Analysis of offgassed water: Calibration and techniquesJames A. Basford Citation: Journal of Vacuum Science & Technology A 12, 1778 (1994); doi: 10.1116/1.579005 View online: http://dx.doi.org/10.1116/1.579005 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/12/4?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Study on guided wave propagation in a water loaded plate with wavenumber analysis techniques AIP Conf. Proc. 1581, 365 (2014); 10.1063/1.4864843 Measurement and calibration techniques used in computer partial pressure analysis J. Vac. Sci. Technol. A 3, 527 (1985); 10.1116/1.572987 Improved and simplified diaphragm cell design and analysis technique for calibration Rev. Sci. Instrum. 54, 1394 (1983); 10.1063/1.1137253 New spectrometer and sample technique for deuterium analysis of enriched water samples Rev. Sci. Instrum. 53, 194 (1982); 10.1063/1.1136925 New pressure calibration technique of manganin gauge by microthermal analysis of mercury Rev. Sci. Instrum. 47, 232 (1976); 10.1063/1.1134593

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Analysis of offgassed water: Calibration and techniques James A Basford Oak Ridge Y-12 Plant, Martin Marietta Energy Systems, Inc., P.O. Box 2009, Oak Ridge, Tennessee 37831-8084

(Received 15 November 1993; accepted 14 February 1994)

Analysis of water in vacuum systems presents large difficulties. Generation of known partial pressures is necessary to calibrate mass spectrometer response for quantitative analysis. For this purpose, two designs of leaks and their calibrations will be described. Two problems of offgas analyses dictated the development of special techniques. One study, examining comparative surface desorption, required rapid sample insertion and data acquisition. The other study, the monitoring of slowly changing offgas rates, required high precision over extended periods. Very constant, elevated temperatures of the entire vacuum system are required to keep the water partial pressure a function of current sample offgassing. Exposure of mass spectrometers to water can cause instabilities to their sensitivities to all gases. Methods of decreasing this effect through ionizer modification and of acquiring good data through special calibration procedures will be described.

I. INTRODUCTION

Water is both ubiquitous and unique. Manufacturing and cleaning processes deposit moisture on surfaces of vacuum system components. Venting vacuum systems with all but the very driest gases results in adsorption of water. Reactions of surface oxides with hydrogen can produce water. Yet these sources of moisture would not present much of a problem if it were not for its very slow ratcs of removal by the system vacuum pumps. From its molecular weight, water would be expected to be pumped slightly faster than neon. That it is not is ascribed to its ability to adsorb tightly to almost every surface it impacts.

This article describes apparatus and techniques which were developed to measure offgassed water quantitatively. Leaks were fabricated and calibrated which provide known, constant leak rates of moisture. Both surface-adsorbed and reaction-product watcr were measured mass spectroscopi­cally under carefully controlled conditions. The effects of changes in mass spectrometer sensitivity correlated with high moisture exposures were minimized.

II. MOISTURE CALIBRATED LEAKS

Introduction of low flow rates of gases into vacuum sys­tems from calibrated leaks is used to calibrate mass spec­trometers. Numerous designs of leaks have been successfully employed for the permanent gases and several techniques are used to determine the flow rates emanating from them.} Physical leak elements such as crimped tubes or drawn cap­illaries are generally not usable to dispense vapors because their small dimensions invite condensation, changing the ef­fective dimensions of the flow element and hence the leak rate. Permeation leak elements are suitable for both perma­nent gases and liquids. Two types of moisture permeation leak are described here.

Water permeates through most plastics at moderate rates. Lightweight leaks, which could be calibrated by measuring the rate of weight loss were fabricated from polypropylene tubing of nominal I/2-in. outside diameter. After sealing one end by melting, they were filled to within about 1 cm of the

top with distilled water and the second end melted closed. Lengths of 3 to 8 cm were used with about 5 cm proving the most convenient. They were placed inside stainless-steel ves­sels attached to a vacuum system. A water bath provided constant, elevated temperature. Removal of the trapped air by permeation required about two weeks.

The use of constant temperature baths is very inconve­nient in nonlaboratory locations. A calibrated leak was made which could be electrically heated. A Conflat-fIanged nipple was closed at one end by a blind flange. After filling with distilled water, a plastic disk leak element was mounted be­tween two copper gaskets on the other end (see Fig. 1). The leak was attached to the vacuum system through a flange­mounted fitting at the leak element end. In this design, the leak element was exposed to the atmosphere at its circumfer­ence. Though of small area, this still allowed significant air permeation into the vacuum system. A second design, shown in Fig. 2, eliminated this problem. The leak element is not exposed to air so that, after an initial period for removal of trapped air, only water passes through the element. Tempera­ture control of the reservoir and element is also improved as the mounting flange is the only good thermal contact. Two wells in the mounting flange provide good thermal contact for the control and monitoring sensors.

III. CALIBRATION

There are several primary methods for calibrating perma­nent gas leaks.} An unsuccessful attempt was made to use the P.:l V method2 for calibration of water leaks. The pressure versus time plot in Fig. 3 is curved and has nonzero inter­cepts, due to the slow accretion of moisture by the apparatus surfaces, making the results impossible to interpret. The pressure drop across an orifice, measured with a spinning rotor gauge, is being used as a primary calibration technique. 3 Long equilibration times are required.

Because of their light weight, the plastic tubing leaks can be calibrated by measuring the rate of weight loss. Yet for leaks with the most useful leak rates, the weight loss is only on the order of a milligram per week. At first, an evacuated electrobalance was used but temperature control of the leak

1778 J. Vac. Sci. Technol. A 12(4), JullAug 1994 0734-2101194112(4)1177814/$1.00 ©1994 American Vacuum Society 1778

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1779 James A. Basford: Analysis of offgassed water

C D

A

FIG. 1. Initial design of moisture calibrated leak with metal reservoir: (A) water reservoir; (B) plastic disk leak element; (C) copper gaskets; (D) leak element support; (El shutoff valve.

was inadequate, giving highly variable leak rates. Weekly removal of the leaks from a temperature-controlled vacuum vessel for weighing was used. Figure 4 shows the calculated flow rate for three leaks calibrated for nearly a year at 40°C. The high initial data scatter is due to the lack of consistency of weighing technique. Refining the technique gave better results toward the end of the calibration period.

The metal-reservoir leaks are calibrated by comparison of its mass spectrometer peaks with those due to lightweight calibrated leaks. To speed the approach to steady-state partial pressures the entire vacuum system must be heated to about 50 °C. Even heating is not important but constant tempera­ture at every point is essential; temperature fluctuations change the surface residence times of the water, changing its partial pressure. Measurements of peak heights of OH+ and H20 + at ml e = 17 and 18, respectively, were made after con­stant signals were attained about 1 h after the leaks were valved in or out. Even with the vacuum system heated, the approach to steady-state partial pressures is slow, requiring high stability of the pump and mass spectrometer. Figure 5 shows the mass spectrometer signals due to H20+ and OH+ of a metal-reservoir leak at several temperatures. The vapor pressure of water, also shown, has a slightly different slope, indicating that the leak rate also depends on the temperature dependence of permeation of the element.

F

FIG. 2. Improved design of a moisture calibrated leak: CA) water reservoir; (B) permeation leak element; (C) leak element support; (D) outer vacuum vessel wall; (E) shutoff valve; (F) them1al sensor wells. The large and small Conflat flanges are 2 3/4- and 1 1/3-in. outside diameter, respectively.

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12 r-- ---I

O.fi

-O.B ~--.---..I ~.- ___ L ~ ___ 1 ______ ..L ___ ._ 1.-_____ -1 ______ I

o 2 6 8 1(1 12 14 Time, ks(;'C'

Flo. 3. Pressure as a functioll of time for PV calibratioll of water leak. The plot must have straight lines passing through the origin to be usable,

IV. MEASUREMENT OF SAMPLE MOISTURE

Measurement of moisture desorbed from sample surfaces requires different techniques than those needed for water out­gassed from within a sample. Surface water is present in very small amounts and is released fairly rapidly while sample outgassing may proceed more slowly and require monitoring for several weeks.

The apparatus diagramed in Fig. 6 was used for measur­ing surface moisture. The pneumatically actuated gate valve allowed rapid sample insertion. The sample chamber was sized to have the minimum volume and surface area neces­sary to contain the sample. A rapid, dry argon purge swept air from the sample chamber when it was open. The thermal contact between the sample and the chamber was minimized to avoid changing initial sample temperature and, hence, the desorption rates. Heat tapes, controlled by a single tempera­ture controller, were wrapped around the entire high vacuum system, maintaining a very constant temperature of about

LMK RATE. MOLls (X iE+lO) 4 - ---- - -------

I ___ ~ ._ .. ___ L _____ .___ _ ___ ..1-____ . ___ ~_~ _ ~

u 100 zoo 300

TiME. IlAYS

FIG. 4. Flow rate of three primary standard leaks. The leaks were weighed about once a week and the weight losses from the previous weighing used to calculate the flow rates, The leaks were maintained at 40 °C except at points A.

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1780 James A. Basford: Analysis of offgassed water

FIG. 5. Flow rate of water from a secondary standard leak at various tem­peratures: A is the mass spectrometer signal due to H20+ and B that due to OR'. The different slope of C, the vapor pressure of water, indicates that some of the flow rate variation with temperature is due to leak element permeation rates.

50°C. From the initial, empty, evacuated condition, the steps were as follows: chamber valve closed, chamber backfilled and purge started, sample valve opened, sample inserted, sample valve closed, purge stopped, mass spectrometer valve closed, chamber valve opened, mass spectrometer valve opened, measurements commenced. This entire sequence was standardized to take 20 s, as~uring repeatable measure­ments of vhtually all desorbed water. Though the chamber contents at atmospheric pressure were dumped directly into the turbomolecular pump, no pump failures were encoun­tered, partly because the chamber-manifold volume ratio was kept small. Because argon was present in significant amounts after measurements were initiated, OH+ was moni­tored rather than H20 + to avoid the interference by 36 A?+ . Figure 7 shows the results of three samples, the area under the curves being proportional to the amount of water des­orbed. Curves A and B are typical of most rough-surfaced samples in this study. Curve C is from a very high surface

c

A

G

FIG. 6. Apparatus for determination of surface moisture: (Al is the sample insertion gate valve; (B) the sample chamber; (C) the argon backfill and purge line; (D) a moisture calibrated leak; (E) the chamber valve; (F) the mass spectrometer; and (0) the turbomolecular pump.

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4 ~g~l~~m~~~~ ________________ _

I

31

100 200 ano Time, min

FIG. 7. Surface moisture analysis. Curves A and B are typical of the rapid moisture removal associated with typical samples. The very high surface area of the whiskers of sample C adsorbed much more water and the nature of that surface held it more tightly.

area sample consisting of large number of whiskers. Its shape indicates that its water was more tightly bound than that of samples A and B.

Monitoring water emission from bulk sample outgassing or slow decomposition requires different techniques. Rapid pumpdowns are not needed, removing some sample con­tainer restrictions. The entire vacuum system must be heated to a very constant temperature for periods of several weeks. Mass spectrometer calibration to water is done under condi­tions which more closely match sample measurement condi­tions, and is hence more accurate. Other outgassed species can be analyzed simultaneously. Monitoring of several samples by multiplexing is possible, although cleanup times of 30-60 min are required to eliminate sample crosstalk

V. SOME EFFECTS OF HIGH WATER PARTIAL PRESSURES

The primary ions fonned from water by electron bom­bardment are H20+, OH+, 0+, and H+, giving mass spectra with peaks at ml e = 18, 17, 16, and 1. There are several other peaks seen when water is present in vacuum systems. ot is formed by a secondary reaction. CO+, and CO; are the prod­ucts of water with carbides on hot filaments and primary water ions with hydrocarbons.

Water also reacts with the metals of the hot filaments, forming oxide surface layers on them which have different work functions than the base metal. This can shift the elec­tron emission pattern, and hence the efficiency of ionization, even though the total electron flux can be maintained by changes in filament current. More important, filament metal can be removed by the formation of oxides, such as the vola­tile WO j from tungsten filaments. Deposition of most of the metal-oxygen species will occur on the first surface encoun­tered, hence line-of-sight deposits are frequently encountered (see Fig. 8). These deposits are almost always oxygen defi­cient and, therefore, electrically conducting. If they cover insulators in the ionizer, as on the left-hand side of Fig. 8 they can (1) overload power supplies, changing potentials of

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1781 James 14.0 Basford: Analysis of offgassed water

FIG. 8. Effect of water-assisted deposition on ionizer insulators. Water reacts with hot filament F to give line-of-sight conductive deposits D. On the left-hand side of the figure, D bridges insulators I, providing a conductance path between F and grid G which is parallel to the emission current The controller interprets the increased current from F to G as an increase in emission current and compensates by reducing filament current which, in turn, decreases true emission current. The shield S on the right-hand side of the figure prevents bridging deposition.

the ionizer components or (2) provide a conduction path be­tween filament and grid. In the latter case, this parallels the current of electrons emitted by the filament, causing the emission control circuitry to decrease the filament current to maintain its sensed current constant. The resulting decrease of true emission current decreases the sensitivity of the mass spectrometer to all gases equally. One instrument exposed to

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high water partial pressures exhibited 70% sensitivity changes. Calibration was maintained by determining the sen­sitivity to 2~e + before every sample and background mea­surement As shown on the right-hand side of Fig. 8, care­fully positioned metal shields were used to block the line-of­sight deposition on insulators, increasing the interval between ionizer insulator replacements. The same difficulties could be encountered in ion gauges.

VI. CONCLUSIONS

The properties of water make its analysis difficult. Two kinds of leaks for quantitative introduction of water into vacuum systems were fabricated. Calibrations of these leaks by primary and comparison methods were described. Analy­ses of surface and of offgassed water were performed using special methods of introducing samples into temperature­controlled vacuum systems. The effects of high water partial pressures on mass spectra and mass spectrometer sensitivity were discussed.

ACKNOWLEDGMENTS

The Oak Ridge Y-12 Plant is managed by Martin Marietta Energy Systems, Inco for the U.S. Department of Energy un­der Contract No. DE-AC05-840R21400.

Ie. D. Ehrlich and J. A. Basford, J. Vac. Sci. Techno!. A 10, 1 (1992). 2M. 'l Iverson and 1. L. Hartley, 1. Vac. Sci. Techno!. 20, 982 (1982). 3M. Boeckmann (personal communication).

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