FIELD OF THE INVENTION:
The present invention relates to a novel method of making silver pads on thin
film samples for measuring electrical conductivity on glass substrate through a
small stamps with very fine line definition.
BACKGROUND OF THE INVENTION:
Amorphous silicon layers are deposited on silicon wafers by PECVD process in
vacuum chamber for fabrication of solar cells. These films are typically very
thin (sub-micron thickness) and conductivity test is a crucial parameter which
affects the optical and electronic properties of a-Si layers for solar cell
application. The accurate measurement of conductivity is very important to
optimize the PECVD process for obtaining best results.
Usually p-type and n-type layers are deposited on normal glass slides for
characterization. For the measurement of electrical conductivity, conducting
paste of Ag is applied as two pads through stencil on the desired thin film
surface. The conductivity measuring probes are connected to Ag paste pads.
For proper measurement, conducting pads on thin film should be uniform
without any smearing. Applying Ag paste on thin film by hand through the
stencil often leads to smearing, resulting in rejection of test samples. If the Ag
pads are not well defined, the conductivity measurements are inaccurate and
this may lead to wrong conclusion.
Due to manual and cumbersome methods of applying the paste by using
stencil, there are chances of variation in size and shape of the Ag paste pads as
shown in Fig. 4.
PRIOR ARTS:
During patent search, there is not such related method of forming conductivity
path on thin film glass samples to measure conductivity. Few patents related to
conductivity are as follows-

United States Patent 51,93,732A by Mario J. Interrante, et al. October 4, 1991
relates generally to apparatus and methods for making simultaneous electrical
connections, and more particularly to making these electrical connections
simultaneously using a new bond tip configuration. Various methods and
processes are being disclosed to simultaneously make electrical connections
between electrical conductor lines or pads. The electrical connection is made by
placing an electrically conductive wire across a pair of electrical lines or pads
that have to be electrically connected and then by using a special tip, both ends
of the electrically conductive wire are simultaneously secured to the two
electrically conductive lines or pads. With the advent of modern day electronics,
the circuits on a PCB (Printed Circuit Board) or modules or chips have become
more and more dense. As a result, the electrical lines or conductor lines and
terminals or pads have become smaller so that more of them can be placed in a
given area.
Object of this invention is to make increasingly smaller electrical connections
between specific pads or lines for the fabrication of functional electrical circuits
and not related to the technique of making conducting path on thin film
samples.
United States Patent 47,24,011A by Gary B. Turner, et al. February 9, 1988
relates Solar cell interconnection by discrete conductive regions. Cells of a thin
film solar module having opposed upper and lower electrodes are connected in
series by a plurality of discrete conductive regions extending between each
upper electrode and the lower electrode of an adjoining cell. In a preferred
embodiment, the opposite electrodes of adjoining cells overlap one another and
one of the electrodes is a transparent conductive pad with a thickened metal-
containing portion to aid in interconnection. In another embodiment, the
conductive regions are formed by applying laser pulses to spaced locations
along the areas of electrode overlap, after which a conductor may be deposited
into cavities formed by the laser pulses.

One object of this invention relates generally to a thin film solar module and,
more particularly, to a method and structure for interconnecting cells by a
plurality of discrete conductive regions extending between each pair of cells to
be interconnected and not related to the technique of making conducting
regions on thin film samples.
United States Patent 7, 4,835,083 by Sakai, et al. May 30, 1989 relates to the
method for patterning electro conductive film and patterned electro conductive
film. An electro conductive film is provided which has a first region constituted
of a lamination layer comprising an electro conductive layer and an insulating
layer, and a second layer having reduced or eliminated electro conductivity.
Another type of electro conductive film is also provided having a first region
constituted of a lamination layer containing a compound having a
polymerizable double bond and comprising an electro conductive layer and an
insulating layer, and a second layer which is modified and having changed
dissolution characteristics. The layer may be formed of a monomolecular film or
monomolecular built-up film. A patterning method by use of the above electro
conductive film is also provided.
This patent is not related to any means for technique of making conducting
path on thin film samples.
United States Patent 7,806,974 by Hwang, et al. October 5, 2010 relates to a
viscosity controllable highly conductive ink composition and method for
fabricating a metal conductive pattern. The highly conductive ink composition
comprises an organic solvent, nanoscale metal particles or metallic-organic
decomposition compounds, and a thermally decomposable organic polymer.
Specifically, since the thermally decomposable polymer can increase the
viscosity of the highly conductive ink composition and be removed by
subsequent thermal treatment, so as to decrease the impact on conductivity by
organic polymer. Therefore, a viscosity-controllable conductive ink composition
is obtained.

The patent does not suggest any means for technique of making conducting
path on thin film samples.
United States Patent 7,473,513 by Park, et al. January 6, 2009 relates to the
Photosensitive metal nanoparticle and method of forming conductive pattern
using the same wherein a self-assembled monolayer of a thiol compound or
isocyanide compound having a terminal reactive group is formed on a surface
of the metal nanoparticle and a photosensitive group is introduced to the
terminal reactive group. The photosensitive metal nanoparticles can easily form
a conductive film or pattern having excellent conductivity upon exposure to UV,
and thus can be applied for conductive polyurethane printer rollers,
electromagnetic interference shielding, etc.
The patent relates, more specifically, relates to a terminal reactive group on its
surface and introducing a photosensitive group through the terminal reactive
group; and a method of forming a conductive pattern using the same and not
related to the technique of making conducting path on thin film samples.
But the conventional process does not provide any uniform Ag paste application
method and hence there is always a risk for smearing the paste on the
conducting path of the sample.
But the present invention eliminates the drawback by overcoming the
drawbacks.
SUMMARY OF THE INVENTION:
A method for uniform printing of silver pads on glass plates for measuring
electrical conductivity comprises the steps of: i) applying a thin layer of silver
(Ag) paste on a glass slide with a spatula; ii) pressing gently the stamp pad on
the surface of glass with the silver (Ag) paste for ensuring pick up of the silver
paste; iii further, pressing of stamp on the desired area on the thin film sample
to print the Ag pads; iv) drying of silver pads along with glass plate in drying
furnace, and thereafter the sample is fixed to a jig and electrical contacts are

taken using pressure probes for the measurement of conductivity under dark
and light condition.
OBJECTS OF THE INVENTION:
It is therefore, the primary object of the present invention to provide a novel
method of uniform printing of silver pads on glass substrate, for measuring
electrical conductivity.
Another object of the present invention is to provide a novel method of uniform
printing of silver pads on glass substrate, where printing of silver pads takes
place through a small stamp with very fine line definition.
Yet another object of the present invention is to provide a novel method of
uniform printing of silver pads on glass substrate, where the thin film is
provided is uniform and on proper distance, so that path should not short due
to smearing on conducting paste.
Further object of the present invention is to provide a novel method of uniform
printing of silver pads on glass substrate, where the stamp is guided while
making Ag pads on the thin film, so that there is no movement of the stamp
which may result in smearing of paste.
Another object of the present invention is to provide a novel method of uniform
printing of silver pads on glass substrate, which is easy for making the
conducting pads on thin film for conductivity measurement.
Yet another object of the present invention is to provide a novel method of
uniform printing of silver pads on glass substrate, which is user friendly
process for making conductivity pad on thin film by stamping method to
measure conductivity of doped and undoped amorphous silicon (a-Si) thin film
on glass substrate.
Further object of the present invention is to provide a novel method of uniform
printing of silver pads on glass substrate, where the process is simple and does
not require any special skill to operate.

Yet another object of the present invention is to provide a novel method of
uniform printing of silver pads on glass substrate, where the stamp will ensure
proper placement on the coated sample which will aid repeatable pad formation
on samples.
Further object of the present invention is to provide a novel method of uniform
printing of silver pads on glass substrate, where the stamp pads are cost
effective and easy to store.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
It is to be noted, however, that the appended drawings illustrate only typical
embodiments of the present subject matter and are therefore not to be
considered for limiting of its scope, for the invention may admit to other equally
effective embodiments. The detailed description is described with reference to
the accompanying figures. Some embodiments of system or methods in
accordance with embodiments of the present subject matter are now described,
by way of example, and with reference to the accompanying figures, in which:
Fig. 1 illustrates thin film of a-Si on glass plate of 2.5 cm x 7.5 cm with Ag pads
of 2mm x 10mm.
Fig 2 illustrates stamp and pattern for conductivity measurement.
Fig 3 illustrates photo of the Ag pads printed using the newly developed stamp.
Fig 4 illustrates smearing of Ag paste on a-Si coated glass slides using hand
operated stencil leading to erroneous data.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS:
The present subject matter relates to a novel method of uniform printing of
Silver (Ag pads) on thin film samples for measuring electrical conductivity.
This process eliminates the smearing of Ag paste as it is not applying by hand.
The pads are generally made by applying silver (Ag) paste using stencil on the
thin film surface on glass substrate.

The conductivity measuring probes are connected to the printed Ag pads. For
proper measurement, conducting pads on thin film should be uniformly defined
without smearing of Ag paste.
The process of making Ag pads comprising the steps of :
i) applying a thin layer of silver paste on a glass slide with a spatula
ii) pressing gently the stamp pad on the surface of glass with the silver
(Ag) paste for ensuring pick up of the Ag paste;
iii) further pressing of stamp on the desired area on the thin film sample
to print the Ag pads;
iv) drying of silver pads along with glass plate in drying furnace at a
temperature of 400°C, and thereafter the sample is fixed to a jig and electrical
contacts are taken using pressure probes for the measurement of conductivity
under dark and light condition.
For measurement of electrical conductivity, conducting paste of Ag is applied in
the form two pads of sufficient width to be able to take out electrical contact for
measurement. Generally, the conductivity of the a-Si film is very low in the
range of 10-4 to 10-11 (Ω-cm)-1. For the measurement of such low
conductivities, very firm and good quality large area contacts are required to be
made using an appropriate conducting paste.
The Figure 1 shows the required design, whereas the thin film of Si is shown on
glass plate of 2.5cm x 7.5 cm with Ag pads of 2mm x 10 mm.
For proper measurement, conducting pads are made on thin film are uniform
and on proper distance, so that both path should not short due to smearing of
conducting paste.
In accordance with another embodiment of the present invention, a small
stamp is designed and fabricated, so that the smearing of Ag paste can be
eliminated or checked substantially.

The stamp is designed for making conducting pads on thin film of a Si.
The required design of the pad is carved in a silicone rubber and is pasted on a
firm wooden block. The projection on the pad is also coated with a PTFE paint
to reduce any smearing of Ag paste during printing process. Further the pattern
surface of stamping pad is made rough using sand paper so that the conductor
paste sticks to the stamp surface firmly. The body of the stamp is allowed to
move up and down with the help of guides so that while stamping the Ag pads
on the a-Si film, the line definition remains fine. The structure of stamp with
handle provides a good support for applying the conducting paste on thin film
substrate. These stamps are small and can be stored in a small space. The
technique of making Ag pads is simple and does not require any special skill to
operate.
The design of stamp and pattern for conductivity measurement is shown in
below Fig.2 and the photo of the Ag pads printed using the newly developed
stamp is shown in fig.3.
The non-limiting advantages are:
1) A simple and clean process for uniform printing of Ag pads on thin film
samples for measuring their electrical conductivity.
2) A process by which there is no smearing of Ag paste by applying hand
pressure on handle by stamping method.
3) A process by which there is no smearing of Ag paste on sample by
using Teflon coating on sides of silicone rubber pads of stamp.
4) A suitable stamp design is provided with good support for applying the
conducting paste on thin film without smearing. This is to eliminate rejection of
samples due to smearing of Ag paste which is generally observed by convention
al methods.
5) The stamp pad is with roughened surface to ensure proper sticking of
Ag paste onto the stamp.

Although embodiments for the present subject matter have been described in
language specific to structural features, it is to be understood that the present
subject matter is not necessarily limited to the specific features described.
Rather, the specific features are disclosed as embodiments for the present
subject matter. Numerous modifications and adaptations of the method of the
present invention will be apparent to those skilled in the art, and thus it is
intended by the appended claims to cover all such modifications and
adaptations which fall within the scope of the present subject matter.

WE CLAIM:
1. A method for uniform printing of silver pads on glass plates for measuring
electrical conductivity comprises the steps of:
i) applying a thin layer of silver (Ag) paste on a glass slide with a spatula;
ii) pressing gently the stamp pad on the surface of glass with the silver
(Ag) paste for ensuring pick up of the silver paste;
iii further, pressing of stamp on the desired area on the thin film sample
to print the Ag pads;
iv) drying of silver pads along with glass plate in drying furnace, and
thereafter the sample is fixed to a jig and electrical contacts are taken using
pressure probes for the measurement of conductivity under dark and light
condition.
2. The method as claimed in claim 1, wherein the silver pads are dried in drying
furnace at 400°C.
3. The method as claimed in claim 1, wherein the designing of said stamp
comprises the steps of:
i) carving of required design in a silicone rubber and pasted on firm
wooden block;
ii) projection the pad is coated mainly on the surface of pad, which is
rough so that the conductor paste sticks to the surface firmly;
iii) providing a handle to have good support for applying the conductivity
paste on thin film substrate.
4. The method as claimed in claim 1, wherein the projection on the pad is
coated with a PTFE (Polytetrafluoroethylene) Paint for reduction of smearing of
silver paste.

5. The method as claimed in claim 1, wherein the surface of stamping pad is
made rough by using sand paper, so that the conductor paste sticks to the
stamp surface firmly.
6. The method as claimed in claim 1, wherein the silver paste is applied in form
of two pads of sufficient dimension to take out electrical contact for
measurement.
7. The method as claimed in claim 1, wherein the dimension of silver pads is
2mm x 10mm.