This paper presents with experimental evidence a novel technology for mass production of three-dimensional hollow or suspended microstructures. The technology does not require any steps involving micro injection molding or microstereolithography. Suspended or hollow microstructures are photolithographically formed at submillimeter resolution by shining a ultraviolet light (UV light) onto a photosensitive resin monomer that has been premixed with an UV-opaque impurity (referred hereafter as “resin-impurity composite”). Generally, the process flow comprises three stages. First, the resin-impurity composite is prepared. Second, an embossing stamp is made. Finally, microstructures are three-dimensionally casted using the embossing stamp from the second stage and, at the same time, hollow or suspended parts are photolithographically patterned and formed by UV exposure. A humidity proof package for RF-MEMS switches has been fabricated and is chosen as an example to demonstrate the feasibility of this technology.