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Micromachining over five axes

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Greg Blackman on the latest scanning solutions and software for micromachining

Scanlab’s XL Scan synchronously controls a 2D scan head and a positioning stage. It can also process 3D workpieces with integrated Z-axis control.

Laser micromachining is now a proven technology, with reliable and reasonably high-power ultrafast pulsed lasers available from a number of suppliers. The control of those pulses is also improving, with new products being developed by scanner firms and laser system manufacturers.

Tadas Kildušis, CEO at Direct Machining Control, a Lithuanian software supplier for laser machines, noted two laser processing trends that DMC has recently been working on: the rise of five-axis laser machining, and the synchronising of galvo scanners with positioning stages. These are borne out by new solutions from scanner provider Scanlab, along with control software Cambridge Technology launched at Photonics West in February. 

 

Five-axis laser machining is a ‘big pressure point for those developing software for laser systems,’ said Kildušis.

One challenge laser integrators and end-users working with five-axis machining with galvo scanning face, he said, is up to five software packages are sometimes necessary to get the part ready. ‘It’s a long process that requires personnel to be trained on high-end programs such as modelling, CAD and 3D texturing software. It’s an ongoing trend to simplify all that for laser machining,’ Kildušis remarked.

Other software that might be involved includes drawing software for 2D texture generation, as well as CAM software for motion path generation. In some cases there is also separate CAM post-processing and machine control software used, according to Kildušis.

‘While we don’t believe this can be simplified down to one solution any time soon, it could be simplified down to two pieces of software,’ he said, such as CAD software – which is used to design the part – and another package that processes the texturing and facilitates everything for the laser machine.

One of the software challenges Kildušis noted when it comes to engineering a five-axis laser machining application, is wrapping a 2D texture on a 2D surface. All wrapping options provide different distortions, he said, so the most suitable option for certain applications must be selected. Alternatively, the structure needs to be generated in 3D from the start, which is also a really complicated task and often different approaches will only work in a limited set of conditions.

One of the biggest applications of five-axis machining is texturing 3D surfaces with a laser, notably moulds. These moulds can be used to make components for consumer electronics, the automotive and aerospace sectors, and parts for medical devices.

Aerotech has recently released a five-axis laser scanner, the AGV5D, for micromachining. 

Scanlab has been supplying a five-axis system for drilling helical holes, one of the applications of which is manufacturing wafer probes. Holger Schlüter, head of business development at Scanlab, said that the market for manufacturing wafer probes is a new one for the company – it has only been selling into this market for a year.

Scanlab’s five-axis system can move the beam in X, Y and Z directions, as well as tilt the laser beam. In this way the system can produce tapered holes, either negatively tapered, no taper at all, or positively tapered. It can make square holes of 25 x 25µm with an aspect ratio of 12.5:1, and this can be used for probing silicon wafers before they are diced into individual chips. The holes the laser makes are the guides for the needles that probe the chip.

‘You need a very stable system for drilling these holes,’ said Schlüter. ‘It’s not only the precision and accuracy of the system, but also the repeatability and long-term stability that has enabled this, because making a probe card interface plate can take up to 60 hours of laser operation.’

The system has been made easier to use through a GUI with which the user can program the path of the laser beam.

Aerotech has also recently released a five-axis laser scanner, the AGV5D, for micromachining medical components, microelectronics, and automotive parts such as fuel injection nozzles. Other firms, including GF with its ML-5 and ML-10 machines, and Lasea with its LS5 system, provide complete five-axis systems for ultrafast laser micromachining.

The German firm Arges even supplies its five-axis Elephant scan head in an eight-axis version – the beam can be moved through XYZ co-ordinates, two beam inclination angles, two polarisation parameters, and a beam attenuation value. Last year Arges was acquired by Novanta, which also owns Cambridge Technology, Synrad, Laser Quantum, and Celera Motion, among other firms active in the medical sector. 

Scan head plus stage

At Photonics West in San Francisco, Cambridge Technology launched version three of its ScanMaster Controller software, which includes the ‘SyncMaster’ feature that synchronises a scan head with any XY stage controller.

‘There is a growing demand for large-area processing with high accuracy and speed in multiple applications, such as micromachining, where customers see a benefit for synchronising the scan head and the XY stage,’ said Bhavesh Bhut, product manager for software and controls at Cambridge Technology. A key advantage of this software release, say the firm, is that it makes synchronisation easier for the user. First, the engineer enters the XY stage and controller specifications, including X- and Y-axis travel distance, calibration data and encoder calibration. Once entered, the software takes care of the synchronisation and communicates directly with the controllers.

Scanlab, together with ACS Motion Control, based in Israel, has developed XL Scan, a scan solution for synchronously controlling a 2D scan head and an XY stage, which gives an almost limitless working area. The XL Scan is ideal for the display market, according to Schlüter, as the scanner works with small laser spots, machining small features, but processing over large areas. Austrian firm Rebeat is using it to make moulds for pressing vinyl records, while there are companies in Germany using XL Scan for large-field processing of glass or PCB board drilling.

To have accurate control of the laser beam over a large area requires third order-limited trajectory planning,’ said Schlüter, which is provided inside the PC. The scanner software has a dynamic-link library (DLL), which the user can access with C, C#, or C++, to give mark and jump commands to the trajectory planning. The software then corrects the beam path in such a way that the scanner and stage can follow the path that has been programmed. In this way the acceleration and jerk of the stage and scanner are not breached.

In terms of the stage, companies such as Siemens or Beckhoff have third order, or higher, limited trajectory planning. But these XY stages have input-process-output (IPO) cycle times of 2ms, so a lot of time to calculate complex paths. Scanlab’s scanners have to operate in 10µs cycle time, Schlüter said, which has recently become possible with the computing power that is now available.

Once third order-limited trajectory is in place, the trajectory is split into a path for the stage and a path for the scanner. This is done with a low-pass filter, which sends slow motions to the stage and the higher frequency motions to the scanner. The trajectory control and the filter design in the software send path co-ordinates to the RTC board, which sends part of the co-ordinate stream to the stage and the other part to the scanner.

‘We rely on the fact that third order-limited motion will be followed exactly by the stage and the scanner,’ Schlüter explained. There is a feedback loop inside the scanner and the stage, but the feedback loop from the scanner to the stage isn’t closed: the system relies on the fact that the scanner and stage can follow the motion perfectly.

‘We have tested this capability and found that the overall accuracy of the system is determined by the accuracy of the scanner and the accuracy of the stage,’ Schlüter added. ‘There is no additional tracking error or additional control error introduced by the combined motion.’

Five-axis machining is able to create tiny holes in materials like silicon nitride. (Image: Aerotech)

Scanlab doesn’t provide a graphical user interface with its software, but ACS Motion Control distributes a GUI for XL Scan. Users can program the system graphically with mark and jump commands.

The XL Scan and similar products are all complex control systems, Schlüter remarked. They require programming knowledge on the side of the user, but this is because there are numerous variables to be considered with the synchronised motion.

‘We are requested to make these scanner systems simpler, but the problem is that we have a cycle time of 10µs, and therefore rely on programming the system in mark and jump commands that are directly sent to the trajectory control or to the RTC, depending on the system,’ Schlüter said. ‘However, we are discussing new ways of programming the RTC card, to make programming the scanner simpler.’

Scanner control is usually a key internal knowhow of the integrator. They have software specialists that know how to program the RTC card.

We have requests from our Chinese customers to provide simpler interfaces,’ Schlüter said. ‘But, the simpler the interface, the less powerful they are. One of the advantages of the RTC card is that it has almost 1,000 commands, which gives a lot of power over controlling how the beam is steered.’

An example of one of these commands is the auto-laser control function, which maintains the spacing of the spots at a constant distance, regardless of the speed of the scanner.

The control command works with an ultrashort pulse-on-demand laser. ‘This increases throughput efficiency,’ said Schlüter, ‘because you can now use the laser while the scanner is accelerating or decelerating.’

XL Scan can achieve accuracy of 10µm under certain conditions, Schlüter explained – with a focal length for the optics of 100mm and a field of view of 50 x 50mm, for example. This is limited to the size of the field of the scanner, because the focal length magnifies any error. 

Scanner syncing

Kildušis, at Direct Machining Control, said that the next challenge for beam control software, will be synchronising multiple scanners and multiple beams.

This is already a trend in additive manufacturing, he said, where there is a push to print larger parts and increase printing speed. Therefore multiple galvo scanners are used at the same time to print a single part.

‘For software, the main challenge is to divide printing trajectories in a smart way, so that all lasers are used as much as possible,’ Kildušis said, making sure that the different galvos operating in the build chamber produce a uniform part.

Another challenge for the beam control software used in additive machines will be to handle the large amounts of data generated when building complex parts, Kildušis noted, for instance when lattice structures are sliced and filled with hatching lines. 

Scanlab has also recently introduced an open interface extension for its scan control systems, which combines process data, such as the signal of a pyrometer, with the co-ordinates of the scanner. The software gives the user a map of the temperature of a part. ‘That is something we think is useful for additive manufacturing,’ Schlüter said.

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