MEMS Vacuum Sensor with Capacitive Motion Beam
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Solution Overview
Problem
It is challenging to accurately measure the vacuum level within a sealed package, such as a hermetically sealed package containing MEMS devices, as external sensors cannot reliably determine the differential pressure between the outside air and the internal vacuum level, which is critical for the proper functioning of these devices.
Innovation Solution
A vacuum level monitor is integrated into the sealed package, featuring a motion beam suspended over sensing and driving electrodes, where the motion beam deflects in response to electrostatic force, and the resulting capacitance change is measured to assess the vacuum level, utilizing the same processing techniques as the MEMS devices themselves.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If external sensors are used to measure pressure, then the measurement can be performed outside the sealed enclosure, but the measurement precision is insufficient because external sensors cannot accurately measure the differential pressure between outside air and internal vacuum level
Solution Approach 1:
The patent merges the sensing function directly into the sealed enclosure by integrating a motion beam sensor structure within the enclosure itself. The motion beam is positioned between the sealed enclosure and reference pressure region, allowing direct measurement of differential pressure without requiring external sensors, thereby achieving accurate internal pressure measurement while maintaining device integration
Solution Approach 2:
The motion beam acts as an intermediary element that transmits mechanical displacement caused by differential pressure into a measurable signal. The motion beam's deflection in response to pressure differential is converted into an electrical signal through capacitive coupling, serving as a mediator between the pressure differential and the measurement system
2Reliability
If the sealed package is hermetically sealed to protect MEMS devices, then the vacuum level can be maintained, but the ability to monitor the vacuum level becomes difficult
Solution Approach 1:
The motion beam structure serves multiple functions: it acts as both a mechanical element responsive to pressure differential and as a capacitive sensor element. The same structure that responds to pressure changes also provides the capacitive coupling necessary for signal detection, eliminating the need for separate monitoring components and simplifying the overall system
Solution Approach 2:
The sealed enclosure's own structural elements (the motion beam and electrodes) are utilized to perform the monitoring function. The motion beam and electrode assembly uses the enclosure's internal pressure differential to generate a measurable capacitive signal, allowing the system to self-monitor without requiring external monitoring equipment
3Manufacturing precision
If smaller feature sizes and tighter tolerances are used in semiconductor devices, then the device performance is improved, but the vacuum seal becomes more prone to degradation from leaks
Solution Approach 1:
The patent implements a monitoring system that detects vacuum level degradation before it becomes critical. By continuously monitoring the differential pressure through the motion beam's capacitive response, the system can identify seal degradation or leaks early, allowing for preventive measures to be taken before the vacuum level drops below operational thresholds
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 solution allows for accurate monitoring of the vacuum level within the sealed package, ensuring the optimal operation of MEMS devices by measuring the capacitance changes caused by the motion beam's deflection, which correlates with the dielectric constant and thus the vacuum level, even in small feature size semiconductor devices.
Implementation Method 1
The motion beam is driven by electrostatic force applied through a driving electrode or electrodes
Implementation Method 2
The capacitance between the motion beam and the fixed sensing electrode or electrodes depends upon the spacing between the motion beam and the sensing electrode
Implementation Method 3
The capacitance also depends upon the dielectric material between the motion beam and the sensing electrode, in this case air or vacuum
Data Source
AI summary
A vacuum sensor for sensing vacuum in a sealed enclosure is provided. The sealed enclosure includes active MEMS devices desired to be maintained in vacuum conditions. The vacuum sensor includes a motion beam anchored to an internal surface in the sealed enclosure. A driving electrode is disposed beneath the motion beam and a bias is supplied to cause the motion beam to deflect through electromotive force. A sensing electrode is also provided and detects capacitance between the sensing electrode disposed on the internal surface, and the motion beam. Capacitance changes as the gap between the motion beam and the sensing electrode changes. The amount of deflection is determined by the vacuum level in the sealed enclosure. The vacuum level in the sealed enclosure is thereby sensed by the sensing electrode.