Khalid Dhima

Flexible composite stamp for thermal nanoimprint lithography based on OrmoStamp

Flexible stamps are common for roll-to-roll processing but less common with planar processing, although they offer a number of benefits as, e.g., an improved conformal contact at reduced pressure. A simple way to realize such a flexible stamp is to use a two layer system with a structured top layer and a flexible backplane. The structured top layer is most easily obtained by molding, the backplane provides the flexibility envisaged. For use in a thermal nanoimprint process, a high thermal stability is required for both. This investigation addresses the preparation of flexible composite stamps with OrmoStamp as the structured top layer and polyimide as the flexible backplane. The process recommended for stamp preparation with OrmoStamp has to be modified to avoid bending after the hard bake that is required after ultraviolet-curing of the material to obtain a high stability of the top layer. Reduction of bending is advised, in particular, for large area stamps, where the hard bake step is in conflict with ...

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...

Guided wrinkling with nanoimprinted SU-8 surfaces

The wrinkling of SU-8 may be useful for microelectromechanical systems and optical applications, but highly regular and periodic wrinkles are the most convenient for such applications. This paper reports wrinkles generated on SU-8 by exposure to 172 nm-wavelength ultraviolet under ozone. To identify the relevant processing parameters, the impact of the SU-8 layer thickness and the effects of the postexposure bake temperature and UV-ozone treatment time are investigated. With 5 μm-thick SU-8 layers, distinct wrinkles are obtained with an ultraviolet-ozone treatment of 3 min and at a postexposure bake temperature of at least 140 °C. To guide the wrinkling, topography is induced on the SU-8 surface via capillary force lithography. By using a stamp with equidistant cavities possessing different widths, the structure size dependence is investigated. Owing to local stress relaxation, single or double wrinkles form along the elevated lines depending upon the line width. Within the gaps between the lines, wrinkle...

Effect of residual stress on replication fidelity with nanoimprint

Relief of residual stress in an imprinted polymer may affect the replication fidelity by leading to recovery. The level of stress induced in the polymer depends on the method of imprint. For example, a “soft” imprint with an elastomeric stamp uses capillary forces to fill the cavities whereas a “hard” imprint with a rigid stamp relies on external pressure. To study the effect of residual stress after imprint, both methods are applied with different imprint times to vary the level of residual stress, as the stress remaining relaxes with imprint time. To visualize the residual stress a temperature treatment is performed after imprint. This temperature treatment allows recovery within a convenient experimental time. A comparison of the shape of the imprinted structures before and after temperature treatment clearly shows that with a hard imprint at short imprint times a considerable amount of stress remains in the polymer, in particular when the residual layer is thin and the imprinted stamp structures are w...

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.

Thermal wrinkling of nanoimprinted SU-8 with masked UV-exposure

When wrinkling can be provided in a controlled way, this has potential for a lot of applications, e.g., in optics or microelectromechanical systems. This paper reports on localized wrinkles generated with an SU-8 layer on silicon; the surface is hardened by a deep-ultraviolet (UV) treatment (172 nm); the size of the wrinkling area is defined by masked UV-exposure. In addition, the SU-8 surface is provided with topography via capillary force lithography beforehand. To identify the interaction between masked UV-exposure and deep-UV treatment several issues are addressed; the size of the wrinkling area, the deep-UV treatment time, and the dose for the masked exposure. Different types of wrinkling are observed, an undulated type, a parallel type, and a zig-zag type. To define a clear wrinkling area an exposure dose of at least 160 mJ/cm2 is advised with a 5 μm-thick SU-8 layer. Then, the wrinkles orient in parallel to the edges of the wrinkling area. The impact of topography on wrinkling within a defined wrin...

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 ...

Method for high temperature nanoimprint of an organic semicrystalline polymer

Performance and functionality of devices prepared from organic semicrystalline polymers strongly depend on the size and the orientation of the ordered crystalline domains, as the conductivity is distinctly different in the different lattice directions. This investigation addresses the potential of thermal nanoimprint to control the size of the ordered domains as well as their orientation with respect to the substrate. Poly-3-hexylthiophene (P3HT) is chosen as one of the prominent semicrystalline polymers available at high quality. In order to control the ordering of the domains freely, the imprint is performed at a temperature beyond the melting point to eliminate the domains already existing after spin-coating and soft-bake. To avoid a degradation of P3HT at such a high temperature under oxygen contact—the imprint system used operates in air—a novel imprint procedure is employed, where a distinct prepressure is applied already during the heating of the imprint stack (stamp/substrate with P3HT layer). To ...