Inline Process Metrology System for the Control of Laser Surface Structuring Processes

Physics Procedia 39 ( 2012 ) 814 – 822

1875-3892 © 2012 Published by Elsevier B.V. Selection and/or review under responsibility of Bayerisches Laserzentrum GmbHdoi: 10.1016/j.phpro.2012.10.105

LANE 2012

Inline process metrology system for the control of laser surface structuring processes

Robert Schmitta,b, Guilherme Mallmanna,*, Kai Winandsa, Mario Pothena aFraunhofer Institute for Production Technology IPT, Steinbachstr. 17, Aachen, Germany

bLaboratory for Machine Tools and Production Engineering WZL, RWTH Aachen University, Aachen, Germany

Abstract

Laser surface structuring is a rapidly developing manufacturing technology. This technology offers a high level of flexibility regarding the geometrical design and a wide range of machinable materials. However, current systems used for laser structuring have a crucial deficit. The existing process control systems have no sufficient feedback infor-mation of the machined surface topology before, during and after the process, not being able to detect errors / devia-tions and so to adapt the structuring strategy while processing. Therefore a novel scanner based inline metrology system has been developed to control the laser surface structuring process by a process monitoring software. © 2012 Published by Elsevier Ltd. Selection and/or review under responsibility of Bayerisches Laserzentrum GmbH Keywords: Laser surface structuring; Inline metrology; Process monitoring; Process control; Frequency domain optical coherence radar; Computer aided manufacturing (CAM); Scan4Surf

1. Introduction and motivation

Laser surface structuring is a rapidly developing manufacturing technology [1] and has already be-come relevant for many industrial applications [2]. In a broad spectrum of industrial branches, for e.g. the automotive, aerospace or power engineering, laser surface structuring is applied to realize technical func-tions like the minimization of friction in tribological systems [3] or to realize innovative product designs. Especially in the tool and die making sector laser surface structuring can either be an alternative for the conventional structuring techniques like chemical etching or a complementary production technology [4].

* Corresponding author. Tel.: +49-241-8904-472 ; fax: +49-241-8904-6472 . E-mail address: [email protected] .

Available online at www.sciencedirect.com

© 2012 Published by Elsevier B.V. Selection and/or review under responsibility of Bayerisches Laserzentrum GmbH

Robert Schmitt et al. / Physics Procedia 39 ( 2012 ) 814 – 822 815

Laser surface structuring bases on the process of material ablation caused substantially by the conver-

sion of laser energy. The process characteristic depends particularly on the available time period for the interaction between the laser radiation and the material after the absorption of the laser radiation. Within dimensions of nanoseconds, linear absorption occurs and the laser energy heats the material locally till it vaporizes. To initialize the laser based material ablation process, high power intensities with more than several Giga-watts per square centimeters are required [5]. To achieve these intensities, on the one hand, pulsed wave laser sources are used, on the other hand the laser beam is focused at the workpiece surface with an effective spot diameter 50 μm or less. Telecentric f-theta lenses are often used for beam focusing. The advantage of such a lens is the characteristic to focus the spot perpendicularly inside a plane field. With a laser scanner the focused laser beam can be moved on the workpiece surface in x-y-direction. An additional component, the dynamic beam expander, expands the raw-beam of the laser source before en-tering the laser scanner. This allows to focus the beam even more and to realize additionally a spot movement in z-direction for a few millimeters. The focal length of such systems is defined as the double Rayleigh length and characterized as an area, where the changes of the intensity distribution is moderate. While laser structuring the position of the focal spot in relation to the part surface should be within a max-imal distance of the focal length to ensure constant results. The programming of the machine tool paths bases on the CAD-model of the part, the digital information about the structure, the targeted depth and the expected material ablation rate. The programming is realized by a special CAM-software. Although the CAD-model includes all information about the part geometry, the pre-machining (e.g. milling or grind-ing) of the part generates in fact an unknown surface topography within the micro range. To ensure a pre-cise laser machining and to compensate these deviations a measurement of the macro geometry of the pre-machined part inside of the machine tool can be an effective solution. However the laser surface structuring of parts, even with modern machine tools, is challenging with regard to the final quality and the needed effort to reach a constant and high product quality. An essential factor is the complexity of the laser process itself. Several process parameters have a direct impact on the machining result. These influ-ences are but not sufficiently described, e.g. by comprehensive process models [6]. This deficit has two significant impacts. One is the time consuming effort to initialize the laser structuring process for machin-ing new product materials. If the process behavior is unknown for a determined material, laser parameters and suitable machining strategies have to be identified in a trial and error testing before machining the real part. Therefore reference geometries are laser structured on a material sample in a first step. The ma-chined structures have to be analyzed outside of the machine tool, one by one and for each laser parame-ter setting as long as a suitable parameter is found. Only then the final machining of the real part can start. The second impact is the uncontrolled machining, especially while machining large surfaces takes several hours. Once the process started it is not possible to detect the reached depth and so to adapt laser parame-ters and the machining strategy while processing the part inside of the machine.

These deficits increases the manufacturing time for laser surface structuring significantly. It also limits the throughput, increases costs for production and the risk to produce parts outside of the specifications. Consequently, the automation and acceleration of these complex and manual procedures are highly de-manded. To face these issues an innovative measuring system has been developed within the project Scan4Surf, funded within the 7th framework program of the European Union. The developed optical measuring system is based on the frequency domain optical coherence radar technique and is able to measure with sub-micrometer accuracy depth and topography of a laser machined part. The system is build-up in a modular way and can be easily integrated into a machine tool for laser surface structuring. With this solution the part can be measured inside of the machine in the original set-up. An additional light beam, the measurement beam, uses precisely the same optical path as the machining laser, utilizing so the system’s scanning unit and the telecentric f-theta lens. This enables the measurement of the macro

816 Robert Schmitt et al. / Physics Procedia 39 ( 2012 ) 814 – 822

geometry before machining and the detection of the machined surface while the laser structuring process is running. The measured machined depth is feedback to the controlling system of the machine, which re-adjusts the process parameters or machining strategy for achieving optimal, reproducible process results.

2. State of the art in laser process monitoring

Laser based machining systems are affected by different machine, workpiece and environmental relat-ed parameters, which influence directly the process stability and product quality. For manufacturing pre-cision parts with tight form tolerances, the tolerable parameter deviations are extreme small. To secure process stability and quality for long production periods is consequently a difficult task. The usage of a process monitoring system is therefore indispensable for laser based precision manufacturing [7]. Most monitoring systems in this field are based on the tracing of process related phenomena, as process related acoustic emission or electromagnetic emission [7]. The process’ acoustic emission is used for example for monitoring the focus position [8] or the machined depth [9]. Furthermore the process’ electromagnetic emission is used for monitoring laser welding processes [10-11] and to control the focus position in laser welding [12], cutting and drilling processes [13]. Another approach is a technique for monitoring the plasma generated electromagnetic phenomena in terms of an induced current on an electrical system [14]. A combination of these monitoring techniques can be used to improve the overall system control robust-ness and stability as a multi-sensor system [15]. However, these methods present a drawback. With these methods the product quality can just indirectly be measured, not being able to determine the depth or ge-ometry of the produced cut, hole, weld or microstructure directly. For this purpose in situ triangulation sensors can be used [16]. Nevertheless an innovative monitoring technique for laser machining based on optical coherence tomography / imaging is able to acquire a high precision depth profile during or after the process. Applications were reported for monitoring laser micromachining [17] and drilling [18-19] as well as to control laser micromachining with [20] and without [21] focusing lens and for laser drilling [22-25]. None of the presented works consider the effects of the optical machine system (scanner unit and the focusing lens) on the measurement beam. These components are always optimized for the machining wavelength. In the case of the scanner mirrors, which are coated in dependency of the laser wavelength, such an optimization leads to reflection artifacts and measurement errors. In the case of the focusing lens, every scanning position presents a different optical path difference (OPD) and a modified measurement beam spot geometry. An in situ 3D scanning of the complete machining area is also not reported.

3. Concept

3.1. Measurement system

The applied metrology technique is based on a design of the low coherence interferometry, the so called Fourier domain optical coherence radar, used for the measurement of distances. A superlumines-cent diode (SLD) is used as a broadband light source in this system, which has a low temporal coherence. In the Fourier domain technique the depth information is gained by analyzing the spectrum of the ac-quired interferogram differently from normal low-coherence interferometers, which use a piezo element to move within the measurement range and to find the maximum interference point. The calculation of the Fourier transformation of the spectrum provides a back reflection profile as a function of the depth. Depth information is obtained by the different interference profiles from different path lengths in both optical paths (reference and measuring paths), where the higher the length difference between reference and measuring path, the higher the resulting interference frequency signals. [26]


The maximum measuring depth, Zmax, of a FD-OCT system is described by [27]:

(1)

and the axial resolution of a FD-OCT is defined as [27]:

(2)

0 is the central wavelength, is the bandwidth, n is the sample’s refractive index and N is the num-ber of elements of the spectrometer detector. The variable lc is the coherence length.

For the measurement of the distance to the machining part surface just the highest back reflection, i.e. the most significant interference spectrum modulation, will be analyzed. In combination with the scan-ning system a 3D form measurement of the workpiece surface can be executed. Therefore a light source with a wavelength near to 1064 nm will be used. This will assure an optimized measurement beam cou-pling to the laser machining beam as well as a small optical aberration due to the system optics. The cou-pling of both beams, machining laser and measuring beam, will be achieved through the usage of a di-chroic mirror (fig. 1). The focusing lens used in the system concept is a telecentric f-theta lens. This lens is wavelength optimized through the insertion of different optical elements, which cause a targeted optical distortion in the lens system. The aim of this optimization is to generate a focal plane for the laser beam, as well as to create a direct proportional relation between the scanning angle and the laser spot position [28]. Therefore the designed optical system of the f-theta lens causes optical aberration in wavelengths, that differ from the machining laser wavelength 1064 nm. This aberration causes systematic errors in the measurement system, such as a distortion of the focal plane, of the optical path and of the spot geometry of the measurement beam. This effects can be partially compensated by the usage of a dispersion com-pensator (fig. 1 (a)). The remaining optical distortion needs to be calibrated and compensated.

Fig. 1. Concept of the inline process monitoring system – {(a) dispersion compensator / glass rod}

NnZ 420max

2044.02cOCTFD lAR


3.1. Automated adaptive laser surface structuring

The Fraunhofer IPT has already developed a digital process chain to generate the required data for the laser surface structuring of a designated part [29]. To machine 3-dimensional microstructures, a series of layers are scanned by the focused laser spot. The number of necessary layers is defined by the current material removal rate, which depends on the process parameter settings. This information has to be identi-fied for each material and can be stored in a database as a virtual “laser tool”. The here proposed adaptive manufacture technique uses the developed measurement system to evaluate the current surface structure and on this basis adapt the manufacturing procedure and the laser parameters.

The adjustment of the adaptive laser process can be carried out after every machined layer by measur-ing the resultant surface topography. Hereby the average ablation on the surface is analyzed by a compar-ison between the achieved and the targeted depth, which enables an optimization of the laser parameters like variation of pulse energy or the focal plane position. Based on this analysis and on the laser tool da-tabase an appropriate adapted strategy is applied for the subsequent layer.

Alternatively the process control is carried out for every laser machined position. Hereby the depth of the ablated cavity is evaluated inline and returned to the system. The control unit adjusts the laser parame-ters for the next position on the surface as well as for the following layers.

In both cases the real removal rate is registered in the virtual laser tool database, building-up a process know-how using this automatic measurement system and process analyses, especially for unknown mate-rials. The achieved know-how can be used easily for similar processes.

The synchronization of the laser and measurement position is carried out similarly to the procedure used to control the laser unit. The scanner control card centralizes the triggering and logging of the cur-rent position of the laser and measurement beam as well as the measurement result.

4. Experimental

4.1. Measurement system prototype

The measurement system prototype was developed considering the optical aberration caused by a tele-centric f-theta lens with a focal length of 80 mm and at the same time to realize the coupling of the meas-urement beam using a dichroic mirror. The measurement wavelength range was defined between 900 nm and 1050 nm, just below the laser wavelength of 1064 nm. In order to analyze the encoded interferometric signals a spectrometer was constructed. The developed spectrometer was designed with a spectral range of 107 nm, which can be adjusted in the absolute frequency range between 900 and 1100 nm depending on the used light source. An indium gallium arsenide (InGaAs) line camera was used as a detector. Stand-ard silicon based detectors present a low quantum efficiency for the applied wavelength range of less than 20%, against values between 60%-80% for InGaAs based detectors [30]. The used light source is a super-luminescent diode with a spectrum of 1017 ± 50 nm and an optical power of 10 mW (fig. 2).


Fig. 2. Measurement system prototype

The theoretical measuring range (max. depth scan) of the developed FD-OCT is 1.31 mm (Eq. 1), us-ing the presented SLD light source. The available measurement range and its linearity was evaluated us-ing a piezo element. The results showed a maximum distance measurement of 1.25 mm using a specimen of aluminum with a technical surface, which simulated a typical workpiece used for the laser structuring. The theoretical axial resolution is 4.58 μm (Eq. 2). Nevertheless the usage of a gauss fitting algorithm increases the axial resolution by calculating a sub-pixel accurate frequency value to find the modulation frequency in the Fourier transformed signal of the light spectrum. The standard deviation of the distance measurement values, acquired in the center of the scanning field, is of 218 nm [31].

4.2. Laser structuring system prototype

The developed measurement system, a beam coupling unit, a scanner and a telecentric f-theta lens compose the prototype of a simple laser structuring system. The scanning unit executes the deflection of the laser and the measurement beam, followed by a telecentric f-theta lens with a focal length of 80 mm (fig. 3). The combination of both elements enables a scanning working field of 30 mm x 30 mm. A 20 W nanosecond pulsed fiber laser with central wavelength at 1064 nm was used as a processing laser. A de-veloped software controls the complete prototype, synchronizing the scanning unit with the measurement system and the laser source.

Fig. 3. (a) prototype of the laser structuring system with beam coupling unit; (b) coupling concept


The concept chosen for the beam coupling is based on an optical filter. The reflective area of the filter

is within the spectrum of the measurement beam and within the transmissive area for the wavelength of the processing laser. The coupling efficiency was evaluated using an optical edge filter for the wavelength 1064 nm. By changing the angle of the edge filter, the edge frequency between reflection and transmis-sion is displaced. At an angle of 23° the edge frequency is adjusted to reflect the wavelength bandwidth of the measurement system and transmit the wavelength of the laser beam (fig. 3 left). An overall cou-pling efficiency of over 95% for the laser beam and over 93% for the measurement beam was evaluated in laboratory tests. These results validate the concept for a integration inside of the existing machine tool.

5. Results

The system prototype was evaluated through a series of measurements in high precision standards (flatness and step standards) as well as in technical and rough surfaces. By measuring a flatness standard the amount of optical aberration introduced by the telecentric f-theta lens in the distance measurement could be investigated. If the thickness changes on the focusing lens, alterations in the optical path differ-ence (OPD) between the measuring and the reference pathway affect the surface form inspection and need to be characterized. The measurement of a flatness standard (Lambda/10) showed a slight distorted plane as expected. For the measurement of an area of 30 mm x 30 mm a parabolic distortion could be detected in x- and y-direction. After the extraction of the plane inclination a maximum deformation of 108 μm in the x axis and of 115 μm in the y axis was measured. This measurement distortion is caused by a thick-ness change of the f-Theta lens in dependency of the scanning angle, leading to a OPD change. A correc-tion of this effect is achieved by a system calibration, which is based on a measurement of the used flat-ness standard. The overall 3D form error is fitted in a x-y polynomial, which is used to correct every measurement point in dependency of its field position.

With regard on typical applications of laser structuring systems, a 2 Euro cent coin and a laser struc-tured injection mold insert were measured (fig. 4). The presented results demonstrate the robustness of the system on the inspection of complex surfaces.

Fig. 4. Measurements of a 20 mm x 20 mm field – (a) 2 Euro cent coin; (b) measurement of a laser structured surface


6. Conclusion and outlook

Within this paper an optical measurement system for inline surface inspection of micro and macro sur-face structures with sub-micron accuracy was described. In addition the potential applications of this technology, especially for an automatic laser parameter setting and an adaptive process control of laser structuring systems, were presented. Measurements on standards and technical surfaces were executed, which validate the potential of this technique for a telecentric surface inspection on laser based machine tools. Future investigations on the optical characterization of the complete system will be carried out, which will determine the overall optical aberration on the measurement system to complete the current calibration solution. In a second step the integration of the measurement system in an existing laser struc-turing machine tool as well as the implementation of an automatic adaptive laser surface structuring pro-cess with a feedback control is aimed.

Acknowledgements

We gratefully acknowledge the financial support by the German Federal Ministry of Education and Research (BMBF) for the project ‘Scan4Surf’ (02PO2861), which is the basis for these achievements.

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Inline Process Metrology System for the Control of Laser Surface Structuring Processes