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Manufacturing of 3D structures for micro-tools using laser ablation
Microelectronic Engineering 57–58 (2001) 775–780 www.elsevier.com / locate / mee
Manufacturing of 3D structures for micro-tools using laser ablation Peter Heyl*, Thomas Olschewski, Roelof W. Wijnaendts Heidelberg Instruments Mikrotechnik GmbH, Tullastr. 2, D-69126, Heidelberg, Germany Abstract A high precision laser ablation machine for 3D structuring has been developed. Laser processing of hard metals and ceramics with UV light (355 nm) was studied and optimised with respect to surface quality. It could be shown that the surface smoothness of the structures produced by laser ablation in metals is equal to that produced by high quality electric discharge milling (EDM). Furthermore, laser ablation is not only restricted to metals. 2001 Elsevier Science B.V. All rights reserved. Keywords: Laser ablation; 3D Structuring; Embossing; Micro structuring
1. Introduction During the last years, especially in the electronic production areas, an increasing tendency for miniaturisation of the structure size has been observed. This is known as micro system technique. Now, production techniques need to be developed which are suitable to follow the requirements of the micro system technique. Traditional manufacturing methods have reached their end. Since the production rate of micro tools is not crucial, laser manufacturing tools can be used. Laser ablation shows clear advantages versus the traditional milling or electric discharge milling(EDM) tools [1]: • • • •
unlimited choice of materials; direct usage of CAD structure data; high geometric flexibility; touchless tool.
HEIDELBERG INSTRUMENTS took part in the PROMPT project (Produktionstechnik zum ¨ Mikropragen und Mikrostanzen metallischer Bauteile) [2] in response to interest from the German ¨ Bildung und Forschung and developed a laser ablation machine. This machine Bundesministerium fur is now known under the brand name LAM 66 and is used for the manufacturing of micro embossing and micro stamping tools. * Corresponding author. Tel.: 149-62-2134-3043 / 60; fax: 149-62-2134-3030. E-mail addresses: [email protected] (P. Heyl), [email protected] (T. Olschewski). 0167-9317 / 01 / $ – see front matter PII: S0167-9317( 01 )00485-3
2001 Elsevier Science B.V. All rights reserved.
776
P. Heyl et al. / Microelectronic Engineering 57 – 58 (2001) 775 – 780
The LAM 66 precisely manufactures microstructures with a minimum feature size of 10 mm, and an accuracy of 1 mm in a working volume of 100 3 100 3 5 mm 3 (xyz).
2. The laser ablation machine The concept of the machine is as follows: the sample is fixed on a motor driven stage which itself is moved under a fixed laser axis. This, in a first approach, allows two-dimensional machining of the sample. Repeating this with different 2D data, a 3D structure will result. This procedure, however, does not allow undercutting of the structures. This limits the possibility of the true 3D machining to 2 ]12 D machining, which is ideal for the production of micro embossing and micro punching tools [2,3]. The sample is fixed on a linear motor driven stage. The positioning is done via laser interferometry in both the x- and y-axis. The chuck may be slightly rotated around the z-axis and inclined along the x- and y-axis for alignment purposes. During machining, the sample is exclusively moved in the xand y-directions. Thus, the precision of the laser interferometer system of 40 nm may be used to its maximum. The machining is performed line by line. The basic design is shown in Fig. 1.
2.1. Laser with external shutter The machine is equipped with an Nd:YAG laser. For machining, the third harmonic frequency at 354.7 nm is used. Pulse energy, as well as repetition rate, may be chosen in advance. During machining, these parameters remain constant. A pre-defined local working raster is created by the delay of the laser pulses and the constant stage velocity. In order to avoid the typical thermal instability of the resonator when turning on the q-switched laser (thermal lensing effect [2]), the laser is operated in the continuous mode. The pulses are switched by means of an external shutter. Therefore, an acousto-optical modulator is positioned in the optical path. The first order of diffraction is used as the working beam, and the non-deviated zero
Fig. 1. LAM schematic design.
P. Heyl et al. / Microelectronic Engineering 57 – 58 (2001) 775 – 780
777
order of the laser is blocked by a ceramic device. The diffraction efficiency of the modulator is about 80%, which is sufficient at high pulse energies.
2.2. Write head adjustable in z-direction The machine was designed with an adjustable write head with a 40 mm objective. A stepper motor moves the laser focus in steps of 1.2 mm in the z-direction. The focal length and aperture of the objective are designed in such a way that one obtains a 5 mm focus diameter at a six times broadening of the initial laser beam. This is small enough for production of micro embossing tools. The focal diameter is 30 mm without broadening the initial laser beam.
2.3. Correction mirror Deviations from the foreseen trajectory of the stage during movement may occur due to vibrations, thus resulting in positioning errors. In order to be able to correct these deviations, it is necessary to move the focus independently from the stage in a range of 40 mm. This is done with an adjustable, piezo-driven correction mirror, which can be tilted along two axes. The tilting of the beam in front of the lens results in a translation of the beam focus on the sample, proportional to the tilt angle. Fig. 2 visualises this effect.
2.4. Control electronics The control electronics consist of a stage controlling unit and a position control and correction unit. The latter also handles the data output. The area of the workpiece is split up into pixels where each pixel equals a laser pulse. Furthermore, the control unit combines the stage movement with the laser frequency, so that at each pixel position a laser pulse, as well as the necessary data are available. This means that, according to the working raster and laser frequency, the stage speed has to be chosen.
Fig. 2. Effect of correction mirror: A tilted beam in front of the writing lense results in a shifted beam on the substrate.
778
P. Heyl et al. / Microelectronic Engineering 57 – 58 (2001) 775 – 780
Finally, the flow of data and laser pulses must be controlled during machining. In parallel, the positioning error must be measured in the x-direction and be corrected.
2.5. Image processing The machine is equipped with a two camera system with an optical path which passes through the working objective. Using the cameras together with an image processing unit, the position of the workpiece can be determined in the co-ordinate system of the machine.
2.6. Data interface In order to obtain flexible manufacturing of 3D microstructures, a specific data interface was developed that makes it possible to directly transform computer designed 3D models (e.g. AutoCAD) into 3D microstructures. Depending on the ablation depth per layer, the 3D model is split up into 2D structure data by layers. This structure data is then converted to 2D machine readable data.
2.7. 3 D rastering Laser ablation is done layer by layer as the sample is moved on the xy-table under the fixed laser spot. The machining is performed in a raster procedure. The work area is divided into parallel lines which are worked successively. Each line in the y-direction is written at a constant x-position. At the end of a y-line, the stage is moved in the x-direction to the next line. Each line is subdivided into pixels. The laser pulse will be shot exactly at the specific moment the stage has the correct pixel position. Overall, the workpiece is divided into several layers, and the layers are worked successively. After a layer is finished, the lens is moved down in z-direction in order to work on the next layer. The z-axis remains fixed during the machining process of a single layer.
3. Results Laser ablation tests were performed on a variety of materials and applications. In addition to experiments with several metals, experience was gained on microstructuring of dielectric materials such as ceramics, diamonds, polyimide and PVC. Since the objective of the PROMT project was to manufacture high quality micro-embossing tools, the work first concentrated on hard metals [2]. In order to avoid strong melt rejections, the initial theoretical focus diameter of 5 mm had to be expanded to 30 mm. In fact, the hole ablated by a single laser pulse still measures about 5 mm in diameter. The intensity, however, is reduced by a factor of 36. In addition, the laser absorption due to plasma formation is reduced [3]. In a limited range above ablation threshold, the ablated volume per pulse grows almost linearly with the laser pulse energy [2,4]. This linearity ends when plasma formation occurs. The surface quality is determined by the fluence (energy per area of a single laser
P. Heyl et al. / Microelectronic Engineering 57 – 58 (2001) 775 – 780
779
Fig. 3. S.E.M.-pictures of example geometries in WC / Co: The quality of the edge (right) can be estimated by comparing the picture to the 20 mm scaling bar. The structures were ablated with a 5-mm X–Y-grid. Ablation depth was approximately 1.3 mm per layer resulting in a total ablation depth of 130 mm.
pulse). In general: the lower the fluence, the better the surface quality. Furthermore, at a given pulse energy and focus size, an appropriate rastering distance must be found. With these optimised settings, an average surface roughness of Ra50.16 mm and an averaged maximum surface deviation of Rz50.7 mm can be reached. Some example geometries in WC / Co (tungsten carbide sinterd in a cobalt matrix) have been viewed with a Scanning Electron Microscope (S.E.M.) and are shown in Figs. 3 and 4. In summary, it could be shown that laser ablation leads to equivalent surface qualities for manufacturing of micro-embossing tools at a higher speed and with more geometrical flexibility compared to the traditional high end electrical discharge milling (EDM) procedures. Furthermore, good quality 2–3 mm steep structures with an angle of 808 are possible in ceramic materials. The higher the ablation per laser pulse, the steeper the wall. This laser ablation technology is ideally used for production of ceramic injection nozzles. In this dimension no other technique leads to equivalent results.
780
P. Heyl et al. / Microelectronic Engineering 57 – 58 (2001) 775 – 780
Fig. 4. S.E.M.-pictures of example geometries in WC / Co. The variety of structures that can be produced by computerdriven laser-ablation is shown. The structures depth is 150 mm.
Other fields of applications could be: injection moulds, moulds for galvanoforming, 2D, 3D laser marking and structuring.
References [1] D. Hellrung, Micro-Structuring by Laser Beam Ablation, Procs. Micro Materials, 1997. [2] Abschlußbericht zum PROMPT-Projekt, BMBF, 1999. [3] P. Heyl, C. Buchner, R. Wijnaendts, Laserablationsmaschine zur Herstellung von Hartmetallwerkzeugen, Procs. Micro Engineering, Stuttgart, 1998. [4] P. Heyl, Mirkostrukturierung von Hartmetall mit Nanosekunden-Laserpulsen, Dissertation, Heidelberg (in press).
Manufacturing of 3D structures for micro-tools using laser ablation Peter Heyl*, Thomas Olschewski, Roelof W. Wijnaendts Heidelberg Instruments Mikrotechnik GmbH, Tullastr. 2, D-69126, Heidelberg, Germany Abstract A high precision laser ablation machine for 3D structuring has been developed. Laser processing of hard metals and ceramics with UV light (355 nm) was studied and optimised with respect to surface quality. It could be shown that the surface smoothness of the structures produced by laser ablation in metals is equal to that produced by high quality electric discharge milling (EDM). Furthermore, laser ablation is not only restricted to metals. 2001 Elsevier Science B.V. All rights reserved. Keywords: Laser ablation; 3D Structuring; Embossing; Micro structuring
1. Introduction During the last years, especially in the electronic production areas, an increasing tendency for miniaturisation of the structure size has been observed. This is known as micro system technique. Now, production techniques need to be developed which are suitable to follow the requirements of the micro system technique. Traditional manufacturing methods have reached their end. Since the production rate of micro tools is not crucial, laser manufacturing tools can be used. Laser ablation shows clear advantages versus the traditional milling or electric discharge milling(EDM) tools [1]: • • • •
unlimited choice of materials; direct usage of CAD structure data; high geometric flexibility; touchless tool.
HEIDELBERG INSTRUMENTS took part in the PROMPT project (Produktionstechnik zum ¨ Mikropragen und Mikrostanzen metallischer Bauteile) [2] in response to interest from the German ¨ Bildung und Forschung and developed a laser ablation machine. This machine Bundesministerium fur is now known under the brand name LAM 66 and is used for the manufacturing of micro embossing and micro stamping tools. * Corresponding author. Tel.: 149-62-2134-3043 / 60; fax: 149-62-2134-3030. E-mail addresses: [email protected] (P. Heyl), [email protected] (T. Olschewski). 0167-9317 / 01 / $ – see front matter PII: S0167-9317( 01 )00485-3
2001 Elsevier Science B.V. All rights reserved.
776
P. Heyl et al. / Microelectronic Engineering 57 – 58 (2001) 775 – 780
The LAM 66 precisely manufactures microstructures with a minimum feature size of 10 mm, and an accuracy of 1 mm in a working volume of 100 3 100 3 5 mm 3 (xyz).
2. The laser ablation machine The concept of the machine is as follows: the sample is fixed on a motor driven stage which itself is moved under a fixed laser axis. This, in a first approach, allows two-dimensional machining of the sample. Repeating this with different 2D data, a 3D structure will result. This procedure, however, does not allow undercutting of the structures. This limits the possibility of the true 3D machining to 2 ]12 D machining, which is ideal for the production of micro embossing and micro punching tools [2,3]. The sample is fixed on a linear motor driven stage. The positioning is done via laser interferometry in both the x- and y-axis. The chuck may be slightly rotated around the z-axis and inclined along the x- and y-axis for alignment purposes. During machining, the sample is exclusively moved in the xand y-directions. Thus, the precision of the laser interferometer system of 40 nm may be used to its maximum. The machining is performed line by line. The basic design is shown in Fig. 1.
2.1. Laser with external shutter The machine is equipped with an Nd:YAG laser. For machining, the third harmonic frequency at 354.7 nm is used. Pulse energy, as well as repetition rate, may be chosen in advance. During machining, these parameters remain constant. A pre-defined local working raster is created by the delay of the laser pulses and the constant stage velocity. In order to avoid the typical thermal instability of the resonator when turning on the q-switched laser (thermal lensing effect [2]), the laser is operated in the continuous mode. The pulses are switched by means of an external shutter. Therefore, an acousto-optical modulator is positioned in the optical path. The first order of diffraction is used as the working beam, and the non-deviated zero
Fig. 1. LAM schematic design.
P. Heyl et al. / Microelectronic Engineering 57 – 58 (2001) 775 – 780
777
order of the laser is blocked by a ceramic device. The diffraction efficiency of the modulator is about 80%, which is sufficient at high pulse energies.
2.2. Write head adjustable in z-direction The machine was designed with an adjustable write head with a 40 mm objective. A stepper motor moves the laser focus in steps of 1.2 mm in the z-direction. The focal length and aperture of the objective are designed in such a way that one obtains a 5 mm focus diameter at a six times broadening of the initial laser beam. This is small enough for production of micro embossing tools. The focal diameter is 30 mm without broadening the initial laser beam.
2.3. Correction mirror Deviations from the foreseen trajectory of the stage during movement may occur due to vibrations, thus resulting in positioning errors. In order to be able to correct these deviations, it is necessary to move the focus independently from the stage in a range of 40 mm. This is done with an adjustable, piezo-driven correction mirror, which can be tilted along two axes. The tilting of the beam in front of the lens results in a translation of the beam focus on the sample, proportional to the tilt angle. Fig. 2 visualises this effect.
2.4. Control electronics The control electronics consist of a stage controlling unit and a position control and correction unit. The latter also handles the data output. The area of the workpiece is split up into pixels where each pixel equals a laser pulse. Furthermore, the control unit combines the stage movement with the laser frequency, so that at each pixel position a laser pulse, as well as the necessary data are available. This means that, according to the working raster and laser frequency, the stage speed has to be chosen.
Fig. 2. Effect of correction mirror: A tilted beam in front of the writing lense results in a shifted beam on the substrate.
778
P. Heyl et al. / Microelectronic Engineering 57 – 58 (2001) 775 – 780
Finally, the flow of data and laser pulses must be controlled during machining. In parallel, the positioning error must be measured in the x-direction and be corrected.
2.5. Image processing The machine is equipped with a two camera system with an optical path which passes through the working objective. Using the cameras together with an image processing unit, the position of the workpiece can be determined in the co-ordinate system of the machine.
2.6. Data interface In order to obtain flexible manufacturing of 3D microstructures, a specific data interface was developed that makes it possible to directly transform computer designed 3D models (e.g. AutoCAD) into 3D microstructures. Depending on the ablation depth per layer, the 3D model is split up into 2D structure data by layers. This structure data is then converted to 2D machine readable data.
2.7. 3 D rastering Laser ablation is done layer by layer as the sample is moved on the xy-table under the fixed laser spot. The machining is performed in a raster procedure. The work area is divided into parallel lines which are worked successively. Each line in the y-direction is written at a constant x-position. At the end of a y-line, the stage is moved in the x-direction to the next line. Each line is subdivided into pixels. The laser pulse will be shot exactly at the specific moment the stage has the correct pixel position. Overall, the workpiece is divided into several layers, and the layers are worked successively. After a layer is finished, the lens is moved down in z-direction in order to work on the next layer. The z-axis remains fixed during the machining process of a single layer.
3. Results Laser ablation tests were performed on a variety of materials and applications. In addition to experiments with several metals, experience was gained on microstructuring of dielectric materials such as ceramics, diamonds, polyimide and PVC. Since the objective of the PROMT project was to manufacture high quality micro-embossing tools, the work first concentrated on hard metals [2]. In order to avoid strong melt rejections, the initial theoretical focus diameter of 5 mm had to be expanded to 30 mm. In fact, the hole ablated by a single laser pulse still measures about 5 mm in diameter. The intensity, however, is reduced by a factor of 36. In addition, the laser absorption due to plasma formation is reduced [3]. In a limited range above ablation threshold, the ablated volume per pulse grows almost linearly with the laser pulse energy [2,4]. This linearity ends when plasma formation occurs. The surface quality is determined by the fluence (energy per area of a single laser
P. Heyl et al. / Microelectronic Engineering 57 – 58 (2001) 775 – 780
779
Fig. 3. S.E.M.-pictures of example geometries in WC / Co: The quality of the edge (right) can be estimated by comparing the picture to the 20 mm scaling bar. The structures were ablated with a 5-mm X–Y-grid. Ablation depth was approximately 1.3 mm per layer resulting in a total ablation depth of 130 mm.
pulse). In general: the lower the fluence, the better the surface quality. Furthermore, at a given pulse energy and focus size, an appropriate rastering distance must be found. With these optimised settings, an average surface roughness of Ra50.16 mm and an averaged maximum surface deviation of Rz50.7 mm can be reached. Some example geometries in WC / Co (tungsten carbide sinterd in a cobalt matrix) have been viewed with a Scanning Electron Microscope (S.E.M.) and are shown in Figs. 3 and 4. In summary, it could be shown that laser ablation leads to equivalent surface qualities for manufacturing of micro-embossing tools at a higher speed and with more geometrical flexibility compared to the traditional high end electrical discharge milling (EDM) procedures. Furthermore, good quality 2–3 mm steep structures with an angle of 808 are possible in ceramic materials. The higher the ablation per laser pulse, the steeper the wall. This laser ablation technology is ideally used for production of ceramic injection nozzles. In this dimension no other technique leads to equivalent results.
780
P. Heyl et al. / Microelectronic Engineering 57 – 58 (2001) 775 – 780
Fig. 4. S.E.M.-pictures of example geometries in WC / Co. The variety of structures that can be produced by computerdriven laser-ablation is shown. The structures depth is 150 mm.
Other fields of applications could be: injection moulds, moulds for galvanoforming, 2D, 3D laser marking and structuring.
References [1] D. Hellrung, Micro-Structuring by Laser Beam Ablation, Procs. Micro Materials, 1997. [2] Abschlußbericht zum PROMPT-Projekt, BMBF, 1999. [3] P. Heyl, C. Buchner, R. Wijnaendts, Laserablationsmaschine zur Herstellung von Hartmetallwerkzeugen, Procs. Micro Engineering, Stuttgart, 1998. [4] P. Heyl, Mirkostrukturierung von Hartmetall mit Nanosekunden-Laserpulsen, Dissertation, Heidelberg (in press).