FIELD OF INVENTION
The present invention relates to an electrochemical process of making thin film
or foil of metal using a low current density technique.
BACKGROUND OF THE INVENTION
Making of thin film and thin foil of metal and alloys is a very pain staking task
and it involves repeated attempts of checking the samples in TEM (Transmission
Electron Microscope) stage and often the result is not so satisfactory. The reason
is that the foil area is not large enough for accurate estimate of the bulk sample.
Nonetheless, the chance of getting thin area for investigation is questionable.
Hence it takes enormous time and this cost money. Often big money is charged
for every hour of usage of TEM. Sometimes it takes many days of efforts to get a
good quality thin foil and thin film.
The traditional technique of thin foil making for TEM was based on the current-
voltage relationship where there is a plateau. The plateau is generally used for
polishing and that is exactly what happens in the beginning of thin foil
preparation when the crests and trough of the thin disk is flattened due to the
difference of local current density. Thereafter, the current density due its high
value for the imposed voltage makes the sample to thin down locally around
places where the current density is the highest and such places are those regions
of the interphase boundaries and it happens due to the large difference of the

standard EMF (Electro Motive Force) and hence it develops a hole and around a
hole a small region gets thin enough to be taken to TEM stage. However since
the current density is very large locally, the thin regions disappears very quickly
if not taken out immediately. Thus it is sometimes a question of luck to get good
foil and thus it is more like trial and error.
However, there is another prior art technique called ion beam milling where the
two phase or multi phase materials with varying density and strength can be
milled layer by layer and thin regions can be generated. Sometimes this milling
could take many weeks for very strong materials such as metal matrix
composites or ceramics or even superalloys. Hence it is also very time
consuming. For ceramic samples this technique is widely used. But the success
rate is very low.
It is further known that thin film is made by suing CVD (Chemical Vapor
Deposition) or PVD (Physical Vapor Deposition) which however is very time
consuming and needs a high vacuum generation. Success rate is very low. And
often the defect density is very high and naturally such film is useless.
OBJECT OF THE INVENTION
It is therefore an object of the invention to propose an electrochemical process
of making thin film or foil of metal using a low current density technique, which
eliminates the disadvantages of prior art.
Another object of the invention is to propose an electrochemical process of
making thin film or foil of metal using a low current density technique, which
substantially reduces the wastage by restricting the defect development.
A further object of the invention is to propose an electrochemical process of
making thin film or foil of metal using a low current density technique, which is
fast and cost-effective.
SUMMARY OF THE INVENTION
Accordingly, there is provided an electrochemical process of making thin film or
foil of metal using a low current density technique, comprising the steps of
preparing a specimen of a thin foil or a film; providing a twizzer having a
conductive end for passing current and enabled to act as an anode; allowing the
specimen to be gripped by the twizzer; providing an electrolyte bath filled with
type of electrolyte corresponding to the type of foil or film being produced;
disposing a mild steel sheet in the electrolyte bath which acts as a cathode;
passing a current between the cathode and anode each being formed of
dissimilar metals and alloys in the electrolyte, wherein the current density in the
circuit is maintained low at the initial phase through restricting the voltage at a
lower value which allows low local current density at first stage followed by
setting of uniform current density, the specimen being periodically checked in an
optical microscope followed by examination of the specimen when becomes
transparent in a transmission electron microscope (TEM) to ensure development
of the desired properties in the produced thin film or foil.
The inventive process adapts a low current density, which takes only few
minutes to produce give extremely good quality thin foil and thin film and does
not need regular monitoring in TEM stage. Instead the quality can be checked in
optical microscope. The inventors during the experimental stages developed
several thin film/thin foil samples of simple metals and alloys and even hard to
work alloys. Thus the efficacy is achieved for the disclosed invention.
The disclosed process is very cheap since it is a simple electrochemical process,
thin film of few mm in length and width to be used in the electronics industry
can be made by the process.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - shows a graphical representation of current density variation against
applied voltage according to the inventive process.
DETAIL DESCRIPTION OF THE INVENTION
For thin foil, the specimen is ground to 0.5mm thick with a diameter of 5mm.
using the inventive technique, even a thicker sample can be considered for
starting material. The advantage of using thick specimen is that it induces less
damage during handling r during cutting process. When the grinding of sample is
done, according to prior art several scratches are generated on the sample.
Development of such scratches according to the invention are eliminated due to
the local current density differences. Thus, the samples become smooth and on
further processing in the bath the samples are thinned down. Alternatively, thin
film can be made from the grown single crystals of metals and alloys and then
sectioning and further processing the sample using the inventive technique, it is
possible to produce thin foil/thin film with a very few defects.
The thick sample is gripped with a metallic twizzer preferably made of a plastic
material with a conductive end for passing current through it. This twizzer holds
the sample and acts as an anode. Then a mild steel sheet is used for cathode
and dipped in an electrolyte bath, the electrolyte being decided corresponding to
the type of metals and alloys or ceramics to be processed as thin film or thin foil.
Unlike the known twin jet technique the inventive process constitutes a simple
dissolution process which is implemented in atomic scale and layer layer by layer,
the atoms of the metal are removed. But the process is very fast.
Thin film also can be made using an alternative embodiment of the inventive
process. First a single crystal can be grown using Bridgman or Czocharlski
method and generally when they are made in that way, very few defects are
introduced. Then the single crystal can be sliced to proper thickness as described
hereinabove, a and then a thin slice can be used for making thin film with low
current density approach. Such thin film can be any size depending on the
formed single crystal dimension.
When the current is passed between the cathode and anode (dissimilar metals
and alloys) in an electrolyte, the current density varies depending on the shape
and size of the sample. The current density versus voltage graph of figure-1
exhibits a four stage curve. The first stage I is the low current density region and
then the regions (Stage II) where the current density shoots up and then it
reaches a plateau (Stage III) and then again it shoots up. This fourth stage (IV)
is where the corrosion happens in metals and alloys. The third stage (III) is the
plateau where the polishing takes place. And in the second (II) dissolution takes
place very rapidly which is very difficult to control. And in the very beginning
stage (I) where the current density is very low, the atomws are removed layer
by layer and thus it gives very thin specimen uniformly over large area. In fact
the local current density difference in ledges (ledges are the atomic steps in a
crystal or in single crystals) also influences which when used can give rise to
defect free single crystal thin film.
In the first stage, for example, the low local current density stage (I), the current
smoothens out the ridges and those are removal from the entire surface. Then
the uniform current density sets in and the sample gets thinned down uniformly.
Voltage is controlled in such a way to allow flow of less than 1-2 milli amperes in
the circuit. Since the voltage is very small and it is around 1-2 Volt, the alloys
containing dispersoids and grain boundaries (II), the local current density is
different and the local standard EMF difference is very small which leads to an
uniform thinning down of the sample. The current density is calculated in the
following way. If the disk is say 5 mm, then voltage is changed such a way that
current that flows in the circuit is equal to current divided by the area exposed.
For example, when 1 mili Ampere flows in the circuit, then the current density is
1 miliamp/cross sectional area of the sample (mm2).
Periodically the specimen is checked in an optical microscope and when the the
thin film or foil becomes transparent, it is examined in TEM (Transmission
Electron Microscope). Thus, thin film and foil can be made easily. This technique
is unique and not reported anywhere.
From the manufacturer point of view, the twin jet polishing technique, the ion
beam milling machine is available and the Bollman technique is also available but
the inventive process is novella and unique.
Advantages
1. There would not be any artifacts in the thin film or thin foil since the initial
thickness is high and it could be of the order of 0.5 mm.
2. The technique takes few seconds to few minutes depending on the initial
thickness of the samples. This would save enormous time and money.
Faithfulness of the structure would be excellent.
3. This technique can be used for difficult to work alloys such as super
alloys, Metal matrix composite and even ceramics.
4. Twin jet or ion beam milling technique is laborious, time consuming and
expensive and success rate is not so good and the area for examination is
very small compared to my technique.
5. This technique can be used for making Epitaxial thin film for directional
properties. All that is needed for that is to grow a single crystal in
preferred direction and slice it and thin it down with the present
technique.
6. Finally the process is very cheap and fast. Bulk samples for industrial
purpose can be made very quickly for electronic packaging applications.
WE CLAIM
1. An electrochemical process of making thin film or foil of metal using a low
current density technique, comprising the steps of:
- preparing a specimen of a thin foil or a film;
- providing a twizzer having a conductive end for passing current and
enabled to act as an anode;
- allowing the specimen to be gripped by the twizzer;
- providing an electrolyte bath filled with electrolyte corresponding to the
type of foil or film being produced;
- disposing a mild steel sheet in the electrolyte bath which acts as a
cathode;
- passing a current between the cathode and anode each being formed of
dissimilar metals and alloys in the electrolyte,
wherein the current density in the circuit is maintained low at the initial
phase through restricting the voltage at a lower value which allows low
local current density at first stage followed by setting of uniform current
density, the specimen being periodically checked in an optical microscope
followed by examination of the specimen when becomes transparent in a
transmission electron microscope (TEM) to ensure development of the
desired properties in the produced thin film or foil.
2. The process as claimed in claim 1, wherein preparing a specimen of thin
foil comprises grinding of the specimen to a predefined thickness and
diameter.
3. The process as claimed in claim 1, wherein preparing a specimen of thin
film comprises growing a single crystal by using Bridgmen or czocharliski
method; and slicing the crystal for the process of making thin film.
4. The process as claimed in claim 1, wherein the current density and the
voltage during electrolyte dipping of the sample, undergoes a four-stage
transformation of the specimen, wherein in the first stage the current
density is very low allowing removal of the atoms layer by layer providing
thin specimen uniformly configured, wherein in the second stage the
current density commencing an increase, wherein in the third stage the
current density reaches a plateau, and wherein in the fourth stage the
current density again shoots up causing corrosion in the metal.
5. An electrochemical process of making thin film or foil of metal using a low
current density technique, as substantially described and illustrated herein
with reference to the accompanying drawings.

The present invention relates to an electrochemical process of making thin film
or foil of metal using a low current density technique, comprising the steps of
preparing a specimen of a thin foil or a film; providing a twizzer having a
conductive end for passing current and enabled to act as an anode; allowing the
specimen to be gripped by the twizzer; providing an electrolyte bath filled with
electrolyte corresponding to the type of foil or film being produced; disposing a
mild steel sheet in the electrolyte bath which acts as a cathode; passing a
current between the cathode and anode each being formed of dissimilar metals
and alloys in the electrolyte, wherein the current density in the circuit is
maintained low at the initial phase through restricting the voltage at a lower
value which allows low local current density at first stage followed by setting of
uniform current density, the specimen being periodically checked in an optical
microscope followed by examination of the specimen when becomes transparent
in a transmission electron microscope (TEM) to ensure development of the
desired properties in the produced thin film or foil.