Variable multibeam tool enables high accuracy and throughput

Miniaturization drives the industry. While feature densities increase due to the smaller structure sizes required in electronics and display manufacturing, laser scanning speed must also increase to hit throughput targets in terms of moves per second for drilling and other manufacturing processes.

On the other hand, ultrashort-pulse lasers are gaining ever-more average power and pulse energy, thanks to continual technological developments. The main obstacle to fully use these high powers for industrial processes is typically the scanning technology, as the pulse separation needs to be high enough to avoid heat accumulation effects on the workpiece.

Two promising approaches for increasing the usable power levels that can be applied include polygon scanning and process parallelization.

Three multibeam scanning concepts

A joint development project between SCANLAB (Puchheim, Germany) and multibeam scanning experts Pulsar Photonics (Herzogenrath, Germany) focuses on parallelization by using multiple beams on one main scanner. This so-called ‘multibeam laser scanning’ configuration allows speeding of laser processes in applications where a high number of identical structures need to be processed with high structure densities—for example, for high-density printed circuit board (PCB) or microLED display manufacturing.

There are different concepts for increasing throughput in industrial laser machining. First, when one laser and one scan head are not sufficient to reach the desired throughput, the process can be scaled by duplicating the machines, including lasers and scanners. This approach, however, is expensive and often ineffective. Furthermore, it does not consider the latest developments in high-energy ultrashort-pulse lasers, such that the full laser power available may not be efficiently used.

A second approach involves arranging one laser with a beam splitter to feed multiple scanners. Still, this approach tends to have issues regarding workpiece accessibility because each scanner has to work on a separate area of the workpiece. If the workpiece is small, additional costs for handling systems apply, as each scanner must process a different workpiece.

The third concept takes a new approach that allows the scanning of multiple beams with one main scanner. In order to have maximum flexibility, each of the individual beams can be turned on and off, as well as steered in a limited area (see Fig. 1).

FIGURE 1. Multibeam parameters and example configurations of multibeam tools. Four individual beams are positioned on the workpiece with a nominal separation distance of approximately 1 mm. The individual beams can be manipulated in a radius of 0.3 mm. Each beam can be turned on and off individually. The focal length is 100 mm.

Parameters of the multibeam engine

For this dynamic multibeam scanning system development to be successful in electronics industry applications, it must meet the following requirements to achieve higher productivity in scanner-based multibeam processes by providing:

Flexibility in the number of laser beams
Flexible spot configurations of the multibeam
Fast switching between different spot configurations
Individual scanning/positioning of each laser sub-beam

Figure 1 shows the main parameters of the multibeam optical engine concept. The demonstrator uses a diffractive optical element (DOE) from Holo/Or (Rehovot, Israel), a sister company of SCANLAB, which splits the laser’s main beam into four identical sub-beams. Accordingly, the multibeam scanner emits four sub-beams with a variable vertical spot spacing between 0.4 and 1.6 mm.

Not only can each sub-beam be switched on and off individually, but all sub-beams can be positioned inside a circle of 0.3 mm radius independently and dynamically. Examples of these ranges are shown in Figure 1. To reach every spot on the workpiece, a combined movement of the main scanner and the individual beams is possible. The next stage of the project will make it possible to increase the number of individual beams.

Multibeam challenges

The distortion caused by the separation of the two scanner mirrors and the distortion introduced by the f-theta lens, in a classical multibeam approach (see Fig. 2a), lead to distortion of the multispot pattern at the edges of the scanner’s field of view. The variable multibeam setup allows for active compensation of the spot field distortion and therefore enables multispot operation even with large scanning angles (see Fig. 2b). Alternatively, by integrating a synchronized mechanical stage (see Fig. 2c), the field of view of the scanner can be decreased to minimize distortion of the multibeam pattern.

FIGURE 2. Multibeam position error in a classical scan head setup (a), in a variable multibeam setup with individual steering of sub beams (b) and in a setup combining the scan head with a synchronized mechanical stage (XL SCAN) (c). In the classical setup (a) the multibeam pattern is twisted and distorted in the corner of the scan field, resulting in a large position error. This distortion can be compensated via individual steering of the sub beams, as shown in (b), resulting in a low position error. If a mechanical stage is synchronized with the scan head, then the distortion and the twisting of the sub beams can be minimized by reducing the deflection angle of the scan head (c). The resulting position error is low. In setup (a) and (b), 80% to 100% of the scan field of the f-theta lens can be used, while in (c), 10% to 20% can be used.

Synchronized mechanical stage integration

SCANLAB and ACS Motion Control (Yokneam, Israel) have proven that integrating a scanner and synchronized mechanical stage enables a new class of accuracy for large-field processing by introducing the XL SCAN concept. Innovative control allows processing of large scan field areas compared to the field of view of the scanner lens. The motion of the beam is split into a high-frequency component, which is passed to the scanner, and a low-frequency component, which is passed to a mechanical stage.

When combined with the variable multibeam concept, this approach further increases position accuracy and throughput on large workpieces. Specifically, multibeam laser scanning eliminates the tradeoff between high positional accuracy at small scan-field sizes and a large field of view for processing large workpiece areas with a lower accuracy. This new approach makes multibeam laser processing particularly attractive for high-speed applications such as laser drilling or laser-induced forward transfer (LIFT).

Figure 3 compares the position accuracy of a single scan head processing a motionless workpiece to the position accuracy of a scan head combined with XL SCAN processing a moving workpiece. Figure 3a shows the absolute position error results, evaluated using a coordinate measurement machine, for a scan head without a synchronized mechanical stage. For larger scanning angles, the position error increases with each point of the sample processed, allowing ample time for the galvanometer scanner to settle into its final position. With this quasi-static approach, a maximum deviation from the set position of 4.0 µm can be achieved.

FIGURE 3. The absolute radial position error is plotted against the set radial position for a standard scan head with a focal length of 100 mm without a synchronized stage (a). The positioning of the spots was carried out with ample time between the shots in order for the galvanometer mirrors to settle into their position. This is a quasi-static approach. The position error was measured using a coordinate measurement machine and increases with the radial position from approx. 2 µm to approximately 4 µm. Also plotted is the absolute position error as a density plot for a combined processing using a scan head with focal length of 100 mm and a synchronized mechanical stage (XL SCAN). The positioning occurs with a processing rate of 2600 Hz and is therefore highly dynamic. 99.99% (4σ) of the 10,000 shots fired at the workpiece show a position error below 3 µm. This shows how the combination with a synchronized mechanical stage can not only increase the processing area, but also improve the position accuracy.

Combining a scan head and a synchronized mechanical stage enables processing the same or even-larger areas by keeping the scanner in the central region of the field, where deviations are smaller than at the edges. With a processing time, which in this case was 150 µs, the processing rate relates to the jump time and variable jump delay according to the following equation:

One can see in Figure 3b that, with a jump delay of 40 µs, a processing rate of 2600 Hz and jump distance (pitch) of 100 µm, a 4σ position error of 3 µm can readily be achieved. This is even lower than the error reported in the quasi-static calibration shown in Figure 3a.

The multiscan engine in combination with a synchronized stage and intelligent control achieve much higher processing rates, at high positional accuracy when compared to single-beam processing. Thus, the tested multibeam optical engine concept offers a powerful solution for high-density drilling applications, answering the future demand for highly productive scan heads in high-density PCB drilling, microLED display manufacturing, and other electronics applications.

Figure 4 shows the simulated results for a variable multibeam setup. In such an approach, a processing rate of well above 10,000 shots per second can be achieved for structures with a spot spacing (pitch) of 100 µm or below. The accuracy achievable should be similar to that achieved in Figure 3b.

FIGURE 4. Processing rate of a multibeam system with four beams based on the results achieved with a single beam system. The processing rate takes a jump delay of 40 µs and a processing time of 150 µs into account. The absolute positioning error achievable in this setup is below 3 µm (4σ).

Outlook

High-energy ultrashort-pulse lasers and the reduction of feature sizes in microelectronics and display manufacturing demand new beam scanning technology developments. The new concept presented here, which allows the individual steering of beams in a multibeam array, offers greater flexibility and higher accuracy in multibeam processing. In combination with intelligent control technology, XL SCAN stands poised to tackle new applications requiring maximum accuracy, throughput, and workpiece size.

ACKNOWLEDGEMENTS

The authors would like to thank Patrick Gretzki and Olga Chemerenko of Pulsar Photonics GmbH and Dr. Felix Lange of SCANLAB GmbH for their contributions to this article.