422 A
A N A LY T I C A L C H E M I S T R Y / O C T O B E R 1 , 2 0 0 3
K ATSU K AWABATA Y OKO K ISHI onsumer demand for smaller electronic devices and more compact integrated circuits has resulted in the need for lower trace metal contamination levels on the surface of silicon wafers and for high-purity chemicals and gases used in the semiconductor manufacturing process. To reduce costs and increase yield, semiconductor manufacturers are making largerdiameter wafers with narrower line widths. Ten years ago, the standards organization Semiconductor Equipment and Materials International (SEMI) deemed that 10-ppb purity levels were adequate for many process chemicals. Today, 100 ppt is typical, and for some of the more critical materials, 10-ppt levels are being proposed. This trend is being driven by initiatives such as the International Technology Roadmap for Semiconductors, which is setting the course for the next generation of electronic devices (1). Traditionally, inductively coupled plasma MS (ICPMS) has been the technique of choice for trace element determinations for many of the high-purity chemicals used in the electronics industry. Unfortunately, for many of the difficult materials, traditional ICPMS does not have the detection capability for all 21 trace metals identified by SEMI, even when spectral background reduction techniques like cool or cold plasma conditions are used. The problem will be compounded further when SEMI approves and publishes the next generation of purity levels in its Book of Semiconductor Standards (BOSS) (2).
C
PerkinElmer Life and Analytical Sciences
However, a novel approach for improving ICPMS detection R OBERT T HOMAS limits has recently been develScientific Solutions oped that uses a dynamic reaction cell (DRC). In this technique, a reaction gas promotes ion–molecule chemistry inside a reaction cell to eliminate argon, matrix, and solvent-based spectral interferences, thereby dramatically improving detection limits for many of the problematic semiconductor ICPMS elements. We will describe the principles of the DRC in detail, explain how the improved detection limit is achieved by comparing it with other background reduction approaches, and provide real-world examples of its capabilities for semiconductor-related samples. Even though ICPMS offers extremely low detection capability, the detection limits for a small group of elements are severely compromised by spectral interferences generated by ions derived from the plasma gas, matrix, or solvent used to get the sample into solution (see “Common spectral interferences in ICPMS” on p 426 A). These polyatomic spectral interferences cannot be avoided in complex semiconductor-related samples, and if low parts-per-trillion detection limits are required, it is imperative to reduce or eliminate the interfering species. For this reason, it is important to understand how these background species are formed and the different approaches that have been used to reduce them. O C T O B E R 1 , 2 0 0 3 / A N A LY T I C A L C H E M I S T R Y
423 A
Cool plasma The first breakthrough to reduce some of the more severe polyatomic overlaps was the use of low-temperature plasma to minimize the argon- and matrix-based polyatomic species that form under normal plasma conditions (1000–1400 W rf power and 0.8–1.0 L/min nebulizer gas flow) (3). By using cool plasma conditions (500–800 W rf power, 1.5–1.8 L/min nebulizer gas flow, and, on some instruments, an increase in sampling depth), ionization in the plasma is changed so that many of these interferences are dramatically reduced and significantly improved detection limits are achieved (4). Typical cool plasma detection limits and background equivalent concentration (BEC) values for a selected group of elements are shown in Table 1. BEC is defined as the apparent concentration for the background signal based on the sensitivity of the element at a specified mass. The lower the BEC value, the easier it is to discern a signal generated by an element from the background. Many analysts in the semiconductor industry believe BEC is a more accurate indicator of the performance of an ICPMS system than the detection limit. Unfortunately, although cool plasma conditions are considered very useful for determining a small group of elements important to the semiconductor industry, their limitations are well documented (3–5). Some of these limitations are based on the fact that less energy is available in cool plasma, so elements that form a strong bond with oxygen or fluoride ions cannot be easily decomposed, and thus detection limits are compromised. Furthermore, elements with high ionization potentials cannot
Table 1. Detection limits and BEC comparison between cool plasma1, collision cell2, and DRC3. Element
Detection limit (ppt)1
39
K
0.5
40
Ca
2
52
Cr
0.2
55
Mn
56 59
BEC (ppt)1
6
Detection limit (ppt)2
BEC (ppt)3
0.27
2.60
Not reported
0.10
0.10
0.2
1.0
0.12
0.12
0.2
0.2
8.0
0.17
0.54
Fe
0.3
0.5
5.3
0.12
0.40
Co
0.2
0.8
2.1
0.04
0.04
25
19
Detection limit (ppt)3
1
Cool plasma detection limits and background equivalent concentration (BEC) values determined in ultrapure water (20 ).
2
Instrument detection limits in potable water measured on a collision cell ICPMS using kinetic energy discrimination (21).
3
Detection limits and BEC values from an ICPMS fitted with a DRC using mass discrimination (13).
424 A
A N A LY T I C A L C H E M I S T R Y / O C T O B E R 1 , 2 0 0 3
be ionized because much less energy is available compared to that of a normal, high-temperature plasma. Elemental sensitivity is severely affected by the sample matrix, so cool plasma often requires the use of standard additions or matrix matching to achieve satisfactory results. Finally, normal plasma conditions must be used in a multielement analysis and, when changing from normal to cool plasma, a stabilization time of ~3 min is needed, which degrades productivity and requires more sample.
Collision–reaction cells Collision cells, pioneered in the late 1980s, were the next major development to reduce polyatomic spectral interferences and improve the performance of quadrupole ICPMS (6). The first commercially available collision cell for ICPMS, based on hexapole technology, was originally developed to study organic molecules with tandem MS because the more collision-induced product species that were generated, the better the chance of identifying the structure of the parent molecule (7 ). However, this very desirable characteristic was a disadvantage in inorganic MS, because secondary reaction-product ions should be avoided. In this approach, ions enter the interface in the normal manner and are extracted (under vacuum) into a collision cell, which is positioned after the ion optics and prior to the mass analyzer. A collision gas such as hydrogen or helium is then bled into the cell, which consists of a multipole, usually operated in the rf-only mode. The rf-only field does not separate the masses but instead focuses the ions, which then collide with molecules of the collision gas. Through a number of different ion–molecule collision mechanisms, polyatomic interfering ions, such as 40Ar+, 40Ar16O+, and 38ArH+, are converted to noninterfering species. For example, hydrogen gas can reduce 38ArH+ spectral interferences when determining 39K+. 38
Ar+ + H2 ↔ 38ArH+ + H+
38
+
ArH+ + H2 ↔ H3 + 38Ar
39 +
K + H2 ↔
39 +
K + H2 (no reaction)
Hydrogen gas converts 38Ar+ and 38ArH+ to the harmless H 3+ and atomic argon but does not react with the potassium. 39K+, free of interferences, emerge from the collision–reaction cell and directed toward the quadrupole analyzer for normal mass separation. However, it is important to recognize that even though the H2 converts the 38Ar+ interference to 38ArH+, it is very difficult to break apart the 38ArH+ bond. As a result, the reaction is extremely slow and therefore the detection limit for 39 + K using a simple collision cell is usually compromised. This example is a simplistic explanation of how a collision cell works. In practice, complex secondary reactions and collisions occur, which generate many undesirable interfering species that, if not eliminated, could potentially lead to additional spectral interferences. Furthermore, if there are any impurities in the
collision gas, they may also form interfering species (8). The products of these unwanted interactions may be rejected by using either kinetic energy (KE) or mass filtering discrimination. The major difference between the two approaches are in the types of multipoles used and their basic mechanism of interference rejection.
Methods of discrimination KE discrimination is typically achieved by setting the collision cell potential slightly more negative than the mass filter potential. This allows the collision-product ions generated in the cell (which have a lower KE as a result of the collision process) to be rejected, while the analyte ions (which leave the cell with a higher KE) are transmitted. The fundamental mechanism of this approach is that there must be a difference of KE between the analyte ion and the collision-product ion. If too many collisions occur, the KE of both the analyte and the collision-product ion becomes too small and they cannot be discriminated. As a result, heavier gases like ammonia and methane, which are more efficient at reducing the ion KE after each collision, are generally not suitable for KE discrimination of a collision cell. This means that light, low-reactivity gases such as hydrogen and helium are the main option, making ion–molecule collisional fragmentation the dominant mechanism of interference reduction. Therefore, even though ion transmission characteristics of a hexapole are considered very good, detection limits are still relatively high because the interference reduction process is much less efficient with low-reactivity gases than with high-reactivity gases (9). This is exemplified in Table 1, which shows typical 3 detection limits for a KE-based collision cell using helium as the collision gas. For most of the elements, detection limits are inferior to the cool plasma detection limits also shown in Table 1. The other approach to rejecting secondary reaction products is to discriminate by mass. Unfortunately, higher-order multipoles (hexapoles, octapoles) cannot be used for efficient mass discrimination because the stability boundaries are diffuse and sequential secondary reactions cannot be easily intercepted. To circumvent this problem, a quadrupole is placed inside the collision–reaction cell and used as a selective bandpass filter. The benefit of this particular design is that highly reactive gases, which are more efficient at interference reduction, can be used (10). One such development of discriminating by mass filtering is called the DRC. Similar in appearance to the hexapole and octapole collision cells, the DRC is a pressurized multipole positioned prior to the analyzer quadrupole. A highly reactive gas such as ammonia is bled into the cell, where the gaseous molecules react with the interfering ions to convert them into either neutral species or species with masses that are different from the analyte. The analyte mass then emerges from the DRC free of interferences and enters the analyzer quadrupole for conventional mass separation. The advantage of using a quadrupole in the reaction cell is that the stability regions are well defined compared with those of a hexapole or an octapole, and
the quadrupole can therefore act as a bandpass filter, not just as an ion guide. Careful optimization of the quadrupole electrical fields prevents unwanted reactions between the gas and the sample matrix or solvent, and every time an analyte and an interfering ion enter the DRC, the bandpass of the quadrupole can be optimized for that specific analysis and then changed on the fly for the next analyte. Figure 1 shows the analyte 56Fe+ and an isobaric interference 40 Ar 16O+ entering the DRC. The reaction gas NH3 reacts with ArO+ to form atomic argon and oxygen together with NH 3+. The quadrupole’s electrical field is then set to transmit the 56 + Fe to the analyzer quadrupole, free of the isobaric interfer-
Cell quadrupole
Gas inlet NH3 40
Ar16O
56
Fe 56
Fe
Analyzer quadrupole
FIGURE 1. Elimination of the ArO+ interference with a DRC. The + + ion–molecule reaction is ArO + NH3 → Ar + O + NH 3 .
ence, 40Ar16O+. In addition, the NH3+ is prevented from reacting further to produce a new interfering ion. The benefit of this approach is that KE discrimination is not required, which allows more ion–molecule reactions to take place. The result is a more efficient removal of the interfering species, with no sacrifice in analyte sensitivity, because unlike KE discrimination, the potential on the quadrupole is lower than the potential on the cell, so both reactive and nonreactive gases can be used. By carefully selecting the reaction gas, the user can take advantage of the different reaction rates of the analyte and the interfering species, for example, eliminating 40Ar+ on 40Ca+ by using NH3 as the reaction gas. The reaction between NH3 gas and 40 + Ar interference, which is predominantly charge exchange, occurs because the ionization potential of NH3 (10.2 eV) is low compared to that of Ar+ (15.8 eV), making the reaction exothermic and fast. On the other hand, the ionization potential of Ca+ (6.1 eV) is significantly less than that of NH3, so that O C T O B E R 1 , 2 0 0 3 / A N A LY T I C A L C H E M I S T R Y
425 A
Common spectral interferences in ICPMS Analyte
Interference
24
Mg+
12 12 +
28
+
Si
C C
14 14 + 12 16 +
N N, C O
31 +
14 16
N OH+
39 +
K
38
ArH+, 37ClH+2
40
Ca+
40
Ar+
P
48 +
16 + 14 16 18 + 32 16 + 29 19 + O 3 , N O O , S O , Si F
51 +
V
35
Cl16O+, 34S16OH+
52
Cr+
40
Ar12C+, 35Cl16OH+, 34S18O+
56
Fe+
40
Ar16O+
63
Cu+
31 16 + P O2
64
Zn+
+ 32 16 + 31 16 S O2 , P O2H+, 32S 2
69
Ga+
37
Cl16O2
74
Ge+
37
Cl37Cl+
75
+
As
40
35
Ar Cl , Ar F O
80
Se+
40
Ar40Ar+
Ti
+
A demanding industry + 40
19 16 +
reaction, which is endothermic, is not allowed to proceed (11). This highly efficient reaction mechanism translates into a significant reduction of the spectral background at mass 40. At the optimum NH3 flow, a reduction in the 40Ar+ background signal of ~8 orders of magnitude is achieved, resulting in a detection limit of 4 is completely free of any chloride-based polyatomic interfermillion cps and 35Cl16OH+ interference by ~1 million cps using ences. The study showed that the majority of chloride-based NH3 as the cell reaction gas. DRC is also very effective in re- polyatomic spectral interferences have been reduced or avoidducing 37Cl16O2+ and 37Cl2+ interferences on 69Ga+ and 74Ge+, ed by using DRC technology. Spike recoveries for all 25 elerespectively. Although NH3 does a fairly good job of reducing ments of interest met the 75–125% range at 50% of the 10040 35 + Ar Cl interference, the sensitivity for 75As is relatively low. ppt level. O C T O B E R 1 , 2 0 0 3 / A N A LY T I C A L C H E M I S T R Y
427 A
Contamination of silicon wafers. Silicon wafers are among the most common materials used in the manufacture of semiconductor devices. To maximize device yields, there is a growing requirement that all analytical techniques be able to handle extremely high concentrations of silicon. Nowhere is this more evident than in the measurement of trace level metal contamination on the surface of silicon wafers by vapor-phase decomposition (VPD) (17 ). In this technique, trace metal contaminants in the decomposed silicon oxide layer are concentrated into a droplet of HF, which is usually collected for analysis with a small amount of HF and H2O2. The sample, typically 200–1000 µL in total volume, is then analyzed by graphite furnace atomic absorption, X-ray fluorescence, or ICPMS (18). Unfortunately, with such large concentrations of silicon in the sample, traditional ICPMS suffers from severe matrix suppression and a multitude of silicon-induced spectral interferences that cannot be addressed by using cool plasma. A recent study of the suppression effects focused on reducing silicon-derived spectral interferences using DRC and then investigated the impact of silicon on the spike recoveries of a group of the most important VPD elements (16). For this experiment, 5 g of bulk silicon was digested with 100 mL of 40% HF/60% HNO3 (1+1). The digested sample was further diluted to reflect silicon levels typically found in VPD samples, based on the thickness of native oxide layers that are up to 50 Å thick and thermally grown layers, which can be as thick as 5000 Å (19). Figure 2, which is a spectral scan of 1 ppb of the analytes in 1000 ppm of silicon, shows how optimization of DRC conditions reduced many of the silicon, fluoride, and solvent- and plasma-based polyatomic interferences. Figure 2a was generated with the instrument in standard mode, and Figure 2b was generated in DRC mode with the use of a cell gas flow of 0.5 mL/min NH3. DRC conditions were then used to evaluate the instrument’s detection limits and spike recovery capability in 1000-ppm silicon. Detection limits for the majority of the 16 elements determined were 1 ppt or better, whereas 5-ppb spike recoveries were all in the 88–99% range. The study also reported detection limits in atoms/cm2, based on a 300-mm wafer, which is a more familiar unit in the semiconductor industry (16). It should be noted that to analyze these types of samples with cool plasma, higher levels of silicon molecular ions are produced, which often necessitates their removal by heating to ensure that silicon-based spectral interferences are kept to a minimum.
(2) (3) (4) (5) (6) (7)
(8) (9) (10)
(11) (12) (13) (14) (15) (16)
(17)
Katsu Kawabata is a semiconductor business development specialist, and Yoko Kishi is an ICPMS product specialist for PerkinElmer. Kawabata’s and Kishi’s research interests include applying DRC technology to the analysis of semiconductor-related materials. Robert Thomas is the principal of Scientific Solutions, a consulting company that specializes in trace metals analysis. Address correspondence about this article to Kawabata at
[email protected].
References (1)
428 A
International Technology Roadmap for Semiconductors; http://public.itrs.net.2001.
A N A LY T I C A L C H E M I S T R Y / O C T O B E R 1 , 2 0 0 3
(18) (19) (20)
(21)
Book of Semiconductor Standards (BOSS); Semiconductor Equipment and Materials International: San Jose, CA, 2002. Jiang, S. J.; Houk, R. S.; Stevens, M. A. Anal. Chem. 1988, 60, 1217. Sakata, K.; Kawabata, K. Spectrochim. Acta, 1994, 49B, 1027. Collard, J. M.; et al. Micro 2002, 20 (1), 39–46. Rowen, T. J.; Houk, R. S. Appl. Spectrosc. 1989, 43, 976. Turner, P.; et al. In Plasma Source Mass Spectrometry: Developments and Applications, Proceedings of the 5th International Conference on Plasma Source Mass Spectrometry; Holland, G., Tanner, S. D., Eds.; Royal Society of Chemistry: Cambridge, England, 1997, p 28. Yamada, N.; Takahashi, J. Poster presentation (FP07) at Winter Conference on Plasma Spectrochemistry, Scottsdale, AZ, 2002. Feldman, I.; et al. Fresenius’ J. Anal. Chem. 1999, 365, 422–428. Tanner, S. D.; Baranov, V. I. In Plasma Source Mass Spectrometry: Developments and Applications, Proceedings of the 6th International Conference on Plasma Source Mass Spectrometry; Holland, G., Tanner, S. D., Eds.; Royal Society of Chemistry: Cambridge, England, 1999. Tanner, S. D.; Baranov, V. I. At. Spectrosc. 1999, 20, 45–52. Tanner, S. D. Spectrochim. Acta 2002, 57B, 1361. Kawabata, K.; Kishi, Y.; Thomas, R. Spectroscopy 2003, 18 (1), 2–9. Bollinger, D. S.; Schleisman, A. J. At. Spectrosc. 1999, 20, 60–63. Ping, C. M.; et al. Micro 2003, 21 (3), 37–42. Kawabata, K.; Kishi, Y. Determination of Various Chemicals Used in the Semiconductor Industry by DRC ICPMS. In Proceedings of the Semiconductor Pure Water and Chemical Conference, Monterey, CA; Balazs, M., Ed.; Semiconductor Pure Water and Chemical Conference: Los Altos, CA, 2001. Shirawa, T.; et al. Semiconductor Fabrication: Technology and Metrology ; ASTM STP 990, 314–323; Gupta, D. C., Ed.; American Society for Testing and Materials: Philadelphia, PA, 1989. Fucsko, J. S.; Tan, S. S.; Balazs, M. K. J. Electrochem. Soc. 1993, 140, 1105–1109. Settembre, G.; Debrah, E. Micro 1998, 16 (6), 79–84. Thermo Elemental Application Note SO28AN—Issue 1, March 2000; www.thermo.com/ eThermo/CDA/Applications/Application_Detail/ 0,1210,PREVIEW-1000002412836-105,00.html. Thermo Elemental Application Note SO27AN—Issue 1, March 2000; www.thermo.com/ eThermo/CDA/Applications/Application_Detail/ 0,1210,PREVIEW-1000002412833-105,00.html.
