U.S. patent number 3,905,883 [Application Number 05/480,472] was granted by the patent office on 1975-09-16 for electrolytic etching method.
This patent grant is currently assigned to Hitachi, Ltd.. Invention is credited to Osamu Asai, Masanobu Hanazono.
United States Patent |
3,905,883 |
Hanazono , et al. |
September 16, 1975 |
Electrolytic etching method
Abstract
A precise pattern figure can be formed on a thin film of
permalloy, iron, nickel, cobalt and copper through an electrolysis
with a mixture of ammonium persulfate and nitric acid electrolyte
under control of the electrode potential. Particularly, a desired
part of an electrode whose surface is in coexistence with aluminum
can be electrolytically etched without damaging the aluminum.
Inventors: |
Hanazono; Masanobu (Hitachi,
JA), Asai; Osamu (Hitachi, JA) |
Assignee: |
Hitachi, Ltd.
(JA)
|
Family
ID: |
13382302 |
Appl.
No.: |
05/480,472 |
Filed: |
June 18, 1974 |
Foreign Application Priority Data
|
|
|
|
|
Jun 20, 1973 [JA] |
|
|
48-68734 |
|
Current U.S.
Class: |
205/666; 205/667;
205/685; 257/E21.309; G9B/5.094 |
Current CPC
Class: |
C25F
3/14 (20130101); H01F 41/14 (20130101); H01L
21/00 (20130101); G11B 5/3163 (20130101); H01L
21/32134 (20130101) |
Current International
Class: |
C25F
3/00 (20060101); C25F 3/14 (20060101); H01F
41/14 (20060101); H01L 21/02 (20060101); H01L
21/3213 (20060101); H01L 21/00 (20060101); G11B
5/31 (20060101); B23p 001/00 () |
Field of
Search: |
;204/129.75,129.8,129.65,129.2 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Tufariello; T. M.
Attorney, Agent or Firm: Craig & Antonelli
Claims
We claim:
1. An electrolytic etching method for etching precise electrical
parts, wherein a 0.1 to 2.5 mol/l concentration ammonium persulfate
- 0.02 to 10 mol/l concentration nitric acid electrolyte is used
and a DC voltage is applied between a cathode electrode of
insoluble metal and an anode electrode including a metal object to
be etched, said metal object being made from a member selected from
the group consisting of iron, nickel, cobalt, copper and alloys
thereof, whereby a constant potential electrolytic etching is
performed by maintaining the anode electrode potential between the
natural electrode potential and the oxygen creation potential of
said objects to be etched.
2. An electrolytic etching method according to claim 1 wherein said
electrolyte comprises a 0.8 to 1.5 mol/l concentration ammonium
persulfate - 0.5 to 8 mol/l concentration nitric acid solution.
3. An electrolytic etching method according to claim 1, wherein
said anode electrode including said metal objects to be etched is
covered with isobutylene rubber, polycinamic vinyle, diazo compound
photoresist films at a part other than that to be etched.
4. An electrolytic etching method according to claim 1, wherein
said anode electrode including said metal objects to be etched is
covered with an aluminum film at a part other than that to be
etched.
5. An electrolytic etching method for etching precise electrical
parts wherein one end of an anode electrode including an object to
be etched is connected to a potentiostat, said object being
constituted by an electrically insulative substrate or aluminum
layer provided thereon with a thin film made from a member selected
from the group consisting of iron, nickel, cobalt and alloys
thereof said thin film being coated with a photoresist or aluminum
pattern, one end of a cathode electrode including an insoluble
metal is connected to said potentiostat, and the other ends of said
anode electrode and said cathode electrode are immersed in a 0.8 to
1.5 mol/l concentration ammonium persulfate 0.5 to 8 mol/l nitric
acid electrolyte, whereby a constant potential electrolytic etching
is performed by maintaining the anode electrode potential between
the natural electrode potential and the oxygen creation potential
of said anode electrode.
6. An electrolytic etching method according to claim 5, wherein
after a constant potential electrolysis is performed by maintaining
the anode electrode potential between the natural electrode
potential of said anode metal and 1.2 volts (relative to SCE), a
noble potential than said natural electrode potential is applied to
said anode electrode immersed in said electrolyte, whereby a
remaining part of metal to be etched which has been electrically
insulated is dissolved.
7. An electrolytic etching method according to claim 1, wherein
said anode electrode including a metal object to be etched
comprises aluminum.
Description
BACKGROUND OF THE INVENTION
1. Field Of The Invention
The present invention relates to an electrolytic etching method
available for preparation of large size integrated circuits
incorporated with a number of highly concentrated elements and
circuits formed on a substrate, formation of electrodes of
semiconductor elements and preparation of other elements and
circuits as well.
2. Description Of The Prior Art
As measures for preparing thin film circuits, vacuum evaporation,
sputtering, plating, chemical etching, electrolytic etching, ion
beam etching and sputter etching are known, and these measures are
used in combination for forming a highly concentrated desired
pattern.
By means of vacuum evaporation or sputtering through a pattern
mask, it is possible to form under a dry process a desired pattern
on a thin film comparatively readily. However, precision in the
pattern figure thus prepared varies widely dependent on precision
in the pattern mask and bonding ability of the mask to an object to
be patterned, and by the usual mask evaporation method the
formation of the pattern is achieved with precision of 20 to 50
.mu.m at most.
A mask pattern of photoresist can have highly precise dimension and
therefore with photoresist it is possible to form a mask pattern
with 1 .mu.m precision. However, when a pattern is formed through
chemical etching by using such a highly precise mask pattern as the
photoresist mask, precision in a completed pattern is reduced to
about 10 .mu.m on account of the irregularity of etching and the
damage of the photoresist mask. Especially, in the chemical etching
it is usual to raise the temperature for the purpose of promoting
the dissolving speed and this often damages the photoresist
film.
As compared to the chemical etching, an electrolytic etching softly
acts on an object to be etched. However, electrolysis creates gases
which are liable to cause the photoresist film to be peeled off,
thus degrading precision in the pattern. Incidentally, a so-called
selective electrolytic etching method wherein only one of two kinds
of metals electrically connected and exposed from the electrode
surface is dissolved has not been used for forming thin film
circuit patterns since it requires a special combination of
different metals and special conditions.
SUMMARY OF THE INVENTION
A principal object of the present invention is to provide an
electrolytic etching method for forming a precise pattern figure on
thin films of permalloy, iron, nickel, cobalt, copper and
molybdenum.
A second object of the present invention is to provide an
electrolytic etching method wherein a metal thin film a part of
which is coated with a photoresist pattern film can be etched
without damaging the photoresist pattern film.
A third object of the present invention is to provide a selective
electrolytic etching method wherein permalloy, iron, nickel,
cobalt, copper, and molybdenum are each electrically connected to
aluminum and under this electrical connection the permalloy, iron,
nickel, cobalt, copper or molybdenum is selectively dissolved.
A fourth object of the present invention is to provide an
electrolytic etching method capable of suppressing the creation of
gases.
A fifth object of the present invention is to provide an
electrolytic etching method wherein etching performance of metals
can be observed during the etching process.
A sixth object of the present invention is to provide measures of
preparing precise electronic parts such as an aluminum strip lead
and thin film magnetic heads.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a schematic representation of one type of electrolytic
etching device embodying the invention.
FIG. 2 is a graphic representation of etching pattern figures in
accordance with different electrolytes and materials of a pattern
mask.
FIGS. 3a to 3d show photographs of etched permalloy films provided
on a glass substrate.
FIG. 4 is a diagramatic representation of preparing process of a
strip lead to which the invention is applied.
FIG. 5 is a perspective view of a thin film magnetic head.
DESCRIPTION OF THE PROFERRED EMBODIMENTS
The present invention contemplates a method of electrolytic etching
with a mixture of ammonium persulfate and nitric acid electrolyte
for dissolving permalloys as well as iron, nickel, cobalt, copper
and alloys thereof. Disclosed herein is an electrolytic etching
method wherein an electro-chemical action is intentionally applied
to an etching reaction. It also involves a case where some
electro-chemical means are added for promoting, or sometimes
suppressing, the chemical etching reaction. The electro-chemical
means is identical to means for changing electrode potential of an
object to be etched from the natural electrode potential, which the
object bears when immersed alone in an electrolyte, to a desired
potential, and it may be constituted, for example, by application
of voltage with an external power source or coupling of the object
to be etched with a different metal.
The electrolyte used herein is an aqueous solution which contains
0.1 to 2.5 mol/l ammonium persulfate [(NH.sub.4).sub.2 S.sub.2
O.sub.8 ] and 0.2 to 10 mol/l nitric acid.
Where an aqueous solution of ammonium persulfate devoid of nitric
acid is used as the electrolyte, the dissolving speed of permalloy
is caused to differ markedly dependent on compositions of the
permalloy and preparing conditions therefor. For example, since the
dissolving speed of a vacuum evaporation deposited film is less
than that of casting material and plated material by 1/10 to 1/l00,
the aqueous solution of ammonium persulfate alone is unsuitable for
electrolyte for making an etching pattern of an object coexistent
with the casting and plating materials.
In addition, the electrolyte of ammonium persulfate aqueous
solution has a comparatively small etching speed and it will tend
to damage anti-etching photoresist films. For example, under an
electrolysis 20 % ammonium persulfate aqueous solution at
50.degree.C temperature, copper and cobalt-vanadium-iron alloy bear
dissolving speeds of about 0.2 .mu.m/min and 1 .mu.m/min,
respectively, while the photoresist film of about 2 .mu.m thickness
can assume the protective ability within about 10 minutes.
Accordingly, with the anti-etching photoresist film the depth of
etching is restricted to less than 2 .mu.m for copper and within 10
.mu.m even for irons.
When the ammonium persulfate aqueous solution is added with nitric
acid, the electrolytic etching speed is markedly increased owing to
the oxidization action of nitric acid and especially difference in
the dissolving speed due to different preparation processes such as
plating or evaporation becomes almost negligible. Further, since
the addition of nitric acid acts to reduce the polarization of an
iron object and to increase the passivating range of aluminum, the
ammonium persulfate aqueous solution added with nitric acid is
advantageous to such a selective etching as to etch only the iron
and copper of metals coexisting with aluminum.
Instead of nitric acid, ferric chloride, cupric chloride, potassium
bichromate, and cerium sulfate are useful as oxidizing acids
adapted to be added to the ammonium persulfate. Since chlorides
induce etch pits on an aluminum surface, other additional
substances than chlorides should be used when it is necessary to
avoid the etch pits on the aluminum surface. To facilitate
oxidization, supplemental injection of oxygen gas and addition of
hydrogen peroxide are effective. The concentration of ammonium
persulfate within the electrolyte is 0.1 to 2.5 mol/l but 0.8 to
1.5 mol/l is preferred to the practical use, at which the solution
assumes pH representation of 1.8 to 1.9. However, since negative
ions HSO.sub.4.sup.- takes place in the dissolving reaction of
metals by ammonium persulfate, it is necessary to select conditions
for easy creation of the negative ions HSO.sub.4.sup.- for the
purpose of facilitating the dissolving reaction of the metals. This
enforces reduction in pH of ammonium persulfate electrolyte to
below 1.6. Ordinary acids such as chloride, sulfuric acid, and
nitric acid may be used for reducing the pH representation.
Especially, for the reason set forth above, oxidizing acids are
preferred to the present invention end.
In ammonium persulfate aqueous solution of 1 mol/l concentration,
the oxygen creation potential (relative to saturated caromel
electrode, hereinafter abbreviated as SCE) is about 1.0 volt. On
the other hand, in order to dissolve a plated coating of 22 %
iron-78 % nickel permalloy at 2 .mu.m/min etching speed in this
solution, it is necessary to maintain the oxygen creation potential
(relative to SCE) about 1.3 volts, naturally resulting in that
oxygen gas taking place at a large amount. However, where nitric
acid concentrated above 0.2 mol/l is added to the 1 mol/l
concentration ammonium persulfate aqueous solution, since the
dissolving speed of permalloy is increased and accompanied by
decrease in the polarization of permalloy, the electrode potential
at the permalloy electrode adapted to be dissolved can be decreased
below the oxygen creation potential, particularly below 1.0 volt
(relative to SCE), thereby enabling etching of the permalloy
without causing the creation of oxygen gas.
For etching the permalloy with electolyte of 0.8 to 1.5 mol/l
ammonium persulfate and 0.5 to 8 mol/l nitric acid, the electrode
potential at permalloy object to be etched may be selected to -0.1
to 0.5 volts (relative to SCE).
In the 1 mol/l concentration ammonium persulfate electrolyte,
aluminum is passivated over a wide range of the oxygen creation
potential of 0.5 to 2.5 volts (relative to SCE). When added with
nitric acid, however, the passivation becomes unstable and the etch
pit potential is observed. As the additional amount of the nitric
acid increases, the etch pit potential decreases, especially, at
the nitric acid concentration of above 10 mol/l a current flow
necessary for maintaining the passivation exceeds 1 mA/cm.sup.2.
Accordingly, it will be seen that the additional amount of the
nitric acid ranges from 0.2 to 10 ml/l to obtain an aluminum etch
pit potential of above 1.7 volts (relative to SCE) and the current
flow necessary for passivation of below 0.5 mA/cm.sup.2. Preferably
it ranges from 0.5 to 8 mol/l.
As understood from the foregoing description, a rapid etching for
magnetic objects such as iron, nickel and cobalt and alloys thereof
as well can be achieved with an electrolyte which contains 0.1 to
2.5 mol/l concentration ammonium persulfate and 0.2 to 10 mol/l
nitric acid under the application of a potential between the
natural electrode potential and the oxygen creation potential to
the object. Under these conditions, gas evolution is avoided, and
it leads to the prevention of the photoresist film from being
peeled off, and so aluminum will not be damaged. In addition to it
the aluminum is maintained in the passivation range, so it will not
be dissolved.
EXAMPLE 1
A permalloy sheet of 30 .mu.m thickness provided with a precise
mask pattern was subjected to an electrolytic etching to examine
the respective preferable kinds of mask material and electrolyte
from the etching figure.
The experiment was made with an electrolytic etching device shown
in FIG. 1. In the figure, numeral 1 designates a permalloy sample
to be etched, which sample constitutes an anode electrode. Numeral
2 designates a platinum cathode electrode in the form of a plate,
which cathode electrode 2 is separated by an unglazed tube 5 such
that gas created therein is prevented to prevail into the
electrolyte but is electrically communicated with the etching
electrolyte through the unglazed tube 5. Numeral 3 disignates a
glass capillary vessel for measuring the electrode potential. There
are further provided electrolyte 4, an electrolytic bath 6, a gas
injection pipe 7 through which gas is injected into electrolyte
with need to control the circumferential conditions thereof a
magnetic stirrer 8 and a stirrer piece 9 adapted for stirring the
electrolyte, saturated KNO.sub.3 solution 10, a salt bridge 11,
saturated KCl solution 12, a saturated calomel electrode 13 and a
potentiostat 14 for measuring the anode potential relative to the
saturated calomel electrode and maintaining the anode potential at
a desired value.
The electrolitic etching in accordance with the present invention
was carried out by maintaining the anode electrode potential and
the oxygen creation potential of the permalloy sample to be etched
with a D.C. voltage being applied between the platinum cathode
electrode and the anode electrode of the permalloy sample.
Listed in Table 1 are permalloy compositions of the sample to be
etched.
Table 1 ______________________________________ Compositions Fe Ni
Mn P S C ______________________________________ Content (weight %)
20.62 78.88 0.36 0.05 0.03 0.01
______________________________________
After polished, washed and dried, the surface of permalloy film was
coated with a photoresist film of a commercial photoresist made of
isobutylene rubber and polycinnamic vinyle and deposited with
aluminum by evaporation, with an uncovered partial figure of 5 .+-.
1 .mu.m width and 50 .mu.m length. Three kinds of electrolytes as
listed in Table 2 were used. With a 30.degree.C temperature
electrolyte and under 0.1 volts (relative to SCE) electrode
potential, the etching was performed to measure relation between
the depth and the width of etched slots.
Table 2 ______________________________________ (mol/l) Electrolyte
No. (NH.sub.4).sub.2 S.sub.2 O.sub.8 HNO.sub.3
______________________________________ 1 0.4 0.5 2 0.85 1 3 1.5 5
______________________________________
Results are illustrated in FIG. 2. In the figure, symbol A
represents a case where No. 2 electrolyte is used with the
protective coating of aluminum evaporation deposition, wherein a
sharp slot is formed having the smallest extension in the width
direction. Symbol B represents a case where No. 2 and No. 3
electrolytes are used with the protective coating of photoresist,
and symbol C represents a case where No. 1 electrolyte is used with
a photoresist film. Symbol D represents a case where No. 2
electrolyte is used with a photoresist film, wherein a chemical
etching is performed without passing a current flow. In chemical
etching the etching speed was less than one-half that of the
electrolytic etching and the dissolved or etched depth remarkably
differs in the direction of width.
EXAMPLE 2
Permalloy was deposited by means of evaporation upon a glass
substrate (No. 7059 glass substrate of Corning) to a thickness of
about 3,000 A and thereupon a permalloy layer was plated to a
thickness of 10 .mu.m. Photoresist film (KMER) was used as the
protective mask and an electrolytic etching was performed under a
constant potential of 0.05 volts (relative to SCE). After the
etching, etched pattern figures were photographed as shown in FIG.
3. FIGS. 3a to 3d correspond to electrolytic etchings with 1 mol/l
ammonium persulfate electrolyte, 1 mol/l ammonium persulfate and
0.1 mol/l nitric acid electrolyte, 1 mol/l ammonium persulfate and
0.5 mole/l nitric acid electrolyte, and 1 mol/l ammonium persulfate
and 1 mol/l nitric acid electrolyte, respectively. Especially, in
the electrolytic etching corresponding to FIG. 3d the anode
potential was maintained at 0.05 volts and then permalloy applied
with a potential of -0.5 volts (relative to SCE) which is lower
than its natural electrode potential was immersed in the
electrolyte for five minutes, wherein island-shaped and
electrically isolated portions were dissolved. It will be
understood from the figure that not only the effect of additional
nitric acid is clearly exemplified in the resulted pattern figure
but also precision in electrolytic etching pattern is further
improved owing to the dissolving ability of electrolyte itself.
EXAMPLE 3
On a glass substrate, a copper evaporation deposition pattern was
formed and thereon a nickel plating layer was placed by about 20
.mu.m thick, thereby to form a multiple-layer as shown in FIG. 4a.
A glass substrate 1, a copper evaporation deposition layer 2 and a
nickel plating layer 3 are illustrated in the figure. The
multiple-layer was subjected to a shadowing evaporation with
aluminum at 30.degree. incident angle to obtain a resulted
structure as illustrated in FIG. 4b. Thereafter, the resulted
structure was electrolytically etched in 1.5 mol/l ammonium
persulfate - 1.5 mol/l nitric acid electrolyte under a constant
electrode potential of zero volt (relative to SCE). Thus, an
aluminum beam lead which takes a shape as shown in FIG. 4c was
prepared.
EXAMPLE 4
In manufacturing process of a thin-film magnetic head for writing
in and reading out recording signals of a magnetic recording
medium, a precise finish of figure is required for increasing
recording density. In FIG. 5, there are provided a ferrite
substrate 1 on which aluminum conductors 2 having an anodic coating
on their surfaces are placed, and an upper permalloy magnetic
member 3 across the aluminum conductors 2 magnetically
communicating with the ferrite substrate 1 at its rear portion. A
magnetic gap between the ferrite substrate and the front portion of
an upper magnetic member serves as a magnetic head. In this
magnetic head, the width d of the upper permalloy magnetic member
equals the track width and therefore the upper permalloy magnetic
member is required to be formed with high accuracy. Upon preparing
the permalloy magnetic member, a permalloy thin film layer having a
larger area than a desired figure and 20 .mu.m thickness was formed
by means of the evaporation and plating, and thereupon a precise
pattern of the desired figure was formed with naphthoquinone.sup..
diazo positive type photoresist coating. A resulted structure was
subjected to an electrolytic etching in 1 mol/l ammonium
persulfate-2 mol/l nitric acid electrolyte under a constant
potential of 0.2 volts (relative to SCE), thereby to obtain the
upper permalloy magnetic member. Errors in the width of the upper
permalloy magnetic member thus obtained were within .+-. 30 .mu.m
(.+-. 15 .mu.m for one side) with respect to a target width of 200
.mu.m.
For comparison, deviations in the width of the upper permalloy
magnetic member prepared by mask evaporation and chemical etching
were measured. When prepared by the mask evaporation, deposition
entering between the mask and the substrate was twice the thickness
of the deposition layer, though fitting precision between the mask
and substrate is responsible for reducing the deviation.
Accordingly, in view of mass-production, obtainable width of the
upper permalloy magnetic member will be about 200 .+-. 50 .mu.m.
When prepared by the chemical etching with the photoresist coating
and 1 mol/l ammonium persulfate - 0.1 mol/l sulfuric acid solution,
the width of the upper permalloy magnetic member was 200 .+-. 40
.mu.m and undissolved portions of island shape were retained.
Further, as compared to the electrolyte etching, edge figure was
not sharp. Effective track width of magnetic head thus obtained was
impaired in response to the poor sharpness of edge figure.
As understood from the foregoing description and applications
referred to in examples, according to an electrolytic etching
method of the invention, it is possible to form precisely figured
etching patterns and further it is possible to form comparatively
readily a desired pattern figure on an object to be etched, with
partly unetched portions if the etching intensity has previously
been measured. Easy washing after completion of electrolyte etching
and mass-treatment of fine and sophisticated objects to be etched
within a short time permit a readily available etching method .
* * * * *