Apparatus and method for the controlled cooling of chemical tanks

An apparatus for the controlled cooling of chemical tanks containing a chemical which must be kept at a first prescribed temperature includes at least one heat-exchanger coil immersed inside a chemical tank and through which there is made to circulate a flow of an inert gas. The apparatus includes a storage system for storing and cooling a prescribed volume of inert gas at a second prescribed temperature lower than the first prescribed temperature, feeding means for feeding the storage means with inert gas coming from a main supply line, and valve means interposed between the storage means and an inlet of the at least one heat-exchanger coil for controlling the supply of cooled inert gas to the heat-changer coil.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an apparatus and method for the controlled 
cooling of chemical tanks. 
2. Discussion of the Related Art 
In microelectronics industry, the manufacturing of integrated circuits 
involves several steps of liquid-based or "wet" etching: the silicon 
wafers are immersed in rather capacious tanks (to allow the simultaneous 
processing of several wafers) containing a chemical solution whose 
composition and temperature depend on the particular material to be 
etched. 
In some cases the etching process is carried out at temperatures higher 
than the room temperature (21.degree. C.): the etching solution must be 
maintained within a temperature range from 28.degree. C. to 38.degree. C., 
for which purpose means for heating the etching solution are necessary. 
There is a class of materials for which the etching process is carried out 
at room temperature. In this class, some materials, such as silicon, react 
with the etching solution generating heat (i.e., the reaction is 
exothermic). If no cooling means are provided, the heat generated by the 
reaction causes an increase in the temperature of the etching solution 
which modifies (increases) the reaction speed, and the etching process 
goes out of control. As a result, several wafers must be rejected. 
Also, as evident, cooling means are necessary when the etching process is 
carried out at temperatures lower than the room temperature. 
According to the known art, the cooling means include teflon coils which 
are immersed in the etching solution and through which water is made to 
flow. The coils are connected to an external tank to which a refrigerating 
system is associated. A temperature sensor immersed in the etching 
solution controls the refrigerating system to maintain the etching 
solution in the desired temperature range. 
The problem is that, with use and handling due to cleaning, microfractures 
are created in the teflon coils, through which the water leaks out 
contaminating the etching solution with ions present in the water. Such 
ions deposit on the silicon wafers and give rise to defects that cause the 
wafers to be rejected. 
In the published European Patent Application No. 507387 a device for 
heating chemical tanks is described which includes a pair of 
heat-exchanger teflon coils, immersed in the etching solution, through 
which an inert gas such as pure nitrogen is made to flow. The coils are 
connected to respective heating units which are fed with the inert gas 
from a main supply line. The use of an inert gas as the heating element 
instead of water assures that, even in presence of microfractures in the 
teflon coils through which the gas can leak, the etching solution is not 
contaminated. 
In view of the state of the art described, it is an object of the present 
invention to provide an apparatus and method for the controlled cooling of 
chemical tanks used for wet etching of semiconductor wafers, which is safe 
and reduces the chance of contamination of the etching solution. 
SUMMARY OF THE INVENTION 
According to the present invention, at least this object is attained by an 
apparatus for the controlled cooling of chemical tanks containing a 
chemical solution which must be kept at a first prescribed temperature, 
comprising at least one heat-exchanger coil immersed inside of a chemical 
tank and through which there is made to circulate a flow of an inert gas, 
comprising a storage system for storing and cooling a prescribed volume of 
inert gas at a second prescribed temperature lower than said first 
prescribed temperature, a feeding system for feeding the storage system 
with inert gas coming from a main supply line, and a valve system 
interposed between said storage means and an inlet of said at least one 
heat-exchanger coil for controlling the supply of cooled inert gas to the 
heat-exchanger coil. 
The apparatus according to the present invention does not pose any problem 
of contamination of the solution, since it uses an inert gas as cooling 
element. Furthermore, as a result the fact that a storage system for 
storing cool inert gas is provided, the cooling apparatus of the present 
invention is capable of rapidly compensating any undesired temperature 
increase in the chemical solution, as a result of, for example the heat 
generated by the etching reaction. To achieve this result, it is 
sufficient to properly dimension the storage system to store a sufficient 
volume of cool inert gas.

DETAILED DESCRIPTION 
With reference to FIG. 1, first and second heat-exchanger coils 1, 2 are 
arranged and intercalated one to the other in the proximity of the walls 
of a chemical tank 3 containing a chemical solution 4 suitable for etching 
silicon wafers 5 (only one shown in the drawing) which in turn contains 
corresponding containers 6. The tank 3 has a typical volume of about 24 l, 
and the solution is, for example, a mixture of hydrofluoric acid and 
ammonium fluoride in the proportion of one to seven by volume. 
The tank 3 is provided with a pump 19 having a corresponding filter 21 for 
the filtered recirculation of the solution 4, and with an associated level 
control 20 connected to an input of a main control unit 13 and suitable 
for stopping the recirculation pump 19 in case of an excessive lowering of 
the level of the solution 4 in the tank 3. 
The coils 1, 2 are made of teflon and are both traversed by a flow of an 
inert gas 23, such as pure nitrogen, that enters into the coils 1, 2 
through respective inlets I1, I2 and leaves the coils 1, 2 from respective 
discharge outlets U1, U2. 
The cooling apparatus comprises a cylindrical container 50 made of 
stainless steel having a volume of about 25 l, inside which there is 
arranged a heat-exchanger coil 51 also made of stainless steel, in a 
position adjacent to the walls of the container 50. The coil 51 is the 
expansion coil of a per-se known refrigerating machine 52. The container 
50 has a coating 59 of thermal insulator material. 
The container 50 has an inlet 60 at its top end which is fed with the inert 
gas 23 coming from a main supply line 55 through a pressure regulator 56 
and a nonreturn valve 57. A pressure gauge 58 measures the pressure of the 
inert gas 23 downstream from the nonreturn valve 57 and just upstream of 
the inlet 60. 
The container 50 has an outlet 61 at its bottom end through which the 
cooled inert gas 23 is fed to the coils 1, 2 respectively through a first 
linearly-operating solenoid valve 62 and a second linearly-operating 
solenoid valve 63, which are individually controlled by a peripheral 
control unit 64. The first and second linearly-operating valves 62 and 63 
are respectively connected to the inlets I1, I2 of the coils 1, 2 through 
respective nonreturn valves 65 and 66. Just downstream from the outlet 61 
of the container 50 there is provided a relief valve 67 which discharges 
the inert gas 23 from the container 50 in case of overpressure. 
The activation of the refrigerating machine 52 is controlled by a 
thermostat 53 externally attached to the wall of the container 50. The 
temperature of the inert gas 23 inside the container 50 is measured by a 
sensor 54 which is connected to the peripheral control unit 64. 
The peripheral control unit 64 communicates with the main control unit 13, 
which is also connected to a temperature controller 14 suitable for 
receiving from a thermocouple 15 signals related to the temperature of the 
solution 4 and for sending to the main control unit 13 a signal E equal to 
the deviation between the detected temperature and the preset temperature. 
The main control unit 13 is further provided with an alarm device 22 
suitable for providing an operator with an acoustical signal in case of 
malfunctions in the apparatus. 
The cooling apparatus according to the preferred embodiment of the present 
invention also comprises a heating device schematically shown in FIG. 1 by 
a block 70 and substantially of the type described in the already 
mentioned European Patent Application No. 507387. The heating device, 
shown in detail in FIG. 2, comprises first and second inert gas heating 
units 11, 12 each including of a shell 31, 32 having a cylindrical shape 
which houses a respective electrical resistance 41, 42 inside it. 
The heating units 11, 12 have respective inlets I11, I12 fed with the inert 
gas coming from the main supply line 55 through a common pressure 
regulator 7 and respective solenoid valves 9, 10, which are individually 
controlled by the main control unit 13. Just downstream from the pressure 
regulator 7, which sets a maximum pressure of about 1 bar, a safety 
pressure switch 18 is inserted in the line feeding the inert gas 23 to the 
solenoid valves 9, 10 and is suitable for transmitting to the main control 
unit 13 a signal for automatically switching the heating units 11, 12 off 
in the case of a pressure fall of the inert gas 23 in the main supply line 
55 so as not to damage the heating elements 11, 12 themselves. 
The heating units 11, 12 are driven by the main control unit 13 and are 
further provided with safety control means 16, 17 of excess temperature, 
which intervene in case of anomalies in the heating units 11, 12. One of 
the two solenoid valves 9 and 10, e.g. the valve 9, is controlled by the 
main control unit 13 in an impulsive manner, while the other valve 10 is 
controlled in a linear manner. 
The heating units 11, 12 have respective outlets U11 and U12 which are 
respectively connected, through respective nonreturn valves 68 and 69, to 
the inlets I1 and I2 of the coils 1 and 2. 
The inert gas 23 coming from the main supply line 55 is fed to the 
container 50 through the pressure regulator 56 and the nonreturn valve 57. 
The pressure regulator 56 limits the pressure downstream from it to a 
value of about 7-8 bar, lower than the pressure of the inert gas 23 in the 
main supply line 55 (typically 10-12 bar). Inside the container 50, the 
inert gas 23 is progressively cooled to the temperature set by the 
thermostat 53 (for example 2.degree.-4.degree. C.). As the inert gas 
cools, the pressure inside the container 50 lowers, and more gas is taken 
in from the main supply line 55, until the pressure inside the container 
50 reaches the value set by the pressure regulator 56. 
Hence, the container 50 works as a reservoir of cool gas for refrigerating 
the etching solution 4, the volume of the container 50 and the temperature 
at which it is kept being chosen so as to store a number of refrigerating 
units at least sufficient to rapidly compensate the heat that can be 
generated by the chemical reaction of etching of the silicon wafers. 
When the temperature of the inert gas inside the container 50, measured by 
the thermal sensor 54, reaches the preset value stored in the peripheral 
control unit 64, the peripheral control unit 64 signals to the main 
control unit 13 a "ready" condition, and also activates a "ready" device 
71 (e.g. a lamp) for the operator. 
For the purpose of controlling the reaction speed and thus the rate of 
removal of the material to be etched, the chemical solution 4 must be 
maintained at a prescribed temperature (T1 in FIG. 3), with maximum 
allowed variations within a narrow temperature range (for example, 
T1+(T2-T1) in FIG. 3). For example, in the case of a room temperature 
etching process, the prescribed temperature T1 is 21 .degree. C., and the 
allowed temperature range is 21 .degree. C. +0.5.degree. C.(i.e. 
T2=21.5.degree. C.). 
After the "ready" condition is signalled to the operator by the peripheral 
control unit 64, the container 6 of silicon wafers 5 is introduced into 
the tank 3 containing the etching solution 4. The etching solution 4 
reacts with the material to be etched. The chemical reaction that takes 
place is, in some cases, exothermic: thus, the heat generated by the 
reaction causes the temperature of the solution 4 to rise. The temperature 
of the solution 4 is detected by the temperature controller 14, which 
supplies the main control unit 13 with the signal E corresponding to the 
deviation of the detected temperature with respect to the preset 
temperature, for example 21.degree. C. for a room temperature etching 
process. If the value of said deviation is positive, the main control unit 
13 sends a command to the peripheral control unit 64 to open the 
linearly-operating valves 62, 63. As long as the deviation does not exceed 
0.5 .degree. C., i.e. as long as the temperature T of the solution 4 does 
not exceed T2 in FIG. 3, only one of the two valves 62, 63 is opened (for 
example the valve 62), proportionally to the amount of the deviation up to 
the maximum opening for T=T2. If instead the deviation from the prescribed 
temperature exceeds 0.5.degree. C., both the valves 62, 63 are opened, the 
valve 62 at its maximum opening and the valve 63 proportionally to the 
difference between the temperature T of the solution and T2, so that the 
temperature of the solution 4 can quickly return into the allowed 
temperature range. FIG. 3 shows the opening characteristics of the valves 
62, 63 with temperature. As cool gas 23 leaves the container 50, the 
pressure in the container 50 decreases, and other gas is taken in from the 
main supply line 55. 
As the temperature of the solution 4 approaches the preset value, the 
valves 62, 63 are progressively closed, again according to the 
characteristic of FIG. 3. 
If, due to the thermal inertia of the system, the temperature of the 
solution decreases below the preset value, the main control unit 13 
determines the activation of the heating units 11, 12. As long as the 
deviation of the detected temperature from the preset temperature is small 
(i.e. the detected temperature is in the allowed range 20.5.degree. 
C.-21.degree. C.), the main control unit 13 impulsively drive the valve 9 
with a pulsed signal having a duty cycle depending on the amount of the 
temperature deviation; the other valve 10 is kept closed. The hot gas at 
the outlet U11 of the heating unit 11 is fed, via the nonreturn valve 68, 
to the inlet I1 of the coil 1. When instead the temperature deviation is 
large (i.e. when the detected temperature falls outside the allowed range 
20.5.degree. C. -21.degree. C.), the valve 9 is kept open, and the valve 
10 is linearly controlled in a manner proportional to the difference 
between the detected temperature and the minimum allowed temperature 
(20.5.degree. C.). The hot gas is fed to both the coils 1, 2, so that the 
temperature of the solution can quickly return to the allowed temperature 
range. 
Having thus described at least one illustrative embodiment of the 
invention, various alterations, modifications, and improvements will 
readily occur to those skilled in the art. Such alterations, 
modifications, and improvements are intended to be within the spirit and 
scope of the invention. Accordingly, the foregoing description is by way 
of example only and is not intended as limiting. The invention is limited 
only as defined in the following claims and the equivalents thereto.