Andre Mayer

Photonic Nanostructures Patterned by Thermal Nanoimprint Directly into Organo-Metal Halide Perovskites

Photonic nanostructures are created in organo-metal halide perovskites by thermal nanoimprint lithography at a temperature of 100 °C. The imprinted layers are significantly smoothened compared to the initially rough, polycrystalline layers and the impact of surface defects is substantially mitigated upon imprint. As a case study, 2D photonic crystals are shown to afford lasing with ultralow lasing thresholds at room temperature.

Study of defect mechanisms in partly filled stamp cavities for thermal nanoimprint control

Self-assembly defects limit the suitability of partial cavity filling in thermal nanoimprint for residual layer minimization. As they represent thermodynamically stable structures already the formation of such defects has to be avoided—because once developed they cannot be removed. One impact parameter for the formation of self-assembly defects is the quality of the stamp surface, in particular its roughness. The authors compared stamps with flat surfaces obtained by potassium hydroxide etching of (110)-Si with stamps prepared by dry-etching, featuring vertical grooves or horizontal scallops (typical of deep-etching in a Bosch process). Under comparable conditions, the stamps with the vertical grooves feature the highest amount of self-assembly defects. In contrast, horizontal scallops avoid defect formation. Thus, the surface quality of the stamp sidewalls is important for a high quality imprint.

Mechanical characterization of a piezo-operated thermal imprint system

Dynamic agitation during imprint is investigated as a means to reduce the imprint temperature of thermoplastic materials. The imprint system used consists of a stepping motor and a piezo unit as the main components. The motor controls the working point of the system, and a sinusoidal motion of the piezo unit (3 piezos at 120° with respect to each other) superimposes an additional dynamic excitation. The system was characterized with respect to its mechanical behavior by exploiting analogies between mechanical and electrical quantities. The results show that the system can be operated at a frequency of up to about 60 Hz and that the piezos act as a dynamic motion source in the system. First imprint experiments with polystyrene at 100 °C indicate the potential of this frequency-assisted imprint technique.

Aspects of hybrid pattern definition while combining thermal nanoimprint with optical lithography

Thermal nanoimprint (T-NIL) is uniquely suitable for combinational lithography because it proceeds without cross-linking and does not rely on changes in properties of the imprint material. Most typical are combinations with other lithography techniques. Combinations with optical lithography may help to circumvent the pattern size dependence of nanoimprint and help to remove the residual layer by a simple development step. This work focuses on a hybrid processing combining T-NIL with optical lithography by using a single resist layer. Critical issues are the imprintability of typical photoresists and the fact that lithography has to be performed over a prepatterned topography. A well-known positive tone photoresist, AZ 1500, is compared with a previously investigated negative tone resist, SU-8. Gel permeation chromatography reveals typical differences of these photoresists compared to typical imprint polymers. The results demonstrate that molecular mobility is a more important indicator for viscosity of photoresists than molecular weight. Additionally, a simple construction method was found to allow prediction of features typically encountered during hybrid patterning.Thermal nanoimprint (T-NIL) is uniquely suitable for combinational lithography because it proceeds without cross-linking and does not rely on changes in properties of the imprint material. Most typical are combinations with other lithography techniques. Combinations with optical lithography may help to circumvent the pattern size dependence of nanoimprint and help to remove the residual layer by a simple development step. This work focuses on a hybrid processing combining T-NIL with optical lithography by using a single resist layer. Critical issues are the imprintability of typical photoresists and the fact that lithography has to be performed over a prepatterned topography. A well-known positive tone photoresist, AZ 1500, is compared with a previously investigated negative tone resist, SU-8. Gel permeation chromatography reveals typical differences of these photoresists compared to typical imprint polymers. The results demonstrate that molecular mobility is a more important indicator for viscosity of ph...

A novel tool for frequency assisted thermal nanoimprint (T-NIL)

Based on the well-known fact that thermoplastic polymers feature a decrease of viscosity at increased frequency we propose a novel tool for frequency assisted thermal nanoimprint. The system is equipped with a stepper motor to drive into contact and to apply a static loading. In addition a piezo-unit is available that allows the superposition of the static load with a dynamic excitation. Detailed analysis of the frequency response of the overall system makes obvious that the frequency range available is limited not only by the frequency cut-off of the piezo amplifier, but also by its power or rather by the limited output current available. As a consequence the maximum frequency at full displacement is only 10 Hz. Nonetheless this should be enough to reduce the viscosity of typical imprint polymers at a low imprint temperature. The measurement system is sensitive enough to detect the small changes induced by the polymeric layer in the imprint stack, when the temperature is raised to typical imprint temperatures. Decay times for the residual force during imprint with a conventional imprint stack could be obtained from a relaxation experiment, where the piezos are used as step-displacement sources. The data are in excellent agreement with values calculated from dynamic rheological characterization experiments.

Preparation of diamond-shaped channels in SU-8 for optical control of the filling state

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.

Recovery prevention via pressure control in thermal nanoimprint lithography

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.

Experimental analysis for process control in hybrid lithography

The positive tone resist AZ 1505 is characterized with respect to hybrid lithography that uses thermal nanoimprint lithography to define the nanometric part of a lithography pattern in a first step, followed by optical lithography to define the micron-scaled patterns in the same resist layer in a second step. The parameters investigated are glass temperature and sensitivity (dose curves), both after thermal loading in a typical imprint sequence. The glass transition of the multicomponent resist formulation is derived from stress measurements and the dose curves are evaluated from development rate monitoring via laser interferometry at 532 nm. The results show that both parameters are affected by thermal loading, but adequate choice of the processing parameters makes it possible to compensate for the thermal degradation of the photoresist as long as the imprint temperature chosen does not exceed 130 °C. Based on the characterization results, successful hybrid lithography and lift-off has been demonstrated ...

Thermal nanoimprint to improve the morphology of MAPbX3 (MA = methylammonium, X = I or Br)

Perovskites have high potential for future electronic devices, in particular, in the field of opto-electronics. However, the electronic and optic properties of these materials highly depend on the morphology and thus on the preparation; in particular, highly crystalline layers with large crystals and without pinholes are required. Here, nanoimprint is used to improve the morphology of such layers in a thermal imprint step. Two types of material are investigated, MAPbI3 and MAPbBr3, with MA being methylammonium, CH3NH3+. The perovskite layers are prepared from solution, and the crystal size of the domains is substantially increased by imprinting them at temperatures of 100–150 °C. Although imprint is performed under atmospheric conditions which, in general, enhances the degradation, the stamp that covers the layer under elevated temperature is able to protect the perovskite largely from decomposition. Comparing imprinting experiments with pure annealing at a similar temperature and time proves this. Furthe...

Flat and highly flexible composite stamps for nanoimprint, their preparation and their limits

To reduce the overall costs, replica stamps have become more and more relevant in the past years. These replicas often are composite stamps with at least two layers, a structured top layer and a backplane. In most cases, the composite stamps are rigid, which limits their use for rolling processes or low-pressure imprints. Flexible composite stamps avoid these limitations. However, due to the flexibility, preparation-induced bending may occur. A concept to avoid such a bending will be introduced and verified by the preparation of a flat and flexible composite stamp consisting of a structured OrmoStamp top layer and a polydimethylsiloxan (PDMS) backplane. To increase the adhesion between both layers, two measures will be adopted: one relies on an increase in the surface energy of the PDMS and the other one on an increase in the interface area. Finally, these flat and highly flexible composite stamps are tested in a critical imprint situation, where particles are dusted to the substrate before imprint. Impri...