Patent ID: 8264668

Claim:
An illumination system, comprising: a depolariser; a light mixing system disposed downstream of the depolarizer in a light propagation direction; and a microlens array arranged upstream of the light mixing system in the light propagation direction, the microlens array comprising a plurality of microlenses arranged with a periodicity; wherein during use: the depolarizer in conjunction with the light mixing system at least partially causes effective depolarisation of polarised light impinging on the depolariser; the depolariser produces a succession of mutually orthogonal polarisation states during use; the succession of mutually orthogonal polarisation states is periodic over its cross-section and can be described by a depolarisation period; the depolarisation period is adapted to the periodicity of the microlens array so that at least two light beams which pass through mutually adjacent microlenses within the microlens array at positions whose spacing from each other corresponds to the periodicity of the microlens array bear mutually orthogonal polarisation states; a contribution due to interaction of the depolariser with the periodicity of the microlens array to a residual polarisation distribution occurring in a pupil plane, which is arranged in the illumination system downstream of the microlens array in the light propagation direction, has a maximum degree of polarisation of not more than 5%; and the illumination system is configured to be used in a microlithographic projection exposure apparatus.