SOI Semiconductor Strain Gauge Thickness Uniformity
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Solution Overview
Problem
Current semiconductor strain gages fabricated on bulk single crystal silicon face challenges in achieving uniform thickness, leading to variations in resistance values and difficulties in post-fabrication handling and packaging due to their small size and sensitivity to environmental variations.
Innovation Solution
The use of Silicon-on-Insulator (SOI) wafers with precisely controlled device layer thickness allows for uniform fabrication and easy release of strain gages from the substrate, enabling improved resistance uniformity and simplified post-fabrication handling through lithographic fabrication and wire bonding, followed by etching the handle wafer without attacking the device layer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If conventional etching and photolithography are used to fabricate strain gages on bulk single crystal silicon, then batch fabrication cost is reduced and productivity increases, but manufacturing precision of gage thickness deteriorates resulting in resistance value variations
Solution Approach 1:
The invention segments the silicon substrate into a device layer and a handle wafer using a buried oxide layer. This segmentation allows the device layer to be precisely controlled in thickness during wafer fabrication, while the handle wafer provides mechanical support. The etching process can then selectively remove the handle wafer without affecting the device layer, ensuring uniform gage thickness across all sensors fabricated on the wafer.
Solution Approach 2:
The device layer thickness is precisely controlled during the wafer fabrication process itself, before any sensor fabrication occurs. This preliminary action of establishing uniform thickness at the wafer level ensures that all subsequent sensors inherit this precision, eliminating the need for post-fabrication thickness adjustment and ensuring consistent resistance values across all devices.
2Productivity
If sensor size is reduced to minimize device bulk, then the number of sensors that can be fabricated per batch increases, but post fabrication handling and packaging complexity increases
Solution Approach 1:
Multiple miniaturized sensors are merged onto a single wafer-scale handle structure. The handle wafer acts as a common support for all sensors, allowing them to be handled, transported, and packaged as a unified assembly rather than as individual components. This merging approach maintains high productivity while dramatically simplifying post-fabrication operations.
Solution Approach 2:
The handle wafer serves as an intermediary structure that mediates between the miniaturized sensors and the external handling/packaging processes. By providing a robust, easily handled substrate, the handle wafer allows small sensors to be manipulated indirectly through the larger, more manageable wafer platform, thus maintaining ease of operation despite reduced sensor size.
3Reliability
If thinning of silicon substrate is increased to achieve desired gage thickness, then strain gage sensitivity improves, but resistance value variation increases due to dimensional variations on the wafer
Solution Approach 1:
The invention segments the substrate thickness control function from the sensor fabrication function. The device layer is fabricated with precise thickness control during wafer production, ensuring uniform sensor properties. The subsequent thinning of the handle wafer to expose the device layer does not affect the already-controlled device layer thickness, thereby maintaining both sensitivity and resistance uniformity.
4Ease of manufacture
If individual sensors are singulated after fabrication, then packaging becomes possible, but handling difficulty and time consumption increase due to small sensor size
Solution Approach 1:
Wires are bonded to the sensors while they are still attached to the handle wafer, before any singulation occurs. This preliminary action of wire bonding at the wafer level eliminates the need to handle and bond wires to each individual small sensor afterward, dramatically reducing the time and complexity of post-singulation packaging operations.
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 method ensures high-yielding semiconductor strain gages with improved resistance uniformity and facilitates easier handling and packaging, reducing yield loss and cost by allowing precise control over device layer thickness and efficient singulation of individual sensors.
Implementation Method 1
Semiconductor strain gages depend on the piezoresistive effects of silicon or germanium. As an electrical conductor is stretched, it becomes longer and narrower, thus increasing its resistance. This piezoresistance effect is used as a measure of applied stress.
Data Source
AI summary
Semiconductor strain gages fabricated on Silicon-on-insulator (SOI) material, and the method of making them. Force sensing elements are uniformly batch-fabricated at wafer level and singulated individually by a wire bonding method. In another method, they are singulated by plucking them off the wafer from their attachment site.