Trace Moisture Research Articles

Trace moisture emissions from heated metal surfaces in hydrogen service

The formation of trace moisture by exposure of dry heated surfaces of 316 L stainless-steel, Restek Silcosteel®, and nickel 1/8 in. outer diameter line segments to purified Ar and H2 was studied using atmospheric pressure ionization mass spectrometry at flow rates of 2 slpm. Prior to H2 exposure, adsorbed moisture was removed by heating incrementally to 500 °C in an argon matrix, where the Restek Silcosteel® material released a maximum of 50 ppb moisture at 300 °C and moisture spikes from the Ni and stainless-steel surfaces reached several 100 ppb. Upon exposure to H2, persistent low ppb moisture emissions due to the reduction of surface oxide species were observed at temperatures as low as 100 °C. Spikes at 300–500 °C ranged from ∼100 ppb for the stainless-steel lines to 400 ppb for the Restek Silcosteel® material. The observed moisture emissions have to be considered as a potential contamination source for high-purity processes utilizing H2 purge at elevated temperatures.

Techniques for the measurement of trace moisture in high-purity electronic specialty gases

The control of water vapor (moisture) contamination in process gases is critical to the successful manufacture of semiconductor devices. As specified moisture levels have become more stringent, there is a growing demand for more sensitive analytical methods. Instrumental methods currently being used or in development for measuring trace moisture at ppbv levels include Fourier transform infrared spectroscopy, tunable diode laser absorption spectroscopy, cavity ringdown spectroscopy, intracavity laser spectroscopy, electron impact ionization mass spectrometry, and atmospheric pressure ionization mass spectrometry. In addition, sensor-based technologies such as oscillating quartz crystal microbalances, and chilled mirror-, capacitor-, and electrolytic-based hygrometers operate in this regime. These approaches are presented and reviewed. As the success of each trace moisture method is dependent on the degree to which the different process gases interfere with the measurement process, important process gas applications of the techniques are highlighted. Advantages and disadvantages as well as future developments and trends are also presented.

Oxygen adsorption on hydrated gold cluster anions: experiment and theory.

The discovery that supported gold clusters act as highly efficient catalysts for low-temperature oxidation reactions has led to a great deal of work aimed at understanding the origins of the catalytic activity. Several studies have shown that the presence of trace moisture is required for the catalysts to function. Using near-atmospheric pressure flow reactor techniques, we have studied humidity and temperature effects on the reactivity of gas-phase gold cluster anions with O2. Near room temperature, the humid source produces abundant gold-hydroxy cluster anions, Au(N)OH(-), and these have a reversed O2 adsorption activity: Nonreactive bare gold clusters become active when in the form Au(N)OH(-), while active bare clusters are inactive when -OH is bound. The binding energies for the stable structures obtained from density functional calculations confirm fully these findings. Moreover, the theory provides evidence that electron-transfer induced by the binding of a OH group enhances the reactivity toward molecular oxygen for odd anionic gold clusters and suppresses the reactivity for the even ones. The temperature dependence of O2 addition to Au(3)OH(-) and Au(4)(-) indicates deviations from equilibrium control at temperatures below room temperature. The effects of humidity on gold cluster adsorption activity support the conclusion drawn for the mechanism of O2 adsorption on "dry" gold cluster anions and provides insight into the possible role of water in the enhanced activity of supported gold cluster catalysts.

Isobutene Polymerization and Isobutene-Isoprene Copolymerization Catalyzed by Cationic Zirconocene Hydride Complexes

The cationic zirconocene trihydrides [Cp‘4Zr2H(μ-H)2]+X-, generated from [Cp‘2ZrH2]2 with various trityl salts of weakly coordinating anions, are powerful initiators for the polymerization of isobutene (IB) and its copolymerization with isoprene (IP) (Cp‘ = C5H4SiMe3). This study is concerned with the quantification of the effects of the counteranion and of trace moisture on IB/IP copolymers and the nature of the initiating species. Polymer molecular weights increase with decreasing anion nucleophilicity in the order X = [B(C6F5)4] ≈ [H2N{B(C6F5)3}2] > [CN{B(C6F5)3}2]. Using [Cp‘4Zr2H3]+ [CN{B(C6F5)3}2]-, high copolymer molecular weights are found (Mw ≈ 5 × 105 g/mol at −35 °C). There is little reduction in either rate or molecular weight on addition of isoprene. Polymer molecular weights are substantially higher than with the Et2AlCl/tBuCl initiator system under identical conditions. Water was shown to be an important chain-transfer agent; substoichiometric quantities of water reduced activity, and copol...

Repeatability and Reproducibility Standard Deviations in the Measurement of Trace Moisture Generated Using Permeation Tubes.

Permeation-tube moisture generators are used in industry as calibrated sources of water vapor and carrier gas mixtures. Measurements were made using three permeation-tube moisture generators of the type used in the semiconductor industry. This paper describes repeatability and reproducibility standard deviations in measurement of moisture concentration from such generators. Repeatability refers to measurements within a system and reproducibility refers to measurements between systems. Two independent methods were used to measure the realized concentration of water vapor. The first measurement, the calculated value, was determined using calibrated permeation rate of permeation-tube and flow rate of dry carrier gas. This is the industrial method of evaluating moisture concentration. The second measurement, the measured value, was determined using the low frost-point generator at the National Institute of Standards and Technology (NIST) and a quartz-crystal-micro-balance. Four pairs of independent measurements for each generator and for six nominal levels in the range from10 nL/L to 100 nL/L were made. The characteristic used to quantify repeatability and reproducibility standard deviations in industrial measurements is the calculated value minus the measured value. Repeatability standard deviation ranges from 1 nL/L to 2 nL/L, approximately. Reproducibility standard deviation ranges from 2 nL/L to 8 nL/L, approximately. The documentary ASTM standard E691-99 was used for both data validation and quantification of the repeatability and reproducibility standard deviations.

Removal characteristics of trace compounds of landfill gas by activated carbon adsorption

The removal characteristics of trace compounds and moisture in raw landfill gas (LFG) were studied. The LFG from the extraction well was saturated with water and moisture was eliminated by physical methods including cyclone-type dehydrator and compressor. The moisture removal efficiency of dehydrator and compressor was above 80%. As the moisture contents of LFG decreased, the toxic compounds like aromatics and chlorinated compounds were effectively removed by using the granular activated carbon. The breakthrough time and adsorption capacity of benzene, toluene, and ethyl benzene decreased rapidly when the relative humidity is over 60%. The effect of moisture was more pronounced at lower adsorbate concentrations tested than at higher concentrations. The breakthrough curves for multi-component mixtures show displacement effects. In the course of competing adsorption, adsorbates with strong interaction force to displace weakly bounded substances. Adsorption by activated carbon is in descending order of xylene, ethylbenzene, toluene, tri or tetrachloroethylene, benzene, carbon tetrachloride and chloroform in LFG, respectively.

Impurities in hydride gases part 1: Investigation of trace moisture in the liquid and vapor phase of ultra-pure ammonia by FTIR spectroscopy

Trace moisture in ammonia is a critical impurity in the growth of epitaxial nitride films. Because moisture is very soluble in the liquid phase of ammonia, moisture in the vapor phase increases dramatically with cylinder use, and is often far higher than the nominal purity specification. A reliable method was developed for sampling and analyzing trace moisture in both liquid-and vaporphase ammonia using FTIR. Analysis of liquid-phase ammonia gives a stable and representative moisture value whereas gas-phase moisture levels strongly depend on sampling time, flow rate, temperature, mixing, and extent of cylinder use. The variation of vapor-phase moisture is discussed in terms of a variable vaporization model with applications to high flow.

Use of ion mobility spectrometry to determine low levels of impurities in gases.

The use of ion mobility spectrometry (IMS) for the determination of trace moisture and oxygen in bulk nitrogen has been explored. IMS utilizes atmospheric pressure ionization to ionize trace impurities in the sample gas. Mobility differences between trace impurity ions are exploited to separate these ions. Our results indicate that an IMS can indeed be used to detect < 1 ppb O2 and H2O in bulk nitrogen. Due to the nature of the interaction between trace moisture and oxygen, a multivariate calibration has to be used to obtain quantitative results, even at levels below 2 ppb.

Efficient Reduction of Fringe Noise in Trace Gas Detection with Diode Laser Multipass Absorption Spectroscopy

The combination of a diode laser source and a multipass cell is attractive for the detection of trace gases. The sensitivity achievable with this system, however, is often limited by etalon fringes arising from path length difference among the optical components. In this paper, we describe a sensitive detection method that minimizes the effect of the fringes. In accordance with the expected widths of the 2f (second-harmonic of the modulation frequency) absorption signals of the target molecule, the base length of a multipass cell is chosen so that the free spectral range of the fringes becomes much narrower than the signal widths. Then, the 2f signal is subjected to multistage smoothing to remove the fringe noise by exploiting its periodicity. As a demonstration of this method, trace moisture in nitrogen is detected using a near-infrared, distributed-feedback diode laser and a multipass cell of 20 m absorption length. The detection limit of the system is found to be about 2.3 ppb, two orders of magnitude better than the limit before fringe elimination.

High precision trace humidity measurements with a fibre-coupled diode laser absorption spectrometer at atmospheric pressure

Laser absorption spectroscopy offers the potential for fast and precise trace moisture detection in gases at atmospheric pressure with a small cross sensitivity towards other molecules. We report on the development and calibration of a fibre-coupled tunable diode laser absorption spectrometer operating in the 1 - 100 ppm humidity range. The spectrometer was tested at three European humidity standards laboratories. The performance of the spectrometer was characterized by monitoring constant water vapour concentrations over several hours, yielding a good long-term stability, reproducibility and accuracy. The standard deviations of the measured water vapour concentrations for values above 5 ppm were below ±2%. Comparisons of the instrument with chilled mirror hygrometers demonstrated its fast response time. The dependences of the signal upon the flow rate and temperature are discussed.

Spatial and temporal variability of nitrogen oxide and methane fluxes from a fertilized tree plantation in Costa Rica

Nitric oxide (NO), nitrous oxide (N2O), and methane (CH4) are naturally produced and consumed by soil biogeochemical processes. Naturally high variation between trace gas fluxes may temporarily increase due to agricultural management. We studied spatial and temporal variability of fluxes in the context of a 3‐year field experiment established to identify and quantify N2O fluxes and controlling factors using automated field measurements. We measured trace gas fluxes, soil temperature, and moisture from fertilized and unfertilized balsa (Ochroma lagopus) plantations. Combining spatial and temporal sampling we evaluate if automatically measured time series of N2O emissions are representative of overall mean fluxes from fertilized loam under balsa. Soil trace gas fluxes were measured manually at 36 randomly distributed sampling locations per plot. Mean plot emissions were evaluated against fluxes measured by seven chambers commonly used for routine bimonthly manual measurements and against N2O emissions measured by two automated chambers at 4.6‐hour sampling intervals. Trace gas fluxes were highly variable over 40 × 40 m plots. Nitrogen oxide fluxes were mainly spatially independent. Fertilization increased nitrogen oxide emissions but did not introduce spatial dependency of flux data. Within about 6 weeks fluxes approached pre‐fertilization level again. Given high spatial variation of nitrogen oxide fluxes we find that automatically measured N2O fluxes represent the nature of the flux response well and are in the range of fluxes indicated by spatial sampling. When soils were relatively dry fertilization inhibited CH4 uptake.

Absorption Spectrometry of Trace Moisture in Ammonia Gas with a 1371 nm Distributed-Feedback Diode Laser

We report on the sensitivity improvement of in the quantitative measurement of trace moisture in ammonia gas. A 1371 nm lnGaAsP distributed-feedback diode laser operating at room temperature was used as the light source. A dual-cell optical configuration was employed in the scheme of tunable diode laser absorption spectrometry. Using signal and reference cells, both 92 cm in length, the interfering effects of absorption lines of the major constituent gas (ammonia) were canceled in a remarkable manner. Other common mode noises including the etalon fringes associated with the wavelength scan were also reduced. The system is capable of detecting trace moisture content of as low as 12 parts per billion. This is an improvement of more than two orders of magnitude compared with the conventional method of single-cell detection.

Development of an IR tunable diode laser absorption spectrometer for trace humidity measurements at atmospheric pressure

The development of a laser diode absorption spectrometer that uses a strong water vapor absorption at 1393 nm is reported. Three spectroscopic techniques were compared in approximately 0.4 m of laboratory air, namely, frequency modulation, wavelength modulation, and two-tone frequency modulation spectroscopy. The first two techniques use a single-frequency modulation at 9.2 GHz and 1 kHz, respectively, generated either by a phase modulator operating at 9.2 GHz or injection current modulation at 1 kHz. The two-tone method requires modulation at two frequencies, in this case 9.19 and 9.21 GHz. It is shown that the two-tone method should provide the highest sensitivity for a trace moisture detection system.

Quantitative Analysis of Trace Moisture in N2 and NH3 Gases with Dual-Cell Near-Infrared Diode Laser Absorption Spectroscopy

This paper demonstrates our optical measurement system based on near-infrared tunable diode laser absorption spectrometry and reports the results of trace moisture determination in nitrogen and ammonia gases. A near-infrared InGaAsP distributed feedback diode laser operating at room temperature was employed as the optical source. We used a dual-cell detection strategy to cancel common mode noise from the diode laser and remove the effect of the residual moisture absorption in the beam path outside the sample cell. We also used this method to successfully eliminate the interfering absorption of matrix gas molecules such as NH(3). The detection limit of H(2)O absorption of 4 ppb in nitrogen and 12 ppb in ammonia was obtained using a single-pass absorption cell of only 92 cm in length and the average results of 10 scan measurements. This system has characteristics of both the high sensitivity and capability of in situ and real-time measurement.

In-Line Moisture Monitoring in Semiconductor Process Gases by a Reactive-Metal-Coated Quartz Crystal Microbalance

A new in-line sensor for trace moisture monitoring has been developed based on a piezoelectric quartz crystal microbalance (QCM) coated with a reactive metal thin film. The properties of the metal coating and QCM allow the sensor to have fast response and fast recovery to moisture in the process gas stream. The properties of the metal-coated QCM also allow the sensor to be compact so it can be in-line in the gas distribution system and readily integrated into the process tools. This paper presents the principle of moisture detection using this technology. The experimental results of accuracy, limit of detection (&amp;lt; 1 ppb), and error analysis of this sensor in parallel with atmospheric pressure ionization mass spectrometry (APIMS) are presented. The experimental setup and procedures are described. The effect of an upstream gas purifier on its rate of response was also studied and the results are presented and discussed. The applications of this technology in semiconductor processing are briefly mentioned.

Trace Moisture Adsorption onto Various Stainless Steel Surface-Investigation of Adsorption Heat and Adsorption Isotherms.

In order to establish a design technology for ultrahigh-purity gas piping systems and to implement effective purging for future ultralarge-scale integration (ULSI) fabrication lines, it is necessary to develop highly accurate simulation technology. We have investigated the heat of adsorption and adsorption isotherms with trace moisture concentrations on various stainless steel surfaces as a means to develop highly accurate simulation technology. We observed that the activation energy of desorption varied as of the surface coverage ratio of moisture increased in the range under monolayer adsorption. We investigated the vibration cycle of adsorbed moisture molecules, and observed that the vibration cycle was 2×10-13s at the adsorbed moisture concentration of 1.4×1014 molecules/cm2 for an Fe2O3 surface. The calculated moisture concentration using the Freundlich adsorption isotherm coincided with the measured concentration in the range of about two orders of magnitude as compared to other adsorption isotherms.

Trace moisture measurement in semiconductor gases

There are quite a few methods for measuring moisture in gases, roughly into two categories: a direct measurement using an alternation amoumt of gaseous property. We have established a technology for trace moisture measurement in gases by optimizing a dew point method, one of the direct categories of measurement. Dew points are defined as the surface temperature at which dew or frost starts to form on a mirror during cooling of the mirror. To obtain moisture concentrations from measured dew points, calculations should be carried out based on a saturation vapor pressure curve for moisture. However equilibrium must be reached. That is, temperatures should be measured when moisture coexists in gas and liquid or solid phase and their partial pressure is equal to each other. Using this principle, measuring the phase changes of moisture in the equilibrium state enables us to measure trace moisture in gases more accurately.

Influence of moisture and oxygen on the formation of cubic phase GaN in halide vapor phase epitaxial growth

Halide vapor phase epitaxy (HVPE) was performed for the growth of GaN films. The hexagonal phase α-GaN films were grown on (0001) and (112¯0) sapphire substrates, using the Ga/HCl/NH3/He system. Reflection high energy electron diffraction (RHEED) and scanning electron microscopy (SEM) were used for the study of phase orientation and surface morphology of the resulting films. Luminescence was assessed by photoluminescence (PL) and local cathodoluminescence (CL) measurements at room temperature. It was observed that the oriented islands of cubic gallium nitride (β-GaN) were formed on the surface of as-grown α-GaN films. Their growth occurred during postgrowth, when the temperature in the deposition zone decreased. It was shown that the growth of the β-GaN islands was caused by the presence of trace moisture and oxygen remained in the gas phase. Apart from the partial pressures of H2O vapor and oxygen the growth conditions of α-GaN films showed a significant effect on the density of the resulting islands. Possible reasons for the formation of β-GaN followed by subsequent growth are discussed.

Tropical Upper-Tropospheric Dry Regions from TOVS and Rawinsondes

Water vapor (WV) radiances from the TIROS-N Operational Vertical Sounder (TOVS) and rawinsondes during January 1979 are intercompared within the equatorial dry zone (EDZ) southwest of Mexico and the subtropical high (STH) southwest of Hawaii. Discrepancies are found when inferring upper-tropospheric moisture over the EDZ from different instrument platforms. This paper reconciles these discrepancies between radiances and operationally measured moisture profiles, quantifies the impact of trace amounts of upper-tropospheri WV on satellite-measured radiances, and summarizes limitations in the use of WV channel data for integrated precipitable water (PW) estimates. Contradictory upper-tropospheric moisture information is obtained over the EDZ when operational rawinsonde data and outgoing longwave radiation (OLR) are compared to TOVS WV radiances. However, no such contradiction is observed over the STH when all three data sources indicate dry conditions in the upper troposphere. This dryness is depicted in histograms of 6.7- and 7.3-µm WV brightness temperatures Tb, where a subset of excessively warm (dry) values exists within the STH but not the EDZ. It is shown that variations in small quantities of subtropopause moisture account for much of the WV Tb, discrepancy between these two regions; this moisture is not reported in operational rawinsonde data or detected in OLR. An upper-layer moisture boundary is defined linking TOVS and rawinsonde observations. This moisture boundary is the pressure level reached when moisture is integrated downward from 300 mb to a PW content of 0.6 mm. Operational rawinsonde limitations require that 300 mb be used for the top of our model atmosphere. Correlation imply that 70% of the upper-boundary elevation pressure (elevation) derived from rawinsonde data can be explained by WV Tb, variations. However, large standard deviations up to 55 mb are still present in boundary pressure estimated from WV radiances. Calculations from a radiative transfer model suggest that trace moisture between 100 and 300 mb, not normally available in operational soundings, causes the discrepancy. Analysis of special ship-launched rawinsondes confirm this hypothesis and demonstrate that up to 50% of the TOVS WV weighting functions can be attributed to tropospheric moisture above 300 mb; associated PW amounts may be as little as 0.2 mm. Accounting for this moisture could reduce errors in moisture boundary locations, estimated from TOVS, to less than 10 mb.

Optimisation of furnace oxidation of Si with respect to negative bias stress instability

A study to observe gaseous impurity ingress into an atmospheric pressure thermal oxidation furnace was carried out using an in-situ probe. Experimental measurements of trace moisture and oxygen deep within the furnace revealed that the level of infiltration into the furnace was critically dependent on the inert gas flow rate during loading. Different process recipes led to different impurity residence times within the furnace once the furnace door was sealed. The level of moisture ingress was directly correlated with the level of negative bias stress instability observed for metal gated MOS capacitors.