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24 April 2003 Advantages of p++ polysilicon etch stop layer versus p++ silicon
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Proceedings Volume 5116, Smart Sensors, Actuators, and MEMS; (2003)
Event: Microtechnologies for the New Millennium 2003, 2003, Maspalomas, Gran Canaria, Canary Islands, Spain
Boron highly doped silicon is now widely used as etch stop layer in MicroElectroMechanical Systems (MEMS) devices fabrication. The present paper shows the advantages of replacing the p++ Si etch stop layer by a p++ polysilicon layer. The etch rate of Tetramethylammoniunhydroxide (TMAH) is measured for LPCVD polysilicon and silicon doped with Boron at concentrations from 8.1018 up to 4.1020 atoms/cm3 which is the Boron solubility limit into Si. TMAH etch being often used during back-end process, selectivity to aluminium is usually needed. The etch selectivity of various TMAH solutions for p++ Si, p++ Poly and aluminium have been measured, from 25 % to 5 % TMAH pure and mixed with silicon powder and ammonium persulfate. Contrarily to silicon, polysilicon is etched isotropically in TMAH solution which constitutes a great advantage when cavities with vertical walls have to be opened. Although the polysilicon etch rate is higher than the silicon one, the selectivity (doped/undoped) is the same for the both materials, allowing identical uses. Another great advantage of polysilicon is that it can be deposited at any process step and does not require clever epitaxy steps or wafer bonding as for silicon. The surface roughness of the etched Poly region is considerably decreased with TMAH mixed with silicon powder and ammonium persulfate mixture compared to pure 25 % TMAH solution. The definition of buried masks in polysilicon layer through Boron implant is the main foreseen application. The p++ Poly buried mask brings solutions for the fabrication of self-aligned double gate MOS, microfluidic or optical networks in MEMS field.
© (2003) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Remy Charavel, Jean Laconte, and Jean Pierre Raskin "Advantages of p++ polysilicon etch stop layer versus p++ silicon", Proc. SPIE 5116, Smart Sensors, Actuators, and MEMS, (24 April 2003);

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