Anal. Chem. 1989, 61, 2589-2592
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Versatile Impedance Matching Network for Inductively Coupled Plasma Spectrometry Akbar Montaser,* Izumi Ishii, and R. H. Clifford Department of Chemistry, George Washington University, Washington, D.C. 20052 S . A. Sinex' and S . G. Capar
Division of Contaminants Chemistry, Food and Drug Administration, Washington, D.C. 20204
INTRODUCTION Impedance matching network (IMN) is the essential component in generating and stabilizing inductively coupled plasmas (ICP) operated at fixed frequency. The chief functions of an IMN are to (a) facilitate plasma formation, in a smooth and rapid fashion, by minimizing the great difference in impedance as the gaseous environment in the plasma torch changes from the nonionized to the ionized stage, (b) ensure the efficient transfer of radio frequency (rf) power from the generator to the plasma, and (c) compensate for real-time variations in impedance as the composition of plasma gas or the sample is altered in spectrochemical measurements. Only a few publications (1-11) have addressed IMN and its modification, probably because Ar ICP has been the most prominent discharge used in spectrochemical analysis (12,13), and generally no change in IMN design was necessary. In the last decade, plasma discharges sustained in other gases have been investigated to an increasing extent due to certain economic and analytical advantages (14, 15). For most fixedfrequency ICP facilities in current use, IMNs possess limited tuning and loading ranges; therefore, either non-argon plasmas cannot be generated or a significant impedance mismatch would exist when the plasma is formed. Thus, it would be relevant to devise a versatile IMN that would allow formation of ICP discharges in gases such as argon, helium, nitrogen, air, and mixed gases by using a single facility. Desirable features for such a system are discussed in this report. The proposed IMN was implemented on two ICP facilities.
IMPEDANCE MATCHING NETWORKS Figure 1A,B shows the circuit diagrams of typical IMNs used currently on almost all crystal-controlled generators. Figure 1C shows the proposed IMN used by us for a 5-kW (Model HFS-5000D generator, RF Plasma Products, Inc., Voorhees, NJ) and a 2.5-kW ICP system (Henry Electronics, Los Angeles, CA). The frequency for the crystal-controlled generators was 27 MHz, but the same IMN could also be used at 41 MHz. Incident power control for generators was provided with automatic power circuitry used commonly in ICP spectrometry. The matching networks for the 5-kW and the 2.5-kW generators were directed by Model AMN-PS-1 and Model AMN-PS-2A controllers (RF Plasma Products, Inc.), respectively. The latter was developed based on the requirements identified in this study and discussed below. Both the conventional and the modified IMNs consisted of three major components: the shunt capacitor (loading capacitor), the series capacitor (tuning capacitor), and the load coil. This design, known as the L configuration, is used on most fixed-frequency ICP systems because of its wide tuning range. The shunt capacitor is normally adjusted manually while the series capacitor is directed by the IMN controller. For the conventional IMN, the shunt capacitors consist of a fixed component, usually 250-500 pF made from four to five 50- to 100-pF ceramic capacitors rated at 5-15 kV (L. S. Jennings, San Jose, CA), and a variable air capacitor. Typical
* To whom correspondence should be addressed.
Present address: Department of Physical Sciences, Prince George's Community College, Largo, MD 20772-2199.
air capacitors used on conventional IMNs have the range of 23-98 pF (rated at 7 kV, Type 153-11-1, Cardwell Condenser Corp., Long Island, NY) and 19-488 pF (rated at 2 kV, Type 154-3-1,Cardwell Condenser Corp.). Such ranges are sufficient to form and stabilize an Ar ICP. For He ICP discharges (16-23) and for mixed-gas plasmas (24-27) sustained in argon-nitrogen, argon-oxygen, and argon-air, shunt capacitance in the range of 400-1000 pF has been necessary to stabilize the plasmas. With the conventional IMN, fixed capacitors must often be inserted in or removed from the shunt circuit to form and stabilize the discharges. Such changes are not only time-consuming but difficult to implement because the free space in most commercial matchboxes is quite limited for mounting additional capacitors or the matchbox is not easily accessible. To eliminate these problems, we replaced the shunt capacitors with a single variable, 25-1000 pF ceramic-envelopevacuum capacitor (Type CVDD-1000-15S, L. S. Jennings). The shunt capacitance could be varied manually with its value tracked by a digital dial. Historically, it would be of interest to mention that in the early days of ICP spectrometry, RF Plasma Products, Inc., used a single capacitor for the shunt circuit to manufacture a few IMNs. The approach was subsequently abandoned because it was too costly and a wide tunging range was not necessary for the Ar ICP. The main electrical stresses on the shunt and the series capacitors are normally current and voltage (4),respectively. Thus, we used a ceramic-envelope vacuum capacitor, rated for high-current usage, for the shunt circuit to prevent excessive heating of the capacitor and the consequent impedance mismatch. In addition, either the warm air inside the matchbox was exhausted with a fan or forced air was fed continuously into the matchbox. The ventilation process enhanced the stability of the forward power, especially for operation above 1.2 kW. For the series circuit, a variable 10-300-pF, 15-kV ceramic-envelope vacuum capacitor was used, similar to that adopted in most commercial IMNs. At forward power levels greater than 2 kW or when the number of turns for the load coil had to be increased to facilitate formation of He plasmas (16-23), voltage on the capacitor was too high and, sometimes, arcing occurred inside the matchbox which could damage the capacitor. Again, proper ventilation for the matchbox reduced changes of overheating and electrical breakdown at high power. For the Ar ICP, the typical load coil, fabricated from l/g in. copper tubing, has an inside diameter of 26-27 mm and consists of 3-3.5 turns. A 3- to 4-turn load coil has been used in studies of mixed-gas plasmas (24-27) at 27 MHz. For both the argon and the mixed-gas ICP discharges, the bottom turn of the load coil is normally grounded. In contrast, He ICPs are formed more easily when the load coil is grounded at the top turn (16-23). Because the ionization energy of helium is higher than that of argon, a higher voltage must be applied to the load coil to generate He ICP discharges (16-23). This may be accomplished either by increasing the number of turns for the load coil or by raising the forward power. The former method has been used by us to generate He ICP discharges (16-23). A coil with a larger number of turns (between 4 to
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5.5 turns) not only increases the voltage for easing plasma ignition but also enhances transfer of rf power to the plasma ( 4 ) . This greater power transfer efficiency is particularly important for He ICP discharges because lower gas temperatures have been measured for helium vs argon plasmas (20). As the coil inductance increases, the magnetic flux density of the coil is enhanced ( 4 ) and a filament-type plasma is formed at the center of the He ICP torch. This filament-type plasma may be converted into an annular discharge by using the injector gas, yet the process can become very sensitive to the gas flow dynamics when coils with seven or more turns are used. Aside from the matchbox, certain modifications are useful for the existing IMN controllers, or in manufacturing the future ones. To generate a variety of ICP discharges on a single facility, the IMN controller must be equipped with an electronic feedback circuit that samples the 50-R transmission line by a phase detector. The positive and negative error signals produced by the detector drive a servo system attached to the series capacitor. If the signal from the phase detector is zero, the system is matched, i.e., reactive impedances en-
countered during plasma operation are transformed to a resistive load. Certain ICP manufacturers use a cam-microswitch mechanism to tune the ICP to preset conditions. Even if a matchbox is modified as suggested in this report, the settings for the cam-switch mechanism must be changed frequently to tune different plasmas for minimum reflected power. Such a task is extremely time-consuming and often frustrates ICP users. Figure 2 shows the general block diagram of one of the controllers (Model AMN-PS-PA) modified in this work for generating various ICP discharges. The general operation of the controller is discussed below. The phase detector (A2) samples the transmission line a t 50 R and generates an error signal that causes the servomotor to turn the series capacitor. The direction of the change depends on the polarity of the phase shift. The error signal is amplified by an operational amplifier (A3) which has a variable gain for adjusting the sensitivity of the feedback loop. This phase gain adjustment is crucial in suppressing plasma oscillation. At low gain, a mismatch is not rapidly compensated, thereby increasing the reflected power and drifts in
ANALYTICAL CHEMISTRY, VOL. 61, NO. 22, NOVEMBER 15, 1989 ERROR AMP PHASE DETECTOR
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spectrometric signal. At high gain, plasma oscillations can be observed and stable plasma operation is not feasible. The signal from the error amplifier (A3) is then directed to the mode selector unit (A4) which allows the operator to select one of the following operations: automatic mode using the phase detector (A2), manual control (A5) using the f switch on the controller, or the remote mode using a switch on the matching network. The preset potentiometer (A6) allows the tuning capacitor to be preset digitally for each plasma when the preset-enable switch (AS) is activated. With this provision, the difference between the tuning conditions under the plasma on/off stages can be minimized, thus facilitating plasma ignition. The position comparator (Ai') receives a signal from the motor position sensor (Al) processed by the position fail detector (A9). Should A9 receive no signal from Al, then a fail light will come on and the motor will not turn the series capacitor. The processed signal from A1 is also amplified by A10 and is directed to a digital meter (All) to provide information on the capacitance of the series capacitor. To prevent damage to the series capacitor at the end of its travel, A9 also feeds the limit detector (A12) which in turn activates the limit break (A13) for stopping the servomotor. The threshold detector switch (A14) detects the signals from A9 and A4 for activating the servomotor under satisfactory conditions. Finally, the motor driver (A15) provides a voltage control that permits adjustment of the speed of the servomotor. Proper adjustment of this control is necessary; otherwise the servomotor will drive the series capacitor past the match point, extinguishing a barely ignited plasma. On the basis of on our experience, an IMN not only should possess a matchbox with sufficient range to tune various ICP discharges but should be equipped with a controller that has provisions for preseting the tuning condition (A6), adjusting
the phase gain (A3), and controlling the speed of the servomotor (A15). Obviously, computer control of these parameters and rf power is desirable. Also, a small fan must be installed on the matchbox to prevent temperature variation. These features are not usually needed for an Ar ICP facility, but they are quite important if ICP users become interested in forming ICP discharges in other gases to extend the domain of samples that can be handled effectively in practice (14). During the last two years, we have successfully used impedance matching networks described above for forming very stable ICP discharges in argon, helium, argon-nitrogen, argon-oxygen, and argon-air. The length of time required to switch over from one plasma to another has been reduced to a few minutes, rather than hours or days. Because plasma gas composition is among the parameters that must be optimized for certain applications, manufacturers of ICP-based instruments are encouraged to adopt the proposed impedance matching system in their future instruments. ACKNOWLEDGMENT We thank H. Tan of GWU, and W. B. Sisson, J. A. Easterling, and W. F.Syner of the Food and Drug Administration (FDA) for their assistance during this work. Special thanks are due to L. West, J. Ott, and R. Spangler of R F Plasma Products, Inc., Voorhees, NJ, for their contributions in implementing our design criteria. LITERATURE CITED (1) (2) (3) (4)
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R E C E ~for D review July 17,1989. Accepted August 31,1989. This research (at GWU) was sponsored in part by the U.S. Department of Energy under Grant No. DE-FG05-87-ER13659. Partial support for A.M. and R.H.C.was provided by the FDA.
