Photoresist Surface Research Articles

Impact of process parameters on the flow behavior and filling accuracy of photoresist in hot embossing for microgroove array

Hot embossing is a cost-effective and efficient method for fabricating micro-nano structures. However, challenges arise from the flow behavior and rebound phenomena of photoresists during hot embossing, leading to issues with filling accuracy and subsequently degrading optical properties. Therefore, elucidating the impact of process parameters on filling accuracy in hot embossing is crucial. Initially, a polydimethylsiloxane (PDMS) soft mold was fabricated by replicating microgroove arrays from a nickel-phosphorus (Ni-P) mold, which was produced using ultra-precision fly-cutting techniques. Subsequently, the microgroove morphology from the PDMS soft mold was transferred onto the photoresist surface through hot embossing. A numerical model for photoresist filling accuracy was established and experimentally validated. The filling mechanism of photoresist in hot embossing was elucidated. Polymer viscoelasticity decreased with increasing temperature, substantially enhancing filling accuracy, followed by a gradual decrease due to rebound phenomena. Additionally, higher embossing forces enhanced the filling ratio, but excessive pressure led to the deformation of the soft mold. Process parameters were optimized through numerical simulation and experimentation, achieving processing errors at the hundred-nanometer level and a filling ratio of 97.3 %.

Strain‐Modulated Hydrogen Production Performance in Monolayer MoS2 Electrocatalysis Nanodevices

AbstractThe integration of flexible micro‐ and nanodevices plays a pivotal role in investigating stress‐enhanced performances and underlying intrinsic mechanisms for two‐dimensional materials. This study presents the fabrication of single‐crystal flexible devices using monolayer MoS2 and its catalytic activities for the hydrogen evolution reaction under stress conditions. A metallic conductive layer was deposited on the photoresist surface via magnetron sputtering, overcoming the challenges associated with lithography on insulating substrates using electron beam lithography (EBL). The results demonstrate optimal etch patterns with a metal modification layer thickness of 10.97 nm. Leveraging this flexible device fabrication process, a single‐layer MoS2 single‐nanosheet flexible micro/nano device was developed and subsequently strain‐modulated (stretched along the zigzag lattice direction with the armchair lattice direction as the axis). A significant enhancement is observed in the electrocatalytic hydrogen evolution performance as the strain increases from 0 % to 0.40 %. Notably, the onset overpotential decreased from 155.6 to 95.7 mV, and the Tafel slope decreased from 175.3 to 98.6 mV dec−1. This study provides new insights into the design and performance of strain devices for two‐dimensional (2D) monocrystalline/polycrystalline materials.

Effect of fabrication process on contact resistance and channel in graphene field effect transistors

Contact resistance, as one of the main parameters that limits the performance of graphene-based transistors, is highly dependent on the metal-graphene contact fabrication processes. These processes are investigated and the corresponding resistances are measured based on the transfer length method (TLM). In fabrication processes, when annealing is done on chemical vapor deposition (CVD)-grown graphene samples that are transferred onto SiO2/Si substrates, the adhesion of graphene to the substrate is improved, and poly methyl methacrylate (PMMA) residues are also reduced. When the metal deposition layer is first applied to the graphene, and then, the photolithography process is performed to define the electrodes and graphene sheet, the graphene-metal contact resistance is better than that in other methods due to the removal of photoresist residues. In fact, by changing the sequence of the fabrication process steps, the direct contact between photoresist and graphene surface can be prevented. Thus, the contact resistance is reduced and conductivity increases, and in this way, the performance of graphene transistor improves. The results show that the fabrication process has a noticeable effect on the transistor properties such as contact resistance, channel sheet resistance, and conductivity.‌ Here, by using the annealing process and changing the order of photolithography processes, a contact resistance of 470 Ω μm is obtained for Ni-graphene contact, which is relatively favorable.

Design of microfluidic radionuclide sensors: Combining microscale 3D printing based on 2-photon polymerization with nanoscale polymer brush scintillators

Development of miniature sensors for radionuclides is a challenging goal. In this paper we have applied specially developed vinyl-functionalized organic fluorophores for fabrication of nanoscale polymer brush scintillator coatings on artificially created 3D mesoporous structures. The applied 3D printing based on 2-photon polymerization is a modern approach, which extends the concept of additive manufacturing to the micro/nano-scale. One of the fundamental limitations of the 3D printing based on 2-photon polymerization (2PP) is a limited range of suitable photo-resins and, thus, limited functionalities of the resulting structures. To address this challenge, various post-processing technological steps, such as overcoating with conformal oxide layers and/or pyrolysis have been suggested. In this study, we demonstrate how chemical modification of 3D printed polymer structures with nanoscale polymer brush scintillators can be applied as a viable technological strategy for microfluidic radionuclide sensors. Application of new polymerizable organic fluorophores allowed “grafting to” or “grafting from” modifications of the photoresist surface with high concentration and precisely controlled location of the active scintillating units within the coating.

Adhesion and collapse of extreme ultraviolet photoresists and the role of underlayers

Pattern collapse and photoresist scumming are major limiting factors to achieve a failure-free process window in extreme ultraviolet lithography. Previous works on this topic have empirically proven the importance of matching photoresist and underlayer surface energy, and the role played by developer liquid in wet development. In this work, we extend those concepts and formulate a figure of merit for the free energy at the exposed and unexposed photoresist-underlayer-developer interfaces. This figure of merit provides a tool to optimize the underlayer surface energy components that best match a given photoresist and developer process. The model is tested against experimental patterning of a chemically amplified resist at pitch 32 nm and pitch 80-nm line spaces, successfully predicting the likelihood of pattern collapse and photoresist scumming. Moreover, we write a quantitative expression for the peeling force acting on photoresist lines owing to unbalanced capillary forces and the threshold energy at which film delamination onsets. It is shown that adhesion and scumming are two manifestations of the same phenomenon at these interfaces.

Study of the Pattern Preparation and Performance of the Resistance Grid of Thin-Film Strain Sensors.

The thin-film strain sensor is a cutting-force sensor that can be integrated with cutting tools. The quality of the alloy film strain layer resistance grid plays an important role in the performance of the sensor. In this paper, the two film patterning processes of photolithography magnetron sputtering and photolithography ion beam etching are compared, and the effects of the geometric size of the thin-film resistance grid on the resistance value and resistance strain coefficient of the thin film are compared and analyzed. Through orthogonal experiments of incident angle, argon flow rate, and substrate negative bias in the ion beam etching process parameters, the effects of the process parameters on photoresist stripping quality, etching rate, surface roughness, and resistivity are discussed. The effects of process parameters on etching rate, surface roughness, and resistivity are analyzed by the range method. The effect of substrate temperature on the preparation of Ni Cr alloy films is observed by scanning electron microscope. The surface morphology of the films before and after ion beam etching is observed by atomic force microscope. The influence of the lithography process on the surface quality of the film is discussed, and the etching process parameters are optimized.

Self-Organized Liquid Crystal Droplets as Phototunable Softmasks.

A single-step self-organized pathway is harnessed to generate large-area and high-density liquid-crystal (LC) microdroplets via rapid spreading of an LC-laden volatile liquid film on an aqueous surfactant bath. The surfactant loading on the water bath and LC loading in the solvent fluid help in tuning the size, periodicity, and ordering of LC microdroplets. Remarkably, the experiments reveal a transition from a spinodal to heterogeneous nucleation pathway of dewetting when the surfactant loading is modulated from below to beyond the critical micellar concentration in the aqueous phase. In the process, a host of unprecedented drop formation modes, such as dewetting and contact-line instability, random ejection, and "fire cracker" toroid splitting, have been uncovered. Subsequently, the LC microdroplets on the air-water interface are employed as photomasks suitable for soft-photolithography applications. Such masks help in the decoration of a host of mesoscale three-dimensional features on the films of photoresists when photons are guided through the LC droplets. In such a scenario, phase transition of LC droplets under solvent vapor annealing is employed to control the movement of photons through drops and subsequently modulate the light exposure on the photoresist surface. Such a simple soft-photolithography setup leads to an array of flattened droplets on a positive resist, while donut features are observed on the negative tone. Remarkably, the orientation of nematogens within 4-cyano-4'-pentylbiphenyl droplets and at the three-phase contact-line provides additional handles in controlling the transmission of photons, which facilitates such a unique pattern formation. A number of low-cost and simple strategies are also discussed to order such soft-photolithography patterns. Importantly, with a minor modification to the same experimental setup, we could also measure the variation in the order parameter of the LC droplet during its phase transitions from the nematic to isotropic state.

Quasi-Random Gratings Enabled by Wrinkled Photoresist Surfaces on a Rigid Substrate.

Micro- and nanoscale surface wrinkling has been widely studied in artificial systems, mostly in soft substrates like polydimethylsiloxane or polystyrene, where the wrinkling dynamics are triggered by thermal stresses or tensile prestrains. Here, we introduce a new wrinkling regime based on photoresist layers on top of a rigid substrate. By introducing a bending deformation, combined with fluorine-based plasma treatment, wrinkles with a characteristic wavelength less than 1 μm can be created. By adding micropatterns on photoresists with standard UV exposure, ordered wrinkles can also be realized. This technique is demonstrated to be applicable in several commercially available photoresists, and the wrinkled patterns can be employed conveniently to create high-aspect-ratio silicon gratings and large-area silicon dioxide membranes. This unique strategy broadens the spectrum of available materials to create wrinkled surfaces in a controllable manner and provides a platform for the easier fabrication of wrinkle-based devices.

Characterization of Surface Variation of Chemically Amplified Photoresist to Evaluate Extreme Ultraviolet Lithography Stochastics Effects

Extreme ultraviolet (EUV) lithography is required for advanced node semiconductor device fabrication. The stochastic effects in EUV lithography are problematic, especially with regards to pattern roughness and defect formation. In this study, we performed atomic force microscopy (AFM) on an EUV photoresist surface to determine the surface roughness, height histogram, line scan, area ratio, and power spectral density (PSD). Polymethyl methacrylate (PMMA) for nonchemically amplified resist (non-CAR), and poly(4-hydroxystyrene)(t-butyl acrylate) copolymer (PHS:tBA) and poly(4-hydroxystyrene)(polystyrene)(t-butyl acrylate) copolymer (PHSPS:tBA) with/ di-(t-butylphenyl)iodonium perfluorobutane sulfonate (TBPI-PFBS)/tetrabutylammonium lactate (TBAL) for chemically amplified resist (CAR) were examined. In this CAR system, the exposure and dark loss contributed to the surface variation of root mean square (RMS) of 1.5 nm and 0.95 nm under a nominal exposure dosage of 8 μC/cm2. The contribution of dark loss was further evaluated from the effects of backbone polymer composition and photoacid generator (PAG) loading. The dark loss induced surface roughness can be attributed to the competition of etch selectivity in the resist components. A skewness of the height histogram and change of correlation in PSD are related to the dark loss induced surface variation.

In situ photografting during direct laser writing in thermoplastic microchannels

A method for in situ photografting during direct laser writing by two-photon polymerization is presented. The technique serves as a powerful approach to the formation of covalent bonds between 3D photoresist structures and thermoplastic surfaces. By leveraging the same laser for both pattern generation and localized surface reactions, crosslinking between the bulk photoresist and thermoplastic surface is achieved during polymerization. When applied to in-channel direct laser writing for microfluidic device fabrication, the process yields exceptionally strong adhesion and robust bond interfaces that can withstand pressure gradients as high as 7 MPa through proper channel design, photoinitiator selection, and processing conditions.

Photoresist-enabled assembly of BN/graphene/BN heterostructure and fabrication of one-dimensional contact electrode

A poly(methyl methacrylate) (PMMA) substrate is easily soluble in acetone and cannot withstand high temperatures, thereby restricting the application of graphene or boron nitride (BN) on it. Furthermore, the assembly mechanism of a BN/graphene/BN heterostructure directly determines the performance of a device. In this paper, we report the single-spin photoresist stacking transfer assembly (SPSTA) of a BN/graphene/BN heterostructure on a PMMA substrate using a photoresist as a support layer. The photoresist served as a protective layer for the retained BN/graphene/BN heterostructure. The excess BN/graphene/BN heterostructure was etched away by oxygen plasma, following which a metal was evaporated on the photoresist surface. As metal is impervious to light, the excellent light transmittance of the PMMA substrate could be utilized. After the photoresist was denatured by ultraviolet light exposure on the back of the substrate, it was dissolved by a sodium hydroxide (NaOH) solution, and a one-dimensional contact of the BN/graphene/BN heterostructure and metal was achieved. Finally, through different testing methods, we found that the SPSTA of the BN/graphene/BN heterostructure yields a smooth morphology and high electrical conductivity with a uniform sheet resistance. We examined the air failure of the BN/graphene/BN heterostructure and found that its SPSTA was stable. Our study realized the transfer of two-dimensional (2D) materials on PMMA substrates for the first time, overcame the membrane surface pollution caused by the traditional BN/graphene/BN heterostructure assembly process, realized the fabrication of BN/graphene/BN heterostructure devices on PMMA substrates for the first time, and offers important insights for the application of graphene and BN or other 2D materials on PMMA substrates.

An approach for one dimensional periodic arbitrary lithography based on Fourier series

In interference lithography, 1-dimensional (1D) Fourier series expansion (FSE) technique can be used to create 1D periodic arbitrary patterns. Since the energy of electric field can change the solubility of photoresist, the required electric field intensity to create the desired pattern should be found. Therefore, in this article, an inequality for the magnitude of electric field on the surface of photoresist is addressed. A solution to this equation is provided with one degree of freedom. Intelligent selection of the parameters corresponding to this degree of freedom enhances the performance of the system seriously. The proposed method confirms that our approach can produce subwavelength resolutions. Based on these achievements, we present a new lithography system that can generate 1D periodic arbitrary patterns.

A polyvinyl alcohol microlens array with controlled curvature on discontinuous hydrophobic surface

In this paper, a facial method was used to fabricate polyvinyl alcohol (PVA) microlens arrays (MLAs) with controllable focal lengths on discontinuous hydrophobic surfaces. To this end, a hydrophobic materials-based solution was spin-coated onto the patterned photoresist surface. After solvent evaporation, a discontinuous hydrophobic surface was obtained by removal of the photoresist. The as-obtained microhole patterned hydrophobic layer modified substrate exhibited hydrophilic and hydrophobic properties inside and outside the microholes, respectively. The scribing of PVA solution allowed its adherence to the hydrophilic area to form a droplet self-assembly due to surface tension. PVA MLAs with radii of 50 μm, 100 μm, 150 μm, 200 μm, 250 μm, 300 μm were obtained to demonstrate the suitability of the proposed method. The curvature of MLAs can be adjusted by controlling the volume of PVA solution in the microholes or changing their sizes. In sum, the proposed PVA MLAs with high optical performances, easy fabrication, and good mechanical properties look promising for future applications in image processing, light extraction, protein detection, light-emitting diodes, sensors, and displays.

Nanohole morphologies on photoresist surface produced by low-energy Ar+ ion bombardment under normal and near-normal incidence

Random nanohole morphologies on a photoresist surface were produced spontaneously by ion bombardment, from normal to near-normal incidence. At an optimized ion energy, the critical parameters (diameter and depth) and uniformity of these nanoholes can be tailored with multi-ion parameters, including ion fluence (i.e. bombardment time), ion flux, and incidence angle. The temporal evolution of the nanohole structures, at normal incidence, showed growth and stabilization regimes. The mass redistribution effect and the distribution of energy deposition led to the transition from nanoholes to nanoripples, which were deduced from the crater shape evolution at low incidence angles. The incidence angle-dependent evolution of the crater shape (or rim) on the resist surface is consistent with the crater function simulation, and confirms the ion bombardment-induced surface atomic currents and mass redistribution. Finally, X-ray photoelectron spectroscopy characterization confirmed ion bombardment-induced decomposition, in particular, graphitization in the surface layer of the resist.

Energy dependence of morphologies on photoresist surfaces under Ar+ ion bombardment with normal incidence

The energy dependence of nanostructures on a photoresist produced by ion bombardment (IB) under normal incidence is studied through atomic force microscopy and time-of-flight secondary ion mass spectroscopy (ToF-SIMS). The energy-dependent morphology evolved from weak islands via nanoholes to a smooth surface on the resist; in particular, nanoholes were produced for a broad energy range of 300–550 eV. The enrichment of light components in the surface layer of the irradiated resist was illustrated owing to the strong decomposition of the photoresist by IB. The energy dependence of the morphologies is explained according to the ToF-SIMS characterization and the existing theoretical models of IB, which is an IB-induced synergy of chemical variation and sputtering. This study extends the IB in inorganic materials into an organic-multicomponent photoresist and provides new insights into the experimental evidence for nanoholes and possible parameters to improve the relevant theoretical model.

Fiber Lithography: A Facile Lithography Platform Based on Electromagnetic Phase Modulation Using a Highly Birefringent Electrospun Fiber.

Lithography plays a key role in advancing manufacturing as well as the semiconductor industry. However, the currently available state-of-the-art lithography methods still require access to expensive tools and facilities. Herein, we suggest a novel lithography method based on electromagnetic phase modulation of ultraviolet using a highly birefringent electrospun fiber to overcome such limitations. The optical birefringent effect, by which the phase of incident ultraviolet electromagnetic fields is retarded when passing through optically anisotropic media, is combined with semicrystalline poly(ethylene oxide) (PEO)-poly(ethylene glycol) (PEG) polymeric microfibers patterned in a programmable form using near-field electrospinning. By positioning the mask between two linear polarizers that are perpendicular to each other, only the UV waves that are passing through the fibers can be selectively utilized to exhibit lithographic property. Therefore, the UV intensity on the photoresist (PR) surface follows the shape of the fiber pattern, enabling precisely controlled patterning of the photoresist. Zero- to two-dimensional key features of lithography are achieved, including straight, curved, array, and isolated patterns. Facile optical alignments without using dedicated alignment marks are successfully demonstrated, as well as various applications including micro- to macroscale serpentine, tree, and antenna circuit patterns on a flexible substrate. The presented approach, packed in a table-top scale, is expected to provide a practical and affordable lithography solution by leveraging the direct-writing capability and tunable optical functionality of polymers, scalability, and the simple optical alignment method.

Evaporation of water droplets on photoresist surfaces – An experimental study of contact line pinning and evaporation residues

We have systematically studied the interaction of ultrapure, de-ionized water droplets with chemically amplified, deep-ultraviolet photoresist layers during evaporation by means of experiments. The contact lines of the evaporating droplets undergo two pinning events. The footprint diameters during pinning D1,2 scale with the initial droplet diameter D0 approximately as D1,2∼D04/3. Evaporated droplets leave a residue behind, generally in the form of an ultrathin layer (order 1–10 nm) with a sub-micron thick mound in the center. We have systematically characterized the residue dimensions as a function of the initial droplet size, the photoresist composition and process conditions. Post-evaporation rinsing steps were found to be unable to completely remove a deposit, depending on how long after droplet evaporation they were performed. Our results indicate that the occurrence of so-called watermark defects might be related to deliquescence induced by ionic residues.

Nanoscale details of liquid drops on 1D patterned surfaces revealed by etching

This paper reports the wetting properties and spatially dependent etch rate variation on the interaction of a dilute potassium hydroxide (KOH):water droplet with a nanopatterned one-dimensional, 500-nm period, grooved photoresist surface. The KOH liquid drop showed a hydrophilic contact angle both along and perpendicular to the grooves and a more significant elongation distortion as compared to a deionized water drop. From the etching of the photoresist lines by the KOH solution, monitored by SEM after the drop was removed, the droplet was in a Cassie–Baxter state with the liquid excluded from the grooves and was pinned at the edge of the grating lines. The etch rate varied with the evaporation rate of the droplet and showed a dependence on the local contact angle with faster etching for smaller contact angles.

Electrical surface charge patterns induced by droplets sliding over polymer and photoresist surfaces

We have studied the generation of surface charge distributions on polymer surfaces due to the deposition, motion and evaporation of liquid droplets. Using liquids with different static dielectric constants, we found that the magnitude of the surface charge conforms well to the Boltzmann relation for the dissociation probability of ions in solution, indicating that the phenomenon is electrokinetic in nature. The origin of the sensitive dependence of the surface charge on the substrate thickness was identified to be an image charge effect. Experiments on polycarbonate and chemically-amplified photoresist yield surface charges of opposite polarity. By varying the photoresist composition, we identified the leaching of negative quencher ions as the reason for the charge inversion.

Evolution of photoresist layer structure and surface morphology under fluorocarbon‐based plasma exposure

AbstractIon bombardment of photoresist materials during plasma etching results in the formation of a surface dense amorphous carbon (DAC) layer that contributes to both etch resistance and the development of surface roughness. Real‐time ellipsometric measurements/analysis reveals that a C4F8‐containing plasma interacts with an Ar‐plasma‐formed DAC layer to produce a modified DAC/fluorocarbon (FC) layer by FC deposition/diffusion of fluorine into the surface. The depletion of the DAC layer via modification and ion bombardment causes the etch rate of the bulk layer to increase. As the modified surface layer is formed, a noticeable decrease in surface roughness decrease is observed. These findings provide an understanding of the mechanisms of atomic layer etching processes in photoresist materials.