Chemistry of inorganic compounds – Extracting – leaching – or dissolving
Reexamination Certificate
1998-04-08
2001-02-06
Dunn, Tom (Department: 1754)
Chemistry of inorganic compounds
Extracting, leaching, or dissolving
C423S352000, C423S24000R, C423S238000, C423S484000, C423S488000
Reexamination Certificate
active
06183720
ABSTRACT:
BACKGROUND OF THE INVENTION
(i) Field of the Invention
The present invention relates to a process and an apparatus for producing high-purity chemicals for the microelectronics industry by dissolving at least one chemical gas in ultrapure water.
(ii) Description of Related Art
In order to produce ultrapure chemicals, such as aqueous ammonia, hydrochloric acid and hydrofluoric acid, it is known to use, respectively, “industrial”-grade anhydrous ammonia gas, gaseous hydrogen chloride and gaseous hydrogen fluoride and to purify them, in particular to purify them of their metallic impurities by scrubbing over a column packed with a solution saturated with the same gas in high-purity deionized water. A technique of this type is, for example, described in Patent Application WO 96/39265.
The technology described in the aforementioned patent application, which has marked an important step forward for allowing delivery to the integrated-circuit production site of the ultrapure chemicals which allow these ever smaller integrated circuits to be fabricated, still has, however, a certain number of drawbacks when a corresponding system is operated on a customer's site, for example an integrated-circuit fabrication (“wafer fab”) plant.
A first problem encountered relates to the dissolving of the gas, which is accomplished, using the technology described in this patent, by injecting it directly into water. This results in a temperature rise and may cause sudden pressure variations due to intense stirring of the liquid. Furthermore, since the gas does not dissolve in the water instantaneously, this generates swirling in the liquid tank, which consequently means that the measurement of the titre of the solution is not always entirely correct.
Another drawback of the process described in this patent is that its operation is not continuous, thereby requiring, when the desired titre or desired concentration is reached, the content of the product container to be transferred to a storage tank (so-called batch process). Furthermore, the use of a heat exchanger as described in this patent application may possibly pose a problem when connecting the heat exchanger in the container for the product formed, in contact with coolant, which may be a source of pollution.
Finally, because of the always limited effectiveness of a mist eliminator placed at the top of a packed column, it is in some cases possible that an aerosol of a solution of the scrubbing liquid with the purified gas can in some situations pass through this mist eliminator, leading to a level of gas purity which may be limited.
SUMMARY OF THE INVENTION
The invention makes it possible to avoid these drawbacks. For this purpose, the process and apparatus according to the invention are essentially characterized by scrubbing the gas, before dissolving it, in at least two scrubbing columns, which are placed in series and preferably provided with packings, as well as by using a packed column in order to dissolve the gas in the water.
The present invention applies more particularly to the production of ultrapure liquid chemicals, such as aqueous ammonia, hydrochloric acid and hydrofluoric acid, but also to any other chemical of this type which may be obtained initially in gaseous form, preferably from a liquid phase.
Preferably, the starting material is a chemical in liquid but anhydrous form such as, for example, anhydrous ammonia in liquid form (for example at a pressure of about 5 bar and at ambient temperature) so as to be able, by vaporizing the product, to recover a vapour from which a certain number of impurities has already been removed, in a manner already described in U.S. Pat. No. 5,496,778. Next, in a first step, the gas obtained, generally after vaporizing the chemical stored in liquid form, is first of all scrubbed and then, in a second step, dissolved in deionized ultrapure water.
With regard to the step of scrubbing the gas, any type of surface may be used, such as trays, but it will be preferable to use packings. As in distillation columns, these surfaces have the purpose of increasing liquid/gas contact so as to increase the exchange between the two substances, liquid and gas. The packings which may be used are, for example, Raschig rings, Pall rings, etc. The purpose of these surfaces is to increase the area of contact between liquid and gas and, according to the invention, the purpose is preferably to increase this contact area by a factor equal to or greater than 4. As a general rule, increasing the contact area means increasing the contact area with respect to the lateral area of the unpacked column (since without any packing in a column the contact between liquid and gas essentially takes place on the lateral surface of this column). Thus, increasing the contact area by four means fitting a number of Raschig rings (or any other surface) whose total contact area is equal to three times the lateral area of the column. However, it will be preferred to increase this contact area by a factor of at least 10. In practice, plastic Raschig rings will be used and a plastic resistant to the chemical which it is desired to produce, such as aqueous ammonia, hydrofluoric acid,, hydrochloric acid, etc., will be chosen. Among suitable plastics are, in general, polyolefins and, preferably, polyethylene and/or polypropylene, which are substituted or unsubstituted, as well as their copolymers. Also suitable, in general, are the products sold by the company DuPont de Nemours under the name “PFA” or perfluoroalkoxy, as well as any type of polytetrafluoroethylene, this being optionally substituted, their copolymers, etc., all these materials being suitable when, in contact with the chemicals used, they do not produce residues, in particular residues of the metallic-elements type which are the main elements that it is important to remove from these ultrapure chemicals intended for the semiconductor industry.
In this gas-scrubbing step, and in the subsequent dissolving step, the flow rate of chemical gas to be scrubbed and then diluted is preferably less than 60 m
3
per hour and preferably between 30 and 45 m
3
per hour while the pressure of this gas will preferably be between about 1 and 3 bar in absolute value (about 0 to 2 bar in relative value).
The minimum packing volume (Raschig or Pall rings) that will preferably be used in all of the 2 or 3 scrubbing columns will be at least 20 liters and preferably at least 40 liters. The flow rate of the scrubbing solution will preferably be at least 5 liters per minute with draining at the bottom of the column collector at about 1 liter per hour.
With regard to the next step, in which the purified gas is dissolved in the deionized ultrapure water, a single column without a mist eliminator will preferably be used, the packing volume being at least one liter, preferably at least 2.5 liters and more preferably at least 4 liters, with a flow rate of dissolving solution, i.e. generally ultrapure deionized water, which is sufficiently high to avoid the column heating up, so as to keep the temperature of this column, in which the gas is dissolved, preferably below 30° C. and more preferably so as to keep the temperature of this column at a temperature close to ambient temperature, i.e. generally between 20° C. and 25° C.
The tank containing the chemical liquid, which at the end of the operation has the desired titre, is generally placed beneath this gas-dissolving column and, in general, the gas is introduced at the base of the column, preventing this gas from coming directly into contact with the chemical liquid in the tank using any suitable means such as, for example, a U-tube, a spiral, etc., while keeping a pressure at the top of the column approximately equal to that above the liquid in the tank so as to prevent the gas from passing through this U-tube or spiral. In this way, the gas follows a forced path towards the top of the column so as to promote liquid/gas exchange and to effect the desired dissolution.
According to a preferred embodiment, the step of purifying the gas before it is dissolved tak
Dulphy Herve
Laederich Thierry
Air Liquide Electronics Labeille
Burns Doane Swecker & Mathis L.L.P.
Dunn Tom
Warn Elin A
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