CMUT Membrane Thickness Variation for Thermal Stress Reduction
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Solution Overview
Problem
The manufacturing of capacitive micro-machined ultrasonic transducer (CMUT) devices often results in a high number of non-functional devices due to membrane warping caused by thermal stress from differences in thermal expansion coefficients between the second electrode and membrane dielectric materials during the cavity formation process, leading to low yields and devices operating outside design tolerances.
Innovation Solution
The method involves forming a membrane portion over the second electrode with a thickness exceeding the sacrificial material, and a further membrane portion extending towards the substrate, ensuring the membrane thickness is at least five times that of the sacrificial material to reduce thermal stress and improve robustness during cavity formation, along with additional sealing and etch stop layers to enhance membrane strength without affecting flexibility.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If the membrane thickness is reduced to improve flexibility and bandwidth, then the device operates better electrically, but the membrane becomes more susceptible to thermal stress and warping during cavity formation
Solution Approach 1:
The patent applies different thicknesses of the second dielectric layer at different locations: a first thickness over the cavity area and a second thickness (greater than the first) over the electrode area. This local variation in thickness provides mechanical support where needed while maintaining the required electrical performance over the cavity.
Solution Approach 2:
The patent forms the complete second dielectric layer structure before removing the sacrificial material to create the cavity. This preliminary formation of the thick dielectric layer ensures the membrane has sufficient strength to withstand the subsequent cavity formation process without warping, while still achieving the desired thin profile over the cavity for electrical performance.
2Ease of manufacture
If standard thickness membrane is used to simplify manufacturing, then the process is easier, but thermal stress during cavity formation causes membrane warping and device failure
Solution Approach 1:
The patent implements a second dielectric layer with spatially varying thickness, being thicker over the electrode region and thinner over the cavity region. This local differentiation provides structural support during manufacturing while maintaining the necessary acoustic and electrical properties in the final device.
Solution Approach 2:
The patent uses a composite structure consisting of multiple dielectric layers with different thicknesses and potentially different material compositions. The second dielectric layer is formed as a composite with varying thickness to simultaneously satisfy mechanical strength requirements during processing and acoustic performance requirements in the final device.
3Reliability
If the second dielectric layer is made thicker to reduce thermal stress, then membrane warping is reduced, but the device complexity and manufacturing steps increase
Solution Approach 1:
The patent forms the second dielectric layer with a thickness profile that is thicker in regions requiring mechanical support (over the electrode) and thinner in regions where acoustic performance is critical (over the cavity). This localized thickness variation optimizes both yield and performance without requiring uniformly thick layers throughout the entire device structure.
Solution Approach 2:
The patent resolves the contradiction by introducing a dimensional variation in the second dielectric layer thickness rather than uniformly increasing thickness across the entire layer. This dimensional approach allows the membrane to have sufficient strength where needed while maintaining the thin profile required for acoustic performance in other areas.
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 improves the yield of acceptable CMUT devices by reducing membrane deformation and maintaining excellent bandwidth and acoustic impedance characteristics, allowing for cost-effective high-yield production while minimizing membrane warping and stress sensitivity.
Implementation Method 1
a cavity formed by the removal of a sacrificial material in between the first electrode and the membrane
Implementation Method 2
membrane warping caused by thermal stress from differences in thermal expansion coefficients between the second electrode and membrane dielectric materials
Data Source
Figure 1~2
Figure 3(a)~3(b)
Figure 4(a)~4(c')
AI summary
Disclosed is a method of manufacturing a capacitive micro-machined ultrasonic transducer (CMUT) device comprising a first electrode (112) on a substrate (110) and a second electrode (122) embedded in an electrically insulating membrane, the first electrode and the membrane being separated by a cavity (130) formed by the removal of a sacrificial material (116) in between the first electrode and the membrane, the method comprising forming a membrane portion (22) on the second electrode and a further membrane portion (24) extending from the membrane portion towards the substrate alongside the sacrificial material, wherein the respective thicknesses the membrane portion and the further membrane portion exceed the thickness of the sacrificial material prior to forming said cavity. A CMUT device manufactured in accordance with this method and an apparatus comprising such a CMUT device are also disclosed.