JP2005063760A - Plasma treatment method and treatment device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a plasma treatment method setting the power-feeding frequency range between electrodes so as to be able to obtain a stable discharge and high output efficiency. <P>SOLUTION: The plasma treatment device has an electrode circuit 1 composed of a secondary coil 22b of a transformer 22 boosting power source voltage, and a pair of electrodes 11, 12 facing each other. A solid dielectric 13 is arranged on at least on of the opposing surfaces of the electrodes 11, 12. The electrode circuit 1 constitutes an LC serial resonance circuit by leakage inductance of the secondary coil and capacitance of the electrodes. The energization frequency of the electrode circuit 1 is set between a resonance frequency when not discharging and a resonance frequency at the time when a space 10a between the electrodes can be deemed as a conductor. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a method and apparatus for forming a plasma by glow discharge or the like in a pressure (substantially normal pressure) environment near atmospheric pressure, for example, and surface-treating an object to be processed such as a semiconductor substrate.

Various methods of performing plasma treatment by glow discharge under a pressure near atmospheric pressure have been proposed (see Patent Documents 1 to 3 below). For example, in Patent Documents 1 and 2, a pair of electrodes are arranged to face each other, and a solid dielectric is placed on the facing surface of at least one of the electrodes. Then, a pulse electric field is applied between the electrodes under a pressure near atmospheric pressure. The frequency of the pulse is preferably set in the range of 0.5 kHz to 100 kHz. Thereby, glow discharge plasma can be formed and stable surface treatment can be performed.
Moreover, in the thing of patent document 3, an alternating current electric field is applied between a pair of electrodes in an air atmosphere. Then, the ratio between the capacitance per unit area of the solid dielectric and the AC frequency is set to 1400 pF / (m ² · kHz) or less.

Japanese Patent Laid-Open No. 10-36537 JP-A-10-154598 JP 2001-284099 A

  In the above-described conventional method, the range of the frequency and the like is indicated by specific numerical values, but these numerical values are meaningful only when the waveform of the electric field, the processing gas, etc. are predetermined, and are not general-purpose. . Further, the output efficiency of the power supply device is not taken into consideration, and the loss may increase.

  In order to solve the above problems, the inventor has intensively studied and considered. That is, an electrode structure composed of a pair of counter electrodes can be considered as a series connection of the impedance of the interelectrode space and the capacitance of the solid dielectric provided on the counter surface of at least one of the electrodes. In general, a transformer is interposed between the electrode structure and the power source, and a hot-side electrode is connected to a secondary coil of the transformer. Since the transformer has a leakage inductance, it can be considered that this and the electrode structure constitute an LC series resonance circuit. As is well known, in the LC series resonance circuit, the output can be maximized when the power source is driven at the resonance frequency. On the other hand, in the electrode structure, the capacitance of the solid dielectric can be easily obtained, but the impedance of the interelectrode space varies depending on the state of the plasma and the like, and it is not easy to directly analyze this.

By the way, the impedance of the inter-electrode space is also constant if it is not discharged. The resonance frequency at this time can be obtained by calculation if the physical properties such as the dielectric constant of the processing gas are known, and of course, it can be easily obtained by actual measurement.
When the discharge starts, it is considered that the impedance of the interelectrode space decreases, and the resonance frequency becomes smaller than that in the non-discharge state.
Further, when the arc discharge is passed through the glow discharge, the interelectrode space can be regarded as a conductor, so that the impedance of the entire electrode structure is considered to be only the capacitance of the solid dielectric. The resonance frequency at this time can be obtained by calculation. Measurement can also be performed by using an equivalent circuit in which the interelectrode space is replaced with a conductor.

  As a result of such research considerations, the following knowledge was obtained. That is, stable glow discharge can be obtained by setting the energization frequency to the electrodes to a range between the resonance frequency when not discharging and the resonance frequency when the inter-electrode space is regarded as a conductor. In this range, there can always be a frequency capable of high output. Based on this finding, the present invention was made.

That is, the present invention includes a plasma processing apparatus provided with an electrode circuit including a pair of electrodes facing each other and an inductor, and a solid dielectric is provided on the facing surface of at least one of the electrodes, and in the space between the electrodes. In this method, plasma treatment is performed by introducing a processing gas and supplying power to the electrode circuit, and the power supply frequency to the electrode circuit during the processing is referred to as a resonance frequency during non-discharge (hereinafter referred to as “first resonance frequency”). )) And a resonance frequency (hereinafter referred to as “second resonance frequency”) when the space between the electrodes can be regarded as a conductor. In addition, it includes a pair of electrodes facing each other and an inductor, and is equipped with an electrode circuit that can be fed from a power source. A solid dielectric is provided on the facing surface of at least one of the electrodes, and a processing gas is introduced into the space between these electrodes In addition, an apparatus for performing plasma processing by supplying power to the electrode circuit, wherein the power supply frequency to the power supply circuit is a resonance frequency when not discharging and a resonance frequency when the inter-electrode space can be regarded as a conductor. A frequency setting means for setting between the two is provided.
As a result, a stable discharge state can be obtained, and the frequency range in which the peak frequency at which high output efficiency is present can be set for general use.

  Here, while causing discharge in the space between the electrodes, the power supply frequency to the electrode circuit is adjusted between the first resonance frequency and the second resonance frequency to obtain a frequency at which the current peaks, and this peak frequency or It is desirable to execute processing at a frequency in the vicinity thereof. As a result, high output efficiency can be obtained with certainty.

  The first resonance frequency may be calculated by using the pair of electrodes as a series connection of a capacitance component of the interelectrode space filled with the processing gas and a capacitance component of the solid dielectric. In addition, the second resonance frequency may be calculated using only the capacitance component of the solid dielectric as the pair of electrodes.

  Instead of the calculation, an electric field having an amplitude less than a threshold value at which discharge occurs is applied between the electrodes by supplying power to the electrode circuit, and the frequency at which the current reaches a peak is adjusted by adjusting the power supply frequency. The resonance frequency may be used. Further, the power supply frequency may be adjusted in a state where the pair of electrodes are brought into contact with each other with the solid dielectric interposed therebetween to eliminate the inter-electrode space, and the frequency at which the current peaks may be set as the second resonance frequency.

The plasma processing apparatus boosts a voltage from a power source with a transformer to supply power to the electrode circuit, and it is preferable that the inductor is constituted by a leakage inductance of the transformer.
The electrode circuit may be configured by adding a real inductor or capacitor to the inductor composed of the leakage inductance and the capacitor composed of the pair of electrodes. As a result, the first and second resonance frequencies can be adjusted, and as a result, the setting range of the power supply frequency during processing can be adjusted.

  Since the present invention performs plasma treatment by glow discharge rather than corona discharge or the like, the discharge portion of the electrode (or dielectric) is preferably planar (hereinafter, this planar portion is referred to as “ This is called the discharge surface.) The distance between the pair of electrodes is preferably substantially constant (the discharge surfaces of the pair of electrodes are parallel to each other). Thereby, arc discharge due to electric field concentration can be prevented and uniform glow discharge can be generated. The distance between the discharge surfaces is preferably 0.5 mm or more and 20 mm or less, and more preferably 1 mm or more and 7 mm or less. The discharge surface may be a curved surface, but a larger curvature is preferable (R = 5 mm or more), and a flat surface is more preferable. In addition, the discharge surface is preferably smooth (smooth). Unevenness and protrusions are not preferable because sparks are conspicuous. The electrode structure satisfying these conditions includes a parallel plate electrode type in which a pair of flat electrodes are opposed in parallel, a roll-concave surface comprising a roll-shaped (cylindrical) electrode and a concave electrode having a cylindrical concave surface along its peripheral surface. Examples thereof include an electrode type and a coaxial cylindrical electrode type including a pair of cylindrical electrodes that are coaxial with each other.

  According to the present invention, a stable discharge state can be obtained, and a frequency range in which a peak frequency at which high output efficiency is present can be set for general use.

Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 schematically shows an atmospheric pressure plasma processing apparatus M according to the present invention. The plasma processing apparatus M includes an electrode structure 10 and an electric field applying unit 20. The electrode structure 10 is composed of a pair of electrodes 11 and 12 facing each other. A solid dielectric 13 is provided on at least one opposing surface of the pair of electrodes 11 and 12. Here, it is provided only on the ground electrode 12, but it may be provided on the hot electrode 11 or on both electrodes 11, 12. A processing gas is introduced into a space 10p between these electrodes 11 and 12 (between the hot electrode 11 and the solid dielectric 13 of the ground electrode 12) by a processing gas introducing means (not shown).

  The electric field applying means 20 has an alternating power source 21 and a transformer 22. The alternating power source 21 is configured to output an alternating voltage having a desired frequency by switching a DC voltage, for example. A frequency adjusting (setting) means 23 is connected to the alternating power source 21, and the output frequency of the alternating voltage can be adjusted (set) by the frequency adjusting unit 23. Instead of the alternating power source 21, a pulse power source that outputs a pulse voltage may be used.

The transformer 22 has a primary coil 22 a connected to the alternating power source 21 and a secondary coil 22 b connected to the electrode 11, and boosts the output voltage of the alternating power source 21 and supplies it to the electrode 11. Yes.
As a result, an alternating electric field is applied to the interelectrode space 10p to cause glow discharge, and the processing gas from the introducing means is turned into plasma. The plasma processing gas is applied to an object to be processed such as a semiconductor substrate so that the surface of the object to be processed is processed. This process is performed under a pressure near atmospheric pressure (substantially normal pressure).

  The electrode circuit 1 is configured by the secondary coil 22 b of the transformer 22 and the electrode structure 10. The coil (inductor) 22b has a leakage inductance L. The electrode structure 10 can be regarded as a capacitor. Therefore, the electrode circuit 1 can be considered as an LC series resonance circuit. The resonance frequency f is expressed by the following equation.

ここで、Ｌは、コイル２２ｂの漏れインダクタンスであり、Ｃは、電極構造１０のキャパシタンスである。

Here, L is the leakage inductance of the coil 22 b and C is the capacitance of the electrode structure 10.

  FIG. 2 is an equivalent circuit of the electrode circuit 1. In the electrode structure 10, the impedance component Zp in the interelectrode space 10 p and the capacitance component Cd in the solid dielectric 13 are connected in series. The impedance component Zp in the interelectrode space 10p is represented by a parallel connection of a capacitance Cp and a resistor R. The capacitance Cd of the solid dielectric 13 is determined by the dimensional shape such as the thickness and cross-sectional area of the solid dielectric 13 and the dielectric constant, and can be easily calculated.

When there is no discharge in the interelectrode space 10p (non-discharge), R = ∞ in the equivalent circuit. Therefore, the capacitance C (= C ₁ ) of the electrode structure 10 is as follows.

非放電時の電極間空間１０ｐのキャパシタンスＣｐは、該空間１０ｐの厚さおよび断面積などの寸法形状、並びに該空間１０ｐ内に満たされた処理ガスの誘電率などの物性に基づいて、容易に算出することができる。ひいては、式（２）により、非放電時のキャパシタンスＣ_１を容易に算出することができる。
また、非放電時の電極回路１の共振周波数ｆ（＝ｆ_１）は、次式で表される。

The capacitance Cp of the inter-electrode space 10p during non-discharge can be easily determined based on the dimensional shape such as the thickness and the cross-sectional area of the space 10p and the physical properties such as the dielectric constant of the processing gas filled in the space 10p. Can be calculated. Hence, the equation (2), the capacitance C ₁ at the time of non-discharge can be easily calculated.
Further, the resonance frequency f (= f ₁ ) of the electrode circuit 1 during non-discharge is expressed by the following equation.

これら式（１ａ）、（２）により、非放電時の電極回路１の共振周波数ｆ_１を容易に算出することができる。以下、非放電時の共振周波数ｆ_１を、適宜「第１共振周波数ｆ_１」という。

From these equations (1a) and (2), the resonance frequency f ₁ of the electrode circuit 1 at the time of non-discharge can be easily calculated. Hereinafter, the resonance frequency f ₁ at the time of non-discharge is appropriately referred to as “first resonance frequency f ₁ ”.

On the other hand, when an arc discharge occurs in the interelectrode space 10p, the interelectrode space 10p can be regarded as a conductor. At this time, R = 0 in the equivalent circuit of FIG. Therefore, the capacitance C (= C ₂ ) of the electrode structure 10 is

となる。
また、アーク放電時の電極回路１の共振周波数ｆ（＝ｆ_２）は、次式で表される。

It becomes.
Further, the resonance frequency f (= f ₂ ) of the electrode circuit 1 during arc discharge is expressed by the following equation.

これら式（１ｂ）、（３）により、アーク放電時の電極回路１の共振周波数ｆ_２を容易に算出することができる。以下、アーク放電時（電極間空間１０ｐを導体と見做せる時）の共振周波数ｆ_２を、適宜「第２共振周波数ｆ_２」という。第２共振周波数ｆ_２は、第１共振周波数ｆ_１より小さい。

These formula (1b), can be calculated by (3), the resonance frequency f ₂ of the electrode circuit 1 during arc discharge easily. Hereinafter, when arc discharge the resonance frequency f ₂ (when considered to the conductor space 10p between the electrodes), referred to as "second resonance frequency f _2." Second resonance frequency _{f 2,} the first smaller resonance frequency _{f 1.}

The first and second resonance frequencies f ₁ and f ₂ can also be obtained by actual measurement. That is, the output voltage of the alternating power source 21 is set so that an electric field having an amplitude less than a threshold value at which discharge occurs is applied between the electrodes 11 and 12. Then, the output frequency (feeding frequency to the electrode circuit 1) is scanned, and the primary side (or secondary side) current of the transformer 22 is measured. The frequency at which the current measurement value reaches a peak is the first resonance frequency f ₁ (see FIG. 6).

Further, as shown in FIG. 3, the electrode structure in which the pair of electrodes 11 and 12 are brought close to each other by the thickness of the interelectrode space 10a so that the solid dielectric 13 is held between them to eliminate the interelectrode space 10a. Make 10 '. This makes it possible to make the circuit equivalent to an arc discharge state (a state in which the interelectrode space 10a can be regarded as a conductor). Then, in the same manner as described above, the output frequency is scanned and current measurement is performed. Frequency This current measurement value becomes peak, the second resonant frequency f ₂ (see FIG. 6).

When the plasma surface treatment is executed by the plasma processing apparatus M, the first and second resonance frequencies f ₁ obtained by the above calculation or measurement by the frequency adjusting means 23 are used to determine the magnitude of the output frequency fp of the alternating power source 21. , F ₂ . That is, it adjusts so that it may be in the range of the following formula.
f ₂ <fp <f ₁ (5)
As a result, stable glow discharge can be caused in the interelectrode space 10a, and good plasma surface treatment can be performed.

Further, a frequency fp _max at which the output efficiency reaches a peak always exists within a range satisfying the expression (5) (see FIG. 6). That is, the relationship of the following formula is established.
f ₂ <fp _max <f ₁ (5 ′)
By setting the frequency to this peak value fp _max , very good output efficiency can be obtained. Note that if the reaction proceeds excessively in the inter-electrode space 10a, the reaction on the surface of the object to be processed may decrease. In such a case, the frequency may be set so as to be shifted from the peak.

The upper and lower limits of the frequency fp, that is, the values of the first and second resonance frequencies f ₁ and f ₂ can be arbitrarily changed. For example, as shown in FIG. 4, a real inductor L ′ or a real capacitor C ′ is interposed in series before or after the electrode structure 10 of the electrode circuit 1, or as shown in FIG. L ″ or a real capacitor C ″ may be provided in parallel with the electrode structure 10. As a result, the first and second resonance frequencies f ₁ and f ₂ can be shifted, and thus the frequency setting range can be changed. Of course, modifications of the electrode circuit 1 are not limited to those shown in FIGS. 4 and 5, and various circuit configurations can be adopted.

  The frequency setting range represented by the above equation (5) can be applied universally regardless of the output waveform, the type of processing gas, the processing content, the apparatus configuration, and the like. That is, the output waveform may be a pulse, a sine wave, or a square wave. Further, it can be applied to various plasma surface treatments such as film formation, etching, cleaning, ashing, and surface modification, and the type of processing gas and the apparatus configuration are not limited. The present invention can be applied to any of a so-called remote type in which the workpiece is disposed outside the inter-electrode space 10a and a so-called direct type in which the workpiece is disposed inside the inter-electrode space 10a. Furthermore, the present invention is not limited to atmospheric pressure plasma processing near atmospheric pressure, and can be applied to reduced pressure plasma processing.

In the same plasma processing apparatus as in FIG. 1, the first and second resonance frequencies f ₁ and f ₂ were measured by the method of the above embodiment. That is, the output voltage of the power source 21 was set to 50 V so that the electric field between the electrodes 11 and 12 was lower than the discharge threshold. Then, the frequency was scanned and the current on the primary side of the transformer 22 was measured. As shown by the one-dot chain line in FIG. 6, a current peak appeared at about 65 kHz (= f ₁ ). Further, when the two electrodes 11 and 12 were brought into contact with each other as shown in FIG. 3 and the current was measured, a current peak appeared at about 20 kHz (= f ₂ ) as shown by a two-dot chain line in FIG.
Note that the current values in FIG. 6 are standardized with the maximum value in each measurement being 100 (the same applies to FIG. 7 described later). Further, the current value at the stage of obtaining the resonance frequencies f ₁ and f ₂ is weak at the peak, and is considerably smaller than that at the later-described glow discharge treatment.

Thereafter, while introducing 100% nitrogen gas as a processing gas into the interelectrode space 10a, the voltage of the power source 21 was set to 250 V, and an alternating electric field was applied between the electrodes 11 and 12. Then, the relationship between frequency and current was measured. As a result, as indicated by the solid line in FIG. 6, a peak of current appeared at 55 kHz (= fp _max ). Thus, it was confirmed that the relational expression f ₂ <fp _max <f ₁ shown in the above formula (5 ′) is satisfied. The primary side current when fp = 55 kHz was 9.2 A, and the input power, that is, the output was 2300 W. When converted per unit area of the electrode, it was 12 W / cm ² .

Further, as shown in FIG. 7, the light emission intensity of the discharge increased in proportion to the output, which was the maximum when fp _max = 55 kHz, and an extremely good and stable glow discharge was confirmed.
In the range of 20 kHz to 65 kHz where the relational expression f ₂ <fp <f ₁ shown in Expression (5) is satisfied, stable discharge could be obtained in the entire interelectrode space 10a. At 65 kHz (= f ₁ ) or more and 20 kHz (= f ₂ ) or less, it is difficult to obtain a desired discharge.

  Furthermore, the glass cleaning ability (contact angle and conveyance speed) was compared between the condition (A) with an output of 2500 W and a frequency of 55 kHz, and the condition (B) with an output of 1200 W and a frequency of 30 kHz. The glass conveyance speed was set to two types, 1 m / min and 2 m / min. In the condition (A), an alternating electric field converted from direct current was applied, whereas in the condition (B), a pulse electric field was applied. As a result, as shown in FIG. 8, it was confirmed that the condition (A) had an equivalent contact angle at twice the conveyance speed as compared to the condition (B), and the processing capability was almost proportional to the output.

When the same apparatus as in Example 1 was used and the processing gas was changed to argon gas and the relationship between frequency and current was measured, almost the same result as in FIG. 6 was obtained. In the range of 20 kHz to 65 kHz where the relational expression f ₂ <fp <f ₁ shown in Expression (5) is satisfied, stable discharge could be obtained in the entire interelectrode space 10a. When the output was increased below 20 kHz, the spark discharge was started. At 65 kHz or higher, arc discharge instantaneously occurred and stable discharge could not be performed.

In the apparatus having the first resonance frequency f ₁ = 190 kHz and the second resonance frequency f ₂ = 75 kHz, nitrogen gas was used as the processing gas, and the frequency fp _max was measured in the same manner as in Example 1. As a result, fp _max = 150 kHz, and it was confirmed that the relational expression f ₂ <fp _max <f ₁ shown in the above formula (5 ′) holds. Needle discharge occurred at 190 kHz or higher. Discharge did not occur at 75 kHz or less.

  Further, the glass cleaning ability (contact angle) is obtained under the condition (C) of an alternating electric field converted from DC at an output of 2000 W, a frequency of 150 kHz, and an output of 1200 W, a frequency of 30 kHz, and a pulse electric field (D). And transport speed). As a result, as shown in FIG. 9, it was confirmed that the condition (C) had the same contact angle at twice the conveyance speed as compared to the condition (D), and the processing capability was almost proportional to the output.

It is a circuit diagram which shows schematic structure of the atmospheric pressure plasma processing apparatus which concerns on one Embodiment of this invention. It is an equivalent circuit diagram of the electrode circuit of the device. It is explanatory drawing of the method of measuring a 2nd resonant frequency in the said apparatus. It is a circuit diagram which shows the modification of the electrode circuit of the said apparatus. It is a circuit diagram which shows the other modification of the electrode circuit of the said apparatus. It is a graph which shows the measurement result of the relationship between the frequency by Example 1, and an electric current. It is a graph which shows the measurement result of the relationship between the output by Example 1, and the light emission intensity of plasma. It is a graph which shows the measurement result of the relationship between the processing conditions by Example 1, and processing capability (contact angle for every conveyance speed). It is a graph which shows the measurement result of the relationship between the processing conditions by Example 3, and processing capability (contact angle for every conveyance speed).

Explanation of symbols

M Atmospheric pressure plasma processing apparatus 1 Electrode circuit 10 Electrode structure 10a Interelectrode space 11 Hot electrode 12 Ground electrode 13 Solid dielectric 20 Electric field applying means 21 Alternating power supply 22 Transformer 22b Secondary coil (inductor)
23 Frequency adjusting means (frequency setting means)
L ′, L ″ real inductor C ′, C ″ real capacitor fp output frequency of alternating power supply during processing (feed frequency to electrode circuit)
fp _max peak frequency

Claims (9)

A plasma processing apparatus having an electrode circuit including a pair of electrodes facing each other and an inductor, and a solid dielectric provided on the facing surface of at least one of the electrodes, and introducing a processing gas into the space between the electrodes A method for performing plasma processing by supplying power to the electrode circuit, wherein a power supply frequency to the electrode circuit during the processing is a resonance frequency during non-discharge (hereinafter referred to as “first resonance frequency”) and the electrode. A plasma processing method, characterized in that it is set between a resonance frequency (hereinafter referred to as a “second resonance frequency”) when the interspace is regarded as a conductor. While causing discharge in the space between the electrodes, the power supply frequency to the electrode circuit is adjusted between the first resonance frequency and the second resonance frequency to obtain the frequency at which the current reaches a peak, The plasma processing method according to claim 1, wherein the processing is performed at a frequency. 2. The first resonance frequency is calculated by using the pair of electrodes as a series connection of a capacitance component of an interelectrode space filled with a processing gas and a capacitance component of a solid dielectric during non-discharge. The plasma processing method as described in any one of. The plasma processing method according to claim 1, wherein the second resonance frequency is calculated using only the capacitance component of the solid dielectric as the pair of electrodes. The electric field having an amplitude less than a threshold value at which discharge occurs is applied between the electrodes and the frequency thereof is adjusted, and the frequency at which the current reaches a peak is defined as the first resonance frequency. Plasma processing method. 2. The frequency according to claim 1, wherein a power supply frequency is adjusted in a state where the pair of electrodes are in contact with each other with a solid dielectric interposed therebetween, and a frequency at which a current reaches a peak is defined as the second resonance frequency. Plasma processing method. The plasma processing apparatus boosts a voltage from a power source with a transformer and supplies power to the electrode circuit, and the inductor is configured by a leakage inductance of the transformer. The plasma processing method in any one. The electrode circuit is configured by adding a real inductor or a capacitor to the inductor composed of the leakage inductance and the capacitor composed of the pair of electrodes, thereby adjusting the first and second resonance frequencies. The plasma processing method according to claim 7. A pair of electrodes facing each other and an inductor are provided, and an electrode circuit that can be fed from a power source is provided. A solid dielectric is provided on the facing surface of at least one of the electrodes, and a processing gas is introduced into the space between the electrodes. An apparatus for performing plasma processing by supplying power to the electrode circuit, wherein the power supply frequency to the power supply circuit is a resonance frequency when the inter-electrode space is not discharged and when the inter-electrode space can be regarded as a conductor. A plasma processing apparatus comprising frequency setting means for setting between a resonance frequency and a resonance frequency.

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