Novel high refractive index positive-tone photosensitive polyimide for microlens of image sensors

A novel high refractive index positive-tone photosensitive polyimide (posi-PSPI) has been successfully developed. The posi-PSPI has a refractive index of 1.78 at 633 nm and good transparency (> 80% at 400 nm). It is prepared from an alkali—soluble transparent polyimide precursor, a diazo-naphthoquinone-type photoactive compound, a surface-modified high refractive index nanosol, and a UV absorbent. The posi-PSPI has a round shape because of the UV absorbent. Further, it displays good sensitivity as well as a good round pattern profile after thermal curing. In addition, the 1% weight loss temperature of the posi-PSPI is 407 °C, which indicates that the posi-PSPI has excellent thermal stability.