CMUT Oxide Layer Charge Buildup Prevention
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Solution Overview
Problem
Capacitive micromachined ultrasonic transducers (CMUTs) face challenges with charge buildup and leakage current during collapse mode operation due to variations in oxide layer characteristics, making predictable operation difficult.
Innovation Solution
A method involving the growth of an undoped epitaxial silicon layer on a doped silicon wafer, followed by thermal oxidation and annealing to disperse dopants into the epitaxial layer, preventing dopant incorporation into the oxide layer and ensuring it remains free of charge, thereby maintaining constant collapse mode voltage and preventing leakage current.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a standard oxide layer is used in CMUTs, then the device structure is simple, but charge buildup and leakage current occur during collapse mode operation
Solution Approach 1:
The oxide layer is segmented into multiple distinct layers: a first oxide layer grown on the substrate, an undoped epitaxial layer, and a second oxide layer grown on the epitaxial layer. This segmentation prevents dopant incorporation into the oxide, eliminating charge buildup and leakage current while maintaining reliable operation.
Solution Approach 2:
An undoped epitaxial layer is grown on the substrate before growing the oxide layer. This preliminary action creates a dopant-free interface that prevents charge buildup in the oxide layer during subsequent operation, ensuring predictable collapse mode performance.
2Reliability
If dopants are present in the oxide layer, then the manufacturing process is simpler, but leakage current increases and insulating behavior deteriorates
Solution Approach 1:
An undoped epitaxial layer is grown on the substrate before growing the oxide layer. This preliminary action creates a dopant-free interface that prevents charge buildup in the oxide layer during subsequent operation, ensuring predictable collapse mode performance.
Solution Approach 2:
The doping concentration parameter is changed by using an undoped epitaxial layer (zero dopant concentration) between the doped substrate and the oxide layer. This parameter change ensures the oxide layer remains free of dopants, maintaining excellent insulating behavior and preventing leakage current.
3Productivity
If the oxide layer varies in characteristics, then the device can be manufactured with standard processes, but operation becomes unpredictable
Solution Approach 1:
An undoped epitaxial layer is grown on the substrate before growing the oxide layer. This preliminary action creates a dopant-free interface that prevents charge buildup in the oxide layer during subsequent operation, ensuring predictable collapse mode performance.
Solution Approach 2:
The oxide layer is grown on a locally undoped epitaxial region rather than on the doped substrate. This local quality difference ensures the oxide layer forms with consistent, predictable characteristics free from dopant-induced variations, while the rest of the device can be manufactured using standard doped substrate processes.
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 ensures the oxide layer exhibits desirable insulating behavior, preventing charge buildup and leakage current, allowing for predictable and stable operation of CMUTs in collapse mode.
Implementation Method 1
growing an undoped epitaxial layer on the highly doped silicon wafer
Implementation Method 2
growing an oxidation layer on the undoped epitaxial layer
Implementation Method 3
annealing the structure to disperse the doping into the undoped epitaxial layer
Implementation Method 4
The membrane, cavity, and an opposing layer together form a capacitor
Data Source
AI summary
Processes for fabricating capacitive micromachined ultrasonic transducers (CMUTs) are described, as are CMUTs of various doping configurations. An insulating layer separating conductive layers of a CMUT may be formed by forming the layer on a lightly doped epitaxial semiconductor layer. Dopants may be diffused from a semiconductor substrate into the epitaxial semiconductor layer, without diffusing into the insulating layer. CMUTs with different configurations of N-type and P-type doping are also described.