Photosensitive polyimide patterning

Figure 19. Process steps for patterning polyimides by wet etching and dry etching and for direct photopatterning of a photosensitive polyimide. The abbreviation PR stands for photoresist.

Direct Patterning of Photosensitive Polyimides. Photosensitive polyi-mides (PSPIs) are recently developed materials that can be directly photo-patterned like a negative photoresist (80,85,88,146-148). The most common PSPIs are polyamic acids that have been esterified with photoreactive alcohols and combined with photoinitiators to form a polymer that will crosslink under exposure to UV radiation and become insoluble. The unexposed material is selectively dissolved in a developer solution, and the patterned film is then cured to convert the cross-linked polyamic acid to a polyimide and drive off the cross-linking groups. 

An all-polyimide microchannel chip was created by patterning two photosensitive polyimide layers, and then by laminating the two layers based on a partially imidized interfacing layer [228,229]. 

Scheme I. Comparison of Patterning Process for Conventional and Photosensitive Polyimides.

H. Itatani, S. Matsumoto, T. Itatani, T. Sakamoto, S. Gorwadkar, and M. Ko-muro. Method for forming polyimide pattern using photosensitive polyimide and composition for use therein. US Patent 6777 159, assigned to PI... 

H. Kikkawa, F. Kataoka, I. Takemoto, J. Tanaka, K. Isoda, S. Uchimura, M. Kaji, and M. Sugiura. Photosensitive polyimide precursor and its use for pattern formation. US Patent 6319656, assigned to Hitachi Chemical Company, Ltd. (Tokyo, JP), November 20, 2001. 

We report on the positive alkali-developable photosensitive polyimides based on an alkali-soluble polyimide precursor as a base polymer and diazonaphthoquinone (DNQ) sensitizer to improve process stability and sensitivity. Polyamic acid ester with pendant carboxylic acid (PAE-COOH) showed good dissolution behavior in aqueous alkali developer. The dissolution rate of PAE-COOH was controlled by the content of pendant carboxylic acid. It was found that a photosensitive system composed of butyl ester of PAE-COOH and a DNQ compound can avoid the residue at the edge of hole patterns (footing) after development, while that of methyl ester of PAE-COOH showed the residue. A DNQ compound containing sulfonamide derived from diaminodiphenylether renders improved sensitivity compared with DNQ compounds derived from phenol derivatives. 

Solvent-Developed Photosensitive Materials. Many of the limitations of the bilayer polyamic acid process have been overcome through the use of photosensitive polyimides (9). Formulated photosensitive products have been developed that result in improved process capability resulting in increased pattern resolution, improved soft bake time and temperature process latitude, and improved lithographic process latitude and throughput while maintaining desired film physical properties. Moreover, the film patterning process requires fewer steps... 

Thermal tempering of the photosensitive or cross-linked polymer gives the polyimide siloxane which has been previously shown to be an excellent candidate as an insulating polymer in electronics. The use of such a directly patternable polyimide for dielectric and passivation applications, particularly in microelectronics, should become increasingly important as polyimides become more widely accepted in the industry. 

Preparation of photocross-linkable furan-containing polyimides was also reported [165]. It was also found that the polymer cross-links with the aid of singlet oxygen. Formations of fine pattern images can be formed. This was taken as clear evidence of the successful photolithography in this photo-curable system that uses Ceo as the photosensitizer [165]. 

Polyimide SBCs can be grouped into four classes based on both the chemistry of their precursors and the processing strategy used to pattern their coatings. These classes are polyamic acids, solvent-soluble poljdmides, solvent-developed, photosensitive materials, and aqueous-base developed, photosensitive materials. 

There are several ways (see Fig. 5) in which polyimides have been used in applications where photoimaging is required to produce a pattern on a substrate (typically silicon, or other materials including polyimide itself) (205). The first method is reactive ion etching (RIE) (includes plasma etching) or photoablation. This method does not require a photosensitive material, only a mask that can withstand the reactive ion gas or photons, and the material to be patterned, which is protected by the mask. In RIE, the reactive ions, generated by an RF field, attack the exposed polyimide to etch out the pattern of the mask. This process can be used with conventional polyimide film and requires few steps, but is not highly accurate and requires a hard (metal, silicon, etc) mask. The other methods... 

As with silicon, polyimide has also been used in IC fabrication processes. Polyimide is used as a dielectric and encapsulant because it provides good planarization, electrical insulation, and resistance to solvents. Polyimide thin films are patterned using standard microfabrication processes such as photolithography and reactive ion etching both photosensitive and nonphotosensitive formulations are available. 

Kasahara, A., Takahashi, A., Oyama, T., Photosensitive sulfonated polyimides utilizing alkaline-developable negative-tone reaction development patterning, J. Photopolym. Sci. Technol., 2011, 24, 269-272. 

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