Maurice Clair, head of process development, Christian Scholz, team leader R2R process development, and Mandy Gebhardt, team leader marketing and public relations, 3D-Micromac
Thin, lightweight and flexible are features that enable applications in numerous areas, including consumer electronics, medical devices, photovoltaics and lighting. The steady growing demand for flexible devices requires new production processes for mass production.
Besides the production of thin-film devices on rigid substrates, the manufacturing of flexible substrates in a roll-to-roll (R2R) process is increasing. However, realising a micromanufacturing technology in an R2R system means transferring requirements from process solutions for rigid substrates to an R2R handling approach for flexible substrates. Conventional R2R systems were developed for mass production methods, such as offset-printing. In most cases, the accuracy requirements are much lower than in laser processing. In addition, the substrate material differs with regards to reduced mechanical and thermal stability.
In contrast to the stage-based systems where a single sheet is fixed onto a vacuum chuck, R2R systems 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.
R2R handling and laser micromachining
R2R systems are a combination of the R2R handling solution and a specific laser micromachining process. As the laser process depends strongly on the actual task and material, it is extremely important that the R2R system provides a seamless integration of the laser sources and their respective beam paths.
The following laser types cover a spectrum of possible applications:
- Excimer laser for thin-film ablation or annealing.
- Nanosecond laser for thin-film annealing or cutting of metal substrates.
- Ultrashort pulse (USP) laser for thin-film patterning or cutting of polymer substrates.
Excimer lasers are usually used if a homogenous exposure of an area combined with a short depth of absorption is required.
Nanosecond lasers are frequently used to ablate thicker layers in a short time because of the available laser power but ablation quality is often compromised due to the thermal load induced based on their pulse duration.
The advantage of USP lasers over nanosecond lasers is their considerably lower heat input into the material. This guarantees much higher selectivity for the scribing process, because almost all of the pulse energy is used for the evaporation and ablation of the exposed material. There is no damage to the foil substrate. As the process is well-controlled, there is the possibility of bulging edges but only in the range of a few tens of nanometres. On the whole, the process window for USP lasers is sufficient to attain a stable ablation process. This is of particular importance for R2R laser structuring.
R2R applications
Processing of thin-film solar cells
R2R processing is increasingly being used to produce lightweight and flexible, thin-film solar cells. These new photovoltaics do not achieve comparable efficiencies to bulk, silicon solar cells, but the fact that they are lightweight opens up new opportunities and use cases. Advanced proprietary laser processes using USP lasers enable monolithic interconnection of solar cells to serially connected submodules. The laser process allows the digital customisation of cell dimensions and electrical output of submodules. Thus, individual application requirements that take into consideration the aesthetics of the cells can be fulfilled. Solar cells are an excellent example of how similar products can require completely different laser machining solutions owing to their designs and fundamental production chains.
An R2R machining area for the processing of flexible sensors using an excimer laser.
3D-Micromac has developed two different types of R2R systems for the production of thin-film solar cells, depending on the basic design and process flow of the cells.
The first type operates in a step and repeat mode. The web is stepped forward and then fixed in its position. Cameras detect either the scribes from the previous step or laser scribes implemented earlier in production and align the next set of scribes to those. In the meantime, another set of cameras undertakes quality control of the previous scribes on width, separation distance and other characteristics. The laser scribes themselves are carried out by a laser source projected towards the web surface by a galvanometer scanner moving on a cross-web axis. Complex algorithms optimise the interaction between the scanner and axis to achieve the maximum throughput for any given solar module layout. This approach provides very high positioning accuracy of the scribes, namely below ±25 µm, and a production capacity of approximately 250,000 m²/annum.
The second type operates in a continuous winding mode. It has been designed for maximised throughput, with process web speeds of up to 3 m/min. Multiple camera systems that, due to the continuous transport, are located upfront are used for alignment and quality control as well as after the actual laser process. A cross-web axis to address the whole web width is not compatible with this approach, therefore multiple laser sources, beam paths and scanner systems were installed to cover the complete width. As this results in a rather complex setup, the actual position accuracy is reduced to ±75 µm. However, on the plus side, this system enables production capacities exceeding 1,500,000 m²/annum. Depending on the underlying deposition technology, this solution can be realised in atmosphere and vacuum.
Laser annealing of a metal substrate
A common R2R application is local or depth-selective laser annealing of thin-film layers for laser-based pinning of magnetic fields, ohmic contact formation or dopant activation, all of which are usually conducted on wafer-based substrates. However, at the request of a customer, 3D-Micromac developed a process that allowed transference to a flexible metal substrate to be achieved.
The initial process was qualified on a sheet-based process flow, but throughput needed to be improved. To achieve the required throughput, two, high-powered nanosecond laser sources were installed. Their individual beams were shaped into lines that were 75 mm long and less than 10 µm wide, while overall homogeneity was kept above the 90 percent threshold typically required for good annealing results. However, this resulted in a very low depth-of-field (DOF), which could potentially reduce the stability of the process. To compensate for this, precise height measurement of the substrate in combination with continuous adjustment of the focal plane during the movement of the laser lines over the substrate was implemented.
As reliability and yield of the process were of the highest importance to the customer, both the laser parameters and beam parameters had to be monitored constantly. A camera-based beam analyser was installed to record beam parameters such as line width, energy distribution and edge steepness. Not only did the camera provide reliable data on the current condition of the beam but also long-time statistical data that was used to monitor the status of the beam-shaping optics.
Lastly, an upscaled web handling system was installed. As the laser lines were moved over the web substrate perpendicular to the web’s transport direction, the web had to be transported in very precise steps so that subsequent line scans would form a fully annealed surface without gaps. Another important factor was that the web could not be touched on the silicon coated top-side at any time. To achieve the highest precision and a positioning accuracy of <10 µm, a large driven vacuum drum was used for web transport.
Laser ablation of sensors
Another key R2R application is the laser ablation of thin-film layers for the production of sensors. Despite being limited to very narrow web widths of below 50 mm, a throughput of more than 500,000 m²/annum can be achieved though very high web speeds of up to 50 m/min.
The ability to process this fast is down to the ablation of thin layers below 100 nm with one single laser pulse from an excimer source on a comparatively 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 repletion rates of the laser sources, which today can achieve up to 150 Hz, meaning that 150 sensors per second can be produced. Time was spent evaluating different quality inspection options for yield control. Eventually, a camera system that uses pattern comparison to deliver 100 percent quality control and resolve features in the range of 5 µm was installed.
A reference pattern for sensor manufacturing processed by excimer laser in an R2R process.
R2R production equipment requirements
The most important aspect in the design of 3D-Micromac’s latest R2R systems was the high-precision transport of the flexible web material so that the laser process could achieve the required accuracy. For narrow web widths up to 520 mm, a solution was developed that incorporated a large diameter, direct-driven, precision manufactured main roller. At a diameter for 300 mm, the main roller perfectly machined the surface with a run-out below 10 µm. To achieve accurate acceleration, deceleration and positioning of the main roller, a highly integrated torque motor was chosen to act as the main drive in the systems. To minimise jerking of the web during acceleration and allow for low web tensions between 15 and 90 N at a web width of 520 mm, friction-reduced dancer rollers were installed. 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 m/min could be achieved. This 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.
For larger 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 coils 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 down. A precise web tension control is also applied, typically with at least two tension zones that are controlled individually in the range between 30 N or 1000 N, depending on the substrate and its properties.
Conclusion
R2R laser micromachining guarantees high throughput and large machining areas for the production of flexible devices. In order to achieve a cost reduction, the machine technology needs to be reliable, sophisticated and scalable. 3D-Micromac has developed and manufactured its microFLEX R2R laser micromachining systems for more than ten years. The integration of different laser sources and wavelengths in various optical setups allows for the processing of thin-film devices on various substrates, such as metal foils, paper and polymers, with substrate widths of up to 1,500 mm.
Furthermore, 3D-Micromac develops and optimises its R2R laser micromachining systems and processes for serial production in close co-operation with the customer. All stages of the process and technology development, from preliminary tests to feasibility and the development of (functional) prototypes to contract manufacturing of the final devices, are undertaken in 3D-Micromacs’ application lab.
3D-Micromac
3D-Micromac’s microFLEX R2R laser micromachining system.