Laser-Induced Selective Heating for MicroLED Bonding
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Solution Overview
Problem
Conventional pick and place processes for manufacturing smaller displays, such as those in smartphones or virtual reality headsets, face issues like thermal expansion and reduced throughput due to the need for heating the entire display substrate, which can cause misalignment and debonding of LEDs.
Innovation Solution
A laser-induced bonding process where a pick and place head aligns LEDs with substrate contact pads, and a laser emits light with a specific wavelength to melt the contact pads directly or indirectly, reducing thermal expansion and improving bonding efficiency by focusing heat on a small area.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the entire display substrate is heated to bond LEDs, then bonding is achieved, but thermal expansion causes misalignment of LEDs with contact pads
Solution Approach 1:
The patent applies localized heating only to the contact pad regions using a laser beam, rather than heating the entire display substrate. This localized approach provides sufficient heat for bonding while minimizing thermal expansion of the overall substrate, thereby maintaining LED alignment precision while achieving reliable bonding.
Solution Approach 2:
The heating process is segmented into discrete localized zones corresponding to individual contact pads. The laser selectively heats only the specific contact pad regions where bonding is required, rather than applying uniform heat across the entire substrate, thus preventing global thermal expansion and alignment issues.
2Reliability
If the entire display substrate is heated for bonding, then bonding is achieved, but the process takes relatively long time reducing throughput
Solution Approach 1:
The heating process is divided into multiple independent localized zones. Multiple lasers or a scanning laser can simultaneously or sequentially heat different contact pad regions, enabling parallel processing and significantly reducing the total bonding time compared to heating the entire substrate uniformly.
Solution Approach 2:
Only the minimum necessary area (contact pads) is heated to the bonding temperature, rather than heating the entire substrate. This partial heating approach achieves the required bonding reliability while minimizing the time and energy required for the process.
3Reliability
If the display substrate is reheated in multiple placement cycles, then bonding is achieved, but previously bonded LEDs may debond
Solution Approach 1:
The laser selectively reheats only the contact pad regions that require bonding in the current placement cycle, while previously bonded contact pads are not exposed to significant heat. This localized approach maintains the stability of previously bonded LEDs while achieving bonding for new placements.
Solution Approach 2:
The heating process is segmented to target only the specific contact pads needing bonding in each placement cycle. This prevents cumulative heating of the entire substrate and avoids thermal stress on previously bonded regions, thereby maintaining bond stability across multiple cycles.
4Productivity
If laser light wavelength is selected to pass through pick-up head and LED, then contact pads are directly heated for bonding, but wavelength must be precisely controlled
Solution Approach 1:
The laser wavelength is carefully selected and controlled to match the absorption characteristics of the contact pad materials while being transparent to the LED materials. This precise parameter selection enables direct heating of contact pads for rapid bonding without damaging the LEDs, achieving both high speed and high precision.
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 method enhances bonding speed, reduces metal oxide growth, and prevents reheating of previously bonded LEDs, leading to faster manufacturing times and improved display substrate alignment.
Implementation Method 1
The wavelength of the laser light and the material used for the contact pads are both selected so that the laser light is capable of melting the contact pads
Implementation Method 2
a laser emits a laser light that is used to induce bonding of the LED contact pads with the corresponding substrate contact pads
Implementation Method 3
the laser light is capable of melting the contact pads without damaging the LED
Implementation Method 4
the pick-up head is formed of polydimethylsiloxane (PDMS) or fused silica, which is transparent or substantially transparent to light with wavelengths between 220 nm and 1200 nm
Data Source
AI summary
A laser is used to induce bonding of LED contact pads with corresponding substrate contact pads on a display substrate. The wavelength of the laser light and the material used for the contact pads are both selected so that the laser light is capable of melting the contact pads. For example, the laser light has a wavelength of between 220 nm and 1200 nm, and the contact pads are formed of a copper-tin oxide (CuSn). Furthermore, the system may be configured to shine the laser light through a number of other components, such as the pick-up head and the LED itself. These materials can be formed of materials that do not absorb the energy of the laser light. Bonding the contacts with a laser in this manner allows for faster heating and cooling times, avoids reheating of previously bonded contact pads, and reduces thermal expansion of the display substrate.