Method and apparatus for performing fine working

A method for performing fine working of a material by electrochemical reaction comprises a two-step scanning operation in which a surface topography of the material is obtained during a first scan which is used to control the position of a probe during a second scan in which an electrochemical reaction is performed. During the first scan, an electrochemical cell is constructed with a four-electrode system, including the probe, a material to be worked, a reference electrode and a counter electrode. The potential of each of the probe and the material to be worked is set so that no electrochemical reaction occurs during the first scan. Data representative of the surface topography is stored and used to control the position of the probe during the second scan in which an electrochemical cell is constructed with a three-electrode system, including the probe, the material, and the reference electrode. The potential of the material with respect to the probe is set such that the electrochemical reaction occurs during the second scan, and the probe is maintained at a distance determined based on the stored topographical data.

BACKGROUND OF THE INVENTION 
The present invention relates to a method of 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. 
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 heretofore 
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 maintain this distance to be constant. 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 for the working precision may be on the order of sub-microns it is 
necessary for the distance between the forward end of the probe and the 
material to worked be also be at a level of sub-microns, it is difficult 
to control such a fine distance with the use of optical means. On account 
of this, 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, a 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 using 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 a 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 to fix the Z-axial 
position of the probe. However, there is a problem in that when 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 is 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 
It is an object of the present invention to provide a method of performing 
fine working that is possible to maintain a fixed distance between a probe 
and a specimen with no Faraday-current effect. 
It is another object of the present invention to provide a method of 
performing fine working in which it 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 object of the present invention to provide a method of 
performing fine working in which it is possible to control easily an 
amount of working by controlling the Faraday current. 
On this account, in the method of performing fine working according to the 
present invention, in order to solve the above-mentioned problems, data 
representing the topgraphy, i.e., inclinations and surface roughness of 
the working region of the material to be worked with respect to which 
working is about to be performed is pre-stored in a memory device and, at 
an actual time of working, the Z-axial position of the probe is controlled 
according to the pre-stored data so that the distance between the probe 
and the material to be worked may be fixed. First, the electrochemical 
cell is constructed with a four-electrode system that comprises the probe, 
the material to be worked, a reference electrode and a counter electrode 
and then the potential of each of the probe and material to be worked is 
set to fall within a range of potentials that causes no electrochemical 
reaction to occur. Then the Z-axial position of the probe is controlled so 
that the tunnel current that flows between the material to be worked and 
the probe may be fixed. While moving the probe along a working line along 
which working is about to be performed, the Z-axial position of the probe 
is continuously stored to thereby store the irregularities and 
inclinations of the surface of the material to be worked. At this time, 
since the potential of each of the probe and material to be worked is set 
to be a level that falls within a range that causes no electrochemical 
reaction to occur, no Faraday current flows with the result that only the 
tunnel current alone can be accurately measured. 
Next, the electrochemical cell is re-constructed as a three-electrode 
system that comprises the probe, the material to be worked and the 
reference electrode, whereby the probe is again moved, while controlling 
this time the Z-axial position thereof to be at the above-mentioned stored 
position or at a position that has been obtained by adding thereto a 
certain fixed offset, along the working line along which measurement has 
been made of the surface configuration of the material to be worked. 
Simultaneously with the re-measurement of the probe, a voltage is applied 
between the probe and the material to be worked to thereby cause the 
occurrence of an electrochemical reaction between the probe and the 
material to be worked. At this time, since the electrochemical cell is 
constructed with a three-electrode system and the distance between the 
probe and the material to be worked is kept fixed, it is possible to 
control easily the Faraday current that flows between the probe and the 
material to be worked. In addition, since at the working time there is no 
need to detect the tunnel current, the distance between the probe and the 
material to be worked can be freely set. For example, if it is desired to 
enlarge the working spot, it is possible to set the distance between the 
probe and the material to be worked to be at a value that is increased as 
the necessity arises.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
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 replaced 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, a 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 outer 
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 outer 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 a 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 and working electrodes and outer electrode 11 
operates as the counter electrode. On the other hand, when the switching 
mechanism 12 has been operated to make a changeover to the working 
electrode potential control mechanism 14, there is constructed the 
electrochemical 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 a preferred method 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 to be started. (step 
1) 
Next, the switching mechanism 12 is operated to make a 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 approach 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 turned "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 1 is being moved along a straight line or curve along 
which working is to be performed on the material to be worked 2, 
measurement is made of the Z-axial position of the probe 1 and the 
thus-measured data is continuously stored in the memory device 8. (step 5) 
After having completed the measurement of the configuration 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 turned "OFF" and the 
Z-axial non-feedback control mechanism 9 is turned "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 a 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) 
Next, 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 process, 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 outer 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 .mu.m 
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.sub.on =30 nA, T.sub.on =0.3 sec. and T.sub.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.times.20 .mu.m 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.