High-speed, ultra-precise laser processing of large substrates

Every large glass substrate, whether used as architectural or functional glass in the solar industry or for manufacturing semiconductors or flat panel displays, is always a combination of two components: a substrate glass and at least one coating. .

Two processes are used for these substrates: selective coating or selective removal of full-surface functional coatings. This article explores ultrafast laser ablation or functionalization of defined layers of large substrates.

The trend toward high-density features or full-surface texturing in conjunction with fast-moving panels or web-based applications challenges established laser scanning solutions. Where lasers can provide satisfactory performance, the traditional approach of multiplexing systems seems to be the way to go. To support and deliver laser material processing on a full industrial scale, LPKF SolarQuipment and the SCANLAB polygon team developed a system concept to validate the process and identify the control functions that need to be adapted. It supports double-sided (top-bottom) synchronized processing of pre-recorded thin film-on-glass patterns.

Wafer and panel processing

The solar power, semiconductor, and flat panel display industries are high-volume manufacturing industries that must process large areas with great precision (see Fig. 1). The larger scan field size compared to conventional scan heads, combined with the improved pulse repetition rate and higher average power of ultrafast lasers and the latest nanosecond pulsed lasers, allows larger areas to be treated with higher throughput.

For ultra-precise processing of large areas using transmissive technology F-optical theta, a stepwise approach can be taken. This approach presents challenges with respect to stitching (overlapping of different scanning passes).

Ultra-precise micromachining requires small focused spot sizes and a full telecentric field of view, for example, a consistent round spot across the entire scan area. But traditional refractive optics using lenses are limited in size due to the gradual increase in cost with increasing image field. This means economical and highly accurate laser scanning heads with refraction F-theta optics have limited scanning fields.

An alternative solution to avoid seams is wide-field mirror optics. The basic technology for this is a polygon scanner system with F-theta mirror optics. It uses a scanning head with a full telecentric scanning field of 300 mm and a scanning field 4 to 5 times wider than that of conventional models. F-theta optics. Such a width eliminates seams for processing wafers with a size of up to 300 mm. Multiple scanheads can be mounted side by side to achieve panel size (1200mm; see Fig. 2).

Choice of laser source

Depending on the coating and the desired scribing result, different laser sources can be used. Treatment direction, wavelength and pulse frequency are optimized for the respective coating to achieve the optimum cost/performance/quality ratio.

Since LPKF systems can place laser units on the top and bottom sides, multilayer structures with layers made up of different materials can also be processed.

Qualitative aspects of LPKF laser tracing

The challenges in achieving a stable production process are manifold. Ultimately, optimum results (quality) and the shortest possible cycle time enter into the economic evaluation of the production process.

The following influencing factors are:

  • For precise assembly with multiple polygon scanners, many factors, including starting clock rate, laser jitter, and a temperature-controlled system enclosure, play a role in reducing tolerances down to the micron.
  • The synchronization of the individual processing units is carried out by a master/slave controller and active correction using integrated sensors. Another aspect is spatial shifting of individual units in the power plane and asynchronous control of individual processing units.
  • Calibrating a multi-head scanner is a daunting task. It is achieved through integrated monitoring of the entire substrate based on pre-registration, temperature effect registration, glass flatness registration, and continuous verification of PU-specific tracing results. These values ​​are converted into optimized trace values ​​in the control module.

Thanks to these complex measures, a high level of quality is guaranteed, even with high data rates. The interaction between the coupled processing units is defined by the measurements presented so that the transitions between the individual scan fields are only a few microns wide.

Overall performance of LPKF treatment

Processing speed plays the most important role after quality. It determines the cycle times and the profitability of the process.

The first decisive value is the speed at which the scanners move the laser beam over the material. Here, up to 170 m/s are achieved over a sweeping width of 300 mm with each system.

Comparable galvo scanners can only generate scan fields of up to 100 mm and achieve speeds of up to 40 m/s – the polygon scanner has an aspect ratio of 1:3 compared to galvo scanners. Polygon technology has a huge throughput advantage, especially for applications with high fill factors. Added to this is a lower cost of ownership due to the 1:3 aspect ratio, with simultaneously higher availability due to fewer processing units.

A demonstration system for process development is available at the LPKF Application Center. First article inspections of substrates up to 1.2 × 1.2 m in size are possible.

LPKF SolarQuipment (Suhl, Germany) is a supplier of thin film solar cell patterning systems with large format laser tracers. Its laser plotters have all the necessary components for highly dynamic glass handling, in-line inspections and control.

SCANLAB has been developing and manufacturing galvanometric scanners and digitizing solutions since its inception in 1990. SCANLAB products turn lasers into precise, highly dynamic and flexible tools that provide the foundation for performing countless processing tasks.

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