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(28) Kelly, J.; McConnel, D.;Ohlligin, C.; Tossi, A.; Klrsh-De Meemaeker, A.; Nasielshl, J. J . Chem. Soc., Chem. Commun. 1987, 1821. (29) Hlrschfeld, T. Appl. Spectrmc. 1977,3 1 , 328.
was performed under the auspices of the U.S. Department of Energy under Contract W-7405-Eng-48. The authors express thanks to Paul Duhamel of the Office of Health and Envi-
Studies of Sputtering Atomizers for Atomic Absorption Spectroscopy David S. Gough,* Peter Hannaford, and R. Martin Lowe
CSIRO Division of Materials Science and Technology, Locked Bag 33, Clayton, Victoria 3168, Australia
Factors lnfluenclng absorptlon wnrltlvity and reproduciblllty have been lnvestlgated for several sputtering atomltenr, Includlng the Analyte Corporatlon Atomsource and a slmllar system reported prevlousty. The enhancement In wnsitlvlty (factor of 3) of the Atomsource over that of the earl& system for given sputterlng cond#kns Is shown to resutt melnly from the longer absorptlon path in the Atomsource. The reproduciMUty is found to be comparable for the varkwr atomizers studied, e.g. about 05-1 % relathre dendard devlatbn for the case of chromlum In low-alloy steel. The presence of water vapor In the argon sputterlng gas at k v e b greater than abml 10 ppm is found to have a deleterkw, effect on the sputtering efflclency and reproduclblllty.
INTRODUCTION The use of cathodic sputtering as a means of atomizing samples for atomic absorption spectroscopy was proposed by Russell and Walsh (1) in 1959, soon after the introduction of the atomic absorption technique. With the sputtering method the sample to be analyzed is made the cathode of a lowpressure rare-gas discharge and subjected to bombardment by energetic rare-gas ions formed in the discharge. Under the action of the ion bombardment, atoms are ejected from the cathode surface, thereby creating an atomic vapor of the cathode material. The f i t reported sputtering atomizer,that of Gatehouse and Walsh (2),required the samples to be in the form of a hollow cathode and to be mounted inside the sputtering chamber. An improved sputtering atomizer described by Gough et al. (3) allowed solid samples with a flat face to be mounted onto the outside of a glass chamber against an insulating disk (called the discharge arrester) and O-ring. The disk had a central hole to confine the discharge to a constant area and a narrow recessed step adjacent to the sample to prevent sputtered material from establishing electrical contact between the sample and inner walls of the chamber. The system utilized a flowing stream of argon t o help remove gaseous impurities from within the chamber,but was not satisfactory for the analysis of readily oxidized metals such as aluminum or zinc. An important advance in the development of sputtering atomizers was to introduce the flowing argon gas as closely as possible to the sample surface. In the system described by Gough (4), which is shown in Figure 1, the gas is admitted into the chamber through a narrow (0.1 mm) annular gap located just below the cathode surface (Figure lb). The pressure developed behind the narrow gap forces the gas to enter the sputtering chamber at 0003-2700/89/0361-1652$01.50/0
high speed. This arrangement has two distinct advantages: (i) the fast gas flow entrains sputtered atoms, greatly reducing lateral and back diffusion, and sweeps them into the light path, resulting in an increased absorption sensitivity of typically an order of magnitude. (ii) The rapid flow of gas at the cathode surface sweeps away gaseous impurities and thus allows metals such as aluminum to be analyzed without difficulty. A newly developed commercial sputtering atomizer, called the Atomsource (Analyte Corp., Grants Pass, OR), also incorporates the principle of high-speed flow of argon at the cathode surface. In this device six jets of argon are directed at the cathode surface to produce a balanced flow of gas that sweeps the sputtered atoms orthogonally away from the surface and into the center of the chamber (see Figure 2). An advantage of the Atomsource is the use of a T-shaped absorption chamber in which the gas flow, with entrained sample atoms, is directed along the light path to increase the absorption sensitivity. The robust design of the Atomsource arrester allows it to be operated at higher powers (factor of about 4) than those of the earlier atomizers, thus increasing the rate of sputtering from the surface of a sample. Applications of the Atomsource have been discussed in a number of recent publications. Ohls (5) has reported a preliminary study of the use of the Atomsource in the analysis of metals and of solutions deposited on metallic cathodes. Kim and Piepmeier (6)have recently reported detailed studies of sample loss rates and discharge conditionsin the Atomsource and also in a simple, single-jet atomizer, These authors also carried out scanning electron microscopy studies of the surface profiles produced under various discharge and flow conditions. Chakrabarti et al. (7) have recently reported studies of the transient atomization of solutions deposited on metallic cathodes, and Winchester and Marcus (8) have used the Atomsource to atomize nonconducting powders. In this paper we report investigations into the physical principles underlying differences in absorption sensitivity and reproducibility of various sputtering atomizers that utilize a high-speed flow of gas at the cathode surface. EXPERIMENTAL SECTION Sputtering Atomizers. A number of glass sputtering chambers were constructed with the three basic configurations shown in Figure 3. The sample mounting arrangement and gas inlet system for these cells is the same as in Figure lb. It consists of a hollow ceramic disk that is sealed by an O-ring between the sample and the main body of the chamber. The argon sputtering gas enters the chamber through the 0.1-mm gap in the arrester and, because of the pressure differential across this gap, is forced into the cell at high speed (- lo4cm/s at the orifice). The rapid @ 1989 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 61, NO. 15, AUGUST 1, 1989
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(a)
Sample (Cathode)
Argon
II
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5
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'Anode
To Pump
(b) 0.5mm
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Figure 3. Glass sputteringatomizers used in this work: (a)straight-tube atomizer, (b) 90' curved-tube atomizer, (c)T-shaped atomizer. Numbers indicate approximately the positions at which absorbance measurements were made (see Figure 4).
/
I
I
4 f3mm kFigure 1. (a) Glass sputtering atomizer as used in ref 4. (b) Details of arrester used with this atomizer and also with atomizers described in Figure 3.
63
Ceramu Arrester
a
Figure 2. (a) Exploded drawing of the Atomsource six-jet pneumatic accelerator/arrester system. (b) Exploded drawing of modified accelerator/arrester system. (c) Internal shape of the Atomsource sputtering chamber. Arrows indicate gas flow.
flow of gas close to the cathode surface sweeps sputtered atoms away from the sample and into the light path at speeds that, for a given pressure, depend on the diameter of the tube forming the sputtering chamber. For most of the work described here tubes of 13 mm i.d. were used with argon flow rates of 0.3-0.4 L/min (at atmospheric pressure), which results in a gas speed through the tube of about 800 cm/s at a pressure of 4 Torr. Absorption measurements were made (a) along the axis of the tubes to determine absorption sensitivities and (b) across the tubes at various distances downstream from the cathode to determine the atom distributionswithin the various types of atomizer. The absorption measurements were made using the strong Cr resonance line at 357.9 nm because this wavelength is readily transmitted through the glass walls of the cells. Other Equipment. The atomic absorption measurementswere carried out with the sputtering atomizers replacing the flame in a Varian-Techtron AA6 spectrometer. The discharge current, argon gas pressure, and timing sequences for the sputtering atomizers were regulated by using the control box and associated microprocessor supplied with the Atomsource. The pumping lines from the cells to the Atomsource control box were identical for all of the sputtering cells used in this work. Cell pressures quoted in this text, except for those in Table 11, are the pressures indicated on the Atomsource control box, but because the pressure sensor is situated inside the control box downstream from the cell, the actual pressure in the cell will be slightly different. An accurate,
absolute pressure gauge, MKS Baratron type 122A, placed at the pumping port of the sputtering chamber indicated that the pressure reading at the control box was high by typically 10%. The control box can be set for a high-current"preburn" sputtering period to clean the sample surface prior to switching to the desired operating current for the "burn" period. A 200 L/min rotary vacuum pump was used with the control box. Water vapor measurements were made with a Dupont Model 303 moisture analyzer. Procedure for Reproducibility Measurements. Reproducibility tests were carried out using Cr determinations on a British Chemical Standards low-alloy steel (BSS 402) as a typical test case. Absorption measurements were made at 357.9 nm, with a hollow-cathode current of 3.5 mA and a monochromator band-pass of 0.3 nm. For these measurements the control box was set for a pressure of 4 Torr, a preburn of 20 s at a current of 40 mA, and a burn of 60 s at a current of 25 mA. A 60-5 burn time was chosen because experience with the various atomizers showed that even after a high-current preburn the absorption can take around 30 s to reach a steady value. Each set of reproducibility measurements consisted of a series of 10 consecutive readings. Between each reading the sample was removed from the atomizer and rubbed with 400-grit emery paper until all traces of the previous burn had been removed. Relative standard deviations were calculated from the 10 individual measurements. RESULTS AND DISCUSSION Atom Distributions. An important feature in the design of sputtering atomizers of the type used in this work is the ability of the fast-flowing argon stream to entrain the sputtered atoms and sweep them away from the cathode and into the region of observation. Atom distributions for both straight and 90' curved cells (Figure 3a,b) have been determined by measuring absorptions across the cells a t various distances downstream from the cathode. The results are shown in Figure 4, where the encircled numbers correspond to the positions shown in Figure 3. The close similarity of the shapes and absorbance values for curve A (straight tube, 12 mm i.d.) and curve B (90' curved tube, 13 mm i.d.) indicates that for these diameters and gas conditions (0.4 L/min, 5 Torr) the gas flow very efficiently sweeps the sputtered atoms around the 90' bend of the curved tube. The slightly lower absorbance values for curve A relative to curve B are attributed to the slightly smaller diameter of the straight tube. The results for the 90' curved tubes of varying diameters (curves B-E) indicate that for the flow conditions used here the optimum diameter is around 13 mm i.d. In wider tubes (curve D) the overall gas speed through the cell is reduced and atoms are less efficiently swept around the bend. In narrower tubes (curves C and E), although the gas speed is higher, the proximity of the walls of the tube results in a larger number of atoms being deposited on the walls. The enhancement in
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Table 11. Comparison of Measured Absorbances in Various Sputtering Atomizers" absoratomizer type
Atomsource (normal six-jet arrester) Atomsource (modified continuous slit arrester) T-piece glass cell previous glass cell (ref 4)
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