Multi-layer microstructure fabrication by combining bulk silicon micromachining and UV-LIGA technology

doi:10.1016/j.mejo.2006.09.003

Microelectronics Journal

Volume 38, Issue 1, January 2007, Pages 120-124

https://doi.org/10.1016/j.mejo.2006.09.003 Get rights and content

Abstract

A novel method for fabrication of multi-layer microstructures of microelectro-mechanical system (MEMS) devices is described. This technique, which combines bulk silicon micromachining technique and UV-LIGA technique can overcome some shape limitations of single technique on complex microstructures. To demonstrate this combination, the SU-8 microstructure fabricated in the etched silicon grooves is presented. In this fabrication process, a SU-8 removal method by fuming sulfuric acid was introduced and a novel type of plastics PETG was tried in microhot embossing process. The proposed fabrication process can be applied to fabricating a high-aspect-ratio microstructure for a large displacement actuator and precision sensors. Moreover, this combined process enables the fabrication of more complex structures, which cannot be fabricated by bulk micromachining or UV-LIGA alone.

Introduction

With the increasing demands on integrated and miniaturized industrial systems, intensified research has been directed towards microelectro-mechanical system (MEMS) technologies. Most MEMS technologies use silicon as the substrate material and a CMOS-compatible fabrication process. Therefore, it can provide low-cost small features by mass production [1], [2], [3]. However, conventional micromachining technologies, i.e. surface micromachining and bulk micromachining technologies, have some difficulties when the device requires complex or three-dimensional (3-D) shapes as well as vertical sidewalls. Some authors have investigated several new methods to realize the 3-D microfabrication. Ling et al. [4] have presented some 3-D microstructures by using the modified SU-8 solutions with reduced PAG concentrations, while great efforts should be made to optimize the process for each modified resist. Bertsch et al. [5] have presented 3-D microfabrication by combining microstereolithography and thick resist UV lithography. This technique can overcome some shape limitations of the planar technologies. However, microstereolithography has the limited resolution and the problems associated with the manipulation and assembling of the obtained polymer structures.

This paper reports a novel micromachining process based on a UV-LIGA process and silicon anisotropic etching to fabricate high-aspect-ratio microstructures (HARMS). Wet chemical anisotropic etching is frequently used for shaping quite intricate silicon microstructures due to its low process cost, simple etch setup, higher etch rate, better surface smoothness, high degree of anisotropy and lower environmental pollution. Although the silicon structures fabricated by anisotropic etching have limited shapes, complex structures can be fabricated by additional UV-LIGA micromachining processes, which can overcome the limited number of possible structures fabricated by an anisotropic wet etching process. The proposed fabrication process can be applied to fabricating a high-aspect-ratio microstructure for a large displacement actuator and precision sensors. Moreover, this combined process enables the fabrication of more complex structures, which cannot be fabricated by bulk micromachining or UV-LIGA alone.

Section snippets

Fabrication process

Bulk silicon micromachining technique is used to fabricate the first layer or other patterns that can be easily made by this technique. Then the upper layers and complex graphics are fabricated by UV-LIGA technique on the patterned silicon wafer. The schematic fabrication process of the proposed microstructure is shown in Fig. 1. First, a silicon wafer is thermally grown to produce a thick silicon dioxide layer. This layer is used as a shield mask for the later etching process. Following

Results and discussion

By using this combining method, some pentagram SU-8 microstructures are fabricated in the square silicon groove as an example. The dimension of square-etched silicon groove is 200 μm×200 μm with the depth of 140 μm. The height of SU-8 photoresist microstructures including the depth of etched groove is 200 μm. The process parameters and its equipment or chemical solvent of each step for these multi-layer microstructures fabrication are listed in Table 1. The SEM picture of SU-8 structures standing

Conclusion

A novel and simple method for realization of 3-D microstructures has been presented. By using this novel micromachining process based on the UV-LIGA process and silicon anisotropic etching process, multi-layer microstructures and complex graphics can be fabricated. The adhesive characteristics between the SU-8 photoresist and the surface of etched silicon substrate and two steps that can thoroughly remove bubbles in the etched silicon groove were studied. Some SU-8 photoresist microstructures

Acknowledgements

The authors acknowledge the financial supports of the National Natural Science Foundation of China (Project code: 50575132) and the key project of Chinese Ministry of Education (No. 0307).

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Multi-layer microstructure fabrication by combining bulk silicon micromachining and UV-LIGA technology

Abstract

Introduction

Section snippets

Fabrication process

Results and discussion

Conclusion

Acknowledgements

Sensors Actuators

Sensors Actuators A

Polymer

Microelectr. Eng.