SU-8 Polymer Research Articles

Vertical Optical Coupling Structure Based on Polymer for Small Size and High-Speed Photodetectors

This letter presents a new collective and passive thin-film 3D vertical optical coupling structure, which is fabricated based on SU-8 polymer. This technology compensates for the alignment errors of the present passive optical coupling solutions. The structure is collectively manufactured on a matrix of small-sized photodiodes on wafer level to vertically couple with a single-mode optical fiber. The proposed structure shows a coupling efficiency between the 10- $\mu \text{m}$ active area diameter photodetector and single-mode fiber of 94% and a 1-dB alignment tolerance of $\pm 9~\mu \text{m}$ at 1550 nm. Its wafer-level mass-production capability and manufacturing process simplicity make this novel coupling approach a potential solution for short- and long-range optical communication applications.

Design and characterization of millimeter-wave micromachined polymer-based cavity filter with resonant cylinders

This paper presents a Ka-band cavity bandpass filter using SU-8 polymer UV lithography technology. The filter has a folded structure with six cavity resonators having resonant cylinders, enabling compact design and improved stopband characteristics. The fabrication of the millimeter- wave filter with a complicated multi-height and high aspect- ratio structure is realized using the proposed methods. A coaxial-rectangular waveguide transition is designed for the measurement process. The measured results of the filter are in good agreement with the simulated ones, with a center frequency of 38 GHz, insertion loss of 0.7 dB, and fractional bandwidth of 4.4%. The stopband rejection reaches 70 dB at 35.5 and 40.5 GHz. The experimental results are discussed and force and thermal stress analysis is conducted to confirm the feasibility of the design and fabrication methods of the cavity filter.

Transient Optical Characteristics of Broad Absorption Band Excitons Modulated by Micro-cavity

The understanding of light-matter interaction within micro-cavity lays the basic groundwork for many future photon-related technologies and applications. We prepared low quality metal-insulator-metal(MIM) micro-cavity consisting massive two-level broad absorption band dye(Nile Red) excitons, which randomly dispersed in SU-8 polymer negative resist matrix and measured their optical characteristics. New binate transmission peaks with large energy separation(so-called Rabi-splitting phenomenon) and their angular anti-crossing behavior in con-sequence of the interaction between dye excitons and confined photons were observed. It was also confirmed that the separated energy can be tuned by changing the doped exciton concentrations. Time-resolved transient absorption measurements showed that such an interaction is indeed a coherent one but rather a strong coupling one and one can modulate such a coherent mechanism by easily adjusting the detuning between dye excitons and confined cavity photons. This work may provide a comprehensive and deep understanding for such massive broad absoprtion band excitons-doped MIM micro-cavities and represent a further step to realize optical cavity-modulated devices in future.

A large free spectral range of 74.92 GHz in comb peaks generated by SU-8 polymer micro-ring resonators: simulation and experiment

The feasibility of integrated SU-8 polymeric optical waveguides on a silicon platform was demonstrated experimentally and supported by simulation. Micro-ring resonator (MRR) configurations were designed and fabricated using SU-8 polymer. Simulation is carried out using the finite-difference time-domain method to improve the design. Both the simulation and experimental results confirm that SU-8 can be successfully used in the optical MRR. Passive MMI-coupled SU-8 single coupler and double coupler with a free spectral range of 74.92 GHz is achieved practically and corresponds with the simulation result. Moreover, other important parameters such as the full width at half maximum, Q factor and finesse are calculated from the simulation and practical results to support the feasibility of the SU-8 polymer MRR.

Low-Cost Microfabrication for MEMS Switches and Varactors

This paper presents a low-cost microfabrication technique for manufacturing radio frequency microelectromechanical system (RF MEMS) switches and varactors without intensive clean-room environments. The fabrication process entails only laser microstructuring technique, non-clean-room microlithography, and standard wet-bench and hot-film emboss of SU-8 and ADEX polymers. MEMS movable structures were fabricated out of 14- $\mu \text{m}$ -thick aluminum foils and suspended above coplanar-waveguide transmission lines, which were implemented on top of FR4 substrates via 5- $\mu \text{m}$ -thick SU-8 dielectric anchors. Both MEMS structures and an FR4 substrate were integrated using micropatterned polymers, developed by using dry-film ADEX and SU-8 polymers, for a composite assembly. An average fabrication yield of higher than 60% was achieved, calculated from 10 fabrication attempts. The RF measurement results show that the RF MEMS devices fabricated by using the novel microfabrication process have good figures of merit, at much lower overall fabrication costs, as compared to the devices fabricated by conventional clean-room process, enabling it as a very good microfabrication process for cost-effective rapid prototyping of MEMS.

Inkjet Printing of Wideband Stacked Microstrip Patch Array Antenna on Ultrathin Flexible Substrates

This paper presents a bandwidth-enhanced, low-cost, compact, inkjet-printed multilayer microstrip fractal patch antenna for integration into flexible and conformal devices. The antenna consists of two layers of patches, with the first layer inkjet-printed directly on a 0.125-mm (only 0.005 of the operating wavelength) Kapton polyimide substrate. On top of it, a 0.12-mm-thick SU-8 polymer is covered. To achieve the desired miniaturization and good impedance match, a Minkowski fractal geometry patch is employed as the second layer inkjet-printed on top of the SU-8 polymer. The proposed antenna has compact dimensions of only <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$22\times 31$ </tex-math></inline-formula> mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and covers 4.79–5.04-GHz frequency spectrum with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$S_{11} &lt; -10$ </tex-math></inline-formula> dB. Moreover, a 2-bit <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1\times 4$ </tex-math></inline-formula> phased array antenna (PAA) is constructed, and its scan ability is estimated to demonstrate its potential application in the true-time-delay flexible PAA systems. The prototype is fabricated and tested for impedance and radiation characteristics. The measured and the simulation results show the superiority of the proposed antenna.

Light-Activated Hybrid Nanocomposite Film for Water and Oxygen Sensing.

Oxygen and water vapor sensing properties are investigated in metal-oxide-hybrid polymer nanocomposite thin films generated by infiltration synthesis, which incorporates molecular ZnO into the matrix of SU-8 polymer, a common negative-tone photoresist. The hybrid thin films display 20-fold higher gravimetric responses to oxygen and water vapor than those of control ZnO thin films in the dark. An additional 50-500% enhanced responses are detected under UV irradiation. The overall enhanced gravimetric response in the hybrid film is attributed to the ZnO molecules distributed in the polymer matrix, whereas the UV enhancement is explained by the light-induced, reversible generation of hydrophilic fluoroantimonic acid from triarylsulfonium hexafluoroantimonate photoacids, which leads to the increased surface potential and adsorption energies for oxygen and water. A gravimetric sensor based on a series of ZnO-infiltrated SU-8 films under UV excitation enables 96% accurate classification of water and oxygen environment with sub 10 mTorr detection limits. The results demonstrate UV-induced fully reversible surface hydrophilicity of ZnO/SU-8 hybrid nanocomposites.

Athermal Narrow-Band Filters Based on Side-Modulated Bragg Gratings

An athermal Bragg grating filter with a narrow bandwidth is proposed and demonstrated. By integrating side-modulated Bragg gratings pillars to silicon-on-insulator slot waveguides, the narrow bandwidth of the wavelength filter can be achieved. With the negative thermo-optic coefficient SU-8 polymer top cladding, the device can be designed to be temperature-insensitive. The proposed Bragg grating filter, with a narrow 3-dB bandwidth around 0.75 nm, can be easily fabricated with only one fully etched step and large corrugation depth. The measured temperature dependence of the fabricated Bragg grating filter is -3 pm/°C.

MEMS 3D Scan Mirror with SU-8 Membrane and Flexures for High NA Microscopy.

We demonstrate a MEMS beam scanner capable of biaxial scanning with simultaneous focus control, for integration into a handheld confocal microscope for skin imaging. The device is based on a dual axis gimbal structure with an integrated largestroke deformable mirror. SU-8 polymer is used to construct both the deformable membrane as well as the torsional hinges for biaxial scanning. The 4 mm diameter mirror can perform raster pattern scanning with a range of +/- 1.5 degrees and Lissajous scanning with a range of +/- 3 degrees (mechanical scan angle), and has a maximum deflection of 9 um for focus control. The design, fabrication and characterization of the opto-mechanical performance of the MEMS device are presented in this paper.

Powering the Internet of Things

Internet connected devices are limited by the performance of microbatteries that power them. In this issue of Joule, Hur et al. develop microbatteries with tall 3D electrodes that have high areal energy densities. These microbatteries were fabricated on a silicon wafer using silicon and polymer processing that are compatible with microelectronics, paving the way for future microbattery and microelectronics integration.

Micro-Lego of 3D SU-8 structures and its application to a re-entrant surface

Deterministic multi-layer micro-scale assembly of SU-8 polymer objects as well as their cohesive interfacial joining strength are investigated. The process of SU-8 photoresist into retrievable individual micro-scale objects is reviewed and the construction of three-dimensional (3D) SU-8 micro-structures via micro-Lego, which involves transfer printing and permanent joining of SU-8 objects on a target site, is demonstrated. Owing to the additive 3D nature of micro-Lego, the demonstrative micro-scale architectures are unattainable through other means such as conventional photolithography. The mechanical joining strength between SU-8 objects assembled by micro-Lego is inspected by probing SU-8 cantilever specimens using a nanoindenter and the results reveal that the joining strength is ~ 0.988 J/m2, which is comparable to wafer scale silicon fusion bonding strength. In order to demonstrate a practical application of SU-8 micro-Lego, a SU-8 re-entrant surface is assembled by one step process. It is shown that the SU-8 re-entrant surface can repel a water droplet while SU-8 is natively hydrophilic with a low intrinsic contact angle (73 ± 5°).

Non-Invasive Millimeter-Wave Profiler for Surface Height Measurement of Photoresist Films

This paper presents a low-cost non-invasive millimeter-wave surface-height measurement sensor of dielectric and polymer films on glass and quartz substrates. The surface-height profiler utilizes near-field resonance measurement technique operating at 96 GHz implemented by using a single complementary split-ring resonator (CSRR) integrated with a tailor-made WR10 rectangular waveguide. By placing a glass or quartz substrate uniformly coated with SU-8 photoresist on top of the CSRR, the thickness of the SU-8 polymer can be extracted based on the reflected and transmitted electromagnetic-wave energy interacting at the electrical boundary between the substrate and polymer film. Uniform single layers of SU-8 polymer with thicknesses from 3 to $13~\mu \text{m}$ coated on top of the glass substrate are measured and characterized. The extracted polymer-film thicknesses from the sensor in this paper show an agreement of higher than 95% as compared with the commercial surface profiler instrument, while offering various advantages, such as non-invasion, ease of measurement setup, low-cost, and miniaturization.

Detection of heart-type fatty acid-binding protein (h-FABP) using piezoresistive polymer microcantilevers functionalized by a dry method

Piezoresistive microcantilever-based sensor platform is being used for the last two decades due to their low cost, rapid response and label-free detection system. In this work, we are reporting a microfabricated piezoresistive SU-8/carbon black (polymer cantilever)-based sensor platform for the detection of a clinically important early-stage cardiac marker, i.e., fatty acid-binding protein. It is a most preferred cardiac marker for the diagnosis of acute myocardial infarction. The embodiment of the sensor is a SU-8 microcantilever chip with an integrated nanoparticle composite (carbon black) as a piezoresistor for on-chip electrical transduction. Prior to improving the sensing and susceptibility towards the specific target biomolecule (i.e., h-FABP), the fabricated SU-8 polymer cantilevers were subjected to tailored functionalization. This includes the use of an in-house dry method of hot wire chemical vapour deposition technique to graft amine groups onto the SU-8 surface. The surface-modified microcantilevers were further integrated with a polydimethylsiloxane liquid flow cell and connected externally with an electrical read-out system. Immobilization of the antibody corresponding to the marker protein on the microcantilever surface and subsequent recording of the signal generated upon the antibody–antigen interaction were carried out inside the liquid flow cell. Using our optimized immobilization protocol with this experimental set-up, we were successfully able to detect h-FABP concentration as low as 100 ng/ml.

Development of micro-ring resonator-based optical bandpass filter using SU-8 polymer and optical lithography

Laterally-coupled SU-8 ridge waveguide-based micro-ring resonator was designed, fabricated and characterized for optical filtering applications. Fabrication was done by optical lithography using a patterned chrome mask of SU-8 ridge waveguide and micro-ring resonator structures, which was replicated onto a plasma enhanced chemical vapour deposited (PECVD) silicon-dioxide layer on a silicon wafer. Optical characterization showed that the fabricated micro-ring resonator had a free spectral range (FSR) of ∼16 nm for a ring radius of 15 μm in TE polarization. A simple semiconductor laser diode and a monochromator were used for the characterization of the device. The measured through port and drop port light output for different wavelengths indicate that the device can be used as an optical filter around 1565 nm centre wavelength, with a 3 dB bandwidth of 5.3 nm and an extinction ratio of ∼10.5 dB.

Demonstration of versatile whispering-gallery micro-lasers for remote refractive index sensing.

We developed chip-scale remote refractive index sensors based on Rhodamine 6G (R6G)-doped polymer micro-ring lasers. The chemical, temperature, and mechanical sturdiness of the fused-silica host guaranteed a flexible deployment of dye-doped polymers for refractive index sensing. The introduction of the dye as gain medium demonstrated the feasibility of remote sensing based on the free-space optics measurement setup. Compared to the R6G-doped TZ-001, the lasing behavior of R6G-doped SU-8 polymer micro-ring laser under an aqueous environment had a narrower spectrum linewidth, producing the minimum detectable refractive index change of 4 × 10-4 RIU. The maximum bulk refractive index sensitivity (BRIS) of 75 nm/RIU was obtained for SU-8 laser-based refractive index sensors. The economical, rapid, and simple realization of polymeric micro-scale whispering-gallery-mode (WGM) laser-based refractive index sensors will further expand pathways of static and dynamic remote environmental, chemical, biological, and bio-chemical sensing.

Formation of Deep-Subwavelength Structures on Organic Materials by Femtosecond Laser Ablation

Femtosecond (fs-) laser induced subwavelength and deep-subwavelength periodic structures have attracted attention due to their sub-diffraction feature size and rich physics of laser-matter interaction. However, the formation mechanisms of fs-laser induced deep-subwavelength structures on organic materials have not been reported nor systematically studied. Herein, based on the degree of laser induced free-carrier excitation, the formation of deep-subwavelength structures by fs-laser ablation was systematically studied on popular materials: negative tone resists SU8 and conductive polymer poly-3, 4-ethylenedioxythiophene: poly-(styrenesulfonate) (PEDOT: PSS). A weak propensity to form deep-subwavelength structures was found on poly-methylmethacrylate and polyvinyl alcohol. The photo-excited electrons forming the surface plasmonic wave contributed to energy localization and absorption, which led to the imprinting of deep-subwavelength structures on polymers. It has been found that materials with $\pi $ -bonds and benzene rings were most susceptible to deep-subwavelength structure formation. Based on a laser-induced surface plasmonic model, the period of regular patterns was estimated and was in a good accordance with experimental results. The proposed formation mechanism of the deep-subwavelength structures on organic materials extends an application field of fs-laser micro/nanomachining of polymers.

A comparative study of tribological and mechanical properties of composite polymer coatings on bearing steel

In this study, epoxy and SU-8 resin along with its composites was coated over bearing steel. The graphite, talc and base oil were used to fabricate the composites using spin-coater. The tribological and surface characterisation tests were performed on both type of polymer and its composite using ball-on-disk setup, optical microscope, 3D optical profilometer, and goniometer, respectively. From the results, it can be seen that epoxy and its composites polymer showed lower coefficient of friction (~0.16), high wear resistance properties (> 102 times), improvement in mechanical properties (elastic modulus and hardness) (~3-4 times) and better hydrophobic nature as compared to SU-8 polymer (~34%) and its composites. Furthermore, a neural network model was developed to determine the influential parameters on specific wear rate and coefficient of friction.

A comparative study of tribological and mechanical properties of composite polymer coatings on bearing steel

In this study, epoxy and SU-8 resin along with its composites was coated over bearing steel. The graphite, talc and base oil were used to fabricate the composites using spin-coater. The tribological and surface characterisation tests were performed on both type of polymer and its composite using ball-on-disk setup, optical microscope, 3D optical profilometer, and goniometer, respectively. From the results, it can be seen that epoxy and its composites polymer showed lower coefficient of friction (~0.16), high wear resistance properties (> 102 times), improvement in mechanical properties (elastic modulus and hardness) (~3-4 times) and better hydrophobic nature as compared to SU-8 polymer (~34%) and its composites. Furthermore, a neural network model was developed to determine the influential parameters on specific wear rate and coefficient of friction.

Characterization of long-range plasmonic waveguides at visible to near-infrared regime

Long-range surface plasmon polariton waveguides composed with thin gold stripes embedded in SU-8 polymer cladding with various stripe widths were fabricated. Material properties of the polymer cladding layer, gold thin film, and the device structures were discussed. Optical properties based on modal propagation were characterized at visible to near-infrared wavelengths. The measured propagation losses of waveguide widths from 3 to 9 μm at 633, 785, and 1550 nm are 7.5-18.8, 6.8-12.5, and 1.9-3.9 dB/mm, respectively. Guiding mode properties such as overlap integrals between the simulated and the measured fields and the polarization extinction ratios of the waveguides with different stripe widths were investigated at the telecommunication wavelength. Good accordance between the measurement and simulation results was presented.

All-organic electro-optic waveguide modulator comprising SU-8 and nonlinear optical polymer.

In this paper we describe the principles of operation as well as the fabrication and testing steps of an all-organic waveguide modulator. The modulator comprises an SU-8 core and an electro-optic host-guest polymer cladding. The polymer properties are tuned in order to achieve single mode operation. We used direct-write laser lithography in two steps for the preparation of the devices. The electro-optic coefficient of the polymer is estimated from observing the modulation of the device operated in push-pull mode.