Ultra Trace Elemental Analysis Through the Use of Cold Plasma on the Thermo Scientific iCAP Qs ICP-MSTomoko Vincent, Thermo Fisher Scientific, Germany

Tech

nica

l No

te 4

31

69

Key WordsCold plasma, semicon, ultra trace

GoalDemonstrate the suitability of the Thermo Scientific™ iCAP™ Qs ICP-MS for ultra-trace elemental analysis of semiconductor samples using a combination of hot and cold plasma conditions.

IntroductionThe iCAP Qs ICP-MS is ideally suited for the analysis of trace element concentrations in aggressive sample matrices. With a high purity, PFA sample introduction system, sapphire injector and Pt tipped cones, samples with high concentrations of mineral acids, for example hydrofluoric and sulfuric, can be analyzed routinely.

In semiconductor manufacturing support applications (for example, quality control of incoming process chemicals or spot sampling of bath solutions), the sample matrix remains aggressive but target concentrations are normally below 10 ng/L (ppt). For many elements, the use of single mode He KED with hot plasma (HP) is sufficient for the suppression of background and matrix induced spectral interferences to allow for reliable measurement at these concentration levels. For some elements however, in particular first and second group metals, as well as some transition metals, analysis using cold plasma (CP) is preferable to hot plasma.

Cold plasma has the following general advantages for the ultra-trace determination of trace elements in semiconductor applications:

1. By running at cold plasma (i.e. lower RF powers, generally < 600 W), the plasma is smaller and elemental contamination from the sample introduction system is thereby reduced, leading to lower BEC (background equivalent concentrations) for common elements such as Na, Al etc.

2. Some background as well as matrix induced interferences are reduced at cold plasma conditions, eliminating the need to run these in KED mode

3. Sensitivities for low ionization potential elements (such as Li and Na) are often higher in cold plasma than in He KED mode

Cold plasma is not recommended for the analysis of all elements however, (for example it gives reduced sensitivities for high ionization potential elements such as Ge) and therefore a combination of hot and cold plasma conditions must be used in routine analysis.

In this note, the hardware that enables the operation of the iCAP Qs ICP-MS in hot and cold plasma mode is described. All the hardware referenced is 100% compatible and interchangeable with the standard instrument configuration.

2 System configurationThe hardware configuration described is specifically designed to operate in either hot or cold plasma modes or a combination of both.

Sample cone The standard iCAP Q ICP-MS sample cone is used in hot and cold plasma applications. Sample cones from different materials are available depending on the application:

• 3600812:NiSampleCone

• 3601289:PtSampleCone

Skimmer coneThe skimmer cone required to operate the iCAP Qs ICP-MS in both hot and cold plasma mode has a specific design that only works in combination with the extraction lens configuration outlined below. Skimmer cones from different materials are available depending on the application:

• 1341420:NiSkimmerConeforColdPlasma

• 1341430:PtSkimmerConeforColdPlasma

Extraction lensA specialized extraction lens system is required to operate the iCAP Qs ICP-MS in cold plasma mode. The cold plasma lens kit consists of two lenses that are fully interchangeable with the standard lens system and are also compatible with hot plasma mode.

Figure 1: Cold plasma lens kit (PN: 1341380).

Both the interface cones and extraction lens system on the iCAP Q ICP-MS are easily accessible on either side of the bench level pop out door as seen in Figure 2.

Figure 2: Access to both the interface cones and extraction lens on the iCAP Q pop out door.

By mounting the interface cones and extraction lens on the same pop out door both items can be replaced at the same time for rapid switching between hardware configurations.

With the cold plasma hardware described above, the iCAP Qs ICP-MS can be used in BOTH hot and cold plasma modes and LabBooks can therefore be processed within a single session. The control software, Thermo Scientific™ Qtegra™ Intelligent Scientific Data Solution™ (ISDS), automatically and reliably switches between the two measurement modes during the analysis of individual samples. By aspirating each sample just once per (combined hot and cold plasma) analysis, sample throughput is maximized.

In order to demonstrate the utility and performance of the iCAP Qs ICP-MS in mixed hot and cold plasma analyses, the instrument was set up with the hardware configuration as shown in Table 1.

Table 1: Instrumental configuration for combined hot and cold plasma analyses

Nebulizer 100 µL PFA nebulizer

Spraychamber Peltier cooled PFA cyclonic (2 °C)

Injector 2.0 mm ID sapphire

Sample cone Pt tipped

Skimmer cone Pt tipped cold plasma skimmer

Extraction lens system Cold plasma

A comparison of the instrument settings in Hot and Cold Plasma modes is shown in Table 2.

Table 2: Summary of hot and cold plasma instrument settings

Setting Hot Plasma Cold Plasma

Extraction lens 1 voltage (v) 0 -35

Extraction lens 2 voltage (v) -120 -270

RF power (W) 1550 550

Nebulizer flow rate (L/min) 0.90 0.74

Improved performance in cold plasmaSodium (23Na) is a low mass, low ionization potential element, the analysis of which is often limited by contamination from the sample introduction system. Typical calibration lines for 23Na in hot and cold plasma are shown in Figures 3a and 3b. As can be seen in Figure 3b, in cold plasma 23Na provides high absolute sensitivities (>30 kcps/ppb) with the sub ng/L (ppt) detection limits (LoD) and background equivalent concentrations (BEC) required in semicon applications.

3

In hot plasma, KED technology is used to minimize background spectral interferences from species such as 40Ar16O at iron (56Fe). Typical calibration lines for 56Fe in hot(KED)andcoldplasmaareshowninFigures4aand4b. AscanbeseeninFigure4b,coldplasmaeffectivelysuppresses any spectral interferences and provides high absolute sensitivities (>17 kcps/ppb), with the sub ng/L (ppt) detection limits (LoD) and background equivalent concentrations (BEC) required in semicon applications.

Long term stability of mixed hot and cold plasma analysesRoutine sample analysis in mixed hot and cold plasma modes requires a robust and reliable switching between plasma supplies such as RF power and gas flow. The swing frequency matching RF generator introduced in the iCAP Q ICP-MS is all solid state with no moving parts and therefore reacts at lightning fast speed to changes in plasma conditions to ensure reliable operation at all times. In another departure from previous ICP-MS systems, the RF generator design does not require a grounded shield for cold plasma operation, improving reliability and minimizing running costs.

Figures 3a and 3b. Calibration curves for 23Na in hot plasma and cold plasma modes respectively.

Figures 4a and 4b. Calibration curves for 56Fe in hot plasma and cold plasma modes respectively.

In order to demonstrate the robustness of the iCAP Q ICP-MS in mixed hot and cold plasma analyses, a 1 µg/L (ppb) solution of various trace metals (in 2% HNO3) was analyzed continuously over twelve hours (Figure5).Ineachofthe246mixedmodeanalysesmadeduring this period, the iCAP Q ICP-MS automatically switched between hot (1550 W) and cold (550 W) plasma settings, with a stabilization time of less than 30 seconds. The RF generator maintains a stable analyte signal over extended measurement periods even when switching between hot and cold plasma in every analysis.

ConclusionThe Thermo Scientific iCAP Qs ICP-MS cold plasma kit, provides hardware that facilitates both cold and hot plasma modes. With the described hardware the iCAP Q ICP-MS has been shown to provide the high sensitivity and low backgrounds required for the ultra-trace analysis of trace elements in semiconductor applications.

In mixed hot and cold plasma applications, the innovative swing frequency matching RF generator provides a robust platform over extended analytical runs.

Tech

nica

l No

te 4

31

69

TN43169_E 06/13C

Africa-Other +27 11 570 1840Australia +61 3 9757 4300Austria +43 1 333 50 34 0Belgium +32 53 73 42 41Canada +1 800 530 8447China +86 10 8419 3588Denmark +45 70 23 62 60

Europe-Other +43 1 333 50 34 0Finland/Norway/Sweden +46 8 556 468 00France +33 1 60 92 48 00Germany +49 6103 408 1014India +91 22 6742 9434Italy +39 02 950 591

Japan +81 45 453 9100Latin America +1 561 688 8700Middle East +43 1 333 50 34 0Netherlands +31 76 579 55 55New Zealand +64 9 980 6700Russia/CIS +43 1 333 50 34 0South Africa +27 11 570 1840

Spain +34 914 845 965Switzerland +41 61 716 77 00UK +44 1442 233555USA +1 800 532 4752

www.thermoscientific.com©2013 Thermo Fisher Scientific Inc. All rights reserved. ISO is a trademark of the International Standards Organization. All other trademarks are the property of Thermo Fisher Scientific Inc. and its subsidiaries. This information is presented as an example of the capabilities of Thermo Fisher Scientific Inc. products. It is not intended to encourage use of these products in any manners that might infringe the intellectual property rights of others. Specifications, terms and pricing are subject to change. Not all products are available in all countries. Please consult your local sales representative for details.

Thermo Fisher Scientific, (Bremen) GmbH Management System Registered is ISO 9001:2008 Certified.

0%

25%

50%

75%

100%

125%

150%

Rel

ativ

e S

ensi

tivi

ty (P

erce

ntag

e)

246 Analyses Over 12 Hours

7Li CP 9Be HP

23Na CP 24Mg CP

39K CP 40Ca CP

51V HP 52Cr CP

55Mn CP 56Fe CP

58Ni CP 59Co CP

63Cu CP 71Ga HP

85Rb CP 88Sr HP

107Ag HP 111Cd HP

115In CP 133Cs HP

137Ba HP 205Tl HP

208Pb HP 209Bi HP

238U HP

Figure 5. Signal stability over 12 hours for a solution of 25 elements (each at 1 µg/L (ppb) in 2% HNO3).

Parts used in this note:

Fisher Optima grade nitric acid A467-500

100 µL concentric PFA nebulizer 1600342

2.0 mm ID sapphire Injector 1323600