An immersion lithography apparatus is provided that includes an energy source, a projection optical system, a stage, a showerhead including an immersion liquid supply device and an immersion liquid discharge device that produces a flow of liquid within an exposure zone, and a cleaning device that cleans a portion of the projection optical system having been contacted with the immersion liquid by means of a cleaning gas. In an embodiment, the cleaning device includes an ozone generation unit produces a flow of ozone into the exposure zone. In embodiments, the apparatus includes a stage that includes a dose sensor and/or an ultra-violet light source. A method for insitu cleaning of a final lens element within an immersion lithography system having an immersion fluid showerhead that provides immersion fluid to an exposure zone of the immersion lithography system is also provided.