RU2551119C1 - Time-of-flight ion spectrometer - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: time-of-flight ion spectrometer contains a vacuum chamber (1) which has inside the series arranged a drift pipe (2) and an ion detector (7); at the inlet and outlet end faces of the drift pipe (2) electrodes (3, 4) are installed, which are transparent for ions and electrically connected with it, in front of the inlet electrode (3) the grounded electrode (5) is placed, the drift pipe (2) is electrically connected to an impulse source of the accelerating voltage (8). Between the output electrode (4) of the drift pipe (2) and the ion detector (7) the additional electrode (9), transparent for ions, electrically connected to a negative output of DC voltage source (10), the second output of which is connected to the vacuum chamber (1) is installed.

EFFECT: improvement of accuracy of measurement of charge and mass structure of plasma ions created by any plasma source at any distance from it.

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Description

The invention relates to the field of charged particle spectrometry and can be used to measure the charge and mass composition of plasma ions.

Known time-of-flight spectrometer [S.P. Gorbunov, V.P. Krasov, I.A. Krinberg, V.L. Paperny. Source of metal ions with a variable velocity ./6th International Conference on Modification of Materials with Particle Beams and Plasma Flows. 23-28 September 2002, Tomsk, Russia, p.67-70], consisting of a plasma source, an accelerating gap, a drift tube and an ion collector located in series in a vacuum chamber. The output diaphragm of the plasma source is the node for introducing the plasma flow into the spectrometer. An accelerating gap is formed between the output diaphragm of the plasma source and the input of the drift pipe, for which a high-voltage output of a pulse source of accelerating voltage of positive polarity is connected to the output diaphragm of the plasma source. The drift pipe is connected to the grounded terminal of the voltage source. When a short pulse of accelerating voltage of positive polarity is applied to the output diaphragm of the plasma source, ions are extracted and accelerated from it. The formed ion beam, when it moves in the drift tube, is divided into separate bunches in accordance with the charge, mass and energy composition of the ions. The mass and charge composition of the beam is determined by the time of arrival of ions on the collector of ions. The fraction of each type of ion in the total beam is determined by the ratio of the peak areas of the current waveform from the collector.

The disadvantage of this spectrometer is that the extraction of ions occurs directly from the source, which is under the accelerating potential. This spectrometer does not allow measuring the charge-mass composition of ions in a free plasma or in a directionally moving plasma stream at any distance from the source.

Also known is a time-of-flight plasma ion spectrometer [RU patent No. 2266587, 06/10/2011] containing a vacuum chamber in which a drift tube and an ion detector are arranged in series, the detected plasma is in contact with the entrance of the drift tube. The spectrometer also includes a pulse source of accelerating voltage, made with negative polarity, its high-voltage output is electrically connected to the drift pipe, the other output is connected to the grounded body of the vacuum chamber. At the input and output ends of the drift pipe, electrodes are installed that are transparent to ions and are electrically connected to the drift pipe. An additional grounded electrode is installed in front of the inlet end of the drift pipe.

The disadvantage of this spectrometer is that when a negative pulse of accelerating voltage is applied to the drift tube, simultaneously with the acceleration of ions in the gap between the grounded electrode and the drift tube, the electrons from the plasma accelerate between the drift tube and the detector. The acceleration of electrons in the gap obeys the laws of formation of particle fluxes in plasma-filled systems. This means that in the energy spectrum of electrons there will be electrons with energies from thermal to maximum corresponding to the accelerating voltage on the drift tube. As can be seen from the oscillogram of the detector current in FIG. 2, the presence of an electronic current leads to a zero line shift and introduces uncertainty in the accuracy of determining the ratio of ions with different charge states in the plasma.

The objective of the invention is to provide a time-of-flight spectrometer of ions with a higher resolution.

The technical result achieved by the invention lies in the possibility of more accurate measurement of the charge-mass composition of plasma ions generated by any plasma source and at any distance from it.

The specified technical result is achieved in that the time-of-flight ion spectrometer, like the prototype, contains a vacuum chamber in which the drift tube and the ion detector are sequentially arranged; electrodes transparent to ions and electrically connected to it are installed at the input and output ends of the drift pipe in front of the input a grounded electrode is placed in the electrode, the drift tube is electrically connected to a pulse source of accelerating voltage, in contrast to the prototype, between the output electrode of the drift tube and the ion detector an additional electrode is installed, transparent for ions, electrically connected to the negative output of a constant voltage source, the second output of which is connected to a vacuum chamber.

Plasma, the charge and mass composition of which is required to be measured with this ion spectrometer, can be created by any method, including vacuum-arc discharge, RF and microwave discharges, various gas plasma sources, and laser radiation.

The invention is illustrated by illustrations, in which: FIG. 1 is a schematic diagram of a time-of-flight ion spectrometer, FIG. 2 is an oscillogram of a spectrum of titanium ions for a prototype, FIG. 3 is an oscillogram of a spectrum of titanium ions obtained with an additional grid electrode.

A time-of-flight spectrometer is installed in a vacuum chamber 1 and consists of a drift pipe 2 made of metal, the input and output of which are closed by electrodes 3 and 4, transparent to ions, for example, metal grids electrically connected to the drift pipe 2. Before entering the drift pipe 2 near the input electrode 3 is installed a grounded electrode 5, transparent to ions, for example a metal grid. The input electrode 3 installed at the inlet of the drift pipe 2 and the grounded electrode 5 form an accelerating gap and may have the shape of a plane or a shape convex outward from the drift pipe 2. The convex shape provides focusing of the generated ion beam. Opposite the output electrode 4, located at the outlet of the drift pipe 2, with a vacuum gap, a charged particle detector 7 is installed. The detector 7 is electrically connected to a system for measuring the current of charged particles. The pulse source of the accelerating voltage of negative polarity, which is the generator of pulse voltages 8, is connected by a high-voltage output to the drift tube 2. The second output of the generator 8 is grounded to the camera 1 and electrically connected to the grounded electrode 5. An additional grid electrode is installed in the gap between the output electrode 4 and the detector 7 9, transparent to ions. A negative DC voltage source 10 is connected to the grid electrode 9, the second terminal of which is connected to the vacuum chamber 1.

Consider the operation of the device on the example of the formation of metal plasma by a vacuum-arc evaporator operating in a continuous mode. The metal plasma enters from the source into the vacuum chamber 1, filling its volume. The analyzed plasma moves along the axis of the drift pipe 2 and is in contact with the input of the drift pipe 2. A voltage pulse of amplitude U and duration τ between the drift pipe 2 and the vacuum chamber 1 is supplied from the pulse voltage generator 8 of negative polarity.

When the bias potential appears on the drift tube 2 between the plasma 6 and the input electrode 3, a potential difference occurs. The appearance of a potential difference between the input electrode 3 and plasma 6 leads to the separation of charges in the plasma and the acceleration of ions from the plasma to the input electrode 3, and electrons in the opposite direction. Accelerated ions pass through the input electrode 3 and fall into the equipotential space of the drift tube 2. In the drift tube 2, the ion beam charge is neutralized by the electrons present in the plasma tube. The beam ions drift in the drift tube 2 with velocities determined by the energy of the ions and their mass. The presence of an additional grid electrode 9 with a constant negative potential between the output electrode 4 and the detector 7 ensures that the gap is cleared of plasma electrons, which eliminates the influence of the electron current in the gap on the measurement of ion current of different charge.

The duration of the high voltage bias pulse from the pulse voltage generator 8 is selected from the following conditions. The minimum duration of the bias pulse is determined by the processes of formation of the accelerating gap between the electrodes 3 and 5 or between the input electrode 3 and the plasma 6 (in the absence of a grounded electrode 5 in the design). First, when applying the bias potential from the pulse voltage generator 8 to the drift tube 2, the accelerating gap is formed due to the displacement of the plasma electrons. This process does not exceed several nanoseconds in time. The ion concentration over the entire formed gap will be the same and will correspond to the initial plasma concentration. In this case, ions located at different distances from the input electrode 3, accelerating in the gap, will receive different energy. As the ions accelerate, a redistribution of the ion concentration over the accelerating gap will occur. The stabilization of the emission boundary will occur either when the width of the accelerating gap becomes equal to the value determined by the law of the second two, or if the emission boundary of the plasma approaches the grounded electrode 5. Depending on the plasma parameters and the magnitude of the accelerating voltage, the stabilization time of the emission boundary can reach from several tens to several hundreds nanoseconds. The pulse duration of the pulse voltage generator 8 is advisable to choose such that it exceeds the stabilization time of the emission boundary τ _c . A fundamentally important condition for choosing the pulse width of the accelerating voltage is the condition for the drift of all accelerated ions of the analyzed plasma, including the fastest, having the highest Z / M _i ratio, during the action of the accelerating voltage, inside the drift tube. This means that accelerated ions will not fall into the braking electric field between the electrode 4 installed at the output of the drift pipe 2 and the detector 7. The maximum energy of ions E entering the drift pipe is determined by their charge and accelerating voltage

, (1) $$E=ZeU$$

, (one)

where Z is the charge of ions;

e is the electron charge;

U is the accelerating voltage.

The highest drift velocity will be for ions with the highest Z / M _i ratio - the highest ratio of ion mass to charge for ions of the analyzed plasma. The drift time of these ions τ _dr is determined by the expression

, (2) $${{\displaystyle \tau }}_{dR}={{\displaystyle \ell }}_{T.D.}\sqrt{\frac{{{\displaystyle M}}_i}{2ZeU}}$$

, (2)

- длина трубы дрейфа;Where $${{\displaystyle \ell }}_{T.D.}$$

- the length of the drift pipe;

M _i is the mass of the ion.

Thus, the pulse width of the accelerating voltage τ _{y is} chosen from the condition:

τ _y <τ _dr , (3)

that is, that the ions with the highest Z / M _i ratio will arrive at the detector 7 after the end of the accelerating voltage pulse and will be detected by the detector 7.

The amplitude of the constant negative voltage at the grid electrode 9 and the distance between the grid electrodes 4, 9 and the detector 7 are selected from the condition that the plasma is completely cleaned of electrons. In other words, the width of the gaps between the electrodes should be less than or equal to the width of the plasma charge separation layer S, determined from the expression:

, (4) $$S=\frac{\sqrt[]{2}}{3}{\lambda }_{D_e}{\left(\frac{2V_0^{}}{T_e}\right)}^{\frac{3}{\mathrm{four}}}$$

, (four)

where λ _De is the Debye screening radius,

T _e is the electron temperature,

V _o is the electrode potential, and the Debye screening radius is determined from the expression:

, (5) $${\lambda }_{D_e}={\left(\frac{{\epsilon }_0{T}_e}{e{n}_s}\right)}^{\frac{\mathrm{one}}{2}}$$

, (5)

where ε _about - the dielectric constant of the vacuum;

T _e is the electron temperature;

e is the electron charge;

n _s is the plasma concentration.

Ions arrive at detector 7 at different times, which is recorded, for example, by an oscilloscope. Subsequently, the mass and charge composition of ions and the percentage ratio of individual ion components in the plasma are determined by the oscillogram.

An example of a time-of-flight spectrometer.

The vacuum arc evaporator works with a cathode made of titanium. The concentration of plasma 6 near the input electrode 3 was (1 · 10 ⁹ -10 ¹⁰ ) ion / cm ³ . The drift tube 2 with a diameter of 100 mm had a length of 600 mm. The gap between the output electrode 4 and the additional grid electrode 9, as well as the gap between the electrode 9 and the detector 7, was chosen equal to 5 mm. A constant negative voltage from a voltage source 10 on a grid electrode 9 with an amplitude of 450 V ensured the complete clearing of the gaps from electrons at a plasma concentration of 10 ¹⁰ ion / cm ³ . In the case when the plasma concentration decreases as the time-of-flight spectrometer moves away from the plasma source, the plasma concentration in the gap between the output electrode 4 and the detector 7 is significantly lower than the plasma concentration between the electrodes 3 and 5. Under these conditions, the DC voltage amplitude at the additional electrode 9 can be reduced to a level determined from formulas (4) and (5). Titanium ions with a charge of i ⁺ , i ⁺² and i ^{+3 are} well separated at a pulse duration of 500 ns and an accelerating negative potential from a pulse voltage generator 8 with an amplitude of 1000 V. The waveform of the current from the detector obtained with an additional grid electrode 9 is shown in FIG. 3. Comparison of the waveforms of FIG. 2. and FIG. 3. shows a significant improvement in the resolution of the spectrometer.

Claims (1)

A time-of-flight plasma ion spectrometer containing a vacuum chamber in which a drift tube and an ion detector are arranged in series, electrodes transparent to ions and electrically connected to it are installed at the input and output ends of the drift tube, a grounded electrode is placed in front of the input electrode, and the drift tube is electrically connected to pulse source of accelerating voltage, characterized in that between the output electrode of the drift tube and the ion detector an additional electrode is installed, transparent for ions, electric ktricheski connected to the negative output of a DC voltage source, a second output of which is connected to the vacuum chamber.

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