Octagonal Spacer Layer for MEMS Sensor Thermal Stress Control
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Solution Overview
Problem
Conventional vibrating structure gyroscopes face performance limitations due to rate bias instability caused by thermal expansion mismatches between materials, leading to quadrature bias variations that affect the accuracy of rate bias measurements.
Innovation Solution
The introduction of an octagonal electrically insulating spacer layer in the MEMS sensor design helps control thermally-induced stress and strain, reducing the frequency split between cos 2θ resonance modes and minimizing quadrature bias variations by compensating for residual stiffness asymmetry.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If conventional rectangular sensor chip design is used, then manufacturing is simplified, but thermal expansion mismatches cause frequency split and quadrature bias variation
Solution Approach 1:
The patent applies asymmetry by transitioning from a conventional rectangular sensor chip design to an octagonal sensor chip design. This geometric change modifies the stress distribution pattern under the chip, reducing the cos 4θ periodicity that causes frequency split between cos 2θ resonance modes. The octagonal shape creates a more uniform stress field that compensates for thermal expansion mismatches between different materials (silicon, Pyrex, Kovar), thereby minimizing quadrature bias variation and improving rate bias stability.
2Manufacturing precision
If laser trimming is used to minimize quadrature bias at room temperature, then initial alignment is improved, but temperature-induced stress changes cause drift
Solution Approach 1:
The patent applies parameter changes by modifying the geometric parameters of the sensor chip from rectangular to octagonal. This geometric parameter change fundamentally alters the stress distribution characteristics, creating a design that is inherently less sensitive to temperature-induced stress changes. The octagonal geometry provides a more uniform stress field that maintains better frequency matching across the operating temperature range, reducing the need for frequent trimming and improving long-term reliability.
3Device complexity
If rectangular chip geometry is used, then stress distribution is simpler, but corner rigidity creates asymmetric stress peaks
Solution Approach 1:
The patent applies asymmetry by replacing the rectangular geometry with an octagonal geometry. This change eliminates the sharp corners of the rectangular chip that create localized stress peaks and asymmetries. The octagonal shape provides rounded transitions that distribute stress more uniformly across the bonding interface, reducing the cos 4θ periodicity in stress distribution and improving the symmetry of the stress field, which in turn reduces frequency split and quadrature bias variation.
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 significantly reduces quadrature bias temperature variation, maintaining the accuracy of rate bias trimming and enhancing the performance of vibrating structure gyroscopes across the operating temperature range.
Implementation Method 1
the different coefficients of thermal expansion for the various materials used in the construction of the MEMS device will induce stresses and strains that will vary with the ambient temperature of the device
Implementation Method 2
When the annular resonator is rotated about an axis normal to its plane, the Coriolis effect causes a secondary vibration in an orthogonal direction that couples energy into the secondary response mode
Implementation Method 3
the annular resonator is typically excited into a cos 2θ resonance mode
Data Source
AI summary
A MEMS sensor comprises a vibrating sensing structure formed from a semiconductor substrate layer (50). The semiconductor substrate layer (50) is mounted on a pedestal comprising an electrically insulating substrate layer (52) bonded to the semiconductor substrate (50) to form a rectangular sensor chip. The pedestal further comprises an electrically insulating spacer layer (54) for mounting the sensor chip to a housing. The electrically insulating spacer layer (54) is octagonal. When the vibrating sensing structure is excited into a cos 2θ vibration mode pair, the quadrature bias arising from any mode frequency split is not affected by changes in temperature as a result of the octagonal spacer layer (54).