FIELD OF INVENTION
The invention relates to an improved stainless steel bubbler, adaptable for the
deposition of front conducting electrode in thin film solar cells.
BACKGROUND OF THE INVENTION
Many semiconducting thin film materials are deposited by Chemical Vapour
Deposition (CVD) process using liquid chemicals as input materials. In CVD
process, the liquids are required to be vaporized and transported to the reaction
zone/chamber. The liquid chemicals are normally vaporized in a bubbler by
keeping at a constant temperature (at boiling point) and inert gas is bubbled
through it so that the chemical vapors are transported to the reaction zone. A
non-openable type SS bubbler is used for the purpose. This type of bubbler is
suitable for chemicals where they don't generate particulates during the
operation.
Optically transparent materials such as glass, plastic, mica etc. are normally
electrical insulators and the conducting materials (films) such as Ag, Al, Cu etc.,
are opaque to visible radiation. In order to make thin-films that are electrically
conducting and transparent to visible radiations, special materials need to be
deposited. Fluorine doped tin oxide is one such material, which is transparent to
visible radiations and is also electrically conducting, and therefore, is called
Transparent Conducting Oxide (TCO). Because of this interesting property, the
transparent conducting oxide (TCO) coated glass substrates are widely used as
front electrode in thin film Photovoltaic (PV) solar cells. An Atmospheric Pressure
Chemical Vapor Deposition (APCVD) method is used for depositing the

Transparent Conductive Oxide (TCO) film on glass substrates. In APCVD process,
the vapors (reactant process gases) of Tin tetrachloride (SnCI4), water (H2O),
methanol (CH3COOH) and Hydrofluoric acid (HF) are allowed to contact a
substrate at an elevated temperature (-550 °C) in an ambient condition of about
one atmospheric pressure to produce a film.
All these chemicals are vaporized using non-openable SS bubblers (sealed
through welding joints). Quite often, it is observed that during the deposition
process of TCO film, the Tin tetrachloride (SnCI4) bubbler get chocked inside due
to formation of particulate material at the outlet of bubbler and as well as at the
diffuser bulb. In addition to this, various pipe joints, welded on the top of the
bubbler also get corroded over a period of time and start leaking. As the bubbler
is a sealed container, it is virtually impossible to remove the particulate material
form the bubbler. Especially during the process, if the bubbler gets chocked, the
process gets interrupted. To clean the bubbler, expensive SnCI4 liquid has to be
drained out through SS tubes to glass bottles by pressurizing the bubbler. This is
a cumbersome exercise. Invariably some vapours do come out and cause
corrosion, as SnCI4 spontaneously reacts with air and produced HCI vapours.
These HCI vapors attack all the SS valves, thin-walled bellows and even the
metallic portions of the mass flow controllers which get corroded over a period of
time and become useless. This cause loss of time and money.
OBJECT OF THE INVENTION
It is therefore an object of the invention to propose an improved stainless steel
bubbler adaptable for the deposition of front conducting electrode in thin film
solar cell.

SUMMARY OF THE INVENTION
According to the invention, an improved bubbler has been configured without
welding any SS pipes to the metal flange. A sealing with viton 'O' ring has been
provided between the two SS flanges. Acid resistant PFA pipefitting has been
suitably modified for fixing and sealing the diffuser bulb and tube made of Teflon
material.
The invention can be used in a semiconductor process industry where highly
corrosive liquids are being used for the deposition of semiconducting materials
by chemical vapour deposition method.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. 1- shows a prior art bubbler
Fig. 2- shows an improved bubbler according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
Various liquid chemicals, namely Tin tetrachloride (SnCI4) water (H2O), methanol
(CH3COOH), Hydrofluoric (HF) acid are used for the deposition of TCO or
Fluorine doped tin oxide film by CVD process. In the deposition process these
chemicals are vaporized in a SS bubbler and transported to reaction zone and
allowed to contact a substrate at an elevated temperature (~550 °C) in an

ambient condition of about one atmospheric pressure to produce the TCO film.
Generally, non-openable type SS bubblers are used for vaporizing the liquids. A
schematic of the conventional SS bubbler (B_) is shown in fig. 1 The bubbler (B)
is fabricated by welding a plate (1) to a cylindrical flask (2). A plurality of (3a to
3d) SS tubes required for temperature sensor (4), vapours OUT (5), drain (6),
including a tube 3(c) with a diffuser (8) for bubbling the liquid, are welded to
the top plate (1). This type of design has some disadvantages especially when it
is used for highly corrosive liquids like Tin tetrachloride. When the liquid inside
the bubbler (B) gets exposed to moisture in atmosphere or moisture in bubbling
gas, its diffuser (8) and outlet tube (3d) get chocked due to formation of white
powder. It is difficult to clear the blockade in this type of sealed bubbler.
Moreover the welded joints are much more prone to corrode over a period of
time and start leaking and bubbler becomes unusable. To over come these
disadvantages a new bubbler has been designed.
As shown in figure 2, the improved stainless steel bubbler (B1) is designed with
a viton 'O' ring sealing (9) between a top flange (1a) and a bottom flange (1b) .
It is openable easily for any maintenance purpose. In this design, the bottom
flange (1b), made of SS-with a grove to hold the viton 'O' ring (9) is welded to a
SS cylindrical flask (2) . Welding is done from bottom of the lower flange (1b) so
that the corrosive liquid inside the bubbler (B1) doesn't percolate to the welding
joint. On a top flange (not shown), PFA tube fittings (10a) are fixed with suitable
threading. A Teflon tube (3c) instead of SS tube is used for bubbling the liquid. A
modified diffuser (12) is designed for bubbling the liquid and constitutes

a part of the Teflon tube (3c) itself. A modified compression PFA fitting (10a) is
used for sealing the tube (3c) used for bubbling. All other tubes (3c,3b,3d) are
also of Teflon material. The viton 'O' ring (9) is placed in between the flanges
(1a,1b), ensures sealing of the bubbler by tightening the flanges (1a,1b) with nut
& bolts. The bubbler (B1) can be disassembled very easily in a few minutes for
cleaning purpose. Since the improved bubbler (B1) is designed without welded
joints exposed to the corrosive chemical, the reliability bubbler (B1) is very high.
The cylindrical flask (2) having a dimension about 180mm , and 280mm height.
The bottom and top flange having a diameter of 180mm with a thickness of 15
mm.

We claim:
1. An openable stainless steel (SS) bubbler adaptable for deposition of front
conducting electrode in thin film solar cell, comprising:
- a bottom flange (1b) having a groove to hold a viton 'O' ring (9)
welded from the bottom to a cylindrical flask (2);
- a plurality of teflon tubes (3a to 3d) fixed to a top flange via
threaded PFA tube fittings (10,10a), one each of the tubes (3a to
3d) at a first end provided with a temperature sensor (4), vapour
out means (5), draining means (6), and input means (7) for the
carrier nitrogen, the distant ends of the tubes (3a to 3c) being
submerged in the highly corrosive liquid inside the cylindrical flask
(2); and
- the viton 'O'ring (9) sand witched between the top and bottom
flanges (1a,1b), to seal the bubbler (B1) by further tightening the
flanges (1a,1b) with fastening means.
2. The bubbler (B1) as claimed in claim 1, wherein the cylindrical flask (2)
comprises stainless steel and configured to have a diameter and height of
180mm and 280mm respectfully.

3. The bubbler (B1) as claimed in claim 1, wherein the top and the bottom
flanges each has a diameter and thickness of 180mm and 15mm respectively.
4. An openable stainless steel (SS) bubbler adaptable for deposition of front
conducting electrode in thin film solar cell as substantially described and
illustrated herein with reference to the accompanying drawings.

The invention relates to an openable stainless steel (SS) bubbler adaptable for
deposition of front conducting electrode in thin film solar cell, comprising a
bottom flange (1b) having a groove to hold a viton 'O' ring (9) welded from the
bottom to a cylindrical flask (2), a plurality of teflon tubes (3a to 3d) fixed to a
top flange via threaded PFA tube fittings (10,10a), one each of the tubes (3a to
3d) at a first end provided with a temperature sensor (4), vapour out means (5),
draining means (6), and input means (7) for the carrier nitrogen, the distant
ends of the tubes (3a to 3c) being submerged in the highly corrosive liquid inside
the cylindrical flask (2); and the viton 'O'ring (9) sand witched between the top
and bottom flanges (1a,1b), to seal the bubbler (B1) by further tightening the
flanges (1a,1b) with fastening means.