GENERATOR FOR SPECTROMETRY

Description

[0001] The present invention relates to an RF plasma generator for generating an inductively coupled plasma in spectrometry having the features of the preamble of the first claim and to a spectrometer having an RF plasma generator according to the invention.

[0002] Inductively coupled plasmas are widely used in the field of spectroscopy, for example to generate ions in mass spectroscopy or to generate a spectrum in optical emission spectroscopy.

[0003] A wide variety of RF plasma generators for generating an inductively coupled plasma have become known from the prior art, such as free-running or frequency-stabilized generators, as well as generators based on high-power tubes and those using semiconductors. Fundamentally, a plasma generator typically comprises a voltage supply device and an oscillator circuit coupled to a load circuit for generating the plasma. The oscillator circuits are, for example, Colpitts, Meißner, or Huth-Kühn circuits. However, the use of half-bridge or full-bridge circuits has also become known. For example, WO2007 / 134363A1 discloses an RF generator for spectroscopy based on the principle of a full-bridge circuit, which uses semiconductor switching elements.

[0004] A device for generating an inductively coupled plasma further comprises one or more induction coils arranged coaxially with a torch into which a gas, often argon gas, is introduced and heated. A high-frequency generator (HF generator) generates a strong alternating electromagnetic field in the coils, which supplies the plasma with the required energy through inductive coupling. Typical plasma temperatures are in the range of 3000 K to 10000 K. Frequencies for generating the alternating magnetic fields, in turn, are in the range of MHz to GHz, often between 10 MHz and 50 MHz. In principle, the RF plasma generators must be capable of providing sufficient power to generate and maintain the plasma. The power is typically in the range of 500 W to 3 kW. Furthermore, high efficiency and high quality are desirable.

[0005] US 2013 / 265810 A1 discloses a high-frequency power supply device for supplying high-frequency power to a load whose impedance fluctuates greatly, maintaining a stable high-frequency current without generating an overcurrent or overvoltage in a driver circuit. A similar device can be found in US 2016 / 066405 A1 or WO 2007 / 134363 A1. EP 2 020 672 A2 discloses a high-frequency generator that provides adjustment of a plasma oscillation frequency.

[0006] Also of great importance is the plasma shape, which depends, among other things, on the prevailing potentials in the vicinity of the plasma, for example, a cone of the respective spectrometer, the shape and mechanical design of the induction coil used, and various properties of the components employed. In order to specifically influence the shape of the plasma, especially the plasma core, it is necessary, for example, to bring prevailing electrical potential shifts to an analytical optimum. In this context, various circuit concepts have been described which, while implementing conditions for an analytically optimal plasma shape, are generally static and cannot be automatically adjusted to changing conditions. Thus, complex adaptation of the components used and the geometry of the plasma coil is typically necessary to achieve optimal plasma shaping.Accordingly, each plasma generator is unique, and its manufacturing process is complex and expensive. However, mass production of plasma generators capable of generating an analytically optimally shaped plasma is very difficult to achieve.

[0007] Based on this, the present invention is based on the object of providing a plasma generator with which an optimally shaped plasma can be generated in a simple manner.

[0008] This object is achieved by the RF plasma generator having the features of the first claim.

[0009] In one possible embodiment, the plasma generator may also comprise a computing unit for controlling the controllable voltage source.

[0010] The provision of a controllable voltage source allows the plasma, particularly the plasma core, to be shaped during continuous operation of the RF plasma generator. By deliberately varying the voltage applied to the load circuit and / or at least a potential difference between the induction coil and a cone of a spectrometer, the plasma, particularly the plasma core, can be deliberately compressed or stretched, or the distance to a cone of the spectrometer can be changed. In this way, the plasma shape can be continuously optimized and, for example, corrected with regard to the analytical optimum. This enables controllable focusing of the ion beam generated.

[0011] An advantage of the present invention is that plasma deformation is possible with a plasma generator according to the invention without any component modifications to the generator. This electronic adjustment option significantly reduces the costs and duration of manufacturing the respective generator, as well as optimizes the analytical performance of the respective device during continuous operation.

[0012] In one embodiment, the oscillator circuit of the RF plasma generator comprises a bridge circuit, in particular a half-bridge circuit or a full-bridge circuit.

[0013] In this respect, it is advantageous if the bridge circuit is a full bridge circuit with four transistors arranged in four branches, wherein each of the transistors has a gate electrode via which it can be supplied with a gate control voltage, and wherein the transistors are switched alternately in order to generate the RF power.

[0014] It is further advantageous if the RF plasma generator comprises a gate control circuit for generating the gate control voltage for switching the transistors.

[0015] Finally, it is advantageous with regard to the gate control circuit if it is designed to set a predeterminable value of a plasma oscillation frequency of a load resonant circuit. The gate control circuit can preferably set the plasma oscillation frequency based on the temporal profile of the gate control voltage; for example, the gate control voltage can be a temporally periodic signal, in particular a square-wave signal, by means of which the transistors can be switched on and off in a suitable manner. Such targeted control of the gate control voltage advantageously allows both compensation for varying plasma loads and compensation for specific component properties. Thus, a reliable, analytically usable plasma can be provided in a simple manner using the RF plasma generator according to the invention.In this context, reference is made to the German patent application with the file number DE 10 2020 106 692.9.

[0016] In a variant according to the invention, the RF plasma generator comprises at least two controllable voltage sources which are arranged in two different branches of the oscillator circuit.

[0017] In the other variant of the invention, the RF plasma generator comprises at least four controllable voltage sources arranged in four different branches of the oscillator circuit. This variant is particularly advantageous in conjunction with an oscillator circuit configured as a full-bridge circuit. In this case, a targeted adjustment of the potentials in all four quadrants is possible.

[0018] The use of several, in particular identical, controllable voltage sources allows a targeted adjustment of the voltage applied to the load circuit and / or at least a potential difference between the induction coil and a cone of the spectrometer.

[0019] The object underlying the invention is further achieved by a spectrometer comprising an RF plasma generator according to the invention according to at least one of the described embodiments.

[0020] It is advantageous if the spectrometer is a mass spectrometer or an optical emission spectrometer.

[0021] It should be noted that the embodiments described in connection with the RF plasma generator according to the invention can also be applied mutatis mutandis to the spectrometer according to the invention and vice versa.

[0022] Further details of the invention are explained with reference to the following figures. They show: Fig. 1 is a schematic representation of an RF plasma generator with a controllable voltage source, Fig. 2 is a schematic illustration of the mode of operation of the controllable voltage source, Fig. 3 is a circuit diagram of an oscillator circuit according to the invention in the form of a full bridge circuit with four transistors and additional inductive elements, and Fig. 4 is a circuit diagram of an oscillator circuit according to the invention according to Fig. 3 and four controllable voltage sources.

[0023] Identical elements are given the same reference numerals in the figures.

[0024] In Fig. 1A schematic block diagram of an RF plasma generator 1 is shown. The generator 1 comprises a voltage supply device 2 in the form of a DC voltage source, which supplies the oscillator circuit 3. The required high-frequency power is generated by the oscillator circuit 3. The oscillator circuit 3 is correspondingly coupled to the load circuit 4 for generating the plasma, which has the induction coil LP (not shown separately here) and a capacitor CP connected in parallel with the induction coil.

[0025] The RF plasma generator 1, in particular the voltage supply device 2, further comprises a controllable voltage source 5, which is arranged in a branch of the oscillator circuit 3 and which is designed to adjust a voltage applied to the load circuit 4 and / or at least a potential difference between the induction coil (not shown here) and a spectrometer (likewise not shown), in particular a cone of the spectrometer.

[0026] In Fig. 2The mode of operation of the controllable voltage source 5 is shown schematically. The shape of the plasma P generated in each case, in particular of the plasma core shown here, depends, among other things, crucially on the potentials U 1 , U 2 between the plasma coil LP and the cone 7, for example a sampler, a spectrometer 6 with which the plasma generator 1 can be used, as well as on the voltage UL applied to the load circuit 4. By specifically adjusting one or more of these potentials, the plasma shape can therefore be specifically influenced. In particular, even dynamic adaptation of the plasma shape during continuous operation is possible.

[0027] An exemplary embodiment of an oscillator circuit 3 is shown in Fig. 3The oscillator circuit 3 comprises a full-bridge circuit of four transistors T1-T4, each of which has a gate electrode E1-E4, via which it can be supplied with a gate control voltage UG. To generate the RF power for the load circuit 4 with the induction coil LP and the capacitor CP, the transistors T1-T4 are connected alternately. An inductive element L1-L4 is optionally connected in series with each of the transistors T1-T4. The inductive elements L1-L4 then form a series resonant circuit with the output capacitors C1-C4 of the transistors T1-T4.

[0028] Fig. 4 shows, by way of example, a preferred variant of the present invention, in which four controllable voltage sources 5a-5d are used. This can be advantageous, for example, when an oscillator circuit 3 configured as a full-bridge circuit is used. The oscillator circuit 3 shown here is similar to the one in Fig. 3 Each of the four branches a, b, c, d comprises a transistor T1-T4 and a controllable voltage source 5a-5d for generating the voltages U s aU sd. Optionally, it would also be possible for the case of Fig. 4 conceivable to use additional series-connected inductors L1-L4, as in the case of Fig. 3 .

[0029] By means of the controlled voltage sources 5a-5d or by means of the voltages U s aU sd, the potentials U 1 , U 2 between the plasma coil LP and the cone 7, as well as the voltage UL applied to the load circuit 4, can be varied. This, in turn, allows the plasma shape, in particular the plasma core, to be specifically influenced, for example, parallel and / or perpendicular to a longitudinal axis I (cf. Fig. 2) can be stretched or compressed by the coil LP. All of this can be accomplished without changing the other components used or the coil geometry. Furthermore, the distance of the plasma core to the cone 7 of the spectrometer 6 can be adjusted. Overall, the focusing of the resulting ion beam can thus be precisely controlled using the voltage sources 5a-5d.

[0030] In other embodiments, other numbers of controllable voltage sources 5 may also be provided. It is advantageous, but by no means absolutely necessary, for the number of branches of the oscillator circuit 3 to correspond to the number of controllable voltage sources 5 used. Reference symbol

[0031] 1 RF plasma generator 2 Voltage supply device 3 Oscillator circuit 4 Load circuit 5, 5a-5d Controllable voltage source 6 Spectrometer 7 Cone T1-T4 Transistors E1-E4 Gate electrodes LP Induction coil CP Capacitance L1-L4 Inductive elements C1-C4 Output capacitances U s a U s d Voltages of the controllable voltage sources U 1 , U 2 Potentials between induction coil and cone PPlasma, plasma core L Longitudinal axis

Claims

1. RF plasma generator (1) for generating an inductively coupled plasma (P) in spectrometry, comprising a power supply (2) with a DC voltage source, an oscillator circuit (3) connected to the power supply (2) for generating RF power, and a load circuit (4) coupled to the oscillator circuit (3) for generating the plasma (P), which load circuit (4) comprises at least an inductor (LP) and a capacitor (CP) connected in parallel to the inductor (LP), characterized in that the voltage supply device (2) further comprises two controllable voltage sources (5) or four controllable voltage sources (5a-5d), which are arranged in two different branches or four different branches (a-d) of the oscillator circuit (3), and that the controllable voltage sources (5, 5a-5d) are configured to adjust a voltage (UL) present at the load circuit (4) and / or at least one potential difference (U1, U2) between the induction coil (LP) and a spectrometer (6).

2. RF plasma generator (1) according to Claim 1, the oscillator circuit (3) comprising a bridge circuit, the bridge circuit being a full bridge circuit with four transistors (T1-T4) which are arranged in four branches (a-d) of the oscillator circuit (3), each of the transistors (T1-T4) having a gate electrode (E1-E4) via which the transistor (T1-T4) can be supplied with a gate control voltage, and the transistors (T1-T4) being connected alternately in order to generate the RF power.

3. RF plasma generator (1) according to claim 2, comprising a gate control circuit for generating the gate control voltage for switching the transistors (T1-T4).

4. RF plasma generator (1) according to claim 3, the gate control circuit being designed to set a predeterminable value of a plasma oscillation frequency of a load oscillating circuit.

5. Spectrometer (6) comprising an RF plasma generator (1) according to at least one of claims 1 to 4.

6. Spectrometer (6) according to claim 5, the spectrometer (6) being a mass spectrometer or an optical emission spectrometer.