[paper] CMOS-First MEMS-last integration

Aron Michael, Ian Yao-Hsiang Chuang, Chee Yee Kwok and Kazuo Omaki

Low-thermal-budget electrically active thick polysilicon for CMOS-First MEMS-last integration

Microsystems & Nanoengineering (2024) 10:75

DOI: 10.1038/s41378-024-00678-5

* UNSW, Sydney, NSW 2052, Australia

Abstract: Low-thermal-budget, electrically active, and thick polysilicon films are necessary for building a microelectromechanical system (MEMS) on top of a complementary metal oxide semiconductor (CMOS). However, the formation of these polysilicon films is a challenge in this field. Herein, for the first time, the development of in situ phosphorus-doped silicon films deposited under ultrahigh-vacuum conditions (~10E−9 Torr) using electron-beam evaporation (UHVEE) is reported. This process results in electrically active, fully crystallized, low-stress, smooth, and thick polysilicon films with low thermal budgets. The crystallographic, mechanical, and electrical properties of phosphorus-doped UHVEE polysilicon films are studied. These films are compared with intrinsic and boron-doped UHVEE silicon films. Raman spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM) and atomic force microscopy (AFM) are used for crystallographic and surface morphological investigations. Wafer curvature, cantilever deflection profile and resonance frequency measurements are employed to study the mechanical properties of the specimens. Moreover, resistivity measurements are conducted to investigate the electrical properties of the films. Highly vertical, high-aspect-ratio micromachining of UHVEE polysilicon has been developed. A comb-drive structure is designed, simulated, fabricated, and characterized as an actuator and inertial sensor comprising 20-μm-thick in situ phosphorus-doped UHVEE films at a temperature less than 500°C. The results demonstrate for the first time that UHVEE polysilicon uniquely allows the realization of mechanically and electrically functional MEMS devices with low thermal budgets.

Fig: Comb-drive fabrication: a Grow oxide; b deposit thick UHVEEPolySi; c electrical pads patterned; d pattern the comb-drive; e backside pattern; f DRIE of UHVEEPolySi; g STS ICP oxide and DRIE from backside; h Remove Cr using O2 plasma; i HF vapor etch

Acknowledgements: The authors wish to acknowledge the Australian National Fabrication Facility (ANFF) NSW node, the School of Photovoltaic & Renewable Energy Engineering (SPREE) and the Electron Microscope Unit at UNSW, where fabrication and film characterization were conducted. In addition, the authors acknowledge the financial support received from the School of Electrical Engineering & Telecommunications (EE) and UNSW Sydney.