# Patent

## Title
Ultraviolet light activated oxidation process for the reduction of organic carbon in semiconductor process water

## Abstract

In a system for decomposing organic compounds in water for use in semiconductor manufacturing, a chemical reactor vessel having a fluid inlet and a fluid outlet, a persulfate anion addition system upstream of the reactor vessel, and a light emitting device contained within the reactor vessel. The light emitting device provides light capable of decomposing persulfate anions.

## Background
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention generally relates to a process for the purification of water used in semiconductor manufacturing. More specifically, the present invention relates to a process that uses ultraviolet activated persulfate to decompose organic compounds in both pure and spent process water streams in semiconductor manufacturing facilities.
2. Background Art
Reducing TOC in water using ultraviolet light activated aqueous persulfate is known. It is an established method of decomposing organic compounds in water and is discussed in, for instance, U.S. Pat. No. 4,277,438, to Ejzak, which teaches a batch process of preparing water samples for the measurement of TOC by: (1) persulfate addition, (2) irradiation with an extreme dose of UV (which also heats the sample) to activate the persulfate to oxidize any TOC to carbon dioxide and water. U.S. Pat. No. 5,443,991, by Godec et al, teaches a similar method.
U.S. Pat. No. 5,571,419, to Obata et al, discloses a method of producing Ultra Pure Water (UPW) having a low concentration of organic matter. The purification method requires the following process for the water to be treated: (1) pH adjustment to less than 4.5, (2) addition of an oxidizing agent (such as a persulfate salt), (3) heating of the water to preferably a minimum of 110° C. and more preferably to 120° to 170° C., and (4) cooling the water to the temperature required for use.
The prior art also includes references showing an advanced oxidation process to destroy organic compounds in wastewater, including U.S. Pat. No. 5,762,808, to Peyton, and U.S. Pat. No. 6,096,283, to Cooper et al.
However, despite improvements in this technology, there remains a need for an improved method of producing a reliable, continuous source of low TOC UPW for the semiconductor industry and other industries that require ultrapure water with controlled total organic carbon. The novelty of the present invention is the UV activation of persulfate salt to produce high oxidation potential radicals at ambient temperature, in non-pH adjusted water to purify UPW prior to discharge from the Point of Distribution (POD), prior to the Point of Connection (POC) (typically labeled as the Point of Use or POU), and to purify spent UPW for reuse on a continuous basis.
The foregoing patents reflect the current state of the art of which the present inventors are aware. Reference to, and discussion of, these patents is intended to aid in discharging Applicants' acknowledged duty of candor in disclosing information that may be relevant to the examination of claims to the present invention. However, it is respectfully submitted that none of the above-indicated patents disclose, teach, suggest, show, or otherwise render obvious, either singly or when considered in combination, the invention described and claimed herein.
DISCLOSURE OF INVENTION
This invention is a process used for the decomposition of carbon-containing compounds in water. This process reduces total organic carbon (TOC) in water through the addition of a persulfate salt upstream of an ultraviolet light source. The ultraviolet light is absorbed by the persulfate—converting the persulfate into sulfate radicals. The sulfate radicals oxidize TOC, converting the contributing compounds into CO2 and mineral salts.
The novelty of this invention is the use of the UV/persulfate oxidation process for the purification of water used in semiconductor manufacturing and the production of UPW in general.
The process uses a standard photochemical reactor either a plug flow (PFR) or a stirred tank (CSTR) or a combination of both. The most cost-effective design is expected to be a CSTR with immersed UV lamps. Multiple reactors can be used in series to improve reagent utilization.
A principal object of the present invention is to provide a novel method of reducing TOC in semiconductor process water.
Other novel features which are characteristic of the invention, as to organization and method of operation, together with further objects and advantages thereof will be better understood from the following description considered in connection with the accompanying drawings, in which preferred embodiments of the invention are illustrated by way of example. It is to be expressly understood, however, that the drawings are for illustration and description only and are not intended as a definition of the limits of the invention. The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming part of this disclosure. The invention resides not in any one of these features taken alone, but rather in the particular combination of all of its structures for the functions specified.



BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be better understood and objects other than those set forth above will become apparent when consideration is given to the following detailed description thereof. Such description makes reference to the annexed drawings wherein:
FIG. 1A is a schematic view showing the inventive advanced oxidation system;
FIG. 1B is a block schematic diagram of a conventional semiconductor industry UPW system showing possible locations where the inventive UV/persulfate advanced oxidation system could be employed in the UPW production system, the point of use, and for reuse.;
FIG. 2 is a graph showing the absorption spectrum for aqueous persulfate;
FIG. 3 is a graph showing the effect of urea concentration on decomposition performance for plug flow reactor, with persulfate at 0.5 ppm and a 0.9 minute residence time; and
FIG. 4 is a graph showing the results of a simulated pilot test of the inventive system to treat spent rinse water.



BEST MODE FOR CARRYING OUT THE INVENTION
Referring to FIGS. 1 through 4, wherein like reference numerals refer to like components in the various views, FIG. 1A is a schematic view showing the elements comprising the inventive UV activated oxidation process for reducing organic carbon in semiconductor process water, generally denominated AOP 100 herein. TOC decomposition performance is controlled by the size of the reaction vessel, the intensity and amount of UV light used, and the amount of persulfate injected. The system configuration includes a source of untreated water 110, which is fed into a chemical reactor vessel 120 through one or more fluid inlets 130. Persulfate from a persulfate anion addition system 140 is also fed into the reactor vessel, the system including a persulfate feed tank 150, a fluid inlet 160, and a feed pump 170. One or more UV lights 180, 190 are disposed within the vessel enclosure, as is a chemical solution mixer 200.
The reactor vessel is preferably a continuous-stirred tank reactor (CSTR). This keeps down cost and increases system simplicity. However, a plug flow reactor (PFR) may be preferred in situations where space (system footprint) is at a premium. Alternatively, a CSTR can be used in series with a PFR 210 having UV lights 220 to increase system efficacy.
The treated water is discharged either directly (in the case of a single reactor system) through one or more discharge outlets 230 for further use, or into the PFR for further treatment and subsequent discharge from a terminal discharge outlet 240. The treated water 250 is sampled at discharge by a TOC analyzer 260, which adjusts and controls the persulfate feed pump 170 according to the needs of the system.
FIG. 1B shows AOP 100 implementation in a typical semiconductor UPW production system 470 when employed in the production of UPW prior to the POD 360 and also prior to the POC 380 to produce ultra low level TOC (i.e. <POD level) for critical POU usage 390. FIG. 1B also shows AOP 100 implementation when employed in the purification of spent UPW 440 from semiconductor fabrication.
Treated reuse UPW 460 can be blended with raw feed water 270 to produce the required blended feed flow 280 to the UPW production system 470. Alternatively, the treated reuse UPW 460 can be blended with UPW at any point within the UPW production system 470. The Pretreatment System treats the blended feed flow by combining various technologies, as required, for suspended solids reduction, initial organic compound reduction, chlorine removal and pH adjustment to produce feed water 300 with the proper characteristics for efficient production of UPW in the Make-Up System 310. The Make-Up System process typically includes Heat Exchange (HX), Ultra-Filtration (UF), Membrane Filtration (MF), 1st Pass Reverse Osmosis (RO), 2nd Pass RO, Electro-deionization (EDI), Ion Exchange (IX), Membrane De-Aeration (MD-A) or Vacuum De-Aeration (VD-A), and UV Sterilization (UV) to produce the required Make-Up water quality 320 with a flow equal or greater than the total average usage at the POU (370 plus 390). Make-Up water stored in DI Storage 330 provides limited backup when the Make-Up System is out of service or when the average POU UPW usage exceeds the capacity of the Make-Up System. UPW from DI Storage is pumped 340 through the Polish System 350 at a flow rate greater than UPW peak usage at the combined POU. UPW in the Polish System is purified using HX, EDI, IX, UV, MF, and UF to produce UPW per POD 360 quality specifications as required at the non-critical POU 370. Certain critical fab processes 390, such as photolithography, require UPW with an impurity level less than produced at the POD 360. To meet this requirement, POU System(s) 410 are installed at the POC(s) to the critical fab process(es). The POU System can include HX, RO, EDI, IX, MD-A, pressure control, UV, MF, and UF, as required to meet fab-specific UPW specifications at the POC(s). The sum of non-critical and critical fab UPW usage equals the combined fab UPW usage. UPW not used at the combined POU is returned 400 to DI Storage. Certain fab UPW usage produces spent UPW water not economically suitable for reuse 420 (due to high levels of suspended solids, chemicals, etc.) that is sent to waste treatment. Most spent UPW 430 is primarily contaminated with TOC that can be purified in a Reuse System 440 and forwarded 460 for blending with raw feedwater 270. Water not meeting specifications for blending 450 is sent to waste treatment.
The AOP 100 can be employed at any point in the UPW production system 470, consisting of the Pretreatment System, the Make-Up System, the DI Storage, and the Polish System.
The preferred location to employ the AOP 100 in the UPW production system 470 is on the product stream of the 1st Pass RO. At this point the feedwater to the AOP 100 has significantly reduced levels of suspended solids, ionized solids, microorganisms, and TOC, all of which enhance the performance of the AOP 100. The TOC in the AOP 100 product should be controlled so that the TOC at the POD 360 and non-critical POU 370 is less than specification. Product water from AOP 100 is directed to following unit processes in the UPW production system 470, where oxidation products from the AOP process are removed.
The AOP 100 can be employed at any point between the POD 360 and the POC 380 to critical POU 390 to reduce TOC to the specification required by POU 390. Additional unit processes, similar to those found in the UPW production system 470 may be employed to remove oxidation products produced in the AOP 100, or to meet other specifications for the POU 390 not achieved with UPW delivered from the POD 360.
The AOP 100 can be employed in the Reuse System 440 to reduce TOC in segregated, spent UPW 430 to produce Reuse UPW 460 suitable for reuse in the UPW System or for other uses. Suspended solids reduction in the reuse feedwater 430 using MF or UF may be employed to improve the efficiency of the AOP 100. Ionized solids reduction using RO, EDI, or IX in part of, or all of the Reuse UPW 460 may be employed to meet use specifications of the Reuse UPW.
As will be immediately appreciated by those with skill in the art, as an alternative or addition to a feed back signal from a TOC/TDS analyzer disposed on the effluent discharge, a feed forward signal to the persulfate addition system may be supplied by a TOC analyzer disposed on the inlet end of the water to be treated. This configuration is straightforward enough to be without need of further illustration.
The performance of the reactor is a function of the following design parameters: (1) residence time; (2) reactor radius (light path length); (3) lamp linear energy density; (4) energy wavelength; (5) persulfate concentration; and, (6) TOC composition and concentration.
The reactor design is selected after determining the amount of persulfate that needs to decompose to effect the required TOC decomposition. The persulfate decomposition fraction and addition rate is established by optimizing the design parameters for capital cost, operating cost and available footprint.
The decomposition of persulfate to sulfate radicals and the subsequent reaction of the radicals with organic compounds is a homogeneous set of reactions. The longer the fluid retention time in the reactor, the greater the amount of activating UV light absorbed. Thus, residence time affects the fraction of the feed persulfate that will be decomposed and, consequently, the amount of organic material that will be oxidized.
Increasing the reactor radius increases the residence time and increases the distance between the lamps and the reactor walls. The incremental reactor volume further from the lamps is less effective for persulfate decomposition due to lower photon flux. However, because the light must pass through more water, more photons are absorbed, resulting in better use of the input UV. The effect of increased reactor radius is then a function of the effective absorption of the water. If the water absorbs little of the light, then increasing the reactor radius has a larger impact on performance. The residence time varies as a square of the radius, but because the view factor scales inversely with the distance from the lamp, photon flux varies as 1/r. Accordingly, if UV power is relatively more expensive than tank size, the tank radius should be set so that little UV light reaches the tank walls.
The effect of light energy input, or linear energy density of the lamp, is also straightforward. As more light is added to the reactor more persulfate will be decomposed. The amount of persulfate decomposed generally scales linearly with energy input. Energy input can be varied by changing the number of lamps used and the choice of lamp. For example, a standard low pressure mercury lamp, used for both photolysis and biological control, has a 254 nm output of about 0.2 W/cm. Amalgam lamps are available that give 0.4 and 0.6 W/cm. And medium pressure mercury lamps typically give 15 W/cm in the 200-260 nm range.
The wavelength of light employed is also a critical design variable. FIG. 2 graphically shows the absorption spectrum 500 for aqueous persulfate As shown in FIG. 2, persulfate absorbs a larger fraction of light as the wavelength is reduced. At 254 nm very little of the light is absorbed. Using 220 nm light would result in much more light absorption for a given path-length. Adjusting the UV to a wavelength where persulfate absorbs stronger improves performance in situations where some fraction of the light is not absorbed before reaching the reactor vessel wall. The extra absorbed light translates directly to more persulfate decomposed and improved performance. A lower limit on wavelength is set by the water absorption of photons which occurs strongly below  ̃190 nm.
For a given reactor design, the TOC decomposition performance depends on the feed rate of persulfate. The amount of persulfate required for a given performance depends on: (a) specific chemicals contributing to TOC; (b) concentration of these chemicals; and (c) TOC decomposition performance required.
The amount of persulfate needed is also affected if there are other chemicals in the water that absorb UV (lowering the effective quantum efficiency of persulfate degradation) or that are oxidized by sulfate radicals (thus competing with TOC for sulfate radicals.) Generally, cleaner water allows for more efficient TOC control.
The method may be demonstrated by the following examples, which are provided for purposes of illustration only, and do not limit the scope of the invention described herein.
EXAMPLE 1
Tests indicate that for a given target chemical the absolute reaction rate is low order in the target chemical concentration. A given persulfate feed rate and UV power generates a number of radicals which, over a narrow target chemical concentration, decompose a fixed amount of the target compound independent of the concentration of the target compound.
FIG. 3 graphically shows the effect of urea concentration on performance for a fixed residence time and persulfate feed rate 600. The data were generated using a plug flow reactor with a 254 nm lamp. The data show that the absolute amount of urea decomposed at 10 ppb was only twice that decomposed at 2 ppb. As with other AOPs, some oxidant-to-target molar ratio will be required to achieve a required performance. This affect contrasts with many chemical processes where the reaction rate scales proportionally to the target chemical concentration.
These data are an example of using the process to remove a relatively difficult to oxidize compound from an ultra-pure water stream.
EXAMPLE 2
A series of tests was conducted to test the stirred tank (CSTR) design concept with simulated spent rinse water. Isopropanol (IPA) was used as the target compound since it has been shown to be difficult to decompose and is a major component of spent rinse water. The objective of the study was to determine if 1-5 ppm TOC could be reduced to <5 ppb.
The results 700 of the pilot study are summarized as FIG. 4. The results can be organized by the three IPA concentrations examined. Note that there is a difference between IPA concentration and TOC concentration; IPA is only 60% carbon so 1.5 ppm IPA is equivalent to 915 ppb TOC.
Low IPA Tests—At the lowest IPA concentration tested (1.5 ppm), the persulfate addition required to achieve the target effluent TOC is reasonable. The effect of increased additive addition on performance is linear up to 100% IPA decomposition with a single 20 minute residence time CSTR. By adding a one minute residence time plug flow reactor (PFR) in series after the CSTR, the performance is improved by decomposing the residual persulfate. In other words, some of the additive is not decomposed in the CSTR and is passed on and decomposed in the PFR. Additive addition of 75 ppm is sufficient to destroy essentially all of the IPA with the two reactors in series.
Moderate IPA Tests—The relationship between persulfate addition and IPA decomposition is also linear at the moderate TA concentration (2.4 ppm) tested. Nearly 100% decomposition is achieved using 200 ppm persulfate using the 20 minute residence time CSTR. The additive requirements are increased somewhat when the residence time of the CSTR is decreased to 15 minutes.
High IPA Tests—At the highest IPA concentration (4.9 ppm) the amount of persulfate required to achieve the 99+% decomposition requirement is significant. The relationship between additive addition and decomposition performance is also not linear. The effect of higher persulfate concentrations on performance is diminished as 100% decomposition is approached. Although this IPA concentration is higher than is expected for the intended application, the ability to attain near complete decomposition for higher IPA concentrations in water gives a desirable flexibility to the process.
Collectively, these tests show that the technology is viable for spent rinse water treatment using a simple CSTR photochemical reactor of sufficient residence time and modest UV energy input.
The disclosure herein shows that the inventive system, in its most essential aspect, is directed to treating semiconductor process water to reduce total organic compounds in the water. The system includes a chemical reactor vessel and a persulfate anion addition system upstream of the chemical reactor vessel, and employs light energy to oxidize the aqueous persulfate. Its preferred use is to treat pure water for semiconductor manufacturing, and it is preferred to employ the system to decompose TOC to less than 5 ppb, and even more preferably below 1 ppb. However, it may also be employed to treat semiconductor-manufacturing wastewater, and in such an implementation, it is used to decompose TOC to a concentration sufficiently low to allow reuse of the water. In each implementation, the persulfate feed rate is controlled by a TOC analyzer based on a feed back signal from effluent TOC analysis. Alternatively, it may be controlled by a feed forward signal from analysis of the TOC in the untreated feed water. In yet another alternative, it may be controlled by both.
Further, in its most essential aspect, the present invention is a method of treating semiconductor process water to reduce total organic compounds, and this method comprises the steps of (1) providing a source of semiconductor manufacturing water with a TOC concentration higher than required for ultrapure uses; (2) mixing aqueous persulfate anions with the semiconductor manufacturing water in a chemical reactor vessel; (3) exposing the persulfate anion and water mixture to ultraviolet light for a predetermined residence time; and (4) discharging the treated water for further treatment required for ultrahigh purity uses. The method is particularly well suited for use when the semiconductor manufacturing water is pure water, and the process is employed to decompose TOC to less than 5 ppb, and even more preferably to less than 1 ppb. Furthermore, the method is especially well suited for use in treating semiconductor manufacturing waste water, and the process is used to decompose TOC to a concentration sufficiently low to allow reuse of the water, and further processing may be effected to remove dissolved solids and dissolved gases.
In effecting the process, the introduction of persulfate into the reaction vessel may be controlled using either a feed back signal from effluent TOC analysis, or a feed forward signal from analysis of the TOC in the untreated feed water, or both. In the former case, the TOC analyzer would be disposed between the source of water to be treated and the chemical reactor vessel, and in the latter, the TOC analyzer would be disposed anywhere downstream of the chemical reactor vessel, including on the discharge outlet itself or anywhere prior to the point of use or re-use.
The foregoing disclosure is sufficient to enable those with skill in the relevant art to practice the invention without undue experimentation. The disclosure further provides the best mode of practicing the invention now contemplated by the inventor.
While the particular apparatus and method herein shown and disclosed in detail is fully capable of attaining the objects and providing the advantages stated herein, it is to be understood that it is merely illustrative of the presently preferred embodiment of the invention and that no limitations are intended concerning the detail of construction or design shown other than as defined in the appended claims. Accordingly, the proper scope of the present invention should be determined only by the broadest interpretation of the appended claims so as to encompass obvious modifications as well as all relationships equivalent to those illustrated in the drawings and described in the specification.

## Claims

1. A method of treating semiconductor manufacturing water to reduce total organic carbon (TOC), the method comprising:
providing a source of semiconductor manufacturing water;
adding a solution of aqueous persulfate to the semiconductor manufacturing water to provide a mixture;
exposing the mixture to ultraviolet light in a chemical reactor vessel for a predetermined residence time to provide a treated water comprising a TOC concentration of less than about 5 ppb;
measuring the TOC concentration of at least one of the semiconductor manufacturing water and the treated water; and
adjusting a rate of aqueous persulfate addition to the semiconductor manufacturing water based on a TOC measurement of the at least one of the semiconductor manufacturing water and the treated water.

2. The method of claim 1, wherein the treated water comprises a TOC concentration of less than about 1 ppb.

3. The method of claim 2, wherein adjusting the rate of aqueous persulfate addition is based on a TOC measurement of the semiconductor manufacturing water.

4. The method of claim 2, further comprising removing dissolved solids and dissolved gases.

5. The method of claim 2, wherein adjusting the rate of aqueous persulfate addition is based on a TOC measurement of the treated water.

6. The method of claim 1, wherein exposing the mixture to ultraviolet light in a chemical reactor vessel comprises exposing the mixture to ultraviolet light in a plug-flow reactor.

7. The method of claim 1, wherein exposing the mixture to ultraviolet light in a chemical reactor vessel comprises exposing the mixture to ultraviolet light in a continuous-stirred tank reactor.

8. The method of claim 7, wherein exposing the mixture to ultraviolet light in a chemical reactor vessel comprises exposing the mixture to ultraviolet light in a plug-flow reactor downstream of the continuous-stirred tank reactor.

9. The method of claim 1, further comprising passing the semiconductor manufacturing water through a reverse osmosis unit prior to adding a solution of persulfate to the semiconductor manufacturing water.

10. The method of claim 1, wherein the method is performed at ambient temperature.