FPA 3000 EX 4

A stepper is a projection exposure tool that prints the circuit pattern of a reticle onto a semiconductor wafer. Steppers of this class are used for the pattern-transfer steps that create the active layers and interconnect wiring of integrated circuits during wafer fabrication.

A photolithography stepper images the pattern of a reticle, also called a photomask, through a projection lens onto a wafer coated with photosensitive resist. The lens and optics form an image of one exposure field at a time rather than exposing the whole wafer in a single flash.

The wafer sits on a precision stage that moves in a step-and-repeat sequence between exposures. After each field is printed, the stage indexes the wafer to the next grid position so that identical die patterns are built up across the wafer surface. The step-and-repeat motion gives the tool class its name.

Photolithography steppers are positioned in the lithography bay of a semiconductor front-end wafer fab. The photoresist-coated wafer is loaded into the stepper, exposed with the reticle pattern, and then moved on to downstream processing.

In the overall process flow, the exposure step sits between resist coating and the pattern-transfer steps of development and etch. A typical integrated circuit requires many lithography layers, and each layer repeats a sequence of coating, exposure, development, and etching. Steppers of this class are used for the definition of device features and for interconnect layers.

KrF steppers are applied to the fabrication of integrated circuits, including logic, memory, and mixed-signal devices. The tools are also used for specialty semiconductor products and for research and development lines that require projection lithography.

In mature production environments, steppers of this class continue to serve products whose design rules match the tool resolution. Additional uses include pilot-line process development and backup capacity for newer equipment in the same fabrication plant.