the setup of pfeiffer and weitkamp the structure height of phase grating g 1 which is needed to obtain the required phase shift is proportional to the photon energy used for 17 5 kev a height of 22 u03bcm is an optimum in the setup proposed by these two authors the period p 1 of phase grating g 1 is close to 4 u03bcm resulting in very high aspect ratios of the structures fig1 d shows a cross section of such a grating the structures are made by wet chemical etching in potassium hydroxide solution as substrates 250 u03bcm thick silicon wafers with 110 orientation are used the grating patterns are exposed using a high precision electron beam lithography process the orientation of the lines is along the 112 direction with a precision of better than 0 1 u00b0 which results in an anisotropic etching with vertical side walls as described in u201c wet etched diffractive lenses for hard x rays u201d j synchrotron radiat 8 2001 pp 1054 1055 by c david e ziegler and b nohammer this process is also used for the fabrication of linear fresnel zone plates the fabrication of absorber grating g 2 is even more challenging firstly period p 2 of the absorber grating has to be.