Laser-assisted chemical micromachining of metals and alloys

Transcription

1 RIKEN Review No. 43 (January, 2002): Focused on 2nd International Symposium on Laser Precision Microfabrication (LPM 2001) Laser-assisted chemical micromachining of metals and alloys Andreas Stephen, Thorsten Lilienkamp, Simeon Metev, and Gerd Sepold BIAS-Bremen Institute of Applied Beam Technology, Germany Three different laser-assisted processes are described and herewith produced components in the fields of micromechanics are presented. First, excimer laser projection technology is modified in order to produce large-area microstructures. It enables the transfer of structures having dimensions much larger than the laser beam section by synchronized scanning of the mask and the substrate. This technology is used to manufacture master structures in polymers for injection molding inserts. The insert tool itself is produced by electroforming of the master leading to a metallic copy. In this way an insert tool is performed for the production of microfluidic components. Its structured area is 2 cm 2 at a resolution better than 3 μm. Second, laser-assisted wet chemical etching using a cw-nd:yag laser is described. The principle of this micromachining method is based on a local thermal activation of chemical etching reactions on the surface of the material. The direct processing of the workpiece resulted in high accuracy microstructuring with smooth surfaces and without any debris or thermal influence on the material properties. Among others one example in the field of applications in micromechanics is the fabrication of superelastic micro-grippers prepared by cutting of temperature sensitive shape memory alloys. The achieved sidewall angle is about 3 degrees and the surface roughness less than 0.4 μm for machined 200 μm thick foils. Third, a combination of the two afore mentioned processes leads to complex shaped microstructures in metallic parts. Thereby, additional microstructures of specific shape, e.g. v-shaped grooves, are machined by laser-assisted wet chemical etching into metallic inserts produced by electroforming of excimer laser machined masters. They are used for hot embossing tools enabling the production of special housings which can be hermetically sealed by ultrasonic welding. Introduction The market for precision metallic microparts is continuously growing and penetrating into new areas of application e.g. molding inserts used for hot embossing of low-cost microfluidic devices in the field of biomedical analysis methods or microtools made of superelastic alloys for medical applications. The differences in size and shape of microstructures contained in a single mold is remarkable for many novel applications. Often, positive as well as negative structures are required. Negative ones can be machined by processes leading to material removal. However, when machining positive structures by such processes the volume of the material to be removed is much bigger than the volume of the structures itself and thus not efficient with respect to processing time. Those metallic structures can be efficiently fabricated by micromachining of polymer substrates leading to a negative master which can be afterwards converted into a positive metallic form by electroplating. Furthermore, many machining processes negatively influence specific material properties. Machining of shape memory alloys, e.g. by electrical discharge machining (EDM), often leads to a reduction or totally loss of the superelastic properties caused by high temperatures of the treatment and/or to insufficient surface qualities for many applications. Lasers can be efficiently applied for micromachining of metals as well as polymers with high resolution and quality. 1) Most address: astephen@uni-bremen.de laser-induced processes for direct micromachining of metals without using masking techniques are based on material removal via evaporation by high intensity pulsed laser irradiation. Consequently, redeposition of evaporated or resolidificated material as well as mechanical and thermal loading of the structured part can negatively influence the functional properties of laser-produced microparts. Such problems can be efficiently avoided by using for material removal the process of laser activation of chemical etching reactions at the interface between the solid and a reactive fluid. 2) Laser-assisted wet chemical etching of metals can be achieved in different solutions of acids or bases using cw-lasers in the visible or near infrared region as e.g. the Nd:YAG laser operating at 1064 nm. 3) The interaction of a focussed laser beam with the material surface being covered with a liquid etchant leads to an enhanced thermochemical etching reaction and thus to a material removal from the irradiated area. In contrast to metals, precise and clean machining of polymers is possible by laser photoablation (photochemical dry etching) due to the specific material properties as low thermal conductivity and high photochemical sensitivity. For an effective micromachining a high absorption of the material as well as short pulses and high repetition rates are necessary. Therefore, excimer lasers are most suitable for machining polymers, e.g. a KrF-excimer laser operating at 248 nm. The aim of this paper is to present some applications and machining results for laser-assisted photo- and thermochemical etching. Furthermore, a combination of both processes linked by an intermediate processing step of electroplating to 56

2 Fig. 1. Setup for laser-assisted photochemical etching. machine complex shaped microstructures is demonstrated. a stylus profilometer (Dektak II ST). Experimental Laser-assisted photochemical etching of polymers For large-area microstructuring of polymer substrates, the laser beam of a KrF-excimer laser operating at a wavelength of 248 nm was homogenized by an array of microlenses inplane with a mask, which was imaged in a projection scale of 5:1 onto the substrate. The mask and the substrate were synchronously translated perpendicular to the laser beam using two computer-controlled xy-translation stages, as shown schematically in Fig. 1. Laser-assisted thermochemical etching of metals A detailed description of the experimental set-up used for laser-assisted thermochemical etching of metals can be found elsewhere. 4) In brief, the beam of a cw-nd:yag laser operating at a wavelength of 1064 nm in its fundamental Gaussian mode at a maximum power of 16 W or alternatively a second one operating at multi-mode conditions with a maximum power of 600 W was focussed to an estimated focal spot diameter of about 20 µm respectively 200 µm on the metal surface, for instance electroplated nickel or superelastic nickel-titanium, immersed in a liquid etchant consisting of 5.6 M H 3 PO 4 and 1.5 M H 2 SO 4. Using a computer-controlled xy-translation stage samples were moved under the focussed beam at speeds ranging from 10 µm/s to 100 µm/s. To perform laser-assisted jet-chemical etching a special liquid phase etching cell is integrated. The cell consists of two parts, a co-axial nozzle assembly and a basin, which are connected to each other by elastomer bellows. The nozzle can be adjusted laterally and in height with respect to the laser beam focus. The etch liquid enters the nozzle tip in such a way that a swirl is given the liquid flow and is injected coaxial to the laser beam directly into the irradiated area. The basin holds the workpiece which is submerged into the etching liquid. The basin is mounted onto computer-controlled xyz-stages allowing a relative movement of the nozzle over a mm 2 area at a resolution of 0.1 µm to position the workpiece with respect to the laser beam. Analytical methods The machined microstructures were investigated by scanning electron microscope tests (SEM, Zeiss DSM 920). Additionally, the roughness of the treated surface was determined by Machining results and applications Laser-assisted photochemical etching of polymers The typical wavelengths of excimer lasers provide photon energies up to 6.4 ev which is sufficient for direct non-thermal cracking of polymer bonds leading to ablative photochemical decomposition (cold ablation). However, this process is characterized by a combination of photochemical and photothermal interactions. Due to the typical pulse duration of several ns which suppresses thermal conduction and the low penetration depth of less than 1 µm the pulse energy is absorbed by a very small material volume. The laser in combination with a demagnifying mask projection system allows the generation of complex miniturised structures. The size of the micromachined area by excimer laser photoablation in a conventional static projection scheme is limited due to the maximum available laser power as well as the maximum size which can be imaged by the objective lens with high resolution. With the developed dynamical projection scheme a sequently machining of small parts of the structure leads to a large-area microstructure of several cm 2 only limited by the travel range of the used stages. The synchronous scanning of the mask and the substrate in opposite direction and perpendicular to the laser beam results in a lateral resolution < 3 µm. Microstructuring polymers was performed at repetition rates of 5 Hz and energy densities of approximately 1 J/cm 2 at the surface of the sample. To avoid redeposition of ablated material the sample was flushed with compressed air while processing. Figure 2 shows a part of a mm 2 sized microstructure in polycarbonate achieved by using the technology of synchronous scanning. The depth of the structure is 60 µm and the angle of the walls approximately 15 degrees. Due to the scanning process a residual waviness of less than 0.5 µm was measured. An application for the manufactured master structure in polymer is to fabricate inserts for hot embossing or injection molding of low-cost microfluidic components in polymers. The insert tool itself can be produced by electroforming or metal injection molding (MIM) of the master leading to a metallic copy. Figure 3 shows an insert tool made of steel produced by MIM of the master shown in Fig

3 Fig. 4. Dependence of etch rate on flow rate and laser power. Fig. 2. Part of a large-area master in polycarbonate. irradiation a sudden increase of the electrical potential reveals an immediate interruption of the etching reaction due to repassivation of the metal surface. In particular, upon laser irradiation a temperature much higher than the boiling point of the liquid can be reached on the metal surface. At such high temperatures etch rates several orders of magnitude higher than at the boiling temperature of the etchant were measured. The measured exponential dependence of the laser-induced etch rate on laser power also supports the thermal nature of the process. Fig. 3. Sintered part in steel made by MIM of the master (MIM by IFAM-Bremen). Laser-assisted thermochemical etching of metals Process fundamentals At room temperature many metals are protected against corrosion by a thin native oxide layer on the surface and behave in many aggressive media like a noble metal. Especially in phosphoric acid negligible corrosion rates < 10 8 µm/s at room temperature for e.g. titanium can be observed. An increase of the temperature results in a shift of the chemical equilibrium towards the formation of soluble metal ions and hydrogen. Time resolved measurements of the electrical potential difference against an electrochemical reference electrode identify two main stages of the etching process: first dissolution of the passivation layer occurs, followed by dissolution of the metal. Localized heating of the passivated metal by focussed laser radiation results in analogy to thermal corrosion to a localized dissolution of the passivation layer followed by chemical etching of the metal. The temporal evolution of the electrical potential under focussed laser irradiation shows strong similarities to the thermal process. After the end of laser The process of laser-assisted wet chemical etching of metals differs substantially from processing in gaseous media due to the ionic nature of the reactants and the electrical conductivity of the substrate. Laser-assisted wet chemical etching benefits from the wide range of available chemical reactions and the high density of reactants in the liquid. Since the density resp. concentration of reactants is influenced by the reaction itself and boiling of the etchant at high temperatures (well below the melting point of the metal) leads to formation of bubbles a fast exchange of the arising reaction products is very essential to avoid saturation effects of the etch rate. In addition, the ionic nature of reactants offers the possibility for an electrochemical enhancement of the reaction. Laser-assisted jet-chemical etching A fast exchange of reactans can be achieved by laser-assisted jet-chemical etching. It leads to an improvement of processing speed as well as quality due to an efficient mass transport and cooling of the workpiece by a direct injection of the etchant into the laser-irradiated area. Additionally, homogeneous flow rates all over the workpiece can be achieved enabling large-area processing. Figure 4 summarizes the dependence of etch rate on flow rate and laser power for nickeltitanium. It shows that etch rates up to µm 3 /s can be achieved for laser powers up to 7 W and a flow rate of 2 m/s. Compared with this, the etch rate at a flow rate of 20 m/s is only half as much. This is due to a higher cooling effect by the liquid jet-stream thus leading to lower thermal activation of the metal resulting in weaker etching reactions. The corresponding shape fidelities are up to 12 µm for a flow rate of 2 m/s and 3 µm for 20 m/s. To achieve such a low shape fidelity for a flow rate of 2 m/s the laser power is reduced to 3 W leading to an etch rate of only µm 3 /s. Thus, an increase of the processing speed at equal qualities can be achieved by simultaneously increasing the laser power 58

4 Fig. 5. Dependence of etch rate on applied voltage. Fig. 6. Dependence of etch depth on laser power and speed. and the flow rate. Laser-assisted jet-electrochemical etching Electrochemical enhancement can be performed by applying an electrical field. The range of voltages leading to laserinduced electrical currents is approximately limited between the cathodic hydrogen formation and the flade potential. Laser-induced currents in this region which are due to etching reactions are caused by thermal activation of the anodic dissolution and/or laser-induced breakthrough of the passivation layer. The corresponding etch rates are shown in Fig. 5 for nickel-titanium using a laser power of 5 W and a flow rate of 10 m/s. The measured voltage without applying an electrical field is 0.15 V and leads to an etch rate of µm 3 /s and a shape fidelity of 10 µm. Compared to this, voltages of 0.3V or 0 V lead to etch rates of µm 3 /s resp µm 3 /s and shape fidelities of 10 µm resp. 2 µm. Thus, the processing speed or quality can be increased by electrochemical enhancement. Cutting of foils An application for laser-assisted thermochemical etching is cutting of metal foils, e.g. to fabricate microtools made of superelastic alloys for medical applications. In this case it is very essential to achieve smooth surfaces inside the cut, nearly perpendicular side walls and to keep the treatment temperature below the transition temperature of the alloy. Otherwise, the specific properties of the superelastic material will be reduced and rather fragile geometries can not be fabricated. In Fig. 6 the dependence of the depth of etched grooves in a 200 µm thick foil on the laser power is represented at different scanning speeds. In the parameter range investigated leading to high accuracy of the treatment an approximately linear dependence of etched depth on laser power is observed. Because of the lateral heat diffusion from the zone of laser action, an aspect ratio of approximately 1 was achieved. One medical application is a micro-gripper made of superelastic nickel-titanium alloy which was fabricated by laserassisted jet-chemical etching. The process was specifically optimised to form the micro-gripper structure from 200 µm thick foils without thermal influence on the materials properties and low surface roughness. As the most essential part, Fig. 7 shows the tip of the micro-gripper. The treat- Fig. 7. Tip of a micro-gripper made of superelastic NiTi alloy (Design by Bartels Mikrotechnik GmbH). ment resulted in high machining qualities with surface roughnesses R a of approximately 0.3 µm and cutting angles of 3 degrees. The competing technologies like laser cutting using a Ti:Sapphire femtosecond laser or electrical discharge machining (EDM) show deficits referring to this. Furthermore, these results can be applied for other new products or the enhancement of the quality of existing products. Structuring with defined shape The realizable structures are not limited in size because laserassisted thermochemical etching is a direct machining process without masking techniques. Thus, the microstructure is generated successively by scanning the laser beam which however results in high processing times. An advantage is the possibility to generate structures with defined shape as for instance v-shaped grooves. Grooves in metals can be generated by moving the workpiece perpendicular to the laser beam. Due to the thermal activation of chemical etching reactions the width and depth of the grooves are determined by the temperature distribution on the surface and the duration of the temperature rise. 5) Therefore, the shape of the groove reflects the intensity dis- 59

5 Fig. 8. Predetermined overlaps for a v-shaped groove. tribution of the incident laser beam. Because of the lateral heat diffusion from the zone of laser action high aspect ratios can not be realised by single scanning of the groove. The width as well as the depth simultaneously increase with increasing laser power. Higher aspect ratios were realized by multiple scanning of the laser beam along the same groove. The groove width is almost independent on the number of scans as the temperature increase is confined to the bottom of the groove. This leads to a continuously increasing depth and, in consequence, to higher aspect ratios. By this method aspect ratios higher than 10 corresponding to side wall angles less than 5 were obtained. 5) An expansion of this method to achieve defined shapes is the multiple scanning along the groove axis with simultaneous lateral shift of every scan. By controlling the overlaps between the laser irradiated areas the etched depth and hence the structure s shape can be determined. 6) Figure 8 shows e.g. the calculated overlaps for a v-shaped groove with defined side angle in dependence on the distance from its center. Fig. 9. V-shaped groove in nickel machined by multiple scanning. Using these parameters a 150 µm deep and 200 µm broad v- shaped groove with a radius of the tip of about 5 µm was fabricated at a scanning speed of 10 µm/s and a laser power of 8 W. Figure 9 shows a SEM micrograph of this groove. The measured surface roughness R a inside the laser irradiated area is less than 1 µm. The overall production time is half an hour for 2 mm length. Using this processing strategy a wide range of precise structures with different shapes can be directly micromachined with a processing speed of approximately 10 4 µm 3 /s using a laser system with high resolution. Much higher processing speeds in the range of 10 6 µm 3 /s however at lower resolution can be achieved using high power lasers. Since the etchant in this case is strongly heated and formation of bubbles occur due to the high incident laser power an efficient injection and cooling of the etchant is necessary to machine with high accuracy and reproducibility as well as to avoid chemical reactions outside the laser irradiated area. Figure 10 shows a cross section of a 250 µm wide and 200 µm deep groove machined with a laser power of 140 W using the technology of laser-assisted jet-chemical etching at a flow rate of 10 m/s and additional cooling of the etchant down to 0 C before it enters the nozzle of the jet. Fig. 10. Cross section of a groove in nickel machined with a laser power of 140 W. Furthermore, a defined shape of the structures can be achieved by forming the intensity distribution of the laser beam on the sample s surface by a projection scheme using a mask of special shape. The main advantage of this method is that the microstructure can be machined with a single scan resulting in a high processing speed. For example, Fig. 11 shows the cross section of a groove achieved by using a rightangled triangle with a length of the hypotenuse of 20 mm projected with a ratio of 100:1 onto the sample. Combination of the laser-assisted processes The principle of the processing route used for the combined method is schematically shown in Fig. 12. It consists of two laser-assisted processing steps in combination with an intermediate step of electroplating: First, a polymer substrate was microstructured by excimer laser photoablation in a dynamical projection scheme. This step offers the possibility of a fast and low-cost fabrication of large-area master structures. Second, by electroforming of nickel the polymeric master is converted into a metallic form. Third, additional structures were micromachined by laser-assisted thermochemical etch- 60

6 Fig. 11. Cross section of a v-shaped groove machined with a high power laser by projection technology. Fig. 13. Large-area master machined by photochemical etching in polycarbonate. Fig. 14. Electroplated insert of the master with additional structures machined by laser-assisted thermochemical etching (Design and electroforming by STEAG microparts GmbH). Fig. 12. Principle of the combined method. ing. These structures show different shapes and heights compared to the microstructures of the master, for instance v- shaped grooves, and are easier to be machined directly into the metallic insert. Furthermore, the overall height of the microstructures can be increased to several 100 µm which is complicated to be fabricated by conventional techniques like lithographic processes. Fourth, the finished metallic mold can be used for a fabrication of several of similar plastic microparts by injection molding or hot embossing resulting in very low processing costs for each part. The combination of laser-assisted photochemical and thermochemical etching leads to complex shaped microstructures in metallic parts. Figure 13 shows a large-area master structure with an overall area of 2 cm 2 machined by photochemical etching of polycarbonate. It is designed for microfluidic ap- plications hermetically sealed in a housing. Figure 14 shows the insert produced by electroforming of the master structure. Additional micromachining by thermochemical etching leads to v-shaped grooves which form energy directors for ultrasonic welding by molding in plastics. They are used to hermetically seal the housing by a cover. There are two 400 µm wide and 200 µm deep v-shaped grooves of 20 mm length shown in Fig. 14. They are located on both sides close to the microfluidic structures. Conclusion Polymers and metals were precisely micromachined by laserassisted processing. A large-area excimer laser photoablation of a polymer substrate was achieved by a developed dynamical projection scheme. As an application this technology was used to manufacture microfluidic master structures for molding inserts which will be produced by electroforming of the 61

7 master leading to a metallic copy. The machined area was several cm 2 at a lateral resolution better than 3 µm. The remaining waviness due to the scanning process was less than 0.5 µm. A precise micromaching process for metals was performed by laser-assisted wet chemical etching. One application in micromechanics is the fabrication of superelastic micro-grippers prepared by cutting of temperature sensitive shape memory alloys. The achieved sidewall angle was about 3 degrees and the surface roughness less than 0.3 µm for machined 200 µm thick foils. Furthermore, various processing strategies for the purpose of machining with defined structure shape were demonstrated by means of a v-shaped groove. Grooves with depths up to 200 µm and dihedral angles of 90 degrees were realized. The determined roughness R a was less than 1 µm. The two different laser-assisted processes were combined using an intermediate processing step of electroplating which converts the polymeric master machined by photochemical dry etching into a metallic mold. The mold was additionally machined by thermochemical etching leading to complex shaped microstructures. The combined processing route was used to fabricate a mold enabling the production of microfluidic devices in special housings which can be hermetically sealed by ultrasonic welding. This work was supported by the German Ministry for Science and Education (project no. 13N7035) and the Brite Euram European project LAMAR (BE ). References 1) S. Metev and V. Veiko: Laser-assisted Microtechnology (Springer, Berlin, 1994); S. Metv and V. Veiko: Laser-assisted Microtechnology, 2nd ed. (Springer, Berlin, 1998). 2) D. Bäuerle: Laser Processing and Chemistry, 2nd ed. (Springer, Berlin, 1996). 3) R. J. von Gutfeld, E. E. Tynan, R. L. Melcher, and S. E. Blum: Appl. Phys. 35, 651 (1979). 4) R. Nowak, S. Metev, and G. Sepold: Mater. Manuf. Proc. 9, 429 (1994). 5) R. Nowak and S. Metev: Appl. Phys. A 63, 133 (1996). 6) A. Stephen, T. Lilienkamp, S. Metev, and G. Sepold: Proc. 1st Euspen Conf., 2 (1999), p