A novel anti-shock silicon etching apparatus for solving diaphragm release problems

This paper presents a novel anti-shock bulk silicon etching apparatus for solving a universal problem which occurs when releasing the diaphragm （e.g. SiNx）, that the diaphragm tends to be probably cracked by the impact of heatinginduced bubbles, the swirling of heating-induced etchant, dithering of the hand and imbalanced etchant pressure during the wafer being taken out. Through finite element methods, the causes of the diaphragm cracking are analysed. The impact of heating-induced bubbles could be the main factor which results in the failure stress of the SiNx diaphragm and the rupture of it. In order to reduce the four potential effects on the cracking of the released diaphragm, an anti-shock hulk silicon etching apparatus is proposed for using during the last etching process of the diaphragm release. That is, the silicon wafer is first put into the regular constant temperature etching apparatus or ultrasonic plus, and when the residual bulk silicon to be etched reaches near the interface of the silicon and SiNx diaphragm, within a distance of 50-80μm （the exact value is determined by the thickness, surface area and intensity of the released diaphragm）, the wafer is taken out carefully and put into the said anti-shock silicon etching apparatus. The wafer＇s position is at the geometrical centre, also the centre of gravity of the etching vessel. An etchant outlet is built at the bottom. The wafer is etched continuously, and at the same time the etchant flows out of the vessel. Optionally, two symmetrically placed low-power heating resistors are put in the anti-shock silicon etching apparatus to quicken the etching process. The heating resistors＇ power should be low enough to avoid the swirling of the heating-induced etchant and the impact of the heating-induced bubbles on the released diaphragm. According to the experimental results, the released SiNx diaphragm thus treated is unbroken, which proves the practicality of the said anti-shock bulk silicon etching apparatus.

A Novel Silicon Etching Method Using Vapor of Tetramethylammonium Hydroxide Solution

Silicon bulk etching is an important part of micro-electro-mechanical system(MEMS) technology. In this work, a novel etching method is proposed based on the vapor from tetramethylammonium hydroxide(TMAH) solution heated up to boiling point. The monocrystalline silicon wafer is positioned over the solution surface and can be anisotropically etched by the produced vapor. This etching method does not rely on the expensive vacuum equipment used in dry etching. Meanwhile, it presents several advantages like low roughness, high etching rate and high uniformity compared with the conventional wet etching methods. The etching rate and roughness can reach 2.13 μm/min and 1.02 nm, respectively. Furthermore,the diaphragm structure and Al-based pattern on the non-etched side of wafer can maintain intact without any damage during the back-cavity fabrication. Finally, an etching mechanism has been proposed to illustrate the observed experimental phenomenon. It is suggested that there is a water thin film on the etched surface during the solution evaporation. It is in this water layer that the ionization and etching reaction of TMAH proceed, facilitating the desorption of hydrogen bubble and the enhancement of molecular exchange rate. This new etching method is of great significance in the low-cost and high-quality micro-electromechanical system industrial fabrication.