Conventional Photoresist Research Articles

Absorption coefficient of metal-containing photoresists in the extreme ultraviolet

The amount of absorbed light in thin photoresist films is a key parameter in photolithographic processing, but its experimental measurement is not straightforward. The optical absorption of metal oxide-based thin photoresist films for extreme ultraviolet (EUV) lithography was measured using an established methodology based on synchrotron light. Three types of materials were investigated: tin cage molecules, zirconium oxoclusters, and hafnium oxoclusters. The tin-containing compound was demonstrated to have optical absorption up to three times higher than conventional organic-based photoresists have. The absorptivity of the zirconium oxocluster was comparable to that of organic polymer-based photoresists, owing to the low absorption cross section of zirconium at EUV. The hafnium-containing resist shows about twice as high absorptivity as an organic photoresist, owing to the significantly higher absorbance of hafnium. From the chemical composition and crystal structure, the density of the spin-coated films was determined. Using the density of the films and the tabulated data for atomic cross section at EUV, the expected absorptivity of these resists was calculated and discussed in comparison to the experimental results. The agreement between measured and expected absorption was fairly good with some substantial discrepancies due to differences in the actual film density or to thickness inhomogeneity due to the spin coating. The developed method here enables the accurate measurement of the EUV absorption of the photoresists and can contribute to the further development of EUV resists and more accurate lithographic modeling.

DREM: Infinite etch selectivity and optimized scallop size distribution with conventional photoresists in an adapted multiplexed Bosch DRIE process

The quest to sculpture materials as small and deep as possible is an ongoing topic in micro- and nanofabrication. For this, the Bosch process has been widely used to achieve anisotropic silicon microstructures with high aspect ratio. Reactive ion etching (RIE) lag is a phenomenon in which etch rate depends on the opening areas of patterns, aspect ratio of the trenches and other geometrical factors. The lag not only gives a non-uniform distribution of scallop size, but it also sets a limit for the maximum achievable aspect ratio. The latter since the mask suffers from persistent erosion. While different kinds of hard masks have been suggested to ensure a longer total etch time, here we report a correctly tuned 3-steps Bosch process – called DREM (Deposit, Remove, Etch, Multistep) – without mask erosion. The erosion-free feature is independent of the type of mask. For example, an aspect ratio of more than 50 is achieved for trenches with 1μm linewidths, while no erosion is observed for 360nm thin standard photoresists. The mechanism behind this extraordinary infinite selectivity is mainly due to the depletion of C4F8 deposition species. Furthermore, a linear ramping of the etch step duration is performed to achieve a uniform distribution of scallop sizes along the etch profile. This outperforms the usual broad scallop size distribution when no parameter ramping is performed and improves the straightness of the etch profile considerably, in addition, it allows an easier way to control periodic sidewall shaping. e.g., a sausage-chain-like feature is demonstrated with an almost perfect periodicity.

Positive‐tone photoresist consisting of a multifunctional acrylate resin that can be patterned after photo‐crosslinking

ABSTRACTA multifunctional acrylate resin having a tertiary ester structure is synthesized using a thiol‐acrylate Michael addition click reaction. By loading both a photo radical initiator and a photo acid generator into this resin, a positive‐tone photoresist, that can be patterned after photocrosslinking (PPaP), is formulated. After the thin film is irradiated with 365 nm light, radical crosslinking proceeds and a solvent resistant “frozen” film can be obtained. The frozen film works as a positive‐tone photoresist. When the frozen PPaP is irradiated with light containing a wavelength much shorter than 300 nm, the ester structure of the exposed region is decomposed by photo‐chemically generated acid and dissolved in an alkaline solution. Furthermore, a photochemical pattern transfer is achieved using the material. By performing the photo exposure on conventional photoresist patterns on the frozen PPaP underlayer, the photoresist patterns are successfully transferred to the underlayer. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 45871.

Thermally Activated Gas-Phase Chemical Reactions in a Single-Wafer Wet Clean Process

A conventional high-dose ion-implanted photoresist strip and clean process flow consists of using a dry remote plasma ashing tool to remove the crust layer and underlying photoresist formed during the implant, followed by sulfuric-peroxide mixture (SPM) and ammonium hydroxide-hydrogen peroxide (SC1) chemistries applied on a wet clean tool to remove the remaining bulk photoresist and implant residues. However, film loss control and residue removal are increasingly difficult to maintain for sub-10 nm devices with conventional approaches. We investigate an alternative sequence that replaces the remote plasma strip with a thermally activated, atmospheric gas-phase oxidation process and replaces the SPM chemistry with a de-ionized water/ozone (DIO3) wet clean. The alternative sequence is entirely processed in a modified single-wafer wet clean chamber. For the oxidation process, a dry O3/O2 gas mixture decomposes and reacts directly with the ion-implanted photoresist wafer that is heated above 300°C from the backside. 9,000 Å/min strip rates are measured on 193 nm photoresist, and 650 Å/min are measured for the implanted crust removal. Based on both X-ray photoelectron spectroscopy (XPS) analysis of the reacted surface as well as the composition of the gas phase products by residual gas analysis (RGA), the crust layer and bulk photoresist are removed by oxidation mechanisms similar to remote oxygen plasma strip processes. Above 350°C, it is found that O2 gas itself reacts with the photoresist but to a lesser extent than the O3/O2 mixture. Reactivity to silicon is lower than a typical remote oxygen plasma based on ~1 Å silicon loss measured on blanket films. This in-situ thermally activated oxidation reaction allows for subsequent wet processing without any queue time delay. The integrated process completely removes 5E15-1E16 at/cm2 high-dose implanted 193 nm photoresist with no residue on patterned substrates. Fig 1: In-situ thermally activated oxidation reaction in a single-wafer wet clean chamber process sequence consisting of (a) a dry ozone-oxygen gas environment, (b) ozone-oxygen decomposition and reaction on a heated wafer surface, (c) ozonated de-ionized water rinse in an N2 environment, (d) ammonium hydroxide-hydrogen peroxide clean in an N2 environment, and (e) a de-ionized water rinse and spin dry. Figure 1

Thermally Activated Gas-Phase Chemical Reactions in a Single-Wafer Wet Clean Process

We investigate an alternative approach to conventional high dose ion-implanted photoresist strip and clean that replaces a remote plasma strip with a thermally activated, atmospheric gas-phase oxidation process and replaces sulfuric-peroxide mixture (SPM) chemistry with a de-ionized water/ozone (DIO3) chemistry. The gas phase oxidation process is first characterized in a mini-test chamber. Then the alternative dry and wet clean sequence is entirely processed in a modified single-wafer wet clean chamber. The atmospheric dry O3/O2 gas mixture decomposes and reacts with the ion-implanted photoresist wafer that is heated above 300 °C from the backside. 9,000 Å/min and 600 Å/min removal rates are measured on 193 nm photoresist and high dose implanted crust, respectively. Silicon reactivity from ellipsometry measurements is lower than a typical remote oxygen plasma based on ~1 Å silicon loss. The integrated process completely removes 5E15-1E16 at/cm2 high dose implanted 193 nm photoresist with no residue remaining on patterned substrates.

Chemical fixation of SO2using norbornene with different side-chain groups and control of the physical properties of poly(olefin sulfone)

Photoresists involving polymer solubility and degradation play a crucial role in the manufacture of integrated circuits by photolithography. Conventional photoresists typically rely on a single switching mechanism based on a change in either polarity or molecular weight of the polymer. Here we report a new type of poly(olefin sulfone) as a photoresist which overcomes the drawback of poor dry etching resistance, solubility and thermal degradation. The effects of different side-chain groups (acetyl, acetoxy and carboxylic acid) of norbornene on the properties of poly(norbornene sulfone)s were analyzed with the aim of developing a new type of photoresist. Thermogravimetry and gel permeation chromatography results revealed that bond breaking occurred at different temperatures, because of the different side-chain groups, and the three side-chain groups affect the solubility and stability of the poly(norbornene sulfone)s. A novel photoresist material based on a poly(olefin sulfone) with different side-chain groups was synthesized and its physical properties examined. © 2017 Society of Chemical Industry

Metal Conductive Surface Patterning on Photoactive Polyimide

Current fabrication methods for metal interconnects and contacts are generally based on conventional photoresist fabrication procedures that require expensive equipment and multiple material/time‐consuming steps. In this work, a photopatternable polyimide is synthesized via the copolymerization of a functional diamine monomer with a 1,4‐dihydropyridine side‐chain which can decompose under UV irradiation into a pyridine group—a promising ligand for palladium ions. After the absorption of palladium ions, the electroless copper plating is carried out to form metal patterns of copper. Copper patterns with smooth boundaries are confirmed by scanning electron microscope and atomic force microscope. Robust interfacial bonding between the copper and the polyimide film is evidenced by Scotch tape adhesion tests. The photopatternable polyimide has the advantages of low Pd consumption, easy operation without expansive equipment. The linear thermal expansion coefficient of the photopatternable polyimide remains close to the one of copper wire, demonstrating the adaptability of the photopatternable polyimide for integrated circuit application. This work presents the approach of (i) the synthesis of a novel photopatternable polyimide and (ii) its application for making flexible conductive metal structures and patterned metal interconnects, which can be expected to have tremendous potential in the field of flexible electronics.

Metallization and electrical characterization of platinum thin film microelectrodes on biocompatible polydimethylsiloxane substrates for neural implants

Microfabrication technology in the biomedical field has provided microelectrode arrays for neural implants with new development opportunities. The need for more complex physiological functions and miniaturization, as well as the use of new materials for more flexible electrodes, can now be satisfied. PDMS (Polydimethylsiloxane) substrates are elastic, biocompatible and permeable to oxygen, in addition to being a highly stable material. However, the implementation of microfabrication techniques such as deposition and patterning processes on these new substrates is not straightforward. This paper will describe the development of a reliable method to metalize thin film microelectrodes on a highly flexible medical grade PDMS layer that is suitable for long-term implantation. Platinum (Pt) microelectrodes were deposited by physical vapor deposition and pattern transfer by lift-off was chosen. Standard photolithography was used to pattern a conventional positive photoresist and was optimized to improve adhesion and to avoid cracks in the resist. The electrical behavior of the metal-polymer interface was analyzed using multiplexed DC measurements. The resistance values of seven samples and a control were acquired sequentially. Special attention was paid to the connectorization from the flexible microelectrodes to a rigid substrate. Measurements were carried out in an air-protected environment as well as in a biological environment designed to mimic the environment of the human body. Long-term stability of the Pt-PDMS interface was strongly influenced by the electrode configuration and its connection. A characteristic electrical behavior was observed for a straight-electrode configuration. This configuration demonstrated significant drift of resistance values of more than 4% during the initial 56h. By contrast, a stable behavior was observed for a loop electrode design, with only small variations of less than ±0.5% caused by thermal fluctuations.

Ion beam etching of multilevel masking layers written by two-photon lithography

Ion beam etching (IBE) provides high surface quality. Finding suitable masking layers is one of the key issues for process optimization. In case of high-intensity and long-term IBE conventional photoresists are not appropriate as masking layers. As an alternative in terms of thermal durability, photoresist masking layers polymerized with two-photon lithography were investigated here and their IBE etch rates were measured. A hard bake (200 °C) lowered them due to higher crosslinking without an alteration of the structure shapes.Two-photon lithography enables the fabrication of multilevel structures which can be etched in one process step. Two types of 3D masking layers were transferred into fused silica to demonstrate this approach. Diffractive structures were chosen because their diffraction efficiency benefits from the high surface quality provided by IBE and it is influenced by fabrication induced deviations of the geometry: 3D line gratings with overlapping photoresist areas are a new approach to avoid delamination problems without the necessity of the integration of unwanted gaps into the resist patterns. Measurements proved good agreement of the diffraction efficiency with simulated results, differ only by 1.14%. The transfer of blazed grating structures illustrated the effect of the angle dependence of the etch rate. The transferred structures showed good agreement with the step heights forecast on the basis of process selectivity.

Selective-area growth and controlled substrate coupling of transition metal dichalcogenides

Developing a means for true bottom-up, selective-area growth of two-dimensional (2D) materials on device-ready substrates will enable synthesis in regions only where they are needed. Here, we demonstrate seed-free, site-specific nucleation of transition metal dichalcogenides (TMDs) with precise control over lateral growth by utilizing an ultra-thin polymeric surface functionalization capable of precluding nucleation and growth. This polymer functional layer (PFL) is derived from conventional photoresists and lithographic processing, and is compatible with multiple growth techniques, precursors (metal organics, solid-source) and TMDs. Additionally, we demonstrate that the substrate can play a major role in TMD transport properties. With proper TMD/substrate decoupling, top-gated field-effect transistors (FETs) fabricated with selectively-grown monolayer MoS2 channels are competitive with current reported MoS2 FETs. The work presented here demonstrates that substrate surface engineering is key to realizing precisely located and geometrically-defined 2D layers via unseeded chemical vapor deposition techniques.

Photoactive Polymers with Benzocyclobutene/Silacyclobutane Dual Crosslinked Structure and Low Dielectric Constant

ABSTRACTLow dielectric photopatternable materials have aroused much interest owing to their potential application as alternatives of conventional photoresists. Although a number of photosensitive groups have been established to construct photopatternable materials for many years, it is still a challenge to introduce them into polymer chains via a facile and control polymerization technique, such as living free radical polymerization. In this work, on the basis of the photoactive silaycyclobutene moiety, a new monomer, 1‐methyl‐1‐(4‐vinylphenyl) silacyclobutane (1‐MVPSCB), was synthesized and demonstrated successful to conduct atom transfer radical polymerization (ATRP). Subsequently, the copolymerization of 1‐MVPSCB with 4‐vinylbenzocyclobutene (4‐VBCB) and styrene produced a kind of benzocyclobutene/silacyclobutane double‐crosslinked polymer (BS‐DCP) with controlled benzocyclobutene/silacyclobutane ratios and low polydispersity index. Owing to the UV/thermally dual crosslinked structure, BS‐DCP possesses photapatternability, high dimensional stability and low dielectric constant (2.37 at 10 MHz). These properties make BS‐DCP a potential photoresist that could be directly used as interconnected dielectrics. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 1920–1928

Use of Hard Mask for Finer (<10 μm) Through Silicon Vias (TSVs) Etching

Through silicon via (TSV) technology holds the promise of chip-to-chip or chip-to-package interconnections for higher performance with reduced signal delay and power consumption. It includes high aspect ratio silicon etching, insulation liner deposition, and seamless metal filling. The desired etch profile should be straightforward, but high aspect ratio silicon etching is still a challenge. In this paper, we investigate the use of etch hard mask for finer TSVs etching to have clear definition of etched via pattern. Conventionally employed photoresist methods were initially evaluated as reference processes, and oxide and metal hard mask were investigated. We admit that pure metal mask is rarely employed in industry, but the etch result of metal mask support why hard mask are more realistic for finer TSV etching than conventional photoresist and oxide mask.

Unconventional Use of a Photoresist as a Nitrogen Gas Generator Forming Transparent Dome‐Shaped Microcavities

The phenomenon that a common positive photoresist (PR) releases nitrogen (N2) gas when exposed to UV light has never been used or received any particular attention. Fabrication process to intentionally trap the generated N2 gas during UV exposure, thereby, to form the microcavity structure is proposed. Various dome‐shaped structures are demonstrated with the proposed process. This process is best suited to make various cavity packages in microelectromechanical systems. Also, microlens arrays are fabricated using plastic replication of the dome‐shaped microcavities. All these results lead to a different view of the conventional positive photoresist and inspire various new applications.

Lift-off patterning of multi-walled carbon nanotube and PEDOT:PSS composite films with fluorinated polymer templates

This paper reports a micro patterning method for multi-walled carbon nanotubes (MWCNTs), whose patterns were defined by a poly(1H,1H,2H,2H-perfluorodecyl methacrylate) (PFDMA) mask in a lift-off process. The PFDMA layer on the patterned polydimethylsiloxane (PDMS) mold was transferred to a polyethylene naphthalate (PEN) film using a Micro-Contact Printing (μCP) technique. In contrast to conventional photoresist masks, the PFDMA polymer template was not damaged by the solvent used for the MWCNT dispersion, isopropyl alcohol (IPA), which enabled the precise and clean deposition of MWCNTs. Moreover, a hydrofluoroether solvent, which was used to remove the PFDMA template, did not show any signal for chemical interactions with the PEN substrate or poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). This orthogonal character led to the successful lift-off patterning of MWCNTs/PEDOT:PSS composite films. The micro-patterned composite films could be examined further as a thin film heater with a bending radius of 15mm, generating enough heat to reach a maximum temperature of 53°C with an applied voltage of 30V.

Fabrication of GaAs/AlAs micro-pillar cavities including low-density InAs quantum dots and their photoluminescence properties

Low-density InAs quantum dots (QDs) were grown by intermittent supply method in molecular beam epitaxy. QD density was controlled in a wide range of 106 to 109 cm−2 by in-situ monitoring of RHEED specular-beam intensity. Low-density InAs QDs were embedded into GaAs/AlAs micro-pillar cavity structure, which was fabricated by anisotropic wet-etching through a conventional photoresist mask. Photoluminescence (PL) spectra of single InAs QD in the micro-cavity were analyzed, and its PL intensity was three times higher than that without the cavity.

Lift-off patterning of Ag nanowire/PEDOT:PSS composite films for transparent electrodes using a fluoropolymer structure

This paper describes a lift-off method of Ag nanowire (Ag NW) patterning using a poly(1H,1H,2H,2H-perfluorodecyl methacrylate) polymer (PFDMA) structure as a mask which is prepared by micro-contact printing. Unlike a conventional photoresist mask, the PFDMA polymer is inert to the dispersion solvent of Ag NW. In addition, the hydrofluoroether solvent used for removing the mask layer of patterned PFDMA films after Ag NW deposition does not chemically affect the polyethylene naphthalate (PEN) substrate or poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) coated on Ag NW layer. In this method, Ag NW/PEDOT:PSS composite films were patterned and the effects of the hot-press method were examined to further improve the electrical and optical properties of the composite films. Moreover, the hot-press method at 110°C has an advantage of applying low pressure to make Ag NW/PEDOT:PSS embedded into PEN films compared to that of pressing samples without heating. The ratio of resistance change of patterned and hot-pressed composite film was only below 1% after repeated bending test.

Polyimide-Damage-Free, CMOS-Compatible Removal of Polymer Residues from Deep Reactive Ion Etching Passivation

A method for removal of passivation polymer residues from deep reactive ion-etching (DRIE) has been systematically investigated in this study. The method combines dry oxygen plasma ashing and conventional photoresist wet stripping. Samples were carefully examined by x-ray photoelectron spectroscopy (XPS), energy-dispersive x-ray spectroscopy (EDX), and study of surface morphology. XPS and EDX analysis showed that the polymer residues consisted mainly of C-O, CF x (x = 1, 2, 3), and C-CF bonds. Optimized oxygen plasma ashing effectively removes most of the fluorocarbon content, except some nano-residues. Subsequent conventional wet stripping in organic solvents could eliminate these stubborn nanoparticles while dissolving the underlying photoresist. Excellent removal is apparent from scanning electron microscopy images. The fluorine content determined by EDX analysis showed that the residues were completely removed. The metal layers, oxide insulator layers, and the polyimide insulators function well after this critical surface treatment. The excellent results show this is an outstanding method for removal of DRIE passivation polymer residues for MEMS fabrication.

Fabricating microfluidic valve master molds in SU-8 photoresist

Multilayer soft lithography has become a powerful tool in analytical chemistry, biochemistry, material and life sciences, and medical research. Complex fluidic micro-circuits require reliable components that integrate easily into microchips. We introduce two novel approaches to master mold fabrication for constructing in-line micro-valves using SU-8. Our fabrication techniques enable robust and versatile integration of many lab-on-a-chip functions including filters, mixers, pumps, stream focusing and cell-culture chambers, with in-line valves. SU-8 created more robust valve master molds than the conventional positive photoresists used in multilayer soft lithography, but maintained the advantages of biocompatibility and rapid prototyping. As an example, we used valve master molds made of SU-8 to fabricate PDMS chips capable of precisely controlling beads or cells in solution.

Progresses and Challenges of EUV Lithography Materials

In the last years the continuous efforts on the development of EUV lithography has allowed to push the lithographic performances of the EUV photoresists on the ASML NXE:3100 full field exposure tool at imec. The latest chemically amplified photoresists can reach an ultimate resolution of 16 nm and 24 nm for line-space (L/S) and dense contacts (CH), respectively, but the major issue on EUV photoresists remains to simultaneously meet resolution, sensitivity, line-edge roughness (LER) for LS and local CD uniformity (LCDU) for CH, suggesting that the desired performance characteristics of EUV photoresists may require the development of new EUV materials. Aiming to this, imec has recently started a new project to look into novel materials for EUV lithography to explore alternative approaches that can offer superior characteristics in photoresist imaging: improved LER and line collapse, high sensitivity and high etch resistance. In this paper we report the first results from the exploration of new EUV alternative materials and the latest results from the conventional EUV photoresist evaluation and process optimization at imec towards the ASML NXE:3300 full field exposure tool.

Directional Oil Sliding Surfaces with Hierarchical Anisotropic Groove Microstructures

A robust directional oil sliding surface is presented by utilizing re-entrant micro-groove arrays inspired from the microgrooves of rice leaf. The overhang micro-groove arrays are shown to provide two primary goals of "omniphobicty" and "anisotropic sliding" with DI water (γlv = 72.1 mN/m) as well as mineral oil (γlv = 28 mN/m) and conventional photoresist.