Development of Electroless- and Electroplating Processes for Lithographic Production of 3D Microstructures
The paper discusses lithographic and plating processes, which are the major components of LIGA techniques. Fabrication of X-ray masks (XRM) and copying of diffractive optical lens are used as examples. The fabrication of XRM is an essential initial stage of LIGA-technology for production of any microstructure with deep channels (up to 200 µm) and a high aspect ratio (up to 100:1). An XRM is an X-ray opaque pattern of the future microstructure supported by an X-ray transparent wafer, for example, a glassy carbon one (Fig. 1). Contrast ratios of the XRM to different absorbents were calculated for the radiation source (Xray synchrotron radiation, a VEPP-3 accelerator, Budker Institute of Nuclear Physics SB RAS, Novosibirsk), and Re and Au were demonstrated to be the best absorbents (Fig. 2). The paper reports the results of the electrodeposition of a Re-Ni alloy (90-95 wt % Re) from sulfate and acetate electrolytes onto XRM blanks. In the first case, if current density is 5-10 А/dm2, the average deposition rate is about 8 µm/h, current efficiency 20-30%, and the final coating thickness 20 µm, the coatings are gray, undulating, very highstressed, and with a continuous network of large microcracks (Fig. 3a). Using an acetate electrolyte allowed to stabilize solution pH during the electrodeposition and to obtain light gray, lower stressed Re-Ni alloy coatings. Nevertheless, microcracks cannot be completely eliminated from a 20-µm-thick cathode deposit even in this case (Fig. 3b). Gold is a more promising absorbent for XRM. It was deposited onto XRM blanks from sulfite-thiosulfate and citrate-cyanide electrolytes. As is not the case with Re, both electrolytes yield continuous and dense Au deposits, which ensure the required high contrast (100-150) if they are 20-30 µm thick. A citrate electrolyte produces higher quality and more finecrystalline Au coatings (Fig. 4). The central problem in copying the flat diffractive element of an optical lens (Fig. 5) was high internal stress inside the electrodeposits (especially Ni ones). It results in local separation of the conducting strike layer (chemical silvering) and distortion of the copy microtopography. A flat copy can be obtained with the required accuracy (Fig. 6) in the following way. First, a thick (40-50 µm), low-stressed copper layer is deposited onto a silver under layer. After that, the flat copper copy is separated from the original (together with the under layer), and a thin (1-2 µm) functional Ni layer is deposited onto its face from a sulfamate electrolyte. The study was conducted under integration project No. 55 of the Siberian Branch of the Russian Academy of Sciences "X-ray LIGA Techniques for Synthesis of 3D Diffraction Structures."