CMUT Isolation Stack for Dielectric Charging Mitigation
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Solution Overview
Problem
Capacitive micromachined ultrasonic transducers (CMUTs) face reliability issues due to dielectric charging problems during operation in collapse mode, leading to electrical short circuits and permanent damage, which conventional solutions like PostCMUTs and spacers have not adequately addressed.
Innovation Solution
A reliable isolation stack is implemented in CMUTs, comprising a silicon dioxide layer and a partially oxidized silicon nitride layer, with optional silicon carbide coating, to prevent short circuits and alleviate dielectric charge buildup by separating the top and bottom electrodes effectively.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If CMUTs operate in collapse mode to achieve higher acoustic power and wider bandwidth, then acoustic performance is improved, but electrical short circuits occur due to dielectric charging problems
Solution Approach 1:
The isolation stack is segmented into multiple distinct layers (silicon dioxide layer, silicon nitride layer, and oxidized nitride layer) rather than using a single dielectric layer. Each layer has specific thickness and material properties designed to address charge trapping independently, collectively preventing electrical short circuits while allowing collapse mode operation
Solution Approach 2:
The patent employs a composite dielectric structure combining different materials (silicon dioxide, silicon nitride, and oxidized nitride) with complementary properties. This composite isolation stack leverages the advantages of each material to resist charge trapping and prevent electrical breakdown during collapse mode operation
2Length of stationary object
If thin dielectric insulation layers are used to maintain small gap between electrodes, then device size is reduced, but charge trapping occurs leading to electrical short circuits
Solution Approach 1:
The patent optimizes the thickness parameters of each isolation layer (silicon dioxide: 10-50 nm, silicon nitride: 50-500 nm, oxidized nitride: 4-10 nm) to achieve the right balance between maintaining a small overall gap for high acoustic performance and providing sufficient insulation to prevent charge trapping and electrical short circuits
3Reliability
If conventional insulation layers are used to prevent electrode contact, then short circuits are avoided, but dielectric charging problems persist
Solution Approach 1:
The isolation stack acts as an intermediary barrier between the top and bottom electrodes, specifically designed with multiple layers that intercept and prevent charge trapping before it can cause electrical short circuits. The complex multi-layer structure disrupts charge accumulation mechanisms that occur in conventional single-layer insulators
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 prevents short circuits and reduces dielectric charge accumulation, enhancing the operational reliability of CMUTs during collapse mode, thereby improving their acoustic performance and longevity.
Implementation Method 1
one or more insulation layers or bumps may be sandwiched between the bottom and top electrode. An electrical reliability issue generally originates substantially from dielectric charging problems in the thin dielectric insulation layers
Implementation Method 2
a first isolation stack layer comprising a silicon dioxide layer, and a second isolation stack layer comprising a partially oxidized silicon nitride on the silicon dioxide layer
Data Source
AI summary
Methods and systems for providing a reliable (i.e., defect mitigated) isolation stack in capacitive micromachined ultrasonic transducers (CMUTs) are disclosed. A capacitive micromachined ultrasonic transducer (CMUT) includes a top electrode and a bottom electrode. The CMUT includes a sidewall between the top electrode and the bottom electrode. The sidewall is configured to separate the top electrode and the bottom electrode by a gap. The CMUT includes an isolation stack part on one or both of a bottom side of the top electrode or a top side of the bottom electrode. The isolation stack part includes a silicon dioxide layer, and a partially oxidized silicon nitride comprising a silicon nitride layer and an oxidized nitride layer.