Plasma treatment device and power source system

Abstract

Provided is technology making it possible to improve processing performance with respect to a plasma treatment in a plasma treatment device. This plasma treatment device comprises: an RF signal generator; and a voltage pulse signal generator that is electrically connected to a bias electrode and is configured so as to generate a voltage pulse signal. The voltage pulse signal has repeating cycles; each of the repeating cycles includes a first period having a repeating sequence of first voltage pulses and a second period having a repeating sequence of second voltage pulses; each of the first voltage pulses included in the sequence of the first voltage pulses has a first duty ratio and a first frequency; each of the second voltage pulses included in the sequence of the second voltage pulses has a second duty ratio and a second frequency; and the first duty ratio is different from the second duty ratio and / or the first frequency is different from the second frequency.

Description

Plasma processing apparatus and power supply system

[0001] SUMMARY OF THE INVENTION Exemplary embodiments of the present disclosure relate to plasma processing apparatuses and power supply systems.

[0002] 2. Description of the Related Art In a plasma processing apparatus, a technique for supplying a DC signal to a lower electrode is disclosed in Japanese Patent Application Laid-Open No. 2003-222999.

[0003] International Publication No. WO2024 / 024594

[0004] The present disclosure provides a technique capable of improving the process performance of plasma processing in a plasma processing apparatus.

[0005] In one exemplary embodiment of the present disclosure, a plasma processing apparatus includes a chamber; a substrate support disposed within the chamber and having a bias electrode; an RF signal generator configured to generate an RF signal to generate a plasma in the chamber; and a voltage pulse signal generator electrically connected to the bias electrode and configured to generate a voltage pulse signal, the voltage pulse signal having repeating cycles, each repeating cycle including a first period having a repeating sequence of first voltage pulses and a second period having a repeating sequence of second voltage pulses, each first voltage pulse included in the first sequence of voltage pulses having a first duty ratio and a first frequency, and each second voltage pulse included in the second sequence of voltage pulses having a second duty ratio and a second frequency, wherein the first duty ratio is different from the second duty ratio and / or the first frequency is different from the second frequency.

[0006] According to one exemplary embodiment of the present disclosure, it is possible to provide a technique that can improve the process performance of plasma processing in a plasma processing apparatus.

[0007] 1 is a diagram for explaining an example of the configuration of a plasma processing system; 2 is a diagram for explaining an example of the configuration of a plasma processing apparatus; 3 is a diagram for explaining an example of the configuration of a substrate support part; 4 is a diagram for explaining an example of the duty ratio and frequency of a voltage pulse in a repeating cycle of a voltage pulse signal; 5 is a diagram for explaining an example of a sequence of a first voltage pulse in a first period; 6 is a diagram for explaining an example of a first voltage pulse; 7 is a diagram for explaining an example of a sequence of a second voltage pulse in a second period; 8 is a diagram for explaining an example of a second voltage pulse; 9 is a diagram for explaining an example of a sequence of a third voltage pulse in a first transition period; 10 is a diagram for explaining an example of a sequence of a fourth voltage pulse in a second transition period;

[0008] Hereinafter, each embodiment of the present disclosure will be described.

[0009] In one exemplary embodiment, a plasma processing apparatus is provided, comprising: a chamber; a substrate support disposed within the chamber and having a bias electrode; an RF signal generator configured to generate an RF signal to generate a plasma in the chamber; and a voltage pulse signal generator electrically connected to the bias electrode and configured to generate a voltage pulse signal, the voltage pulse signal having repeating cycles, each repeating cycle including a first period having a repeating sequence of first voltage pulses and a second period having a repeating sequence of second voltage pulses, each first voltage pulse in the first sequence of voltage pulses having a first duty ratio and a first frequency, and each second voltage pulse in the second sequence of voltage pulses having a second duty ratio and a second frequency, wherein the first duty ratio is different from the second duty ratio and / or the first frequency is different from the second frequency.

[0010] In one exemplary embodiment, each of the repeating cycles includes a first transition period having a plurality of third voltage pulses between the first period and the second period, and when the first duty ratio is different from the second duty ratio, the duty ratio of the plurality of third voltage pulses gradually changes from the first duty ratio to the second duty ratio, and when the first frequency is different from the second frequency, the frequency of the plurality of third voltage pulses gradually changes from the first frequency to the second frequency.

[0011] In one exemplary embodiment, the first transition period includes a repeating sequence of third voltage pulses, and the duty ratio and / or frequency of the third voltage pulses included in the sequence of third voltage pulses changes stepwise for each sequence of third voltage pulses.

[0012] In one exemplary embodiment, each repeating cycle includes, after the second period, a second transition period having a plurality of fourth voltage pulses, wherein if the first duty ratio is different from the second duty ratio, the duty ratio of the plurality of fourth voltage pulses gradually changes from the second duty ratio to the first duty ratio, and if the first frequency is different from the second frequency, the frequency of the plurality of fourth voltage pulses gradually changes from the second frequency to the first frequency.

[0013] In one exemplary embodiment, the second transition period includes a repeating sequence of fourth voltage pulses, and the duty ratio and / or frequency of the multiple fourth voltage pulses included in the sequence of fourth voltage pulses changes stepwise for each sequence of fourth voltage pulses.

[0014] In one exemplary embodiment, the second duty cycle is greater than the first duty cycle and / or the second frequency is greater than the first frequency.

[0015] In one exemplary embodiment, the second duty cycle is less than the first duty cycle and / or the second frequency is less than the first frequency.

[0016] In one exemplary embodiment, the first sequence of voltage pulses has a first voltage level and the second sequence of voltage pulses has a second voltage level, the first voltage level and the second voltage level being equal.

[0017] In one exemplary embodiment, the first sequence of voltage pulses has a first voltage level and the second sequence of voltage pulses has a second voltage level, the first voltage level being different from the second voltage level.

[0018] In one exemplary embodiment, the first voltage level and the second voltage level have negative polarity.

[0019] In one exemplary embodiment, a power supply system for use in a plasma processing apparatus is provided, the power supply system including a voltage pulse signal generator configured to generate a voltage pulse signal having repeating cycles, each repeating cycle including a first period having a repeating sequence of first voltage pulses and a second period having a repeating sequence of second voltage pulses, each first voltage pulse in the first sequence of voltage pulses having a first duty ratio and a first frequency, and each second voltage pulse in the second sequence of voltage pulses having a second duty ratio and a second frequency, wherein the first duty ratio is different from the second duty ratio and / or the first frequency is different from the second frequency.

[0020] In one exemplary embodiment, each of the repeating cycles includes a first transition period having a plurality of third voltage pulses between the first period and the second period, and when the first duty ratio is different from the second duty ratio, the duty ratio of the plurality of third voltage pulses gradually changes from the first duty ratio to the second duty ratio, and when the first frequency is different from the second frequency, the frequency of the plurality of third voltage pulses gradually changes from the first frequency to the second frequency.

[0021] In one exemplary embodiment, the first transition period includes a repeating sequence of third voltage pulses, and the duty ratio and / or frequency of the third voltage pulses included in the sequence of third voltage pulses changes stepwise for each sequence of third voltage pulses.

[0022] In one exemplary embodiment, each repeating cycle includes, after the second period, a second transition period having a plurality of fourth voltage pulses, wherein if the first duty ratio is different from the second duty ratio, the duty ratio of the plurality of fourth voltage pulses gradually changes from the second duty ratio to the first duty ratio, and if the first frequency is different from the second frequency, the frequency of the plurality of fourth voltage pulses gradually changes from the second frequency to the first frequency.

[0023] In one exemplary embodiment, the second transition period includes a repeating sequence of fourth voltage pulses, and the duty ratio and / or frequency of the multiple fourth voltage pulses included in the sequence of fourth voltage pulses changes stepwise for each sequence of fourth voltage pulses.

[0024] In one exemplary embodiment, the second duty cycle is greater than the first duty cycle and / or the second frequency is greater than the first frequency.

[0025] In one exemplary embodiment, the second duty cycle is less than the first duty cycle and / or the second frequency is less than the first frequency.

[0026] In one exemplary embodiment, the first sequence of voltage pulses has a first voltage level and the second sequence of voltage pulses has a second voltage level, the first voltage level and the second voltage level being equal.

[0027] In one exemplary embodiment, the first sequence of voltage pulses has a first voltage level and the second sequence of voltage pulses has a second voltage level, the first voltage level being different from the second voltage level.

[0028] In one exemplary embodiment, the first voltage level and the second voltage level have negative polarity.

[0029] Hereinafter, each embodiment of the present disclosure will be described in detail with reference to the drawings. In each drawing, the same or similar elements are designated by the same reference numerals, and redundant explanations will be omitted. Unless otherwise specified, the positional relationships, such as up, down, left, and right, will be described based on the positional relationships shown in the drawings. The dimensional ratios in the drawings do not represent actual ratios, and the actual ratios are not limited to the ratios shown in the drawings.

[0030] <Example of Plasma Processing System> FIG. 1 is a diagram illustrating an example of the configuration of a plasma processing system. In one embodiment, the plasma processing system includes a plasma processing apparatus 1 and a control unit 2. The plasma processing system is an example of a substrate processing system, and the plasma processing apparatus 1 is an example of a substrate processing apparatus. The plasma processing apparatus 1 includes a plasma processing chamber 10, a substrate support 11, and a plasma generation unit 12. The plasma processing chamber 10 has a plasma processing space. The plasma processing chamber 10 also has at least one gas supply port for supplying at least one processing gas to the plasma processing space and at least one gas exhaust port for exhausting gas from the plasma processing space. The gas supply port is connected to a gas supply unit 20 (described later), and the gas exhaust port is connected to an exhaust system 40 (described later). The substrate support 11 is disposed in the plasma processing space and has a substrate support surface for supporting a substrate.

[0031] The plasma generating unit 12 is configured to generate plasma from at least one processing gas supplied into the plasma processing space. The plasma generated in the plasma processing space may be capacitively coupled plasma (CCP), inductively coupled plasma (ICP), electron-cyclotron-resonance plasma (ECR plasma), helicon wave plasma (HWP), surface wave plasma (SWP), or the like. Various types of plasma generators may be used, including alternating current (AC) plasma generators and direct current (DC) plasma generators. In one embodiment, the AC signal (AC power) used in the AC plasma generator has a frequency in the range of 100 kHz to 10 GHz. Thus, AC signals include radio frequency (RF) signals and microwave signals. In one embodiment, the RF signal has a frequency in the range of 100 kHz to 150 MHz.

[0032] The control unit 2 processes computer-executable instructions that cause the plasma processing apparatus 1 to perform various processes described in this disclosure. The control unit 2 may be configured to control each element of the plasma processing apparatus 1 to perform various processes described herein. In one embodiment, part or all of the control unit 2 may be included in the plasma processing apparatus 1. The control unit 2 may include a processing unit 2a1, a storage unit 2a2, and a communication interface 2a3. The control unit 2 may be implemented by, for example, a computer 2a. The processing unit 2a1 may be configured to read a program from the storage unit 2a2 and execute the read program to perform various control operations. This program may be stored in the storage unit 2a2 in advance or may be acquired via a medium when needed. The acquired program is stored in the storage unit 2a2 and read from the storage unit 2a2 by the processing unit 2a1 for execution. The medium may be various storage media readable by the computer 2a or a communication line connected to the communication interface 2a3. The processing unit 2a1 may be a CPU (Central Processing Unit). The storage unit 2a2 may include a random access memory (RAM), a read-only memory (ROM), a hard disk drive (HDD), a solid state drive (SSD), or a combination thereof. The communication interface 2a3 may communicate with the plasma processing apparatus 1 via a communication line such as a local area network (LAN).

[0033] The following describes a configuration example of a capacitively coupled plasma processing apparatus as an example of the plasma processing apparatus 1. Fig. 2 is a diagram for explaining a configuration example of a capacitively coupled plasma processing apparatus.

[0034] The capacitively coupled plasma processing apparatus 1 includes a plasma processing chamber 10 (also simply referred to as the "chamber"), a gas supply unit 20, a power supply 30, and an exhaust system 40. The plasma processing apparatus 1 also includes a substrate support 11 and a gas inlet. The gas inlet is configured to introduce at least one process gas into the plasma processing chamber 10. The gas inlet includes a showerhead 13. The substrate support 11 is disposed within the plasma processing chamber 10. The showerhead 13 is disposed above the substrate support 11. In one embodiment, the showerhead 13 forms at least a portion of the ceiling of the plasma processing chamber 10. The plasma processing chamber 10 has a plasma processing space 10s defined by the showerhead 13, a sidewall 10a of the plasma processing chamber 10, and the substrate support 11. The plasma processing chamber 10 is grounded. The showerhead 13 and the substrate support 11 are electrically insulated from the housing of the plasma processing chamber 10.

[0035] The substrate support 11 includes a main body 111 and a ring assembly 112. The main body 111 has a central region 111a for supporting a substrate W and an annular region 111b for supporting the ring assembly 112. A wafer is an example of a substrate W. The annular region 111b of the main body 111 surrounds the central region 111a of the main body 111 in a plan view. The substrate W is disposed on the central region 111a of the main body 111, and the ring assembly 112 is disposed on the annular region 111b of the main body 111 so as to surround the substrate W on the central region 111a of the main body 111. Therefore, the central region 111a is also called a substrate support surface for supporting the substrate W, and the annular region 111b is also called a ring support surface for supporting the ring assembly 112.

[0036] In one embodiment, the main body 111 includes a base 1110 and an electrostatic chuck 1111. The base 1110 includes a conductive member. The conductive member of the base 1110 can function as a lower electrode. The electrostatic chuck 1111 is disposed on the base 1110. The electrostatic chuck 1111 includes a ceramic member 1111a and an electrostatic electrode 1111b disposed within the ceramic member 1111a. The ceramic member 1111a has a central region 111a. In one embodiment, the ceramic member 1111a also has an annular region 111b. Note that the annular region 111b may also be provided by another member surrounding the electrostatic chuck 1111, such as an annular electrostatic chuck or an annular insulating member. In this case, the ring assembly 112 may be disposed on the annular electrostatic chuck or the annular insulating member, or may be disposed on both the electrostatic chuck 1111 and the annular insulating member. Furthermore, at least one RF / DC electrode coupled to an RF power supply 31 and / or a DC power supply 32, which will be described later, may be disposed within the ceramic member 1111a. In this case, the at least one RF / DC electrode functions as a lower electrode. When a bias RF signal and / or a DC signal, which will be described later, is supplied to the at least one RF / DC electrode, the RF / DC electrode is also called a bias electrode. Note that the conductive member of the base 1110 and the at least one RF / DC electrode may function as multiple lower electrodes. Furthermore, the electrostatic electrode 1111b may function as a lower electrode. Therefore, the substrate support 11 includes at least one lower electrode.

[0037] The ring assembly 112 includes one or more annular members. In one embodiment, the one or more annular members include one or more edge rings and at least one cover ring. The edge rings are formed of a conductive or insulating material, and the cover rings are formed of an insulating material.

[0038] The substrate support 11 may also include a temperature adjustment module configured to adjust at least one of the electrostatic chuck 1111, the ring assembly 112, and the substrate to a target temperature. The temperature adjustment module may include a heater, a heat transfer medium, a flow passage 1110a, or a combination thereof. A heat transfer fluid such as brine or a gas flows through the flow passage 1110a. In one embodiment, the flow passage 1110a is formed in the base 1110, and one or more heaters are disposed in the ceramic member 1111a of the electrostatic chuck 1111. The substrate support 11 may also include a heat transfer gas supply configured to supply a heat transfer gas to a gap between the backside of the substrate W and the central region 111a.

[0039] The showerhead 13 is configured to introduce at least one process gas from the gas supply unit 20 into the plasma processing space 10s. The showerhead 13 has at least one gas supply port 13a, at least one gas diffusion chamber 13b, and multiple gas inlets 13c. The process gas supplied to the gas supply port 13a passes through the gas diffusion chamber 13b and is introduced into the plasma processing space 10s from the multiple gas inlets 13c. The showerhead 13 also includes at least one upper electrode. In addition to the showerhead 13, the gas inlet may also include one or more side gas injectors (SGIs) attached to one or more openings formed in the sidewall 10a.

[0040] The gas supply unit 20 may include at least one gas source 21 and at least one flow controller 22. In one embodiment, the gas supply unit 20 is configured to supply at least one process gas from a corresponding gas source 21 to the showerhead 13 via a corresponding flow controller 22. Each flow controller 22 may include, for example, a mass flow controller or a pressure-controlled flow controller. Additionally, the gas supply unit 20 may include at least one flow modulation device that modulates or pulses the flow rate of the at least one process gas.

[0041] The power supply 30 includes an RF power supply 31 coupled to the plasma processing chamber 10 via at least one impedance matching circuit. The RF power supply 31 is configured to supply at least one RF signal (RF power) to at least one lower electrode and / or at least one upper electrode. This generates a plasma from at least one processing gas supplied to the plasma processing space 10s. Therefore, the RF power supply 31 can function as at least a part of the plasma generation unit 12. Furthermore, by supplying a bias RF signal to the at least one lower electrode, a bias potential is generated on the substrate W, thereby attracting ion components in the formed plasma to the substrate W.

[0042] In one embodiment, the RF power supply 31 includes a first RF generating unit 31a and a second RF generating unit 31b. The first RF generating unit 31a is coupled to at least one lower electrode and / or at least one upper electrode via at least one impedance matching circuit and is configured to generate a source RF signal (source RF power) for plasma generation. In one embodiment, the source RF signal has a frequency in the range of 10 MHz to 150 MHz. In one embodiment, the first RF generating unit 31a may be configured to generate multiple source RF signals having different frequencies. The generated one or more source RF signals are supplied to at least one lower electrode and / or at least one upper electrode.

[0043] The second RF generator 31b is coupled to at least one lower electrode via at least one impedance matching circuit and is configured to generate a bias RF signal (bias RF power). The frequency of the bias RF signal may be the same as or different from the frequency of the source RF signal. In one embodiment, the bias RF signal has a frequency lower than the frequency of the source RF signal. In one embodiment, the bias RF signal has a frequency in the range of 100 kHz to 60 MHz. In one embodiment, the second RF generator 31b may be configured to generate multiple bias RF signals having different frequencies. The generated one or more bias RF signals are supplied to at least one lower electrode. In various embodiments, at least one of the source RF signal and the bias RF signal may be pulsed.

[0044] The power supply 30 may also include a DC power supply 32 coupled to the plasma processing chamber 10. The DC power supply 32 includes a first DC generator 32a and a second DC generator 32b. In one embodiment, the first DC generator 32a is connected to the at least one lower electrode and configured to generate a first DC signal. The generated first DC signal is applied to the at least one lower electrode. In one embodiment, the second DC generator 32b is connected to the at least one upper electrode and configured to generate a second DC signal. The generated second DC signal is applied to the at least one upper electrode.

[0045] In various embodiments, the first and second DC signals may be pulsed. In this case, a sequence of voltage pulses is applied to at least one lower electrode and / or at least one upper electrode. The voltage pulses may have a rectangular, trapezoidal, triangular, or combination thereof pulse waveform. In one embodiment, a waveform generator for generating the sequence of voltage pulses from the DC signal is connected between the first DC generator 32a and at least one lower electrode. Thus, the first DC generator 32a and the waveform generator constitute a voltage pulse generator. When the second DC generator 32b and the waveform generator constitute a voltage pulse generator, the voltage pulse generator is connected to at least one upper electrode. The voltage pulses may have either positive or negative polarity. Furthermore, the sequence of voltage pulses may include one or more positive voltage pulses and one or more negative voltage pulses within one period. The first and second DC generating units 32a and 32b may be provided in addition to the RF power supply 31, or the first DC generating unit 32a may be provided instead of the second RF generating unit 31b.

[0046] The exhaust system 40 may be connected to, for example, a gas exhaust port 10e provided at the bottom of the plasma processing chamber 10. The exhaust system 40 may include a pressure regulating valve and a vacuum pump. The pressure in the plasma processing space 10s is regulated by the pressure regulating valve. The vacuum pump may include a turbomolecular pump, a dry pump, or a combination thereof.

[0047] In one embodiment, as shown in FIG. 3 , the substrate support 11 has a bias electrode 200. The bias electrode 200 is an example of a lower electrode. The bias electrode 200 is disposed within an electrostatic chuck 1111. The bias electrode 200 may be a conductive base 1110. In one embodiment, the plasma processing apparatus 1 has a voltage pulse signal generator 210 and an RF signal generator 211. The voltage pulse signal generator 210 may be an example of a first DC generating unit 32a. The voltage pulse signal generator 210 may constitute a part or all of a power supply system used in the plasma processing apparatus 1. The RF signal generator 211 may be an example of a first RF generating unit 31a.

[0048] The voltage pulse signal generator 210 is configured to generate a voltage pulse signal. The voltage pulse signal generator 210 is electrically connected to the bias electrode 200. The voltage pulse signal generator 210 can supply the generated voltage pulse signal to the bias electrode 200.

[0049] 4, the voltage pulse signal has a repeat cycle C(n) (n is an integer) that is repeated a given number of times. In one embodiment, each repeat cycle C(n) includes a first period T1, a first transition period S1, a second period T2, and a second transition period S2, in this order. The first transition period S1 may be shorter than the first period T1 and the second period T2. The second transition period S2 may be shorter than the first period T1 and the second period T2.

[0050] In one embodiment, as shown in FIG. 5, a first period T1 of the voltage pulse signal has a repeating sequence of first voltage pulses SC1. In the first period T1, the sequence of first voltage pulses SC1 may be intermittently repeated a given number of times. For example, the first period T1 has sub-repeating cycles CT1. Each of the sub-repeating cycles CT1 includes a first sub-period T1-1 having the first sequence of voltage pulses SC1 and a reference voltage level (v ｒｅｆ１ The sequence of first voltage pulses SC1 includes a plurality of first voltage pulses P1. The plurality of first voltage pulses P1 have a first pulse voltage level (v1) and a reference voltage level (v ｒｅｆ１ ) alternately and repeatedly. ｒｅｆ１ The absolute value of the reference voltage level (v) is smaller than the absolute value of the first pulse voltage level (v). The plurality of first voltage pulses P1 have a constant first pulse voltage level (v). In one embodiment, the first pulse voltage level (v) has a negative polarity. ｒｅｆ１ ) has a zero voltage level. In one embodiment, each of the plurality of first voltage pulses P1 may have a rectangular pulse waveform. Note that each of the plurality of first voltage pulses P1 may have various pulse waveforms other than rectangular, such as trapezoidal, triangular, or a combination thereof.

[0051] As shown in FIG. 4 , the first voltage pulse P1 in the first period T1 has a first duty ratio DU1 and a first frequency HE1. The first duty ratio DU1 and the first frequency HE1 of the multiple first voltage pulses P1 in the first period T1 may be constant. The first duty ratio DU1 is in the range of 1% to 99%. The first frequency HE1 is in the range of 100 kHz to 60 MHz. FIG. 6 is a diagram illustrating an example of the first voltage pulse P1. In one embodiment, the first duty ratio DU1 in FIG. 6 is the ratio of the ON time (the time during which the first pulse voltage level (v1) is applied) b1 to the period a1 of the first voltage pulse P1. In one embodiment, the first frequency HE1 corresponds to the reciprocal of the period a1.

[0052] In one embodiment, as shown in FIG. 7, the second period T2 of the voltage pulse signal has a repeating sequence of second voltage pulses SC2. In the second period T2, the sequence of second voltage pulses SC2 may be intermittently repeated a given number of times. For example, the second period T2 has sub-repeating cycles CT2. Each of the sub-repeating cycles CT2 includes a third sub-period T2-1 having the second sequence of voltage pulses SC2 and a reference voltage level (v ｒｅｆ２ The sequence of second voltage pulses SC2 includes a plurality of second voltage pulses P2. The plurality of second voltage pulses P2 have a second pulse voltage level (v2) and a reference voltage level (v ｒｅｆ２ ) alternately and repeatedly. ｒｅｆ２ The absolute value of the reference voltage level (v) is smaller than the absolute value of the second pulse voltage level (v2). The plurality of second voltage pulses P2 have a constant second pulse voltage level (v2). In one embodiment, the second pulse voltage level (v2) has a negative polarity. ｒｅｆ２ ) has a zero voltage level. In one embodiment, each of the plurality of second voltage pulses P2 may have a rectangular pulse waveform. Note that each of the plurality of second voltage pulses P2 may have various pulse waveforms other than rectangular, such as trapezoidal, triangular, or a combination thereof.

[0053] As shown in FIG. 4 , the second voltage pulse P2 in the second period T2 has a second duty ratio DU2 and a second frequency HE2. The second duty ratio DU2 and the second frequency HE2 of the multiple second voltage pulses P2 in the second period T2 may be constant. In one embodiment, the second duty ratio DU2 is greater than the first duty ratio DU1. In one embodiment, the second frequency HE2 is greater than the first frequency HE1. The second duty ratio DU2 is in the range of 1% to 99%. The second frequency HE2 is in the range of 100 kHz to 60 MHz. FIG. 8 is a diagram illustrating an example of the second voltage pulse P2. In one embodiment, the second duty ratio DU2 in FIG. 8 is the ratio of the ON time (the time during which the second pulse voltage level (v2) is applied) b2 to the period a2 of the second voltage pulse P2. In one embodiment, the second frequency HE2 corresponds to the reciprocal of the period a2. The period a2 of the second voltage pulse P2 is longer than the period a1 of the first voltage pulse P1.

[0054] In one embodiment, as shown in FIG. 4, the first transition period S1 of the voltage pulse signal is a transition period between the first period T1 and the second period T2. The first transition period S1 has a repeating third sequence of voltage pulses SC3 as shown in FIG. 9. In the first transition period S1, the third sequence of voltage pulses SC3 may be intermittently repeated a given number of times. For example, the first transition period S1 has sub-repeating cycles CS1. Each of the sub-repeating cycles CS1 includes a first sub-transition period S1-1 having the third sequence of voltage pulses SC3 and a reference voltage level (v ｒｅｆ３ The third sequence of voltage pulses SC3 includes a plurality of third voltage pulses P3. The plurality of third voltage pulses P3 have a third pulse voltage level (v3) and a reference voltage level (v ｒｅｆ３ ) alternately and repeatedly. ｒｅｆ３The absolute value of the third pulse voltage level (v3) is smaller than the absolute value of the third pulse voltage level (v3). The plurality of third voltage pulses P3 may have a constant third pulse voltage level (v3). In one embodiment, the third pulse voltage level (v3) has a negative polarity. ｒｅｆ３ ) has a zero voltage level. In one embodiment, each of the plurality of third voltage pulses P3 may have a rectangular pulse waveform. Note that each of the plurality of third voltage pulses P3 may have various pulse waveforms other than rectangular, such as trapezoidal, triangular, or a combination thereof.

[0055] As shown in Fig. 4, the duty ratio and frequency of the plurality of third voltage pulses P3 during the first transition period S1 gradually change from a first duty ratio DU1 and a first frequency HE1 to a second duty ratio DU2 and a second frequency HE2. In one embodiment, the duty ratio and frequency of the plurality of third voltage pulses P3 may increase stepwise for each third voltage pulse sequence SC3 shown in Fig. 9. That is, the plurality of third voltage pulses P3 included in each third voltage pulse sequence SC3 may have a constant duty ratio and frequency, and the duty ratio and frequency of the (k+1)th (k is an integer greater than or equal to 1) third voltage pulse sequence SC3 may be greater than the duty ratio and frequency of the immediately preceding (k)th third voltage pulse sequence SC3.

[0056] In one embodiment, as shown in FIG. 4, the second transition period S2 of the voltage pulse signal is a transition period after the second period T2 (between the second period T2 and the first period T1 of the next repeating cycle C(n)). The second transition period S2 has a fourth sequence of voltage pulses SC4 repeatedly, as shown in FIG. 10. In the second transition period S2, the fourth sequence of voltage pulses SC4 may be intermittently repeated a given number of times. For example, the second transition period S2 has sub-repeating cycles CS2. Each of the sub-repeating cycles CS2 includes a third sub-transition period S2-1 having the fourth sequence of voltage pulses SC4 and a reference voltage level (v ｒｅｆ４The fourth sequence of voltage pulses SC4 includes a plurality of fourth voltage pulses P4. The plurality of fourth voltage pulses P4 have a fourth pulse voltage level (v4) and a reference voltage level (v ｒｅｆ４ ) alternately and repeatedly. ｒｅｆ４ The absolute value of the fourth pulse voltage level (v4) is smaller than the absolute value of the fourth pulse voltage level (v4). The plurality of fourth voltage pulses P4 may have a constant fourth pulse voltage level (v4). In one embodiment, the fourth pulse voltage level (v4) has a negative polarity. ｒｅｆ４ ) has a zero voltage level. In one embodiment, each of the plurality of fourth voltage pulses P4 may have a rectangular pulse waveform. Note that each of the plurality of fourth voltage pulses P4 may have various pulse waveforms other than rectangular, such as trapezoidal, triangular, or a combination thereof.

[0057] As shown in Fig. 4 , the duty ratio and frequency of the plurality of fourth voltage pulses P4 during the second transition period S2 gradually change from the second duty ratio DU2 and the second frequency HE2 to the first duty ratio DU1 and the first frequency HE1. In one embodiment, the duty ratio and frequency of the plurality of fourth voltage pulses P4 may decrease stepwise with each fourth voltage pulse sequence SC4 as shown in Fig. 10 . That is, the plurality of fourth voltage pulses P4 included in each fourth voltage pulse sequence SC4 may have a constant duty ratio and frequency, and the duty ratio and frequency of the (k+1)th (k is an integer greater than or equal to 1) fourth voltage pulse sequence SC4 may be smaller than the duty ratio and frequency of the immediately preceding (k)th fourth voltage pulse sequence SC4.

[0058] The first to fourth pulse voltage levels (v1 to v4) in the first to fourth voltage pulse sequences SC1 to SC4 may be equal to or different from each other.

[0059] 3 may be electrically connected to the lower electrode or the upper electrode of the substrate support 11. The RF signal generator 211 is configured to generate a source RF signal for generating plasma and supply it to the lower electrode or the upper electrode. As shown in FIGS. 5, 7, 9, and 10, the source RF signal may be supplied in synchronization with the first to fourth voltage pulse sequences SC1 to SC4 of the voltage pulse signal.

[0060] <Example of Plasma Processing> Plasma processing is performed in the plasma processing apparatus 1. The plasma processing includes an etching process in which a film on a substrate W is etched using plasma. In one embodiment, the plasma processing is performed by the control unit 2 in the plasma processing apparatus 1.

[0061] In the plasma processing apparatus 1 shown in FIG. 2, first, the substrate W is carried into the chamber 10 by a transport arm, placed on the substrate support portion 11 by a lifter, and held on the substrate support portion 11 by suction.

[0062] Next, a processing gas is supplied to the plasma processing space 10s through the shower head 13 by the gas supply unit 20. The processing gas supplied at this time includes a gas that generates activated species necessary for etching the substrate W.

[0063] A source RF signal for generating plasma is generated by an RF signal generator 211 and supplied to the upper electrode and / or the lower electrode. A voltage pulse signal for attracting ion components in the plasma to the substrate is generated by a voltage pulse signal generator 210 and supplied to the bias electrode 200 of the substrate support 11. In this way, plasma is generated from the processing gas on the substrate support 11 in the plasma processing space 10s, and the substrate W is etched.

[0064] In plasma processing of each substrate W, the voltage pulse signal has a repeat cycle C(n) that is repeated a given number of times, as shown in Fig. 4. During a first period T1 of each repeat cycle C(n), a sequence SC1 of a plurality of first voltage pulses is intermittently and repeatedly supplied, as shown in Fig. 5. As shown in Fig. 4, during the first period T1, each first voltage pulse P1 included in the sequence SC1 of first voltage pulses has a first duty ratio DU1 and a first frequency HE1.

[0065] In the first period T1, the first duty ratio DU1 and the first frequency HE1 of the first voltage pulse P1 are relatively small compared to the second duty ratio DU2 and the second frequency HE2 of the second voltage pulse P2 in the second period T2. That is, the OFF time of the first voltage pulse P1 is lengthened. This prevents plasma ions from being attracted to the surface of the substrate, suppresses etching of the film on the substrate surface, and promotes the formation of a protective film on the substrate surface.

[0066] During the second period T2 of each repeat cycle C(n), a sequence of second voltage pulses SC2 is intermittently and repeatedly supplied, as shown in Fig. 7. During the second period T2, each second voltage pulse P2 included in the sequence of second voltage pulses SC2 has a second duty ratio DU2 and a second frequency HE2, as shown in Fig. 4.

[0067] In the second period T2, the second duty ratio DU2 and the second frequency HE2 of the second voltage pulse P2 are relatively large compared to the first duty ratio DU1 and the first frequency HE1 of the first voltage pulse P1 in the first period T1. That is, the OFF time of the second voltage pulse P2 is shortened. This increases the ion flow rate in the plasma, supplying more ions to the surface of the substrate and accelerating etching of the film on the substrate surface.

[0068] During the first transition period S1 of each repeat cycle C(n), a sequence SC3 of third voltage pulses is intermittently and repeatedly supplied, as shown in Fig. 9. During the first transition period S1, the duty ratio and frequency of the third voltage pulse P3 gradually change from a first duty ratio DU1 and a first frequency HE1 to a second duty ratio DU2 and a second frequency HE2, as shown in Fig. 4.

[0069] During the second transition period S2 of each repeat cycle C(n), a sequence of fourth voltage pulses SC4 is intermittently and repeatedly supplied, as shown in Fig. 10. As shown in Fig. 4, during the second transition period S2, the duty ratio and frequency of the fourth voltage pulse P4 gradually change from the second duty ratio DU2 and the second frequency HE2 to the first duty ratio DU1 and the first frequency HE1. Then, a repeat cycle C(n) consisting of the first period T1, the first transition period S1, the second period T2, and the second transition period S2 is repeated a given number of times.

[0070] According to this exemplary embodiment, a plasma processing apparatus 1 includes a chamber 10, a substrate support 11 having a bias electrode 200, an RF signal generator 211 configured to generate an RF signal to generate a plasma in the chamber 10, and a voltage pulse signal generator 210 electrically connected to the bias electrode 200 and configured to generate a voltage pulse signal. The voltage pulse signal generated by the voltage pulse signal generator 210 has a repeating cycle C, each of which includes a first period T1 having a repeating sequence of first voltage pulses SC1 and a second period T2 having a repeating sequence of second voltage pulses SC2, wherein each first voltage pulse P1 included in the first sequence of voltage pulses SC1 has a first duty ratio DU1 and a first frequency HE1, and each second voltage pulse P2 included in the second sequence of voltage pulses SC2 has a second duty ratio DU2 and a second frequency HE2. The first duty ratio DU1 is different from the second duty ratio DU2, and the first frequency HE1 is different from the second frequency HE2. This causes the OFF time (ON time) of the voltage pulse to vary between the first period T1 and the second period T2, making it possible to control the substrate processing results. As a result, it is possible to improve the process performance of the plasma processing.

[0071] During the first transition period S1, the duty ratio and frequency of the plurality of third voltage pulses P3 gradually change from a first duty ratio DU1 and a first frequency HE1 to a second duty ratio DU2 and a second frequency HE2. Changing the duty ratio and frequency of the voltage pulses between the first period T1 and the second period T2 causes a change in impedance, including that of the plasma, in the plasma processing apparatus 1, which may result in the generation of reflected waves due to the impedance change. By gradually changing the duty ratio and frequency of the voltage pulses from the first duty ratio DU1 and the first frequency HE1 to the second duty ratio DU2 and the second frequency HE2 during the first transition period S1, the generation of reflected waves due to the impedance change can be suppressed. This stabilizes the plasma and improves plasma processing performance.

[0072] During the second transition period S2, the duty ratio and frequency of the plurality of fourth voltage pulses P4 gradually change from the second duty ratio DU2 and the second frequency HE2 to the first duty ratio DU1 and the first frequency HE1, thereby suppressing the generation of reflected waves due to impedance fluctuations, thereby stabilizing the plasma and improving the plasma processing performance.

[0073] In the above embodiments, the second duty ratio DU2 is greater than the first duty ratio DU1, and the second frequency HE2 is greater than the first frequency HE1, but the second duty ratio DU2 may be smaller than the first duty ratio DU1, and the second frequency HE2 may be smaller than the first frequency HE1.

[0074] In the above embodiment, the first voltage pulse P1 in the first period T1 and the second voltage pulse P2 in the second period T2 have different duty ratios and frequencies, but either one of them may be different. That is, the first duty ratio DU1 and the second duty ratio DU2 may be different and the first frequency HE1 and the second frequency HE2 may be the same, or the first duty ratio DU1 and the second duty ratio DU2 may be the same and the first frequency HE1 and the second frequency HE2 may be different.

[0075] In the above embodiment, a capacitively coupled plasma device has been described as an example, but the present invention is not limited thereto and may be applied to other plasma devices. For example, an inductively coupled plasma device may be used instead of the capacitively coupled plasma device. In this case, the inductively coupled plasma device includes an antenna and a lower electrode. The lower electrode is disposed within the substrate support, and the antenna is disposed above or at the top of the chamber. In one embodiment, the RF signal generator 211 is electrically connected to the antenna, and the voltage pulse signal generator 210 is electrically connected to the lower electrode.

[0076] Embodiments of the present disclosure further include the following aspects.

[0077] (Supplementary Note 1) A plasma processing apparatus comprising: a chamber; a substrate support disposed within the chamber and having a bias electrode; an RF signal generator configured to generate an RF signal to generate a plasma in the chamber; and a voltage pulse signal generator electrically connected to the bias electrode and configured to generate a voltage pulse signal, the voltage pulse signal having repeating cycles, each repeating cycle including a first period having a repeating sequence of first voltage pulses and a second period having a repeating sequence of second voltage pulses, each first voltage pulse included in the first sequence of voltage pulses having a first duty ratio and a first frequency, and each second voltage pulse included in the second sequence of voltage pulses having a second duty ratio and a second frequency, the first duty ratio being different from the second duty ratio and / or the first frequency being different from the second frequency.

[0078] (Supplementary Note 2) The plasma processing apparatus of Supplementary Note 1, wherein each of the repeating cycles includes a first transition period having a plurality of third voltage pulses between the first period and the second period, and when the first duty ratio is different from the second duty ratio, the duty ratio of the plurality of third voltage pulses gradually changes from the first duty ratio to the second duty ratio, and when the first frequency is different from the second frequency, the frequency of the plurality of third voltage pulses gradually changes from the first frequency to the second frequency.

[0079] (Supplementary Note 3) The plasma processing apparatus according to Supplementary Note 2, wherein the first transition period includes a repeated sequence of third voltage pulses, and the duty ratio and / or frequency of the plurality of third voltage pulses included in the sequence of third voltage pulses changes stepwise for each sequence of the third voltage pulses.

[0080] (Supplementary Note 4) The plasma processing apparatus according to any one of Supplementary Notes 1 to 3, wherein each of the repeating cycles includes, after the second period, a second transition period having a plurality of fourth voltage pulses, and when the first duty ratio is different from the second duty ratio, the duty ratio of the plurality of fourth voltage pulses gradually changes from the second duty ratio to the first duty ratio, and when the first frequency is different from the second frequency, the frequency of the plurality of fourth voltage pulses gradually changes from the second frequency to the first frequency.

[0081] (Supplementary Note 5) The plasma processing apparatus according to Supplementary Note 4, wherein the second transition period includes a repeated sequence of fourth voltage pulses, and the duty ratio and / or frequency of the plurality of fourth voltage pulses included in the sequence of fourth voltage pulses changes stepwise for each sequence of fourth voltage pulses.

[0082] (Supplementary Note 6) The plasma processing apparatus according to any one of Supplementary Notes 1 to 5, wherein the second duty ratio is greater than the first duty ratio and / or the second frequency is greater than the first frequency.

[0083] (Supplementary Note 7) The plasma processing apparatus according to any one of Supplementary Notes 1 to 5, wherein the second duty ratio is smaller than the first duty ratio and / or the second frequency is smaller than the first frequency.

[0084] (Supplementary Note 8) The plasma processing apparatus according to any one of Supplementary Notes 1 to 7, wherein the first sequence of voltage pulses has a first voltage level, the second sequence of voltage pulses has a second voltage level, and the first voltage level and the second voltage level are equal.

[0085] (Supplementary Note 9) The plasma processing apparatus according to any one of Supplementary Notes 1 to 7, wherein the first sequence of voltage pulses has a first voltage level, the second sequence of voltage pulses has a second voltage level, and the first voltage level is different from the second voltage level.

[0086] (Supplementary Note 10) The plasma processing apparatus according to Supplementary Note 8 or 9, wherein the first voltage level and the second voltage level have negative polarities.

[0087] (Supplementary Note 11) A power supply system for use in a plasma processing apparatus, comprising: a voltage pulse signal generator configured to generate a voltage pulse signal, the voltage pulse signal having repeating cycles, each of the repeating cycles including a first period having a repeating sequence of first voltage pulses and a second period having a repeating sequence of second voltage pulses, each first voltage pulse included in the sequence of first voltage pulses having a first duty ratio and a first frequency, each second voltage pulse included in the sequence of second voltage pulses having a second duty ratio and a second frequency, the first duty ratio being different from the second duty ratio and / or the first frequency being different from the second frequency.

[0088] (Supplementary Note 12) The power supply system of Supplementary Note 11, wherein each of the repeating cycles includes a first transition period having a plurality of third voltage pulses between the first period and the second period, and when the first duty ratio is different from the second duty ratio, the duty ratio of the plurality of third voltage pulses gradually changes from the first duty ratio to the second duty ratio, and when the first frequency is different from the second frequency, the frequency of the plurality of third voltage pulses gradually changes from the first frequency to the second frequency.

[0089] (Supplementary Note 13) The power supply system according to Supplementary Note 12, wherein the first transition period includes a repeated sequence of third voltage pulses, and a duty ratio and / or a frequency of the plurality of third voltage pulses included in the sequence of third voltage pulses changes stepwise for each sequence of third voltage pulses.

[0090] (Supplementary Note 14) The power supply system of any one of Supplementary Notes 11 to 13, wherein each of the repeating cycles includes, after the second period, a second transition period having a plurality of fourth voltage pulses, and when the first duty ratio is different from the second duty ratio, the duty ratio of the plurality of fourth voltage pulses gradually changes from the second duty ratio to the first duty ratio, and when the first frequency is different from the second frequency, the frequency of the plurality of fourth voltage pulses gradually changes from the second frequency to the first frequency.

[0091] (Supplementary Note 15) The power supply system according to Supplementary Note 14, wherein the second transition period includes a repeated sequence of fourth voltage pulses, and a duty ratio and / or a frequency of the plurality of fourth voltage pulses included in the sequence of fourth voltage pulses changes stepwise for each sequence of fourth voltage pulses.

[0092] (Supplementary Note 16) The power supply system according to any one of Supplementary Notes 11 to 15, wherein the second duty ratio is greater than the first duty ratio and / or the second frequency is greater than the first frequency.

[0093] (Supplementary Note 17) The power supply system according to any one of Supplementary Notes 11 to 15, wherein the second duty ratio is smaller than the first duty ratio and / or the second frequency is smaller than the first frequency.

[0094] (Supplementary Note 18) The power supply system described in any one of Supplementary Notes 11 to 17, wherein the first sequence of voltage pulses has a first voltage level, the second sequence of voltage pulses has a second voltage level, and the first voltage level and the second voltage level are equal.

[0095] (Supplementary Note 19) The power supply system of any one of Supplementary Notes 11 to 17, wherein the first sequence of voltage pulses has a first voltage level, the second sequence of voltage pulses has a second voltage level, and the first voltage level is different from the second voltage level.

[0096] (Supplementary Note 20) The power supply system according to Supplementary Note 18 or 19, wherein the first voltage level and the second voltage level have negative polarity.

[0097] The above embodiments are described for the purpose of explanation and are not intended to limit the scope of the present disclosure. Various modifications can be made to the above embodiments without departing from the scope and spirit of the present disclosure. For example, some components in one embodiment can be added to other embodiments. Also, some components in one embodiment can be replaced with corresponding components in other embodiments.

[0098] 1...plasma processing apparatus, 10...chamber, 10s...plasma processing space, 11...substrate support, 200...bias electrode, 210...voltage pulse signal generator, 211...RF signal generator, T1...first period, T2...second period, S1...first transition period, S2...second transition period, SC1...first voltage pulse sequence, SC2...second voltage pulse sequence, P1...first voltage pulse, P2...second voltage pulse, DU1...first duty ratio, HE1...first frequency, DU2...second duty ratio, HE2...second frequency, W...substrate

Claims

1. A plasma processing apparatus comprising: a chamber; a substrate support disposed within the chamber and having a bias electrode; an RF signal generator configured to generate an RF signal to generate a plasma in the chamber; and a voltage pulse signal generator electrically connected to the bias electrode and configured to generate a voltage pulse signal, the voltage pulse signal having repeating cycles, each repeating cycle including a first period having a repeating sequence of first voltage pulses and a second period having a repeating sequence of second voltage pulses, each first voltage pulse in the first sequence of voltage pulses having a first duty ratio and a first frequency, and each second voltage pulse in the second sequence of voltage pulses having a second duty ratio and a second frequency, the first duty ratio being different from the second duty ratio and / or the first frequency being different from the second frequency.

2. The plasma processing apparatus of claim 1, wherein each of the repeating cycles includes a first transition period having a plurality of third voltage pulses between the first period and the second period, and when the first duty ratio is different from the second duty ratio, the duty ratio of the plurality of third voltage pulses gradually changes from the first duty ratio to the second duty ratio, and when the first frequency is different from the second frequency, the frequency of the plurality of third voltage pulses gradually changes from the first frequency to the second frequency.

3. The plasma processing apparatus according to claim 2, wherein the first transition period includes a repeated sequence of third voltage pulses, and the duty ratio and / or frequency of the plurality of third voltage pulses included in the sequence of third voltage pulses changes stepwise for each sequence of third voltage pulses.

4. The plasma processing apparatus of claim 2, wherein each of the repeating cycles includes, after the second period, a second transition period having a plurality of fourth voltage pulses, and when the first duty ratio is different from the second duty ratio, the duty ratio of the plurality of fourth voltage pulses gradually changes from the second duty ratio to the first duty ratio, and when the first frequency is different from the second frequency, the frequency of the plurality of fourth voltage pulses gradually changes from the second frequency to the first frequency.

5. The plasma processing apparatus according to claim 4, wherein the second transition period includes a repeated sequence of fourth voltage pulses, and the duty ratio and / or frequency of the plurality of fourth voltage pulses included in the sequence of fourth voltage pulses changes stepwise for each sequence of fourth voltage pulses.

6. The plasma processing apparatus according to any one of claims 1 to 5, wherein the second duty ratio is greater than the first duty ratio and / or the second frequency is greater than the first frequency.

7. The plasma processing apparatus according to claim 1, wherein the second duty ratio is smaller than the first duty ratio and / or the second frequency is smaller than the first frequency.

8. The plasma processing apparatus of claim 1, wherein the first sequence of voltage pulses has a first voltage level, the second sequence of voltage pulses has a second voltage level, and the first voltage level and the second voltage level are equal.

9. The plasma processing apparatus of claim 1, wherein the first sequence of voltage pulses has a first voltage level, the second sequence of voltage pulses has a second voltage level, and the first voltage level is different from the second voltage level.

10. The plasma processing apparatus according to claim 8 or 9, wherein the first voltage level and the second voltage level have negative polarities.

11. A power supply system for use in a plasma processing apparatus, comprising: a voltage pulse signal generator configured to generate a voltage pulse signal, the voltage pulse signal having repeating cycles, each of the repeating cycles including a first period having a repeating sequence of first voltage pulses and a second period having a repeating sequence of second voltage pulses, each first voltage pulse included in the sequence of first voltage pulses having a first duty ratio and a first frequency, each second voltage pulse included in the sequence of second voltage pulses having a second duty ratio and a second frequency, the first duty ratio being different from the second duty ratio and / or the first frequency being different from the second frequency.

12. The power supply system of claim 11, wherein each of the repeating cycles includes a first transition period having a plurality of third voltage pulses between the first period and the second period, wherein if the first duty ratio is different from the second duty ratio, the duty ratio of the plurality of third voltage pulses gradually changes from the first duty ratio to the second duty ratio, and if the first frequency is different from the second frequency, the frequency of the plurality of third voltage pulses gradually changes from the first frequency to the second frequency.

13. The power supply system according to claim 12, wherein the first transition period includes a repeating sequence of third voltage pulses, and the duty ratio and / or frequency of the plurality of third voltage pulses included in the sequence of third voltage pulses changes stepwise for each sequence of third voltage pulses.

14. The power supply system of claim 12, wherein each of the repeating cycles includes, after the second period, a second transition period having a plurality of fourth voltage pulses, wherein if the first duty ratio is different from the second duty ratio, the duty ratio of the plurality of fourth voltage pulses gradually changes from the second duty ratio to the first duty ratio, and if the first frequency is different from the second frequency, the frequency of the plurality of fourth voltage pulses gradually changes from the second frequency to the first frequency.

15. The power supply system according to claim 14, wherein the second transition period includes a repeated sequence of fourth voltage pulses, and the duty ratio and / or frequency of the plurality of fourth voltage pulses included in the sequence of fourth voltage pulses changes stepwise for each sequence of fourth voltage pulses.

16. The power supply system according to any one of claims 11 to 15, wherein the second duty ratio is greater than the first duty ratio and / or the second frequency is greater than the first frequency.

17. The power supply system according to any one of claims 11 to 15, wherein the second duty ratio is smaller than the first duty ratio and / or the second frequency is smaller than the first frequency.

18. The power supply system of claim 11, wherein the first sequence of voltage pulses has a first voltage level, and the second sequence of voltage pulses has a second voltage level, and the first voltage level and the second voltage level are equal.

19. The power supply system of claim 11, wherein the first sequence of voltage pulses has a first voltage level, and the second sequence of voltage pulses has a second voltage level, and the first voltage level is different from the second voltage level.

20. The power supply system of claim 18 or 19, wherein the first voltage level and the second voltage level have negative polarity.

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