HKUST develops new MicroLED transfer printing process

Foreign media reported that recently, a research team at the Hong Kong University of Science and Technology (Guangzhou) developed a new Micro LED transfer printing process, which is based on a dynamically programmable transfer head and uses local heating to control the viscosity of the polymer.

Image source: "International Journal of Extreme Manufacturing"

Researchers said that this new tool can selectively process devices with a variety of different geometries, solving key issues in the process of building complex microsystems. The research team demonstrated that the transfer system is capable of selectively sorting and transferring functioning Micro LEDs with dimensions of 45 × 25 microns and arranging them into customized layouts without degrading their performance.

During the study, the researchers successfully transferred semiconductor chips, copper films with a thickness of 90 nanometers, and spherical polystyrene beads with a diameter of 50 microns. These components are placed with extremely high accuracy, with positional offsets of less than 0.7 microns and rotational errors of less than 0.04 radians.

To build the transfer system, the research team formulated a special polymer that undergoes a sharp physical transformation at 44 degrees Celsius, from a hard plastic state to a rubber state. The team coated the polymer on an array of individually controllable microheaters.

During the transfer process, the research team pressed the stamp against the component array, activating specific heaters that melted a target area of 50 microns in size on the polymer in about 60 milliseconds, allowing it to adhere to the selected chip. The polymer then cools and hardens naturally in about 40 milliseconds, physically locking the chip. When it's time to move the component to a new location, the heater is triggered again to soften the polymer to release the chip. This temperature-driven mechanism provides a pick-to-release adhesion strength ratio of over 190 to 1.

Currently, the research team is studying how to expand the scale of the microheater array. This presents a challenge: densely packed heaters can cause thermal crosstalk, where heat leaks into adjacent pixels. To solve this problem, the researchers plan to use thinner polymer layers and introduce active matrix driver circuits, similar to the architecture used in commercial flat-panel TVs, to manage large-scale arrays without overly complex wiring. (Compiled by LEDinside)

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