Thick Oxide CMUT Cavity for Low Frequency Ultrasonic Operation
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Solution Overview
Problem
Current capacitive micromachined ultrasonic transducers (CMUTs) face challenges in scaling up to accommodate low frequency ultrasonic applications, such as airborne ultrasound, due to difficulties in forming thick oxide structures necessary for larger cavity sizes and thicker membranes, which are required for proper operation at frequencies below 20 MHz.
Innovation Solution
The development of a CMUT design utilizing thick oxide layers to increase the size of the cavity, allowing for the reception and transmission of low-frequency ultrasonic waves, involves growing a thick oxide layer on the semiconductor substrate using high-pressure oxidization and subsequent etching to create a post structure that surrounds a cell region, enabling a larger vertical cavity for improved frequency compatibility.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If conventional oxide formation processes are used, then standard cavity sizes are achieved, but low frequency operation below 20 MHz cannot be supported
Solution Approach 1:
The patent applies parameter changes by modifying the oxidation process conditions (pressure, temperature, time) to grow thicker oxide layers than conventionally achieved. This enables the cavity size to be increased for low frequency operation while maintaining manufacturing control through optimized process parameters.
Solution Approach 2:
The patent transitions from planar cavity structures to vertically extended cavities by growing thick oxide posts that extend upward from the substrate. This vertical dimension increase allows larger cavity volume for low frequency operation without increasing lateral footprint.
2Adaptability or versatility
If thicker membranes are used for low frequency operation, then frequency compatibility improves, but structural integrity and vibration characteristics become difficult to maintain
Solution Approach 1:
The patent uses composite material structures combining silicon membranes with thick oxide support structures. This composite approach allows the membrane to be optimized for vibration characteristics while the oxide provides structural support, maintaining reliability at low frequencies.
Solution Approach 2:
The patent employs curved or domed membrane geometries instead of flat membranes. This curvature provides structural strength while allowing the membrane to vibrate effectively at low frequencies, maintaining both integrity and frequency compatibility.
3Volume of stationary object
If larger cavity sizes are created, then low frequency reception and transmission improve, but conventional post oxide structures cannot accommodate the required size
Solution Approach 1:
The patent changes the oxidation parameters (high pressure, extended time, elevated temperature) to grow much thicker oxide posts than conventional processes allow. This enables large cavity volumes to be manufactured using standard semiconductor processing techniques.
Solution Approach 2:
The patent performs preliminary oxidation to grow the thick oxide posts before forming the cavity. This preliminary action creates the structural foundation that accommodates the large cavity volume required for low frequency operation.
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 CMUT to effectively operate at lower frequencies by increasing the volume of the cavity, accommodating the requirements for airborne ultrasonic imaging applications, while maintaining the necessary structural integrity and vibration characteristics.
Implementation Method 1
growing a thick oxide layer on the semiconductor substrate using high-pressure oxidization
Implementation Method 2
an ultrasonic wave causes conductive structure 430 to vibrate. The vibration varies the capacitance across the first and second capacitor plates, thereby generating an electrical signal that varies as the capacitance varies.
Implementation Method 3
conductive structure 430 to vibrate
Implementation Method 4
an alternating electrical signal applied across the biased first and second capacitor plates causes conductive structure 430 to vibrate which, in turn, transmits ultrasonic waves
Implementation Method 5
Acoustic dampening structure 140 absorbs and dampens the ultrasonic waves in semiconductor substrate 410
Data Source
AI summary
A capacitive micromachined ultrasonic transducer (CMUT), which has a conductive structure that can vibrate over a cavity, utilizes a thick oxide layer to substantially increase the volume of the cavity which, in turn, allows the CMUT to receive and transmit low frequency ultrasonic waves. In addition, the CMUT can include a back side bond pad structure that eliminates the need for and cost of one patterned photoresist layer.