rus
Issue #5/2015
T.Kuntz, T.Roch, T.Gofman, H.Fedina, V.konovalov, D.Ulyanov, A.Lazagni
Precision Laser Interference Engraving with The Help of High-energy Q-switched Lasers
Precision Laser Interference Engraving with The Help of High-energy Q-switched Lasers
Direct precision laser interference engraving permits to create periodic circuits on the surface of different materials during a single material operation. The method can be used to produce tailor made surfaces and safety sytems protecting against counterfeit products.
S
urfaces with controlled topographic characteristics have shown in the past to provide enhanced surface properties in comparison to surfaces with a "random" roughness. Several examples of surfaces with an ordered topography (e. g. periodic surface structure) can be found on the surfaces of different plants and animals, resulting from several thousand years of evolution [1]. In this way, nature has shown to be the best technologist to overcome any survival challenge.
The field of bio-inspiration is emerging as one of the most innovative areas of science today. In this frame, laser based technologies can provide the required technological and economical aspects to reproduce such surfaces [2, 3].[2]
An innovative solution for high resolution surface patterning of periodic structures in a one step process is Direct Laser Interference Patterning (DLIP) [4, 5]. This process is based on creation of interference patterns by overlapping two or more coherent high power pulsed laser beams on a surface of a material. If laser intensity of interference pattern nodes exceeds ablation threshold the surface of the material gets modified. That way sub-micron periodic features could be created on surfaces of varies materials which could be potentially used in industrial, biophysical and medical applications. As an example DLIP could be successfully employed for product counterfeit protection. High resolution DLIP allows to apply complex security hologram-like elements on various technological surfaces which are extremely difficult to duplicate.
A basic optical diagram of DLIP experimental setup is shown in Fig.1. The laser beam exiting from the laser system aperture passes through a focusing lens and a mask for beam shaping. Depending on the intended structure geometry on the material, the resulting beam is split into two or more sub-beams by beam splitting optics (Fig. 1 illustrates a 2-beam setup). The following arrangement of mirrors merges the split sub-beams at a specific incident angle on the material surface which consequences the interference effect [4]. Depending on the incident angle and the laser beam wavelength, a periodic intensity pattern of energy maxima and minima follows. Fig. 2a and Fig. 2b illustrate the resulting laser intensity pattern for a two – and three-beam setup, respectively. The dimensionality of the resulting pattern is determined by the number N of used laser beams. A (N-1) – dimensional pattern follows by interference of N ≤ 4. Thus, a high level of degrees of freedom with respect to the shape and dimension of the structure follows which only depend on the specific DLIP setup.
Due to the relatively simple structure of the DLIP setup, a wide range of structural types (line-, pillar – and hole-like pattern) as well as periods (material-dependent; 150 nm to 30 µm) can be realized. The advantage of DLIP is that a homogeneous structuring of the material takes place over the entire surface covered by the interfering laser beams. Hence, depending on the laser pulse energy, several cm 2 can be structured within a single process step when compared to direct laser writing. If the structurable area extends the laser spot size, displacement units can be employed to scan the material to be patterned. Such displacement units are, for example, specific axis systems (x, y, z-direction).
The DLIP technology is very flexible with respect to the employed laser light wavelength where different wavelengths from IR down to UV are available for an ablative modification of surfaces. A basic requirement for DLIP is a sufficiently high coherence length of the beam source as well as the absorption of the laser light through the material. In principle, the DLIP technology can process almost all materials which absorb laser light with a wavelength between deep UV (~ 0.26 µm) and IR (~ 1 µm). The material structuring largely dependent on the laser pulse duration, the employed laser wavelength and the specific material properties (such as absorption depth and thermal diffusivity) where the ablation processes can be either photochemical, photothermal or photophysical. DLIP process has been tested on many different materials such as stainless steel grades (304, 304L, 316, 316L), titanium alloys, aluminum alloys, hard metals, nickel and chrome materials, other steel materials such as 100Cr6 and non-metallic materials including ceramics, glass, sapphire and various types of polymers (PET, PEEK, PC, PP, PS).
The DLIP technology also enables a 3D machining of surfaces due to the volumetric extend of the interference pattern. As a single-process method, DLIP enables the production of high resolution and sufficiently complex security features for product protection, which can be implemented without an extensive pre – and post-treatment of the work pieces and without the need of clean room conditions. As a result, DLIP can be easily integrated in existing production lines.
Being a contactless processing method DLIP allows for an optimal process control since the material modification occurs at the material surface and without an introduction of additional material. The energy input by the laser radiation occurs locally at the material surface and typically shows a negligible influence on the underlying solid-state or substrate material. Furthermore, the non-contact machining of DLIP even allows material processing under specific conditions (such as underwater). On the other hand, industrial application of the DLIP technology will only be possible if compact optical-head solutions are available and the applicability of the technology is demonstrated in an operational environment. In the last years, three different interference patterning optical concepts have been developed at Fraunhofer IWS (Germany) [6]. These optical heads offer the possibility not only to process planar surfaces but also complex three dimensional parts.
One of these possibilities, namely 4th Generation of DLIP optical heads, was developed to achieve high resolution and flexibility. This optical heads are equipped with mobile components that permit to control the intercepting angle between two laser beams fully automatically. The principle of operation of this DLIP-head can be described as follows. Firstly, all laser beams required to obtain the interference pattern (2, 3 or 4) are focused on the substrate obtaining a circular pixel with a diameter ranging from 25 μm to 300 μm, corresponding to resolutions of 1016 and 85 DPI (Dots Per Inch), respectively [4]. Within such a pixel, the interference pattern intensity is transferred in the materials surface. If the spatial pattern period of the pixels is varied, different functionalities can be obtained (e. g. different optical color under a specific observation angle). This principle is shown in Fig. 3 on a structured PET foil processed with TECH-263 (λ=263 nm) nanosecond pulsed TECHNOLOGY-series laser by LASER-COMPACT (Russia) (pulse energy 50 µJ). In this example, three different structure periods have been utilized to treat the polymer surface. Fig. 4 shows another example, a structured Nickel plate. It was processed by 4th Generation DLIP system with TECH-1053 (λ=1053 nm) of TECHNOLOGY-series laser.
The examples depicted in Fig. 3 and 4 require not only high resolution translational stages but also a software solution permitting the fully automatized processing of the material’s surface. In addition, the maturity of the DLIP technology can only be demonstrated not only by developing optical heads, but also a prototype-system which capable to operate in an operational environment. Such a prototype (DLIP-μFAB) including a 4th generation DLIP optical head, IR or UV laser systems, translational stages and automatized software is shown in Fig. 5. This system has been already tested over 500 h under ambient conditions (no clean room, no protective gases, non-controlled room temperature). The software platform, also developed at IWS, permits to automatically control the laser system, the optical head as well as the translational stages (Fig. 5b). The system has been classified as Laser Class 1 based on German laser safety regulations (by the Technischer Ьberwachungsverein, TЬV).
High resolution DLIP technology imposes stringent requirements on the laser used. First of all the pulse energy should be high enough to exceed material ablation threshold over the whole beam spot, typically 100—500 µJ at 1 µm and 10—50 µJ at 0.26 µm. Then beam quality has to be perfect, essentially a TEM00 mode beam is required. Coherence length of longer than 0.5 cm at 1 µm (0.15 cm at 0.26 µm) is needed to get good interference contrast. In addition, in order to achieve high process throughput the laser pulse repetition rate needs to be sufficiently high (1–5 kHz). Good pulse-to-pulse stability is also important because it has direct impact on pattern quality. Laser head dimensions should be kept small in order to fit into the system’s optical head. Last, but not the least, laser cost of operation must be relatively low, which means low purchase price and high reliability.
Lasers of TECHNOLOGY series by LASER-COMPACT perfectly fit the above criteria. These lasers are based on actively Q-switched DPSS laser technology and second, third and forth harmonics generation. TECHNOLOGY-series lasers are known for their rugged design, high stability and reliability. Compact and powerful these lasers are new alternatives to the large, complicated and expensive high-power lasers. Fig. 6. shows a head of TECHNOLOGY laser.
Conclusions
The results reported here demonstrate the high level of maturity of the DLIP technology. If high resolution is required (e. g. holographic pixels with more than 1000 DPI resolution), compact solutions permitting to control and change patterns period automatically are available. These optical heads can be used with high energy solid state lasers as the Lasers of TECHNOLOGY series by LASER-COMPACT which provide enough coherence length to obtain interference patterns.
Acknowledgement
A.Lasagni and T.Roch acknowledges the Bundesministerium fьr Bildung und Forschung (BMBF) and the German Research Foundation (DFG) for financial support (Verbundfцrderprojekt "Laser Interference High Speed Surface Functionalization" (FKZ 13N13113) and Schwerpunktprogramm "Trockenumformen-Nachhaltige Production durch Trockenbearbeitung in der Umformtechnik" (SPP 1676)). This work was also partially supported by the Fraunhofer-Gesellschaft under Grant No. Attract 692174.
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[1]1 Фраунгоферовский институт материалов и лучевых технологий IWS, Винтербергштрассе 28, 01277 Дрезден, Германия.
2 Институт технологий машиностроения, Дрезденский Технический университет, Георг-Баер-штрассе.1, 01069 Дрезден, Германия
3 ЛАЗЕР-КОМПАКТ, ООО "Лазер-экспорт", ул. Введенского 3, Москва, 117342, Россия
* ulyanov@laser-export.com
[2]1 Fraunhofer Institute for Material and Beam Technology IWS, Winterbergstrasse 28, 01277 Dresden, Germany.
2 Institute for Manufacturing Technology, Technical University of Dresden, George-Baehr-Str.1, 01069 Dresden, Germany.
3 LASER-COMPACT, Laser-export Co. Ltd., Vvedenskogo St. 3, Moscow, 117342, Russia.
*Corresponding author, ulyanov@laser-export.com.
urfaces with controlled topographic characteristics have shown in the past to provide enhanced surface properties in comparison to surfaces with a "random" roughness. Several examples of surfaces with an ordered topography (e. g. periodic surface structure) can be found on the surfaces of different plants and animals, resulting from several thousand years of evolution [1]. In this way, nature has shown to be the best technologist to overcome any survival challenge.
The field of bio-inspiration is emerging as one of the most innovative areas of science today. In this frame, laser based technologies can provide the required technological and economical aspects to reproduce such surfaces [2, 3].[2]
An innovative solution for high resolution surface patterning of periodic structures in a one step process is Direct Laser Interference Patterning (DLIP) [4, 5]. This process is based on creation of interference patterns by overlapping two or more coherent high power pulsed laser beams on a surface of a material. If laser intensity of interference pattern nodes exceeds ablation threshold the surface of the material gets modified. That way sub-micron periodic features could be created on surfaces of varies materials which could be potentially used in industrial, biophysical and medical applications. As an example DLIP could be successfully employed for product counterfeit protection. High resolution DLIP allows to apply complex security hologram-like elements on various technological surfaces which are extremely difficult to duplicate.
A basic optical diagram of DLIP experimental setup is shown in Fig.1. The laser beam exiting from the laser system aperture passes through a focusing lens and a mask for beam shaping. Depending on the intended structure geometry on the material, the resulting beam is split into two or more sub-beams by beam splitting optics (Fig. 1 illustrates a 2-beam setup). The following arrangement of mirrors merges the split sub-beams at a specific incident angle on the material surface which consequences the interference effect [4]. Depending on the incident angle and the laser beam wavelength, a periodic intensity pattern of energy maxima and minima follows. Fig. 2a and Fig. 2b illustrate the resulting laser intensity pattern for a two – and three-beam setup, respectively. The dimensionality of the resulting pattern is determined by the number N of used laser beams. A (N-1) – dimensional pattern follows by interference of N ≤ 4. Thus, a high level of degrees of freedom with respect to the shape and dimension of the structure follows which only depend on the specific DLIP setup.
Due to the relatively simple structure of the DLIP setup, a wide range of structural types (line-, pillar – and hole-like pattern) as well as periods (material-dependent; 150 nm to 30 µm) can be realized. The advantage of DLIP is that a homogeneous structuring of the material takes place over the entire surface covered by the interfering laser beams. Hence, depending on the laser pulse energy, several cm 2 can be structured within a single process step when compared to direct laser writing. If the structurable area extends the laser spot size, displacement units can be employed to scan the material to be patterned. Such displacement units are, for example, specific axis systems (x, y, z-direction).
The DLIP technology is very flexible with respect to the employed laser light wavelength where different wavelengths from IR down to UV are available for an ablative modification of surfaces. A basic requirement for DLIP is a sufficiently high coherence length of the beam source as well as the absorption of the laser light through the material. In principle, the DLIP technology can process almost all materials which absorb laser light with a wavelength between deep UV (~ 0.26 µm) and IR (~ 1 µm). The material structuring largely dependent on the laser pulse duration, the employed laser wavelength and the specific material properties (such as absorption depth and thermal diffusivity) where the ablation processes can be either photochemical, photothermal or photophysical. DLIP process has been tested on many different materials such as stainless steel grades (304, 304L, 316, 316L), titanium alloys, aluminum alloys, hard metals, nickel and chrome materials, other steel materials such as 100Cr6 and non-metallic materials including ceramics, glass, sapphire and various types of polymers (PET, PEEK, PC, PP, PS).
The DLIP technology also enables a 3D machining of surfaces due to the volumetric extend of the interference pattern. As a single-process method, DLIP enables the production of high resolution and sufficiently complex security features for product protection, which can be implemented without an extensive pre – and post-treatment of the work pieces and without the need of clean room conditions. As a result, DLIP can be easily integrated in existing production lines.
Being a contactless processing method DLIP allows for an optimal process control since the material modification occurs at the material surface and without an introduction of additional material. The energy input by the laser radiation occurs locally at the material surface and typically shows a negligible influence on the underlying solid-state or substrate material. Furthermore, the non-contact machining of DLIP even allows material processing under specific conditions (such as underwater). On the other hand, industrial application of the DLIP technology will only be possible if compact optical-head solutions are available and the applicability of the technology is demonstrated in an operational environment. In the last years, three different interference patterning optical concepts have been developed at Fraunhofer IWS (Germany) [6]. These optical heads offer the possibility not only to process planar surfaces but also complex three dimensional parts.
One of these possibilities, namely 4th Generation of DLIP optical heads, was developed to achieve high resolution and flexibility. This optical heads are equipped with mobile components that permit to control the intercepting angle between two laser beams fully automatically. The principle of operation of this DLIP-head can be described as follows. Firstly, all laser beams required to obtain the interference pattern (2, 3 or 4) are focused on the substrate obtaining a circular pixel with a diameter ranging from 25 μm to 300 μm, corresponding to resolutions of 1016 and 85 DPI (Dots Per Inch), respectively [4]. Within such a pixel, the interference pattern intensity is transferred in the materials surface. If the spatial pattern period of the pixels is varied, different functionalities can be obtained (e. g. different optical color under a specific observation angle). This principle is shown in Fig. 3 on a structured PET foil processed with TECH-263 (λ=263 nm) nanosecond pulsed TECHNOLOGY-series laser by LASER-COMPACT (Russia) (pulse energy 50 µJ). In this example, three different structure periods have been utilized to treat the polymer surface. Fig. 4 shows another example, a structured Nickel plate. It was processed by 4th Generation DLIP system with TECH-1053 (λ=1053 nm) of TECHNOLOGY-series laser.
The examples depicted in Fig. 3 and 4 require not only high resolution translational stages but also a software solution permitting the fully automatized processing of the material’s surface. In addition, the maturity of the DLIP technology can only be demonstrated not only by developing optical heads, but also a prototype-system which capable to operate in an operational environment. Such a prototype (DLIP-μFAB) including a 4th generation DLIP optical head, IR or UV laser systems, translational stages and automatized software is shown in Fig. 5. This system has been already tested over 500 h under ambient conditions (no clean room, no protective gases, non-controlled room temperature). The software platform, also developed at IWS, permits to automatically control the laser system, the optical head as well as the translational stages (Fig. 5b). The system has been classified as Laser Class 1 based on German laser safety regulations (by the Technischer Ьberwachungsverein, TЬV).
High resolution DLIP technology imposes stringent requirements on the laser used. First of all the pulse energy should be high enough to exceed material ablation threshold over the whole beam spot, typically 100—500 µJ at 1 µm and 10—50 µJ at 0.26 µm. Then beam quality has to be perfect, essentially a TEM00 mode beam is required. Coherence length of longer than 0.5 cm at 1 µm (0.15 cm at 0.26 µm) is needed to get good interference contrast. In addition, in order to achieve high process throughput the laser pulse repetition rate needs to be sufficiently high (1–5 kHz). Good pulse-to-pulse stability is also important because it has direct impact on pattern quality. Laser head dimensions should be kept small in order to fit into the system’s optical head. Last, but not the least, laser cost of operation must be relatively low, which means low purchase price and high reliability.
Lasers of TECHNOLOGY series by LASER-COMPACT perfectly fit the above criteria. These lasers are based on actively Q-switched DPSS laser technology and second, third and forth harmonics generation. TECHNOLOGY-series lasers are known for their rugged design, high stability and reliability. Compact and powerful these lasers are new alternatives to the large, complicated and expensive high-power lasers. Fig. 6. shows a head of TECHNOLOGY laser.
Conclusions
The results reported here demonstrate the high level of maturity of the DLIP technology. If high resolution is required (e. g. holographic pixels with more than 1000 DPI resolution), compact solutions permitting to control and change patterns period automatically are available. These optical heads can be used with high energy solid state lasers as the Lasers of TECHNOLOGY series by LASER-COMPACT which provide enough coherence length to obtain interference patterns.
Acknowledgement
A.Lasagni and T.Roch acknowledges the Bundesministerium fьr Bildung und Forschung (BMBF) and the German Research Foundation (DFG) for financial support (Verbundfцrderprojekt "Laser Interference High Speed Surface Functionalization" (FKZ 13N13113) and Schwerpunktprogramm "Trockenumformen-Nachhaltige Production durch Trockenbearbeitung in der Umformtechnik" (SPP 1676)). This work was also partially supported by the Fraunhofer-Gesellschaft under Grant No. Attract 692174.
--------------------------------------------------------------------------------
[1]1 Фраунгоферовский институт материалов и лучевых технологий IWS, Винтербергштрассе 28, 01277 Дрезден, Германия.
2 Институт технологий машиностроения, Дрезденский Технический университет, Георг-Баер-штрассе.1, 01069 Дрезден, Германия
3 ЛАЗЕР-КОМПАКТ, ООО "Лазер-экспорт", ул. Введенского 3, Москва, 117342, Россия
* ulyanov@laser-export.com
[2]1 Fraunhofer Institute for Material and Beam Technology IWS, Winterbergstrasse 28, 01277 Dresden, Germany.
2 Institute for Manufacturing Technology, Technical University of Dresden, George-Baehr-Str.1, 01069 Dresden, Germany.
3 LASER-COMPACT, Laser-export Co. Ltd., Vvedenskogo St. 3, Moscow, 117342, Russia.
*Corresponding author, ulyanov@laser-export.com.
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