Device for heating a chemical tank with an inert heat exchange fluid using linear and impulsive control

A device for heating chemical tanks includes at least one heat exchanger coil immersed inside a chemical tank. An inert gas, such as nitrogen, is heated and made to flow through the coil to heat a solution in the chemical tank to the desired temperature, which is sensed by a sensor. If microfractures form in the heat exchanger coil, the inert gas simply bubbles out of the solution and does not adversely impact the purity of the solution. In addition, the bubbling of the gas indicates the presence of a microfracture. A second heating coil is provided, and also provides heated inert fluid for heating the solution in the tank. The fluid flow through the first coil is controlled linearly to maintain the temperature at the desired temperature. The fluid flow through the second coil is controlled impulsively to quickly return the solution to the desired temperature after a temperature transient occurs, such as after a relatively cool object is immersed into the solution.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a device for heating chemical tanks. 
2. Description of the Prior Art 
In microelectronics manufacturing, one technique for etching silicon slices 
involves immersing them in a solution. The solution can be, for example, a 
heated mixture of dilute hydrofluoric acid and ammonium fluoride in the 
proportion of one to seven by volume. In a typical etch process, 
twenty-four liters of solution must be maintained within a temperature 
range of 28.degree. C. to 38.degree. C., with a tolerance of plus or minus 
1.degree. C., according to the type of process to which the silicon is 
subjected. 
According to the prior art, heating of the mixture takes place through the 
use of Teflon coated electrical resistance heaters, or of corrugated coils 
made of Teflon, both immersed in the solution. The coils use hot water, 
generated by automatic external heaters, as the heating element. A 
temperature sensor immersed in the solution controls the external heater 
so as to maintain the desired temperature of the solution. 
The application of the systems described above has some drawbacks. 
Insofar as the heating systems based on Teflon coated resistances are 
concerned, it is known that the protective coating deteriorates with use 
and handling due to cleaning. Microfractures are created in the Teflon 
coating, which allow the etching solution to directly contact the metal of 
the resistance. As a consequence, the solution become contaminated with 
metal ions. 
Insofar as the heating systems based on hot water coils are concerned, 
microfractures are created in the corrugated Teflon coil as a result of 
use and handling due to cleaning. Heating water infiltrates into the 
etching solution through the microfractures, contaminating it with ions of 
various kinds present in the heating water itself. 
The contaminating ions introduced into the solution by the heating systems 
described above deposit themselves on the silicon slices contained in the 
etching tank. This damages the integrity of the slices, and increases the 
number of rejects. The increase in rejects can become a quite significant 
factor. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a heating 
system for a solution that is reliable and avoids any form of 
contamination. 
Another object of the present invention is to provide a heating system 
which enhances the uniformity of the temperature of the solution. 
Therefore, according to the present invention, a device for heating 
chemical tanks includes at least one heat exchanger coil immersed inside a 
chemical tank. An inert gas, such as nitrogen, is heated and made to flow 
through the coil to heat a solution in the chemical tank to the desired 
temperature, which is sensed by a sensor. If microfractures form in the 
heat exchanger coil, the inert gas simply bubbles out of the solution and 
does not adversely impact the purity of the solution. In addition, the 
bubbling of the gas indicates the presence of a microfracture. A second 
heating coil is provided, and also provides heated inert fluid for heating 
the solution in the tank. The fluid flow through the first coil is 
controlled linearly to maintain the temperature at the desired 
temperature. The fluid flow through the second coil is controlled 
impulsively to quickly return the solution to the desired temperature 
after a temperature transient occurs, such as after a relatively cool 
object is immersed into the solution.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In summary, a heating device for chemical tanks comprises at least one heat 
exchanger coil located in the proximity of the walls and immersed inside a 
chemical tank containing a solution to be heated. The coil is made to 
circulate a flow of inert gas. 
Nitrogen, for example, can be used as the inert gas. In this manner, in the 
case of microfractures in the Teflon coils, the presence of the inert gas 
introduced into the solution does not jeopardize the composition and the 
purity of the solution itself. In addition, immediate visualization of the 
loss is allowed by the bubbling of the inert gas in the solution through 
the microfractures. The silicon slices are not contaminated by the 
introduction of nitrogen into the solution. 
As an alternative to the use of a single heat exchanger coil, it is 
possible to provide two heat exchanger coils. Pure nitrogen is fed in at 
the inlet of the device, and flows through both heat exchanger coils. At 
the inlet to the pair of coils is connected a pair of heating units. Each 
heating unit is suitable for receiving nitrogen under pressure. One 
heating unit is driven by a control unit in a linear manner for 
maintaining and operating temperature of the solution. The second coil is 
driven in an impulsive manner for the recovery of said operating 
temperature from transient conditions. 
Such a twin coil system allows a very fine and stable adjustment of the 
temperature of the solution, with attainable maximum deviations and 
absolute values equal to one-half .degree.C. The control unit is also 
suitable for driving a pair of solenoid valves for controlling the flow of 
nitrogen at the inlet into the pair of heating units. The control unit can 
interrupt the flow of nitrogen in order to switch the device off. 
With reference now to the FIGURE, first and second heat exchanger coils X, 
1 are arranged and intercalated with respect to each other in the 
proximity of the walls of a chemical tank 3. Tank 3 contains a solution 4 
suitable for etching silicon slices 5 contained in corresponding 
containers 6. (Only one silicon slice 5 is shown in the drawing.) The 
solution 4 can be, for example, a mixture of HF (hydrofluoric acid) and 
NH.sub.4 F (ammonium fluoride) in the proportion of one to seven by 
volume. For present purposes, this solution must be maintained within a 
given temperature interval. In one particular example, 24 liters of the 
solution must be maintained within the temperature from 28.degree. C. to 
38.degree. C. with a tolerance of plus or minus 1.degree. C. 
The coils 1, 2 are made of Teflon. A flow of pure nitrogen 23 is fed in an 
inlet I to the device. Immediately downstream from the inlet I there is a 
pressure regulator 7 for controlling the pressure of the nitrogen 23, and 
a pressure sensor 18 for measuring the pressure. 
In succession there is a pair of solenoid valves 9, 10 suitable for 
regulating the flow of nitrogen 23. The valves 9, 10 are also suitable for 
directing the flow of nitrogen to the respective inlets I11, I12 of first 
and second heating units 11, 12. The outlets U11, U12 of the heating units 
11, 12 are directed into the coils 1, 2. Thus, the outlets U11 and U12, 
respectively, are connected to the inlets I1 and I2. Each heating unit 11, 
12 consists of a shell 31, 32 having a cylindrical shape which houses 
electrical resistances 41, 42 within. The resistances 41, 42 perform the 
function of heating the flow of nitrogen 23 to the preset temperature. 
The device also comprises control logic 13. The control logic 13 is 
connected to a temperature controller 14 which is suitable for receiving 
signals from a thermocouple 15. These signals from the thermocouple 15 
indicate the temperature of solution 4. The controller 14 sends to the 
control logic 13 a signal proportional to the deviation between the 
detected temperature and the preset temperature. 
Control logic 13 is also connected to the solenoid valves 9, 10 for 
controlling, and possibly interrupting, the flow of nitrogen 23 toward the 
first and second heating units 11, 12. It is also connected to the first 
and second heating units 11, 12 themselves. Preferably, the first heating 
unit 11, connected to the first coil 1, is suitable for being driven by 
the control unit 13 in a linear manner. This provides for maintaining the 
preset temperature in the solution 4. The second heating unit 12, 
connected to the second coil 2, is suitable for being driven by the 
control unit 13 in an impulsive manner. This provides for the prompt 
recovery of the preset temperature of the solution 4 in the case of a 
transient lowering of the temperature 4. This can occur, for example, at 
the moment when the container 6 with the slices 5 to be processed is 
introduced into the solution 4. The set of two independent drives (linear 
and impulsive) of the heating units 11 and 12 allows a more uniform 
temperature of the solution 4 to be maintained. It is possible for such a 
device to maintain the temperature of the solution 4 with maximum 
deviations and absolute values equal to one-half .degree.C. 
Safety control devices 16, 17 are interposed between the control logic 13 
and the heating units 11, 12. Safety control devices 16, 17, protect 
against excess temperatures, and intervene in case of anomalies in the 
heating elements 11, 12. Safety pressure sensor 18 is inserted in the line 
feeding nitrogen to the solenoid valves 9, 10 and is connected to the 
control unit 13. Pressure sensor 18 transmits a signal to the control unit 
13 for automatically switching off the heating units 11, 12 in case of a 
pressure drop of nitrogen 23 in the line. The prevents damage to the 
heating elements 11, 12. 
The tank 3 is provided with a pump 19 for filtered recirculation of the 
solution 4 through filter 21. An associated level control 20 is connected 
to the control unit 13, which stops the recirculation pump 19 in case of 
an excessive lowering of the level of the solution 4. Finally, the control 
unit is provided with an alarm device 22 for providing an operator with an 
acoustical signal in case of malfunctions in the device. 
Initially, a container 6 with one or more silicon slices 5 to be processed 
is positioned on the bottom of the tank 3. The tank 3 is filled with a 
solution 4 of, for example, the type described above. Then nitrogen 23 is 
fed at the inlet I of the device and taken to a given pressure through the 
pressure regulator 7. Following the path indicated by the arrows, the 
nitrogen 23 flows through the solenoid valves 9, 10. These valves 9, 10 
are driven by the control unit 13, which adjusts the flow of nitrogen 23 
at the input to the heating units 11, 12. 
Simultaneously, the control unit 13 drives the operation of the heating 
units 11, 12 as described above. Control unit 13 takes into account 
figures relating to the temperature deviations originating from the 
temperature controller 14. In particular, with the exclusion of an initial 
transitory period, the heating unit 11 is operated in a linear manner for 
maintaining the preset temperature in the solution 4. Heating unit 12 is 
driven in an impulsive manner for compensating for temperature transients 
in the solution 4 such as those that occur during the introduction of one 
or more contains 6 with the silicon slices 5 to be processed. The nitrogen 
23 flowing through the heating units 11, 12 is heated up, and provided to 
the inlets I1 and I2 of the coils 1, 2 at the respective outlets U11, U12. 
During circulation in the coils 1, 2, the nitrogen cools down and give up 
heat to the solution 4. 
During operation of the device, the temperature of the heating units 11, 12 
is kept under control by the safety control devices 16, 17. Any anomalies 
are immediately signaled to the control unit 13, which signals the anomaly 
to the operator using the buzzer 22. In a similar fashion, the safety 
pressure switch 18 controls the pressure of the nitrogen 23. In case of a 
sudden drop in pressure, it transmits a signal to the control unit 13 
suitable for causing the heating units 11, 12 to be switched off 
automatically and for signaling the anomaly with the alarm device 22. 
The pump 19 and corresponding filter 21 cause a continuous filtered 
recirculation of the solution 4. In the event that the level of the 
solution 4 is lowered, as signaled by the level controller 20 to the 
control unit 13, recirculation is stopped and the event is signaled to the 
operator by the alarm device 22. 
It should be noted that the device described above, in addition to allowing 
the heating of the solution contained in a chemical tank with minimum heat 
deviations, is also protected against possible micro losses from the 
coils. Since an inert gas is used, the composition and purity of the 
solution are not jeopardized, and the occurrence of such micro losses are 
immediately visualized by the bubbling of the nitrogen. Thus, the 
described device can be used in all those applications where a controlled 
heating of solutions is required, without running the risk of 
contaminating the solutions themselves. 
To the described embodiment it is naturally possible to make numerous 
variations without going beyond the scope of the claimed invention. For 
example, it is possible to provide more than two heat exchanger coils, 
connected to an equal number of heating units, according to the mass to be 
heated and to the temperature of the solution. 
While the invention has been particularly shown and described with 
reference to a preferred embodiment, it will be understood by those 
skilled in the art that various changes in form and detail may be made 
therein without departing from the spirit and scope of the invention.