FIELD OF THE INVENTION
The invention generally relates to technique for removal of waste toxic and
pyrophoric gases from the scrubbers and vacuum deposition system used for the
production of amorphous silicon thin film solar cells. More particularly the present
invention relates to an improved flap switch and a sequence of operation of the
switch for detection of failure of a waste gas removal system.
BACKGROUND OF THE INVENTION
The manufacturing process for production of thin film solar cells requires the use
of toxic and pyrophoric gases like silane and other dopant gases. The safe
disposal of the diluted gases is done through a combination of gas scrubber and
an exhaust system into the atmosphere. The gas scrubbers are connected to the
exhaust system through a duct of suitable material. The operation of the exhaust
system is extremely important as any failure of the exhaust system can lead to
explosion of the scrubber and the gasses may leak into the working area, which
can be fatal to the human beings. A number of interlocks are available
commercially to detect the failure of the exhaust system but these are not fool
proof and failsafe.
The exhaust system is used to remove waste gases from the toxic gas scrubber
and the deposition system. Highly toxic and pyrophoric gases are used for the
deposition of thin film of semiconductors like amorphous silicon, using plasma
enhanced chemical vapour deposition
(PECVD) system for the production of amorphous silicon thin film solar cells. The
main gas, which is used in huge quantity, is silane and is toxic and pyrophoric in
nature. The gas ignites itself, the moment it comes in contact with oxygen,
which is available in plenty in the atmosphere. Other gases like diborane and
phosphene are also used for the production of thin film solar cells and are also
highly toxic and pyrophoric in nature. These gases are used as dopants and are
used in small quantities. The mixture of these gases with silane is used in a
PECVD system and hardly 30-40% gas is used for the deposition. Rest of the
quantity of gases goes as waste and needs to be scrubbed to avoid atmospheric
pollution. Normally the silane gas is diluted with nitrogen at the inlet of the
vacuum pump to more than 100 times to bring its concentration below 1% so
that it does not catch fire during scrubbing as the scrubber remains at
atmospheric pressure. The scrubber is connected to an exhaust system, which is
kept continuously ON to keep the scrubber and the gas pipes connected to pump
outlet, continuously under negative pressure. The exhaust system keeps
removing the gases out of the scrubber. The efficiency of the chemical scrubber
is normally 98-99%. This means, some part of the gas still goes out into
atmosphere, which is within acceptable limits. If, for any reason, the exhaust
fails or the suction becomes weak, the waste gases will start collecting in the
gas pipeline and the scrubber. After some time the gases will start coming out of
the scrubber. As scrubbers are designed to work with exhaust continuously ON,
accumulation of gases can become dangerous and can lead to explosion of the
scrubber and toxic gases can be harmful to human beings. The failure of the
exhaust system can be detected by a number of methods i.e. by sensing the
failure of power supply to the exhaust, the rotor shaft's movement of motor or
blower etc. The suction of the exhaust can fail because of following reasons:
1. Failure of power supply to the exhaust system.
2. Jamming or non-operation of the motor.
3. Jamming and non-operation of the exhaust/blower rotor.
4. Breaking of driving belt (in case it is a belt driven exhaust are used).
5. Puncture or breaking of exhaust duct.
6. Choking of exhaust ducts either at inlet or outlet.
7. Breaking of blades of the blower section of the exhaust.
Detection of the exhaust failure for reasons given above at si. Nos. 1 to 4 is
possible by detecting the failure of rotation of the rotor of the motor and the
blower, which is commonly available commercially. Detection of the failure due
to reasons given from number 5-7 is not possible by detecting the failure of
rotation of the rotor of the motor and blower. The best method is to detect the
failure of actual flow of exhaust suction at the point of use.
SUMMARY OF THE INVENTION
Accordingly, there is provided a fool proof but simple interlock flap switch has
been invented which is capable of detecting the full or even partial failure of the
exhaust system as to strike "Emergency Power Off" (EPO) mode in a control
panel of the vacuum deposition system. The
5
EPO mode automatically shuts off the input valves of all the toxic gases of the
vacuum chamber and sounds a visual alarm to the operator for further actions.
Accordingly, the sensitiveness of a micro switch has been enhanced by attaching
a flap on the micro switch, and a sequence has been defined. The flap switch is
fixed to a parallel exhaust entry duct to detect the part or total failure of the
exhaust system. On detection of exhaust failure, the flap switch moves up and
triggers the EPO through a micro switch.
OBJECT OF THE INVENTION
It is therefore an object of the invention to propose a flap switch to detect failure
of an exhaust system, connected to the scrubber of a vacuum deposition system.
Another object of the invention is to propose a process of operation of the flap
switch to detect failure of the exhaust system.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
FIGURE 1 - Schematically illustrates the operation of a flap switch
according to the invention for a fail-safe detection of an exhaust system in
vacuum deposition systems.
DETAILED DESCRIPTION OF THE INVENTION
As shown in Figure 1, the present invention employs a spring-loaded micro
switch (5) and a flap (6), which is mounted on a branch of a suction duct (2b') of
an exhaust system (2) having a motor (2a), and a blower (2b) vacuum
deposition system (1) used for the deposition of thin films comprises at least one
vacuum pump (la) and connected to the exhaust system (2) via a scrubber (3).
The vacuum deposition system (1) is controlled by an electronic control cubicle
(4) from where all the inlet and outlet gas valves of the vacuum deposition
system (1) are controlled remotely using pneumatic pressure. A number of safety
interlocks (7) are available on the control cubicle (4). These safety interlocks (7)
show a red light when triggered and create an emergency power off (EPO) which
puts the vacuum deposition system (1) in a safe shutdown mode and alerts the
operator.
The length of lever of micro the switch (5) is extended using the flap (6)
(200mm long and 20mm wide made of plastic sheet of 0.5 mm thick) to enhance
the sensitivity of the micro (5) switch and is adjustable from 200 to 250 mm. The
width of the flap (6) and its length is adjusted such that the flap switch (5,6)
remains normally in Position A, (see fig.l) irrespective of whether the exhaust
system (2) is ON or OFF. This is possible due to the spring action of the micro
switch (5). The micro switch (5) has NO (normally open) and NC (normally
closed) contacts. The NC contacts of the micro (5) switch are connected through
a two core wire (7a) to the safety interlock (7) of the control cubicle (4) and the
exhaust interlock (7) shows RED when the flap switch (5, 6) is in Position A. It is
assumed that the exhaust system (2) of the scrubber (3) is put OFF daily after
the deposition process is
finished. The red signal on the exhaust interlock (7) confirms that the exhaust
system (2) has been put OFF. Next day when the vacuum deposition (1) system
is put ON and the exhaust system (2) is still OFF, the operator gets a red signal
on the safety interlocks (7) of the exhaust system (2) on the control cubicle (4)
and cannot start the toxic gases as the electronic control will not allow to enable
the gas buttons as long as any interlock (7) is showing red. This sequence
confirms that the detection system of exhaust failure OK and can show red signal
when the flap switch (5,6) is in position A. The operator has to turn the exhaust
system (2) ON and goes to the flap switch (5,6). The size and length of the flap
(6), as explained earlier, is adjusted such that the flap switch (5,6) is not
activated automatically on putting the exhaust (2) ON. The operator pushes the
flap switch (5,6) to position B to activate it and the flap (6) sticks to the mouth
of the exhaust duct (2b') due to suction pressure. The flap (6) sits on the mouth
of the exhaust duct (2b') only if the exhaust (2) is running and suction is of
proper intensity. This step makes the NC contacts open. The operator goes to
the control cubicle (4) and resets the safety interlocks (7). The red light will go
green as the interlock switch (7) is shifted to open position (Position B). This
sequence confirms following:
1. The exhaust is running, is not weak and is OK.
2. The connecting wires of the flap switch are OK.
3. The operation of the flap switch is OK.
4. The exhaust failure detection system is working in perfect condition.
The operator can now start the process and use the toxic gases safely. If, for
any reason cited above, the suction fails or becomes weak, the flap switch (5,6)
will get automatically detached from the mouth of the exhaust duct (2b') due to
the spring action and goes to position A, the NC position of the micro switch.
This will immediately strike the EPO at the control cubicle (4) and all the toxic
gases will stop automatically and the operator will get a visual signal to take
suitable action for safe shut down of the deposition system (1).
The flap switch and sequence of operation according to the invention can be
used in photovoltaic thin film solar cell manufacturing industry to detect the
failure of exhaust system.

WE CLAIM :
1. A process for a fail-safe detection of failure of an exhaust device in a
vacuum deposition system, the system comprising at least one vacuum
pump operably connected to a scrubber which is connected to exhaust
device, the exhaust device having at least ope each motor and blower,
and a suction duct; an electronic control device remotely controlling
the system by sensing the pressure at the inlet of exhaust duct and
controlling the valves, the control device having a plurality of safety
interlocks and via a two-core wire connected to a spring loaded
microswitch having a lever, and at least one NO and NC contacts, and
disposed on said exhaust duct, wherein the microswitch is provided
with a flap which extends the length of said lever and enables the
micro-switch to operate with enhanced sensitivity, the process
comprising the steps of:
- manually changing the normal position (A) of the flap switch to a
position B to activate the exhaust device, and the flap getting stuck
on the outlet of the exhaust duct due to suction pressure;
- resetting the safety interlocks by manipulating the control device;
- the flap switch repositioned at location (A) in case of any failure of
the exhaust device enabling the emergency power off (EPO) to
activate which in turn deactivates the toxic gas valves.
A process for a fail-safe detection of failure of an exhaust device in a
vacuum deposition system as substantially described and illustrated
herein with reference to the accompanying drawings.

The invention relates to a process for a fail-safe detection of failure of
an exhaust device in a vacuum deposition system, the system
comprising at least one vacuum pump operably connected to a
scrubber which is connected to exhaust device, the exhaust device
having at least one each motor and blower, and a suction duct; an
electronic control device remotely controlling the system by sensing
the pressure at the inlet and exhaust duct and controlling the valves,
the control device having a plurality of safety interlocks and via a two-
core connected wire to a spring loaded microswitch having a lever, and
at least one NO and NC contacts, and disposed on said exhaust duct,
wherein the microswitch is provided with a flap which extends the
length of said lever and enables the micro-switch to operate with
enhanced sensitivity, the process comprising the steps of manually
changing the normal position (A) of the flap switch to a position B to
activate the exhaust device, and the flap getting stuck on the outlet of
the exhaust duct due to suction pressure; resetting the safety
interlocks by manipulating the control device; the flap switch
repositioned at location (A) in case of any failure of the exhaust device
enabling the emergency power off (EPO) to activate which in turn
deactivates the toxic gas valves.