Negative Tone Photoresist Research Articles

Nanoimprinting and backside ultraviolet lithography for fabricating metal nanostructures with higher aspect ratio.

This paper introduces an innovative approach to increasing the aspect ratio of metal nanostructures fabricated using nanoimprint lithography (NIL). Although conventional NIL and metal lift-off processes can fabricate metal nanostructures, the achievable aspect ratio is often limited by the inherent constraints of NIL. In this study, we demonstrate that for an ultraviolet (UV) transparent substrate, metal nanostructures patterned via NIL can serve as a photomask. A negative-tone photoresist (PR) layer was then deposited on top of the patterned metal nanostructures. By illuminating the substrate from the backside with UV light and subsequently developing the PR, PR structures complementary and self-aligned to the metal layer were obtained. This enabled a second round of metal deposition and lift-off, thereby increasing the height of the metal structures and enhancing the aspect ratio. Experimentally, we demonstrated that this method can improve the aspect ratio from less than 1.0 to as high as 2.1. This paper also addresses the further developments and potential applications of this technique.

Spatial and Bidirectional Work Function Modulation of Monolayer Graphene with Patterned Polymer "Fluorozwitterists".

Understanding the electronic properties resulting from soft-hard material interfacial contact has elevated the utility of functional polymers in advanced materials and nanoscale structures, such as in work function engineering of two-dimensional (2D) materials to produce new types of high-performance devices. In this paper, we describe the electronic impact of functional polymers, containing both zwitterionic and fluorocarbon components in their side chains, on the work function of monolayer graphene through the preparation of negative-tone photoresists, which we term "fluorozwitterists." The zwitterionic and fluorinated groups each represent dipole-containing moieties capable of producing distinct surface energies as thin films. Kelvin probe force microscopy revealed these polymers to have a p-doping effect on graphene, which contrasts the work function decrease typically associated with polymer-to-graphene contact. Copolymerization of fluorinated zwitterionic monomers with methyl methacrylate and a benzophenone-substituted methacrylate produced copolymers that were amenable to photolithographic fabrication of fluorozwitterist structures. Consequently, spatial alteration of zwitterion coverage across graphene yielded stripes that resemble a lateral p-i-n diode configuration, with local increase or decrease of work function. Overall, this polymeric fluorozwitterist design is suitable for enabling simple, solution-based surface patterning and is anticipated to be useful for spatial work function modulation of 2D materials integrated into electronic devices.

Ultradurable Embedded Physically Unclonable Functions.

Physically unclonable functions (PUFs) have attracted growing interest for anticounterfeiting and authentication applications. The practical applications require durable PUFs made of robust materials. This study reports a practical strategy to generate extremely robust PUFs by embedding random features onto a substrate. The chaotic and low-cost electrohydrodynamic deposition process generates random polymeric features over a negative-tone photoresist film. These polymer features function as a conformal photomask, which protects the underlying photoresist from UV light, thereby enabling the generation of randomly positioned holes. Dry plasma etching of the substrate and removal of the photoresist result in the transfer of random features to the underlying silicon substrate. The matching of binary keys and features via different algorithms facilitates authentication of features. The embedded PUFs exhibit extreme levels of thermal (∼1000 °C) and mechanical stability that exceed the state of the art. The strength of this strategy emerges from the PUF generation directly on the substrate of interest, with stability that approaches the intrinsic properties of the underlying material. Benefiting from the materials and processes widely used in the semiconductor industry, this strategy shows strong promise for anticounterfeiting and device security applications.

Synthesis of pentameric chlorotin carboxylate clusters for high resolution EUV photoresists under small doses.

This work reports the synthesis and characterization of a novel pentameric tin chloro cluster, (vinylSn)3Sn2Cl5O2(OH)2(t-BuCO2)6 (1), and explores its application as an efficient negative-tone photoresist in a 1 : 2 weight ratio blend with [(n-BuSn)12O14(OH)6](BF4)2 (2). Through e-beam lithography, a small high-resolution pattern (HP = 20 nm) is achieved for the blend photoresist (3) at a dose of 2080 μC cm-2. Additionally, EUV lithography demonstrates the development of a high-resolution pattern (HP = 16 nm) at an EUV dose of 70 mJ cm-2. Mechanistic studies by reflective FTIR indicate a significant decomposition of Sn-carbon and SnO2(t-Bu) moieties starting at J = 35 mJ cm-2, which is accompanied by growth of the Sn-O absorption intensity. A collapse of the cluster frameworks of clusters (1) and (2) is observed at J > 70 mJ cm-2. High-resolution X-ray photoelectron spectroscopy (HRXPS) reveals that low EUV light predominantly decomposes Sn-butyl and Sn-Cl bonds. As EUV doses increase, primary photolytic reactions involve cleavage of Sn-butyl, Sn-O2CBut, and Sn-vinyl bonds. Notably, the photolytic decomposition of Sn-Cl bonds is distinctive, with only two out of five bonds being cleaved, even at high EUV doses, resulting in a break in film growth at J = 27-35 mJ cm-2 in the EUV contrast curve. Moreover, HRXPS analysis suggests that radical propagation on the vinyltin end of the blend is unlikely, providing concise mechanistic insights into the photochemical processes governing the behavior of this advanced photoresist.

Fabrication of Silver Iodide (AgI) Patterns via Photolithography and Its Application to In-Situ Observation of Condensation Frosting.

This study introduces an innovative photolithography-based method for patterning ionic and inorganic particle materials such as silver iodide (AgI). Conventional methods lack precision when patterning powdered materials, which limits their applicability. The proposed method stacks layers of a particle material (AgI) and negative-tone photoresist for simultaneous ultraviolet exposure and development, resulting in well-defined AgI patterns. The sintering process successfully removed binders from the material layer and photoresist, yielding standalone AgI patterns on the Si substrate with good adhesion. The pitch remained consistent with the design values of the photomask when the pattern size was changed. In-situ observation of condensation frosting on the patterns was conducted, which confirmed the practicality of the developed patterning process. This versatile method is applicable to large areas with a high throughput and presents new opportunities for modifying functional surfaces.

Synthesis and characteristics of maleic anhydride-based photopolymers with pendant POSS groups for negative tone photoresists

Synthesis and characteristics of maleic anhydride-based photopolymers with pendant POSS groups for negative tone photoresists

On the origin and evolution of hotspots in multipatterning processes

Understanding the origins and propagation of defects and hotspots in patterning processes used for semiconductor fabrication is of paramount importance in managing yield. In this paper, results from physics-based simulators to model lithography and dry etch processes are presented and compared to experimental results. These models are used to study different types of hotspots and defects observed in a litho-etch-litho-etch (LELE) multipatterning process. At each pass of the LELE flow, patterns are printed into a SiO2 collecting layer using a trilayer film stack comprised of a negative tone photoresist layer, a spin-on-glass layer (SOG), and a spin-on-carbon layer (SOC). After both passes of the LELE process, the patterns in the SiO2 collecting layer will be transferred to a TiN hardmask prior to final etch into an underlying dielectric. The SOG and SiO2 layers are etched using fluorocarbon plasma, while the SOC layer is etched with an H2/N2 plasma generated in a capacitively coupled plasma source. A pinching hotspot is observed during the single litho-etch pass in a region where two features are placed very close and the image contrast is low. However, for some lithography process conditions, this hotspot is rectified by subsequent etch steps and does not always transfer as a defect into the SiO2 layer. The quenching of the hotspot occurs primarily during the etching of the SOC layer due to the aspect ratio-dependent etching (ARDE) effect. A bridging hotspot is also observed at lithography during the single litho-etch pass at high exposure doses. This hotspot, on the other hand, is exacerbated by the etch steps because of the ARDE effect. Hotspots are also identified that originate from overlay errors between photomasks exposed during first and second passes of the LELE process. The etch bias generated during etching of the SOG layer is crucial to ensure that the overlay-related hotspot does not translate to the SiO2 layer. The extent of etch bias in the SOG etch step is critical and can be tuned by adjusting the neutral to ion flux ratio during that etch step. Increasing the flux ratio improves the process window for the overlay defect; however, when the ratio is higher than approximately 20% of the nominal value, a different defect type is formed in the SOG layer due to the inverse ARDE effect that propagates downstream to the SiO2 layer.

Synthesis of a PCST-containing acrylic polymers and its application in negative-tone photoresist

The process design and synthesis of new acrylate photoresist is still a challenge due to the limited resolution and sensitivity of resist. In this report, acrylate derivatives with different p-chlorostyrene (PCST) contents were prepared. The effects of different monomer contents on the film-forming properties and sensitivity of the polymers were discussed. The introduction of PCST significantly improves the thermal stability and film-forming properties of the polymers, and reduces the polydispersity (PDI) of the polymers to a certain extent, facilitating the achievement of high sensitivity and resolution of chemically amplified resists (CARs). Under optimized conditions, a final linewidth of 105 nm can be obtained. This new acrylate photoresist with good thermal stability and high resolution is also expected to be used in the next generation of lithography.

An Evaluation of Bath Life Effects on Photoresist Removal for Wafer Level Packaging

With the emergence of 3D integration and wafer level packaging, the pillar bumping process has become a critical processing step. As the process has matured, significant efforts have been made for optimization in terms of both production and cost. This is especially true for photoresist stripping, since thick photoresists are needed to pattern high aspect ratios and must be removed in subsequent steps to expose underlying metal films. In this study, the removal of a thick negative tone photoresist with solder pillar plated wafers were investigated using a dimethyl sulfoxide blend with a quaternary ammonium hydroxide. Emphasis was placed on exploring the feasibility of an extended bath life beyond 7 days. Coupon level beaker tests were used to provide insight into the swelling and dissolution mechanisms with fresh and aged chemistry. Additionally, several chemistries were compared to examine the effect of the quaternary ammonium hydroxide. Based on these findings, chemical aging studies were performed on 300 mm wafers using a highly customizable single wafer processing tool that combined soaking and a high-pressure spray processing to potentially prolong the bath age even further. In addition to stripping efficiency at extended bath life, the compatibly with pillar structures and seed layers is investigated. Results from the study highlight potential methods to extend bath life while maintaining photoresist stripping efficiency.

Metasurface-Enabled Holographic Lithography for Impact-Absorbing Nanoarchitected Sheets.

Nanoarchitected materials represent a class of structural meta-materials that utilze nanoscale features to achieve unconventional material properties such as ultralow density and high energy absorption. A dearth of fabrication methods capable of producing architected materials with sub-micrometer resolution over large areas in a scalable manner exists. A fabrication technique is presented that employs holographic patterns generated by laser exposure of phase metasurface masks in negative-tone photoresists to produce 30-40 µm-thick nanoarchitected sheets with 2.1 × 2.4 cm2 lateral dimensions and ≈500 nm-wide struts organized in layered 3D brick-and-mortar-like patterns to result in ≈50-70% porosity. Nanoindentation arrays over the entire sample area reveal the out-of-plane elastic modulus to vary between 300 MPa and 4 GPa, with irrecoverable post-elastic material deformation commencing via individual nanostrut buckling, densification within layers, shearing along perturbation perimeter, and tensile cracking. Laserinduced particle impact tests (LIPIT) indicate specific inelastic energy dissipation of 0.51-2.61 MJ kg-1 , which is comparable to other high impact energy absorbing composites and nanomaterials, such as Kevlar/poly(vinyl butyral)(PVB) composite, polystyrene, and pyrolized carbon nanolattices with 23% relative density. These results demonstrate that holographic lithography offers a promising platform for scalable manufacturing of nanoarchitected materials with impact resistantcapabilities.

Influence of Counteranions on the Performance of Tin-based EUV Photoresists

A positive-tone extreme ultraviolet (EUV) photoresist was obtained via the exchange of the hydroxide ions of tin-oxo-hydroxo cages with tetrakis(pentafluorophenyl)-borate. Thin films were obtained after spin coating on silicon substrates and exposed to line patterns in interference lithography at 92 eV (13.5 nm). For low doses and development with ethylbenzene positive tone patterns were obtained. In contrast, the exchange of the hydroxide ions of tin-oxo-hydroxo cages with tetrakis(4-methylphenyl)-borate or tetrakis[3,5-bis(1,1,1,3,3,3-hexafluoro-2-methoxy-2-propyl)phenyl]-borate led to the negative-tone photoresist behaviour that has previously been observed for tin-oxo cages with all other counterions. In situ exposures at 92 eV and subsequent X-ray Photoelectron Spectroscopy were performed on a film of tin-oxo-hydroxo cages with tetrakis(pentafluorophenyl)-borate anions to probe the chemical changes induced by the EUV exposure. This shows that the C-F bonds of the anions are relatively stable.

Design and manufacture of an all-polymeric integrated multimode interferometer for visible photonics.

This work demonstrates an integrated multimode interferometer (MMI) based on a fully polymeric platform and optimized for visible range operation. The dimensions of a 2×2 MMI are first calculated analytically and simulated using finite elements method. The devices are manufactured using two layers of negative tone photoresists. The top layer is patterned by e-beam lithography demonstrating the adaptability of this material, naturally designed to respond to UV radiation. Fabrication tolerance was smaller than 100 nm. Devices were optically characterized with a 635 nm input source and the best performance for a 3 dB power splitter was found at an interferometric cavity dimension of 10.5 × 190.68 µm. Other interferometric lengths were characterized to establish a process design kit that allows future use of this platform in more complex photonic integrated circuits architectures.

Enhanced Solubility of Zirconium Oxo Clusters from Diacetoxyzirconium(IV) Oxide Aqueous Solution as Inorganic Extreme‐Ultraviolet Photoresists

AbstractMetal oxo clusters have drawn a great deal of attention as the extreme‐ultraviolet (EUV) photoresists. However, industrial implementation in the next‐generation lithography demands further improvements in their sensitivity and resolution. In this study, a zirconium oxo cluster with methacrylate and acetate ligands was synthesized from diacetoxyzirconium(IV) oxide aqueous solution for use as EUV photoresist materials. The obtained Zr oxo cluster, consisting of a Zr core and methacrylate and acetate ligands, was similar in size to Zr6O4(OH)4(methacrylate)12 cluster, but much more soluble in a variety of organic solvents. Open‐frame exposure tests revealed that the Zr oxo cluster served as a negative tone photoresist with a sensitivity greater than that of Zr6O4(OH)4(methacrylate)12 cluster. At a dose of 22 mJ/cm2 in the absence of photoacid generators, the Zr oxo cluster film resolved a pattern 15 nm in width, which is sufficiently low to meet the current technological requirements.

Stress-strain response of polymers made through two-photon lithography: Micro-scale experiments and neural network modeling

Photopolymerization is the governing chemical mechanism in two-photon lithography, a multi-step additive manufacturing process. Negative-tone photoresist materials are widely used in this process, enabling the fabrication of structures with nano- and micro-sized features. The present work establishes the relationship among the process parameters, the degree of polymerization, and the nonlinear stress-strain response of polymer structures obtained through two-photon polymerization. Honeycomb structures are fabricated on a direct laser writing system (Nanoscribe) making use of different laser powers for two widely applicable, commercially available resins (IP-S and IP-Dip). The structures are then tested under uniaxial compression to obtain the corresponding stress-strain curves up to 30% strain. Raman spectroscopy is used to correlate the degree of conversion achieved upon different laser exposures of the base photoresist material with the selected mechanical properties (Young’s modulus, tangent modulus, deformation resistance) after polymerization. Significant differences are recorded in the observed constitutive responses. Higher degrees of conversion result in higher elastic moduli and strength at large strains. Moreover, it is found that the IP-Dip resin yields higher degrees of conversion for the same laser power compared to the IP-S resin. A neural network model is developed for each resin that predicts the stress-strain response as a function of the degree of conversion. For each material, an analytical form of the identified constitutive response is provided, furnishing basic formulas for engineering practice. • Fabricated micro-honeycombs from negative-tone photoresists using direct laser writing (Nanoscribe). • Employed Raman spectroscopy to determine the degree of conversion. • Characterized effect of laser power on the degree of conversion during two-photon polymerization. • Performed more than 140 micro-compression experiments in-situ to determine stress-strain response. • Developed neural network model to predict the stress-strain response of the additively fabricated polymer structures.

Biocompatibility of SU-8 and Its Biomedical Device Applications.

SU-8 is an epoxy-based, negative-tone photoresist that has been extensively utilized to fabricate myriads of devices including biomedical devices in the recent years. This paper first reviews the biocompatibility of SU-8 for in vitro and in vivo applications. Surface modification techniques as well as various biomedical applications based on SU-8 are also discussed. Although SU-8 might not be completely biocompatible, existing surface modification techniques, such as O2 plasma treatment or grafting of biocompatible polymers, might be sufficient to minimize biofouling caused by SU-8. As a result, a great deal of effort has been directed to the development of SU-8-based functional devices for biomedical applications. This review includes biomedical applications such as platforms for cell culture and cell encapsulation, immunosensing, neural probes, and implantable pressure sensors. Proper treatments of SU-8 and slight modification of surfaces have enabled the SU-8 as one of the unique choices of materials in the fabrication of biomedical devices. Due to the versatility of SU-8 and comparative advantages in terms of improved Young’s modulus and yield strength, we believe that SU-8-based biomedical devices would gain wider proliferation among the biomedical community in the future.

Negative-tone molecular glass photoresist for high-resolution electron beam lithography.

A low molecular weight organic compound containing bis-phenol A backbone (BPA-6OH) is reported as a negative-tone photoresist. This material has a high glass transition temperature and excellent thermal stability. A good contrast, well-resolved line pattern around 73.4 nm and sensitivity of 52 µC cm−2 can be received for negative-tone molecular glass photoresist upon exposure in electron beam lithography system. It indicates that the negative-tone molecular glass photoresist is one of the promising candidates for use in electron beam lithography.

Self-Assembled Monolayers Assisted all Wet Metallization of SU-8 Negative Tone Photoresist

The present work investigates the application of a Self-Assembled Monolayer (SAM) on the widely used negative photoresist SU-8. (3-Aminopropyl) trimethoxysilane (APTMS) is employed to form SAMs on the surface of the polymer through wet silanization in ethanol. The treatment process of SU-8 resin surface is optimized to achieve a well-formed, high quality SAM. Wettability measurements, atomic force microscopy (AFM) and infrared (IR) spectroscopy are employed to follow and optimize the silanization process. Following silanization, the resulting SAMs are employed as adhesion layers for electroless plated metallic layers. For this purpose, the superior affinity of APTMS amine terminal groups towards Pd2+ ions is exploited to activate the surface. Metallic coatings such as Cu, Ni–P or Co–Ni–P are thus deposited by mean of autocatalytic deposition. The surface of metallized samples is analyzed using AFM, scanning electron microscopy (SEM) and glow discharge optical emission spectroscopy (GDOES). Finally, Ni–P is deposited on a micropatterned SU-8 surface to demonstrate the potential of the presented metallization approach for microfabrication.

Lithographic Performance of Aryl Epoxy Thermoset Resins as Negative Tone Photoresist for Microlithography.

Photoresists (or photo-resins) are the main and most important raw material used for lithography techniques such as deep X-ray (DXRL), ultraviolet (UVL), deep-UV (DUVL), and extreme UV (EUVL). In previous work, we showed how complicated could be the synthesis of the resins used to produce photoresist. In this study, we follow up on the strategy of tuning deep and macro levels of properties to formulate photo-resins. They were developed from a primary basis, using epoxy resins, a solvent, and a photoinitiator in several concentrations. The formulations were evaluated initially by the UVL technique, using a squared pattern of 2.3 mm2. The most suitable compositions were then studied in a pattern structure varying from 50 down to 1 µm width, applying UVL and DUVL. The patterned structures were compared with the chemical composition of the photo-resins. Considering the deep level of properties, polydispersion, and epoxidation degree were evaluated. Regarding the macro level of properties, the concentration of photoinitiator was studied. Promising results have been achieved with the control of the deep and macro levels methodology. By means of UV lithography, it was possible to note, for a large feature size above 2.0 mm2, the formulations presented good quality structures with a broad range of epoxidation degrees and photoinitiator concentrations, respectively from 3 to 100% (mol·molpolymer−1) and from 10 to 40% (mol·molpolymer−1). For structures smaller than 50 µm width, the composition of the photo-resins may be restricted to a narrow range of values regarding the formulation. The results indicate that the polydispersion of the oligomers might be a significant property to control. There is a tendency to better outcome with a low polydispersity (resins P1 and P2). Regarding UV and deep-UV irradiation, the best results were achieved with UV. Nevertheless, for DUV, the sensitivity seems to be more intense, leading to well-defined structures with over-exposure effects.

Effect of high strain rates and temperature on the micromechanical properties of 3D-printed polymer structures made by two-photon lithography

Two-photon lithography (TPL) enables the fabrication of metamaterials exhibiting unprecedented mechanical performance. Such lattice structures consist of micron-scale geometric features and are designed to achieve their envisioned properties through stretching and bending of individual trusses. Here, we present for the first time a comprehensive study of the effect of temperature and strain rate on the mechanical properties of micron-sized 3D printed polymers made by TPL using Nanoscribe’s negative tone photoresist IP-Dip. A strong strain rate dependency was identified for the yield strength, which increased fourfold in compression from 68 MPa to 230 MPa as the strain rate was raised from 7e−4 s−1 to 600 s−1. This trend was also seen in tension. The elastic modulus was found to increase with strain rate in the quasistatic regime but was constant at 3.2 GPa for strain rates of 0.7 s−1 and above. Even slightly elevated temperatures of 80°C led to a significant drop in the yield strength and elastic modulus, though the observed reduction was less severe at higher strain rates. Further, a model based on the strain rate-temperature superposition principle is reported that allows the extrapolation of the mechanical properties even beyond the conditions tested here.

Near infrared photopolymer for micro‐optics applications

AbstractNear infrared (NIR) activable photopolymers suitable for versatile fabrication of micro‐optical elements were developed. The first main objective of this article is to show that these new photopolymers can be used for microfabrication and investigate the parameters governing the microfabrication process. The impact of photonic, physico‐chemical, and chemical parameters is discussed. High quality microstructures with a good control over their size and shape are demonstrated. The second main objective is to show practical examples of microlenses and waveguides implemented on single core and multiple core optical fibers, VCSELs, and glass slides are then presented. The NIR photosensitivity of this negative tone photoresists allows using the device source itself as to start the crosslinking process, which constitutes a convenient approach for micro‐optics self‐positioning on NIR sources and justifies the interest of such NIR photopolymer for the fabrication micro‐optical elements and optical interconnects.