Photosensitive polyimides (PSPIs) are attracting much attention as an insulating materials for microelectronic applications. PSPIs can be directly patterned, simplify processing steps and do not need a photoresist to be used in the micro-lithographic stage, nor a toxic etchant. This review introduces the recent development of PSPIs with emphasis placed on the chemistry responsible for images.

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SU-8 Resists are used widely in the development and fabrication of MEMS devices [1-4]. The resist's images are the product of photochemical and thermal cationic processes and result in vertical sidewalls and high aspect ratio features. These are among SU-8's most desirable attributes. The process, which creates the desirable images, however, generates considerable internal stress, which produces cracked features. It would be beneficial, therefore, to investigate modifications of the SU-8 resist formulation which incorporate materials that will attenuate stress cracking without significant image degradation.
Work reported previously by IBM focused on compositions containing SU-8 targeted for applications in which lithographic performance could be sacrificed for high flexibility [5]. In this paper we present our results of studies directed toward the selection of materials compatible with SU-8, which will significantly reduce cracking and also retain high aspect ratios images and vertical sidewalls. To this end, we examined mono and polyfunctional epoxides from the following chemical classes; siloxanes, urethanes, cycloaliphatics, a bicycloaliphatic copolymer with phenol, bisphenol-A derivatives, a long chain aliphatic polyol and some long chain aliphatic esters.
This work allowed us to identify compositions having improved crack resistance and adhesion over standard SU-8 resist, without sacrificing lithographic performance, which may provide benefits for applications such as a dielectric barrier.

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Bicyclo[2.2.1]heptane-2-exo, 3-exo, 5-exo-tricarboxyl-5-endo-acetic dianhydride (4a, BSDA) and its 5-endo-carboxyl-5-exo-acetic isomer (4b) with an alicyclic and spiro structure were prepared from a Diels-Alder adduct of itaconic anhydride with cyclopentadiene via three steps. Fully alicyclic polyimides were synthesized by polycondensations of an alicyclic diamine, 2, 5(6)-bis(aminomethyl)bicyclo[2.2.1]heptane (BBH) with four kinds of cycloaliphatic dianhydrides including BSDA in HMPA. Although inherent viscosities of the poly(amic acid) were somewhat low, the polymer solutions gave the free-standing polyimide films after curing. The films were soluble in organic polar solvents such as DMAc and NMP.

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The low absorptivity of triarylsulfonium salts above 320nm limits their utilization of longer wavelength emission from light sources. Herein we report the synthesis and photoactivity of anthracene-bound sulfonium salts, in which the sulfonium salt is insulated from the anthracene chromophore by a saturated chain. The salts efficiently absorb radiation between 330-410nm owing to the strong anthracene absorptivity in this spectral region. The efficiency of utilization of the absorbed radiation for generating cationic initiators is strongly dependent on the substituents attached to the sulfonium ion. Evidence is presented in support of generation of initiators for cationic polymerization by photoinduced electron transfer from the anthracene chromophore to the sulfonium group, wherein the efficiency of electron transfer is dependent on the sulfonium ion substituents. A new synthesis of sulfonium salts is also provided, which occurs by direct alkylation of diaryl, dialkyl and arylalkyl sulfides by oxiranes in the presence of strong acids having non-nucleophilic anions.

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A new aqueous developable positive tone photodefinable polyimide was prepared from poly(hydroxy amic acid) and naphthoquinonediazide for semiconductor stress buffer coat, which exhibits excellent photolithographic properties in high resolution, wide focus latitude and good stability for process holding time, with equivalent film properties to conventional non-photodefinable polyimide. Through dry etching evaluation of the SiN passivation layer, it was confirmed that the cured polyimide pattern works as an etching mask.

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The fully alicyclic polyimides were prepared by a 2-step polycondensation of 2, 5(6)-bis(aminomethyl)bicyclo[2.2.1]heptane with cycloaliphatic dianhydrides such as bicyclo[2.2.1]heptane-2, 3, 5, 6-tetracarboxylic dianhydride. These polyimides possessed a 5% weight-loss temperature range of 422-459°C in N₂, and 411- 416°C in air. The polyimides had the Tg's over 200°C, especially those of polyalicyclic polyimides were over 250°C. The free-standing polyimide films exhibited cutoffs at a wavelength shorter than 235nm, and are colorless. The averaged refractive indices were 1.52-1.54 and the optical anisotropies, birefringencies, were nearly zero. The dielectric constants that were evaluated from the refractive index were 2.55-2.73.

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By applying a certain load directly with a micro cantilever tip, a elastic deformation of resist micro pattern adhering on a substrate can be characterized quantitatively. By combining with finite element analysis of resist pattern deformation, Young's modulus of micro resist pattern of 190nm width and 770nm height formed by KrF excimer laser lithography can be obtained. Young's modulus of resist pattern is estimated in the range of 4 to 5 GPa, which is similar to those of typical polymer materials.

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Enol ether derivative of poly(6-vinyluracil) was synthesized. The polymer released completely the tent-butyl groups above 190°C and gave poly(6-vinyluracil) which was stable up to 350°C. The resist film as spin coated from chloroform solution containing photo-acid generator released completely the tert-butyl groups after UV light irradiation followed by post exposure baking. Solubility of the obtained polymer containing uracil unit was significantly different from the original polymer containing alkoxy units in both polar (TMAH aq) and nonpolar (anisole) solvents as developer. The resist film was found to have high sensitivity and high contrast as both positive and negative type deep-UV resist materials.

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146-nm Light induced photolysis of poly(vinylphenol) (PVP) thin film (50-60nm) and vapor phase silylation of the irradiated PVP film were studied. On irradiation at 146nm under nitrogen atmosphere, insolubilization of PVP film was observed and the efficiency increased with molecular weight of PVP. A crosslinking mechanism was proposed based on the FT-IR spectroscopy which showed the decrease of the peaks due to O-H (3360cm-1), C=C (1515cm-1) and C-O (1242cm-1) bonds on irradiation. Vapor phase silylation of PVP thin film was carried out using dimethylsilyldimethylamine (DMSDMA) as a silylatiog reagent. The silylation of PVP was inhibited on irradiation at 146nm. The inhibition was enhanced when molecular weight of PVP increased.

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