Link to the article: https://www.nature.com/articles/s41467-024-52661-3
The graphene proposed in this paper as an intermediate isolation layer can be optimized for reusability, and if the intermediate layer is turned into a substance such as photoresist, will wet peeling also be a feasible solution? Device contamination is a problem, and other more optimized materials, low cost and reusable can also be considered.
summary
Laser peeling (LLO) of ultra-thin polyimide (PI) films is critical in the manufacture of ultra-thin displays. However, conventional LLO technologies face the challenge of causing mechanical and electrical damage to integrated devices when separating ultra-thin PI membranes. Here, we propose a graphene-assisted laser stripping (GLLO) method to address these challenges. The GLLO method demonstrates improved manufacturability and peel quality by integrating chemical vapor deposition (CVD)-grown graphene at the interface between the transparent carrier and the ultra-thin PI film. In particular, the GLLO method significantly mitigates the plastic deformation of the PI film and minimizes the carbonaceous residue remaining on the support. The effect of graphene is attributed to three factors: enhanced interfacial UV absorption, lateral thermal diffusion, and reduced adhesion, and this mechanism has been verified by experiments and numerical simulations. Finally, it was demonstrated that the GLLO method was able to separate ultra-thin organic light-emitting diode (OLED) devices without sacrificing performance. We believe this work will pave the way for the use of CVD graphene in a variety of laser manufacturing applications.
Background and main content of the study
Since the discovery of laser ablation-driven separation in organic polymers, the laser peel (LLO) process has been widely used in the manufacture of flexible electronics. Among the various polymers that can be processed with LLO, polyimide (PI) is considered the most suitable substrate material for manufacturing flexible displays due to its excellent thermal stability. In this regard, PI films with thicknesses of 10 to 100 μm have been used to demonstrate bendable and rollable properties. At the same time, in stretchable display applications, reducing the thickness of the PI film can help improve stretchability and mechanical reliability. In addition, next-generation applications, such as implantable and wearable photonic medical devices, require the use of ultra-thin (sub-5μm) substrate thicknesses to allow conformal contact with soft and curvilinear surfaces based on their extreme flexibility. However, the extreme flexibility makes the PI film susceptible to mechanical deformation during the LLO process, resulting in wrinkling and cracking, leading to photonic device failure.
Two representative methods have been developed to reduce laser damage to PI films in the LLO process. The first method is to place a sacrificial layer between the PI film and the glass carrier, including amorphous gallium oxide (α-GaOx), amorphous silicon (α-Si), and lead zirconate titanate (PZT). While these sacrificial layers reduce thermal damage to flexible substrates and thin-film devices, significant mechanical deformation during the LLO process still hinders the successful peeling of ultra-thin PI films. In addition, the deposited sacrificial layer cannot be reused after the LLO process, resulting in increased manufacturing costs. The second method is to directly ablate the PI film by controlling the laser irradiation parameters, such as multiple irradiation of low energy density lasers and optimizing the laser type and beam shape. These methods are able to reduce plastic deformation during LLO, but it remains challenging to successfully separate ultra-thin PI films through low-throughput and single-pass LLO processes for high-throughput manufacturing.
In this paper, we hypothesize that chemical vapor deposition (CVD)-grown graphene, a two-dimensional nanostructured carbon material that can be processed over a large area, has advantages in the LLO process due to its unique optical, thermal, adhesive, and geometric properties. Specifically, high planar thermal conductivity, UV absorption, and lubricity can lead to laser ablation without causing thermal and mechanical damage. In addition, CVD graphene not only enables large-area integration, but also allows LLO performance to be programmed by controlling the number of integrated layers. Based on these assumptions, a graphene-supported laser stripping (GLLO) method was developed by integrating a graphene layer at the interface between the ultra-thin PI film and the glass carrier. The PI film process window and peel quality of the traditional LLO and GLLO methods were compared to demonstrate the effectiveness of the integrated graphene, and the role of the graphene layer in the ablation process was clarified through comprehensive experiments and numerical simulations. In addition, the applicability of the GLLO method was verified by demonstrating ultra-thin organic light-emitting diode (OLED) devices.
Figure 1: Schematic diagram of the process and characteristics of separating ultra-thin polyimide (PI) films from the present invention and the traditional laser peeling method. aGraphene laser stripping (GLLO) process. bConventional laser stripping (LLO) process.
GLLO process
Figure 1 illustrates the flow and characteristics of GLLO versus traditional LLO methods. The main difference between the two methods is the insertion of CVD-grown graphene at the interface between the transparent glass carrier and the PI film. The flow of the GLLO method is shown in Figure 1a. Glass carrier-graphene-PI film structure samples are prepared by transferring a graphene layer to a carrier and then spin-coating the PI film. In this process, graphene layers are transferred layer by layer in a roll-to-roll fashion, which controls the number of integrated layers (Supplementary Figure 1). The thickness of the spin-coated PI film (target peel material) is fixed at 2.9 μm (Supplementary Figure 2), which is much thinner than reported in previous LLO studies. In this regard, the presence of a graphene layer on the glass carrier does not significantly affect the thickness of the prepared PI film (Supplementary Figure 3). Detailed information on sample preparation is provided in the Methods section. After sample preparation, the stripping process was performed using a 355 nm diode-pumped solid (DPSS) laser system (Supplementary Figure 4). DPSS layer systems offer advantages over excimer laser systems in terms of cost competitiveness, high reliability, and precision-manufactured beam quality. As a result, the integrated graphene layer enables the successful peeling of the 2.9 μm thick PI film without significant plastic deformation and carbonaceous PI residues.
In the comparison experiment, a traditional LLO process was performed using a sample consisting of a glass carrier and a 2.9 μm thick PI film, which was identical to the GLLO method, except without a graphene layer (Figure 1b). In this case, successful peeling of ultra-thin PI films proved to be extremely challenging. For example, partial separation of the PI membrane was observed at low laser energy densities, while microscale wrinkles or rupture were observed at high laser energy densities. In addition, traditional LLO methods result in a thick carbonaceous PI residue left behind after the process, hindering the recovery of expensive glass carriers.
Figure 2: Performance of traditional LLO and GLLO methods. aSchematic diagram of peeling off an ultra-thin PI film using ultraviolet (UV) laser irradiation. The results of this process are divided into partial separation (pink, upper half-filled symbol), wrinkled (red, lower half-filled symbol), successful peel (green, right half-filled symbol), and fractured (black, left half-filled symbol) (b, c) Peel results vs. (b) laser energy density and scan spacing for conventional LLO and (c) GLLO methods. The green area indicates the process window for a successful stripping. Light microscopy (OM) image of each PI film with laser energy density (fixed scan spacing of 15 μm) after the peeling process. The images in the first and second rows correspond to the traditional LLO and GLLO methods, respectively. Scale bar: 300 μm. e Surface roughness of independent PI films separated using conventional LLO and GLLO methods at each laser energy density. The inset shows the 3D surface topography of the PI film (scale factor: 5). Error bars indicate the standard deviation obtained from the sample (n≥8). fMeasured thickness of carbonaceous residue on glass supports, and (g) Raman spectra analyzed on glass supports after each treatment. The shaded area highlights the difference in Raman peaks between the graphene-integrated thin residue in the GLLO method and the thick residue in the traditional LLO method.
Figure 3: Role and foaming behavior of graphene layers during ablation. aThe ablation mechanism of the traditional LLO method, and (b) the shape of the high and low diameter blisters obtained. Sharp blisters introduce a high mechanical strain. c The role of the graphene layer in the ablation process of the GLLO method, and (d) the resulting high and low diameter blister shapes. Smooth blisters can reduce mechanical strain. (e) blister height and (f) diameter measured by e and f as a function of laser energy density and graphene layers. Error bars represent the standard deviation obtained from the sample (n =7). Representative images of blisters at laser energy densities of 110.9 and 79.2 mJ/cm2 (scale factor: 120). The shape and size of the blisters were measured using confocal microscopy experiments.
Figure 4: Mechanism validation. UV-Vis absorption spectra of each layer of graphene. bThe relationship between the peel strength of the prepared PI film and the number of graphene layers in the bottom layer. Error bars represent the standard deviation obtained from the sample (n = 4). The inset shows the experimental setup. cTemperature distribution of graphene layers during ablation. The distribution results were obtained using a simplified simulation model. Temperature distribution in the vertical direction (d) and temperature distribution at the glass(graphene)-PI interface (e). The distribution results were obtained using a full-scale simulation model. The inset illustrates the direction of temperature data extraction in the simulation model, with the gray dotted line indicating the threshold ablation temperature. The colored area in (d) represents the change in the material along the vertical distance.
Figure 5: Application of the GLLO method in ultra-thin organic light-emitting diode (OLED) devices. a Schematic diagram of OLED structure. bCompare photographs and OM images of peel results from traditional LLO and GLLO methods. Scale bar: 200 μm. c, d Electrical properties of OLEDs manufactured before and after the GLLO process: (c) current density-voltage-brightness characteristics and corresponding (d) current efficiency-current density curves. e OLED operation under severe mechanical deformation.
Figure 6: Reusability of graphene-integrated carriers. aSchematic diagram of the renewable process of graphene integrated carrier. b, cChanges in electrical properties of OLEDs manufactured in the first and third cycles before and after the GLLO process. b. Current density-voltage-brightness characteristics and the corresponding (c) current efficiency-current density curves.
discuss
This article describes the significant impact of CVD-grown graphene on the LLO process. The role of the graphene layer in the GLLO process is divided into three factors: enhanced UV absorption at the interface, lateral diffusion of heat, and reduced adhesion. These factors alter the blistering behavior, thereby reducing plastic deformation during ablation. The proposed mechanism is verified by experimental and numerical studies. The performance and applicability of the GLLO approach are discussed below.
In terms of performance, the GLLO method successfully peels off PI films that are much thinner than previous LLO studies, providing a wide process window and minimal plastic distortion. In addition, due to the presence of a graphene layer, the GLLO method requires only a single irradiation with a low energy density laser with a wide scan spacing, allowing for a higher throughput process compared to multiple low energy density laser irradiation previously reported. At the same time, the photon stripping (PLO) method, which uses high-intensity light pulses from flash lamps instead of lasers, and uses relatively thick metal light-absorbing layers, such as Mo and W/Ti alloys, also enables high-yield processes. However, the PLO process requires a much higher light energy density than the LLO process. This high light energy density can cause damage to PI films below 10 microns with typical molecular structures, as well as to integrated devices consisting of brittle electrodes and encapsulation layers35. With these characteristics in mind, it can be concluded that the GLLO method can achieve high throughput and precise peeling of ultra-thin PI films.
Regarding the applicability of the GLLO method, the results show that OLEDs fabricated on a 2.9 μm thick PI substrate can be successfully separated without mechanical damage and degradation of electrical performance, which is a challenge for traditional LLO methods. With regard to the reusability of graphene integrated carriers, the separation of ultra-thin OLED devices fabricated on reusable carriers was achieved using slightly higher laser energy density irradiation. Although experiments demonstrated early reusability, a slight decrease in OLED brightness was observed after the GLLO process. The degradation in LLO performance may be attributed to the thin carbonaceous PI residue remaining on the support, which covers the graphene layer. Although the GLLO method proposed in this paper reduces the thickness of carbonaceous PI residues by approximately 92.8% compared to traditional LLO methods, it is expected that further research is still needed on the full reusability of graphene-integrated supports. To improve the performance, reusability, and industrial applicability of LLOs, several efforts are worthwhile, such as optimizing laser radiation conditions and beam distribution, as well as improving sandwich materials. Finally, we believe this work will open up new possibilities for utilizing CVD-grown graphene in laser manufacturing applications such as emerging displays, wafer-level packaging, and energy harvesting devices.
From: 2D Materials
Reprinted by Chen Changjun of the Yangtze River Delta G60 Laser Alliance