WO2011116968A2

WO2011116968A2 - System and method for laser-beam based processing

Info

Publication number: WO2011116968A2
Application number: PCT/EP2011/001481
Authority: WO
Inventors: Oliver Haupt; Frank Siegel
Original assignee: Laser Zentrum Hannover E.V.
Priority date: 2010-03-24
Filing date: 2011-03-24
Publication date: 2011-09-29
Also published as: WO2011116968A3

Abstract

In a first aspect, the present invention relates to a system or device allowing fast, efficient and flexible laser-beam based processing. Further, the present invention provides a method for laser-beam based processing, e.g. in the field of high-precision laser patterning of micro structures on large-scale surfaces.

Description

System and Method for Laser-Beam based Processing

In a first aspect the present invention relates to a system for laser-beam based processing, comprising a laser source providing a raw laser beam, a diffractive optical element, a laser scanner and a focussing lens. In a second aspect the present invention relates to a method for laser-beam based processing in which such a system is used. In a third aspect the present invention relates to a laser scanner assembly that can be used in a system according to the first aspect of the present invention.

Such a system and method is for example used in the field of high precision laser patterning of microstructures on large-scale surfaces. The laser source, which preferably is a pulsed laser, generates a raw laser beam which by some optical elements is led onto the surface of the work piece, on which a microstructure is to be generated.

The interaction of pulsed laser radiation with matter is determined by pulse duration, pulse fluence and specific properties of the material such as the density, the heat conductivity, etc. These parameters affect the quality and process efficiency of micro machining applications on macroscopic areas. For micro machining applications on metals and semiconductors it is well known, that pulse duration in the sub-nanosecond (ns) range is necessary to achieve high surface quality as well as precise structure geometries [1 , 2, 3, 4]. Picosecond (ps) pulse duration in the range of 10 ps is regarded as useful to minimize thermal damage and burr. Compared to nanosecond pulses on one hand, the achievable ablation rates are clearly lower, leading to increased processing time. On the other hand, the quality of the ablated structures, as well as the process resolution, is close to femtosecond (fs) ablation when using qualified process parameters. After all, picosecond pulse duration promises to be the best compromise between quality and industrial usage. Beside this quality approach both, femtosecond and picosecond pulse duration, show very low ablation rates in the range of several ten to hundred nanometers per pulse. This characteristic ablation behavior of ultra-short pulses is based on absorption and energetic interactions within the material lattice. It leads to increased processing times of several hours and minimal area throughput, which is not industrially applicable.

The increased demand for high area throughput micro machining applications on macroscopic areas results in the development of new high- power ultra-short pulse laser sources during the last years [2, 3, 4]. In contrast to higher available laser power with corresponding high pulse energies and repetition rates, the processes are often limited by the damage threshold. Especially when a high quality laser ablation is required and/or low damages can be tolerated, the available laser power exceeds the demand by orders of magnitude [5, 6].

The maximum efficiency of current laser sources with high power can be achieved applying multi-spot technology. By using a DOE the incident laser beam is separated into several laser beams, each with the characteristics of the original beam except the power [7]. Each of these beams will be called "split beam" in the following. There is already one industry proven application where a DOE is used to cut Silicon wafers. For this technological application a fixed optical setup with a fast linear axis is used but it is limited in speed and flexibility [8].

Manufacturing of the diffractive optical elements for high laser power intensities is technically possible and established. Due to the diffraction of the DOE an incoming raw laser beam can be split into an amount of M x N split beams, leading to M x N laser spots on the surface of the work piece. Possible amounts are for example 1x3, 1x7, 5x5, 1x81 or 15x15 split beam and laser spots. The beam splitting is based on diffraction and can be realized by amplitude or phase diffraction. Amplitude DOEs can be easier produced due to a partial light absorbing layer. Phase DOEs are based on complex micro structures on transparent materials, where different optical length causes beam splitting. Due to less absorption phase DOEs have a higher efficiency. In Figure 3 the resulting array, in this case a 1x3 spot array, of a phase splitting DOE is shown. It shows the measured energy distribution before and after beam splitting using a 1x3 DOE.

The minimal efficiency of phase DOEs is approximately 70% for an AR coated element [7]. A combination of fused silica substrates with common AR coatings leads to high power thresholds of about 10 J/cm². This fulfills the requirements for high peak power ultra short laser pulses and is the base for high area throughput micro machining.

A known application is using a DOE and a fixed mounted optical system to split an incident laser beam into a one-dimensional laser spot line. This set-up is used to generate a step-shaped laser ablation for cutting of crystalline silicon and other semiconductors [8]. The optical system is fixed and the relative linear motion between the laser beam and the sample is realized by linear axes. The processing speed is restricted by the maximum possible speed and dynamics of the axes. Further, the stationary optical system is restricted to a one-dimensional group of spots which leads to a limited flexibility during the processing.

Further, a laser processing device depicts a double or multiple laser spot on the material using a galvanometer scanner. This set-up uses for example a stationary mounted DOE or a refractive beam splitter. The split laser beams are guided by a necessary mirror arrangement where the split beams are coupled into the scanner specifically that all incident laser beams are crossed between the two mirrors of the scanner [16]. This is a complex set-up using beam splitters and mirrors to process a work piece with two or more laser spots with a fixed distance between the spots.

DOEs can also be used for beam shaping and beam separation [7, 15]. Due to the compact shape and miniaturization with a corresponding light weight, DOEs can be used for different laser processes and within optical sensors [12]. Beside transmissive DOEs diffractive silicon mirrors can be used for high power lasers [10]. Parallel processing is possible with refractive micro lens arrays which are generating multi-spots or rather multi-lines on the work piece [9]. A stationary complex optical system is necessary to make the parallel processing possible. Further, two lens arrays one behind each other and a focusing lens are necessary for a multi-spot arrangement.

First approaches to realize parallel processing used a splitting of an incident laser beam into multiple spots by spatial light modulators (SLM). Disadvantageously SLM-s are restricted in resolution and power density. Further, a complex optical system is necessary using a stationary lens and mirror set-up [11]. Another advantage of DOEs is the thin structure with a corresponding low weight. DOEs can be made of different optical materials for each desired wavelength and will have an enormous potential in the near future [12].

DE 10 2007 037 133 A1 discloses a system in which a laser beam is led to a DOE and is split in this DOE into a plurality of split beams, each of which is used to form microstructures on the surface of a work piece.

Due to the DOE each laser pulse leads to a material ablation on the surface of the work piece at a plurality of different positions. The pattern of these ablations is determined by the kind of DOE, which is used. If the desired structure on the surface of the work piece for example consists of a periodic repetition of this pattern, the work piece itself has to be moved in relation to the system, since no laser scanner is provided for in the system. This is disadvantageous especially for large work pieces and reduces the patterning speed.

The same is true for the EP 1 063 048 B1. This document also discloses a system in which a laser beam which is provided by a laser source is led onto a diffractive optical element and split into a plurality of split beams. Whenever the DOE is used in the optical path of the laser beam, no laser scanner is used. Since again no laser scanner is provided for in the disclosed systems the work piece has to be moved in order to produce the desired structures. In addition, in both systems the DOE has to be changed when a new microstructure is to be patterned on the surface of the work piece.

From JP 2007-268600 a system for laser-beam based processing is known which comprises both a diffractive optical element and a laser scanner. In order to get the system flexible in the sense that different structures can be patterned on the surface of the work piece the laser has to be bent by bend mirrors before leading the laser beam to the DOE and the laser scanner. Due to this setup it is possible to adjust the distance between two adjacent laser spots on the surface of the work piece by moving the DOE in the direction of the incident laser beam. Hence the DOE has to be positioned very accurately, which makes the system complicated and time consuming to prepare.

It is therefore an object of the present invention to provide a system and a method for laser-beam based processing which is easier and faster to handle.

This problem is solved by a system for laser-beam based processing, comprising a laser source providing a raw laser beam, a diffractive optical element, a laser scanner and a focussing lens wherein the system is characterized in that the diffractive optical element is placed in front of the laser scanner in the raw beam of the laser source and is capable of dividing the raw beam into a plurality of split beams.

A raw laser beam in the context of the present invention as usual is a laser beam having the same or a better mode factor M² as the laser beam provided directly from the laser source. Preferably the mode factor M² is smaller than 1.5 for raw laser beams. M² is the ratio of the beam parameter product of the real beam and the beam parameter product of an ideal beam. The beam parameter product is the divergence angle multiplied with the radius of the beam at its narrowest point. Hence, by placing the diffractive optical element directly in the raw beam of the laser source, both the beam in front of the diffractive optical element and the split beams behind it are easier to handle since no complex focussing optic is needed. A simple focussing lens positioned between the laser scanner and the surface of the work piece as known from the art is sufficient to get optimal sizes of the laser spots on the surface of the work piece.

Since a raw laser beam has the same or a better mode factor as the laser beam provided directly from the laser source, the beam itself is essentially parallel. Hence, the position of the diffractive optical element along the path of the laser beam has no or very little influence. This ensures that the system according to the present invention is easy to handle and the setting and preparation is not time consuming.

The definition of a raw laser beam given above also includes laser beams which are widened up by some optical elements which are placed in the optical path of the laser beam between the laser source and the diffractive optical element. As long as the widened laser beam, i. e. the laser beam having a larger diameter than the one being provided directly from the laser source, has a divergence angle which is small enough to fulfil the constraint concerning the beam parameter product mentioned above, also this widened beam is a raw laser beam in the sense of the present invention.

The diameter of the raw laser beam provided from the laser source is preferably equal to or larger than 2 mm and can for example be 16 mm. This is preferably measured directly at the surface of the DOE.

The diffractive optical element can preferably be a one-dimensional or a two-dimensional diffractive optical element. This means that the splitting of the incoming raw laser beam into the plurality of split beams can be realized in one or two directions leading to a one- or two-dimensional pattern of laser spots on the surface of the work piece.

In a preferred embodiment of the present invention the system is characterized in that the diffractive optical element is mounted in a pivotable manner. This means that it can be rotated and the rotation axis is preferably parallel to the incident raw laser beam. If the DOE for example generates a one-dimensional laser spot line on the surface of the work piece, a rotation of the DOE results in a rotation of the laser spot line. Usually the laser spot line will be moved on the surface of the work piece during patterning by using the laser scanner. With this setup it is possible to generate parallel recesses on the surface of the work piece. Every laser spot generates one of these linear recesses when being moved in one direction on the surface of the work piece. When the DOE is rotated around an axis which is parallel to the direction of the raw laser beam which is led onto the DOE, also the laser spot line on the surface of the work piece is rotated. If now this rotated line is scanned over the surface of the work piece this also leads to linear parallel recesses on the surface. Now the distance between two of these parallel recesses has changed. The maximum distance is reached, when the laser spot line on the surface of the work piece is perpendicular to the direction of scanning. Then also the linear parallel recesses on the surface of the work piece are perpendicular to the laser spot line. If the DOE is rotated by an angle which is smaller than 90°, the distance between two parallel linear recesses is decreased. When the DOE is rotated by exactly 90°, also the laser spot line is rotated by 90°, which means that it is parallel to the direction of the scanning. This leads to one single recess, which is much deeper, since every laser spot in the laser spot line leads to ablation of material in this single linear recess. A further rotation of the DOE leads to an again increasing distance between the parallel linear recesses on the surface of the work piece.

In a preferred embodiment the laser scanner is a galvanometer or a polygon scanner. The laser scanner then comprises at least two mirrors. With this set up it is easily possible to move the laser spots of the plurality of split beam to every position needed on the surface of the work piece within a defined working area. Said laser scanner preferably comprises in addition a focusing lens which may be part of the laser scanner or which may be separated from said laser scanner.

Preferably the laser source provides laser pulses. In a still preferred embodiment the laser source provides picosecond laser pulses. The length of such a pulse is preferably no longer than 50 picoseconds.

With the system according to the present invention a fast and flexible multi-spot microprocessing is possible. For this, of course, the laser scanner, which can for example be a galvanometer scanner, has to be fast enough. Using this combination, the available laser power as well as the continuously increasing repetition rates of the pulsed laser can be completely applied to the work piece. Therewith, the necessary step from serial single spot processing to high throughput multi-spot machining is done and finally industrial micro machining applications can be addressed.

In a method for a laser-beam based processing according to the present invention one of the above mentioned systems is used. The method further comprises the steps of providing a raw laser beam by the laser source, splitting the raw laser beam into a plurality of split beams by using the diffractive optical element and focussing the plurality of split beams using the focussing lens onto a work piece such that an array of laser spots is generated on the surface of the work piece. In a preferred embodiment the array of laser spots is a one-dimensional or two- dimensional array of MxN laser spots.

In a preferred embodiment of the present invention the method further comprises the step of using the laser scanner in order to move the array of laser spots on a surface of the work piece. In a preferred embodiment this leads to parallel line-shaped recesses on the surface of the work piece by ablation of material. Preferably the distance between two parallel recesses is adapted by rotating the diffractive optical element. The laser source preferably provides laser pulses. Line-shaped recesses can be obtained by moving the mirrors of the laser scanner between two laser pulses only a little bit, such that the spots at the second position overlap with the spots at the first position, so that connected recesses occur which lead to the desired line-shaped design. Of course the spots can also be moved in a different way leading to a different pattern of recesses.

A laser fluence H per laser pulse in each laser spot on the surface of the work piece is chosen such that an ablation behaviour of the material of the work piece is in an optical penetration zone.

One of the main goals of the present invention is to provide a system and a method which can easily be handled and which efficiently uses the available laser power provided by the laser source. Therefore, it is necessary to investigate the ablation behaviour of the corresponding materials of the work piece and the laser parameters using different pulse durations, for example picosecond or femtosecond pulses. A study of the ablation effects when using multi-spot DOE in comparison to single spots is investigated and discussed. A theoretical analysis of the optimal set of parameters for efficient parallel processing with high-power lasers is provided.

First, the average ablation rate against the fluence per pulse has been determined. The results, in this case for crystalline Silicon and Aluminum are shown in Figure 1. Figure 1 shows the average ablation rate a_P in dependence of the laser fluence per pulse H_P for crystalline Silicon and Aluminum using picosecond pulse duration.

Analog to the ablation behavior of metals for femtosecond pulses, the use of picosecond pulses leads to two characteristic ablation regimes. The solid-lined curves (optical penetration zone) in Figure 1 can be approximately described by Equation 1 :

( H λ

o_P = d_opl - \n - (1)

Equation 1 characterizes ablation within a fluence regime where heat conduction beyond the optical penetration depth d_opt can be neglected. The material characteristic value H_th specifies the ablation threshold which depends mainly on the pulse duration and the laser wavelength. Within this "optical penetration zone" the ablation rate ap for crystalline Silicon as well as Aluminum could not exceed 0.1 μιη/pulse. Generally, the maximum ablation rate within the optical penetration zone for semiconductors and metals using picosecond pulses is about 0.1 pm/pulse or below. However, the achievable edge and surface quality is comparable to results of experiments done with femtosecond pulse duration.

The dotted curves in Figure 1 describe the "thermal penetration zone" and are given by Equation 2:

(2)

Within this thermal penetration zone, a significant part of the absorbed pulse energy is conducted by electron diffusion beyond the optical penetration depth d_opt and leads to further ablative interaction with the surrounding material. Due to the hot electron diffusion the extent of the ablation regime is called thermal penetration depth or electron diffusion length d_th. The advantages of higher ablation rates up to 1 pm/pulse and above is coupled with a clearly reduced surface quality and the formation of burr as well as melting zones along the irradiated areas. The material specific fluence H_trans indicates the transition fluence from the optical to the thermal penetration zone.

Preferably the laser fluence H per laser pulse in each of the laser spot on the surface of the work piece is smaller than 4 J/cm². For raw material, such as for example crystalline Silicon and Aluminum the transition fluencies H_trans has been determined to be between 3 and 4 J/cm², corresponding to pulse energies E_P between 10 μϋ and 50 pJ for a common laser spot diameter. Below this transition fluence the ablation is characterized by high surface and edge quality with minimal ablation rates, resulting in the demand of parallel processing. Above the transition fluence clearly higher ablation rates by a factor of 5 and more can be achieved. Simultaneously thermal impact and melt formation are increased leading to reduced structure quality.

For serial processing it can be seen that up-scaling of area throughput regarding the increase of ablation rates per pulse is limited on one hand side by the used laser fluence. This limiting factor leads to the result, that the available laser power can not be fully applied to the material surface.

Another interesting parameter for choosing the optimal energy density and energy in each of the laser spots on the surface of the work piece is the ablation efficiency per pulse. Beside the discussed ablation rates a_p the ablation efficiency Δ_ρ also indicates the necessity of parallel processing strategies. Efficiency in this case is defined by the ablation rate referred to the used fluence, given by Equation 3.

(3)

Figure 2 shows the calculated ablation efficiency for crystalline Silicon and Aluminum according to given ablation rates from Figure 1. For both materials the highest efficiency can be observed for lowest fluence values in the range of few 100 mJ/cm² and clearly below the transition fluence area.

This characteristic behavior has also been observed in other materials like Copper, Tungsten, Germanium or Silicon Carbide. It can be explained by increasing losses resulting from heat conduction and plasma based shielding effects for higher fluencies. Figure 2 shows the ablation efficiency Δ_Ρ in dependence of the laser fluence H_P for crystalline Silicon and Aluminum using picosecond pulse duration.

To sum up, the best ablation quality can be achieved for low fluencies below the material specific transition fluence and, simultaneously, highest ablation efficiency is given within this range of fluence. Therefore, high area throughput micro machining requires new and innovative machining strategies to overcome these given process limitations.

The present invention bases on the described knowledge. Only by using the combination of a DOE and a galvanometer or polygon scanner and placing the DOE in front of the laser scanner in the raw beam of the laser source a flexible, precise and fast processing on macroscopic areas is possible. This leads to a considerable reduction of process times and an efficient use of expensive laser sources and their available output power. With the present invention the ablation efficiency can be increased by using multi-spot technology based on DOEs combined with fast galvanometer scanners. This leads to an increased area throughput by factor of 10 and above with corresponding reduction of processing time.

The employment of DOEs in the raw laser beam in front of the laser scanner is easy and practical. It depends on the application which DOE is applicable and has to be used. Where entire surfaces have to be structured or modified, a 2D element is in many cases much more efficient than a 1 D element. In contrast, where deep grooves or defined structures have to be generated a 1 D element can be more useful. For further increase in flexibility of the entire system, a path adjustment or correction is necessary. This means the DOE must be mounted rotatable and angle- operated by the scanner control.

In the following two exemplary applications for micro machining on macroscopic areas using multi-spot technology are shown. The preparation of crystalline Silicon solar cells for photovoltaic applications (λ=515 nm, τ_Ρ = 7.3 ps) as well as the extensive micro structuring of metallic compressor blades for fluidic purposes (λ=1064 nm, τ_Ρ=11.8 ps). Here, λ denotes the wavelength of the laser beam and τ_Ρ is the pulse duration of the laser pulses.

According to the third aspect of the present invention, a laser scanner assembly comprises a holder for a diffractive optical element and a laser scanner, which comprises at least one mirror. The diffractive optical element is capable of separating an incident raw laser beam into an array of M x N split beams. Either M or N can also be chosen to be one, resulting in a one-dimensional array of split beams. Each of the split beams is separated from the adjacent split beams by a separation angle. According to the present invention a distance between the holder for the diffractive optical element and an input aperture of the laser scanner is chosen such, that every split beam passes the input aperture. Hence, every split beam the incident raw laser beam is separated into enters the laser scanner and can be used for the desired processing. Usually this distance is the same as the distance between the DOE and the input aperture.

With an assembly according to the third aspect of the present invention it can be achieved, that DOE and laser scanner are compatible and interoperable with each other in a very easy way. By using such an assembly in a system according to the first aspect of the present invention the system is very easy to handle. No complicated calibration or adjusting of the different devices is necessary.

In a preferred embodiment the laser scanner assembly is characterized in that a distance between the holder for the diffractive optical element and the at least one mirror is chosen such that every split beam, that passes the input aperture of the laser scanner is reflected by the at least one mirror. Again this distance usually is the same as the distance between the DOE and the mirror.

In a typical assembly the distance between the input aperture of the laser scanner and the mirror can for example be 40 mm to 60 mm. The separation angle a can for example be 0.14°. For a 1 x 7 DOE, separating an incident raw laser beam into a 1 x 7 array of split beams, this results in an angle between the two outer split beams of 0.84°. In a distance of 40 mm behind the DOE this means a distance of 580 μιη between these to split beams. In a distance of 60 mm, the distance between the two split beams is 880 μητι and in a distance of 100 mm behind the DOE the distance between the two outer split beams is 1466 μηι.

For a 1 x 81 DOE with a separation angle of 0.13° the angle between the two outer split beams is 10.4 °, resulting in a distance between the two split beams of 7.2 mm at 40 mm, of 10.92 mm at 60 mm and of 18.2 mm at 100 mm behind the DOE. If a 5 x 5 DOE with a separation angle of 1.35° is used, the angle between the outer split beams in both directions is 5.4°, resulting in a distance between the two split beams of 3.76 mm at 40 mm, of 5.66 mm at 60 mm and of 9.42 mm at 100 mm behind the DOE.

In a preferred embodiment the DOE can be mounted in the holder in a pivotable manner.

Example 1 : SOLAR CELLS (CRYSTALLINE SILICON)

Crystalline Silicon solar cells are particularly sensitive to damages due to laser processing which leads to lower efficiencies and more breakage. On the other hand there are some laser based processes which improve the efficiency of the solar cell. Laser edge isolation, laser drilling for Metal Wrap Through (MWT) or Emitter Wrap Through (EWT) are established technologies. Another approach is selective opening of the dielectric layer for direct metal deposition or opening of etching layers. The processes are often limited in processing speed with corresponding long cycle times.

On a solar cell with the dimensions of 156x156 mm² approximately 1.1 -10⁸ single spots have to be ablated to generate a high light trapping structure due to a subsequent etching process. Using a common laser setup with a single spot and a scanner the processing time exceeds 10 minutes which is not industrially applicable. For the successful process implementation a fast and damage reduced ablation is necessary. In Figure 1 the ablation behavior for crystalline Silicon shows an optical and thermal regime which causes a damage reduced (free) ablation in the optical regime. With a measured spot diameter of dB=23.4 μι^η the measured transition pulse energy is

μϋ. This pulse energy is much lower than the maximum pulse energy of the laser system. To evaluate the use of DOEs, a comparison of single spot, 3 spot, and 7 spot elements has been done. The pulse energy was kept constant in front of the DOE, which means the pulse energy of each split beam. This means tripled pulse energy for the 3 spot element compared to the single spot. For the 7 spot element the pulse energy was increased by a factor of 7 compared to a single spot. In Figure 5 lines are scribed in crystalline Silicon with a 7 spot DOE. These are the line-shaped parallel recesses mentioned earlier. The measured distance between two spots/lines a=692 pm. The distance between the lines is given by the separation angle of the DOE and the focal length of the scanner lens. Here, the distance can be determined to a=692 pm. This line gap indicates that adjoining lines do not influence each other.

When DOEs are used, losses occur depending on the structure and number of spots. In Figure 6 the comparison of the single spot, 3 spot DOE and 7 spot DOE shows that the average depth of the scribed lines aa from Figure 5 varies. The 3 spot element shows the same average depth which means the efficiency is high and losses can be neglected. The line depth for the 7 spot element is much lower which means the efficiency is decreased and can be given with approximately 70%. This has to be taken into account when using multi-spot DOEs. Generally, regarding each scribed line, i.e. each line-shaped recess, the same ablation behavior can be found for single spot and multi-spot elements. Beside this, the efficiency is constant for different pulse energies as shown in Figure 6.

To compare and analyze the ablation behavior of each single spot, parallel lines, i.e. parallel line-shaped recesses, were scribed and measured. The normalized values for different pulse energies are shown in Figure 7.

The line-shaped recesses show that there is a tendency to higher ablation for the centered laser spots of the DOE corresponding to higher intensities within these spots. By using pulse energies in the optical regime a more symmetrical ablation can be obtained as shown for the pulse energy of

This example shows that DOEs are efficient tools for parallel processing where low energies in each single spot are required. For fast laser processing of solar cells it can be seen how efficient DOEs can be implemented. As described before, for opening the AR coating, 1 -10⁸ spots have to be processed on a complete solar cell. It was obtained that a pulse energy of Ep.ispo 0.67 pJ and spot diameter of ds=23.4 pm leads to a damage free and high quality successful ablation of the SiN_x-layer. As shown in Figure 8, ablated spots before and after etching using CP4 are generating a light trapping surface.

The spots are scribed with a feed rate of v_f=1200 mm/s limited by the scanner performance and a repetition rate of fp=40 kHz to generate spots with a distance about 15 μιτι. These process parameters are leading to a laser processing time of approximately 22 minutes for a 156x156 mm² solar cell. This area throughput of about aA=18. mm²/s is much to low from an industrial point of view.

Commercial DOEs are available with 81 spots in one line. The laser system delivers pulse energy of approximately 70 pJ at 400 kHz. By calculating the single ablation with a pulse energy of E_{P-1 S}pot⁼0.67 μϋ and an efficiency of a DOE of approximately 75%, 81 spots can be generated parallel resulting in an overall pulse energy of the laser of Ep.8i_Spots⁼72 pJ. Using the 81 spot DOE the processing time theoretically can be reduced from 22 minutes to 16.3 seconds, at least theoretically. Furthermore, a factor of 3 is possible for scanning speed optimization due to available repetition rates. The combination of maximum spot numbers and optimized scanning speed leads to a calculated processing time of 5.4 seconds and an area throughput of about aA=4400 mm²/s for a 156x156 mm² solar cell. The limiting factor is the maximum scribing speed of common laser scanning systems. In combination the use of a DOE, the flexibility of a laser scanner and a high power high repetition rate laser source the processing time as well as the process efficiency can be increased by orders of magnitude, which means by a factor of 100 and above in the near future.

EXAMPLE 2: RIBLETS ON COMPRESSOR BLADES

The second example of multi-spot machining is the laser-based generation of extensive Riblet-structures on compressor blades. Riblets are channel- structures aligned in stream wise direction on passed surfaces. Such structures have proven to reduce skin friction and wall shear stresses in turbulent flow up to 10% compared to smooth surfaces, for instance on air planes and wind energy plants [13, 14]. The aimed geometry and dimensions, shown in Figure 9, were calculated by project partners of the applicant (Institute of Turbomachinery and Fluid Dynamics, TFD, Hannover, German Aerospace Center, DLR, Berlin). Figure 9 shows calculated ideal riblet geometry and dimensions as well as compressor blade profile NACA 6510 [13].

The flow conditions were simulated for the chosen profile, a NACA 6510 compressor profile made of chromium steel X20Cr13 with a chord length of 90 mm and 120 mm width. Figure 10 shows the ablation behavior and resulting rates for X20Cr13 as well as characteristic Riblet quality.

The exemplary Riblet-structures in Figure 10 have been realized using a single focused laser beam by scanning the surface line by line. The period of the Riblet-structures is simply defined by the distance between the lines, which is in the order of the spot diameter.

The SEM picture Figure 10.B shows the result for a fluence H_P clearly above the transition fluence. For this fluence the required ablation depth of 20 μιη could be achieved by an area throughput of about aA=0.1 mm²/s. The influence of the high fluence is in particular observable by the completely molten material of the Riblet tips. Such structures have very low stability and will be rapidly destroyed under high thermal or mechanical stress. The Riblets in Figure 10.A have been generated using a fluence H_P in the range of transition between the two regimes. Here the required ablation depth of 20 pm could be achieved with a resulting area throughput of nearly 0.07 mm²/s. Thermal damages like melting phases along the irradiated area, especially on the Riblet tips, have been avoided.

The scribing speed of about 200 mm/s was limited by the 50 kHz maximum repetition rate to achieve a pulse overlap of about 90%. The achieved area throughput between 0.07 mm²/s and 0.1 mm²/s for a single spot process resulting in a processing time of about 30 h to 40 h to complete the NACA 6510 profile. This is at least one order of magnitude to slow for industrial demands. Here, the target value for the upper side (suction side) of a NACA 6510 is about 1 h, which means an area throughput of a_A=3 mm²/s.

To fulfill these industrial demands, the usage of multi-spot machining has been successfully adapted for the generation of Riblet-surfaces. Initially, the 3 spot element, given in Figure 3, was used. The pulse energy for single spot ablation below the transition fluence, in this case E_P-i_spot=12.5 p^j, has been tripled to a value of about E_P-3_Sp₀ts=37.5 pJ. Three parallel lines instead of one single line were scribed, analog to Figure 5. The area throughput has been increased by the factor of 3, to approximately 0.21 mm²/s.

Current picosecond lasers deliver repetition rates up to 1 MHz and pulse energies of more than 1.0 mJ. On the market DOEs with up to 81 spots (1x81) or 225 spots (15x15), respectively, are available. By calculating the single ablation with a necessary pulse energy of about E_{P-1 spo}t=12.5 pJ and an efficiency of the DOE of approximately 75%, 81 spots can be generated parallel, leading to pulse energy of

-35 mJ. 225 spots require pulse energy of

mJ, which is entirely conceivable for future ultra-short pulse laser sources. Beside the number of spots for increased area throughput, a factor of 5 to 20 is possible for scanning speed optimization due to available repetition rates. Here, the limiting factor is the maximum scribing speed of common laser scanning systems. This allow preliminary and theoretically but realistic estimations of achievable area throughput of about 5.0 mm²/s and above, which surpasses the mentioned industrial needs.

Figure 4a and 4b each show a system according to an embodiment of the present invention. This setup ensures a maximum in flexibility and high scanning velocities. The DOE is in both cases placed directly in front of the scanner as a fixed element meaning in front of the moving mirrors. The distance between the spots behind the DOE is given by the separation angle of the DOE and the focal length f.

The DOE (1) shown in Figure 4a and Figure 4b is placed directly in front of the laser scanner (2) in the incident raw laser beam (3) of a laser source (4). The incident laser beam is separated into an amount of split beams (5) under a separation angle (6) which is given by the diffractive grating of the DOE. The distance (7) between the laser spots (8) is a result of the separation angle and the focal length (9) of the focusing lens (10). The separated beams are reflected by a first mirror (11) onto a second mirror (12) which are responsible for X- and Y movement of the laser spots (8).

The described devise offer the advantage of the beam splitting technology which guarantees that a adjusted pulse energy in each spot leading to an optimized processing result while the whole available laser output power of the laser is used. Due to this effect the described timely parallel process is possible as in each spot a timely parallel process is performed. Supplementary examples show a combination of DOEs and refractive optical elements or a combination of diffractive optical elements to adapt the laser spot arrangement for an optimized parallel processing. Examples of combined optical elements in front of a laser scanner are shown in Figure 11 , where the resulted beam intensities are shown. Figure 11 shows exemplary intensity profiles (raw beam, 1 D- and 2D-DOEs, combined DOEs as well combination of DOE and cylindrical lens)

A further option to use a DOE in front of a laser scanner to gain a maximum in flexibility uses a rotary optical element (1a) which can be turned in dependence of the vector of the laser scanner, which is shown schematically in Figure 12. Figure 12 shows a pivotable DOE in front of the scanner which allows to vary the distance between the laser spots and the line-shapes parallel recesses, resulting from scanning in a scanning direction, in a direction perpendicular to a scanning direction in dependence of the angle a=f((p).

Current laser scanners are already equipped with an output of exact position and the vector of the laser beam in the scanning field. This can be used to turn the optical element or guide the laser beam. Further, with the rotary optical element the distance between the laser spots can be adapted between nearly zero (7a) and the maximum (7) given by the separation angle. The invented device provides a high flexible and fast laser processing tool for different applications and materials.

The system and method according to the present invention allow to reduce processing time and an efficient use of expensive laser sources and the available output power. In particular, the system and method for multi spot processing enables the exploitation of the full laser power.

With the process and the system a timely parallel process is possible whereby each spot allows a timely parallel process since each spots have the same properties enabling the same processing step. At the same time the system is easy and fast to prepare and to handle while being flexible and effective.

References

[1] Ali, M.; Wagner, T.; Shakoor, M.; Molian, P. A.: Review of laser nanomachining; In: Journal of Laser Applications, Vol. 20, Nr. 3, 169-184, 2008

[2] Weiler, S.: Scheibenlaser fiir die Industrie; In: Laser und Photonik,

Ausgabe 5 2009, 62-65, 2009

[3] Klimt, B.: Picosecond lasers: the power of cold ablation; In:

Technology tutorial, optics and laser Europe, OLE July/August

2009, 19-21 , 2009

[4] Raciukaitis, G.; et al.: Use of high repetition rate and high power lasers in microfabrication: How to keep the efficiency high?; In: Journal of Laser Micro / Nanoengineering JLMN, 186-1191 , Received: July 4th, 2008, Accepted: November 17th, 2009

[5] Haupt, O.; et al.: Improved Laser Edge Isolation Of Crystalline Solar Cells Using A High Power Picosecond Laser; In: Proc. of ICALEO, M903, 2009

[6] Siegel, F.; Klug, U.; Kling, R.: Extensive Micro-Structuring of Metals using Picosecond Pulses - Ablation Behavior and Industrial Relevance; In: Journal of Laser Micro / Nanoengineering JLMN, 104-110, Received: June 16th, 2008, Accepted: May 28th, 2009

[7] Golub, M. A.: Laser Beam Splitting - By diffractive optics; In: Optics and Photonics News, Optical society of America, 36-41 , 2004

[8] Chall, P.: ALSI^'s Low Power Multiple Beam Technology for High Throughput and Low Damage Wafer Dicing; In: Proc. of 65^th Laser Material Conference, 211-215, 2006

[9] Zimmermann, M.; et al.: Refractive Micro-optics for Multi-spot and Multi-line Generation; In: Proc. of LPM, 2008

[10] Ciofini, M.; Lapucci, A.; Lolli, S.: Diffractive optical components for high power laser beam sampling; In: J. Opt. A, Pure Opt. 5, 186- 191 , 2003

[11] Liu, D.; et al: Ultrafast parallel laser processing of materials for high throughput manufacturing; In: Proc. of LAMP, 2009

[12] Brinkmann, M.; et al.: Diffraktive Optische Elemente erobern den Markt; In: Optik und Photonik, April 2009

[13] Oehlert, K.; Seume, J. R.: Exploratory experiments of machined riblets on compressor blades; In: Proceedings of FEDSM2006 - 2006 ASME Joint U.S. - European Fluids Engineering Summer Meeting, July 17-20, Miami, FL; FEDSM 2006 - 98093, 2006

[14] Seume, J. R.; et al: Exploratory Experiments on Machined Riblets for 2-D Compressor Blades; In: Proceedings of ASME International Mechanical Engineering Congress and Exposition, DVD, Seattle, USA, 2007

[15] Holo/Or Ltd.: www.holoor.com/Diffractive Optics

Publications/Laserperforation.htm

[16] Innolas Systems GmbH, EP 2143518 A1

Claims

1. A system for laser-beam based processing, comprising

a laser source (4) providing a raw laser beam (3), a diffractive optical element (1 )

a laser scanner (2) and

a focussing lens (10)

characterized in that the diffractive optical element (1 ) is placed in front of the laser scanner (2) in the raw beam (3) of the laser source (4) and is capable of dividing the raw beam (3) into a plurality of split beams (5).

2. A system according to claim 1 characterized in that the diffractive optical element (1 ) is a one-dimensional or two- dimensional diffractive optical element.

3. A system according to claim 1 or 2 characterized in that the diffractive optical element (1 ) is mounted in a pivotable manner.

4. A system according to one of the claims 1 to 3 characterized in that the laser scanner (2) is a galvanometer or a polygon scanner.

5. A system according to one of the claims 1 to 4 characterized in that the laser scanner (2) comprises at least two mirrors (1 1 , 12).

6. A system according to one of the claims 1 to 5 characterized in that the focussing lens (10) is part of the laser scanner (2).

7. A system according to one of the claims 1 to 6 characterized in that the laser source provides laser pulses.

8. A system according to claim 7 characterized in that the laser source (4) provides picosecond laser pulses.

9. A system according to claim 8, characterized in that a pulse duration is no longer than 50 ps.

10. A method for a laser-beam based processing in which a system according to one of the claims 1 to 9 is used, comprising the steps of providing a raw laser beam (3) by the laser source (4), splitting the raw laser beam (3) into a plurality of split beams (5) by using the diffractive optical element (1 ), focussing the plurality of split beams (5) using the focussing lens (10) onto a work piece such that an array of laser spots (8) is generated on the work piece.

1 1. A method according to claim 10 characterized in that the array of laser spots (8) is a one-dimensional or two- dimensional array of M x N laser spots (8).

12. A method according to claim 10 or 1 1 characterized in that it further comprises the step of using the laser scanner (2) in order to move the array of laser spots (8) on a surface of the work piece.

13. A method according to claim 12 characterized in that moving the array of laser spots (8) on the surface of the work piece leads to parallel line shaped recesses on the surface of the work piece by ablation of material.

14. A method according to claim 13 characterized in that the distance between two parallel recesses is adapted by rotating the diffractive optical element (1 ).

15. A method according to one of the claims 10 to 14 characterized in that the laser source (4) provides laser pulses.

16. A method according to claim 15 characterized in that a laser fluence H per laser pulse in each laser spot is chosen such that an ablation behaviour of the material of the work piece is in an optical penetration zone.

17. A method according to claim 16 characterized in that the laser fluence H is smaller than 4 J/cm².

18. A laser scanner assembly comprising a support for a diffractive optical element (1 ) and a laser scanner (2) which comprises at least one mirror (1 1 , 12), wherein the diffractive optical element (1 ) is capable of splitting an incident raw laser beam (3) into an array of M x N split beams (5), each of which is separated from an adjacent split beam (5) by a separation angel (a) characterized in that a distance between the holder for the diffractive optical element (1 ) and an input aperture of the laser scanner (2) is chosen such, that every split beam (5) passes the input aperture.

19. A laser scanner assembly according to claim 18, characterized in that a distance between the holder for the diffractive optical element (1 ) and the at least one mirror (1 1 , 12) is chosen such, that every split beam (5) that passed the input aperture is reflected by the at least one mirror (1 1 , 12).

20. A laser scanner assembly according to claim 18 or 19, characterized in that the diffractive optical element is mountable in a pivotable manner in the holder.

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