The application of nickel or nickel-containing alloys as stamp materials employed in nanoimprint lithography (NIL) can be regarded as an industrially highly relevant solution. In opposite to silicon-based stamp materials the highly ionic character of the oxidized nickel surface favors a strong ionic linkage between fluorinated alkyl phosphoric acid derivatives and the nickel stamp surface. This fact can be utilized to produce wear-resistant self-assembling monolayer films acting as anti-adhesion coatings [1].
However, the oxidized/ hydroxidized Ni surface is a highly complex system with respect to catalytic activities. Especially the heating-cooling cycles of an industrial NIL process in air humidity will continuously reduce the quality of the anti-adhesion films. Heating will initiate the desorption of oxygen and hydrogene containing species from the surface, having the capacity to break bonds within the phosphoric acid based anti-adhesion film. Cooling in air humidity will lead to a growth of a new oxide/ hydroxide layer due to the chemisorption of water and oxygen from air [2].
X-ray photoelectron spectroscopy (XPS) as well as imprint tests have been performed in order to investigate both the heat stability of nickel stamp surfaces and the durability and reliability of three different phosphoric acid derivatives immobilized as anti-adhesion coatings onto these surfaces.
The XPS investigation demonstrates that heat treatment of a Ni stamp provokes a transition from a Ni2O3 and Ni(OH)2 dominated surface into a NiO dominated surface, which creates damages of the P-O-C linkages within the phosphoric acid molecules. On the other hand the ionic Ni-O-P linkages have been observed to be quite stable. Both imprint tests and XPS investigation underline that anti-adhesion films comprising fluorinated phosphonate derivatives demonstrate better wear-resistances than phosphate based films, composed of molecules having an O bridge between the phosphorous atom and the alkyl chain.
[1] M. Keil, M. Beck, G. Frennesson, E. Theander, E. Bölmsjö, L. Montelius, J. Vac. Sci. Technol. B 22 (2004) 3283.
[2] J.C. de Jesús, J. Carrazza, P. Pereira, F. Zaera, Surf. Sci. 397 (1998) 34.
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