Method and apparatus for electrochemical fine working of materials

Abstract: An electrochemical cell is constructed with the use of a four-electrode system that comprises a probe 1, material to be worked 2, reference electrode 10 and outer electrode 11. Respective potentials of the probe 1 and material to be worked 2 are set to be at potentials that cause no electrochemical reaction to occur. Then, the Z-axial position of the probe 1 is controlled so that the tunnel current that flows between the probe 1 and the material to be worked 2 may be fixed. Then, while the probe 1 is being moved along a working line, the Z-axial position of the probe 1 is stored continuously in a memory device 8 whereby storage is performed of the irregularities configuration and inclinations of the material to be worked 2. When working is performed, the electrochemical cell is reconstructed in the form of a three-electrode system that comprises the probe 1, material to be worked 2, and reference electrode 10, whereby, the probe 1 is again moved along the working line while controlling the Z-axial position of the probe 1 to be kept at the stored position or a position that has been obtained by adding thereto a certain offset. Simultaneously, a voltage is applied between the probe 1 and the material to be worked 2 to thereby perform the working operation along the working line through an electrochemical reaction.

Description: The present invention relates to a method and apparatus for performing fine working which is directed, in metal industries, electronic industries, etc., to performing fine working in a solution through an electrochemical reaction by the use of a probe having a fine tip. US 5308974 describes a scanning probe microscope which uses stored data to control the vertical position of the microscope probe over a surface. The scanning operation is conducted in two phases. In the first phase the probe is scanned over the surface to obtain topographical data, which is stored. The stored topographical data is then used to control the vertical height of the probe above the surface in the second phase, during which a parameter of the surface other than photometry, such as surface temperature, is measured, or a task such as the manipulation of DNA strand is carried out. As a method of performing working in a liquid through an electrochemical reaction by the use of a probe having a fine tip, there has hitherto been reported a method of performing working by the use of an electrochemical scan type tunnel microscope. In a method of performing fine working which is directed to approaching a probe having a fine tip to the surface of a material to be worked and thereby performing fine working by utilizing an electrochemical reaction that occurs between the two, in order to enhance the working precision it is important to decrease the distance between the probe and the material to be worked and control this distance to be kept fixed. If the distance between the probe and the material to be worked increases, the working area inconveniently widens. Also, if the distance between the probe and the material to be worked varies during the working operation, it is difficult to shape the worked configuration as predetermined. Since in order that the working precision may be on an order of sub-microns it is necessary that the distance between the forward end of the probe and the material to be worked be also at a level of sub-microns, it is difficult to control such a fine distance with the use of optical means. On this account, if measurement is performed of the tunnel current that flows between the forward end of the probe and the material to be worked, it becomes possible to control such a fine distance with a high precision relatively easily. While the conventional method of performing fine working that uses an electrochemical scan type tunnel microscope is also arranged to make feedback control of the probe-to-specimen distance by the use of this tunnel current, it involves several problems. First, there is pointed out the respect that when an electrochemical reaction is caused to occur between the probe and the material to be worked, the Faraday current (electrolytic current) flows between the two. It is difficult to determine whether the current that flows between the probe and the material to be worked is a tunnel current or Faraday current. Also, in the method of making a feedback control of the probe-to-working-material distance by the use of the tunnel current, there is the problem that when an electrochemical reaction occurs with the result that the Faraday current flows, the distance between the probe and the material to be worked inconveniently varies with the result that the worked configuration diverges from the predetermined configuration. In order to avoid the occurrence of this problem, there can be also considered the use of a method to make the feedback control ineffective at the time of performing the working operation and make the Z-axial position of the probe fixed. However, there is the problem that in a case where working is continuously performed while the probe is being moved, since the distance between the material to be worked and the probe is very short, the probe inconveniently collides with the material to be worked due to the surface roughness thereof, surface inclinations thereof, etc. Also, in a case where the feedback control is performed with the use of a tunnel current, the distance between the material to be worked and the probe must be a magnitude of distance that enables the detection of the relevant tunnel current. Namely, the degree of freedom with which the relevant distance can be set was not high. Also, since in the process of an electrochemical reaction the amount of reaction is proportionate to the value of the Faraday current, in order to adjust the amount of working it is important to control the Faraday current that flows between the probe and the material to be worked. In the conventional electrochemical scan type tunnel microscope, generally, the probe and the material to be worked operate respectively as working electrodes and an electrochemical cell is constructed with a four-electrode system that comprises these working electrodes and reference and counter electrodes added thereto. In the case of this construction, although the potential of each of the probe and the material to be worked can be independently set, the cell basically is constructed with a main purpose placed on controlling the electrochemical reactions that occur between the probe and counter electrode and between the material to be worked and counter electrode. This means that the cell construction is not made so as to control precisely the Faraday current between the probe and the material to be worked. For this reason, there arises also the problem that it is difficult to adjust the amount of working. SUMMARY OF THE INVENTION With a view of overcoming or at least reducing the above problems, the present invention provides a method and apparatus for fine working as set forth in claims 1 and 3, respectively. It is a feature of the present invention to provide a method of performing fine working that is possible to control a distance between a probe and a specimen to be kept fixed with no Faraday-current effect. It is another feature of the present invention to provide a method of performing fine working that is possible to set the distance between the probe and the specimen to be at a large distance that disables the detection of the tunnel current, with the result that the degree of freedom for setting such distance is high. It is a further feature of the present invention to provide a method of performing fine working that is possible to control easily an amount of working by controlling the Faraday current. In accordance with a first aspect of the invention a method of performing fine working in which a material to be worked is electrochemically worked in a liquid by the use of a probe having a fine tip, comprises the steps of: constructing an electrochemical cell in a form of a four-electrode system that comprises the probe having the fine tip, the material to be worked, a reference electrode and a counter electrode; setting each potential of the probe having the fine tip and the material to be worked to the value at which no electrochemical reaction occurs; controlling the distance between the tip of the probe and the material to be worked whilst maintaining a constant tunnel current between the material to be worked and the probe having the fine tip; storing the z-axial position of the tip of the probe in order to store a configuration of the material to be worked; re-constructing the electrochemical cell in the form of a three-electrode system that comprises the probe having the fine tip, the material to be worked, and the reference electrode; controlling the z-axial position of the tip of the probe to be at the stored position thereof or a position which has been obtained by adding a certain fixed offset to the stored position, and working the material to be worked by electrochemical reaction. Said working the material may include applying any one of a constant voltage or a constant voltage pulse or constant current or constant current pulse between the probe and the material. In accordance with a second aspect of the invention, there is provided an apparatus for carrying out a method of performing fine working as set forth above, comprising: a probe having a fine tip, means for moving the probe having the fine tip across the surface of a material to be worked in a liquid for causing an electrochemical reaction; means for selectively applying voltage between the probe having the fine tip and the material to be worked, as part of a three-electrode system which includes a reference electrode, or between the probe having the fine tip, the material to be worked and a counter electrode, as part of a four-electrode system which includes said reference electrode; means, as part of said four-electrode system, for measuring a tunnel current between the probe having the fine tip and the material to be worked when each potential of the probe having the fine tip and the material to be worked is set to the value at which no electrochemical reaction occurs, means for controlling the distance between the tip of the probe and the material to be worked in dependence on the tunnel current, and means for storing information relating to the z-axial position of the probe; means, as part of said three-electrode system, for applying voltage between the probe having the fine tip and the material to be worked, wherein the probe having the fine tip, the material to be worked and the reference electrode in said three-electrode system are immersed in said liquid so as to cause the occurrence of an electrochemical working; and means for switching from the means for measuring a tunnel current to the means for applying voltage between the probe having the fine tip and the material to be worked. BRIEF DESCRIPTION OF DRAWINGS Fig. 1 is a block schematic diagram of an apparatus according to the present invention; Fig. 2 is a flowchart of performing fine working according to the present invention; and Fig. 3 is a photograph showing an example wherein a pattern is formed on a thin film of chromium by the use of the method of performing fine working according to the present invention. DETAILED DESCRIPTION OF THE INVENTION An embodiment of a method of performing fine working according to the present invention will now be explained with reference to the drawings. Fig. 1 is a view illustrating an example of an apparatus for performing fine working that has been constructed for the purpose of executing the method of performing fine working according to the present invention. A probe 1 and a material to be worked 2 are immersed in an electrolytic solution 3 and are disposed in such a manner as to oppose each other. The probe 1 is installed on a probe driving mechanism 4 that is movable with high precision in the X, Y and Z directions. While in this embodiment a mechanism that comprises a plurality of combined piezoelectric elements is used as the probe driving mechanism 4. Such mechanism is not a constituent element that is indispensable for the method of performing fine working according to the present invention and this mechanism can be substituted for by another mechanism that has a similar function. Further, the probe driving mechanism 4 is connected to a probe position control mechanism 5. The probe position control mechanism 5 comprises in the interior thereof an X/Y axes control mechanism 6 for controlling the horizontal position of the probe, a Z-axis feedback control mechanism 7 for controlling the Z-axial position of the probe 1 so that the tunnel current that flows between the probe 1 and the material to be worked 2 may be fixed, memory device 8 which is connected to the Z-axis feedback control mechanism 7 and which can continuously record therein the variation in the Z-axial position of the probe 1 during the feedback control and from which the thus-recorded data can be again read out, and a Z-axis non-feedback control mechanism 9 for controlling the Z-axial position according to the data from the memory device 8. Also, within the electrolytic solution 3 there are installed a reference electrode 10 which in the electrochemical measurement serves as a reference for the electrode potential and an counter electrode 11 which in the electrochemical measurement serves as an electrode for applying a potential. The probe 1, material to be worked 2, reference electrode 10 and counter electrode 11 are connected through a switching mechanism 12 to one of a tunnel current measuring mechanism 13 that includes a measuring electrode potential control mechanism and a working electrode potential control mechanism 14. The signal from the tunnel current measuring mechanism 13 is input to the above-mentioned Z-axis feedback control mechanism 7. When the switching mechanism 12 has been operated to make changeover to the tunnel current measuring mechanism 13, there is constructed the electrochemical cell with a four-electrode system wherein the probe 1 and material to be worked 2 operate respectively as the working electrodes and counter electrode 11 operates as the counter electrode. On the other hand, when the switching mechanism 12 has been operated to make changeover to the working electrode potential control mechanism 14, there is constructed the elecrochemical cell with a three-electrode system wherein the probe 1 operates as the counter electrode and the material to be worked 2 operates as the working electrode. Fig. 2 is a flowchart of performing fine working according to the present invention. When performing working, first, the probe 1 is moved by the X/Y axes control mechanism 6 to the position at which the working of the material to be worked 2 is wanted to be started. (step 1) Next, the switching mechanism 12 is operated to make changeover to the tunnel current measuring mechanism 13 and then the potential of each of the probe 1 and material to be worked 2 is set to be in a range of potentials that causes no electrochemical reaction to occur between the two. (step 2) Then, the Z-axial position of the probe 1 is varied slowly to thereby make approach of the probe 1 to the material to be worked 2. (step 3) At this time, while the tunnel current that flows between the probe 1 and the material to be worked 2 is being measured by the use of the tunnel current measuring mechanism 13, approach is made of the probe 1 to the material to be worked 2 until the value of the tunnel current becomes a value that is prescribed. After the value of the tunnel current has become the prescribed value, the Z-axis feedback control mechanism 7 is made "ON" to thereby make feedback control of the Z-axial position of the probe 1 so as for the tunnel current to be kept fixed. (step 4) Next, while the probe I is being moved along a straight line or curve along which working is performed of the material to be worked 2, measurement is made of the Z-axial position of the probe 1 and the thus-measured data are continuously stored in the memory device 8. (step 5) After having completed the measurement of the conguration of the surface portion of the material to be worked 2 that extends along the working straight line or curve, the probe 1 is returned to the foremost position of the region to be worked. Next, the Z-axial feedback control mechanism 7 is made "OFF" and the Z-axial non-feedback control mechanism 9 is made "ON" so as for the Z-axial position of the probe 1 to be controlled according to the data from the memory device 8. (step 6) Further, the switching mechanism 12 is operated to make changeover to the working electrode potential control mechanism 14. An electrochemical cell in a three-electrode system is constructed with the probe 1, material 2 and the reference electrode 10. (step 7) And, the probe 1 is moved along the same surface portion configuration as that mentioned previously. (step 8) During this movement, while controlling the Z-axial position of the probe 1 according to the data from the memory device 8 so that the distance between the probe 1 and the material to be worked 2 may be fixed, an appropriate level of voltage is applied between the probe 1 and the material to be worked 2 by means of the working electrode potential control mechanism 14. (step 9) Then, according to the level of the voltage applied, and the kind of the electrolytic solution 3 used, at this time, the surface of the material to be worked is etched, or conversely substances are precipitated thereon by electric sedimentation along the locus that has been traced by the probe 1. (step 10) By repeating this, the material to be worked 2 can be finely worked into a predetermined configuration. At this time, by adding a certain offset to the data on the stored Z-axial position of the probe 1, it is possible to freely set the distance between the probe 1 and the material to be worked to be at a magnitude of distance that falls outside a range that enables the detection of the tunnel current and thereby select the size of the working spot and the depth of the working. Also, it is possible to use, as the method of applying a voltage when performing working, a method of applying a constant voltage continuously (constant voltage mode), a method of applying a voltage pulse continuously (constant-voltage pulse mode), a method of applying a constant current while controlling the current that flows so that this current may be kept constant (constant current mode), a method of applying a constant-current pulse while performing control so that this constant-current pulse may be applied (constant-current pulse mode), etc. Fig. 3 is a photograph that has been taken when having observed by means of the scan type tunnel microscope the result that had been obtained by etching a thin film of chromium on a glass substrate by the use of the above-mentioned method. Chromium is deposited by sputtering on the glass substrate to a thickness of 200 nm and the resulting glass substrate is used as the material to be worked 2. An aqueous sulfamic-acid solution of 0.1 mol/l was used as the electrolytic solution 3, a platinum-iridium alloy wire whose tip end was sharpened by electrolytic etching and whose portion that excluded the tip end was clad by resin was used as the probe 1, a platinum plate was used as the counter electrode 11, and a saturated silver/silver chloride electrode was used as the reference electrode 10. First, while under the conditions wherein the tunnel current = 0. 3 nA the probe 1 is being moved along' a straight line having a length of 20 µm at a speed of 200 nm/sec., the Z-axial position of the probe 1 is stored and measurement is thereby made of the surface configuration of the chromium thin film that extends along the same straight line. Next, while along this straight line the probe 1 was being moved at a position that had been obtained by adding an offset of 20 nm to the stored data, control was performed so that during this movement of the probe a current pulse of I on = 30 nA, T on = 0.3 sec. T off = 1.0 sec. was applied continuously between the probe 1 and the material to be worked 2 in the constant-current pulse mode. And, the working that corresponds to this straight line was repeated at 200 nm intervals whereby a square pattern of 20 x 20 µm was formed finally. The depth of the region thus etched is approximately 100 nm. As mentioned above, according to the method of performing fine working of the present invention, it is possible to control the distance between the probe and the specimen to be kept fixed with no Faraday-current effect being had thereon. In addition, it is also possible to set the distance between the probe and the specimen to be at a large distance that disables the detection of the tunnel current, with the result that the degree of freedom for setting such distance is high. Also, since the electrochemical cell is constructed with a three-electrode system, it is also possible to control easily the amount of working by controlling the Faraday current.

Claims: A method of performing fine working in which a material (2) to be worked is electrochemically worked in a liquid (3) by the use of a probe having a fine tip (1), comprising the steps of: constructing an electrochemical cell in a form of a four-electrode system that comprises the probe having the fine tip, the material to be worked, a reference electrode and a counter electrode; setting each potential of the probe having the fine tip and the material to be worked to the value at which no electrochemical reaction occurs; controlling the distance between the tip of the probe and the material to be worked whilst maintaining a constant tunnel current between the material to be worked and the probe having the fine tip; storing the z-axial position of the tip of the probe in order to store a configuration of the material to be worked; re-constructing the electrochemical cell in the form of a three-electrode system that comprises the probe having the fine tip, the material to be worked, and the reference electrode; controlling the z-axial position of the tip of the probe to be at the stored position thereof or a position which has been obtained by adding a certain fixed offset to the stored position, and working the material to be worked by electrochemical reaction. A method as claimed in claim 1, in which said working the material includes applying any one of a constant voltage or a constant voltage pulse or constant current or constant current pulse between the probe and the material. Apparatus for carrying out a method of performing fine working as claimed in claim 1, comprising: a probe having a fine tip (1), means (6) for moving the probe having the fine tip across the surface of a material to be worked (2) in a liquid (3) for causing an electrochemical reaction; means (14, 13) for selectively applying voltage between the probe (1) having the fine tip and the material (2) to be worked, as part of a three-electrode system which includes a reference electrode (10), or between the probe (1) having the fine tip, the material (2) to be worked and a counter electrode (11), as part of a four-electrode system which includes said reference electrode (10); means (13), as part of said four-electrode system, for measuring a tunnel current between the probe (1) having the fine tip and the material (2) to be worked when each potential of the probe (1) having the fine tip and the material (2) to be worked is set to the value at which no electrochemical reaction occurs, means (7) for controlling the distance between the tip of the probe (1) and the material (2) to be worked in dependence on the tunnel current, and means (8) for storing information relating to the z-axial position of the probe (1); means (14), as part of said three-electrode system, for applying voltage between the probe (1) having the fine tip and the material (2) to be worked, wherein the probe (1) having the fine tip, the material (2) to be worked and the reference electrode (10) in said three-electrode system are immersed in said liquid (3) so as to cause the occurrence of an electrochemical working; and means (12) for switching from the means (13) for measuring a tunnel current to the means (14) for applying voltage between the probe (1) having the fine tip and the material (2) to be worked.

Inventors: ATAKA TATSUAKI, SAKUHARA TOSHIHIKO, SUDA MASAYUKI, ATAKA, TATSUAKI, SAKUHARA, TOSHIHIKO, SUDA, MASAYUKI, Ataka, Tatsuaki, c/o Seiko Instruments Inc, Sakuhara, Toshihiko, c/o Seiko Instruments Inc, Suda, Masayuki, c/o Seiko Instruments Inc
Applicants: SEIKO INSTR INC, SEIKO INSTRUMENTS INC.
Application year: 1997
Main classification: G01N27
IPC codes: C25D21, B81C5, C25F3, G01N27, G11B9, C25F7, B23H3, G01Q70