WO2000000815A1

WO2000000815A1 - Method and apparatus for the measurement of surface tension-related characteristics of liquids

Info

Publication number: WO2000000815A1
Application number: PCT/US1999/014205
Authority: WO
Inventors: Donald C. Grant; Mark R. Litchy
Original assignee: Grant Donald C; Litchy Mark R
Priority date: 1998-06-26
Filing date: 1999-06-25
Publication date: 2000-01-06
Also published as: AU4959899A

Abstract

The present invention provides a practical, reliable, quick, accurate method and system for determining a surface tension-related characteristic of a liquid-containing composition, wherein the surface tension-related characteristic is determined from bubble point measurements. The approach of the present invention is accurate, because bubble point measurements are consistently sensitive to even small changes in surface tension-related characteristics of a composition. Such an approach is also relatively quick as a bubble point pressure profile can be determined for a sample in seconds, which makes such an approach suitable for on-line use.

Description

METHOD AND APPARATUS FOR THE MEASUREMENT OF SURFACE TENSION-RELATED CHARACTERISTICS OF LIQUIDS

FIELD OF INVENTION

This invention relates to a method and apparatus for accurately measuring the dynamic and static surface tension characteristics of a liquid by examining the bubble point curve ofthe mixture The techniques ofthe present invention can also be used as a feedback control in the preparation or monitoring of admixture quality within exacting specifications The invention is especially suitable for use on-line in industrial processes

BACKGROUND ON THE INVENTION

Chemical solutions requiring controlled surface tensions are used in a wide range of industries, such as the semiconductor, raw chemical, pulp and paper, pharmaceutical, ink jet printer, petroleum, and medical industries The surface tensions of these solutions often must meet precise specifications For example, in the process of fabricating microelectronic devices, such as forming semiconductor integrated circuits on semiconductor wafers, surfactant containing solutions have been used to increase the performance ofthe photolithography process

In a typical photolithographic process, a coating of photosensitive photoresist is applied onto the wafer After the photoresist is coated onto the wafer, the coating is exposed through a photomask The photomask has a desired pattern that selectively allows light to pass through the mask and expose selective portions ofthe photoresist coating After exposure, the photoresist coating is developed using an aqueous based developer solution If the photoresist is of a so-called negative type, the developer solution dissolves and washes away the unexposed regions ofthe photoresist coating If the photoresist is of a so-called positive type, the developer solution dissolves and washes away the exposed regions In either case, it is imperative that the entire surface ofthe photoresist coating is exposed to the developer

Ensuring that all regions ofthe photoresist coating are exposed to the developer can be difficult because the photoresist coating may not wet uniformly with the developer solution To promote uniform exposure ofthe photoresist coating to the developer, the developer may include a surfactant The surfactant reduces the surface tension ofthe developer solution so that the developer solution more easily wets the surface of the photoresist coating In many applications, the use of a developer solution including a surfactant has been shown to improve the contrast between the unexposed and exposed regions of the photoresist coating, allowing smaller features to be resolved

A typical photoresist developer solution contains precise concentrations of a base and a surfactant in water within exacting specifications to ensure that the developing process is accurate and repeatable Even a slight deviation in surfactant and/or base concentration from the specification can adversely affect the developing process, hence the resolution of the features being created by the photolithography process Historically, such developer solutions were purchased pre-blended, ready-to-use, and having a specified concentration of base and surfactant However, the use of pre-blended, ready- to-use developer solutions requires the end user to incur the cost of shipping and handling ofthe product in a dilute form, to incur the effects of shipping and handling upon the product purity and concentration, to rely upon a certificate of analysis for concentration verification (no real time monitoring of developer and surfactant concentration is realized), and to provide large storage areas for the pre-blended product

One alternative to purchasing a pre-blended, ready-made developer solution is to purchase the components of the developer solution individually and then prepare the developer solution on-site For instance, a concentrated, aqueous mixture of the base and surfactant could be purchased in bulk and stored on-site When developer is needed, the concentrated mixture of base and surfactant could be diluted with an appropriate diluent, e g , water, in precise amounts effective to meet the corresponding specification This would allow the end user to reduce the risk of contamination, realize a significant cost savings for shipping and handling, realize a significant cost savings on the raw materials, reduce required storage space, enjoy more flexibility to customize developer solutions for different applications, and have the capability to monitor base and surfactant concentrations in real-time For this alternative approach to work, however, it is imperative that the concentrated surfactant/base mixture be mixed with proper amounts of the diluent in order to ensure that the resultant developer solution meets the required specifications Of course, this imperative is not just pertinent to the microelectronics industry or to preparing developer solutions, the same challenge is posed in any instance in which mixtures containing surfactants (or other ingredients that affect surface tension) are to be mixed on-site according to precise specifications If on-site mixing is performed, it will be necessary to verify the concentration ofthe mixture What is needed, therefore, is a practical, quick, reliable, accurate way to monitor concentration of these kinds of mixtures

SUMMARY OF THE INVENTION

The present invention provides a practical, reliable, quick, accurate method and system for determining a surface tension-related characteristic of a liquid-containing composition, wherein the surface tension- related characteristic is determined from bubble point measurements The approach ofthe present invention is accurate, because bubble point measurements are consistently sensitive to even small changes in surface tension-related characteristics of a composition Such an approach is also relatively quick, because a bubble point pressure profile can be determined for a sample in seconds, which makes such an approach suitable for on-line use

The present invention has many beneficial uses in applications in which it is desirable to characterize a chemical in terms of surface tension or a surface tension-related characteristic For example, the invention can be used to determine the critical micelle concentration (CMC) of surfactant solutions The CMC is the concentration above which surface molecules form aggregates Knowledge ofthe CMC is very useful in predicting the ability ofthe surfactant to act as a detergent At the CMC the surface tension and other physical properties of a surfactant solution often undergo striking alterations in many characteristics including detergency, osmotic pressure, surface tension, conductivity, interfacial tension, and the like

In another example, if a correlation between composition or the like, on one hand, and bubble point characteristics, on the other hand, is known, then measurements of bubble point characteristics may be used to monitor the composition and/or quality of liquids to verify that the liquids meet desired specifications As another example, using such a correlacion, bubble point measurements may be used as feedback to help control the mixing of precise, relative amounts of two or more bulk chemicals This application ofthe present invention facilitates mixing of any chemicals for which suiface tension, and hence bubble point measurements, vary as a function of composition Because the surface tension of a photoresist developer solution is dependent upon surfactant concentration, the present invention is particularly suitable for preparing photoresist developer solutions whose base and surfactant concentrations must meet extremely strict specifications

In one aspect, the present invention relates to a method of characterizing a liquid-containing composition A body having an upstream portion and a downstream portion is provided, wherein the body comprises at least one pathway providing open communication between the upstream and downstream portions The body is wetted with the liquid-containing composition A pressure measurement indicative ofthe ability of the wetted body to resist a pressure acting against the wetted body is obtained A surface tension-related characteristic ofthe liquid-containing composition is determined from information comprising said pressure measurement

In another aspect, the present invention relates to a method of characterizing a photoresist developing solution A body having an upstream portion and a downstream portion is provided, wherein the body comprises at least one pathway providing open communication between the upstream and downstream portions The body is wetted with the photoresist developing solution A pressure measurement is obtained indicative ofthe ability of the wetted body to resist a pressure acting against the wetted body A surface tension-related characteristic ofthe photoresist developing solution is determined from information comprising said pressure measurement In yet another aspect, the present invention relates to a method of combining first and second chemical components to form an admixture with a desired composition The method includes the steps of

(a) combining an amount ofthe first chemical component and an amount of the second chemical component to form an admixture comprising said first and second components,

(b) wetting a body with the admixture, wherein the body has an upstream portion and a downstream portion, wherein the body comprises at least one pathway providing open communication between the upstream and downstream portions, and wherein the body is wetted with a sufficient amount ofthe admixture to inhibit open communication between the upstream and downstream portions, (c) obtaining a pressure measurement indicative ofthe ability ofthe wetted body to resist a pressure acting against the wetted body,

(d) using information comprising the pressure measurement to determine if the admixture contains relatively too little of one ofthe first and second components,

(e) if the admixture contains relatively too little of said one component, then

(1) adjusting the composition ofthe admixture by adding a corrective amount of the one component to the admixture, and (n) repeating at least steps (b) through (e) until the admixture has the desired composition In still yet another aspect, the present invention relates to an apparatus for measuring a surface tension-related characteristic of an admixture The apparatus included a gas source comprising a supply of gas, a housing, comprising a porous filter provided in the housing in a manner effective to define an upstream chamber and a downstream chamber, wherein the gas source is in fluid communication with the upstream chamber, a supply of the admixture in fluid communication with the filter so that the filter is capable of being wetted by the admixture, and a pressure measuring device disposed in the apparatus at a position effective to measure a pressure parameter indicative of a pressure in the upstream chamber

In yet another aspect, the invention can be used to measure the dynamic surface tension of a solution in which the surface tension is measured as a function of surface age Surface age at the time of measurement can be controlled by changing the time period over which the surface tension measurement is made BRIEF DESCRIPTION OF THE DRAWINGS

The above mentioned and other advantages ofthe present invention, and the manner of attaining them, will become more apparent and the invention itself will be better understood by reference to the following description ofthe embodiments ofthe invention taken in conjunction with the accompanying drawings, wherein

Fig 1 is a schematic diagram of an apparatus according to the present invention

Fig 2a is a schematic diagram of a portion ofthe body shown in Fig 1 having passageways that are open

Fig 2b is a schematic diagram ofthe portion shown in Fig 2a having passageways that are at least partially filled with admixture sample

Fig 2c is a schematic diagram ofthe portion shown in Fig 2b at the bubble point Fig 3 is a plot of upstream chamber pressure versus time for the mode of operation shown in Figs 2a through 2d

Fig 4 is a bubble point calibration curve relating P_max of a wetted body to the surfactant concentration of an admixture determined according to the present invention Fig 5 is a flowchart of a methodology ofthe present invention in which bubble point data is used as a feedback control to accurately blend two components

Fig 6 shows a system ofthe present invention for determining surface tension-related characteristics of an admixture Fig 7 shows a porous membrane filter that can be used as a body in the system of Fig 6

Fig 8 shows a tube that can be used as a body in system of Fig 6

Fig 9 shows a bundle of several tubes of Fig 8 that can be used as a body in system of Fig 6 Fig 10 shows a porous-walled tube that can be used as a body in system of Fig 6

Fig 1 1 shows a methodology for operating system of Fig 6

Fig 12 is a plot of surfactant concentration vs maximum pressure for the experiments described in Example 1

Fig 13 is a plot of surface tension vs maximum pressure for the experiments described in Example 1

Fig 14 is a plot of surfactant concentration vs maximum pressure described in Example 2 Fig 15 is a plot of surfactant/base solution concentration vs maximum pressure for the experiments described in Example 3

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED

EMBODIMENTS The embodiments ofthe present invention described below are not intended to be exhaustive or to limit the invention to the precise forms disclosed in the following detailed description Rather the embodiments are chosen and described so that others skilled in the art may appreciate and understand the principles and practices ofthe present invention Figs 1 through 2d schematically show how the principles ofthe present invention may be used to characterize the surface tension and/or a surface tension-related characteristic of an admixture sample quickly, economically, and with repeatable accuracy based upon measurements relating to the bubble point (to be explained further below) ofthe sample Because the principles ofthe present invention as shown in Figs 1 through 2d are readily applicable to on-line usage, the present invention may be incorporated into feedback control systems and methodologies when blending ingredients together in industrial processes The present invention may also be used on-line or off- line for quality control purposes to ensure that liquid chemicals being processed, packaged, stored, or otherwise handled meet desired specifications

Referring first to Fig 1, apparatus 10 includes body 12 supported in housing 14 by support frame 16 Housing 14 is thus divided into upstream chamber 18 and downstream chamber 20 by the body 12 Body 12 incorporates at least one, and more preferably a plurality of pathways 22 When clear, I e , not blocked by a sample, pathways 22 provide open communication between upstream chamber 18 and downstream chamber 20 Admixture sample source 30 is coupled to the upstream chamber 18 When a sample to be analyzed is supplied to upstream chamber 18 of housing 14 from admixture sample source 30, the sample supplied to the upstream chamber 18 wets the body 12 by filling and blocking all of pathways 22 Open communication between the upstream chamber 18 and the downstream chamber 20 of housing 14 is inhibited as a result The wetted body 12 thus has a greater ability to resist a pressure acting against the wetted body as compared to when body 12 is unwetted In the practice of the present invention, the ability of body 12 to resist such pressure when wetted can be quantified through pressure measurements (as will be described below), and these pressure measurements can then be used to accurately characterize one or more surface tension-related characteristics ofthe sample used to wet body 12

Pressurized fluid source 24 is also coupled to the upstream chamber 18 of the housing 14 Pressurized fluid source 24 supplies upstream chamber 18 with pressurized fluid, typically air, or an inert gas such as He, Ar, N₂, combinations of these, or the like Body 12 is first wetted with the sample Pressurized fluid is then delivered to the upstream chamber 18 causing pressure in the chamber 18 to increase Alternately, the liquid in chamber 20 may be drained prior to blowing the liquid out of pathway 22 through drain line 19A As the pressure in chamber 18 increases, a "bubble point" pressure will be reached for the body 12 wetted with the liquid sample The bubble point occurs when the pressure in chamber 18 is sufficiently high to overcome the surface tension of the sample such that the sample in at least one pathway 22 is blown out so that open communication between upstream chamber 18 and downstream chamber 20 is restored through at least one pathway 22 In other words, the bubble point is the point at which bubbles are formed downstream of body 12

Just after the bubble point is reached, sample still remains in some ofthe other pathways 22, causing the pressure in chamber 18 to continue to increase as more pressurized fluid enters upstream chamber 18 As the pressure increases, more sample is blown from additional pathways 22 Eventually, a maximum pressure is reached within chamber 18 when a sufficient number of the pathways 22 have been cleared After this maximum pressure condition is reached, the pressure in upstream chamber 18 decreases even though more pressurized fluid continues to enter upstream chamber 18 because sufficient pathways are opened to allow the pressurized fluid to escape more quickly than it enters The profile of pressure vs time (which includes the bubble point and maximum pressures) developed in upstream chamber 18 as pressurized fluid is delivered to chamber 18 is referred to herein as the "bubble point pressure profile" for that sample

Pressure transducer 38 is operationally coupled to apparatus 10 to measure a parameter indicative ofthe pressure in the upstream chamber 18

Pressure transducer 38 thus may be used to measure the bubble point pressure profile as pressurized fluid flows into upstream chamber 18 Downstream chamber 20 includes outlet 40 through which materials conveyed through housing 14 can be discharged through discharge line 28 The operation of apparatus 10 when determining bubble point characteristics of a sample is illustrated in Figs 2a through 2d Figs 2a through 2d are detailed schematic views of portion 42 of the body 12 Fig 2a shows portion 42 before body 12 is wetted with the sample to be tested Pathways 22a, 22b, and 22c are fully open in the sense that no sample material occupies pathways 22a, 22b, and 22c Fluid communication between upstream chamber 18 and downstream chamber 20 is not inhibited by any pathway blockage at this point Thus, when pressurized fluid in the form of a gas flows into upstream chamber 18 when pathways 22a, 22b, and 22c are open, the pressurized fluid is capable of flowing from upstream chamber 18 through pathways 22a, 22b, and 22c into downstream chamber 20

In contrast, Fig 2b shows the same portion 42 of body 12 located in housing 14, except that portion 42 has been wetted with admixture sample portions 44a, 44b, and 44c from the admixture sample source 30 The admixture sample portions 44a, 44b, and 44c thus occupy at least a portion of pathways 22a, 22b, and 22c, respectively, and prevent open communication between upstream chamber 18 and downstream chamber 20 In other words, sample portions 44a, 44b, and 44c block unimpeded passage from upstream chamber 18 to downstream chamber 20 that might otherwise occur through pathways 22a, 22b, and 22c Consequently, as shown in Fig 2b, when pressurized fluid is caused to flow into upstream chamber 18, the admixture sample portions 44a, 44b, and 44c in pathways 22a, 22b, and 22c, respectively, resist the flow of pressurized fluid through pathways 22a, 22b, and 22c and into downstream chamber 20 This causes the pressure in upstream chamber 18 to increase As the surface tension ofthe sample portions increases, the ability of sample portions 44a, 44b, and 44c to resist the pressurized fluid also increases Accordingly, a body 12 wetted with a sample having higher surface tension will have a greater ability to resist pressure Consequently, such a body 12 will provide a bubble point pressure profile that has higher bubble point and higher P_ma values

Fig 2c shows portion 42 when the bubble point pressure has been reached so as to overcome surface tension and clear the admixture sample from at least one (e g , 22b) ofthe pathways 22a, 22b, and 22c Not all of pathways 22 are cleared at the bubble point, however Thus, for example, admixture sample portions 44a and 44c remains in pathways 22a and 22c, which causes the pressure in chamber 18 to continue to increase as the flow of pressurized fluid continues to enter chamber 18 Eventually, the pressure in chamber 18 reaches a maximum pressure when the admixture sample portions 44a and 44c in a sufficient number of pathways 22a and 22c are cleared

Fig 3 shows a representative bubble point pressure profile 60 of upstream chamber pressure versus time corresponding to wetted body 12 described in Figs 1 and 2a through 2d Initially, after body 12 is wetted with sample, but before pressurized fluid is caused to flow into upstream chamber 18, upstream chamber 18 is at a relatively low, base-line pressure as shown in curve region 62 However, as shown by curve region 64, once pressurized fluid is caused to flow into upstream chamber 18 at a controlled rate from time t₀, the upstream chamber pressure steadily rises and reaches a pressure P_op corresponding to the bubble point ofthe sample Curve region 66 shows that the chamber pressure further increases for a period after the bubble point has been reached, eventually reaching a maximum pressure P_maλ Fig 3 shows that P_bp is less than P_maλ, with the exact pressure differential depending upon factors such as the rate at which pressurized fluid is delivered to the upstream chamber 18, the nature ofthe sample being tested, temperature, and the like Curve region 68 shows the pressure decrease within chamber 18 after the maximum pressure Pm_ax has been reached as a result ofthe clearing of a significant portion of pathways 22

The present invention is based, at least in part, upon the appreciation that the features of a bubble point profile of a sample are generally a function ofthe surface tension characteristics ofthe sample Advantageously, therefore, one or more features of bubble point pressure profile 60 can be correlated to surface tension-related characteristics of sample 44 This allows any such characteristics to be easily determined from bubble point measurements Any one or more features ofthe bubble point profile may be used to determine such characteristics, including the bubble point itself, Pmax, or the

In a preferred embodiments, P_maλ is the most preferred bubble point feature to use when determining surface tension characteristics, because P_maλ can be more easily measured with accuracy and consistency at a given temperature For example, if body 12 of Fig 1 were to be wetted with the same admixture sample 44 and retested a number of times at the same temperature, substantially the same P_maλ value would be obtained with each test

P a for a body 12 having pathways 22 of diameter d is related to the surface tension σ of admixture sample 44 by the following formula

(1) P_max - kσcos(θ) /d

where k is a constant and θ is the contact wetting angle between sample 44 and the surface of body 12 When such a surface is substantially fully wettable by sample 44, cos(θ) is substantially equal to 1 and formula (1) reduces to the following

where k₂ is a constant equal to k divided by d In other words, the surface tension σ of admixture sample 44 is directly proportional to P_maλ when P_max is measured using a particular, wettable body 12 having diameter d pathways Therefore, it can be appreciated that altering the surface tension, or a surface tension-related characteristic, of admixture sample 44, such as surfactant concentration, would cause P_maλ to change in a manner depending upon what surface tension-related characteristic is altered and to what degree that characteristic is altered This correspondence provides an excellent way to quickly and accurately measure surface tension-related characteristics of a sample 44 using P_ma measurements

The relationship between

and surface tension-related characteristics can be described more concretely with respect to characterizing the surfactant concentration of an aqueous photoresist developer solution containing stringently specified concentrations of a base, a surfactant, and a diluent such as water Such a photoresist developer solution may be formed on demand by blending precise amounts of two or more components in amounts effective to meet the corresponding composition specifications A typical blending protocol, for instance, may specify that an amount of a surfactant or surfactant containing chemical (hereinafter referred to as Component A) is to be blended into another component (hereinafter referred to as Component B) to provide a photoresist developer solution containing a precise amount ofthe surfactant, e g , 1000 ppm +/- 30 ppm surfactant on a weight basis The present invention provides an effective feedback strategy in which P_maχ measurements can be used to precisely control the blending process Specifically, when blending Component A into Component B, the surface tension of the blend will tend to change, e g , decrease in some instances, as the concentration of Component A, and hence the concentration ofthe surfactant, in the blend increases In other words, the surface tension ofthe blend is a characteristic that is related to the surfactant concentration ofthe blend Also, as suggested by formula (2), as the surface tension changes (due to the increase in surfactant concentration), P_max changes in a corresponding fashion as well Therefore, inasmuch as P_max of the blend is also a function of the surface tension, the P_maλ ofthe blend may be monitored as an indirect way to determine when precisely enough of Component A has been added to Component B to meet the desired specification

Indeed, if one were to determine P_maλ for blends of Components A and B as a function of the composition, e g , surfactant concentration, ofthe resultant blend, then reference data in the form of bubble point calibration curve 70 of Fig 4 would be obtained Calibration curve 70 allows the concentration ofthe surfactant in an AB blend to be quickly and easily determined as soon as P_max for the blend is measured For example, if a blend of A and B was determined to have a P_max of P,, calibration curve 70 shows that the blend must have a surfactant concentration of C, Depending upon whether the measured concentration is too high or too low to be within the specification, blending could be continued until the measured P_max indicated that the corresponding surfactant concentration satisfied the specification The ability to correlate P_maλ, or other measured bubble point characteristic, of a sample to surfactant concentration allows the principles of the present invention to be incorporated into a feedback and/or quality control methodology to help form blends whose surface tension is a function of composition For example, Fig 5 shows a flowchart of a preferred methodology 80 ofthe present invention in which bubble point data are used in a feedback control loop to accurately blend Components A and B In step 82, an amount of Component A is blended into Component B In step 84, P_max for the blend resulting from step 82 is then measured In step 86, the measured P_max is compared to reference data, e g , a calibration curve such as the calibration curve 70 of Fig 4, or a mathematical expression thereof, in order to determine if the blend meets the applicable specification If the blend meets the specification, the chemical can be released for further handling in step 88 If not, then the measured P_ma can be used to determine if the blend has too much or too little of Component A If too much of Component A is included in the blend, then the blend can be diluted with additional Component B in step 90, after which the revised blend is tested again in accordance with steps 84 and 86 If too little of Component A is included in the blend, then additional Component A can be added in step 92, after which the revised blend is tested again in accordance with steps 84 and 86 Methodology 80 can also be easily modified to blend three or more components For example, three or more components may be blended in a predetermined order of mixing steps, wherein each step has a corresponding, predetermined P_max to indicate when the blended product of each such step meets specifications For each of the mixing steps, methodology 80 would be carried out to create a blend meeting the specification for that step The predetermined order of mixing steps could include, for example, a first mixing step wherein two ofthe components could be combined to form a blend using methodology 80 Then, each ofthe remaining components could be added individually to the blended product ofthe previous mixing step according to methodology 80 in subsequent mixing steps

The above methodology 80 of Fig 5 has been described in connection with preparation of a photoresist developer solution from two or more components for use in the semiconductor industry It can be readily appreciated that the same methodology can be used to prepare, or monitor the quality of, any chemical whose surface tension is a function of composition, particularly but not exclusively chemicals containing surfactants

Further, it can also be readily appreciated that methodology 80 is not limited solely to characterizing surfactant concentration of chemicals by measuring P_maχ Reference data representative of a bubble point calibration curve can be obtained for any surface tension-related characteristic as a function of any measured bubble point characteristic Consequently, the present invention is particularly useful for monitoring those surface tension-related characteristics of a chemical that might be difficult to measure directly by other conventional analytic techniques but are easily determined indirectly in accordance with this invention from information comprising bubble point measurements and suitable reference data providing such characteristics as a function of bubble point measurements As used herein, a "surface tension-related characteristic" refers to any characteristic of an admixture sample whose variation causes corresponding changes in the surface tension ofthe sample For example, in addition to surfactant concentration, other surface tension-related characteristics of an admixture sample include surface tension itself, admixture temperature, solvent mix ratio (e g , the solvent mix ratio of isopropyl alcohol in water), salt concentration, or the like Strictly, the P_max of a sample is also a surface tension-related characteristic

Fig 6 shows one preferred embodiment of a system 110 of the present invention for determining surface tension-related characteristics of a sample of an admixture from bubble point measurements System 1 10 is particularly suitable for determining the surfactant concentration of an admixture sample that is an aqueous photoresist developer solution containing stringently specified concentrations of a base and a surfactant System 110 includes body 112 supported in housing 114 upon support frame 116 Housing 1 14 is thus divided into upstream chamber 1 18 and downstream chamber 120 by the body 1 12 Body 1 12 has an upstream surface 122 facing chamber 1 18 and a downstream surface 124 facing downstream chamber 120 Preferably, surfaces 122 and 124 are substantially nonreactive with the admixture to be analyzed It is also preferable thai surfaces 122 and 124 are substantially wettable by the admixture to be analyzed so that the value of cos(θ) in formula (1) above is substantially about 1 For example, when system 1 10 is used to analyze an admixture that is polar (e g , an aqueous photoresist developer solution), preferably, surfaces 122 and 124 should be hydrophilic Hydrophilic embodiments of body 1 12 may be formed entirely from one or more hydrophilic materials, may bear a hydrophilic coating, or the like Suitable hydrophilic bodies 1 12 include those comprising a polyamide, a fluoπnated polymer coated with a hydrophilic coating, a polysulfone, a cellulose resin, and a polyolefin coated with a substantially hydrophilic coating Conversely, when system 1 10 is used to analyze an admixture that is nonpolar (e g , many organic, nonaqueous materials), surfaces 122 and 124 can be hydrophobic Hydrophobic embodiments of body 1 12 may be formed entirely of hydrophobic materials, may bear a coating of hydrophobic materials, or the like Representative hydrophobic embodiments of body 1 12 include those comprising fluoπnated polymers or polyolefins Body 1 12 preferably also has mechanical properties so that body 1 12 resists deformation when subjected to the expected range of pressures encountered during bubble point testing

Body 1 12 incorporates at least one, and more preferably, a plurality of, pathways 126 that provide open communication between upstream chamber 1 18 and downstream chamber 120 The average diameter, d, of pathways 126, e g , pores, through body 1 12 is generally selected so that bubble point characteristics of samples to be tested can be reliably determined If pathways 126 are too large, then the bubble point and

will occur at a pressure that is too low to be easily measured with repeatable accuracy If too small, then the pressure developed in the upstream chamber 1 18 before reaching the bubble point can be so great as to cause mechanical problems (e g , deformation ofthe body 1 12) As general guidelines when testing aqueous photoresist developer solution, the average diameter, d, of the pathways 126 is in the range from 0 1 micrometers to 10 0 micrometers, preferably 0 2 micrometers to 1 0 micrometers Preferably, the bubble point of the filter/sample combination to be tested is in the range from about 800 torr (about 15 psig) to 10,000 torr (about 200 psig), more preferably from about 1000 torr (about 20 psig) to about 6000 torr (about 120 psig) Body 1 12 may also have any of a variety of suitable structures and can be made in a multitude of ways including phase inversion, drilling, machining, track-etch, etc Body 1 12 can be rigid or flexible

For example, Fig 7 shows a preferred embodiment of body 1 12 in the form of microporous membrane filter disc 220 Microporous membrane filter disc 220 has a first major surface 222 that corresponds to upstream surface 122 and a second major surface (not shown) that corresponds to the downstream surface 124 A plurality of pores are formed in filter disc 220 and provide open communication between the major surfaces The pores are represented schematically by the designation 226 Pores 226 correspond to pathways 126 Fig 8 shows a tube 236 that also can be used in system 1 10 as a body 1 12 Tube 236 has an inlet end 238 corresponding to upstream surface 122 and a discharge end 240 corresponding to the downstream surface 124 Lumen 242 within tube 236 provides open communication between inlet end 238 and discharge end 240 and corresponds to pathway 126

Fig 9 shows a bundle 237 of several tubes 236 that also can be used as a body 1 12 in system 1 10 As in Fig 8, each tube 236 of Fig 9 has an inlet end 238, a discharge end 240, and a lumen 242 within tube 236 The several lumens 242 provide open communication between inlet end 239 and outlet end 241 and corresponds to pathways 126

Fig 10 shows a porous-walled tube 250 that also can be used in system 1 10 as a body 1 12 Porous-walled tube 250 has an exterior surface 252 and an interior surface 254, wherein exterior surface 252 corresponds to the upstream portion 1 18 and interior surface 254 corresponds to the downstream portion 120 Pores 256 formed in the wall of tube 250 provide open communication between exterior surface 252 and an interior surface 254 and correspond to pathways 126 Alternatively, tube 250 could be arranged within the housing 1 14 so that interior surface 254 corresponds to the upstream portion 1 18 and exterior surface 252 corresponds to the downstream portion 120 Referring again to Fig 6, gas supply circuit 130 is in fluid communication with the housing 1 14 and body 1 12 and provides pressure regulated gas to upstream chamber 1 18 ofthe housing 1 14 Gas supply circuit 130 includes pressurized gas supply 132 that is coupled to an inlet 136 of a conventional mass flow controller 134 The gas supplied from regulated gas supply 132 preferably should be substantially nonreactive with the admixture being analyzed as well as nonreactive with body 1 12 and other components of system 1 10 The gas preferably is air or an inert gas such as He, Ar, N₂, combinations, or the like Mass flow controller 134 regulates the flow rate of gas supplied from regulated gas supply 132 to upstream chamber 1 18 The pressure and flow rate are selected so that P_max of samples being tested can be determined quickly and accurately The precise flow rate to be used will depend on a variety of factors, including the nature of body 1 12, the nature ofthe samples being tested, the temperature at which testing is carried out, and the like For instance, if the flow rate is too high, the pressure within upstream chamber 1 18 may increase so quickly that P_max cannot be determined with good resolution On the other hand, if the flow rate is too low, it may take too long to measure P_maχ without justification for the delay Balancing these concerns and as general guidelines, the gas is delivered at a flow rate allowing P_maχ to be determined over a time period ranging from fractions of a second to a few minutes, preferably 1 second to about 60 seconds Alternatively, bubble point measurements preferably may be determined by delivering gas at a flow rate in the range of 0 01 standard liter per minute (slpm) to 5 slpm, preferably 0 05 slpm to 0 5 slpm Bubble point measurements taken at various flowrates with the same solution may be taken to measure dynamic surface tension

Mass flow controller 134 also has an outlet 138 that is coupled to three-way valve 140 Three-way valve 140 directs pressurized gas either to the upstream chamber 1 18 of housing 1 14 via supply line 142 or to vent 144 Vent 144 is of conventional design and vents gas from gas supply 132 when pressurized gas is not being delivered to upstream chamber 1 18 so that gas supply 132 can operate continuously Alternatively, the gas supply 132 can be run only when pressurized gas is being supplied to upstream chamber 1 18, which would require gas supply 132 to be shut down when pressurized gas is not being delivered to upstream chamber 1 18 However, typical mass flow controllers 134 require time to stabilize when a flow of gas from gas supply 132 is first initiated, which tends to introduce inaccuracies into system 1 10 while mass flow controller 134 is stabilizing Thus, when system 1 10 is used on-line, it is preferable to avoid having to intermittently start and stop the flow of gas from pressurized gas supply 132

Supply line 142 connects valve 140 to gas inlet 146 leading to upstream chamber 1 18 ofthe housing 1 14 A pressure measuring device 148 is operationally coupled to supply line 142 (as shown) and/or upstream chamber 1 18 so as to be able to measure a characteristic indicative ofthe pressure within upstream chamber 1 18 Pressure measuring device 148 can be a conventional 0 to 100 psig (about 0 to 5100 torr) pressure transducer such as Part No 68971-10 commercially available from Cole Palmer of Vernon Hills, Illinois, although it is to be understood that any device that measures a characteristic indicative of pressure can be used

Admixture supply circuit 150 including admixture supply 152 is in fluid communication with and provides admixture sample to upstream chamber 1 18 so that body 1 12 can be wetted with an admixture sample for bubble point analysis Preferably, when an aqueous photoresist developer solution is being analyzed, the admixture supply circuit 150 also includes a re- circulation loop 154 that allows the admixture to continuously recirculate when the admixture is not being delivered to the housing 1 14 Re-circulation loop 154 includes a pump 156 that motivates admixture from supply 152 to the inlet of three-way valve 158 One outlet of valve 158 is coupled to admixture inlet 162 of upstream chamber 1 18, and the other outlet of valve 158 is coupled to recirculation line 154 Thus, admixture is drawn from supply 152 by pump 156 to valve 158, at which point the admixture can be re-circulated back to the supply 152, sent to the upstream chamber 1 18 of housing 1 14, or both Supply circuit 150 need not include pump 156 if the admixture supply 152 is sufficiently pressurized to cause admixture to circulate through loop 154 and/or travel to upstream chamber 1 18 without recourse to additional motivating means Moreover, admixture supply circuit 150 need not include re-circulation line 154 at all By-pass circuit 163 is in fluid communication with the upstream chamber 1 18 of housing 1 14 and allows gas and/or admixture, as appropriate, to be discharged from upstream chamber 1 18 without having to pass through body 112 By-pass circuit 163 is designed so that fluids discharged from upstream chamber 1 18 either flow to drain 172 through a restπctor 174 on line 173 or flow to drain 172 unrestricted through line 175 Directing discharged materials through restπctor 174 is particularly desirable to provide a slow release of pressurized gas from upstream chamber 1 18 after P_ma is reached during bubble point measurements The slow release of pressure helps reduce pressure shocks that might otherwise occur Supply line 164 couples by-pass circuit 163 to by- pass outlet 166 of upstream chamber 1 18 Supply line 164 runs from outlet 166 to a two-way valve 168 that is also coupled to the inlet of a three-way valve 170 Valve 170 has one outlet coupled to the drain 172 through the restπctor 174, line 173, and line 177 Valve 170 has a second outlet coupled directly to the drain 172 via unrestricted lines 175 and 177 Drain 172 is also in fluid communication with an outlet 176 from downstream chamber 120 of housing 1 14

Control system 178 includes controller 179 and a plurality of control lines 180 operationally coupled to mass flow controller 134, valve 140, pressure measuring device 148, valve 158, valve 168, and valve 170 Controller 179 comprises appropriate hardware and/or software componentry (e g , microprocessors, memory, program instructions, etc ) to operate and control the various devices attached to the control lines 180 and to receive and analyze data from the pressure measuring device 148 Fig 1 1 is a flow chart showing a preferred methodology 200 for operating system 1 10 of Fig 6 In step 202, any previously tested sample remnants or the like are flushed from upstream chamber 1 18 and supply line 159 leading to chamber 1 18 Although any suitable flushing material could be used to carry out step 202, system 1 10 is advantageously configured so that the admixture to be tested from admixture source 152 is used to carry out flushing To accomplish this, control system 178 sets valve 158 so that admixture is caused to flow into chamber 1 18 at a suitable flow rate, e g , 1 ml/min to 2000 ml/min, preferably 20 ml/min to 100 ml/min To prevent any gas from gas supply 132 from flowing into chamber 1 18 at this time, valve 140 is set so that the gas is vented through vent 144 To allow the admixture to be discharged from upper chamber 1 18 through port 166, valve 168 is opened and valve 170 is set so that discharged material flows to drain 172 via lines 164, 175, and 177 without restriction Flushing occurs until system 1 10 is adequately flushed for carrying out the sample analysis The length ofthe flushing period will depend upon a variety of factors, including the nature ofthe admixture, the flow rate of the admixture during flushing, and the like For example, when the admixture is an aqueous photoresist developer solution flowing at 40 ml/min at about room temperature, flushing may occur for 10 seconds to 10 minutes, preferably 10 seconds to 1 minute, more preferably about 30 seconds

In step 204, body 1 12 is purged of any previous sample remnants that may remain in pathways 126 and is also wetted with a sufficient amount of new admixture to carry out sample analysis To purge and wet body 1 12, valve 168 is closed so that admixture being delivered to chamber 1 18 cannot flow into by-pass circuit 163, but, instead, is forced to go through pathways 126 of body 1 12 into downstream chamber 120, and then to drain 172 via port 176 and discharge line 179 The conditions under which purging and wetting occur will depend upon a variety of factors including the nature of body 1 12, the nature of the admixture, the flow rate ofthe admixture, and the like For example, when the admixture is an aqueous photoresist developer solution, step 204 may be carried out by flushing 10 ml to 100 ml, more preferably 20 ml to 50 ml, of admixture through body 1 12

Once body 1 12 is wetted with admixture, sample testing may then be carried out pursuant to step 206 Valve 158 is set so that no more admixture is delivered to chamber 1 18 Then, responsive to mass flow controller 134, valve 140 is opened so that a flow of gas from gas supply 132 flows into chamber 118 Preferably, the gas flow is steady and consistent throughout the analysis to make it easier to interpret the measurements obtained from the analysis As gas is delivered, one or more pressure measurements are obtained using pressure measuring device 148 More preferably, a sufficient number of pressure measurements are made such that a bubble point pressure profile for the admixture is obtained from which P_ma for the sample can be determined

Once the desired pressure measurements are obtained, the flow of gas into chamber 1 18 can be stopped by setting valve 140 to vent the gas through vent 144 Any pressure remaining in chamber 1 18 can be controllably released by setting valve 170 so that the gas discharged from chamber 1 18 flows to dram 172 through restπctor 174 and then opening valve 168

In step 208, one or more surface-tension related characteristics may then be accurately determined using the pressure measurement(s) from step 206 In preferred embodiments, this is accomplished by using the pressure measurement(s) and suitable reference data, such as a graph, look-up table, or numerical expression, that gives one or more surface-tension related characteristics as a function of the pressure measurement s) Preferably, the reference data correspond to a bubble point pressure profile, such as bubble point calibration curve 70 of Fig 4

Advantageously, methodology 200 can be incorporated into a quality control system for monitoring the quality of chemicals having characteπstιc(s) that vary with surface tension Methodology 200 can also be incorporated into a feedback system to be used in connection with mixing chemical components together as described, for instance, in connection with methodology 80 of Fig 5

The present invention will now be further described with reference to the following examples

Example 1

This example shows how maximum pressure (P_maχ) for a particular admixture sample can be used to characterize the concentration of a surfactant in aqueous, basic solutions A nylon, porous membrane filter obtained from Cole- Parmer Instrument Company under the trade designation Nylon membrane and having an average pore size of 0 45 micrometers was used in an apparatus configured in accordance with Figure 6 The average maximum pressure (P_max) of aqueous solutions respectively comprising 0, 43, 65, 83, 195, 288, 495, and 1016 parts per million (ppm) of a surfactant in an aqueous solution of 2 4% by weight tetramethylammonium hydroxide (TMAH) in ultrapure, deionized water was measured with this apparatus The surfactant used in the measurements was commercially available under the trade designation "PLURONIC L62D" from BASF Corporation

For each measurement, the system was purged of old test solution with the new solution to be tested, after which the filter was wetted by flushing 20 mL ofthe test sample through the filter Next, the upstream surface ofthe filter was pressurized with compressed air at a flow rate of 100 standard cm³/min (seem) to determine P_ma Each measurement took about 135 seconds, so there were about 25 data points per hour A total of 20 measurements were made for each sample The Pmax for each sample was deemed to be the average of hese measurements In the course of making these measurements, it was observed that the system quickly responded to a step changes in surfactant concentration Generally, P_max was within 95% ofthe average P_max within 5 measurements ( -10 minutes) after a step change measurements of maximum pressure measurement were very repeatable over time For example, the following table gives the average maximum pressure and standard deviation for each sample

The surfactant concentration was plotted as a function of average maximum pressure of each sample The resultant curve is shown in Fig 12 and is essentially a calibration curve for this system using this filter A regression analysis was performed on the data to produce a formulaic representation of the calibration data The equation is as follows

[Cs] = 0 448 exp(109 4/(P_ma - 1 1 4)) (0

wherein [Cs] is the concentration in ppm of the L62D surfactant in the aqueous 2 4% TMAH solution This equation can be used in the present invention to determine surfactant concentration for a given measured P_ma For example, given a sample of unknown L62D surfactant concentration in 2 4% TMAH, the actual surfactant concentration can be determined by measuring P_maλ and then using the calibration curve in Fig 12 or the calibration equation (1) to determine the L62D surfactant concentration in 2 4% TMAH

Furthermore, the maximum pressure measurements can be correlated to static surface tension by the curve shown in Fig 13 Fig 13 shows surface tension as a function of P_max for the samples of this example A regression analysis was performed on the data in Fig 13 to produce a formulaic representation of the calibration data The equation is as follows

Surface tension (dyne/cm) = [0 301 - 2 74 x 10^" 4-]1-1

This equation can be used in the present invention to determine the surface tension for a given measured P_max For example, given a sample of unknown L62D surfactant concentration in 2 4% aqueous TMAH, the surface tension can be determined by measuring P_ma and then using the calibration curve in Fig 13 or equation (2) to determine the corresponding surface tension

Example 2

This example demonstrates how a calibration curve relating P_max and surfactant concentration was determined for a different surfactant in an aqueous solution of 2 4% by weight TMAH in water A nylon, porous membrane filter obtained from Cole-Parmer Instrument Company under the trade designation Nylon membrane and having an average pore size of 0 45 micrometers was used in the apparatus of Fig 6 The maximum pressure (P_maχ) of aqueous solutions respectively comprising 0, 53, 199, 275, 407, and 484 ppm, respectively, of a surfactant commercially available as Surfynol 440 from Air Products in the 2 4% TMAH solution were measured with the apparatus

For each measurement, the system was purged of old test solution with the solution to be tested, and then the filter was wetted by flushing 20 mL ofthe test sample through the filter Next, the upstream surface of the filter was pressurized with compressed air at a flow rate of 100 seem to determine P_max The average Surfynol 440 surfactant concentrations in the 2 4 % TMAH solution are plotted as a function of P_max in Fig 14 A regression analysis was performed on the data to produce a formulaic representation ofthe calibration data The equation is as follows [C_surf] = 2 124xl0⁴exp(-0 176 P_maχ) - 2 956x10⁵exp(4 319 P_max) (3) wherein C_SUrf is the concentration ofthe surfactant in the solution in terms of ppm This equation can be used in the present invention to determine surfactant concentration for a given measured P_maχ For example, given a sample of unknown Surfynol 440 surfactant concentration in a 2 4% TMAH solution, the actual surfactant concentration can be determined by measuring P_max, and then using the calibration curve in Fig 14 or equation (3) to determine the Surfynol 440 concentration in the 2 4% TMAH solution

Example 3

This example demonstrates how a calibration curve relating P_max and surfactant concentration was determined for a third surfactant in an aqueous solution of 2 4% by weight TMAH in water A nylon, porous membrane filter obtained from Cole-Parmer Instrument Company under the trade designation Nylon membrane and having an average pore size of 0 45 micrometers was used in the apparatus of Fig 6 In this example, P_max was measured for samples comprising various amounts (0 to 100%>) of a surfactant/base solution in a base solution The surfactant/base solution was commercially obtained under the trade designation ADC-01 1 and includes an unknown weight percent of a surfactant in an aqueous solution containing 2 4% weight percent TMAH in water The ADC-011 solution is commercially available from Moses Lake Industries, Inc The base solution contained 2 4% by weight of TMAH in water The maximum pressure of these solutions was measured using the apparatus in Fig 6

For each measurement, the system was purged of old test solution with the solution to be tested, and then the filter was wetted by flushing 20 mL of the test sample through the filter Next, the upstream surface ofthe filter was pressurized with compressed air at a flow rate of 100 seem to determine P_max The average ADC-011 concentrations in the 2 4 % TMAH solutions at each concentration are plotted as a function of P_max in Fig 15 A regression analysis was performed on the data to produce a formulaic representation ofthe calibration data The equation is as follows

[C_ADC] = -9 543 + 1 550x10⁵exp(-0 2662 P_max) (4)

wherein [C_ADC] IS the weight percent of the ADC-01 1 solution in each sample

This formulaic representation can be used in the present invention to determine surfactant concentration for a given measured P_max For example, given a sample of unknown ADC-01 1 concentration, the actual concentration can be determined by measuring P_maχ, and then using the calibration curve in Fig 15 or equation (4) to determine the ADC-01 1 concentration

Other embodiments of this invention will be apparent to those skilled in the art upon consideration of this specification or from practice of the invention disclosed herein Various omissions, modifications, and changes to the principles and embodiments described herein may be made by one skilled in the art without departing from the true scope and spirit of the invention which is indicated by the following claims

Claims

WHAT IS CLAIMED IS

1 A method of characterizing a liquid-containing composition, comprising the steps of (a) providing a body having an upstream portion and a downstream portion, wherein the body comprises at least one pathway providing open communication between the upstream and downstream portions, (b) wetting said body with the liquid-containing composition, (c) obtaining a pressure measurement indicative ofthe ability ofthe wetted body to resist a pressure acting against the wetted body, and (d) determining a surface tension-related characteristic ofthe liquid-containing composition from information comprising said pressure measurement

2 The method of claim 1, wherein step (b) comprises wetting the body with a sufficient amount ofthe liquid-containing composition to block the pathway and inhibit open communication between the upstream and downstream portions

3 The method of claim 2, wherein step (c) comprises obtaining said pressure measurement while pressurizing the upstream portion relative to the downstream portion

4 The method of claim 3, wherein step (c) comprises carrying out said pressurizing under conditions such that the pressure measurement comprises a maximum pressure value 5 The method of claim 1 , wherein

(1) the method further comprises the step of providing reference data comprising a correlation for determining the surface tension-related characteristic at least in part from the pressure measurement, and

(u) the information of step (d) further comprises said reference data

6 The method of claim 1, wherein the body is positioned in a housing in a manner to define an upstream chamber proximal to the upstream portion and a downstream chamber proximal to the downstream portion, wherein step (c) comprises causing a fluid to flow into the upstream chamber ofthe housing to cause the pressure ofthe upstream chamber to increase until a maximum pressure is reached at which communication between the upstream and downstream chambers is restored, and wherein said pressure characteristic corresponds to said maximum pressure

7 The method of claim 1 wherein said body is a porous membrane filter having a first major surface corresponding to said upstream portion and a second major surface corresponding to the downstream portion

8 The method of claim 1 , wherein the body is at least one tube having an inlet end corresponding to said upstream portion and a discharge end corresponding to said downstream portion

9 The method of claim 1 , wherein the body is a porous walled tube having an interior surface and an exterior surface, and wherein one of said interior and exterior surfaces corresponds to the upstream portion and the other of said interior and exterior surfaces corresponds to the downstream portion 10 The method of claim 1 , the surface of the body is sufficiently hydrophilic so that the cosine ofthe bubble point contact angle between the body and the amount of liquid wetting the body is substantially about 1

1 1 The method of claim 1, wherein the body is a microporous filter having an average pore diameter in a range of from about 0 1 micrometer to about 10 micrometer

12 The method of claim 1 , wherein said liquid-containing composition comprises a plurality of liquid samples having a variation in surfactant concentration from sample to sample, step (b) comprises wetting said body with said plurality of samples a corresponding plurality of times, respectively, step (c) comprises obtaining a pressure measurement for each sample that is indicative ofthe ability ofthe correspondingly wetted body to resist a pressure acting against such correspondingly wetted body, and step (d) comprises determining a critical micelle concentration from information comprising said plurality of pressure measurements

13 The method of claim 1 , wherein the pressure measurement is a maximum pressure

14 The method of claim 1, wherein step (d) comprises determining a dynamic surface tension characteristic

15 The method of claim 1 , wherein step (d) comprises determining a critical micelle concentration characteristic 16 A method of characterizing a photoresist developing solution, comprising the steps of

(a) providing a body having an upstream portion and a downstream portion, wherein the body comprises at least one pathway providing open communication between the upstream and downstream portions,

(b) wetting said body with the photoresist developing solution,

(c) obtaining a pressure measurement indicative ofthe ability of the wetted body to resist a pressure acting against the wetted body, and

(d) determining a surface tension-related characteristic of the photoresist developing solution from information comprising said pressure measurement

17 The method of claim 16, wherein the photoresist developing solution comprises a concentration of a surfactant and wherein the surface tension- related characteristic corresponds to said concentration

18 The method of claim 16, wherein step (b) comprises wetting the body with a sufficient amount of the liquid-containing composition to block the pathway and inhibit open communication between the upstream and downstream portions

19 The method of claim 18, wherein step (c) comprises obtaining said pressure measurement while pressurizing the upstream portion relative to the downstream portion 20 The method of claim 19, wherein step (c) comprises carrying out said pressurizing under conditions such that the pressure measurement comprises a maximum pressure value

21 The method of claim 16, wherein

(l) the method further comprises the step of providing reference data comprising a correlation for determining the surfactant concentration as a function ofthe pressure characteristic, and (n) the information of step (d) further comprises said reference data

22 The method of claim 16, wherein the body is positioned in a housing in a manner to define an upstream chamber proximal to the upstream portion and a downstream chamber proximal to the downstream portion, wherein step (c) comprises causing a fluid to flow into the upstream chamber ofthe housing to cause the pressure of the upstream chamber to increase until a maximum pressure is reached at which an amount of the photoresist developing solution wetting the body is discharged from the body such that open communication between the upstream and downstream chambers is restored, and wherein said pressure characteristic corresponds to said maximum pressure

23 The method of claim 22, wherein said body is a porous membrane filter having a first major surface corresponding to said upstream portion and a second major surface corresponding to the downstream portion

24 The method of claim 22, wherein the body is a porous filter having an average pore diameter in a range of from about 0 1 micrometer to about 1 micrometer 25 The method of claim 16, wherein said liquid-containing composition comprises a plurality of liquid samples having a variation in surfactant concentration from sample to sample, step (b) comprises wetting said body with said plurality of samples a corresponding plurality of times, respectively, step (c) comprises obtaining a pressure measurement for each sample that is indicative ofthe ability ofthe correspondingly wetted body to resist a pressure acting against such correspondingly wetted body, and step (d) comprises determining a critical micelle concentration from information comprising said plurality of pressure measurements

26 The method of claim 16, wherein the pressure measurement is a maximum pressure

27 The method of claim 16, wherein step (d) comprises determining a dynamic surface tension characteristic

28 The method of claim 16, wherein step (d) comprises determining a critical micelle concentration characteristic

29 A method of combining first and second chemical components to form an admixture with a desired composition, said method comprising the steps of

(a) combining an amount of the first chemical component and an amount ofthe second chemical component to form an admixture comprising said first and second components,

(b) wetting a body with the admixture, wherein the body has an upstream portion and a downstream portion, wherein the body comprises at least one pathway providing open communication between the upstream and downstream portions, and wherein the body is wetted with a sufficient amount of the admixture to inhibit open communication between the upstream and downstream portions, (c) obtaining a pressure measurement indicative of the ability of the wetted body to resist a pressure acting against the wetted body,

(e) if the admixture contains relatively too little of said one component, then

(l) adjusting the composition of the admixture by adding a corrective amount of the one component to the admixture, and

(n) repeating at least steps (b) through (e) until the admixture has the desired composition

30 The method of claim 29, wherein the admixture having the desired composition is a photoresist developer

31 An apparatus for measuring a composition characteristic of an admixture, comprising

(a) a gas source comprising a supply of gas, (b) a housing, comprising a porous component provided in the housing in a manner effective to define an upstream chamber and a downstream chamber, wherein the gas source is in fluid communication with the upstream chamber, (c) a supply ofthe admixture in fluid communication with the porous component so that the porous component is capable of being wetted by the admixture, and

(d) a measuring device disposed in the apparatus at a position effective to measure a pressure parameter indicative of a pressure in the upstream chamber

32 The apparatus of claim 31 , further comprising a controller comprising information representative of a correlation between the measured pressure parameter and the composition characteristic and further comprising program instructions enabling the controller to determine the composition characteristic from information comprising the correlation and the measure pressure parameters

33 The apparatus of claim 31 , wherein said porous component is a porous membrane filter

34 The apparatus of claim 31 , wherein said porous compoint is a porous membrane filter that has an average pore diameter in a range from about 0 1 μm to lOμm