Liqun Du

Effects of ultrasonic agitation on adhesion strength of micro electroforming Ni layer on Cu substrate

Micro electroforming is an important technology, which is widely used for fabricating micro metal devices in MEMS. The micro metal devices have the problem of poor adhesion strength, which has dramatically influenced the dimensional accuracy of the devices and seriously limited the development of the micro electroforming technology. In order to improve the adhesion strength, ultrasonic agitation method is applied during the micro electroforming process in this paper. To explore the effect of the ultrasonic agitation, micro electroforming experiments were carried out under ultrasonic and ultrasonic-free conditions. The effects of the ultrasonic agitation on the micro electroforming process were investigated by polarization and alternating current (a.c.) impedance methods. The real surface area of the electroforming layer was measured by cyclic voltammetry method. The compressive stress and the crystallite size of the electroforming layer were measured by X-ray Diffraction (XRD) method. The adhesion strength of the electroforming layer was measured by scratch test. The experimental results show that the imposition of the ultrasonic agitation decreases the polarization overpotential and increases the charge transfer process at the electrode-electrolyte interface during the electroforming process. The ultrasonic agitation increases the crystallite size and the real surface area, and reduces the compressive stress. Then the adhesion strength is improved about 47% by the ultrasonic agitation in average. In addition, mechanisms of the ultrasonic agitation improving the adhesion strength are originally explored in this paper. The mechanisms are that the ultrasonic agitation increases the crystallite size, which reduces the compressive stress. The lower the compressive stress is, the larger the adhesion strength is. Furthermore, the ultrasonic agitation increases the real surface area, enhances the mechanical interlocking strength and consequently increases the adhesion strength. This work contributes to fabricating the electroforming layer with large adhesion strength.

Enhancing the adhesion strength of micro electroforming layer by ultrasonic agitation method and the application

Micro electroforming is widely used for fabricating micro metal devices in Micro Electro Mechanism System (MEMS). However, there is the problem of poor adhesion strength between micro electroforming layer and substrate. This dramatically influences the dimensional accuracy of the device. To solve this problem, ultrasonic agitation method is applied during the micro electroforming process. To explore the effect of the ultrasonic agitation on the adhesion strength, micro electroforming experiments were carried out under different ultrasonic power (0W, 100W, 150W, 200W, 250W) and different ultrasonic frequencies (0kHz, 40kHz, 80kHz, 120kHz, 200kHz). The effects of the ultrasonic power and the ultrasonic frequency on the micro electroforming process were investigated by polarization method and alternating current (a.c.) impedance method. The adhesion strength between the electroforming layer and the substrate was measured by scratch test. The compressive stress of the electroforming layer was measured by X-ray Diffraction (XRD) method. The crystallite size of the electroforming layer was measured by Transmission Electron Microscopy (TEM) method. The internal contact surface area of the electroforming layer was measured by cyclic voltammetry (CV) method. The experimental results indicate that the ultrasonic agitation can decrease the polarization overpotential and increase the charge transfer process. Generally, the internal contact surface area is increased and the compressive stress is reduced. And then the adhesion strength is enhanced. Due to the different depolarization effects of the ultrasonic power and the ultrasonic frequency, the effects on strengthening the adhesion strength are different. When the ultrasonic agitation is 200W and 40kHz, the effect on strengthening the adhesion strength is the best. In order to prove the effect which the ultrasonic agitation can improve the adhesion strength of the micro devices, micro pillar arrays were fabricated under ultrasonic agitation (200W, 40kHz). The experimental results show that the residual rate of the micro pillar arrays is increased about 17% by ultrasonic agitation method. This work contributes to fabricating the electroforming layer with large adhesion strength.

Fabrication of SU-8 moulds on glass substrates by using a common thin negative photoresist as an adhesive layer

A method for fabricating SU-8 moulds on glass substrates is presented. A common thin negative photoresist was coated on the glass slide as an adhesive layer, and then SU-8 was patterned on the adhesive layer. The presence of the adhesive layer improved the lifetime of a SU-8 mould from a few cycles to over 50 cycles. Moreover, the fabrication of the adhesive layer is quite simple and no additional equipment is required. The effects of the adhesion behavior of the negative photoresist and SU-8 on substrates on the durability of the SU-8 mould were investigated. The work of adhesion of the common thin negative photoresist on glass was 51.2 mJ m−2, which is 22.5% higher than that of SU-8 on silicon and 32.3% higher than that of SU-8 on glass. The abilities of the method for replicating high-aspect-ratio microstructures were also tested. One SU-8 mould with 60 × 60 array micropillars with aspect ratios lower than 3 could be used to cast at least 20 polydimethylsiloxane devices.

Study on Improving Thickness Uniformity of Microfluidic Chip Mold in the Electroforming Process

Electroformed microfluidic chip mold faces the problem of uneven thickness, which decreases the dimensional accuracy of the mold, and increases the production cost. To fabricate a mold with uniform thickness, two methods are investigated. Firstly, experiments are carried out to study how the ultrasonic agitation affects the thickness uniformity of the mold. It is found that the thickness uniformity is maximally improved by about 30% after 2 h electroforming under 200 kHz and 500 W ultrasonic agitation. Secondly, adding a second cathode, a method suitable for long-time electroforming is studied by numerical simulation. The simulation results show that with a 4 mm width second cathode used, the thickness uniformity is improved by about 30% after 2 h of electroforming, and that with electroforming time extended, the thickness uniformity is improved more obviously. After 22 h electroforming, the thickness uniformity is increased by about 45%. Finally, by comparing two methods, the method of adding a second cathode is chosen, and a microfluidic chip mold is made with the help of a specially designed second cathode. The result shows that the thickness uniformity of the mold is increased by about 50%, which is in good agreement with the simulation results.

Research of megasonic electroforming equipment based on the uniformity of electroforming process

Megasonic has obvious advantages in overcoming the limitations of electroforming process based on its low cavitation effect, high sound intensity and acoustic streaming. In this paper, megasonic was employed to achieve uniform electroformed layer in electroforming process. Impedance values, resonant frequencies were measured in order to get a high-efficiency megasonic source. Considering the directions of acoustic radiation and combining with other functional modules, an integrated megasonic electroforming equipment was designed and set up. Then, nickel was electroformed on copper substrates without megasonic wave, with single directional megasonic wave and with bidirectional alternating megasonic wave, respectively. The planeness value (PV) of electroformed layer is 15.03u202fμm without megasonic agitation, and the PV of electroformed layer is 15.36u202fμm with single directional megasonic wave radiation. Bidirectional alternating megasonic wave assisted electroforming has an outstanding performance on the uniformity of electroformed layer, which achieves the lowest planeness value (PVu202f=u202f10.91u202fμm) of all the electroforming experiments. Besides, the bidirectional megasonic wave assisted electroforming can achieve better surface quality than other conditions too.

Thermoelectric effect on electroosmotic flow in microchannel

The present work reports the thermoelectric effect on electroosmotic flow (EOF) in microchannel. The thermoelectric (TE) effect on EOF was analyzed and the switch process of fluids controlled by TE unit was numerically simulated. The mathematical model was established that is composed with electrokinetics fluid, liquid-solid phase transition, and heat transfer. The microfluidic chip with the integrated TE unit was fabricated that includes the microchannel layer, heat conducting layer and refrigerating layer. As the transition from the liquid phases to the solid phase can significantly decrease electrical conductivity of electrolyte, the electrical current is available to make sure the status of phase transition. From the results, the controlling integrated TE unit is effective to close/open the EOF in microchannel. The freezing phenomena were described by results from numerical simulation and experiment.

Numerical simulations and electrochemical experiments of the mass transfer of microvias electroforming under ultrasonic agitation

This paper explores the mass transfer mechanism of microvias electroforming under ultrasonic agitation by numerical simulations and electrochemical experiments. Firstly, the velocity distribution of electroforming solution inside the microvias under ultrasound treatment is simulated by COMSOL Multiphysics software. The ultrasonic frequency is that of 120u202fkHz. The ultrasonic powers are 100u202fW, 200u202fW, 300u202fW and 400u202fW, respectively. The simulation results indicate that the mean liquid velocity inside the microvias increases with the increasing of acoustic power. In addition, under a certain ultrasonic power, the mean liquid velocity will decrease with increasing the distance between microvias and transducer, the aspect ratio of microvias and the distance between cathode and central position. Secondly, electrochemical experiments are presented to investigate the effect of ultrasonic agitation on the electrode kinetics of microvias electroforming. It is found that ultrasonic treatment decreases the thickness of diffusion layer, increases the limiting diffusion current densities and further enhances the mass transfer of microvias electroforming. Compared with the silent condition, the diffusion layer thicknesses with the acoustic power of 100u202fW, 200u202fW, 300u202fW, 400u202fW are decreased by 50.0%, 64.1%, 69.3% and 74.5%, respectively. Finally, according to the results above, the 200u202f×u202f200 metal micro-column array structures are fabricated by ultrasonic electroforming under the condition of 120u202fkHz and 200u202fW. The metal micro-column is 250u202fμm high and has a diameter of 80u202fμm. The results show that ultrasonic electroforming can enhance the mass transfer of microvias electroforming, and further solve the problem of porous structure in electroforming layer. This work contributes to expanding the application of ultrasonic agitation in the microvias electroforming.

Reducing the residual stress in micro electroforming layer by megasonic agitation

In order to reduce the large residual stress in micro elelctroforming layer, megasonic assisted electroforming is proposed here. Micro electroforming experiments were performed with and without megasonic agitation, respectively. Four different megasonic power densities were applied to investigate the influence of megasonic agitation on reducing the residual stress. The residual stress was measured by X-ray diffraction (XRD) method. Experiment results show that the residual stresses fabricated with megasonic agitation are less than that fabricated without megasonic. When the megasonic power density is 2u202fW/cm2, the residual stress can be the minimum value of -125.7u202fMPa, reduced by 60% in comparison with the value of -315.1u202fMPa electroformed without megasonic agitation. For exploring the mechanism of megasonic agitation on reducing the residual stress, the dislocation density and crystal orientation were calculated by the single-line Voigt profile analysis and Relative Texture Coefficient (RTC) method, respectively. The diameters and distributions of pits on the surface of electroforming layer were observed by the STM-6 tool microscope and counted by the Image-Pro Plus software. It reveals that one hand of the mechanism is the acoustic streaming produced by megasonic can strengthen the motion of dislocation in crystal lattice and makes the crystal lattices grow towards the equilibrium shape, which is benefit to crystallization with low residual stress. When the megasonic power density is 2u202fW/cm2, the dislocation density increases to be the maximum value of 8.09u202f×u202f1015u202fm-2 and the difference between RTC(1u202f1u202f1) and RTC(2u202f0u202f0) decreases to be zero, which is consistent with the residual stress results. The other hand is that the stable cavitation produced by megasonic can provide residual stress release points during the electroforming process.