Advanced Photoresist Research Articles

Photochemically Activated 3D Printing Inks: Current Status, Challenges, and Opportunities.

3D printing with light is enabled by the photochemistry underpinning it. Without fine control over the ability to photochemically gate covalent bond formation by the light at a certain wavelength and intensity, advanced photoresists with functions spanning from on-demand degradability, adaptability, rapid printing speeds, and tailored functionality are impossible to design. Herein, recent advances in photoresist design for light-driven 3D printing applications are critically assessed, and an outlook of the outstanding challenges and opportunities is provided. This is achieved by classing the discussed photoresists in chemistries that function photoinitiator-free and those that require a photoinitiator to proceed. Such a taxonomy is based on the efficiency with which photons are able to generate covalent bonds, with each concept featuring distinct advantages and drawbacks.

A Critical Review of Lithography Methodologies and Impacts of Topography on 2.5-D/3-D Interposers

This paper analyzes the lithography design rules in package foundry and wafer foundry and reviews the major lithography techniques for package redistribution layer (RDL) fabrication for panel level 2.5D/3D interposers, fan-out packages and heterogeneous integration. The techniques surveyed in this paper are- contact aligners, projection aligners/steppers, laser direct writing (LDW) and laser ablation, capable of resolving routing line and space (L/S) of 0.8-1.5 μm with aspect ratio (AR) ≤ 5 and creating microvia with diameter of 1.5-2.0 μm at via pitch ≤5μm. The biggest challenge of advanced packaging is scaling the critical dimensions (CDs) on package to keep up with the pace of scaling of the bump pitch. In addition, there is a critical need for patterning fine lines-and-spaces with high aspect ratio to have low resistance traces. These high AR ultra-fine lines and ultra-fine pitch vias are crucial for meeting the needs of high-bandwidth die-to-die interconnections at high input/output (I/O) densities. All these challenges are mainly driven by lithography. With the development of advanced photoresists (PRs), the resolution factor K1 in projection lithography reduces from 0.66 to 0.39, improving the resolution by 40% without the negative impacts on the depth-of-focus (DOF). This paper also discusses the specific lithography challenges associated with topography of multi-layer RDL as well as their impacts on the fabrication of fine features. The fine pitch microvias can be a solution for scaling the I/O pitch down to 5-10 μm as a bumpless way to connect copper pads of known-good-dies to known-good- substrates in fan-out packages.

Advanced Photoresists: Development, Application and Market

Photoresist refers to the corrosion-resistant film material whose solubility changes when irradiated or radiated by ultraviolet light, electron beam, ion beam, X-ray, etc. Engraving technology is a micromachining technology that uses the solubility change of photoresist under ultraviolet light or electron beam to transfer the pattern designed on the mask to the exposed substrate. With the continuous improvement of light sources for semiconductor processing, from G line and I line to KrF(248 nm) and then to ArF(193 nm), the matching photoresist is constantly changing to meet the requirements of sensitivity, transparency and etching resistance. Nowadays, extreme ultraviolet (EUV) lithography has become recognized as the next-generation lithography technology. This paper mainly introduces the development history, classification and application of photoresists and the related characteristics, principles and process flow of photoresists in different periods and systems. At the same time, the market situation of photoresists at home and abroad is introduced and analyzed, and the prospect of domestic development is also discussed.

Sumitomo expands advanced photoresists

Sumitomo Chemical will add production lines for argon fluoride and extreme ultraviolet photolithography resists at its plant in Osaka, Japan. The expansion builds on other advanced photoresist inve...

Sumitomo expands advanced photoresists

Sumitomo expands advanced photoresists

Japan hits South Korea with controls on key chemicals

The Japanese government has imposed controls on exports to South Korea of three categories of materials that are critical to producing semiconductors and organic light-emitting diode (OLED) displays. The materials are advanced photoresists, hydrogen fluoride, and fluorinated polyimides. The controls will likely affect the operations of Korean chipmakers because of Japan’s near monopoly on advanced photoresists needed to produce the latest generations of chips. Japan is also a major producer of the grades of HF and fluorinated polyimides used in electronic manufacturing. Two South Korean firms, SK Hynix and Samsung, are among the world’s five largest and most sophisticated producers of computer chips, particularly memory chips. “Production of semiconductors in Korea will be affected,” says Mikiya Yamada, a chemical and textile analyst at Mizuho Securities, a large Japanese stockbroker. South Korean firms also dominate the production of OLED displays. The measures reflect a deterioration in the “relationship of trust” between

Cleaving Direct‐Laser‐Written Microstructures on Demand

Using an advanced functional photoresist we introduce direct-laser-written (DLW) 3D microstructures capable of complete degradation on demand. The networks consist exclusively of reversible bonds, formed by irradiation of a phenacyl sulfide linker, giving disulfide bonds in a radical-free step-growth polymerization via a reactive thioaldehyde. The bond formation was verified in solution by ESI-MS. To induce cleavage, dithiothreitol causes a thiol-disulfide exchange, erasing the written structure. The mild cleavage of the disulfide network is highly orthogonal to other, for example, acrylate-based DLW structures. To emphasize this aspect, DLW structures were prepared incorporating reversible structural elements into a non-reversible acrylate-based standard scaffold, confirming subsequent selective cleavage. The high lateral resolution achievable was verified by the preparation of well-defined line gratings with line separations of down to 300 nm.

Advanced photoresist technologies by intricate molecular brush architectures: Diblock brush terpolymer‐based positive‐tone photoresist materials

The reported diblock brush terpolymer displays the fundamental advantages of high macromolecular and regiochemical control in simultaneous top-down/bottom-up construction of advanced positive-tone photoresist materials. By selective partition the substrate adhesion, surface-active migration, and chemically-reactive deblocking functional compositions into the cylindrical polymer brush architectures, large-areas of vertical alignment of the polymers within thin films are realized. The bottom-up premeditated system enables effective top-down lithography to produce near-molecular pixel resolution (2–3 molecules) at low operation dosage.

Progress Towards Production Worthy EUV Photoresists: Balancing Litho, Outgassing and OOB Performance

Implementation of EUV Lithography (EUVL) for device high volume manufacturing (HVM) requires advanced photoresists capable of meeting the criteria of advanced logic and memory design rules. To achieve the level of performance required, resists must show excellent performance in terms of resolution, LWR (or CDU) and sensitivity. In addition, resists must meet the outgassing criteria required for HVM on the NXE toolset. Lastly, it is anticipated that resists with low OOB sensitivity will also be required. In this paper, we describe our progress in all of these areas. Based on our results, we believe we are on track to deliver production worthy resists for the EUVL era.

The preparation of a novel polymeric sulfonium salt photoacid generator and its application for advanced photoresists

With poly(4-hydroxylstyrene) (PHS) as a raw material, a new type of polymeric photoacid generator (PAG) is synthesized with sulfonium perfluoroalkyl sulfonate groups bonded onto part of the benzene rings. The phenolic hydroxyl groups are esterified to increase the solubilities of the polymeric sulfonium salts in organic solvents. Upon irradiation to 254 nm light, the sulfonium units in the polymer effectively decompose to generate sulfonic acid. The photolysis is confirmed by UV spectroscopy and the generation of sulfonic acids which catalyze the decomposition of the t-BOC protection group of the polymeric PAGs. Making use of the polymeric PAGs, positive-tone chemically amplified 248-nm photoresists can be formed by them and ester acetal polymer with high acidolytic activity. The photolithography performance of the photoresist was evaluated using a KrF laser exposure system with high photosensitivity and resolution. This novel kind of polymeric sulfonium salt PAG is applicable for advanced chemically amplified resist materials.

Molecular Structure of Advanced Photoresists

This article reviews the development of lithography technology and photoresists as well as their molecular structure for 193 nm and extreme ultravioletlithography(EUVL,the next generation lithography).Especially,we describe the recent research and development of EUV photoresists in detail for the purpose of being conducive to the domestic research on advanced photoresists.

Plasma-polymer interactions: A review of progress in understanding polymer resist mask durability during plasma etching for nanoscale fabrication

Photolithographic patterning of organic materials and plasma-based transfer of photoresist patterns into other materials have been remarkably successful in enabling the production of nanometer scale devices in various industries. These processes involve exposure of highly sensitive polymeric nanostructures to energetic particle fluxes that can greatly alter surface and near-surface properties of polymers. The extension of lithographic approaches to nanoscale technology also increasingly involves organic mask patterns produced using soft lithography, block copolymer self-assembly, and extreme ultraviolet lithographic techniques. In each case, an organic film-based image is produced, which is subsequently transferred by plasma etching techniques into underlying films/substrates to produce nanoscale materials templates. The demand for nanometer scale resolution of image transfer protocols requires understanding and control of plasma/organic mask interactions to a degree that has not been achieved. For manufacturing of below 30 nm scale devices, controlling introduction of surface and line edge roughness in organic mask features has become a key challenge. In this article, the authors examine published observations and the scientific understanding that is available in the literature, on factors that control etching resistance and stability of resist templates in plasma etching environments. The survey of the available literature highlights that while overall resist composition can provide a first estimate of etching resistance in a plasma etch environment, the molecular structure for the resist polymer plays a critical role in changes of the morphology of resist patterns, i.e., introduction of surface roughness. Our own recent results are consistent with literature data that transfer of resist surface roughness into the resist sidewalls followed by roughness extension into feature sidewalls during plasma etch is a formation mechanism of rough sidewalls. The authors next summarize the results of studies on chemical and morphological changes induced in selected model polymers and advanced photoresist materials as a result of interaction with fluorocarbon/Ar plasma, and combinations of energetic ion beam/vacuum ultraviolet (UV) irradiation in an ultrahigh vacuum system, which are aimed at the fundamental origins of polymer surface roughness, and on establishing the respective roles of (a) polymer structure/chemistry and (b) plasma-process parameters on the consequences of the plasma-polymer interactions. Plasma induced resist polymer modifications include formation of a thin (∼1–3 nm) dense graphitic layer at the polymer surface due to ion bombardment and deeper-lying modifications produced by plasma-generated vacuum ultraviolet (VUV) irradiation. The relative importance of the latter depends strongly on initial polymer structure, whereas the ion bombardment induced modified layers are similar for various hydrocarbon polymers. The formation of surface roughness is found to be highly polymer structure specific. Beam studies have revealed a strong ion/UV synergistic effect where the polymer modifications introduced at various depths by ions or ultraviolet/UV photons can interact. A possible fundamental mechanism of initial plasma-induced polymer surface roughness formation has been proposed by Bruce et al. [J. Appl. Phys. 107, 084310 (2010)]. In their work, they measured properties of the ion-modified surface layer formed on polystyrene (PS) polymer surfaces, and by considering the properties of the undamaged PS underlayer, they were able to evaluate the stressed bilayer using elastic buckling theory. Their approach was remarkably successful in reproducing the wavelength and amplitude of measured surface roughness introduced for various ion bombardment conditions, and other variations of experimental parameters. Polymer material-dependent VUV modifications introduced to a depth of about 100 nm can either soften (scission) or stiffen (cross-linking) this region, which produce enhanced or reduced surface roughness.

Photoresist Latent and Developer Images as Probed by Neutron Reflectivity Methods

Photoresist materials enable the fabrication of advanced integrated circuits with ever-decreasing feature sizes. As next-generation light sources are developed, using extreme ultraviolet light of wavelength 13.5 nm, these highly tuned formulations must meet strict image-fidelity criteria to maintain the expected performance gains from decreases in feature size. However, polymer photoresists appear to be reaching resolution limits and advancements in measurements of the in situ formed solid/solid and solid/liquid interface is necessary. This Review focuses on the chemical and physical structure of chemically amplified photoresists at the lithographic feature edge at length scales between 1 nm and 100 nm. Neutron reflectivity measurements provide insight into the nanometer-scale composition profiling of the chemical latent image at an ideal lithographic line-edge that separates optical resolution effects from materials processing effects. Four generations of advanced photoresist formulations were examined over the course of seven years to quantify photoresist/photoacid and photoresist/developer interactions on the fidelity of lithographic features. The outcome of these measurements complement traditional resist design criteria by providing the effects of the impacts of the photoresist and processing on the feature fidelity. These physical relations are also described in the context of novel resist architectures under consideration for next-generation photolithography with extreme-ultraviolet radiation.

Photoresist modifications by plasma vacuum ultraviolet radiation: The role of polymer structure and plasma chemistry

While vacuum ultraviolet (VUV) photon irradiation has been shown to significantly contribute to material modifications of polymers during plasma exposures, the impact of radiation-induced material alterations on roughness development during plasma processing has remained unclear. The authors have studied the interaction of the radiation of Ar and C4F8/Ar plasma discharges with 193 and 248 nm advanced photoresists (PRs). Optical filters were used to vary the radiation exposure wavelength range in the ultraviolet (UV) and VUV emission spectra. This enables clarification of the respective roles of plasma photon radiation wavelength and PR polymer structure on the chemical and structural changes produced in the materials. Chemical changes in polymer composition at the film surface and in the material bulk were determined by vacuum transfer x-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy. Morphological changes, film thickness reduction, and changes in surface and pattern morphology were characterized by ellipsometry, scanning electron microscopy, and atomic force microscopy. The exposure of methacrylate based 193 nm PR to photon radiation in the UV/VUV spectral range (λ=112–143 nm) leads to detachment and removal of oxygen containing polymer pendant groups to a depth of about 200 nm. This causes changes in the polymer structure by chain scission, significant film thickness reduction, and reduced pattern critical dimensions and line edge roughness. Chain-scission reactions and residual detached polymer pendant groups are expected to effectively soften layers of 193 nm PR. In contrast to 193 nm PR, styrene based 248 nm PR was found to be significantly more stable under plasma-produced irradiation due to the low oxygen content, low ester linkage concentration, and absence of lactone. Small thickness reduction, reduced oxygen loss, and cross-linking were observed in the surface region of 248 nm PR. Radiation-induced material modifications of both PR materials decreased with increasing photon wavelength in Ar discharges. Increasing modification of 193 nm PR was observed for increasing photon flux at higher wavelengths (λ=143–300 nm) by the emission characteristic of fluorocarbon containing plasmas. In C4F8/Ar plasma, the authors observed strongly increased loss of oxygen at the film surface and in the COC and CO lactone bonds in the material bulk along with film thickness reduction compared to pure Ar discharges. These modifications are directly relevant to plasma processes used for pattern transfer, which often contain fluorocarbon species.

Real-time studies of surface roughness development and reticulation mechanism of advanced photoresist materials during plasma processing

Surface roughness development of photoresist (PR) films during low pressure plasma etching has been studied using real-time laser light scattering from photoresist materials along with ellipsometric and atomic force microscopy (AFM) characterization. We show that evolution of the intensity of light scattered from a film surface can be used to study the development of surface roughness for a wide range of roughness starting from subnanometer to few hundred nanometers. Laser light scattering in combination with ellipsometry and AFM is also used to study the reticulation mechanism of 193 and 248 nm PRs during argon plasma processing. We employ a three-layer model (modified layer, rough layer, and bulk film) of the modified PR surface (193 and 248 nm PRs) to simulate and understand the behavior of ellipsometric Ψ-Δ trajectories. Bruggeman’s effective medium approximation is employed to study the roughness that develops on the surface after reticulation. When the glass transition temperature of the organic materials is reached during Ar plasma processing, the PR films reticulate and roughness develops rapidly. Roughness development is more pronounced for 248 nm PR than for 193 nm PR. Simulation of Ψ-Δ shows that the growth of roughness is accompanied by strong expansion in the materials, which is stronger for 248 nm PR than 193 nm PR. The leading factors responsible for reticulation are found to be compressive stress that develops in the modified surface layer as it is created along with strong molecular chain motion and expansion of the material when the temperature is increased past the glass transition temperature. Reticulation leads to a significantly different surface morphology for 248 nm PR as compared to 193 nm PR and can be related to differences in molecular structure and composition leading to different responses when a modified surface layer is formed by ion bombardment accompanying plasma etching.

Graded Spin-on Organic Bottom Antireflective Coating for High NA Immersion Lithography

A spin-on organic Graded Bottom AntiReflective Coating (GBARC) was developed to enable appropriate reflectivity control during advanced photoresist patterning using 193nm immersion lithography up to 1.35 NA (limits of hyper-NA lithography using water as immersion fluid). This novel technology constitutes a simplified reflectivity control approach over more conventional schemes such as dual-layer BARCs, trilayer stacks or single layer BARCs. The GBARC layer is formed upon vertical phase separation of optically tuned polymeric components present in a GBARC casting solution during the spin-coating and post-applied baking steps. The substrate surface energy is found to dictate the direction of the spatial assembly for any given phase-separating polymer blend combination. The index of refraction (n193) of the GBARC polymeric components is approximately matched to that of the photoresist and substrate layers for optimum reflectivity control, while the extinction coefficient (k193) increases gradually towards the substrate. Analytical characterization of the varying optical constants throughout the GBARC film is achieved by Variable Angle Spectroscopic Ellipsometry (VASE) which allows for a quantitative description of the optical gradient present within the GBARC layer. The optical gradient sharpness is shown to be impervious to total GBARC thickness variations at constant relative polymer mass loading as well as changes in the relative mass loading of polymeric components at fixed total GBARC thickness. Also, the link between image profile afforded by various levels of reflectivity control and the mechanical stability of photoresist nanostructures is investigated at 1.2NA by designing GBARC formulations with variable residual reflectivity. Substrate reflectivity in excess of 1% is shown to limit the focus-exposure window due to pattern collapse. Last, adhesion of 193 nm photoresist patterns to a GBARC film is compared to a commercially available single layer BARC. Enhanced adhesion is demonstrated in the case of the GBARC surface, despite the low surface energy that typifies GBARC layers

AirGaps for Ultra Low-k Interconnects Fabricated with Hardened Sacrificial Polymer

Polymers based on norbornene have been used for low-k dielectric materials, advanced photoresists, and as sacrificial materials in the formation buried air-gaps. In this study, the hardness of thin films of butylpolynorbornene - triethoxysilylpolynorbornene copolymer was enhanced by exposure to electron beam radiation. The effect of electron energy, dose, and temperature on the thermal and mechanical properties of the irradiated copolymer films was investigated using dynamic thermal gravimetric analysis and nanoindentation. The hardness of the copolymer thin films was increased by electron beam irradiation; however, the thermal stability was degraded.

Electron-beam hardening of thin films of functionalized polynorbornene copolymer

Polymers based on norbornene have been used for low-k dielectric materials, advanced photoresists, and as sacrificial materials in the formation buried air-gaps. In this study, the hardness of thin films of butylpolynorbornene-triethoxysilylpolynorbornene copolymer was enhanced by exposure to electron beam radiation. The effect of electron energy, dose, and temperature on the thermal and mechanical properties of the irradiated copolymer films was investigated using dynamic thermal gravimetric analysis and nanoindentation. The hardness of the copolymer thin films was increased by electron beam irradiation; however, the thermal stability was degraded. Electron beam exposure caused bond cleavage that led to beneficial cross linking and subsequent (undesirable) oxidation. The degree of oxidation depends on the electron energy and dose. By using a mixture of 2 vol.% H2 and 98 vol.% N2 as the electron-beam process gas and a post exposure heating in prior to air exposure, the oxidation of the copolymer was reduced, and the thermal stability was improved.

Investigation of surface modifications of 193 and 248nm photoresist materials during low-pressure plasma etching

Plasma-based pattern transfer of lithographically produced nanoscale patterns in advanced photoresist materials is often accompanied by photoresist surface roughening and line edge roughening due to factors which are not well understood. We have studied the evolution of surface roughening in prototypical 193 and 248nm photoresist materials during plasma processing as a function of plasma operating parameters. We used real-time ellipsometry and mass spectrometry, along with atomic force microscopy, x-ray photoemission spectroscopy and time-of-flight secondary ion mass spectrometry in an effort to understand the morphological and chemical changes of the photoresist materials as a function of plasma–surface interactions parameters, e.g., maximum ion energy, total energy flux, and plasma chemistry, and photoresist material. A comparison of 248nm photoresist with 193nm photoresist shows that significantly more surface roughness is introduced in the 193nm photoresist for most plasma processing conditions investigated. We also find a dramatic dependence of surface roughening on the chemistry of the plasma process, e.g., for Ar–C4F8 a modified photoresist surface layer with an extent of about 50nm is produced in 193nm photoresist, whereas for C4F8 discharges the surface modification is much less for otherwise similar conditions. We show that one important reason for these differences may be ion-enhanced selective volatilization of carbonyl groups of the 193nm photoresist polymer backbone which is absent for the 248nm material, along with modulation of the ion-interaction with the photoresist material by fluorocarbon surface passivation.

Advanced Photoresist for High-Density Groove-Recording Disc

In high-density mastering process, the relationship between the groove noise of the groove-recording disc and the molecular weight of the photoresist was examined. With a decrease in the molecular weights of novolak resin and the photoactive compound used, the shape of the groove edge became smooth and the groove noise value was reduced. By optimizing the molecular weights of novolak resin and the photoactive compound, the groove noise value is reduced by 3.5 dB at a track pitch of 0.32 µm.