William M. Geiger

Gas chromatography (GC) coupled to inductively coupled plasma–mass spectrometry (ICP-MS) is a powerful and flexible analytical solution that is well-established for measuring organometallic compounds such as organotin, organomercury, and organolead in environmental samples, foodstuffs, and consumer products. GC–ICP-MS is also used for industrial applications, such as monitoring catalyst poisons and environmental contaminant elements in petrochemical processing and plastics manufacturing. The semiconductor industry uses a range of high-purity specialty gases and volatile liquids as precursors in wafer substrate production, and for processes such as plasma etching and deposition of thin films by chemical vapor deposition (CVD). Semiconductor manufacturers are constantly developing new integrated circuit (IC) chips with smaller sizes, higher speeds, lower power consumptions, and greater transistor densities. This trend means that contaminants must be controlled at even lower levels in process chemicals and precursors. In this article, we show how GC–ICP-MS, particularly using triple quadrupole or tandem ICP-MS (ICP-MS/MS), enables determination of the lowest levels of contaminants in the specialty gases and volatile liquids used to make the most advanced electronic devices.

Integrated circuit (IC) chips play an increasingly important role in modern life, supporting technologies from consumer electronics and self-driving vehicles to the high-speed communications and servers used in accelerated computing and artificial intelligence (AI). The manufacturing of advanced semiconductor chips requires high-purity materials, metals, and a range of process chemicals, such as ultrapure acids and solvents. Wafer manufacturers and chip fabrication plants (FABs) also use a variety of bulk and electronic specialty gases (ESGs) and volatile liquids throughout the manufacturing process (1,2). For example, silane (SiH

) and chlorosilanes are used as feedstocks for gas purification of silicon to the nine to eleven nines purity used to make semiconductor wafers. Nine nines (9N) Si is 99.9999999% pure, meaning there is only 1 part per billion (ppb) of total impurity elements in the solid silicon crystal. Alternative, non-silicon wafers and films are often made from “III-V compound” semiconductors, such as gallium arsenide (GaAs) and indium GaAs (InGaAs), which may use arsine gas (AsH3) as a precursor together with trimethyl gallium (TMG).

Fabricating an advanced IC chip involves upwards of a thousand steps where thin layers of conducting and insulating material are alternately deposited and etched to create the millions of transistors and other features that make up the circuit. Dopant elements are added to modify the conductivity of the silicon substrate or surface film. High-purity gases are used extensively in these “front end” IC fabrication processes. For example, phosphine (PH

) is used to dope the required level of phosphorus into thin-film transistor (TFT) displays. Diborane (B

) is used for boron doping and ion implantation, for growing metal-boron thin films, and as a component of chemical vapor deposition (CVD) of boron-phosphate-silicate-glass (BPSG) films. Fluorocarbon (FC) gases, such as 2-fluorobutane and hexafluoro butadiene, are also used in the electronics industry (for example, in dry etching and for plasma enhanced chemical vapor deposition [PECVD]). The high reactivity and low toxicity of some FC gases means they are also used to clean reaction chambers between fabrication processes. In a typical semiconductor FAB, high-purity gases can make up the second largest material cost after high-purity silicon.

Controlling the level of contaminants in specialty gases is critical to ensuring acceptable chip yield and the performance and reliability of the finished semiconductor device. To understand why contamination is so important, it is useful to consider that advances in semiconductor device technology are driven by reductions in scale. Packing more transistors into a smaller space improves the speed and performance of the chip. The latest chips have circuit features and line widths measured in single nanometers (nm, one millionth of a millimeter), and the presence and position of individual atoms in the silicon crystal lattice can affect the electrical properties of the device. This makes the chips highly susceptible to contamination, so controlling the levels of impurities in the high-purity gases used during fabrication is fundamental to the production process.

Controlling the levels of dopant elements and metallic contaminants in substrate materials, process chemicals, and gases, is a particular industry concern. Dopant elements are deliberately added to the silicon substrate to make it “semiconducting” and control the electrical properties, as illustrated in Figure 1. Silicon has four valence electrons, and pure silicon forms a crystal lattice in which all these electrons are shared with the adjacent silicon atoms. As a result, pure silicon is a poor conductor of electricity. N-type (negative) dopants are Group V elements such as phosphorus, arsenic, antimony, and bismuth, which have five valence electrons, so they add one free electron to the silicon lattice. P-type (positive) dopants are Group III elements, which have three valence electrons, so they contribute a “missing” electron or a positive “hole” in the lattice. The level of doping varies depending on the intended properties, but low-level doping can add as few as one dopant atom per 100 million silicon atoms, so controlling contamination for these elements is critical.

FIGURE 1: Schematic showing: (a) Pure silicon crystal lattice; (b) silicon doped with phosphorus, an 

-type dopant element; (c) silicon doped with boron, a 

ICP-MS instruments are already widely used in the semiconductor industry to monitor the levels of ultratrace contaminants in high-purity materials and process chemicals. Contaminants in high-purity gases often need to be controlled at lower levels than can be achieved using traditional gas chromatography (GC) detectors, which may not have sufficient selectivity for the required analysis to begin with. Also, for some matrices and compounds, standards are unstable or not available, making calibrating and quantifying contaminants difficult.

As a detector for GC, ICP-MS, particularly with a triple quadrupole or tandem mass spectrometer (MS/MS), can overcome many of the limitations of other GC detectors. GC–ICP-MS can detect and quantify almost every element at low levels in gases and volatile liquids. The technique gives element and isotope-specific analysis, and is highly selective. As a result, matrix components and other compounds have little to no impact on the signal for the target analyte. GC–ICP-MS also gives practically the same elemental signal regardless of the compound, so non-specific or “compound independent” calibration (CIC) can be used, simplifying the quantification of compounds that are unstable or for which standards are not available. GC–ICP-MS is a versatile and sensitive technique that enables semiconductor manufacturers to reliably quantify trace-level contaminants in the wide range of specialty gases and volatile liquids used in semiconductor manufacturing.

Examples of different electronic gas analysis applications are presented in this work. The details of the specific GC columns and methods used for the different sample types and compounds measured are provided in the discussion. Specialty electronic gas analysis using GC–ICP-MS often requires a special approach to sample introduction and analysis, as many of the gases are toxic, flammable, and pyrophoric (spontaneously combust on contact with air or moisture). Silicon compounds such as silanes, siloxanes, and chlorosilanes decompose to form silica, which would deposit in a standard GC injector. Flammable and pyrophoric gases can be analyzed directly from the cylinder, using a switch- ing valve and loop to deliver the required sample volume directly to the column in the GC injector port. A 10-port sampling valve provides the option to automatically add a second gas as an online internal standard, as illustrated in Figure 2. In a similar way, volatile liquids such as trichlorosilane (TCS) can be sampled and injected directly into the GC–ICP-MS system using a high-pressure switching valve (3).

FIGURE 2: Direct sampling of flammable and pyrophoric gases for analysis by GC–ICP-MS, using a 10-port valve to allow online addition of an internal standard to the sample gas. Showing the (a) load and (b) inject sequences.

A further consideration when GC– ICP-MS is used to analyze samples such as silanes, chlorosilanes, AsH

) is that these compounds are inorganic. This means the matrix is not easily decomposed and volatilized in the ICP instrument, unlike with hydrocarbon matrices. To successfully analyze these inorganic matrices, a low-volume Deans Switch microfluidic flow control device was used to divert the matrix to vent, preventing excessive matrix load from reaching the ICP. The Deans Switch was positioned in the GC oven in the flow path between the column exit and the ICP-MS transfer line. Once the retention time of the matrix peaks were established using a low volume injection, the Deans Switch was used to divert the matrix to vent during the GC separation.

Both single and triple quadrupole (QQQ) Agilent ICP-MS systems were used for elemental detection. ICP-QQQ is a tandem MS configuration (MS/MS); as a result, it supports the use of reactive cell gases to give more effective control of spectral overlaps. In many cases, analysis can be performed on either type of mass spectrometer, with ICP-MS/MS typically giving detection limits (DLs) that are approximately an order of magnitude lower or more, depending on the element. Because the ICP-MS plasma is “dry” (no aqueous solution), many spectral backgrounds are much lower with GC–ICP-MS than with the conventional liquid sample introduction. As a result, non-typical ICP-MS elements, such as silicon and sulfur, can be measured at their major isotopes using single quadrupole ICP-MS. However, lower DLs are possible if an ICP-MS/MS method is used with a reactive cell gas to further reduce spectral overlaps. For more typical ICP-MS elements, such as germanium and arsenic, part per trillion (ppt) level DLs are routinely achievable. As a multi-element detector, GC–ICP-MS can be used to analyze hydride gas contaminants such as SiH

 at trace level (single ppb down to 10 ppt DLs) in other ultrapure hydride gases (4).

The sulfur compounds, hydrogen sulfide (H

S) and carbonyl sulfide (COS), were determined as ppb-level contaminants in PH

 using a 60-m (2 x 30 m) x 0.53 mm i.d. x 20 μm film mega-bore Plot U GC column at 50 

C with helium carrier gas at 17.5 psig. Sulfur was measured as the 

P matrix peak. The chromatogram is illustrated in Figure 3, demonstrating the complete separation of the sulfur compounds from the PH

 gas. The shaded region of the time plot shows when the PH

 matrix was diverted to vent using the Deans Switch.

FIGURE 3: GC–ICP-MS/MS analysis of trace sulfur compounds in phosphine (PH

 matrix to vent, avoiding excessive matrix loading to the ICP. Sulfur compounds at 25 ppb each. ICP-MS/MS compound DLs were approximately 1.5 ppb.

 matrix is difficult for organic MS as the two compounds both have their large molecular ion peak at 

 34 and their fragmentation patterns are similar. As illustrated in Figure 4, the only major difference in the GC–TOF-MS spectra is the 

 31, which is not useful for quantifying sulfur. Although H2S is separated from the PH

 matrix peak chromatographically, the tail of the matrix peak causes a raised background for H

S. By contrast, low-level determination of H

 matrix is straightforward using ICP-MS, which is a mass-specific elemental detector that determines 

 when measured by GC–TOF-MS. Mass-specific elemental detection of 

P, enabling interference-free, trace-level analysis of H

 is a critical high-purity process gas, in which sulfur compounds and other hydride contaminants must be monitored at trace levels. However, PH

 also needs to be measured as a trace-level contaminant in other gases and volatile liquids used in the semiconductor industry, to avoid unwanted contamination by phosphorus, an 

-type dopant element. Dichlorosilane (DCS) is used as a precursor for high-purity silicon crystal growth, and in combination with ammonia (NH

) to grow silicon nitride (SiN) thin films by CVD. The purity of the silicon crystal or SiN film depends on the purity of the precursor gases, so contaminants including PH

 must be monitored at ppb levels in the DCS. PH

 was determined in DCS by GC–ICP-MS/MS using a 60 m x 0.32 mm x 5.0 μm film DB-1 column using helium carrier gas at 20 psig. The oven program had a starting temperature of 35 

C/min ramp and a final temperature of 220 

Figure 5 shows overlaid chromatograms for a PH

 being determined as a trace contaminant in DCS. The PH

 contaminant was quantified at 4.8 ppb. The use of ICP-MS/MS with O

 47]) enabled low-level phosphorus analysis with minimal contribution from the silicon matrix.

FIGURE 5: Overlaid GC–ICP-QQQ chromatograms showing trace PH

 contaminant quantified at 4.8 ppb in DCS matrix, overlaid with PH

GC–ICP-MS was used to measure higher boranes as trace impurities in high-purity diborane (B

) using a 60 m x 0.32 mm x 1.5 m Rtx-Volatiles column. The oven program used a 35 

C/min ramp and a final temperature of 240 

C. Tolerances for diborane purity are extremely tight (<1% variation), so accurate measurement of higher boranes is essential for production control. Standards are not available or stable for higher boranes, but GC–ICP-MS provides a solution, as illustrated in Figure 6. The chromatogram shows the separation and quantification of tetraborane (B

 matrix. The higher boranes were quantified at single ppm levels in 2% B

 using CIC against a diborane standard. Quantification by CIC is based on the consistent response for elemental boron and a simple mathematical correction for the different amounts of boron in the different compounds. ICP-MS also offers isotope specific measurement, which is beneficial in this application, as some uses of borane require B

 that is isotopically enriched up to 99.99% B-11. The 

B isotopic abundances can easily be measured using GC–ICP-MS.

FIGURE 6: Tetra and pentaborane quantified at single ppm levels in 2% B

 sample using compound independent calibration with B

ICP-MS and ICP-QQQ are well-established techniques that are widely used in the semiconductor industry for measuring ultratrace levels of elemental contaminants in wafer substrate materials, metals, and process chemicals. Combining ICP-MS with GC extends the use of the technique to monitoring contaminants in bulk, specialty gases, and volatile liquids. These sample types require special GC sampling approaches, as many of the samples are toxic, flammable, or pyrophoric. Such samples can be handled safely using an appropriate switching valve or loop injection gas sampling system.

As a sensitive, multi-elemental detector for GC, ICP-MS gives low detection limits, mass-specific measurements, and excellent selectivity. GC coupled to a triple quadrupole ICP-MS system enables trace-level analysis of compounds containing elements traditionally considered difficult to determine by ICP-MS, such as silicon and phosphorus. The high temperature of the ICP ion source means that ICP-MS gives nearly equal elemental response regardless of the compound’s composition. This enables the use of compound independent calibration, which is useful when 

compound-specific calibration standards are unstable or unavailable.

Trace Analysis of Specialty and Electronic Gases

(2) Linde. Electronic Special Gases Index. 

High-Pressure Liquid Injection Device for Agilent Gas Chromatographs

(4) Geiger, W. M.; McElmurry, B.; Anguiano, J.; Kelinske, M. 

Determination of Trace Impurities in Electronic Grade Arsine by GC-ICP-QQQ

 is with Consolidated Sciences (CONSCI) Ltd., in Pasadena, Texas. 

 are with Agilent Technologies, Inc. Direct correspondence to 

Articles

Manufacturing advanced electronic devices requires the production of high-quality semiconductors and integrated circuit chips. In this article, the authors explain how GC, when coupled with ICP-MS, enables the detection of elements that are essential in semiconductor production.

GC-ICP-MS for Process and Quality Control in Semiconductor Manufacturing

Inductively coupled plasma–mass spectrometry has been investigated as a detector for gas chromatography and applied to analysis of trace impurities in specialty and electronic gases. Several key attributes, including high sensitivity, near-universal nature, ease of interfacing to gas chromatography, tolerance to matrix gases, and use of wet-plasma conditions to facilitate indirect calibration for a gas species for which a gas standard is unavailable, are discussed. Examples such as sulfur speciation in hydrocarbon gases at low part-per-billion levels demonstrate its use for specialty gas analysis. A more detailed case study is given on detection of trace impurities in arsine, including silane, carbonyl sulfide, hydrogen sulfide, germane, and hydrogen selenide. Under optimized conditions, detection of germane is possible down to 5 ppt.

As microelectronic devices become more complex and the features shrink, there has been increased demand for higher purity process gases. Some impurities, even if present at trace levels, can become incorporated into deposited layers and affect device performance significantly. Similar purity levels are required in other industrial specialty gas applications, such as for polymer manufacture, where trace impurities in monomer raw materials can affect catalyst activity significantly. This need to measure impurities at parts-per-billion and even parts-per-trillion levels has driven the development and use of highly sensitive and selective instrumental methods such as gas chromatography with inductively coupled plasma–mass spectrometry (GC–ICP-MS) and atomic emission detection (GC–AED).

ICP-MS originally was designed primarily to replace atomic absorption and ICP-optical emission instrumentation for analysis of metals in aqueous and organic matrices. However, its potential as a chromatographic detector was clear from the outset. Indeed, as the technique developed and matured, so too has the number of publications in which it has been used, as both a detector for liquid chromatography (LC) and GC (1–4). Like the AED, the ICP-MS can measure, with few exceptions, all the elements on the periodic chart. However, for most elements it typically offers greater sensitivity than the AED. The AED is the object of comparison because it is the only other commercially available GC detector that has similar specificity, selectivity, and sensitivity characteristics as ICP-MS (5,6). Table I compares the detection limits of the two technologies.

Table I: Comparison of detection limits for ICP-MS and AED 

As a detection method, ICP-MS cannot provide a satisfactory replacement for the analysis of those elements that are in the atmosphere, such as N, O, C, and of course Ar, because it is an open system. Furthermore, it cannot measure fluorine at all due to fluorine's ionization potential. However, ICP-MS is comparable to other sulfur detection methods and about the same for chlorine, phosphorus, and silicon as AED. AED excels at elemental carbon measurement and is satisfactory for measuring fluorine. For most other elements, ICP-MS is just short of spectacular.

The excellent sensitivity of ICP-MS is complemented by the fact that it not only has high specificity over carbon-bearing matrices, but it is also very tolerant of other matrices. Venting is only required to avoid torch contamination as in the case of metallic deposition from metal hydride gases or simply to keep the torch pristine for subsequent work. It is relatively easy to interface a GC system with ICP-MS for volatile gas analytes, as shown in Figure 1. The heated transfer line becomes more important for less-volatile components, as transfer from the column to the plasma must occur at elevated temperatures to ensure that species remain volatilized in the gas phase until they reach the plasma.

The instrumentation used to generate the data used in this article consisted of a Thermo Electron X-Series ICP-MS system coupled with a Thermo Electron Focus gas chromatograph (both from Thermo Fisher Scientic, Waltham, Massachusetts). The gas samples were injected via a switching valve and a 400-μL sample loop into a 0.53-μm i.d. capillary GC column that was held isothermal during the analysis. The stationary phase was selected to optimize analyte separation. Column temperatures ranged from 35 °C to 70 °C, and the detector interface was set 10 °C above the column temperature. In addition, the column effluent was mixed with heated argon make-up gas to aid in transfer of the components into the ICP-MS system. A vacuum could also be applied at this point to remove undesirable components (such as the matrix peak) before entering the plasma (Figure 1). The ICP-MS system was operated using a nebulizer flow rate of 0.68 L/min, an auxiliary gas flow rate of 0.70 L/min, and cool gas flow rate of 13.5 L/min. Standard wet plasma tune conditions were employed to tune the ICP-MS system. Detection was achieved using single ions for P (

The chromatograms in Figures 2–4 illustrate a few of the routine measurements that can be made using GC–ICP-MS that demonstrate equivalent or better sensitivities than GC–AED. They also show the tolerance the plasma has for the matrix as well as its selectivity with respect to carbon. Sulfur speciation in hydrocarbon matrices can be accomplished at low parts-per-billion levels with very little interference. Figure 2 shows the minimal matrix contribution from a propylene sample. The only effect is a small baseline depression from the propane and propylene matrix. This also exemplifies use of collision cell technology. Under normal plasma operating conditions, there is a great deal of OO

  ion formation leading to interference with sulfur (OO

  both have the same mass of 32). A technique to reduce this interference is to add a small amount of oxygen in the collision cell after initial ion formation to form SO

 , thus forming mass 48 and eliminating the interfering OO

  (mass 32) from the indigenous oxygen. The collision cell approach varies and goes by different names according to each manufacturer but is somewhat analogous to chemical ionization used in organic mass spectrometers.

The lack of matrix interference and generally inherent high sensitivity of the ICP-MS system allows for extremely low detection limits in some applications, such as the analysis of arsine impurities in propylene as shown in Figure 3. Propylene exhibits virtually no signal, and subsequent sample analyses show no deterioration in response for arsine. This represents a detection limit for arsine approximately 10 times that which can be achieved with GC–AED. Figure 4 illustrates the worse case matrix interference when measuring phosphine in propane/propylene mixture. However, selectivity is still >100,000:1 P to C.

An ICP-MS system can be run in two modes when interfaced with a gas chromatograph — wet plasma or dry plasma. In dry plasma mode, the GC column effluent is connected directly to the torch and introduced with a make-up gas at a flow rate high enough to penetrate the plasma. Optimization of the ICP-MS system operating conditions can be accomplished using Xe (or other dopants) in the make-up gas. In wet plasma mode, a wet aerosol from the nebulization system is merged with the GC effluent before its entry into the plasma. Time and space do not allow for a detailed discussion of the advantages of both modes, but wet plasma has two advantages when analyzing low boiling simple matrices. First, it allows for a more robust plasma, since aspiration of water introduces an oxygen source and allows for operation at full power. The other advantage is that it allows us to indirectly calibrate for a gas species for which we might not have a gas standard. For example, if we wanted to determine the vapor phase concentration of mercury in the gas phase and only had a standard of a brominated compound, we would start by presenting known aqueous phase bromine and mercury standards to the instrument to determine the responses, as shown in Figure 5. The ICP-MS response to a 1-ppm or 12.3 μmol/μL Br solution was 200,000 counts, which converts to 16,260 counts/μmol/μL versus a blank. In the same way, the ICP-MS response to a 100 ppb or 0.495 μmol/μL Hg solution was 650,000 counts or 1,313,131 counts/μmol/μL versus a blank.

Performing the math in equation 1, we determine a relative response factor of 80.7.

The next step would be to analyze a known brominated hydrocarbon and the unknown mercury vapor, as demonstrated in Figure 6.

Completing the math in equation 2 we find that the unknown mercury is 9.3 ppb. 

A thorough investigation of this strategy has not been made, but rough estimates have shown the accuracy to be within 50%. This normally would not be considered particularly good, but when dealing with concentrations for which standards are virtually unavailable at parts-per-billion and parts-per-trillion levels, it becomes very palatable.

In the mid-1990s we were presented with the problem of determining the concentration of germane impurity in bulk arsine. At that time detection limits were about 30 ppb using GC–MS and about 5–10 ppb using GC–AED or GC with reductive gas detection (RGD). This meant that detection limits were close but not quite good enough to make a measurement with confidence. So, a packed-column GC system was interfaced to an older ICP-MS system for the analysis. Coupling the two instruments together, it was possible to prove that the product contained 7 ppb germane. At the time, it was believed this hyphenated approach was a unique circumstance and that such a marriage of technologies was far too expensive to be used in any routine fashion. Times change, demands are greater, and specifications that were once 10 or 1 ppm are now parts-per-billion or parts-per-trillion levels. Many polymer-grade gas specifications are 10 ppb or lower for metal hydrides, and purities for highly pure electronic gases are even more demanding. This is beyond the reach of almost any other measurement strategy.

Arsine is an important gas used for metalorganic chemical vapor deposition (MOCVD) of GaAs and AlGaAs epitaxial films in the manufacture of high-speed microelectronic and optoelectronic devices. Since the purity of epitaxial films is critical to the performance of devices, microcontamination from O-, C-, S-, Si-, Se-, and Ge-containing species that cause unintentional doping, lattice defects, and oxide formation must be minimized. Unfortunately, critical contamination levels are not always above the detection limits of standard instrumentation. So, to ensure that these are avoided in the arsine process gas, development of sensitive analytical methods is required. Advances have been made to determine the carbon content indirectly by determining the amount of C

 hydrocarbons. Flame ionization detection has been used historically to determine these but is limited to ~50 ppb without preconcentration. Application of pulsed discharge helium ionization detection has brought this detection limit down to a more acceptable range of 4–8 ppb. The hydride gas impurities SiH

S can all be determined at parts-per-billion and parts-per-trillion levels using GC–ICP-MS. Figures 7–12 demonstrate SiH

The detection limits for silane, phosphine, carbonyl sulfide, and hydrogen sulfide are equivalent or only slightly better than what one could obtain using GC–AED as a comparison. It so happens that these are acceptable detection limits for these particular contaminants at this time. However, this is not the case for germane. 

For certain devices, the critical contamination level of germane in arsine can be at the low single digit parts-per-billion level, which is just beyond the practical reach of GC–AED. Germanium is an n-type impurity of concern in GaAs and AlGaAs epi layers and is particularly detrimental to high-speed transistor devices such as high electron mobility transistors (HEMTs) and field-effect transistors (FETs). It has also been positively identified in GaAs layers using secondary ion MS (SIMS) analysis and its presence increases leakage currents and reduces the gain of transistors, which is undesirable. The common source of this contaminant is indigenous presence in the bulk arsine. Although GC–AED sensitivity for germane is generally acceptable, greater sensitivity is required in certain circumstances. Work on germane involved instrumental modifications designed to enhance sensitivity. In the course of this work, a linearity study was made on germane.

Figure 9 is a log–log plot of area counts vs. germane concentrations. Sample injections were made in triplicate at concentrations of 190 ppt, 1.9 ppb, 19 ppb, 190 ppb, and 1 ppm. Response was linear over five orders of magnitude. The initial effort using standard conditions resulted in a detection limit of 43 ppt based on 3 sigma in a population of seven. Figure 10 represents chromatograms of spiked and unspiked arsine. After instrument modification and retuning, a detection limit of ~5 ppt was established. 

GC–ICP-MS analysis of hydrogen selenide has also been investigated. This analyte presented a different challenge and learning curve due mainly to its limited chemical stability. H

Se is one of the most labile metal hydrides. Thus, handling of standards and sample introduction are still proprietary. It also was a more difficult target since its elution order was after arsine instead of before as for the other hydrides. Once again, due to the forgiving nature of the ICP-MS plasma, it was possible to make this measurement. Figure 11 shows chromatograms of pure arsine with and without H

Even under highly optimized conditions we were only able to achieve a detection limit of ~0.8 ppb H

Se. Investigation of an aqueous sample containing 200 ppb selenium and germanium (Figure 12) showed that Ge had about 10 times better response than Se. It is hypothesized that the higher detection limit is due to less efficient ionization, additional possible interference from krypton present in the argon, and 

GC coupled with ICP-MS offers ultratrace sensitivity for many volatile metallic compounds, particularly those compounds of interest in semi-conductor manufacturing. In addition, ICP-MS is also technically competitive for detecting sulfur-containing compounds. Its element selectivity, cross-calibration capability, and ability to handle introduction of difficult matrix gases make it flexible and powerful as a chromatographic detection method and one that is relatively easy to interface to GC for gas analysis.

Since ICP-MS can be used in wet plasma mode, it can fill a role as a GC detection method while maintaining full ICP-MS analysis capability for aqueous samples.

From the point of view of development of ICP-MS as a GC detection method, areas that still need attention include reducing the overall complexity and dimensions of the instrumentation, improving software to truly integrate operation and data handling aspects, and investigating cross-calibration correlation techniques further. 

 is with Consolidated Sciences, Ltd., Pasadena, Texas; 

 is with Matheson Tri-Gas, Inc., Longmont, Colorado; 

(1) J.C.A. Wuilloud, R.G. Wuilloud, A.P. Vonderheide, and J.A. Caruso, 

The authors discuss the use of ICP-MS as an effective chromatographic detection method that is relatively easy to interface to gas chromatography for gas analysis.

William M. Geiger

Articles

GC-ICP-MS for Process and Quality Control in Semiconductor Manufacturing

ICP-MS: A Universally Sensitive GC Detection Method for Specialty and Electronic Gas Analysis