Explores the latest advances and applications of specialtyand electronic gas analysis The semiconductor industry depends upon a broad range ofinstrumental techniques in order to detect and analyze impuritiesthat may be present in specialty and electronic gases, includingpermanent gases, water vapor, reaction by-products, and metalspecies. Trace Analysis of Specialty and Electronic Gasesdraws together all the latest advances in analytical chemistry,providing researchers with both the theory and the operatingprinciples of the full spectrum of instrumental techniquesavailable for specialty and electronic gas analysis. Moreover, thebook details the advantages and disadvantages of each technique,steering readers away from common pitfalls. Featuring contributions from leading analytical and industrialchemists, Trace Analysis of Specialty and Electronic Gasescovers a wide range of practical industrial applications. The bookbegins with the historical development of gas analysis and thenfocuses on particular subjects or techniques such as: * Metals sampling and ICP-MS analysis * Improvements in FTIR spectroscopy * Water vapor analysis techniques * New infrared laser absorption spectroscopy approaches * GC/MS, GC/AED, and GC-ICP-MS techniques * Gas chromatography columns * Atmospheric pressure ionization mass spectrometry Lastly, the book examines gas mixtures and standards that arecritical for instrument calibration. There are also two appendicesoffering information on fittings and material compatibility. With its thorough review of the literature and step-by-stepguidance, Trace Analysis of Specialty and Electronic Gasesenables researchers to take full advantage of the latest advancesin gas analysis. Although the book's focus is the semiconductor andelectronics industry, analytical chemists in other industriesfacing challenges with such issues as detection selectivity andsensitivity, matrix gas interference, and materials compatibilitywill also discover plenty of useful analytical approaches andtechniques.
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Half Title page
List of Figures
List of Tables
Chapter 1: Introduction to Gas Analysis: Past and Future
1.1 The Beginning
1.2 Gas Chromatography
1.3 Ion Chromatography
1.4 Mass Spectrometry
1.5 Ion Mobility Spectrometry
1.6 Optical Spectroscopy
1.7 Metals Analysis
1.8 Species-Specific Analyzers
1.10 The Future
Chapter 2: Sample Preparation and ICP–MS Analysis of Gases for Metals
2.2 Extraction of Impurities Before Analysis
2.3 Direct Analysis of ESGs
Chapter 3: Novel Improvements in FTIR Analysis of Specialty Gases
3.1 Gas-Phase Analysis Using FTIR Spectroscopy
3.2 Gas-Phase Effects on Spectral Line Shape
3.3 Factors That Greatly Affect Quantification
3.4 Future Applications
Chapter 4: Emerging Infrared Laser Absorption Spectroscopic Techniques for Gas Analysis
4.2 Laser Absorption Spectroscopic Techniques
4.3 Applications of Semiconductor LAS-Based Trace Gas Sensor Systems
4.4 Conclusions and Future Trends
Chapter 5: Atmospheric Pressure Ionization Mass Spectrometry for Bulk and Electronic Gas Analysis
5.2 APIMS Operating Principle
5.3 Point-to-Plane Corona Discharge Ionization
5.4 Factors Affecting Sensitivity in Point-to-Plane Corona Discharge APIMS
5.5 Applications of Point-to-Plane Corona Discharge APIMS in Bulk and Electronic Gases
5.6 Nickel-63 Beta Emitter APIMS
5.7 Specialty Gas Analysis Application: Determination of Oxygenated Impurities in High-Purity Ammonia
Chapter 6: GC/MS, GC/AED, and GC–ICP–MS Analysis of Electronic Specialty Gases
Chapter 7: Trace Water Vapor Analysis in Specialty Gases: Sensor and Spectroscopic Approaches
7.2 Primary Standards for Water Vapor Measurement
7.3 Sensor Technologies
7.4 Spectroscopic Methods
Chapter 8: Gas Chromatographic Column Considerations
8.2 Column Considerations with Packed Columns
8.3 Primary Selection Criteria for Capillary Columns
8.5 The Future
Chapter 9: Gas Mixtures and Standards
9.2 Definition of Gas Standards
9.3 Cylinders and Valves: Sizes, Types, and Material Compositions
9.4 Preparation Techniques for Gas Standards
9.5 Pressure Restrictions and Compressibility Considerations
9.6 Multicomponent Standards: General Considerations
9.7 Cylinder Standard Stability Consideration
9.8 Liquefied Compressed Gas Standards: Preparation Differences and Uses
9.9 Cylinder Standard Alternatives
9.10 Dilution Devices and Calibration Uses
Appendix A: Cylinder and Specialized Fittings
A.1 Cylinder Fittings
A.2 Specialized Fittings
Appendix B: Materials of Construction
B.1 Tubing, Transfer Lines, and Other Hardware
B.2 FTIR Materials
TRACE ANALYSIS OF SPECIALTY AND ELECTRONIC GASES
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Library of Congress Cataloging-in-Publication Data:
Trace analysis of specialty and electronic gases / edited by William M. Geiger, Consolidated Sciences, Houston, TX, Mark W. Raynor, Matheson, Advanced Technology Center, Longmont, CO. pages cm Includes bibliographical references and index. ISBN 978-1-118-06566-2 (cloth) 1. Gases—Analysis. 2. Trace elements—Analysis. 3. Gases—Spectra. I. Geiger, William M., 1948–II. Raynor, Mark W., 1961– QD121.T73 2013 543—dc23 2012050030
FLORIAN ADLER, JILA, National Institute of Standards and Technology, and University of Colorado, Department of Physics, Boulder, CO
KRIS A. BERTNESS, National Institute of Standards and Technology, Boulder, CO
MARTINE CARRE, Air Liquide, Jouy-en-Josas, France
DANIEL R. CHASE, Matheson, Advanced Technology Center, Longmont, CO
KEVIN C. COSSEL, JILA, National Institute of Standards and Technology, and University of Colorado, Department of Physics, Boulder, CO
DANIEL COWLES, Air Liquide Balazs NanoAnalysis, Dallas, TX
DARON DECKER, Agilent Technologies, Pearland, TX
WILLIAM M. GEIGER, Consolidated Sciences, Pasadena, TX
TRACEY JACKSIER, Air Liquide, Newark, DE
SUHAS N. KETKAR, Air Products and Chemicals, Allentown, PA
ROBERT LASCOLA, Savannah River National Laboratory, Aiken, SC
RAFAL LEWICKI, Rice University, Houston, TX
BARBARA MARSHIK, MKS Instruments, Inc., Methuen, MA
SCOTT MCWHORTER, Savannah River National Laboratory, Aiken, SC
GLENN M. MITCHELL, Matheson, Advanced Technology Center, Longmont, CO
JORGE E. PÉREZ, CIC Photonics, Inc., Albuquerque, NM
MARK W. RAYNOR, Matheson, Advanced Technology Center, Longmont, CO
LEONARD M. SIDISKY, Supelco, Bellefonte, PA
KOHEI TARUTANI, Air Liquide Japan, Ibaraki, Japan
FRANK K. TITTEL, Rice University, Houston, TX
STEPHEN VAUGHAN, Custom Gas Solutions, Durham, NC
JUN YE, JILA, National Institute of Standards and Technology, and University of Colorado, Department of Physics, Boulder, CO
LIST OF FIGURES
1.1 Sampling apparatus used for sampling from a furnace.
1.2 Small sampling tube for gas sampling in mines.
1.3 Simmance Abady Combustion Recorder with output reel.
1.4 GC system used in Prior’s work in 1945–1947.
1.5 Separation of ethylene and acetylene.
2.1 Filtration system setup.
2.2 Metal impurity content in filters prior to filter washing.
2.3 Liquid-phase hydrolysis system setup.
SUHAS N. KETKAR
Air Products and Chemicals, Allentown, Pennsylvania
It is useful to look at the historical developments that contributed to the growth of analytical techniques, with a particular emphasis on the analysis of gases, not with the intent to provide a comprehensively detailed history of gas analysis, but to offer a peek into the development of the rudimentary techniques that began in the eighteenth century and how they evolved to the sophisticated, sensitive analyzers of the twenty-first century. This development has been influenced by discoveries and developments in all branches of science and engineering. Many of the analytical techniques discussed here are detailed in later chapters, especially those that have found application in the analysis of specialty and electronic gases.
The beginnings of gas analysis can be traced back to the mid-eighteenth century during the period when the constituents of air were discovered. At that time air was considered to be indivisible. Johann Beecher and George Stahl postulated the presence of phlogiston to explain how air initially supports combustion but later gets saturated with phlogiston and does not support combustion. In 1750, Joseph Black discovered carbon dioxide during a reaction of magnesium carbonate and sulfuric acid. He demonstrated that the gas produced during this reaction was the same as that produced from combustion and that this gas was not atmospheric air. The experiments that Black carried out are thought to have introduced the art of gas analysis .
In 1774, Joseph Priestley discovered oxygen. He indicated that he had discovered an air that was five or six times as good as common air, and called it “dephlogisticated air.” Although he had concluded that this air constituted about 20 % of atmospheric air, he was not willing to give up his belief in the phlogiston theory. The following year Antoine Lavoisier made quantitative measurements of Priestley’s experiment to propose the laws of conservation of mass as well as the theory of combustion. Lavoisier’s careful measurements of the weights of the reactants and products in a chemical reaction are regarded as one of the first truly quantitative measurements of a chemical reaction. Lavoisier also investigated the composition of water and air. Using volumetric measurements during experiments by heating mercury, he concluded that air was one-fifth oxygen. This was the beginning of the use of the volumetric method for gas analysis and the first quantitative measurements made on gases.
The beginning of the industrial revolution in the nineteenth century created a need for the analysis of gas composition in mining and, specifically, the iron industry, which relied on the combustion of charcoal, or coke. Sampling apparatus was developed to obtain samples, which were shipped to central laboratories for analysis. One of the earliest examples of such apparatus is from 1885 (see Figure 1.1) .
Figure 1.1 Sampling apparatus used for sampling from a furnace .
The sampling apparatus was made of a metal alloy, usually copper or iron. The gas sample flows through tube A, which is kept cool by flowing water through concentric tube C. Sample tube A is connected to a sampling tube and an aspirator. The sampling tube can be removed and sent to a laboratory for analysis. For gas sampling in mines, small sampling tubes were used (Figure 1.2).
Figure 1.2 Small sampling tube for gas sampling in mines .
These tubes were made of fusible glass and were evacuated in a laboratory using mercury pumps and sealed. Once inside the mine, the seals were opened to obtain the atmospheric sample inside the mine and then resealed. Similar sampling tubes and apparatus were developed to capture gases dissolved in liquids as well as gases formed during chemical reactions.
Analytical methods based on selective absorption and combustion were used to quantitatively measure the composition of the gases in the sample. In the late nineteenth and early twentieth centuries, it was recognized that shipping samples to a laboratory for analysis was a time-consuming process and that many industries needed faster analyses. Early in the twentieth century, Alexander Wright and Company of London, England, introduced the Simmance Abady Precision Combustion Recorder, which was capable of automatically measuring the percentage of carbon dioxide in flue gases as fast as every 3 minutes . This is one of the first commercial instruments capable of making quantitative gas measurements automatically and displaying the results on a recorder in real time. One of these early instruments, along with the output measured is shown in Figure 1.3.
Figure 1.3 Simmance Abady Combustion Recorder with output reel .
Although the Simmance Abady CO2 recorder was able to provide measurements every 3 minutes, there was a need to obtain measurements at even faster rates. In 1907, Strache et al. described the Autolysator, which was able to provide carbon dioxide concentration at any instant of time . In 1906, Haber described the gas refractometer, which used light refraction, for measurement of the composition of gases. Carl Zeiss of Jena, Germany, introduced a commercial gas refractometer .
By 1910, volumetric methods had been established for the measurement of gases, which included oxygen, hydrogen, nitrogen, ammonia, hydrogen chloride, chlorine, carbon monoxide, nitric oxide, silicon tetrafluoride, and acetylene. These gases are used routinely in the semiconductor fabrication industry today. Although the methods developed in the early twentieth century were not as sophisticated or sensitive as the techniques available today, they nonetheless laid the foundation for rapid development in the field of gas analysis. Gas analysis methods, developed to meet the needs of the industrial revolution, also found applications in other fields. The method for the analysis of carbon dioxide found application in the field of medicine for the analysis of expired breath. In 1920, August Krogh described an apparatus to measure carbon dioxide in expired breath to an accuracy of 0.001 % .
In the early twentieth century, reactive methods relying on color-changing reactions were being used for the detection of inorganic compounds. An early application of this to gas detection was the use of lead acetate to detect hydrogen sulfide gas. This method used a filter paper dipped in a solution of lead acetate. Upon exposure to hydrogen sulfide the filter paper turns black, due to the formation of lead sulfide. Detectors based on similar principles are still being used in industrial applications, with coated paper tapes available for the detection of a variety of gases. In 1937, the Draeger Company of Lubeck, Germany, introduced the first detector tubes for mobile gas detection. The tubes were packed with solid granules coated with lead acetate solution to enable the detection of hydrogen sulfide. Sample gas was flowed through the detector tube for a fixed amount of time. The length of the packing that changed color gave an indication of the concentration of the gas. Further development led to the availability of variety of tubes, each designed to detect a particular gas. These tubes are still widely used. Further refinements have integrated the colorimetric concept to an optical system capable of enabling the direct reading of a concentration value for the gas being detected.
In the late nineteenth century, books were published in Germany which collected the results of the experimental work on gas analysis of Bunsen, Winkler, and Hempel. In 1902 the Macmillan Company published Gas Analysis by Walther Hempel, which was translated and expanded by Louis Monroe Dennis, who published his own book, Gas Analysis, in 1913 . The accuracy of the methods, which were mainly volumetric based, relied primarily on the skill of the analyst. To aid the analyst, these books went into great detail about not only the methods but also the construction of apparatus for the sampling and analysis of gases. Emphasis was placed on covering minute details about the apparatus, details that one might find trivial in hindsight. For example, almost two pages are devoted to the subject of the lubrication of stopcocks used in gas analysis apparatus. Detailed recipes are given for making lubricants using commonly available ingredients such as Vaseline and paraffin. These books were almost certainly used as reference guides by scientists working in laboratories performing gas analysis.
In 1920, H. A. Daynes reported the development of the Katharometer, an instrument to give an automatic indication of the presence of small quantities of hydrogen in air . This instrument was based on measuring the heat loss from two heating coils surrounded by the gas. The much higher thermal conductivity of hydrogen as compared to air resulted in the Katharometer being very sensitive to small amounts of hydrogen in air. In 1935, GOW–MAC Instruments of Newark, New Jersey, introduced an automotive engine analyzer . This instrument used a thermal conductivity detector to measure carbon dioxide in engine exhaust. This led to the development of binary gas analyzers which were used by the U.S. Navy during World War II. Thermal conductivity detectors played a key role in the development of gas chromatography and its application to gas analysis. Thermal conductivity binary gas analyzers are still used for the measurement and control of binary gas mixtures used in the semiconductor industry (e.g., phosphine in hydrogen, arsine in hydrogen). In 1989, Jonathan Stag disclosed the use of acoustic measurements to determine the composition of a binary fluid mixture. Commercial analyzers based on this are available and are finding use in real-time control of binary mixtures produced using dynamic blending.
The technique of chromatography owes its existence to the pioneering work of Ramsay, who used adsorbents to separate mixtures of gases and vapors. Tswett used adsorption to separate plant pigments and has been credited for coining the word chromatography, meaning writing in color. Chromatography as a technique to analyze gases was developed in the mid-1940s by Erika Cremer and her student, Fritz Prior. Prior’s doctoral dissertation from 1947 describes the first chromatograph used to analyze for gaseous samples . Although rudimentary by today’s standards, it had all the components of the modern gas chromatograph (GC). The gas sample was introduced using a gas burette, hydrogen was used as the carrier gas, and silica gel and activated carbon were used in the column, which was kept at a constant temperature by submerging it in a liquid bath. A thermal conductivity cell was used as a detector, and was coupled to a recorder, which served as the output device. The original setup and a sample chromatogram are shown in Figures 1.4 and 1.5 [10,11].
Figure 1.4 GC system used in Prior’s work in 1945–1947. A, Adsorbent for purification of the carrier gas (hydrogen); B, sample inlet system; C, buret containing mercury with niveau glass for sample introduction; D, Dewar flasks; E, separation column (1 cm OD containing in a 20 cm length, silica gel or activated charcoal); F, thermal conductivity detector; 1 to 8 are glass stopcocks. A vacuum pump was connected to the system at stopcock 8. With kind permission from Springer Science+Business Media: , fig. 1.
Figure 1.5 Separation of ethylene and acetylene. x-axis, time (min); y-axis, galvanometer deflection. With kind permission from Springer Science+Business Media: , fig. 3.
The need for more sensitive analysis, not just for gases, spurred innovation in all components of the simple gas chromatograph developed by Cremer and Prior. Gas burettes gave way to gastight syringes, which were replaced by metallic injection valves with elastomeric seals. Multiport rotary valves were introduced which could accommodate column switching and other methods that were developed for gas chromatography. To meet the need for lower impurity levels in semiconductor gases, better more compatible elastomers were used. Diaphragm valves, which rely on metal-to-metal seals, became available for use. These valves have exceptionally long lifetimes, which is an added advantage, and they are used routinely in process monitoring. In 1957, Marcel Golay proposed using very long (>90m) tubes of narrow diameter (0.25 μm) lined with a very thin liquid film which would significantly improve the separating power of a chromatographic column. These capillary columns, or Golay columns, drastically improved the speed of chromatographic analysis by enabling the separation of multiple impurities in a short time . Molecular sieves (or zeolites) of different pore sizes began to be used for the analysis of gases. The 1980s saw the development of porous polymer packings for columns such as Porapak (Millipore Corporation), Chromosorb (Johns Manville Corporation), and HayeSep (Hayes Separation, Inc.) which found widespread acceptance. The HayeSep A column found application in the air separation industry since it could separate nitrogen, oxygen, argon, and carbon monoxide at room temperature. The analysis of reactive and corrosive gases was facilitated by the use of precolumns and back-flush techniques, which prevented the sample gases from attacking the downstream components of the gas chromatograph [13–15]. The needs of the space program led to the development of micro-GCs to monitor for contaminants in spacecraft air.
Due to their ruggedness, capacity, and simple separation requirements, packed columns continue to be used for a wide variety of applications in the gas industry and other industrial and process environments, despite the advancement of capillary columns. The capillary column proposed by Golay in 1957 drastically improved the speed of chromatographic analysis, but perhaps more important, it provided higher plate counts, making it a more efficient column capable of separating close-boiling analytes in complex mixtures. These capillary columns are also known as open tubular columns, and with the traditional use of a liquid film coating as the stationary phase, are sometimes referred to as wall-coated open tubular (WCOT) columns. A solid stationary phase can also be achieved in these columns by applying a thin coating of a solid material. These types of columns, known as porous layeropen tubular (PLOT) columns, have found widespread applications in gas analysis. Molecular sieve-coated PLOT columns are useful in the separation of permanent gases, whereas alumina- or divinylbenzene-coated PLOT columns have found use in the separation of C1 to C10 hydrocarbons in specialty gases. In some applications for the analysis of electronic specialty gases the GC is outfitted with two columns, one with a molecular sieve stationary phase and the other with a divinylbenzene stationary phase to analyze for all impurities that are of interest. In 1979, S. C. Terry proposed a gas chromatograph integrated on a 2-inch silicon substrate in 1979 [16,17]. This miniaturization resulted in not only in a very small footprint, but also in a faster analysis time and reduction in the amount of gas needed for analysis, all of which were requirements of the space program. In 1995, Microsensor Technologies, Inc. introduced a portable GC based on this concept.
Along with advances in column technology was the development of specialized ionization detectors in the late 1950s. Work by J. E. Lovelock demonstrated the use of argon metastable atoms for detecting impurities in the effluent of a GC . This detector used ionized argon atoms, created either by using a discharge or electrons from a beta emitter. The ions collide with the argon atoms to produce metastable atoms that have an energy of 11.8 eV. Through collisions, these metastable atoms can transfer their energy to impurity molecules, which subsequently can be detected. To be ionized and detected, the impurity molecules have to have an ionization potential (IP) below 11.8 eV. Argon could also be replaced by helium to take advantage of the higher energy of metastable helium, which is 19.8 eV as opposed to the 11.2 eV of metastable argon. Commercially available helium ionization detectors using the radioactive decay of tritium (3H) as the ionizing source are referred to as helium ionization detectors (HIDs). When an electric discharge replaces the radioactive source, the detectors are referred to as discharge ionization detectors (DIDs). Improved performance was obtained by pulsing the discharge, and these are referred to as pulsed discharge ionization detectors (PDIDs). These detectors are very sensitive and find universal use in the analysis of gases.
Lovelock’s work on ionization also led to the development of the electron capture detector (ECD) . These detectors typically use the beta emission from a 63Ni foil. The electrons, which are thermalized due to collisions, can easily attach to impurities that have a large electron capture cross sections. These detectors, therefore, are very sensitive to halogenated compounds. In semiconductor applications, a GC-ECD is used to measure sub-part per billion (ppb) levels of iron carbonyl [Fe(CO)5] and nickel carbonyl [Ni(CO)4] in carbon monoxide, which is used in etching applications. This is a particularly interesting detector, since it has a high correlation of sensitivity to compounds that are biologically harmful.
During this time, the flame ionization detector (FID) was developed and today is the most widely used gas chromatographic detector . An FID uses a hydrogen/air flame to ionize the analyte molecules. The analyte molecules are ionized due to the thermal emission from microscopic carbon particles that are generated during the combustion process. FIDs are sensitive to molecules containing C–H bonds. Just like the use of thermocouple detectors for continuous gas analysis, FIDs are also widely used for continuous monitoring of hydrocarbons in gas streams.
An unusual but surprisingly useful method of detection developed in the early 1950s has become a staple in the high-purity bulk gas business. This detector is based on the reduction of mercury oxide to elemental mercury by the reducing analyte. The first application, for the determination of ethylene, involved detection of the reduced mercury based on the extent of blackening of a sensitized strip . Quantification was also possible using the strong mercury ultraviolet (UV) absorption at 253.7 nm. In the late 1980s, Trace Analytical introduced a GC system utilizing a reduction gas detector to achieve sub-ppb detection of hydrogen and carbon monoxide in bulk inert gases of interest to the semiconductor fabrication industry.
In the late 1950s, the flame photometric detector (FPD) (or the emissivity detector, as it was called then) was developed . It was not as sensitive as the ionization detectors that were introduced around the same time. However, the FPD was very sensitive to compounds containing phosphorus or sulfur, which chemiluminesce in a flame. Pulsing of the flame in the pulsed flame photometric detector (PFPD) provides much higher sensitivities and selectivities for the detection of phosphorus and sulfur. The PFPD is also used for the detection of compounds containing nitrogen, arsenic, tin, selenium, germanium, tellurium, and others.
In the late 1980s, Hewlett-Packard introduced a GC with an atomic emission detector (AED). The AED utilizes elemental emission lines generated in a microwave induced plasma to characterize the constituents in the plasma. With GC/AED the components separated by the GC column, in sequence enter the helium plasma, where they are atomized and excited. The atomic emission lines specific to the elements present in the molecule are detected with a spectrophotometer and used to identify and quantify the components. GC/AED provides a selective approach to impurity detection and is used most commonly for the analysis of impurities in hydride gases such as phosphine, arsine, and silane. Meanwhile, over a number of years from the 1960s on, what had earlier been stand-alone instruments were being interfaced with gas chromatographs to become powerful hyphenated technologies, such as GC/MS and GC–ICP–MS.
As the name implies, ion chromatography is used to separate ions in an aqueous sample. Ions in a solution have different affinities to the resins in a chromatographic column, which leads to the separation of these ions. Ion chromatography as a general analytical technique was established after Small and others at Dow Chemical Company demonstrated the use of stripper columns to suppress the background due to the chromatographic eluent . This suppression of the eluent ions enabled the use of conductivity detection to detect ions of interest. Ion chromatography is used to analyze for anions in the hydrolysis solutions of specialty gases. The anions detected can then be quantified and related back to the impurities present in the specialty gas. This technique has found use in the semiconductor industry, where there are strict requirements for acidic and basic contaminants such as hydrogen chloride, nitric acid, sulfur dioxide, and ammonia in clean room air. Impingers are used to sample clean room air, and the solution is analyzed using ion chromatography . A similar technique is used to determine acidic and basic contaminants in purge gases used for 157-nm lithographic tools.
Mass spectrometry had its origin in the pioneering work of J. J. Thomson. During his experimental work using cathode ray tubes in 1910, he discovered that under the influence of a magnetic and an electrostatic field, charged particles followed different parabolic paths, which are dependent on their charge-to-mass ratio. The apparatus he used was referred to as Thomson’s parabola spectrograph. By 1912, this apparatus had demonstrated the existence of stable isotopes by measuring 20Ne and 22Ne. F. W. Aston, who was working in the Cavendish Laboratory along with Thomson, is credited with coining the term mass spectrograph. During his career Aston discovered 212 of the 287 naturally occurring isotopes.
Advances in vacuum technology and detectors led to the commercial introduction of mass spectrometers. The first commercial mass spectrometers used a combination of electrostatic and magnetic fields to disperse ions of different masses. The sample was ionized using electron impact ionization, and either electric or magnetic scanning was used to separate ions of different mass-to-charge ratios. These mass spectrometers were very large in size and were typically installed in central laboratories where the samples could be sent for analysis. Gas analysis was limited to relatively high concentrations, and complex mixtures required solving simultaneous equations for measurements. Mass spectrometers found increased use in the field of gas analysis with the advent of quadrupole mass spectrometers. In the early 1950s, Wolfgang Paul and his co-workers developed a mass spectrometer based on the motion of charged particles in a quadrupolar electric field . The field was generated by utilizing a combination of radio-frequency (RF) and direct-current (DC) fields applied to four cylindrical rods. The opposite rods were connected together and applied with a voltage of
The placement of cylindrical rods at the corners of a square was chosen such that the radius of the rods was 1.144 times the radius of the circle inscribed between the rods. This placement resulted in the generation of a quadrupolar field, and the trajectory of charged particles in such a field is described by the Mathieu equation. The ratio of the RF and DC voltages and the peak amplitude of the RF voltage determine the mass-to-charge ratio of the ions that would be on stable trajectories and detectable by the detector. In a strict sense, these are not spectrometers but, rather, filters. Nonetheless, these devices are described routinely as quadrupole mass spectrometers. Compared to magnetic mass spectrometers, quadrupole mass spectrometers are small in size and enjoy widespread use in physics and chemistry laboratories. Mass spectrometers typically use electron impact ionization in ionizing a sample. In this form of ionization, typically, 70 eV electrons are used for the ionization and only about 1.0 × 10−6 of the sample molecules are ionized. In gas analysis applications, even with such low ionization efficiency, quadrupole mass spectrometers have found use for detecting impurities in the low-part per million ppm) and even the ppb range. Some cryogenic air separation plants use these types of mass spectrometers to monitor and control the purity of the gases that are produced. As indicated previously, the use of gas chromatography with electron impact ionization mass spectrometry (EI–MS) has also become widespread for trace impurity analysis.
Atmospheric pressure ionization sources were developed many years later than were electron impact ionization sources. The first reported coupling of an ionizer operated at atmospheric pressure to a mass spectrometer was demonstrated by Horning et al. from the Baylor College of Medicine, who observed that significant sensitivity improvement over other sources could be achieved . This discovery was a milestone in the development of mass spectrographic application for detecting impurities in the sub-ppb range in both air monitoring and high-purity gas analysis applications. In 1976, Kambara and Kanomata published work demonstrating the use of atmospheric pressure ionization mass spectrometry (APIMS) for the analysis of impurities in nitrogen at the sub-ppb level. They used a novel approach of low-pressure collisions to overcome the issues with cluster formation that are present in high-pressure ionization sources . Hitachi introduced a commercial APIMS instrument, based on the work of Kambara and Kanomata, to analyze for sub-ppb impurities in gases. Accurate determination of impurities in gases at the sub-ppb level required gas standards at these levels. This was accomplished by using dynamic dilution of ppm-level gas standards, which could easily be made using gravimetric techniques. Air Liquide patented a dynamic dilution system based on double dilution, and Praxair patented a single-dilution system capable of achieving high dilution ratios [28,29]. Air Liquide teamed up with VG Gas Analysis, and Praxair paired with Extrel Corporation, to develop commercial APIMS instrumentation tailored for sub-ppb analysis of inert gases. APIMS instrumentation is used widely in the semiconductor fabrication industry to certify bulk inert gases to meet specifications at part per trillion ppt) levels. Early APIMS applications of gas analysis were limited to nitrogen, argon, helium, and gases with a higher IP than the typical impurities of significance to the industry. Novel approaches were used to overcome some of these limitations. Clustering reactions were utilized to detect low levels of nitrogen in argon and water in oxygen [30,31]. Another novel solution to utilize proton transfer reactions was developed to detect impurities in oxygen .
Drift cells were used to study gas-phase ion molecule reactions in the 1950s, and by the late 1960s, a Plasma Chromatograph, an instrument based on the drift cells, was introduced. This instrument used an atmospheric pressure ionizer which was coupled to a drift cell operating at atmospheric pressure. Ions moving under the influence of a field gradient in the drift cells were separated according to their mobilities. This technique is very sensitive, and ion mobility spectrometers based on this have found widespread use in environmental monitoring, and in chemical warfare agent and explosives detection. Most airports are equipped with ion mobility-based instruments for explosive detection. Research at Air Products and Chemicals, Inc. led to the development of ion mobility spectrometers for detecting trace levels of impurities in gases [33,34]. In the late 1990s, SAES of Milan, Italy, using this licensed technology, introduced an ion mobility spectrometer to detect sub-ppb impurities in bulk inert gases for the semiconductor industry.
The origins of optical spectroscopy can be traced back to Isaac Newton, who in the late seventeenth century, demonstrated that light from the sun could be separated into a series of colors and is thought to have coined the word spectrum . His original apparatus, which used a prism as a dispersing unit, could be thought of as the first spectrometer. The eighteenth and nineteenth centuries heralded significant developments. In 1729, Pierre Bougeur observed that the amount of light that could pass through a liquid decreased with increasing thickness; In 1760, Johann Heinrich Lambert published the law of absorption, which was based partially on Bougeur’s observations; and in 1776, Alessandro Volta demonstrated that sparking different materials produced different colors. Volta used this to identify some gases by the color of their emissions, which can be thought of as the beginning of emission spectroscopy. In 1851, M. A. Masson produced the first spark emission spectroscope. In the following year, August Beer published a paper extending the absorption law of Lambert to solutions. The law of absorption, known as the Beer-Lambert law, is the basis for determining the concentration of different constituents in a sample in absorption spectroscopy. The first commercially available quartz prism spectrograph was introduced by Adam Hilger, Ltd., of London.
Nondispersive infrared (NDIR) spectroscopy is used widely in gas analysis. In 1937–1938, Luft and Lehrer, working in the laboratories of the German chemical company I. G. Farbenindustrie, developed a NDIR-based process analyzer that was used widely by the German chemical industry. Use of optical filters, tuned to the impurity of interest, resulted in instruments that were targeted the detection of a single gas species. Miniaturization has resulted in compact analyzers that have the ability to detect multiple impurities and have found widespread use in industry. With the advent of minicomputers, Fourier transform infrared spectroscopy (FTIR)–based instrumentation became available for gas analysis. In this instrument, a broadband light source is used in a Michelson interferometer. The sample cell is in one arm of the spectrometer. The Fourier transform of the recorded interferogram is related to the absorption characteristics of the sample. The advantage of the FTIR instrument is its ability to record the entire spectrum of a sample without the need for a dispersing element, thereby allowing determination of multiple impurities using a single instrument. Advances in this technology have led to the development of instruments that can detect impurities at levels in the low-ppb range .
The development of tunable diode lasers led to the introduction of instruments based on traditional absorption spectroscopy as well as cavity ring-down spectroscopy (CRDS) for the analysis of gases. The development of quantum cascade lasers (QCLs) led to the availability of light sources covering the range of 4 to 12 μm . This has provided the sources needed for the detection of a broad range of gases. A variety of manufacturers have targeted different applications spanning the range from traditional gas analysis to environmental analysis to expired breath analysis.
Atomic emission spectroscopy using inductively coupled plasma (ICP) was developed for multielemental analysis of liquid samples in the 1960s. The Applied Research Laboratories in the United States introduced the first commercial instrument in 1974. Issues with optical interferences in emission spectroscopy, especially for samples with high matrix concentrations, led to the development of ICP sources suitable to be coupled to mass spectrometers [38,39]. Both Sciex, Inc. in Canada and VG Isotopes in the U.K. introduced ICP–MS instruments for the analysis of elemental impurities in liquid samples in the early 1980s. Hutton et. al. reported the first gas-phase determination of elemental impurities in 1990 using an ICP–MS instrument . In 1992, Jahl and Barnes reported the use of a sealed ICP source coupled to an emission spectrometer for determining elemental impurities in specialty gases . To meet the needs of the semiconductor industry, methods were developed for trapping the elemental impurities for subsequent analysis on an ICP–MS instrument. Hydrolyzable gases were dissolved in water to obtain aqueous samples, and nonhydrolyzable gases were bubbled through acids or bases to trap the elemental impurities. Inert gases, where the elemental impurities were particulate in nature, were filtered and the filter digested in acid. Coupling of GCs to an ICP–MS enabled the determination of gas-phase metallic impurities.
The reactivity of oxygen and the corrosivity of water vapor (moisture) have detrimental effects in many applications. This is particularly the case for the application of gases in the semiconductor industry. This has led to tight specifications for oxygen and moisture in all the gases used in semiconductor fabrication. The specifications vary by the application and range from ppt to ppm levels. Analytical instruments have been developed which are targeted specifically to the determination of oxygen or water.
Electrochemical Analyzers The use of electrochemical cells to study the oxidation and reduction of a material under the influence of an electrical stimulation dates back to the 1880s, when Kohlrausch described a two-electrode cell with platinum electrodes . In 1952, Hersch developed an electrochemical cell with a platinum cathode and a lead anode in a potassium hydroxide solution acting as an electrolyte . The depolarization of the cathode by the oxygen in the sample gas passing through the solution gives rise to a galvanic current, which is proportional to the concentration of oxygen in the gas in contact with the cathode. Researchers at Monsanto further refined this concept to make a rugged instrument for the monitoring of trace oxygen in petrochemical plant streams .
Electrochemistry, the study of the chemical response of a system to an electrical stimulation, is used routinely to study the oxidation and reduction that a material undergoes during this electrical stimulation. Many analytical techniques based on this have been developed for the study of solutions and surfaces. Two techniques based on an application of electrochemistry have been used for the analysis of oxygen and water in gases. In 1960, Keidel described the development of a coulometric analyzer for determining trace quantities of oxygen . This analyzer relied on the quantitative electrochemical conversion of oxygen to electron current. Due to the quantitative conversion, Faraday’s law could be used to convert to oxygen concentration the current measured. Moreover, since no current was generated in the absence of oxygen, there was very little background.
In the 1970s, Teledyne introduced an analyzer based on a sealed electrochemical cell, and Delta F Corporation introduced a coulometric analyzer for the measurement of trace oxygen. Refinements to these are still in use. The coulometric analyzer is currently capable of measuring oxygen in inert gases at levels as low as 100 ppt.
Analyzers based on the use of solid-state electrolytes were introduced in the 1960s. It had been recognized since the late nineteenth century that some solids begin to conduct electricity at elevated temperatures. It was also recognized that doped zirconia conducts oxygen ions at elevated temperatures. Panson and Ruka of Westinghouse Electric Corporation patented an “oxygen gauge” which used a solid-state oxygen sensor . The sensor was based on mixed valence oxides that exhibited a resistance change at temperatures as low as 350 °C when the oxygen partial pressure changed. Sensors based on these are used widely in the automotive industry. Applications in the gas industry are limited since coexisting impurities such as hydrogen, carbon monoxide, and methane interfere with the determination of oxygen.
The presence of two unpaired electrons gives rise to the paramagnetic properties of oxygen. Oxygen’s large paramagnetic susceptibility has been exploited to determine low oxygen levels. Linus Pauling used the large paramagnetic susceptibility of oxygen to build an “oxygen meter” in response to the needs of the U.S. military. The meter consisted of a torsion balance with two hollow glass spheres. In the 1940s, under a contract from Cal Tech, A. O. Beckman, a staff member there, started manufacturing these instruments for the U.S. military. A variety of instruments based on the paramagnetic susceptibility of oxygen are available commercially. The coexistence of other paramagnetic gases, such as nitrous oxide, carbon dioxide, and water, in the sample gas exhibit small interference effects.
The paramagnetic analyzer is not very sensitive, and is typically not used for oxygen measurements below 0.1%. Nonetheless, it is the analyzer used most commonly for analyzing oxygen impurity in fluorine-containing laser gas mixes that are used for photolithography.
In 1958, F. A. Keidel of DuPont patented an “apparatus for water determination” . This apparatus utilized a film of a hygroscopic material, phosphorus pentoxide (P2O5), which became electrically conductive when wet. On the application of an electrical voltage, the current measured gave an indication of the amount of moisture absorbed according to Faraday’s law. Meeco of Warrington, Pennsylvania, licensed this technology from Dupont and introduced a moisture analyzer for gases using a phosphorus pentoxide cell. Initially, the analyzers were used to measure moisture in fluorocarbons; however, the approach soon found applications in the natural gas industry, where corrosion in pipelines was a major issue. With the growth of the semiconductor industry in the 1980s, it found itself the leader in the analysis of moisture in semiconductor gases. As the specifications for moisture for semiconductor gases approached the low- and sub-ppb levels, analyzers based on spectroscopic techniques gained prominence.
Laser Spectroscopy–Based Moisture Analyzers A number of technological advances have led to the development of laser spectroscopy–based moisture analyzers. The main advance occurred due to the needs of the long-haul telecommunications industry, which resulted in the development of low-cost tunable diode lasers in the region 1.3 to 2 μm. The wavelength of these diodes [typically, gallium arsenide (GaAs)–based devices] can be varied by varying the temperature and/or the diode current. Since water has an absorption line in the near infrared at 1.39253 μm, and quartz has regions of high transparency in the near infrared near 1.33 and 1.55 μm, these GaAs laser diodes could be employed for moisture detection in gases. Two approaches have been developed for moisture analyzers using diode lasers. The first uses traditional absorption spectroscopy. Analyzers based on this use a low-pressure absorption cell, typically at approximately 100 torr, to avoid issues with pressure broadening [48,49]. Measurements have been performed at sub-ppb levels in both inert and corrosive gases used in the semiconductor industry utilizing this approach. Delta F Corporation introduced an instrument based on tunable diode laser spectroscopy (TDLAS) to detect moisture in inert gases. The second approach used is cavity ring-down spectroscopy (CRDS) [50–53]. In this approach, the absorption cell is a high-finesse optical cavity. A pulse of light is injected into this cavity and the time decay of the transmitted light is measured. The exponential time decay depends on the concentration of the absorbing species, which in this case is water vapor, via the relation
where c is the speed of light, d is the cell length, R is the mirror reflectivity, N is the molecular density (concentration), σ is the absorption cross section, τ is the ring-down time, and v is the frequency.
Thus, a time measurement can be used to measure the concentration. This approach has some advantages over absorption spectroscopy. The cell can be operated at atmospheric pressure since pressure broadening has minimal impact on the measurement. Changes in the intensity of the diode laser have no effect since the concentration measured is based on the decay time. However, the degradation of the cavity mirrors will be detrimental to the measurement sensitivity. Tiger Optics introduced a CRDS-based instrument to measure sub-ppb levels of moisture in inert gases. Both the TDLAS- and the CRDS-based instruments have been developed further to be able to determine moisture in corrosive gases. Excellent review articles are available which have compared the performance of moisture analyzers based on different techniques [54–57].
The history of gas sensors can be traced back to the observation by Wagner and Hauffe in 1938 that atoms and molecules can interact with semiconductor surfaces and influence the conductivity. In 1962, Seiyama et al. described the first semiconductor-based gas sensor which relied on changes in the resistivity. In 1968, the Taguchi gas sensor, based on the change in conductivity of a tin oxide film, was introduced in Japan for the detection of combustible and reducing gases . Advances in microelectromechanical system (MEMS) technology have enabled the miniaturization of sensors based on this and other technologies. Sensors based on changes in conductivity, mass, optical properties, and work function of the sensing element are widely available. In the post-9/11 world, a lot of focus has been placed on the detection of biological and chemical weapons. Funding has been available to develop instrumentation to detect chemical and biological agents in ambient air. Two types of piezoelectric sensors, one based on surface acoustic waves (SAWs) and the other based on quartz crystal microbalance (QCM), have been developed for this application. By changing the active adsorbing film on these sensors, it is anticipated that they will find widespread use in the specialty gas industry.
The successful demonstration of a solid-state transistor at Bell Labs in 1947, followed by the development of the integrated circuit by Kilby at Texas Instruments, started the electronic revolution. The path of the semiconductor industry following Moore’s law has resulted in ever-shrinking dimensions, increasing computation prowess, and decreasing costs of electronic devices. Sensors, often referred to as electronic noses, have greatly benefited by this development [59,60]. Computational power available in a small package has led to the use of arrays of MEMS-based SAW sensors as well as QCM sensors to detect multiple coexisting impurities. This has been accomplished by developing and characterizing the adsorption properties of coatings and using fast computation to deconvolute the response of the sensor arrays to multiple impurities. This trend is likely to continue and enable the detection of virtually any molecule in trace quantities.
Development of laser-based spectroscopic methods continues, including cavity-enhanced techniques such as frequency-comb spectroscopy, a broadband approach allowing for detection of multiple species in a matrix with sensitivities matching or exceeding FTIR.
The need for field-portable analytical instruments for environmental monitoring has also led to the miniaturization of instruments that would have been unfathomable considering the size of original devices. Miniature FTIR instruments as well as miniature quadrupole mass spectrometers have been developed [61,62].
The analysis of specialty and electronic gases is a niche market for sensor and analytical instrument developers. New developments in sensor technology to meet the growing needs of homeland security and environmental monitoring will eventually lead to the availability of these systems to meet the needs of the specialty gas industry. One can envision the value (price/performance) of these devices to reach a point where they are ubiquitous in the specialty gas industry. It is possible to imagine, in a not too distant future, a sensor integrated into every container of a specialty gas that will provide the user with real-time information on the quality (purity). It is also possible to imagine a widespread array of sensors in a semiconductor fabrication facility that monitors the quality of the gas in the entire gas distribution system. With the availability of real-time purity information of the gases used in semiconductor processing, one can envision the use of feedback loops to fine-tune the operating conditions of processing instruments based on the quality of the gases.
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TRACEY JACKSIER,1 KOHEI TARUTANI,2 MARTINE CARRE3
1Air Liquide, Newark, Delaware
2Air Liquide Japan, Ibaraki, Japan
3Air Liquide, Jouy-en-Josas, France
The presence of trace metal impurities in electronic specialty gases (ESGs) can modify the electrical properties of the insulating or conducting layers in a semiconductor device. The importance of controlling these impurities in the etching and deposition processes has increased as the feature sizes have decreased and become more complex. Typical metal levels today are around 10 parts per billion (ppb), but can reach below 10 parts per trillion (ppt) for specific metals in certain critical gases.
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