Multi-layer porous polymeric membrane and related filters and methods

Abstract

To provide a method of performing filtration to remove particles from a liquid such as a semiconductor processing fluid, a filter product, and a semiconductor processing filtration system.SOLUTION: Described are: assemblies of porous polymeric filter membranes that include two polymeric filter membranes in series, i.e., as part of a membrane assembly; filter components and filter products that include the two polymeric filter membrane layers; methods of assembling the polymeric filter membranes, filter components, and filter products; and methods of using the polymeric filter membranes to remove particles from a liquid such as a semiconductor processing liquid fluid.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The following description relates to an assembly including two polymeric filtration membranes in series, i.e., as part of a membrane assembly, and also to a filter component and a filter product including the two polymeric filtration membranes, and also to a method of assembling the polymeric filtration membranes, the filter component, and the filter product, and to a method of using a polymeric filtration membrane to remove particles from a liquid such as a semiconductor processing fluid.

Background Art

[0002] Polymeric filtration membranes are used to remove unwanted substances from fluids having various applications. Fluids include water, liquid industrial solvents and processing fluids, industrial gases used in manufacturing or processing, and liquids having medical or pharmaceutical applications. Contaminants and impurities removed from the fluid include particulate contaminants, microorganisms, and dissolved chemical species such as ionic species.

[0003] Two or more types of filtration membranes can be used together to remove various types of contaminants or impurities from a single liquid. A particular type of filtration membrane removes contaminants from a liquid via a sieving mechanism, thereby mechanically preventing contaminant particles larger than the pores of the membrane from passing through the membrane and being captured within the membrane so as not to pass through the membrane. Other membranes function via a non-sieving mechanism, whereby impurities smaller than the pores of the membrane are attracted to the membrane surface via chemical or electrostatic interactions, retained within the membrane, and do not leak from the membrane. By combining non-sieving membranes with sieving membranes, various contaminants of different sizes can be removed from a liquid containing particles of various sizes.

[0004] The use of filters to remove contaminants from liquids is particularly important in the semiconductor processing industry where even minimal impurities, sometimes in the nanoscale size range, can cause defects in ultra-small electronic devices. This applies to many of the fluids used during processing in semiconductors and ultra-small electronics technologies, such as the photoresist solutions used in photolithography.

[0005] In photolithography processing, a photosensitive (or radiation-sensitive) photoresist solution is used to form a pattern coating on a surface. The photoresist solution is coated as a thin layer on the substrate surface, and a pattern mask is used to cover part of the coated surface. The unmasked areas of the photoresist solution are exposed. Then, a solvent called a developer is applied to the surface of the photoresist solution. In the case of a "positive-type photoresist", the exposed portions of the photoresist are degraded by light, and the developer dissolves and removes the areas of the exposed photoresist coating, leaving the pattern coating where the mask was placed. In the case of a "negative-type photoresist", the exposed portions of the photoresist are strengthened by light (either polymerization or cross-linking), and the developer dissolves and removes only the masked areas of the photoresist that were not exposed, leaving the pattern coating in the areas not covered by the mask.

[0006] Conventional photoresist solutions may contain three different components, namely a polymer resin (a binder that gives physical properties such as adhesion, chemical resistance, etc.), a sensitizer (having a photoactive compound), and a solvent. Unfortunately, photoresist solutions also contain a significant amount of unwanted contaminant particles that can potentially end up on the surface of the semiconductor substrate, and those contaminants can cause defects during subsequent processing. The contaminants can be particles or suspended or dissolved chemical species and can have a size range that includes particles in the nanometer scale size.

[0007] In the semiconductor manufacturing industry, there is a continuous need for advancing filtration technology that effectively removes a high percentage of contaminants of various types and sizes from liquids such as photoresist solutions.

SUMMARY OF THE INVENTION

[0008] A filter assembly, a product, and a method of using a first polymeric filtration membrane, also referred to as a “coarse” membrane, and a second polymeric filtration membrane, also referred to as a “tight” membrane, are described below. The first (coarse) polymeric filtration membrane has, on average, relatively large pores and a lower bubble point compared to a second polymeric filtration membrane having relatively small pores and a higher bubble point. The first or “coarse” polymeric filtration membrane also has a relatively large thickness compared to the smaller thickness of the second or “tight” polymeric filtration membrane.

[0009] Polymeric filtration membranes are useful for removing contaminants from the flow of liquid through the membrane, for example, by first passing the liquid through the first (coarse) polymeric filtration membrane and then through the second (tight) polymeric filtration membrane.

[0010] According to examples of methods and membranes, and not limited to this specification, the first (coarse) polymeric filtration membrane can be a polyolefin membrane prepared by a phase inversion technique, and the second (tight) polymeric filtration membrane can be a polyolefin membrane prepared by a stretching technique. Generally, phase inversion polyolefin filtration membranes have a greater thickness and a more complex microporous structure than stretched polyolefin filtration membranes, and phase inversion polyolefin filtration membranes can exhibit a greater retention capacity and a greater holding capacity for a wider range of particles of various sizes.

[0011] It is difficult to make a polymeric filtration membrane that has a smaller pore size but does not cause a significant loss of the membrane's flow rate. The stretched polyolefin filtration membrane can be very thin, such as having a thickness of less than 10 microns, but also has a relatively small pore size and may achieve a useful combination of both a very dense (small) pore size and good flow rate. However, the thinner stretched membrane has a limited effective surface area and a small holding capacity because the space within the thinner membrane is small.

[0012] According to the embodiments described herein, a first (coarse) polymeric filtration membrane that can be prepared by the phase inversion method and a second (dense) polymeric filtration membrane that can be prepared by the stretching method can be used together in series to remove contaminants from a liquid. The coarse polymeric filtration membrane can remove relatively large-sized contaminants from a liquid containing contaminants of various sizes, large and small. The dense polymeric filtration membrane can remove relatively small-sized particles or dissolved contaminants that can pass through the first (upstream) polymeric filtration membrane. The combination of the two polymeric filtration membranes is effective in removing contaminants of various types and sizes, including particulate contaminants that can be relatively large in size and dissolved or suspended chemical molecules that may be relatively small in size, from the liquid passing through both the first polymeric filtration membrane and the second polymeric filtration membrane.

[0013] In one aspect, this specification relates to a method of filtering a semiconductor processing liquid. The method includes passing a liquid through a first polymeric filtration membrane having an average bubble point of less than 120 pounds per square inch and a thickness of at least 20 microns, measured using ethoxy-nonafluorobutane (HFE7200), and passing the liquid through a second polymeric filtration membrane having an average bubble point of greater than 120 pounds per square inch and a thickness of less than 20 microns, measured using ethoxy-nonafluorobutane (HFE7200).

[0014] According to another aspect, the present specification relates to a filter product useful for removing contaminants from semiconductor processing fluids. The filter product includes a first polymeric filtration membrane having an average bubble point of less than 120 pounds per square inch as measured using ethoxy-nonafluorobutane (HFE7200) and a thickness of at least 20 microns, and a second polymeric filtration membrane having an average bubble point greater than 120 pounds per square inch as measured using ethoxy-nonafluorobutane (HFE7200) and a thickness of less than 20 microns.

Brief Description of the Drawings

[0015]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Modes for Carrying Out the Invention

[0016] The drawings are schematic and not necessarily to scale.

[0017] The following describes a polymer filtration membrane (or, as used herein, more simply, a "membrane", "filtration membrane", etc.), an assembly containing two or more polymer filtration membranes including a "first" (or "coarse") polymer filtration membrane and a "second" (or "dense") polymer filtration membrane, a filter product containing the first and second polymer filtration membranes, and related methods of using the polymer filtration membrane assembly and filter product, particularly for removing contaminants from semiconductor processing liquids. Here, the first polymer filtration membrane may also be referred to as the "first membrane" or "coarse membrane", and the second polymer filtration membrane may also be used to refer to the "second membrane" or "dense membrane".

[0018] As described, the filter product and filtration method use an arrangement of two different polymer filtration membranes, i.e., at least two different "membranes" or "membrane layers" arranged in series, to perform the filtration step. This arrangement may also be referred to as a "laminated" arrangement or "multilayer membrane assembly" that includes two different polymer filtration membranes, and in some cases, may include additional filtration membranes or support layers. The polymer filtration membranes may be assembled as a laminated composite membrane, in which case the first and second polymer filtration membranes are joined together by lamination (without using a separate adhesive), or the polymer filtration membranes may not be adhered, i.e., may not be laminated.

[0019] The first polymeric filtration membrane or "coarse membrane" is adapted to remove relatively large-sized contaminants from a liquid containing contaminants of various sizes. The coarse membrane has pores with a relatively large pore diameter compared to the smaller pores of the dense membrane. The dense membrane may have smaller pores and a smaller thickness compared to the large thickness of the coarse membrane. A combination of two polymeric filtration membranes is effective in removing various types of particles, including particulate contaminants that may be relatively large in size and dissolved or suspended chemical molecules that may be relatively small in size, from a liquid passing through both the first and the second polymeric filtration membranes. The dense membrane is adapted to remove and separate relatively small-sized particles or dissolved contaminants that may pass through the coarse membrane.

[0020] According to an example of a method and a filter assembly, the liquid can be made to flow first through a coarse membrane having a large thickness and large pores and then through a dense membrane having a small thickness and small pores. An assembly including a combination of a coarse upstream polymeric filtration membrane and a dense downstream polymeric filtration membrane can be useful in removing contaminants of various sizes from a liquid. Desirably, an assembly including both a coarse membrane and a dense membrane is effective in removing a greater amount (higher percentage) of contaminants (e.g., of a particular size) from the liquid compared to the amount of contaminants removed by filtering the liquid using only the dense membrane and the amount of contaminants removed by filtering the liquid using only the coarse membrane. The performance of a polymeric filtration membrane in removing contaminants of a particular size from a liquid can be measured in terms of "retention" and evaluated by a retention test that removes standard particles (or "test particles") from a sample liquid having a known concentration of particles under controlled conditions.

[0021] According to certain membrane assemblies and methods, contaminants of various sizes can be removed from a liquid containing contaminants using a membrane assembly that includes at least one coarse membrane and at least one dense membrane. The liquid can be any type of liquid that has a significantly high purity but contains contaminants of a wide range of sizes for which filtration is desired. Examples of such liquids include various types of liquids used in the processing of semiconductors or microelectronic devices, which types of liquids are referred to herein as "semiconductor processing fluids." Semiconductor processing fluids include organic solvents such as water or isopropyl alcohol (one of many other organic and inorganic chemical substances including organic solvents and organic or inorganic acids and bases; see below); reactive substances such as reactive monomers, oligomers, or polymers, including but not limited to those used in photosensitive materials such as photoresist solutions; cleaning liquids; liquids used as acids or bases, or liquids containing one or more of them. A specific example of a semiconductor processing fluid is a "photoresist solution" that includes a polymer resin, a sensitizer (having a photoactive compound), and a solvent.

[0022] The coarse polymer membrane and the dense polymer membrane each have two opposing surfaces or opposing "sides" and a membrane thickness between those two opposing surfaces. The pores of the polymer filtration membrane are located throughout the thickness of the membrane and allow the flow of liquid from one side of the membrane, through the thickness of the membrane, to the opposite side of the membrane and through the opposite side of the membrane. This type of membrane is sometimes referred to as a "coarse pore" membrane.

[0023] A coarse pore membrane can be in the form of a thin film or sheet of a porous polymeric material having a coarse pore structure that includes a polymeric matrix defining a relatively uniform thickness and a large number of coarse "cells" or "pores". The coarse cells can be referred to as openings, pores, channels, or passageways that are mostly interconnected between adjacent cells to allow fluid to flow through the thickness of the membrane from one side of the membrane to the other. The passageways can provide tortuous tunnels or passageways through which the liquid to be filtered needs to pass. Contaminants in the form of particles or dissolved or suspended molecular chemical species contained in the liquid passing through the polymeric filter membrane are captured by or within the membrane based on either size (i.e., the "sieving" mechanism) or chemical or electrostatic attraction between the membrane surface and the contaminants (i.e., the "non-sieving" mechanism).

[0024] Polymeric filtration membranes can be made from one or in combination from known polymeric materials useful for making coarse pore membranes and can be described in terms of physical properties such as thickness, morphology, pore size, porosity, etc. Examples of useful polymer materials include polyolefins such as polyethylene (PE) (including polyethylene of various molecular weights such as ultra-high molecular weight polyethylene (UPE), including homopolymers, copolymers, and blends) and polypropylene (PP) (including its copolymers and blends). Other polymers that can be used to form polymeric filtration membranes include polyethers such as polyetheretherketone (PEEK), polysulfone, polyamide (e.g., nylon), fluoropolymers (including fluorinated and fully fluorinated substituted polymers), polyester (e.g., polyethylene terephthalate), as well as other types of polymers and copolymers known to be useful for preparing polymeric filtration membranes.

[0025] The term "polyethylene" refers to a polymer having a linear molecular structure of repeating -CH2-CH2- units, in part or substantially. Polyethylene can be made by reacting a monomer composition that contains, consists of, or consists essentially of ethylene monomers. Thus, a polyethylene polymer can be a polyethylene homopolymer prepared by reacting monomers that consist of, or consist essentially of, ethylene monomers. Alternatively, a polyethylene polymer can be a polyethylene copolymer prepared by reacting a combination of ethylene and non-ethylene monomers that contains, consists of, or consists essentially of ethylene monomers and one or more additional monomer species such as another alpha-olefin monomer, for example, butene, hexene, or octene, or combinations thereof. For polyethylene copolymers, the amount of ethylene monomer used to produce the copolymer can be any useful amount, for example, at least 50%, at least 60%, at least 70%, at least 80%, or at least 90% (by weight) of ethylene monomer relative to the total weight of all monomers (ethylene monomer and non-ethylene monomers) in the monomer composition used to prepare the ethylene copolymer, compared to the non-ethylene monomers.

[0026] The term "polypropylene" refers to a polymer having a molecular structure of repeating -CH-CH(CH3)- units, in part or substantially. Polypropylene can be made by reacting a monomer composition that includes, consists of, or consists essentially of propylene monomers. Thus, a polypropylene polymer can be a polypropylene homopolymer prepared by reacting monomers that consist of, or consist essentially of, propylene monomers. Alternatively, a polypropylene polymer can be a polypropylene copolymer prepared by reacting a combination of propylene and non - propylene monomers, which includes, consists of, or consists essentially of propylene monomer and one or more additional monomer species such as another alpha - olefin monomer, e.g., ethylene, butene, hexene, or octane, or combinations thereof, such as alkenes. For a polypropylene copolymer, the amount of propylene monomer used to form the copolymer can be at least 50%, at least 60%, at least 70%, at least 80%, or at least 90% (by weight) of the propylene monomer based on the total weight of all monomers (propylene monomer and non - propylene monomers) in the monomer composition used to prepare the propylene copolymer, compared to the non - propylene monomers.

[0027] As used herein, a composition (e.g., a polymer composition) described as "consisting essentially of" a particular component or a specified combination of components is a composition that contains that component or specified combination of components and other materials in minor or trace amounts less than, for example, 3 wt% or less, 2 wt% or less, 1 wt% or less, 0.5 wt% or less, 0.1 wt% or less, or 0.05 wt% or less of any other component or combination of components.

[0028] For example, a polymer prepared from monomers that "consist essentially of" an olefin, ethylene, or propylene monomer is a polymer prepared from an olefin monomer, an ethylene monomer, or a propylene monomer, respectively, and other monomer materials in small or trace amounts less than trace amounts, for example, any other monomer at 3 wt% or less, 2 wt% or less, 1 wt% or less, 0.5 wt% or less, 0.1 wt% or less, or 0.05 wt% or less.

[0029] As another example, a polymer composition or polymer film described as "consisting essentially of" a polyolefin, polyethylene, or polypropylene is a polymer composition or polymer film containing, respectively, a polyolefin (including copolymers), polyethylene (including copolymers), or polypropylene (including copolymers), and various polymer species in small or trace amounts less than trace amounts, for example, any other polymer species at 3 wt% or less, 2 wt% or less, 1 wt% or less, 0.5 wt% or less, 0.1 wt% or less, or 0.05 wt% or less.

[0030] Examples of assemblies of polymer filtration membranes can include two polymer filtration membranes arranged in series, allowing a liquid to first pass through a first (upstream) polymer filtration membrane that is a coarse membrane as described, and then through a second (downstream) polymer filtration membrane that is a dense membrane as described. The first (coarse) polymer filtration membrane is relatively thick compared to the second (dense) polymer filtration membrane and has pores of a relatively larger size compared to the second polymer filtration membrane.

[0031] The first (coarse) polymeric filtration membrane can be made of any useful polymer, such as polyolefins (e.g., polypropylene, polyethylene, or their copolymers), polyetheretherketone (PEEK), polysulfone, polyamide (e.g., nylon), fluoropolymers (including fluorinated and fully fluorinated substituted polymers), polyester (e.g., polyethylene terephthalate), and other types of polymers and copolymers that can be formed into a coarse polymeric filtration membrane. According to an example of a membrane assembly, the first polymeric filtration membrane may have a relatively large thickness compared to the second membrane, a relatively large pore size (on average) compared to the second polymeric filtration membrane, a relatively long measured "flow time" compared to the second polymeric filtration membrane, and a lower bubble point compared to the second polymeric filtration membrane.

[0032] The first polymeric filtration membrane can be a type of coarse pore membrane useful as a polymeric filtration membrane, for example, a polymeric filtration membrane useful for removing contaminants from semiconductor processing liquids. The first polymeric filtration membrane has physical properties (e.g., pore size, porosity, and bubble point) and filtration characteristics (e.g., retention and flow time or flow rate), and it is possible for the first polymeric filtration membrane to be useful for removing unwanted contaminants from a liquid stream, either alone as a polymeric filtration membrane or in combination with a second (dense) polymeric filtration membrane as part of a filtration membrane assembly.

[0033] Useful first polymer filtration membranes can be of a type sometimes described as ultraporous membranes, nanoporous membranes, microporous membranes, etc. These polymer filtration membranes are generally effective at removing unwanted contaminants (e.g., particulate matter or dissolved chemical species or both) from a liquid stream. The useful average pore size can be selected based on one or more factors including fluid flow rate, pressure, pressure drop considerations, viscosity considerations, and the amount and type of contaminants (e.g., dissolved metal ions or particulate contaminants in the liquid) in the liquid being filtered. Examples of useful first polymer filtration membranes include various membranes known to be useful for microfiltration applications that are commercially available and considered to be microporous, ultraporous, or nanoporous. Examples of the first polymer filtration membrane can have an average pore size in the range of about 0.001 micron to about 5 or 10 microns (1 nanometer to about 5,000 or 10,000 nanometers (nm)), for example, 0.01 to 0.8 microns (10 nanometers to 800 nanometers).

[0034] Useful or preferred first polymer filtration membranes can be considered to have a relatively uniform pore size and degree of pore symmetry or may have a relatively non-uniform pore size and degree of pore asymmetry. The polymer filtration membrane can be isotropic or anisotropic with respect to pore size, symmetric or asymmetric, "skinned" or "skinless", or combinations thereof. A polymer filtration membrane having pores of substantially uniform size and uniformly distributed throughout the thickness of the membrane is often referred to as "isotropic" or "uniform". In comparison, an anisotropic (also known as "asymmetric") membrane can be considered to have a form in which a pore size gradient exists across the membrane. For example, an anisotropic or asymmetric membrane may have a porous structure with relatively large pores on one membrane surface and relatively small pores on the other membrane surface, and the pore structure varies along the thickness of the membrane. The term "asymmetric" is often used interchangeably with the term "anisotropic".

[0035] The first polymeric filtration membrane can be in the form of a sheet having a relatively uniform thickness, for example, in the range of 20 to 300 microns, for example, 30 microns or 50 microns to 150 microns, 200 microns, or 250 microns.

[0036] The first polymeric filtration membrane may have any porosity that enables the polymeric filtration membrane to function effectively as described herein. Examples of the first polymeric filtration membrane can have a relatively high porosity, for example, in the range of 40 to 80%. As used herein, the "porosity" (sometimes referred to as void fraction) of a polymeric filtration membrane is a measure of the space in the polymeric filtration membrane that is empty (i.e., "void") as a proportion of the total volume of the polymeric filtration membrane, and is calculated as the ratio of the volume of voids in the polymeric filtration membrane to the total volume of the polymeric filtration membrane. A body with 0% porosity is completely solid.

[0037] Examples of polymeric filtration membranes that can be useful as the first polymeric filtration membrane and have the physical properties as described include polyolefin (e.g., polyethylene and polypropylene) membranes prepared by known methods including melt casting, thermally induced phase separation (TIPS), solvent evaporation induced phase separation (SEIPS), and phase inversion methods using specific examples including nonsolvent induced phase separation.

[0038] The first polymeric filtration membrane can be treated or coated to improve the removal of dissolved or suspended contaminants in a liquid by a non-sieving filtration mechanism by adding either a cationic or anionic ionic group to the surface of the polymeric filtration membrane.

[0039] The bubble point is an understood property of the polymeric filtration membrane and is related to the pore size of the polymeric filtration membrane. A relatively high bubble point suggests relatively small pores in the polymeric filtration membrane, and a relatively low bubble point suggests a relatively large pore size in the polymeric filtration membrane. The measurement of the bubble point is based on the premise that, for a particular fluid and pore size of a polymeric filtration membrane having a certain wettability, the pressure required to extrude air bubbles through the pores of the polymeric filtration membrane is inversely proportional to the size of the pores. The porosimetry bubble point test method measures the pressure required to push a gas (e.g., air) through the wet pores of the membrane. The bubble point test is a well-known method for determining or estimating the pore size of a membrane. An example of a useful average bubble point of the first polymeric filtration membrane as described is measured using the test method described herein (using ethoxy-nonafluorobutane (HFE-7200) as the wetting solvent and a temperature of 22° C.) and can be in the range of less than 130 psi, e.g., 50 psi or from 60 psi to 110 psi or 120 psi or less.

[0040] In combination with a desirable bubble point and filtration performance (e.g., as measured by the retention described herein), the first polymeric filtration membrane can exhibit a useful flow rate of liquid through the polymeric filtration membrane. The rate of flow of liquid through the polymeric filtration membrane can be measured by converting it to a flow rate or a flow time (the reciprocal of the flow rate). As described, the first polymeric filtration membrane can have a useful or relatively low flow time, preferably in combination with a bubble point within the range as described and good filtration performance (e.g., as measured by a retention test). A useful example of the first (coarse) polymeric filtration membrane is one where the measured flow time, measured using isopropyl alcohol (IPA) as described herein, can be less than about 15,000 seconds or less than 13,000 seconds, e.g., less than 10,000 seconds, less than 5,000 seconds, or less than 1,000 seconds.

[0041] The level of effectiveness of a polymeric filtration membrane for removing contaminants (including dissolved or suspended chemical molecules) from a liquid can be measured in one manner as "retention". Retention, taking into account the filtration performance of the polymeric filtration membrane, generally refers to the total amount of one or more contaminants removed from the contaminant-containing liquid (under filter-using conditions, or under test conditions) relative to the total amount of one or more contaminants present in the liquid before passing the liquid through the polymeric filtration membrane. Thus, the "retention" value of a polymeric filtration membrane is a ratio, and a polymeric filtration membrane with a higher retention value (higher ratio) is relatively effective in removing contaminants from the liquid, while a polymeric filtration membrane with a relatively low retention value (lower ratio) is less effective in removing contaminants from the liquid.

[0042] When measured under test conditions, an example of a first (coarse) polymeric filtration membrane can show a retention exceeding 98% or 99% at a monolayer coverage of 1.0%, a retention exceeding 97% or 98% at a monolayer coverage of 2.0, a retention exceeding 98% or 99% at a monolayer coverage of 2.0, a retention exceeding 95%, 96%, or 97% at a monolayer coverage of 3.0, and a retention exceeding 94% or 95% at a monolayer coverage of 4.0, each measured using the test method described herein. See Figure 5.

[0043] The second polymeric filtration membrane is thinner compared to the first polymeric filtration membrane and has smaller-sized pores compared to the first polymeric filtration membrane. The second polymeric filtration membrane can be made of any useful polymer, preferably prepared from polyolefins such as polypropylene, polyethylene including copolymers, or blends of two or more polyolefins or polyolefin copolymers. Certain examples of the second polymeric filtration membrane include, consist of, or consist essentially of a polymer that is a polyolefin such as polypropylene, polyethylene, or a blend of polyethylene and polypropylene.

[0044] The second polymeric filtration membrane of the assembly can consist of a single polymeric filtration membrane or can alternatively include two or more “sub-layers,” in which case the second polymeric filtration membrane can be considered a “multi-layered” membrane that includes two or more sub-layers. The second polymeric filtration membrane can be in the form of a sheet having a relatively uniform thickness. The second polymeric filtration membrane of the assembly that consists of a single filtration layer (a single polymeric filtration membrane) can have a thickness of less than 10 microns, less than 15 microns, or less than 20 microns, and can, for example, have a thickness in the range of 0.1 to 10 microns, or 0.2 to 5 microns. The second polymeric filtration membrane that includes two or more sub-layers can include sub-layers having thicknesses within these ranges and can have a total thickness within these ranges.

[0045] The second “dense” polymeric filtration membrane is smaller than the pores of the first “coarse” polymeric filtration membrane and, for example, has pores that enable the second polymeric filtration membrane to function effectively as the dense layer of the filter assembly as described, to remove relatively small contaminants from the liquid that are not removed by the liquid flowing through the first “coarse” polymeric filtration membrane upstream of the second “dense” polymeric filtration membrane. In some examples of the second polymeric filtration membrane, or in examples of the sub-layers of the second membrane, the second polymeric filtration membrane or its sub-layers can have an average pore diameter in the range of about 0.001 microns to about 0.1 microns (1 nanometer to about 500 (nm)), for example, 0.01 to 0.05 microns (10 to 50 nanometers), or 0.001 to 0.005 microns (1 to 5 nanometers).

[0046] It can be useful as the second polymer filtration membrane, and examples of polymer filtration membranes having the physical properties as described can be prepared by any useful technique (e.g., extrusion, melt casting, or phase inversion techniques) to prepare a polyolefin membrane, followed by uniaxially or biaxially stretching the membrane to reduce its thickness by known methods. According to one technique, a second polymer filtration membrane containing (including, consisting essentially of, or consisting of) polyethylene (including copolymers), polypropylene (including copolymers), or blends thereof can be prepared by extruding a polymer (polyolefin) resin containing a diluent to form a polyolefin sheet, stretching the sheet, and then following steps of extraction and annealing.

[0047] The second polymer filtration membrane can be treated or coated to improve the removal of dissolved or suspended contaminants in a liquid by a non-sieving filtration mechanism by adding either a cationic or anionic ionic group to the surface of the membrane.

[0048] Useful or preferred second polymer filtration membranes can have physical properties (e.g., pore size, porosity, and bubble point) and filtration characteristics (e.g., retention and flow time or flux), whether in the form of a single layer or a number of sub-layers, and it is effective to enable the second polymer filtration membrane to be useful as a single polymer filtration membrane for removing unwanted contaminants from a liquid stream or, as described, in combination with a second (dense) polymer filtration membrane as part of an assembly of two or more polymer filtration membranes including a first (coarse) polymer filtration membrane in combination with another polymer filtration membrane.

[0049] The bubble point of the second (dense) polymeric filtration membrane of an assembly (comprising, consisting essentially of, or consisting of) a first (coarse) polymeric filtration membrane and a second (dense) polymeric filtration membrane may preferably be greater than the bubble point of the first (coarse) polymeric filtration membrane. Examples of useful bubble points of the second (dense) polymeric filtration membrane as described, measured using the test method described in the Examples section herein, can be greater than 100 psi, for example, in the range of 120 - 300 psi, for example, in the range of 130 - 200 psi.

[0050] The second polymeric filtration membrane can have any porosity that enables the polymeric filtration membrane to be effective when used as the second polymeric filtration membrane described herein. Examples of the second polymeric filtration membrane can have a relatively high porosity, for example, in the range of 40 - 80%.

[0051] The second polymeric filtration membrane can preferably be useful or have a relatively short flow time in combination with a bubble point that is relatively high and has good filtration performance when measured alone. The second polymeric filtration membrane can have a flow time that is less than the flow time of the first polymeric filtration membrane used with the second polymeric filtration membrane. Examples of the second polymeric filtration membrane can have a measured flow time that, when measured using IPA as described below, is less than about 15,000 seconds or less than 11,000 seconds, or less than 10,000 seconds, less than 9,000 seconds, or less than 5,000 seconds, or less than 1,000 seconds.

[0052] Examples of the second polymeric filtration membrane can exhibit a retention of greater than 97% or 98% at 1.0% monolayer coverage, greater than 95% or 96% at 2.0 monolayer coverage, greater than 96% or 97% at 2.0 monolayer coverage, greater than 93% or 94% at 3.0 monolayer coverage, and greater than 91% or 92% at 4.0 monolayer coverage, each measured at a useful flow rate through the membrane using the tests described in the Examples section. See Figure 4.

[0053] A membrane assembly including a first polymer filtration membrane and a second polymer filtration membrane in series may have a combined thickness of two polymer filtration membranes equal to the combined thickness of the first and second polymer filtration membranes of the assembly, without any additional polymer filtration membranes, filtration elements, or support layers that may be used in combination with the first and second polymer filtration membranes of the assembly. Examples of values for the thickness of the membrane assembly may be comparable to the thickness of the first polymer filtration membrane of the assembly, for example, in the range of 20 to 300 microns, for example, 30 microns or 50 microns to 150 microns, 200 microns, or 250 microns.

[0054] A membrane assembly, such as the first and second polymer membranes that make up the assembly, can also be evaluated in terms of the properties of the assembly, such as the bubble point, retention, and flow time.

[0055] The bubble point of an assembly (comprising, consisting essentially of, or consisting of) the first and second polymer filtration membranes may preferably be greater than the bubble point of the first (coarse) polymer filtration membrane. According to an example of the membrane assembly, the assembly may have a bubble point (average bubble point) comparable to the bubble point of the second (dense) polymer filtration membrane of the assembly. For example, the average bubble point, measured in pounds per square inch, is within 10% of the average bubble point of the second (dense) membrane. Examples of the average bubble point of the membrane assembly may be greater than 100 psi, for example, in the range of 120 to 300 psi, for example, in the range of 130 to 200 psi.

[0056] The retention of an assembly comprising a first (coarse) polymeric filtration membrane and a second (dense) polymeric filtration membrane (comprising, consisting essentially of, or consisting of) can be good compared to the retention of each of the first and second polymeric filtration membranes when measured individually. This can be shown during use of the membrane assembly to remove contaminants from a semiconductor processing fluid such as a photoresist solution, or by testing under controlled conditions, for example by measuring the retention of test contaminants present in a test solution containing test particles of a known concentration under controlled conditions. Examples of test particles are G25 particles made of polystyrene with an average particle diameter of 3 to 25 nanometers. Other types and sizes of test particles may also be suitable for individually measuring the retention of the assembly and of each polymeric filtration membrane.

[0057] Examples of assemblies comprising a single first polymeric filtration membrane and a single second polymeric filtration membrane (comprising, consisting essentially of, or consisting of) can exhibit measured retention values exceeding 99% at a monolayer coverage of 1.0%, retentions exceeding 98% or 99% at a monolayer coverage of 2.0, retentions exceeding 98% at a monolayer coverage of 3.0, and retentions exceeding 98% at a monolayer coverage of 4.0, each measured at a useful flow rate through the membrane using the tests described in the Examples section. See FIGS. 4 and 5.

[0058] A membrane assembly comprising a first polymeric filtration membrane and a second polymeric filtration membrane (comprising, consisting essentially of, or consisting of) may have a flow time comparable to that of either or both of the first polymeric filtration membrane of the assembly or the second polymeric filtration membrane of the assembly when measured separately. The assembly can have a useful or relatively low flow time, for the membrane assembly, for example, in combination with a bubble point that is preferably relatively high and has good filtration performance, as described herein. An example of a membrane assembly can have a measured flow time of less than 20,000 seconds when measured using IPA as described herein.

[0059] A membrane assembly comprising at least a first (coarse) polymeric filtration membrane and a second (dense) polymeric filtration membrane can be incorporated into a filter product and used to filter liquids such as semiconductor processing liquids. The assembly can include a first polymeric filtration membrane and a second polymeric filtration membrane arranged in series between an inlet and an outlet of the filter product. The assembly can include or be used in conjunction with one or more additional membranes or support structures such as support layers or frames. In a preferred use, the first polymeric filtration membrane is upstream of the flow of liquid through the assembly and the second polymeric filtration membrane is downstream. The upstream side of the second polymeric filtration membrane faces the downstream side of the first polymeric filtration membrane. The liquid first flows through the first polymeric filtration membrane and then through the second polymeric filtration membrane.

[0060] Referring to FIG. 1, an assembly 100 of a first (coarse) polymeric filtration membrane 102 and a second (dense) polymeric filtration membrane 104 in series is described. The coarse polymeric filtration membrane 102 is upstream with respect to the flow of the liquid 110 to the upstream surface of the coarse polymeric filtration membrane 102. The liquid 110 (see arrow) flows to the first polymeric filtration membrane 102 having a greater thickness and a greater pore size compared to the second polymeric filtration membrane 104. Contaminants present in the liquid 110 are removed by the pores of the first (coarse) polymeric filtration membrane 102. After flowing through the polymeric membrane 102, the liquid flows to the second (dense) polymeric filtration membrane 104 having a smaller thickness and relatively small-sized pores. Smaller contaminants remaining in the liquid 110 after the liquid 110 has passed through the first polymeric filtration membrane 102 can be removed by the second polymeric filtration membrane 104 by either a sieving mechanism or a non-sieving mechanism. The filtrate 112 (see arrow) passes from the downstream surface of the second polymeric filtration membrane 104.

[0061] The assembly 100 of FIG. 1 consists of the first polymeric filtration membrane 102 and the second polymeric filtration membrane 104, and it is shown that there are no additional filtration membranes or support membranes or structures that may be included during the use of the example of the assembly 100 although not shown. Also, although the second polymeric filtration membrane 104 is described as consisting of a single polymeric filtration membrane layer, in some cases, it may be made to include two or more sub-layers described herein.

[0062] The polymer filtration membranes 102 and 104 may be separated from each other, i.e., not adhered, but the downstream surface of the first polymer filtration membrane 102 is disposed facing, adjacent to, or in contact with the upstream surface of the second polymer filtration membrane 104. If desired but not essential, the two polymer filtration membranes may be adhered by a lamination step at their opposing surfaces (the exit surface of the first polymer filtration membrane attached to the inlet surface of the second polymer filtration membrane), whereby the surfaces of the two polymer filtration membranes come into contact with each other and adhere to each other at high temperature and slight pressure. The temperature and pressure conditions of the optional lamination step are mild so as to avoid significantly affecting the morphology and filtration performance characteristics (flow time, retention) of the first polymer filtration membrane or the second polymer filtration membrane. The lamination step can be carried out without disposing an additional material such as an adhesive between the surfaces of the two polymer filtration membranes.

[0063] The membrane assembly can be incorporated into a filter product such as a filter cartridge (removable filter cartridge) that supports the membrane assembly in a liquid flow, or a filter housing that includes any structure that supports the membrane assembly such that a liquid fluid can flow from the inlet side of the membrane assembly through the membrane assembly, through the membrane assembly, and out the outlet side of the membrane assembly. The filter housing may include an inlet on one side of the supported membrane assembly that allows liquid to enter the filter housing when the membrane assembly is supported by the housing, and an outlet on a second side of the membrane assembly that allows the fluid to exit the filter housing after passing through the membrane assembly.

[0064] The terms "inlet" and "outlet" may refer to a defined opening of the filter housing or a defined conduit (tube, pipe, or their fitting) or a specific structure such as a component of the filter housing (e.g., support frame, core, cage, etc.), or may refer to a space (the "inlet space" or "outlet space") located on either side of the membrane assembly when supported by a filter housing that accommodates the fluid flow into and out of the membrane assembly. The terms "inlet" and "outlet" generally refer to a space on either side of the membrane assembly that accommodates the fluid flow into the membrane assembly, through the membrane assembly, and out of the membrane assembly when the membrane assembly is supported by a filter housing.

[0065] The filter housing includes a membrane assembly supported to allow fluid to pass through the membrane and can be a component of a larger filtration system that supplies a filtered liquid chemical to a tool or process for manufacturing a microelectronic or semiconductor device. The filter assembly or filter housing may include one or more various additional materials and structures that support the membrane assembly within the filter housing to smooth the fluid flow through the membrane assembly. The membrane assembly supported by the filter housing can be of any useful shape, such as a cylinder with pleats, a cylindrical pad, a (flat) cylindrical sheet without one or more pleats, a pleated sheet, etc.

[0066] An example of a filter structure (e.g., a removable filter cartridge) including a membrane assembly can be in the form of a replaceable filter cartridge including a cylindrical filter element with pleats, which includes a rigid or semi-rigid core that supports the pleated membrane assembly inside (the inlet or outlet) of the pleated membrane assembly, a rigid or semi-rigid cage that supports or surrounds the outside (the outlet or inlet) of the pleated membrane assembly outside the membrane assembly, and one or more additional components that may include optional end pieces or "packs" located at each of the two opposite ends of the pleated membrane assembly. The replaceable filter cartridge is considered to have an inlet on one side of the membrane assembly and an outlet on the opposite side of the membrane assembly, and the inlet or outlet can be on either side of the membrane depending on the direction of the liquid flow through the membrane assembly.

[0067] As an example, FIG. 2 shows a filter product (replaceable filter cartridge) 230, which includes a pleated cylindrical part (e.g., filter element) 210 and an end piece 222, and has other optional structural components not specifically shown. The cylindrical part 210 includes the membrane assembly 212 described herein and has pleats. The end piece 222 is attached (e.g., "inserted") to one end of the cylindrical filter part 210. The end piece 222 can preferably be made of a melt-processable polymeric material such as a thermoplastic resin fluoropolymer. A core (not shown) can be disposed in the internal opening or space ("inlet") 224 of the pleated cylindrical part 210, and a cage (not shown) can be disposed in the external opening or space ("outlet") of the pleated cylindrical part 210. Thus, the cartridge includes an inlet and an outlet (e.g., with a core, cage, or other support structure) on each of the inlet (upstream) side of the membrane and the outlet (downstream) side of the membrane assembly 212, and the positions of the inlet and outlet are determined according to the direction in which the liquid causes a flow through the membrane assembly 212, but the cartridge 230 can be installed in a filter housing and can also have an inlet and an outlet. A second end piece (not shown) can be attached (inserted) to the second end of the pleated cylindrical part 230. The resulting replaceable filter cartridge 230, having two opposed inserted ends, an optional core, and a cage, can then be disposed in a larger filter housing that includes an upstream housing inlet for receiving the liquid flowing into the filter membrane and a downstream housing outlet for discharging the liquid that has passed through the membrane assembly. The housing is configured such that a certain amount of liquid enters the housing at the housing inlet and necessarily passes through the membrane assembly 212 before exiting the filter housing at the housing outlet.

[0068] The components of the filter housing can be of any useful and desirable size, shape, and material, and are preferably fluorinated or non-fluorinated polymers such as nylon, polyethylene, or fluorinated polymers such as poly(tetrafluoroethylene-co-perfluoro(alkyl vinyl ether)), TEFLON® perfluoroalkoxy alkane (PFA), perfluoromethyl alkoxy (MFA), or another suitable fluoropolymer (e.g., perfluoropolymer).

[0069] Referring to Figure 3, the filter product 250 includes a housing that contains an interchangeable filter cartridge 230 therein. The filter product 250 includes a two-component housing that includes a bowl 252 attached to a base 254 at the open end of the bowl 252. In some cases, the bowl is removably engaged with the base 254 to allow the bowl 252 to separate from the base 254 and allow the removable filter cartridge 230 to be positioned within or removed from the interior of the housing. The assembled housing includes an inlet 256, an inlet space 260 inside the cartridge 230, an outlet space 262 between the cartridge 230 and the bowl 252, and an outlet 258. The removable filter cartridge 230 can be installed inside the bowl 252 in an arrangement that allows liquid to flow into the filter product 250 through the inlet 256, into the inlet space 260, then through the filter assembly 212 of the cartridge 230, into the outlet space 262, and then out of the filter product 250 through the outlet 258.

[0070] The membrane assembly as described is useful in a method for filtering, purifying, or removing unwanted substances (contaminants) from a liquid chemical to produce a high-purity liquid chemical, particularly useful for industrial processes that require the input of liquid chemicals having a very high level of purity. Generally, the liquid chemical can be any of various useful commercially available liquid chemicals of a type useful for any industrial or commercial application. Specific examples of the membrane assembly and filter products as described can be used, for example, in the purification of liquid chemicals used in semiconductor photolithography methods, wet etching or cleaning steps, methods for forming spin-on glass (SOG), backside anti-reflective coating (BARC) methods, etc., for filtering liquid solvents or other processing solutions used in the manufacture of semiconductors or microelectronics.

[0071] Examples of some specific and non-limiting solvents (including cleaning liquids) that can be filtered using the membrane assembly as described include n-butyl acetate (nBA), isopropyl alcohol (IPA), ethyl 2-ethoxyacetate (2EEA), xylene, cyclohexanone, ethyl lactate, methyl isobutyl carbinol (MIBC), methyl isobutyl ketone (MIBK), isoamyl acetate, undecane, propylene glycol methyl ether (PGME), propylene glycol monomethyl ether acetate (PGMEA), or any mixture thereof, such as a mixture of PGME and PGMEA, or concentrated or diluted ammonium hydroxide, hydrogen peroxide, hydrochloric acid, HF, sulfuric acid, another peroxide solution, or combinations thereof, such as a combination of ammonium hydroxide and hydrogen peroxide, or a combination of hydrochloric acid and hydrogen peroxide.

[0072] A more specific example of a semiconductor processing fluid is a "photoresist solution" that includes a polymer resin, a sensitizer (also known as an "inhibitor" that regulates the photochemical process), and a solvent.

[0073] Examples of photoresist solutions include reactive curable polymer resins that can cure in the presence of a sensitizer and in the presence of electromagnetic irradiation. The type of irradiation can be deep ultraviolet irradiation, near ultraviolet irradiation, electron beam irradiation, or X-ray irradiation. Examples of specific reactive polymers known to be useful in photoresist solutions include polymethacrylate (PMMA) resins, isoprene, polyalkyl aldehydes, diazonaphthoquinone (DNQ) resins, novolak resins (phenol-formaldehyde polymers), epoxy resins, out-of-stoichiometry thiol-ene (OSTE) polymer resins, and hydrogen silsesquioxane (HSQ) resins. Various solvents and photosensitizers are known to be useful in photoresist solutions containing various types of curable polymer resins. The polymer resin, solvent, and sensitizer can each be used in specific amounts effective for use in a particular photoresist solution.

[0074] Examples of unwanted contaminants that may be present in the photoresist solution and can desirably be removed by a filtration step using the membrane assembly as described include particulate contaminants that can be particles of reactive polymers, dissolved metal ions ("metals"), or other particulate or dissolved contaminants.

[0075] The terms "metal" and "metal ion" are used in a manner consistent with their use in the fields of chemistry and semiconductor processing and refer to molecular species that include metal ions, metal atoms, or metal complexes. Metal contaminants can be neutral, negatively charged, or positively charged metal species or can include them. Some examples of specific metals or metal ions that can be removed from the photoresist solution include Na, K, Ca, Fe, Mg, Al, Cr, Ni, and Zn.

[0076] Particulate contaminants in the photoresist solution can include reactive monomeric, oligomeric, or polymeric polymers having a particle size in the range of 0.01 to 0.2 microns.

[0077] The membrane assembly, or the first membrane or the second membrane, can be tested for retention, bubble point, and flow time, either individually or together, according to the following test methods.

[0078] Retention test method using test particles The level of effectiveness of a polymeric filtration membrane or membrane assembly in removing unwanted substances (i.e., "contaminants") from a liquid can, in one manner, be measured as "retention". Retention generally refers to the amount of contaminants removed from a liquid containing contaminants relative to the total amount of contaminants initially present in the liquid before the liquid passes through the membrane or membrane assembly. The "retention" value of a polymeric filtration membrane or membrane assembly is a ratio, and a polymeric filtration membrane with a higher retention value (higher ratio) is relatively effective in removing contaminants from the liquid, while a polymeric filtration membrane with a lower retention value (lower ratio) is relatively less effective in removing contaminants from the liquid.

[0079] Retention can be measured by measuring a number of test particles removed from the liquid stream by a polymeric filtration membrane or membrane assembly placed in the liquid stream. By one method, retention can be measured by passing a sufficient amount of an aqueous test solution containing 0.1% Triton X-100 and 8 ppb of polystyrene particles (e.g., G25 spherical polystyrene particles with a nominal diameter of 3 - 25 nanometers) through a membrane to achieve 1% monolayer coverage of the membrane at a constant flow rate and collecting the permeate. The concentration of polystyrene particles in the permeate can be calculated from the absorbance of the permeate. Then, the particle retention is calculated using the following equation based on the particle concentrations in the feed and the filtrate. JPEG2025109159000002.jpg23170

[0080] Bubble point test method using HFE-7200 One way to determine the bubble point of a polymeric filtration membrane is to immerse a sample of the polymeric filtration membrane in a liquid having a known surface tension to wet it, and apply a gas pressure to one side of the sample. The gas pressure is increased gradually. To measure the bubble point, a 47 mm polymeric filtration membrane disk is placed in a holder, and a highly permeable spunbond nonwoven fabric (PGI Inc.) is placed downstream as a support layer. Air is pressurized through the holder and measured as a function of pressure. Thereafter, HFE-7200 (3M), a low surface tension liquid, is introduced downstream to wet the membrane. Air is pressurized through the holder again, and the air flow is measured as a function of pressure.

[0081] The minimum pressure at which gas flows through the sample is called the initial bubble point.

[0082] The pressure at which the ratio of the air flow through the wet membrane to the air flow through the dry membrane is 0.5 is called the average bubble point. If the membrane produces two different pressure values based on which side of the membrane faces the air, the lower measured value is used as the average bubble point.

[0083] Flow time test method The ease of liquid passage through a polymeric filtration membrane or membrane assembly, e.g., the flow rate of liquid through the membrane or membrane assembly, can be evaluated as the measured "flow time", which is the reciprocal of the flow rate. The flow time can be defined as the time required to pass 500 milliliters of fluid (in this case, isopropyl alcohol) at 14.2 psi through a porous membrane or membrane assembly having a surface area of 13.8 cm 2 .

Example

[0084] Table 1 shows a comparison of the average bubble point, flow time, and thickness of two examples of a first (coarse) polymeric filtration membrane, one example of a second (dense) polymeric filtration membrane, and Example 1 of an assembly including both the first (coarse) polymeric filtration membrane and the second (dense) polymeric filtration membrane. TIFF2025109159000003.tif54170TIFF2025109159000004.tif54170

[0085] The graphs of FIGS. 4 and 5 show the particle retention data of the membranes and membrane assemblies of Tables 1 and 2, tested using 8 amounts of G25 PSL test particles and 0.1% Triton X-100 surfactant, expressed in parts per million in deionized water, according to the retention test method described herein.

[0086] FIG. 4 shows the measured retention values for two different assemblies (“Assembly 1” and “Assembly 2”) of a first (coarse) polymeric filtration membrane and a second (dense) polymeric membrane. Assembly 1 is a non-layered assembly consisting of a coarse polymeric filtration membrane (“first layer”) made of stretched UPE and a dense polymeric filtration membrane (“second layer”). Assembly 2 is a non-layered assembly consisting of a coarse polymeric filtration membrane (“first layer”) made of stretched UPE and a dense polymeric filtration membrane (“second layer”). The measured retention for each of Assembly 1 and Assembly 2 is at least 98% for the test particles at particle loadings correlating to single layers of 1.0, 2.0, 3.0, and 4.0.

[0087] FIG. 4 also shows the measured retention values for a dense polymeric filtration membrane (“Example 2 of the dense membrane”), a stretched UPE membrane, tested alone. The measured retention for Example 2 of the dense membrane is approximately 98% for the test particles at particle loadings correlating to single layers up to 1.5 and less than 98% at high particle loadings.

[0088] FIG. 5 shows the measured retention values for Assembly 2 and compares those values to the measured retention values for two different coarse polymeric filtration membranes (“Example 3 of the coarse membrane” and “Example 4 of the coarse membrane”). Example 3 of the coarse membrane is a melt cast membrane. Example 4 of the coarse membrane is a melt cast membrane.

[0089] The measured retention for Assembly 2 is at least 98% for the test particles at particle loadings correlating to single layers of 1.0, 2.0, 3.0, and 4.0.

[0090] The measured retention force for Example 3 of the coarse membrane is approximately 98% for the test particles with a particle loading that correlates to a monolayer up to 1.5 or about 2.0, and less than 98% at high particle loadings.

[0091] The measured retention force for Example 4 of the coarse membrane is approximately 98% for the test particles with a particle loading that correlates to a monolayer up to about 2.5, and less than 98% at high particle loadings.

Claims

Claim 1 A method for filtering a semiconductor processing liquid, comprising: passing the liquid through a first polymeric filtration membrane having an average bubble point of less than 120 pounds per square inch measured using ethoxy-nonafluorobutane (HFE7200), and a thickness of at least 20 microns ; and passing the liquid through a second polymeric filtration membrane having an average bubble point of greater than 120 pounds per square inch measured using ethoxy-nonafluorobutane (HFE7200), and a thickness of less than 20 microns . A method as described above. . Claim 2 The method according to claim 1, wherein the method of passing the liquid through both the first polymeric filtration membrane and the second polymeric filtration membrane removes a greater amount of contaminants from the liquid compared to both the method of passing the liquid through only the first polymeric filtration membrane and the method of passing the liquid through only the second polymeric filtration membrane. Claim 3 The method according to claim 1, wherein the liquid is a photoresist solution comprising a polymeric resin, a sensitizer, and a solvent. Claim 4 The method according to claim 3, wherein the photoresist solution contains metal contaminants. Claim 5 The method according to claim 3, wherein the photoresist solution contains contaminant particles having a particle size in the range of 0.01 to 0.2 microns. Claim 6 The method according to claim 1, wherein the first polymeric filtration membrane is a melt-cast polyolefin membrane. Claim 7 The method according to claim 1, wherein the first polymeric filtration membrane has a thickness in the range of 20 to 300 microns. Claim 8 The method according to claim 1, wherein the second polymeric filtration membrane is a drawn polyolefin membrane. Claim 9 The method according to claim 1, wherein the second polymeric filtration membrane has a thickness in the range of 0.1 to 10 microns. Claim 10 A filter product useful for removing contaminants from a semiconductor processing fluid, comprising: a first polymeric filtration membrane having an average bubble point of less than 120 pounds per square inch measured using ethoxy-nonafluorobutane (HFE7200), and a thickness of at least 20 microns ; and a second polymeric filtration membrane having an average bubble point of greater than 120 pounds per square inch measured using ethoxy-nonafluorobutane (HFE7200), and a thickness of less than 20 microns . A filter product as described above. . Claim 11 The filter product according to claim 10, wherein the first polymeric filtration membrane is a melt-cast polyolefin membrane. Claim 12 The filter product according to claim 10, wherein the first polymer filtration membrane has a thickness in the range of 20 to 300 microns.

13. The filter product according to claim 10, wherein the second polymer filtration membrane is a stretched polyolefin membrane.

14. The filter product according to claim 10, wherein the second polymer filtration membrane has a thickness in the range of 0.1 to 10 microns.

15. The filter product according to claim 10, comprising a frame that supports the first polymer filtration membrane and the second polymer filtration membrane.

16. The filter product according to claim 10, which is a replaceable filter cartridge.

17. A housing including an inlet, an inlet space, an outlet, and an outlet space, adapted such that a liquid can flow through the inlet into the inlet space, then through the first polymer filtration membrane, then through the second polymer filtration membrane, and then into the outlet space, the filter product according to claim 10 comprising the housing.

18. The filter product according to claim 10, which provides better retention of G25 spherical polystyrene particles compared to both the retention force of only the first polymer filtration membrane and the retention force of only the second polymer filtration membrane.

19. The filter product according to claim 10, showing a measured retention of at least 98% based on a 4.0% single layer using G25 spherical polystyrene particles with a nominal diameter of 5 to 15 nanometers.

20. The filter product according to claim 10, showing a measured retention of at least 98% based on a 3.0% single layer using G25 spherical polystyrene particles with a nominal diameter of 5 to 15 nanometers.

21. A semiconductor processing filtration system, comprising the filter product according to claim 17, a semiconductor processing fluid source connected to the inlet, and a semiconductor processing apparatus connected to the outlet The semiconductor processing filtration system.

22. The semiconductor processing filtration system according to claim 21, wherein the semiconductor processing fluid is a photoresist solution.

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