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Chapter One : Requirements for Ultrapure Water Semiconductor technology for 0.1 to 1 m pattern formation and the 10 nm or less thin film. Impurities should neither exist nor generated during the production process. Pure water is used as medium for the removal of contaminants on wafer surface. Pure water produced by ion exchange resin as early as 1945. In modern ultrapure water requirement demand a pureness of parts per billion to trillion level. 1. Effect of wafer surface defects on VSLI With finer pattern, minute defects and trace impurities affect the characteristics and reliability of the device. It damages the wafer substrate and dislocates the edge of the fine patterns. (Refer to Table 2 Page 28) As VLSI becomes more integrated, the production process becomes more complex. For 4-16M RAM, more than 200 process steps are required. Each process has cleaning step and the device repeatedly rinsed. The water of ultrapure quality is therefore critical to keep wafers from contamination. The ultrapure qualities include : resistivity, bacteria count, total organic carbon (TOC), fine particles, silica, metal and concentration of ionic impurities. 2. Requirements for ultrapure water quality There are two kinds of contamination on the wafer surface. The visible contaminant – mainly particles and the invisible contaminant - organic and inorganic materials ; the heavy metal absorbed on the wafer surface as atoms or molecules. Yield proportional to exp –D , where D is the defect density. If there is n critical patterning steps, Yield proportional to exp –nD These mean the yield decreases exponentially as the number of absorbed particles and the number of steps increase. Particle adhered on surface had exponential effect on yield for lithographic process. The largest possibly allowed particle size depends on the line width W. The alignment accuracy is W/5 and the particle size should not be larger than ½ of that, i.e. the largest permitted particle size is W/10 . Table : Acceptable particle size Device Min Line Width Max acceptable (Dram) (m) particle size (m) 1M 1.0 0.1 4M 0.8 0.08 16 M 0.6 0.06 64 M 0.4 0.04 Metallic contamination leads to crystalline defects and oxide defect. The absorption on surface must not exceed 109 atoms per cm2. It works out to be less than 30 ppt in the ultrpure water. 3. Properties of ultrapure water Water is an excellent solvent that could dissolve many substances. As the integration level improves, the pure water must be of higher purity in view of the yield and reliability of semi-conductors. Theoretical pure water properties : a. Resistivity = 18.24 M.cm b. Electrical conductivity = 0.05482 S cm-1 at 25oC (see Fig 1 on page 48) c. Surface tension – the surface tension reduces as the temperature rises, cleaning efficiency improves. The weakened surface tension makes easy for the penetration into smaller space for cleaning. d. Viscosity – also reduces as temperature increases. Effect is like reduction of surface tension. e. SiO2 solubility – see figure 10 in page 57 f. Air, Oxygen and Nitrogen solubility – see figure 8 in page 55 Highly integrated semiconductors needs high quality water of conductivity above 18 M.cm and with no particles, organic materials, bacteria, silica and others. (1ppb of Fe++ will drop it to 17 M.cm . The quality of ultrapure water is measured by ppt. With this high purity, it makes measurement very unrepeatable because of the elution. Measurements could be different between morning and afternoon. 4. SI wafer cleaning with ultrapure water Cleaning means to remove unwanted substances that adhered or generated on a surface and to recover the cleanliness of the surface. Mechanism of contaminants adhered on surface are : a. Physical adsorption b. Electrical force c. Chemical adsorption d. Mechanical force In Si wafer cleaning, only mechanical and water dissolve method. Pure water is a better solvent to polarized substances than the non-polarized substances. There are some ionic substances much easily dissolved in water than others. (see table 2 to 4 in page 58-59). Cleaning of adhered contaminants – physical and chemical method. Chemical method for removal of organic materials is by organic solvent and subsequently cleaned by ultra pure water. Physical cleaning – ultra sound, high pressure jet blow with fine particle ice. Electrical and chemical action - Watermark method – It is a way to investigate the purity of ultrapure water. A drop of water is left to dry on wafer to see the mark left. Chapter 2 : Ultrapure Production System Impurities in water could not be removed by a single method. It involves pretreatment, primary treatment, secondary treatment (ultrapure system or subsystem), pipe system and wastewater reclamation. The primary treatment system – comprises a RO unit, a deaerator and an ion exchange unit. The secondary treatment (Ultrapure system or subsystem) – comprises of ultraviolet sterilization, oxidation, ion exchange and ultrafiltration. A pipe system – transport the ultrapure water to the point of use without degrading the water. In short an ultrapure treatment system comprises of : a. Pretreatment System b. Primary treatment system c. Secondary treatment system and d. Distribution system. (see figure 1 of page 71 and table 1 of page 72) For the choice of RO + IX special attention needed to the pretreatment system to avoid fouling and scaling of RO membrane. The IX + RO is ideal for the prevention of RO membrane fouling because the IX has taken care of the ions that cause trouble to the membrane. For the cost reduction measure, the RO + IX is more generally used. (see table 2 for the different type of fouling and scaling ) 1. Water source and quality : Water from river – From both underground and surface water. Surface water contains suspended matter, inorganic salts from underground water. When river flows through forest or peat bogs it dissolves organic matters. Sewer and industrial water may also be mixed into river water. The river water quality is unstable and therefore varied with time and wet weather. Water from lake – The lake water quality depends on the quality of the inflow of river water. It is significantly effected by the biological activities in detention. The pH level may change due to the consumption of dissolved CO2. The water stratifies within a lake, shortage of oxygen at bottom of lake which leads to dissolution of iron and manganese and formation of H2S under anaerobic condition. Taking water from bottom of lake is not ideal. Underground water – Suspended matter is low. Quality of underground water depends on the layers of ground it flows through. The lime stone layer hardens the water and the volcanic stone causes high silica concentration. It contains limited oxygen. Sometimes, it contains Ferro compound and manganese and H2S. (see table 4 in page 74 for quality of water from different water sources) City water – most city water complies with the WHO standard. Yet its quality varies depending on where the water came from. (see table 5 for water quality in the US) 2. Pretreatment system : Pretreatment must take care of the softening of water, removal of heavy ions and the algae. Pretreatment unit : a. Coagulation, settling and filtration – When industrial water with turbility of more than 10, this method is used. b. Coagulation : the normal sand filtration method removes particles of more than 10-50m. Most of the particles in water must be coagulated first and later removed by filtration. c. Most particle in water is negatively charged. The charges should be neutralized before they could be coagulated. Particles of charges between – to 10 mV can be coagulated by inter molecular forces (van der Waals forces). d. Coagulated particles could be linked to make larger particles. The salt of Al and Fe goes through hydrolysis in water to be positively charged hydroxide polymer. PAC or [Al2(OH)nCl6-n]m, alum [Al2S(SO4)3.18H2O], and Fe2(SO4)3 are often used as coagulants. The process allows quick mixing of the coagulants with the particles. Then left for 20 to 40 minutes for the formation of large flocs. (see table 7 in page 77 for specifications of the PAC) e. Settling – the size of the coagulation decides the time for settling and could be guided by the Stroke’s law. Larger the diameter quicker is the settle. For water with low turbility, it takes longer time for settling and might be the microfloc filtration would be better than settling/precipitation method. Tube settler and blankets settler to increase the settling speed. (see figure 4,5 in page 79-80) f. Filtration – sand filter or multimedia filtration are used (see figure 6 and 7 in page 80-81). The particles is filtrated at 3-5 m/day. Particles do not settle in the basin with coagulant but are filtered through the biofiltration membrane formed on the surface of the filtration basin. It uses biochemical treatment and is good for city water but not for water of high turbidity. The method needs large space and hence not suitable for industrial water. Rapid filtration use sand of 0.3-0.7 mm diameter and filter velocity is 5-10 m/hr. It does not use biofiltration membrane and therefore must be used with coagulants settling. It removes particles of 10-50m. After the trapping of the particles, there is increase in pressure drop for water passing through the filter bed. Backwash (5-10 minutes) is applied when back pressure reaches 1-5 mH2O. To improve filtration efficiency, multimedia sand filter is used. (see figure 8 in page 81 for 3 types of filter). g. Microfloc and filtration – applied to water of low turbidity. Flocculent is injected directly to the pipe supplying water to the filter and microflocs are formed and are filtered directly (for under ground water and lake water). This method also used as pretreatment for RO in order to reduce the SDI of city water. The flocculent have to be carefully and accurately applied, it is therefore more suitable for less variation of incoming water quality. Generally a dual filter media (0.4-0.6 mm sand and 0.7-1.5 mm sand) are used and must be back washed by the combination of both air and water. (see figure 10 in page 83 for the microfloc filtration system) h. Other treatment : softening, pH adjustment and dispersant injection 2.1 Softening : Cold Lime-Soda Method : The hardness material precipitates in such a form as CaCO3 and Mg(OH)2 at room temperature by injecting hydrated lime, soda ash and alum to raise the pH to 10-11. Ca(HCO3)2 + Ca(OH)2 2CaCO3 + 2 H2O CaSO4 + NaCO3 2CaCO3 + Na2SO4 Mg(HCO3)2 + Ca(OH)2 Mg(OH)2 + 2CaCO3 + 2 H2O see figure 11 in page 84 The hardness is not totally removed. There is still 35 mg/L by the lime soda method. Ion exchange method : It applies sodium cycle cation-exchange method. 2R-SO3Na + CaCl2 (R-SO3)2Ca + 2 NaCl softening (R-SO3)2Ca + 2NaCl 2R-SO3Na + CaCl regeneration The capacity for 1 L cation rasin is 30-50 g CaCO3. Sodium chloride solution or seawater is used for regeneration. The ion exchange is good for the pretreatment for RO membrane. It prevents scaling formation 2.2 Silica removal : It happens in water from volcanic area where the solubility is 100 mg/L at 25oC. In RO process when the concentration in excess of its solubility, it can be rapidly deposited in the present of iron or aluminum, and forms very hard scale which is hard to be cleaned and led to the quick replacement of the cartridge. One can apply ion exchange + RO for the silica removal. It can also be done by coprecipitation using a flocculent like alum. Cold lime-soda method mentioned before could also be used for silica removal. When Magnesium precipitates in the form of magnesium hydroxide at high pH, silica coprecipitates. Since the silica removal efficiency drops with decrease magnesium hardness, MgO or dolomite [MgCa(CO3)2] added to maintain the hardness. Silica precipitation depends also on the pH. (see Figures 12, 13 & 14) Using Aluminum salt : Silica can be removed by adding a large amount of such coagulants as PAC and alum. (see figures 15, 16) . The silica removal depends on the amount of PAC and the pH level. To protect the RO membrane from silica, the pH keeps at 7-8 will be appropriate for PAC is used. 2.3 Iron and manganese removal : It is one of the most important steps in pretreatment. Iron has important role in fouling in RO unit. Under ground water with reduced atmosphere iron dissolved in the form of ferrous. When iron is oxidized to form ferric, its solubility decreases and exits as suspension matter. It is easily removed with coagulation and filtration. If the dissolved iron and manganese, the removal is not necessary for the RO. However, if the water is stored in tank, it should be oxidized and removed. The oxidation agents are oxygen, chlorine and potassium permanganate. The oxidation rate may be slow in low pH, especially for the manganese where catalyzer is used. Item Fe (mg/L) Mn(mg/L) Cl2 o.63 1.29 * KmnO4 0.94 1.92 Table 8 Required oxidative reagent for Fe and Mn oxidation *catalyzer required - FeOOH is effective catalyse for iron and manganese sand coated with manganese oxide are useful for manganese. (see figure 17) 2.4 Activated carbon filtration: 2 major functions – (1) remove organic (2) Residue chlorine. For synthetic RO that does not feature chlorine resistance, the activated carbon filtration is employed for chlorine removal. Water without chlorine may promote bacteria growth and may lead to fouling of RO membrane. Chapter 3 : Primary treatment System : I Outline a. General : The primary Treatment system is located in between the pretreatment system and the substystam. It produces primary pure water. The system is large and comprises of very tall recovery ion exchange tower, a vacuum degasifier tower (VDG tower), a RO unit that often caused noise and vibration, and a large tank.. The objective of the primary treatment system is to ensure the supply of consistent quality of water to the subsystem despite the fluctuation in the quality of water supply. (see Figure 1 of page 97) b. Targetted quality of water and current level of water quality : The table 1 give the targetted water quality and the type of VSLI. It may be out dated but slight difference for 1 to 1 Mbit device. II Typical Flow and Characteristics 1. The older type of primary treatment system Development : The primary treatment system comprises of the following : Membrane system : RO unit ; ultrafiltration unit (UF); membrane filter (MF) Ion exchange system : Two-bed ultrapure water unit (2B tower) Mixed bed ion exchange tower (MB tower) Deaeration system : Degasifier tower (DG tower) Vacuum degasifier tower (VDG tower) Others : Pump, tank, ultraviolet sterilizer (UV), chemical supply unit. See figure 2 : some system diagrams. 1.1 Ion exchange unit only : When pure water was first installed in 1960, rsistivity regarded as important indicator of water quality (10-16 M at 25oC), the 2-B 3T or the MB was used only for removal of ion. Later, 1970, the requirement improved and combined 2B3T with MB were used. Depending on the quality, multiple bed ion exchange like the 4B5T instead of the 2B3T. The TOC and the particle problem are not able to be resolved here. 1.2 Front-stage RO system : In the late 70’s, ultra-filtration began to be used as the final filter. Replacing membrane filter in removing particles and bacteria at point of use. Initially either RO or UF was used. Later the RO used in the primary treatment and UF for the subsystem. The font stage RO removes all the ions in the supply water which dramatically decreased on the load on the ion exchange and increased the volume of water produced. The water quality also drastically improved because the RO also removes the particles, bacteria, silica and the TOC. The problem is the fouling and scaling of membrane that reduces production and the fluctuation of quality. This is dealt with chemical treatment for the control of FI and SDI values. The ion exchange efficiency could be improved by having more multiple bed and multiple tower could be considered. 1.3 Middle-stage and two-stage RO systems : In the 80’s, the mid-pressure and low pressure complex membrane RO were developed. The front end RO is replaced by the low-pressure complex membrane RO, It is effective in reducing energy cost and improving the removal efficiency of TOC and silica. It prevents colloidal silica generation. It is less efficiency in the oxidative agent and fouling resistance and thus cannot be sterilized using soda hypochlorite except by adopting intermittent sterilization. The other problem is that it is electrically absorb foulants that clogging the membrane. Reducing FI and SDI index or reduce the recovery ratio might solve the problem. 1.4 Later in the mid 80’s, the low pressure complex membrane is use in the middle stage RO between the 2B3T or the MB units. The middle stage RO is used for raw water with high silica content. In the 90’s, the low pressure RO membrane is used to replace the middle stage RO to stabilize water quality and suppress TOC. Two-stage RO are also used to stabilize and keep low the TOC. The variation will be (a) 1 RO to supply to the next RO directly. (b) First RO produce water in tank and used for the 2nd RO. (c) In between the 2 RO’s there is an ion exchange process. 2. Recent primary treatment system : 2.1 Two-Bed + RO + MB : In addition, it comprises of a vacuum degasifier, an ultraviolet sterilization, micro-filtration and an heat exchanger. (see figure 3 and table 2 of page 103). Municipal water is processed in the pretreatment system by chemical injection before being supplied to the primary treatment system. The pretreated water also sterilized with NaClO. The 2B3T produces deionized water. The residual Chlorine in water is reduced by NaHSO3 . The heat exchanger is to cool the water to 25oC before water supplies to the RO system. A prefilter is to reduce the particles and also to prevent the leak of resin from the 2B3T before entering the RO unit. To prevent bacteria from entering the RO, the Utraviolet (UV) sterilizer is installed just before the RO. The low pressure (15kg/cm2) complex spiral RO membrane is used for water of low conductivity. It effectively targeted for the removal of particle, TOC, bacteria and colloidal materials at the recovering ratio of 85%. After the Ro the Toc could be < 0.03 g/L and the particle reduced to 0 – 1 particle per ml. The MB removes almost all the ion to the resistivity of > = 17 M.cm . The vacuum degasifier made of stainless steel operates at 20-30 mmHg to remove all dissolved oxygen to lower than 50-100 g/L. The processed water then flows through the UV sterilizer and the post filter and stored in a primary pure water tank. To prevent absorption of CO2, the tank is purged with N2. The advantages of the system are : (a) The 2B3T placed at the front stage prevent the water quality fluctuation that affect the later stage. (b) material eluted from the 2B3T can be removed by the RO. (c) After the 2B3T then RO reduces the silica and hardness which allows the RO to operate at higher recovery ratio. (d) The regeneration of the MB is long enough to ensure the quality for long period of time. 2.2 RO + 2B3T + MB + RO : This system employed processes similar to the preceding system except the RO is placed upstream and another one in the down stream of the ion exchange for the following reasons : (a) It prevents the contamination of organic material and colloidal materials for the 2B3T because they are reduced by the RO. (b) Due to the RO the ion loading on the 2B3T is low and is capable for expansion should the water volume increase. I(see figure 4 and table 3 in page 105) Industrial water is fed to this system as raw water. The sludge contact clarifier and the gravity sand filtration unit are installed in the pretreatment section. Since the quality of water fluctuates greatly, the pH is constantly adjusted. The water after the pretreatment is stored in the filtered water tank to be fed into the primary treatment system. The temperature of the water is adjusted to 25oC by a plate heat exchanger. The water flows through a spool prefilter before entering the RO module. This low pressure spiral RO module made of cellulose acetate. The removal efficiency of NaCl is not very good but that of MgSO4 is as high as 98%. It is capable of removing 90% of TOC and 80% of ions. The pH level is controlled by injection of H2SO4. NaClO injection kills bacteria. The water is then store in a RO tanks and then treated in a 2B3T unit. To prevent oxidative reagent from entering the unit, NaHSO3 as reducing agent is injected. The Vacuum degasifier is mounted on the 2B3T unit to remove both dissolved CO2 and the O2 (<= 100g/L). The cation-exchange tower use a strongly acid cation exchange resin gel and the anion-exchange tower used a strongly basic anion-exchange resin gel. The result water quality is resistivity is 1-5 M.cm and silica content is <= 0.01-0.04 mg/L. The water is stored in a deionizes tank after treatment by UV sterilizer. Next the water is send to a MB unit, the TOC level drops from 0.23 mfC/L to 0.12 mgC/L. The water now to be treated to >17 M.cm flows to a the guard filter and UV sterilizer to fed the RO unit. RO of low pressure complex spiral module operated at the same condition mentioned before. The quality of the water is as high as ultrapure. (see figure 5 of page 107) 2.3 Two-stage RO + MB system : The raw water feed is the well water, which has stable quality and low turbidity. The well water flows to the chemical injected sand filtration and store in a tank for the primary treatment system. The water flows through a spool prefilter and than through the 2-stage RO system. The RO is equipped with the following chemical injection system. (a) sterilization chemicals (b) pH adjustment (NaOH) (c) scale prevention chemicals The silica becomes an ion under basic condition can be removed by RO TOC including organic acid can be removed more easily under alkali (basic) condition. Under basic condition CO2 becomes bicarbonate ion for removal by RO. The removal efficiency of the low pressure complex RO membrane is higher under the alkali pH level. Conversely, the alkalinity may induce scaling cause by hardness materials. The acrylic acid polymer instead of the phosphoric acid is used because the former is more effective in the prevention of scaling. The quality of water after the 2-stage RO is extremely good in terms of TOC, particles and bacteria. The resistivity is improved by a small cartridge anion polisher and a cartridge mixed bed (MB) polisher. The reason is that water after the 2-stage RO with low level of ion and the expected life cycle of the cartridge will be long. The water then stored after the final filter for supplying to the subsystem. Chapter 4 Utrapure Water System - Secondary treatment I Introduction : The aim is to upgrade the quality of water from the primary treatment system. II Latest Secondary Treatment System 1. Water quality (see table 1 of page 139) : Depending on the wafer type. The 4 M DRAM will require higher quality of water than that of the 256 k DRAM. 2. Structure of the system : (see figure 5 of page 140) The structure of the primary treatment system depends on the type of raw water supply. The quality of the water after the primary treatment should be of (a) Resistivity of > 15 M.cm (b)trace particle of 0.1 m at < 100 per ml (c) TOC < 50 ppb (d) SiO2 < 10 ppb. The secondary system is therefore required to upgrade to the level of ultrapure. 3. The secondary treatment system only comprises of pure water tank, a pump, a heat exchanger, an ultraviolet (UV) oxidation unit, an ion exchange polisher and an ultrafiltration unit. The ultrapure water is immediately supply to the point of use through a pipe system. Always 20-30% (in Japan ) (in US 100%) more than the capacity needed. Water not in use (excess) will be circulated back to the secondary system for treatment. If the operation of the supply system is suspended, the pipe and the secondary treatment system have to be sterilized before restarting ensuring the systems are free from bacteria. The UV oxidation unit operates in low pressure decomposes organic materials which stabilise and suppress the TOC concentration in ultrpure water to < 10 ppb. III Function of each unit 1. UV sterilization unit : UV light of wave length of 253 nm by mercury lamp eliminates bacteria in ultrapure water. It is superior then the chemical sterilization because it does not introduce chemical that change the water quality. The irradiation density (W.cm-2) x Irradiation time (s) = ultraviolet irradiation volume (Ws.cm-2). With 50,000 – 100,000 (Ws.cm-2), 99.9% of bacteria will be eliminated in ultrapure water. For some kinds of mold, more irradiation volume still be needed for achieving the same level of killing. 2. UV Oxidation Unit : The low pressure UV is smaller than the normal UV and it does not require the oxidative agent of H2O2 and O3. It decompose the organic mater using UV of 185 nm. The organic mater is decomposed to CO2 and intermediate product like the organic acid. The later reduces the reisitivity and to be treated by the anion ion exchange unit. The unit also radiates 253 nm wave length, it is therefore also used for the bacteria sterilization. The TOC level is related to the volume of the irradiation. It is therefore able to control the TOC level to the expected value by using the correct irradiation volume and hence guarantee the ultrapure water quality. The irradiation capacity deteriorates with time. It needs replacement regularly to ensure the quality of the water. 3. Ion exchange polisher : It is to remove the remaining ions of concentration is a few ppb. It comprises of mixed bed unit with anion and cation exchange resin of high purity. The consumption is low even for long period of time (replacement might be once a year) If UV oxidation unit is employed, the anion exchange is used to remove the organic material. 4. Ultrafiltration unit : It is placed in the final stage and to remove all the particles that is leak through the primary treatment or the generation in the secondary treatment. It also eliminates the bacteria all together to prevent contamination of the piping system. There are spiral and hollow fiber elements. The membrane is made of polysulfone and epoxy is used as adhesive. Both materials are suitable for hot water sterilization (80 – 90oC with 1% of H2O2). The hollow fiber is more commonly used than the spiral wound type. Due to the difficulty in removing in contaminant from the inner part of the hollow fiber filter, the external pressurize type is more prevalent. In inner pressure type filtered 90% of water and rejected about 10%. IV Change in water quality in secondary treatment system 1. Bacteria : Action must be taken to prevent bacteria from growing. This includes the design stage to prevent dead end. 0.5 – 1% H2O2 hot water of 80oC for 4-6 hours. Bacteria will be killed and still stick to the pipe wall and will be released slowly that increase the particle count of the ultrapure water. In US O3 sterilization is employed. 2. Resistivity : Water kept in the secondary system and the piping system trace of ions will have eluted that contaminate the water that reduces resistivity. The water must be continuously recirculated to prevent the building up of contaminants. CO2 dissolved in water reduces the resistivity significantly in pure water tank. N2 was used to seal the tank from other dissolvable gases. 3. The UV sterilization reduces the resistivity. The change is subtle and often overlooked. It is reverted after the ion exchange polisher. There is also leak of polymer from the porous ion exchange resin but will be absorb by the ultrafiltration. It will cause drastic reduction of water volume in short period of time. The resistivity could be > 18M.cm and 70% achieve 18.21 18M.cm . 4. TOC : Water kept in the secondary system and the piping for too long, TOC rises resulted from the contaminants from the organic materials from piping material, tank material and the ion-exchange resin. A UV oxidation unit capable of removing the TOC to 10 ppb.
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