Ralph Aschenbach | Coherent: Can laser transfer overcome crosstalk to handle sub-5-micron microLEDs with street widths down to two microns?
18:24 - 20:10
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Summary of the clip:
Can laser transfer overcome crosstalk to handle sub-5-micron microLEDs with street widths down to two microns?
The crosstalk question is central when laser transfer targets devices with two-micron street widths. From the epi wafer, LEDs stick very rigidly, so the process requires high energy density. That raises the risk of breaking the LED and forces a larger street width. As a result, two-micron streets cannot be reached directly from the epi wafer, even if sub-10-micron devices are already possible.
Working from the lift process on a temporary carrier changes the energy-density requirement. The carrier-based process can use lower energy density, making the system less sensitive and reducing the risk of LED breakage. The remaining issue is the neighbor: when it sees some UV light, will it start to move or break? That neighbor response defines crosstalk, not just the LED being transferred.
The practical conclusion is that two-micron street widths are not reachable from the epi wafer. They are possible from the temporary carrier because that process is less sensitive. This distinction separates the wafer-level transfer constraint from the carrier-level laser transfer capability. It also explains why lower energy density matters for neighbors exposed to UV light and why crosstalk limits tight-pitch microLED laser transfer.
In this short video, you can learn:
* From the epi wafer, two-micron street widths cannot be reached because high energy density is needed and the LEDs stick rigidly.
* The temporary-carrier lift process can use lower energy density, making the system less sensitive to crosstalk.
* When a neighboring LED sees UV light, it may move or break, which is the crosstalk effect that matters.
📋 **Clip Abstract** Laser transfer faces a crosstalk limit from the epi wafer that prevents two-micron street widths. Moving to a temporary carrier lowers energy density, reduces sensitivity and makes two-micron transfer possible while neighbor LEDs remain the key UV-exposure risk.
About the speaker:
* Speaker: Ralph Aschenbach
* Company: Coherent
* Event: Eindhoven 2026
* Location: High Tech Campus, Eindhoven
#LaserTransfer, #EpiWaferTransfer, #TemporaryCarrierLift, #TwoMicronStreetWidth, #MicroLEDDisplays, #DisplayManufacturing
This is a highlight of the presentation:
Fast and flexible - how lasers can solve the current technical bottlenecks for the mass production of microLED displays
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05:05 - 06:26
How does UV transfer S adapt for data communication instead of TV panels?
How does UV transfer S adapt for data communication instead of TV panels?
The TV market will still need some time for micro LED, while many people now discuss micro LED in data communication. The tool requirements for data communications are slightly different, but the basic technology remains the same. That is why this product was put on our roadmap. It is called UV transfer S, where S stands for selectivity and flexibility. Precision is even higher.
The target for this tool is one micron placing accuracy. It comes with the same degree of automatization and fab integration. The optical concept differs because requirements on the stage system are even a little bit higher to reach that one micron placing accuracy, and the approach is very flexible. It uses a faster laser.
It comes with a faster laser and with the agility of a scanner system combined with the robustness of a mask imaging system. Test results are presented for this tool. The targets are restated: what are they again? The system must be very fast. That speed requirement sits alongside one micron placing accuracy, selectivity and flexibility.
In this short video, you can learn:
* UV transfer S is on the roadmap because data communication requirements differ slightly while the basic technology is the same.
* The tool targets one micron placing accuracy with higher stage-system requirements, the same automatization and fab integration.
* It comes with a faster laser, combining scanner agility with mask imaging robustness, and the system must be very fast.
📋 **Clip Abstract** Micro LED for TV still needs time, so attention turns to data communication, where tool requirements differ slightly but the basic technology is unchanged. The roadmap product UV transfer S targets one micron placing accuracy, adds selectivity and flexibility, and pairs a faster laser and scanner agility with mask imaging robustness for a system that must be very fast.
About the speaker:
* Speaker: Ralph Aschenbach
* Company: Coherent
* Event: Eindhoven 2026
* Location: High Tech Campus, Eindhoven
#UVTransferS, #SelectivityFlexibility, #OneMicronPlacingAccuracy, #MaskImagingRobustness, #MicroLEDDisplays, #DataCommunication
15:55 - 18:08
Does the reported microLED transfer yield include electrical functionality, or only optical inspection and physical placement?
Does the reported microLED transfer yield include electrical functionality, or only optical inspection and physical placement?
In this process, yield is not a functional test. The LEDs are not lit up. There is no electrical test of the LED after transfer. Instead, the check is purely optical inspection: the LED must have landed, must not be broken, and its positioning must be within specification. Photoluminescence testing exists at other companies and was presented elsewhere, but it is not implemented in this machine.
The transfer discussed here is made to a temporary carrier, not to the final display. That is why this inspection approach is used for this testing. The reported yield belongs to this process step alone; it is not the overall process yield. Defects from the backplane, errors from the soldering process, and other downstream issues are outside its scope. The figure therefore reflects placing accuracy and physical transfer yield.
The questioner asks what the point is of transferring to something not functional, and stresses that electrical contact must be formed and the LED must become functional after transfer. The answer confirms that. Yet this yield does not cover that electrical outcome; it remains a physical transfer and placement metric. The LED not being lit up means no functional test is performed.
In this short video, you can learn:
* The yield definition uses optical inspection only: landed, unbroken, and positioning within specification, not electrical functionality.
* The transfer is to a temporary carrier, not the final display, so the reported yield is only for that process step.
* Backplane defects and soldering-process errors are outside this yield figure, which does not represent overall process yield.
📋 **Clip Abstract** This clip clarifies that the reported transfer yield is measured by optical inspection of landing, breakage and placement accuracy, not by lighting up the microLEDs. The transfer goes to a temporary carrier, so backplane defects, soldering errors and electrical functionality remain outside that process-step yield.
About the speaker:
* Speaker: Ralph Aschenbach
* Company: Coherent
* Event: Eindhoven 2026
* Location: High Tech Campus, Eindhoven
#MicroLEDTransfer, #OpticalInspection, #TemporaryCarrier, #PlacementAccuracy, #MicroLEDDisplays, #ARDisplays
07:01 - 09:07
Can high-speed microLED transfer reach five nines and 1.7-micron placement accuracy across 120,000 LEDs?
Can high-speed microLED transfer reach five nines and 1.7-micron placement accuracy across 120,000 LEDs?
The tool currently achieves a yield of about 9.98, described as very close to the 9.999 target. The presented error map shows faults spread more or less randomly, and the root cause of most errors is not machine quality: particle contamination or issues with the donor material are named instead. The requirement set is future-proofing — processing LEDs down to five microns with street widths down to two microns.
Placement accuracy is direction-dependent. In one direction the tool reaches 1.7 microns, already very close to target; in Y it sits far below, at one micron. The test sample was assembled from roughly 120,000 LEDs. A slight dependency on process speed remains: at very high process speed, especially in Y, results fall a little out of spec, and slowing down brings them closer.
Work on the tool is not finished: small optimization continues over the next two months. The target is to get the blue bar down to the red bar, and to ensure that the dependency on the dynamics is reduced. Yield and placement accuracy are both being pushed toward the five-nines level and two-micron street widths.
In this short video, you can learn:
* Yield currently sits at about 9.98 against a 9.999 target, with errors spread randomly and attributed mainly to particle contamination or donor-material issues, not machine quality.
* Placement accuracy is 1.7 microns in one direction and one micron in Y, described as far below, with a slight process-speed dependency at high speed.
* The data comes from a sample of roughly 120,000 LEDs, and two more months of tool optimization target the blue bar coming down to the red bar.
📋 **Clip Abstract** The high-speed microLED transfer tool reaches a yield of about 9.98 against a 9.999 target, with errors randomly distributed and blamed on particle contamination or donor material rather than machine quality. Placement accuracy is 1.7 microns in one direction and one micron in Y on a 120,000-LED sample, with a residual process-speed dependency that two months of optimization aim to remove.,
About the speaker:
* Speaker: Ralph Aschenbach
* Company: Coherent
* Event: Eindhoven 2026
* Location: High Tech Campus, Eindhoven
#ErrorMap, #PlacementAccuracy, #FiveNinesYield, #DonorMaterial, #MicroLEDDisplays, #MassTransfer