Phenyl-bridged polysilsesquioxane positive and negative resist for electron beam lithography

We present and characterize an organic-inorganic hybrid sol-gel material, phenyl-bridged polysilsesquioxane (ph-PSQ), for use as a new high resolution resist for electron beam lithography (EBL). The resist has a unique characteristic as the only positive tone silica-based resist available for EBL. Exploring the processing parameters has revealed that it is possible to switch the behaviour from negative to positive tone by application of a post-exposure bake (PEB). Based on the results from micro-FTIR spectroscopy, a description of the tone switching mechanisms is proposed. The negative tone behaviour is explained by the etch rate difference between silanol groups and cross-linked silica, present in unexposed and in exposed areas of the films, respectively. In the case of positive tone, after a PEB, the etch rate difference between a thermally densified cross-linked silica network and cage-like silica structures allows us to reveal the pattern. Contrast and sensitivity are estimated under different processing conditions, and the significant parameters for line edge roughness minimization are pointed out. Dense patterns down to 25 nm half-pitch and isolated structures down to 30 nm are demonstrated, exploiting the positive tone, and dense patterns down to 60 nm half-pitch are demonstrated in the negative tone. Etching selectivities in fluorinated gases for ph-PSQ nanostructures on silicon substrates are 1-9 for the positive tone and 1-12 for the negative tone.