Investigation of the adhesive strength of PMMA structures on substrates obtained by deep X-ray lithography

The adhesive strength of PMMA structures obtained by deep X-ray lithography with synchrotron radiation is a decisive factor for effective structure replication in subsequent electroforming processes. The adhesion depends on various factors, in particular on the interaction of radiation and secondary electrons at the boundary resist/substrate. We have determined the adhesion strength of PMMA microcolumns as a function of the dose deposited below the mask absorber by breaking tension measurements applying a new experimental method. Care has been taken to separate degradation effects in the resist due to the primary photon beam from the loss of adhesion at the boundary substrate/resist caused by secondary electrons. This has been studied quantitatively by utilizing different substrate materials like glassy carbon, wet oxidized titanium, copper and gold. Monte Carlo simulations have been carried out to simulate the deposited dose at the surface layer of different substrates. The corresponding results are supplemented by deep lithography experiments, which may be used as a means to directly conclude the deposited resist dose from secondary electrons emitted from the substrate surface. All the irradiation experiments have been performed with specially designed masks for deep X-ray lithography at the storage ring DCI (Orsay/France).