reduction of carbon-based interferences in organic...

www.spectroscopyonl ine.com14 Spectroscopy 19(9) September 2004

Reduction of Carbon-BasedInterferences in OrganicCompound Analysis by DynamicReaction Cell ICP MSThe authors show that dynamic reaction cell ICP MS can eliminate a number of argon- and carbon-based polyatomic interferences, allowing the determination of many critical elements in problematic organic compounds found in the semiconductor industry.

Yoko Kishi, Katsu Kawabata, Haiying Shi, and Robert Thomas

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Inductively coupled plasma mass spectrometry (ICPMS) traditionally has been an indispensable analyticaltool for the semiconductor industry because of its abil-ity to determine analytes simultaneously at the sub-

part-per-trillion (ppt) level in various process chemicals.However, it is extremely important to address certain poten-tially problematic areas when analyzing organic solventsdirectly, including:

Viscosity and volatility of organic solvent. The viscosity andvolatility of organic solvents have a significant effect onthe efficiency of sample introduction and ICP stability. Ahigher viscosity solvent causes inefficient nebulization,which requires dilution either with water, if it is watermiscible, or with some other organic solvent if it is notmiscible with water. On the other hand, volatile organicsolvents have a higher vapor pressure and most probablywould result in the plasma being extinguished. A cooledspray chamber might help to reduce the vapor pressure,but it cannot solve this problem completely becausesome organic solvents need to be chilled to an extremelylow temperature. An alternative solution is to use anoptimized sample uptake rate, where virtually the entiresample is nebulized into the plasma. In fact, a cooledspray chamber is not very critical with a well-controlledsample uptake rate.

Deposition of carbon on the interface cones. If organic sol-vents are aspirated directly into the plasma, carbon willbe deposited on the tip of the interface cones. This can beavoided by adding a small amount of oxygen into theinjector gas flow between the spray chamber and thetorch. The amount of oxygen depends upon the organicsolvent being analyzed, but it is typically 3–7% of the

nebulizer gas flow. For example, longer chain hydrocar-bons require more oxygen in order to burn the carbon atthe tip of the cones. The amount of oxygen is very criti-cal, because an insufficient amount can cause deposition

Organic Compound Analysis


of carbon at the cones, while too muchcan cause erosion and result in shortercone lifetime. Because of this harsh,oxygen-rich environment, platinumcones are recommended as opposed tothe traditional nickel cones.

Compatibility of sample introductiondevice. The compatibility of sampleintroduction devices with organic sam-ples also is a very critical issue becausemany of the solvents actually will dis-solve the peristaltic pump tubing. Othermaterials are more resistant, but theyusually are not clean enough for ppt-level determinations by ICP MS. Inorder to avoid contamination from theperistaltic pump tubing, self-aspirationusing a concentric nebulizer fitted withnarrow diameter perfluoroalkane (PFA)tubing is recommended for the analysisof organic solvents.

Reduction of spectral and matrix interfer-ences. Polyatomic spectral interferencesand matrix suppression effects due to car-bon species also are problematic issues inthe analysis of organic solvents by ICPMS. There are several approaches used toeliminate carbon-based interferences

such as CC+ and ArC+

using a quadrupole ICPMS system (1, 2, 3). Acool plasma is one way toreduce these interferencesas well as to minimize thebackground from con-taminants of the conematerial. However, be-cause of its lower temper-ature, a cool plasma can-not ionize many elements

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Figure 2. Calibration curves and statistical data for (a) 24Mg,(b) 52Cr, and (c) 27Al in 100% isopropyl alcohol.

Table I. DRC instrumental parameters and sample introduction components.Argon plasma gas 18 L/min

Argon auxiliary gas 1.2 L/minRF power 1600 W

Torch Quartz with 2 mm i.d. injectorNebulizer Concentric nebulizer (Meinhard TQ-30-A3)

Argon nebulizer gas 0.6–1.0 L/minArgon makeup gas 0.0–0.3 L/min

Oxygen gas 0.03–0.07 L/minSpray chamber Quartz Cyclonic (Elemental Scientific Inc.)

Reaction gas NH3: 0–1 mL/minCones Ni base with Pt insert



sufficiently, such as boron, that have highionization potential. In addition, the coolplasma approach suffers from matrixsuppression effects because most ele-ments cannot be ionized completely in alow-temperature plasma (4). This prob-lem is more significant for organic sam-ples with high molecular weight, suchas N-methyl pyrrolidone (NMP) andphotoresist, because the cool plasma

doesn’t have enough energy to decom-pose them. A cooler plasma tempera-ture is more desirable to reduce bothpolyatomic ion interferences and con-taminants from the cone material, buta hotter plasma temperature most def-initely is required to minimize matrixsuppression and to efficiently decom-pose organic compounds.

Collision cells using multipoles and

low reactive gases also have proved use-ful in reducing polyatomic interfer-ences in ICP MS (5). However, thisapproach necessitates the use of kineticenergy discrimination to remove theunwanted collisional by-product ionsthat are formed inside the cell. This typ-ically is achieved by setting the collisioncell potential slightly more negativethan the mass filter potential, whichmeans the lower kinetic energy colli-sion-product ions (as a result of the col-lision process) are rejected, while theanalyte ions are transmitted.Unfortunately, this produces a loss ofsensitivity, which is more significant forlighter elements because those ions tendto lose more kinetic energy per colli-sion, compared with the heavier ions(6). The problem is exaggerated withorganic samples because of the largenumber of carbon-, oxygen-, andargon-based by-product ions that areformed. For these reasons, instrumentsthat use collision cells with kineticenergy discrimination often have torevert to using cool plasma conditionsto achieve the desired detection whenanalyzing organic matrices (7).

Dynamic reaction cell. The dynamicreaction cell (DRC™) is anotherapproach to reducing polyatomic spec-tral interferences that doesn’t discrimi-nate by kinetic energy (3). Thisapproach has been described previous-

Table II. Quantitative results for 100% isopropyl alcohol. A value of zero in the NH3 gas flow column indicates thatthe element was determined in standard mode with no gas flow used (N.D. = less than DL).

Element m/z BEC DL (ppt) NH3 flow Element m/z BEC (ppt) DL (ppt) NH3 flow (ppt) (mL/min) (mL/min)

Li 7 N.D. 4.0 0 Cu 63 6.6 4.2 0.3

B 11 270 50 0 Zn 64 5.8 3.9 0.3Na 23 8.0 3.2 0 Ga 69 N.D. 6.7 0.6

Mg 24 N.D. 2.4 0.6 As 75 14.5 6.1 0Al 27 6.0 4.9 0.6 Mo 98 N.D. 1.8 0

K 39 49.0 1.7 0.6 Sr 88 4.1 1.7 0.3Ca 40 2.7 1.8 1.0 Cd 114 N.D. 0.8 0

Ti 48 N.D. 0.6 0.3 In 115 18.0 6.1 0V 51 1.3 1.2 0.3 Sn 120 N.D. 0.6 0

Cr 52 5.5 3.0 0.6 Sb 121 1 2 0Mn 55 2.2 1.4 0.6 Ba 138 1.3 0.7 0

Fe 56 2.4 0.9 0.6 Tl 205 N.D. 1.5 0Co 59 0.7 0.3 0.3 Pb 208 N.D. 0.6 0

Ni 58 2.1 1.9 0.3 Bi 209 N.D. 0.2 0

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C C2CN Ar ArH ArC Ar2

Figure 3. Overlaid spectra of NMP blank in standard (green) and DRC (black) modes.

September 2004 19(9) Spectroscopy 19

ly in the literature for the analysis ofsemiconductor materials (8, 9). Thedynamic reaction cell is a pressurizedquadrupole positioned before the ana-lyzer quadrupole. A highly reactive gas,such as pure ammonia, is supplied tothe cell, where ion molecule chemistryoccurs. Through various mechanisms,the highly reactive gaseous moleculesreact with the interfering ions to con-vert them either into species of differ-ent mass than the analyte or into neu-tral species. The analyte mass thenemerges from the dynamic reaction cellfree of its interference and enters theanalyzer quadrupole for conventionalmass separation. The advantage ofusing a quadrupole in the reaction cellis that the ion stability regions are welldefined, as opposed to those of a hexa-pole or an octapole. As a result, thequadrupole inside the reaction cell canbe operated with a relatively narrowermass transmission window, not just asan ion guide. Therefore, optimizing thequadrupole prevents unwanted reac-tions between the gas and the samplematrix or solvent, which potentiallycan lead to new interferences, while notaffecting transmission of analyte ions.Every time an analyte and interferingion enter the dynamic reaction cell, thebandpass of the quadrupole automati-

cally is optimized for that specificanalysis and then changed on-the-flyfor the next analyte. The other majoradvantage of the dynamic reaction cellis that it can be operated using hotplasma conditions, which offer severalbenefits for the analysis of organic sol-vents, as described earlier.

ExperimentalTo exemplify the benefits of dynamicreaction cell ICP MS for the analysis ofcomplex organic samples, the follow-ing semiconductor process chemicalswere analyzed:

● Isopropyl alcohol

Table III. Analysis of various organic compounds in different organic solvents. Elements in red were determined under DRC conditions using NH3 as the reaction gas, while elements in black were determined under

standard mode conditions. (The precision values for all quantitative data in this table were 1–4% RSD for the three replicates.)

Element m/z 3% Photoresist 10% ARC

in NMP in MMP in ethyl lactate in PGMEA in PGME in NMP

Li 7 <0.004 1.02 102 0.017 0.997 98 <0.01 0.92 92 <0.003 0.96 96 1.35 2.40 105 - - -

Na 23 0.21 1.13 92 0.28 1.13 85 0.7 1.83 113 1.77 2.94 117 1.61 2.74 113 0.63 1.58 95

Mg 24 0.06 1.05 99 0.06 1.02 96 <0.01 0.94 94 0.15 1.27 112 0.90 1.85 95 <0.04 1.00 100

Al 27 0.12 1.04 92 0.04 0.96 92 <0.03 0.98 98 0.17 1.13 96 2.50 3.35 85 0.01 1.03 102

K 39 0.16 1.15 99 0.08 1.01 93 0.38 1.44 106 0.44 1.47 103 1.39 2.32 93 0.04 1.11 107

Ca 40 0.54 1.71 117 0.16 1.17 101 <0.1 1.0 100 0.49 1.67 118 0.79 1.77 98 0.27 1.39 112

Cr 52 4.9 5.84 94 <0.009 1.02 102 <0.07 1.08 108 0.19 1.32 113 1.27 2.12 85 0.59 1.73 114

Mn 55 0.07 1.01 94 <0.002 1.01 101 0.01 1.09 108 0.05 1.17 112 0.99 2.04 105 0.02 1.25 123

Fe 56 3.2 4.25 105 0.11 0.97 86 0.29 1.26 97 0.72 1.75 103 2.89 3.77 88 0.14 1.26 112

Ni 60 0.43 1.26 83 <0.01 1.05 105 0.05 1.20 115 0.07 1.18 111 2.26 3.40 114 - - -

Cu 63 0.82 1.79 97 <0.005 1.05 105 0.07 1.12 105 0.18 1.35 117 2.03 2.96 93 0.07 1.21 114

Zn 64 3.43 4.50 107 0.32 1.24 92 0.25 1.28 103 2.92 4.05 113 1.70 2.85 115 - - -

Pb 208 <0.00 10.93 93 <0.001 1.04 104 <0.004 1.17 117 0.02 0.95 93 0.10 0.93 83 - - -

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Be (3.3%)B (1.7%)Na (1.1%)As (1.6%)Mo (2.8%)Pd (1.8%)Cd (2.3%)Sn (1.5%)Sb (3.8%)W (1.3%)Au (1.3%)Pb(1.4%)Bi (2%)

Figure 4. A 15-h long-term precision run of 0.5 ppb analytes in PGMEA using standardmode. (Both DRC and standard condition elements were determined in the same multi-element run, but are shown separately for clarity.)

Unspk Spk Rec(ppb) (ppb) (%)






20 Spectroscopy 19(9) September 2004 www.spectroscopyonl ine.com


● N-Methyl pyrrolidone (NMP)● Propylene glycol monomethyl ether

acetate (PGMEA) ● Propylene glycol methyl ether

(PGME) ● Methyl methoxypropionate (MMP) ● Ethyl lactate● Photoresist ● Antireflective coating (ARC)

The instrumentation used for thisexperiment was an ELAN DRC II(PerkinElmer-SCIEX, Concord, Ontario,CA). Instrumental parameters and sam-ple introduction components are shown

in Table I.

Results and DiscussionRemoval of interferences. It is well recog-nized that the dynamic reaction cell canremove the argon plasma-related inter-ferences such as Ar+, ArH+, and ArO+

completely, using pure NH3 as a reac-

tion gas (3). For this reason, it wasdecided to investigate the use of NH3gas for the reduction of carbon-relatedinterferences such as CC+, CN+, andArC+ on the determination of Mg, Al,and Cr. The optimization plots for24Mg and 52Cr in isopropyl alcohol areshown in Figure 1. The x axis representsthe NH3 cell gas flow rate, while the yaxis is the signal intensity. The steepslope of the isopropyl alcohol blankshows that both the CC+ and ArC+

interferences are reduced by four to fiveorders of magnitude. At a flow rate ofapproximately 0.6 mL/min of NH3 gas,

the slopes of the isopropyl alcoholblank and 1 ppb spiked isopropyl alco-hol become parallel, which indicatesthat approximately a ppt level of impu-rity of Mg and Cr exists in the isopropylalcohol blank. The dynamic bandpasstuning of the DRC rejects NH3 ionsgenerated in the cell immediately,

which prevents undesirable side reac-tions from taking place (3). As a result,the analyzer quadrupole always is at amore negative potential compared tothe reaction cell, so that it doesn’t usethe difference in kinetic energy to dis-criminate the harmful species. All ionsleaving the cell are introduced into theanalyzer quadrupole mass filter, whichis a critical difference between the DRCapproach and the collision cell.

Table II represents the results of theisopropyl alcohol analysis by themethod of standard additions, togetherwith the NH3 gas flows used (a value ofzero indicates that the element wasdetermined in standard mode). Thebackground equivalent concentration(BEC) and detection limit (DL) werecalculated based upon the isopropylalcohol blank signals and three timesthe standard deviation of blank iso-propyl alcohol, together with the slopeobtained from the calibration curves.Three of the calibration curves (withstatistical data), for 0, 20, 60, 100, and200 ppt 24Mg, 52Cr, and 27Al in a100% isopropyl alcohol, are shown inFigure 2. It should be emphasized thatisopropyl alcohol was chosen for thistest because it represents one of themost important organic solvents usedin the semiconductor manufacturingprocess. As a result, the SemiconductorEquipment and Materials International(SEMI) standards organization setsextremely high purity levels for thischemical. Because of its high purity, noclean organic solvent was available for acalibration blank and as a result, themethod of standard additions was usedfor quantitation.

Cell gas optimization. Some elementsshowed slightly higher BEC valuesbecause of impurities in the isopropylalcohol, as confirmed by the cell gasoptimization graphs. In order toachieve the best BEC for each analytein a multielement run, three differentNH3 gas flows (0.3, 0.6, and 1.0

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AI (1.8%)K (1.9%)Ca (2.2%)Ti (2.6%)V (1.6%)Cr(1.4%)

Mn (1.5%)Fe (1.1%)Co (1.6%)Ni (2.5%)

0

Cu (1.6%)Zn (2.4%)

Figure 5. A 15-h long-term precision run of 0.5 ppb analytes in PGMEA using DRC-mode conditions. (Both DRC and Standard condition elements were determined inthe same analytical run, but are shown separately for clarity.)

AN ICP MS SYSTEM OPERATING UNDER OPTIMIZED SAMPLING CONDITIONS CAN

MINIMIZE THE AMOUNT OF UNDESIRABLE CARBON-BASED SPECIES ENTERING THE MASS

SPECTROMETER WHEN ASPIRATING ORGANIC MATRICES.

www.spectroscopyonl ine.com


mL/min) were used, as shown in TableII. Once the measurement is initiated,the DRC vent valve is closed and thegas flow is adjusted to the highest flowin the method. First, the data acquisi-tion for the elements with the highestcell gas flow (for example, Ca) are per-formed automatically. Then the cell gasflow is reduced to the second highestflow, and the data acquisition per-formed automatically after 15 s of sta-bilization time. Once the data acquisi-tion for all elements in the DRC modeis completed, the vent valve is openedto evacuate NH3 gas from the cell, andthe data acquisition for all elements inthe standard mode is performed after15 s of stabilization time. For the

analysis of all the elements listed inTable I, it takes about 2 1/2 min forthree replicates using a 1-s integrationtime for each element.

Controlling the cell gas flow with thevent valve is essential to the successfuloperation of the DRC. When the stan-dard mode is applied, the cell gas isstopped and the vent valve is opened,which evacuates NH3 gas from the cell.This allows the pressure inside the cellquickly to become the same as themain quadrupole mass filter. Withouthaving the vent valve, the pressure inthe cell would become higher, eventhough no gas is introduced. Thiswould reduce the mean free path of theions and cause formation of by-prod-uct ions in the cell by the collisionalprocess. Therefore, to eliminate by-product ions with the collision cellapproach, kinetic energy discrimina-tion must be used every time, even forelements determined in the non-colli-sion gas mode (6).

Analysis of various organic compounds.The same DRC conditions for isopropylalcohol were used for other organicmatrices such as NMP, PGMEA, PGME,MMP, photoresist, and antireflective

coating. Because the DRC conditionsare independent of matrix, the samecondition was used for all the solvents.Figure 3 is the overlaid spectra of asample of NMP in standard and DRCmodes. The DRC mode at 0.5 mL/minof NH3 gas flow can eliminate themajority of argon- and carbon-basedspectral interferences — although toachieve the best BEC values, the threedifferent cell gas flows, shown in TableII, are preferred.

Figures 4 and 5 show 15-h long-termprecision runs for 0.5 ppb of 27 ana-lytes in PGMEA using standard modeand DRC-mode conditions respective-ly. This test was performed overnight bycontinually aspirating the sample con-

tained in a capped, large-volume PFAbottle. (An autosampler was not usedfor this test because solvent evaporationin open containers would have been aproblem during the time period.) Bothsets of elements were determined in thesame multielement run with no recali-bration, but are shown separately forclarity. This kind of stability test meas-ures the capability of an ICP MS systemto run organic samples unattendedwithout carbon deposits building up onthe cones. Running an instrument for12–15 h of daily use is considered fairlytypical for a high-throughput semicon-ductor lab.

Table III is a summary of the quanti-tative results (average of three repli-cates) of a number of photoresists andanti-reflective coatings diluted withvarious organic solvents.Concentrationvalues of the organic compounds weremeasured against external aqueousstock standard solutions (PEPure,PerkinElmer Life and AnalyticalSciences) made up in the organic sol-vent (which also was used as the blank).Whereas the reported spike recoverydata were generated by making spikedadditions directly in the actual samples,

Circle 17

USING PURE NH3 AS THE REACTION GAS AND ONE SET

OF DRC CONDITIONS, THE FULL SUITE OF

SEMICONDUCTOR ELEMENTS CAN BE DETERMINED IN MANY

OF THE ORGANIC ELEMENTS USED IN THE INDUSTRY.

Circle 18

all sample preparation and dilutionswere performed gravimetrically in PFAbottles. It should be noted that theseorganic chemicals are not as well-regu-lated by the SEMI standards organiza-tion as isopropyl alcohol and as a resultthere are no published elemental speci-fications or guidelines in the SEMI Bookof Semiconductor Standards. For thisreason, the list of analytes represents atypical suite of elements that are usedwithin the industry.

SummaryIt has been demonstrated that an ICPMS system operating under optimizedsampling conditions can minimize theamount of undesirable carbon-basedspecies entering the mass spectrometer,when aspirating organic matrices. Italso has been shown that by using adynamic reaction cell, the argon- andcarbon-based polyatomic spectralinterferences can be reduced signifi-cantly, allowing for the determinationof many of the traditionally problem-atic elements by ICP MS with good

spike recovery and precision. Usingpure NH3 as the reaction gas and oneset of DRC conditions, the full suite ofsemiconductor elements can be deter-mined in many of the organic com-pounds used in the industry. In addi-tion, running with hot plasma condi-tions allows for the determination ofelements with both high and low ion-ization potential in a multielementrun, with the minimum of matrix sup-pression, even for complex organicmatrices.

References1. F. Meyer, J. White, and M. Radle,

Semiconductor International (Website), June 1999.

2. E. McCurdy and D. Potter, SemiconductorInternational (Web site), October, 2001.

3. S. Tanner and V. Baranov, Plasma SourceMass Spectrometry (The Royal Societyof Chemistry, Cambridge, 1999), p. 46.

4. K. Kawabata and Y. Kishi, Proceedingsfrom Semiconductor Pure Water andChemical Conference, Los Altos, CA, pp.241–253 (2001).

5. I. Feldman, N. Jakubowski, C. Thomas,and D. Stuewer, Fresenius Journal ofAnalytical Chemistry 365, 422–428(1999).

6. N. Yamada and J. Takahashi, WinterConference on PlasmaSpectrochemistry, Scottsdale, AZ,January, 2002.

7. J. Takahashi, K. Youno, and K. Mizobuchi,Proceedings from Semiconductor PureWater and Chemical Conference, LosAltos, CA, pp. 110–121(2004).

8. K. Kawabata Y. Kishi, and R. Thomas,Analytical Chemistry 75(9), 423A,2003.

9. J.M. Collard, K. Kawabata, Y. Kishi, and R.Thomas, Micro, (1) 39–46, 2002. ■

Katsu Kawabata is semiconductorbusiness development specialist forPerkinElmer Instruments. E-mail: [email protected]. Yoko Kishiis ICP MS product specialist for the Semiconductor Business Unit of PerkinElmer.Haiying Shi is an analytical chemist forBrewer Science (Rolla, MO).Robert Thomas is principal of

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