JP2001023968A - Plasma processing system and plasma cleaning method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a plasma processing system, capable of reducing charge up damage applied to a semiconductor element during plasma processing. SOLUTION: A high-voltage electrode 9 and a ground electrode 10 are disposed on a cylindrical dielectric reaction tube 7 so as to be opposed to each other, the tube having one end thereof opened as a purging port 12. A mixed gas, where O2 is added to a noble gas, is introduced into the tube 7 as a plasma generating gas 8. An AC electric field is applied across the electrodes 9 and 10, so that a glow discharge is generated within the tube 7 under atmospheric pressure. An atomized plasma 13 is spouted out of the port 12 of the tube 7, whereby the plasma is sprayed onto a substrate 15 for processing which has a semiconductor element 14 mounted thereon. In this plasma processing system, O2 concentration of the gas 8 is set at 0.8-2.0%. The oxygen gas adsorbs electrons within the plasma 13, so that the oxygen gas itself becomes negative ions, and these negative ions act to decrease the electron density within the plasma 13.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to improving the bonding property, improving the adhesion and wettability with a sealing resin, and forming a film by modifying the surface of an electronic component such as a semiconductor device with plasma. TECHNICAL FIELD The present invention relates to a plasma processing apparatus that generates plasma used for plasma processing such as plasma processing, and a plasma cleaning method using the same. This is applied to cleaning of the surface of the semiconductor device, and aims at reducing damage to the semiconductor element.

[0002]

2. Description of the Related Art Conventionally, attempts have been made to perform plasma processing under atmospheric pressure. For example, Japanese Unexamined Patent Publication No.
No. 1, JP-A-3-241739 and JP-A-1-30
Japanese Patent No. 6569 discloses that a pair of electrodes are arranged in a discharge space in a reaction tube, a dielectric is provided between the electrodes, and the discharge space is filled with a plasma generating gas mainly containing a rare gas such as He or Ar. Then, there has been proposed a plasma processing method in which an object to be processed is placed in a reaction vessel and an AC electric field is applied between the electrodes. Also, JP-A-4-358076, JP-A-4-212253, and JP-A-4-21908.
Japanese Patent Application Laid-Open No. 2 (1999) -1999 proposes performing plasma processing by blowing plasma generated by glow discharge under atmospheric pressure onto a workpiece in a jet shape. Also, Japanese Patent Laid-Open No. 9-1
In JP-A-67775, as the plasma-generating gas, a gas obtained by adding an inert gas selected from He, Ar, Kr and Xe, or O ₂ , CF ₄ , N ₂ , CO ₂ , SF ₆ , reactive gas selected from the CHF _3,
It is described that plasma treatment is performed using a gas containing at least O, N, F, and Cl.

[0003]

However, in the above-described plasma processing method and apparatus, the plasma processing apparatus focuses on the surface modification of a semiconductor element, and particularly, is applied to a substrate to be processed such as an electronic circuit board. The charge-up phenomenon when plasma processing is performed with a semiconductor element mounted is not considered at all. Irradiation of the semiconductor element with plasma causes charge-up, which causes a problem in the characteristic of the semiconductor element.

Hereinafter, the charge-up will be described with reference to FIG. FIG. 4 is a cross-sectional view of the semiconductor element 14 showing a gate electrode 2 formed on a silicon single crystal substrate 1.
FIG. The charge-up is performed by injecting the charged particles 3 in the plasma into the wiring 6 electrically connected to the gate electrode 2 of the semiconductor element 14 and accumulating in the gate oxide film 5 through the wiring 6 and the gate electrode 2. It is a phenomenon that occurs. The passivation film 4 is formed on the wiring 6 in order to protect the surface of the semiconductor element 14, and the passivation film 4 is formed at a position corresponding to the bonding portion 20 where wire bonding of the wiring 6 is performed. The bonding portion 20 is directly exposed to plasma during cleaning (during plasma processing).
The bonding part 20 plays a role of an antenna for picking up the charged particles 3 in the plasma.

If the above-mentioned charge-up phenomenon proceeds excessively, the physical characteristics of the gate oxide film 5 will be affected. Specifically, as a result of a change in the physical characteristics of the gate oxide film 5, in the case of a MOSFET (field effect transistor), gm (conductance), Vth (threshold voltage), and the like change. This is a phenomenon called charge-up damage.

As a method of evaluating the charge-up damage, there is a method of measuring a shift amount of VFB (flat band voltage) before and after plasma processing using an MNOS (metal-silicon nitride-silicon oxide film-silicon) element. A method of measuring the time until a breakdown occurs between a metal and silicon by applying a constant current to an oxide film (Q
bd evaluation method) and the like, and by these methods, the presence or absence and size of damage can be known by the amount of change in the oxide film life before and after plasma irradiation.

The present invention has been made in view of the above points, and a plasma processing apparatus and a plasma processing apparatus capable of reducing charge-up damage applied to a semiconductor element during plasma processing by the composition of a plasma generating gas. And a plasma cleaning method.

[0008]

In the plasma processing apparatus according to the first aspect of the present invention, a high-voltage electrode 9 and a ground electrode 10 are connected to a cylindrical dielectric reaction tube 7 having one side opened as an outlet 12. The mixed gas obtained by adding O ₂ to a rare gas is introduced into the reaction tube 7 as a plasma generating gas 8 so as to face the reaction tube 7 so as to face each other.
By applying an AC electric field during zero, a glow discharge is generated in the reaction tube 7 under the atmospheric pressure, and the jet-shaped plasma 13 is blown out from the outlet 12 of the reaction tube 7 to mount the semiconductor element 14. In the plasma processing apparatus that sprays on the substrate 15 to be processed, the O ₂
The concentration is set to 0.8 to 2.0%.

A plasma processing apparatus according to a second aspect of the present invention is characterized in that, in addition to the configuration of the first aspect, He and Ar are used as the rare gas of the plasma generating gas 8. .

According to a third aspect of the present invention, in addition to the configuration of the second aspect, the ratio of He is set to He.
And 20 to 50% of the total flow rate of Ar and Ar.

According to a fourth aspect of the present invention, there is provided a plasma processing apparatus according to any one of the first to third aspects.
The distance between the outlet 12 of the reaction tube 7 and the substrate 15 on which the semiconductor element 14 is mounted is set to 2 to 10 mm.

According to a fifth aspect of the present invention, there is provided a plasma cleaning method, wherein a plasma 13 blown from the plasma processing apparatus according to any one of the first to fourth aspects is blown onto a substrate 15 on which a semiconductor element 14 is mounted. The surface modification of the substrate 15 is performed.

[0013]

Embodiments of the present invention will be described below.

FIG. 1 shows an example of a plasma processing apparatus according to the present invention. This plasma processing apparatus is formed by providing a high-voltage electrode 9 and a ground electrode 10 in contact with the outer periphery of a reaction tube 7 and arranging the high-voltage electrode 9 and the ground electrode 10 so as to face up and down. , A discharge space 21 is formed between the high-voltage electrode 9 and the ground electrode 10.

The reaction tube 7 is formed of a high melting point dielectric material (insulating material) in a cylindrical shape, and its upper end is opened as a gas inlet 25. Further, a nozzle portion 22 having a tapered structure narrowed down to a tapered shape such that the diameter thereof becomes smaller toward the lower side is formed at a lower portion of the reaction tube 7, and a lower surface of the nozzle portion 22 which is a lower end surface of the reaction tube 7 is formed. Is provided with an outlet 12. If the diameter of the outlet 12 is formed to be substantially the same as the diameter of the reaction tube 7 without providing the nozzle portion 22, it is difficult to accelerate the flow rate of the jet-shaped plasma (plasma jet) 13 blown out from the outlet 12 However, as described above, by forming the lower part of the reaction tube 7 as the nozzle portion 22 which is narrowed down to a small diameter, the flow rate of the jet-like plasma 13 can be accelerated without reducing the volume of the discharge space 21. Before the reactive gas active particles such as short-lived radicals are extinguished, the plasma 13 can be made to reach and irradiate the workpiece 30, and the plasma processing of the workpiece 30 can be performed efficiently. Things. Semiconductor element 1
In the case where the substrate 15 on which the substrate 4 is mounted is the substrate 30, in order to obtain a flow rate of the plasma 13 suitable for cleaning the surface, the outer peripheral surface of the nozzle 22 and the nozzle 2 are required.
It is preferable that the taper angle formed between the outer peripheral surface of the reaction tube 7 other than 2 is 10 to 30 °.

The opening area of the outlet 12 is formed to have a size corresponding to the area of a perfect circle having a diameter of 0.1 to 5 mm. If the opening area of the outlet 12 is too smaller than the above range, the processing range of the plasma 13 to be blown out becomes too small, and the plasma processing of the workpiece 30 takes a long time. If the opening area of the opening 12 is too large, the processing range of the plasma 13 to be blown out becomes too large, and
0 may not be subjected to local plasma processing.

The dielectric constant of the dielectric material forming the reaction tube 7 is an important factor in lowering the temperature in the discharge space 21. Specifically, as the insulating material, glass such as quartz, alumina, and yttria partially stabilized zirconium is used. And a ceramic material.

The high voltage electrode 9 and the ground electrode 10 are formed of a metal material, for example, copper, aluminum, brass, stainless steel having high corrosion resistance (such as SUS304) or the like. Also,
The two electrodes 9 and 10 have the same shape and are formed in an annular shape (ring shape), and the inner diameter of the high-voltage electrode 9 and the ground electrode 10 is formed substantially the same as the outer diameter of the reaction tube 7. The inner peripheral surfaces of the high-voltage electrode 9 and the ground electrode 10 are surfaces that come into contact with the reaction tube 7,
The surface roughness represented by the arithmetic average roughness of this surface is 10 to 100
It is set to 0 μm. By setting the surface roughness of the surface in contact with the reaction tube 7 to 10 to 1000 μm in this manner, uniform discharge in the discharge space 21 can be achieved. This is considered to be because, when viewed from a microscopic viewpoint, a very fine aggregate of microdischarges is formed, and transfer to an arc is hindered. If the surface roughness of the inner peripheral surface of the high voltage electrode 9 and the ground electrode 10 is less than 10 μm, there is a possibility that the discharge becomes difficult, and if the surface roughness of the inner peripheral surface of the high voltage electrode 9 and the ground electrode 10 exceeds 1000 μm. In addition, nonuniform discharge may occur. As described above, physical processing such as sand blasting can be employed as a process for roughening the inner peripheral surfaces of the high voltage electrode 9 and the ground electrode 10. The arithmetic average roughness Ra (μm) when the surface roughness is expressed in the form of y = f (x) is defined by the following equation (1) in JIS B0601.

[0019]

(Equation 1)

Then, the high voltage electrode 9 and the ground electrode 10 are inserted into the reaction tube 7 and attached to the outer periphery of the reaction tube 7, and the inner peripheral surfaces of the high voltage electrode 9 and the ground electrode 10 are brought into contact with the outer peripheral surface of the reaction tube 7. Deploy. The high-voltage electrode 9 is connected to a power supply 11 for generating an AC electric field, and is connected to the ground electrode 1.
0 is grounded. The ground electrode 10 is disposed so as to be located below the high-voltage electrode 9 and above the nozzle portion 22, that is, between the outlet 12 and the high-voltage electrode 9. As a result, the ground electrode 10 is located closer to the workpiece 30 than the high voltage electrode 9, that is, the high voltage electrode 9 is located farther from the workpiece 30 than the ground electrode 10, The arc discharge does not easily fly from the high-voltage electrode 9 to the object 30, and damage to the object 30 due to the arc discharge can be prevented.

The distance between the high-voltage electrode 9 and the ground electrode 10 (the distance L between the lower end of the high-voltage electrode 9 and the upper end of the ground electrode 10) is 3 to 20.
It is preferably set to mm. High voltage electrode 9 and ground electrode 1
If the interval L of 0 is less than 3 mm, a short circuit may occur between the high-voltage electrode 9 and the ground electrode 10 outside the reaction tube 7 and the discharge may not occur in the discharge space 21, and the discharge space 21 is narrow. This may make it difficult to efficiently generate the plasma 13. If the distance between the high-voltage electrode 9 and the ground electrode 10 exceeds 20 mm, the discharge space 21
As a result, there is a fear that it becomes difficult to generate a discharge, and it is difficult to efficiently generate the plasma 13.

As the plasma generating gas 8, a mixed gas of a rare gas (inert gas) and a reactive gas is used. As the rare gas, He (helium), Ar (argon), Ne (neon), Kr (krypton), or the like can be used. However, in consideration of discharge stability and economic efficiency, He and Ar are used. Is preferred. O ₂ (oxygen) can be used as a reaction gas.

The O ₂ concentration is set to 0.8 to 2.0% with respect to the total amount of the plasma generating gas 8. O ₂ concentration of 0.8
%, The electron adsorption effect of oxygen is reduced,
Since the charged particle density in the jet-like plasma (plasma jet) 13 is increased, the mobility of electrons is larger than that of ions, and the life is longer, the charged particles are more likely to fall on the object 30 to be discharged. (Thunder) may occur and the object 30 may be damaged. On the other hand, if the O ₂ concentration exceeds 2.0%, the flow rate of the plasma generating gas 8 increases, the effect of adsorbing electrons by oxygen in the discharge space 21 decreases, and the lifetime of electrons in the plasma 13 increases, As the density increases, many charged particles fall on the semiconductor element 14 mounted on the substrate 15 to be processed, and charge-up damage is likely to occur. And O
_{2 If the} concentration exceeds 2.0%, the flow rate of the plasma generating gas 8 increases, the time spent in the discharge space 21 decreases, and high-frequency energy is not efficiently transmitted.
There is a possibility that the discharge property is reduced.

When both He and Ar are used as the rare gas, the ratio of He is set to 20 to the total flow rate of He and Ar.
Preferably, it is set to 50%. The ratio of He is He and A
If the total flow rate of r is less than 20%, the dielectric breakdown voltage becomes high, and the discharge performance may decrease.
If the total flow rate of Ar and Ar exceeds 50%, the cleaning performance may be reduced.

When performing plasma processing using the plasma processing apparatus A formed as described above, first, as shown by an arrow in FIG. And a high-frequency voltage is applied from a power supply 11 to the high-voltage electrode 9, and a discharge space 2 between the high-voltage electrode 9 and the ground electrode 10 is introduced.
1 is applied with a high-frequency AC electric field. A glow discharge is generated in the discharge space 21 under atmospheric pressure by the application of the AC electric field, and the plasma generating gas 8 is turned into plasma by the glow discharge to generate the plasma 13 containing the plasma active species. The plasma 13 is continuously discharged downward from the outlet 12, and the plasma 13 is blown onto the surface of the workpiece 30 disposed below the outlet 12. Thus, the plasma processing of the processing target 30 can be performed.

In the present invention, the outlet 1 of the reaction tube 7
It is preferable to set the distance L between 2 and the surface of the object 30 (the surface of the substrate 15) to be 2 to 10 mm. Distance L
If the distance L is less than 2 mm, the proportion of electrons that disappear in the plasma 13 before reaching the workpiece 30 may decrease, and it may be difficult to suppress charge-up damage. If the distance L exceeds 10 mm, , The plasma active species in the plasma 13 disappear before reaching the workpiece 30,
There is a possibility that the effect of the plasma treatment may not be sufficiently obtained.

In the present invention, the frequency of the applied AC electric field is preferably set to 1 kHz to 200 MHz. If the AC frequency is less than 1 kHz, the discharge in the discharge space 21 cannot be stabilized, and the plasma processing may not be performed efficiently. When the AC frequency exceeds 200 MHz, the temperature of the plasma 13 in the discharge space 21 rises remarkably, and the life of the reaction tube 7, the high-voltage electrode 9, and the ground electrode 10 may be shortened. There is a risk that the size and size will increase.

In the present invention, the applied power applied to the discharge space 21 is preferably set to 20 to 3500 W / cm ³ . When the applied power applied to the discharge space 21 is 2
If it is less than 0 W / cm ³ , plasma cannot be generated sufficiently. Conversely, if the power applied to the discharge space 21 exceeds 3500 W / cm ³ , stable discharge cannot be obtained. There is fear. The density of the applied power (W / cm ³ ) is defined by (applied power / discharge space volume).

In the present invention, the plasma generating gas 8
Since setting the O ₂ concentration from 0.8 to 2.0%, itself by the electrons in the plasma 13 of oxygen gas is adsorbed becomes negative ions acts to reduce the electron density in the plasma 13. Generally, electrons have a higher mobility than ions and a longer lifetime than ions. Therefore, when blowing out a plasma jet, charge-up is caused by electrons. Therefore, by adding oxygen gas to the plasma generating gas 8, negative ions of oxygen are
By the time it reaches 0, the negative ions of oxygen do not affect the charge-up. As a result, electrons that reach the surface of the processing object 30, which is a cause of charge-up, can be reduced. Even when the processing target substrate 15 on which the semiconductor element 14 is mounted as the processing object 30 is subjected to plasma processing, the charge is reduced. It is possible to suppress the occurrence of up damage and reduce the characteristic failure of the semiconductor element 14.

In the present invention, since both the high-voltage electrode 9 and the ground electrode 10 for applying an AC electric field to the discharge space 21 are provided outside the reaction tube 7, the high-voltage electrode 9 and the ground electrode 10 are provided.
Are not directly exposed to the plasma 13, so that they can be prevented from being sputtered by the plasma 13 and not corroded by the reaction gas, and the high-voltage electrode 9 and the ground electrode 10 can be damaged. And the life can be extended. Moreover, since no impurities are generated by sputtering or corrosion, the object to be treated 30 even if used for a long period of time.
Can be prevented from being contaminated by impurities.

Furthermore, since the high-voltage electrode 9 and the ground electrode 10 are arranged so as to be substantially parallel to the direction of introduction of the plasma generating gas, that is, the high-voltage electrode 9 and the ground electrode 10 are arranged vertically and opposed to each other. The direction of the AC electric field generated in the plasma 21 and the flow directions of the plasma generating gas and the plasma 13 can be made substantially coincident with each other, and active species of the plasma 13 can be efficiently generated. By changing the distance between the electrode 9 and the ground electrode 10, the size of the discharge space 21 can be easily changed, and the amount of plasma 13 generated can be easily adjusted.

[0032]

The present invention will be described below in detail with reference to examples.

(Example 1) A plasma processing apparatus as shown in FIG. 1 was formed by installing a high-voltage electrode 9 on the upper side and a ground electrode 10 on the lower side on the outer periphery of a reaction tube 7 made of a dielectric material. The nozzle portion 22 at the lower portion of the reaction tube 7 has a shape narrowed down in a tapered shape, and the outlet 12 is formed on the lower surface of the nozzle portion 22.

As the object 30 to be processed, the semiconductor element 14
Was used. Substrate to be processed 15
And a gate oxide film of a MOSFET as a semiconductor element 14, respectively.

Then, He gas is introduced into the reaction tube 7 from the gas inlet 25.
Is supplied at a rate of 1 liter / minute, Ar at a rate of 3 liter / minute, and O ₂ gas mixed at a rate of 30 cc / minute.
A plasma 13 is generated by applying a high frequency of 56 MHz,
The plasma 13 is blown out from the outlet 12 in a jet shape, and the plasma 13 is supplied to the workpiece 30 arranged 10 mm below the outlet 12 for 5 seconds to perform plasma processing (surface modification of the substrate 15 to be processed). Was done.

In the semiconductor element 14 mounted on the substrate 15 after the plasma processing and the semiconductor element 14 mounted on the substrate 15 before the plasma processing, damage due to charge-up is reduced. Qbd
The comparison was made based on the difference in service life due to evaluation. As a result, as shown in FIG. 2, it was found that there was almost no change (decrease) in the life of the gate oxide film before and after the plasma treatment, and the damage due to charge-up was reduced. (Example 2) In Example 1, the distance L between the outlet 12 and the workpiece 30 was 1 mm, 2 mm, 5 mm, and 10 mm.
Is changed, the Q of the gate oxide film of the semiconductor element 14 is changed.
How the bd life changed was measured. As a result, as shown in FIG.
If the distance L of 2 is not less than 2 mm, the life of the gate oxide film is almost the same as that of the plasma-untreated product, and is 10 mm.
There is almost no difference, and it can be seen that the charge-up damage is reduced.

[0037]

As described above, according to the first aspect of the present invention, a reaction tube of a cylindrical dielectric having one side opened as a blowout port has a high voltage electrode and a ground electrode opposed to each other. A gas mixture of rare gas and O ₂ is introduced into the reaction tube as a plasma-generating gas, and an AC electric field is applied between the high-voltage electrode and the ground electrode. 0.8 to generate glow discharge, the plasma processing apparatus for spraying the substrate to be processed to a semiconductor device blows a jet-like plasma is mounted from outlet of the reaction tube, the O ₂ concentration of the gas for plasma generation to the
Since it is set to 2.0%, the oxygen gas adsorbs the electrons in the plasma to become negative ions and acts to lower the electron density in the plasma. Therefore, a semiconductor element is mounted as the object to be processed. Even when the substrate to be processed is subjected to the plasma processing, the occurrence of charge-up damage can be suppressed, and the characteristic failure of the semiconductor element can be reduced.

According to the second aspect of the present invention, He and Ar are used as the rare gases of the plasma generating gas.
A stable glow discharge can be generated.

According to the invention of claim 3 of the present invention, the ratio of He is set to 20 to 50% of the total flow rate of He and Ar, so that even if the substrate on which the semiconductor element is mounted is subjected to plasma processing, the charge is increased. It is possible to suppress the occurrence of damage and reduce the characteristic failure of the semiconductor element.

According to the fourth aspect of the present invention, since the distance between the outlet of the reaction tube and the substrate on which the semiconductor element is mounted is set to 2 to 10 mm, the distance until the plasma shet reaches the object to be processed is set. By increasing the time, the rate at which electrons in the plasma are extinguished can be increased and the electron density near the surface of the object can be reduced, and the substrate on which the semiconductor element is mounted as the object is subjected to plasma processing. Even in such a case, it is possible to suppress the occurrence of charge-up damage and reduce the characteristic failure of the semiconductor element.

According to a fifth aspect of the present invention, the surface of the substrate to be processed is modified by spraying the plasma blown from the plasma processing apparatus of the first to fourth aspects onto the substrate on which the semiconductor element is mounted. Therefore, oxygen gas adsorbs electrons in the plasma to become negative ions by themselves, and acts to reduce the electron density in the plasma.
Even when a substrate to be processed on which a semiconductor element is mounted is subjected to plasma processing, the substrate to be processed can be cleaned while suppressing the occurrence of charge-up damage and reducing defective characteristics of the semiconductor element. is there.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing an example of an embodiment of the present invention.

FIG. 2 is a graph showing Qbd evaluation of Example 1 of the above.

FIG. 3 is a graph showing Qbd evaluation of Example 2 of the same.

FIG. 4 shows a semiconductor device (near a gate electrode of a MOSFET).
FIG.

[Explanation of symbols]

 Reference Signs List 7 Reaction tube 8 Plasma generation gas 9 High-voltage electrode 10 Ground electrode 12 Blow-out port 13 Plasma 14 Semiconductor element 15 Substrate to be processed

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yasushi Sawada 1048 Kadoma Kadoma, Osaka Pref.Matsushita Electric Works, Ltd. 72) Inventor Kazuya Kitayama 1048 Kadoma Kadoma, Osaka Prefecture Inside Matsushita Electric Works Co., Ltd. 1048 Kadoma, Kazuma, Osaka Prefecture F-term (reference) 5F004 AA06 AA13 AA14 BA03 BB11 BB18 BB28 BD07 DA00 DA01 DA16 DA18 DA22 DA23 DA25 DA26 DB00 EB08

Claims (5)

[Claims] 1. A high-voltage electrode and a ground electrode are arranged outside a reaction tube of a cylindrical dielectric, one side of which is opened as an outlet, so as to face each other, and O ₂ is added to the rare gas. The mixed gas is introduced into the reaction tube as a gas for plasma generation, and by applying an AC electric field between the high-voltage electrode and the ground electrode, a glow discharge is generated in the reaction tube at atmospheric pressure,
In a plasma processing apparatus in which a jet-like plasma is blown out from an outlet of a reaction tube and blown onto a substrate to be processed on which a semiconductor element is mounted, the O ₂ concentration of a plasma generation gas is set to 0.8 to 2.0%. A plasma processing apparatus, comprising: 2. The method according to claim 1, wherein the rare gas of the plasma generating gas is H.
The plasma processing apparatus according to claim 1, wherein e and Ar are used. 3. The ratio of He is 20 times the total flow rate of He and Ar.
3. The plasma processing apparatus according to claim 2, wherein the setting is set to 50%. 4. The plasma processing apparatus according to claim 1, wherein the distance between the outlet of the reaction tube and the substrate on which the semiconductor element is mounted is set to 2 to 10 mm. . 5. A plasma cleaning method, wherein plasma is blown from a plasma processing apparatus according to any one of claims 1 to 4 onto a substrate on which a semiconductor element is mounted, thereby modifying the surface of the substrate. Method.