JPH07235535A - Formation of insulating film - Google Patents

Abstract

PURPOSE:To form an insulating film, which is superior in step coverage and of which the content of a hydrocarbon radical component is reduced. CONSTITUTION:A plasma CDV method is conducted using organic Si compound gas having an Si-N bond, an SiN insulating film 4 is formed and thereafter, a plasma treatment is conducted in an atmosphere of post-treatment gas consisting of molecules having N atoms, such as N2 gas and NH3 gas, and hydrocarbon group components contained in the film 4 are removed. Or the plasma CDV method is conducted using the mixed gas of the organic Si compound gas having the Si-N bond and organic nitrogen compound gas, such as dimethylhidrazine gas and phenylhydrazine gas, and the film 4 is formed while the hydrocarbon group components contained in the film 4 are removed. When this film formation is applied to the formation of a passivation film and an interlayer insulating film, the insulation properties of the passivation and interlayer insulation is ensured and the passivation and the interlayer insulating film is superior also in water resistance and corrosion resistance. As a result, a highly reliable semiconductor device can be manufactured.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a silicon nitride (SiN) type insulating film or a silicon oxynitride (SiON) type insulating film used as a final protective film or an interlayer insulating film of a wafer in a semiconductor device, for example. The present invention relates to a method for forming an insulating film, which is formed by a chemical vapor deposition (hereinafter referred to as CVD) method using an organic Si compound as a source gas.

[0002]

2. Description of the Related Art Conventionally, a SiN-based insulating film has been widely used as a final protective film of a wafer, that is, a so-called passivation film. When forming this SiN-based insulating film, plasma CV is used so as not to damage the low-melting-point material layer such as aluminum (Al) -based wiring already formed.
Film formation is performed at a low temperature by the D method. As a source gas, conventionally, silane (SiH ₄ ) / ammonia (NH
₃ ) Mixed gas, SiH ₄ / nitrogen (N ₂ ) mixed gas, etc. have been used.

However, the SiN film thus formed
The coverage of the system insulating film cannot follow the increase in the surface step of the substrate due to the miniaturization of the semiconductor device or the multi-layer wiring. FIG. 3 shows a wafer in which the SiO-based interlayer insulating film 2 and the Al-based wiring 3 are formed on the Si substrate 1, and the SiN-based insulating film 14 is formed thereon. Since the step coverage is poor, the void 15 is formed. Also, cracks are likely to occur in such a SiN-based insulating film 14.

As a method for improving the above step coverage, it has been proposed to control the plasma state by the dual frequency method. This is because, in a parallel plate electrode of a plasma CVD apparatus, a low frequency RF voltage of several hundred kHz is applied to the electrode on the side where the wafer is placed, and MHz is applied to the other electrodes.
A high-frequency RF voltage of the order is applied, and an attempt is made to increase low-energy ion bombardment and improve coverage. However, even with this method, it is not possible to sufficiently cope with the miniaturization of the pattern and the increase of the surface step, and the conformal film formation has not been achieved yet.

Therefore, SiN which is more excellent in coverage
As a method of forming a system insulating film, it has been proposed to perform CVD using an organic Si compound as a source gas. here,
The organic Si compound means [(CH ₃ ) ₂ N] ₄ Si,
[(CH ₃ ) ₂ N] ₃ SiH, [(CH ₃ ) ₂ N] ₂ S
It is a compound such as iH _{2 having} Si atoms, N atoms, C atoms, and H atoms as main constituent elements and having a bond between a silicon atom and a nitrogen atom (Si—N bond). When a film is formed using this as a raw material gas, the existence of the above Si—N bond makes it possible to efficiently form a SiN-based insulating film. In addition, since the hydrocarbon group is cleaved from the organic Si compound during the film formation, the intermediate product that retains the Si—N bond is easily polymerized, and the fluidity becomes high. It is believed that a system insulating film can be formed.

[0006]

However, in the SiN type insulating film formed by using the organic Si compound as described above, the insulation resistance is deteriorated due to the residual hydrocarbon groups, and the water resistance and the corrosion resistance are deteriorated. There is concern that the property may deteriorate.
If such a SiN-based insulating film is applied as a passivation film and an interlayer insulating film, the reliability of the semiconductor device may be reduced.

Therefore, the present invention has been proposed in view of such conventional circumstances, and provides a method for forming an insulating film capable of suppressing the incorporation of a hydrocarbon group while ensuring excellent coverage. With the goal.

[0008]

Means for Solving the Problems The inventors of the present invention have made intensive studies to achieve the above-mentioned object, and as a result, have proposed the following two methods.

A first invention is a film forming step of forming an insulating film on a substrate by a CVD method using an organic Si compound having a Si--N bond, and a post-treatment gas containing at least N atom in the molecule. And a post-treatment step of performing plasma treatment on the insulating film in the atmosphere.

The post-treatment gas containing at least N atoms in the molecule functions to remove the hydrocarbon group component taken into the insulating film. As the post-treatment gas, in addition to N ₂ , hydrogen azide (N ₃ H), NH ₃ , an ammonia derivative, a hydrazine (N ₂ H ₄ ), a hydrazine derivative such as methylhydrazine, or the like having H atoms in the molecule. Compounds also containing can be used. Since the latter compound gas is more easily decomposed in plasma than the N ₂ gas, it is considered that the action of extracting the hydrocarbon group component from the insulating film is stronger.

In order to further reduce the hydrocarbon group component, the insulating film may be formed to a desired film thickness by alternately repeating the film forming process and the post-treatment process a plurality of times. As a result, the hydrocarbon group component can be sufficiently removed not only in the surface layer portion of the insulating film but also in the deep layer portion.

On the other hand, the second invention is to form an insulating film on a substrate by a CVD method using a mixed gas containing an organic Si compound having a Si--N bond and an organic nitrogen compound.

Of course, in combination with the above-mentioned first invention, after forming an insulating film with a small incorporation of hydrocarbon group components by film formation using the organic nitrogen compound as described above, it is possible to The hydrocarbon group component in the surface layer portion of the insulating film may be further reduced by plasma treatment in an atmosphere of a post-treatment gas containing at least N atoms. Further, the insulating film may be formed by repeating the film formation using the organic nitrogen compound as described above and the plasma treatment under the atmosphere of the post-treatment gas containing at least N atoms in the molecule.

The organic nitrogen compound is preferably a hydrazine derivative having at least one alkyl group or phenyl group.

In any of the above-described methods for forming an insulating film, an organic Si compound having a Si--N bond is used as a source gas during film formation. As the organic Si compound, an azide group (hereinafter, -N) is used. ₃ units) and -N
R ₂ (However, R is a hydrocarbon group having 1 or more carbon atoms.)
A structure in which a group is bonded to a Si atom is preferable. In particular, when -NR ₂ is bonded to the Si atom, the intermediate product produced by the cleavage of the hydrocarbon group during film formation is likely to be polymerized, and the fluidity is enhanced, so that the effect of improving the coverage is obtained. Excellent in.

Further, when an alkyl group (hereinafter referred to as -R 'group) and / or an alkoxyl group (hereinafter referred to as -OR "group) is bonded to the above Si atom, these are formed at the time of film formation. Cleavage of the —R ′ group and the —OR ″ group increases the fluidity of the intermediate product to be produced and improves the coverage.

The organic Si having the above Si--N bond
The compound may have a structure in which a fluorine (F) atom is bonded to a Si atom. When the F atoms are bonded, the polymerization reaction at the time of film formation is promoted, the coverage is improved, and the dielectric constant of the formed insulating film can be reduced.

The H atom directly bonded to the Si atom is
Unlike the above-mentioned —NR ₂ group, —N ₃ group, —R ′ group, and —OR ″ group, it does not exhibit the effect of maintaining the Si—N bond to improve the film formation efficiency and the effect of improving the coverage. So
It is better not to bond even one Si atom, or to make a small number even if bonded.

Furthermore, an organic Si compound having a Si--Si bond may be used to improve the film formation rate. This makes it possible to increase the flow rate of the source gas supplied during film formation and to increase the film formation rate without increasing the power applied to the electrodes of the plasma CVD apparatus, thus increasing the uptake of impurities, Throughput can be improved without causing a problem such as deterioration of coverage.

Therefore, among the organic Si compounds that can be used as the source gas, those having the most desirable structure can be represented by the following general formula (1).

Si _n (NR ₂ ) _w (N ₃ ) _x (R ′) _y (OR ″) _z F _v (1) (where v, w, x, y, z are v + w + x + y + z =
2n + 2, 0 ≦ w ≦ 2n + 2, 0 ≦ x ≦ 2n + 2, 1 ≦
w + x ≦ 2n + 2, 0 ≦ y ≦ 2n + 1, 0 ≦ z ≦ 2n +
1, an integer satisfying 0 ≦ v ≦ 2n + 1, and n is an integer of 1 or more. Further, R, R'and R '' represent a hydrocarbon group having 1 or more carbon atoms. The carbon skeleton of the hydrocarbon group represented by R, R ′ and R ″ is not particularly limited, and may be saturated hydrocarbon or unsaturated hydrocarbon. In each case, a linear, branched, or cyclic carbon skeleton is conceivable, but any of these may be used, and examples thereof include a methyl group, an ethyl group, and a cyclopentadienyl group. .

In the above general formula (1), when z = 0, that is, when the organic Si compound is one in which the --OR '' group is not bonded, this is used as a source gas, and an oxygen-based gas is added to this. If they are not mixed, a SiN-based thin film will be formed. On the other hand, when x ≧ 1, that is, when the compound is an organic Si compound having an —OR ″ group bonded, when this is used as a source gas, a trace amount of O atoms is taken in during film formation, so that an oxygen-based gas is used. The SiON-based thin film can be formed without using the above.

Further, when the film is formed, it is possible to perform the process at a low temperature without damaging the already formed low melting point material layer such as the Al-based wiring, so that plasma is generated in the reaction chamber. It is suitable to perform CVD. The plasma CVD apparatus used may be a parallel plate type plasma CVD apparatus, or a magnetic field microwave plasma CVD (E which can generate high density plasma under low pressure).
CR-CVD equipment and inductively coupled plasma CVD (IC
P-CVD) equipment, helicon wave plasma CVD equipment, T
It may be a CP-CVD apparatus.

When forming a film by plasma CVD,
In addition to the desired insulating film components, intermediate products, by-products, undissociated components of the raw material gas, etc. are also present in the vicinity of the substrate surface, so to prevent these impurity components and particles from being taken into the film. The plasma may be generated intermittently.

[0025]

When the present invention is applied and an organic Si compound having a Si-N bond is used as a source gas, a hydrocarbon group component is preferentially cleaved from the organic Si compound during film formation,
Since the chemical species in which the Si-N bond is retained are bonded to each other,
The intermediate product is polymerized and exhibits fluidity. And
This improves the step coverage of the insulating film.

Further, after the film formation, when the plasma treatment is performed in the atmosphere of the post-treatment gas containing N atoms in the molecule, the hydrocarbon group component taken into the insulating film is extracted by the N atoms. If a gas containing further H atoms in the molecule is used as this post-treatment gas, the H atoms also play a role of extracting the hydrocarbon group component in the insulating film, so that the hydrocarbon group component taken into the insulating film is removed. It becomes possible to further reduce.

When the insulating film is formed by alternately repeating the film forming step and the post-treatment step a plurality of times, the hydrocarbon group component is sufficiently removed not only in the surface layer portion of the insulating film but also in the deep layer portion. Becomes an insulating film.

The gas containing at least N atoms in the molecule functions to extract the hydrocarbon group component even when supplied at the time of forming the insulating film, but a large amount of inorganic gas such as N ₂ gas or NH ₃ gas is used. When supplied, the hydrocarbon group component may be extracted before the intermediate product produced from the organic Si compound is polymerized to reduce the fluidity. On the other hand, when the organic nitrogen compound gas is used, the hydrocarbon group component can be extracted while maintaining good fluidity.

Particularly, the hydrazine derivative having an alkyl group or a phenyl group is excellent in a catalytic action for promoting a polymerization reaction of an intermediate product, an effect of introducing an N atom into this product, an effect of extracting a hydrocarbon group component and the like. Not only
The presence of the alkyl group and the phenyl group also has the effect of reducing the probability of attachment of the intermediate product to the substrate, and therefore, good step coverage can be maintained.

[0030]

EXAMPLES The method for forming an insulating film according to the present invention will be described below with reference to specific examples. Here, a SiN-based insulating film or S is used as a passivation film on the Al-based wiring.
An example of forming an iON-based insulating film will be described.

In each of the following examples, a parallel plate type plasma CVD apparatus was used as the CVD apparatus. In this plasma CVD apparatus, a wafer is placed on the lower electrode and RF power is applied to the upper electrode. In addition, the temperature of the wafer can be adjusted by providing a heater on the lower electrode. On the other hand, the upper electrode is a shower electrode for uniformly supplying the source gas onto the substrate.

Example 1 In this example, trisdimethylaminosilylazide [(CH ₃ ) ₂ N] ₃ SiN ₃ was used as the organic Si compound.
After forming a film using Si, a post-treatment is performed using N ₂ to form Si.
An N type insulating film was formed.

Specifically, first, a plasma CV under the following conditions is formed on a wafer having a SiO type interlayer insulating film 2 and an Al type wiring 3 formed on a Si substrate 1 as shown in FIG.
The SiN insulating film 4 was formed by D to a film thickness of 1 μm.

Plasma CVD conditions Source gas: [(CH ₃ ) ₂ N] ₃ SiN ₃ Flow rate 100 sccm RF power: 350 W (13.56 MHz) (applied to the upper electrode) Pressure: 1200 Pa Wafer temperature: 200 ° C. Electrode distance: 10 mm Film-forming time: 60 seconds The obtained SiN-based insulating film 4 was formed as shown in FIG.
It had excellent step coverage with no voids or cracks.

Subsequently, a plasma treatment under the following conditions was performed as a post treatment.

Plasma processing conditions Post-processing gas: N ₂ flow rate 100 sccm RF power: 350 W (13.56 MHz) Pressure: 1330 Pa Wafer temperature: 400 ° C. Electrode distance: 10 mm By this processing, SiN-based insulating film 4 is contained. The hydrocarbon group component was reduced.

Further, an annealing treatment was performed under the following conditions.

Annealing conditions Introduced gas: The above raw material gas diluted with N ₂ gas containing 3% H ₂ Flow rate 8000 sccm Annealing time: 60 minutes Pressure: Atmospheric pressure Annealing temperature: 400 ° C. The passivation film made of the film 4 is completed.

Here, a corrosion test was conducted on the wafer on which the passivation film made of the SiN-based insulating film 4 was formed. The conditions of this corrosion test are shown below.

Corrosion Test Conditions Hydrochloric acid concentration: 5% Test time: 5 minutes Solution temperature: 25 ° C. As a result of this corrosion test, no corrosion was observed on the Al-based wiring 3. From this, the SiN formed as described above
It was found that the system insulating film 4 has good water resistance and corrosion resistance.

Example 2 In this example, bisdimethylaminosilyldiazide [(CH ₃ ) ₂ N] ₂ Si (N ₃ ) ₂ was used as the organic Si compound.
And after film formation using a mixed gas of NH _3, to form the SiN-based insulating film by performing a post-treatment using NH _3.

Specifically, a raw material gas of [(CH ₃ ) ₂ N] ₂ Si (N ₃ ) ₂ :
Plasma CV under the same conditions as in Example 1 except that the flow rate was changed to 100 sccm and NH ₃ was changed to 50 sccm.
The SiN insulating film 4 was formed by D to a film thickness of 1 μm. The SiN-based insulating film 4 thus formed is
It had excellent step coverage with no voids or cracks.

Then, the post-treatment gas is NH _{3 at a} flow rate of 100.
Plasma processing was performed in the same manner as in Example 1 except that the sccm was changed and the wafer temperature was changed to 150 ° C. By this treatment, the hydrocarbon group component contained in the SiN-based insulating film 4 could be reduced.

[0044] Further, the raw material gas 3% H ₂ containing N ₂
An annealing process was performed in the same manner as in Example 1 except that a gas diluted with a gas was used to complete the passivation film made of the SiN-based insulating film 4.

Here, when a corrosion test was performed on the wafer on which the passivation film made of the SiN-based insulating film 4 was formed in the same manner as in Example 1, the Al-based wiring 3
No corrosion was found on the. From this, it was found that the SiN-based insulating film 4 exhibits good water resistance and corrosion resistance.

Example 3 In this example, a film-forming process and a post-treatment process with N ₂ H ₄ were alternately repeated to form a SiN-based insulating film.

Specifically, the SiN insulating film 4 was formed on the same wafer as in Example 1 by plasma CVD under the same conditions as in Example 2 except that the film formation time was changed to 6 seconds.
The film was formed to a film thickness of 00 nm. The SiN-based insulating film 4 thus formed had no step or void and was excellent in step coverage.

Subsequently, the post-treatment gas was N ₂ H _{4 with a} flow rate of 10
Plasma treatment was performed in the same manner as in Example 2 except that the treatment time was changed to 0 sccm and the treatment time was set to 60 seconds. By this treatment, the hydrocarbon group component contained in the SiN-based insulating film 4 could be reduced.

After that, the film forming process and the post-treatment process are alternately repeated 9 times, and finally the SiN film having a film thickness of 1 μm is formed.
The system insulating film 4 was formed. As a result, the SiN-based insulating film 4 in which the hydrocarbon group component was reduced was formed from the surface layer portion to the deep layer portion.

[0050] Further, the raw material gas 3% H ₂ containing N ₂
An annealing process was performed in the same manner as in Example 1 except that a gas diluted with a gas was used to complete the passivation film made of the SiN-based insulating film 4.

Here, when a corrosion test was performed on the wafer on which the passivation film made of the SiN-based insulating film 4 was formed in the same manner as in Example 1, the Al-based wiring 3
No corrosion was found on the. From this, it was found that the SiN-based insulating film 4 exhibits good water resistance and corrosion resistance.

Example 4 In this example, tetradimethylaminodiflorodisilane [(CH ₃ ) ₂ N] ₄ Si ₂ F ₂ was used as the organic Si compound.
And a dimethylhydrazine (CH ₃ ) ₂ N ₂ H ₂ mixed gas were used to form a SiN-based insulating film.

Concretely, on the same wafer as in Example 1, the raw material gas was [(CH ₃ ) ₂ N] ₄ Si ₂ F _{2 at a} flow rate of 100 sccm, and (CH ₃ ) ₂ N ₂ H _{2 at a} flow rate of 50 sc.
The SiN insulating film 4 was formed to a thickness of 1 μm by plasma CVD under the same conditions as in Example 2 except that the thickness was changed to cm.

The SiN-based insulating film 4 thus formed had no voids or cracks, was excellent in step coverage, and had a reduced hydrocarbon group component.

[0055] Further, the raw material gas 3% H ₂ containing N ₂
An annealing process was performed in the same manner as in Example 1 except that a gas diluted with a gas was used to complete the passivation film made of the SiN-based insulating film 4.

Here, when a corrosion test was conducted on the wafer on which the passivation film made of the SiN-based insulating film 4 was formed in the same manner as in Example 1, the Al-based wiring 3
No corrosion was found on the. From this, it was found that the SiN-based insulating film 4 exhibits good water resistance and corrosion resistance.

Example 5 In this example, bisdimethylaminobisethoxydiflorodisilane [(CH ₃ ) ₂ N] ₂ was used as the organic Si compound.
Si ₂ (OC ₂ H ₅ ) F ₂ and diethyl hydrazine (C ₂
A SiON-based insulating film was formed using a mixed gas of H ₅ ) ₂ N ₂ H ₂ .

Specifically, a raw material gas of [(CH ₃ ) ₂ N] ₂ Si ₂ (OC ₂ H) was placed on the same wafer as in Example 1.
₅ ) F ₂ : Flow rate 100 sccm, (C ₂ H ₅ ) ₂ N ₂ H
₂ : Example 1 except that the flow rate was changed to 50 sccm
A SiON-based insulating film 5 was formed to a thickness of 1 μm by plasma CVD under the same conditions as described above.

The SiON-based insulating film 5 thus formed was free of voids and cracks, excellent in step coverage, and reduced in hydrocarbon group component.

[0060] Further, the raw material gas 3% H ₂ containing N ₂
An annealing process was performed in the same manner as in Example 1 except that a gas diluted with a gas was used to complete the passivation film made of the SiON-based insulating film 5.

Here, when a corrosion test was conducted on the wafer on which the passivation film made of the SiON type insulating film 5 was formed in the same manner as in Example 1, no corrosion was found on the Al type wiring 3. It was From this, it was found that the SiON-based insulating film 5 has good water resistance and corrosion resistance.

Example 6 In this example, a SiN insulating film was formed using a mixed gas of tetradimethylaminosilane [(CH ₃ ) ₂ N] ₄ Si and phenylhydrazine C ₆ H ₅ NHNH ₂ as an organic Si compound. Filmed

Specifically, a raw material gas of [(CH ₃ ) ₂ N] ₄ Si and a flow rate of 100 was placed on the same wafer as in Example 1.
sccm, C ₆ H ₅ NHNH ₂ : plasma C under the same conditions as in Example 1 except that the flow rate was changed to 50 sccm.
The SiN insulating film 4 was formed by VD to a film thickness of 1 μm.

The SiN-based insulating film 4 thus formed had no voids or cracks, was excellent in step coverage, and had a reduced hydrocarbon group component.

[0065] Further, the raw material gas 3% H ₂ containing N ₂
An annealing process was performed in the same manner as in Example 1 except that a gas diluted with a gas was used to complete the passivation film made of the SiN-based insulating film 4.

Here, when a corrosion test was performed on the wafer on which the passivation film made of the SiN-based insulating film 4 was formed in the same manner as in Example 1, the Al-based wiring 3
No corrosion was found on the. From this, it was found that the SiN-based insulating film 4 exhibits good water resistance and corrosion resistance.

As described above, Examples 1 to 3 and Example 4
The methods of forming an insulating film shown in FIGS. 6 to 6 are different in that the hydrocarbon group component is removed during the post-treatment or the hydrocarbon component is removed during the film formation. SiN-based insulating film 4 or SiO whose amount is suppressed
The N-type insulating film 5 can be provided. Further, since these insulating films also exhibit good coverage, they were suitable as passivation films.

Although the example in which the method for forming an insulating film according to the present invention is applied has been described above, the present invention is not limited to the above-described embodiments. For example, the present invention may be applied to form an interlayer insulating film in addition to the passivation film. Further, the formation of the SiN-based insulating film 4 or the SiON-based insulating film 5 according to Examples 4 to 6 may be combined with the post-treatment in Examples 1 to 3. Further, the type of raw material gas for forming the SiN-based insulating film 4 or the SiON-based insulating film 5, the film forming conditions, and the wafer configuration can be changed as appropriate.

[0069]

As is apparent from the above description, application of the present invention makes it possible to form an insulating film having excellent step coverage and a reduced content of hydrocarbon group components.

Therefore, the insulating film formed by applying the present invention ensures the insulating property and is excellent in water resistance and corrosion resistance. Therefore, when this is used as a passivation film or an interlayer insulating film, it is possible to form a highly reliable semiconductor device in which deterioration of device characteristics is prevented.

[Brief description of drawings]

FIG. 1 shows a process of manufacturing a semiconductor device to which the present invention is applied, in which a SiO-based interlayer insulating film and A are formed on a Si substrate.
It is a schematic diagram which shows the cross section of the wafer in which 1 type | system | group wiring was formed.

FIG. 2 is a schematic diagram showing a state in which an insulating film is formed on the wafer of FIG.

FIG. 3 is a schematic view showing a cross section of a wafer on which a SiN-based insulating film is formed by a conventional method.

[Explanation of symbols]

 1 Si substrate 2 SiO-based interlayer insulating film 3 Al-based wiring 4 SiN-based insulating film 5 SiON-based insulating film

Claims (6)

[Claims] 1. A film forming step of forming an insulating film on a substrate by a chemical vapor deposition method using an organic silicon compound having a bond between a silicon atom and a nitrogen atom, and containing at least a nitrogen atom in the molecule. And a post-treatment step of performing a plasma treatment of the insulation film in an atmosphere of a post-treatment gas. 2. The method for forming an insulating film according to claim 1, wherein the post-treatment gas contains hydrogen atoms in its molecule. 3. The method for forming an insulating film according to claim 1, wherein the film forming step and the post-treatment step are alternately repeated a plurality of times. 4. An insulating film is formed on a substrate by chemical vapor deposition using a mixed gas containing an organic silicon compound having a bond between a silicon atom and a nitrogen atom and an organic nitrogen compound. And a method for forming an insulating film. 5. The method for forming an insulating film according to claim 4, wherein a hydrazine derivative having at least one alkyl group or phenyl group is used as the organic nitrogen compound. 6. The method for forming an insulating film according to claim 1, wherein the film formation is performed while generating plasma.