Off-axis Epitaxial Lift Off Wafer Orientation
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Solution Overview
Problem
The epitaxial lift off (ELO) process is limited by stress-induced corner cracking and diffusion constraints in thin film devices, particularly when using nearly rectangular or square growth wafers, which impede the removal of epitaxial films without tearing and reduce process throughput.
Innovation Solution
An off-axis epitaxial lift off process is employed, where the growth wafer edges are oriented at an angle other than the natural cleavage plane, allowing corners to be etched at a faster rate than edges, reducing stress and cracking by creating a rounded geometry that facilitates smoother separation of the epitaxial film from the wafer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If the growth wafer has edges oriented parallel or perpendicular to the flat cut (on-axis orientation), then the manufacturing process is simple and straightforward, but corner cracks develop during etching due to increased stress concentration
Solution Approach 1:
The patent applies asymmetry by rotating the wafer edges to an off-axis orientation (e.g., 45 degrees) relative to the flat cut, breaking the symmetric on-axis alignment. This asymmetric orientation causes corners to etch at different rates than edges, distributing stress more evenly and preventing corner crack formation while maintaining manufacturing simplicity.
Solution Approach 2:
The patent applies local quality by creating different etch rates at different locations (corners vs. edges) through the off-axis orientation. The crystallographic orientation causes the corners to experience faster etching rates compared to the edges, locally modifying the etching behavior to reduce stress concentration and prevent cracking at critical corner regions.
2Reliability
If the sacrificial layer is etched away using a wet chemical process, then the epitaxial film can be separated from the growth wafer, but the process speed is limited by diffusion constraints and reactant delivery to the etch front
Solution Approach 1:
The patent applies spheroidality by creating a rounded geometry at the etch front through off-axis orientation. Instead of a flat or sharp etch front, the curved geometry at the corners facilitates better reactant delivery and byproduct removal, reducing diffusion constraints and increasing etch process throughput while maintaining complete film separation.
Solution Approach 2:
The patent applies dimensionality change by transitioning from a two-dimensional planar etch front to a three-dimensional curved etch front geometry. The off-axis orientation creates vertical component to the etching at corners, forming a more complex three-dimensional etch front that improves mass transport and reduces diffusion limitations.
3Stability of the object's composition
If the epitaxial film is maintained in its deposited geometry during etching, then the film structure is preserved, but a narrow and long opening forms that severely limits the overall process speed
Solution Approach 1:
The patent applies dynamics by allowing the epitaxial film geometry to dynamically change during the etching process. The off-axis orientation causes the film to bend and conform as the sacrificial layer is removed, transforming from a rigid maintained geometry to a dynamic adapting geometry that follows the etch front, thereby preventing narrow opening formation and increasing throughput.
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 reduces corner cracking and increases the yield of the ELO process by modulating the etch rate, ensuring faster corner etching relative to edges, resulting in less stress and improved film separation efficiency.
Implementation Method 1
The sacrificial layer is etched away via a wet chemical process
Implementation Method 2
The growth wafer has edges oriented in a direction other than that of the natural cleavage plane. The corners of the growth wafer are etched at a faster rate relative to the edges.
Data Source
AI summary
Embodiments described herein provide processes for forming and removing epitaxial films and materials from growth wafers by epitaxial lift off (ELO) processes. In some embodiments, the growth wafer has edge surfaces with an off-axis orientation which is utilized during the ELO process. The off-axis orientation of the edge surface provides an additional variable for controlling the etch rate during the ELO process and therefore the etch front may be modulated to prevent the formation of high stress points which reduces or prevents stressing and cracking the epitaxial film stack. In one embodiment, the growth wafer is rectangular and has an edge surface with an off-axis orientation rotated by an angle greater than 0° and up to 90° relative to an edge orientation of <110> at 0°.