Thickness Of SU-8 Research Articles

Fabrication of monolithic SU-8 microneedle arrays having different needle geometries using a simplified process

Transdermal drug delivery using hollow microneedle enables creating small, wearable and minimally invasive closed-loop system. Polymer hollow microneedles are preferable because they are cost-effective and easy to manufacture. SU-8 is chosen for creating the hollow microneedles as it is a biocompatible photopolymer with robust mechanical properties. Previously reported SU-8 microneedles either use melt casting process for coating SU-8 which is laborious or do not have monolithic structures, thereby making these mechanically weak and difficult to integrate. To the best of our knowledge, for the first time, we report the use of a single-step spin coating process to achieve the desired thickness of SU-8 while using UV lithography to create a monolithic microneedle array. Three types of microneedles were fabricated with outer dimensions varying from 90 to 180 μm, and lumen dimensions ranging from 60 to 80 μm and needle height of 600 μm. These needles are fabricated in a 10×10 array with a platform thickness of 300 μm. Geometrical, mechanical and fluid flow characterisations are carried out for the fabricated arrays. We report the use of a non-destructive evaluation method to characterise the lumen of the fabricated microneedles. The fabricated needles are robust and offer low resistance to fluid flow. The triangular needles can withstand a bending load of 0.2 N and an axial load of 0.7 N. The needles with circular lumen offer least resistance to fluid flow of 0.2 Pa-min μL−1.

Thermo-mechanical and tribological properties of SU-8/h-BN composite with SN150/perfluoropolyether filler

In this study, SU-8 and its composites are fabricated by blending 10 wt.% hexagonal boron nitride (h-BN) fillers with/without lubricants, such as 10 wt.% base oil (SN150) and 20 wt.% perfluoropolyether (PFPE). The thickness of SU-8 and its composites coating is fabricated in the range ∼100–105 ±m. Further, SU-8 and its composites are characterized by a 3D optical profilometer, atomic force microscopy, scanning electron microscopy, a thermal gravimetric analyzer, a goniometer, a hardness tester, and an optical microscope. Under a tribology test performed at different normal loads of 2, 4, and 6 N and at a constant sliding speed of 0.28 m/s, the reduction in the initial and steady-state coefficient of friction is obtained to be ∼0.08 and ∼0.098, respectively, in comparison to SU-8 (∼0.42 and ∼0.75), and the wear resistance is enhanced by more than 103 times that of pure SU-8. Moreover, the thermal stability is improved by ∼80–120 °C, and the hardness and elastic modulus by ∼3 and ∼2 times that of pure SU-8, respectively. The SU-8 composite reinforced with 10 wt.% h-BN and 20 wt.% PFPE demonstrated the best thermo-mechanical and tribological properties with a nano-textured surface of high hydrophobicity.

Comparison Study on SU-8 and Its Composites

In this paper, SU-8 is an industrially useful photo resist polymer for micro-fabrication because of its unique UV sensitive curing property. It is also used as a structural material for micro-machines such as micro-electro mechanical systems (MEMS). However, it has poor tribological and mechanical properties which make SU-8 inferior to Si, the mainstay MEMS material today. In this paper, we report the fabrication of SU-8 nanocomposites which are self-lubricating and have better mechanical properties. The liquid lubricant i.e., perfluoropolyether (PFPE) and nanoparticles such as SiO₂, CNTs, and graphite were added into SU-8 for this purpose. These self-lubricating SU-8 + PFPE and SU-8 + PFPE + nanoparticle composites have shown a reduction in the initial coefficient of friction, increased wear life and the mechanical properties such as the elastic modulus and the hardness have increased. We have used perfluoropolyether (PFPE) as in-situ lubricant filler to SU-8. This composite material has shown highly superior friction and wear performance over pristine SU-8 and any such alternative material for this application. We have shown that the mechanical property can also be enhanced by this method if we utilize nano-particles for further reinforcement. This self-lubricating SU-8+PFPE composite can be used for the fabrication of MEMS applications requiring no external lubrication, and also these composites can find applications in many tribological components of traditional machines. The thickness of SU-8 and its composites coating is fabricated in the range ∼100–105 ±m. Further, SU-8 and its composites are characterized by a 3D optical profilometer, atomic force microscopy, scanning electron microscopy, a thermal gravimetric analyzer, a goniometer, a hardness tester, and an optical microscope. Under a tribology test performed at different normal loads of 2, 4, and 6 N and at a constant sliding speed of 0.28 m/s. The SU-8 composite reinforced with 10 wt.% h-BN and 20 wt.% PFPE demonstrated the best thermo-mechanical and tribological properties with a nano-textured surface of high hydrophobicity.

Reduced distortion in high-index microsphere imaging by partial immersion.

We experimentally demonstrate that pincushion distortion is obvious when using a BaTiO3 glass (BTG) microsphere fully immersed in ethanol or SU-8 2002 resist to image a Blu-ray disk with sub-diffraction features. The distortion is related to the layer thickness of the immersing medium. For a BTG microsphere partially immersed in the SU-8 resist where the SU-8 thickness is around 4/5 the diameter of the microsphere, its distortion decreases dramatically, but it can still clearly resolve the Blu-ray disk. For such a partially immersed microsphere, the calculated position of the photonic nanojet is outside the microsphere and close to the object, indicating the microsphere has a super-resolution imaging property, and the distortion simulated by ZEMAX is decreased.

Design, simulation and analysis of a digital RF MEMS varactor using thick SU-8 polymer

This paper presents the design, simulation and analysis of a 5-bit RF MEMS digital variable capacitor using thick SU-8 polymer to achieve both high capacitance ratio (varvec{C}_{varvec{r}}) and Q-factor. The proposed varactor design consists of five capacitive shunt switches loaded over a co-planar waveguide (CPW) line. This results 32 capacitance steps ranging from 102.23 fF to 3.57 pF (varvec{C}_{varvec{r}} = 35). The high capacitance ratio of the varactor is achieved by implementing SU-8 polymer as the base structure of the capacitor due to its low dielectric constant (varvec{varepsilon}_{varvec{r}} = 4) hence reducing the overall minimum capacitance (varvec{C}_{{varvec{min}}}). The optimization of the SU-8 thickness has been carried out to get the optimum capacitance ratio and Q-factor. Moreover, the inclusion of semi-elliptical slot at the ground of CPW and variable sizes of CPW line further reduce the parasitic capacitance. To improve the reliability of the variable capacitor design, a new aluminium (Al) stopper design is used to prevent contact between the beams and the pull-down electrodes. Mechanical simulations and analysis have been carried out to investigate the effects of in-plane residual stress and stress gradient on the spring constant and the initial displacement of the proposed horizontal truss fixed–fixed beam structure. Compared to the conventional doubly clamped solid beam design, the proposed beam has less sensitivity to in-plane residual stress. This makes the pull in voltage to be less affected by the stress due to the fabrication. The simulated pull-in and lift-off voltages are 30 and 25 V respectively while the estimated switching time for the beam is 4.64 μs. The overall size of the varactor is 740 μm × 653 μm. The proposed RF MEMS varactor could be integrated in reconfigurable filters, phase shifters and matching networks targeting future multi-standard wireless communication systems.

A user-friendly guide to the optimum ultraviolet photolithographic exposure and greyscale dose of SU-8 photoresist on common MEMS, microsystems, and microelectronics coatings and materials

We provide here a user-friendly guide to find the optimumi-line (365 nm) photolithographic exposure dose of an arbitrary thickness of SU-8 on various substrate materials and thin film coatings used in MEMS, microsystems and microelectronics technologies: semiconductors, 2D materials (graphene and MoS2) plastics, glass, metals and ceramics.

Low auto-fluorescence fabrication methods for plastic nanoslits.

Plastic nanofluidic devices are becoming increasingly important for biological and chemical applications. However, they suffer from high auto-fluorescence when used for on-chip optical detection. In this study, the auto-fluorescence problem of plastic nanofluidic devices was remedied by newly developed fabrication methods that minimise their auto-fluorescence: one by depositing a gold (Au) layer on them, the other by making them ultra-thin. In the first method, the Au layer [minimum thickness is 40 nm on 150 μm SU-8, 50 nm on 1 mm polyethylene terephthalate (PET), and 40 on 2 nm polymethyl methacrylate (PMMA)] blocks the auto-fluorescence of the polymer; in the second method, auto-fluorescence is minimised by making the chips ultra-thin, selected operating thickness of SU-8 is 20 μm, for PET it is 150 μm, and for PMMA it is 0.8 mm.

Complex Permittivity Measurements in 1–30 GHz Using a MEMS Probe

An open-ended coplanar waveguide (CPW) structured probe was implemented for the measurement of dielectric properties of liquids using microelectromechanical systems (MEMS) technology. The MEMS probe, fabricated on sapphire substrate, consists of an open-ended CPW, where the conducting layers were made using Cr/Al. A dielectric layer using SU-8 was added to protect the metal layer. An aperture at the end of the probe was added where the CPW contacts with the material under test. The size of the probe is $180~\mu \textrm {m}~\textrm {(width)}\times 660.2~\mu \textrm {m}~\textrm {(thickness)} \times 6000~\mu $ m (length), aperture size is $100~\mu \textrm {m} \times 100~\mu $ m, and thickness of SU-8 is 10 $\mu $ m. The permittivities of ethanol-water mixtures measured using the MEMS probe in the 1–30 GHz agreed well with the values presented in the literature. The low cost, small, lightweight, and portable probe described in this paper can be used to measure dielectric properties of very small volume of samples. [2014-0050]

Three-step lithography to the fabrication of vertically coupled micro-ring resonators in amorphous silicon-on-insulator

A simple method to fabricate vertically coupled micro-ring resonators in amorphous silicon-on-insulator is created by a three-step lithography process. First, the linear loss at 1.55 μm of the a-Si:H film is calculated to be 0.2±0.05 dB/cm. Then, the bottom line waveguide of Su-8 with a flat top surface of 300 nm is created by etching. The thickness of Su-8 can easily be controlled by the etching time. Finally, by opening the window pattern and etching several layers, the first layer marks made by electron beam lithography are found with a 50 nm resolution, and the high quality of the micro-ring resonator is demonstrated.

Effects of post exposure bake temperature and exposure time on SU-8 nanopattern obtained by electron beam lithography

SU-8 is a photoresist imaged using UV rays. However, we investigated the characteristics of an SU-8 nanopattern obtained by electron beam lithography (EBL). In particular, we studied the relationship between post-exposure bake (PEB) temperature and exposure time on an SU-8 nanopattern with a focus on phase transition temperature. SU-8 residue was formed by increasing both PEB temperature and exposure time. To prevent the formation of this, Monte Carlo simulation was performed; the results of such simulation showed that decreasing the thickness of SU-8 can reduce the amount of residue from the SU-8 nanopattern. We confirmed that decreasing the thickness of SU-8 can also prevent the formation of residue from the SU-8 nanopattern with EBL.

A simulation model on photoresist SU-8 thickness after development under partial exposure with reflection effect

The negative photoresist SU-8 is often used to fabricate thick micro structures. It was known that reflection from the interface between SU-8 and the substrate surface would affect the thickness after development and profile of SU-8 structures. However, the model to predict the thickness after development of SU-8 under reflection effect has not been reported yet. Here, a simulation model to predict SU-8 thickness after development under partial exposure with reflection effects is proposed. Two kinds of SU-8 micro structures with different exposure dosages and coated thickness are fabricated on glass substrates to demonstrate the capability of the proposed model. For different exposure dosages or coated SU-8 thickness, the maximum difference between simulated and experimental results is shown to be less than 3.1%, which verifies the accuracy of the proposed model.

실험 및 유한요소해석에 의한 SU-8 박막의 Tribological 특성 연구

In this study, two-dimensional finite element models were developed and experiments were conducted using an atomic force microscope to investigate the tribological characteristics of an SU-8 layer coated on a patterned wafer for microsystem applications. The results revealed that both the adhesion and the friction forces measured by the atomic force microscope were lower for the SU-8 coated surface than for the bare silicon surface. This is attributed to the hydrophobicity of SU-8. Another important result derived from the finite element analysis was the critical load required to fracture the SU-8 film with respect to the thickness. The critical loads for thicknesses of 200, 400, and 800 nm were approximately 13, 22, and 28 mN, respectively, which corresponded to a Hertzian contact pressure of 1.2-1.8 GPa. These results will aid in the design of a suitable SU-8 thickness for microsystem components that are in contact with one another.

A Numerical Study on the Mechanical Characteristics of Zinc Oxide-Based Transparent Thin Film Transistors

Zinc Oxide (ZnO) based Thin Film Transistors (TFTs) have been fabricated and analyzed to investigate mechanical characteristics regarding the stress, strain and deformation of electro circuits using the Finite Element Method (FEM). As the best compromise between the stretching and bending abilities, the coating thickness of SU-8 can be as important for bendability as a neutral mechanical plane. The neutral mechanical plane in electro circuits was designed for obtaining flexibility, e.g., bendability, in a previous numerical study. After that, through experimental validation, we observed what degree of SU-8 thickness was attributable for improved mechanical stability. The results suggest that not only numerical but also experimental measurements of the deformation and SU-8 coating thickness in electro circuits are useful for enhancing structural stability.

Fabrication of Hierarchical Pillar Arrays from Thermoplastic and Photosensitive SU‐8

By exploiting the thermoplastic and photosensitive nature of SU-8 photoresists, different types of hierarchical pillar arrays with variable aspect ratios are fabricated through capillary force lithography (CFL), followed by photopatterning. The thermoplastic nature of SU-8 enables the imprinting of micropillar arrays with variable aspect ratios by CFL using a single poly(dimethylsiloxane) mold, simply by tuning the initial film thickness of SU-8 on a substrate. The pillar array is subsequently photopatterned through a photomask, followed by post-exposure baking above the glass transition temperature (T(g)) of SU-8. The pillars in the exposed region become highly crosslinked and, therefore, neither soluble nor able to reflow above T(g), whereas the pillars in the unexposed regions can reflow and flatten out. Two developing strategies are investigated after UV exposure of the SU-8 pillar arrays including i) solvent development and drying and ii) thermal reflow to create bilevel hierarchical structures with short pillars and single-level, dual-scaled, high-aspect-ratio (up to 7.7) pillars in a microdot array, respectively.

Lithography: Fabrication of Hierarchical Pillar Arrays from Thermoplastic and Photosensitive SU‐8 Small 6/2010

The cover image illustrates an array of high-aspect-ratio (up to 7.7) polymer pillars from SU-8 photoresists imprinted by capillary force lithography (CFL), followed by photopatterning and a thermal reflow process. Because of the thermoplastic nature of SU-8, pillars with variable aspect ratios are created through CFL using a single poly(dimethylsiloxane) membrane as the mold, simply by tuning the initial film thickness of SU-8 on a substrate. The pillars are subsequently photopatterned through a photomask with a square dot array, followed by post-exposure bake above the glass-transition temperature (Tg) of SU-8. The pillars in the exposed region become highly crosslinked, and thus neither soluble nor reflowable, whereas the pillars in the unexposed regions can reflow and flatten out above Tg. In this way, a single-level, dual-scaled, high-aspect-ratio SU-8 pillar array is “developed” in the exposed region without the use of solvent. Such high-aspect-ratio pillar arrays may find applications in biomimetic superhydrophobic surfaces and dry adhesion, microfluidics, diagnosis, tissue engineering, force sensing, and actuation. For more information, please read the Full Paper “Fabrication of Hierarchical Pillar Arrays from Thermoplastic and Photosensitive SU-8” by S. Yang et al., beginning on page 768.

Control of the out-of-plane curvature in SU-8 compliant microstructures by exposure dose and baking times

The effect of processing conditions on the curvature of SU-8 cantilevers up to 6400 µm long is determined by experimental methods. Our observations suggest that a zero curvature condition can be achieved by controlling the exposure dose and post-exposure baking time for any given SU-8 thickness. The curvature of SU-8 cantilevers processed with different exposure doses and post-exposure baking times has been measured and reveals that the gradient of crosslinking density throughout the SU-8 film is the cause of out-of-plane stress. A general model for the SU-8 cantilever curvature is developed which explains the behavior of SU-8 structures that have been processed with different conditions and can be used to predict the general effect on the SU-8 cantilever curvature for different SU-8 thicknesses and process parameters.

CHARACTERIZATION AND APPLICATION OF PAG DILUTED SU-8

The concentration of photoacidgenerator (PAG) in SU-8 determines the UV absorption and, therefore, is a limiting factor of the applicable SU-8 thickness. The motivation of this work is to improve SU-8 process control and expand the application scope of SU-8 by modifying its PAG concentration. In this paper we present the experimental results on lithographic properties of SU-8 as the function of PAG concentration (varying up to 2 orders of magnitude). It includes determining the minimum bottom dose and minimum effective energy density for x-ray and UV lithography of SU-8, respectively, and the UV absorption spectra of SU-8. It is fund that as the PAG concentration reduced, SU-8 has lower sensitivities in UV and x-ray lithography, and lower UV absorption coefficients. By using PAG diluted SU-8 taller microstructures with vertical sidewall can be obtained with UV lithography. Based on the large difference in lithographic sensitivities between normal and PAG diluted SU-8, we have developed a new process concept for making 3D microstructures. The 3D microstructures fabricated based on this new process, including microcantilever and microchannel, will be presented.