image: Figure 1 | Concept and experimental results of TCGC-assisted self-aligned laser transfer printing. a, Schematic illustration of the stamp with TCGC: (i) Schematic of the self-aligned mechanism of TCGC. (ii) Conversion of an asymmetric light intensity input to an even heat output by TCGC. (iii) Composition of TCGC: the upper layer is graphene with ordered atomic arrangement and high phonon transport efficiency; the lower layer is amorphous carbon with disordered atomic structure and low phonon transport efficiency. (iv) Schematic of thermal homogenization by light absorption and directional heat conduction through TCGC. b, Schematic illustration of the pick-up and printing process of SALT: (i) Heated stamp is in contact with the chip. (ii) The stamp picks up the chip from the donor substrate using negative pressure. (iii) IR laser irradiation enables chip transfer from the stamp. c, Schematic illustration of chip transfer errors caused by irradiation deviations in conventional laser transfer techniques. d, Adhesion strength test results at different temperature states. e, Comparison of chip transfer accuracy under IR laser offset irradiation, conventional approaches (without TCGC) and this work. f, SEM images of different patterned chips (triangles, circles and squares) transferred by SALT.
Credit: YongAn Huang et al.
In the realm of novel micro/nano-electronics manufacturing, the massive integration of microchips (such as MicroLEDs) onto heterogeneous substrates is essential for realizing high-performance displays and flexible electronics. Among the various transfer techniques, laser-assisted transfer technique is regarded as the most promising solution for industrial production due to its high speed, massive parallelism, and selective adhesion capabilities. However, current techniques require precise alignment between the laser beam and the chip during high-speed scanning, as even a slight deviation can lead to chips missing their target or experiencing severe transfer errors.
Now, in a new paper published in Light: Science & Applications, a research team, led by Professor YongAn Huang from State Key Laboratory of Intelligent Manufacturing Equipment and Technology, Huazhong University of Science and Technology, China, has developed a breakthrough solution: Self-aligned Laser Transfer (SALT) technique. This new method eliminates the need for precise laser-to-chip alignment by engineering the transfer stamp to "self-correct" irradiation deviations.
Concept of TCGC-assisted self-aligned laser transfer printing
The core innovation lies in the team's Thermal Conductivity Gradient Carbon (TCGC) stamp. By utilizing excimer laser self-limited carbonization on polyimide, the researchers created a unique layered structure embedded within the stamp: an upper graphene layer with excellent in-plane thermal conductivity and a lower layer of amorphous carbon (Fig. 1). The TCGC converts asymmetric light input into uniform heat output under non-uniform/misaligned infrared laser irradiation, whereas the upper graphene layer absorbs heat from the lower amorphous carbon and rapidly conducts heat laterally, ensuring uniform heat distribution of the underlying adhesive layer. This guarantees synchronous chip release at all adhesive sites, mitigating the transfer errors that plague conventional methods.
“The SALT has enabled the heterogeneous integration and selective transfer of diverse micro-objects with varying shapes, sizes, and patterns onto various challenging surfaces, demonstrating reversible adhesion switchability of ~650, rapid response time (~30 ms), excellent size compatibility (from 100 μm to 1 mm), and high tolerance for irradiation deviations (transfer accuracy < 5 μm under a 30% beam offset).” the researchers added.
Self-alignment mechanism through directional photothermal regulation
The study provides an in-depth investigation into the mechanism of this self-alignment. Through system simulations and experiments, the team has proved that the gradient structure (graphene-amorphous carbon) manages temperature far better than conventional homogeneous materials (Fig. 2).
Specifically, the top graphene layer in the irradiated region extracts a considerable amount of heat from the AC layer and rapidly conducts it laterally to the non-irradiated area. This ingenious strategy of interface thermal management via intentionally directing the heat into top graphene layer through a gradient decrease in thermal conductivity maximizes the effect of thermal homogenization. Additionally, through an in-depth investigation of the relationship between absorbance of TCGC, laser offset degree and the transfer error, the self-alignment ability can be quantitatively predicted.
“The statistical result of average transfer error is 11.1 µm, closely matching the predicted precision (i.e., 10 µm). Besides, for non-offset irradiation, the process exhibits a very high accuracy of 4.6 µm, significantly superior to the previous laser non-contact transfer process. Moreover, when the offset degree of the laser spot is 30%, the transfer accuracy remains below 5 µm according to the prediction model.” they stated.
Programmable transfer of microchips for flexible full-color display
The comprehensive capability of SALT has been demonstrated by selectively transferring microchip arrays with various materials and dimensions onto diverse substrates (Fig. 3). Furthermore, the batch selective integration of RGB MicroLED chips onto flexible circuit boards and the fabrication of programmable display devices demonstrate the promising prospects of the SALT technique for flexible MicroLED display integration.
“Demonstrations in programmable transfer printing of RGB MicroLED chips for flexible display illustrate the self-aligned and batch-selective capabilities of SALT, highlighting its immense potential for developing advanced electronic systems.” the researchers forecast.
Journal
Light Science & Applications
Article Title
Gradient-graphene-enabled Directional Photothermal Regulation for Self-aligned Laser Transfer Printing