KR20190048234A - Impedance Matching Method of RF Power - Google Patents

Abstract

An impedance matching method of RF power according to an embodiment of the present invention comprises: a step of providing variable frequency RF power to a load through an impedance matching network; a step of measuring reflected power while sweeping a driving frequency of the variable frequency RF power; a step of extracting a minimum reflected wave driving frequency at which the reflected residual power is minimized; a step of fitting a reflectance, which is the ratio of the incident power and reflected power, to the difference between the driving frequency and the minimum reflected wave driving frequency with a quadratic function around the minimum reflected wave driving frequency and extracting the actual resistance of the load and the capacitance component of the load; a step of calculating a value of a first capacitance of a first variable reactance element and a value of a second capacitance of a second variable reactance element constituting the impedance matching network at a target driving frequency by using the actual resistance of the load, the capacitance component of the load, and the previously determined inductance component of the load; and a step of adjusting the first variable reactance element with the calculated value of the first capacitance of the first variable reactance element and adjusting the second variable reactance element with the calculated value of the second capacitance of the second variable reactance element, and changing the driving frequency of the variable frequency RF power to the target driving frequency.

Description

TECHNICAL FIELD [0001] The present invention relates to an impedance matching method for RF power,

Field of the Invention [0002] The present invention relates to an RF power source and an impedance matching network, and more particularly, to a method of impedance matching of a plasma that is a time-varying load.

Conventionally, a value corresponding to an RF reflection coefficient is obtained with an RF sensor in an RF impedance matching network, and variable elements of an RF impedance matching network are driven through a calculation algorithm implemented in the RF impedance matching network to perform impedance matching .

Conventional impedance matching networks perform impedance matching in the ignition and sustain phase of plasma, which is a time-varying load. Conventional impedance matching networks are focused on uniformly supplying RF power in terms of time. When the cycle time is several tens of seconds, the duration of the plasma ignition step is less than 1 second and the retention time is several tens of seconds, so it is important to keep the reflection coefficient below a certain value during the holding time. Also, since the drive frequency is fixed, it is important to have an RF sensor technology that accurately measures the reflection coefficient at a fixed drive frequency.

On the other hand, when variable frequency matching is performed, the value of the RF sensor may be changed. In addition, stable ignition algorithms through precise variable changes are important in the ignition phase. In performing variable frequency matching, precise measurement of the reactance value of the elements of the impedance matching network is not required. Therefore, it is difficult to manage the history of changes in characteristics of plasma, which is a load.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an impedance matching method capable of simultaneously performing impedance matching while monitoring changes in load characteristics.

A method of impedance matching RF power according to an embodiment of the present invention includes: providing RF power to a load through a variable frequency RF power source through an impedance matching network; Measuring reflected power while sweeping a driving frequency of the variable frequency RF power supply; Extracting a minimum reflected wave driving frequency at which the residual reflection power becomes minimum; A reflection coefficient, which is a ratio of incident power to reflected power, is fitted to the difference between the driving frequency and the minimum reflected wave driving frequency with a quadratic function around the minimum reflected wave driving frequency, and the capacitance of the load ; The value of the first capacitance of the first variable reactance element constituting the impedance matching network at the target drive frequency and the value of the second capacitance of the first variable reactance element constituting the impedance matching network at the target drive frequency are calculated using the actual resistance of the load, the capacitance of the load, and the already determined inductance component of the load. Calculating a value of a second capacitance of the two-variable reactance element; And adjusting the first variable reactance element with the calculated value of the first capacitance of the first variable reactance element and adjusting the second variable reactance element with the calculated value of the second capacitance of the second variable reactance element And changing the driving frequency of the variable frequency RF power supply to the target driving frequency.

In one embodiment of the present invention, the first variable reactance element is adjusted by the calculated value of the first capacitance of the first variable reactance element and the calculated value of the second capacitance of the second variable reactance element The step of adjusting the second variable reactance element and changing the driving frequency of the variable frequency RF power supply to the target driving frequency may be performed with the variable frequency RF power removed.

In one embodiment of the present invention, the load may generate a capacitively coupled plasma and periodically generate a capacitively coupled plasma.

The impedance matching method according to an embodiment of the present invention can detect a characteristic change of a load by extracting a real resistance component of a load and a capacitance component of the load.

1 is a circuit diagram illustrating an impedance matching network according to an embodiment of the present invention.
FIG. 2 is a circuit diagram showing a serial variable element and a load of the impedance matching network of FIG. 1 as one element.
Fig. 3 is a circuit diagram showing the circuit of Fig. 2 in terms of admittance.
4 is a flowchart illustrating an impedance matching method according to an embodiment of the present invention.
5 is a timing diagram illustrating an impedance matching method according to an embodiment of the present invention.

The impedance matching method according to an embodiment of the present invention can achieve fast matching through frequency modulation. The reflectance is fitted as a quadratic function of the difference between the driving frequency and the minimum reflective driving frequency. Thus, the real resistance component and the capacitance component of the load are extracted. When the impedance of the load is extracted, the variable reactance of the impedance matching network is calculated through the analytical equation at the target driving frequency. Accordingly, the real resistance component and the capacitance component of the load directly related to the plasma process can be used for the history management of the plasma equipment.

The present invention relates to measuring frequency modulation and power reflectivity, adjusting the variable reactance of an impedance matching network, and setting the driving frequency to a target driving frequency. The frequency can be modulated to find the minimum reflective drive frequency at which the power reflectance is minimum, where the power reflectance is fitted to a quadratic function (parabola) of the drive frequency difference based on the minimum reflective drive frequency. Using the coefficients of this fitting function, the real resistance of the load and the capacitance component of the load are extracted. When the impedance of the load is fully known, the reactances of the variable reactance elements constituting the impedance matching network at the target driving frequency are determined through an interpolation equation. Thus, by adjusting the reactances of the variable reactance elements, the impedance matching is completed at the target drive frequency.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS The advantages and features of the present invention and the manner of achieving them will become apparent with reference to the embodiments described in detail below with reference to the accompanying drawings. However, the present invention is not limited to the embodiments described herein but may be embodied in different forms. Rather, the embodiments disclosed herein are provided so that the disclosure can be thorough and complete, and will fully convey the concept of the invention to those skilled in the art, and the invention is only defined by the scope of the claims.

Like reference numerals refer to like elements throughout the specification. Accordingly, although the same reference numerals or similar reference numerals are not mentioned or described in the drawings, they may be described with reference to other drawings. Further, even if the reference numerals are not shown, they can be described with reference to other drawings.

1 is a circuit diagram illustrating an impedance matching network according to an embodiment of the present invention.

Referring to FIG. 1, a variable frequency RF power source 110 supplies RF power to a load 130 through an impedance matching network. The impedance matching network 120 and the variable frequency RF power source 110 vary the driving frequency f and the variable reactance elements 122 and 124 to perform impedance matching. The variable frequency RF power source 110 measures reflected waves or reflected power.

The impedance of the first variable reactance element 122 connected in parallel to the variable frequency RF power supply 110 is represented by a ₁ (f). The impedance of the second variable-reactance element 124 connected in series with the load 130 is represented by ia ₂ (f). Further, the impedance of the load is expressed by r (f) + ic (f). r (f) is the actual resistance of the load at the driving frequency f, and c (f) is the reactance of the load at the driving frequency f.

The processing unit 140 receives the reflectance or the reflected power according to the frequency in the variable frequency RF power source 110 and calculates the first capacitance of the first variable reactance element constituting the impedance matching network at the target driving frequency f _T And the value (C2_cal) of the second capacitance of the second variable reactance element to the first variable reactance element and the second variable reactance element, respectively. The first variable reactance element is changed to the first capacitance value C1_cal and the second variable reactance element is changed to the second capacitance value C2_cal.

FIG. 2 is a circuit diagram showing a serial variable element and a load of the impedance matching network of FIG. 1 as one element.

2, the impedance of the first variable-reactance element 122 is a ₁ (f), and the total impedance 230 of the second variable-reactance element and the load is represented by r (f) + ib (f) . b = a + c is _2.

Fig. 3 is a circuit diagram showing the circuit of Fig. 2 in terms of admittance.

Referring to FIG. 3, A = 1 / a, G = r / (r ² + b ² ), and B = b / (r ² + b ² ).

The reflection coefficient (Γ) is given by

[Equation 1]

Here, Z is the total impedance viewed from the output of the variable frequency RF power supply 110 to the load. Z ₀ is the characteristic impedance of the transmission line. Typically, Z ₀ is 50 ohms. z is the total impedance normalized to the characteristic impedance (Z ₀ ). g is the normalized total admittance.

The reflectance (| Γ | ² ) is given by

&Quot; (2) "

In order to perform the impedance matching, the second reactance (a ₂₎ and the driving frequency of the first reactance (a ₁₎ and a second variable reactance element 124, a first variable reactance element 122 with respect to the time-varying load 130 (f) is adjusted so that the reflected power is minimized. Further, the drive frequency is controlled to be close to the target drive frequency f _T already set.

That is, the ultimate goal of impedance matching is to minimize reflected power or reflectivity. Specifically, the target drive frequency (f _T) and G (f _T) = 1, A (f _T) from the target drive frequency (f _T) is B (f _T) if they match, the reflected power is minimized. That is, these conditions are expressed as follows.

&Quot; (3) "

(R (f ₀ )) of the load 130 and the reactance c (f ₀ ) of the load 130 at the minimum reflected wave driving frequency f ₀ at which the reflected wave or reflectance is minimum, The first reactance (a ₁ ) of the first variable reactance element and the second reactance (a ₂ ) of the second variable reactance element for better matching at the frequency (f _T ) are extracted.

Specifically, when the actual resistance (r) of the load is sufficiently smaller than 1 (r << 1), G and B are expressed as follows. Typically, when the load 130 is a plasma, the field resistance is less than 1 ohm. Since the room resistance is less than 1 ohm, the normalization room resistance represented by r is less than 0.02 (r < 0.02 < 1)

&Quot; (4) &quot;

Further, the condition (1 + G) ² &quot; (AB) ² is satisfied under the condition that the impedance matching is almost made, and therefore, the reflectance (|? | ² ) is expressed as follows.

&Quot; (5) &quot;

When the condition A = B is satisfied, the local minimum value at the current driving frequency (f ₀ ) is expressed as follows.

&Quot; (6) &quot;

When Taylor Expansion is performed around the minimum reflected wave driving frequency (f ₀ ), it is expressed as follows.

&Quot; (7) &quot;

Here, Δ _f, m, and K is defined as:

&Quot; (8) &quot;

m? _f << 1, if G (f) = G ₀ = approximate to a constant, it is expressed as follows.

&Quot; (9) &quot;

Therefore, the reflectance (| Γ | ²⁾ is a Δ with a minimum at _f = f ₀ f Is a quadratic function of.

The first variable reactance element 122 may include a first inductor having a fixed first inductance L ₁ and a first variable capacitor having a first capacitance C ₁ . The second variable reactance element 124 may include a second inductor having a fixed second inductance L ₂ and a second variable capacitor having a second capacitance C ₂ . In addition, the reactance of the load 130 may include an inductance component L _f and a capacitance component C _r . The inductance component L _f of the load 130 may be due to the power supply line. Also, the capacitor component C _r of the load 130 may be caused by the sheath of the capacitor-coupled plasma electrode. The inductance component L _f of the load 130 may be determined by measurement or calculation with a fixed value. In this case, the reactance components (a, b) of the impedance can be expressed as follows. Here,? Is the driving angular frequency.

&Quot; (10) &quot;

The derivative of the reactance component (a, b) of the impedance with respect to the driving frequency f is given as follows.

&Quot; (11) &quot;

Therefore, the reflectance (|? | ² ) is expressed as follows.

&Quot; (12) &quot;

Here, x, Q, and M are defined as follows.

&Quot; (13) &quot;

That is, the reflectance (| Γ | ²⁾ to _{x (= (ff 0) /} f 0) fitting (fitting) can be obtained with respect to the Q value. The capacitance component (C _r ) of the load 130 can be obtained under the condition of a ₀ = b ₀ at the minimum reflected wave driving frequency (f ₀ ). In addition, the actual resistance r of the load 130 can be obtained from the Q value.

That is, when the actual resistance r of the load 130 is extracted and the capacitance component C _{r of} the load 130 is extracted, the impedance of the load 130 is determined. That is, the inductance component L _f of the load 130 is already measured or given by calculation.

When the impedance of the load 130 is determined, the first variable reactance (C ₁ ) and the second variable reactance (C ₂ ) of the impedance matching network circuit at the target driving frequency (f _T ) are analytical problems in which a solution exists. The interpretation solution is disclosed in "Microwave Engineering", David M. Pozar, page 282.

Thus, the first variable reactance element is adjusted so as to have the determined first variable reactance _{C1_cal} , and the second variable reactance element is adjusted to have the determined second variable reactance _{C2_cal} . Thus, the impedance matching is completed at the target drive frequency f _T.

On the other hand, since the load is a time-varying load, it is possible to repeat the above operation repeatedly with a constant cycle.

4 is a flowchart illustrating an impedance matching method according to an embodiment of the present invention.

5 is a timing diagram illustrating an impedance matching method according to an embodiment of the present invention.

Referring to FIGS. 4 and 5, a method of impedance matching RF power includes: providing a variable frequency RF power source 110 with RF power to a load through an impedance matching network 120; Measuring reflected power while sweeping the driving frequency of the variable frequency RF power supply 110 (S11); Extracting a minimum reflected wave driving frequency at which the reflected power is minimized (S12); The reflectance, which is the ratio of the incident power to the reflected power, is fitted to the difference between the driving frequency and the minimum reflected wave driving frequency with a quadratic function around the minimum reflected wave driving frequency, and the actual resistance of the load 130 and the load 130 A step (S13) of extracting a capacitance component of the received signal; The first variable reactance component constituting the impedance matching network 120 at the target drive frequency using the actual resistance of the load 130, the capacitance component of the load 130, and the previously determined inductance component of the load 130 Calculating (S14) the value of the first capacitance of the element (122) and the value of the second capacitance of the second variable reactance element (124); And the first variable reactance element (122) _adjusts the first variable reactance element (122) by a calculated value (C _{1_cal} ) of the first capacitance of the first variable reactance element (122) ( _S15 ) of adjusting the second variable reactance element 124 to a calculated value C _{2_cal of} the variable frequency RF power source 110 and changing the driving frequency of the variable frequency RF power source 110 to the target driving frequency f _T .

(C _{1_cal} ) of the first capacitance of the first variable reactance element 122 and the second capacitance C _{2_cal of} the second variable reactance element of the second variable reactance element ₁₂₂ And the step of changing the driving frequency of the variable frequency RF power supply 110 to the target driving frequency f _T may include the step of controlling the variable frequency RF power supply 110 to adjust the second variable reactance element 124, And is performed in a state where it is removed.

The load 130 generates a capacitively coupled plasma and periodically generates a capacitively coupled plasma.

While the present invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, It will be understood. It is therefore to be understood that the above-described embodiments are illustrative and non-restrictive in every respect.

110: Variable Frequency RF Power
120: Impedance Matching Network
122: first variable reactance element
124: second variable reactance element
130: Plasma load

Claims (3)

Providing variable frequency RF power to the load through an impedance matching network;
Measuring reflected power while sweeping a driving frequency of the variable frequency RF power supply;
Extracting a minimum reflected wave driving frequency at which the reflected power is minimized;
A reflection coefficient, which is a ratio of incident power to reflected power, is fitted to the difference between the driving frequency and the minimum reflected wave driving frequency with a quadratic function around the minimum reflected wave driving frequency, and the capacitance of the load ;
The value of the first capacitance of the first variable reactance element constituting the impedance matching network at the target drive frequency and the value of the second capacitance of the first variable reactance element constituting the impedance matching network at the target drive frequency are calculated using the actual resistance of the load, the capacitance of the load, and the already determined inductance component of the load. Calculating a value of a second capacitance of the two-variable reactance element; And
The first variable reactance element is adjusted by the calculated value of the first capacitance of the first variable reactance element and the second variable reactance element is adjusted by the calculated value of the second capacitance of the second variable reactance element And changing the driving frequency of the variable frequency RF power supply to the target driving frequency. The method according to claim 1,
The first variable reactance element is adjusted by the calculated value of the first capacitance of the first variable reactance element and the second variable reactance element is adjusted by the calculated value of the second capacitance of the second variable reactance element And changing the driving frequency of the variable frequency RF power source to the target driving frequency is performed in a state where the variable frequency RF power source is removed. The method according to claim 1,
Wherein the load generates a capacitively coupled plasma and periodically generates a capacitively coupled plasma.

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