Study On Thickness Uniformity Of Electroformed Microfluidic Chip Mold

Microfluidic chip is a main device to realize the miniaturization, integration and portability of the analytical instrument. With the development of micro-electromechanical processing technology, the fabrication technology of microfluidic chips is becoming diversified and mature. And microfluidic chip molds play an indispensable role in mass production of polymer microfluidic chips. Among different-material molds, the nickel mold fabricated by micro-electroforming has become the first choice, due to its advantages of long service life and good surface quality. However, this kind of mold has the problem of uneven thickness, which finally influences the performance of chips due to the bad dimensional accuracy of the mold. Hence, in this paper, by using two kinds of microfluidic chip mold as studying objects, two methods of electroforming with an ultrasonic agitation and a second cathode are investigated by the experiment and the finite element simulation. The research work includes the following aspects:(1) The effect of the ultrasonic agitation on the diffusion layer thickness was studied. During the electroforming process, by promoting the motion of reactive ions to reduce the thickness of the diffusion layer on the surface of microstructures, the ultrasonic agitation can improve the thickness uniformity of electroforming layer. Therefore, to better analyze how the ultrasonic agitation affects the thickness uniformity of microfluidic chip molds, the impact of the ultrasonic agitation on the diffusion layer thickness was studied. And by measuring the nickel cathodic polarization curve, the diffusion layer thickness was obtained. It is found that the ultrasonic agitation can reduce the thickness of the diffusion layer. With the ultrasonic frequency increased, the diffusion layer firstly increased and then decreased. And the increase of the ultrasonic power leads to the decrease of the diffusion layer thickness.(2) The influence of the ultrasonic electroforming on the thickness uniformity of a circle-style microfluidic chip mold was investigated. The thickness uniformity of the mold is represented by the thickness uniformity of structure units in 5 regions. The thickness uniformity of structure uints was quantified by the nonuniformity equation. The results show that the thickness uniformity of the mold can be increased by the ultrasonic agitation, and the thickness uniformity of the small-linewidth structural unit can be more obviously improved. With 40kHz/250W utlrasond exerted, the thickness uniformity of 50?m, 100?m,200?m, 400?m and 700?m structural unit are increased by 67.4%,32%,11.4%,13.4% and 12.8%, respectively.(3) By using a butterfly-style microfluidic chip mold as the studying object, how the ultrasonic parameters affect the thickness uniformity of the mold was investigated by two groups of electroforming experiment. The results show that the ultrasonic agitation can improve the thickness uniformity of the mold. And the influence of the ultrasonic parameters on the thickness uniformity is consistent with that on the thickness of the diffusion layer. With 200kHz/500W ultrasonic agitation used, the thickness uniformity of the mold is improved by 32.3% after 2-hour electroforming.(4) With the butterfly-style microfluidic chip mold above used as the studying object, the impact of a ringed second cathode on the thickness uniformity of the mold was investigated by using the finite element software COMSOL Multiphysics. And then, based on the simulation results and experiment equipment, a second cathode is specially designed to fabricate a mold, and an experiment was made to verify the simulation results. The simulation results show that the thickness uniformity of the mold can be improved by adding a ringed second cathode. With the distance between the second cathode and the microstructure of the mold decreasing, the thickness uniformity firstly decreases and then increases. With the cathode width increasing, the thickness uniformity decreases. Meanwhile, with the electroforming time extended, the thickness uniformity is more obviously improved. The experiment result shows that the specially designed second cathode improves the thickness uniformity of the mold by 51.5% after 22-hour electroforming, which is in a good agreement with the simulation result.