Surface Treatments of Mold and Substrate

The key for a successful transfer reUes on the surface energy differences between the polymer, the mold and the substrate. The mold should have a lower surface energy than the substrate, so that the polymer film will adhere more strongly to the latter and could therefore be easily detached from the mold [8, 9]. This gets more critical as molds with small feature sizes are used. The larger contact area causes larger adhesive force with the polymer and difficult pattern transfer. 

The situation is more complex in DM I, because two molds are used, which should have different surface energies. The difference must be sufficient to allow one mold to separate cleanly from the 3D structures still attached to the second mold. Additionally, in the case of supported structures, the surface energy of the second mold must also allow an easy transfer to the final substrate [27]. For molds with similar pattern profiles, a strategy of employing different silane treatments for each 

A specific situation occurs in DTM, where substrates are required to have surface oxides, in order to form strong adhesive bonds to a UVO-treated PDMS. When that is not the case, surface modifications such as functionalization with self-assembled monolayers or deposition of thin (5-20 nm) adhesion layers of silica allows a wider range of materials to be employed as substrates for DTM [31]. 

One drawback of several TP processes is the presence of a residual layer after imprinting, due to the polymer being present not only in the trenches, but also on the protrusions of the mold. Patterns transferred to the substrates will, in such cases, be connected by a thin film of the material, as it is not fully removed from the raised features of the mold. Typically, these layers are 100 nm thick in pTM [2, 34], while in some variants of RNi much thinner layers (on the order of 4-5 nm) have 

In order to work efficiently, the mechanical properties of the layer should be tailored. A thin residual layer reduces the shear stress needed to induce failure, while the use of a brittle polymer allows for a cleaner failure of the layer and, consequently, smoother surface and sidewalls of the final transferred structures than if a ductile polymer is used. The quality of the process depends as well on mold design molds with sharp edges induce failure of the residual layer in a specific location, while molds where protrusions edges are not sharp lead to nonspecific layer failure and, consequently, patterned features with rough edges [31]. 

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