Study On Etching Current Control System Of Photo-electrchemical Etching For Macroporous Silicon

Macroporous silicon is a kind of porous silicon with the pore diameter of microns.The fabrication methods of macroporous silicon include deep reactive ion etching and photo-electrochemical etching.Compared with the deep reactive ion etching,the photo-electrochemical etching has many advantages,such as the high aspect ratio,controlled pore diameter,inclined pore and low cost.The main technical problem of photo-electrochemical etching for macroporous silicon is how to control the morphology and diameter of pore,effectively.To fabricate the macroporous silicon with a specific structure for complex application,the etching current should adjust based on the requirements of the pore diameter.In this paper,the control principle and system of etching current were investigated.The influence factors and correction curves of the etching current were explored.The effects of factors on the etching current were investigated,such as temperature,electrolyte diffusion,light and surfactant.The correction curve of etching current was obtained by establishing the electrolyte diffusion model and calculating the dark current.In the electrolyte diffusion model,the diffusion equation was obtained by quasi-static approximation.Based on the boundary conditions,the relationship between the HF concentration at the tip of the pore and the pore depth was obtained by solving diffusion equations.Based on Taylor approximation and differential equation,the effect of electrolyte diffusion on the etching rate and pore diameter were analyzed,and the critical current density was obtained.For the dark current,the darkness I-V characteristic curves of the sample after etching with different surfactants were measured,and the dark currents were obtained from the I-V characteristic curves.For the anionic surfactant,the dark current can be neglected,getting the correction curve of etching current.The control system of etching current was designed to setting and controlling the process of etching current.In the system hardware,the circuit design and FPGA programming of each module of the etching current control system were mainly studied.The system hardware consists of FPGA,communication interface,temperature module,agitation module and light control model.System hardware used the PWM drive Buck circuit controls the light current.The system software is based on LabVIEW,which is mainly used to communicate with system hardware,set experimental parameters,display and record experimental data,and so on.Based on the deduced correction curve of etching current,a self-made etching current control system was used to fabricate high aspect ratio macroporous silicon with uniform or periodic aperture.