US20120085038A1

US20120085038A1 - Method for manufacturing porous sheet and porous sheet manufactured by the method

Info

Publication number: US20120085038A1
Application number: US13/377,792
Authority: US
Inventors: Seong-Uk Jeong; Byeong-In Ahn; Young-ji Tae; Keong-Yeon YOON
Original assignee: LG Chem Ltd
Current assignee: LG Chem Ltd
Priority date: 2009-06-10
Filing date: 2010-06-10
Publication date: 2012-04-12
Also published as: TW201109375A; WO2010143899A9; KR20100132933A; JP5748747B2; WO2010143899A3; WO2010143899A2; KR101234313B1; JP2012529553A; TWI466930B

Abstract

The present invention provides a method for producing a porous sheet, including the steps of a) producing a polymer resin sheet containing an object to be processed by supercritical fluid extraction which is dissolved in supercritical fluid; and b) injecting the supercritical fluid into the polymer resin sheet to extract the object to be processed by supercritical fluid extraction that is contained in the polymer resin sheet, thereby forming pores in the polymer resin sheet, and a porous sheet produced by the same.

Description

TECHNICAL FIELD

The preset invention relates to a method for producing a porous sheet using a supercritical fluid extraction (SFE) method, and a porous sheet produced by the same.
This application claims priority from Korean Patent Application No. 10-2009-0051598 filed on Jun. 10, 2009 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety.

BACKGROUND ART

Chemical-mechanical polishing (CMP) processes are used in the manufacturing of microelectronic devices to form flat surfaces on semiconductor wafers, field emission displays, and many other microelectronic substrates. For example, the manufacture of semiconductor devices generally involves the formation of various process layers, selective removal or patterning of portions of those layers, and deposition of additional process layers on the surface of a semiconducting substrate to form a semiconductor wafer. The process layers can include, by way of example, insulation layers, gate oxide layers, conductive layers, and layers of metal or glass, etc. It is generally desirable in certain steps of the wafer process that the uppermost surface of the process layers be planar, i.e., flat, for the deposition of subsequent layers.
Chemical-mechanical polishing (CMP) processes are used to planarize process layers wherein a deposited material, such as a conductive or insulating material, is polished and removed to planarize the wafer for subsequent process steps.
In a typical chemical mechanical polishing (CMP) process, a wafer is mounted upside down on a carrier in a CMP tool. A force pushes the carrier and the wafer downward toward a polishing pad. The carrier and the wafer are rotated above the rotating polishing pad on the CMP tool's polishing table. A polishing composition (also referred to as a polishing slurry) generally is introduced between the rotating wafer and the rotating polishing pad during the polishing process. The polishing composition typically contains a chemical that interacts with or dissolves portions of the uppermost wafer layer(s) and an abrasive material that physically removes portions of the layer(s). The wafer and the polishing pad can be rotated in the same direction or in opposite directions, whichever is desirable for the particular polishing process being carried out. The carrier also can oscillate across the polishing pad on the polishing table.
Polishing pads used in the chemical mechanical polishing (CMP) process are manufactured using both soft and rigid pad materials, which include polymer-impregnated fabrics, microporous films, cellular polymer foams, non-porous polymer sheets, and sintered thermoplastic particles.
A pad containing a polyurethane resin impregnated into a polyester non-woven fabric is illustrative of a polymer-impregnated fabric polishing pad. Microporous polishing pads include microporous urethane films coated onto a base material. These polishing pads are closed cell, porous films. Cellular polymer foam polishing pads contain a closed cell structure that is randomly and uniformly distributed in all three dimensions.
Recently, in most of polishing pads, porous sheets having closed-pores are used in the pad, in which the pores control fluidity of polishing slurry to improve process efficiency. Therefore, it is important to uniformly disperse the pores in the polishing pad.
An example of the production method of polishing pads is to produce pads by adding hollow polymeric microelements to a polymeric matrix, as described in Korean Patent No. 10-0191227.
However, the hollow polymeric microelements have shells with a thickness of several microns, and problematically, these shells generate scratches on a polishing object, for example, a wafer during chemical mechanical polishing (CMP) process.

DISCLOSURE

Technical Problem

The present invention provides a method for producing a porous sheet, which forms pores having excellent uniformity and dispersibility, reduces generation of scratches during the process, and improves process efficiency, and a porous sheet produced by the same.

Technical Solution

The present invention provides a method for producing a porous sheet, including the steps of a) producing a polymer resin sheet containing an object to be processed by supercritical fluid extraction which is dissolved in supercritical fluid; and b) injecting the supercritical fluid into the polymer resin sheet to extract the object to be processed by supercritical fluid extraction that is contained in the polymer resin sheet, thereby forming pores in the polymer resin sheet.
The present invention provides a porous sheet produced by the production method according to the present invention.
The present invention provides a polishing pad containing the porous sheet according to the present invention.

Advantageous Effects

According to the present invention, provided is a method for producing a porous sheet, capable of forming pores having excellent uniformity and dispersibility in a sheet. In addition, pores can be formed without remaining residue in the sheet, because the method is performed by extracting a material dissolved in supercritical fluid. Therefore, during polishing process, generation of scratches on a polishing object, wafer due to residue can be reduced, and the process efficiency can be improved.

DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating the method for producing a porous sheet according to the present invention;

FIG. 2 is a SEM photograph of the porous sheet according to Example 1 of the present invention;

FIG. 3 is a graph showing the experimental results of Example 1 and Examples 3 to 5; and

FIG. 4 is a SEM photograph of the sheet according to Comparative Example.

EMBODIMENTS OF THE INVENTION

The method for producing a porous sheet according to the present invention includes the steps of a) producing a polymer resin sheet containing an object to be processed by supercritical fluid extraction which is dissolved in supercritical fluid; and b) injecting the supercritical fluid into the polymer resin sheet to extract the object to be processed by supercritical fluid extraction that is contained in the polymer resin sheet, thereby forming pores in the polymer resin sheet.
In step a), the object to be processed by supercritical fluid extraction may be one material selected from aromatic compounds, aliphatic hydrocarbons and aliphatic alcohols.
Herein, examples of the aromatic compounds may include naphthalene, anthracene, chrysene and pentacene. The aliphatic hydrocarbons may be C₇to C₁₀aliphatic hydrocarbons, but are not limited thereto, specific examples thereof may include mineral oil, octane, decane, and dodecane. Examples of the aliphatic alcohols may include heptanol, nonanol and dodecanol.
Among them, it is preferable that the object to be processed by supercritical fluid extraction may be naphthalene or octane, but is not limited thereto.
The object to be processed by supercritical fluid extraction that is contained in the polymer resin sheet may have a round or oval shape, but is not limited thereto.
In step a), the object to be processed by supercritical fluid extraction may be contained in the polymer resin sheet in an amount of 5 to 50% by weight, and more preferably 20 to 40% by weight.
The polymer resin sheet of step a) may include a polymer resin selected from the group consisting of polyurethane, thermoplastic elastomer, polyolefin, polycarbonate, polyvinyl alcohol, nylon, elastomeric rubber, styrene-based copolymer, polyaromatics, fluoropolymer, polyimide, cross-linked polyurethane, cross-linked polyolefin, polyether, polyester, polyacrylate, elastomeric polyethylene, polytetrafluorethylene, polyethylene terephthalate, polyarylene, polystyrene, polymethylmethacrylate, copolymers thereof, block copolymers thereof, mixtures thereof and blends thereof. Herein, the polyurethane resin is a most preferable material for forming the polishing pad because it is excellent in abrasion resistance. In addition, a polyurethane polymer having desired physical properties can be easily obtained by changing its raw material composition. These properties of polyurethane are suitable for forming the polishing pad.
Step a) may include the steps of a1) mixing the object to be processed by supercritical fluid extraction with a polymer resin or a precursor; and a2) curing the mixture of step a1).
In step a1), the mixing conditions of the object to be processed by supercritical fluid extraction and the polymer resin or precursor may vary depending on impeller speed and reactor temperature in a reactor. In addition, a curing agent may be used during the mixing process, simultaneously. Herein, the impeller speed and reactor temperature may be controlled variously, but the preferred impeller speed may be 200 to 3000 rpm and the preferred reactor temperature may be 40 to 70° C.
In step a1), one or more chain extenders selected from the group consisting of 1,4-butanediol, 4,4′-methylenebis(2-chloroaniline), ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,2-butanediol, 1,6-hexanediol, neopentyl glycol, 1,4-cyclohexanediol, 1,4-cyclohexanedimethanol, 2,2,4-trimethylpentanediol, hydroquinone, bis (2-hydroxyethyl) hydroquinone, 4,4′-dihydroxybiphenyl, bisphenol A, bisphenol F, and mixtures thereof may be further added.
In step a2), the curing process may be performed at 70 to 100° C. for 4 to 48 hrs.
In step a), in the case of producing a sheet using a polyurethane resin, the polyurethane may be prepared by an organic polyisocyanate, a polyurethane prepolymer, a polyol compound and a chain extender.
Herein, the organic polyisocyanate of 1 to 20% by weight, the polyurethane prepolymer of 10 to 88% by weight, the polyol compound of 10 to 88% by weight and the chain extender of 1 to 50% by weight may be included.
Examples of the organic polyisocyanate may include aromatic diisocyanates such as 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, 2,2′-diphenylmethane diisocyanate, 2,4′-diphenylmethane diisocyanate, 4,4′-diphenylmethane diisocyanate, 1,5-naphthalene diisocyanate, p-phenylene diisocyanate, m-phenylene diisocyanate, p-xylylene diisocyanate, and m-xylylene diisocyanate; aliphatic diisocyanates such as ethylenediisocyanate, 2,2,4-trimethylhexamethylene diisocyanate, and 1,6-hexamethylene diisocyanate; and cycloaliphatic diisocyanates such as 1,4-cyclohexane diisocyanate, 4,4′-dicyclohexylmethane diisocyanate, isophorone diisocyanate, hydrogenated m-xylylene diisocyanate, and norbornane diisocyanate. Mixtures of one or two or more thereof may be used. However, the organic polyisocyanate is not limited thereto.
The organic polyisocyanate includes not only the diisocyanate compounds described above but also multifunctional (trifunctional or more) polyisocyanate compounds. As the multifunctional isocyanate compounds, Desmodule-N (manufactured by Bayer Ltd.) or a series of diisocyanate adduct compounds under the trade name of Duranate (Asahi Kasei Corporation) are commercially available.
The polyol compound is exemplified by high-molecular weight polyols such as polyether polyol, polyester polyol, polycarbonate polyol, and acryl polyol. In addition to the high-molecular weight polyols, low-molecular weight polyols may be also used. These polyol compounds may be used in mixtures of two or more thereof. However, the polyol compound is not limited thereto.
The ratio of the organic polyisocyanate, polyol compound and chain extender may vary depending on molecular weight of each compound or use of the polyurethane produced by these compounds (e.g., polishing pad), and desired properties. To obtain a polishing pad having desirable polishing properties, the number of isocyanate groups of organic polyisocyanate to total number of functional groups of the polyol compound and chain extender (the number of active hydrogen groups of hydroxyl group, amino group, etc.) may preferably range from 0.95 to 1.15, and more preferably from 0.99 to 1.10. Meanwhile, the ratio of high molecular weight component to low molecular weight component in the polyol compound may be determined by the properties required for polyurethane produced therefrom.
In step a), in the case of producing a sheet using the polyurethane resin, the polyurethane resin to be a matrix may be produced by application of urethane technology such as a melting method, a solution method or the like. In consideration of cost and operating environment, the melting method is preferable.
Any one of one-shot and prepolymer methods may be employed for the production of polyurethane. However, in terms of physical properties of produced polyurethane, suitable is a prepolymer method, in which an isocyanate-terminal prepolymer from organic polyisocyanate and polyol compound is previously synthesized and a chain extender is reacted therewith.
Meanwhile, any commercially available isocyanate-terminal prepolymer that is produced from organic polyisocyanate and polyol compound may be used, if suitable in the present invention, and also applicable to the production of polyurethane by the prepolymer method. The isocyanate-terminal prepolymer having a molecular weight of approximately 800 to 5000 may be suitable in terms of processibility and physical properties.
In the prepolymer method, the isocyanate-terminal prepolymer is an isocyanate group-containing compound, and the chain extender (if necessary, polyol compound) is an active hydrogen group-containing compound. In the one shot process, the organic polyisocyanate is an isocyanate group-containing compound, and the chain extender and polyol compound is an active hydrogen group-containing compound.
In the production of polyurethane, stabilizers such as antioxidants, surfactants, lubricants, pigments, fillers, antistatic agents, and other additives may be added to the polyurethane stock solution, if necessary.
In step b), the method of injecting supercritical fluid into the polymer resin sheet produced in step a) may be performed by pressurized gas injection process involving the use of high pressures to force supercritical fluid into the polymer resin sheet.
The supercritical fluid injected into the polymer resin sheet in step b) will be described in detail.
Herein, supercritical fluid means a material, having characteristics of both liquid and gas at usual temperature and pressure, being at a critical state above supercritical point, that is, at which a liquid cannot be distinguished from a gas at high temperature and pressure called a supercritical point, because a chemical can no longer be vaporized.
The supercritical fluid is produced by applying increasing temperature and pressure to gas, the temperature and pressure being sufficient to maintain the fluid in a supercritical state.
The gas may be hydrocarbon, chlorofluorocarbon, hydrochlorofluorocarbon (e.g., Freon), nitrogen, carbon dioxide, carbon monoxide or combinations thereof.
The preferred gas is nonflammable gas, for example, gas having no C—H bonds. More preferably, the gas is nitrogen, carbon dioxide or combinations thereof. Most preferably, the gas is carbon dioxide or gas containing carbon dioxide.
It is preferable that the gas is converted into supercritical gas before injection into the polymer resin sheet.
If the gas is carbon dioxide, the temperature is over 31° C. and the temperature ranges from 7 MPa (about 1000 psi) to 35 MPa (about 5000 psi) (e.g., 19 MPa (about 2800 psi) to 26 MPa (about 3800 psi)).
The supercritical fluid of step b) may preferably include one or more selected from supercritical carbon dioxide, supercritical isobutane, supercritical butane, supercritical propane, supercritical pentane, and supercritical nitrogen.
Step b) may be performed at a pressure of 50 to 300 atm and at a temperature of 25 to 120° C., and preferably at a pressure of 70 to 200 atm and at a temperature of 30 to 80° C. Step b) may be performed in a supercritical equipment known in the art.
In step b), a supercritical fluid extraction method may be performed by mixing with acetone, alcohol or the like.
Herein, when supercritical fluid is injected, the solvent may be added at the same time, or the solvent is previously put in a supercritical reactor, and then mixing process is performed. The solvent may vary depending on the object to be processed by supercritical fluid extraction that is contained the polymer resin sheet. For example, acetone, alcohol, or hexane is a material capable of well dissolving the object to be processed by supercritical fluid extraction.
As such, when the supercritical fluid of step b) is injected into the polymer resin sheet produced in step a), the supercritical fluid dissolves the object to be processed by supercritical fluid extraction that is contained in the sheet, thereby forming pores within the polymer resin sheet produced in step a) without residues. Herein, the formed pores may be closed pores, in which the closed pore means a pore that does not connected to other pores.
In step b), the pores formed within the polymer resin sheet may have a round or oval shape, but is not limited thereto.
In step b), the pores formed within the polymer resin sheet may have an average diameter of 80 micrometers or less, preferably 5 to 50 micrometers, and more preferably 10 to 30 micrometers. Herein, the average diameter of the pores denotes the mean values of lines, several of which pass through the center of circles from their circumference.
In step b), the sheet having the pores formed therewithin may have a density of 0.5 to 1 g/cm³, preferably 0.6 to 1 g/cm³, and more preferably 0.7 to 0.9 g/cm³.
In step b), the polymer resin sheet having the pores formed therewithin may have a porosity of 50% or less, preferably a porosity of 10 to 50%, and more preferably a porosity of 20 to 40%.
As such, according to the present invention, pores may be formed within the polymer resin sheet without residues (see FIG. 2). However, in the known high pressure gas foaming methods, when the foaming process is performed by injecting into the cured, namely, cross-linked polyurethane, there is a problem that foaming may not occur depending on the degree of cure. In polyurethane being not cross-linked or having a low modulus, foaming occurs by pressurized gas injection, but practically, it is difficult to show properties suitable for CMP pad. When the foaming process is performed in polyurethane having a high degree of cure by pressurized gas method, foaming does not occur or a polymeric matrix can be broken (see FIG. 4).
Meanwhile, the present invention provides a porous sheet produced by the above described method.
The porous sheet according to the present invention may be used as a polishing pad. The porous sheet may be used as a polishing pad, singly and a plurality of the porous sheets are laminated and used as a polishing pad. In addition, other film attached to the porous sheet may be used as a polishing pad.
Hereinbelow, the present invention will be described in detail with reference to FIG. 1.
As shown in FIG. 1, the prepolymers of polyurethane are mixed with aliphatic hydrocarbon, naphthalene or fish oil which is dissolved in supercritical fluid, and well dispersed. This prepolymer mixed solution is reacted with 1.4-butanediol or 4,4′-methylenebis(2-chloroaniline) as a chain extender to form polymer chains. The resultant is cured in a 100° C. oven for 4 hrs to 6 hrs in order to form a desired shape through a cast. The cured polyurethane is put in a supercritical equipment to extract a solute. Herein, CO₂may be used as a supercritical fluid, and supercritical fluid extraction may be performed by mixing with acetone or alcohol. Specifically, a material such as acetone or alcohol is put into a polyurethane sheet to be extracted, simultaneously. That is, a suitable amount of acetone or alcohol is put into the supercritical fluid extraction equipment, and then the supercritical fluid extraction equipment is closed, followed by injection of CO₂to a desired pressure. Alternatively, injection of CO₂into the supercritical equipment is performed at the same time.

EXAMPLES

Hereinafter, the present invention will be described in more detail with reference to Examples.

Example 1

In Example 1, prepolymer and octane were first mixed with each other to prepare a mixed solution. L325 (manufactured by Chemtura, NCO % 9.17%) was used as a prepolymer, and octane was used as an object to be processed by supercritical fluid extraction which is dissolved in supercritical fluid, that is, as a porogen.
First, 1000 g of prepolymer and 400 g of octane was put in a 50° C. reactor, and mixed with each other from 5 min. In this case, octane was dispersed in prepolymers to form spherical liquid-drops within the prepolymers. Thereafter, 260 g of MOCA (methylenebis(2-chloroaniline)) was added thereto, and then mixed. The prepared polyurethane mixture was poured into a cast, and gelated at a room temperature for 1 hr, and then cured in a 100° C. oven for 24 hrs.
The cured polyurethane mixture was cut in a thickness of 3 mm, and placed in the supercritical fluid extraction equipment. Temperature of the supercritical fluid extraction equipment was set at 45° C., and carbon dioxide was pressurized into the equipment, and the pressure was maintained at 150 bar. The mixture was maintained in the equipment for 1 hr, and then depressurized. After the polyurethane sample was taken out of the equipment, and left at room temperature for 1 hr, and then placed in 100° C. oven for 1 hr.
The prepared sample had a density of 0.802 g/cm³and a Shore D hardness of 50. A SEM photograph of the sample is shown in FIG. 2. The storage modulus and tan delta of the sample are shown in FIG. 3.
As such, when a porous sheet is produced by supercritical fluid extraction (SFE) according to the present invention, pores can be formed without remaining residue in the sheet because the method is performed by extracting a material dissolved in supercritical fluid. Therefore, generation of scratches on a polishing object due to residue of the sheet during polishing process can be reduced, and the process efficiency can be improved.

Example 2

In Example 2, prepolymer and MOCA were first mixed, and then octane was mixed therewith.
First, as a prepolymer, 1000 g of L325 (manufactured by Chemtura, NCO % 9.17%) was put in a reactor and the temperature was maintained at 50° C. 260 g of MOCA previously dissolved was put and mixed at 1000 rpm for 30 sec, and then 400 g of octane was mixed therewith.
The prepared polyurethane mixture was poured into a cast, and gelated at a room temperature for 30 min, and then cured in a 100° C. oven for 16 hrs.
The cured polyurethane mixture was cut in a thickness of 3 mm, and placed in the supercritical fluid extraction equipment. Temperature of the supercritical fluid extraction equipment was set at 40° C., and carbon dioxide was pressurized into the equipment, and the pressure was maintained at 100 bar. The mixture was maintained in the equipment for 2 hrs, and then depressurized. After the polyurethane sample was taken out of the equipment, and left at room temperature for 1 hr, and then placed in 100° C. oven for 1 hr.

Example 3

In Example 3, in order to produce a pad having high modulus, H₁₂MDI was added.
The experiment was performed in the same manner as in Example 1, except that H₁₂MDI was added to L325 to prepare prepolymers of NCO % 9.7%. The storage modulus and tan delta of the pad produced in Example 3 are shown in FIG. 3.

Example 4

In Example 4, in order to produce a pad having high modulus, H₁₂MDI was added.
The experiment was performed in the same manner as in Example 1, except that H₁₂MDI was added to L325 to prepare prepolymers of NCO % 11%. The storage modulus and tan delta of the pad produced in Example 4 are shown in FIG. 3.

Example 5

In Example 5, in order to produce a pad having high modulus, H₁₂MDI was added.
The experiment was performed in the same manner as in Example 1, except that H₁₂MDI was added to L325 to prepare prepolymers of NCO % 12%. The storage modulus and tan delta of the pad produced in Example 5 are shown in FIG. 3.

Example 6

First, 1000 g of prepolymer, 300 g of octane, and 100 g of dodecane were put in a 50° C. reactor, and mixed with each other for 2 min. In this case, octane and dodecane were dispersed in prepolymers to form spherical liquid-drops (1 to 100 micrometers) within the prepolymers. Thereafter, 260 g of MOCA (methylenebis(2-chloroaniline)) was added thereto, and then mixed. The prepared polyurethane mixture was poured into a cast, and gelated at a room temperature for 1 hr, and then cured in a 100° C. oven for 24 hrs. After curing process, the experiment was performed in the same manner as in Example 1. A pore of the produced pad had a diameter of 10 to 70 micrometers.

Example 7

In the same manner as in Example 1, the cured pad sheet was put in the supercritical fluid extraction equipment. The temperature and pressure were maintained at 50° C. and 150 bar, respectively. After 1 hr, the pressure was eliminated, and the sheet was taken out, and CO₂was completely removed therefrom in a 100° C. oven. The produced pad had a density of 0.75 g/cm³.
The experimental results of Example 1 and Examples 3 to 5 (storage modulus and tan delta measured) are shown in FIG. 3, and the measurement method of storage modulus and tan delta of FIG. 3 will be described in detail. The storage modulus and tan delta were measured using DMA8000 (manufactured by PerkinElmer) at a frequency of 1.5 Hz and amplitude of 0.05 mm, scanned at the temperature from -−0° C. to 100° C.
As shown in FIG. 3, the storage modulus of the sample (NCO % 9.17%) produced in Example 1 was found to have 396.5 MPa at 25° C.
The storage modulus of the sample, of which NCO % was increased to 9.7, 11, or 12% by addition of H₁₂MDI, was found to have 446.8, 580.3, 698.4 MPa, respectively, indicating that the increase of NCO % can improve the storage modulus.
Meanwhile, as shown in FIG. 3, tan delta tends to reduce as the storage modulus increases.

Comparative Example

First, as a prepolymer, 1000 g of L325 (manufactured by Chemtura, NCO % 9.17%) was put in a 50° C. reactor, and mixed for 5 min. Thereafter, 260 g of MOCA (methylenebis(2-chloroaniline)) was added thereto, and then mixed. The prepared polyurethane mixture was poured into a cast, and gelated at a room temperature for 1 hr, and then cured in a 100° C. oven for 24 hrs.
The cured polyurethane mixture was cut in a thickness of 3 mm, and placed in a supercritical foaming equipment. Temperature of the supercritical foaming equipment was set at 45° C., and carbon dioxide was pressurized into the equipment, and the pressure was maintained at 150 bar. The mixture was maintained in the equipment for 1 hr, and then depressurized. After the polyurethane sample was taken out of the equipment, and left at room temperature for 1 hr, and then placed in 100° C. oven for 1 hr.
A SEM photograph of the sheet according to Comparative Example was taken. As a result, cracks were found, as shown in FIG. 4. It was found that upon foaming polyurethane having high degree of cure by the pressurized gas method, the polymeric matrix is broken, as shown in FIG. 4.

Claims

1. A method for producing a porous sheet, comprising the steps of:

a) producing a polymer resin sheet containing an object to be processed by supercritical fluid extraction which is dissolved in supercritical fluid; and

b) injecting the supercritical fluid into the polymer resin sheet to extract the object to be processed by supercritical fluid extraction that is contained in the polymer resin sheet, thereby forming pores in the polymer resin sheet.

2. The method according to claim 1, wherein the object to be processed by supercritical fluid extraction contains one or more selected from the group consisting of aromatic compounds, aliphatic hydrocarbons and aliphatic alcohols.

3. The method according to claim 1, wherein the polymer resin sheet of step a) contains a polymer resin selected from the group consisting of polyurethane, thermoplastic elastomer, polyolefin, polycarbonate, polyvinyl alcohol, nylon, elastomeric rubber, styrene-based copolymer, polyaromatics, fluoropolymer, polyimide, cross-linked polyurethane, cross-linked polyolefin, polyether, polyester, polyacrylate, elastomeric polyethylene, polytetrafluorethylene, polyethylene terephthalate, polyarylene, polystyrene, polymethylmethacrylate, copolymers thereof, block copolymers thereof, mixtures thereof and blends thereof.

4. The method according to claim 1, wherein step a) includes the steps of a1) mixing the object to be processed by supercritical fluid extraction with a polymer resin or a precursor; and a2) curing the mixture of step a1).

5. The method according to claim 4, wherein in step a1), one or more chain extenders selected from the group consisting of 1,4-butanediol, 4,4′-methylenebis(2-chloroaniline), ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,2-butanediol, 1,6-hexanediol, neopentyl glycol, 1,4-cyclohexanediol, 1,4-cyclohexanedimethanol, 2,2,4-trimethylpentanediol, hydroquinone, bis (2-hydroxyethyl) hydroquinone, 4,4′-dihydroxybiphenyl, bisphenol A, bisphenol F, and mixtures thereof are further added.

6. The method according to claim 4, wherein in step a2), the curing process is performed at 70 to 100° C. for 4 to 48 hrs.

7. The method according to claim 1, wherein in step a), the object to be processed by supercritical fluid extraction is contained in the polymer resin sheet in an amount of 5 to 50% by weight.

8. The method according to claim 1, wherein the supercritical fluid of step b) contains one or more selected from the consisting of supercritical carbon dioxide, supercritical isobutane, supercritical butane, supercritical propane, supercritical pentane, and supercritical nitrogen.

9. The method according to claim 1, wherein the step b) is performed at a pressure of 50 to 300 atm and a temperature of 25 to 120° C.

10. The method according to claim 1, wherein in step b), the pores formed within the polymer resin sheet have an average diameter of 80 micrometers or less.

11. The method according to claim 1, wherein in step b), the sheet having the pores formed within the polymer resin sheet may have a density of 0.5 to 1 g/cm3.

12. The method according to claim 1, wherein in step b), the polymer resin sheet having the pores has a porosity of 50% or less.

13. A porous sheet produced by the method according to claim 1.

14. A porous sheet having pores, which is formed by extraction of an object to be processed by supercritical fluid extraction contained in a polymer resin sheet.

15. The porous sheet according to claim 14, wherein the object to be processed by supercritical fluid extraction contains one or more selected from the consisting of aromatic compounds, aliphatic hydrocarbons and aliphatic alcohols.

16. A polishing pad including the porous sheet according to claim 13.

17. A polishing pad including the porous sheet according to claim 14.