Pattern transfer accuracy in deep x-ray lithography - Calculation and experimental results

Deep X-ray lithography with synchrotron radiation (DXRL) represents the technological core of the LIGA technique, which is a modern microfabrication technology facilitating the high volume production of micro products from a huge variety of materials. Since several applications make use of the high structure accuracy obtained in the primary lithography process, the demands of a detailed investigation of structure accuracy limiting aspects came to rise. Therefore, thorough theoretical and experimental research has been undertaken in order to understand the different radiation effects influencing the shape of the side walls and the lateral resolution to be obtained. Physical effects like diffraction, divergence of the synchrotron radiation beam, photo and Auger electrons, fluorescence and scattering have been calculated and are condensed in a computer code. The calculation results are discussed in detail with respect to LIGA mask production by x-ray lithography as well as for deep x-ray lithography applications. The model can be partially extended to new irradiation techniques like tilted and rotated exposures. Different absorber gradients due to various tilt angles have to be taken into account and the resulting dose contour lines resulting from inclined irradiations are compared with experimental data. In order to enhance the normal shadow printing process and to realize shaping in the third dimension, previous studies used the aligned multiple exposure technique realizing step like structures. We will discuss a novel approach using 500 mu m thick Beryllium mask blanks with free standing absorber structures (gold) on open windows for alignment purposes. First results show an overlay accuracy of about 0.4 mu m using an internal alignment system installed in a DEX 2 JENOPTIK exposure apparatus.