LIGA is a well-established process to fabricate metallic micro parts with high resolution, high precision and very low sidewall
roughness by means of X-ray lithography and electroplating. Typical mask substrate materials, e.g. beryllium, carbon based foils,
Si3N4 or SiC show different disadvantages such as low X-ray transparency or high toxicity or high prices or low conductivity or high
thermal expansion or surface porosity causing X-ray scattering. Due to the amorphous structure of vitreous carbon this mask material
proved to significantly reduce the amount of side wall striations, leading to extremely smooth pattern sidewalls. For the fabrication of
X-ray masks, PMMA with its unique features such as high aspect ratio patterns with high precision, exhibits low sensitivity and the
layers preparation is not easy. SU-8, an epoxy-based UV and X-ray sensitive, chemically amplified negative tone photoresist exhibits
high aspect ratio patterns with vertical sidewalls. The difficult remove of the resist after the electroplating process significantly
hinders the inspection of the fabricated X-ray mask. We present the suitability of an UV sensitive, chemically amplified, aqueous-alkaline
developable, and easy removable positive tone photoresist, XP mr-P 15 AV for the fabrication of X-ray masks by means of
UV lithography on vitreous carbon substrates.
The production of X-ray masks is one of the key techniques for X-ray lithography and the LIGA process. Different ways for the fabrication of X-ray masks has been established. Very sophisticated, difficult and expensive procedures are required to produce high precision and high quality X-ray masks. In order to minimize the cost of an X-ray mask, the mask blank must be inexpensive and readily available. The steps involved in the fabrication process must also be minimal. In the past, thin membranes made of titanium, silicon carbide, silicon nitride (2-5μm) or thick beryllium substrates (500μm) have been used as mask blanks. Thin titanium and silicon compounds have very high transparency for X-rays; therefore, these materials are predestined for use as mask membrane material. However, the handling and fabrication of thin membranes is very difficult, thus expensive. Beryllium is highly transparent to X-rays, but the processing and use of beryllium is risky due to potential toxicity. During the past few years graphite based X-ray masks have been in use at various research centers, but the sidewall quality of the generated resist patterns is in the range of 200-300 nm Ra. We used polished graphite to improve the sidewall roughness, but polished graphite causes other problems in the fabrication of X-ray masks. This paper describes the advantages associated with the use of polished graphite as mask blank as well as the fabrication process for this low cost X-ray mask. Alternative membrane materials will also be discussed.