SU-8 Polymer Research Articles

Planar waveguide to vertical mode couplers covering the visible to IR spectral range

Planar 45o turning mirrors with metal coating embedded in SU8 polymer waveguides enable waveguide to vertical mode coupling across a broad range of wavelengths from the visible to IR. The fabrication of these 2.5D structures is achieved using relatively simple grayscale lithography in thin film resists, compatible with standard planar lithography methods. Mirror losses of <1 dB are measured from 516 -1630 nm, and direct coupling to single mode fibre is achieved.

Suppression of cracking induced by epitaxial strain relaxation using a relaxation absorber during the transfer of epitaxial thin films of anatase-type Nb:TiO2

This work developed a technique for fabricating flexible epitaxial thin films of anatase-type Nb:TiO2 (epitaxial TNO) via a transfer process. In our prior work, epitaxial TNO deposited on LaAlO3 (001) single crystal substrates was lifted off and affixed to flexible polymeric sheets. However, many cracks occurred on the surfaces of the transferred thin films, such that the conductivity of the epitaxial TNO was drastically decreased. This loss of conductivity was attributed to the interruption of current paths by the cracks. These numerous cracks were linear and in most cases were oriented either parallel to the [100]anatase∥[100]LAO direction or parallel to the [010]anatase∥[010]LAO direction (where the subscripts anatase and LAO represent the materials for each Miller index). Therefore, cracking likely occurred as a consequence of the relaxation of epitaxial strain and a return to the original axial lengths of the TNO during the transfer process. In the present work, the surface of epitaxial TNO was coated with the ductile polymer SU-8 3050 as a relaxation absorber prior to the transfer. This modified transfer process greatly reduced crack formation in the TNO such that the conductivity of the thin film was improved by two orders of magnitude. Thus, pre-coating with a ductile relaxation absorber was shown to effectively suppress the cracking of flexible epitaxial TNO.

SU-8-meta-phenylenediamine-conjugated thin film for temperature sensing.

Polymers have distinctive optical properties and facile fabrication methods that have been well-established. Therefore, they have immense potential for nanophotonic devices. Here, we demonstrate the temperature-sensing potential of SU8-meta-phenylenediamine (SU8-mPD), produced by epoxy amination of the SU-8 polymer. Its properties were examined through a series of molecular structural techniques and optical methods. Thin layers have demonstrated optical emission and absorption in the visible range around 420 and 520 nm, respectively, alongside a strong thermal responsivity, characterized by the 18 ppm °C-1 expansion coefficient. A photonic chip, comprising a thin 5-10 μm SU8-mPD layer, encased between parallel silver and/or gold thin film mirrors, has been fabricated. When pumped by an external light source, this assembly generates a pronounced fluorescent signal that is superimposed with the Fabry-Pérot (FP) resonant response. The chip undergoes mechanical deformation in response to temperature changes, thereby shifting the FP resonance and encoding temperature information into the fluorescence output spectrum. The time response of the device was estimated to be below 1 s for heating and a few seconds for cooling, opening a new avenue for optical sensing using SU8-based polymers. Thermoresponsive resonant structures, encompassing strong tunable fluorescent properties, can further enrich the functionalities of nanophotonic polymer-based platforms. This article is part of the theme issue 'Celebrating the 15th anniversary of the Royal Society Newton International Fellowship'.

Influence of SU-8 curing parameters on the terahertz absorption characteristics

A plethora of polymers are excellent candidates that can be utilized to build various terahertz (THz) based systems for sensing and microfluidics. SU-8 is a versatile epoxy-based polymer with excellent properties, such as biocompatibility and good mechanical properties. Nevertheless, the impact of curing parameters on the THz absorption characteristics of SU-8 remains uncertain. This study explores the impact of various curing conditions on the THz absorption properties of SU-8. The aim is to establish a correlation between THz absorption and the polymer's cross-linking characteristics. Therefore, three key curing parameters have been examined: time, temperature, and ultraviolet (UV) exposure dose. The SU-8 samples that are cured under different conditions are examined using THz Time-Domain Spectroscopy (THz-TDS) to estimate the THz absorption coefficient. Next, the swelling experiment is conducted to evaluate the polymer cross-linking of the cured samples. The results show that curing conditions and, thus, cross-linking routines significantly influence THz wave propagation and attenuation within SU-8 samples. The findings recommended using an optimal curing dose for SU-8 spanning 1240 mJ/cm2 to 1860 mJ/cm2 which results in a relatively lower absorption coefficient while still maintaining a higher state of cross-linking. This research establishes a crucial link between SU-8 processing and its THz response. It can lay the foundation for tailored SU-8 polymer with optimized THz transmission for novel and customized biosensing and microfluidic devices.

Analyzing the evanescent field ratio of ridge waveguide based on different material platforms for sensing applications

Abstract In this study, a comprehensive numerical investigation focusing on a ridge waveguide composed of various material platforms is undertaken, including silicon-on-insulator (SOI), silicon nitride (SiN), and SU-8 polymer. In the realm of photonic sensing devices, the evanescent field assumes paramount significance in facilitating light-matter interactions. The primary objective of this study is to furnish a comparative analysis discerning the suitability of these material platforms for sensing applications. The findings reveal distinct characteristics among the different material platforms. SOI emerges as a standout contender, exhibiting an augmented evanescent field ratio that substantially enhances light-matter interaction. Following closely, SiN-based waveguides demonstrate commendable performance in this regard. Conversely, SU-8 polymer waveguides, characterized by larger dimensions, present limitations in evanescent field confinement, resulting in the lowest sensitivity among the materials investigated. Hence, the selection of the appropriate material platform and waveguide type emerges as a critical consideration in the design phase of photonic sensors. This decision significantly influences the sensor’s performance and efficacy in real-world applications.

Electrowriting of SU-8 Microfibers.

As microfiber-based additive manufacturing (AM) technologies, melt electrowriting (MEW) and solution electrowriting (SEW) have demonstrated efficacy with more biomedically relevant materials. By processing SU-8 resin using MEW and SEW techniques, a material with substantially different mechanical, thermal, and optical properties than that typically processed is introduced. SU-8 polymer is temperature sensitive and requires the devising of a specific heating protocol to be properly processed. Smooth-surfaced microfibers resulted from MEW of SU8 for a short period (from 30 to 90 min), which provides the greatest control and, thus, reproducibility of the printed microfibers. This investigation explores various parameters influencing the electrowriting process, printing conditions, and post-processing to optimize the fabrication of intricate 3D structures. This work demonstrates the controlled generation of straight filaments and complex multi-layered architectures, which were characterized by brightfield, darkfield, and scanning electron microscopy (SEM). This research opens new avenues for the design and development of 3D-printed photonic systems by leveraging the properties of SU-8 after both MEW and SEW processing.

Fabrication of SU-8 polymer micro/nanoscale nozzle by hot embossing method

Electrohydrodynamic-jet printing (E-jet printing) is a direct-writing technology for manufacturing micro-nano devices. To further reduce the inner diameter of the nozzle to improve the printing resolution, a large-scale manufacturing method of SU-8 polymer micro/nanoscale nozzle by means of a process combining UV exposure and hot embossing was proposed. To improve the adhesive strength between the UV mask and SU-8, the influence of the oxygen plasma treatment parameters on the water contact angles of the UV mask was analyzed. The effect of hot embossing time and temperature on the replication precision was studied. The influence of UV exposure parameters and thermal bonding parameters on the micro and nanochannel pattern was investigated. The SU-8 polymer nozzles with 188 ± 3 nm wide and 104 ± 2 nm deep nanochannels were successfully fabricated, and the replication precision can reach to 98.5%. The proposed manufacturing method of SU-8 polymer nozzles in this study will significantly advance the research on the transport properties of nanoscale channels in E-jet nozzles and facilitate further advancements in E-jet based applications.

Thin microfilters with tunable pores expedite blood filtration

Membranes fabricated from biocompatible polymer SU-8 are durable, effective, and simple to manufacture.

Area-Selective Growth of Zinc Oxide Nanowire Arrays for Piezoelectric Energy Harvesting.

In this work, we present the area-selective growth of zinc oxide nanowire (NW) arrays on patterned surfaces of a silicon (Si) substrate for a piezoelectric nanogenerator (PENG). ZnO NW arrays were selectively grown on patterned surfaces of a Si substrate using a devised microelectromechanical system (MEMS)-compatible chemical bath deposition (CBD) method. The fabricated devices measured a maximum peak output voltage of ~7.9 mV when a mass of 91.5 g was repeatedly manually placed on them. Finite element modeling (FEM) of a single NW using COMSOL Multiphysics at an applied axial force of 0.9 nN, which corresponded to the experimental condition, resulted in a voltage potential of -6.5 mV. The process repeated with the same pattern design using a layer of SU-8 polymer on the NWs yielded a much higher maximum peak output voltage of ~21.6 mV and a corresponding peak power density of 0.22 µW/cm3, independent of the size of the NW array. The mean values of the measured output voltage and FEM showed good agreement and a nearly linear dependence on the applied force on a 3 × 3 µm2 NW array area in the range of 20 to 90 nN.

In-situ SEM micropillar compression and nanoindentation testing of SU-8 polymer up to 1000 s−1 strain rate

In situ micropillar compression and nanoindentation were performed on photolithographically fabricated SU-8 polymer inside a scanning electron microscope (SEM), covering seven orders of strain rates from 10-3 to 103 s−1. The extracted mechanical properties – modulus, hardness, yield strength, strain rate sensitivity (SRS) exponent – were systematically compared from both microscale tests and showed excellent agreement. No change in deformation mechanism was observed at high strain rates. We report a novel experimental protocol for high strain rate nanoindentation, comprising of input profile smoothing, that minimizes resonance amplitude during unloading and allows reliable extraction of modulus and hardness using standard Oliver-Pharr analysis. To the best of our knowledge, this is the first report of mechanical properties of SU-8 beyond 1 s−1 strain rate using nanoindentation based tests.

SU8 polymer derived high capacity and performance anode material for secondary and flexible Li-ion batteries: Experimental and first principle study

This study provides a novel and sustainable protocol for the synthesis of SU8 polymer-based pattern, un-pattern carbon nanofibers (CNF) using an electrospinning process followed by a photolithography technique. Scanning electron microscopy (SEM) analysis of pattern, un-pattern CNFs shows that the average diameter of CNF is within the range of 60–140 nm. Transmission electron microscopy (TEM) studies disclose the average size of CNF exists within the range of 40–150 nm. The first-principle calculations predict that the gravimetric energy and power densities of Li/CNF half cells are 26.96 kW h g−1 and 26.96 kW g−1, respectively. The corresponding theoretical volumetric energy and power densities are 0.07 W h cm−3 and 0.07 W cm−3, respectively. Electrochemical analysis demonstrates the initial discharge capacities of pattern, un-pattern Li/CNF half cells at 100 mA g−1 are 3450 and 2429.8 mA h g−1, respectively. The gravimetric energy density of pattern, un-pattern Li/CNF half cells are within the range of 1800–1366.76 W h kg−1, and subsequent power densities are in the span of 519.63–434.22 kW g−1. The theoretical gravimetric energy and power densities of CNF/LiMn0.33Ni0.33Co0.33 (LMNC) full cells are 2.9 kWh kg−1 and ∼ 2.9 kW kg−1, respectively. Their theoretical volumetric energy and power densities are 1.96 W h cm−3 and 1.96 W cm−3, respectively. Electrochemical analysis discloses that the gravimetric energy and power densities of pattern CNF/LMNC full cells are in the range of 43.18–42.4 kW h kg−1 and 51.82–50.90 kW kg−1, respectively. The gravimetric energy and power densities of un-pattern CNF/LMNC full cells are 42.07–41.59 W h kg−1 and 50.48–49.92 kW kg−1, respectively.

Detailed Study of the Correlation between Cross-Linking of Thick SU-8 and UV-NIR Optical Transmission/Photoluminescence Spectroscopy.

SU-8 polymers are promising materials for various applications due to their low cost, excellent thermal stability, and outstanding mechanical properties. Cross-linking of SU-8 is a crucial process that determines the properties of the materials. This study investigates the effect of cross-linking of free-standing SU-8 films on optical transmission and PL emission under various curing conditions. Our findings show that an increase in the cross-linking density reduces optical transmission and causes a red shift of the PL emission band peaks. By directly measuring the optical response of the isolated SU-8, we remove any uncertainty due to the substrate's presence. Moreover, we show that optical transmission and PL spectroscopy are two non-distractive techniques that can be employed to monitor the curing of the SU-8. This finding enhances our understanding of the cross-linking process in SU-8 and paves the way to further enhance the properties of the SU-8 polymer for various electronics and optoelectronics applications.

Oxygen plasma assisted room temperature bonding for manufacturing SU-8 polymer micro/nanoscale nozzle

Abstract Electrohydrodynamic-jet (E-jet) printing is an on-demand additive manufacturing method that allows various functional materials to be directly deposited on the target substrate. Many theoretical and experimental results indicate that E-jet has scale effect, and reducing the inner diameter of the nozzle can effectively improve printing resolution. Herein, a method for fabricating SU-8 polymer micro/nanoscale nozzle by oxygen plasma assisted room temperature bonding was proposed. It can prevent channel deformation and blockage caused by excessive bonding temperature. The surface modification parameters of silicon nano-mold were optimized, the influence of hot embossing parameters on the replication precision of nano patterns was investigated, the effect of UV lithography parameters on the micro and nanochannel was analyzed, the relationship between the oxygen plasma treatment parameters on the contact angle of SU-8 was revealed, the influence of bonding pressure on the morphology of the nanochannel was discussed, and the bonding principle at room temperature was deeply analyzed. This method can fabricate SU-8 polymer micro/nanoscale nozzle with low cost and high precision, and provide a new idea for the realization of room temperature bonding to manufacture SU-8 polymer nozzle.

Design, fabrication and characterization of SU-8 and PMMA grating couplers

Grating couplers utilize light diffraction to achieve vertical coupling between waveguides and optical fibers. We proposed using SU-8 or polymethyl methacrylate (PMMA) polymer materials as the waveguide core layer and polydimethylsiloxane (PDMS) as the cladding layer for the grating coupler and completed the fabrication of both uniform and convergent grating couplers. We studied the effect of waveguide layer thickness, etching depth, and grating period on the uniformity of grating coupling performance. The influence of the convergence angle on the coupling performance of the convergence grating is investigated, and the optimal parameters are finally determined. The experimental results show that the coupling efficiency of the SU-8 uniform grating coupler reaches 30.1% and the 3 dB bandwidth is 70 nm when the grating period length is 4μm. For the SU-8 convergent grating coupler, the coupling efficiency reaches 40.0%, and the bandwidth is 65 nm when the convergence angle is 20°. The coupling efficiency of the PMMA grating coupler is lower than that of the SU-8 grating coupler; the coupling efficiency reaches 29.0% when the grating period of the uniform grating coupler is 4μm and 35.1% when the convergence angle of the convergent grating coupler is 20°.

(Invited) Gallium Oxides Devices for Grid-Scale Power Electronics and High Power Switched Mode RF Amplifiers

Within a decade of the first demonstration of a MESFET device, monoclinic β-Ga2O3 (Ga2O3) devices have made incredible progress in breakdown voltages, power device figure of merit and high-speed performance. It has emerged as a promising ultra-widebandgap semiconductor for next generation power, GHz switching and RF applications. In this talk, we will present an overview of our studies on fundamental transport properties, kilovolt class power MOSFETs and aggressively scaled RF devices. The large bandgap of Ga2O3 leads to a high critical field strength. This high field strength in combination with demonstrated room temperature mobility and calculated electron velocity leads to a higher Baliga’s Figure of Merit (BFoM) and Johnston Figure of Merit (JFoM) than current commercially available widebandgap technologies. Additionally, the large bandgap also enables high temperature operation and radiation hardness making it attractive for space applications such as Mars and Venus missions.In the first part, we will discuss the deep insights gained from the first-principles based transport calculations in Ga2O3. We will discuss the limits to low field mobility, saturation velocities and impact ionization coefficients. The impact of the plasmon-phonon coupling on the phonon limited transport will be discussed. We will present the experimental signatures of plasmon-phonon coupling and anti-screening behavior in ionic gated accumulation layers.Next, I will describe our work on the lateral MOSFETs with improved field plate design and beyond-kV breakdown. Temperature dependent analysis and device simulation suggest an extrinsic breakdown mechanism outside the channel. A simple and yet effective SU-8 polymer passivation technology was developed that results in significant improvement in breakdown voltages. The higher field strength of the SU-8 polymer enables a significant increase in breakdown voltage to 8 kVs in lateral MOSFETs. However, these devices show a high Ron, which is due to the depletion caused by RIE of the channel. We will present the use of ultra-high vacuum annealing techniques to improve the on-resistance of the devices still maintaining the multi-kilo-volt rating of the devices.Due to its high JFoM, Ga2O3 is a potential candidate for high power density RF amplifiers. We will also present the performance advancement that can be obtained in RF power performance using Ga2O3 technology. We will present the challenges in the technology and potential solutions. We will present the experimental DC and pulsed measurements on laterally scaled MOSFETs with 100 nm gate length. Severe DC-RF dispersion was observed which is attributed to the RIE damage and surface states on the oxide. Passivation of the devices shows reduced current collapse. Using aggressive gate lengths and gate-source spacing scaling, (AlxGa1-x)2O3/Ga2O3 heterostructure FETs were fabricated that show record high RF performance. The device transconductance was limited by the regrowth interface resistance. Ex-situ silicon nitride passivation was shown to be effective to reduce DC-RF dispersion. Recently, thin channel MOSFETs with deep-sub-micron gate lengths and source/drain regrowth have demonstrated devices with simultaneously low on resistance, high frequency, high voltages.

Dual-layer optical encryption fluorescent polymer waveguide chip based on optical pulse-code modulation technique

Information encryption technique has broad applications in individual privacy, military confidentiality, and national security, but traditional electronic encryption approaches are increasingly unable to satisfy the demands of strong safety and large bandwidth of high-speed data transmission over network. Optical encryption technology could be more flexible and effective in parallel programming and multiple degree-of-freedom data transmitting application. Here, we show a dual-layer optical encryption fluorescent polymer waveguide chip based on optical pulse-code modulation technique. Fluorescent oligomers were doped into epoxy cross-linking SU-8 polymer as a gain medium. Through modifying both the external pumping wavelength and operating frequency of the pulse-code modulation, the sender could ensure the transmission of vital information is secure. If the plaintext transmission is eavesdropped, the external pumping light will be switched, and the receiver will get warning commands of ciphertext information in the standby network. This technique is suitable for high-integration and high-scalability optical information encryption communications.

Microarray biochip fabricated on silicon nanowires/carbon dots heterostructures for enhanced viral DNA detection

Recent studies have shown that human oncoviruses are responsible for 10% of cancer cases worldwide. While PCR assays are highly sensitive and specific, microarrays offer outstanding performance for detecting multiple targets simultaneously. However, in microarray systems, accurate detection of DNA viruses relies on the type of microarray platform, while obtaining high-quality, fluorescent-labelled single-stranded DNA bares an equal contribution.Our study demonstrates a proof-of-concept of a microarray platform for enhanced DNA detection. First, we assembled 3D microarray chips on silicon nanowires (SiNWs) substrates using a one-step metal-assisted chemical etching (1-MACE) technique, coated them with SU-8 polymer, and ornated with carbon quantum dots (CDs) in either poly(diallyldimethylammonium) PDDA or poly(ethyleneimine) PEI solutions. Second, we obtained high-quality labelled HPV 16 target ssDNA through classic PCR and asymmetric PCR with a nested primer labelled with Cy5, then hybridised the ssDNA on the SiNWs/SU-8/CDs platforms.The fluorescent signal of hybridised DNA was strongest on the SiNWs/SU-8 substrates, and adding CDs further enhanced the signal. The best signal intensities (4.303, 3.7) and coefficients of variation (3.12%, 2.59%) were achieved by combining HPV 16 probes co-immobilised with CDs functionalised with PDDA. Our study conclusively demonstrates the vast potential of this microarray platform for improved DNA detection.

WMS-based near-infrared on-chip acetylene sensor using polymeric SU8 Archimedean spiral waveguide with Euler S-bend

SU8 is a cost-effective polymer material that is highly suitable for large-scale fabrication of waveguides. However, it has not been employed for on-chip gas measurement utilizing infrared absorption spectroscopy. In this study, we propose a near-infrared on-chip acetylene (C2H2) sensor using SU8 polymer spiral waveguides for the first time to our knowledge. The performance of the sensor based on wavelength modulation spectroscopy (WMS) was experimentally validated. By incorporating the proposed Euler-S bend and Archimedean spiral SU8 waveguide, we achieved a reduction in the sensor's size by over fifty percent. Leveraging the WMS technique, we evaluated the C2H2 sensing performance at 1532.83 nm for SU8 waveguides of lengths 7.4 cm and 13 cm. The limit of detection (LoD) values were 2197.1 ppm (parts per million) and 425.5 ppm, respectively, with an averaging time of 0.2 s. Furthermore, the experimentally obtained optical power confinement factor (PCF) closely approximated the simulated value, with a value of 0.0172 compared to the simulated value of 0.016. The waveguide loss is measured to be 3 dB/cm. The rise time and fall time were approximately 2.05 s and 3.27 s, respectively. This study concludes that the SU8 waveguide exhibits significant potential for high-performance on-chip gas sensing in the near-infrared wavelength range.

Synthesis of Metal/SU-8 Nanocomposites through Photoreduction on SU-8 Substrates.

The paper presents a simple, fast, and cost-effective method for creating metal/SU-8 nanocomposites by applying a metal precursor drop onto the surface or nanostructure of SU-8 and exposing it to UV light. No pre-mixing of the metal precursor with the SU-8 polymer or pre-synthesis of metal nanoparticles is required. A TEM analysis was conducted to confirm the composition and depth distribution of the silver nanoparticles, which penetrate the SU-8 film and uniformly form the Ag/SU-8 nanocomposites. The antibacterial properties of the nanocomposites were evaluated. Moreover, a composite surface with a top layer of gold nanodisks and a bottom layer of Ag/SU-8 nanocomposites was produced using the same photoreduction process with gold and silver precursors, respectively. The reduction parameters can be manipulated to customize the color and spectrum of various composite surfaces.

Stadium-type resonator sensor based on a multi-mode waveguide with mode discrimination phenomenon.

In this work, we present a multi-mode resonator based on SU-8 polymer and experimentally verify that the resonator showed mode discrimination can be used as a sensor with high performance. According to field emission scanning electron microscopy (FE-SEM) images, the fabricated resonator shows sidewall roughness which is canonically considered to be undesirable after a typical development process. In order to analyze the effect of sidewall roughness, we conduct the resonator simulation considering the roughness under various conditions. Mode discrimination still occurs even in the presence of sidewall roughness. In addition, waveguide width controllable by UV exposure time effectively contributes to mode discrimination. To verify the resonator as a sensor, we perform a temperature variation experiment, which results in a high sensitivity of about 630.8 nm/RIU. This result shows that the multi-mode resonator sensor fabricated via a simple process is competitive with other single-mode waveguide sensors.