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Low-thermal-budget electrically active thick polysilicon for CMOS-First MEMS-last integration
Aron Michael1, Ian Yao-Hsiang Chuang1, Chee Yee Kwok1
1UNSW, Sydney, NSW 2052 Australia.
Microsystems & Nanoengineering
|June 10, 2024
Summary
Researchers developed new in situ phosphorus-doped silicon films using ultrahigh-vacuum electron-beam evaporation (UHVEE). This process enables the creation of thick, electrically active polysilicon films for microelectromechanical systems (MEMS) with low thermal budgets.
Area of Science:
- Materials Science
- Electrical Engineering
- Mechanical Engineering
Background:
- Microelectromechanical systems (MEMS) require low-thermal-budget, electrically active, and thick polysilicon films for fabrication on complementary metal-oxide-semiconductor (CMOS) integrated circuits.
- Current methods face challenges in achieving these desired polysilicon film properties.
Purpose of the Study:
- To report the development of in situ phosphorus-doped silicon films deposited via ultrahigh-vacuum electron-beam evaporation (UHVEE) for the first time.
- To investigate the crystallographic, mechanical, and electrical properties of these novel polysilicon films.
- To demonstrate the feasibility of fabricating functional MEMS devices using these films.
Main Methods:
- Deposition of in situ phosphorus-doped silicon films using UHVEE under ~10⁻⁹ Torr.
- Characterization using Raman spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), and atomic force microscopy (AFM).
- Mechanical property evaluation via wafer curvature, cantilever deflection, and resonance frequency measurements.
- Electrical property assessment through resistivity measurements.
- Micromachining and fabrication of a comb-drive structure for MEMS actuator and sensor applications.
Main Results:
- Achieved fully crystallized, low-stress, smooth, thick, and electrically active polysilicon films with low thermal budgets (<500°C).
- Demonstrated highly vertical, high-aspect-ratio micromachining capabilities.
- Successfully designed, simulated, fabricated, and characterized a MEMS comb-drive structure using 20-μm-thick films.
- Compared properties of phosphorus-doped films with intrinsic and boron-doped UHVEE silicon films.
Conclusions:
- UHVEE polysilicon uniquely enables the realization of mechanically and electrically functional MEMS devices with low thermal budgets.
- The developed process overcomes previous limitations in polysilicon film formation for MEMS-on-CMOS applications.
- This advancement paves the way for more integrated and efficient MEMS devices.
