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Microfabrication of the High Aspect Ratio Microstructure

Author: ChenShaoJun
Tutor: LiYiGui
School: Shanghai Jiaotong University
Course: Microelectronics and Solid State Electronics
Keywords: ICP-RIE aspect ratio orthogonal test microneedle array LIGA mask compensation
CLC: TN305.93
Type: Master's thesis
Year: 2011
Downloads: 185
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Abstract

Although there are many methods to fabricate high-aspect-ratio micro-structure, there are only two dominating processes recently. One is ICP-RIE, called Inductively Coupled Plasma Reactive-ion Etching; the other is LIGA process, which is based on X-ray lithography technology.With the appearance of the fourth generation of semiconductor packaging, TSV (Through Silicon Via), it needs higher aspect ratio microstructure. TSV technology mostly used the ICP-RIE process, and using MEMS technics in future main packaging would speed up MEMS industry. In this thesis, we do research in the etching principle, and etching state of the ICP-RIE, using the Al as mask material, based on the 4 inch STS multiplex etcher. In this system, there were lots of parameters, such as platen power, etching/passivation time cycle, pressure and so on. By adjusting each system parameter, it got the etching rate of Si/SiO2 vs. each parameter, and the selectivity vs. each parameter. And using a set of system process parameters based on the orthogonal test, we can fabricate perfect aspect ratio and control the sidewall verticality well. Based on these parameters, we can achieve the results: aspect ratio was greater than 25:1 and the angle of sidewall can be controlled in the range of 90°±1°. The etching rate was about 2.5μm/min calculated though final micro-structure.Fabrication of high aspect ratio micro-structure using LIGA process was quite different process compared with ICP-RIE. The light sourceof LIGA was the very high energy X-ray. Considerating the mask making difference between LIGA and ICP-RIE, it should do some changes. Taking the micro-needle for example in this thesis, we developed a mask compensation method used in accurately manufacturing LIGA devices.However, there were some differences between actual micro-needle and model because of the sidewall etching. In order to eliminate the difference, a LIGA mask compensation methods was proposed by analyzing and sμmmarizing the experamental data. For simplified the model, we selected a typical pyramid solid core micro-needle. We thought that with PMMA depth increase, the absorptivity of X-ray in PMMA was changed and when LIGA mask absorbed the X-ray energy, lateral diffusion occurs.There were also many other factors which affected the micro-needle structure, for example, Fresnel diffraction, and thermal stress in the developer.In the process of compensation lithography mask, we took a horizontal coordinate discretization method. By simulation of each unit of time, the vertical etching depth of micro-needles each abscissa point, we can optimize the mask compensated shape. The vertical etching of the non-linear change was obtained by different coefficients of the longitudinal position of simulation. While the horizontal spread of the energy can be reflected to the adjacent point depth on the vertical displacement by trigonometric functions and add together. So we can calculate the shape of lithography mask. After the cycle comparison of block rate of the mask, we can get the optimization value of the mask shape.From the result of the simulation, the LIGA mask compensation method had counteracted the distortion most by considering the relationship between distributing of X-ray absorbed energy and shape of the microstructure. The shape of the LIGA mask change from isceles triangle to oval, it can strengthen the rigidity of the microneedles. The microneedle was made of PMMA, if needed to replication using master mold, we have to fabricate the Ni needle modle. This method was also used in other LIGA fabrication. We confirmed the method was effective by fabrication PMMA microneedle with channel.

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