EP0603485B1 - Electrolytic etching process - Google Patents

Abstract

Precision etching small areas of a substrate in which the cathodically connected substrate is contacted with an electrolytic soln. and the areas to be etched are contacted by a micro carbon fibre electrode connected as the anode.

Description

The invention relates to a method for electrolytic etching according to the preamble of the first claim.

Such a process is known, for example, from the publication by D. M. Allen entitled "The principles and practice of photochemical machining and photoetching", published by Adam Hilger, Bristol and Boston. In Chapter 4 (Etching, Etchants and Etching Machines) under 4.7 (Electrolytic etching) on page 104 a device for electrolytic etching is shown and briefly described. This device comprises a vessel which is filled with an electrolytic solution into which the sample to be etched is immersed. The sample is connected as an anode. An electrode, which is connected as a cathode, is also immersed in the electrolyte solution. The passage of current in this electrochemical cell causes an anodic dissolution. It is further stated that this technique is usually only used for materials that are difficult to etch, such as ceramics, precious metals, rust-free amorphous alloys and superalloys.

It is easy to ascertain that the device shown and the method carried out with it can only be used for relatively large areas to be etched away. They are unsuitable if individual areas of small samples are to be etched off.

The object of the invention is therefore to remedy this disadvantage of the known method. A method of the type mentioned at the outset is to be proposed, with which microscopic areas of a sample can also be etched off, wherein the etching can be carried out selectively over other components or spatial areas of the sample if necessary.

The object is achieved by the marked feature of the first claim. Preferred embodiments of the method according to the invention are specified in the dependent claims.

In electrolytic etching, metals are dissolved by electrolysis, whereby chemical reactions are forced on electrodes using a current source. If you switch a sample to be etched as an anode - as in the above-mentioned publication - and bring it into contact with an electrolyte solution in which there is a cathode as counter electrode, then the anode / electrolyte solution / run at the phase interfaces due to the potential difference between the two electrodes. Cathode electrochemical reactions, z. B. is oxidized at the anode metal and goes into solution as a metal ion. Corresponding reducing reactions take place at the cathode, e.g. B. the discharge of hydrogen ions or metal ions.

In general, in the case of electrolytic etching, the sample to be etched is switched as an anode, the oxidizing effect of the electrical current being exploited. However, the reverse circuit can also be selected, in which the sample to be etched represents the cathode. With such a circuit z. B. metals indirectly, d. H. under the influence of the reagents formed on the cathode, attacked and dissolved. An example is the generation of OH⁻ ions, which cause an increase in the pH value and can be corrosive to alkali-sensitive metals such as aluminum. With such a circuit, substances other than metals can also be attacked.

It is therefore only necessary to apply different potentials to the sample to be etched, which is switched as the first electrode, and to the second electrode. In addition to direct current, alternating current or others can also be used depending on the problem periodic currents are impressed as electrode potentials. Potentiostatic etching (at constant potential) and galvanostatic etching (at constant current) are further special features.

It is not necessary for the sample to be etched to be completely immersed in the electrolytic solution. In microstructure technology in particular, it is advantageous if a small amount, for example a drop, of the electrolyte solution is applied only to that point on the sample which is to be etched off. Here, the first electrical potential can be applied to the entire sample, while the second electrical potential is applied to an electrode which is immersed in the drops of the electrolyte solution.

Particularly in microstructure technology, special requirements are placed on the etching steps. Since very filigree, in part overhanging and only adhering to small base structures with structural details on a micrometer scale are produced with the help of etching technology, the etching process must be carried out very gently and yet as quickly and effectively as possible. In any case, mechanical damage to the microstructures must be avoided. This must be taken into account in particular when designing the electrode.

It is therefore essential to the invention that an electrically conductive micro-carbon fiber is used as the electrode. Micro-carbon fiber is understood to mean a carbon fiber that has a diameter of less than 100 μm, but preferably a diameter of less than 20 μm. The length of the micro carbon fibers to be used can be chosen almost arbitrarily. It must at least be dimensioned in such a way that a structural base to be etched, which is surrounded by microstructures, is reached. On the other hand, the length can be up to two centimeters. Micro-carbon fibers that are longer than 5 mm are generally not necessary. Micro-carbon fibers which are suitable for the process according to the invention are commercially available as yarns or individual fibers in various lengths. Their electrical conductivity is approximately 1.5 · 10⁻³ Ωcm; it is about an order of magnitude lower than the conductivity of mercury.

Such micro-carbon fibers have a number of advantages that are particularly important in microstructure technology:
Due to the high dimensional stability, the fiber maintains its elongated shape and does not irreversibly deform. Therefore, an exact positioning of the fiber is possible; after a change of position, it can be reproducibly returned to the original position even if it has bent due to a collision with the sample. If fine microstructures are touched unintentionally, there is no risk of mechanical damage, since the fiber is deflected elastically by an obstacle without transmitting damaging mechanical forces. Micro-carbon fibers are also resistant to highly aggressive etching media, in which they are not attacked and therefore cannot contaminate the etching medium. They can also be easily attached to a conventional wire and quickly replaced. After all, micro-carbon fibers in various lengths and thicknesses are commercially available; Such fibers can also be adapted to special treatment steps by tapering or tapering them by electrochemical removal. In addition, by applying insulating varnish to the fiber and cutting the fiber after drying the varnish, so-called disc electrodes can be produced, which are only electrically conductive on the front side and therefore have a more selective influence on the etching process.

The sample to be etched can consist of a single material. This material must then be electrically conductive so that it can be etched electrolytically. The sample can consist, for example, of a thin metal plate into which a fine hole is to be etched at a defined point. In this case, a small drop of a suitable electrolyte solution, such as an acid, is applied to the site and the electrolyte solution is contacted with the micro-carbon fiber. Under the influence of the electrical potential difference, the etching process takes place very quickly at this point, so that as a result the metal plate has the desired hole at this point, while the adjacent areas are practically not attacked. As mentioned above, other electrically conductive materials such as. B. electrically conductive plastics are etched.

In microstructure technology, the task is often to selectively detach one of these substances from a sample that consists of a composite of several materials in predetermined areas. In this case, the procedure can be the same. This technique is particularly advantageous when sputtered or vapor-deposited materials are to be removed in places, since dusting and vapor deposition often result in particularly inert and inert layers that are difficult to attack with wet-chemical etching processes.

Should aluminum be selective with the method according to the invention compared to other substances, e.g. B. plastics such as polyimide or metals such as copper or titanium are etched off, preferably a solution of an iodide salt, e.g. B. an alkali metal iodide salt such as potassium iodide. An even faster etching of the aluminum and thus an even greater protection of the other substances is possible if alcohols such as methanol or isopropanol are added to the solution of the iodine salt. Such an electrolytic solution has however, a lower surface tension, which is why the applied drops tend to run. In the case of longer etching times, the electrolyte solution must be replenished, as evaporation losses occur.

A fundamental advantage of the method according to the invention is that the etching process can be observed under the microscope. For this purpose, the micro-carbon fiber is connected to a conventional lead wire in such a way that the view under the microscope is not obscured by the comparatively large lead wire. The connection to the lead wire can be done with the help of a conductive lacquer, e.g. B. with conductive silver lacquer.

The invention is explained in more detail below with reference to figures and examples of implementation.

1 shows a preferred embodiment of the method according to the invention.

2 shows the experimental setup for carrying out the method according to the invention.

1 shows a sample in the form of a metal disk 1, into which a hole is to be etched using the method according to the invention. The disc 1 is switched as an anode via the electrical lead 2. An electrolyte drop 3 is applied to the location of the sample to be etched. The cathode consists of the electrical lead wire 4, at the end of which a micro-carbon fiber 5 is attached with the aid of silver conductive lacquer.

The experimental setup shown in FIG. 2 was designed for etching processes on microstructures. The electrolytic etching process can be observed here under a microscope 6. The sample 1 to be etched is placed on the work table 7 of the microscope 1 placed; it is connected via the electrical lead 2 to the positive pole of a controllable power supply 8. An electrolyte drop 3, into which the micro-carbon fiber 5 is immersed, is applied to the sample 1 at the location to be etched. The micro-carbon fiber 5 is connected via the electrical lead wire 4 to the negative pole of the controllable power supply 8. The lead wire 4 and thus the micro-carbon fiber 5 can be moved in all spatial directions with the aid of a micromanipulator 9.

Example 1: Alternative microelectrodes

With the aid of the experimental setup shown in FIG. 2, alternative microelectrodes were examined for their suitability. For this purpose, pieces of approx. 1 cm in length were cut from a stainless steel wire with a diameter of 25 µm, a nickel wire with a diameter of 10 µm and a gold wire with a diameter of 17 µm and connected to the electrical lead wire 4 with silver conductive lacquer. The lead wire consisted of a silver-plated copper wire. The free-standing thin wire ends were shortened to 3 mm and used for electrolytic etching tests.

The stainless steel wire and the nickel wire were wound on a spool by the manufacturer (Goodfellow) and remained strongly bent after unwinding; they could no longer be bent into a straight shape. For this reason, the lead wire had to be bent for each individual piece of wire so that the structural base could be touched by microstructured plates under microscopic control. In addition, plastic bends occurred in the event of unintentional contact with the microstructures, so that it was no longer possible to continue working precisely because precise optical control was no longer possible. With bent wires, it was not possible to penetrate into the structure base of cavities between closely adjacent microstructures.

All of these difficulties did not arise if the same was done with commercially available, about 7 µm thick and 5 - 7 mm long micro-carbon fibers.

Example 2: Etching experiments with the method according to the invention on wafers

Wafers were obtained as the sample to be etched, on which there was a dusted-on, 1.0 μm thick aluminum layer and strips of approximately 35 μm thick copper layers. The experiments served to find an electrolytic solution for the electrolytic etching of aluminum that was as selective as possible. For this purpose, droplets of electrolyte were selectively applied to predetermined locations on the wafer in such a way that both the aluminum and part of the copper strips were wetted. During the electrolytic etching tests, it was then checked under a microscope whether there was a visible attack on the copper or whether copper ions formed on the micro-carbon fiber connected as the cathode due to possible dissolution of the copper were reflected again as metal.

With regard to experiments to be carried out later, in which aluminum should be etched selectively in the presence of polyimide, the use of alkaline electrolyte solutions was dispensed with.

The following electrolyte solutions were examined:

3% nitric acid; 10% K₃ [Fe (CN) ₆] solution in water; a solution of 20% K₃ [Fe (CN) ₆] and 3% KNO₃; a solution of 5 ml glycol and 250 mg tetrabutylammonium tetrafluoroborate; a solution of 500 mg K₃ [Fe (CN) ₆], 30 mg 1-H-benzotriazole and 100 mg KNO₃; Potassium iodide in concentrations of 10, 20 and 30% and a solution of 10% potassium iodide with 1.8% 1-H-benzotriazole. It was found that aluminum with the Potassium iodide solutions particularly quickly and selectively etched over copper. An attack on copper could not be detected with these solutions.

As mentioned, the etching rate can be increased further with potassium iodide solutions, however evaporation losses occur and the surface tension drops. Nevertheless, such electrolyte solutions are very suitable for etching off small amounts of aluminum.

Example 3: Etching off sacrificial layers in the manufacture of micro valves

The samples to be etched were produced in the following way: Using microtechnology methods, an approximately 3 µm thin titanium layer was built up on a later removable substrate. The titanium layer was surrounded by a circular, raised edge made of copper with a diameter of 1400 µm, 940 µm or 460 µm. At the center of the circle, an opening was made in the titanium layer, which later served as a valve opening. An approximately 1.0 µm thick aluminum layer was dusted on the titanium layer within the raised edge. The aluminum layer was in turn covered with an approx. 2 µm thin polyimide layer, from which several radially symmetrical, circular areas with a diameter of a few 10 µm were removed using microtechnology processes. These areas later served as valve openings in the polyimide layer. The selective etching of the sacrificial layer made of aluminum should create a cavity, so that a free connection between the individual valve openings in the titanium or polyimide layer (inlet and outlet openings) was created. Both the titanium and polyimide layers served as valve membranes. The etching process could only be initiated through the circular areas in the polyimide layer. Since the polyimide layer is transparent, the etching process on the underlying aluminum layer can be followed optically well; the end point of the etching can be precisely defined. This can prevent the etching solution from creeping under the titanium layer and causing premature detachment of the titanium layer.

To carry out the etching step, the electrically contacted sample was placed on the microscope table as shown in FIG. 2 and a drop of an electrolyte solution was pipetted on. The electrolytic solution consisted of an aqueous solution of 10% KJ and 1.8% 1-H-benzotriazole. The micro-carbon fiber (diameter 7 µm) was then adjusted with the help of the micromanipulator to one of the circular areas close above the aluminum layer. A low DC voltage (......... volts) was then set on the controllable power supply unit until a noticeable hydrogen gas development was detectable on microscopic observation on the carbon fiber connected as cathode. As a rule, the aluminum did not dissolve when the power was switched on, but required an induction period which was between about 1 and 5 minutes and which depended on the degree of passivation of the aluminum.

The beginning of the aluminum etching and the progress of the etching process were always clearly recognizable, because the titanium that emerged during the free etching formed a good contrast. Depending on the size of the area to be etched, the etching times including the induction period were between approx. 5 and 15 minutes. The voltages required for the electrolytic etching process were in a range between 1 and 1.5 V, the associated currents between 10 and 50 µA. Current densities cannot be specified because the area changed continuously during the etching process. By using voltages up to 2 V, the etching speed could be increased significantly, but for safety reasons with regard to the selectivity of the etching towards copper was waived. Compared to wet chemical etching with 5% sulfuric acid, the process according to the invention achieved an increase in speed by a factor of 60 to 180.

Another interesting fact has been observed that it is possible to shorten the induction period before the actual aluminum etching by creating a short circuit. If you bring the micro-carbon fiber briefly onto the aluminum surface and thus generate a short-circuit current, then the etching process starts at this point within a very short time. It is also possible to induce the etching process at other points where no etching is taking place by generating a short circuit.

In the case of the underetching described here, the electrolytic etching proceeds very evenly in all directions. After the electrolytic etching, the samples were rinsed well with dilute sulfuric acid to prevent the formation of insoluble basic aluminum compounds as a residue. During the etching process, the copper structures in contact with the electrolyte solution were either only very slightly or not visibly attacked.

Claims (9)

1. Method for electrolytic etching, wherein

   a) a first electrical potential is applied to a sample to be etched,

   b) the sample to be etched is brought into contact with an electrolytic solution,

   c) an electrode is introduced into the electrolytic solution,

   d) a second electrical potential is applied to the electrode, the first potential differing from the second potential, characterised in that an electrically conductive micro-carbon fibre is used as the electrode.
2. Method according to claim 1, characterised in that the sample to be etched is brought into contact with electrolytic solution by a drop of the electrolytic solution being applied to the sample.
3. Method according to claim 1 or 2, characterised in that the sample to be etched is formed from at least two different substances, of which one substance is selectively etched from the remaining substance or substances.
4. Method according to claim 3, characterised in that the substance, which is selectively to be etched from the remaining substances, has been sprayed-on (sputtered-on).
5. Method according to claim 3 or 4, characterised in that the substance, which is selectively to be etched from the remaining substances, is aluminium.
6. Method according to claim 5, characterised in that the electrolytic solution contains water and an iodide salt.
7. Method according to claim 6, characterised in that the iodide salt is an alkali metal iodide.
8. Method according to claim 6 or 7, characterised in that an alcohol is added to the electrolytic solution.
9. Method according to claim 8, characterised in that the alcohol is methanol and/or isopropanol.

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