WO1996029100A1

WO1996029100A1 - Filtering proton-accepting molecular contamination from air for use in a clean environment

Info

Publication number: WO1996029100A1
Application number: PCT/US1996/003971
Authority: WO
Inventors: Michael A. Joffe
Original assignee: Extraction Systems, Inc.
Priority date: 1995-03-23
Filing date: 1996-03-22
Publication date: 1996-09-26

Abstract

An air filter comprising: adsorbent surface structure (80) (e.g., activated carbon particles, activated carbon fibers, zeolite, or silica gel) impregnated with oxalic acid to remove ammonia and other proton-accepting molecular contamination from air flowing through the air filter; and an air permeable support structure having an input face exposed to receive a flowing stream of air for use in a clean environment and having an output face for delivering filtered air for use in a clean environment, the support structure retaining the oxalic acid-impregnated adsorbent surface structure for exposure to the air stream flowing therethrough. An air filter fabrication method is also disclosed. Clean environments (e.g., a deep UV photolithography station) substantially free of ammonia and other proton-accepting molecular contamination and a method for cleaning such clean environments are also described. A filtering system including two or more of the above-mentioned filters stacked in series is also described. Embodiments may include detectors for detecting the amounts of selected molecular contaminants upstream and downstream of the filter (or filtering system) for determining when the filter should be replaced based on the efficiency of the filter.

Description

FILTERING PROTON-ACCEPTING MOLECULAR CONTAMINATION FROM AIR FOR USE IN A CLEAN ENVIRONMENT

Background This invention relates to filtering proton- accepting molecular contamination from air for use in a clean environment.

Filtering proton-accepting molecular contamination (e.g., ammonia) from air is important in many environments, including semiconductor fabrication clean environments, museums, archives, petrochemical plants, refineries, waste-water treatment facilities, airport terminals, office buildings in urban areas, and hospitals. Ammonia is corrosive and is known to react with other airborne species to form corrosive aerosols and electrolytes. The presence of even low levels of corrosive gases and vapors threatens cultural property and capital equipment including computer control systems, manufacturing tools, electrical systems, and facility mechanical equipment. It is particularly important to filter proton-accepting molecular contamination (e.g., ammonia) from environments that must remain clean, e.g., semiconductor device manufacturing environments. For example, it has been realized that low levels of ammonia contamination can impose severe limitations on further reduction of device geometry and improvement of device performance. Ammonia has also been found to cause wafer hazing (clouding) by forming precipitates on wafer surfaces. Such wafer hazing significantly degrades the wafer surface, impeding successful completion of subsequent process steps.

Summary In one aspect, the invention features an air filter comprising: adsorbent surface structure impregnated with oxalic acid to remove ammonia and other proton-accepting molecular contamination from air flowing through the air filter; and an air permeable support structure having an input face exposed to receive a flowing stream of air for use in a clean environment and having an output face for delivering filtered air for use in a clean environment, the support structure retaining the oxalic acid-impregnated adsorbent surface structure for exposure to the air stream flowing therethrough.

Embodiments of the invention may include one or more of the following features. The adsorbent surface structure preferably includes porous material (e.g., activated carbon particles or fibers, zeolite, or silica gel) . In some embodiments the porous material is in the form of adsorbent particles that have sizes in the range of 4 X 8 or 20 X 140 (depending on the features of the support structure) ; in some embodiments the sizes of the adsorbent particles are preferably 20 X 70, and more preferably 30 X 40 U.S. mesh size. The oxalic acid- impregnated adsorbent surface structure is preferably impregnated with oxalic acid at a doping level of 5% to 50% by weight, and preferably with a doping level of about 25%. The support structure preferably comprises a pleated, air permeable web of non-woven fibrous material defining a matrix of interstices, the oxalic acid- impregnated adsorbent structure being in the form of porous particles that are distributed in the web and are bound in the interstices of the matrix in a manner preventing loss, to air flowing through the filter, of particles in quantity substantially detrimental to the clean environment. Some embodiments include a casing formed from material of low vapor pressure so that the casing does not contribute gas-phase contamination to air flowing through the air filter, wherein the pleated web has edges that are potted within the casing. The above- defined air filter is preferably characterized in operation by losing to air flowing therethrough no more than 100,000 particles per cubic foot of air when the air is flowing at 2000 cubic feet per minute through a filter of four square foot face area. The above-defined air filter is preferably characterized in operation by producing no more than 0.8 inches W.G. (200 Pa) of pressure drop to air flowing therethrough at a rate of 2000 feet per minute through a filter of four square foot face area. In some embodiments the above-defined filter is formed by a method comprising the steps of: providing a non-woven carrier material having a top surface and comprising thermo-plastic fibers of lower fiber density relative to the fiber density of the resultant composite; under dry conditions, applying oxalic acid-impregnated adsorbent particles to the top surface of the carrier material; agitating the carrier material until the oxalic acid-impregnated adsorbent particles penetrate the top surface of the carrier material and become distributed through the depth of the carrier material; heating the carrier material and applied oxalic acid-impregnated adsorbent particles; and calendering the heated carrier material with the oxalic acid-impregnated adsorbent particles distributed therein; wherein the heating and calendering steps are performed for a period of time and under a pressure selected to be sufficient for the oxalic acid-impregnated adsorbent particles to become retained within the heated and calendered carrier material to form a calendered composite having an open fibrous structure of the given fiber density with the surfaces of the distributed oxalic acid-impregnated adsorbent particles being substantially exposed for contact with air passing through the calendered composite, the resulting non-woven air filter composite being characterized by a pressure drop sufficient for use as an air filter. Embodiments may also include one or more of the following features. The carrier material is preferably embossed. The carrier material is also preferably pleated. The oxalic acid-impregnated adsorbent particles are preferably applied by fluidizing the particles on a vibrator tray. The oxalic acid-impregnated adsorbent particles are preferably spilled from the vibrator tray into a roller system from which the particles fall onto the carrier material. The non-woven carrier material that is provided preferably comprises polyester fibers. The carrier material, with the oxalic acid-impregnated adsorbent particles distributed therein, is preferably heated to an average temperature of 300-400°F during processing. The carrier material, with the oxalic acid- impregnated adsorbent particles distributed therein, is preferably heated by radiative heating of the carrier material. The carrier material, with the oxalic acid- impregnated activated carbon particles distributed therein, is preferably subjected to an average pressure of 1000-1500 psi during processing.

In another aspect, the invention features a filtering system comprising two or more filters, as defined above, stacked in series to increase the efficiency of the filtering system. In still another aspect, the invention features a filtering system comprising: a filter as defined above; a first detector positioned upstream of the input face of the filter for detecting an amount of a selected molecular contaminant upstream of the filter; and a second detector positioned downstream of the output face of the filter for detecting an amount of the selected molecular contaminant downstream of the filter; whereby the detected amounts of the selected contaminant upstream and downstream of the filter are used to determine the efficiency of the filter. In another aspect, the invention features a clean environment that is substantially free of ammonia and other proton-accepting molecular contamination comprising: a processing station that releases ammonia or other proton-donating molecular contamination into air flowing therepast; an air system for filtering air from adjacent the processing station, the air system comprising an air filter as defined above.

In yet another aspect, the invention features a deep UV photolithography station comprising: a first deep UV lithographic tool for exposing chemically amplified photoresist to a selected pattern of UV light; a second deep UV lithographic tool for developing chemically amplified photoresist exposed to the selected pattern of UV light; an air system for filtering air from adjacent the first and second lithographic tools, the air system comprising an air filter as defined above.

Embodiments may include a recirculating air system or a make-up air system for drawing air from an atmosphere that may be subject to molecular contamination including ammonia or other proton-donating contamination; the recirculating and make-up air systems each comprising an air filter as defined above.

In another aspect, the invention features a method for removing ammonia and other proton-accepting molecular contamination from a flowing stream of air for use in a clean environment, the method comprising the steps of: providing a flowing stream of air for use in a clean environment; positioning in the air stream adsorbent surface structure impregnated with oxalic acid to remove ammonia and other proton-accepting molecular contamination from air flowing therepast; retaining the oxalic acid-impregnated adsorbent particles with an air permeable support structure having an input face exposed to receive a flowing stream of air for use in a clean environment and having an output face for delivering filtered air for use in a clean environment, the support structure retaining the oxalic acid-impregnated adsorbent surface structure for exposure to the air stream flowing therethrough; and delivering to the clean environment air that has flowed through the oxalic acid-impregnated adsorbent surface structure retained by the air permeable support structure. In some embodiments the adsorbent surface structure includes activated carbon particles having sizes in the range of 4 X 140 U.S. mesh size. Embodiments may includes one or more of the following advantages. Some reagents used to remove ammonia contain metals (e.g., zinc, which changes the electrical properties of semiconductor device) or phosphorus (an electrical dopant for silicon) , which are known to degrade semiconductor fabrication processes. Oxalic acid (C₂H₂0₄) , on the other hand, contains only carbon, hydrogen, and oxygen and, thus, does not by itself produce contamination detrimental to clean room processes (e.g., deep UV photolithography). We have found that filters that include oxalic acid-impregnated activated carbon particles effectively remove ammonia and other proton-accepting molecular contamination from clean environments, enabling reduction of wafer hazing and preventing degradation of chemically amplified photoresist, without generating process-degrading contamination. We have discovered that oxalic acid is readily incorporated into adsorbent surface structure (e.g., porous material, such as, activated carbon particles and fibers, zeolite, and silica gel) to provide effective removal of ammonia and other proton-accepting molecular contamination from air under clean room conditions (e.g., an average temperature of 68-70° F and an average relative humidity of 40-60%) , without generating molecular contamination that is detrimental to clean room processes (e.g., deep UV photolithography). We have further discovered that the ammonia removal efficiency of oxalic acid-impregnated activated carbon particles is not detrimentally degraded during the fabrication of a preferred fabric-based support structure, described in detail below.

Other features and advantages will become apparent from the following description and from the claims.

Description Figs. 1 and IA are diagrammatic perspective and side views of a clean room, respectively.

Fig. 2 is a schematic view of an air filter for removing ammonia and other proton-accepting molecular contamination from air for use in a clean environment. Fig. 3 is a graph of ammonia concentration plotted as a function of time for five filtering systems including activated carbon beds respectively impregnated with different reagents.

Fig. 4 is a diagrammatic side view of an air handling system.

Fig. 5 is a diagrammatic side view of an air handling system.

Figs. 6-6B are perspective views of a fabric-based filter fabrication process. Figs. 7-7B are diagrammatic side views of a semiconductor bearing a layer of photoresist at different stages of a photolithographic patterning process.

Referring to Figs. 1 and IA, inside a clean room 10, molecular contaminants (too small to be shown) in recirculating air streams 12 are removed by chemically active molecular air filters 14, at least some of which having air filtering beds selected for effective targeted removal of proton-accepting molecular contamination (e.g., ammonia) from the air streams in a manner reducing self-contamination and cross-contamination of process tools 16 (e.g., a conventional semiconductor photolithography station 17, although other processing stations are contemplated including etch, chemical vapor deposition, thin film deposition, developing, epitaxy and diffusion stations) . Such process-limiting molecular contaminants may be released into the recirculating air streams by process tools 16, by technicians who operate the process tools, and by cleaning solutions. Each processing station 16 is associated with a recirculating air handling system that generates recirculating air streams 12 (e.g., with air blowers 18). An air stream 12 follows a path that includes, a process station 16, a floor 20, a common air plenum 22, a molecular air filter 14, and a high efficiency particulate air (HEPA) filter 24 used to remove particulate contamination (e.g., dust, lint, and other debris) from the air stream. Floor 20 has air passages 26 to allow air streams 12 to pass therethrough. The air stream recirculation rate is on the order of 10 interchanges per minute (on the order of 2000 cu.ft. per minute) to enable thorough filtering of the clean room air, even when the molecular filters are near the end of their respective service lives (i.e., when a filter has a low efficiency (e), defined by the formula

where C_upβtrβam is the concentration of molecular contamination upstream of the filter and C_downatcβam is the concentration of molecular contamination downstream of the filter. A high recirculation rate can compensate for the inevitable decrease in efficiency. For example, after 10 air cycles a filter with a 30% efficiency may reduce the level of air contamination by 99%. A make-up air handling system 40 is used to replace the air removed from the vicinity of the processing stations by the exhaust systems with air from, e.g., a sub-fab area. Air blower 42 generates the make- up air stream 44. A HEPA filter 46 is located downstream of the blower to prevent fine particulate contaminants from entering the common air plenum 22. An efficient particulate bag filter 48 (e.g., e « 70%) is located upstream of the HEPA filter to reduce particulate loading of the HEPA filter. A chemically active air filter 50, which has a particulate removal efficiency of about 30%, is located at an inlet port of the make-up air handling system to prevent premature particulate loading of the bag and HEPA filters and to filter proton-accepting molecular contamination from the incoming make-up air. Filtering Proton-Acceptinα Molecular Contaminants

Referring to Fig. 2, an air filter 14 for removing ammonia and other proton-accepting molecular contamination from air for use in a clean environment includes activated carbon particles 52 impregnated with oxalic acid and having sizes in the size range of 20 X 140 U.S. mesh size, more preferably in the size range of 20 X 70 U.S. mesh size, and still more preferably in the size range of 30 X 40 U.S. mesh size, and an air permeable support structure 54. The support structure has an input face 56 exposed to receive an upstream air flow 58 and an output face 60 for delivery of a filtered downstream air flow 62.

The effectiveness of the molecular filter directly depends on the reactivity of the reagent selected to remove the targeted molecular contaminant. Some of the most highly reactive reagents exhibit some vapor pressure under normal operating conditions and, therefore, tend to generate and release into air streams molecular components that are detrimental to many clean room processes. Also, some reactive reagents react with molecular contamination to produce unstable by-products that may sublime at normal operating temperatures (e.g., 68-70° F for a typical clean room) . Among the important factors that should be considered in the selection of an activated carbon impregnate (reagent) for use in removing ammonia and other reactive proton-accepting molecular contamination from air inside a clean environment are the following. The reagent should be readily incorporated into the adsorbent surface structure, such as, activated carbon particles, without reducing the effective surface area of the particles, e.g., by blocking the pores in the surfaces of the particles. The reagent should not form particles that are easily lost to the air flowing through the filters at a high rate (e.g., on the order of 2,000 cu. ft. per minute or greater) . In the event that the reagent-impregnated particles are released into the air flowing through the filter, the reagent should not be a source of process-limiting contamination. The reagent should be a solid in the operating temperature range of the clean environment. The reagent should not react with the filter structure. The reagent should also retain its adsorptive effectiveness after exposure to process conditions during the fabrication of the filter (e.g., temperature and humidity conditions of the fabrication process) . Furthermore, to achieve a desired filtering efficiency and at least a minimal desired filter service life, the reagent should readily react with ammonia and other proton-accepting molecular contamination under the ambient thermodynamic and chemical conditions of the clean environment.

We have discovered that oxalic acid is readily incorporated into adsorbent surface structure (e.g., porous materials, such as, activated carbon particles and fibers, zeolite, and silica gel) to provide effective removal of ammonia and other proton-accepting molecular contamination from air under clean room conditions (e.g., an average temperature of 68-70° F and an average relative humidity of 40-60%) , without generating molecular contamination that is detrimental to clean room processes (e.g., deep UV photolithography). We have further discovered that the ammonia removal efficiency of oxalic acid-impregnated activated carbon particles is not detrimentally degraded during the fabrication of a preferred fabric-based support structure, described in detail below.

Referring to Fig. 3, in one study, 16 cm long tubes, each having a volume of 20 ml, were filled with activated carbon particles, in the size range of 20X140 U.S. mesh size and respectively impregnated with different reagents (oxalic acid, citric acid, sulfuric acid phosphoric acid, and zinc chloride) . Air having an ammonia concentration of 120 parts per million (ppm) was flowed through each of the tubes at a flow rate of 4.36 liters per minute. The monitored ammonia concentration downstream of the particle-filled tubes are shown by the ammonia breakthrough plots in Fig. 3. In this study, the oxalic acid-impregnated carbon particles retained their ammonia removal efficiency longer than carbon particles impregnated with citric acid, zinc chloride, or sulfuric acid; only the phosphoric acid-impregnated carbon particles retained their ammonia removal efficiency for a longer period of time. Some reagents used to remove ammonia contain metals (e.g., zinc, which changes the electrical properties of semiconductor device) or phosphorus (an electrical dopant for silicon) , which are known to degrade semiconductor fabrication processes. Oxalic acid (C₂H₂0₄) , on the other hand, contains only carbon, hydrogen, and oxygen and, thus, does not by itself produce contamination detrimental to clean room processes (e.g. , deep UV photolithography) . We have found that filters that include oxalic acid-impregnated activated carbon particles effectively remove ammonia and other proton-accepting molecular contamination from clean environments, enabling reduction of wafer hazing and preventing degradation of chemically amplified photoresist, without generating process-degrading contamination.

Air Handling System

In the design of the recirculating and make-up air handling systems for clean environments, it is desirable to achieve the lowest practical level of particulate and molecular contamination. However, a compromise must be reached between the contamination level and the cost of the clean room. To achieve the lowest practical level of air contamination additional air filters could be added in series. This, however, would add to the total cost of the clean room in at least two ways. First, additional filters would cause an increase in pressure drop (i.e., resulting in reduction in the volume of air per unit time flowing in air stream 12 through filtering system 14) in the air handling systems and larger air blowers would be required to make up the loss in pressure. Second, the increase in the size of the blowers and the additional space taken up by the additional filters would require a larger and therefore more expensive clean room.

Referring to Fig. 4, a pleated fabric-based air filter 68 containing oxalic acid-impregnated activated carbon particles is located directly upstream of a HEPA filter 24 (i.e., there is no intervening filter between the HEPA filter and the air filter that would cause additional pressure drop to the air stream) inside the recirculating air systems of the clean room. Air filter 68 has typical dimensions of 24 inches by 24 inches in face area by 12 inches in depth. A particulate bag filter 74 is positioned directly upstream of chemical filters 68 to remove particulates from the air streams that would tend to cover the adsorbent surfaces of the adsorbent particles, reducing their filtering effectiveness.

As shown in Fig. 5, in another embodiment, the oxalic acid-impregnated activated carbon particles are contained within a rack and tray filter 80. Rack and tray filter 80 is formed from an air permeable perforated metal container, which is filled with adsorbent material and has typical dimensions of 24 inches by 24 inches in face area by 29 inches in depth. A particle filter 82 (e.g., a bag filter) is located downstream of rack and tray filter 80 to capture the released particulate matter and to prevent rapid loading of HEPA filter 84, which is expensive and difficult to replace. An additional particulate filter 86 is employed directly upstream of the rack and tray filter 80 to further remove particulates from the air streams. In one embodiment, the oxalic acid-impregnated activated carbon particles are in the size range of 4 X 8 U.S. mesh size. Selection of Adsorbent Surfaces The adsorbent surfaces selected for impregnation with oxalic acid are preferably in the form of porous adsorbent material, such as, activated carbon particles or fibers (woven or non-woven) , zeolite, and silica gel; although, other adsorbent surface structure may be useful.

Adsorption-based filters operate on the principles of diffusion and the diffusion gradient. Diffusion delivers the molecular contaminant to the surface of the adsorbent particles and provides the mechanism by which the contaminant penetrates the exterior surface of the adsorbent material. Diffusion in the airstream is a passive process whereby an area of high contaminant concentration moves to an area of relatively lower concentration on the adsorbent surface. The diffusion gradient is the concentration difference between a region of higher concentration (in the airstream or on the adsorbent surface) and a region of lower concentration (on the adsorbent surface) . The larger the diffusion gradient, the faster the movement process occurs. At low concentrations (less than 10 ppm) , the diffusion gradient is low and diffusion into the adsorbent material occurs at a rate much lower than the rate at which the filter loses efficiency (i.e., its ability to adsorb contaminants) . We have realized, therefore, that when designing molecular air filtration systems for low concentration environments it is important to have a large surface area of adsorbent material exposed to the constituent air streams. This is accomplished, e.g., by using very small adsorbent particles that have a very high surface area to mass ratio. This is especially important in chemisorptive systems in which molecular contaminants react to form solids at the surface of the adsorbent particles. Solids are less mobile and do not migrate appreciably into the particle, even under high contaminant concentrations. Increasing the available adsorbent surface area by using small adsorbent particles provides a higher performing system at a lower cost. Preferred sizes for activated carbon particles for use in a clean environment are in the range of 4 X 8 or 20 X 140, more preferably 20X70, and still more preferably 30 X 40 U.S. mesh size. Impregnating Carbon Particles with Oxalic Acid

In a preferred embodiment, activated carbon particles are impregnated with oxalic acid at the selected doping level by the following process. A known weight of activated carbon particles of a selected size range (e.g., 4 X 8 or 20 X 140 (depending on the associated support structure) and preferably 50X140, and more preferably 30 X 40 U.S. mesh size) is loaded into a soaking container. A known weight of oxalic acid is mixed with filtered, de-ionized water in quantity sufficient to completely dissolve the oxalic acid. The oxalic acid solution is then pumped into the soaking container. The weight of the carbon particles and the weight of the oxalic acid is adjusted to achieve the selected doping level (e.g., 5-50% by weight). The activated carbon particles are left in the soaking container until the liquid in the soaking container dries.

Fabric-Based Filter Embodiment

Referring generally to Figs. 6-6B, in a preferred fabric-based air filter embodiment, the air permeable support structure 54 is formed from a dense, needled, non-woven polyester batting 90 (or some other thermo¬ plastic material) of about fifteen denier and having a thickness of about k inch (Fig. 6) . The batting is spray-bonded to a loose non-woven polyester batting 92 of approximately six denier and having a thickness of approximately 1 inch. After bonding, the resulting carrier material has two distinct layers and a thickness of approximately 0.8 inch. In an alternative embodiment, the non-woven carrier is formed from a polyester batting that is needled on one side, forming a single polyester batting having a dense layer on one side and a total thickness of about 0.8 inch. Adsorbent particles impregnated with oxalic acid are applied to the carrier material from a vibrating support that evenly distributes the particles in the polyester batting. The resulting particle-loaded carrier material is heated and calendered under controlled conditions. The dry processing of the non-woven polyester batting, which includes the combination of the fluidized bed particle deposition process, the inherent stratification of the batting's density, and the even distribution of the adsorbent particles as well as the stratification of the adsorbent particle size, allows for a fabric architecture having an increased bed depth at a very low pressure drop, which is highly desirable due to its high first pass efficiency coupled with its low operating cost.

As shown in Fig. 6A, adsorbent particles 100, impregnated with oxalic acid, are poured onto a polyester batting 102 from a fluidized bed 104, the batting 102 is agitated using a roll bar 106 that agitates the batting in a direction perpendicular to the length of the batting. This agitation insures that the adsorbent particles 100 are distributed in the depth of the batting 102. The agitation causes the smaller particles to migrate furthest from the batting surface while the larger particles remain nearer the surface thereby providing a stratification of the adsorbent particles in the depth of the polyester batting. An increased bed depth of adsorbent particles increases residence time, increases exposure of the adsorbent particle surfaces, provides a low pressure drop, and increases the lifetime of the filter.

Batting 102, with adsorbent particles 100 distributed therein, is then exposed to two zones 108, 110 of radiant infrared energy at different respective temperatures. In a first zone 108, the heating energy is set to a relatively high temperature (e.g., about 550° F) and is directed toward the dense non-woven backing 112. In the second zone 110, the heating energy is set to a relatively low temperature (e.g., about 325° F) and is directed to the loose non-woven surface 114. The adsorbent particles are heated to an overall average temperature of about 250-350°F.

The infrared energy is not substantially absorbed by the fibers of the batting, and is instead, preferentially absorbed by the adsorbent particles, which act as black-body absorbers. This causes the adsorbent particles to adhere to the batting at points where the particles contact the batting. This procedure avoids the necessity of raising the temperature of the entire batting to a point at, or near, the melting point of the polyester batting, which could cause the batting to melt and collapse thereby encasing the particles and destroying their chemical activity.

Batting 102 is then calendered using a pair of calender rolls 116, 118. The first of these rolls 116 can be temperature-controlled which allows the heating and calendering steps to be carried out at a steady temperature of around 110-115°F, and prevents overheating and subsequent melting of a cover sheet 122 that may be provided over the backing layer 112, and prevents over- calendering of the batting. When the temperature- controlled roller 116 is used, the pressure at which the batting is calendered can be lowered from 3000-5000 psi to about 1000-1500 psi, and preferably 1000 psi across a twenty-six inch long roller (about 38 psi per inch of roller) , as a result of the steady temperature maintained during calendering. Higher calendering pressures would crush the adsorbent particles, forming dust that cannot be retained in the filter composite. Therefore, the ability to use lower pressures in the calendering step is very desirable in preventing the destruction of the carbon particles contained in the batting, and formation of carbon dust. A non-woven cover sheet 122 helps to maintain the carbon in the batting is preferably calendered with the batting 102. The material is also preferably conducted over an upper roller 124 to facilitate cooling the carrier material before the filter is processed further. The composite is preferably pleated. The pleated structure is placed in a containment structure such that the crease of the fold is perpendicular to the air flow. A wire mesh 120 is preferably calendered with the batting to help maintain the pleated shape of the filter material. The presence of the wire mesh 120 in the filter material also enables the filter material to be embossed before pleating. Referring to Fig. 6B, in one embodiment, the pleated filter structure 125 is framed within a formaldehyde-free prelaminate-coated hardboard (e.g., Masonite") casing 127 with dimensions of 24 inches by 12 inches in face area by 12 inches in depth. This size permits two filters seated side-by-side, as shown, with a combined face area of 24 inches by 24 inches, to be easily retro-fitted into conventional clean room air handling systems. The materials chosen for the construction of casing 127 are chosen to have a low vapor pressure so that the casing does not contribute gas-phase contamination to the clean room. The filter is potted inside the casing so that the higher density fibers are downstream the lower density fibers. In this configuration, any larger carbon particles that may become unbound from the lower density fibers will be caught by the downstream higher density fibers. The ends of the pleated structure are potted into the casing with a foamed polyamide hot-melt adhesive film 129. The polyeunide adhesive and the formaldehyde-free casing are selected because they do not off-gas into the clean room after they have been installed. The two end flaps, which would normally be loose in conventional pleated structures, are also sealed using the same polyamide adhesive. The filter and frame form a single disposable filter unit.

For further details regarding the construction of such fabric-based molecular air filters refer to co- pending application Serial No. 08/161,931, filed December 2, 1993, which is herein incorporated by reference.

Other embodiments are within the scope of the claims.

Example 1 - Deep UV Photolithography We have discovered that even trace quantities, e.g., on the order of parts per trillion (ppt) of certain molecular contaminants can severely limit many clean environment processes. For example, photolithography systems operating in the deep ultraviolet wavelength range (e.g., 248 nm or 193 nm) can be process-limited by the presence of about 600 ppt of proton-accepting contaminants, such as ammonia and NMP.

Referring to Figs. 7-7B, in deep UV photolithography process, a substrate 130 (e.g., a semiconductor wafer) bearing a layer of photoresist 132 is patterned by selectively illuminating regions 134, 136 of photoresist layer 132 with deep UV radiation 138, which permits devices to be fabricated with submicron features (e.g., about 0.25-0.5 μm) . As shown in Figs. 7A and 7B, if molecular contamination 140, which includes, e.g., a proton acceptor (e.g., a base such as ammonia and other proton- accepting molecular contamination) , is deposited on the surface of photoresist layer 132, the resulting photoresist pattern 142 (Fig. 5B) will be degraded from the desired pattern in a manner catastrophic for subsequent processing steps, thereby severely reducing the fabrication yield of the process. When UV light illuminates photoresist, an acid group is formed which begins the polymerization of the photoresist. When a proton acceptor deposits on the surface of the photoresist it reacts with the acid group, causing the polymerization process to terminate, preventing controlled polymerization of the photoresist. To remove ammonia and other proton-accepting molecular contamination from air in the vicinity of a deep UV photolithography process tool, the associated air handling system is fitted with an air filter that includes activated carbon particles in the size range of 20 X 140, preferably 20 X 70, and more preferably 30 X 40 U.S. mesh size impregnated with oxalic acid at a doping level of 5-50% by weight and contained in a fabric-based air permeable support structure, as described above. In an alternative embodiment, oxalic acid- impregnated activated carbon particles in the size range of 4 X 8 U.S. mesh size are retained in a rack and tray support structure, as described above.

Example 2 - Wafer Storage Area To remove ammonia and other proton-accepting molecular contamination from a wafer storage area to prevent hazing or clouding of semiconductor surfaces, the associated air handling system is fitted with an air filter that includes adsorbent material (e.g., activated carbon particles, zeolite, or silica gel) impregnated with oxalic acid at a doping level of 5-50% by weight and contained in a fabric-based air permeable support structure or in a rack and tray support structure, as described above.

Example 3 - Wafer Hazing Prevention in General To remove ammonia and other proton-accepting molecular contamination from the vicinity of a process tool that releases ammonia or other proton-accepting molecular contamination, the associated air handling system is fitted with an air filter that includes porous adsorbent material (e.g., activated carbon, zeolite, or silica gel) impregnated with oxalic acid at a doping level of 5-50% by weight and contained in a fabric-based air permeable support structure or in a rack and tray support structure, as described above. This embodiment substantially prevents hazing or clouding of semiconductor surfaces and further reduces self- contamination and cross-contamination of the process tools within the clean room.

Example 4 - HQneygQmfr s ruc res To remove ammonia and other proton-accepting molecular contamination from the vicinity of a process tool that releases ammonia or other proton-accepting molecular contamination, the associated air handling system is fitted with an air filter that includes porous particles (e.g., activated carbon, zeolite, or silica gel) impregnated with oxalic acid at a doping level of 5- 50% by weight and retained on a honeycomb support structure (formed from, e.g., cardboard or ceramic material) . The particles adhere to the honeycomb support structure by a wet slurry process know in the art.

It is also contemplated that each processing station may be self-contained (a so-called "mini- environment") and include its own air handling system, instead of sharing a common clean environment with other processing stations. Along these lines, as shown in Fig. IA, an air filter 150 (as described above) including oxalic acid-impregnated adsorbent material retained by an air permeable support structure, is installed inside a processing station 146, which has an independent air handling system.

One or more individual air filters, described above, may be stacked in series to form a filtering system have a higher overall efficiency.

In some embodiments, a filter monitor that includes detection surfaces positioned upstream and downstream of the filter (or stacked filter system) to measure the efficiency of the filter to determine when the filter should be replaced, as described in U.S. Serial No. 08/365,213, filed December 28, 1994, which is herein incorporated by reference. In some other embodiments, ion mobility spectroscopy is used to detect ammonia upstream and downstream of the filter (or stacked filtering system) to determine the efficiency of the filter. In still other embodiments, gas chromatography is used to detect ammonia upstream and downstream of the filter (or stacked filtering system) to determine the efficiency of the filter. In some clean environments, a filter should be replaced when the efficiency drops below values of 90-99%. Still other embodiments are within the scope of the claims.

Claims

1. An air filter for removing ammonia and other proton-accepting molecular contamination from air for use in a clean environment, the filter comprising adsorbent surface structure impregnated with oxalic acid to remove ammonia and other proton-accepting molecular contamination from air flowing through the air filter, and an air permeable support structure having an input face exposed to receive a flowing stream of air for use in a clean environment and having an output face for delivering filtered air for use in a clean environment, the support structure retaining the oxalic acid- impregnated adsorbent surface structure for exposure to the air stream flowing therethrough.

2. The air filter of claim 1 wherein the oxalic acid-impregnated adsorbent surface structure is impregnated with oxalic acid at a doping level of 5% to 50% by weight.

3. The air filter of claim 1 wherein the oxalic acid-impregnated surface structure is formed from porous adsorbent material

4. The air filter of claim 3 wherein the porous material includes oxalic acid-impregnated activated carbon particles.

5. The air filter of claim 4 wherein the oxalic acid-impregnated activated carbon particles are in the size range of 4 X 140 U.S. mesh size.

6. the air filter of claim 5 wherein the oxalic acid-impregnated activated carbon particles are in the size range of 30 X 40 U.S. mesh size.

7. The air filter of claim 3 wherein the porous material includes oxalic acid-impregnated zeolite particles.

8. The air filter of claim l wherein the support structure comprises a pleated, air permeable web of non- woven fibrous material defining a matrix of interstices, the oxalic acid-impregnated adsorbent surface structure being in the form of porous particles that are distributed in the web and are bound in the interstices of the matrix in a manner preventing loss, to air flowing through the filter, of particles in quantity substantially detrimental to the clean environment.

9. The air filter of claim 8 further comprising a casing formed from material of low vapor pressure so that the casing does not contribute gas-phase contamination to air flowing through the air filter, wherein the pleated web has edges that are potted within the casing.

10. The air filter of claim 1 formed by a process comprising the steps of: providing a non-woven carrier material having a top surface and comprising thermo-plastic fibers of lower fiber density relative to the fiber density of the resultant composite; under dry conditions, applying oxalic acid- impregnated adsorbent particles to the top surface of the carrier material; agitating the carrier material until the adsorbent particles penetrate the top surface of the carrier material and become distributed through the depth of the carrier material; heating the carrier material and applied oxalic acid-impregnated adsorbent particles; and calendering the heated carrier material with the oxalic acid-impregnated adsorbent particles distributed therein; wherein the heating and calendering steps are performed for a period of time and under a pressure selected to be sufficient for the oxalic acid-impregnated particles to become retained within the heated and calendered carrier material to form a calendered composite having an open fibrous structure of a selected fiber density with the surfaces of the distributed oxalic acid-impregnated adsorbent particles being exposed for contact with air passing through the calendered composite, the resulting non-woven air filter composite being characterized by a pressure drop sufficient for use as an air filter.

11. The air filter of claim 1 characterized in operation by losing to air flowing therethrough no more than 100,000 particles per cubic foot of air when the air is flowing at 2000 cubic feet per minute through a filter of four square foot face area.

12. The air filter of claim 1 characterized in operation by producing no more than 0.8 inches W.G. (200 Pa) of pressure drop to air flowing therethrough at a rate of 2000 feet per minute through a filter of four square foot face area.

13. A filtering system comprising two or more filters, as defined in claim 1, stacked in series to increase the efficiency of the filtering system.

14. A filtering system comprising a filter as defined in claim 1, a first detector positioned upstream of the input face of the filter for detecting an amount of a selected molecular contaminant upstream of the filter, and a second detector positioned downstream of the output face of the filter for detecting an amount of the selected molecular contaminant downstream of the filter, whereby the detected amounts of the selected contaminant upstream and downstream of the filter are used to determine the efficiency of the filter.

15. A clean environment that is substantially free of ammonia and other proton-accepting molecular contamination comprising: a processing station that releases ammonia or other proton-donating molecular contamination into air flowing therepast; an air system for filtering air from adjacent the processing station, the air system comprising adsorbent surface structure impregnated with oxalic acid to remove ammonia and other proton-accepting molecular contamination from air flowing through the air system, and an air permeable support structure having an input face exposed to receive a flowing stream of air for use in a clean environment and having an output face for delivering filtered air for use in the clean environment, the support structure retaining the oxalic acid- impregnated adsorbent surface structure for exposure to the air stream flowing therethrough.

16. A deep UV photolithography station comprising a first deep UV lithographic tool for exposing chemically amplified photoresist to a selected pattern of UV light; a second deep UV lithographic tool for developing chemically amplified photoresist exposed to the selected pattern of UV light; an air system for filtering air from adjacent the first and second lithographic tools, the air system comprising adsorbent surface structure impregnated with oxalic acid to remove ammonia and other proton-accepting molecular contamination from air flowing through the air system, and an air permeable support structure having an input face exposed to receive a flowing stream of air for use in a clean environment and having an output face for delivering filtered air for use in the deep UV photolithography station, the support structure retaining the oxalic acid-impregnated adsorbent surface structure for exposure to the air stream flowing therethrough.

17. The deep UV photolithography station of claim 16 wherein the adsorbent surface structure includes oxalic acid-impregnated activated carbon particles having sizes in the range of 4 X 140 U.S. mesh size.

18. A method for forming an air filter for removing ammonia and other proton-accepting molecular contamination from air for use in a clean environment, the method comprising the steps of: providing a non-woven carrier material having a top surface and comprising thermo-plastic fibers of lower fiber density relative to the fiber density of the resultant composite; under dry conditions, applying oxalic acid- impregnated adsorbent particles to the top surface of the carrier material; agitating the carrier material until the oxalic acid-impregnated adsorbent particles penetrate the top surface of the carrier material and become distributed through the depth of the carrier material; heating the carrier material and applied oxalic acid-impregnated adsorbent particles; and calendering the heated carrier material with the oxalic acid-impregnated adsorbent particles distributed therein; wherein the heating and calendering steps are performed for a period of time and under a pressure selected to be sufficient for the oxalic acid-impregnated adsorbent particles to become retained within the heated and calendered carrier material to form a calendered composite having an open fibrous structure of the given fiber density with the surfaces of the distributed oxalic acid-impregnated adsorbent particles being substantially exposed for contact with air passing through the calendered composite, the resulting non-woven air filter composite being characterized by a pressure drop sufficient for use as an air filter.

19. The method of claim 18 further comprising the step of embossing the carrier material with the oxalic acid-impregnated adsorbent particles distributed therein.

20. The method of claim 18 further comprising the step of pleating the carrier material with the oxalic acid-impregnated adsorbent particles distributed therein.

21. The method of claim 18 wherein the step of applying oxalic acid-impregnated adsorbent particles comprises the step of fluidizing the particles on a vibrator tray.

22. The method of claim 18 further comprising the step of spilling the oxalic acid-impregnated adsorbent particles from the vibrator tray into a roller system from which the particles fall onto the carrier material.

23. The method of claim 18 wherein the non-woven carrier material that is provided comprises polyester fibers.

24. The method of claim 18 wherein the heating step comprises heating the carrier material, with the oxalic acid-impregnated adsorbent particles distributed therein, to an average temperature of 300-400°F.

25. The method of claim 24 wherein the heating step comprises radiative heating of the carrier material, with the oxalic acid-impregnated adsorbent particles distributed therein.

26. The method of claim 18 wherein the calendering step comprises applying to the carrier material, with the oxalic acid-impregnated adsorbent particles distributed therein, an average pressure of 1000-1500 psi.

27. A method for removing ammonia and other proton-accepting molecular contamination from a flowing stream of air for use in a clean environment, the method comprising the steps of: providing a flowing stream of air for use in a clean environment; positioning in the air stream adsorbent surfaces impregnated with oxalic acid to remove ammonia and other proton-accepting molecular contamination from air flowing therepast; retaining the oxalic acid-impregnated adsorbent surface structure with an air permeable support structure having an input face exposed to receive a flowing stream of air for use in a clean environment and having an output face for delivering filtered air for use in a clean environment, the support structure retaining the oxalic acid-impregnated adsorbent surface structure for exposure to the air stream flowing therethrough; and delivering to the clean environment air that has flowed through the oxalic acid-impregnated adsorbent surface structure retained by the air permeable support structure.