EUV RADIATION BEAM POWER REDUCTION

摘要

According to a first aspect of the present invention, there is provided a method of reducing EUV radiation power incident upon a paltcrning device of a lithographic apparatus, the lithographic apparatus comprising a first array of mirrors and a second array of mirrors, the first array of mirrors being configured to receive EUV radiation and to reflect the EUV radiation as sub-beams of radiation towards the second array of mirrors, wherein the method comprises rotating mirrors of the first array such that at least some of the sub-beams of radiation are incident on mirrors of the second array at positions which provide reduced transmission of the sub-beams of radiation to the patterning device. Advantageously, the method reduces the power incident upon the patterning device without modifying the power output from an EUV radiation source (it may be difficult to control the power output from an EUV radiation source). The sub-beams of radiation may be moved from starting positions on the mirrors of the second array which provide maximum transmission of the sub-beams of radiation. The sub-beams of radiation may be moved from starting positions on the mirrors of the second array which arc at centers of the mirrors of the second array. The reduced transmission may be provided without parts of the sub-beams of radiation falling outside of the mirrors of the second array. The reduced transmission may be provided with parts of the sub-beams of radiation falling outside of the mirrors of the second array. Some sub-beams of radiation may be moved in a first direction across the mirror; of the second array and other sub-beams of radiation may be moved in a second direction across the mirrors of the second array. The second direction may be opposite to the first direction. The rotations of the mirrors of the first array may be performed based upon a predetermined transmission calibration. All of the mirrors of the first array may be rotated. According to a first aspect of the present invention, there is provided a lithographic apparatus comprising an illumination system which comprises a first array of mirrors and a second array of mirrors, the first array of mirrors being configured to receive EUV radiation and to reflect the EUV radiation as sub-beams of radiation towards the second array of mirrors, wherein the lithographic apparatus further method comprises a controller configured to rotate mirrors of the first array such that at least some of the sub-beams of radiation are incident on mirrors of the second array at positions which provide reduced transmission by the illumination system of the sub-beams of radiation. Advantageously, the lithographic apparatus reduces the power transmitted by the illumination system without modifying the power output from an EUV radiation source (it may be difficult, to control the power output from an EUV radiation source).The controller may be configured to rotate mirrors of the first array such that the sub-beams of radiation move from starting positions on the mirrors of the second array which provide maximum transmission of the sub-beams of radiation. The controller may be configured to rotate mirrors of the first array such that the reduced transmission is provided with parts of the sub-beams of radiation falling outside of the mirrors of the second array. The controller may be configured to rotate mirrors of the first array such that some sub-beams of radiation arc moved in a first direction across the mirrors of the second array and other sub-beams of radiation are moved in second direction across the mirrors of the second array. The second direction may be opposite to the first direction. The controller may be configured to rotate all of the mirrors of the first array. The lithographic apparatus may further comprise a memory which stores predetermined transmission calibration data as a function of rotation of the mirrors of the first array. According to a third aspect of the invention there is pr