carefully chosen, since the ability to manipulate the factors that govern the nature of the phase transformation to the extent that full control of the microstructure is possible remains to be demonstrated. However, trends in characteristics such as film orientation and columnar versus uniaxial grains have certainly already been achieved.120... 

The experimental results described above show that the gas-permeability properties of thin glassy polymer films (submicrometer in thickness) are more time- or history-dependent than much thicker films (the bulk state for example, 50 pm or thicker) seem to be. This is manifested in terms of physical aging over a period of 1 year and more. The observed permeability values for the current thin films are all initially greater than the reported bulk values but approach or become less than these values after a few days or weeks, depending on the thickness. After a year, the thin films may be as much as four times less permeable than the reported bulk values. Selectivity increases with aging time, as might be expected from a densification process. 

It is known that the maximum thickness of sol-gel films is limited by the generation of tensile stress in the films during the densification... 

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