1. Field of the Invention
The present invention relates to a lithographic apparatus and a device manufacturing method.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. The lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs), flat panel displays, and other devices involving fine structures. In a conventional lithographic apparatus, a patterning means, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC (or other device), and this pattern can be imaged onto a target portion (e.g., comprising part of one or several dies) on a substrate (e.g., a silicon wafer or glass plate) that has a layer of radiation-sensitive material (e.g., resist). Instead of a mask, the patterning means may comprise an array of individually controllable elements that generate the circuit pattern.
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning” direction), while synchronously scanning the substrate parallel or anti-parallel to this direction.
The conventional lithographic apparatus delivers a beam to the substrate through a lens assembly in which each of the lenses is arranged in series along the beam projection direction. The lens component closest to the substrate is a single lens through which all of the beam passes.
An alternative design approach uses a series of lenses arranged along the beam path, but the lens component closest to the substrate is in the form of a two dimensional array of small lenses. Each of the small lenses focuses a respective part of the beam onto a respective part of the substrate. Lithography systems using this design are generally referred to as microlens array imaging systems or MLA systems.
In the lithographic apparatus incorporating the MLA systems, it is possible to rely upon an array of individually controllable elements that provide a black or white effect. An individual element either reflects a beam directly towards an individual lens of the MLA (“white”) or directs light away from the lens of the MLA array (“black”). The effect is the same as turning ON or turning OFF a beam component directed towards that single lens, which delivers either a maximum intensity beam or a zero intensity beam.
It is desirable have a gray-tone capability, which is the capability of delivering to the substrate a light intensity intermediate a maximum and a zero intensity. A gray-tone capability is desirable in lithographic apparatus relying upon MLA arrays. It has been proposed to provide gray-tone capability by progressively adjusting the position of individual mirrors in a mirror array to progressively reflect light away from the center of a respective lens in the lens array. A single beam of light from a single mirror of a mirror array is progressively displaced relative to a respective lens of the lens array.
In a lithographic apparatus in which a single mirror is used to reflect light to a single lens of the array and that mirror is progressively displaced to direct the reflected beam progressively away from that single lens, not only the intensity of the beam reaching the substrate is modulated. For example, in MLA systems in which the pupil of the projection optics is imaged on the substrate, the pupil is asymmetrical with respect to the beam of light which illuminates a single lens of the array. Thus, both the intensity and position of the spot of illumination at the substrate are modulated as a result of deflection of the mirror element. In MLA systems in which the displaceable mirror elements are imaged on the substrate, the beam reaching the substrate from an individual mirror will not be telecentric, The non-telecentricity will vary with displacement of the mirror, resulting in the position of the spot of illumination at the substrate varying with focus.
Therefore, what is needed is an improved lithographic apparatus and device manufacturing method that can be used in a microlens array imaging system to provide gray-tone capability.