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On-Line Monitoring of Hydrogen Peroxide in CMP Slurry

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2. <strong>Hydrogen</strong> <strong>Peroxide</strong> Monitor Design And PerformanceOptical absorption spectroscopy is an accurate, reliable, cost effective means <strong>of</strong> measur<strong>in</strong>g chemicalconcentrations <strong>in</strong> liquids or gases [1, 2, 3]. Absorption spectroscopy is widely used for autonomous, on-l<strong>in</strong>e, processmonitor<strong>in</strong>g <strong>in</strong> a number <strong>of</strong> <strong>in</strong>dustries, <strong>in</strong>clud<strong>in</strong>g: chemical solvent, pharmaceutical, polymer and paper manufactur<strong>in</strong>g,natural gas, jet fuel and metals ref<strong>in</strong><strong>in</strong>g, power plant and automobile emissions monitor<strong>in</strong>g. In these <strong>in</strong>dustrialapplications, absorption spectroscopy can provide simultaneous measurement <strong>of</strong> one or more, liquid or gas species.Recently this author described the use <strong>of</strong> UV absorption spectroscopy for measurement <strong>of</strong> the BTA(benzotriazole) concentration <strong>in</strong> the <strong>CMP</strong> polisher r<strong>in</strong>se [3]. BTA is a copper corrosion <strong>in</strong>hibitor, which is widelyused <strong>in</strong> copper <strong>CMP</strong> processes. This effort provided a first demonstration <strong>of</strong> on-l<strong>in</strong>e monitor<strong>in</strong>g <strong>of</strong> the chemicalconstituents <strong>of</strong> a <strong>CMP</strong> process, us<strong>in</strong>g absorption spectroscopy.Figure 1 displays BTA concentration data, obta<strong>in</strong>ed from cont<strong>in</strong>uous, autonomous, on-l<strong>in</strong>e monitor<strong>in</strong>g <strong>of</strong> a<strong>CMP</strong> polisher r<strong>in</strong>se chemical delivery system, with a <strong>Slurry</strong>Alert ® precision UV spectrometer. The chemical deliverysystem was <strong>in</strong>itially run for a few days at a BTA concentration <strong>of</strong> 328 ppm, the BTA monitor was disconnected fromthe data collection system for a couple <strong>of</strong> days, then reconnected. The data record <strong>in</strong>dicates f<strong>in</strong>e tun<strong>in</strong>g <strong>of</strong> the BTAconcentration output by the CDU operator, over the range <strong>of</strong> 320-333 ppm, with the f<strong>in</strong>al week <strong>in</strong>dicat<strong>in</strong>g somedecrease <strong>in</strong> the CDU’s ability to control to a fixed set po<strong>in</strong>t. The first 1/3 <strong>of</strong> the data record, where the CDU exhibitstight control at 328-329 ppm, then later at 332-333 ppm, demonstrates the excellent stability and low noise <strong>of</strong> the<strong>Slurry</strong>Alert BTA monitor. The resolution <strong>of</strong> the BTA monitor exceeds 1 ppm at 330 ppm, or better than 0.3% <strong>of</strong>read<strong>in</strong>g,212


Figure 1: BTA concentration versus time, as measured <strong>in</strong> a <strong>CMP</strong> polisher r<strong>in</strong>se chemical deliverysystem. For the horizontal time axis, with each division represents 1 day; for the verticalconcentration axis, which ranges from 300 to 350 ppm, with each division represents 5 ppm.Optical measurement <strong>of</strong> the BTA or H 2 O 2 concentration at the CDU or <strong>in</strong> the <strong>CMP</strong> polisher r<strong>in</strong>se is lessdifficult than mak<strong>in</strong>g that measurement after the chemicals have been added the slurry. Application <strong>of</strong> absorptionspectroscopy requires the identification <strong>of</strong> strong absorption bands for each chemical species to be measured. Ingeneral, the spectral signature <strong>of</strong> each absorption band is unique to that species, s<strong>in</strong>ce it’s a function <strong>of</strong> molecularstructure. After chemical species are added to the <strong>CMP</strong> slurry, their absorption bands can be masked by the highoptical ext<strong>in</strong>ction (scatter<strong>in</strong>g plus absorption) <strong>of</strong> the slurry particles. Cerni [4] described a method <strong>of</strong> correct<strong>in</strong>g forthe particle ext<strong>in</strong>ction, such that absorption spectroscopy can be used for accurate measurement <strong>of</strong> chemicalconcentrations <strong>in</strong> <strong>CMP</strong> slurry. However, this method is limited to spectral regions where the particle ext<strong>in</strong>ction islow to moderate and does not dom<strong>in</strong>ate the liquid chemical absorption. For most <strong>CMP</strong> slurries, this means that themethodology described by Cerni [4] is limited to the visible and near <strong>in</strong>frared spectral regions [5]. BTA and H 2 O 2 donot exhibit any significant absorption <strong>in</strong> the visible or near <strong>in</strong>frared, rather they exhibit strong UV absorption, asshown <strong>in</strong> Figures 2 and 3. However, most <strong>CMP</strong> slurries are opaque at these wavelengths (


very small portion <strong>of</strong> the clear liquid (permeate) from the <strong>CMP</strong> slurry circulat<strong>in</strong>g <strong>in</strong> the global loop, pass thatpermeate through the <strong>Slurry</strong>Alert for measurement <strong>of</strong> it’s chemical composition, then return the permeate to the slurryday tank [6]. These cross flow or membrane filters are <strong>of</strong>ten used <strong>in</strong> <strong>CMP</strong> wastewater reclamation, where the goal isto extract most <strong>of</strong> the liquid from the waste slurry, to concentrate it prior to disposal.1250 260 270 280 290 300 310 320 330 340 35010.90.90.80.8TRANSMISSION0.70.60.50.40.3H 2 O 2 Concentration0.5%1.0%2.5%5.0%0.70.60.50.40.30.20.20.10.10250 260 270 280 290 300 310 320 330 340 350WAVELENGTH (nm)0Figure 3: Measured transmission spectra for H 2 O 2 concentrations <strong>of</strong> 0.5 – 5.0%.A simplified version <strong>of</strong> the microprocessor controlled <strong>CMP</strong>-CLS measurement sequence proceeds as follows:(1) A portion <strong>of</strong> the slurry circulat<strong>in</strong>g <strong>in</strong> the global loop is diverted to flow through a <strong>CMP</strong> membrane or crossflow filter. A small portion <strong>of</strong> the slurry clear liquid (permeate) is cont<strong>in</strong>uously extracted from the slurryflow, by virtue <strong>of</strong> the fact that the global loop is at some pressure above ambient.. The cont<strong>in</strong>uous permeateflow is directed through the <strong>Slurry</strong>Alert UV spectrometer, allow<strong>in</strong>g for measurement <strong>of</strong> the H 2 O 2concentration.(2) After exit<strong>in</strong>g the <strong>Slurry</strong>Alert, the permeate flow is directed to a reservoir. When the permeate reservoirbecomes full, the permeate flow from the filter is momentarily halted, such that the reservoir can be emptied<strong>in</strong>to the slurry day tank. Empty<strong>in</strong>g <strong>of</strong> the reservoir <strong>in</strong>terrupts spectrometer sampl<strong>in</strong>g for only a few secondsevery 10 m<strong>in</strong>utes.(3) Additionally, every 5 m<strong>in</strong>, a one second reverse pressure pulse is directed at the cross flow filter, to clear it’spores or membrane, thus prevent<strong>in</strong>g the filter from clogg<strong>in</strong>g.The permeate reservoir holds less than 0.1 L, whereas the slurry global loop and day tank typically conta<strong>in</strong> 200 -800 L. Hence extraction <strong>of</strong> the permeate from the global loop, changes the slurry solids content by less than 0.05% <strong>of</strong>the target value, which is an amount too small to be measured. S<strong>in</strong>ce the permeate reservoir is periodically emptied214


ack <strong>in</strong>to the day tank, chemical measurement via optical means does not result <strong>in</strong> any changes to the physical orchemical properties <strong>of</strong> the slurry circulat<strong>in</strong>g <strong>in</strong> the global loop.A major cause <strong>of</strong> ma<strong>in</strong>tenance problems for the automated, on-l<strong>in</strong>e chemical titration systems, is failure <strong>of</strong>sampl<strong>in</strong>g valves and sample handl<strong>in</strong>g apparatus, by cont<strong>in</strong>uous exposure to the very abrasive slurry particles. The<strong>CMP</strong>-CLS elim<strong>in</strong>ates this problem, by plac<strong>in</strong>g all the sampl<strong>in</strong>g valves on the permeate side <strong>of</strong> the flow, thus isolat<strong>in</strong>gthem from the abrasive slurry particles. The only element which is exposed to the abrasive slurry particles, is thecross flow filter, which has no mov<strong>in</strong>g parts and is specifically designed for this application. Frequent, <strong>in</strong>termittentreverse flush<strong>in</strong>g prevents filter clogg<strong>in</strong>g. Furthermore the <strong>Slurry</strong>Alert has no mov<strong>in</strong>g parts which are exposed to theslurry particles or the permeate. This optical spectroscopy method <strong>of</strong> chemical monitor<strong>in</strong>g can therefore providehigher reliability and lower ma<strong>in</strong>tenance costs. This optical measurement technique does not require consumption <strong>of</strong>chemical reagents, which provides a further, significant reduction <strong>in</strong> cost <strong>of</strong> ownership, when compared to on-l<strong>in</strong>echemical titration systems.Cerni [3] provided an example <strong>of</strong> a calibration methodology and response function for a BTA monitor based onUV absorption spectroscopy. Figure 4 displays similar results for measurement <strong>of</strong> the H 2 O 2 concentration <strong>in</strong> water.The Y-axis represents the logarithm <strong>of</strong> the ratio <strong>of</strong> transmissions measured at 2 wavelengths; this allows forimplementation <strong>of</strong> the differential absorption measurement technique [2, 3]. A total <strong>of</strong> 10 standard H 2 O 2 solutionsplus DI water were used <strong>in</strong> the calibration procedure; these data are plotted <strong>in</strong> Figure 4 as crosses. The standardsolutions were generated through precision dilution <strong>of</strong> a s<strong>in</strong>gle bottle <strong>of</strong> reagent grade H 2 O 2 <strong>of</strong> known concentration.The H 2 O 2 concentration range <strong>of</strong> 0 - 1% is well covered by T1 and T2, while the H 2 O 2 concentration range <strong>of</strong> 1 - 6%is well covered by T2 and T3. The response functions are derived by fitt<strong>in</strong>g the data with smooth curves.Log(Trans Ratio)0-0.1-0.2-0.3-0.4-0.5-0.6-0.7-0.8-0.9-1-1.120031208 DataLog(T1/T2)Log(T2/T3)0 1 2 3 4 5 6 7H2O2 Concentration (%)Figure 4: The <strong>Slurry</strong>Alert-H 2 O 2 response functions and calibration data. T1, T2 andT3 refer to transmissions measured at 250, 280 and 310 nm respectively.215 PacRim-<strong>CMP</strong> 2004


After the calibration procedure was completed, and the response functions def<strong>in</strong>ed, three <strong>of</strong> the H 2 O 2standard solutions were measured a second time. The results are displayed <strong>in</strong> Table 1, which provides a quantitative<strong>in</strong>dication <strong>of</strong> the accuracy <strong>of</strong> the calibration procedure and the <strong>in</strong>strument repeatability.Table 1: Measurement <strong>of</strong> 3 H 2 O 2 standard solutions <strong>of</strong> known concentrationH 2 O 2 concentration (% by mass)Error as a % <strong>of</strong> read<strong>in</strong>gStandard Value Measured Value Curve Fit Error Measurement Error Total Error0.1006 0.1028 +1.7% +0.5% +2.2%0.943 0.9470 +0.2% +0.2% +0.4%4.95 4.896 -0.3% -0.8% -1.1%Figure 5 displays results <strong>of</strong> circulat<strong>in</strong>g a H 2 O 2 sample closed loop through the <strong>Slurry</strong>Alert, with a Teflondiaphragm pump, for a period <strong>of</strong> 29 hr. These data provide an example <strong>of</strong> the stability <strong>of</strong> the <strong>in</strong>strument, plus it’sexcellent signal-to-noise level. Dur<strong>in</strong>g the f<strong>in</strong>al hour, a small amount <strong>of</strong> DI water was added to the 4 liter reservoir,result<strong>in</strong>g <strong>in</strong> a 1.4% drop <strong>in</strong> the <strong>in</strong>dicated concentration, from 3.765% to 3.714%. It’s clear from the data, that achange <strong>of</strong> less than 0.1% <strong>in</strong> concentration can easily be detected. Note that these data do not show any H 2 O 2decomposition. However when slurry is added to the H 2 O 2 , and the mixture pumped closed loop <strong>in</strong> a slurrydistribution system, H 2 O 2 decomposition is easily detected; the slurry appears to act as a catalyst for thisdecomposition reaction.43.9H 2O 2(%)3.83.73.60 5 10 15 20 25 30TIME (hr)Figure5: H 2 O 2 concentration versus time for a simulated closed loop distribution system.216


known. The Teflon bellows pump was then started, and the mixture cont<strong>in</strong>uously circulated. The H 2 O 2 concentration<strong>of</strong> the cont<strong>in</strong>uously circulat<strong>in</strong>g slurry was then subjected to a sequence <strong>of</strong> changes, by add<strong>in</strong>g a precisely knownamount <strong>of</strong> H 2 O 2 , slurry and DI water to the reservoir. The results <strong>of</strong> this test are shown <strong>in</strong> Figure 7. Of note is thevery low noise level, plus the high precision and accuracy <strong>of</strong> the <strong>Slurry</strong>Alert. Table 2 provides a tabular comparison<strong>of</strong> the measured H 2 O 2 concentration as determ<strong>in</strong>ed by the <strong>Slurry</strong>Alert, versus the calibration standard values. Theaccuracy <strong>of</strong> this spectroscopy-based <strong>in</strong>strument is remarkable, and comparable to a well ma<strong>in</strong>ta<strong>in</strong>ed chemical titrationsystem, operated by experienced personnel.HYDROGEN PEROXIDE (%)1.81.71.61.51.41.31.21.11.00.90.80.70.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5TIME (hr)Figure 7: A <strong>Slurry</strong>Alert accuracy and repeatability test, for measurement <strong>of</strong> the H 2 O 2concentration <strong>of</strong> W2000 slurry. The 5 slurry mixtures, <strong>in</strong> chronological order, wereknown to conta<strong>in</strong> 1.002, 1.502, 1.600, 1.701 and 1.500% H 2 O 2 by weight.Table 2: A tabular comparison <strong>of</strong> the data plotted <strong>in</strong> Figure 7.H 2 O 2 Concentration (% mass)Measurement ErrorCalibration Standard <strong>Slurry</strong>Alert H 2 O 2 Error % <strong>of</strong> Read<strong>in</strong>g1.002 1.001 -0.001 0.1%1.502 1.504 +0.002 0.1%1.600 1.604 +0.004 0.2%1.701 1.708 +0.007 0.4%1.500 1.511 +0.011 0.7%The data plotted <strong>in</strong> Figure 5 demonstrated that when H 2 O 2 is cont<strong>in</strong>uously pumped though a chemicaldistribution system, for more than 24 hr, no decomposition is observed. However when H 2 O 2 is mixed with silicaslurry, then cont<strong>in</strong>uously pumped through a slurry distribution system, decomposition is rapid and easily apparent.218


Data from this latter test case is shown <strong>in</strong> Figure 8; here H 2 O 2 was added to Cabot SS-25 silica slurry, diluted to 10%solids. The H 2 O 2 concentration <strong>in</strong> the slurry decays from 1.86% to 1.62%, <strong>in</strong> approximately 72 hr. The datademonstrate the need for cont<strong>in</strong>uous monitor<strong>in</strong>g <strong>of</strong> the H 2 O 2 concentration <strong>in</strong> the slurry global distribution loop, toprevent that concentration from fall<strong>in</strong>g outside <strong>of</strong> specified <strong>CMP</strong> process limits.219 PacRim-<strong>CMP</strong> 2004


Figure 8: <strong>Slurry</strong>Alert measurements <strong>of</strong> the H 2 O 2 concentration (<strong>in</strong> slurry) versus time. For the horizontal time axis, eachdivision represents 4 hr, with the total time period encompass<strong>in</strong>g 72 hr. The vertical concentration axis rangesfrom 1.0% to 2.0%, with each division represent<strong>in</strong>g 0.1%.3. SummaryThe majority <strong>of</strong> <strong>CMP</strong> slurries require an oxidiz<strong>in</strong>g agent, and hydrogen peroxide (H 2 O 2 ) is by far, the mostwidely used oxidiz<strong>in</strong>g agent. <strong>Hydrogen</strong> peroxide is naturally unstable and decomposes with time, hence it is widelyaccepted that the H 2 O 2 concentration <strong>in</strong> the slurry global loop must be frequently measured, such that additional H 2 O 2can be added, when the concentration falls outside <strong>of</strong> specified <strong>CMP</strong> operat<strong>in</strong>g limits.Chemical titration systems are now widely used to measure the H 2 O 2 concentration <strong>in</strong> <strong>CMP</strong> slurry. Theautomated on-l<strong>in</strong>e titration systems have several disadvantages, <strong>in</strong>clud<strong>in</strong>g high <strong>in</strong>itial cost, high ma<strong>in</strong>tenance, highcost <strong>of</strong> ownership, chemical reagent consumption and chemical waste stream creation. The optical absorptionspectroscopy system described <strong>in</strong> this paper, <strong>of</strong>fers significant improvements <strong>in</strong> all <strong>of</strong> the above areas, and providescont<strong>in</strong>uous on-l<strong>in</strong>e monitor<strong>in</strong>g <strong>of</strong> the H 2 O 2 concentration <strong>in</strong> the slurry global loop. Optical absorption spectroscopy isan accurate, reliable, cost effective means <strong>of</strong> measur<strong>in</strong>g chemical concentrations <strong>in</strong> liquid or gases. Absorptionspectroscopy is widely used for autonomous, on-l<strong>in</strong>e, process monitor<strong>in</strong>g <strong>in</strong> a number <strong>of</strong> <strong>in</strong>dustries, <strong>in</strong>clud<strong>in</strong>g:chemical solvent, pharmaceutical, polymer and paper manufactur<strong>in</strong>g, natural gas, jet fuel and metals ref<strong>in</strong><strong>in</strong>g, powerplant and automobile emissions monitor<strong>in</strong>g.220


After chemical species are added to the <strong>CMP</strong> slurry, their absorption bands can be masked by the highoptical ext<strong>in</strong>ction (scatter<strong>in</strong>g plus absorption) <strong>of</strong> the slurry particles. <strong>Hydrogen</strong> peroxide exhibits strong absorption <strong>in</strong>the UV spectral region, however it is here that most <strong>CMP</strong> slurries are opaque, mak<strong>in</strong>g implementation <strong>of</strong>conventional absorption spectroscopy methods impractical or impossible. Very recently, this measurement problemwas solved by comb<strong>in</strong><strong>in</strong>g a <strong>Slurry</strong>Alert ® UV precision spectrometer with an <strong>in</strong>novative <strong>CMP</strong> cont<strong>in</strong>uous liquidsampler (<strong>CMP</strong>-CLS). The <strong>CMP</strong>-CLS utilizes a cross flow or membrane filter to cont<strong>in</strong>uously extract a very smallportion <strong>of</strong> the clear liquid (permeate) from the <strong>CMP</strong> slurry circulat<strong>in</strong>g <strong>in</strong> the global loop. The permeate then flowsthrough the <strong>Slurry</strong>Alert for chemical composition measurement, after which the it is returned to the slurry day tank.Chemical measurement via optical means does not result <strong>in</strong> any changes to the physical or chemical properties <strong>of</strong> theslurry circulat<strong>in</strong>g <strong>in</strong> the global loop.A major cause <strong>of</strong> ma<strong>in</strong>tenance problems for automated slurry samplers, is failure <strong>of</strong> sampl<strong>in</strong>g valves andsample handl<strong>in</strong>g apparatus, by cont<strong>in</strong>uous exposure to the very abrasive slurry particles. The <strong>CMP</strong>-CLS elim<strong>in</strong>atesthis problem, by plac<strong>in</strong>g all the sampl<strong>in</strong>g valves on the permeate side <strong>of</strong> the flow, thus isolat<strong>in</strong>g them from theabrasive slurry particles. The only element which is exposed to the abrasive slurry particles, is the cross flow filter,which has no mov<strong>in</strong>g parts and is specifically designed for this application. Frequent, automated, reverse flush<strong>in</strong>gprevents filter clogg<strong>in</strong>g.The data demonstrate that UV spectroscopy can provide measurement <strong>of</strong> the H 2 O 2 concentration over the fullrange <strong>of</strong> <strong>in</strong>terest to <strong>CMP</strong> users (0.1 – 5.0%), with a resolution <strong>of</strong> better than 1% <strong>of</strong> read<strong>in</strong>g, and an accuracy <strong>of</strong> 2% <strong>of</strong>read<strong>in</strong>g. This is comparable to well ma<strong>in</strong>ta<strong>in</strong>ed chemical titration systems, when operated by experienced personnel.Furthermore, the spectroscopy-based H 2 O 2 monitor <strong>of</strong>fers lower <strong>in</strong>itial cost, lower cost <strong>of</strong> ownership, and noconsumables, when compared to the automated, on-l<strong>in</strong>e chemical titration systems.4. References[1] Nelson, L. and T.A. Cerni, 1989: Method <strong>of</strong> and Apparatus for Measur<strong>in</strong>g Vapor Density. U.S. Patent No. 4,874,572.[2] Cerni, T.A., 1994: An Infrared Hygrometer for Atmospheric Research and Rout<strong>in</strong>e <strong>Monitor<strong>in</strong>g</strong>. J. Atmospheric andOceanic Technology, April, 1994.[3] Cerni, T.A., 2003: <strong>On</strong>-<strong>L<strong>in</strong>e</strong> <strong>Monitor<strong>in</strong>g</strong> <strong>of</strong> BTA <strong>in</strong> Copper <strong>CMP</strong>. Semiconductor Pure Water and ChemicalsConference.[4] Cerni, T.A., 2003: Spectroscopic Measurement <strong>of</strong> the Chemical Constituents <strong>of</strong> a <strong>CMP</strong> <strong>Slurry</strong>. U.S. Patent No.6,709,311.[5] Cerni, T. A., 2000: <strong>CMP</strong> Process Control Us<strong>in</strong>g Spectroscopic <strong>On</strong>-<strong>L<strong>in</strong>e</strong> <strong>Monitor<strong>in</strong>g</strong>. Symp. Contam<strong>in</strong>ation Free Mfg.,Semicon West 2000, pp L1.[6] Cerni, T.A., 2003: Optical Measurement <strong>of</strong> the Chemical Constituents <strong>of</strong> an Opaque <strong>Slurry</strong>. U.S. Patent Applicationsubmitted 12/31/2003.221 PacRim-<strong>CMP</strong> 2004

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