An ionization chamber chip, a nano-aperture ion source, a proton beam writing system, and a method of fabricating an ionization chamber chip. The method comprises the step of providing a first substrate comprising a first depression formed in a back surface thereof; providing a backing element attached at the back surface of the first substrate such that a chamber is formed comprising at least the first depression; forming a gas inlet in the first substrate in fluid communication with the chamber; and forming a first aperture structure in the first substrate in fluid communication with the chamber.