MEMS Cap Layer Surface Finish via Barrier and Refill Epitaxy
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Solution Overview
Problem
Microelectromechanical system (MEMS) devices face challenges in maintaining a smooth surface on their cap layers due to polysilicon migration, leading to surface roughness, which affects device performance.
Innovation Solution
The use of a barrier layer, such as Aluminum Oxide, is implemented to prevent polysilicon migration, combined with epitaxial growth and etching processes to form a smoothed cap layer, and refill material deposition to seal openings and maintain surface smoothness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If epitaxial growth is used to form the cap layer, then the cap layer can be formed with good material quality, but polysilicon migration occurs causing surface roughness
Solution Approach 1:
A barrier layer is introduced as an intermediary between the substrate and the cap layer. This barrier layer prevents polysilicon from migrating from the substrate up to the cap layer surface, thereby maintaining surface smoothness while allowing the cap layer to be formed with good epitaxial quality.
Solution Approach 2:
The barrier layer is deposited in advance before the cap layer is formed through epitaxial growth. This preliminary action establishes a protective interface that prevents polysilicon migration during subsequent processing steps, ensuring surface smoothness is maintained throughout the fabrication process.
2Ease of operation
If openings are etched through the cap layer for sacrificial layer removal, then access to the sacrificial layer is achieved, but the cap layer surface continuity is disrupted
Solution Approach 1:
The sacrificial layer is temporarily removed through the openings to allow membrane release, and then the openings are refilled with additional epitaxial material. This process discards the sacrificial layer when needed and recovers the cap layer continuity by sealing the openings, maintaining both operational access and structural integrity.
Solution Approach 2:
After the sacrificial layer is removed through the openings, the openings are immediately refilled with epitaxial material to restore the cap layer continuity. This ensures that the cap layer remains a continuous protective and functional layer throughout the device operation, maintaining structural stability.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution results in a MEMS device with reduced surface roughness on the cap layer, enhancing device performance and reliability.
Implementation Method 1
The use of the barrier layer prevents the polysilicon on the underside of the cap layer from migrating and forming a rough surface
Implementation Method 2
A material such as a polysilicon layer is epitaxially grown in the etched structure and on the barrier layer, defines a cap layer
Implementation Method 3
Vapor hydrofluoric acid (vHF) is used to remove the sacrificial layer under the plurality of holes, forming a gap
Data Source
AI summary
A method of fabricating a MEMS device includes performing an atomic layer deposition (ALD) process to deposit a barrier layer such as Aluminum Oxide (AI2O3) having a thickness on a sacrificial layer deposited on a substrate. A portion of the barrier layer is removed to form an etched structure defined as a trench. An epi-polysilicon cap layer is epitaxially growth on the barrier layer and the entire etched structure. A portion of the epi-polysilicon cap layer has been removed to form a plurality of openings. The sacrificial layer is etched away leaving a cavity below the etched openings. A refill epi-polysilicon layer is epitaxially grown in the openings and seals the entire openings after a gap is formed between the cap layer and the substrate.