Study of X-ray lithographic conditions for SU-8 by Fourier transform infrared spectroscopy

There is growing interest in the use of chemically-amplified resists (CARs) such as SU-8 in the field of microelectromechanical systems (MEMS) research. This is due to its outstanding lithographic performance and its ability for use in the fabrication of stable structures with very high aspect ratio. However, it is important to control the processing conditions for optimum results in the desired application. In this investigation, the thickness (10-25 μm) of SU-8 resist film, due to different spin coating speeds on silicon wafers, was measured using Fourier transform infrared (FT-IR) spectroscopy. The effect of thermal-initiated cross-linking at various temperatures (95-160 °C) for 15 min baking time on the 25 μm SU-8 resist was studied by monitoring the 914 cm⁻¹ absorption peak in the FT-IR spectrum. Results of the experiments showed that the onset of thermal-initiated cross-linking begins at about 120 °C. Furthermore, 25 μm SU-8 resist was optimized for X-ray lithographic applications by studying the cross-linking process of the resist under different conditions of post-exposure bake (PEB) temperatures. The exposure dose of soft X-ray (SXR) irradiation with energies about 1 keV from a dense plasma focus (DPF) device was fixed at 2500 mJ/cm² on the resist surface. Results showed that the optimum processing conditions consisted of an intermediate PEB at 65 °C for 5 min, with the PEB temperature ramped up to 95 °C over 1.5 min and then followed by a final PEB at 95 °C for 5 min. The scanning electron microscopy (SEM) images showed SU-8 test structures successfully imprinted, without affecting the resolution, and with aspect ratios of up to 20:1 on 25 μm SU-8 resist.     

Wet releasing and stripping SU-8 structures with a nanoscale sacrificial layer

This paper presents a new method of wet releasing SU-8 structures using very thin Omnicoat^™ as sacrificial layer and NMD-3 developer as releasing solution. Four-inch sized SU-8 structures with different pattern and thickness of 65 μm have been successfully released from silicon substrate by this method. Experiments show that Omnicoat^™ layer is very helpful to strip SU-8 molds in which metal structures are electroplated. SU-8 can be removed directly using remover if there is an Omnicoat^™ layer between the substrate and SU-8 and the electroplated structures is with large dimension. In the case of electroplated structures with small feature size, SU-8 can be removed by combining wet stripping and plasma etching. Nickel micro coils with linewidth of 10 μm and aspect ratio of 4.1 have been fabricated. The analysis about the influence of Omnicoat^™ layer’s thickness on integrity of SU-8 micro coils is also given in our work.     

Microcontact printing of biomolecular gratings from SU-8 masters duplicated by Thermal Soft UV NIL

Thermal Soft UV nanoimprint lithography (NIL) was performed to replicate nanostructures in SU-8 resist. The SU-8 resist was structured with a PDMS stamp molded against an original silicon master which comported gratings of lines (500 nm width/1 μm pitch). The patterns obtained in SU-8 were used in a second step as a template for PDMS molding of daughter stamps. Pattern transfer quality and dimension control were achieved on these second generation PDMS stamps using AFM measurements. As a final validation of the whole duplication processes, these second generation PDMS stamps were finally employed to perform μCP of streptavidin molecules on a glass slide activated by plasma O₂ treatment. AFM observation and fluorescence microscopy reveal that molecular patterns produced with SU8-molded PDMS stamps are not discernable from those obtained with a PDMS stamp directly molded on the original silicon master. Coupling Thermal Soft UV NIL and microcontact printing opens a new method for generating a large quantity of SU-8 templates on which functional PDMS stamps can be replicated in a reduced time. We thus propose a functional duplication process for soft-lithography implementation which may further reduce the cost of this technology for industrial development.     

Compact LED based nanoimprinter for UV-NIL

UV-based nanoimprint lithography (UV-NIL) is a cheap and fast way to imprint patterns ranging from nanometres to micrometres. However, commonly used equipment can be expensive and require a clean room infrastructure. Here we present the design and testing of a simple UV-NIL system based on a light emitting diode. The current design permits imprints of 10 × 10 mm² in size using a 25 × 25 mm² master. This printer can be used in a semi-clean environment such as a laminar flow bench. The imprinter was used to imprint photoresists as well as UV sensitised hydrogels. The best results were obtained using SU-8 photoresist with features down to 50 nm in size, only limited by the imprint master. Patterns in SU-8 resist were also transferred into silicon substrates by reactive ion etching demonstrating its full potential as a lithographic tool.     

TiO2-poly(4-vinylphenol) nanocomposite dielectrics for organic thin film transistors

We report on the fabrication of organic thin film transistors (OTFTs), which operate at low voltages, by incorporating a nanocomposite gate insulator material consisting of titania (TiO₂) nanoparticles used as fillers and poly(4-vinyl phenol) (PVP) used as matrix. The surface of the nanoparticles was modified by the ligands, 4-hydroxybenzoic acid, to enhance their compatibility with the polymer. The structure of the ligand is similar to that of the repeat units in the polymer. Once the nanoparticles were homogeneously dispersed in the polymer matrix, they were immobilized by cross-linking PVP with poly(melamine-co-formaldehyde) methylated/butylated (cross-linker). Consequently, no significant aggregation of the nanoparticles, even at a concentration of 31 wt%, was found in the nanocomposites, as observed by transmission electron microscopy (TEM). As a result, the nanocomposite exhibited a low leakage current density (∼10⁻⁸ A/cm⁻²). With an increase in the concentration of TiO₂ nanoparticles added, the dielectric constant of the nanocomposites also increased proportionately as compared to that of pristine PVP. The performance of the OTFTs in terms of the charge carrier mobility, on/off ratio, threshold voltages, and hysteresis was evaluated. In addition, the relationship between the concentration of TiO₂ nanoparticles and the device performance is discussed in detail.     

Fabrication and application of silicon-reinforced PDMS masters

A new molding process is developed in this work to generate a silicon (Si)-reinforced polydimethylsiloxane (PDMS) master of a 4 in wafer size using an SU-8 mold. The reinforced PDMS master is applied to pattern a conducting polymer, poly-3-hexylthiophene (P3HT), which is normally dissolved by a non-polar solvent. PDMS is usually patterned by a molding process, in which PDMS is first coated on and then peeled off from a rigid mold. However, in the new molding process, the Si-reinforced PDMS master is rigid but the SU-8 mold is flexible, and the SU-8 mold is first placed on and then peeled off from the rigid PDMS master. In such a way, a reinforced PDMS master of a size as large as a 4 in wafer can be produced. Meanwhile, a new way of obtaining free-standing, large SU-8 structures is presented. PDMS swells when it gets exposed to non-polar solvents. This swelling makes PDMS not suitable for patterning materials, which are usually dissolved by non-polar solvents, e.g., P3HT. In this work, we demonstrate that, with the reinforcement of a Si plate, the swelling effect in generating this specific type of materials is much reduced, and good patterns can be produced.     

Conception of optical integrated circuits on polymers

The authors present a successful design, realisation and characterisation of single-mode TE₀₀-TM₀₀ rib optical waveguides composed of SU-8 polymer. For the simulation, a generic software package that provides an interactive and graphical environment for analysis by polarised Semi-Vectorial Finite Difference (SVFD) method of all kinds of integrated optical waveguides, such as buried channel, raised strip, rib, embedded, or ridge waveguides, has been implemented and tested. In this method we have taken into account the terms due to the interface between each layer. After realisation of various single mode optical waveguides on SU-8 polymer and Spin on Glass (SOG) like straight, S-bends, Y-junctions, Mach-Zehnder (MZ) interferometers, the linear absorption coefficient of energy α^TE-TM of such rib waveguides have been measured and estimated, respectively, near 0.32 and 0.46 cm⁻¹ for both optical modes TE₀₀ and TM₀₀ on Si/SiO₂/SU-8 structures. These values yield optical losses of 1.36 and 2.01 dB/cm. Optical losses ascribed to Si/SiO₂/SOG/SU-8 microstructures have been evaluated to 2.33 and 2.95 dB/cm for both polarisations. Hence, as a crucial step for designing polymer components devoted to microsensors applications (pressure, heat transfert), the SU-8 polymer appears as a promising candidate for integrated optics with low optical losses.     

Patterning of SiO2 nanoparticle-PMMA polymer composite microstructures based on soft lithographic techniques

Soft lithography and self-assembly provide powerful means of organizing colloidal solution of synthesized nanoparticles (NPs) for a wide variety of application. Pattern transfer of silicon dioxide (SiO₂) nanoparticles-polymethylmethacylate (PMMA) nanocomposite was investigated using two such soft lithographic techniques, micro molding in capillaries (MIMIC) and micro transfer molding (μTM) using an elastomeric stamp in Polydimethyl siloxane (PDMS). Nanocomposite periodic arrays of 20 μm wide and 10 μm deep lines with 10 μm spacing were obtained over approximately 1 cm² area on silicon substrates by μTM and MIMIC using a 3 wt.% monodisperse silica nanoparticles (∼338 ± 2 nm) in polymethyl methacrylate (PMMA) solution. In addition, free standing nanocomposite self-standing films of centimeter size were also manufactured by μTM. Single line of nanocomposite could also be obtained using MIMIC with a lower concentration of silica NPs (0.25 wt.%) in PMMA.     

Extended lifetime of pentacene thin-film transistor with polyvinyl alcohol (PVA)/layered silicate nanocomposite passivation layer

We prepared a novel organic passivation material to protect organic thin-film transistors (OTFTs) from H₂O and O₂ using a polyvinyl alcohol (PVA)/hydrophilic layered silicate (HLS) nanocomposite system. Using up to a 3 wt% layered silicate to PVA weight concentration, a highly homogeneous nanocomposite solution was prepared. In addition, a PVA/HLS nanocomposite solution formed a smooth film layer by the spin-coating method with RMS surface roughness of about 1 nm. An OTFT device with PVA passivation showed a decrease of 23% in field effect mobility after passivation. However, a pentacene TFT device with PVA/HLS nanocomposite passivation showed no significant initial performance drop after passivation, and mobility was even slightly improved. Pentacene TFT-passivated PVA/HLS showed 2860 h of TFT lifetime (the time required to reduce the mobility by one-half of the initial mobility after passivation), which is almost twice the lifetime of pentacene TFT with PVA passivation (1320 h). We propose that a layered silicate containing PVA nanocomposite film can be used as an effective organic passivation layer in an OTFT.     

Facile Single‐Precursor Synthesis and Surface Modification of Hafnium Oxide Nanoparticles for Nanocomposite γ‐Ray Scintillators

Inorganic nanoparticles/polymer nanocomposites provide a low cost, high performance alternative for gamma scintillation. However, inorganic nanoparticles used thus far suffer from either moderate atomic numbers or low band gaps, limiting the gamma stopping power and photoelectron production in these systems. Here, a highly efficient, facile single‐precursor synthesis protocol is reported for hafnium oxide nanoparticles with an average diameter of 5 nm. The nanoparticle surface is further functionalized for the fabrication of highly transparent bulk‐size nanocomposite monoliths (2 mm thick, transmittance at 550 nm >75%) with nanoparticle loadings up to 40 wt% (net hafnium wt% up to 28.5%). Using poly(vinyltoluene) as the matrix, 2‐(4‐tert‐butylphenyl)‐5‐(4‐biphenylyl)‐1,3,4‐oxadiazole and 1,4‐bis(5‐phenyl‐2‐oxazolyl)benzene as the cascade fluors, and hafnium oxide nanoparticles as the gamma sensitizer, the nanocomposite monolith of 1 cm diameter and 2 mm thickness is fabricated capable of producing a full energy photopeak for 662 keV gamma rays, with the best deconvoluted photopeak energy resolution <8%.     

Bending characteristics of SU-8

This study characterises SU-8's material properties under large deformations in out-of-plane bending. A novel test setup is presented to deform the SU-8 to high levels, and a mathematical model presented that accurately predicts the results. This test shows that hard-baked SU-8, with a Young's modulus tested at 1.9 GPa, is capable of undergoing stresses of up to 120 MPa before failure and in some cases will undergo plastic deformation.     

玻璃纤维增强SU-8胶技术

SU-8胶是一种能够以低成本制作高深宽比微结构的负性光刻胶,在非硅微电子机械系统(MEMS)领域具有广阔的应用前景。针对SU-8胶作为微结构材料时,其弹性模量和断裂强度较低的特性,采用透明度高的微细玻璃纤维作为增强相材料,制备玻璃纤维/SU-8复合材料。开发出一种既能够增强成型后SU-8微结构机械强度,又不影响SU-8可直接光刻成型能力的改性技术。通过光学显微镜和扫描电子显微镜(SEM)对该复合材料的微观结构进行了分析和表征。同时,利用拉伸实验对复合材料的机械性能进行测试。实验结果表明,在不改变紫外光刻电铸(UV-LIGA)的光刻-显影工艺特征的条件下该复合材料的弹性模量(1 613 MPa)、断裂伸长率(1.63%)和断裂强度(26.7 MPa)均比纯SU-8胶材料有较大提升。     

微桥法镍膜的弹性模量和残余应力测量

利用MEMS技术制作了不同尺寸的镍(Ni)膜微桥结构样品。采用纳米压痕仪XP系统测量了微桥载荷与位移的关系,并结合微桥力学理论模型得到了两种不同尺寸的Ni膜的弹性模量和残余应力。结果表明,两种不同尺寸的Ni膜的弹性模量结果一致,为190 GPa左右,但是残余应力变化较大。与采用纳米压痕仪直接测得的带有硅(Si)基底的Ni膜弹性模量186.8 7.5 GPa相比较,两者符合较好。     

Mechanical characterization of aluminium nanofilms

The mechanical properties (Young’s modulus, hardness, wear resistance) of aluminium nanofilms on silicon substrate are studied. Size effect on these mechanical properties are exhibited. Young’s modulus, hardness and wear resistance increases when the thickness is reduced. Experimental investigations have been led by atomic force microscopy (AFM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and nanoindentation. Compared to the bulk values, hardness and wear resistance of one aluminium nanofilm (thickness = 100 nm) have increased by a factor ∼7 whereas the Young’s modulus only increased by a term ∼15%. By comparing mechanical properties between high and low melting point materials, we conclude that high melting point materials have a decreasing behaviour of the Young’s modulus with size whereas low melting point materials have an increasing one.     

Three-dimensional SU-8 sub-micrometer structuring by electron beam lithography

We demonstrate electron beam lithography on the negative tone electron resist SU-8 to fabricate self-supporting three-dimensional structures in sub-micrometer range. Applying SU-8 thin films spin cast on glass substrates and forming layers of 1 μm thickness, the structuring is performed in a two step process. First, the SU-8 film is exposed for supporting structures down to the substrate, a second exposure step with accordingly modified parameters leads to elevated structures. Applications as microscale shadow masks for evaporation based deposition processes and microfluidics are discussed.     

Fabrication and application of metallic nano-cantilevers

A new approach was developed in this work to fabricate metallic nano-cantilevers using a one-mask process and a deep reactive ion etch (DRIE) technique. 40-nm-thick Al and 70-nm-thick Au cantilevers of lengths from 5 μm and widths in the range of 200-300 nm were fabricated on a silicon substrate. The silicon underneath the suspended beams was completely etched. Short Al nano-cantilevers were used to find local residual stress induced in rapid thermal oxidation and the oxidized spots according to the deflection profiles of the nano-cantilevers. The deflection profiles were determined with the aid of a scanning electron microscope (SEM). Compared with a single feedback in the existing cantilever-based static methods, i.e., the deflection of the open end of a cantilever, the whole deflection profile provides more information regarding the effect of surface stresses on a cantilever.     

Formation of aluminum tunnel pits arrayed using SU-8 masks with UV-assisted thermal imprint lithography

For high capacitance aluminum electrode was selectively etched with square of tunnel pits. SU-8 photoresist as etching mask was patterned on aluminum by UV-assisted thermal imprint lithography and then surface area of aluminum was increased by electrochemical etching, where tunnel pits were generated regularly and were approximately 20 μm in length, 3.5 μm in width, resulting in 10⁶ tunnels per cm² of surface. Consequently, the capacitance of the dielectric showed an increase of up to four times higher than the unpatterned surface.     

Exposure parameters for MeV proton beam writing on SU-8

Proton beam writing was performed on a lithographic resist to determine the main parameters required to achieve the minimum feature size, maximum pattern lateral density and maximum aspect ratio. A 2.5 MeV proton beam focused to sizes between 1.5 and 2.5 μm was used to expose SU-8 negative resist. The number of protons per pixel was varied in the exposure of SU-8 with thicknesses between 5 and 95 μm. Patterns consisting of single pixels, single-pixel lines and multi-pixel areas with different densities were fabricated. The smallest structures achieved were posts 1.5 μm in diameter with 4:1 structure-space ratio in 15 μm thick resist and the highest aspect ratio structures of 20:1 in 40 μm resist were produced. It was found that the minimum feature size depended only on the beam size, and ±10% post size accuracy could be achieved within 40-70% variation of the number of protons. MeV proton beam allows a direct fabrication of complex shapes without a mask in single-step irradiation and, in addition, no proximity correction is needed. We present examples of MeV proton beam written single and multi-pixel microstructures with easily reproducible high aspect ratios and densities.     

Research of mechanical stresses in irradiated tin-doped silicon crystals

An optical method of registration of mechanical stresses in undoped and tin-doped silicon samples is offered. Influence of electron irradiation on energy 5 MeV and high-temperature treatment at a 723 K on residual stresses in a silicon lattice was analyzed in the paper. The proposed method is based on а modulation of polarization of laser radiation transmitted through the anisotropic area and the definition of its anisotropy parameters by means of this modulation. The modulation polarimetry technique is an express method with high detection and resolution. The method allows identifying residual stresses in samples in absolute units with a resolution of 1·10⁻⁴ MPa.     

Molecular‐Level Dispersion of Graphene into Poly(vinyl alcohol) and Effective Reinforcement of their Nanocomposites

Despite great recent progress with carbon nanotubes and other nanoscale fillers, the development of strong, durable, and cost‐efficient multifunctional nanocomposite materials has yet to be achieved. The challenges are to achieve molecule‐level dispersion and maximum interfacial interaction between the nanofiller and the matrix at low loading. Here, the preparation of poly(vinyl alcohol) (PVA) nanocomposites with graphene oxide (GO) using a simple water solution processing method is reported. Efficient load transfer is found between the nanofiller graphene and matrix PVA and the mechanical properties of the graphene‐based nanocomposite with molecule‐level dispersion are significantly improved. A 76% increase in tensile strength and a 62% improvement of Young's modulus are achieved by addition of only 0.7 wt% of GO. The experimentally determined Young's modulus is in excellent agreement with theoretical simulation.