Tribology issues in nanoimprint lithography

Novel thermoplastic polymers with improved release properties for thermal NIL

Demolding, the process to separate stamp from molded resist, is most critical to the success of ultraviolet nanoimprint lithography (UV-NIL). In the present study we first investigated adhesion and demolding force in UV-NIL for different compositions of a model UV-curable resist containing a base (either tripropyleneglycol diacrylate with shorter chain length or polypropyleneglycol diacrylate with longer chain length), a cross-linking agent (trimethylolpropane triacrylate) and a photoinitiator (Irgacure 651). The demolding force was measured using a tensile test machine after imprinting the UV resist on a silicon stamp. In general, the shorter monomer shows a larger demolding force. Decreasing the cross-linking agent content from 49 to 0 wt% results in a decreased adhesion force at the resist/stamp interface thereby facilitating the demolding. Demolding stress in general is mainly generated due to shrinkage of the resist in the UV curing step and also adhesion and friction at the stamp/resist interface in the subsequent demolding step. In the second part of this study the effect of resist compositions on the stress generation was studied by numerical simulation of the curing and demolding steps in UV-NIL. Input parameters required for the simulation were determined experimentally. As the cross-linking agent content increases the fracture strength of the resist also increases. At the same time, shrinkage stress due to cure and also adhesion at the stamp/resist interface both increase. By normalizing the overall maximum local stress by the fracture stress of the resist, we found that there is an optimum for the cross-linking agent content that leads to the most successful imprinting. In the third part of our study a simple method was developed to obtain the polymerization shrinkage stress exerted on the sidewalls of resist/stamp interface in UV NIL. This method is based on the measurements of demolding force which is the sum of adhesion and friction forces. The mean polymerization shrinkage stress on the sidewalls can readily be decoupled from overall demolding force by independently measuring the friction coefficient, adhesion force, and geometries of stamp structures. The polymerization shrinkage stress on the sidewalls is overall larger than adhesion and increases by adding more cross-linking agent to the resist composition.

Nanoimprint Lithography (NIL) has demonstrated its value in manufacturing nanostructures with extremely tight tolerances for the optics industry. Doing NIL process on single samples in the lab has been displayed for a long time in industry and academia. Scaling up from one good sample to full wafer and wafer to wafer uniformity for many wafers in a row, has some obstacles to overcome. In this paper, we discuss the process repeatability of manufacturing nanostructures starting from basic line/space through various other periodic and meta structures. Demonstrating the manufacturing capability using NIL for pattering also includes the other related process steps such as thin films and etching. We discuss key metrology, process control characterization, and process stability which include thin film RI uniformity, master to print replication uniformity, and post etch structural critical dimensions uniformity (CDU). Our NIL process has achieved 1.0nm σ on line widths at 45nm. This precision of replication includes all variation introduced from multiple stamps, wafer to wafer prints, and multiple sites within the wafer. This level of process control at the masking layer needs to be maintained for the finished structure.

A theoretical study of resolution in nanoimprint lithography (NIL) has been carried out using molecular dynamics (MD) simulation. We have performed a MD simulation for glass NIL, monitored the friction force during entire NIL process and evaluated the deformed shapes of glass patterns after the mold releasing. The resolution in NIL is governed by the maximum tensile stress acting on the glass, which is induced by the friction force during the mold releasing. Based on the distribution of average number density of atoms in the molded glass, the ultimate resolution in the glass NIL has been proved to be 0.4 nm.

Nanoimprint lithography (NIL), as a non-conventional pattern transfer technique, presents unique competitiveness in the fabrication of semiconductor structures for medium-to-low resolution, cost-sensitive, and high-volume applications, owing to its advantages in high resolution, high throughput, low cost, and scalability. Compared with conventional lithography, NIL achieves high-precision pattern transfer beyond the optical diffraction limitation by directly imprinting and transferring patterns through mechanical contact, without relying on complex optical systems, thereby significantly reducing equipment and operational costs. This article systematically reviews the fundamental principles of NIL and its major process types, including thermal nanoimprint lithography (T-NIL), ultraviolet nanoimprint lithography (UV-NIL), and roller nanoimprint lithography (R-NIL), elaborating on the characteristics and applicable scenarios of each in terms of resolution, process conditions, substrate compatibility, throughput, and cost. Furthermore, a multi-dimensional comparison is conducted to analyze NILs competitive positioning within the current universal lithography technology landscape, while potential future development directions are outlined. Although NIL still faces challenges such as limited mold lifetime and pattern flexibility, advancements in material engineering, process optimization, and technology integration are expected to broaden its industrialization prospects.

Nanoimprint lithography (NIL) has been broadly applied in the fabrication of nano-patterned polymer films for cost-efficiency and high through-put; however, the intrinsic tradeoff between mechanical strength and residual stress of polymer films significantly limits the NIL resolution while the harsh processing conditions limit its versatile applications to different substrates. Herein, 1 nm metal oxide cluster, phosphotungstic acid (PTA), is used to complex with polyvinyl alcohol (PVA) for high-resolution NIL that can be operated at large-scale and mild conditions. The ultra-small size of PTA enables dense supramolecular interaction with PVA for the diminished crystallinity and accelerated chain dynamics that help relax the residual stress during film casting. Meanwhile, the PTAs serve as supramolecular crosslinkers for the increased modulus of the films (ca. 2 GPa), providing promising dimensional stability required for high-resolution NIL. Simple casting the aqueous blend on a master mold successfully gives a residual stress-free film with sub-2 nm resolution at wafer scale (>100 cm2). The mild processing at ambient condition permits broad NIL applications to diverse substrates, e.g., integrated circuit chips, compact discs, PET nano gratings and even delicate bio-surfaces.

Low cost, rapid fabrication of durable molds of grating arrays for nanoimprint lithography

Superhydrophobic polymeric films with hierarchical structures produced by nanoimprint (NIL) and plasma roughening

ABSTRACTTo investigate template releasing process in nanoimprint lithography, template releasing energy (i.e. surface energy between the template and the resist polymer) in various releasing conditions is evaluated using multi-axial controlled releasing system. The releasing energy is in proportion to the surface free energy of the template, but does not depend on the velocity of releasing. Also, a peeling mode where the template is released from a single side and a lift-off mode where the template is removed in the perpendicular direction to the resist are examined. The result shows that the releasing energy by peeling mode is lower than that by lift-off modes.

In recent years extensive research on CMAs (complex metallic alloys) were performed. Among many interesting qualities of complex metallic alloys are its low surface energy and high wear resistance, both making CMA compounds very appealing as a coating layer. The AlCuFeB compound is of particular interest since the addition of boron heps reduce friction forces. nano-imprint lithography as well as other manufacturing techniques need a solution to tackle problems related to stick-slip effects, and damage of molds and substrates, which are all effects of friction and adhesion forces on the interface of contacting materials. Complex metallic alloy compounds could be seen as a possible coating solution, reducing the surface forces, therefore ensuring lower failure rate during the manufacturing process as well as giving opportunity to create more complicated structures. This study focuses on properties of AlCuFeB CMA coatings, explores the possibilities of manufacturing such films, discusses their properties and points to the possible use of this compound in nano-imprint lithography. Most measurements in this study were performed with an atomic force microscope (AFM) acting as a profilometer, tribometer and tool to measure adhesion forces.

Polymer photonic band-gaps fabricated by nanoimprint lithography

Electrochemical transducers are frequently used to electrochemically deposit, synthesize and/or sense chemical compounds in material science, chemistry and biology. Traditionally this is done in fairly large volumes; generally several milliliters to even full production scale tanks, however, for high throughput measurements and combinatorial experiments smaller volumes are generally preferred. To probe small volumes small electrodes with preferable high density are needed. Presented in this paper is how to fabricate interdigitated electrodes by electron beam lithography (EBL) and also how to make large quantities by nanoimprint lithography (NIL) to enable combinatorial studies of cells or materials in microsystems (Lab-on-a-Chip Systems).Interdigitated electrodes are shown to have additional advantages compared to simple disk or rod electrodes, such as being able to perform redox cycling experiments. The collection efficiency for the electrodes with pitch of 400 nm and width of 200 nm fabricated by EBL has been found to be round 87% when characterized electrochemically using ferro-/ferricyanide. The shape of the cyclic voltammograms for the electrodes are also in good agreement with the theoretical expectations for ultramicroelectrodes. Hence, these electrodes should be ideal candidates for combinatorial and high-throughput studies based on electrochemical methods.Furthermore, a fabrication process based on nanoimprint lithography (NIL) is demonstrated, this could potentially ease fabrication and reduce cost of devices. The NIL process is based on thermal imprinting in PMMA 950k and LOR 0.7A.

In this work, thermal nanoimprint lithography (NIL) is used on full 8 in. silicon wafers to imprint a thin PS-b-PMMA block copolymer (BCP) layer. The authors show that the imprinted BCP layer can thermally self-organize after the NIL process or during the NIL process itself, depending on experimental conditions used. Self-organized imprinted features with good graphoepitaxy alignment are obtained with a cylindrical BCP. Nevertheless, a standard fluorinated silane mold treatment is shown not to be neutral to the BCP. Then, if the line features do not have a thickness exactly commensurable to the natural self-organizing period of the polymer l0, a surface wetting layer is observed.

Electroosmosis-driven nanoimprint lithography (NIL) is an emerging nanopatterning approach which has shown great potential and unique strength in large-area nanoimprinting, fabricating high-aspect-ratio micro/nanostructures, patterning fragile substrates and non-flat surface. However, compared to the conventional pressure-driven NIL and the electrocapillary force-driven UV-imprinting, there is a big difference for such novel electroosmotic flow driven imprinting process. This paper investigated the mechanism, influencing factors and laws of polymer flow and filling behavior for the electroosmosis-driven nanoimprinting. Based on the principle of electroosmosis-driven micro flows, a theoretical model estimated body force was established. And the model of filling velocity and the model of filling time were derived. Furthermore, the dynamic filling process of electroosmosis-driven NIL, influence and laws of polymer flow and filling behavior regarding the imprint process parameters, the feature patterns and the properties of polymer were revealed by numerical simulation using COMSOL Multiphysics simulation software. These findings are valuable in providing a theoretical basis for electroosmosis-driven nanoimprinting, optimizing imprinting process and further enhancing the performance of the new patterning method.

The centrifuge technique, which has been previously used in adhesion experiments, has been modified for use in single particle friction studies. Both flat compacted surfaces and large single particles were used as substrate surfaces to allow assessment of drug-drug, drug-drug carrier and drug carrier-drug carrier friction forces. Particle size, particle shape and surface roughness were identified as main factors influencing the change from a static into a dynamic friction process and the division between friction due to adhesion and ploughing. The forces of adhesion and friction were found to be proportional to the reversible energy of adhesion. The ratio between the force of adhesion and the press-on force applied and the ratio between the force of friction and the press-on force can be related to the yield stress and the reduced Young's modulus of the materials in contact.