In addition to being used for pattern transfer, the negative photoresist SU-8 is widely used as a structural material in microelectromechanical systems (MEMS). Due to its good photopatternability, SU-8 has lower manufacturing costs than many other materials, but its mechanical properties are relatively weak to some extent, which limits its performance. The mechanical properties of epoxy-like SU-8 can be enhanced by adding micro- or nano-fillers such as carbon nanotube, clay, and SiC nanowire, which have superior elastic modulus. In this study, SiC nanowires were used to improve the mechanical properties of SU-8 while the SU-8 retains its photopatternability. The SiC nanowires were uniformly dispersed in SU-8 by stirring and ultrasonication. SU-8 materials with different SiC nanowire contents were fabricated into dog bone samples by lithography. The elastic modulus, storage modulus, and damping factor of the samples were measured by the Dynamic mechanical analysis (DMA) Q800. The experiment result shows that the rigidity and toughness increased, and the damping reduced. The 2 wt% SiC nanowires-reinforced SU-8 had a 73.88% increase in elastic modulus and a 103.4% increase in elongation at break. Furthermore, a spring component made by SiC-doped SU-8 could withstand greater acceleration. The SiC nanowires-reinforced SU-8 has the potential to meet higher requirements in the design and manufacture of MEMS and greatly reduce the manufacturing costs of MEMS devices.

  • SiC nanowires were uniformly dispersed in SU-8 by stirring and ultrasonication.

  • The mechanical properties of SU-8 were significantly improved by compounding it with SiC nanowires.

  • SiC-doped SU-8 still features photopatternability, which facilitates rapid formation of structural materials.

As a negative photoresist, SU-8 is increasingly used as a structural material for microelectromechanical systems (MEMS) and microfluidics in addition to being used for pattern transfer.1 Because of its low manufacturing cost, good chemical stability, and thermomechanical properties,2 SU-8 has advantages over other materials in some cases. Moreover, its good biocompatibility3 makes it suitable for the fabrication of microfluidic devices.

As SU-8 is being used as a structural material in more and more cases, its performance, due to its properties such as mechanical properties, is getting increasing attention. In some cases, its mechanical properties still need to be enhanced to meet higher requirements. The mechanical properties of epoxy resin have been improved to some extent by adding micro- or nano-fillers.4–6 Similarly, many studies have improved the mechanical properties of SU-8 by adding fillers such as GO,7 single-wall carbon nanotubes, gold nanospheres8 and multi-walled carbon nanotubes.9,10 However, the improvement is small, and the addition of these materials will greatly affect the electrical insulation of SU-8.

The research shows that compared to micro-fillers, nano-fillers can more remarkably improve the mechanical properties of the composite due to larger interfacial areas.11 

Silicon carbide nanowires have been regarded as an excellent reinforcement material for epoxy resin due to their high tensile strength, high elastic modulus, good thermal stability, high plasticity,12,13 high glass transition temperature,14,15,16 and good chemical stability. Through molecular dynamics simulations, it has been found that the nano-sized SiC incorporated into the epoxy significantly improves the glass transition temperature and elastic modulus of the epoxy.15 In Ref. 16, the tensile test results showed that the tensile strength of polyurethane incorporated with nano-sized SiC was 2.2 times higher than that of pure polyurethane. In an experiment, Jitendra K. et al17 greatly improved the mechanical properties of SU-8 by adding carbon fillers, but this scheme causes SU-8 to lose its photopatternability and electrical insulation.

In this study, β-SiC nanowires were used to compound SU-8, allowing the composite to achieve better mechanical properties by photolithography. The influence of the SiC nanowires on the mechanical properties of SU-8 was studied. For this purpose, a tensile test and dynamic mechanical analysis (DMA) were carried out. The elastic modulus, elongation at break, storage modulus, and glass transition temperature (Tg) of the SU-8 were analyzed. The mixture ratio of the SiC nanowires to the SU-8 was optimized by analyzing the mechanical properties.

The SiC nanowires (β-SiC, Changsha Sinet Advanced Materials Co. Ltd.) shown in Fig. 1, with a length of 50–100 μm and diameter of 0.1–0.6 μm, were used. The SU-8 negative photoresist was purchased from MicroChem (USA).

Fig. 1.

SEM image of β-SiC nanowires.

Fig. 1.

SEM image of β-SiC nanowires.

Close modal

Good dispersion of nano-fillers in the matrix is crucial, as it can effectively improve the mechanical properties. In general, nano-fillers are prone to agglomeration, which will result in weak interaction between the nanofiller and the matrix, thus making it difficult to effectively improve the mechanical properties or even reduce mechanical properties.11 Therefore, taking more time to achieve uniform dispersion of SiC nanowires within the SU-8 is necessary.

Silicon carbide nanowires were directly dispersed in the range of 0.5 wt%–2.5 wt% in the SU-8 by full mixing and ultrasonic bath to reach a good dispersion of nanowires in the SU-8. The dispersion of SiC nanowires in the SU-8 was analyzed by a digital microscope and scanning electron microscope. The dispersion condition is shown in Fig. 2. As can be seen, the SiC nanowires were uniformly dispersed in the horizontal and vertical directions of the SU-8. The uniformly distributed SiC nanowires can achieve effective load transfer and prevent stress concentration.18 As the content increased, the nanowires distribution became increasingly dense. The aggregation of SiC nanowires occurred until the content reached 2.5%, which easily induced voids in the composites11 and affected performance improvement. SU-8 uniformly dispersed with SiC nanowires was used to prepare samples.

Fig. 2.

Dispersion conditions of SiC nanowires in SU-8: (a) 3D super digital microscope photograph of different SiC nanowire contents; (b) SEM image of vertical distribution (etched sample side).

Fig. 2.

Dispersion conditions of SiC nanowires in SU-8: (a) 3D super digital microscope photograph of different SiC nanowire contents; (b) SEM image of vertical distribution (etched sample side).

Close modal

A sketch of the main fabrication process is depicted in Fig. 3 and described as follows:

  • A three-inch glass substrate was chosen and cleaned in a standard style.

  • Positive photoresist AZ4620 was spin-coated on the glass and substrate as a sacrificial layer after thermal treatment at 100 °C.

  • Prior to coating, a vacuum was used to remove the air that mixed with SU-8 during the mixing process. Afterward, the SU-8 was spin-coated on the sacrificial layer.

  • The substrate was kept in a horizontal position at room temperature to level off the SU-8 layer and release stress. Then, this substrate was soft-baked on a hot plate. After the SU-8 was naturally cooled, it was exposed to ultraviolet light using a mask, which defined the test sample. The post-baked substrate was then cooled naturally.

  • The SU-8 layer was ultrasonic, developed in a propylene glycol monomethyl ether acetate (PGMEA) bath.

  • The SU-8 test samples were soaked in isopropanol and water successively.

Fig. 3.

Samples fabrication flow.

Fig. 3.

Samples fabrication flow.

Close modal

The shape and dimensions of the specimen are according to the dog bone model in the literature10,19,20 and the requirements of the test equipment. As shown in Fig. 4, the length, width, and thickness of the sample are 10 mm, 2 mm, and 0.25 mm respectively.

Fig. 4.

(a) Layout of sample on the mask; (b) dimensions of sample.

Fig. 4.

(a) Layout of sample on the mask; (b) dimensions of sample.

Close modal

As shown in Fig. 6(a), many nanowires exist in the sidewalls of structures formed by lithography. To remove these excessive nanowires to make the size of the sample more accurate, the samples were treated by reactive-ion etching (RIE). The gases used in the RIE process are SF6 and oxygen. By optimization of the RIE parameters such as etching power, pressure, and gas flow, the nanowires in sidewalls were completely removed, and the matrix was not affected, as shown in Fig. 6(b).

The SiC-doped SU-8 was proven to have an advantage over pure SU-8 in MEMS devices. This is demonstrated by Fig. 7, which shows a spring component made of SiC-doped SU-8 and developed using photolithography.

Scanning electron microscopy was used to study the surface morphology and the fracture surface of the samples with different SiC loadings. A thin gold coating was applied to improve the conductivity for good observation.

There are many ways to test the mechanical properties of materials, such as tension and nanoindentation. For viscoelastic materials such as SU-8, the choice of test methods may affect the test accuracy. In this situation, a tension test is more suitable.21 The tensile diagram is shown in Fig. 5. The mechanical properties of the pure SU-8 and SiC nanowires-reinforced SU-8 were analyzed by a dynamic mechanical analyzer (DMA-Q800, TA Instruments, Inc., USA).

Fig. 5.

Tensile test schematic.

Fig. 5.

Tensile test schematic.

Close modal
Fig. 6.

Sidewalls of sample before (a) and after (b) RIE.

Fig. 6.

Sidewalls of sample before (a) and after (b) RIE.

Close modal
Fig. 7.

Spring component made with SiC-doped SU-8.

Fig. 7.

Spring component made with SiC-doped SU-8.

Close modal

The tensile test process involved pulling samples at a tensile speed of 15 μm/min in the “DMA strain rate” mode of Q800 until the sample is pulled off.

The parameters for DMA are amplitude, temperature, and frequency. The specimen was stretched by 0.05% before the test was started to remove the initial buckling. At an amplitude of 30 μm and a frequency of 1 Hz, the temperature was raised from 25 °C to 200 °C at a rate of 5 °C/min.

To avoid the contingency, the above test results were derived from the average results of repeated tests.

The effect of SiC nanowires addition on the elastic modulus and elongation at break of the SU-8 is illustrated in Table 1 and Fig. 8, respectively.

Table 1.

Elastic modulus and elongation at break of samples of various SiC contents.

Content (SiC:SU-8 wt%)Elastic modulus (MPa)Percentage increase (%)Elongation at break (%)Percentage increase (%)
0.0 1288.08 1.47 
0.5 1429.82 11.00 1.74 18.37 
1.0 1660.05 28.88 1.97 34.01 
1.5 1902.17 47.67 2.81 91.16 
2.0 2239.66 73.88 2.99 103.40 
2.5 1802.60 39.94 2.63 78.91 
Content (SiC:SU-8 wt%)Elastic modulus (MPa)Percentage increase (%)Elongation at break (%)Percentage increase (%)
0.0 1288.08 1.47 
0.5 1429.82 11.00 1.74 18.37 
1.0 1660.05 28.88 1.97 34.01 
1.5 1902.17 47.67 2.81 91.16 
2.0 2239.66 73.88 2.99 103.40 
2.5 1802.60 39.94 2.63 78.91 
Fig. 8.

Stress of samples of various contents vs. strain.

Fig. 8.

Stress of samples of various contents vs. strain.

Close modal

As the strain increases, the stress rises rapidly and then slowly increases. In the initial phase of the curve (nearly linear region), the slope of the curve represents the elastic modulus of the sample. When the strain increases to the limit, the sample breaks. The maximum x value of the curve represents the elongation at break of the sample. To study the mechanism of pure SU-8 and SiC-reinforced SU-8, the fracture surfaces of broken samples were comparatively observed by scanning electron microscopy (SEM). The sectional views in Fig. 9 were obtained when the samples were stretched to break.

Fig. 9.

The fracture surface of the tensile samples with different SiC loadings: (a) pure SU-8; (b) SiC:SU-8 = 0.5 wt%; (c) SiC:SU-8 = 1.5 wt%. SiC nanowires are marked with dotted box.

Fig. 9.

The fracture surface of the tensile samples with different SiC loadings: (a) pure SU-8; (b) SiC:SU-8 = 0.5 wt%; (c) SiC:SU-8 = 1.5 wt%. SiC nanowires are marked with dotted box.

Close modal

3.1.1. Increase in rigidity

The rigidity of a material is usually measured by its elastic modulus. The elastic modulus, which is the ratio of stress to strain, is calculated and presented in Table 1. As shown in Table 1, the elastic modulus of SU-8 was enhanced with the increase in SiC nanowires addition amount until 2 wt%, from which the elastic modulus dropped with further SiC addition. Compared with other samples, 2 wt% SiC nanowires-reinforced SU-8 had a 73.88% increase in elastic modulus, achieving maximum improvement in elastic modulus, which also means the greatest rigidity improvement.

As shown in Fig. 9, the SiC nanowires homogeneously dispersed in SU-8 could transfer stress when force was applied to the sample, allowing the material to withstand great forces and resist deformation.

3.1.2. Increase in toughness

Toughness is the resistance of a material to breaking when subjected to a force that causes it to deform. The toughness of a film is generally characterized by elongation at break.

With the increase in the addition amount of SiC nanowires, the elongation at break of SU-8 increased. Elongation at break of the samples containing 2 wt% SiC nanowires increased by 103.40%, achieving maximum improvement in elongation at break, which also means the greatest toughness improvement.

The micrograph of pure SU-8 shown in Fig. 9(a) indicates a typical fractography feature of brittle fracture behavior, thus accounting for the low fracture toughness of the unfilled SU-8.22 Much rougher fracture surfaces are shown in Fig. 9(b) and (c).

In Fig. 10, SiC nanowires interact with epoxy chain. In this region, the nanowires bridge the cracks together, reducing the force received by the crack tip and suppressing the crack from continuing to expand and achieving the toughening effect.23 Nanowires' toughening contribution increases with an increase in nanowires content.24 

Fig. 10.

SiC nanowires preventing crack propagation.

Fig. 10.

SiC nanowires preventing crack propagation.

Close modal

3.1.3. Reduction in rigidity and toughness

When the SiC content was increased to 2.5 wt%, a hole (Fig. 11) arising from pulling off was observed, which indicates an agglomeration of SiC nanowires. The inhomogeneous dispersion of SiC nanowires would cause stress concentration and weaker bonding between the nanowires and the matrix, and the agglomeration of SiC nanowires would result in the crack formation when the SU-8 is under stress. Therefore, when the added content of SiC nanowires was higher than 2 wt%, the elastic modulus and elongation at break decreased.

Fig. 11.

The fracture surface of a sample containing 2.5 wt% SiC nanowires.

Fig. 11.

The fracture surface of a sample containing 2.5 wt% SiC nanowires.

Close modal

Fig. 12 shows the typical storage modulus-temperature curves and damping factor (tan δ)-temperature curves of samples of each SiC nanowires content. The ratio of the loss modulus to storage modulus in a viscoelastic material is defined as tan δ, which provides a measure of damping in the material.

Fig. 12.

(a) Storage modulus of samples of various contents vs. temperature; (b) damping factor of samples of various contents vs. temperature.

Fig. 12.

(a) Storage modulus of samples of various contents vs. temperature; (b) damping factor of samples of various contents vs. temperature.

Close modal

For all samples with different SiC nanowire contents, the storage modulus decreased with increasing temperature. At the same temperature, the storage modulus increased as the SiC nanowires content increased until the content reached 2 wt%. When the content reached 2.5 wt%, the storage modulus began to decrease. It is inferred that the rigidity of SU-8 increased with the increase of SiC nanowires content until the content reached 2 wt%. Thus, 2 wt% is the most suitable content to improve SiC nanowires-reinforced SU-8 rigidity.

In the damping factor-temperature curve, the damping factors of all samples first increased and then decreased with increasing temperature. The damping factor of samples containing SiC nanowires was much lower than that of pure SU-8, which means the SiC nanowires reduced the damping of the SU-8.

The x-coordinate value of the curve peak represents the glass transition temperature (Tg) of the sample. The Tg of the pure SU-8 sample was between 150 °C and 160 °C. The Tg of the sample with 0.5 wt% content was almost the same as that of pure SU8. When the content continued to increase to more than 1%, the Tg of the sample increased to 200 °C or higher.

From the previous results, a conclusion can be obtained that SiC nanowires addition into the SU-8 increased the rigidity and Tg of SU-8 and reduced the damping of SU-8. The combination of SiC nanowires and matrix strongly increased the rigidity of the SU-8 and reduced the energy dissipation.

The spring structure in Fig. 7 is a component of the inertial sensor, and the results show that the SiC-doped SU-8 can withstand greater acceleration than the pure SU-8.

This paper presents a novel approach to enhance the mechanical properties of SU-8 by compounding it with SiC nanowires. The SiC nanowires were uniformly dispersed in the SU-8 by full stirring and ultrasonic bath. RIE treatment made the size of the SU-8 structure containing nanowires formed by lithography more accurate. Uniformly dispersed SiC nanowires successfully enhanced the SU-8 mechanical properties while retaining the photopatternability and electrical insulation. By optimizing the mixture ratio of SiC nanowires to SU-8, the 2 wt% SiC-reinforced SU-8 had a 73.88% increase in elastic modulus and a 103.4% increase in elongation at break, achieving maximum improvement in rigidity and toughness. In addition, the SU-8 storage modulus and glass transition temperature improved.

In addition, doped SU-8, which is capable of forming a spring by lithography (lower cost and higher precision) and has high mechanical properties, has advantages over metal materials and pure SU-8.

According to Yu et al,25 β-SiC nanowires have strong absorption in the whole ultraviolet range (200–400 nm); this will inevitably affect the photocuring of SU-8 containing SiC nanowires. When the SU-8 containing SiC nanowires was exposed to ultraviolet light, part of the ultraviolet light was absorbed by the nanowires, and thus, the ultraviolet light could not effectively reach the expected depth quickly, and as a result, the actual curing thickness was less than the ideal value. Thus, the thickness of the sample containing more than 2.5 wt% nanowires is not ideal. To ensure that the dimensions of the test samples were the same, the samples with SiC content greater than 2.5 wt% were not prepared and tested. Moreover, the aspect ratio of photocured SU-8 structure containing SiC nanowires was lower than that of pure SU-8. Therefore, a balance needs to be achieved between mechanical properties and aspect ratio.

In summary, SiC nanowires-reinforced SU-8, which has a good photopatternability, electrical insulation, and better mechanical properties, has the potential to meet higher requirements in the design and manufacturing of MEMS.

None.

The authors would like to thank supports from the Shanghai Professional Technical Service Platform for Non-Silicon Micro-Nano Integrated Manufacturing and Project funded by China Postdoctoral Science Foundation (No. 2018M630440).

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Guifu Ding was born in 1963. He received his B.Sc. and the M. Sc. degree from Fudan University, Shanghai, China, in 1984 and 1987, respectively. He is now a professor and the vice-director of the National Key Laboratory of Science and Technology on Micro/Nano Fabrication, Research Institute of Micro/Nano Science & Technology, Shanghai Jiao Tong University, Shanghai, China. He is also currently with the Key Laboratory for Thin Film and Microfabrication of Ministry of Education, Shanghai Jiao Tong University. His main research interests include the nano materials and the design, simulation and fabrications of the MEMS/NEMS devices, in particular, the micro-fabrication technologies for non-silicon devices.

Yu Yang received his B.Sc. degree from Nanjing University of Science and Technology, Nanjing, China, in 2017. He is currently working towards the M.Sc. degree in Electronic Science and Technology at the National Key Laboratory of Science and Technology on Micro/Nano Fabrication, Shanghai Jiao Tong University, Shanghai, China. His research interests are MEMS devices.

Liyan Lai received the B.Sc. degree from Nanchang Hangkong University, China, in 2011. She received the M.Sc. degree from Guilin University of Technology, China, in 2014. She is currently working towards the Ph.D. degree in microelectronics and solid state electronics at the National Key Laboratory of Science and Technology on Micro/Nano Fabrication, Shanghai Jiao Tong University, China. Her research interests include the design, simulation, and fabrication of MEMS/NEMS devices.

Ting Chen received her B.Sc. degree from Nanjing University of Science and Technology, China, in 2017. She is currently working towards the M.Sc. degree in electronics and communication engineering at the National Key Laboratory of Science and Technology on Micro/Nano Fabrication, Research Institute of Micro/Nano Science & Technology, Shanghai Jiao Tong University, China. Her research interests are development of small ion trap based on MEMS technology.