Process-to-process recycling of high-purity water from semiconductor wafer backgrinding wastes

Resources, Conservation and Recycling 41 (2004) 119–132

Process-to-process recycling of high-purity water from semiconductor wafer backgrinding wastes Ming Wu, Darren Sun∗ , Joo Hwa Tay School of Civil and Environmental Engineering, Nanyang Technological University, Block N1 #1C-89, Nanyang Avenue, Singapore 639798, Singapore Received 8 January 2003; accepted 10 September 2003

Abstract The semiconductor integrated circuits (IC) manufacturing processes consume a huge amount of high-purity water (HPW) at different grades of quality. In order to conserve natural resources and reduce production costs, a novel process-to-process water recycling system was developed from this study. The pilot-scale system, consisting of ultrafiltration (UF) followed by a polishing loop, was able to produce low grade HPW (4.5 m3 /h) from the wastewater discharged from wafer backgrinding and sawing processes with an overall recovery rate of 90%. The reclaimed water was directly supplied to other IC assembly processes such as electroplating and marking that consumed HPW of lower purity. The wastewater treatment system was integrated with the HPW system on site to achieve the best recycling practice. In comparison with the conventional recycling methodologies, process-to-process recycling offered a few advantages including: higher recovery rate; improved quality performance; minimized operating issues; and reduced return of investment (ROI) period. In comparison with the HPW production unit cost ranging from US$ 2.38–5.28 m−3 , the reclaimed wastewater unit cost was US$ 0.85 m−3 only. The system has been operated in a continuous mode for the past 2 years without any quality issue being raised. © 2003 Elsevier B.V. All rights reserved. Keywords: Semiconductor; Wastewater; Process-to-process recycling; Ultrafiltration; Ion-exchange

1. Introduction Manufacturing of semiconductor integrated circuits (IC) consists of a few steps in sequence—wafer backgrinding, sawing, die attach, wire bonding, encapsulation, electroplating, ∗

Corresponding author. Tel.: +65-6790-6237. E-mail address: [email protected] (D. Sun).

0921-3449/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.resconrec.2003.09.003

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trim and form, and marking. The assembly process consumes a huge amount of high-purity water (HPW) and resultantly generates as much wastewater as the input. Hence, recycling of wastewater offers tremendous benefits from both reduced cost of ownership and environmental liability. For the past few years, progressive water recycling targets were set year-by-year for the semiconductor industry. For example, the latest International Technology Roadmap for Semiconductor aimed at reducing HPW consumption rate from current 6–8 to 4–6 m3 per wafer by 2007 (ITRS, 2001). The industrial owners have taken radical initiative and great effort to achieve these targets (Chiarello et al., 2000; Kekre et al., 2001; Klusewitz and McVeigh, 2002; Martyak, 1999; Veltri et al., 2000; Wu, 2002). A few optimizing strategies that are commonly used in reducing the overall HPW demand are summarized below. 1.1. Enhancement of HPW productivity A semiconductor grade HPW purification process basically consists of pre-treatment, reverse osmosis (RO), and polishing stages. Based on this preliminary design, various system configurations can be developed for different applications (Franken, 1999). With the latest technologies, the overall water recovery rate of a HPW system is around 65–75% (Farmen and Tan, 2002), implying 25–35% of the feedwater is rejected during HPW production. Since a modern wafer fab consumes 3000 to 10,000 cubic meter HPW per day, considerable savings can be expected if the RO reject water is recycled. There are two methodologies to enhance the water recovery rate: selecting higher recovery RO membranes and recirculating RO reject either internally or externally (Byrne, 2002). However, enhancement of the water recovery rate is limited by the concentration of impurities that can be tolerated in the RO permeate. Direct reuse of RO reject water in industrial cooling systems has been also reported (You et al., 2001), which may require further chemical treatment in the cooling towers. 1.2. More silicon with less water This concept involves direct reduction of HPW input by utilizing alternative rinsing and cooling mechanisms such as plasma cleaning and air-drying in the manufacturing processes, which may lead to 80% reduction of water consumption (Allen and Hahn, 1999). Another minimization technology is to use ozone-based HPW to improve the cleaning efficiency (Mertens and Heyns, 2000). Although these novel technologies have been developed over quite a few years, they are still not widely implemented in the industry. 1.3. Recycling for non-process applications This recycling system normally includes a two-step process: chemical precipitation followed by ultrafiltration (UF) or nanofiltration. Whenever necessary, an RO unit should also be considered, depending on the compositions of the wastewater to be recycled. The reclaimed water can be used for non-process purpose such as cooling towers, scrubbers, and air-compressing systems. This matured technology is, nowadays, widely used in the semiconductor industry nowadays (Okazaki et al., 2000).

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1.4. Recycling as HPW system feed water After proper treatment, the reclaimed water can be blended with raw water and fed to a HPW system, which reduces the unit production cost and improves HPW quality at the point-of-use (Bonisolli and Lynn, 2001). This is mainly because that the reclaimed source water supplied to the HPW make-up system is monitored to control unwanted constituents to known values. However, a major issue associated with such a recycling concept is the accumulation of silica in the HPW system. As most of the semiconductor IC chips are made from silicon wafers, the wastewater is normally silica enriched. When the treated water is repeatedly reused in the same HPW system, it may lead to high silica contents in the final HPW product. It should be noted that the silica concentration must be kept at sub-ppb level with the latest IC assembly technologies (ASTM, 1999). Another potential problem is silica fouling of the RO membranes, which may eventually result in a reduced permeate flux and therefore increase the operating cost. It has been reported that silica is a very adherent foulant depositing on the RO membrane surface that can only be removed by strong cleaning solutions such as HF (Sheikholeslami et al., 2001). In this study, the authors developed a process-to-process recycling concept and applied this concept to the direct reclamation of high-purity water from the wastewater generated from wafer backgrinding and sawing processes. In comparison with the conventional recycling methodologies, this novel recycling system achieved a higher recycling rate at a lower cost with minimized quality risks and operating problems.

2. Material and methods 2.1. High-purity water requirements Semiconductor IC chips can be manufactured in “leaded” or “Ball Grid Array” (BGA) packages. When leaded and BGA packages are manufactured, a few processes consume high-purity water. Based on different grades of the purity required, HPW can be classified into four categories as shown in Table 1. Classes I and II are termed as high grade HPW, which is consumed by wafer related processes such as wafer bumping, etching, backgrinding, sawing, etc. Some other indirect users such as failure analysis (FA), reliability test (RT) and quality assurance (QA) consume very small amount of high grade HPW. Classes III and IV are termed as low grade HPW, which is consumed by IC related processes such as electroplating, singulation, marking, etc. The detailed quality specifications of Classes I–IV HPW are summarized in Table 1 based on the standard HPW guidelines (ASTM, 1999). 2.2. Backgrinding and sawing processes The wastewater collected from a backgrinding machine (Disco DGF 841, Japan) was used in this study. The backgrinding process consists of three steps, i.e., rough grinding, fine grinding and cooling. Blades with different grit size #600 and #2000 were used during rough grinding and fine grinding, respectively. After grinding, the wafer was transferred into DVCOLLC vacuum/coolant unit (Disco Corporation, Japan) for cooling. During this

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Table 1 Quality requirements of HPW at different grades of purity Parameter

UOM

Class I

Class II

Class III

Class IV

Resistivity TOC Bacteria Silica

M
cm ppma cfu/mlb ppm

18.0 0.01 0.1 0.003

16.0 0.5 1 0.02

12.0 1.0 10 0.5

0.5 2.0 100 1.0

Ions Chloride Fluoride Sulfate Ammonium Copper Potassium Sodium

ppm ppm ppm ppm ppm ppm ppm

0.001 0.001 0.001 0.001 0.001 0.001 0.001

0.03 – 0.01 – 0.01 0.01 0.01

a b

0.05 – 0.05 – 0.02 0.02 0.05

1.0 – 0.5 – 0.5 0.5 1.0

ppm: parts per million. cfu/ml: colony forming units per milliliter.

three-step process, high-purity water (Type E-2, ASTM, 1999) was continuously injected through a nozzle for flushing and cooling of the wafer. After mounting on the ring, the wafer was transferred to a sawing machine (Disco DAD 341, Japan) for dicing into the desired size by dual cut. Again high-purity water was used for cooling and flushing of the wafer during sawing. 2.3. Wastewater characterization An understanding of the nature of wastewaters is essential in the design and operation of collection, treatment, and disposal facilities and in the engineering management of effluent quality. In this work, the wastewater was characterized in terms of its physical and chemical compositions. Particle size and zeta potential: particle size of the waste streams was investigated by Mastersizer Microplus, Version 2.18 (Malvern, UK). The analyzer measures particle size distribution based on the principle of laser diffraction in the range of 0.05–550 ␮m. Zeta potential of the waste stream was investigated by Zetasizer 3000 (Malvern, UK). The analyzer measures the zeta potential of particles from 0.005 to 30 ␮m in ideal and complex matrices with high dissolved solids contents based on the method of laser doppler velocimetry (LDV). Cation and anion, silica monitoring: cations and anions were analyzed by ICP-MS (Agilent 7500A, USA) based on USEPA Method 6020. All samples were filtered with a 0.45 ␮m filter prior to the ICP-MS tests and all the experiments were carried out in a Class 100k cleanroom. An online silica analyzer (HACH Series 5000, USA) was installed to semi-continuously monitor silica concentration in water. The analyzer utilizes the heteropoly blue method (also called the molybdenum method) adapted from the Standard Methods (APHA, 1995) with an analysis range of 0–5000 ␮g/l. Total suspended solids (TSS) were measured using the method described in Standard Methods (APHA, 1995) Section 2540D. Sample size was varied to yield between 50 and

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Fig. 1. Schematic diagram of the simplified HPW recycling system.

100 mg residues. Conductivity and turbidity were measured by a conductivity meter (MYRON 758-09-420D, USA) and HACH Turbidimeter with AquaTrend interface (HACH 1720D, USA), respectively. Total organic carbon was measured by a TOC analyzer (Shimatzu 5000, Japan). 2.4. Ultrafiltration recycling system A pilot-scale wastewater recycling system was designed and constructed according to the schematic diagram as shown in Fig. 1. The system was operated in a continuous operating mode for 24 months. In this system, the wastewater produced from the wafer grinding and sawing process was collected in a tank then pumped into the ultrafiltration membrane modules for treatment. The on/off status of the feed pump was controlled by a level switch (four levels) installed in the collection tank. A high water level would trigger the pump to start transferring wastewater into the system until the low water level was reached. The system capacity was 4.5 m3 /h with an overall water recovery rate of 90% (5% as concentrate reject and 5% as backwash). Excessive waste DI water was diverted into the existing wastewater plant for treatment (high–high level triggered). A low-low level was also set in the collection tank to prevent the feed pump from running dry. The UF module contained NTU-3250-C4K hollow fiber membranes (Nitto Denko Corporation, Japan) with a nominal molecular weight cut-off (NMWCO) of 20,000. A pre-UF filter (50 ␮m) was also installed to prevent large particles from damaging the UF membranes. The polishing stage contained an ion-exchanger (IX) followed by an UV disinfecting unit. The ion-exchanger had an effective volume of approximately 0.1 m3 and was filled with IR120 and IR420 resins (Rohm & Hass, USA) mixed in the ratio of 2:3. The UV reactor (Aquafine MP-2-SL, USA) was installed at the point-of-use for bacterial control. The maximum flow capacity

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was 5.5 m3 /h. The electrically powered lamps (low-pressure) emitted UV irradiation at wavelengths of approximately 254 nm with a guaranteed minimum dosage of 30 mJ/cm2 after 4400 h operation. Operation and maintenance of the process were controlled and monitored by a programmable logical control system with a graphic touch screen (GLC). The GLC (Science Gate GP77, Japan) was able to receive 4–20 mA signals from the online monitoring instrument and control the operation accordingly. Historical operation and maintenance data could be stored in the GLC. 3. Results 3.1. Wastewater characteristics Fig. 2 shows a typical particle size distribution of the wastewater, ranging from 0.06 to 7.72 ␮m. It should be noted that PSD results did vary from one to another during the tests. This might have been due to that the samples used for the tests were mixed wastewater collected from both grinding and sawing processes at different times. Zeta potential measurement results revealed that the wastewater was negatively charged, around −33 mV (Fig. 3). Although it has been reported that zeta potential varies with pH (Tay et al., 2002), it might not be applicable in this study as the wastewater was always near neutral pH value. Other characteristics are summarized in Table 2. As shown in the feed water column (Table 2), the wastewater produced from backgrinding and sawing processes is ready for recycling. Although the wastewater contains high concentrations of fine particles (TSS as high as 600 mg/l) resulting in high turbidity, it has a low organic contents and concentration of ions. 3.2. Ultrafiltration performance As mentioned above, the UF modules were operated in a continuous mode except for the chemical cleaning, which was performed every 4 months. The system was monitored in term Accumulative % 100.00 80.00

%

60.00 40.00 20.00 0.00 0.01

0.10

1.00 Particle size ( m )

Fig. 2. Particle size distribution of the backgrinding and sawing wastewater.

10.00

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Fig. 3. Zeta potential of the backgrinding and sawing wastewater.

of incoming wastewater conductivity and outgoing permeate turbidity. Whenever influent conductivity was greater than 5 ␮s/cm or effluent turbidity was greater than 1 NTU, the wastewater was diverted to the existing chemical precipitation plant. Fig. 4 shows turbidity monitoring data of the feed water and UF permeate over a period of 6 months. The UF Table 2 Summary of wastewater and reclaimed HPW quality Parameter

UOM

Feed water

UF permeate

Point-of-use

Resistivity Turbidity TSS TOC Bacteria Silica

M
cm NTU mg/l ppm cfu/ml ppm

>0.9 110–490 130–600 <2.0 180–400 20–500

>0.9 <0.0033 0 0.2–0.4 <30 <70

>12.0 or >16.0 – – 0.5–0.9 <0.1 <0.5

Ions Chloride Fluoride Sulfate Ammonium Copper Potassium Sodium

ppm ppm ppm ppm ppm ppm ppm

<0.05 <0.01 <0.21 NDa <0.09 <0.04 <0.16

<0.05 <0.01 <0.21 ND <0.09 <0.04 <0.16

<0.03 <0.01 <0.10 ND <0.01 <0.04 <0.10

a

ND: not detected.

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Fig. 4. Turbidity trend before and after the UF module.

membranes showed a very high TSS removal efficiency at 99.998% on average. Although the feed water turbidity fluctuated from 110 to 490 NTU, the permeate turbidity varied within a narrow range, 0.0027–0.0033 NTU, implying a stable performance of the module. 3.3. Final water product UF membranes are ineffective in the removal of dissolved solids and ions, mainly due to their large pore size (Guo et al., 2001). This was evidenced by little change of the resistivity before and after the UF module (UF permeate column in Table 2). Hence, two sets of ion-exchanger (one operation, one standby) were included in the recycling system as a polishing stage. The resistivity of the final water product was monitored online with a low limit setting. Once the low limit was reached, the ion-exchanger was regenerated. The resistivity trend over a period of 6 months is plotted in Fig. 5. During the first 3 months (Phase I), the low resistivity limit was set at 16.0 M
cm to meet Class II HPW requirement. The ion-exchanger was able to maintain the resistivity above the limit for about 20 days. In the Phase II study, the low resistivity limit was set at 12.0 M
cm to meet Class III

Fig. 5. Resistivity trend of the UF permeate and final water product.

M. Wu et al. / Resources, Conservation and Recycling 41 (2004) 119–132 1.000

18

0.800

16

0.600 14 0.400 12

Silica (ppm)

Resistivity (Meg ohmcm)

127

0.200

10

0.000 1

3

5

7

9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 Day Resistivity

Silica

Fig. 6. Silica trend of the final water product.

HPW requirement. The ion-exchanger was able to maintain the resistivity above the limit for about 28 days. 3.4. Silica control Silica in HPW may cause particulate induced defects and watermarks on the wafer surface if its concentration is out of control. It is therefore important to ensure a very low silica concentration in the high-purity water reclaimed from the recycling system to avoid contamination. Fig. 6 shows monitoring records of the silica and resistivity trend over 40 days. Although the resistivity was above 16.0 M
cm for about 20 days, silica concentration failed to meet the Class II specification after 8 days (>0.02 ppm). At the time when the polishing ion-exchanger was regenerated on day 21, silica concentration in the reclaimed HPW was as high as 0.45 ppm while the resistivity was still above 16 M
cm, implying that resistivity change was totally insensitive to silica increase within that range. This observation indicated that regeneration of the ion-exchangers should be based on silica concentration rather than resistivity trend if the reclaimed water is reused as Class II HPW. Alternatively, a lower silica concentration of the final water product can be achieved by either reducing the IX surface-loading rate (SLR) or increasing the total volume of the resins. However, both actions may lead to a reduced unit HPW recovery rate and therefore higher operational cost. 3.5. Microorganism control Bacterial growth in a HPW system is another key factor that will have direct impacts on the quality of the final water product. High bacterial count in HPW may cause organic and particle-induced defects of semiconductor wafers. Although it has been reported that 99–99.9% of bacteria can be effectively removed by ultrafiltration (Linden et al., 2002), reproduction of bio-cells in the permeate tank and distribution pipes after the UF module should be taken into consideration. Das (2001) has summarized the two mechanisms of cell self-repair: (a) two-step photoreactivation process involving the formation of an enzyme–dimer complex and conversion of the enzyme–dimers into thymine monomers

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Fig. 7. Total bacterial count in feedwater, UF permeate, and storage tank.

resulting in a reversal of the photochemical damage; and (b) dark repair involving the excision of dimers that is similar to the repair of cell damage caused by non-photochemical agents. In this HPW recycling system, the UF membrane achieved 90% removal efficiency of total bacterial count (TBC). However, higher TBC was observed in the permeate tank due to the reproduction of bio-cells (Fig. 7). In order to meet the Class II grade of HPW, an ultraviolet (UV) disinfecting unit should be installed to control bacterial growth. As shown in Table 2 (point-of-use column), TBC at point-of-use was as low as 10 cfu/100 ml after the UV unit, implying that 2–3 log inactivation of the microorganisms was achieved. 3.6. Fouling potential One of the major problems in pressure driven membrane processes is the reduction of the flux to far below the theoretical capacity due to membrane fouling. In this case, the wastewater fed to the recycling system contained a high concentration of silica in soluble, colloidal and suspended forms and there was no pre-treatment prior to the UF membranes. A study was therefore performed to evaluate its fouling potential on the membranes. Boerlage (2001) developed a Modified Fouling Index (MFI, s/l2 ) for this purpose. The MFI is based on cake filtration theory, whereby, particles are retained on a membrane during filtration by a mechanism of surface deposition. The MFI is defined as the gradient of the linear region found in the plot of t/V versus V from the general cake filtration equation for constant pressure: t ηRm ηαCb = + V V PA 2 PA2 where V (l) is filtrate volume, t (s) the filtration time, α the specific resistance of the cake deposited and Cb (mg/l) the concentration of particles in the feedwater, Rm (kPa) the membrane resistance, which can be measured by clean water filtration (CWF), P (kPa) the transmembrane pressure gradient and η (Pa s) is the dynamic viscosity. At the beginning of filtration, the membrane resistance controls the rate of flow. With accumulation of the particles on the membrane surface, a proportion of the pressure drop is absorbed by the deposited layer, and eventually the Rm becomes negligible. Although Boerlage (2001) suggested that

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129

300 MFI-UF (s/l2)

250 200 150 100 50 0 0

50

100

150

200

250

300

350

400

Time (min) Transmembrane pressure:

0.1 MPa

Temperature: 22 0C

Fig. 8. Measurement of wastewater fouling potential on the membranes.

the MFI test should be conducted under standardized conditions: 13 kDa polyacrylonitrile membrane at 2 bar transmembrane pressure and 20 ◦ C temperature, the authors reckoned that the test should simulate the actual membrane process and the working environment so that the results could be directly applied in the full-scale system. In this study, a flat-sheet UF membrane (surface area = 60 cm2 ) with exactly the same properties as the pilot-scale membranes was used to determine the MFI. After 260 min stabilization, the MFI index reached 158 s/l2 , approximately, as shown in Fig. 8. In comparison with MFI indices (up to 900 s/l2 ) reported in the literature (Dillon et al., 2001), the wastewater showed less fouling tendency on the membranes used in this application.

4. Discussion The results obtained from this study indicate that the developed recycling system is able to reclaim high-purity water from the wastewater produced from semiconductor wafer backgrinding and sawing processes. Although the reclaimed HPW meets the Class II grade quality specifications as shown in Table 1 and can be reused back in the same processes, the quality risks might not be manageable. The two major concerns arise from high silica content and bacterial reactivation in the reclaimed HPW. It can be seen from Fig. 6 that the polishing mixed bed is able to maintain a low silica concentration (<0.02 ppm) for about 8–9 days only and its subsequent increase in concentration up to 0.45 ppm does not cause resistivity drop. Normally, only the resistivity is monitored online as an indicator of HPW quality. Hence, the unpredictable silica increase may cause potential contamination of the wafers being processed. Microorganism control is another issue. Although an UV disinfecting unit is able to control bacterial growth at the point-of-use, it causes an increase in TOC concentration (point-of-use column in Table 2). It should be noted that UV energy converts bacteria into pyrogens rather than removes them from the system (Hargy, 2001). High TOC content in HPW is also a quality threat to the semiconductor wafers. Based on these observations, reuse of the treated water in the backgrinding and sawing processes might not the best practice due to the associated quality risks. Since the

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Fig. 9. Schematic diagram of the integrated wastewater recycling system.

semiconductor IC assembly processes consumes HPW at different grades of purity, an integrated recycling plan should be able to minimize the risks (Fig. 9). As shown in the schematic diagram, the first level wastewater produced from the wafer backgrinding and sawing processes contains less impurities and therefore can be recovered by the process-to-process recycling system. The reclaimed HPW is supplied to an IC assembly process such as electroplating that requires a lower grade of HPW. The second level wastewater produced from the electroplating process that contains more impurities is recycled by a conventional process. The reclaimed water can be reused for non-process applications such as in cooling towers and scrubbers. This integrated configuration has been proven cost-effective with minimized quality risks. For the past 2 years, the system was in a continuous operation mode without any quality issue being reported. In comparison with the conventional recycling strategies, it could be argued that the UF permeate should be blended with the virgin HPW feedwater as a simple reuse option. However, it should be noted that process-to-process recycling offers a few advantages, which are not attainable by the conventional methods: • Higher recovery rate: The process-to-process recycling system is able to achieve an overall 90% water recovery rate. If the UF permeate is used as HPW feedwater, the RO unit will further reject 25–35% of the feedwater. • Improved quality performance: The integrated system forms a semi-closed HPW supply–treatment–reuse loop in between a few assembly processes to avoid accumulation of impurities in the HPW system. As mentioned above, repeated reuse of the reclaimed water as HPW feedwater may eventually cause increased silica concentration in HPW.

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• Minimized operating issues: The MFI test results clearly indicate that the silica-enriched wastewater has a low fouling tendency on the UF membranes. In comparison with the HPW purification process, a few components are excluded from this simplified recycling system such as multi-media filter, carbon filter, RO, TOC reducer, etc. • Reduced return of investment (ROI) period: It has been reported that the unit cost to produce semiconductor grade HPW is about US$ 2.38–5.28 m−3 (including US$ 0.66 m−3 as the raw water price) depending on the size of the HPW system and local energy and labor cost (Farmen and Tan, 2002; Klusewitz and McVeigh, 2002). The simplified recycling system is able to produce low grade HPW for the semiconductor IC assembly processes at a very low cost around US$ 0.85 m−3 only (detailed cost breakdown is not shown here), which reduces the ROI period.

5. Conclusion When the semiconductor industry aims at less HPW consumption rate per wafer, it is necessary to develop a more practicable and cost-effective recycling plan for the wafer fabrication and IC assembly processes with minimized quality risks. Sometimes, the industrial owner might not be fully convinced by the fact that the rewards associated with recycling outweigh the risks, they need a solid guarantee of zero quality risk simply because quality is everything to the latest semiconductor technologies. While the wafer size increases to 300 mm and the line width shrinks to 0.13 ␮m, any impurity spike in the HPW may cause breakdown of the whole manufacturing processes. A process-to-process recycling system was developed from this study. In comparison with the conventional recycling systems, this simplified process reclaims water that can be directly reused as low grade HPW. When the wastewater recycling system is integrated with the HPW system, it actually bypasses the RO unit (Fig. 9), which results in a very low recycling cost. The process-to-process water recycling system has been in a continuous operation mode for the past 2 years without any major operational and quality issues being observed.

Acknowledgements The authors would like to thank Dr. Frank Wilson for his valuable comments on this R&D study.

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