Micro LED Evaluation Wafer for In-Wafer Transient Testing
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Solution Overview
Problem
Existing techniques fail to effectively evaluate the current-carrying and transient properties of AlGaInP-based micro LEDs in an epitaxial wafer state without disconnection, as they require complex processing and different thermal conditions, leading to inaccurate quality assessment.
Innovation Solution
A wafer structure comprising a GaAs substrate with a micro LED of 100 µm or less, an upper electrode pad on a pad base connected through an insulator, and a lower electrode pad, allowing direct evaluation of current-carrying and transient properties without disconnection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If micro LEDs are mounted on drive substrate for evaluation, then device functionality can be tested, but transfer information contaminates the quality assessment and process time increases
Solution Approach 1:
The patent extracts the micro LED from the complex transfer and mounting process by evaluating it directly in its original epitaxial wafer state. The evaluation structure allows current application and transient property measurement without removing the micro LED from the wafer, thereby eliminating transfer-related contamination and reducing evaluation time.
Solution Approach 2:
The patent performs preliminary evaluation of micro LED transient properties directly on the epitaxial wafer before any transfer or mounting operations. The evaluation structure is formed in advance on the wafer, allowing quality assessment to be completed before the micro LED leaves the wafer state, thus preventing subsequent process variables from affecting the measurement.
2Reliability
If micro LEDs are mounted on drive substrate, then device testing is possible, but thermal design differences mask the true epitaxial layer quality
Solution Approach 1:
The patent changes the thermal parameter by using an evaluation structure with high thermal conductivity (diamond or copper substrate) that closely matches the thermal conditions of the original TO-18CAN mounting. This allows the transient property measurement to reflect the true epitaxial layer quality rather than being influenced by the poor thermal design of the drive substrate.
3Ease of manufacture
If conventional LED evaluation methods are used for micro LEDs, then existing processes can be maintained, but the small size of micro LEDs makes wire bonding impossible
Solution Approach 1:
The patent transitions from the conventional three-dimensional wire bonding approach to a two-dimensional planar evaluation structure. The current application electrodes are formed in the same plane as the micro LED on the epitaxial wafer, eliminating the need for vertical wire bonding and making the process feasible for sub-100 μm micro LEDs.
4Reliability
If deep etching is performed to access DH layer for side-face wiring, then electrical connection is possible, but disconnection occurs due to uneven surface and metal coverage difficulty
Solution Approach 1:
Instead of etching down to access the DH layer from the top surface, the patent inverts the approach by forming the evaluation structure and current application electrodes on the top surface of the epitaxial wafer. The micro LED remains in its original position with the DH layer intact, and electrical connection is achieved through planar electrodes rather than deep-trench wiring.
Data Source
Figure 1~2
Figure 3A
Figure 3B
AI summary
A wafer for micro LED characteristic evaluation includes a GaAs substrate, a micro LED having a side of 100 µm or less on the GaAs substrate, a pad base adjacent to the micro LED, an upper electrode pad on the micro LED and the pad base, and a lower electrode pad on the GaAs substrate in the vicinity of the micro LED. The micro LED and the pad base are coupled through an insulator. Thus, the present invention provides a wafer for micro LED characteristic evaluation that enables evaluation of current-carrying properties by forming a micro LED-size device in an epitaxial wafer state as it is, and enables evaluation of transient properties without disconnection under environmental testing.