Method of manufacturing a platinum tip

In the production of a tunneling probe tip having a sharp point by utilizing electropolishing, the present invention has enabled the production of a platinum tunneling probe tip always produceable in an identical conical form having high point sharpness without scattering in the shape caused by the anisotropy of crystal face with respect to the rate of polishing during polishing of a platinum material through adoption of a two-stage polishing process, which comprises subjecting a platinum tunneling probe tip material to a primary polishing for primary shaping of the tunneling probe tip material into a tunneling probe tip form, and then subjecting the shaped material to a secondary polishing for sharpening of the point with a mixed acid comprising sulfuric acid, nitric acid, and phosphoric acid.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a process for producing a platinum 
tunneling probe tip having a sharp point for use in FE-SEM, STM, and an 
ion beam electrode. 
2. Description of the Related Art 
The production of a tunneling probe tip having a sharp point by 
electropolishing has heretofore been conducted on tungsten, TiC, tantalum, 
etc. With respect to platinum, it is known that the polishing can be 
conducted by electrolysis with some cyanide solution (see FIG. 1). 
However, with respect to the electropolishing of platinum, since the 
crystal grain of a platinum tunneling probe tip material is large as 
opposed to tungsten, TiC, tantalum, etc., the grain boundary is 
selectively polished, which raises problems with poor reproducibility, 
such as occurrence of scattering, in the shape due to the anisotropy of 
the crystal face with respect to the rate of polishing and difficulty in 
sharpening the point. 
SUMMARY OF THE INVENTION 
It is the primary object of the present invention to provide a method of 
manufacturing a platinum tunneling probe tip having a sharp point always 
produceable in an identical form through elimination of the 
above-mentioned drawback. 
In accordance with the present invention, there is provided a method of 
manufacturing a platinum tunneling probe tip comprising subjecting 
platinum material to a mechanical polishing, or to primary 
electropolishing with a sodium cyanide or potassium cyanide solution and 
subjecting the treated platinum material to secondary electropolishing 
milling with a mixed acid comprising sulfuric acid, nitric acid, and 
phosphoric acid. 
In the above-described electropolishing, the point of a platinum tunneling 
probe tip material is sharpened to a certain extent by the first stage 
electrolysis with a sodium cyanide or potassium cyanide solution to be 
formed into a sharp point, and the point is further sharpened by the 
second stage electrolysis with a mixed acid, which makes it possible to 
produce a platinum tunneling probe tip with high reproducibility.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present inventors have made various studies on electropolishing of a 
platinum tunneling probe tip material and, as a result, have found that 
electrolysis with a mixed acid comprising sulfuric acid, nitric acid, and 
phosphoric acid can provide excellent polishing finish with less influence 
of crystal grin of the platinum tunneling probe tip material. In view of 
the fact that the platinum tunneling probe tip cannot be sharpened by the 
above electropolishing unless the tip has a point sharpened to a certain 
extent, the present invention adopted a two-stage electropolishing process 
in the electrolysis wherein the electropolishing with the above-described 
mixed acid is conducted after a primary electropolishing with a sodium 
cyanide or potassium cyanide solution. 
The present invention will now be described in more detail with reference 
to the following examples. 
EXAMPLE 1 
A 0.3 mm .phi.-pure platinum wire as a tunneling probe tip material was 
degreased and washed with water. Then, the point portion of the tunneling 
probe tip material was immersed in a KCN solution to conduct first-stage 
electropolishing. 
(First-stage electrolysis conditions) 
electrolyte: 1M KCN 
electrolyzing voltage: 2-20 V AC 
plate: platinum 
electrolyzing time: 3-20 min 
After washing with water, a second-stage electropolishing was conducted 
with the following mixed acid to produce a platinum tunneling probe tip 
having a sharp point. 
(Second-stage electrolysis conditions) 
Composition of electrolyte: 
sulfuric acid: 1 volume 
nitric acid: 1 volume 
phosphoric acid: 1 volume 
electrolyzing voltage: 2-20 V AC 
plate: platinum 
electrolyzing time: 1-60 min 
In the electropolishing, it is possible to arbitrarily regulate the 
sharpness of the point of the tunneling probe tip by varying the 
electrolyzing voltage and time. 
The platinum tunneling probe tip thus formed had a high 
point-reproducibility and exhibited a stable tunneling probe tip 
performance (see FIG. 2). 
EXAMPLE 2 
A 0.3 mm .phi.-pure platinum wire as a tunneling probe tip material was 
degreased and washed with water. Then, the point portion of the tunneling 
probe tip material was immersed in a NaCN solution to conduct first-stage 
electropolishing. 
(First-stage electrolysis conditions) 
electrolyte: 1M NaCN 
electrolyzing voltage: 2-20 V AC 
plate: platinum 
electrolyzing time: 3-20 min 
After washing with water, second-stage electropolishing was conducted with 
the following mixed acid to produce a platinum tunneling probe tip having 
a sharp point. 
(Second-stage electrolysis conditions) 
Composition of electrolyte: 
sulfuric acid: 1 volume 
nitric acid: 1 volume 
phosphoric acid: 2 volumes 
electrolyzing voltage: 2-20 V AC 
plate: platinum 
electrolyzing time: 1-60 min 
In the electropolishing, it is possible to arbitrarily regulate the 
sharpness of the point of the tunneling probe tip by varying the 
electrolyzing voltage and time. 
As with the platinum tunneling probe tips formed in Example 1, the platinum 
tunneling probe tips thus formed had a high point reproducibility and 
exhibited a stable tunneling probe tip performance. 
EXAMPLE 3 
A 0.3 mm .phi.-platinum tunneling probe tip material was subjected to a 
first-stage electropolishing with a KCN solution in the same procedure as 
that of Example 1, and then subjected to a second-stage electropolishing 
with the following mixed acid. 
(Second-stage electrolysis conditions) 
Composition of electrolyte: 
sulfuric acid: 10% 
nitric acid: 5% 
phosphoric acid: 10% 
electrolyzing voltage: 2-20 V AC 
plate: platinum 
electrolyzing time: 1-60 min 
In the electropolishing, it is possible to arbitrarily regulate the 
sharpness of the point of the tunneling probe tip by varying the 
electrolyzing voltage and time. 
As with the platinum tunneling probe tips formed in Example 1, the platinum 
tunneling probe tips thus formed had a high point reproducibility and 
exhibited a stable tunneling probe tip performance. 
With respect to the electrolysis conditions according to the present 
invention, the concentration of the electrolyte for the first-stage 
electrolysis (sodium cyanide or potassium cyanide) may be in any range 
besides the concentration used in the above-described Examples so long as 
the other conditions are properly selected. However, the concentration of 
the electrolyte is preferably 0.3 to 5M from the viewpoint of the rate of 
polishing. Although it is preferred that the mixed acid be free from water 
and comprise sulfuric acid, nitric acid, and phosphoric acid, an excellent 
polishing effect can be attained as far as the mixed acid comprises at 
least 10% of sulfuric acid, at least 5% of nitric acid, and at least 10% 
of phosphoric acid. Further, it is also possible to conduct surface 
regulation through cathode electrolysis with an acid solution after the 
second-stage electropolishing according to the present invention. 
In a preferred embodiment, the platinum base material comprises at least 
99.9% of Pt and 0.005% or less of copper with the balance being 
unavoidable ingredients and has a crystal grain diameter regulated to 10 
to 200 .mu.m from the viewpoint of removing the factors inhibiting the 
polishing to form the point of a tunneling probe tip with a crystal grain 
having a suitable size. 
With respect to the method of producing a tunneling probe tip having a 
sharp point after a mechanical polishing or primary electropolishing for 
shaping a tunneling probe tip material into a macroscopic tunneling probe 
tip, a secondary electropolishing milling with a polishing solution 
composed of sulfuric acid nitric acid, and phosphoric acid is conducted. 
Various elements besides Pt are present as unavoidable ingredients in the 
platinum material and they remarkably spoil the electropolishing 
performance through selective dissolution thereof during electropolishing, 
although this tendency depends upon the content and kind of the element. 
In particular, Cu has a large influence of this type, and it is therefore 
preferred that the Cu content be 0.005% or less. 
The polishing solution comprising sulfuric acid, nitric acid, and 
phosphoric acid is suited for fine polishing, and it is difficult to 
directly shape a tunneling probe tip material into a tunneling probe tip 
form by making use of this polishing solution. Therefore, it is necessary 
to previously shape the tunneling probe tip material into a macroscopic 
tunneling probe tip form through mechanical polishing or electropolishing 
in a cyanide bath or the like. 
EXAMPLE 4 
A 0.3 mm .phi.-platinum wire was prepared from a platinum material 
comprising at least 99.9% of Pt and 0.005% of Cu with the balance being 
unavoidable ingredients such as Mn, Mg, Fe, Zn, Pd, Au, Ag and Rh through 
a step of wire drawing. The platinum wire was heat treated in vacuo at 
700.degree. C. for 30 min, and then gradually cooled to prepare a platinum 
tunneling probe tip material having a crystal grain diameter of 10 to 100 
.mu.m. The platinum tunneling probe tip material was shaped by mechanical 
polishing into a macroscopic tunneling probe tip form having a polishing 
angle of 30.degree.. Thereafter, the shaped tunneling probe tip material 
was degreased and washed with water and then subjected to electropolishing 
with the following polishing solution. 
Polishing solution: 
sulfuric acid-nitric acid-phosphoric acid 
(1:1:1) 
Electrolysis conditions: 
voltage: 2-20 V AC 
time: 1-60 min 
plate: platinum 
The tunneling probe tip thus formed had a sharp point and exhibited 
excellent performance when used as a platinum tunneling probe tip for STM. 
EXAMPLE 5 
A 0.3 mm .phi.-platinum wire was prepared from a platinum material 
comprising at least 99.9% of Pt and 0.0025% or less of Cu with the balance 
being unavoidable ingredients such as Mg, Mn, Fe, Zn, Pd, Au, Ag and Rh 
through a step of wire drawing. The platinum wire was heat treated in 
vacuo at 800.degree. C. for 30 min and then gradually cooled to prepare a 
platinum material having a crystal grain diameter of 10 to 150 .mu.m. The 
point portion of the platinum tunneling probe tip material thus prepared 
was immersed in the following polishing solution and subjected to 
electropolishing to shape the material into a macroscopic tunneling probe 
tip form. 
Polishing solution: 
1M KCN solution 
Electrolysis conditions: 
voltage: 2-20 V AC 
plate: platinum 
time: 3-20 min 
Thereafter, the shaped tunneling probe tip material was washed with water, 
and subjected to electropolishing with a polishing solution comprising 
sulfuric acid, nitric acid and phosphoric acid in the same manner as that 
of Example 1. 
The tunneling probe tip thus formed had a sharp point and exhibited 
excellent performance when used as a platinum tunneling probe tip for STM. 
EXAMPLE 6 
Three kinds of 0.3 mm .phi.-platinum wires were prepared from platinum 
materials each having a Pt content of 99.9% and different in the contents 
of the remaining Cu and unavoidable ingredients (see Table 1) through a 
step of wire drawing. 
The heat treatment, electropolishing with a potassium cyanide solution, and 
further electropolishing with a sulfuric acid-nitric acid-phosphoric acid 
solution were conducted by making use of the above-prepared platinum wires 
as the tunneling probe tip material in the same manner as that of Example 
2. The shape of the point of each platinum tunneling probe tip thus formed 
was observed. As a result, it has been found that the tunneling probe tips 
prepared by using platinum materials each having a Pt content of 99.9% or 
more and a Cu content of 0.005% or less (materials A and B) according to 
the present invention had a very sharp point and exhibited excellent 
performance when used as a tunneling probe tip for STM. By contrast, the 
tunneling probe tip prepared by using a material having a Cu content 
exceeding 0.005% (material C) had no sharp point because of high rate of 
polishing. 
TABLE 1 
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Chemical ingredients of Platinum material 
tunneling probe tip material 
chemical unit: % 
ingredients A B C 
______________________________________ 
Pt 99.9 99.9 99.9 
Cu 0.00135 0.00046 0.00701 
Mn 0.000015 0.000028 0.000015 
or less or less 
Fe 0.00059 0.00323 0.00081 
Mg 0.000041 0.00015 0.0000045 
or less 
Zn 0.000540 0.00049 0.00047 
Pd 0.00428 0.00225 0.00366 
Ba 0.000003 0.00048 0.000003 
or less or less 
Au 0.00160 0.00197 0.00244 
Rh 0.0009 0.0009 0.0009 
or less or less or less 
others balance balance balance 
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Although the present embodiments were described with reference to the 
Examples wherein a polishing solution comprising sulfuric acid, nitric 
acid and phosphoric acid in a ratio of 1:1:1 was used, the same polishing 
performance can be attained when the polishing solution comprises at least 
10% of sulfuric acid, at least 5% of nitric acid and at least 10% of 
phosphoric acid. 
As described above, the present invention enables production of a platinum 
tunneling probe tip with a high reproducibility and a high point sharpness 
which has not been attained in the prior art. 
The present invention exhibits a particularly excellent effect when used 
for the fabrication of a platinum tunneling probe tip for STM.