JPH0593300A - Electrolytic etching method - Google Patents

Abstract

PURPOSE:To uniformize the electrolytic current distribution in the etching region by using the surface of a solid to be worked as the working electrode and a probe-shaped electrode inserted into a capillary having an open tip as a counter electrode and allowing an electrolyte to flow from the open tip of the capillary. CONSTITUTION:The solid surface of a material to be etched set in an electrolytic cell 10 is used as the working electrode WE, a capillary 8 having an open tip is opposed to the solid surface with a gap in between, and a probe-shaped electrode is inserted into the capillary 8 as a counter electrode CE. The electrolytic cell is filled with an electrolyte, the working electrode WE, the counter electrode CE and a reference electrode RE are connected to a potentiostat 11, the electrolyte is allowed to flow from the open tip of the capillary 8 and circulated by a microtube 9, an anode current pulse is impressed from the potentiostat 11, and electrolytic etching is conducted. Consequently, a small etched hole almost corresponding to the open tip of the capillary 8 is formed in the solid surface on the working electrode WE even without using a mask.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrolytic etching method as an electrochemical microfabrication method, and more particularly to an electrolytic etching method using as a counter electrode a probe-like electrode having at least its tip inserted in a capillary having an open tip. Regarding

[0002]

2. Description of the Related Art In recent years, various silicon microfabrication methods have been developed which originated in electronic device manufacturing technology, and various microactuators and sensors have been tried to be miniaturized. As for such fine processing technology, although laser processing and electric discharge processing are being actively developed as new precision processing methods, photolithography and wet anisotropic etching are mainly used as polishing / cutting means. There are many combinations.

In the field of electrochemistry, electroplating and electrolytic etching have been widely used as precision surface processing techniques. For example, as a product obtained by using such a processing technique, a net blade of an electric razor, a stamper of a CD, and the like can be given.

The advantages of these electrochemical techniques include not only the common advantages of etching techniques such as the ability to simultaneously produce a large amount of products by simultaneous processing but also the wide choice of the type of solution used and the reaction. There are advantages such as easy control and allowing the reaction to occur under mild conditions.

On the other hand, the disadvantages of these electrochemical methods are that a special electrode for processing is required, that the object to be processed must have a conductive surface, and that there are general drawbacks of the wet method. There are some problems in processing accuracy that can be seen. From this point of view, the conventional electrochemical method has been mainly used for the treatment of a relatively large surface, and it is not always suitable for the treatment of a local minute surface. The technology is not established.

From a different point of view, the problems associated with performing electrochemical processing using microelectrodes are consistent with the advantages of microelectrodes often used in electrochemical analysis these days.

That is, as the area of the electrode is reduced, the abnormal current density in the peripheral area of the electrode, that is, the so-called edge effect becomes more effective, and the current distribution near the electrode surface has a radial spread.

From the standpoint of electrochemical analysis, this creates advantageous conditions such as reducing the influence of liquid resistance and increasing the current value per unit area.

However, in terms of surface processing,
This phenomenon causes a large non-uniformity of the current distribution, and at the same time, greatly expands the processing region, resulting in a significant reduction in processing accuracy and efficiency. In extreme cases, surface processing is completely impossible.

[0010]

Problems to be Solved by the Invention The improvement of the current distribution is
This can be achieved by reducing the distance between the work surface that is the working electrode and the microelectrode that is the counter electrode. Therefore, when the electrode is made minute, the tip of the electrode must be brought closer to the processed surface in order to perform the processing corresponding to the miniaturization.

Except in the case where an extremely small number of atoms are moved as in an experiment using an STM (scanning tunneling microscope), when a certain amount of processing is performed, a dissolution precipitation reaction accompanying the processing causes The surface shape of the microelectrode changes every moment. Further, at the same time, it is possible that the processed surface is contaminated by the reaction products of the microelectrodes, so generally speaking, it can be said that the processing using the microelectrodes has an extremely difficult problem to solve.

As a method for solving the above-mentioned difficult problems, the present inventors have introduced an etching method (hereinafter, abbreviated as "capillary etching method") in which a probe-shaped microelectrode is inserted into a capillary and this is used as a counter electrode. There is also something to do). That is, copper is used as the material to be etched, which is the working electrode, and a glass capillary electrode having a fine opening tip as a counter electrode (a platinum wire in which a counter electrode is inserted into the glass capillary) is brought into close contact with it and closed. A copper electrode part is created and electrolytic etching is performed using a sodium nitrate solution as an electrolytic solution (supporting electrolyte solution).

In this electrolytic etching method, an electrolytic current flows in the capillaries to obtain a uniform electric current with uniform directionality. Therefore, minute holes having the same size as the inner diameter of the capillaries can be formed in the copper surface. Generally speaking, in isotropic electrolytic etching on a polycrystalline copper plate, etc.,
Peripheral over-etching (side etching) as an edge effect has become a serious problem. In order to overcome this drawback, it is indispensable to make the current distribution near the edge surface uniform. From this point of view, it is possible to obtain the above results by preventing the current distribution from becoming nonuniform by the capillary. Further, it has been clarified that this electrolytic etching method can prevent the contamination of the electrode by the electrolytic reaction product.

However, in the above-mentioned etching method, a copper salt of nitric acid is generated in the capillary along with the progress of electrolytic etching, and this is deposited in the end, and the tip of the capillary is clogged. However, there is a problem that the etching reaction rate becomes slower in the supersaturated state. For this reason, this electrolytic etching method has no potential as a practical industrial fine processing technology.

The present invention has been made under the technical background as described above, and an object thereof is to provide an electrolytic etching method having a possibility of development as a practical industrial fine processing technology.

[0016]

[Means for Solving Problems] As a result of further diligent study on the above capillary etching method,
The inventor of the present invention has found that the above object can be achieved by the following electrolytic etching method, and has completed the present invention.

That is, according to the present invention, a solid surface to be processed is used as an operating electrode, and a probe-like electrode having at least its tip inserted into a capillary having an opening tip is used as a counter electrode, and at least the opening of the capillary is used. An electrolytic etching method is provided, which is performed while flowing an electrolytic solution through a tip.

The present invention will be described in detail below. The feature of the capillary etching method of the present invention resides in that the electrolytic solution is caused to flow through the opening tip of the capillary. In order to realize this, in order to realize this, the electrolytic solution inside and outside the capillary is fluidized and connected. It should be in a state.

Specifically, one method is to provide a gap between the solid surface as the working electrode and the peripheral surface of the tip opening of the capillary. Further, if at least one notch is provided in the peripheral surface portion of the tip opening of the capillary, for example,
Even if the peripheral surface of the tip end opening of the capillary is brought into contact with the surface of the working electrode solid, the flow communication can be realized (of course, a gap may be provided in this case).

The size of the above-mentioned gap generally varies depending on the shape, size, etc. of the tip opening of the capillary. For example, about 0.01 μm to 20 μm, preferably about 0.1 μm to 10 μm. And it is sufficient. Generally, the larger the tip opening, the larger the gap.

When at least one notch is provided, its shape and size may be selected according to the shape and size of the capillary, and is not particularly limited.
In this case, if the effect caused by the disturbance of the current distribution due to the cutout portion can be neglected, the electrolytic etching operation may be performed while rotating the capillary.

The capillary is not particularly limited as long as it does not interact with the electrolytic solution and can suppress the spread of the current distribution, and various materials can be used.
For example, acrylic hollow plastic fibers or glass capillaries can be used. These capillaries themselves may be used as the tip opening,
A small hole may be formed in a plastic or glass plate, and the small hole may be attached to the tip of the capillary as described above.

A jig made of plastic or the like may be used to secure the position fixing of the capillary in the electrolytic solution. Such a jig is provided with one or more grooves on the side facing the working electrode so as to communicate with an external electrolytic solution so as not to hinder the flow of the electrolytic solution through the tip opening of the capillary.

The cross-sectional shape of the capillaries and the shape of the tip opening of the capillaries are not particularly limited, and may be triangular,
Although various shapes such as a square, a polygon, a circle, an ellipse, a rectangle and the like can be adopted, the circle is the most general and preferable. Further, the capillary may or may not be tapered toward the tip opening.

The size of the opening of the tip of the capillary is not particularly limited because it depends on the purpose of etching and the like, but is preferably about 0.1 μm to 2 mm,
The thickness is more preferably about 1 μm to 100 μm, and further preferably 3 μm to 50 μm in consideration of the ease of forming the tip opening. It is not preferable to make the size of the tip opening too small, because it becomes difficult to process the capillary, and if the tip opening is too large, side etching is likely to occur. It approaches anisotropic etching in which the tip opening becomes smaller.

Examples of the counter electrode material to be inserted into such a capillary include carbon, platinum, gold stainless steel, nickel, copper and lead. These wires may be inserted in a straight line into the capillary, for example,
It may be inserted in the form of a spring.

As the working electrode to be processed, copper, nickel, aluminum, stainless steel, silicon, platinum,
Examples thereof include gold, but are not limited to these.

As the reference electrode, a silver / silver chloride electrode, a copper / copper sulfate electrode, an antimony electrode, a calomel electrode or the like can be used.

The electrolyte used in the electrolytic solution varies depending on the material of the working electrode, but may be sodium nitrate, potassium nitrate,
Examples thereof include sodium fluoride, potassium fluoride, ferric chloride, potassium chloride, sodium chloride, nitric acid, hydrochloric acid and sulfuric acid.

If desired, an additive (smoothing agent) such as phosphoric acid, thiourea, saccharin or lead acetate may be added to the electrolytic solution.

The distance between the counter electrode and the working electrode inserted in the capillary is not particularly limited, but may be, for example, 3 mm to 2 cm, preferably 5 mm to 1 cm. Generally, if the above distance is long, the liquid resistance becomes large and the reaction tends to be difficult to control.

During the electrolytic etching operation, the capillaries may be rotated to make the etching end face uniform, and in particular, in the case of a peripheral surface of the tip opening having no notch, it is not necessary to rotate the capillaries.

The flow direction of the electrolytic solution flowing through the opening end of the capillary may be from the inside of the capillary to the outside of the capillary, or vice versa. In the former case,
It is possible to prevent the counter electrode from being contaminated by the electrolytic reaction product, and in the latter case, there are respective characteristics that the electrolytic reaction rate can be accelerated.

The flow rate of the electrolytic solution cannot be generally stated because the optimum rate varies depending on the size of the opening at the tip of the capillary, the size of the gap between the working electrode and the counter electrode, and the like, together with the above factors, the purpose of etching, etc. It should be decided in consideration of. Generally, if the flow rate is too low, the etching efficiency (etching amount with respect to the total amount of electricity used) decreases, but if it exceeds a certain flow rate, the etching efficiency tends to be substantially constant.

Whether or not to use, for example, a photolithographic mask on the solid surface to be processed as the working electrode depends on the case. When the gap between the working electrode and the counter electrode is small, the etching hole can be formed with a clean contour without a mask. If the gap between the working electrode and the counter electrode is large, overetching becomes large, and therefore a mask may be used in some cases.

In carrying out the electrolytic etching operation, it is preferable to use a current pulse or a potential pulse. The pulse method is effective in smoothing the etched surface, can reduce side etching (overetching) of etching small holes, and can generally improve etching efficiency in many cases.

The current pulse is generally easier to control the pulse height constant, and has an advantage in improving the etching efficiency. The potential pulse has an advantage in that the selectivity of the reaction can be improved.

There are an oxidation current pulse and a reduction current pulse as the current pulse, but the potential is set to zero and the electrolytic etching does not proceed only with the reduction current pulse. However, if the reducing current pulse is supplied with a constant reducing current and the same state as that of the reducing current pulse is supplied as a result, or if the reducing current pulse is supplied with a constant oxidizing current, the above effect can be expected. Regarding such a point, it can be considered in the case of the potential pulse as in the case of the current pulse.

At this time, the duty ratio (ratio of the oxidation current pulse to the reduction current time) has no great effect in terms of action and effect except when it is extremely large or small.

Especially when the solid surface to be processed is a material which tends to form a passive film, the use of current pulse or potential pulse greatly contributes to the improvement of etching efficiency.

For example, when electrolytic etching is performed using copper that does not form a passivation film as a working electrode and a 3M sodium nitrate solution as an electrolytic solution, a reducing current is not passed but an oxidizing current pulse (anode pulse) is passed. In this case, when the pulse height is very small (1 mA or less), the etching efficiency improves as the pulse height increases, but in other pulse height regions, the etching efficiency depends mostly on the height of the oxidation current pulse. Since it is not performed (the copper surface is considered to be a fully active surface), the effect of improving the etching efficiency by using the current pulse is not so large, but there is an effect of reducing the side etching especially on the short wavelength side.

For example, when nickel which forms a passivation film is used as a working electrode and a 3M sodium nitrate solution is used as an electrolytic solution, a current is not applied to the reducing side, and an oxidizing current pulse (anode current pulse) is applied. Although the etching efficiency improves as the height increases, the same amount of etching requires several tens of times as much electricity as copper. It is considered that, although the etching is largely restricted by the passivation film on the nickel surface, a part of the film is destroyed by the anode current pulse and the etching proceeds.

When a cathode current pulse (reduction current pulse) is alternately applied to the anode current pulse, etching is performed while reducing the passivation film, and the etching efficiency tends to be further improved. In this case, the time until the passivation film that has been reduced and disappeared is reoxidized and regenerated is
It is considered to be on the order of 10 ms (microseconds), and thus the wavelength of the cathode current pulse should be shorter in order to exert the passivation film reduction effect. However, at wavelengths shorter than about 2 ms, the opposite effect due to electric double layer charging occurs, and the etching efficiency tends to decrease. Therefore, in the nickel etching as described above, an anode current pulse wavelength in the range of about 2 ms to about 20 ms is preferable because the reforming of the partially reduced passivation film during the off pulse competes with the etching.

In such electrolytic etching of nickel or the like, the passivation film is less likely to be formed in a state of insufficient dissolved oxygen. Therefore, as the depth of the small holes increases as the etching progresses, the oxygen diffusion is delayed and the etching rate increases. Tend to be accelerated. That is, the state is such that pitting is intentionally caused. In this case, over-etching (side etching) is small due to the existence and formation of the passivation film.

Further, in the case of electrolytic etching of a metal such as nickel which forms a passivation film, if halogen ions are present in the etching environment, chemical destruction (pitting corrosion) of the passivation film is likely to occur, resulting in small holes. Portions other than (pits) are covered with a passivation film, and cathodic reduction of oxygen occurs on the entire surface, forming a local cell and accelerating anode dissolution in the pits. Once pits are generated, halogen ion concentration and pH change occur inside the pits, and pit growth continues.

From the above, in the case of electrolytic etching of a metal forming a passivation film such as nickel, it is preferable that halogen ions such as chlorine ions coexist in the supporting electrolyte solution. As the halogen ion source, for example, sodium salts, potassium salts, etc. of halogen can be used. The coexistence of halogen ions dramatically improves the etching efficiency.

[0047]

[Function] By using a capillary in which a probe electrode is inserted as a counter electrode, the distribution of the electrolytic current can be made uniform, and even if a mask is not used, the tip surface of the capillary can be almost formed on the solid surface as the working electrode. Corresponding etched pores can be formed.

Since the electrolyte flows through the tip opening of the capillary into which the probe electrode as the counter electrode is inserted,
It is possible to prevent the electrolytic reaction product from accumulating / precipitating in the capillary, clogging the tip opening of the capillary, or contaminating the counter electrode, and the electrolytic reaction product becomes supersaturated in the etching region, It is possible to prevent the deceleration of the etching rate from occurring, and it is extremely feasible as an industrial fine processing method.

[0049]

EXAMPLES The present invention will be described more specifically and in detail below with reference to the accompanying drawings, but the present invention is not limited to the examples. FIG. 1 is an explanatory diagram of an experimental apparatus for carrying out the electrolytic etching method of the present invention, and is a diagram schematically illustrating one used in the following examples. The depiction of "capillary electrode 8" (described later) in this figure is inaccurate, but this is because the drawing is simplified, and the following description will provide a sufficient understanding of what the actual capillary electrode used in the examples is. Should be done.

EXAMPLE A tip-capillary glass tube having a probe electrode used as a counter electrode (CE) for electrolytic etching (here,
This is referred to as "capillary electrode 8") was manufactured as follows.

The tip of a glass tube having an outer diameter of 2 mm, an inner diameter of 1 mm and a length of 90 mm was processed into a capillary tube having an outer diameter of about 5 μm, and the tip surface was polished to be smooth. Next, a plastic bifurcated plug was attached to the rear part of the glass tube, and a platinum wire (counter electrode CE) with a diameter of 0.2 mm was inserted into the glass tube from one of the ports, and the resin was bonded and potted with epoxy resin. At the other end, a silicone rubber tube 7 for causing the electrolytic solution sent by a microtube pump 9 [MP-3, manufactured by Tokyo Rika Kikai Co., Ltd.] to flow out from the capillary tube is similarly bonded with an epoxy resin. It was potted.

Using a micromanipulator, the tip of the capillary electrode 8 thus manufactured is 1 μm from the surface of the etching sample as the working electrode (WE).
It was set at a position slightly above. The distance between the working electrode (WE) and the counter electrode (CE) was about 5 mm.

On the other hand, a silver / silver chloride reference electrode was used as a reference electrode (RE) for regulating and measuring the potential of the etching sample that was subjected to etching as the working electrode (WE).

The above three electrodes are potentio-galvanostats (hereinafter simply referred to as "potentiostats") 11 [2000, manufactured by Toho Giken Co., Ltd., which is a device for regulating the potential or the current during the etching operation. ]
Connected to. Further, a function generator 12 [FG-02, Toho Giken Co., Ltd.], which is a device for operating the potentiostat 11 in a potential pulse or current pulse state, was connected to the potentiostat 11.

As the etching sample, a copper foil having a thickness of 10 μm as an example of a non-corrosion resistant metal and a nickel foil having a thickness of 10 μm as an example of a corrosion resistant metal were used.

As an electrolytic solution, 3 is used for etching copper foil.
A M sodium nitrate solution was used, and a 3M sodium nitrate + 0.5M sodium chloride solution was used for etching the nickel foil. The liquid temperature was kept constant at 300K.

By filling the electrolytic bath 10 with the electrolytic solution and letting the microtube pump 9 flow out from the tip opening of the capillary electrode 8 (flow rate: about 3 ml / hr), etching is always performed with a new electrolytic solution. .. The electrolytic solution passes from the electrolytic cell 10 through the microtube 7 ′, returns to the microtube pump 9, and circulates.

After the electrolytic bath 10 was filled with the electrolytic solution, the etching sample (WE) and the capillary electrode 8 were installed, and then the potentiostat 11 and each electrode were connected.

For electrolytic etching, an anode current pulse having a wave height of −10 μA and a wavelength of 500 ms generated from the potentiostat 11 through the function generator 12 was used with a duty ratio of 50% to the cathode current time. I was there. Here, the xt recorder 13 [R302V, manufactured by Rika Denki Co., Ltd.] recorded a change in potential.

The results of electrolytic etching as described above are as follows. It was confirmed by a scanning electron microscope (SEM) that a circular small hole having a diameter of about 5 μm penetrated through both the etched copper foil and nickel foil.

In the case of copper foil, the shape of the small holes was almost a perfect circle, and almost no overetching was observed. On the other hand, in the case of nickel foil, the small holes are circular with a slightly distorted shape,
Although almost no over-etching was observed, unevenness due to the appearance of grain boundaries was observed on the side wall.

In each case, the electrolytic etching rate was about 2 to 3 μm. This is much faster than the dry etching method, and it is possible to further improve the electrolytic etching rate by the method of the present invention.

In the case of electrolytic etching of a copper foil, it was found that if the electrolysis time is too long, for example, about 30 minutes, "drip" will occur on the side wall of the small hole.

[0064]

According to the electrolytic etching method of the present invention,
The distribution of the electrolytic current in the etching region can be made uniform, and even if a mask is not used, it is possible to form an etching small hole substantially corresponding to the tip opening of the capillary on the solid surface as the working electrode.

Since the electrolyte flows through the tip opening of the capillary into which the probe electrode as the counter electrode is inserted,
It is possible to prevent the electrolytic reaction product from accumulating / precipitating in the capillary, clogging the tip opening of the capillary, or contaminating the counter electrode, and the electrolytic reaction product becomes supersaturated in the etching region, It is possible to prevent the deceleration of the etching rate from occurring, and it is extremely feasible as an industrial fine processing method.

In the case of the dry etching method, the maximum etching depth is limited to about 10 μm because the resist layer as a mask is also etched at the same time. In the electrolytic etching method of the present invention, however, etching far deeper than this is possible. It is possible.

In the method of the present invention, it is considered that the electrolytic etching operation can be performed while scanning the tip of the capillary by controlling the movement of the capillary, for example, by a computer. As a cutting / polishing means for micromachining, a complicated shape can be used. It is expected that it can be used for manufacturing micromachines, printed wiring boards, semiconductor devices of some kind, and the like.

[Brief description of drawings]

FIG. 1 is a schematic explanatory view of an example of an apparatus for carrying out the electrolytic etching method of the present invention.

[Explanation of symbols]

 WE Working electrode CE Counter electrode RE Reference electrode 7 Micro tube 7'Micro tube 8 Capillary electrode 9 Micro tube pump 10 Electrolyzer 11 Potentiostat 12 Function generator 13 xt recorder

Claims (3)

[Claims] 1. A solid electrode to be machined is used as a working electrode, and a probe electrode having at least its tip inserted into a capillary having an open tip is used as a counter electrode.
An electrolytic etching method, which is performed while flowing an electrolytic solution through an opening end of the capillary. 2. The electrolytic etching method according to claim 1, wherein a gap is provided between the solid surface and an opening tip of the capillary. 3. The electrolytic etching method according to claim 1, wherein the method is performed by using a current pulse or a potential pulse.