METHOD OF OPERATING A PIEZOELECTRIC PLASMA GENERATOR

Abstract

A method of operating a piezoelectric plasma generator including applying an input signal to a piezoelectric transformer of the piezoelectric plasma generator. An absolute value of a peak amplitude of the input signal is periodically reduced and increased to a level smaller and larger than an ignition voltage of the plasma generator, such that plasma generation periodically collapses.

Claims

1. A method of operating a piezoelectric plasma generator, comprising the step of applying an input signal to a piezoelectric transformer of the piezoelectric plasma generator, wherein an absolute value of a peak amplitude of the input signal is periodically reduced and increased to a level smaller and larger than an ignition voltage of the plasma generator, such that plasma generation periodically collapses.

2. The method of claim 1, wherein a duty cycle, which is the proportion of on-time in which the absolute value of the peak amplitude is larger than the ignition voltage during one period of oscillation of the absolute value of the peak amplitude, is adjusted during operation of the plasma generator.

3. The method of claim 2, wherein a parameter correlated to an energy input in the substrate is measured during operation of the plasma generator, wherein the duty cycle is adjusted depending on the measured energy input.

4. The method of claim 1, wherein the absolute value of the peak amplitude of the input signal switches between a high level and a low level, wherein the low level of the absolute value of the peak amplitude is zero.

5. The method of claim 1, wherein the absolute value of the peak amplitude of the input signal switches between a high level and a low level, wherein the low level of the absolute value of the peak amplitude is above zero.

6. The method of claim 1, wherein the absolute value of the peak amplitude oscillates in accordance with a continuous envelope curve.

7. The method of claim 1, wherein the absolute value of the peak amplitude during off-time, in which the absolute value of the peak amplitude is smaller than the ignition voltage, is at least for most of the off-time above zero.

8. The method of claim 1, wherein the input signal is based on a base signal having a first frequency, wherein the base signal is modulated by a modulating signal having a second frequency, the second frequency being lower than the first frequency.

9. The method of claim 1, wherein the second frequency is at most 1/20 of the first frequency.

10. The method of claim 1, wherein after an off-time, in which the absolute value of the peak amplitude is smaller than the ignition voltage, the first frequency is adjusted to the resonance frequency of the plasma generator.

11. A piezoelectric plasma generator comprising a piezoelectric transformer and a control circuit for operating the plasma generator according to the method of claim 1.

12. The piezoelectric plasma generator of claim 11, wherein the control circuit is configured for providing an input signal to the piezoelectric transformer, wherein the control circuit comprises a base signal generator for generating a base signal having a first frequency, a modulating signal generator for generating a modulating signal having a second frequency, the second frequency being smaller than the first frequency, and a signal mixer for mixing the base signal with the modulating signal such than an input signal is provided such that an absolute value of a peak amplitude of the input signal is periodically reduced and increased to a level smaller and larger than an ignition voltage of the plasma generator.

13. The piezoelectric plasma generator of claim 1, wherein the control circuit further comprises a measuring device for measuring a parameter related to an energy input provided by the plasma generator to a plasma-treated substrate, wherein the modulation signal is adjusted depending on the measured energy input.

14. The piezoelectric plasma generator of claim 1, wherein the control circuit is configured to adjust a duty cycle, which is the proportion of on-time in which the absolute value of the peak amplitude is larger than the ignition voltage in one period of oscillation of the absolute value of the peak amplitude, is adjusted during operation of the plasma generator.

15. The piezoelectric plasma generator of claim 1, comprising a measuring device for measuring a parameter related to a shift of the first frequency from a resonance frequency of the plasma generator.

Description

[0036] Further features, refinements and expediencies become apparent from the following description of the exemplary embodiments in connection with the figures.

[0037] FIG. 1 shows a schematic illustration of a piezoelectric transformer for a piezoelectric plasma generator,

[0038] FIG. 2A, 2B, 2C, 2D show examples of different base signals,

[0039] FIGS. 3A, 3B, 3C show examples of different modulating signals,

[0040] FIG. 4 shows an input signal for operating a piezoelectric transformer according to a first embodiment,

[0041] FIG. 5 shows a further example of a modulating signal,

[0042] FIG. 6 shows an input signal for operating a piezoelectric transformer according to a further embodiment,

[0043] FIG. 7 shows a further example of a modulating signal,

[0044] FIG. 8 shows an input signal for operating a piezoelectric transformer according to a further embodiment,

[0045] FIG. 9 shows a schematic circuit diagram of a piezoelectric plasma generator according to an embodiment.

[0046] In the figures, elements of the same structure and/or functionality may be referenced by the same reference numerals. It is to be understood that the embodiments shown in the figures are illustrative representations and are not necessarily drawn to scale.

[0047] FIG. 1 shows a piezoelectric transformer 1 in a perspective view. The piezoelectric transformer 1 can be used in a plasma generator for generating a plasma, in particular a non-thermal low pressure plasma or an atmospheric pressure plasma or a high pressure plasma. A piezoelectric transformer 1 is an embodiment of a resonance transformer, which is based on piezoelectricity and, in contrast to conventional magnetic transformers, forms an electromechanical system. For example, the piezoelectric transformer 1 is a Rosen-type transformer.

[0048] Alternatively, other types of piezoelectric transformers can be used.

[0049] The piezoelectric transformer 1 has a first region 2 that is an input region and a second region 3 that is an output region, wherein the direction from the first region 2 to the second region 3 defines a longitudinal direction z. The first region 2 comprises an input-side end region 4 and the second region 3 comprises an output-side end region 5.

[0050] In the first region 2, the piezoelectric transformer 1 comprises internal electrodes 6, 7 to which an alternating voltage can be applied. The internal electrodes 6, 7 extend in the longitudinal direction z of the piezoelectric transformer 1. The internal electrodes 6, 7 are stacked alternately with a piezoelectric material 8 in a stacking direction x, which is perpendicular to the longitudinal direction z. The piezoelectric material 8 is polarized in the stacking direction x.

[0051] The internal electrodes 6, 7 are arranged inside the piezoelectric transformer 1 between layers of piezoelectric material 8 and are also referred to as internal electrodes. The piezoelectric transformer 1 comprises a first side surface 9 and a second side surface 10, which is opposite the first side surface 9. On the first and second side surfaces 9, 10 external electrodes 11, 12 are arranged. The internal electrodes 6, 7 are alternately connected to one of the external electrodes 11, 12.

[0052] The second region 3 comprises a piezoelectric material 13 and is free of internal electrodes. The piezoelectric material 13 in the second region 3 is polarized in the longitudinal direction z. The piezoelectric material 13 of the second region 3 can be the same material as the piezoelectric material 8 of the first region 2.

[0053] The piezoelectric materials 8 and 13 differ in their respective polarization direction. In particular, in the second region 3 the piezoelectric material 13 is formed into a single monolithic layer, which is completely polarized in the longitudinal direction z. Thus, the piezoelectric material 13 in the second region 3 has only one single polarization direction.

[0054] Via the external electrodes 11, 12 a low alternating voltage can be applied between adjacent internal electrodes 6, 7 in the first region 2. Due to the piezoelectric effect of the piezoelectric material 8 the alternating voltage applied on the input side is converted into a mechanical oscillation. Consequently, when an alternating voltage is applied to the electrodes 6 in the first region 2, a mechanical wave that generates an output voltage in the second region 3 by means of the piezoelectric effect is formed within the piezoelectric materials 8, 13.

[0055] A high electrical voltage is generated between the output-side end region 5 and the end of the electrodes 6, 7 of the first region 2. This also creates a high potential difference between the output-side end region 5 and the surroundings of the piezoelectric transformer 1, sufficient to generate a strong electric field that ionizes a surrounding medium and causes the generation of a plasma. The field strength that is required for the ionization of the atoms or molecules or for the generation of radicals, excited molecules or atoms in the surrounding medium is referred to as the ignition field strength of the plasma. An ionization occurs if the electric field strength on the surface of the piezoelectric transformer 1 exceeds the ignition field strength of the plasma. The voltage at which the ignition field strength is achieved is called ignition voltage, in the following.

[0056] The piezoelectric transformer 1 can be used for generating a plasma in a variety of fields of application. In particular, the piezoelectric transformer 1 can be used for a plasma treatment of a surface. The surface can be part of a human body such as a finger. Alternatively, the treatment object can be any object having a surface comprising a material that for instance is to be cleaned and/or modified by a plasma treatment. In particular, the piezoelectric transformer 1 can be part of a hand-held device that needs not to be placed inside a gas chamber together with the treatment object.

[0057] FIGS. 2A, 2B, 2C and 2D show different base signals S.sub.base, i.e., basic signal shapes of a voltage U over time t provided to the external electrodes 11, 12 for generating a plasma.

[0058] The frequency f.sub.base of the base signals S.sub.base may correspond to the resonance frequency of the piezoelectric transformer. The resonance frequency depends not only on internal factors of the transformer such as the geometry of the transformer but also on external factors such as a load established by the ignited plasma interacting with the substrate.

[0059] Furthermore, the resonance frequency may also depend on the temperature of the transformer, for example.

[0060] A control circuit may register a shift between current and voltage and change the base signal so that current and voltage show nearly 0° phase shift. Alternatively or additionally, the field strength at the output region can be measured by a field probe and the frequency of the input signal can be adjusted such that a maximum field strength is achieved. In this case, the frequency of the base signal

[0061] S.sub.base corresponds to the resonance frequency.

[0062] The resonance frequency may be below 100 kHz. As an example, the resonance frequency may be not higher than 99 kHz. The resonance frequency may be at least 10 kHz. The resonance frequency may be in a range from 10 kHz to 90 kHz, for example. In specific embodiments, the resonance frequency may be about 50 kHz.

[0063] A base signal S.sub.base may have a saw-toothed shape as shown in FIG. 2A, a rectangular shape as shown in FIG. 2B, a triangular shape as shown in FIG. 2C or a sinusoidal shape as shown in FIG. 2D. Other shapes of base signals S.sub.base are possible.

[0064] Input voltages may be in the range of a few Volts while the output voltage at the tip of the transformer may be in the range of several kilo-Volts. As an example, a peak-to-peak input voltage U.sub.pp, i.e., the distance between positive and negative peak amplitudes .sub.Apeak may be in the range of 12 to 24 V and an output voltage may be up to 30 kV, for example. The absolute value of the peak amplitudes |A.sub.peak| is at a constant level.

[0065] During operation of the transformer so-called streamers may occur at the corners of the output-side end region, in the area of the ignited plasma. When these streamers hit the surface of a sensitive substrate, such as thin fabric or skin, local burn marks may occur. Accordingly, sensitive substrates may be damaged, which is an undesirable effect. Furthermore, the thermal power can lead to an undue increase of temperature in the substrate, which can damage the substrate.

[0066] In order to avoid local high temperature caused by such streamers, the absolute value of the peak amplitude A.sub.peak of an input signal provided to the transformer can periodically be reduced and increased to a level smaller and larger than ignition voltage of the plasma generator. A reduction of the absolute value of the peak amplitude .sub.Apeak has the effect that the high local power density leading to a damage is reduced. In particular, a leakage current can be achieved, fulfilling also DIN specification DIN EN 60601-1 [3].

[0067] A resulting modulated input signal can be achieved by modulating the base signal, for example one of the base signals S.sub.base shown in FIGS. 2A to 2D, with a modulating signal.

[0068] FIGS. 3A, 3B and 3C show different embodiments of modulating signals S.sub.mod having pulse shapes. The pulse signal shapes differ in their duty cycles DC. The duty cycle DC is the proportion of “on-time” T.sub.on in which for the resulting modulated input signal the absolute value of the peak amplitude is larger than the ignition voltage in one period of oscillation of the peak amplitude. The pulse signal shapes oscillate between a level of 1 and 0. The length of a pulse at a level of 1 corresponds to the “on-time”, the time between such pulses corresponds to the “off-time”.

[0069] The frequency of the modulating signal S.sub.mod is smaller than the frequency of the base signal S.sub.base A maximum frequency of the modulating signal may be 1/20 of the resonance frequency of the plasma generator. Thus, with a resonance frequency in the range of 10 kHz to 100 kHz, the maximum frequency of the modulating signal Smod is between 0.5 kHz and 5 kHz.

[0070] In order to dynamically adjust the frequency of the base signal S.sub.base such that it is near the resonance frequency of the plasma generator, the duty cycle DC has to be sufficiently large in order to obtain a sufficient number of periods of the base signal. At a frequency of the modulating signal S.sub.mod of 0.5 kHz the duty cycle DC may be at least 0.5% and at a frequency of 5 kHz at least 5%. In this case, at least ten full periods of a base signal S.sub.base with a frequency of 50 kHz are present in each duty cycle DC.

[0071] In FIG. 3A, the modulating signal S.sub.mod has a duty cycle DC of 20%, in FIG. 3B the modulating signal S.sub.mod has a duty cycle of 50% and in FIG. 3C the modulating S.sub.mod has a duty cycle of 80%. A base signal S.sub.base can be modulated by such a pulse modulating signal S.sub.mod by a switch which is periodically closed and opened, for example. As an example, a transistor may be used for switching the voltage.

[0072] FIG. 4 shows an input signal S.sub.in resulting from a base signal S.sub.base having a sinusoidal shape as shown in FIG. 2D and being periodically being switched on and off according to a modulating signal S.sub.mod as shown in FIG. 3C. Thus, the absolute value of a peak amplitude |A.sub.peak| switches between the absolute value of the peak amplitude of the base signal and value of zero.

[0073] The resulting modulated signal S.sub.mod can be calculated by multiplying the base signal S.sub.base with the modulating signal S.sub.mod, for example. A phase shift may be applied to ensure that the modulated signal S.sub.mod is always increased starting from zero voltage.

[0074] In order to make the ignition of plasma easier after an off-time T.sub.off, the off-time T.sub.off should not be too long. As an example, a suitable duration of the off-time is 10 ms or shorter. In some embodiments, 5 ms may be an upper limit for the off-time.

[0075] The plasma generator may be operated such that the duty cycle DC is adjusted such that a desired amount of energy input to a substrate can be achieved. Such an adjustment can be made dynamically during operation, such that the duty cycle varies during operation.

[0076] The average energy emitted from the plasma generator depends on the duty cycle and frequency of the modulating signal

[0077] S.sub.mod When the duty cycle is high, the emitted energy is high. When the duty cycle is low, the emitted energy is low.

[0078] Adjusting the duty cycle enables controlling a maximum energy transfer and a maximum patient leakage current without changing geometric distances, adding additional dielectric barrier and/or changing the process media, for example.

[0079] According to an embodiment, a parameter corresponding to an energy input in a substrate or a substrate surface is determined. Depending on the determined value, the duty cycle can be adjusted such that the average energy over time is increased or decreased.

[0080] When switching on the base signal again, the frequency f.sub.base of the base signal S.sub.base can be re-adjusted to the resonance frequency. For this aim, a parameter corresponding to a shift of the frequency from the resonance frequency may be obtained and the frequency of the base signal is re-adjusted such that it corresponds to the resonance frequency. Such a re-adjustment can be done in each cycle when the base signal is switched on again. At a frequency of the modulating signal of 5 kHz, a re-adjustment will be done every 200 ps, accordingly.

[0081] FIG. 5 shows a further embodiment of a pulse-shaped modulating signal S.sub.mod. In this embodiment, the modulating signal S.sub.mod oscillates between levels of 1 and 0.5.

[0082] FIG. 6 shows the resulting input signal S.sub.in obtained by a sinusoidal base signal S.sub.base modulated by the modulating signal S.sub.mod of FIG. 5. During the off-time T.sub.off the absolute value of the peak amplitude |A.sub.peak| is not zero but half the amplitude of the absolute value |A.sub.peak| during on-time T.sub.on. During the off-time T.sub.off the absolute value of the peak amplitude |A.sub.peak| is smaller than the ignition voltage V.sub.ig and plasma generation is stopped.

[0083] Other levels of the pulse-shaped modulating signal S.sub.mod are possible. However, the low level should be low enough such that the input voltage is lower than the ignition voltage and the plasma collapses. The low level may be chosen high enough to maintain an oscillation of the component, such that the next ignition starts at a lower ignition voltage and can be reached by only a slight increase in input voltage. By such a “warm” restart, the mechanical stress on the component can be reduced and the reliability can be significantly increased.

[0084] Such a modulation has the advantage that an oscillating motion of the piezoelectric transformer is upheld between the high pulses.

[0085] FIG. 7 shows a further example of a modulating signal S.sub.mod in which the signal continuously oscillates in difference to switching between fixed levels as shown in FIGS. 3A to 3C and FIG. 5. The modulating signal S.sub.mod has a shape of an absolute value of a sinusoidal oscillation. The shown continuous oscillation is suitable for maintaining a continuous oscillation of the piezoelectric transformer.

[0086] FIG. 8 shows an embodiment of an input signal S.sub.in, wherein the absolute value of the peak amplitude |A.sub.peak| continuously oscillates. The input signal S.sub.in is based on a sinusoidal base signal modulated by the modulating signal S.sub.mod shown in FIG. 7. The course of the peak amplitude |A.sub.peak| follows an enveloping curve having the shape of the modulating signal S.sub.mod.

[0087] A duty cycle DC of the resulting amplitude-modulated input signal S.sub.in is also here the “on-time” T.sub.on in which the absolute value of the peak amplitude |A.sub.peak| of the input signal S.sub.in is larger than the ignition voltage and plasma is generated, in relation to the length of a full period of oscillation of the absolute value of the peak amplitude, i.e.

[0088] the sum of the “on-time” T.sub.on and the “off-time” T.sub.off, in which the absolute value of the peak voltage is smaller than the ignition voltage V.sub.ig.

[0089] Also in this embodiment, the peak amplitude |A.sub.peak| during off-time may be such that plasma generation collapses during off-time, but at the same time, oscillation of the piezoelectric transformer is maintained during off-time. The peak amplitude |A.sub.peak| is for most of the off-time above zero. In particular, the input voltage U (t) has several periods of oscillation during off-time, wherein in a majority of the periods, the peak amplitude |A.sub.peak| is above zero. In the shown embodiment, the peak amplitude |A.sub.peak| is in the vicinity of zero for only a single period during off-time.

[0090] FIG. 9 shows a piezoelectric plasma generator 14 comprising a control circuit 15 and a piezoelectric transformer 1.

[0091] The control circuit 15 comprises a base signal generator 16 supplying a base signal, e.g. one of the base signals shown in FIGS. 2A to 2D. The control circuit 15 further comprises a modulating signal generator 17, in which a modulating signal is defined and a signal mixer 18 mixing, e.g., scaling, the base signal with the modulating signal such that a modulated input signal is generated.

[0092] The control circuit 15 further comprises a measuring device 19 which determines a parameter of the plasma generator 14 during operation. The measuring device 19 may determine a shift of the resonance frequency from the frequency of the base signal. The measuring device 19 may alternatively or additionally determine an energy input into a substrate and/or a current flow.

[0093] The measurement results of the measuring device may be provided to the base signal generator 16 such that the frequency of the base signal can be periodically adjusted to the resonance frequency.

[0094] Furthermore, the measurement results of the measuring device 19 may be provided to the modulating signal generator 17. The modulating signal generator 17 may adjust the duty cycle of the modulating signal in order to dynamically lower or increase the energy input into a substrate or a current flow.

[0095] In some embodiments, the input signal may be completely shut off depending on the measurement results. As an example, the input signal may be shut off when the energy input in a substrate is too high and/or too low.

REFERENCE NUMERALS

[0096] 1 piezoelectric transformer

[0097] 2 first region

[0098] 3 second region

[0099] 4 input-side end region

[0100] 5 output-side end region

[0101] 6 first internal electrode

[0102] 7 second internal electrode

[0103] 8 piezoelectric material

[0104] 9 first side surface

[0105] 10 second side surface

[0106] 11 first external electrode

[0107] 12 second external electrode

[0108] 13 piezoelectric material

[0109] 14 piezoelectric plasma generator

[0110] 15 control circuit

[0111] 16 base signal generator

[0112] 17 modulating signal generator

[0113] 18 signal mixer

[0114] 19 measuring device

[0115] 20 substrate

[0116] z longitudinal direction

[0117] x stacking direction

[0118] S.sub.in input signal

[0119] S.sub.base base signal

[0120] S.sub.mod modulating signal

[0121] f.sub.base frequency of base signal (first frequency)

[0122] f.sub.mod frequency of modulating signal (second frequency)

[0123] A.sub.peak peak amplitude

[0124] |A.sub.peak| absolute value of peak amplitude

[0125] U.sub.PP peak-to-peak voltage

[0126] V.sub.ig ignition voltage

[0127] T.sub.on on-time

[0128] T.sub.off off-time

[0129] T.sub.cycle cycle-time

[0130] DC duty cycle