Flash Imprint Lithography Research Articles

Nanoimprint with CO<sub>2</sub> Ambient

In Jet and Flash Imprint Lithography (JFIL), ambient gas is trapped between the resist, the substrate and the mold. The volume of the trapped ambient gas is estimated about 9.7 ~ 21.5% of the resist volume. It takes time for the bubbles to disappear in the closed space. By using carbon dioxide as the atmospheric gas and applying an organic layer such as Spin on Carbon (SOC) to the top surface layer of the substrate, it is proposed by theoretical calculation that the gas disappearance time is shortened and the throughput is improved. When the thickness of the SOC layer on the substrate was 5 nm or more, it was indicated that the gas disappearance time was shorter in carbon dioxide ambient than in helium ambient.

Improvement of productivity by spin-coating and flash imprint lithography

To improve the productivity of nanoimprint lithography (NIL) in semiconductor manufacturing, we have developed spin-coating and flash imprint lithography (SC-FIL). Using a newly developed SC-FIL resist, we imprinted a 300-mm-wide whole wafer including partial fields. The cross-sectional image showed a well-shaped half-pitch dense line with a width of 26 nm. The mix-and-match overlay accuracy (3σ) was 3.9 nm in the X direction and 3.4 nm in the Y direction. Assuming Washburn’s model of capillary flow, we identified the unique defect-generation mechanism in SC-FIL and hence optimized the SC-FIL process for high throughput and low defect density. After optimizing the NIL, the multimodule NZ2C system with four imprint heads is expected to achieve a throughput of 124 wafers per hour and a defectivity of only 0.005 defects per cm2.

Enabling nanoimprint simulator for quality verification: process-design co-optimization toward high-volume manufacturing

Computational technologies are still in the course of development for nanoimprint lithography (NIL). Only a few simulators are applicable to the nanoimprint process, and these simulators are desired by device manufacturers as part of their daily toolbox. The most challenging issue in NIL process simulation is the scale difference of each component of the system. The template pattern depth and the residual resist film thickness are generally of the order of a few tens of nanometers, whereas the process needs to work over the entire shot size, which is typically of the order of several hundred square millimeters. This amounts to a scale difference of the order of 106. Therefore, in order to calculate the nanoimprint process with conventional fluid structure interaction simulators, an enormous number of meshes is required, which results in computation times that are unacceptable. We introduce a process simulator which directly inputs the process parameters, simulates the whole imprinting process, and evaluates the quality of the resulting resist film for jet and flash imprint lithography process. To overcome the scale differences, our simulator utilizes analytically integrated expressions which reduce the dimensions of the calculation region. In addition, the simulator can independently consider the resist droplet configurations and calculate the droplet coalescence, thereby predicting the distribution of the non-fill areas which originate from the trapped gas between the droplets. The simulator has been applied to the actual NIL system and some examples of its applications are presented here.

Investigating the interplay of lateral and height dimensions influencing neuronal processes on nanogrooves

In this work, nanogroove dimensions as a design input parameter for neuronal differentiation and neurite outgrowth in brain-on-a-chip (BOC) applications are investigated. Soft lithography in polydimethylsiloxane (PDMS) is used extensively in organ-on-a-chip applications to create environments for in vitro models. As such, here it is used to fabricate cell culture substrates with nanogrooved patterns. Using a newly developed analysis method, the effect of the nanogrooved, biomimetic PDMS substrates is compared with lateral and height variations within the nanometer range as measured by means of atomic force microscopy (AFM). PDMS culture substrates were replicated from a cyclic olefin copolymer template, which was fabricated either directly by thermal nanoimprinting from a jet and flash imprint lithography (J-FIL) resist pattern (process I) on a polished silicon wafer or via an intermediate reactive ion etched all-silicon mold (process II) that was fabricated by using the J-FIL resist pattern as in process I as a mask. To study the interplay between the lateral and height dimensions of nanogrooves on the differentiation process of SH-SY5Y cells, which are a well-established model for neuronal cells that form networks in culture, the authors first characterized the feature sizes of the PDMS substrates received from both processes by AFM. On average, nanogrooved patterns from process I had a 1.8 ± 1.1% decrease in pattern period, a 15.5 ± 12.2% increase in ridge width compared to the designed dimensions, and a height of 95.3 ± 10.6 nm. Nanogrooved patterns for process II had a 1.7 ± 1.7% decrease in pattern period, a 43.1 ± 33.2% increase in ridge width, and a height of 118.8 ± 13.6 nm. Subsequently, they demonstrated that neurite outgrowth alignment was particularly strong if the pattern period was 600 nm or 1000 nm with the additional constraint for these patterns that the ridge width is <0.4 times the pattern period. Increasing pattern height increased the fraction of differentiated cells within the cell culture and increased neurite length, but had no direct impact on outgrowth alignment. This study forms the basis for optimization in the bottom-up engineering of neuronal network architecture, for which specific patterns can be selected to assist in neuronal cell differentiation and direct neurite growth and alignment. Such organized neuronal networks can aid in the design of in vitro assay systems for BOC applications by improving biological response readouts and providing a better understanding of the relationship between form and function of a neuronal network.

In situ characterization of enhanced thermal performance by periodic nanostructures on the surface of a microchannel

In situ formation of nanofins (i.e. isolated and dispersed precipitation of nanoparticles) on flow conduits induces changes to the surface morphology for heat exchange, increasing effective surface area and enhancing their cooling performance. Furthermore, the existence of these nanostructures has a significant influence on heat transfer, as opposed to the thermo-physical properties (e.g. thermal conductivity) of cooling fluids enhanced by addition of nanoparticles. This study conducted an in situ analysis exploring the effects of nanoscale surface modifications on forced convective heat transfer and thermal performance. A numerical heat transfer analysis, based on the conductive/convective heat transfer between a fin base and deionized water (DIW), was performed with the assumption that periodic nanostructures existed on the heated microchannel surface. Predictions of enhanced thermal performance (between 37% and 143%) from the resultant increase in the effective heat transfer area were validated for flows over these nanostructures (artificial nanofins) fabricated by SFIL (step and flash imprint lithography) inside a microchannel. Polydimethylsiloxane microchannels are bonded to a silicon wafer containing thin-film thermocouple arrays deposited onto artificial nanofins, which had been fabricated a priori for the in situ characterization of thermal performance. It was found that the convective heat transfer Nusselt number (Nu) increased from 61% to 110%. Additionally, a theoretical analysis of the pressure drop was also successfully achieved for a comprehensive understanding of the heat transfer characteristics at the fluid-wall interface of a microchannel.

A review of nanoimprint lithography for high-volume semiconductor device manufacturing

Abstract Imprint lithography has been shown to be a promising technique for the replication of nanoscale features. Jet and flash imprint lithography (J-FIL) [jet and flash imprint lithography and J-FIL are trademarks of Molecular Imprints, Inc.] involves the field-by-field deposition and exposure of a low-viscosity resist deposited by jetting technology onto the substrate. The patterned mask is lowered into the fluid, which then quickly flows into the relief patterns in the mask by capillary action. After this filling step, the resist is cross-linked under UV radiation, and then the mask is removed, leaving a patterned resist on the substrate. There are many criteria that determine whether a particular technology is ready for wafer manufacturing. Included on the list are overlay, throughput, and defectivity. The most demanding devices now require an overlay of better than 4 nm, 3σ. Throughput for an imprint tool is generally targeted at 80 wafers/h. Defectivity and mask life play a significant role relative to meeting the cost of ownership (CoO) requirements in the production of semiconductor devices. The purpose of this paper is to report the status of throughput and defectivity work and to describe the progress made in addressing overlay for advanced devices. To address high-order corrections, a high-order distortion correction (HODC) system is introduced. The combination of applying magnification actuation to the mask and temperature correction to the wafer is described in detail. Examples are presented for the correction of K7, K11, and K17 distortions as well as distortions on actual device wafers.

Nanoscale Magnification and Shape Control System for Precision Overlay in Jet and Flash Imprint Lithography

Jet and flash imprint lithography steppers have demonstrated unprecedented capability for patterning of sub-25-nm features for semiconductor manufacturing. A critical requirement for such patterning is the ability to overlay one layer of a device to a previously printed layer. In this paper, the design and development of a nanoprecision mask magnification/shape control system (MSCS) for the unique requirements of imprint-based overlay is presented. Imprint specific topics such as in-liquid overlay and distortions, and on-tool overlay metrology are discussed. The MSCS presented here has demonstrated 10-nm mix and match overlay (mean + 3 sigma) capability that approaches performance of state-of-the-art photolithography tools.

A hybrid algorithm for solving inverse problems in elasticity

Abstract The paper offers a new approach to handling difficult parametric inverse problems in elasticity and thermo-elasticity, formulated as global optimization ones. The proposed strategy is composed of two phases. In the first, global phase, the stochastic hp-HGS algorithm recognizes the basins of attraction of various objective minima. In the second phase, the local objective minimizers are closer approached by steepest descent processes executed singly in each basin of attraction. The proposed complex strategy is especially dedicated to ill-posed problems with multimodal objective functionals. The strategy offers comparatively low computational and memory costs resulting from a double-adaptive technique in both forward and inverse problem domains. We provide a result on the Lipschitz continuity of the objective functional composed of the elastic energy and the boundary displacement misfits with respect to the unknown constitutive parameters. It allows common scaling of the accuracy of solving forward and inverse problems, which is the core of the introduced double-adaptive technique. The capability of the proposed method of finding multiple solutions is illustrated by a computational example which consists in restoring all feasible Young modulus distributions minimizing an objective functional in a 3D domain of a photo polymer template obtained during step and flash imprint lithography.

Planarizing material for reverse-tone step and flash imprint lithography

Reverse-tone step and flash imprint lithography (S-FIL/R) requires materials that can be spin-coated onto patterned substrates with significant topography and that are highly planarizing. Ideally, these planarizing materials must contain silicon for etch selectivity, be UV or thermally curable, and have low viscosity and low volatility. One such unique material, in particular, a branched and functionalized siloxane (Si-12), is able to adequately satisfy the above requirements. This paper describes a study of the properties of epoxy functionalized Si-12 (epoxy-Si-12) as a planarizing layer. An efficient synthetic route to epoxy-Si-12 was successfully developed, which is suitable and scalable for an industrial process. Epoxy-Si-12 has a high silicon content (30.0%), low viscosity (29 cP at 25°C), and low vapor pressure (0.65 Torr at 25°C). A planarizing study was carried out using epoxy-Si-12 on trench patterned test substrates. The material showed excellent planarizing properties and met the calculated critical degree of planarization (critical DOP), which is a requirement for a successful etch process. An S-FIL/R process using epoxy-Si-12 was demonstrated using an Imprio® 100 (Molecular Imprints Inc., Austin, Texas) imprint tool. The results indicate that epoxy-Si-12 works very well as a planarizing layer for S-FIL/R.

Active wafer shape modulation using a multi-actuator chucking system

Jet and Flash Imprint Lithography has proven to be a viable alternative to optical lithography for fabrication of sub 30nm nanostructures for large volume semiconductor manufacturing. Machine throughput, overlay and process defectivity that meet and exceed the International Technology Roadmap for Semiconductors (ITRS) are essential for commercial viability of any new lithography technology. Jet and Flash Imprint Lithography uses an inkjet head to dispense a grid of liquid drops on the wafer surface to match the volume requirements of the pattern being imprinted. Wafer shape modulation has been shown to increase imprinting speed significantly by reducing air bubble trapping in the drop interstitial sites. A wafer shape modulation chuck that can address arbitrary field locations and sizes on a wafer with a novel actuation scheme that minimizes the number of actuators while increasing imprinting speed and reducing process defects significantly is presented.

Imprint lithography provides topographical nanocues to guide cell growth in primary cortical cell culture

In this paper, we describe a technology platform to study the effect of nanocues on the cell growth direction in primary cortical cell culture. Topographical cues to cells are provided using nanoscale features created by Jet and Flash Imprint Lithography, coated with polyethylenimine. We investigated nanoscaffolds with periodicities ranging from 200nm to 2000nm, and found that the samples with a period between 400nm and 600nm and a height of 118nm showed highly ordered regions of neurites in a newly formed network with a preferential alignment tendency for astrocytes. Live/dead staining results showed that different materials, such as silicon, glass, and imprinted resist are rendered biocompatible by coating with polyethylenimine. This coating therefore allows for a free choice of scaffold materials and promotes good cell-substrate adhesion. From our results we conclude particular length scales of nanoscaffold can impose a degree of order on cell spreading behavior in a complex cellular brain-on-a-chip network, which could thus be used to emulate ordered brain regions and their function in vitro.

Fast Graph Transformation based Direct Solver Algorithm for Regular Three Dimensional Grids

This paper presents a graph-transformation-based multifrontal direct solver with an optimization technique that allows for a significant decrease of time complexity in some multi-scale simulations of the Step and Flash Imprint Lithography (SFIL). The multi-scale simulation consists of a macro-scale linear e lasticity model with thermal expansion coefficient and a nano-scale molecular statics model. The algorithm is exemplified with a photopolimerization simulation that involves densification of a polymer inside a feature followed by shrinkage of the feature after removal of the template. The solver is optimized thanks to a mechanism of reusing sub -domains with similar geometries and similar material properties. The graph transformation formalism is used to describe the algorithm - such an approach helps automatically localize sub-domains that can be reused.

Reactive fluorinated surfactant for step and flash imprint lithography

One of the major concerns with nanoimprint lithography is defectivity. One source of process-specific defects is associated with template separation failure. The addition of fluorinated surfactants to the imprint resist is an effective way to improve separation and template lifetime. This study focuses on the development of new reactive fluorinated additives, which function as surfactants and also have the ability to chemically modify the template surface during the imprint process and thereby sustain a low surface energy release layer on the template. Material screening indicated that the silazane functional group is well suited for this role. The new reactive surfactant, di-(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl)silazane (F-silazane) was synthesized and tested for this purpose. The material has sufficient reactivity to functionalize the template surface and acceptable stability (and thus shelf-life) in the imprint formulation. Addition of F-silazane to a standard imprint resist formulation significantly improved template release performance and allowed for significantly longer continuous imprinting than the control formulation. A multiple-imprint study using an Imprio® 100 tool confirmed the effectiveness of this new additive.

Hard disk drive thin film head manufactured using nanoimprint lithography

The lithographic requirements for the thin film head (TFH) industry are comparable to the semiconductor industry for certain parameters such as resolution and pattern repeatability. In other aspects such as throughput and defectivity, the requirements tend to be more relaxed. These requirements match well with the strengths and weaknesses reported concerning nanoimprint lithography (NIL) and suggest an alternative approach to optical lithography. We demonstrate the proof of concept of using NIL patterning, in particular Jet and Flash™ Imprint Lithography (J-FIL™) (Imprio, Jet and Flash Imprint Lithography, and J-FIL trademarks are the property of Molecular Imprints, Inc.), to build functional TFH devices with performance comparable to standard wafer processing. An Imprio™ 300 tool from Molecular Imprints, Inc. (MII) was modified to process the AlTiC ceramic wafers commonly used in the TFH industry. Templates were produced using commercially viable photomask manufacturing processes and the AlTiC wafer process flow was successfully modified to support NIL processing. Future work is identified to further improve lithographic performance including residual layer thickness uniformity, wafer topography, NIL→NIL overlay, and development of a large imprint field that exceeds what is available in optical lithography.

Evolutionary Multi-Agent Computing in Inverse Problems

The paper tackles the application of evolutionary multi-agent computing to solving inverse problems. High costs of fitness function call become a major difficulty when approachingthese problems with population-based heuristics, however evolutionary agent-based systems (EMAS)turn out to reduce the fitness function calls, which makes them a possible weapon of choicefor them. The paper recalls the basics of EMAS and describes the considered problem (Step and Flash Imprint Lithography),later showing convincing results, that EMAS turns out to be more effective than classical evolutionary algorithm.

Ultraviolet curable branched siloxanes as low-k dielectrics for imprint lithography

A methacrylate-based, UV curable branched siloxane (MA-Si-12) was designed for use as a low-k dielectric that can be directly patterned by step and flash imprint lithography. MA-Si-12 has the properties of low viscosity (20 cP) and low vapor pressure (1.3 Torr), which make the material suitable for imprint processes based on an inkjet dispensing system. However, the mechanical strength of UV cured MA-Si-12 is too low (0.4 GPa Young's modulus by nanoindentation) for use with chemical mechanical polishing. Therefore, a methacrylate-based, UV curable cyclic siloxane (8-ring) was synthesized to be blended with the siloxane and improve the modulus. A formulation consisting of a mixture of MA-Si-12 and 8-ring (50 wt. % respectively) provides patterned structures with a modulus of 2.0 GPa. This formulation still has a viscosity (19 cP) and vapor pressure (1.3 Torr) low enough for inkjet dispensing. The UV cured resin has a dielectric constant of 2.8 at 1 MHz, low water absorption (0.7 wt. % after 24 h), and low UV cure shrinkage (5.6%). It provides high resolution, high fidelity imprint patterns. Mixing or blending of branched and cyclic siloxanes is a valuable approach to the design of imprint resist formulations with low dielectric constants.

Fabrication of Three-Dimensional MIS Nano-Capacitor Based on Nanoimprinted Single Crystal Silicon Nanowire Arrays

We report fabrication of single crystalline silicon nanowire based-three-dimensional MIS nano-capacitors for potential analog and mixed signal applications. The array of nanowires is patterned by Step and Flash Imprint Lithography (S-FIL). Deep silicon etching (DSE) is used to form the nanowires with high aspect ratio, increase the electrode area and thus significantly enhance the capacitance. High-! dielectric is deposited by highly conformal atomic layer deposition (ALD) Al2O3 over the Si nanowires, and sputtered metal TaN serves as the electrode. Electrical measurements of fabricated capacitors show the expected increase of capacitance with greater nanowire height and decreasing dielectric thickness, consistent with calculations. Leakage current and time-dependent dielectric breakdown (TDDB) are also measured and compared with planar MIS capacitors. In view of greater interest in 3D transistor architectures, such as FinFETs, 3D high density MIS capacitors offer an attractive device technology for analog and mixed signal applications.

Defect reduction for semiconductor memory applications using jet and flash imprint lithography

Acceptance of imprint lithography for manufacturing will require demonstration that it can attain defect levels commensurate with the defect specifications of high-end memory devices. Defects occurring during imprinting can generally be broken into two categories; random defects and repeating defects. Examples of random defects include fluid phase imprint defects, such as bubbles, and solid phase imprint defects, such as line collapse. Examples of repeater defects include mask fabrication defects and particle induced defects. Previous studies indicated that soft particles cause nonrepeating defects. Hard particles, on the other hand, can cause either permanent resist plugging or mask damage. In a previous study, two specific defect types were examined; random nonfill defects occurring during the resist filling process and repeater defects caused by interactions with particles on the substrate. We attempted to identify the different types of imprint defect types using a mask with line/space patterns at dimensions as small as 26 nm. An Imprio 500 twenty-wafer per hour development tool was used to study the various defect types. The imprint defect density was reduced nearly four orders of magnitude, down to ∼4/cm 2 in a period of two years following the availability of low defect imprint masks at 26-nm half-pitch. This reduction was achieved by identifying the root cause of various defects and then taking the appropriate corrective action.

DFB LDs at DWDM wavelengths fabricated by a novel nanoimprint process for mass production and tolerance simulation

DWDM distributed feedback laser diodes from 1540 nm to 1560 nm were manufactured on one single wafer by novel high throughput, low cost nanoimprint lithography (NIL). Thirteen ITU channels of 200 GHz spacing are obtained. The process includes two main steps: first, step flash imprint lithography (SFIL) was used to reduce the cost of a large stamp. Then soft stamps were applied to improve imprint uniformity on a large area. Errors were analyzed and tolerances to DFB-LD performance have been simulated.

Mask replication using jet and flash imprint lithography

The jet and flash imprint lithography (J-FILTM) process uses drop dispensing of UV curable resists to assist high resolution patterning for subsequent dry etch pattern transfer. The technology is actively being used to develop solutions for memory markets including flash memory and patterned media for hard disk drives. It is anticipated that the lifetime of a single template (for patterned media) or mask (for semiconductors) will be on the order of 104 to 105 imprints. This suggests that tens of thousands of templates/masks will be required to satisfy the needs of a manufacturing environment. Electron-beam patterning is too slow to feasibly deliver these volumes, but instead can provide a high quality master mask which can be replicated many times with an imprint lithography tool. This strategy has the capability to produce the required supply of "working" templates/masks. In this paper, we review the development of the mask form factor, imprint replication tools, and the semiconductor mask replication process. A PerfectaTM MR5000 mask replication tool has been developed specifically to pattern replica masks from an e-beam written master. Performance results, including image placement, critical dimension uniformity, and pattern transfer, are covered in detail.