A principle of deposition of ultra low and uniform stress absorber for X-ray mask

We investigate the deposition of ultra low and uniform stress absorbers for highly accurate X-ray masks. We calculate the maximum overlay error between two masks having different absorber coverages as parameters of absorber stress peak-to-valley (pv) value and distribution shape in order to establish the criteria of absorber stress; which suggested that less than 10 and 6MPa uniformity was required for 30 and 50mm windows respectively to achieve the mask overlay of less than 5 nm. Then we perform several experiments to optimize the sputtering conditions from the viewpoint of the geometric relation between a sputtering target and mask substrates. We observe that the target-substrate (T-S) distance in the axial direction changes only the average stress, but does not affect the uniformity, while the T-S distance in the parallel direction affects both the average and distribution. A very uniform stress absorber of +/-3 MPa for a 25 mm square area is deposited 60 mm away from the target center. From further experiments, we establish a simple but important principle that the linear stress distribution on the stage deposited without rotation cancels out by rotating the substrate, and as a result the absorber stress becomes sufficiently uniform.