Graphene-Enhanced Laser Lifting Technology for Ultrathin Displays

Abstract

Laser lift-off (LLO) of ultrathin polyimide (PI) films is crucial in the manufacturing of ultrathin displays. However, traditional LLO techniques face challenges in separating ultrathin PI films without causing mechanical and electrical damage to integrated devices. This paper presents a graphene-assisted laser lift-off (GLLO) method to address these challenges. By integrating chemical vapor deposited (CVD) graphene at the interface between a transparent carrier and the ultrathin PI film, this method enhances processability and lifting quality. Specifically, the GLLO method significantly reduces plastic deformation of the PI film and minimizes carbon residues on the carrier. The role of graphene is attributed to three factors: enhanced ultraviolet absorption at the interface, lateral thermal diffusion, and reduced adhesion. This mechanism is validated through experiments and numerical simulations. Ultimately, the GLLO method successfully separates ultrathin organic light-emitting diode (OLED) devices without compromising performance. We believe this work paves the way for utilizing CVD graphene in various laser-based manufacturing applications.

Introduction

Since the discovery of laser ablation-driven organic polymer separation, the laser lift-off (LLO) process has been widely adopted in the manufacturing of flexible electronics. Among various polymers suitable for LLO, polyimide (PI) is considered the most suitable substrate material for flexible displays due to its excellent thermal stability. In this context, bending and curling characteristics have been demonstrated using PI films with thicknesses ranging from 10 to 100 micrometers. Moreover, in applications involving stretchable displays, reducing the thickness of PI films enhances stretchability and mechanical reliability. Next-generation applications, such as implantable and wearable photonic medical devices, require ultrathin substrate thicknesses (less than 5 micrometers) for conformal contact with soft and curved surfaces due to their high flexibility. However, this extreme flexibility makes PI films susceptible to mechanical deformation during the LLO process, leading to wrinkling and cracking, which can cause photonic device failure.

Two representative methods have been developed to reduce laser damage to PI films during the LLO process. The first method involves placing a sacrificial layer between the PI film and the glass substrate, which can include amorphous gallium oxide (α-GaOx), amorphous silicon (α-Si), and lead zirconate titanate (PZT). Although these sacrificial layers reduce thermal damage to the flexible substrates and thin-film devices, significant mechanical deformation during LLO still hinders successful separation of ultrathin PI films. Additionally, the deposited sacrificial layers cannot be reused after the LLO process, leading to increased manufacturing costs. The second method is to directly ablate the PI film by controlling laser irradiation parameters, such as low-flux laser irradiation multiple times and optimizing laser types and beam shapes. While these methods reduce plastic deformation during the LLO process, achieving low-flux and single-pass separation of ultrathin PI films to promote high-throughput manufacturing remains challenging.

In this paper, we propose that CVD-grown graphene, a large-area processable two-dimensional nanostructured carbon material, has advantages in the LLO process due to its unique optical, thermal, adhesive, and geometric properties. Specifically, its high in-plane thermal conductivity, ultraviolet absorption, and lubricity can facilitate laser ablation without thermal and mechanical damage. Moreover, CVD graphene not only enables large-area integration but also allows for programmable LLO performance by controlling the number of integrated layers. Based on these assumptions, we developed a graphene-assisted laser lift-off (GLLO) method by integrating graphene layers at the interface between ultrathin PI films and glass substrates. We compared the process windows and lifting quality of PI films using traditional LLO and GLLO methods to demonstrate the effectiveness of integrated graphene, elucidating the role of graphene layers during the ablation process through thorough experiments and numerical simulations. Additionally, the applicability of the GLLO method was validated by showcasing ultrathin organic light-emitting diode (OLED) devices.

Results

GLLO Process
Figure 1 illustrates the processes and characteristics of GLLO (Graphene-Assisted Laser Lift-Off) and traditional LLO (Laser Lift-Off) methods for separating ultrathin polyimide (PI) films. The primary difference between the two methods is the insertion of CVD-grown graphene at the interface between the transparent glass substrate and the polyimide (PI) film. Figure 1a describes the procedure of the GLLO method. By transferring the graphene layer onto the substrate and spin-coating the PI film, samples with a glass substrate-graphene-PI film structure were prepared. During this process, graphene layers were transferred in a roll-to-roll manner to control the number of integrated layers (Supplementary Figure 1). The thickness of the spin-coated PI film (target lifting material) was fixed at 2.9 micrometers (Supplementary Figure 2), which is significantly smaller than the thicknesses reported in previous LLO studies. Notably, the presence of the graphene layer on the glass substrate had no significant impact on the thickness of the PI film being manufactured (Supplementary Figure 3). Detailed sample preparation information is provided in the “Methods” section. After sample preparation, the lifting process was performed using a 355 nm diode-pumped solid-state (DPSS) laser system (Supplementary Figure 4). Compared to excimer laser systems, the DPSS laser system offers advantages in cost competitiveness, high reliability, and precise beam quality for manufacturing. As a result, the integrated graphene layer enabled successful lifting of 2.9 micrometer thick PI films without noticeable plastic deformation and carbon residues.

Figure 1: Schematic of the proposed and traditional laser lift-off methods for separating ultrathin polyimide (PI) films.

a. Graphene-assisted laser lift-off (GLLO) process.
b. Traditional laser lift-off (LLO) process.

In comparative experiments, the traditional LLO process used the same samples as the GLLO method, consisting of a glass substrate and a 2.9-micrometer thick PI film, but lacked the graphene layer (Figure 1b). In this case, successfully lifting the ultrathin PI film was highly challenging. For example, partial separation of the PI film was observed under low laser flux conditions, while high laser flux conditions resulted in micro-wrinkling or cracking. Additionally, the traditional LLO method left a thick layer of carbonaceous PI residues after the lifting process, hindering the recycling of the expensive glass substrate.

GLLO Process Windows and Lifting Quality
The process windows and lifting quality of GLLO and traditional LLO methods were studied (Figure 2). The LLO process was achieved by irradiating the glass substrate-graphene-PI film samples with ultraviolet laser pulses (Figure 2a). In this experiment, the parameters of laser flux and scanning distance were controlled in the ranges of 63.4-158.5 mJ/cm² and 7.5-150 µm, respectively. Detailed experimental settings of the laser system can be found in the “Methods” section. To investigate LLO performance, the lifting area was controlled to 1.5×1.5 mm², with four layers of graphene integrated for the GLLO process. The results of each LLO process were categorized as partial separation, wrinkling, cracking, and successful lifting. The standard for successful lifting was defined as separation of the PI film without micro-wrinkling, cracking, and partial adhesion. Detailed explanations for each category and representative images can be found in Supplementary Figure 5.

Figure 2: Performance of traditional LLO and GLLO methods.

a. Schematic of the lifting process of ultrathin PI films using ultraviolet (UV) laser irradiation. Process outcomes are categorized into partial separation (pink, upper half-filled symbol), wrinkling (red, lower half-filled symbol), successful lifting (green, right half-filled symbol), and cracking (black, left half-filled symbol) (b, c) Effects of laser flux and scanning distance on lifting outcomes for (b) traditional LLO and (c) GLLO methods. The green area indicates the process window for successful lifting. d. Optical microscopy (OM) images of the PI film after the lifting process at each laser flux while maintaining a fixed scanning distance of 15 µm. The images in the first and second rows correspond to traditional LLO and GLLO methods, respectively. Scale bar: 300 µm. e. Variation of surface roughness of the independent PI films post-lifting using traditional LLO and GLLO methods with respect to laser flux. Insets show the 3D surface morphology of the PI films (scale factor: 5). Error bars represent standard deviation obtained from samples (n≥8). f. Measured thickness of carbonaceous residues on the glass substrate, and (g) Raman spectral analysis of the glass substrates after each process. The shaded areas highlight the differences in Raman peaks between the thin residues in the GLLO method and the thick residues in the traditional LLO method.

Figure 2b shows the lifting results of the traditional LLO process without the introduction of the graphene layer. The results emphasize that the process window for successful lifting is extremely narrow. At a scanning distance of 15 µm (indicated by the blue arrow in Figure 2b), the ultrathin PI film could only be successfully separated without noticeable wrinkling when the laser flux was 110.9 mJ/cm². In other cases, laser flux below 110.9 mJ/cm² led to partial separation, while higher flux caused wrinkling of the separated PI film due to plastic deformation during the ablation process. The optical microscopy (OM) images in the first row of Figure 2d confirm the aforementioned lifting behaviors. More detailed analyses of the lifting results using the traditional LLO process can be found in Supplementary Figure 6.

In contrast, the GLLO process achieved broader process windows with significantly improved lifting quality (Figure 2c). Under the same scanning distance (15 µm), a laser flux of 63.4 mJ/cm² enabled successful lifting without any mechanical deformation. Moreover, the process could be extended to a laser flux of 158.5 mJ/cm² without the occurrence of micro-wrinkling or cracking, which was unattainable using the traditional LLO method. The optical microscopy images shown in the second row of Figure 2d reveal a smooth PI film surface after the GLLO process, with less than 0.3 µm surface roughness (Figure 2e). Additionally, a distinct decrease in the thickness of carbonaceous residues on the glass substrate (Figure 2f) and significantly reduced intensity of D and G peaks in Raman spectra (Figure 2g) were observed, indicating a lower density of carbon residues compared to the traditional LLO method. The results demonstrated that integrating the graphene layer in the GLLO process effectively enhanced lifting quality by significantly mitigating plastic deformation and carbon residue accumulation.

Mechanism of Graphene-Assisted Laser Lift-Off
We investigated the physical mechanisms underlying the GLLO method and its role in enhancing lifting quality. First, the UV absorption characteristics of CVD-grown graphene were analyzed using UV-visible spectroscopy (Supplementary Figure 7). The results demonstrated that the incorporation of the graphene layer significantly enhances UV absorption in the PI film compared to the traditional LLO method, thus facilitating effective energy transfer from the laser pulse to the PI film during the ablation process.

The enhanced UV absorption can be attributed to the highly conjugated sp2 structure of graphene, which promotes efficient energy absorption across a range of UV wavelengths, especially in the 200-400 nm region (Figure 3a). Moreover, with the introduction of the graphene layer, the absorption peak of the PI film shifts, indicating a distinct transition of energy absorption dynamics (Supplementary Figure 7). Consequently, graphene not only improves the energy transfer from laser pulses to the PI film but also leads to lateral thermal diffusion during the LLO process, thus promoting enhanced thermal stability.

Figure 3: Mechanisms of the GLLO method.

a. UV absorption spectra of CVD-grown graphene and PI films, demonstrating enhanced absorption in the UV range with the introduction of graphene. b. The thermal distribution in the PI film after laser irradiation, indicating improved thermal diffusion with graphene integration. c. Adhesion analysis of the PI film and graphene layer at various laser fluxes. d. Comparison of lifting quality with and without graphene, showcasing enhanced lifting performance. e. Lateral heat diffusion dynamics of laser-irradiated PI films, where the presence of graphene enhances thermal distribution, leading to more uniform heating and improved lifting results.

To quantify the thermal diffusion properties, we conducted finite element simulations of the thermal distribution within the PI film during the LLO process (Figure 3b). The simulation results demonstrated that the lateral heat diffusion is significantly improved in the presence of the graphene layer, which serves as an efficient thermal conductor, effectively spreading heat away from the laser-irradiated region. This phenomenon enhances the overall thermal stability of the PI film during the lifting process, minimizing the risk of mechanical deformation.

The third mechanism contributing to the GLLO process is the reduced adhesion between the graphene layer and the PI film compared to conventional substrates (Figure 3c). We measured the adhesion force between the PI film and glass substrate, with and without the graphene layer, under various laser fluxes. The results indicate that the introduction of graphene results in a marked reduction in adhesion forces, which further facilitates easier lifting of the PI film. By optimizing the adhesion characteristics, the GLLO process achieves effective separation of ultrathin films without compromising their structural integrity.

The experimental outcomes collectively demonstrate that the GLLO method effectively integrates three critical factors: enhanced UV absorption, improved thermal diffusion, and reduced adhesion, contributing to the successful separation of ultrathin PI films during the laser lift-off process.

Conclusion
In conclusion, the proposed graphene-assisted laser lift-off (GLLO) method presents a promising approach for effectively separating ultrathin polyimide (PI) films with high quality. By incorporating chemical vapor deposited (CVD) graphene at the interface, the GLLO method overcomes the limitations of traditional LLO techniques, which often result in mechanical deformation and carbon residue accumulation. Our findings indicate that the integration of graphene significantly enhances UV absorption, facilitates thermal diffusion, and reduces adhesion, leading to successful lifting of ultrathin films without compromising their performance. This work not only advances the manufacturing capabilities of ultrathin displays but also opens up avenues for utilizing CVD graphene in various laser-based applications, providing a pathway for future innovations in flexible and stretchable electronics.

1. Role of Graphene Layers:
   • The effects attributed to graphene layers include:
     • Enhanced UV absorption at the interface.
     • Lateral thermal diffusion of heat.
     • Reduced interfacial adhesion between materials.
2. Mechanisms Explained:
   • The graphene layer improves the absorption of photothermal energy, shifting the ablation point from within the PI (polyimide) film to the graphene-PI interface, which helps minimize carbonaceous residue thickness.
   • The high in-plane thermal conductivity of graphene disperses absorbed photothermal energy horizontally, widening the narrow pyrolysis zone, which helps smooth out initially formed bubbles.
   • The presence of graphene reduces molecular interactions between the glass substrate and the PI film, lowering interfacial adhesion and facilitating the lateral expansion of interface cracks, resulting in larger bubble diameters.
3. Experimental Observations:
   • The study measured bubble geometry by firing a single UV laser pulse at samples with varying numbers of graphene layers and observing significant differences in bubble height and diameter, especially with energy densities exceeding 100 mJ/cm².
   • Graphene layers were found to enhance UV absorption significantly, validating their role in photothermal processes.
4. Thermal Analysis and Simulations:
   • Finite Element Analysis (FEA) simulations demonstrated that the temperature distribution varied with the number of graphene layers, indicating that multiple graphene layers enhance the thermal diffusion properties during the ablation process.
   • The maximum temperatures observed were significantly different when comparing multiple layers of graphene versus none, confirming the enhanced light absorption at the graphene-PI interface.
5. Application in OLED Manufacturing:
   • The GLLO method was applied to produce OLEDs on ultra-thin PI substrates (2.9 µm thick) without causing mechanical damage or electrical performance degradation, which is a notable improvement over traditional LLO methods.
   • The study also evaluated the reusability of graphene-integrated glass substrates for OLED production, noting some decline in brightness after multiple cycles likely due to residual carbonaceous PI.

Conclusion

The research highlights the advantageous effects of incorporating graphene layers in laser lift-off processes, demonstrating potential applications in advanced manufacturing techniques for electronic devices like OLEDs. The findings suggest that graphene can significantly improve the efficiency and outcomes of these processes, paving the way for more reliable and high-performance electronic components.