ELSEVIER Desalination 161 (2004) 223-233

Development of a practical model for capacity evaluation of ultrapure water systems

Ming Wu”, Darren Sunb*, Joo Hwa Tayb “ST Assembly Test Services Ltd, 5 Yishun Street 23, Singapore 768442

‘Environmental Engineering and Research Center, School of Civil and Environmental Engineering, Nanyang Technological University, BLK Nl #IC-89 Nanyang Ave, Singapore 639798

Tel. +65 6790-6273; Fax +6.5 6791-0676; email: DDSun@ntu.edu.sg

Received 3 October 2002; accepted 25 August 2003

Abstract Modem ultrapure water (UPW) systems designed for the semiconductor manufacturing industry are very complex

nowadays, especially when wastewater recycling is part of the configuration. In this paper, we developed a practical model for system evaluation by correlating the actual UPW consumption rate, rl, with the velocity of water level changes (v) in the storage tank. It was found that the UPW consumption rate is a sole function of the velocity of water level changes in the water storage tank that could be described by the following equation: q = 1 - Kv. The model was then applied to a standardized UPW system to examine its accuracy and found good agreement between the estimated and actual results. When process-to-process recycling is included in the model, the equation is still valid; however, the system-related coeffkient K decreases with increasing recycling capacity. By properly estimating the engineering parameters of an UPW system, a generic q-v graph can also be plotted based on this equation. This rl-v graph provides accurate UPW flowrate information as the variation of water levels in the DI storage tank is monitored online, which can be easily used for capacity evaluation of an UPW system operating with or without wastewater recycling at the outset of any UPW expansion project.

K~JGVO&S: Semiconductor; IC; Assembly; UPW; Capacity; 7)-v model; Recycling

1. Introduction for the next 10 to 15 years, after which the

The life-cycle of semiconductor integrated circuits (IC) is typically 18 months and the semi- conductor industry will move forward at this pace

industry treads perilously close to physical device limitations that appear insurmountable [ 11. As the wafer size increases to 300 mm and the line width shrinks to 0. I3 pm, there are two emerging requirements for ultrapure water (UPW), i.e., the

*Corresponding author. concentration of impurities is limited to the

001 l-9164/04/$- See front matter 0 2004 Elsevier Science B.V. All rights reserved PII:SOOll-9164(03)00703-3

224 M. Wu et al. /Desalination I61 (2004) 223-233

sub-ppb level whilst total UPW consumption is doubled [2]. In order to meet these new quality and quantity requirements, it is essential to upgrade the existing UPW system and to expand its capacity frequently. Franken [3] summed up a few typical UPW designs commonly used in the semiconductor industry. These designs were then further optimized by other researchers to include wastewater recycling and reuse [4-6] .Nowadays, it is generally agreed that an optimized UPW designed for IC chip manufacturing should have the following features: l a reliable RO/DI system to produce con-

tinuous UPW products at low-grade purity from Types E-4 to E-3 [7]

to introduce 300 mm and wafer bumping tech- nologies into its IC assembly processes and it faced these questions: what is the peak and average loading of the UPW system? Is the existing system able to cope with additional UPW demand from the new processes? If not, how much additional capacity is needed? Actually these are the frequently asked questions by the industry. In this paper, we developed a simple and reliable model to evaluate the capa- city of an existing UPW system with or without wastewater recycling. The UPW system in STATS was used to illustrate the model.

l several segregated polishing loops for the 2. Theory various end users at different grades of purity ranging from Types E-2 to E- 1.2 [7] and

l wastewater recycling as an indispensable part of the UPW system.

For a given UPW system, the actual con- sumption flowrate (Q,) at a time can be calcu- lated by subtracting the UPW return flowrate (QJ from the supply flowrate (es):

Although this system has been reported to be cost effective with minimized environmental impact by instituting wastewater recycling [8], the design is rather complex (Fig. 1). Prior to any expansion, it is difficult to determine the actual utilization rate of an UPW system with such a configuration. Sometimes, analysis of the waste- water recycling network itself may require specialized software and skills that may not be available to practicing engineers [9]. Because the system cannot be evaluated by a practicable and reliable means at an acceptable level of accuracy, designers tend to oversize the system to ensure that it can accommodate the expected UPW demand with a safe buffer coefficient. An over- sized UPW results in increased capital invest- ment, up to 200%, as well as increased operating and maintenance cost after the commissioning [lOI.

dQ, = dQ,-dQ, (1)

The UPW supply and return flow rates vary with time, depending on the actual number of end users. When more machines are in operation, the supply flowrate will increase while the return flowrate reduces. For an UPW system with n end users, the minimum RO permeate flowrate (Q,) shall be greater than or, at least, equal to the total UPW demands:

Qp 2 I$ (dQsi-dQti) (2)

ST Assembly Test Services Ltd (STATS), an IC assembly and test service provider located in Singapore, adopted the above-mentioned design for its UPW system. In 200 1, the company started

In order to estimate minimum demands of the RO permeate using the above formulas with acceptable accuracy, we need to monitor flowrate online so that the peak and average loading can be identified. For n sub-systems, at least 2n flow meters with output signals should be used. This procedure will be tedious and expensive with inevitable errors. It should be noted that the UPW

M. Wu et al. /Desalination 161 (2004) 223-233 225

HH H

Ve EL M

kL

Rae Water Tank RIAlIUntts : MV~terTank r.....,

piiq . . . ..--I y-$-J+

Fig. 1. Optimized UPW system with process-to-process recycling.

return flow might not be fully pressurized in the pipe, so we might not be able to measure the return flowrate accurately using an inline flow meter.

We can also calculate the required RO capa- city by summing up the specified flowrate of an individual machine in the manual, then multiply by the total number of that type of machines, mi. For a few different types of machines, the following equation will give the total flowrate:

Qt = 0 (Qs, mI+Qs,m,+...+Qpn) ” (3)

An overall diversion factor(u) is included in the equation as the machines may not be all operating at full capacity at any time. Although a diversion factor of 0.7-0.8 is commonly used in

the semiconductor industry, it can vary from system to system. To avoid an undersized UPW design that is not able to provide a continuous flow, u is always set at the high end. This will again lead to an oversized UPW design in most cases.

3. Materials and methods

The UPW system in STATS, as schematized in Fig. 1, was used for this study. The system was properly segregated with or without point-of-use (POU) polishing to produce UPW at different grades of purity to satisfy the different process requirements. The capacity of the RO units was 98.15 m3/h (permeate flow). In order to minimize bacteria growth in the piping system, the distri- bution piping was looped back either to the DI

226 M. Wu et al. /Desalination 161 (2004) 223-233

storage tank or the raw water tank so that a continuous flow through the piping could be maintained. One of the return lines was diverted to the raw water tank because carbon dioxide (CO,) was dosed in the supply line to reduce DI water resistivity. High DI water resistivity may cause electrostatic discharge on the wafers in some of the assembly processes. Waste DI water recycling systems were included in the design. Part of the recycled water (14.72 m3/h) was pure enough for direct DI water supply. The total volume of the DI water storage tank was 107.97 m3 with a bottom area (S) of 19.63 m* and total height (H,) of 5.50 m. The variation ofwater levels in the tank was monitored by a SCADA system. Operation of the RO units and pumps was interlocked with the level controls in the storage tank. There were five levels set in the DI water tank to control the system. When the storage tank emptied to the middle level (M), the RO units automatically started up and kept on running over a period of time (from 0 to f,, called the RO-On cycle). When the storage tank reached the high level (H), the RO units auto- matically shut off, after which the DI tank would continually supply UPW to the end users over a period of time (from t, to t,, called the RO-Off cycle) until the RO units started up again. These were the two UPW supply cycles in the system. The DI water stored in the tank between the middle (M) and high (H) water levels referred to the effective water storage capacity (V, in Fig. 1). The high-high (HH) and low (L) water levels were the alarm limits while the low-low (LL) level was the pump protection limit. When the LL level was triggered, the DI water pumps would stop.

4. Results and discussion

4.1. RO and DI tank capacity

In the UPW system as shown in Fig. 1, the general mass balance of UPW will exist:

Assuming that water leakage/evaporation effect can be ignored and there is no internal recycling (water recycling will be discussed in a later stage), we can rewrite the above equilibrium in the following differential form:

e,dt-2 (Qsi-Q,)dt = dy i=l

(5)

During the RO-On cycle (from time 0 to f,), the total volume of RO permeate (V,) is equal to the volume of UPW consumed plus the quantity of DI water stored in the tank:

(6)

Assuming the effective volume of the DI water storage tank (V,) is a constant, which is normally true, and let dye = Sdh where S is the cross- sectional area of the tank, the following equation is derived from Eq. (5):

2 (Qsi - Q,)dt = Q,dt -Sdh i=l

(7)

If both sides of Eq. (7) are divided by dt, the following equation can be obtained:

2 (Qsi-Qd)= Qp-S$ i=i

(8)

In Eq. (8), dhldt refers to the velocity (v) of water level changes in the DI tank. We can split this equation into two for the RO-On and RO- Off cycles, respectively, with the following con- ditions: during the RO-On cycle from time 0 to t,, Qp is a constant; during the RO-Off cycle from time t, to tl, Qp = 0.

M. Wu et al. /Desalination 161 (2004) 223-233 227

During the RO-On cycle, the water level is expected to rise, v = dhldt > 0. During the RO- Off cycle, the water level is expected to drop: v = dhldt -C 0.

4.2. 7-v graph

The overall UPW utilization rate q is defined as the ratio of actual UPW flowrate,

2 (Q,,-Q ,) n , over the RO permeate flowrate, Q,,. i=l

When 0 I t < t,, Eq. (9.1) can be rearranged to:

(10) As mentioned above, S and Qp are constants in a given UPW system, so we let

S/Q, = K (11) where K (h/m) is termed as the system-related coefficient. Therefore, we can simplify Eq. (10) to:

rl=l-Kv (12)

The equation indicates that at any time during the RO-On cycle, the overall UPW consumption rate is a sole function of the velocity of water level changes in the DI storage tank. This rl-v relationship will always be valid in any design similar to the UPW system shown in this study. Once the RO capacity and DI tank volume are known, the overall UPW utilization rate rl can be plotted vs. v, called the q-v graph, which pro- vides accurate UPW flowrate information as the variation of water levels in the DI storage tank is monitored online. Hence v can be easily deter- mined from the data collected with sufficient accuracy.

In order to plot a generic q-v graph by using Eq. (12), we should estimate the range of K values. Although daily UPW consumption of

0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 6.00 9.00 10.00

v (mill)

Fig. 2. Generic q-v graph.

228 M. Wu et al. /Desalination 161 (2004) 223-233

different semiconductor companies may vary vastly from tens to over 10,000 m3/d, the Kvalue varies within a narrow range, typically from 0.05 to 1.00 (h/m) only. For any UPW system equipped with a DI water storage tank, the total volume (V,) and total height (H,) of the tank must be known. Because S = K/H,, we can rewrite Eq. (10) as:

v, 1

q=l-epHrv (13)

In the above equation, V, /QP refers to the ratio of buffer capacity over RO capacity. Normally, this ratio ranges from 0.5-l .O, which implies that RO units will automatically start up at 0.5-1.0 hour intervals. A too long RO-Off cycle (more than an hour) will induce biological fouling of the membranes whereas a too short RO-Off cycle (less than half an hour) will shorten the life span of the RO feed pumps (too frequent start-up). Another factor that will affect the range of K values is H,. Normally the total height of a storage tank is designed from 1 to 10 m. Once we identify the ranges of these two factors, K can be estimated in between 0.05 to 1 .O.

The q-v graph is plotted based on Eq. (12) (Fig. 2). At any time when v is determined by analyzing water level changes in the DI storage tank, the actual UPW utilization rate at that moment can be identified instantaneously using the graph. An example is given in the later part of this paper to illustrate how to apply the graph to an UPW system.

4.2. Wastewater recycling capacity

Recycling plays an important role in water conservation in the semiconductor industry. Total recycling rates ranging from 40% to 80% of the UPW consumed have been reported recently [ 11,121. Although we can use the recycled water for non-process purposes such as cooling tower

and scrubber make-up [6], or blend with virgin UPW feedwater streams [ 131, the best practice is still process-to-process recycling, which directly reclaims the wastewater as UPW for the IC assembly processes [8]. Full-scale process-to- process wastewater recycling systems have been widely implemented in the semiconductor indus- try in the past few years [ 141. Hence, if the reclaimed water is directly used for the IC manufacturing processes that by-passes the RO units, we should take the wastewater recycling capacity into account. However, if the reclaimed water is used in non-process applications or blended with RO feedwater streams, it will not affect the UPW capacity.

We can take internal process-to-process recycling as “generation within system” as shown in Eq. (4). Assuming 0 percent used UPW can be directly recycled for production use, which by- passes the RO units (as shown in Fig. I), the increased RO permeate Qp’ can be expressed as:

Qp’ =(l+6)Qp (14)

Substitute Qp’ into Eq. (11) and combine it with Eq. (12), and the following equation is obtained:

rl=l-K’v (15)

where K’ is the modified system-related coeffi- cient and IS = S/Q,‘.

From Eqs. (14) and (15), we can conclude that the rl-v relationship is still valid when a process- to-process recycling system is integrated into the existing UPW system configuration; however, the system-related coefficient decreases with increas- ing recycling capacity. At the 8 percent recycling rate, K’ = Kl( 1 + 0).

4.3. A case study The design and operation parameters of the

UPW system with STATS are summarized in Table 1. The existing system was designed based

Table 1

M. Wu et al. /Desalination 161 (2004) 223-233 229

Design and operation parameters of the existing UPW system

Parameters Remarks

RO capacity, m’/h DI storage tank, m3

Total volume

Effective volume Waste DI recycling capacity, m3/h

System-related constant K, h/m Modified system-related constant K’, h/m Total UPW demand, m3/h

Process 1 (Type E1.2)

98.15

107.97

64.78 14.72

0.20 0.17

49.17

Process 2 (Type El) 27.10 Process 3 (Type E2) 6.55 Process 4 (Type E4) 14.14

Sub-total 96.96 Utilization rate, % 98.79 Utilization rate, % 85.90 Process 5 (new process) 30.17 Total 127.13 Future utilization rate, % 129.53 Future utilization rate, % 112.63

on the total UPW demand as specified in the manuals of process equipment and was almost fully utilized (98.79%). After a wastewater recycling system was installed, the overall utili- zation rate was reduced to 85.90%. With the expected UPW demand from the new processes, the system would be short of about 13% in capacity. From these results, it seemed that new RO units should be purchased to expand the system in order to cope with the additional UPW demands.

We applied the q-v graph developed from this study to the system for a more accurate survey of the actual UPW utilization rate. Water level changes in the DI water storage tank were continually recorded at 6-min intervals by the SACADA system. Records taken over a 12-h

-

Permeate flowrate at 75% recovery

Bottom area = 19.63 m2, total height = 5.50 m

M and H water level settings: 1.0 and 4.3 m Direct us as RO/DI permeate

Computed from Eq. ( 10) Computed from Eq. (13)

Sum of UPW consumption as specified in the manuals for each process, diversion factor (0.75-0.95) included

Sub-total of the existing system Without the waste DI recycling system With the waste DI recycling system Type El .2 Including existing and future demand Without the waste DI recycling system With the waste DI recycling system

period should be sufficient to chart the actual flow pattern since the company only operated two shifts per day. The recycling system was switched off for the first 6 h in order to compare the utilization rates with and without recycling.

Fig. 3 is a printout from the SCADA system showing the variation of water levels during a 12-h shift. Since half-hourly readings were indicated in the graph, we could easily compute v using v = dhldt. During the RO-On cycle (v>O), rl was determined from the rl-v graph on a half- hourly basis (K = 0.20 and 0.17 when the system was operated with and without recycling, respectively). During the RO-Off cycle, the half- hourly UPW consumption flowrate could be directly calculated using Eq. (9.2). In Fig. 4, flowrate and system utilization rates were plotted

230 M. Wu et al. /Desalination 161 (2004) 223-233

History

4.30 Without Recycling 4 m

A 4.09 A 1 A -PI! A 39F===d

Fig. 3. Typical h-t graph printed out from the online monitoring system.

UPW System UtilizAon Rate and Flowrate

Fig. 4. Estimated flowrate and system utilization rate on a half-hourly basis.

against time. It can be seen from Fig. 4 that the peak loading was about 76.00 m3/h while the average loading was about 60.00 m3/h on that shift. The actual utilization rates varied from 43.00% to 67.00% only, which is much lower than the estimated utilization rates as listed in Table 1. One might argue that the average utili- zation rate of the system could also be estimated from the RO-Offhours over the overall operating hours (Fig. 3). However, it should be noted that the method developed in this study provides a more accurate estimation within the RO-On and RO-Off cycles. For example, if we just simply use the RO-On and RO-Off hours over the

operating hours in the analysis, the peak loading appeared in Fig. 4 will be leveraged due to the “averaging” effect. Actually, the shorter the time selected for study, the better accuracy achieved. In this case, the data at 6-min intervals could be used to determine the real-time flow pattern per shift.

In order to confirm the flow pattern per shift, we studied the monitoring records of water level changes in the DI storage tank for ten shifts using the same method. Statistical results showed a very constant flow pattern per shift (Table 2). We therefore concluded that the maximum utilization rate of the system was about 70% only. The

Table 2

M. Wu et al. / Desalination I61 (2004) 223-233 231

Statistical analysis of system utilization rate of ten shifts

Shift Flowrate, m3/h

Peak Average

1 75.65 60.50 2 79.48 64.15 3 75.32 50.16 4 72.77 60.87 5 74.06 62.96 6 69.68 60.55 7 67.36 60.20 8 73.18 61.48 9 74.93 65.86 10 71.14 56.33 Variationb 73.36 zk 2.77 61.21 f 2.16

‘Utilization rate of UPW system with recycling. bVariation range computed at 99% confidence level.

system should be able to cope with the additional UPW demands from the new processes, which accounted for about 28% of the existing system capacity. Based on these results, we advised the company not to expand the UPW system by adding in new RO units. With the DI water storage tank as buffer capacity, the existing RO/DI units should be able to handle the peak loading. However, the polishing loop producing Type E-l.2 UPW should be expanded because the new processes required UPW at that grade. After a minor upgrading of the polishing loop in 200 1, the system has been supplying UPW to the production lines in the company for the past year without any shortage in capacity.

5. Conclusions

As the semiconductor industry strives for “low-cost wafer production” to be competitive in the market, maintain margins and market share, it is essential to reduce capital costs as well as operating and maintenance costs. An optimized UPW system design will definitely help achieve

Utilization ratea 9 % -

Peak Average

67.02 53.60 70.42 56.84 66.73 52.41 64.47 53.93 65.62 55.78 61.72 53.65 59.68 53.34 64.84 54.47 66.39 58.34 63.03 49.91 64.99 f 2.46 54.23 f 1.92

this goal. Prior to any upgrading and expansion work, the utilization rate of an UPW system should be carefully examined by studying real- time flowrate such as peak hourly consumption and daily average loading. Inaccurate flow infor- mation may result in increased capital investment and land wastage in the initial stages as well as increased operating and maintenance costs later on. A “simplified” methodology with acceptable accuracy for evaluating real-time utilization rate of an UPW system should be available to practicing engineers in the industry. The generic TJ-v graph developed from this study can be easily used for the analysis. For a given UPW system, the utilization rate at a time can be inherently correlated to velocity of water level changes in the DI water storage tank using q = 1 -Kv, where K is a system-related coefficient that can be determined from the ratio of the cross-sectional area of the storage tank over RO permeate flowrate. Although instantaneous velocity can be determined from the h-t graph, the average velocity at a 30-min interval is sufficient for estimation of the peak and average flowrate on a

232 M. Wu et al. /Desalination I61 (2004) 223-233

half-hourly basis. When a semiconductor com- pany maintains stable production, the UPW consumption pattern will not vary much from shift to shift. This can be verified by repeating the analysis for a few shifts. When direct process-to-process wastewater recycling is included in an UPW system, the q-v relationship is still valid. However, the values of system- related coefficient K will decrease, implying an increased RO/DI capacity. From the example given in this study, it can be seen that accurate measurement of the real-time UPW consumption rate is critical to an optimized UPW design.

6. Symbols

dh

H

HH

H,

K K’

L

LL

M

m

n

QO

QfJ QP'

QF

-

-

-

-

- -

-

-

-

-

-

-

- -

-

Water level change in the DI water storage tank, m High level setting in the DI water storage tank, m High-high level setting in the DI water storage tank, m Total height of the DI water storage tank, m System-related coefficient, h/m Modified system-related coefficient with recycling, h/m Low level setting in the DI water storage tank, m Low-low level setting in the DI water storage tank, m Middle level setting in the DI water storage tank, m Number of machines consuming UPW Number of sub-systems at point-of- use Actual UPW consumption flowrate, m3/h RO permeate flowrate, m3/h RO permeate flowrate with recy- cling, m3/h UPW return flowrate, m3/h

Qs - s -

4 - 4 - v -

ve -

VP -

v, -

Greek

(3 - 0 -

rl -

UPW supply flowrate, m3/h Bottom area of the DI water storage tank, m* Moment when the RO units start up Moment when the RO units shut off Velocity of water change in the DI water storage tank, m/h Effective storage volume in between M and H levels in the DI water tank, m3 Total volume of RO permeate during the RO-On cycle, m3 Total volume of the DI water storage tank, m3

Diversion factor Ratio of recycling capacity over the UPW capacity, % UPW system utilization rate, %

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PI

PI

[31

[41

[51

[61

[71

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