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Roll-to-roll laser processing of flexible thin-film devices

By Maurice Clair, head-Process Development; Christian Scholz, product specialist-Roll-to-Roll Systems; and Mandy Gebhardt, team leader-Marketing/PR, 3D-Micromac AG


In the manufacturing of flexible thin-film devices, precision, throughput and machining quality on ever-smaller structures are playing an important role. This paper will give a brief overview of different case studies where roll-to-roll (R2R) laser processing achieves new dimensions in terms of precision, quality and process efficiency. One example application is the ablation of thin-film layers for medical sensors. In this case, the on-the-fly laser ablation takes place by using an excimer laser and mask projection. The layout of the products is adaptable by various projection masks. The high-repetition rate of the excimer laser allows the production of up to 150 sensors/sec. Innovative laser micro-processes paired with sophisticated machining concepts from this firm fulfill exactly all requirements for such sensitive and advanced devices.


 

Introduction

Thin, lightweight and flexible – these features enable applications in numerous areas, such as consumer electronics, medical devices, photovoltaics and lighting. The steadily growing demand for flexible-electronics devices requires new production processes for mass production.

 

Besides the production of thin-film devices on rigid substrates, manufacturing of flexible substrates in a roll-to-roll (R2R) process is increasing. However, realizing a micro-production technology in a R2R system means transferring requirements known from process solutions for rigid substrates to a handling approach whose development has been dominated by mass-production methods such as printing while also transferring processes to the linked requirements such as reduced mechanical and thermal stability of the substrate material.

 

In contrast to the stage-based systems where a single sheet is fixed onto a vacuum chuck, R2R tools have to transport the web material either continuously or in a step-and-repeat manner without elongating the material due to high values or sudden changes in web tension.

 

Laser micromachining and R2R processing:

A winning combination

Roll-to-roll processes are a combination of the handling solution with the specific laser process. As the laser processes depend strongly on the actual task and material, it is of high importance that the R2R tool provide a seamless integration of the laser sources and their respective beam paths. The following examples show a spectrum of possible applications.

·         Excimer laser – Thin-film ablation or annealing

·         Nanosecond laser – Thin-film annealing or cutting of metal substrates

·         Ultrashort pulsed laser – thin-film patterning or cutting of polymer substrates

 

Excimer lasers with their specific properties are often used when a homogenous exposure of an area combined with a short depth of absorption is required.

 

Nanosecond lasers are often used to ablate thicker layers in a short time because of the available laser power but frequently with compromises in the ablation quality due to the thermal load induced based on their pulse duration.

 

The advantage of ultrashort pulse lasers compared to nanosecond lasers lies in a considerably lower heat input into the material. This guarantees a much higher selectivity for the scribing process, because almost the entire pulse energy is used for the evaporation and ablation of the exposed material. There are no damages to the film substrate. The bulging of the scribing edges is partly well within only a few 10s of nanometers. On the whole, the process window for the use of ultrashort pulsed lasers is sufficient to attain a stable ablation process. This is of particular importance with regard to the R2R laser structuring.

 

Application areas for R2R laser processing

Possible use cases for roll-to-roll applications are given when flexible, lightweight and large-area substrates have to be processed with high quality and maximum throughput. An example is laser cutting of RFID antennae. Conventional RFID antennae are manufactured by a wet-etching process. A new sustainable way for producing antennae is a high-reliability, digital, R2R laser process. An NS laser and a galvo scanner structure the aluminum layer on paper without any damage to the substrate. Use of this laser technology ensures a higher accuracy, as well as smaller and repeatable patterns. According to the customer, the dry process and speed of laser technology provides for a production speed up to 10X faster than that of etching. The technology brings new dimensions to substrate-material choices, exactness, cost efficiency and sustainability to the growing RFID and IoT markets.

 

A further example is the laser processing of flexible thin-film photovoltaics. Because lasers are an ideal tool for the structuring of single layers with a thickness between a few nano- and micrometers, they offer some important advantages in comparison to photolithographic-etching processes or mechanical scribing. Expensive, inflexible exposure masks and the use of environmentally harmful chemicals become obsolete. A constant wear of scribing needles, ever-changing scribing parameters and thus necessary maintenance intervals are non-existent for laser ablation. The needed electrical isolation of the single layers is achieved by means of selective laser ablation.

 

Advanced proprietary laser processes using ultrashort pulsed lasers enable monolithic interconnection of solar cells to serially connected submodules. The laser process allows the customization of cell dimensions and electrical output of submodules. Thus, individual application requirements with consideration of the aesthetics of the cells can be fulfilled.

 

Laser ablation for sensor production

Another application for R2R processing is the laser ablation of thin-film layers for sensor manufacturing. Despite being limited to very narrow web widths below 50 mm, an annual throughput of more than 500,000 m² can be achieved simply by high web speeds up to 50 mpm.

 

The ability to process this fast is the ablation of thin layers below 100 nm with one single laser pulse from an excimer source on comparably large area of approximately 45 x 15 mm. A positive side effect is that because of the ablation mechanisms at work the metal is selectively ablated from a polymer substrate without damaging it. The combination of an excimer laser source with a mask-projection system resolves features down to 5 µm despite the large area exposed. The productivity of this ablation process scales with the available repetition rates of the laser sources which today can achieve up to 150 Hz, meaning that 150 sensors/sec can be produced. The high speed is not only highly cost-efficient, but it also poses the risk that in case of a failure a lot waste is produced in a short time. A QC camera system based on pattern comparison was installed providing a 100 % quality control of the produced sensors, also resolving features in the range of 5 µm (see Figure 1).

 




FIGURE 1. A reference pattern for sensor manufacturing processed by excimer laser in a R2R process

Requirements for laser-based R2R production equipment

The main design aspect in this firm’s new R2R platforms is the high-precision transport of the flexible web so that the laser process can achieve the required accuracy. For narrow web widths up to 520 mm, a solution was developed by use of a large-diameter, direct-driven, precision-manufactured main roller. At a diameter of 300 mm, this roller features a machined surface with run-out below 10 µm. To achieve accurate acceleration, deceleration and positioning of the main roller, a highly integrated torque motor acts as the main drive in the tool. To minimize jerking of the web during acceleration and also to allow for low web tensions between 15 and 90 N at a web width of 520 mm, friction-reduced dancer rollers are used. With all these features combined and a control scheme to take full advantage of the component’s capabilities, a web transport accuracy of <2 µm at a web speed of 6 mpm is achieved. That means the web is never more than 2 µm off its supposed position at any time, thereby providing an excellent basis for high-precision laser processes (see Figure 2).





FIGURE 2. A R2R machining area for the processing of flexible sensors using an excimer laser. The laser system provides continuous on-the-fly processing for machining speeds up to 50 mpm.

 

For wider webs up to 1,500 mm, two further platforms were developed mainly focusing on the challenges introduced by the increased web width, namely the weight of the rolls and the waviness of the material. To reduce the impact of the waviness of the material, several approaches have been tested and are available depending on the overall requirements of the laser process. Typical solutions are spreading rollers combined with either air bearings or vacuum units to introduce a cross-tension into the web to flatten it. A precise web-tension control also is applied, typically with at least two tension zones that are controlled individually in the range between 30 N and 1,000 N – depending on the substrate and its properties.

 

Conclusion

Roll-to-roll laser machining guarantees high throughput and large machining areas for the production of flexible thin-film electronic devices. To achieve cost reduction for producing of flexible devices, the machine technology needs to be reliable, sophisticated and scalable. This firm has developed and manufactured its systems for R2R laser scribing for more than 10 years (see Figure 3). The integration of different laser sources and wavelengths, as well as various optical setups allow the processing of thin-film devices on various substrates (metal foils, polymer films and paper) with web widths up to 1,500 mm.

 

Maurice Clair, head-Process Development at 3D-Micromac AG (Chemnitz, Germany), studied micro-production technology at Chemnitz Technical University and joined 3D-Micromac in 2006. He can be reached at +49-371-40043-67, email: clair@3d-micromac.com, www.3d-micromac.com 

 

Christian Scholz, product specialist-Roll-to-Roll Systems at 3D-Micromac, studied Physical Science and Technology at Hochschule Mittweida and joined 3D-Micromac in 2008 for the development of roll-to-roll laser processes. He is now working as Team Leader-Process Development as well. Christian can be reached at +49-371-40043-66, email: scholz@3d-micromac.com

 

Mandy Gebhardt, , team leader-Marketing/PR at 3D-Micromac, studied business administration/business information technology at University of Corporate Education Glauchau. She joined 3D-Micromac in 2006 and since then has been responsible for marketing communications and public relations. Mandy can be reached at +49-371-40043-26, email: gebhardt@3d-micromac.com





FIGURE 3. The 3D-Micromac system is a highly modular, roll-to-roll laser system for industrial manufacturing for flexible thin-film electronic devices. The modular concept allows the system design to be easily adapted to converters’ requests.

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