Electric resistance heating devices – Heating devices – Tank or container type liquid heater
Reexamination Certificate
2000-01-21
2001-07-03
Leung, Philip H. (Department: 3742)
Electric resistance heating devices
Heating devices
Tank or container type liquid heater
C352S072000, C352S072000, C396S573000
Reexamination Certificate
active
06256455
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to an apparatus and a method for preventing fire in a liquid holding tank that is heated by resistive heating elements and more particularly, relates to an apparatus and a method for preventing fire in a liquid holding tank which is filled with a flammable liquid and is heated by resistive heating elements to a temperature of at least 80° C. wherein a fire would otherwise occur when the tank is ruptured to expose the heating elements.
BACKGROUND OF THE INVENTION
In the fabrication of semiconductor integrated circuit (IC) devices, hundreds of fabrication steps must be performed on a semi-conducting substrate in order to complete the fabrication of the devices. The hundreds of processing steps may include cleaning, deposition, etching, buffer coating and various other necessary steps. In these fabrication steps, a variety of process chemicals, including liquids and gases must be used in different processing machines and be transported from their storage tanks to the machines. A large number of these process liquids are of high viscosity and short shelf life and therefore their transportation between a reservoir and a process machine must be carefully controlled. Deterioration or premature reaction of these process liquids can result in poor quality products and unnecessary machine down time which in turn lead to a decrease in process yield.
One of the process liquids that requires delicate handling is a photoresist stripper liquid. A photoresist stripper liquid, in order to be effective in stripping a photoresist layer from an IC device, must be heated to higher than room temperature, i.e. to as high as 80° C. or preferably to as high as 115° C. A typical photoresist liquid is highly flammable and therefore must be carefully controlled during the heating process. For instance, a widely used commercial photoresist stripper of ACT®-690 which contains various low boiling point and high boiling point components is shown in FIG.
1
. The low boiling point components, i.e. those having a boiling temperature of up to 100° C., includes dimethyl sulfide, IPA and acetone. The high boiling point components include dimethyl sulfoxide, N-methyl pyrolidone and methyl ethyl alcohol.
A conventional process tank for holding and heating a photoresist liquid is shown in FIG.
2
. The process tank
10
is constructed by an inner tank
20
, an outer tank
30
and an overflow tank
22
for the inner tank
20
. The overflow tank
22
may be advantageously formed integral with the inner tank
20
to prevent overfill of the inner tank
20
. The inner tank
20
is filled with a photoresist stripper liquid
24
which is heated by resistive heating elements
26
positioned at near the bottom
28
of the inner tank
20
. The resistive heating elements
26
are normally constructed of a metallic heating element embedded in an insulating ceramic coating (not shown). During a regular heating mode, the surface of the resistive heating elements reaches a temperature between about 800° C. and about 1000° C. When the resistive heating elements
26
are properly controlled by a process controller (not shown), the photoresist stripper liquid
24
can be suitably heated to a temperature of about 125° C., or to a temperature of at least 80° C. For more efficient photoresist removal, the higher temperature of 115° C. is more preferred. The inner tank
20
is further equipped with a fluid level indicator
32
which senses the level of the photoresist liquid being stored in the inner tank
20
. The overflow tank
22
is further equipped with a set of level sensors
34
for sensing a high level, a normal level and a low level of the photoresist liquid in the overflow tank
22
.
Surrounding the inner tank
20
and the overflow tank
22
for holding the photoresist stripper liquid, is an outer tank
30
which is equipped with a water inlet
36
such that the tank is filled with water
38
to a level that at least covers the bottom
28
of the inner tank
20
. The use of deionized water is more preferred. The level of water
38
being held in the outer tank
30
is controlled by a fluid level indicator
40
to ensure that the bottom
28
of the inner tank
20
is always immersed in the water
38
. The upper level of water
38
is controlled by a drain pipe
42
which drains away overfilled water
38
in the outer tank
30
. The outer tank
30
is further equipped with a manual drain valve
44
which is used to completely drain the outer tank
30
when maintenance or cleaning of the tank is required. The inner tank
20
is supported by a plurality of supports
46
positioned on the bottom panel
48
of the outer tank
30
. The outer tank is further equipped with an ultrasonic vibration device (not shown) such that ultrasonic vibration can be transmitted to the inner tank
20
, i.e. thus to the photoresist stripper liquid
24
, to facilitate mixing and to achieve a more uniform temperature in the stripper liquid. The ultrasonic vibration from the outer tank
30
is transmitted to the inner tank
20
by water
38
that contacts at least the bottom
28
of the inner tank
20
.
The sidewalls
18
and the bottom wall
28
of inner tank
20
should be constructed in a high temperature resistant material for holding the heated, highly flammable photoresist liquid
24
. A suitable material that is chemically inert, dimensionally stable for holding such photoresist stripper liquid heated to a temperature of 115° C. is quartz. The use of a ceramic material such as quartz for forming the inner tank
20
presents a serious problem of breakage due to the fragile nature of quartz upon impact or vibration. When the quartz inner tank
20
breaks or fractures, the photoresist stripper liquid
24
leaks out of the inner tank
20
into the outer tank
30
and mixes with water
38
to be drained out through drain pipe
42
to a fluid level
16
that is below the resistive heating elements
26
and thus leaving the heating elements
26
exposed. When the resistive heating elements
26
are no longer submerged in liquid
24
, the high surface temperature of the heating elements, i.e. as high as 1000° C., immediately causes a fire potential by igniting residual photoresist stripper liquid on the heating elements
26
, or by igniting the residual vapor left in the inner tank
20
even after the photoresist liquid
24
is substantially drained away. Any such fire would cause a disastrous effect since a number of photoresist stripper tanks may be positioned close to each other, and furthermore, the inner tank
20
normally has a capacity of about 50 liters of the highly flammable liquid. The potential for a severe fire that is difficult to control is therefore very high upon a breakage of the inner tank and must be protected.
It is therefore an object of the present invention to provide an apparatus for preventing fire in a liquid heating tank that does not have the drawbacks or shortcomings of the conventional apparatus.
It is another object of the present invention to provide an apparatus for preventing fire in a liquid heating tank which holds a highly flammable photoresist stripper liquid.
It is a further object of the present invention to provide an apparatus for preventing fire in a liquid heating tank that utilizes resistive heating elements which would be exposed upon a breakage of the tank and cause a fire.
It is another further object of the present invention to provide an apparatus for preventing fire in a liquid heating tank that utilizes resistive heating elements by providing a smaller buffer tank positioned below the liquid heating tank which is equipped with an air solenoid valve for shutting off a drain pipe and thus stopping the photoresist stripper liquid/water mixture from draining from an outer tank.
It is still another object of the present invention to provide an apparatus for preventing fire in a liquid heating tank that utilizes resistive heating elements for heating flammable liquid which is capable of keeping resistive heating elements submerged in the flammable
Campbell Thor
Leung Philip H.
Taiwan Semiconductor Manufacturing Company Ltd
Tung & Associates
LandOfFree
Apparatus and method for preventing fire in a liquid heating... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Apparatus and method for preventing fire in a liquid heating..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Apparatus and method for preventing fire in a liquid heating... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2512789