TW201338923A - Polishing pad and method of manufacturing the same - Google Patents

Abstract

Polishing pad and method of manufacturing the same, the method including: (a) mixing materials for forming a polishing layer; (b) mixing at least two from among inert gas, a capsule type foaming agent, a chemical foaming agent, and liquid microelements that are capable of controlling sizes of pores, with the mixture in (a) so as to form two or more types of pores; (c) performing gelling and hardening of the mixture generated in (b) so as to form a polishing layer including the two or more types of pores; and (d) processing the polishing layer so as to distribute micropores defined by opening the two or more types of pores on a surface of the polishing layer.

Description

Polishing pad and method of manufacturing same

The present invention relates to a polishing pad and a method of manufacturing the same, and more particularly to a polishing pad that can efficiently collect and supply a polishing slurry, and a method of manufacturing the polishing pad.

The chemical mechanical planarization/polishing (CMP) process has been widely used for planarization of semiconductor devices, and with increasing wafer diameter, high integration density, micro-line width, and multilayers. The trend of the line structure, the CMP process has become more important.

In the CMP process, the polishing speed and the flatness of the wafer are important, and the performance of the CMP process depends on the conditions of the CMP equipment and also on the polishing liquid and the polishing pad belonging to the consumable member. which performed. Specifically, the polishing pad allows the polishing liquid to be applied while the polishing pad is in contact with the surface of the wafer, and the polishing liquid is evenly dispersed onto the wafer, and the abrasive particles and the protrusions of the polishing pad are contained in the polishing liquid ( Prominence) causes physical wear.

In this case, in order for the slurry to flow smoothly, the slurry must saturate the polishing pad to directly contact the surface of the wafer. For this, on the surface of the polishing pad A technique for forming a micro-hole (e.g., a pore) is disclosed in U.S. Patent No. 5,578,362 and the like.

In this regard, it is very important to maintain the surface of the polishing pad in a state of being filled with the slurry to increase the role of the polishing pad in the CMP process and to promote its performance. Therefore, grooves of various shapes are formed in the polishing pad to form a large amount of slurry flow, and micropores are formed on the surface of the polishing pad by opening a microporous material (as above) Said).

Among them, a technique of forming grooves into various patterns has been developed; however, the technique related to a plurality of holes for forming micropores is still limited, and is limited to the use of a method of forming predetermined holes.

That is to say, according to the related art, a method of forming a plurality of holes has both advantages and disadvantages. In reality, considering these advantages and disadvantages, the CMP process is adjusted when it is used.

However, as semiconductor processes must be more miniaturized and more complex, to meet this need, CMP processes also require an improved technique for forming multiple holes.

The present invention provides a polishing pad which can maximize the performance of polishing and flattening by collecting and using a polishing liquid while performing a chemical mechanical planarization/grinding (CMP) process, and the present invention also provides a polishing pad for manufacturing the same. method.

According to an aspect of the present invention, there is provided a polishing pad which performs a polishing process by being in contact with a surface of an article to be polished and moving upon contact. The polishing pad includes an abrasive layer, wherein the abrasive layer includes two or more holes, the size of which is determined by using at least two of an inert gas, a capsule type foaming agent, a chemical agent, and a liquid microelement. Controlling, the polishing pad further includes micro-holes, and the micro-holes are The two or more holes distributed on the surface of the abrasive layer are defined by opening.

According to another aspect of the present invention, there is provided a method of manufacturing a polishing pad, the method comprising (a) mixing a material for forming an abrasive layer; (b) an inert gas capable of controlling a pore size, a capsule type foaming agent , at least two of the chemical blowing agent and the liquid micro-unit are mixed with the mixture in (a) to form two or more kinds of pores; (c) the mixture produced in (b) is gelled and hardened To form an abrasive layer comprising two or more holes; and (d) treating the abrasive layer to disperse the microvoids by two or more on the surface of the abrasive layer A variety of holes are defined by opening.

1‧‧‧ grinding equipment

3‧‧‧ platform

5‧‧‧ polishing head

7‧‧‧矽 wafer

11‧‧‧Nozzles

13‧‧‧Slurry

100‧‧‧ polishing pad

110‧‧‧Support layer

120‧‧‧Abrasive layer

130‧‧‧Polymer subject

141‧‧‧ first hole

141‧‧‧liquid micro unit

142‧‧‧Second hole

141’, 142’‧‧‧ micro holes

160‧‧‧ surface

170‧‧‧ Beam

S100S110, S120‧‧‧ steps

The above and other features and advantages of the present invention will become more apparent from the detailed description of embodiments of the invention.

1 is a cross-sectional view of a polishing pad in accordance with an embodiment of the present invention.

2 is a scanning electron microscope (SEM) enlarged photograph of a cross section of the polishing layer of the polishing pad shown in FIG. 1.

Figure 3 is a schematic illustration of a polishing apparatus employing the polishing pad of Figure 1.

4 is a flow chart illustrating a method of making an abrasive layer of a polishing pad in accordance with an embodiment of the present invention.

5 and 6 are SEM photographs of a hole formed in a surface of an abrasive layer including a hole formed of an inert gas and a hole formed by a liquid micro cell, in accordance with an embodiment of the present invention.

Fig. 7 compares the following two: the polishing efficiency of the polishing pad formed using the method according to the present invention (Experimental Example 2 and Experimental Example 3) and the polishing efficiency of the polishing pad formed according to the related art.

The term "and/or" used herein includes any and all combinations of one or more of the associated listed items.

The invention will be described more fully hereinafter with reference to the accompanying drawings.

1 is a cross-sectional view of a polishing pad 100 in accordance with an embodiment of the present invention, and FIG. 2 is a scanning electron microscope (SEM) enlarged photograph of a cross section of the polishing layer 120 of the polishing pad 100 shown in FIG. 1, and FIG. 3 is A schematic view of a polishing apparatus 1 employing the polishing pad 100 of FIG.

Referring to FIG. 1, in accordance with the present embodiment of the present invention, a polishing pad 100 includes a support layer 110 and an abrasive layer 120. The support layer 110 is used to secure the polishing pad 100 to the platform 3, as shown in FIG. In order to correspond to the force applied to the wafer 7 (i.e., the object to be polished, which is loaded on the head 5 facing the platform 3), the support layer 110 is made of a material having a stability such that The support layer 110 is capable of supporting the polishing layer 120 formed on the support layer 110 with uniform elasticity with respect to the germanium wafer 7. Therefore, the support layer 110 is mainly made of a non-porous, solid, and uniform elastic material, and has a lower hardness with respect to the abrasive layer 120 formed on the support layer 110.

In addition, at least a portion of the support layer 110 is transparent or translucent, and thus the light beam 170 for detecting the surface flatness of the object to be polished may pass through the support layer 110. In FIG. 3, the object to be polished is the silicon wafer 7, and the layer to be polished of the silicon wafer 7 is a metal layer or an insulating layer. However, various forms of the substrate can be used as the object to be polished, for example, a substrate on which a thin film transistor liquid crystal display (TFT-LCD) is formed, a glass substrate, a ceramic substrate. And polymer plastic substrate. Further, the polishing pad 100 can be fabricated without the support layer 110.

Furthermore, although the polishing pad 100 presented in FIG. 3 is circular (and thus suitable for the rotary type grinding apparatus 1), the polishing pad 100 may be modified into various shapes such as a rectangle, a square, or the like according to the shape of the polishing apparatus 1.

As shown in FIG. 3, the polishing layer 120 directly contacts the tantalum wafer 7 as an object to be polished. The abrasive layer 120 can be formed by mixing or chemically bonding a predetermined material used to form the abrasive layer. That is, the polymer body 130 constituting the polishing layer 120 is composed of various conventional components, and the description of these conventional materials and molding materials will be omitted.

The abrasive layer 120 may include a plurality of holes of two or more. The size of the two or more plurality of holes is controlled by at least two selected from the group consisting of an inert gas, a capsule type blowing agent, a chemical blowing agent, and a liquid micro unit.

That is to say, the types of holes can be distinguished from each other by the method of forming the holes. According to the present invention, the polishing layer 120 includes at least two types of pores selected from the group consisting of pores formed of an inert gas, pores formed by a capsule type foaming agent, pores formed by a chemical foaming agent, and liquid A hole formed by a trace unit.

Here, the holes may be formed, and the sizes thereof may be distinguished from each other depending on the kind thereof; however, the aspect of the invention is not limited thereto.

Hereinafter, as an example of the present invention, it is assumed that two kinds of holes (ie, a plurality of first holes and a plurality of second holes) are formed in the polishing layer 120, and specifically, a plurality of first holes are caused by A liquid microcell is formed, and a plurality of second pores are formed by an inert gas.

In this regard, when the polishing layer 120 includes a plurality of first holes formed of liquid micro-cells, it is formed by mixing materials for forming the polishing layer (as described above). The material may correspond to a hydrophilic polymer host (hereinafter referred to as "polyolefin polymer-containing hydrophilic polymer host") 130 containing a polyolefin diol (polyalkylene glycol).

That is, the polishing layer 120 may include a plurality of first holes 141 formed of an embedded liquid micro-cell and a plurality of second holes 142 belonging to the pores (including the embedded inert gas), both of which are evenly It is distributed in a predetermined region of the hydrophilic polymer main body 130 containing the polyolefin diol.

In this manner, a practical example of a plurality of first holes (liquid microelement pores) 141 and a plurality of second holes (pores) 142 included in the polishing layer 120 is as shown in FIG.

The plurality of micro holes 141' and the plurality of micro holes 142' having the open-cell microstructure are defined by the plurality of first holes 141 and the plurality of second holes 142, and are uniformly disposed directly on the wafer 7 Contacted in the abrasive layer surface 160.

Here, a plurality of micro holes 141 ′ having an open-cell microstructure and defined by a plurality of first holes 141 and a plurality of second holes 142 and a plurality of micro holes 142 ′ are indicated when embedded in the polishing layer 120 . When the liquid trace unit or the inert gas leaks to the outside, the region including the liquid trace unit or the inert gas remains in the micropores 141' and the micropores 142', so that predetermined materials from the outside can be collected in the regions.

The plurality of first holes 141 and the plurality of second holes 142 embedded in the polishing pad 100 are subjected to wear during the polishing process, continuously exposed to the surface of the polishing layer 160, and the micro holes 141' and the micro holes 142' are formed. The micropores 141' and the micropores 142' are substituted by the slurry 13. Therefore, since only the polymer body 130 is present in the surface 160 of the polishing layer, uneven wear of the polishing pad 100 does not occur, and the silicon wafer 7 as the object to be polished can be uniformly polished.

The polyolefin polymer-containing hydrophilic polymer host 130 may be formed of a material that is insoluble in the polishing liquid 13 (chemical solution used for planarization). For example, as shown in FIG. 3, the polyolefin polymer-containing hydrophilic polymer body 130 is formed of a material that is infiltrated by a polishing liquid 13 (performed through the nozzle 11 of the polishing apparatus 1).

The polyolefin polymer-containing hydrophilic polymer host 130 can be formed by chemically or physically mixing a material for forming a polymer host, a hydrophilic material, and a polyolefin diol compound.

Here, the polymer body 130 formed of a material for forming a polymer body may be formed of a material selected from the group consisting of polyurethane, polyether, polyester, and poly. Polysulfone, polyacryl, polycarbonate, polyethylene, polymethylmetacrylate, polyvinylacetate, polyvinyl chloride, poly A polyethyleneimine, a polyethersulfone, a polyetherimide, a polyketone, a melamine, a nylon, a hydrocarbon fluoride, or a combination thereof.

The polyolefin polymer-containing hydrophilic polymer host 130 is formed by chemically or physically mixing a hydrophilic material with a polyolefin diol compound and a polymer host 130.

The hydrophilic material may be polyethylene glycol, polyethylene propylene glycol, polyoxyethylene alkylphenolether, polyoxyethylene alkylether, polyethylene glycol. Polyethylene glycol fatty acid ester, polyoxyethylene alkylamine ether, glycerol fatty acid One selected from the group consisting of glycerine fatty acid ester, sugar fatty acid ester, sorbitol fatty acid ester, or a combination thereof.

The polyolefin diol compound may be one selected from the group consisting of an alkylene oxide added to an aqueous or active hydrogen-containing compound or a combination thereof.

The foregoing materials for forming the abrasive layer 120 may include various materials different from the foregoing materials.

The embedded liquid micro-unit forming the first hole 141 is formed of a liquid material which is incompatible with the hydrophilic polymer main body 130 containing the polyolefin diol, that is, an aliphatic mineral oil or an aromatic An aromatic mineral oil, a silicon oil having no hydroxyl group at the molecular end, soybean oil, coconut oil, palm oil, cotton seed oil, A material selected from the group consisting of camellia oil, hardened oil, and combinations thereof.

The first pores 141 formed of the embedded liquid micro-cells may be dispersed in the polyolefin polymer-containing hydrophilic polymer body 130 in a microspherical shape. The average diameter of the spheres can be between 1 μm and 30 μm, for example between 2 μm and 10 μm. The diameter of the sphere within the foregoing range is optimal for collecting and applying the slurry 13. However, the diameter of the sphere may vary depending on the kind of the polishing liquid 13, and the size of the embedded liquid micro-unit 141 may also vary.

The shape of the first hole 141 formed by the embedded liquid smear unit (that is, the average diameter of the sphere and the concentration of the sphere) can be simply and more varied by changing the hydrophilicity of the hydrophilic polymer body containing the polyolefin diol. Adjust the style.

In addition, the shape of the first hole 141 formed by the embedded liquid micro-unit can be The adjustment is simply and multi-patterned by adjusting the weight fraction of the liquid material (100 parts by weight of the material for forming the polymer body). For example, in order to form the first hole 141 by a desired shape of the embedded liquid micro unit, the material for forming the polymer main body is 100 parts by weight (that is, based on 100 parts by weight of the polyurethane), and 20 weight is used. The portion is 50 parts by weight, more preferably 30 parts by weight to 40 parts by weight.

The size and concentration of the first hole 141 formed by the embedded liquid micro-cell and the micro-hole 141' defined by the first hole 141 can be adjusted by adjusting the hydrophilic property of the hydrophilic polymer body containing the polyolefin diol. And/or the amount of liquid material is adjusted simply and in multiple styles. Therefore, depending on the type of the object to be polished and/or the type of the polishing liquid 13, it is possible to manufacture the polishing pad 100 having various polishing performances.

The second hole 142 is formed by injecting an inert gas, a capsule type foaming agent or a chemical foaming agent.

Here, the inert gas may be a chemically stable gas having a valence of 0, that is, helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe) or krypton (Rn). ). Further, in addition to the Group 8 element of the periodic table, the inert gas may be any gas that does not react with the polymer host, that is, a gas that does not participate in the polyurethane reaction, such as N ₂ .

A blowing agent which is mixed with a predetermined material and evaporated or reacted under the action of heat to produce a large amount of bubbles can be mainly classified into a chemical foaming agent and a physical foaming agent.

In the chemical foaming agent, foaming is produced by carbon dioxide, and carbon dioxide is produced by reacting with water using the activity of an isocyanate group, and therefore, water is used as a foaming agent. In a physical blowing agent, bubbles are formed by generating heat of reaction, which is generated by injecting a gas or using a decomposable or evaporative blowing agent, and therefore, the physical blowing agent does not participate in high Molecular reaction. The kind and characteristics of these blowing agents are well known, and thus detailed description thereof will be omitted.

The second hole 142 is formed on the polishing layer 120 by mixing an inert gas or various foaming agents (capsule-type foaming agent or chemical foaming agent). The radius of the second hole 142 may be larger than the radius of the first hole 141. Preferably, the second hole 142 is formed such that its volume corresponds to 10 times the volume of the first hole 141.

Hereinafter, a method of manufacturing the polishing layer 120 of the polishing pad 100 according to an embodiment of the present invention will be described with reference to FIG.

First, the material for forming the polishing layer 120 is mixed (S100). In detail, the material for forming the polyolefin polymer-containing hydrophilic polymer body 130 described above may be mixed with the material for forming the polishing layer 120 (S100).

Here, the material for forming the polyolefin polymer-containing hydrophilic polymer main body 130 is produced by mixing or reacting a hydrophilic material with a polyolefin diol compound and a material for forming a polymer main body. Mixing or reaction can be carried out by stirring.

In the mixing process, a liquid material such as mineral oil is mixed with a material for forming a polymer body. In this case, an inert gas such as Ar may be injected or replaced with a predetermined blowing agent.

The amount of the liquid material after mixing and the amount of the inert gas can be adjusted depending on the size of the pore to be formed and according to the kind thereof.

Next, gelation and hardening are performed (S510). That is, the mixture is injected into a cast having a predetermined shape, followed by solidification by gelation and hardening. The gelation is carried out at 80 ° C to 90 ° C for 5 minutes to 30 minutes, and the hardening is carried out at 80 ° C to 120 ° C for 20 hours to 24 hours. However, process temperatures and times can be varied to provide optimum conditions.

Finally, the structure having a predetermined shape obtained by the hardening is processed (S520). The resulting structure is taken off the cast, cutting, surface treatment (surface Treatment) and cleaning. First, the hardened resulting structure is taken out of the mold and cut into a predetermined thickness and shape. It is apparent that the abrasive layer 120 can be formed into a sheet shape by any method, and in order to increase the yield, for example, a casting or extrusion method known in the field of polymer sheet manufacturing can be used. Grooves of various shapes may be formed in the surface of the polishing layer 120 so that the polishing liquid 13 can be uniformly applied throughout the working surface of the polishing layer 120.

After the cleaning process is performed, the polishing layer 120 is completed. During the cleaning process, the embedded liquid micro-cells 141 exposed on the surface of the polishing layer 120 flow out, and thus, the micropores 141' of the openings are distributed on the surface 160 of the polishing layer. Here, the embedded liquid micro-unit 141 can be removed from the abrasive layer surface 160 using a liquid detergent.

The polishing pad 100 may be composed only of the polishing layer 120. However, if necessary, the support layer 110 can be fabricated using a method well known in the art of fabricating the polishing pad 100 and bonded to the polishing layer 120 to complete the polishing pad 100.

Further details of the invention will be described by way of explanation of specific experimental examples. The details not described below are because they can be technically inferred from those having ordinary knowledge in the art, and thus are omitted. Obviously, the scope of the present invention is not limited to the following experimental examples.

<Experimental Example 1>

50 kg of polytetramethylene glycol (molecular weight 1000), 50 kg of polyethylene glycol (molecular weight 1000) and 52 kg of toluendiisocyanate (toluendiisocyanate) were placed in a 200 kg reactor. The reaction was carried out at 70 ° C to 80 ° C for 4 hours to 5 hours, so that the final product had an NCO content of 11.0%. Viscosity of the obtained isocyanate prepolymer (viscosity) is 6,900 cPs (25 ° C).

<Experimental Example 2>

100 kg of the isocyanate prepolymer prepared in Experimental Example 1, 46 kg of mineral oil (manufactured by Seojin Chemical Co., Ltd., hereinafter referred to as KF-70), and 33 kg of MOCA at a mixing head of 5000 rpm ( The mixing head) is mixed and ejected using a casting machine. In this case, an inert gas (i.e., Ar gas) was placed in the mixing head at a volume ratio of 10%.

Immediately thereafter, the mixture was poured into a rectangular mold. Gelation was then carried out for 30 minutes, after which hardening was carried out in an oven at 100 ° C for 20 hours.

The hardened mixture is demolded and the surface of the hardened mixture is cut to form an abrasive layer of the polishing pad.

Figure 5 presents a scanning electron microscope (SEM) photograph of a hole formed on the surface of the abrasive layer.

The polishing performance and the flattening performance of the obtained polishing pad are shown in Fig. 7 (the polishing pad according to the present embodiment is referred to as "hybrid pore 1").

<Experimental Example 3>

100 kg of the isocyanate prepolymer prepared in Experimental Example 1, 46 kg of mineral oil (manufactured by Seojin Chemical Co., Ltd., hereinafter referred to as KF-70), and 33 kg of MOCA were mixed at a mixing head of 5000 rpm. After that, use a casting machine to shoot. In this case, an inert gas (i.e., Ar gas) was placed in the mixing head at a volume ratio of 20%.

Immediately thereafter, the mixture was poured into a rectangular mold. Gelation was then carried out for 30 minutes, after which hardening was carried out in an oven at 100 ° C for 20 hours.

The hardened mixture is demolded and the surface of the hardened mixture is cut to form an abrasive layer of the polishing pad.

Figure 6 presents a scanning electron microscope (SEM) photograph of a hole formed on the surface of the abrasive layer.

Fig. 7 shows the polishing performance and the flattening performance of the obtained polishing pad (the polishing pad according to the present embodiment is referred to as "mixing hole 2"). The "solid-state capsule hole" in Fig. 7 indicates that different types of composite holes (i.e., the first hole and the second hole) are not generally used as in the present invention, but only a single solid capsule hole is used (here, the solid capsule may represent a hollow The polishing pad of the micropowder, and the "liquid microcell hole" in Fig. 7 also means that different types of composite holes (i.e., the first hole and the second hole) are not generally used as in the present invention, but only a single liquid trace is included. The polishing pad of the unit.

When using composite holes of different sizes, the smaller first hole collects a small amount of the grinding liquid particles, thereby performing precise grinding, while the larger second hole collects a large amount of the polishing liquid particles at a time, thereby enabling high grinding. Speed is processed.

In this respect, the abrasive layer of the polishing pad contains different kinds of holes at the same time, so that more complicated grinding work can be performed.

Two holes are formed in the foregoing embodiment. However, the aspect of the invention is not limited to this as described above.

That is, the polishing pad according to the present invention may include three or more of a hole formed of a liquid micro cell, a hole formed of a solid capsule, a hole formed by injecting an inert gas, and a hole formed of a chemical foaming agent. Hole. The size of the holes depends on the kind thereof, and may be changed as described above, depending on the kind of material used to form the holes, or may be changed in order to increase the grinding efficiency.

Specifically, in the method of forming a hole, the size of the hole may be made of a mixed material The concentration of the material or the reaction temperature is controlled, and different types of pores do not necessarily have different sizes.

As described above, according to the present invention, a plurality of (two types) of holes (rather than a single type of holes) are controlled, so that the collection and application of the slurry can be efficiently performed, and a more excellent CMP polishing performance can be achieved. This enables a more complex CMP process required for miniaturized semiconductor processes.

While the invention has been particularly shown and described with reference to the exemplary embodiments of the present invention, it will be understood by those of ordinary skill in the art Various forms or details can be changed.

100‧‧‧ polishing pad

110‧‧‧Support layer

120‧‧‧Abrasive layer

130‧‧‧Polymer subject

141‧‧‧ first hole

141’, 142’‧‧‧ micro holes

142‧‧‧Second hole

160‧‧‧ surface

Claims (7)

A method of manufacturing a polishing pad comprising: (a) mixing a material for forming an abrasive layer; (b) at least one of an inert gas capable of controlling a pore size, a capsule type foaming agent, a chemical foaming agent, and a liquid micro unit Both are mixed with the mixture in (a) to form two or more holes; (c) gelling and hardening the mixture produced in (b) to form a hole including the two or more kinds of holes An abrasive layer; and (d) treating the abrasive layer to disperse micropores defined by opening the two or more holes on a surface of the abrasive layer . The method for producing a polishing pad according to claim 1, wherein the inert gas is in a group consisting of a group 8 element of the periodic table and a gas which does not react with the material for forming the polishing layer. Elected. The method for producing a polishing pad according to claim 1, wherein the liquid material constituting the liquid micro unit comprises aliphatic mineral oil, aromatic mineral oil, eucalyptus oil having no hydroxyl group at the molecular end, soybean oil, and coconut. At least one selected from the group consisting of oil, palm oil, cottonseed oil, camellia oil, hardened oil, or a combination thereof. An abrasive pad that performs a polishing process by contacting a surface of an article to be polished and moving upon contact, the polishing pad comprising an abrasive layer, wherein the abrasive layer comprises two or more holes, The size of two or more holes is controlled by using at least two of an inert gas, a capsule type blowing agent, a chemical agent, and a liquid micro unit, and by opening the two or more holes The defined micropores are distributed on the surface of the abrasive layer. The polishing pad according to claim 4, wherein the inert gas is selected from the group consisting of a group 8 element of the periodic table and a gas which does not react with a material for forming the polishing layer. The polishing pad according to claim 4, wherein the liquid material constituting the liquid micro unit comprises aliphatic mineral oil, aromatic mineral oil, eucalyptus oil having no hydroxyl group at the molecular end, soybean oil, coconut oil, palm. At least one selected from the group consisting of oil, cottonseed oil, camellia oil, hardened oil, or a combination thereof. The polishing pad of claim 4, wherein the polishing layer comprises a plurality of first holes formed by the liquid micro-cells and a plurality of second holes having a relatively large size, and the plurality of The two holes are formed by using at least one of injecting the inert gas, injecting the capsule type foaming agent, and injecting the chemical foaming agent.