ISFET pH Sensor Bonding Layer for Stress Compensation
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Conventional pH sensors face accuracy limitations due to mechanical stresses induced by wide pressure and temperature variations in deep-sea environments, leading to measurement errors that affect their ability to accurately monitor ocean health and global warming risks.
Innovation Solution
A pH sensor design featuring an ion sensitive field effect transistor (ISFET) die bonded to a substrate with a bonding layer that induces a counteracting force to mitigate pressure and temperature-induced mechanical stresses, maintaining piezoresistance changes below 0.5% across a wide range by utilizing materials with anisotropic mechanical properties and strategically oriented bonding agent strips.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If conventional pH sensors are used in deep-sea environments with wide pressure and temperature variations, then the sensors can operate across different depths, but measurement accuracy deteriorates due to mechanical stresses inducing piezoresistance errors
Solution Approach 1:
The patent applies parameter changes by selecting substrate and bonding layer materials with specific coefficients of thermal expansion (CTE) and elastic moduli that differ from the ISFET die properties. By carefully choosing materials with appropriate CTE values (e.g., CTE substrate between 2-10 ppm/°C and CTE bonding layer between 5-15 ppm/°C) and elastic moduli ratios, the sensor structure compensates for thermal and mechanical stresses, maintaining piezoresistance stability across wide temperature and pressure ranges while preserving pH measurement accuracy.
Solution Approach 2:
The patent employs composite materials by combining the ISFET die with a substrate and bonding layer having specific material properties. The substrate is made from materials like silicon, silicon carbide, or gallium arsenide with controlled CTE, while the bonding layer uses materials such as metal alloys or ceramic compounds with tailored elastic moduli. This composite structure creates a mechanically compliant system that reduces stress-induced piezoresistance errors while maintaining operational versatility across deep-sea conditions.
2Device complexity
If the ISFET die is directly bonded to the substrate without a bonding layer, then the structure is simpler, but mechanical stresses from pressure and temperature changes cause larger piezoresistance variations
Solution Approach 1:
The patent introduces a bonding layer as an intermediary component between the ISFET die and the substrate. This bonding layer, made from materials with specific elastic moduli (ratio of 0.1-10 relative to the substrate) and CTE values, acts as a mechanical buffer that distributes and mitigates stress concentrations. The intermediary layer prevents direct stress transmission to the fragile ISFET die, maintaining piezoresistance stability while adding minimal structural complexity.
3Temperature
If materials with matched CTE are used for substrate and bonding layer, then thermal stress is minimized, but the ability to counteract mechanical stresses is reduced
Solution Approach 1:
The patent applies local quality by assigning different CTE values to the substrate and bonding layer materials. The substrate has a CTE in the range of 2-10 ppm/°C while the bonding layer has a higher CTE of 5-15 ppm/°C. This differential CTE design creates localized thermal expansion differences that generate counteracting mechanical forces to offset external pressure and temperature stresses on the ISFET die, while still maintaining overall thermal compatibility.
Solution Approach 2:
The patent uses the bonding layer's higher CTE as a counterweight mechanism. When temperature changes occur, the bonding layer expands or contracts more than the substrate, generating internal stresses that oppose and compensate for external mechanical stresses applied to the ISFET die. This counteracting force mechanism maintains piezoresistance stability across wide temperature and pressure variations.
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 effectively reduces piezoresistive errors, maintaining accurate pH measurements despite environmental stress, thereby enhancing the reliability of ocean pH monitoring.
Implementation Method 1
the bonding layer or the substrate or both are configured such that the volume change induces a counteracting force that opposes at least a portion of the environmental force
Implementation Method 2
the counteracting force is configured to maintain the change in piezoresistance of the ISFET die from the drain to the source to less than 0.5% over the pressure and temperature range
Data Source
AI summary
Embodiments described herein provide for a pH sensor that is configured for use over a pressure and temperature range. The ISFET die of the pH sensor is bonded to the substrate of the pH sensor with a bonding layer that is disposed between the substrate and the ISFET die. The pressure and temperature change across the pressure and temperature range generates an environmental force in the pH sensor. Further, the substrate or the bonding layer or both change volume over the pressure and temperature range, and the substrate or the bonding layer or both are configured such that the volume change induces a counteracting force that opposes at least a portion of the environmental force. The counteracting force is configured to maintain the change in piezoresistance of the ISFET die from the drain to the source to less than 0.5% over the pressure and temperature range.