MEMS Rotor Bottom Coating for Mass and Yield
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Solution Overview
Problem
Thin device wafers used in MEMS devices are prone to sensitivity issues due to small rotor mass, and increasing rotor weight by enlarging its surface area reduces manufacturing yield and chip density.
Innovation Solution
A method involving the deposition of a dense material layer on the rotor surface before bonding to a handle wafer, increasing the rotor's mass without enlarging its surface area, using materials like gold, copper, or tungsten with higher densities and controlled thicknesses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If the device wafer is made thinner to reduce structural irregularities and improve manufacturing yield, then manufacturing precision and yield improve, but the rotor mass decreases leading to reduced sensitivity and signal stability
Solution Approach 1:
The rotor structure is segmented into multiple functional layers: a base layer formed in the thin device wafer and additional mass layers deposited on top. This segmentation allows the rotor to benefit from the thin wafer's manufacturing advantages while adding mass through separate deposition processes, resolving the contradiction between thin wafer requirements and sufficient rotor mass.
Solution Approach 2:
The rotor is constructed as a nested structure where multiple material layers are deposited sequentially on the base rotor structure. Each layer contributes to the overall mass while maintaining the compact footprint, enabling the rotor to achieve sufficient mass without increasing surface area or requiring a thicker wafer.
2Weight of moving object
If the rotor surface area is increased to increase its mass, then rotor mass increases, but the number of MEMS chips that fit on one device wafer decreases reducing productivity
Solution Approach 1:
Instead of increasing rotor mass by expanding the rotor's footprint in the planar dimensions (which would reduce chip density), the patent adds mass in the vertical dimension through multiple deposited layers. This dimensional transition allows high mass with small footprint, maintaining both productivity and rotor mass.
3Manufacturing precision
If a thin device wafer is used to reduce etching irregularities, then manufacturing precision improves, but the rotor mass becomes too small causing sensitivity to environmental fluctuations
Solution Approach 1:
The rotor mass is built up through preliminary deposition of multiple layers before the wafer is thinned or bonded to the handle wafer. This preliminary mass accumulation ensures sufficient rotor mass is achieved while still allowing the use of a thin device wafer for etching, thereby maintaining both etching quality and signal 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
This approach allows for the use of thin device wafers with heavier rotors, enhancing signal stability and maintaining high manufacturing yields by balancing rotor mass and surface area, suitable for applications like accelerometers and gyroscopes.
Implementation Method 1
depositing a bottom coating layer on at least a part of the rotor bottom surface before the device wafer is bonded to the handle wafer
Implementation Method 2
depositing a bottom coating layer on at least a part of the rotor bottom surface before the device wafer is bonded to the handle wafer
Implementation Method 3
bonding a bottom surface of a silicon device wafer onto the top surface of the handle wafer
Data Source
Figure 1a~1c
Figure 1d~3a
Figure 3b
AI summary
This disclosure describes a comprising a handle wafer and a device wafer which is bonded to the handle wafer. The handle wafer comprises a cavity and the device wafer comprises a mobile rotor part above the cavity. A bottom coating layer covers at least a part of the bottom surface of the rotor.