Clean-Room Controls and Optimization B. G. LIPTÁK

(1985, 1995, 2005)

SEMICONDUCTOR MANUFACTURING The process, or production area of a typical semiconductor 2 manufacturing laboratory is between 100,000 and 200,000 ft 2 (9,290 and 18,580 m ). The market value of the daily production from this relatively small area is well over $1 million. Plant productivity is increased if the following conditions are maintained in the process area: 1. 2. 3. 4.

No drafts (+0.02 in. H2O ±0.005 in. H2O, or 5 Pa ±1.3 Pa) No temperature gradients (72°F ±1°F, or 22°C ±0.6°C) No humidity gradients (35% RH ±3%) No airflow variations (60 air changes/hr ±5%)

Therefore, the goal is to continuously maintain these conditions through the accurate control of these parameters. The secondary goal is to conserve energy. The sensors and control loop configurations required to meet both of these goals will be described in this section.

Service core

Clean work aisle

Clean-rooms are used for experimental studies of a wide variety. Testing and analysis laboratories, used in the medical, military, and processing industries, are all designed to operate as clean-rooms. The control and optimization of clean-rooms, therefore, is an important subject to many industries. The traditional method of maintaining air cleanliness is by filtering. Standard HEPA filters are capable of removing up to 99.97% of impurities at 0.3 micrometers, while ULPA filters are efficient up to 99.9995% at 12 micrometers. In this section, besides filtering, other air cleanliness control methods will also be discussed. The production of semiconductors is one of several processes that require a clean-room environment. Optimization of the clean-room control system will drastically reduce both the energy cost of operation and the cost associated with the manufacturing of off-spec products, because plant productivity is maximized if drafts are eliminated in the clean work aisle. This is because drafts can stir up the dust in the workstation area, and if the dust settles on the product, production losses result.

Service core

INTRODUCTION

Clean work aisle

8.14

Process area (clean room)

Rows of work stations (production tools with exhaust hoods) Corridor (isolates clean-room area from offices)

FIG. 8.14a The configuration of a semiconductor manufacturing clean-room with rows of workstations and a corridor surrounding the whole area.

In order to prevent contamination through air infiltration from the surrounding spaces, the pressure in the clean-room must be higher than that of the rest of the building. As shown in Figure 8.14a, the clean-room is surrounded by a perimeter corridor. In order to protect against the in-leakage of dirty air from the corridor, the clean-room pressure is controlled above it. The clean-room is made up of rows of workstations. These rows are also referred to as zones. A typical semiconductor producing plant has approximately 200 workstations, which are also called subzones. Each subzone faces the clean work aisle, and behind it is the service core. Subzone Optimization As shown in Figure 8.14b, air is supplied to the subzone through filters located in the ceiling in front of the workstation. The total flow and the temperature of this air supply are both controlled. The evacuated exhaust air header is connected to the lower hood section of the workstation. It pulls in all the air and chemical fumes that are generated in the workstation and safely exhausts them into the atmosphere. In order to make sure that none of the toxic fumes will spill into the work aisle, clean air must enter the workstation at a velocity of about 75 fpm (0.38 m/s). The air that is not pulled in by the exhaust system enters the service core, where some of it is recirculated back into the workstation by a local fan. The rest of the air is returned from the service core by the return air header. A damper in this header (RAR-1) is modulated to control the pressure (DPS-1) in the clean work aisle. 1753

© 2006 by Béla Lipták

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Control and Optimization of Unit Operations

Cold air

Hot air CAD 2

Return air

Exhaust air

Mixing box

HAD 2

FC

FC 4

FO

TAD 4

HBD 6 RHT 3

RAR 1 M

DPS 1

Filter

M

Supply air

Corridor reference

ZSC 5

EAD 5

Filter Ceiling

Supply air

Door switch Fan Filter Operable sash

TC 2 Set at 72°F ± 1°F (22°C ± 0.6°C)

Hood

DPS-1 set at 0.02" H2O ± 0.005" H2O (5 Pa ± 1.5 Pa) FC-4 set to provide the sum of exhaust plus 60 air changes per hour RHT-3 is kept at 35% RH ± 3% RH

Floor

Service core

Work station

Clean work aisle

FIG. 8.14b Workstation controls maintain comfort while guaranteeing that all toxic chemical fumes are removed from the work area.

The filter shown in Figure 8.14b can be a standard HEPA filter, which is efficient up to 99.97% at 0.3 micrometers, or it can be an ULPA filter, which is efficient up to 99.9995% at 12 micrometers. Pressure Controls The pressure in the corridor is the reference for the clean-room pressure controller DPS-1 in Figure 8.14b. This controller is set to maintain a few hundredths of an inch of positive pressure relative to the corridor. The better the quality of building construction, the higher this set point can be, but even with the lowest quality buildings, a setting of approximately 0.02 in. H2O (5 Pa) can easily be maintained. At such near-atmospheric pressures, air behaves as if it were incompressible, so the pressure control loop shown in Figure 8.14b is both fast and stable. When the loop is energized, DPS-1 quickly rotates the return air control damper (RAR-1) until the preset differential is reached. At that point, the electric motor stops rotating the damper, and it stays at its last opening. This position will remain unaltered as long as the air balance in the area remains the same: return airflow = supply airflow − (exhaust airflow + pressurization loss) 8.14(1)

© 2006 by Béla Lipták

When this airflow balance is altered (for example, as a result of a change in exhaust airflow), it will cause a change in the space pressure, and DPS-1 will respond by modifying the opening of RAR-1. Elimination of Drafts Plant productivity is maximized if drafts are eliminated in the clean work aisle. Drafts would stir up the dust in this area, which in turn would settle on the product and cause production losses. In order to eliminate drafts, the pressure at each workstation must be controlled at the same value. Doing so eliminates the pressure differentials between stations and therefore prevents drafts. When a DPS-1 unit is provided to control the pressure at each workstation, the result is a uniform pressure profile throughout the clean-room. PDS-1 usually controls the pressure in the clean work aisle on the “process” side of the workstation. Yet, it is important to make sure that all points, including the service aisle, are under positive pressure. Because the local circulating fan within the workstation draws the air in from the service core and discharges it into the clean work aisle, the pressure in the service core will always be lower than that on the process side (see Figure 8.14b). On the other hand, it is possible for localized vacuum zones to evolve in the service core. This could cause contamination by allowing air infiltration into the service core. To prevent this from happening, several solutions have been proposed. One possibility is to have DPS-1 control the service core pressure. This is not recommended, because a draft-free process area can be guaranteed only if there are no pressure gradients on the work aisle side; this can only be achieved by locating the DPS-1 units on the work aisle side of the workstations. Another possibility is to leave the pressure controls on the work aisle side but raise the set point of DPS-1 until all the service cores in the plant are also at a positive pressure. This solution cannot be universally recommended either, because the quality of building construction might not be high enough to allow operation at elevated space pressures. The pressurization loss in badly sealed buildings can make it impossible to reach the elevated space pressure. Yet another solution is to install a second DPS controller, which would maintain the service core pressure by throttling the damper HBD-6. This solution will give satisfactory performance but will also increase the cost of the control system by the addition of a few hundred control loops (one per workstation). A more economical solution is shown in Figure 8.14b. Here, a hand-operated bypass damper (HBD-6) is manually set during initial balancing. This solution is reasonable for most applications, because the effect of the workstation fan is not a variable, and therefore a constant setting of HBD-6 should compensate for it. Temperature Controls The temperature at each workstation is controlled by a separate thermostat, TC-2 in Figure 8.14b.

8.14 Clean-Room Controls and Optimization

1755

Relative humidity transducer 2.3" (58 mm)

1.0" (25 mm)

Temperature transducer

Threaded probe body 3 (18 −20 thread)

Duct-mount

Shoulder O-ring Wall-mount

FIG. 8.14c Polystyrene surface conductivity humidity element. (Courtesy of General Eastern.)

This temperature control adjusts the ratio of cold air to hot air within the supply air mixing box to maintain the space temperature. Unfortunately, conventional thermostats cannot be used if the temperature gradients within the clean-room are to be kept within ±1°F of 72°F (−0.6°C of 22°C). Conventional thermostats cannot meet this requirement for either measurement accuracy or control quality. Even after individual calibration, it is unreasonable to expect an error of less than ±2 or 3°F (±1 or 1.7°C) in the overall loop performance if HVAC-quality thermostats are used. Part of the reason for this is the fact that the “offset” cannot be eliminated in plain proportional controllers such as thermostats (Section 7.9). Thus, in order for the thermostat to move its output from the mid-scale value (50–50% mixing of cold and hot air), an error in room temperature must first develop. This error is the permanent offset. The size of this offset error for TC-2 in Figure 8.14b for the condition of maximum cooling can be determined as follows: offset error =

spring range of CAD 2(thermostat gain)

8.14(2)

Assuming that CAD-2 has an 8–13 PSIG (55–90 kPa) spring (a spring range of 5 PSI, or 34.5 kPa) and that TC-2 is provided with a maximum gain of 2.5 psi/°F (31 kPa/°C), the offset error is 1°F (0.6°C). Under these conditions, the space temperature must permanently rise to 73°F (22.8°C) before CAD-2 can be fully opened. The offset error will increase as the spring range increases or as the thermostat gain is reduced. Sensor and set-point dial error are always additional to the offset error. Therefore, in order to control the clean-room temperature within ±1°F (±0.6°C), it is necessary to use an RTD-type or a semiconductor transistor-type temperature sensor and a proportional-plus-integral controller, which will eliminate the

© 2006 by Béla Lipták

offset error. This can be most economically accomplished through the use of microprocessor-based shared controllers that communicate with the sensors over a pair of telephone wires that serve as a data highway. Humidity Controls The relative humidity sensors are located in the return air stream (RHT-3). In order to keep the relative humidity in the clean-room within 35% RH ±3% RH, it is important to select a sensor with an error lower than ±3% RH. The repeatability of most relative humidity sensors (see Section 8.32 in the first volume of this handbook) is approximately ±1% RH. Figure 8.14c illustrates the polystyrene surface conductivity-type humidity element. These units are available for duct mounting and can be used for clean-room control applications if they are individually calibrated for operation at or around 35% RH. Without such individual calibration, they will not perform satisfactorily, because their off-the-shelf inaccuracy, or error, is approximately ±5% RH. The controller associated with RHT-3 is not shown in Figure 8.14b, because relative humidity is not controlled at the workstation (subzone) level but at the zone level (as mentioned earlier, a zone is a row of workstations). The control action is based on the relative humidity reading in the combined return air stream from all workstations within that zone. Flow Controls The proper selection of the mixing box serving each workstation is of critical importance. Each mixing box serves the dual purpose of providing accurate control of the total air supply to the subzone and of modulating the ratio of “cold” and “hot” air to satisfy requirements of the space thermostat TC-2. The total air supply flow to the subzone should equal 60 air changes per hour plus the exhaust rate from subzone. This total air supply rate must be controlled within ±5% of actual

1756

Control and Optimization of Unit Operations

flow by FC-4, over a flow range of 3:1. The rangeability of 3:1 is required because as processes change, their associated exhaust requirements will also change substantially. FC-4 in Figure 8.14b can be set manually, but this setting must change every time a new tool is added to or removed from the subzone. The setting of FC-4 must be done by individual in-place calibration against a portable hot wire anemometer reference (Sections 2.2 and 2.13 in Volume 1). Settings based on the adjustments of the mixing box alone (without an anemometer reference) will not provide the required accuracy. Some of the mixing box designs available on the market are not acceptable for this application. Such unacceptable designs include the following: •

• •

“Pressure-dependent” designs, in which the total flow will change with air supply pressure. Only “pressureindependent” designs can be considered, because both the cold and the hot air supply pressures to the mixing box will vary over some controlled minimum. Low rangeability designs. A 3:1 rangeability with an accuracy of ±5% of actual flow is required. Selector or override designs. In these designs either flow or temperature is controlling on a selective basis. Such override designs will periodically disregard the requirements of TC-2 and will thus induce upsets, cycling, or both.

If the mixing box is selected to meet the aforementioned criteria, both airflow and space temperature can be accurately controlled. Optimization of the Zones Each row of workstations shown in Figure 8.14a is called a zone, and each zone is served by a cold deck (CD), a hot deck (HD), and a return air (RA) subheader. These subheaders are frequently referred to as fingers. The control devices serving the individual workstation (subzones) will be able to perform their assigned control tasks only if the “zone finger” conditions make it possible for them to do so. For example, RAR-1 in Figure 8.14b will be able to control the subzone pressure as long as the ∆P across the damper is high enough to remove all the return air without requiring the damper to open fully. As long as the dampers are throttling (neither fully open nor completely closed), DPS-1 in Figure 8.14b is in control. PIC-7 in Figure 8.14d is provided to control the vacuum in the RA finger and thereby to maintain the required ∆P across RAR-1. PIC-7 is a nonlinear controller with a fairly wide neutral band. This protects the CD finger temperature (TIC-6 set point) from being changed until a sustained and substantial change takes place in the detected RA pressure. Similarly, the mixing box in Figure 8.14b will be able to control subzone supply flow and temperature as long as its dampers are not forced to take up extreme positions. Once a

© 2006 by Béla Lipták

CD finger

RA finger

RHT 3

FC RHC

Control signal to Figure 8.14 f

HWS RHCV

B

FB FB RHIC SP = 3 35% RH Note # 5

>

HD finger

A

TIC 6

SP L

Note # 3 HIC 8

H All other sub-zones within zone to mixing box TC 2

FB Int R/A DPC 5 D/A NL PIC 7 Note #4 FB

> : High selector

SP = 100%

Cold deck (CD) finger Return air (RA) finger > : High limit

INT Note R/A # 2 DPC FB 4

Note #1

To mixing SP = 100% box Typical sub-zone (see figue 8.14b)

TC 2

Hot deck (HD) finger

< : Low selector < : Low limit DPC : Damper position controller NL : Nonlinear controller D/A : Direct acting controller FB : External feedback signal to R/A : Reverse acting controller eliminate reset windup INT: intergral only controller, set for ten times the integral time in PIC-7 or RHIC-3

FIG. 8.14d Typical zone control system that is under envelope control. Note 1: This is the opening of the most-open hot air damper. HAD-2 (shown in Figure 8.14b) which is fully open when the TC-2 output is minimum. Note 2: This is the opening of the most-open cold air damper. CAD-2 (shown in Figure 8.14b) is fully open when the TC-2 output is maximum. Note 3: HIC-8 sets the maximum limit for the CD temperature at approximately 70°F (21°C). Note 4: As vacuum increases (and pressure decreases), the output of PIC-7 also increases. Note 5: The controller action is D/A in summer, R/A in winter.

damper is fully open, the associated control loop is out of control. Therefore, the purpose of the damper position controllers (DPCs) in Figure 8.14d is to prevent the corresponding dampers from having to open fully. Lastly, the relative humidity in the return air must also be controlled within acceptable limits. Therefore, at the zone level there are five controlled, or limit variables but only one manipulated variable:

Limit or Controlled Variables RA pressure RA relative humidity Max. CAD opening Max. HAD opening Max. TIC-6 set point

Manipulated Variable TIC-6 set point

8.14 Clean-Room Controls and Optimization

Constraint #1 Cold deck finger temperature is at its maximum limit of about 70°F (21°C)

Constraint # 3 Return air excess is signaled by PIC-7 (RA finger pressure high)

Constraint # 2 Most open cold air damper (CAD-2) of all mixing boxes within this zone is fully open

increased, more CD air will be needed to accomplish the required cooling. Increasing the ratio of humidity-controlled CD air in the zone supply (HD humidity is uncontrolled) also brings the zone closer to the desired 35% RH set point. In this envelope control system, the following conditions will cause an increase in the TIC-6 set point: •

Response is to decrease cold deck flow by decreasing TIC-6 set point Control envelope for determining cold deck supply temperature

Controlled variable

TIC-6 set point

Response is to increase cold deck flow by increasing TIC-6 set point

Constraint #5 Most open hot air damper (HAD-2) of all mixing boxes within this zone is fully open Constraint #6 Constraint #4 Return air shortage is Return air humidity error being signaled by PIC-7 in RA finger is too high (RA finger pressure low)

FIG. 8.14e For optimized control of the cold deck temperature, a constraint envelope-based control system can be used.

•

•

Envelope Optimization Whenever the number of control variables exceeds the number of available manipulated variables, it is necessary to apply multivariable envelope control. This means that the available manipulated variable (TIC-6 set point) is not assigned to serve a single task but is selectively controlled to keep many variables within acceptable limits—within the “control envelope” shown in Figure 8.14e. By adjusting the set point of TIC-6, it is possible to change the cooling capacity represented by each unit of CD air. Because the same cooling can be accomplished by using less air at lower temperatures or by using more air at higher temperatures, the overall material balance can be maintained by manipulating the set point of the heat balance controls. Return air humidity can similarly be affected by modulating the TIC-6 set point, because when this set point is

© 2006 by Béla Lipták

Return air shortage detected by a drop in the RA finger pressure (increase in the detected vacuum) measured by PIC-7. An increase in the TIC-6 set point will increase the CD demand (opens all CAD-2 dampers in Figure 8.14b), which in turn lowers the HD and, therefore, the RA demand. Hot air damper in mixing box (HAD-2 in Figure 8.14b) is near to being fully open. This, too, necessitates the aforementioned action to reduce HD demand. RA humidity does not match the RHIC-3 set point. This condition also necessitates an increase in the proportion of the CD air in the zone supply. Because the moisture content of the CD air is controlled, an increase in its proportion in the total supply will help control the RH in the zone.

On the other hand, the following conditions will require a decrease in the TIC-6 set point: •

• •

Figure 8.14d shows the control loop configuration required to accomplish the aforementioned goals. It should be noted that dynamic lead/lag elements are not shown and that this loop can be implemented in either hardware or software.

1757

Return air excess detected by a rise in RA finger pressure (drop in the detected vacuum) measured by PIC-7. Cold air damper in mixing box (CAD-2 in Figure 8.14b) is fully open. CD finger temperature exceeds 70°F (21°C). This limit is needed to keep the CD always cooler than the HD.

Plantwide Optimization In conventionally controlled semiconductor plants, both the temperature and the humidity of the main CD supply air are fixed. This can severely restrict the performance of such systems, because as soon as a damper or a valve is fully open (or closed), the conventional system is out of control. The optimized control system described here does not suffer from such limitations, because it automatically adjusts both the main CD supply temperature and the humidity so as to follow the load smoothly and continuously, never allowing the valves or dampers to lose control by fully opening or closing. The net result is not only increased productivity but also reduced operating costs. A semiconductor production plant might consist of two dozen zones. Each of the zones can be controlled as shown in Figure 8.14d. The total load represented by all the zones is followed by the controls shown in Figure 8.14f. The overall control system is hierarchical in its structure: The subzone controls in Figure 8.14b are assisted by the zone controls in

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Control and Optimization of Unit Operations

Setpoint to HWS temperature (TIC-11)

FB2

D/A Least-open RHCV VPC 5 Y = SP

%x

VPA 5

100



80 60 40 20

SP D/A AMC TIC 1 PID

−5

FB2

0

5

10 15

f(x)

SP %Y

All other zones

Δ

Y Set at 95% x