Search

Layers from fluorinated trichlorosilanes are in widespread use for the prevention of sticking in nanoimprint. It is generally assumed that these layers are monolayers, bonded to the substrate, and internally cross-linked. The authors have investigated a gas phase deposition process for such layers in detail, varying the amount of antisticking agent and the deposition time in a vacuum process. The quality of the layers was determined from the layer thickness and the contact angle was measured. In accordance with some recent literature, claiming that formation of dense cross-linked monolayers from fluorocarbon-trichlorosilanes is not possible, the results indicate that the deposited layers are not monolayers. Instead the authors suggest that they are even thinner disordered but cross-linked layers. Obviously monolayer formation is not required for sticking prevention.

In a thermal imprint process, contact angles may evolve in two situations, between the stamp and the polymer, when a cavity remains unfilled, or, in cases where the polymer locally dewets the substrate, between the substrate and the polymer. For two polymers of different polarity, such contact angles are determined experimentally and compared to values calculated from surface energy data. In doing so, the specific temperature dependence of surface energies of a thermoplastic polymer as well as the typical course of an imprint process, where cooling times often exceed the imprint times, are considered. The differences between experimental and theoretical values are critically discussed. The results indicate that the respective adhesive layers govern the surface status of the substrate and the stamp as well.

In order to investigate the nonuniformity occurring below wider patterns during thermal nanoimprint lithography, the pressure situation is analyzed in detail. A balance of vertical forces shows that the gas pressure within the cavities is negligible, whereas the effective pressure acting on the stamp structures in contact to the polymer is increased compared to the externally applied pressure. This effective pressure has a high hydrostatic component, which may result in local stamp deformation. Imprint with a low molecular weight polymer provides evidence that the stamp structures become compressed under high pressure, resulting in a nonuniform residual layer beneath the imprinted patterns. In contrast, bending deformation of single stamp structures is found to be negligible. Pressure reduction is effective to reduce stamp compression, improving the uniformity of the residual layer. With typical imprint polymers of medium molecular weight, however, pressure reduction reduces the overall imprint depth.

Imprints into thin thermoplastic layers in thermal imprint often go along with phenomena such as physical self-assembly and recovery, which impair the resulting replication quality. To avoid recovery, in particular within larger structures, a polymer with extremely low Mw (polystyrene with a molecular weight of 1kg∕mol) is investigated. In contrast to expectations, the polymer develops a strong recovery. This occurs at such low viscosities that the imprint is also impaired by strong physical self-assembly effects. The imprint is further hampered by the fact that, due to the very low glass-transition temperature, a layer preparation without prebake is required. In addition the suitable temperature window for imprint is extremely small (T≈40°C) so that process control is hardly feasible. Polystyrene with such low Mw is not suitable for thermal imprint.

To improve control during the filling of the channels of a microfluidic device with the fluid under investigation, a new concept is presented: channels with a diamond-shaped cross section. These channels easily allow one to distinguish an unfilled channel from a filled channel by simple optical microscopy. The idea is based on the fact that incident light is totally reflected with unfilled channels, whereas it is mostly transmitted with filled channels. Preparation of such channels in thin SU-8 layers on a glass was performed via a double replication of a Si template with undercut trenches. Functionality tests with filling materials spanning the range of refractive indices of typical organic media provided an excellent contrast between filled and unfilled channels, thus demonstrating the capacity of this concept.

Three dimensional linear structures with definite undercuts of different sizes were replicated in the first step in an elastomeric material and, in the second replication step, in a negative tone resist. Undercut masters for the first step were prepared from Si by laser ablation and anisotropic wet etching. For the second step the elastomeric replica, featuring the inverse undercut, served as a master. The first replication was performed by molding or imprinting, where the prepolymer of the elastomer was cross-linked within the master in an oven or in a thermal press, respectively. The second replication of the undercut elastomeric structures into the negative tone resist was performed successfully by molding. Comparison of the Si masters with the second replication revealed shrinkage-induced differences. Undercuts of up to 60% of the overall pattern width were successfully replicated.

To address the importance of the parameter glass transition temperature Tg for the choice of an adequate process temperature in a thermal nanoimprint process, three polymers with different values of Tg were investigated with respect to their imprint behaviors (poly)vinyl-chloride, (poly)styrene, and (poly)methyl-methacrylate. In order to evaluate the imprint results obtained at different temperatures, the volume of the polymer squeezed into the cavities was observed, and the temperature margins for the appearance of typical pattern size dependent failure types were derived. It turned out that despite the comparable molecular weight of these materials, the imprint temperature has to be chosen at different levels above Tg. Thus, Tg is not sufficient to characterize imprint polymers, and, in addition to the molecular weight, the viscosity at a definite temperature should be known. Tg alone can only indicate a lower limit for the process temperature.

Thermal imprint into polymer layers that are thin compared to the pattern height allows for imprints with extremely low and uniform residual layers where lift-off works without previous dry etching. The authors report about possible restrictions of this method such as unintended self-assembling and recovery of polymer underneath the imprinted structures. Both can be attributed to structure size properties and temperature influence and hinder a subsequent successful lift-off. Taking the example of two different temperatures and two different initial layer thicknesses the authors investigate their impact on the resulting structure shape and potential defects. In addition, to qualitatively estimate the residual layer thickness the authors apply lift-off and discuss the results with focus on the usability of this approach as a lithography technique.