Ablation-Driven Selective Laser Direct Mass Transfer for Micro-LEDs with High Speed, Enhanced Gap Tolerance, and High Precision

The gist The ablation-driven direct laser chip transfer method, termed D-DLAT, establishes a robust paradigm for high-speed manipulation and heterogeneous integration of micro-optoelectronic devices by directly ablating a polyimide dynamic release layer to actively propel micro-LED chips toward the receiver via ablation shock

Problem and Motivation

Micro light-emitting diodes (micro-LEDs) offer superior performance for next-generation displays, yet their scalable manufacturing is severely bottlenecked by the mass transfer process from donor to receiver substrates. The existing laser-induced forward transfer (LIFT) methods are restricted by low throughput, limited selectivity, and long reaction times. The fundamental requirements for the mass transfer process are summarized as follows:

1. High precision: The overall displacement during transfer must not exceed 10% of the dimension of the chip.

2. High throughput: The transfer reaction time should be minimized to maximize the overall efficiency of production.

3. Full selectivity: The process must be capable of transferring any specific chips to any designated positions while leaving the remaining chips on the donor unaffected.

D-DLAT Mechanism and Optimization

The D-DLAT method utilizes a defocused laser beam to directly ablate the PI dynamic release layer, which generates a highly confined, ultrafast transient shockwave within a timescale of ∼ 1 ns. This direct ablation generates an active propulsion that fundamentally alters traditional indirect laser transfer dynamics. Key optimization criteria established for a favorable ablation profile include:

1. A large diameter of the inscribed circle, to ensure a high efficiency of coverage during the ablation.

2. High circularity, which represents the uniformity and the robustness of the process.

3. No penetration by the laser, to minimize the potential damage to the chips.

The optimal parameters for subsequent transfer experiments were determined by evaluating these criteria across various defocus depths (DFD) and pulse energies, establishing that the 300 µm DFD and 10 µJ pulse energy is utilized as default for all subsequent transfer experiments.

Performance Metrics Across Chip Sizes

The D-DLAT method demonstrates a wide range of successful transfers, facilitating the transfer of micro-LEDs with dimensions ranging from approximately 600 µm down to 35 µm. The performance comparison with alternative laser transfer methods shows that the D-DLAT method achieves "a minimum transferable chip size of 35 µm, a minimum placement error of <1 µm, and a maximum transfer gap of ≥50 µm. Furthermore, for smaller chips (35 × 20 × 5 µm), the CTG for higher-energy transfers is considerably larger than that for lower-energy transfers," with the CTG exceeding 50 µm, measuring more than ten times the chip height.

Transfer Modes and Selectivity

The method is compatible with both single-pulse and multi-pulse transfer modes. For large chips, where a single pulse is insufficient to cover the surface, an array of pulses is necessary to accomplish the transfer of the chip. The scanning path optimization involves three patterns—Spiral Side to center (STC), Side first then spiral (SFS)—to ensure the final ablation occurs near the center of the chip, achieving the highest precision. Full selectivity is demonstrated through patterned transfer demonstrations, where both re-pitching and patterned transfer are effectively achieved for chips measuring 35 × 20 × 5 µm.

Damage and Performance Evaluation

Damage evaluation confirmed that structural damage in single-pulse transfers is often due to the collision between the chip and the receiver, as the foam material demonstrates a modulus of merely 1 to 10 kPa compared to glass slide. The D-DLAT process maintains acceptable photoelectric performance, showing that the brightness and EQE of the transferred chips shows a maximum decrease of ≈ 35%, but remains the same trend and order of magnitude as the reference chips.

Conclusion

In conclusion, an ablation-driven direct laser chip transfer method for the mass transfer of microLEDs, termed D-DLAT, is reported and systematically investigated for the first time. This method demonstrates the capability to transfer micro-LED chips with high precision, large gap tolerance, and full selectivity. Direct laser ablation of the DRL propels micro-sized chips toward the receiver, enabling high-precision transfer across large gaps. The defocused laser energy profile facilitates mild ablation and prevents laser penetration through the DRL, mitigating potential laser-induced damage to the chips while simultaneously improving transfer precision and the overall success rate. Future work will explore beam shaping to enlarge the ablation area for single-pulse transfer of larger chips.

How it works

The D-DLAT process involves several key steps, including substrate fabrication, laser control, and transfer execution. The source substrate is fabricated by spin-coating a 10 µm-thick PI layer onto the glass substrate followed by baking and curing procedures. Subsequently, an RIE treatment is applied to etch off the residual PI between the chips, forming separate PI DRLs directly beneath them.

The core of the transfer mechanism relies on laser interaction with a dynamic release layer (DRL). The D-DLAT method uses a defocused laser beam to directly ablate this layer, which generates a highly confined, ultrafast transient shockwave within a timescale of ∼ 1 ns. This shockwave actively propels the micro-LEDs toward the receiver, fundamentally altering traditional indirect laser transfer dynamics.

Key Performance Characteristics

The D-DLAT method achieves superior performance metrics compared to existing techniques, as summarized in Table 2. Specifically, it achieves a minimum placement error of "<1 µm and a maximum transfer gap of ≥50 µm. In the single-pulse transfer mode, the method actively propels chips towards the receiver, achieving a minimum transferable chip size of 35 µm, a minimum placement error of <1 µm, and a maximum transfer gap of ≥50 µm," which exceeds ten times the chip height.

Selective Transfer Capabilities

The method exhibits full selectivity through precise control over relative motion and laser scanning patterns. For example, re-pitching of chips measuring 570 × 270 × 400 µm is achieved by controlling the relative motion to change the pitch from an original pitch of 0.72 × 0.38 mm on the donor to a new pitch of 2 × 2 mm on the receiver. This capability is crucial for fabricating advanced full-color display panels.

Damage Mitigation

To prevent structural damage, the laser pulse energy is carefully controlled, as the D-DLAT method facilitates mild ablation and prevents laser penetration through the DRL. Damage evaluation showed that in single-pulse transfer, an increase in laser energy corresponds to a higher flight velocity of the chips. Furthermore, for small chips, reducing pulse energy to 6 µJ results in chip fracturing is completely eliminated.

How it works

The D-DLAT process involves several key steps, including substrate fabrication, laser control, and transfer execution.

1. Improved chip placement accuracy through cross-correlation algorithms: The system can precisely quantify translational displacement by calculating a normalized cross-correlation coefficient (Equation S4) to determine the translational mismatch is determined from the location of the maximum NCC value (Equation S5). This allows for sub-micron error quantification, moving beyond limitations of direct image processing.

2. Enhanced transfer robustness via optimized laser parameters: The system can dynamically select optimal ablation parameters by evaluating a set of criteria, specifically prioritizing A large diameter of the inscribed circle, High circularity, and No penetration by the laser to define a favorable ablation profile (Table 1). This ensures that the process achieves a minimum placement error of less than 1 µm and minimizes risks like direct damage.

3. Adaptive scanning path generation for larger chips: For chips exceeding the ablation diameter, the system can automatically generate complex laser scanning paths, such as Spiral Side to center (STC) or Side first then spiral (SFS), to ensure the final ablation occurs near the center and mitigate torque-induced displacement. This is validated by testing patterns that achieve the highest precision across varying transfer gaps.

4. Gap tolerance management for high-yield manufacturing: The system can utilize a defined critical transfer gap (CTG) as a performance metric, enabling it to determine if a specific gap meets the threshold for success—e.g., a CTG exceeding the chip thickness is essential for full-color integration, thus optimizing throughput based on wafer flatness and color requirements.

5. Damage avoidance through energy control: When dealing with small chips where structural damage is a risk, the system can adjust pulse energy dynamically to prevent fracturing, as demonstrated by reducing energy to 6 µJ in favor of damage-free transfers, thereby ensuring chip functionality and yield.