Y Layouni

G Girardin

A Benilov

J Etzkorn

V Semet

P Morin

M Cabrera

Micro EDM milling with automated electrochemical fabrication of the cylindrical microtool

Keywords: microelectrode, electrochemistry, micromachining, micro EDM, electrical discharge machining

Micro electrical discharge machining in the milling configuration (µEDM milling) allows machining of complex microparts in a flexible way by controlling the erosion process along the 3D path of a thin cylindrical microtool. This technique is attracting growing interest for the processing of mesoscopic and/or high aspect ratio parts. A bottleneck in the development of this technology is the fabrication of thin diameter tools with large aspect ratios. In this paper, we introduce an innovative µEDM milling machine along with a machine entirely developed in our laboratory used to form microtools by electrochemical fabrication. The fabrication process is performed automatically according to a method derived from the technology originally developed for the fabrication of Scanning Tunnelling Microscope (STM) tips. Examples of fabricated tungsten microtools with 4 to 10 µm diameters over a 300 to 800 µm lengths are presented along with preliminary data about the machining of microfluidic channels in stainless steel and titanium.

Introduction

Micro EDM milling (µEDM milling) allows machining of complex microparts in a flexible way by controlling the erosion process along the 3D path of a thin cylindrical microtool [START_REF] Pham | Micro-EDM-recent developments and research issues[END_REF]. This technique is attracting growing interest in microtechnology for the processing of mesoscopic and/or high aspect ratio parts, especially when the parts are made in material difficult to machine by other means (EDM is compatible with all kinds of electrically conducting materials). Examples of such parts are stainless steel micro-reactors for microchemistry, micromolds in hard metals for injection molding, and biocompatible titanium objects for medicine. Conventional microfabrication processes such as Reactive Ion Etching, Focused Ion Beam, and LIGA are not optimized for such materials. Contrary to µEDM milling, these technologies are expensive and require skilled operators. However, their resolution is superior to µEDM milling. Therefore, it is important to improve the machining resolution of µEDM milling.

A bottleneck in improving the resolution of µEDM milling is the fabrication of the cylindrical microtools. Such tools are traditionally made by reverse EDM, wire EDM, WEDG or micromilling [START_REF] Pham | Micro-EDM-recent developments and research issues[END_REF][START_REF] Lim | A study on the machining of high-aspect ratio micro-structures using micro-EDM[END_REF][START_REF] Masuzawa | Wire electro-discharge grinding for micro-machining[END_REF] but these processes require dedicated expensive machinery. The best tools that have been reported have diameters between 2 to 5 μm with a residual stress which is not negligible [START_REF] Kawakami | Study on Factors Determining Limits of Minimum Machinable Size in Micro EDM[END_REF]. However, in commercial µEDM milling machines, the tool diameter is much higher. Therefore, it is necessary to develop new ways to easily obtain microtools with controlled geometry, thin diameter, and high aspect ratio.

Electrochemical dissolution is among the different techniques which may be considered for this purpose because it is a fast and low cost technique. However, the tool geometry is not ideal. The reason is that electrochemical dissolution naturally leads to a conical shape which requires complex compensation during the fabrication in order to obtain a cylindrical shape [START_REF] Lim | Fabrication of arbitrarily shaped microelectrodes by electrochemical etching[END_REF].

Previously we have suggested to prepare the tool with a low cost electrochemical fabrication process according to a method derived from the technology originally developed for the fabrication of Scanning Tunnelling Microscope (STM) tips [START_REF] Benilov | Micro and nano electrical discharge machining in microfluidics and micro nanotechnology[END_REF]. These tips can be made with the "drop off" effect [START_REF] Melmed | The art and science and other aspects of making sharp tips[END_REF], which leads to a conical micrometric shape with an apex with a few atoms. Such a shape at micrometric scale is interesting but irrelevant for EDM milling due to the low aspect ratio. However, as we shall see below, we have found a way to adapt the process to change the geometry of the device to better match our purpose.

In this paper, we describe an innovative µEDM milling machine with automated in situ electrochemical fabrication of the microelectrode tool [START_REF] Cabrera | Machine et procédé d'usinage d'une pièce par micro-électroérosion[END_REF]. The drop-off technique has been avoided in a simple and innovative manner so as to obtain cylindrical ultrathin microtool with very high aspect ratio [START_REF] Cabrera | Procédé et dispositif de fabrication d'un cylindre de diamètre micrométrique[END_REF]. Examples of fabricated tungsten microelectrodes and preliminary data about the micromachining of microfluidic channel of stainless steel and titanium are reported. First trials of µEDM performed in dry air demonstrate that the machining resolution is in the micrometric range.

Experimental set up

Overview

The machine developed in our laboratory has basically two functions: to fabricate the cylindrical tool by electrochemical dissolution of a metallic rod and to machine parts by μEDM milling with this microtool.

The principle of operation is shown in Fig. 1. The machine has two vats which are displaced by a XY translation stage. Firstly, the vat for the tool fabrication, which is filled with an electrolyte, is placed below the rod. A vertical Z stage raises the stage and the rod is immerged in the electrolyte. A voltage is then applied and the rod is progressively made thinner, as explained below, so as to obtain the target diameter. When this is achieved, the stage is lowered and the EDM vat, with the part to machine, is placed below the tool. The part is then machined by µEDM milling with deionised water. When the tool is worn, the stage is lowered and the electrochemical vat is placed again below the tool for tool regeneration. This involves removing the worn tool by electrochemistry and introducing a new section of the rod by a feeding system (still in development) in order to repeat the tool fabrication process. In the present machine configuration, all translation stages are based on step by step motors with 1.25 µm halfstep resolution.

Microtool fabrication

The experimental set up for the tool fabrication is described in more detail in Fig. 2. A tungsten rod is immersed in a vat containing the electrolyte (5 Mol/L NaOH with 50% glycerol). The vat contains a cylindrical platinum grid used for the counter-electrode, a supporting dielectric liquid (non-mixable with the electrolyte), and a small container filled with Gallium-Indium. To start the process, a 250 µm diameter rod and 20 mm height, is vertically translated until electrical contact is made with the Ga-In (Fig 2a) The objective of the tool fabrication process is to greatly reduce the diameter of the rod while conserving its cylindrical geometry. As mentioned above, electrochemical etching of a rod leads to a conical shape unless a specific voltage waveform is applied. The applied voltage waveform is shown in Fig. 3. Thin cylindrical tools can be obtained by applying alternating voltage pulses with a 50 Hz frequency between the rod and the counter-electrode with appropriate time of application TON, rest time TOFF, and current amplitude. The length of the tool is directly related to the height of the electrolyte. An example of a fabricated tungsten microtool with 10 µm diameter over an 800 µm length is provided in Fig. 4. Typical process parameter values are shown in Table 1 for the above mentioned tungsten rod. Fig. 5 shows a view of a second tungsten microtool with a 4 µm diameter over a 300 µm length. During θON time, if the tool is close enough to the part, the fast discharge of the capacitance CM leads to the machining of the part through creation of a microplasma. With our setup, the duration of the plasma is about 400 ns as shown in Fig. 7. Only a few micro discharges are possible during each θON depending on the ratio between RMCM and θON, which limits the amount of energy passing trough the tiny microtool.

µEDM milling machine

Examples of machining

Resolution of the µEDM machining in dry EDM

Fig. 10 illustrates the potential of this process with an example of EDM machining with a single micro discharge performed on p+ silicon in dry air (not deionized water) with a platinum-iridium tool. Machining resolution in the micrometric range is thus demonstrated with this simple experiment. Therefore, an improved version of the µEDM milling machine is in development in our laboratory to allow taking full advantage of this resolution.

Conclusion

A new method for the fabrication of thin cylindrical microtools has been reported with successful application to µEDM milling. The fabrication of the tool by electrochemical dissolution is straightforward and performed with simple equipment allowing the tool fabrication and the µEDM milling to be incorporated in the same machine. Preliminary results on the machining of stainless steel and titanium parts have been provided. Future work will concern the characterization of the tool fabrication process. The repeatability of the etching will be evaluated with a 10 nm resolution LSM-500S Mitutoyo laser micrometer. Preliminary results show consistent data with the values of Fig. 4 and5. The fabrication time of the tool can be significantly reduced, probably down to 10 minutes, but this requires careful optimization. To increase productivity, parallel fabrication of microtools will be considered. Future work will also concern the improvement of the µEDM milling machine to take full advantage of the potential of the process.

Figure 1 .

 1 Figure 1. (a) Machine showing the electrochemical fabrication of the tool. (b) µEDM milling of the part.

  . The electrochemical dissolution process is then started. The lower part of the rod (in the dielectric) is unchanged during the process whilst the intermediate part (in the electrolyte) is made thinner (Fig2b). The resulting shape resembles a dumb-bell. When the target tool diameter is obtained, the rod is translated upward in order to place the lower part of the dumb-bell at the air-electrolyte interface (Fig2c). A cut off voltage is then applied for a short duration in order to remove the lower part of the dumb-bell and free the microtool, which is now ready for the µEDM process (Fig2d).

Figure 2 .

 2 Figure 2. Principle of microtool fabrication. (a) Initial setup of the fabrication process. (b) Etching occurs only on the section of the rod exposed to the electrolyte. (c) Bottom section of the dumb-bell is removed. (d) Tool fabrication is complete.

Figure 3 .

 3 Figure 3. Voltage pulses applied between the tool and counter-electrode voltage.

Figure 4 .

 4 Figure 4. SEM view of a fabricated tungsten microtool with 10 µm diameter over an 800 µm length.

Table 1 .Figure 5 .

 15 Figure 5. Image of a second tungsten tool (diameter estimated by optical microscope).

Fig. 6

 6 Fig. 6 is a simplified view of the electronics of the

Figure 6 .

 6 Figure 6. Electronics of the µEDM milling machine.

Figure 7 .

 7 Figure 7. Voltage VBG across capacitance showing 2 discharges during θON (100 V UM, 10 nF CM, 50 µs θON, 100 µs θOFF) .

Fig. 8

 8 Fig. 8 and 9 show examples of microfluidics channels machined respectively in polished 20-by-20by-0.6 mm stainless steel 316 Ti and unpolished titanium using tungsten microtools similar to the one in Fig. 4. The channel width and depth are respectively approximately 40 µm and 15 µm, for stainless steel 316 Ti and 25 µm and 8 µm for titanium. Preliminary experiments have shown that the tool wear ratio is in the 0.1-10% range depending on the machining parameters (voltage, average gap value, flow of deionised water, etc.).

Figure 8 .

 8 Figure 8. SEM view of approximately 40 µm wide and 15 µm deep channels in stainless steel 316 Ti. Machining parameters: 100 V UM, 100Ω RM, 10 nF CM, 5 µm/s speed for XYZ axis.

Figure 9 .

 9 Figure 9. Optical microscope view of 25 µm wide and 8 µm deep channels in bulk titanium. Machining parameters: 100 V UM, 100Ω RM, 10 nF CM, 5 µm/s speed for XYZ axis.

Figure 10 .

 10 Figure 10. Crater formed after a single spark on p+ (0.005 Ω•cm) silicon (4.7 nF CM, 30V UM) (a) SEM image, (b) AFM image, (c) AFM profile.

Acknowledgements

This work was financed by grant L356 of Lyon Science Transfert (with support from the PRES of Lyon, the ANR, the FEDER, the Région Rhône Alpes and the Grand Lyon) and the FUI ConProMi. G. Girardin acknowledges the support of the CNRS and DGA for a PhD grant.


G Girardin

Y Layouni

P Morin

M Cabrera
email: michel.cabrera@univ-lyon1.fr

MICRO EDM WITH THE IN SITU ELECTROCHEMICAL FABRICATION AND REGENERATION OF THE TUNGSTEN MICROELECTRODE TOOL

Keywords: microelectrode, electrochemistry, micro EDM, microelectrical discharge machining, micromachining

A new process for the fabrication of cylindrical microelectrode tools with high aspect ratio for microelectrical discharge machining (µEDM) is reported. Tungsten micro-tools with a diameter ranging from 1 µm to 26 µm and aspect ratio ranging from 15 to 20 are obtained in about 30 minutes with a low cost and automated technique. This one is based on the electrochemical etching of the tool material with a process which has fully been integrated in a milling µEDM machine. As proof of interest, preliminary examples of µEDM machining are also reported with the 26 µm diameter tool. After µEDM machining, fast and automated in situ tool regeneration is also demonstrated. This may be of great help to simplify the tool wear compensation algorithm during µEDM milling.

INTRODUCTION

The improvement of machining technique like micro electrical discharge machining (µEDM) is still required in Microfabrication when 3D mesoscopic and/or high aspect ratio (i.e. the ratio of the part height with its lateral dimension) parts are needed. In addition, it is known that µEDM is compatible with all kinds of electrically conducting materials, which opens the way to machine unconventional (non-silicon) materials in Micro-technology. Examples of application of growing importance are the machining of stainless steel for the chemical micro-reactors and hard metals for the micromolds for injection molding. Processes like Reactive Ion Etching, Focused Ion Beam and LIGA are expensive and/or not optimized for these materials [START_REF] Benilov | Micro and nano electrical discharge machining in microfluidics and micro nanotechnology[END_REF].

However, µEDM has still to be improved in terms of resolution. This is of paramount importance in the case of µEDM milling, which allows machining 3D parts by controlling the path of a cylindrical microelectrode tool. In this case, the miniaturization of the tool is a key factor for the machining resolution.

The manufacturing of theses tools has still to be largely improved in terms in tool diameter and aspect ratio. These tools are traditionally made by reverse EDM [START_REF] Lim | A study on the machining of high-aspect ratio micro-structures using micro-EDM[END_REF], wire EDM, WEDG [START_REF] Masuzawa | Wire electro-discharge grinding for micro-machining[END_REF] or micromilling but this requires a dedicated machine and the tool diameter is still limited at best to the 2-5 µm range with important residual stress [START_REF] Kawakami | Study on Factors Determining Limits of Minimum Machinable Size in Micro EDM[END_REF]. Therefore, in practice 10-20 µm diameter tools are often used.

In this article, we shall report a new process for the fabrication of a microelectrode tool and its integration in a milling µEDM machine under development in our laboratory [START_REF] Cabrera | Machine et procédé d'usinage d'une pièce par micro-électroérosion[END_REF]. Our machine is made of two subssystems: -The tool fabrication place -The milling µEDM machine itself. The tool which is typically a tungsten rod (250 µm diameter -20 mm height) is first prepared in the tool fabrication place and then translated to the µEDM machining place for further use. After machining, the tool can be returned to the tool fabrication place. In other respects, it is well known that tool wear in an important problem during the µEDM milling [START_REF] Lim | A study on the machining of high-aspect ratio micro-structures using micro-EDM[END_REF]. Therefore fast in situ tool regeneration will be also demonstrated in this work.

TOOL FABRICATION

The fabrication of the tool is based on the electrochemical etching of a cylindrical tungsten electrode. This process is derived from the techniques originally developed for preparing the tips of Scanning Tunneling Microscopes (STM) or Atomic Force Microscopes (AFM). The process has been modified to avoid the conical shapes with atomic apex needed for STM and AFM. Our goal was to obtain the cylindrical microelectrode tools with high aspect ratio which are necessary for the µEDM milling.

SET UP

The process is based on the electrochemical etching of the tungsten rod. This one is immersed in a vat filled with NaOH electrolyte at 1 Mol/L (mixed with 50% glycerol) together with a cylindrical counter-electrode grid placed around the rod. The vat contains also a supporting dielectric liquid (non miscible with the electrolyte) and also a small container filled with Gallium-Indium. The electrical contact is performed by putting in contact the end of the rod with the Ga-In (Figure 1).

Figure 1: Electrochemical cell

By applying a pulse voltage (50Hz) with alternate polarities from a few tens of volts to few volts with appropriate time of application (T ON ) and duty cycle (T OFF / T ON ) (Figure 2), a homogeneous etching of the tungsten rod is observed.

Figure 2: Pulse voltage applied with TON-TOFF phases

Electronic hardware and Labview interface have been developed to perform successive etching steps as reported in Table 1. with the control of applied voltage U, application time T ON , duty cycle and etching time. The result is the progressive thinning of the rod as depicted schematically in Figure 3.

Table 1: The electrochemical etching of the tungsten rod in 4 steps

Before Step 1, the rod is vertically translated until the electrical contact (Figure 3a). At the end of Step 1, the lower part of the rod (in the dielectric) is unchanged whilst the intermediate part (in the electrolyte) is made thinner (Figure 3b). Of course the upper part of the rod in the air is unchanged. So altogether the rod shape is like a dumb-bell. Then steps 2-4 are performed with a series of decreasing voltages to thin down the central part of the rod (Figure 4c) whilst keeping its cylindrical shape. At the end, once the central part has reached the targeted diameter, the rod is translated upward in order to place the lower part of the dumb-bell in the electrolyte. A cut off is applied with a small voltage during a short time in order to remove the lower part of the dumb-bell (Figure 4d).

EXPERIMENTAL RESULTS

Figure 4 shows the experimental results according to the parameters of Table 1:

-The original rod (Figure 4a).

-The rod after Steps 1-4 (Figure 4b).

-The tool after final cut off showing a diameter of 26 ± 2 µm along 400 µm (Figure 4c). This tool fabrication process is fully automated and takes about 30 minutes. It is performed without any mechanical machining in a small electrochemical reactor (5x3x3 cm) with simple and low cost electronics.

It is important to note that Figure 4 (which is included here only for a good understanding of the process) is a collection of about 40 pictures per view. For each view, the electrode is translated up outside the vat so as to take a picture in the air with a Zoom 6000 Navitar with long working distance objective and ½" Sony CCD camera.

All measurements (diameter, length) are performed at the highest magnification.

Figure 4: (a) Original rod (b) After the 4 etching steps of Table 1 (c) After cut-off

EDM Machining

With the tool shown in Figure 4c, µEDM machining was performed with a RC circuit for discharge generation (Figure 5). The voltage applied between the electrode and the piece (stainless steel 316 Ti) was 100V. The rod was covered with a flow of water, which served as a dielectric. The electrode was moved downward and the part was moved horizontally by stepper motors with a resolution of 2.5 µm. At this stage of development (proof of concept), only a set of 3 grooves was machined per trial.

Figure 5: RC-circuit for µEDM machining

The grooves made with the electrode are shown in Figure 6a. The height of the grooves has been estimated to 50±5 µm with an Olympus optical microscope BX51M. The width at the bottom of the part is about 26±1 µm (close to the tool diameter) whilst the width at the top of the part is 70±1 µm. As explained above, the electrode can be regenerated by placing it back to the tool fabrication place with the stepper motor. The regeneration phase is performed with ±7V at 50 Hz (Ton: 0.3s; Toff: 3s). The result is shown in Figure 7b. The conical effect has been significantly reduced: the tool diameter is now 20±1 µm over 600 µm.

Figure 7: (a) Tool after first µEDM machining showing tool wear (b) Regenerated micro-tool (c) Tool after second µEDM machining

The tool of Figure 7b has been re-used for a second µEDM machining step with the results shown in Figure 6b. The width of the machined grooves has been reduced: 56±1 µm at the top and 20±1 µm at the bottom of the grooves. After machining the tool has a diameter of 20±1 µm over 400 µm. (Figure 7c).

MICROMETRIC TOOLS

By adjusting the parameters of table 1, it is possible to decrease the tool diameter. As a first example Figure 8 shows an optical view of tool with 4±0.5 µm diameter and 320 µm length. Figure 9 shows a SEM view of the tip of the tool. In addition, it is possible also to obtain tools with a diameter in the micrometric range. Figure 10 shows a tool with a diameter of about 1 µm and a length of 15 µm. At this scale no residual stress was observed probably because of the chemical nature of the fabrication process (electrochemical dissolution).

CONCLUSION

Different examples of manufacturing of tungsten microtools have been reported with diameter ranging from 1 µm to 26 µm and aspect ratio ranging from 15 to 20. Preliminary examples of µEDM machining have been also reported with the 26 µm diameter tool. Further optimization will be necessary to take full advantage of the miniaturized tools for µEDM machining.

However, these tools have been obtained in 30 minutes with a process which has fully been integrated in our milling µEDM machine. The tool fabrication is a low cost and automated technique performed with simple electronics. This opens new perspectives especially in the field of µEDM milling which requires very thin and long tools which are extremely fragile. Future research will concern the fabrication of tools with submicrometric diameter as well as the increase of the aspect ratio. The question of the residual stress after fabrication of the tools will be also investigated.

In addition, in situ electrochemical regeneration of the tool has been also demonstrated in this work. It is known that the tool wear is of paramount importance in the milling EDM process and is difficult to compensate because of the difference of surface between the tool and the part. Therefore fast and automated regeneration of the tool could be of great help to simplify the tool wear compensation algorithm.

Figure 3 :

 3 Figure 3: Schematic view of the rod during the different etching steps

Figure 6 :

 6 Figure 6: (a) View of the grooves after µEDM machining with the tool of Figure 4c (optical microscope) (b) View of another set of grooves after machining with the tool of Figure 7b

Figure 8 :Figure 9 :

 89 Figure 8: Optical view of a tungsten electrode with 4 µm diameter

Figure 10 :

 10 Figure 10: Optical view of a tungsten tool with a diameter in the micrometric range over 15 µm.

ACKNOWLEDGEMENT

The work was financed by grant L356 of Lyon Science Transfert (with support from the PRES of Lyon, the ANR, the FEDER, the Région Rhône Alpes and the Grand Lyon). G. Girardin acknowledges the support of the CNRS and DGA for a PhD grant.