Correction of blur variation in a multi-beam writer

A method for computing an exposure pattern for exposing a desired pattern on a target in a charged-particle multi-beam processing apparatus, in which a particle beam is directed to and illuminates a pattern definition device comprising an aperture array composed of a plurality of blanking apertures through which said particle beam penetrates for writing said desired pattern by exposing a multitude of pixels within an exposure area on the target by means of said particle beam impinging on the target according to a nominal beam direction, the target being oriented along a nominal target plane substantially perpendicular to said nominal beam direction,

• wherein a writing process for writing said desired pattern comprises imaging of the blanking apertures onto the target in each of a sequence of exposure intervals, thus generating a corresponding plurality of aperture images, wherein said imaging of blanking apertures onto the target involves a blur,
• the method taking into account an elevation of the target within the exposure area relative to the nominal target plane, as well as a dependence of the blur upon such elevation of the target according to an elevation dependence function,
• wherein the desired pattern is provided as a graphical representation composed of a multitude of image elements in a correspondence to said multitude of pixels in the exposure area on the target,
• wherein in the pattern definition device said plurality of blanking apertures is arranged in a predetermined arrangement defining mutual positions of the blanking apertures, each blanking aperture being selectively adjustable with regard to a dose value to be exposed through the respective blanking aperture onto a corresponding aperture image generated on the target during a respective exposure interval,
• the method comprising:
• (i) measuring an elevation of the target at a reference point within the exposure area, said elevation representing a local shift of the target from the nominal target plane at the location of said reference point, and also determining a value of pattern density around said reference point,
• (ii) determining, based on said elevation dependence function, a local blur value, said local blur value representing the actual value of blur corresponding to the elevation of the target determined in the previous step, plus an additional blur calculated as a function of the pattern density around the respective reference point,
• (iii) determining, based on said local blur value and a given target blur level, an additional correction blur value and calculating a convolution kernel using said correction blur value, said kernel describing a mapping from an image element of the graphical representation to a group of pixels, said group of pixels being centered around a nominal position of said image element, wherein said kernel represents a point spreading function realizing said correction blur value, and
• (iv) calculating a nominal exposure pattern as a pixel raster graphics defined on the multitude of pixels, by convolution of the graphical representation with the convolution kernel in at least a region of the exposure area which includes said reference point, said nominal exposure pattern being suitable to create a nominal dose distribution on the target;
• wherein the convolution calculated in step (iv) corresponds to introducing said correction blur into the nominal exposure pattern, said correction blur increasing the blur of the exposure pattern from the local blur value to said target blur value.