Method for Keyhole-Free High-Aspect-Ratio Trench Refill by LPCVD

Area of Science:

• Materials Science
• Mechanical Engineering
• Electrical Engineering

Background:

• Micro-electromechanical systems (MEMS) utilize micro-machined structures, including high-aspect-ratio trenches, for various applications.
• Keyholes, or voids, frequently form within these trenches during fabrication, negatively impacting device functionality and reliability.

Purpose of the Study:

• To explain the mechanisms of keyhole formation in MEMS trenches.
• To present a novel method for etching positively-tapered high-aspect ratio trenches to prevent keyhole formation.
• To optimize trench sidewall profiles for void-free refilling.

Main Methods:

• A two-step Bosch-based plasma etching process was employed.
• Key process parameters including cycle time, platen power, and process pressure were systematically varied.
• Scallop dimensions and sidewall profile angles were analyzed to understand their correlation with trench taper.

Main Results:

• The study demonstrates a direct relationship between the etch step's cycle time and the resulting scallop height and sidewall profile angle.
• Lowering the etch step time was found to effectively control the chemical flux, leading to the desired positive trench taper.
• Achieved a positive sidewall taper of 88° to 89°, crucial for preventing keyholes after subsequent low-pressure chemical vapor deposition (LPCVD) refilling.

Conclusions:

• Optimizing the Bosch process parameters, specifically the etch cycle time, is critical for controlling trench sidewall profiles.
• The presented method successfully enables the fabrication of keyhole-free, positively-tapered high-aspect ratio trenches.
• This advancement is vital for enhancing the performance and reliability of MEMS devices relying on refilled trench structures.