Design for nanoimprint lithography: total layout refinement utilizing NIL process simulation

Novel thermoplastic polymers with improved release properties for thermal NIL

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.

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.

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

Three-dimensional diamond-like carbon mold fabricated by focused-ion-beam chemical vapor deposition (FIB–CVD) using a precursor of phenanthrene has been applied to a nanoimprint lithography (NIL) process. The mold has a very smooth surface, which is one of the advantages for the fabrication of optical devices. FIB–CVD uses a Ga+ beam control system which convert three-dimensional computer aided design data into the scanning signal and is capable of fabricating any shape of optical device mold surface such as a microlens. The microlens molds have been replicated onto poly(methylmethacrylate) (PMMA) and hydrogen silsequioxane using NIL. A microlens of polydimenthyl siloxane has been fabricated by replication using the PMMA reversal mold fabricated by NIL using the FIB–CVD mold.

ABSTRACTDiamond like carbon (DLC) films were deposited on Si and then patterned to form 40 nm features as nanoimprint templates. A plasma enhanced chemical vapor deposition (PECVD) system with CH4 precursor was used to deposit DLC films on Si and quartz substrates. Then these films were characterized using Raman spectroscopy, atomic force microscopy (AFM), nanoindentation, and contact angle measurement. By varying the RF power and pressure of the PECVD, DLC films with good uniformity, smooth surfaces (<0.2 nm RMS), low surface energy (∼40 mJ/m2), and high hardness (∼22 GPa) were achieved. Nanoimprint lithography and liftoff process were used to pattern Cr mask on DLC films. An inductively coupled plasma (ICP) etching process was performed with CF4 to transfer the patterns into the DLC films to form nanostructured template for nanoimprint. Water contact angles on the patterned DLC templates were measured and it was stable at about 70° under thermal annealing at 180 °C for more than 12 hours. With these DLC templates, UV and reversal UV nanoimprint lithography were carried out on SU-8 at typical imprint conditions and then the fidelity of pattern-transfer was investigated. These experimental results indicate that DLC is an excellent material for nanoimprint templates because of its high wear resistance, robust low surface energy, UV transparency, and ease of patterning.

Polymer photonic band-gaps fabricated by nanoimprint lithography

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.

Computational study of material optimization on de-molding process in nanoimprint lithography

Optical metasurfaces, consisting of subwavelength‐scale meta‐atom arrays, hold great promise of overcoming the fundamental limitations of conventional optics. Due to their structural complexity, metasurfaces usually require high‐resolution yet slow and expensive fabrication processes. Here, using a metasurface polarimetric imaging device as an example, the photonic structures and the Nanoimprint lithography (NIL) processes are designed, creating two separate NIL molds over a patterning area of > 20 mm2 with designed Moiré alignment markers by electron‐beam writing, and further subsequently integrate silicon and aluminum metasurface structures on a chip. Uniquely, the silicon and aluminum metasurfaces are fabricated by using the nanolithography and 3D pattern‐transfer capabilities of NIL, respectively, achieving nanometer‐scale linewidth uniformity, sub‐200 nm translational overlay accuracy, and <0.017 rotational alignment error while significantly reducing fabrication complexity and surface roughness. The micro‐sized multilayer metasurfaces have high circular polarization extinction ratios as large as ≈20 and ≈80 in blue and red wavelengths. Further, the metasurface chip‐integrated CMOS imager demonstrates high accuracy in broad‐band, full Stokes parameter analysis in the visible wavelength ranges and single‐shot polarimetric imaging. This novel, NIL‐based, multilayered nanomanufacturing approach is applicable to the scalable production of large‐area functional structures for ultra‐compact optic, electronic, and quantum devices.

Nanoimprint lithography (NIL) got large attention when it was introduced 1, 2 since it de-scribed the possibility for a low cost parallel nanotechnology. It got even larger attention when it was reported that sub 10 nm features on a Si stamp could be replicated into a resist layer, which in turn had a profile allowing a lift-off process to be possible.3,4 More reports about NIL technology followed and soon also devices were reported made with NIL. 5, 6, 7 This development inspired a few research groups to initiate projects around NIL technology. Also related techniques such as micro contact printing and its relatives were further developed at this time. 8, 9 Several different nanoimprint machines, set-ups and processes were reported. Examples of this rich development are for instance the roller imprint, 10 Step and flash imprint lithography, 11 described multi-layer resist methods. 12 An interesting observation is that in most of the works reported so far (except the early works in ref. 2,3 above) the feature sizes have rather been closer to the 100 nm domain than to the 10 nm domain. Furthermore in most of the NIL experiments reported the de-scribed were made with stamps having areas of only one or a few square centimetre(s).1, 2, 3, 4, 5, 6, 7 In order to meet the demand from industries, there was thus a need to develop a large area NIL process.13 Here two alternatives exists, either use a full wafer single printing step procedure 14, 15, 16 or to utilize a step & repeat procedure.17 Still up to now, more than seven years ago after its introduction, very few reports of real large wafer scale NIL exists. The largest size of the wafers successfully printed is 6 inch 14, 15 and for these experiments two kinds of stamps has been used. The stamp was either a metal CD-master stamp or an de-scribed defined electroplated metal stamp having regions with lines at the nanometer scale distributed at few locations over the wafer. The printed pattern definition in the resist to be accurate.

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.

Nanoimprint lithography (NIL) is one of the most promising technologies for nanofabrication because it can create nano- and microscale structures and devices in a cost-effective manner. In the NIL process, a mold with patterns on its surface comes in contact with a polymer film on a substrate. The patterns are transferred to the polymer film and then the mold is separated from the film. Mechanical contact between the mold and the polymer film, and between the film and the substrate, is inevitable. In some cases, during the separation process, adhesion and friction forces at the interfaces can deform and fracture the transferred patterns and detach the polymer film from the substrate. Thus, controlling the adhesion and friction between the materials in contact is very important in achieving a successful pattern transfer and making the NIL process a robust nanofabrication technique. Many theoretical and experimental research efforts have been made to clarify the tribological phenomena in NIL and to reduce defects due to adhesion and friction. This article describes the tribological problems encountered and reviews the related research.

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.

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.