AlN Nucleus Layer Reflectivity Control for HEMT Current Collapse
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Solution Overview
Problem
High electron mobility transistors (HEMTs) made of nitride semiconductor materials exhibit a drift in drain current due to current collapse, which is attributed to the quality of the aluminum nitride (AlN) nucleus forming layer on silicon carbide (SiC) substrates, leading to instability in device performance.
Innovation Solution
A process involving the measurement of reflectivity using a monitoring beam with a wavelength between 350 to 720 nm to control the growth of the AlN nucleus forming layer, ensuring it falls within a specific range to enhance crystal quality and prevent island formation, thereby reducing current collapse.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the AlN nucleus forming layer is grown thicker to improve crystal quality, then the drain current drift is reduced, but the growth time increases and productivity decreases
Solution Approach 1:
The patent implements in-situ reflectivity measurement during AlN layer growth to provide real-time feedback on layer thickness and quality. The normalized reflectivity ratio (R1/R0) serves as a feedback parameter to monitor growth progression, allowing precise control of the AlN layer thickness to achieve optimal crystal quality without excessive growth time, thus resolving the contradiction between reliability and productivity.
Solution Approach 2:
The patent changes the monitoring parameter from direct thickness measurement to normalized reflectivity ratio (R1/R0), which provides a more sensitive indicator of AlN layer formation quality. By optimizing the wavelength range (350-720 nm) and using normalization against initial substrate reflectivity, the method achieves precise control of crystal quality while minimizing required growth thickness and time.
2Manufacturing precision
If the AlN layer thickness is precisely controlled to improve crystal quality, then current collapse is reduced, but the manufacturing complexity increases due to additional monitoring requirements
Solution Approach 1:
The patent employs in-situ reflectivity measurement that utilizes the growth chamber's existing optical path and materials. The monitoring beam reflects off the growing AlN layer surface, and the reflectivity change inherently provides information about layer thickness and quality without requiring external complex measurement systems. The system essentially monitors itself during the growth process.
Solution Approach 2:
The patent exploits optical reflectivity changes (analogous to color changes) of the AlN layer during growth. By monitoring the reflectivity ratio at specific wavelengths (350-720 nm), the system detects subtle changes in the layer's optical properties that correlate with crystal quality and thickness, providing a simple yet effective means of precision control without complex instrumentation.
3Reliability
If the growth temperature is optimized to improve AlN crystal quality, then the drain current drift is suppressed, but the energy consumption increases
Solution Approach 1:
The patent performs preliminary optimization of the AlN layer growth conditions, including temperature and thickness control, before proceeding to subsequent device fabrication steps. By using in-situ reflectivity monitoring to ensure optimal AlN layer quality from the outset, the method prevents the need for rework or additional corrective steps that would consume more energy later, thus reducing overall energy consumption while maintaining high reliability.
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 process effectively suppresses the drift in drain current by improving the crystal quality of the AlN nucleus forming layer, resulting in a more stable HEMT performance by optimizing the thickness and growth temperature of the AlN layer.
Implementation Method 1
measuring first reflectivity of a semiconductor substrate made of silicon carbide (SiC) by a monitoring beam with a wavelength from 350 to 720 nm
Implementation Method 2
growing a nucleus forming layer made of aluminum nitride (AlN) on the semiconductor substrate using a metal organic chemical vapor deposition (MOCVD) technique
Data Source
AI summary
A process of forming an epitaxial substrate is disclosed, where the epitaxial substrate includes a nucleus forming layer made of aluminum nitride (AlN) grown on a substrate made of silicon carbide (SiC). The process includes steps of: (1) first measuring the first reflectivity R0 of a surface of the SiC substrate, (2) growing the nucleus forming layer made of AlN as measuring second reflectivity R1 of a grown surface of the AlN nucleus forming layer, and (3) ending the growth of the AlN nucleus forming layer when a ratio R1/R0 of the reflectivity enters a preset range.