Multi-layer porous polymeric membrane and related filters and methods

Abstract

This application relates to multilayer porous polymer membranes and related filters and methods. The invention describes: a porous polymer filter membrane assembly comprising two polymer filter membranes connected in series, i.e., as part of a membrane assembly; a filter assembly and filter product comprising two polymer filter membrane layers; a method for assembling the polymer filter membranes, the filter assembly, and the filter product; and a method for removing particles from a liquid, such as a semiconductor processing liquid fluid, using the polymer filter membranes.

Description

Multilayer porous polymer membranes and related filters and methods The following description relates to an assembly comprising two polymer filter membranes connected in series (i.e., as part of a membrane assembly); also relates to a filter assembly and a filter product comprising two polymer filter membranes; also relates to methods of assembling polymer filter membranes, filter assemblies and filter products; and relates to a method of using polymer filter membranes to remove particles from a liquid such as a semiconductor processing fluid. Polymeric filtration membranes are used to remove unwanted materials from fluids in a range of applications. These fluids include water, liquid industrial solvents and process fluids, industrial gases used in manufacturing or processing, and liquids with medical or pharmaceutical uses. Contaminants and impurities removed from a fluid include particulate contaminants, microorganisms, and dissolved chemicals such as ions. Two or more types of filter membranes can be used together to remove different types of contaminants or impurities from a single liquid. Certain types of filter membranes remove contaminants from a liquid via a sieving mechanism, whereby contaminant particles larger than a pore in a membrane are mechanically prevented from passing through the membrane and become trapped within the membrane, unable to pass through. Other membranes function via a non-sieving mechanism, whereby impurities smaller than a pore in a membrane are attracted to the membrane surface via a chemical or electrostatic interaction and are held within the membrane without escaping. By combining a non-sieving membrane with a sieving membrane, both larger and smaller contaminants can be removed from a liquid containing particles of different sizes. Using filters to remove contaminants from liquids is particularly important in the semiconductor processing industry, where even the smallest impurities (sometimes in the nanometer size range) can cause a defect in a microelectronic device. This is true for many fluids used during semiconductor and microelectronics processing, one example being a photoresist solution used in lithography. In lithography, a light-sensitive (or radiation-sensitive) photoresist solution is used to form a patterned coating on a surface. The photoresist solution is applied as a thin layer to a substrate surface and a patterned mask is used to cover a portion of the coated surface. The unmasked areas of the photoresist solution are exposed to light. A solvent called a developer is then applied to the surface of the photoresist solution. For a "positive photoresist," the portions of the photoresist exposed to light are degraded by the light and the developer dissolves away the areas of the photoresist coating exposed to light, leaving behind a patterned coating where the mask was placed. For a "negative photoresist," the portions of the photoresist exposed to light are strengthened (polymerized or cross-linked) by light, and the developer will only dissolve away the masked areas of the photoresist that were not exposed to light, leaving behind a patterned coating in the areas not covered by the mask. Conventional photoresist solutions can contain three different components: a polymer resin (a binder that provides physical properties such as adhesion and chemical resistance), a sensitizer (which contains a photoactive compound), and a solvent. Unfortunately, a photoresist solution will also contain a certain amount of unwanted contaminant particles. These contaminant particles can be located on the surface of a semiconductor substrate, where they can create defects during subsequent processing. Contaminants can be particles or suspended or dissolved chemical substances and can have a range of sizes, including nanometer-sized particles. The semiconductor manufacturing industry continues to demand ever-improving filtration technologies, including the efficient removal of high percentages of contaminants of varying types and sizes from liquids such as photoresist solutions. The following describes filter assemblies, products, and methods that utilize a first polymeric filtration membrane, referred to as an "open" membrane, and a second polymeric filtration membrane, referred to as a "compact" membrane. The first (open) polymeric filtration membrane has, on average, relatively larger pores and a lower bubble point, compared to the second polymeric filtration membrane, which has relatively smaller pores and a higher bubble point. The first or "open" polymeric filtration membrane also has a relatively greater thickness, compared to the smaller thickness of the second or "compact" polymeric filtration membrane. Polymeric filter membranes are used to remove contaminants from a liquid stream passing through the membrane, for example by passing the liquid first through a first (open) polymeric filter membrane and then through a second (tight) polymeric filter membrane. According to exemplary methods and membranes, and without limiting the present description, the first (open) polymeric filtration membrane can be a polyolefin membrane prepared by a phase inversion technique, and the second (compact) polymeric filtration membrane can be a polyolefin membrane prepared by a stretching technique. Generally speaking, 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 greater retention capacity for a wider range of particle sizes and stronger retention capacity. It is difficult to produce polymer filtration membranes with smaller pore sizes without significantly compromising the membrane's flow rate. Stretched polyolefin filtration membranes can be very thin (e.g., having a thickness of less than 10 microns while also having relatively small pore sizes) and can achieve a useful combination of very tight (small) pore sizes and good flow rates. However, thinner stretched membranes have lower retention capacity due to the limited effective surface area and smaller spaces within the thinner membrane. According to the examples described herein, a first (open) polymer filter membrane, which can be produced by a phase inversion process, and a second (compact) polymer filter membrane, which can be produced by a stretching process, can be used in series to remove contaminants from a liquid. The open polymer filter membrane can remove relatively large contaminants from a liquid containing both larger and smaller contaminants. The compact polymer filter membrane can remove relatively small particulate or dissolved contaminants that can pass through the first (upstream) polymer filter membrane. The combination of the two polymer filter membranes effectively removes contaminants of varying types and sizes, including relatively large particulate contaminants and relatively small dissolved or suspended chemical molecules, from a liquid passing through the first and second polymer filter membranes. In one aspect, the present disclosure relates to a method for filtering a semiconductor processing liquid, comprising: passing the liquid through a first polymeric filter membrane having an average bubble point of less than 120 psig, as measured using ethoxylated nonafluorobutane (HFE7200), and a thickness of at least 20 microns; and passing the liquid through a second polymeric filter membrane having an average bubble point of greater than 120 psig, as measured using ethoxylated nonafluorobutane (HFE7200), and a thickness of less than 20 microns. According to another aspect, the present description relates to a filter product for removing a contaminant from a semiconductor processing fluid. The filter product comprises: a first polymer filter membrane having an average bubble point of less than 120 pounds per square inch measured using ethoxylated nonafluorobutane (HFE7200) and a thickness of at least 20 microns; and a second polymer filter membrane having an average bubble point of greater than 120 pounds per square inch measured using ethoxylated nonafluorobutane (HFE7200) and a thickness of less than 20 microns. The following describes: polymeric filter membranes (or, as used more simply herein, "membranes," "filter membranes," or the like); assemblies comprising two or more polymeric filter membranes, including a "first" (or "open") polymeric filter membrane and a "second" (or "compact") polymeric filter membrane; filter products comprising the first polymeric filter membrane and the second polymeric filter membrane; and related methods of using the polymeric filter membrane assemblies and filter products, particularly for removing contaminants from semiconductor processing fluids. Herein, a first polymeric filter membrane is sometimes referred to as a "first membrane" or an "open membrane," and a second polymeric filter membrane is sometimes referred to as a "second membrane" or a "compact membrane." The filter products and filtration methods described herein utilize a configuration of two different polymeric filter membranes, i.e., at least two different "membranes" or "membrane layers" arranged in series, to perform a filtration step. This configuration, sometimes referred to as a "stacked" configuration or a "multi-layer membrane assembly," comprises two different polymeric filter membranes and, optionally, additional filter membranes or support layers. The polymeric filter membranes can be assembled into a laminated composite membrane, wherein the first and second polymeric filter membranes are bonded together by lamination (without the use of a separate adhesive), or the polymeric filter membranes can be unattached (i.e., non-laminated). The first polymeric filter membrane, or "open membrane," is adapted to remove relatively large contaminants from a liquid containing contaminants ranging in size from larger to smaller. The open membrane has pores of relatively larger pore size, compared to the smaller pores of a tight membrane. The tight membrane may have pores of relatively smaller size and thickness, compared to the greater thickness of the open membrane. The combination of two polymeric filter membranes effectively removes different types of particles from a liquid passing through either the first or second polymeric filter membrane, including particulate contaminants that may be relatively large in size and dissolved or suspended chemical molecules that may be relatively small in size. The tight membrane is adapted to remove and collect relatively small particles or dissolved contaminants that may pass through the open membrane. According to exemplary methods and filter assemblies, a liquid can be caused to flow first through an open membrane having a greater thickness and larger pores, and then through a tight membrane having a smaller thickness and smaller pores. An assembly comprising a combination of an open upstream polymer filter membrane and a tight downstream polymer filter membrane can be used to remove a range of contaminants of different sizes from a liquid. Desirably, an assembly comprising both an open membrane and a tight membrane can effectively remove a greater amount (a higher percentage) of contaminants (e.g., contaminants of a particular size) from a liquid compared to the amount of contaminants removed by filtering the liquid using the tight membrane alone and the amount of contaminants removed by filtering the liquid using the open membrane alone. The effectiveness of a polymer filter membrane in removing contaminants of a particular size from a liquid can be measured in terms of "retention rate" and can be evaluated by a retention rate test of removing standard particles (or "test particles") from a sample liquid having a known particle concentration under controlled conditions. According to certain specific membrane assemblies and methods, a membrane assembly comprising at least one open membrane and at least one tight membrane can be used to remove contaminants of various sizes from a liquid containing contaminants. The liquid can be any type of liquid that is very pure but contains contaminants of a range of sizes that it is desired to filter out. Examples of such liquids include various types of liquids used in semiconductor or microelectronic device processing, which are referred to herein as "semiconductor processing fluids." Semiconductor processing fluids include liquids that are used as or contain one or more of: water or organic solvents such as isopropyl alcohol (as well as many other organic and inorganic chemicals, including organic solvents and organic or inorganic acids and bases, see below); reactive materials such as reactive monomers, oligomers, or polymers, including but not limited to those used in photosensitive materials such as photoresist solutions; cleaning solutions; acids or bases; and others. A specific example of a semiconductor processing fluid is a "photoresist solution" comprising a polymer resin, a sensitizer (having a photoactive compound), and a solvent. Each of open polymeric membranes and tight polymeric membranes has two opposing surfaces or "sides" and a membrane thickness between the two opposing surfaces. The pores of a polymeric filtration membrane are located throughout the thickness of the membrane, allowing a liquid to flow through the thickness of the membrane from one side of the membrane to and through the opposite side of the membrane. This type of membrane is sometimes referred to as an "open pore" membrane. Open pore membranes can be in the form of a thin film or sheet of porous polymer material having a relatively uniform thickness and an open pore structure comprising a polymer matrix defining a large number of open "cells" or "pores." The open cells may be referred to as openings, pores, channels, or passages, most of which are interconnected between adjacent cells to allow fluid to flow through the thickness of the membrane from one side of the membrane to the other. These passages can provide a tortuous tunnel or path through which a liquid to be filtered must pass. Contaminants in the form of particulates or dissolved or suspended molecular chemicals contained in a liquid passing through a polymer filter membrane are trapped by or within the membrane based on size (i.e., a "sieving" mechanism) or based on chemical or electrostatic attraction between the membrane surface and the contaminants (i.e., a "non-sieving" mechanism). A polymeric filtration membrane can be made from a polymeric material or a combination of polymeric materials known to be useful for making open pore membranes and can be described with respect to physical properties such as thickness, morphology, pore size, porosity, etc. Examples of useful polymeric materials include polyolefins such as polyethylene (PE) (including polyethylene of various molecular weights, such as ultra-high molecular weight polyethylene (UPE), and including homopolymers, copolymers, and blends) and polypropylene (PP) (including copolymers and blends thereof). Other polymers that can be used to form a polymeric filtration membrane include polyethers such as polyetheretherketone (PEEK), polysulfones, polyamides (e.g., nylon), fluoropolymers (including fluorinated and perfluorinated polymers), polyesters (e.g., polyethylene terephthalate), and other types of polymers and copolymers known to be useful for making polymeric filtration membranes. The term "polyethylene" refers to a polyethylene having partially or substantially repeating -CH 2-CH A polymer having a linear molecular structure of 2-units. Polyethylene can be made by reacting a monomer composition comprising monomers including, consisting of, or consisting essentially of ethylene monomers. Thus, a polyethylene polymer can be a polyethylene homopolymer prepared by reacting monomers consisting of, consisting essentially of, ethylene monomers. Alternatively, a polyethylene polymer can be a polyethylene copolymer prepared by reacting a combination of ethylene and non-ethylene monomers comprising, consisting of, or consisting essentially of ethylene monomers and one or more additional types of monomers (such as another α-olefin monomer, for example, an olefin such as butene, hexene, or octene, or a combination thereof). For a polyethylene copolymer, the amount of ethylene monomer relative to non-ethylene monomers used to produce the copolymer can be any useful amount, such as at least 50, 60, 70, 80, or 90 weight percent ethylene monomer per the total weight of all monomers (ethylene monomer and non-ethylene monomers) in the monomer composition used to prepare the ethylene copolymer. The term "polypropylene" refers to a polypropylene having partially or substantially repeating -CH-CH(CH 3)-a polymer having a molecular structure of units. Polypropylene can be made by reacting a monomer composition comprising monomers that include, consist of, or consist essentially of propylene monomers. Thus, a polypropylene polymer can be a polypropylene homopolymer prepared by reacting monomers that consist of, consist essentially of, or consist of propylene monomers. Alternatively, a polypropylene polymer can be a polypropylene copolymer prepared by reacting propylene with a combination of non-propylene monomers, the combination of propylene and the non-propylene monomer comprising, consisting of, or consisting essentially of propylene monomers and one or more additional types of monomers (such as another α-olefin monomer, for example, an olefin such as ethylene, butene, hexene, or octane, or a combination thereof). For a polypropylene copolymer, the amount of propylene monomer relative to non-propylene monomers used to produce the copolymer can be any useful amount, such as, an amount of at least 50, 60, 70, 80, or 90 weight percent propylene monomer per the total weight of all monomers (propylene monomer and non-propylene monomers) in a monomer composition used to prepare the propylene copolymer. As used herein, a composition (e.g., a polymeric composition) described as consisting "essentially of" a specified ingredient or a combination of specified ingredients is a composition that contains the specified ingredient or a combination of specified ingredients and no more than minor or insignificant amounts of other materials (e.g., no more than 3, 2, 1, 0.5, 0.1, or 0.05% by weight of any other ingredient or combination of ingredients). For example, a polymer prepared from monomers that "consist essentially of" olefin, ethylene, or propylene monomers is a polymer prepared from olefin monomers, ethylene monomers, or propylene monomers, respectively, and no more than minor or insignificant amounts of other monomeric materials (e.g., no more than 3, 2, 1, 0.5, 0.1, or 0.05 weight percent of any other monomer). As another example, a polymer composition or a polymer film described as consisting "essentially of" polyolefin, polyethylene, or polypropylene is a polymer composition or a polymer film that contains polyolefin (including copolymers), polyethylene (including copolymers), or polypropylene (including copolymers), respectively, and no more than a small or insignificant amount of a different type of polymer (e.g., no more than 3, 2, 1, 0.5, 0.1, or 0.05 weight percent of any other type of polymer). An exemplary assembly of polymer filtration membranes may include two polymer filtration membranes arranged in series to allow liquid to flow first through a first (upstream) polymer filtration membrane, which is described as an open membrane, and then through a second (downstream) polymer filtration membrane, which is described as a tight membrane. The first (open) polymer filtration membrane is relatively thicker than the second (tight) polymer filtration membrane and has relatively larger pores than the second polymer filtration membrane. A first (open) polymeric filtration membrane can be made from any useful polymer, such as polyolefins (e.g., polypropylene, polyethylene, or copolymers thereof), polyetheretherketone (PEEK), polysulfone, polyamide (e.g., nylon), fluoropolymers (including fluorinated and perfluorinated polymers), polyesters (e.g., polyethylene terephthalate), and other types of polymers and copolymers that can form an open polymeric filtration membrane. According to exemplary membrane assemblies, a first polymeric filtration membrane can have a relatively greater thickness than the second membrane, a relatively larger pore size (on average) than the second polymeric filtration membrane, a relatively higher measured "flow time" than the second polymeric filtration membrane, and a lower bubble point than the second polymeric filtration membrane. The first polymeric filter membrane can be an open cast membrane of a type that can be used as a polymeric filter membrane (for example, a polymeric filter membrane that can be used to remove contaminants from a semiconductor processing liquid). A first polymeric filter membrane has physical properties (e.g., pore size, porosity, and bubble point) and filtration properties (e.g., retention and flow time or flow rate) that allow the first polymeric filter membrane to be used alone as a polymeric filter membrane to remove undesirable contaminants from a liquid stream, or as part of a filter membrane assembly in combination with a second (compact) polymeric filter membrane. A useful first polymeric filtration membrane can be of a type sometimes described as an ultraporous membrane, a nanoporous membrane, a microporous membrane, or the like. Such polymeric filtration membranes are generally effective in removing undesirable contaminants (e.g., particulate material or dissolved chemicals, or both) from a liquid stream. A useful average pore size can be selected based on one or more factors including: fluid flow rate, pressure, pressure drop considerations, viscosity considerations, the amount and type of contaminants in the liquid to be filtered (e.g., dissolved metal ions or particulate contaminants in the liquid). Examples of a useful first polymeric filtration membrane include various membranes that are commercially available and known to be useful in microfiltration applications, including polymeric filtration membranes that are considered microporous, ultraporous, or nanoporous. Exemplary first polymeric filtration membranes can have an average pore size in a range from about 0.001 micrometer to about 5 micrometers or 10 micrometers (1 nanometer to about 5,000 nanometers or 10,000 nanometers (nm)), such as from 0.01 micrometer to 0.8 micrometer (from 10 nanometers to 800 nanometers). A useful or preferred first polymer filtration membrane can be considered to have relatively uniform pore size with a degree of pore symmetry, or relatively non-uniform pore size with a degree of pore asymmetry. A polymer filtration membrane can be isotropic or anisotropic (with respect to pore size), symmetric or asymmetric, "skinned" or "unskinned," or a combination thereof. A polymer filtration membrane having substantially uniform pore size distributed uniformly throughout the thickness of the membrane is often referred to as "isotropic" or "homogeneous." In contrast, an anisotropic (also known as "asymmetric") membrane can be considered to have a morphology in which a pore size gradient exists across the membrane. For example, an anisotropic or asymmetric membrane can have a porous structure with relatively larger pores on one membrane surface and relatively smaller pores on another membrane surface, where the pore structure varies along the thickness of the membrane. The term "asymmetric" is often used interchangeably with the term "anisotropic." The first polymeric filtration membrane may be in the form of a sheet having a relatively uniform thickness, such as a thickness in a range from 20 to 300 microns, for example from 30 or 50 microns to 150, 200 or 250 microns. A first polymer filtration membrane can have any porosity that will allow the polymer filtration membrane to function as described herein. Exemplary first polymer filtration membranes can have a relatively high porosity, such as a porosity in the range of 40% to 80%. As used herein, the "porosity" (sometimes also referred to as the void fraction) of a polymer filtration membrane is a measure of the percentage of void (i.e., "empty") space in the polymer filtration membrane to the total volume of the polymer filtration membrane and is calculated as the fraction of the void volume of the polymer filtration membrane to the total volume of the polymer filtration membrane. A host system having a porosity of zero percent is completely solid. Examples of polymer filtration membranes that can be used as the first polymer filtration membrane and have the physical properties described include polyolefin (e.g., polyethylene and polypropylene) films prepared by known methods (including melt casting and phase inversion methods), with specific examples including thermally induced phase separation (TIPS), solvent evaporation induced phase separation (SEIPS), and non-solvent induced phase separation. A first polymeric filtration membrane can be treated or coated by adding ionic groups (cationic or anionic groups) to the surface of the polymeric filtration membrane to improve the removal of dissolved or suspended contaminants in a liquid by a non-sieving filtration mechanism. The bubble point is a known property of polymeric filter membranes and is related to the pore size of the polymeric filter membrane. A relatively high bubble point indicates that the pores of a polymeric filter membrane are relatively small, and a relatively low bubble point indicates that the pore size of a polymeric filter membrane is relatively large. A bubble point measurement is based on the following premise: for a specific fluid and pore size, the pressure required to force a bubble through the pores of a polymeric filter membrane when the polymeric filter membrane is continuously wetted is inversely proportional to the pore size. A porosity bubble point test method measures the pressure required to force a gas (e.g., air) through the wetted pores of a membrane. A bubble point test is a well-known method for determining or estimating the pore size of a membrane. Examples of useful average bubble points for a first polymeric filtration membrane as described, measured using the test method described herein (using ethoxynonafluorobutane (HFE-7200) as a wetting solvent and a temperature of 22° C.), can be less than 130 psi, e.g., within a range of from 50 psi or 60 psi to 110 psi or 120 psi. In combination with a desired bubble point and filtration efficiency (e.g., as measured by retention as described herein), a first polymeric filtration membrane can exhibit a useful flow rate of liquid through the polymeric filtration membrane. The flow rate of liquid through a polymeric filtration membrane can be measured in terms of flow rate or flow time (which is the inverse of flow rate). A first polymeric filtration membrane as described can have a useful or relatively low flow time, preferably in combination with a bubble point within the range described and good filtration efficiency (e.g., as measured by a retention test). Useful examples of first (open) polymeric filtration membranes can have a measured flow time of less than approximately 15,000 seconds or less than 13,000 seconds (e.g., less than 10,000 seconds, 5,000 seconds, or 1,000 seconds) as measured using isopropyl alcohol (IPA) as described herein. The effectiveness of a polymeric filter membrane in removing contaminants (including dissolved or suspended chemical molecules) from a liquid can be measured in one manner as "retention." Retention, as it relates to the filtration efficiency of a polymeric filter membrane, generally refers to the total amount of one or more contaminants removed from the liquid (when using the filter or under test conditions) relative to the total amount of the one or more contaminants present in the liquid before the contaminant-containing liquid was passed through the polymeric filter membrane. Thus, the "retention" value of a polymeric filter membrane is a percentage, where a polymeric filter membrane with a higher retention value (higher percentage) is relatively more effective at removing a contaminant from a liquid, while a polymeric filter membrane with a relatively lower retention value (lower percentage) is less effective at removing a contaminant from a liquid. As measured under the test conditions, examples of the first (open) polymeric filter membrane can exhibit a retention rate exceeding 98% or 99% for a monolayer coverage of 1.0%; a retention rate exceeding 97% or 98% for a monolayer coverage of 2.0%; a retention rate exceeding 98% or 99% for a monolayer coverage of 2.0%; a retention rate exceeding 95%, 96%, or 97% for a monolayer coverage of 3.0%; and a retention rate exceeding 94% or 95% for a monolayer coverage of 4.0%; each of which is measured using the test method described herein. See Figure 5. The second polymeric filtration membrane is thinner than the first polymeric filtration membrane and has smaller pores than the first polymeric filtration membrane. The second polymeric filtration membrane can be made of any useful polymer and is preferably made of a polyolefin (such as polypropylene, polyethylene (including copolymers thereof)) or a mixture of two or more polyolefins or polyolefin copolymers. Specific 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 mixture of polyethylene and polypropylene. A second polymeric filtration membrane of an assembly may consist of a single polymeric filtration membrane or, alternatively, comprise two or more "sub-layers," in which case the second polymeric filtration membrane can be considered a "multi-layer" membrane comprising two or more sub-layers. The second polymeric filtration membrane may be in the form of a sheet having a relatively uniform thickness. A second polymeric filtration membrane of an assembly consisting of a single filtration layer (single polymeric filtration membrane) may have a thickness of less than 10, 15, or 20 microns, for example, a thickness within a range of 0.1 to 10 microns or 0.2 to 5 microns. A second polymeric filtration membrane comprising two or more sub-layers may include several sub-layers each having a thickness within these ranges, and may have a total thickness within these ranges. The second "compact" polymeric filter membrane has smaller pores than the first "open" polymeric filter membrane, and this allows the second polymeric filter membrane to be effectively used as a compact layer of a filter assembly as described, for example, to remove relatively small contaminants from a liquid that have not been removed by a liquid flowing through a first "open" polymeric filter membrane upstream of the second "compact" polymeric filter membrane. In some exemplary second polymeric filter membranes or exemplary sublayers of a second membrane, a second polymeric filter membrane or a sublayer thereof can have an average pore size in a range of from about 0.001 μm to about 0.1 μm (1 nm to about 500 nm), for example, from 0.01 μm to 0.05 μm (10 nm to 50 nm), or from 0.001 μm to 0.005 μm (1 nm to 5 nm). Examples of polymeric filtration membranes useful as the second polymeric filtration membrane and having the physical properties described above can be prepared by known methods of preparing a polyolefin film by any useful technique (e.g., extrusion, melt casting, or phase inversion techniques), followed by uniaxial or biaxial stretching of the film to a reduced thickness. By one technique, a second polymeric filtration membrane comprising polyethylene (including copolymers), polypropylene (including copolymers), or mixtures thereof (including, consisting essentially of, or consisting of polyethylene, polypropylene, or mixtures thereof) can be prepared by extruding a polymer (polyolefin) resin containing a diluent to form a polyolefin sheet and stretching the sheet, followed by extraction and annealing steps. A second polymeric filtration membrane can be treated or coated by adding ionic groups (cationic or anionic) to the surface of the membrane to improve the removal of dissolved or suspended contaminants in a liquid by a non-sieving filtration mechanism. A useful or preferred second polymer filter membrane (whether in the form of a single layer or in the form of multiple sublayers) can have certain physical properties (e.g., pore size, porosity and bubble point) and filtration properties (e.g., retention and flow time or flow rate) that effectively allow the second polymer filter membrane to be used alone as a polymer filter membrane to remove undesirable contaminants from a liquid stream, or as a polymer filter membrane in combination with another polymer filter membrane as part of an assembly of two or more polymer filter membranes as described, the assembly comprising a first (open) polymer filter membrane combined with a second (compact) polymer filter membrane. The first (open) polymer filter membrane and the second (compact) polymer filter membrane of an assembly comprising (including, essentially consisting of, or consisting of) a first (open) polymer filter membrane and a second (compact) polymer filter membrane can preferably have a bubble point greater than the first (open) polymer filter membrane. Examples of useful bubble points for the second (compact) polymer filter membrane as described herein, measured using the test method described in the Examples section, can be greater than 100 psi, for example, in a range of from 120 psi to 300 psi, for example, in a range of from 130 psi to 200 psi. A second polymeric filtration membrane can have any porosity that will allow the polymeric filtration membrane to be effective when used as a second polymeric filtration membrane described herein. Exemplary second polymeric filtration membranes can have a relatively high porosity, such as a porosity in a range of from 40% to 80%. When measured alone, a second polymeric filtration membrane can have a useful or relatively low flow time, preferably combined with a relatively high bubble point and good filtration efficiency. A second polymeric filtration membrane can have a flow time that is less than the flow time of a first polymeric filtration membrane used with the second polymeric filtration membrane. An exemplary second polymeric filtration membrane can have a flow time measured using IPA as described below of less than approximately 15,000 seconds, less than 11,000 seconds, or less than 10,000 seconds, 9,000 seconds, 5,000 seconds, or 1,000 seconds. The exemplary second polymeric filter membrane can exhibit a retention rate exceeding 97% or 98% for a monolayer coverage of 1.0%; a retention rate exceeding 95% or 96% for a monolayer coverage of 2.0%; a retention rate exceeding 96% or 97% for a monolayer coverage of 2.0%; a retention rate exceeding 93% or 94% for a monolayer coverage of 3.0%; and a retention rate exceeding 91% or 92% for a monolayer coverage of 4.0%; each of which is measured at a useful flow rate through the membrane using the test described in the Examples section. See Figure 4. A membrane assembly comprising a first polymeric filtration membrane and a second polymeric filtration membrane connected in series can have a total thickness of the two polymeric filtration membranes that is equal to the combined thickness of the first polymeric filtration membrane and the second polymeric filtration membrane, excluding any additional polymeric filtration membranes, filter elements, or support layers that may be used in combination with the first and second polymeric filtration membranes of the assembly. Example thickness values ​​for a membrane assembly can be comparable to the thickness of a first polymeric filtration membrane of the assembly, for example, in a range from 20 μm to 300 μm, for example, from 30 μm or 50 μm to 150 μm, 200 μm, or 250 μm. As with each of a first polymeric filtration membrane and a second polymeric membrane comprising a membrane assembly, the assembly can also be evaluated with respect to properties of the assembly such as bubble point, retention, and flow time. The bubble point of an assembly comprising a first polymer filter membrane and a second polymer filter membrane (including, consisting essentially of, or consisting of the first and second polymer filter membranes) can preferably be greater than the bubble point of the first (open) polymer filter membrane. According to exemplary membrane assemblies, an assembly can have a bubble point (average bubble point) comparable to the bubble point of a second (compact) polymer filter membrane of the assembly, for example, an average bubble point within 10% of the average bubble point of the second (compact) membrane measured in pounds per square inch. Exemplary average bubble points for a membrane assembly can be greater than 100 psi, for example, within a range of 120 psi to 300 psi, for example, within a range of 130 psi to 200 psi. When the retention rate of each of a first (open) polymer filter membrane and a second (compact) polymer filter membrane is measured individually, the retention rate of an assembly comprising (including, consisting essentially of, or consisting of) the first and second polymer filter membranes can be superior to the retention rates of both the first and second polymer filter membranes. This effect can be demonstrated during the use of the membrane assembly to remove contaminants from a semiconductor processing fluid, such as a photoresist solution, or can be demonstrated by testing under controlled conditions, such as by measuring the retention rate of a test contaminant in a test solution containing a known concentration of test particles. An example of a test particle is G25 particles made of polystyrene with an average particle diameter ranging from 3 nanometers to 25 nanometers. Other types and sizes of test particles are also suitable for measuring the retention rate of the assembly and the retention rate of each polymer filter membrane individually. An exemplary assembly comprising a single first polymeric filter membrane and a single second polymeric filter membrane (comprising, consisting essentially of, or consisting of) can exhibit a measured retention value exceeding 99% for a monolayer coverage of 1.0%; a retention exceeding 98% or 99% for a monolayer coverage of 2.0; a retention exceeding 98% for a monolayer coverage of 3.0; and a retention exceeding 98% for a monolayer coverage of 4.0, each measured at a useful flow rate through the membrane using the test described in the Examples section. See Figures 4 and 5. A membrane assembly comprising (comprising, consisting essentially of, or consisting of) a first polymeric filtration membrane and a second polymeric filtration membrane can have a flow time comparable to the flow time of either or both of the first polymeric filtration membrane or the second polymeric filtration membrane of the assembly when each is measured independently. The assembly can have a useful or relatively low flow time, preferably in combination with a relatively high bubble point and good filtration efficiency, such as those described herein for a membrane assembly. An exemplary membrane assembly can have a measured flow time of less than 20,000 seconds, as measured using IPA, as described herein. A membrane assembly comprising at least a first (open) polymer filter membrane and a second (tight) polymer filter membrane can be incorporated into a filter product and used to filter a liquid, such as a semiconductor processing liquid. The assembly can comprise the first polymer filter membrane and the second polymer filter membrane arranged in series between an inlet and an outlet of the filter product. The assembly can comprise or be used in conjunction with one or more additional membranes or support layers or support structures (such as a frame). In preferred use, the first polymer filter membrane is located upstream and the second polymer filter membrane is located downstream relative to a flow of liquid through the assembly. An upstream side of the second polymer filter membrane faces a downstream side of the first polymer filter membrane. A liquid first flows through the first polymer filter membrane and then flows through the second polymer filter membrane. Referring to FIG1 , an assembly 100 is illustrated, comprising a first (open) polymer filtration membrane 102 and a second (compact) polymer filtration membrane 104 connected in series. The open polymer filtration membrane 102 is positioned upstream relative to a liquid flow 110 entering the upstream surface of the open polymer filtration membrane 102. Liquid 110 (see arrow) flows into the first polymer filtration membrane 102, which has a greater thickness and larger pore size than the second polymer filtration membrane 104. Contaminants present in the liquid 110 are removed by the pores of the first (open) polymer filtration membrane 102. After flowing through polymer membrane 102, the liquid flows into the second (compact) polymer filtration membrane 104, which has a smaller thickness and relatively smaller pore size. After the liquid 110 has passed through the first polymer filtration membrane 102, smaller contaminants remaining in the liquid 110 can be removed by the second polymer filtration membrane 104 via a sieving mechanism or a non-sieving mechanism. The filtrate 112 (see arrow) passes through the downstream surface of the second polymer filtration membrane 104 . The assembly 100 of FIG1 is shown as being comprised of a first polymeric filtration membrane 102 and a second polymeric filtration membrane 104, without additional filtration membranes or support membranes or structures, which, although not illustrated, may be included during use of the example assembly 100. Furthermore, the second polymeric filtration membrane 104 is illustrated as being comprised of a single polymeric filtration membrane layer, but may optionally be fabricated to include two or more sub-layers, as described herein. The polymer filtration membranes 102 and 104 can be separate from each other (i.e., not attached), but arranged so that the downstream surface of the first polymer filtration membrane 102 faces, is adjacent to, or contacts the upstream surface of the second polymer filtration membrane 104. If desired, but not required, the two polymer filtration membranes can be attached at their opposing surfaces (the outlet surface of the first polymer filtration membrane attached to the inlet surface of the second polymer filtration membrane) by a lamination step in which the surfaces of the two polymer filtration membranes are brought into contact with each other and adhered together under high temperature and slight pressure. The temperature and pressure conditions of an optional lamination step are moderate so as not to significantly affect the morphology and filtration performance properties (flow time, retention) of either the first or second polymer filtration membranes. The lamination step can be performed without placing an additional material (such as an adhesive) between the surfaces of the two polymer filtration membranes. The membrane assembly can be incorporated into a filter product, such as a filter cartridge that supports the membrane assembly within a liquid flow (a removable filter cartridge), or a filter housing that includes any structure that supports the membrane assembly to allow a liquid fluid to flow through the membrane assembly from an inlet side of the membrane assembly, through the membrane assembly, and out of the membrane assembly at an outlet side. A filter housing can include an inlet on one side of the supported membrane assembly and an outlet on a second side of the membrane assembly, the inlet allowing fluid to enter the housing when the membrane assembly is supported by the filter housing, and the outlet allowing fluid to exit the filter housing after passing through the membrane assembly. The terms "inlet" and "outlet" may refer to a specific structure, such as a defined opening or a defined conduit (a tube, a pipe, or a joint thereof) of a filter housing or a component of a filter housing (e.g., a support frame, a core, a cage, etc.), or may refer to a space (an "inlet space" or an "outlet space") on either side of the membrane assembly when supported by the filter housing that accommodates fluid flowing into or out of the membrane assembly. The terms "inlet" and "outlet" may generally refer to the space on either side of the membrane assembly that accommodates fluid flowing into, through, and out of the membrane assembly when the membrane assembly is supported by the filter housing. A filter housing includes a membrane assembly supported to allow fluid to pass through the membrane and may be a component of a larger filtration system that supplies filtered liquid chemicals to a tool or process used to manufacture microelectronic or semiconductor devices. The filter assembly or filter housing may include one or more of a variety of additional materials and structures that support the membrane assembly within a filter housing to facilitate fluid flow through the membrane assembly. The membrane assembly supported by the filter housing may have any useful shape, such as a pleated cylinder, a cylindrical pad, one or more non-pleated (flat) cylindrical sheets, a pleated sheet, and other shapes. An example of a filter structure (e.g., a removable filter cartridge) comprising a membrane assembly can be in the form of a replaceable filter cartridge comprising a pleated cylindrical filter element comprising the membrane assembly with additional components, which may include one or more of the following: a rigid or semi-rigid core supporting the pleated membrane assembly at an interior side (inlet or outlet) of the pleated membrane assembly; a rigid or semi-rigid cage supporting or surrounding an exterior side (outlet or inlet) of the pleated membrane assembly at an exterior side of the pleated membrane assembly; and optional end pieces or "pucks" located at each of two opposing ends of the pleated membrane assembly. A replaceable filter cartridge is considered to have an inlet on one side of the membrane assembly and an outlet on an opposite side of the membrane assembly; depending on the direction of liquid flow through the membrane assembly, an inlet or an outlet can be located on either side of the membrane. As an example, FIG2 shows a filter product (replaceable filter cartridge) 230 comprising a pleated cylindrical assembly (e.g., filter element) 210 and an end piece 222, with other optional structural components not specifically shown. Cylindrical assembly 210 includes a membrane assembly 212 as described herein and is pleated. End piece 222 is attached (e.g., "potted") to one end of cylindrical filter assembly 210. End piece 222 is preferably made of a melt-processable polymer material (e.g., a thermoplastic fluoropolymer). A core (not shown) can be positioned at the interior opening or space ("inlet") 224 of pleated cylindrical assembly 210, and a cage (not shown) can be positioned around the exterior opening or space ("outlet") of pleated cylindrical assembly 210. Thus, the filter cartridge includes an inlet and an outlet (e.g., at a core, a 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, wherein the locations of the inlet and outlet depend on the direction in which a liquid is caused to flow through the membrane assembly 212 when the filter cartridge 230 is installed in a filter housing, which may also have an inlet and an outlet. A second end piece (not shown) can be attached ("potted") to the second end of the pleated cylindrical element 210. The resulting replaceable filter cartridge 230, having two opposing potted ends and an optional core and cage, can then be placed into a larger filter housing that includes an upstream housing inlet for receiving liquid flowing into the filter membrane and a downstream housing outlet for draining liquid that has passed through the membrane assembly. The housing is configured such that a metered amount of liquid entering the housing at the housing inlet must pass through the membrane assembly 212 before exiting the filter housing at the housing outlet. The components of a filter housing can be of any useful and desired size, shape, and material, and can preferably be a fluorinated or non-fluorinated polymer such as nylon, polyethylene, or a fluorinated polymer such as poly(tetrafluoroethylene-co-perfluoro(alkyl vinyl ether)), TEFLON® perfluoroalkoxyalkane (PFA), perfluoromethylalkoxy (MFA), or another suitable fluoropolymer (e.g., a perfluoropolymer). 3 , a filter product 250 includes a housing containing a replaceable filter cartridge 230 within an interior thereof. The filter product 250 comprises a two-component housing comprising a bowl 252 attached to a base 254 at an open end thereof. Optionally, the bowl 252 can be removably engaged with the base 254 to allow the bowl 252 to be separated from the base 254, thereby allowing a removable filter cartridge 230 to be placed into or removed from the interior of the housing. The assembled housing comprises an inlet 256, an inlet space 260 located at an interior side of the filter cartridge 230, an outlet space 262 between the filter cartridge 230 and the bowl 252, and an outlet 258. The removable filter cartridge 230 can be located within one of the bowls 252, and one of the configurations of the removable filter cartridge allows liquid to flow into the inlet 256, flow into the inlet space 260, then pass through the membrane assembly 212 of the filter cartridge 230, flow into the outlet space 262, and then discharge from the filter product 250 through the outlet 258. A membrane assembly as described can be used to filter a liquid chemical to purify or remove unwanted materials (contaminants) from the liquid chemical, particularly in a process for producing a high-purity liquid chemical that can be used in an industrial process requiring a liquid chemical input having a very high purity level. Generally speaking, the liquid chemical can be any of a variety of useful commercial liquid chemicals of a type useful in any industrial or commercial application. Specific examples of the membrane assembly and filter products as described can be used to purify a liquid chemical used in a semiconductor or microelectronics fabrication application, such as for filtering a liquid solvent or other processing solution used in a semiconductor lithography process, a wet etching or cleaning step, a process for forming spin-on-glass (SOG), a backside antireflective coating (BARC) process, and the like. Some specific non-limiting examples of solvents (including cleaning solutions) that can be filtered using a membrane assembly as described include: n-butyl acetate (nBA), isopropyl alcohol (IPA), 2-ethoxyethyl acetate (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 a mixture of any of these, 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 a combination of these, such as a combination of ammonium hydroxide and hydrogen peroxide, or a combination of hydrochloric acid and hydrogen peroxide. A more specific example of a semiconductor processing fluid is a "photoresist solution," which includes a polymer resin, a sensitizer (also called a "inhibitor," which regulates the photochemical process), and a solvent. An exemplary photoresist solution comprises a reactive curable polymer resin that can be cured in the presence of electromagnetic radiation and a sensitizer. The type of radiation can be: deep ultraviolet radiation, near ultraviolet radiation, electron beam radiation, x-ray radiation. Examples of specific reactive polymers known to be useful in photoresist solutions include polymethacrylate (PMMA) resins, isoprene, polyalkyl aldehydes, diazonaphthoquinone (DNQ) resins, novolac resins (phenol-formaldehyde polymers), epoxy resins, non-stoichiometric thiol-ene (OSTE) polymer resins, and hydrogen silsesquioxane (HSQ) resins. Various solvents and photosensitizers are known to be useful in photoresist solutions containing different types of curable polymer resins. Each of the polymer resin, solvent, and sensitizer can be used in a specific amount effective in a specific photoresist solution. Examples of unwanted contaminants that may be present in a photoresist solution and that may be desired to be removed by a filtration step using a thin film assembly as described include particulate contaminants, which may be reactive polymer particles, dissolved metal ions ("metals"), or other particulate or dissolved contaminants. The terms "metal" and "metal ion" are used consistently with their usage in the chemical and semiconductor processing arts and refer to a molecular species comprising a metal ion, metal atom, or a metal complex. A metal contaminant can be or comprise a neutral, negatively charged, or positively charged metal species. Some examples of specific metals or metal ions that can be removed from a photoresist solution include Na, K, Ca, Fe, Mg, Al, Cr, Ni, and Zn. Particle contaminants in a photoresist solution may include reactive monomers, oligomers, or polymerized polymers having a particle size in a range from 0.01 microns to 0.2 microns. A membrane assembly or a first membrane or a second membrane can be tested individually or collectively for retention, bubble point, and flow time according to the following test methods. Retention rate test method using test particles The degree of effectiveness of a polymeric filter membrane or a membrane assembly in removing unwanted substances (i.e., "contaminants") from a liquid can be measured in one manner as "retention." Retention generally refers to the amount of contaminant removed from a liquid relative to the total amount of contaminant initially present in the liquid before the contaminant-containing liquid is passed through the membrane or membrane assembly. The "retention" value of a polymeric filter membrane or a membrane assembly is a percentage, wherein a polymeric filter membrane with a higher retention value (higher percentage) is relatively more effective at removing a contaminant from a liquid, while a polymeric filter membrane with a relatively lower retention value (lower percentage) is relatively less effective at removing a contaminant from a liquid. Retention can be measured by measuring the amount of test particles removed from a liquid stream using a polymeric filter membrane or a membrane assembly placed in the liquid stream. By one approach, retention can be measured by passing a sufficient amount of test aqueous solution at a constant flow rate through a membrane containing 0.1% Triton X-100 and 8 ppb polystyrene particles (e.g., G25 round polystyrene particles having a nominal diameter of 3 nm to 25 nm) to achieve 1% monolayer coverage of the membrane and collecting the permeate. The concentration of polystyrene particles in the permeate can be calculated based on the absorbance of the permeate. The particle retention is then calculated based on the particle concentrations in the feed and filtrate using the following equation: Particle Retention use HFE-7200 Bubble Point Test Method A method for determining the bubble point of a polymer filter membrane is used. A sample of the polymer filter membrane is immersed in a liquid with a known surface tension and wetted by the liquid, and a gas pressure is applied to one side of the sample. The gas pressure is gradually increased. To measure the bubble point, a 47 mm polymer filter membrane disc is placed in a holder and a high-permeability spunbond nonwoven fabric (PGI) is placed downstream as a support layer. Air is pressurized through the holder and measured as a function of pressure. Then, low surface tension fluid HFE-7200 (3M) is introduced to the downstream side to wet the membrane. Air is again pressurized through the holder and air flow is measured as a function of pressure. The minimum pressure at which a gas flows through a sample is called an initiation bubble point. The pressure at which the ratio of the air flow rate of the wet film to the air flow rate of the dry film is 0.5 is called the average bubble point. If a film produces two different pressure values ​​based on the side of the film facing the air, the lower measured value is used as the average bubble point. Flow Time Test Method The ease with which a liquid passes through a polymeric filtration membrane or a membrane assembly (e.g., the flow rate of a liquid through a membrane or a membrane assembly) can be evaluated as a measure of "flow time," which is the inverse of the flow rate. Flow time can be defined as the time it takes for 500 ml of fluid (in this case, isopropyl alcohol) to pass through a 13.8 cm diameter container at 14.2 psi. 2 The time required to form a porous film or a film assembly with a certain surface area. Examples Table 1 shows a comparison of the average bubble point, flow time and thickness of two examples of the first (open) polymeric filtration membrane, a single example of the second (compact) polymeric filtration membrane and an example of an assembly comprising a first (open) polymeric filtration membrane and a second (compact) polymeric filtration membrane. surface 1 surface 2 The graphs of Figures 4 and 5 show the particle retention data for the films and film assemblies of Tables 1 and 2, as tested according to the retention test method described herein using G25 PSL test particles at 8 parts per million in deionized water and 0.1% Triton X-100 surfactant. Figure 4 shows the measured retention values ​​for two different assemblies ("Assembly 1" and "Assembly 2") of a first (open) polymeric filter membrane and a second (compact) polymeric filter membrane. Assembly 1 is a non-laminated assembly consisting of an open polymeric filter membrane ("first layer") and a compact polymeric filter membrane made from stretched UPE ("second layer"). Assembly 2 is a non-laminated assembly consisting of an open polymeric filter membrane ("first layer") and a compact polymeric filter membrane made from stretched UPE ("second layer"). The measured retention of each of Assemblies 1 and 2 was at least 98% for test particles at particle loadings associated with monolayers of 1.0, 2.0, 3.0, and 4.0. FIG4 also shows the measured retention values ​​for a separately tested compact polymer filtration membrane ("Example Compact Membrane 2"), which is a stretched UPE membrane. For test particles at particle loadings associated with monolayers as high as 1.5, the measured retention of Example Compact Membrane 2 was approximately 98%, and for higher particle loadings, the measured retention was below 98%. FIG5 shows the measured retention values ​​for Assembly 2 and compares them with the measured retention values ​​for two different open polymeric filtration membranes ("Example Open Membrane 3" and "Example Open Membrane 4"). Example Open Membrane 3 is a melt-cast membrane. Example Open Membrane 4 is a melt-cast membrane. Assembly 2 had a measured retention of at least 98% for the test particles at particle loadings associated with monolayers of 1.0, 2.0, 3.0, and 4.0. The measured retention of the exemplary open film 3 was approximately 98% for the test particles at particle loadings associated with monolayers up to 1.5 or about 2.0, and was below 98% for higher particle loadings. The measured retention of the exemplary open film 4 was approximately 98% for the test particles at particle loadings associated with up to about 2.5 monolayers, and was below 98% for higher particle loadings. 100: Assembly 102: First (open) polymer filter membrane / polymer membrane / polymer filter membrane 104: Second (tight) polymer filter membrane / polymer filter membrane 110: Liquid flow / liquid 112: Filtrate 210: Pleated cylindrical assembly / cylindrical assembly / cylindrical filter assembly 212: Membrane assembly 222: End piece 224: Internal opening or space (inlet) 230: Filter product (replaceable filter cartridge) / filter cartridge / removable filter cartridge 250: Filter product 252: Bowl 254: Base 256: Inlet 258: Outlet 262: Outlet space FIG1 shows an example of an assembly comprising a first (open) polymer filtration membrane and a second (compact) polymer filtration membrane in series. FIG. 2 shows an example of a filter product comprising a first (open) polymer filter membrane and a second (compact) polymer filter membrane. FIG. 3 shows an example of a filter product comprising a first (open) polymer filter membrane and a second (compact) polymer filter membrane contained in a filter housing. 4 and 5 show retention data for the described polymer filtration membrane assemblies and comparative polymer filtration membranes. The figures are schematic and not necessarily drawn to scale. 100: Assembly 102: First (open) polymer filter membrane / polymer membrane / polymer filter membrane 104: Second (compact) polymer filter membrane / polymer filter membrane 110:Liquid flow / liquid 112: Filtrate

Claims

1. A method for filtering a semiconductor-processed liquid, the method comprising: The liquid is passed through a first polymer filter membrane having an average bubble point of less than 120 psi (measured using ethoxynonfluorobutane (HFE7200)) and a thickness of at least 20 micrometers; and the liquid is passed through a second polymer filter membrane having an average bubble point of greater than 120 psi (measured using ethoxynonfluorobutane (HFE7200)) and a thickness of less than 20 micrometers, wherein the first polymer filter membrane and the second polymer filter membrane form a layered filter membrane, wherein the first polymer filter membrane is prepared by a phase inversion technique and the second polymer filter membrane is prepared by a stretching technique.

2. The method of claim 1, wherein the method of passing the liquid through both the first polymer filter membrane and the second polymer filter membrane removes a greater amount of contaminants from the liquid compared to both a method of passing the liquid through the first polymer filter membrane and a method of passing the liquid through the second polymer filter membrane alone.

3. The method of claim 1, wherein the liquid system is a photoresist solution, the photoresist solution comprising: Polymer resins, sensitizers, and solvents.

4. The method of claim 3, wherein the photoresist solution includes a metallic contaminant.

5. The method of claim 3, wherein the photoresist solution comprises contaminant particles having a particle size in the range of 0.01 micrometers to 0.2 micrometers.

6. The method of claim 1, wherein the first polymer filter membrane has a thickness in the range of 20 micrometers to 300 micrometers.

7. The method of claim 1, wherein the second polymer filter membrane has a thickness in the range of 0.1 micrometers to 10 micrometers.

8. A filter product for removing a contaminant from a semiconductor processing fluid, the filter product comprising: A first polymer filter membrane having an average bubble point of less than 120 psi (measured using ethoxynonfluorobutane (HFE7200)) and a thickness of at least 20 micrometers; and a second polymer filter membrane having an average bubble point of greater than 120 psi (measured using ethoxynonfluorobutane (HFE7200)) and a thickness of less than 20 micrometers, wherein the first polymer filter membrane and the second polymer filter membrane form a layered filter membrane, wherein the first polymer filter membrane is prepared by a phase inversion technique and the second polymer filter membrane is prepared by a stretching technique.

9. The filter product of claim 8, wherein the first polymer filter membrane has a thickness in the range of 20 micrometers to 300 micrometers.

10. The filter product of claim 8, wherein the second polymer filter membrane has a thickness in the range of 0.1 micrometers to 10 micrometers.

11. The filter product of claim 8, comprising a frame, wherein the first polymer filter membrane and the second polymer filter membrane are supported by the frame.

12. The filter product as requested in item 8, wherein the filter product is a replaceable filter element.

13. The filter product of claim 8, wherein the filter product includes a housing including an inlet, an inlet space, an outlet and an outlet space, which is adapted to allow liquid to flow through the inlet into the inlet space, then through the first polymer filter membrane, then through the second polymer filter membrane, and then into the outlet space.

14. The filter product of claim 8, wherein the filter product provides a better retention rate of G25 round polystyrene particles compared to the retention rates of the first polymer filter membrane alone and the second polymer filter membrane alone.

15. The filter product of claim 8, wherein based on the use of a 4.0% monolayer of G25 round polystyrene particles with a nominal diameter of 5 to 15 nanometers, the filter product exhibits a measured retention rate of at least 98%.

16. The filter product of claim 8, wherein based on the use of a 3.0% monolayer of G25 round polystyrene particles with a nominal diameter of 5 to 15 nanometers, the filter product exhibits a measured retention rate of at least 98%.

17. A semiconductor processing filtration system, comprising: The filter product of request item 13 includes a semiconductor processing fluid source connected to an inlet and a semiconductor processing device connected to an outlet.

18. The semiconductor processing filtration system of claim 17, wherein the semiconductor processing fluid contains a photoresist solution.

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