On-line analytical measurements Ultrapure water

1 on-line analytical measurements

1.1 conductivity/resistivity
1.2 sodium
1.3 dissolved oxygen
1.4 silica
1.5 particles
1.6 non-volatile residue
1.7 toc

1.7.1 organic oxidation methods toc analysis

on-line analytical measurements
conductivity/resistivity

in pure water systems, electrolytic conductivity or resistivity measurement common indicator of ionic contamination. same basic measurement read out in either conductivity units of microsiemens per centimeter (µs/cm), typical of pharmaceutical , power industries or in resistivity units of megohm-centimeters (mohm•cm) used in microelectronics industries. these units reciprocals of each other. absolutely pure water has conductivity of 0.05501 µs/cm , resistivity of 18.18 mohm•cm @ 25 °c, common reference temperature these measurements compensated. example of sensitivity contamination of these measurements 0.1 ppb of sodium chloride raises conductivity of pure water 0.05523 µs/cm , lowers resistivity 18.11 mohm•cm.

ultrapure water contaminated traces of carbon dioxide atmosphere passing through tiny leaks or diffusing through thin wall polymer tubing when sample lines used measurement. carbon dioxide forms conductive carbonic acid in water. reason, conductivity probes permanently inserted directly main ultrapure water system piping provide real-time continuous monitoring of contamination. these probes contain both conductivity , temperature sensors enable accurate compensation large temperature influence on conductivity of pure waters. conductivity probes have operating life of many years in pure water systems. require no maintenance except periodic verification of measurement accuracy, typically annually.

sodium

sodium first ion break through depleted cation exchanger. sodium measurement can detect condition , used indicator cation exchange regeneration. conductivity of cation exchange effluent quite high due presence of anions , hydrogen ion , therefore conductivity measurement not useful purpose. sodium measured in power plant water , steam samples because common corrosive contaminant , can detected @ low concentrations in presence of higher amounts of ammonia and/or amine treatment have relatively high background conductivity.

on-line sodium measurement in ultrapure water commonly uses glass membrane sodium ion-selective electrode , reference electrode in analyzer measuring small continuously flowing side-stream sample. voltage measured between electrodes proportional logarithm of sodium ion activity or concentration, according nernst equation. because of logarithmic response, low concentrations in sub-parts per billion ranges can measured routinely. prevent interference hydrogen ion, sample ph raised continuous addition of pure amine before measurement. calibration @ low concentrations done automated analyzers save time , eliminate variables of manual calibration.

dissolved oxygen

advanced microelectronics manufacturing processes require low single digit 10 ppb dissolved oxygen (do) concentrations in ultrapure rinse water prevent oxidation of wafer films , layers. in power plant water , steam must controlled ppb levels minimize corrosion. copper alloy components in power plants require single digit ppb concentrations whereas iron alloys can benefit passivation effects of higher concentrations in 30 150 ppb range.

dissolved oxygen measured 2 basic technologies: electrochemical cell or optical fluorescence. traditional electrochemical measurement uses sensor gas-permeable membrane. behind membrane, electrodes immersed in electrolyte develop electric current directly proportional oxygen partial pressure of sample. signal temperature compensated oxygen solubility in water, electrochemical cell output , diffusion rate of oxygen through membrane.

optical fluorescent sensors use light source, fluorophore , optical detector. fluorophore immersed in sample. light directed @ fluorophore absorbs energy , re-emits light @ longer wavelength. duration , intensity of re-emitted light related dissolved oxygen partial pressure stern-volmer relationship. signal temperature compensated solubility of oxygen in water , fluorophore characteristics obtain concentration value.

silica

silica contaminant detrimental microelectronics processing , must maintained @ sub-ppb levels. in steam power generation silica can form deposits on heat-exchange surfaces reduces thermal efficiency. in high temperature boilers, silica volatilize , carry on steam can form deposits on turbine blades lower aerodynamic efficiency. silica deposits difficult remove. silica first readily measurable species released spent anion exchange resin , therefore used trigger anion resin regeneration. silica non-conductive , therefore not detectable conductivity.

silica measured on side stream samples colorimetric analyzers. measurement adds reagents including molybdate compound , reducing agent produce blue silico-molybdate complex color detected optically , related concentration according beer-lambert law. silica analyzers operate on automated semi-continuous basis, isolating small volume of sample, adding reagents sequentially , allowing enough time reactions occur while minimizing consumption of reagents. display , output signals updated each batch measurement result, typically @ 10 20 minute intervals.

particles

particles in upw have presented big problem semiconductor manufacture, particle landing on silicon wafer can bridge gap between electrical pathways in semiconductor circuitry. when pathway short-circuited semiconductor device not work properly; such failure called yield loss, 1 of closely watched parameters in semiconductor industry. technique of choice detect these single particles has been shine light beam (a laser) through small volume of upw , detect light scattered particles (instruments based on technique called laser particle counters or lpcs). semiconductor manufacturers pack more , more transistors same physical space, circuitry line-width has become narrow , narrower. result, lpc manufacturers have had use more , more powerful lasers , sophisticated scattered light detectors keep pace. line-width approaches 10 nm (a human hair approximately 100,000 nm in diameter) lpc technology becoming limited secondary optical effects, , new particle measurement techniques required.

non-volatile residue

another type of contamination in upw dissolved inorganic material, silica. silica 1 of abundant mineral on planet , found in water supplies. dissolved inorganic material has potential remain on wafer upw dries. once again can lead significant loss in yield. detect trace amounts of dissolved inorganic material measurement of non-volatile residue commonly used. technique involves using nebulizer create droplets of upw suspended in stream of air. these droplets dried @ high temperature produce aerosol of non-volatile residue particles. measurement device called condensation particle counter counts residue particles give reading in parts per trillion (ppt) weight.

toc

total organic carbon commonly measured oxidizing organics in water co2, measuring increase in co2 concentration after oxidation or delta co2, , converting measured delta co2 amount “mass of carbon” per volume concentration units. initial co2 in water sample defined inorganic carbon or ic. co2 produced oxidized organics , initial co2 (ic) both defined total carbon or tc. toc value equal difference between tc , ic.

organic oxidation methods toc analysis

oxidation of organics co2 commonly achieved in liquid solutions creation of highly oxidizing chemical species, hydroxyl radical (oh•). organic oxidation in combustion environment involves creation of other energized molecular oxygen species. typical toc levels in upw systems methods utilize hydroxyl radicals in liquid phase.

there multiple methods create sufficient concentrations of hydroxyl radicals needed oxidize organics in water co2, each method being appropriate different water purity levels. typical raw waters feeding front end of upw purification system raw water can contain toc levels between 0.7 mg/l 15 mg/l , require robust oxidation method can insure there enough oxygen available convert carbon atoms in organic molecules co2. robust oxidation methods supply sufficient oxygen include following methods; ultraviolet light (uv) & persulfate, heated persulfate, combustion, , super critical oxidation. typical equations showing persulfate generation of hydroxyl radicals follows.

s2o8 + hν (254 nm) → 2 so2• , so2 • + h2o → hso4 + oh •

when organic concentration less 1 mg/l toc , water saturated oxygen uv light sufficient oxidize organics co2, simpler oxidation method. wavelength of uv light lower toc waters must less 200 nm , typically 184 nm generated low pressure hg vapor lamp. 184 nm uv light energetic enough break water molecule oh , h radicals. hydrogen radicals react create h2. equations follow:

h2o + hν (185 nm) → oh• + h • , h • + h • → h2

different types of upw toc analyzers

ic (inorganic carbon) = co2 + hco3 + co3

tc (total carbon) = organic carbon + ic

toc (total organic carbon) = tc – ic

h2o + hν (185 nm) → oh• + h •

s2o8 + hν (254 nm) → 2 so2 •

so2 • + h2o → hso4 + oh •