Piezoelectric Micromachined Ultrasonic Transducer Trench Stress Reduction
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Solution Overview
Problem
Existing methods for producing piezoelectric micromachined ultrasonic transducers result in wide and flat undercuts in the trench created during the manufacturing process, which can lead to local stress overloads and inaccuracies in the transducer plate dimensions.
Innovation Solution
A method involving the creation of a circumferential trench on a silicon substrate, partial filling with a passivation layer, and subsequent epitaxial growth of a polysilicon layer, followed by precise trenching and isotropic silicon etching to form a funnel-shaped second trench, allowing for accurate definition and reduction of stress on the transducer plate.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a trench is created through the carrier substrate using conventional trenching methods, then the trench can be formed to reach the polysilicon layer, but the trench develops wide and flat undercuts that cause local stress overloads and dimension inaccuracies
Solution Approach 1:
A first circumferential trench is created around the transducer element position before depositing the passivation layer. This preliminary trench defines the future transducer plate boundaries and allows the passivation layer to be deposited precisely within the intended area, preventing lateral material spread that would cause undercuts and stress concentrations.
Solution Approach 2:
A passivation layer is deposited over the carrier substrate and into the first circumferential trench, serving as an intermediary material that seals the trench and defines the transducer plate area. This passivation layer acts as a barrier that prevents subsequent polysilicon deposition from spreading laterally, thereby eliminating the formation of wide undercuts and associated stress concentrations.
2Manufacturing precision
If the passivation layer is deposited to fill the first circumferential trench, then the transducer plate area is precisely defined, but additional process steps are required
Solution Approach 1:
The passivation layer serves multiple functions simultaneously: it acts as a sealing material for the first circumferential trench, defines the transducer plate area boundaries, and serves as a mask for subsequent polysilicon deposition. By combining these functions into a single material layer, the process achieves high precision without proportionally increasing complexity.
Solution Approach 2:
The first circumferential trench is created in advance before polysilicon deposition, and the passivation layer is deposited to fill it. This preliminary structuring establishes precise boundaries that guide all subsequent processing steps, ensuring that the transducer plate forms with accurate dimensions while the passivation layer remains in place to prevent lateral spreading throughout the process.
3Manufacturing precision
If a second trench is created entirely through the carrier substrate to the passivation layer, then the oscillatable transducer plate is formed with accurate lateral dimensions, but the process requires precise control to avoid undercuts
Solution Approach 1:
The first circumferential trench is created around the transducer element position before any polysilicon deposition occurs. This preliminary trench, combined with the subsequently deposited passivation layer, establishes boundaries that guide the second trench formation process, ensuring that the oscillatable transducer plate is released with accurate lateral dimensions while preventing the formation of harmful undercuts.
Solution Approach 2:
The passivation layer deposited into the first circumferential trench serves as an intermediary that defines the transducer plate area and prevents lateral material spread during second trench formation. This intermediary layer allows the second trench to be created with precise control, achieving accurate lateral dimensions without the stress concentration problems associated with conventional trenching methods.
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 enables the precise manufacturing of transducer plates with improved lateral dimensions and reduced stress, preventing undercuts and steps, thereby enhancing the reliability and performance of the microelectromechanical oscillation system.
Implementation Method 1
a passivation layer is applied to the first surface of the first carrier substrate and, in this process, the first circumferential trench is at least partially filled with the passivation layer
Implementation Method 2
a first polysilicon layer grows onto the passivation layer and/or the first surface of the carrier substrate. In particular, the first polysilicon layer grows epitaxially onto the passivation layer and/or the first surface of the carrier substrate
Implementation Method 3
a second trench is created entirely through the carrier substrate in the direction of the transducer element. In this case, the second trench extends to the passivation layer
Data Source
AI summary
A method for producing a microelectromechanical oscillation system. A carrier substrate having a first surface is provided. A circumferential first trench is produced, which extends from the first surface at least partially through the carrier substrate. A passivation layer is applied to the first surface of the first carrier substrate and the first circumferential trench is at least partially filled with the passivation layer. A first polysilicon layer is grown on the passivation layer and/or the first surface of the carrier substrate. A transducer element of the microelectromechanical oscillation system is arranged on a second surface of the first polysilicon layer. A second trench is produced through the carrier substrate in the direction of the transducer element, which extends up to the passivation layer so that the oscillatable transducer plate of the microelectromechanical oscillation system is produced adjacent to the second trench using the first polysilicon layer.