Customer Application Brief Electronics Ultra-High Purity .
Customer Application BriefElectronicsUltra-High Purity Gas FiltrationElectronics ManufacturingIntroductionThe microelectronics industry continues to improve the design and cost of integrated circuits by increasing speeds, reducing powerconsumption, and leveraging economies of scale by moving to larger substrates like 300mm wafers for semiconductor manufacturingand video displays in excess of 42 inches. As the trend towards smaller line-widths and the demand for dry processing steps hasincreased so, has the importance of contamination control for CVD, Etch, Epitaxial, Ion Implant, RTP, PVD, and Utility gases used forthe manufacturing of electronic devices.This Application Brief reviews the particle capture mechanisms, concept of most penetrating particle size (MPPS), log reduction value(LRV), and performance of PTFE, Nylon, and polypropylene filter cartridges used in Ultra-High Purity (UHP) bulk gas applications.Benefits include superior particle reduction efficiency 0.003 µm, high flow rates, low pressure drops, longer filter lifetime, and the useof smaller and less costly stainless steel housings.The ProcessThe manufacturing of an integrated circuit (IC) includes many steps which are either performed as a dry (gas) or wet (chemical) process.The focus of this application brief is on the delivery of bulk gas to dry processing steps like those used during CVD, Etch, Epitaxial,Ion Implant, RTP, and PVD. Bulk gas like nitrogen is typically generated on site or delivered by tube trailers. In both cases, the gas isdelivered at very high flow rates (up to 25,000 SCFM/707,921 SLPM) and high operating pressures (up to 150 psig or 10.34 bar) as itis required to supply most of the equipment in the manufacturing plant. Filtration steps are critical in achieving the necessary particlespecifications (see Figure 1).Inside ElectronicManufacturing FacilityPOUGasCabinetOutside Electronic Manufacturing FacilityTube TrailerVMBBulk SS HousingBulk Storage TanksFigure 1 — Typical Gas Filtration Process
The ProblemWhile gas suppliers provide ultra-high purity gases, the end user must be able to maintain that purity level while reducing anycontamination from the gas distribution system. Maintaining gas purity is a somewhat difficult task for the electronics devicemanufacturer. Some issues of concern include exposure (leaks) of the gas distribution system to the atmosphere, moisture, high initialor upstream pressure drop, and downtime. A low pressure drop at the bulk filtration stage is absolutely critical since most downstreamcomponents such as valves, pressure regulators, pressure gauges, pressure transducers, filters, purifiers, and mass flow controllerswill only increase pressure drop. Delivering a consistent line pressure to the tool will minimize out-of-spec conditions and allow forrepeatability of the process. In addition, IC manufacturers desire a long lifetime for the filter cartridges as replacing them will expose thegas distribution system to the atmosphere thereby increasing downtime and decreasing throughput.Particle Capture Mechanisms– While gas filters differ in materials of construction, from one supplier to the next, the mechanismsfor capturing particles are the same. The particle reduction efficiency of a filter used in gas service is substantially different from oneused in liquid service. The primary particle reduction mechanism involved with liquid filtration is that of mechanical sieving. BecauseFigure 2 — Particle Capture Diagramof the influence of the fluid stream there is a tendency for particles, particularlyElectrostaticsmall particles, to follow the flow path of the fluid or the path of least resistance InterceptionDepositionthrough the filter media. This, the filter rating is indicative of the actual poresize of the filter membrane. In gas filtration, other particle capture mechanisms(figure 2) such as interception, electrostatic deposition, inertial impaction, anddiffusion are in effect. Interception is the main mechanism for reduction ofparticles larger than the pore size of the filter membrane. A certain portion ofparticles which have diameters smaller than the pore size rating of the filter,will be reduced by partial plugging of the pores by previously deposited contaminants. The reduction of particles by interception becomes more effectiveFlowas the particle size increases. Diffusion involves the reduction of particlesmuch smaller than the pore size rating of the filter. These particles interact withthe gas stream because of their relative size and kinetic energy. The resultantrandom movement of such small particles is termed “Brownian Motion”. Thisrandom movement of particles from the gas stream increases the probability ofcontact with the filter media and subsequent reduction. This particles capturemechanism is extremely efficient at the reduction of submicron particles but asDiffusionthe particle size increases it becomes less effective. Particles are deposited onImpactionthe filter by inertial impaction due to the finite mass and momentum of particles as the gas changes direction when it flows around the filter media. Particle retention by inertial impaction becomes more effectiveas particle size increases. Particles captured by electrostatic deposition occur when the electrical charges on either the filter or particle,or both, create attractive electrostatic forces.Concept of Most Penetrating Size (MPPS) – Of all of the particle capture mechanisms, Diffusion and Interception are the mostimportant for the reduction of submicron particles in the vicinity of the most penetrating particle size. The MPPS is described as theparticle size that has the greatest potential for penetration through the filter. The combination of diffusion and interception leads toa parabola shaped particle penetration curve in the vicinity of the MPPS (figure 3). The MPPS is dependent upon other factors suchas gas flow rate, filtration surface area, and differential pressure. For example, maximum penetration decreases as the gas flow rateis decreased with a slight increase to the MPPS. The MPPS for membrane filters, such as 3M Purification’s LifeASSURE PFS,LifeASSURE EMC, and LifeASSURE IMC filters is in the range of 0.04 - 0.08 µm, and 0.1 – 0.3 µm for non-woven media filters,such as the Betafine PEG series filters, Betafine PG series filters.Log Reduction Value – Log reduction value is a term used to describe filters which have a low penetration and high particle reductionefficiency. LRV is expressed as:LRV -log (Upstream Particle Concentration/Downstream Particle Concentration)A filter with a particle retention efficiency of 99.9% has a penetration of 0.001 and an LRV of 3. Penetration is described as the fractionalamount of particles that are not captured by the filter. Typically critical electronic applications, such as point-of-use require particleretention efficiency at 99.9999% (6 LRV).2
Particle Reduction Ratings for Gas Filters– Over the years particle reduction ratings forFigure 3 — MPPS DiagramCapture byInterceptionCapture byDiffusionMaximumPenetrationPenetrationgas filters have gotten finer and finer not because of advances in filtration technology butbecause of the ability of newer analytical equipment to detect smaller particles. Gas ratingsfor filters are determined by challenging the filter with an aerosol and using Laser ParticleCounters and Condensation Nucleus Counters to detect particles in the effluent gas stream.Typically the filters are subjected to a high concentration of a sodium chloride aerosol which iseither poly-dispersed or mono-dispersed by the use of a classifier. Laser particle counters arecapable of detecting particle sizes down to 0.1 micron while providing a particle sizedistribution for the particles detected. Condensation nucleus counters are capable of measuringparticles with diameters 0.003 µm which includes the MPPS range for all 3M Purificationfilters discussed above. Table 1 compares the particle reduction ratings, log reductionvalues, and recommended flow rates for 3M Purification filters containing PTFE, Nylon 6,6,and polypropylene filter media.MostPenetratingParticle SizeMostPenetratingParticle Sizeat aDecreasedVelocityParticle SizeTable 1. Particle Reduction Efficiency for Different Types of Filer Materials3M Purification FilterLiquid FiltrationFIlter Material Reduction Rating(µm)Gas FiltrationReduction Rating(µm)Gas ParticleReductionEfficiency (%)Filter SurfaceArea (ft2)RecommendedFlow Rate (slpm)LifeASSURE FPPTFE0.20 0.003 99.9999997.03000LifeASSURE PFSPTFE0.20 0.003 99.99999911.04800LifeASSURE IMCNylon0.20 0.003 99.99999911.04000LifeASSURE EMCNylon0.45 0.003 99.99999911.04300Betafine PEG Series FiltersBetafine PBG Series FiltersBetafine PPG & PTG Series Filterspolypropylene0.20 0.003 99.9999995.32300Hydrophobic Vs Hydrophillic Filter Membranes- Free gas flow with litter resistance requires that the pores of the filter membranenot wet with water or moisture. Filter membranes that repel or push water away are termed “hydrophobic” and filters that prefer todissolve or wet more readily in water than non-polar solvents are more commonly referred to as “hydrophilic”. Membrane wetting canresult in filter pore blockage, which in turn restricts free gas flow and increases pressure drop across the filtration system. In order toprevent pore blockage, an extremely hydrophobic filter membrane should be used to prevent wetting with water or moisture. Graph 1shows the relative hydrophobicity (expressed as a reciprocal of surface tension) of three different filter materials that are commonly usedin gas applications. As can be seen, PTFE offers the highest level of hydrophobicity ensuring superior flow characteristics.NOTE: Hydrophilic filters can be used if a gas drying or purification system is located upstream of the filter.3
Filtration Surface Area- The flow rating, pressure drop, andlifetime of a pleated cartridge filter are often dictated by theaccessible surface area. Conventional pleated (Figure 4) filtersmay offer a large gross surface area, but when the media ispacked into the cartridge, only part of the surface area is usedresulting in both flow restrictions and limited contaminantholding capacity. The “blind” or unused area commonly occursnear the inside diameter where the pleats are most tightly compressed. 3M Purification filters with Advanced Pleat Technology(Figure 4) construction contain a staggered and stepped pleatconfiguration, which provides more open space between thepleats. This novel technology maximizes particle loading byincreasing the open area at the inside diameter, while the shorterstepped pleats take advantage of existing open space closer tothe outside diameter of the cartridge. The result is a fully usedsurface area that provides lower pressure drop and superior filterlife.Advanced Pleat TechnologyDesignStandard PleatDesignFigure 4 — Advanced Pleat Technology DesignAir Filter Hydrophobicity0.050.040.03PVDF0.02PTFE1/Critical Surface Tension (dynes/cm)0.06PolypropyleneLow Pressure Drop- Advanced Pleat Technology constructionincreases filtration surface area by as much as 35% when comparedto equivalent size competitive filter cartridges with a standard pleatdesign. This results in a significantly lower pressure drop whichreduces the demand on the distribution system and allows forhigher viscosity fluids to be delivered more efficiently, providing aconsistent supply of gas to downstream components and theprocess tool. This novel construction delivers a higher per cartridgeflow rate at the same pressure drop when compared to competitivefilters containing a standard pleat design. This can reduce filtrationcosts two ways: Graph 2 provides the typical flow rate vs.pressure drop data for LifeASSURE FP filters with anoptimized pleat design. With an inlet pressure of 30 psig (2.07 bar)and a differential pressure of 1 psid (0.07 bar) across both filtersthe flow rate difference between the two filters is quite significant.LifeASSURE FP filters provide a flow rate of 53 SCFM (1500slpm) while LifeASSURE PFS filters deliver 141 SCFM (4000slpm) which equates to a flow rate increase of 1.66 times.0.010Graph 1 — Comparison of Hydrophobic MembranesReduced Filter Change-Out Frequency- For existingAverage Air Flow vs Differential Pressure@ 30 psig Inlet Pressureapplications at a given flow rate, filters with more surface area percartridge have a lower flux (flow per unit surface area) than filterswith less surface area. Since filter lifetime is inversely proportional to flux (lower flux longer life) in most applications. 3MPurification filters with optimized pleat construction provide longerlife and require fewer change-outs.3.5 UREASS3.0LifeilterFP FPressure (psid)2.52.01.5 S FilterURE PF1.0LifeASS0.50010002000300040005000Flow (slpm)Graph 2 —LifeASSURE FP & LifeASSURE PFSFilter Flow Rates @ 21 C (70 F)4Particle Size - Micrometers
Lower Filter Housing Costs- A filter that can deliver a higher flow rate at the same pressure drop when compared to equivalent sizecompetitive products will reduce filtration costs because fewer filters are required for the system. In turn, this allows for the use ofsmaller and less costly stainless steel filter housings which reduce initial capital investment; filter change-out time, hold-up volume, andtotal cost-of-ownership for the life of process. Graph 3 provides the typical air flow rate vs. pressure drop data for 10” LifeASSURE FP, LifeASSURE PFS, LifeASSURE EMC, LifeASSURE IMC, and the Betafine PEG series filters, Betafine PBG seriesfilters, and Betafine PPG & PTG series filters.Quality ManufacturingAverage Air Flow vs Differential PressurePressure (psid)The objective of a gas filter is to retain particles while not shedding,@ 30 psig Inlet Pressurereleasing, or extracting any contaminants back into the gas stream.ersFilt3.5esterseris FilFor this reason, all 3M Purification electronics grade membrane filterserieGSEPFP S3.0EenURtafiASSare manufactured using the most advanced welding techniques- noBersLifeC Filte2.5E IMSSURsLifeAC FilterMadhesives, binders, or surfactants are used in the process- and allE2.0SURELifeASrslte1.5Series Ficartridges are bagged and sealed in a clean environment to ensureRE PFSLifeASSU1.0superior downstream cleanliness out of the package. In addition,0.5all UHP grade filters like those listed in the table (primary filter0400010002000300050000recommendation) below are 100% integrity tested, rinsed with 18megohm DI water, and dried with UHP gas to reduce the potential forFlow (slpm)particle shedding and downtime during system start-up. Ionic, organic,and metallic contaminants can extract from competitive filters thatGraph 3. — Air Flow Comparison @ 21 C (70 F)do not contain the proper materials of construction and or manufacturingtechniques resulting in extended purge times or not being able to meet particle specifications. All membrane filter cartridgesare built to exacting specification to provide a consistent quality of gas to downstream components and the membranemanufacturing facilities operate with an ISO certified quality system. Table 2. 3M Purification Filter Recommendations for Bulk Gas FiltrationBulk Gas TypePrimary Filter RecommendationAlternative Filter RecommendationAirLifeASSURE PFSLifeASSURE FPBetafine PEG Series FiltersBetafine PBG Series FiltersBetafine PPG & PTG Series FiltersArgonLifeASSURE PFSLifeASSURE FPCarbon MonoxideLifeASSURE PFSLifeASSURE FPCarbon DioxideLifeASSURE PFSLifeASSURE FPClean Dry Air (CDA)LifeASSURE PFSLifeASSURE FPBetafine PEG Series FiltersBetafine PBG Series FiltersBetafine PPG & PTG Series FiltersHeliumLifeASSURE PFSLifeASSURE FPHydrogenLifeASSURE PFSLifeASSURE FPNitrogenLifeASSURE PFSLifeASSURE FPLifeASSURE PFSLifeASSURE FPBetafine PEG Series FilterBetafine PBG Series FiltersBetafine PPG & PTG Series FiltersOil Free Air (OFA)5
Summary1. The filtration of a gas is substantially different than the filtration of a liquid. In gas filtration, other particle capture mechanisms such asinterception, diffusion, electrostatic deposition, and inertial impaction are in effect. Testing by CNC has verified the retention of particles 0.003 microns by LifeASSURE FP, LifeASSURE PFS, LifeASSURE EMC, LifeASSURE IMC, and Betafine PEG seriesfilters, and Betafine PG series filters.2. Major reductions in particles can directly be associated to proper bulk gas filtration, thereby ensuring protection of downstreamcomponents such as piping, valves, regulators, pressure transducers, and mass flow controllers. Providing a consistent gas quality todownstream components ensures repeatability of the manufacturing process and reduces the potential for killer defects to occur on theelectronic device.3. Membrane wetting can result in filter pore blockage, which in turn restricts free gas flow and increases pressure drop across thefiltration system. In order to prevent pore blockage, an extremely hydrophobic filter membrane like PTFE should be used to preventwetting with water or moisture. LifeASSURE PFS and LifeASSURE FP ultra-high purity gas filter cartridges both contain PTFEmembrane to reduce the potential for moisture bypassing the filter.4. 3M Purification filters with optimized pleat designs like Advanced Pleat Technology construction can increase flow by as much as1.66 times when compared to equivalent size competitive filter cartridges. This translates into fewer filters being required and changedout for the life of the system while also reducing the size and cost of the housing.5. All UHP grade filters like those listed in the table (primary filter recommendation) above are 100% integrity tested, rinsed with 18megohm DI water, and dried with UHP gas to reduce the potential for particle shedding and downtime during system start-up.6. Ionic, organic, and metallic contaminants can extract from competitive filters that do not contain the proper materials of constructionand or manufacturing techniques resulting in extended purge times or not being able to meet particle specifications.7. Utilizing high efficiency filters as recommended in Table 1 and 2 above will provide the peak level of performance needed for years oftrouble free bulk gas service.References1. Rubow, K.L. 1981. “Submicron Aerosol Filtration Characteristics of Membrane Filters”, Ph.D. Thesis, University of Minnesota,Mechanical Engineering Department, Minneapolis, MN.2. Rubow, K.L., and Liu, B.Y.H. 1986. “Characteristics of Membrane Filters for Particle Collection”, Fluid Filtration: Gas, Volume 1,ASTM STP 975, R.R. Raber, Ed., American Society for Testing and Materials, Philadephia, 1986.Acknowledgement3M Purification Inc. would like to thank Pureron Japan for their contribution to this application brief.Other Related Reference LiteratureReference Title/Description3M Purificaton Inc. Literature ID.LifeASSURE EMC Product LiteratureLITZRLAEMC1LifeASSURE IMC Product LiteratureLITZROPT1LifeASSURE PFS Filter CartridgesLITCM2LifeASSURE FP Filter CartridgesLITMRFP1Betafine PEG Series FiltersLITPXLEL3M ZWB & ZWC Series Filter HousingsLITZRH1063M ZVS & ZMS Series Filter HousingsLITZRH1046
LifeASSURE PFS PTFE filter cartridges are highly retentive naturally hydrophobic PTFE filters that have been specifically designedto meet the needs of high purity bulk gas and air applications. Utilizing 3M Purification’s optimized pleat construction; LifeASSUREPFS filter cartridges are able to deliver a high per cartridge flow rate at the same pressure drop when compared to equivalent sizecompetitive filters containing a standard pleat design. A filter that can deliver a higher flow rate at the same pressure drop whencompared to equivalent size competitive products will reduce filtration costs because fewer filters and smaller metal housings arerequired for the system. LifeASSURE PFS filters are ideally suited for flow rates up to 4800 slpm or 170 scfm per 10
LifeASSURE EMC, and LifeASSURE IMC filters is in the range of 0.04 - 0.08 µm, and 0.1 – 0.3 µm for non-woven media filters, such as the Betafine PEG series filters, Betafine PG series filters. Log Reduction Value – Log reduction value is a term used to describe filters which have a low penetration and high particle reduction
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