Determination of Ultratrace Elements inSemiconductor Grade TMAH Developer byHigh Resolution ICP-MS

SummaryHigh-resolution sector-field ICP-MS in both hot and coldplasma operating conditions is used to determine sub-ng/glevels of metals in semiconductor grade 0.3 N TMAHdeveloper. Instrumental sensitivity in the TMAH matrix isidentical to that in dilute nitric acid with > 1000 cps perpg/g In. High resolution was used to determine theexistence of matrix induced polyatomic interferences. Theinterferences found were of sufficient severity that highresolution had to be used for the analysis of ten of thefifteen trace metals elements investigated. By means ofcomputer controlled switching between low and highresolution and hot and cold plasma conditions, inter-ference-free analysis was possible during a single analysis.Detection limits in TMAH ranged from 0.1 pg/g for Li to9 pg/g for Cu.

Introduction0.3 N TMAH (tetramethyl ammonium hydroxide,(CH3)4NOH) is used during the lithographic process insemiconductor industry. A direct ICP-MS technique wouldprovide a useful quality control for pg/g and sub-pg/gmetal concentrations in TMAH but this has provendifficult to realize due to the existence of significantmatrix-derived interferences as well as non-spectralinterferences. While cold plasma has been shown to beeffective in reducing argon based interferences, it is evenmore prone to matrix suppression than hot plasma andother polyatomic interferences, previously not foundunder hot plasma conditions, may be preferentiallyformed.

However, the use of cold plasma does provide anadditional benefit of a reduced background equivalentconcentration (BEC) for elements with low first ionizationpotentials.

Because the Thermo Scientific ELEMENT 2 has beenshown to be more resistant to matrix effects thanquadrupole ICP-MS[1], sensitivities in complicatedmatrices, such as organic solvents, can approach those inwater. In this application report, the suitability of theELEMENT 2 with its high mass resolution, highsensitivity and hot and cold plasma capabilities will beassessed for the direct analysis of 0.3 N TMAH.Analytically determined concentration data will bereported as well as limits of detection for the fifteenelements measured.

ExperimentalThe sample introduction equipment used and instrumentaloperating conditions are shown in Table 1. Sampleanalysis was carried out completely unattended using a CETAC ASX-100 autosampler and software controlledswitching between hot and cold plasma conditions andresolution settings. The spreadsheet display from theThermo Scientific ELEMENT 2 Sequence Editor is shownin Figure 1.

SAMPLE INTRODUCTION EQUIPMENT AND OPERATING PARAMETERS

100 L/min self-aspirating PFA concentric nebulizer (Micro-Flow PFA-100, ESI, Omaha, NE, USA)PFA spray chamberDemountable quartz torch with sapphire injectorNi sampling and skimmer cones

HOT PLASMA CONDITIONS

Forward power: 1030 WSample gas flow: 0.940 L/minFocus Lens: - 820 v

COLD PLASMA CONDITIONS

Forward power: 725 WSample gas flow: 1.000 L/minFocus Lens: - 1000 v

NOTE

All other instrument settings are identical for the two plasma conditions.

Table 1: Sample introduction equipment and instrumental operatingconditions used for the analysis of 0.3 N TMAH.

Key Words

ELEMENT 2

High Resolution ICP-MS

Semiconductor Analysis

TMAH

ApplicationNote: 30072

Sensitivity in the TMAH MatrixThe Thermo Scientific ELEMENT 2, with a specifiedsensitivity of > 1 Mcps per ng/g for 115Indium in lowresolution and detector dark noise of < 0.2 cps, has beenshown to be ideally suited for the determination of ultra-trace metal impurities in high purity water. Because theELEMENT 2 has been shown to be more resistant tomatrix effects than quadrupole ICP-MS[1], instrumentalperformance in TMAH was expected to be comparable tothat obtained in water. The sensitivity under hot plasmaoperating conditions for 0.3 N TMAH solution spikedwith 100 pg/g of a multi-elemental solution containing Li,In and U is shown in Figure 2. Instrumental sensitivity inTMAH is identical to that in dilute nitric acid with >1000 cps per pg/g In. These high sensitivities allow theELEMENT 2 to analyze TMAH at low pg/g levelswithout any sample preparation, thus ensuring a highsample throughput in a routine laboratory environment.

Figure 2: Instrumental sensitivity under hot plasma operating conditions for 0.3 N TMAH solution spiked with 100 pg/gof a multi-elemental solution containing Li, In and U.

Figure 1: Sequence table from the Thermo Scientific ELEMENT 2 Sequence Editor.

Identification of Polyatomic InterferencesAs part of method development, a TMAH sample wasscanned for interferences under both hot and cold plasmaconditions. Matrix induced interferences were identifiedthat made the use of medium resolution (R = 4000)necessary for interference-free quantification of ten of thefifteen elements examined (Table 2).

ISOTOPE PLASMA CONDITIONS INTERFERENCES IDENTIFIED

24Mg Hot 12C227Al Cold 12C15N, 13C14N39K Cold 38ArH, H218OH316O44Ca Cold 12C16O2, 30Si14N, 12C2H614N47Ti Hot 15N16O2, 14N16O2H52Cr Cold 40Ar12C55Mn Cold 40Ar15N, 12CH14N3, (H216O)3H56Fe Cold 40Ar16O, 12C213CH514N60Ni Cold 12C2H614N16O66Zn Hot 40Ar12C14N

Table 2: Elements analyzed, plasma operating conditions and polyatomicinterferences identfied in 0.3 N TMAH.

For example, the direct analysis of manganese at m/z 55 under hot plasma conditions is complicated byinterferences from 40Ar15N and 40Ar14NH generated by thesample matrix (Figure 3). Argon based interferences canbe reduced or even eliminated by cold plasma parame-ters[2]. However, the use of cold plasma does not ensureinterference-free analysis as, with even relatively simplematrices, new interferences that are not generated withhot plasma conditions can be shown to occur. Forexample, Figure 4 shows the polyatomic interferencesobserved at the nominal mass of 56 a.m.u., the majorisotope of Iron, under both hot and cold plasma con-ditions. As expected the 40Ar16O interference at m/z 56 isgreatly reduced with cold plasma, but is not completelyremoved. A new additional interference, preferentiallyformed under cold plasma conditions is seen, clearlydemonstrating that high resolution is necessary, even withcold plasma, to guarantee interference free analysis.

ISOTOPE PLASMA RESOLUTION CONCENTRATION DETECTION CONDITIONS (pg/g) LIMIT (pg/g)

7Li Cold LR 13.4 0.1211B Hot LR 177 8.6823Na Cold LR 55.2 4.7563Cu Cold LR 13.5 9.16208Pb Cold LR 2.29 0.7424Mg Hot MR 12.8 8.1627Al Cold MR 33.5 1.3139K Cold MR 21.3 0.4444Ca Cold MR 135 0.8747Ti Hot MR 67.0 6.0752Cr Cold MR 11.7 2.1655Mn Cold MR 4.94 1.1656Fe Cold MR 37.4 8.3960Ni Cold MR 7.71 6.8966Zn Hot MR 42.5 4.78

Table 3: Concentration data for the 0.3 N TMAH sample, as well asdetection limits for the fifteen elements determined.

Figure 3: Medium resolution (R = 4000) spectrum of manganese (m/z 55)showing 40Ar15N and 40Ar14NH interferences generated by the TMAH matrix.

Figure 4: Medium resolution (R = 4000) of iron (m/z 56) showing thedifference polyatomic interferences observed under hot and cold plasmaconditions.

40Ar15N

40Ar14NH

55Mn

56Fe

40Ar16O

40Ar16O

56Fe

HOT PLASMA

COLD PLASMA

ResultsFully quantitative analysis of fifteen elements in a 0.3 NTMAH sample was performed using a standard additioncalibration in the sample matrix with spike concentrationsof 10 to 200 pg/g, depending on the element. An internalstandard (500 pg/g Rh) was added to all samples. Figure 5shows the addition calibration line obtained for Na (lowresolution and cold plasma) and Fe (medium resolutionand cold plasma) in the TMAH matrix. Here the benefitof a low BEC using cold plasma operating conditions canbe seen, allowing low pg/g concentrations to be deter-mined accurately. Concentration data and detection limitsfor the fifteen elements determined by direct analysis of0.3 N TMAH are presented in Table 3. The detectionlimits were calculated as the analyte concentrationequivalent to 3 times the standard deviation from tenreplicate on-peak analyses of the 0.3 N TMAH blank.

ConclusionsThe Thermo Scientific ELEMENT 2 is shown suitable forthe direct, routine quantification of trace elementalconcentrations at the pg/g level in 0.3 N TMAH. Samplematrix induced interferences specific to either hot or coldplasma operating conditions, are shown to precludeaccurate low concentration level analysis with quadrupoleICP-MS. The use of medium resolution (R = 4000) istherefore necessary for the interference free analysis of ten(Mg, Ti and Zn for hot plasma and Al, K, Ca, Cr, Mn, Feand Ni with cold plasma) of the fifteen elementsdetermined. The high sensitivity in the sample matrix andlow dark noise of the ELEMENT 2 enable sub pg/gdetection limits.

References[1] Non-spectral interferences caused by a saline water matrix in quadrupoleand high resolution ICP-MS. Rodushkin I. and Klockare, D., JAAS, 1998,13, 159-166[2] Order of magnitude improvement in sector-field ICP-MS performanceusing a Pt guard electrode. Wiederin D. and Hamester M. 1998 WinterConference on Plasma Spectrochemistry, Scottsdale, AZ, 5-10/1/1998, USA

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Figure 5: Calibration lines obtained for Na (low resolution, cold plasma) and Fe (medium resolution, coldplasma) in the TMAH matrix.

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