Multi-Sacrificial Layer MEMS Fabrication for Planar Surfaces
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Solution Overview
Problem
The use of standard photoresist sacrificial layers in microelectromechanical system (MEMS) devices is limited by fluid dynamic characteristics, leading to non-planar surfaces and high temperature dry stripping processes that can cause defects and reduce the efficiency of MEMS devices.
Innovation Solution
A method involving multiple sacrificial layers with different removal rates and thicknesses is employed, where a first sacrificial layer is removed faster than a second layer, and a blanket layer is patterned to maintain the second layer, allowing for the formation of a variable capacitor with a torsional hinge and step junction, reducing the impact of fluid dynamics and thermal budgets.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a standard photoresist sacrificial layer is used, then the manufacturing process is simple, but the surface topography becomes non-planar due to fluid dynamic characteristics
Solution Approach 1:
The sacrificial layer is divided into multiple layers with different materials and removal rates. The first sacrificial layer (e.g., oxide) is removed faster than the second sacrificial layer (e.g., nitride), allowing differential removal that compensates for fluid dynamic variations and achieves a planar surface topology.
2Productivity
If a high temperature dry strip process is used to remove the sacrificial layer, then the sacrificial layer can be effectively removed, but thermal damage occurs to the MEMS structure
Solution Approach 1:
The invention changes the material composition parameters of the sacrificial layers to have different removal rates. This allows the use of lower temperature removal processes that are selective to the first sacrificial layer, avoiding the high temperature thermal damage that would occur with conventional single-layer photoresist removal.
3Device complexity
If a single sacrificial layer is used, then the process is straightforward, but removal rate variations cause defects in the MEMS structure
Solution Approach 1:
The sacrificial layer is segmented into multiple layers with different materials (e.g., oxide and nitride) that have different removal rates. This segmentation allows controlled differential removal that compensates for process variations, resulting in a planar surface and reduced defects in the final MEMS structure.
Solution Approach 2:
The invention uses composite sacrificial layers made of different materials with complementary properties. The first layer (oxide) removes faster and defines the primary geometry, while the second layer (nitride) removes slower and provides a planarizing effect, together creating a defect-free surface.
Data Source
AI summary
MEMS devices and methods for utilizing sacrificial layers are provided. An embodiment comprises forming a first sacrificial layer and a second sacrificial layer over a substrate, wherein the second sacrificial layer acts as an adhesion layer. Once formed, the first sacrificial layer and the second sacrificial layer are patterned such that the second sacrificial layer is undercut to form a step between the first sacrificial layer and the second sacrificial layer. A top capacitor electrode is formed over the second sacrificial layer, and the first sacrificial layer and the second sacrificial layer are removed in order to free the top capacitor electrode.