Cap Bonding Structure for Backside Absolute Pressure Sensors
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Solution Overview
Problem
Current designs for backside absolute pressure sensors face structural failures due to weak bonding between the cap and the pressure sensing element, primarily caused by poor adhesion of silicon nitride films, which also allows mobile ions to migrate, leading to output instability and degradation of the silicon nitride layer during fabrication.
Innovation Solution
A bonding mechanism comprising a silicon dioxide layer between the polysilicon and silicon nitride films, with an oxide layer providing superior adhesion and protection against over-etching, enhancing cap bond strength and maintaining the robustness of the silicon nitride layer to block mobile ions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a layer of silicon nitride film is incorporated between polysilicon and silicon dioxide to block mobile ions, then output instability is reduced, but adhesion between bonding films deteriorates causing weak cap bond strength
Solution Approach 1:
A silicon oxide layer is introduced as an intermediary between the silicon nitride film and polysilicon layer. This intermediate layer improves adhesion between the poorly bonding silicon nitride and polysilicon, while the silicon nitride continues to block mobile ions from reaching the silicon oxide and silicon surface, thus resolving both the reliability and strength issues
2Reliability
If the thickness of silicon nitride film is increased to provide robust passivation against mobile ions, then ion blocking capability is improved, but film integrity deteriorates due to tensile stress causing cracks
Solution Approach 1:
The bonding mechanism is segmented into multiple thin layers (silicon dioxide, silicon nitride, and polysilicon) rather than using a single thick silicon nitride layer. This segmentation reduces tensile stress in each individual layer, preventing cracks while maintaining effective ion blocking capability through the combined thickness of the layered structure
3Ease of manufacture
If dry etching steps are used during wafer fabrication, then cavity formation is achieved, but silicon nitride film is over-etched reducing its thickness below protective threshold
Solution Approach 1:
The silicon nitride film is deposited with an initial thickness greater than 0.1 μm (typically less than 0.2 μm) to provide a cushion against over-etching during subsequent dry etching steps. This excess thickness compensates for the etching loss, ensuring that the residual thickness remains above the 0.1 μm threshold needed for effective mobile ion blocking
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 achieves a robust cap bond strength greater than the cap material, minimizing ion migration and fabrication-induced degradation, resulting in stable and reliable pressure sensor performance.
Implementation Method 1
Silicon nitride has proven to be an efficient dielectric layer to block mobile ions from getting into the silicon oxide and silicon surface
Implementation Method 2
an oxide layer deposited between the polysilicon and silicon nitride films... this oxide layer provides superior adhesion
Implementation Method 3
the oxide layer provides superior adhesion and protects the silicon nitride from over-etching
Implementation Method 4
the bonding between the cap and the pressure sensing element is not always robust enough to maintain the hermetic bonding between the cap and the pressure sensing element
Data Source
AI summary
A pressure sensor includes a pressure sensing element having a diaphragm, a cavity, and bridge circuitry connected to the diaphragm. A top surface is formed as part of the pressure sensing element such that at least a portion of the top surface is part of the diaphragm, and the plurality of piezoresistors are located on the top surface. A cap is bonded to the top surface through the use of a plurality of layers. One of the layers is a silicon dioxide layer, another layer is a silicon nitride layer, another layer is an oxide layer, and another of the layers is a polysilicon layer. The plurality of layers provides proper bonding between the cap and the top surface of the pressure sensing element.