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Creating a low-cost, ultra-clean environment
Precision Engineering Journal of the International Societies for Precision Engineering and Nanotechnology 26 (2002) 122–127
Technical Note
Creating a low-cost, ultra-clean environment V. Anghel, D.G. Chetwynd* Centre for Nanotechnology and Microengineering, University of Warwick, Coventry, CV 4 7AL, UK Received 9 January 2001; accepted 26 January 2001
Abstract This paper describes a way to create a cleanroom environment with a limited budget, which may be useful to organizations requiring small amounts of very high quality surface processing. After a brief review of some general principles, it discusses the design, including practical constraints, and installation of a small facility in the University of Warwick. Based around standard bought-in items, it achieves cleanroom Class 10 000 (ISO Class 7) with local enclosures of better than Class 10 (ISO Class 4) air quality. © 2001 Elsevier Science Inc. All rights reserved.
1. Introduction Contamination control and cleanroom technology is an essential tool for the manufacture of a great variety of quality products: semiconductors, liquid crystal displays, sterile pharmaceutical drugs, vaccines and medical devices, precision mechanical instruments and high-quality food products and beverages, to name but a few. The world-wide market value of products manufactured within cleanrooms and with the help of contamination control procedures already totals many hundred billion U.S. dollars a year and this value continues to grow at an impressive pace. Developments in advanced technologies based on miniaturization and high precision manufacturing impose ever higher requirements on the degree of cleanliness of production environments. Computer and optical industries as well as semiconductor and pharmaceutical industries around the world, needing ultra-pure working space, are purchasing new cleanrooms at the rate of 6,000 per year, according to the World Cleanrooms Projects report by the McIlvaine Company [1]. The USA has the most semiconductor cleanrooms, South East Asia is the leading operator of disc drive cleanrooms and Europe has the highest number of pharmaceutical cleanrooms. But a high degree of cleanliness means high cost. Prices in the year 2000 can go as high as $US 60 million for a 60 000 ft2 (⬃6 000 m2) Class 1 (ISO Class 3) cleanroom for * Corresponding author. Tel.: ⫹44-24-7652-3121; fax: ⫹44-24-76418922. E-mail address: [email protected] (D.G. Chetwynd).
chip manufacture [1]. These prices are prohibitive for small manufacturers using super-precise surfaces in small quantities, such as those working in replication processes. However, because only small quantities are needed, the economics are different and more manual operations can be accepted. Further, simpler processes may allow different approaches, making it easier to remove the human operator from the controlled environment. In the most sophisticated systems this is desired to avoid contamination; in simple ones it permits the use of conditions unpleasant for humans. Portable cleanrooms exist on the market for a lower price but with most models the size is too small to accommodate the necessary equipment or instrumentation and they usually consist of a plastic curtain and a HEPA filter only. It should be born in mind that when a portable room is setup it is absolutely necessary to provide a clean environment where the portable room should stand, otherwise the filter will clog up quickly and lose its efficiency.
2. Cleanrooms in general A suitable way to define a cleanroom could be that of an enclosure in which the air purity is controlled and maintained to a pre-determined level. Frequently this is accomplished by continuously “flushing” the cleanroom with highly filtered air that is forced in through High Efficiency Particulate Air (HEPA) filters. HEPA filters can prevent over 99.997% of particles measuring greater than 0.3 m in size from entering the cleanroom. This is remarkable considering that the outside air we normally breathe may con-
0141-6359/02/$ – see front matter © 2001 Elsevier Science Inc. All rights reserved. PII: S 0 1 4 1 - 6 3 5 9 ( 0 1 ) 0 0 0 9 3 - 9
V. Anghel, D.G. Chetwynd / Precision Engineering 26 (2002) 122–127
123
Fig. 1. Typical particulate contaminants—size and visibility (after Barrett, [2]).
tain up to 5 million suspended particles of dust, smog, and pollen in one cubic foot of air (⬃0.03 m3). The dimensions of particles that are of concern to cleanroom operators are of the order of 10 m or less. Inevitably, those that are most numerous are also the smallest and mostly invisible to the naked eye under normal conditions of illumination (see Fig. 1) [2]. People are considered to be the greatest source of contamination in a cleanroom. By simply walking around, the human body can generate 5 to 10 million skin, hair, dirt and clothing particles every minute. Thus the reduction of particulate contamination from people is a paramount condition for the successful operation of cleanrooms; hence the need to wear a cleanroom suit, overshoes, mask, gloves, and cap. Cleanrooms are classified according to the quality of the cleanroom environment with several standards covering this issue [3], for example: Y ISO 14664 [4], regarding cleanrooms and associated controlled environments, Y US FED STD-209E, 1992 [5], which replaces US FED STD-209D, 1988. However, the requirements set in different standards are not fully comparable. When comparing the UD FED STD-209 and ISO 14664 –1, the cleanroom classes are approximately in agreement, but in general, the requirements set in the ISO standard are more stringent than those set in the US standard. The “traditional” approach and still used by, e.g., US standards define class number as the permissible number of particles greater than 0.5 m diameter per cubic foot of air. ISO classes are ranking numbers based on similar notions. Common cleanroom status levels are Class 10 000, Class 1,000 and Class 100 according to Fed. Standard 209D corresponding to classes 7, 6, and 5 according to ISO EN 14644 –1. Cleanrooms of Class 10 and Class 1 (ISO Classes 4 and 3, respectively) are used in the silicon wafer/microchip industry. With contamination control requirements becoming more stringent, there is increased demand for a consistent standard for cleanrooms and particle tests. This will probably be achieved when all parts from ISO 14664 are finally
published (parts 2 and 4 are at present in the Draft International Standards, DIS, stage). To maintain the air purity, continuous recycling with some “fresh” air is done through appropriate laminar air flow filters. However, none of the above Standards sets the requirements for the number of air changes in the cleanrooms and 20 changes per hour is arbitrarily considered as a minimum requirement [3] for Class 10 000 (ISO Class 7) cleanliness, for example. Many more air changes are required to maintain a room at Class 100. To prevent ingress of unwanted air and air-borne particulates cleanrooms are normally slightly pressurized (some mm H2O) above atmospheric pressure. Another aspect is the environment within the cleanroom regarding temperature and humidity control. Since people dressed in suitable attire (non-shedding suits, caps, face masks, footwear, gloves) have to work in such rooms, the temperature and humidity need to be controlled; usually between 15–18°C and 25– 40% RH, respectively. Precise temperature and humidity control is frequently necessary to maintain thermal and chemical stability of components.
3. Starting point The work described in this paper was part of a European research project [6] concerned mainly with the development of calibration artifacts for surface topographic instruments, which also included the specification of safe cleaning procedures for master and working standards. The target is to produce conditions approaching those obtained from expensive automated cleaning plants within a budget acceptable to users such as metrology laboratories. This task required the design and construction at accessibly low cost of a pure environment for testing the validity of the cleaning procedures. A clean manufacturing facility is a complex system incorporating the building, supporting facilities and process tools. Adapting an existing facility to a cleanroom status is very different from starting everything from the scratch. One has to make the most of what is available and also find
124
V. Anghel, D.G. Chetwynd / Precision Engineering 26 (2002) 122–127
Fig. 2. View of the 2 Hepaire cabinets and the air conditioning unit.
solutions to problems that would not arise if money were available for purchasing a completely new commercial installation. The conditions at Warwick were those offered by a 1960s style building of reinforced concrete and glass. Rooms and laboratories provided with suspended lightweight ceilings were divided by semi-permanent coated mild steel panel partitions. The laboratory prior to conversion to a surface cleaning unit was part of a three-room suite which previously had been used for sample preparation, lapping and polishing. The suite was provided with two wall-mounted Hepaire filtering units but they were out of use because both the input and output filters were blocked by abrasive and polishing compounds. Thus up-grading a small room (3.5 ⫻ 3 ⫻ 2.5 m3) to a Class 10 000 (ISO Class 7) facility was a real challenge bearing in mind the type of construction, the cleanliness status and the small budget available. One of the main problems was the air circulation in the space between the lightweight ceiling panels and the concrete beams, which was carrying particulates and could contaminate the room air. On the other hand there was space between the steel panels and the edge of the ceiling, which again would allow air-borne particulates to ingress the room.
4. The up-grade debate Considerations of budget and room geometry indicated that the best that could be achieved was to add two more filter units, making four in total. The two additional filters were contained within basic laminar flow cabinets (approximately 500 ⫻ 700 mm2 table area). In our case, the choice was governed by the availability of second-hand units but the approach is quite cost-effective even if new ones must be acquired. Initially the intention was to bring the whole room to Class 100, but careful analysis of the number of air changes obtainable led to a different strategy. Calculations for the whole room showed the need for 90 and 62.5 air
changes/hr (at 50% flow capacity) for using four and three filter units, respectively. If one unit was used to feed a tunnel of 3.5 ⫻ 0.9 ⫻ 1 m3 with clean air from the other units, the number of air changes in the tunnel would be 213 per hour at 50% flow capacity (the actual tunnel sizes were dictated by the room and convenient bench widths). Better overall quality could be obtained by using three units to clean the room air and the fourth one to feed an enclosed tunnel (housing all the cleaning, drying and inspection equipment) with air filtered to Class 100 (ISO Class 5).
5. Room modifications Three filtering units were used for cleaning the air inside the room. Two Hepaire laminar air flow cabinets were mounted on the wall. In our case there was only one possible place to put the filters and that was the solid wall where the air conditioning unit was also mounted (as seen in Fig. 2). Because of this limitation, the air flow inside the room was not laminar but this was not so critical since all the important work was done inside the purpose built tunnel which is providing a laminar air flow. New filters were installed in the Hepaire units, both for the air input and output. The pre-filters are cardboard cased V pleat of 380 ⫻ 380 ⫻ 50 mm3 nominal dimensions. The output filters are Class 100 quality (99.997% efficiency) and their dimensions are 610 ⫻ 610 ⫻ 66 mm3. The third filter unit, a WHS cabinet, was adapted to bring fresh air from outside the room. The air is drawn from the corridor area through a plastic pipe which is connected to the pre-filter via a box partly covering it. Thus about 50% of the air intake of this filter unit comes from outside the room. It is worth mentioning that the corridor area is always kept in a spotless condition and has little traffic. Keeping the intact cabinet provided a secondary work area that was cleaner than the room average condition. The filters in this unit were also replaced with new ones of the same quality
V. Anghel, D.G. Chetwynd / Precision Engineering 26 (2002) 122–127
125
Fig. 3. Cleaning sequence inside the enclosed tunnel.
and efficiency as those installed in the Hepaire units for both input and output. An air conditioning system had to be installed because our room suffered a large and varying thermal input from adjacent parts of the building. It may not be needed elsewhere. To avoid air contamination from unfiltered air, the room was sealed all the way around the ceiling and all the steel panel edges were sealed to the aluminum support frame by the use of silicone sealant. This was done to slightly pressurize the room in keeping with best cleanroom practice, and to maintain the best air quality. Other tasks to ensure the best environmental conditions included repainting of the walls, trunking of the electrical wires and fitting a circuit breaker within the tunnel and thoroughly wiping all the surfaces including the ceiling and the walls. A small changing room was created by partitioning walls. It provides the only entrance to the cleanroom as well as storing the overall suits and slippers, etc. “Tack mats” on the changing room floor and inside the entrance area to the cleanroom remove particulates from the soles of slippers while entering the cleanroom. With 62 air changes/hour at 50% operating capacity, the room is well above the requirements (20 air changes/hour) for a cleanroom Class 10 000 (ISO Class 7). Bearing in mind that this air forms the input for the enclosed tunnel and that inside the tunnel the air changes are of the order of a couple of hundreds, we believe that the air quality we have achieved is more than suitable.
6. The flow tunnel system Working surfaces for the clean tunnel (with cupboards and drawers below) were built from good quality standard
plastic-coated kitchen furniture units. Care was taken to seal the top bench edges to the room walls with silicone sealant. A mask was also built to cover the area under the sink. Since it was made out of wood the whole surface needed to be painted and then thoroughly sealed all around the edges. The idea was to build an enclosed tunnel containing high purity air around an ultrasonic cleaning unit. The pre-cleaning, drying and inspection stages were to be performed in the confined space as well (see Fig. 3) where there is a continuous laminar flow of air of at least Class 100 (ISO Class 5) purity. Packaging and sealing required in a later stage of the project would also take place in the same enclosure. The tunnel setup is shown in Fig. 4. Commercial double glazed windows in PVC-U frames built to order were used for the top and front of the tunnel mainly because of the good quality obtained for a reasonably low price. The front and the top were silicone sealed to the benches and to the existing plastic windows separating the laboratory from the adjacent room. The filter unit used to improve the air quality inside the tunnel was built inhouse by using the component parts from a Slee filter cabinet. The air fan was vertically mounted in the left hand side of the tunnel. The air is drawn from the room through a cardboard V pleat pre-filter and blown into the tunnel enclosure through a Class 100 filter of the size 914 ⫻ 457 ⫻ 50 mm3. The cleaning sequence starts from the far end of the enclosure (i.e., the right hand side) in the opposite direction of the laminar flow to ensure that the last stages (packaging and sealing) will be performed in the area closest to the filter where the air is the most pure. The sample is transported inside the enclosure from one stage to the next by suspending and manually sliding it along a rail. Access is provided through several small doors
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V. Anghel, D.G. Chetwynd / Precision Engineering 26 (2002) 122–127
Fig. 4. Double glazed class 10 laminar air flow tunnel.
that are opened and closed in sequence to minimize contamination. The last door is kept slightly open to exhaust the air and maintain the flow directionality, but is closed when the tunnel is not in use. The flow rate, 213 changes per hour at 50% flow, is much higher than could be used in an occupied room and results in Class 10 (ISO Class 4) conditions in the tunnel. Contamination could be reduced further by using gloves fixed in the window panels instead of opening doors and by motorizing the rail transport. However, this seems uneconomic since no back-flow of air towards the cleanest end has been detected when using the doors. Table 1 Cleanroom evaluation results Cleanroom air data Instrument used – Airflow Air Velocity Meter model LCA6000 UK Location Air velocity (m s⫺1) ROOM Target ⫽0.45 WHS unit ⫽0.44 Hepaire unit 1 ⫽0.45 Hepaire unit 2 ⫽0.47 TUNNEL Pre US station ⫽0.47 US unit ⫽0.53 Drying/inspection station ⫽0.53 Differential pressure measurement Instrument used – Druck Differential Pressure Indicator, model DPI 705, UK Differential pressure between room and main building ⫽ 0.1 mm H2O Differential pressure between tunnel and pre-US door open ⫽2.2 mm H2O – door open 50 mm ⫽1.0 mm H2O – door open 100 mm ⫽0.3 mm H2O – door open fully Particles per cubic foot of air greater than 0.5 m Instrument used – LASAIR model 310 from Particle Measuring Systems Inc. USA - Under air conditioning unit ⫽2400 - Above US unit inside tunnel ⫽4 - WHS unit (200 mm from filter, ⫽6 200 mm above bench) - WHS unit (400 mm from filter, ⫽1 200 mm above bench) - 500 mm below Hepaire 2 ⫽2200 - 500 mm below Hepaire 1 ⫽2300
7. Measurement of cleanroom and tunnel status It was deemed imperative that the evaluation of the cleanroom and tunnel status should be carried out by an independent evaluator. This was done with the room in normal operating conditions with the evaluator inside the room. The results of the cleanroom status evaluation are given in Table 1.
8. Conclusions Very expensive and very effective cleanrooms are available on the market but they are only affordable to largescale industries. This paper has shown that cheaper options can be achieved, with quite impressive outputs. For general guidance the capital outlay on filters, double glazed windows, benches and cabinets and a second hand laminar air flow cabinet was a little over £3 000 ($4500). Based on a modest capital outlay, a small cleaning facility was built which can provide Class 10,000 (ISO Class 7) air environment. It also has a self contained tunnel enclosure in which all the operations are conducted i.e., cleaning, drying, inspection and packaging which is in better than Class 10 (ISO Class 4) condition.
Acknowledgments The authors wish to thank Mr. Rob Smith, Managing Director of Speedfam UK Ltd. for generously donating the Slee Class 100 (ISO Class 5) laminar air flow unit to the project. The work was supported by EU grant SMT4-CT972 176.
References [1] Anon. News report by McIlvane Company. Cleanroom Technology June 2000; p. 5. [2] Barret GFC. Stamping out particulates. Cleanroom Technology October 1997; p. 11.
V. Anghel, D.G. Chetwynd / Precision Engineering 26 (2002) 122–127 [3] Winter BR, Holmgren H. A filter guide for cleanrooms in the pharmaceutical industry. Cleanroom Technology August 2000; p. 24. [4] Draft International Standard ISO/DIS 14644-1, Cleanrooms, and associated controlled environments—Part 1: classification of air cleanliness. Geneva: International Organisation for Standardisation, December, 1996.
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[5] U.S. Federal Standard 209E, Airborne particulate cleanliness classes for cleanrooms and clean zones. Washington, September 1992. [6] “Calibration Standards for Surface Topography Measuring Systems down to Nanometric Range”, European Union grant SMT4-CT972176, 1997–2000.
Technical Note
Creating a low-cost, ultra-clean environment V. Anghel, D.G. Chetwynd* Centre for Nanotechnology and Microengineering, University of Warwick, Coventry, CV 4 7AL, UK Received 9 January 2001; accepted 26 January 2001
Abstract This paper describes a way to create a cleanroom environment with a limited budget, which may be useful to organizations requiring small amounts of very high quality surface processing. After a brief review of some general principles, it discusses the design, including practical constraints, and installation of a small facility in the University of Warwick. Based around standard bought-in items, it achieves cleanroom Class 10 000 (ISO Class 7) with local enclosures of better than Class 10 (ISO Class 4) air quality. © 2001 Elsevier Science Inc. All rights reserved.
1. Introduction Contamination control and cleanroom technology is an essential tool for the manufacture of a great variety of quality products: semiconductors, liquid crystal displays, sterile pharmaceutical drugs, vaccines and medical devices, precision mechanical instruments and high-quality food products and beverages, to name but a few. The world-wide market value of products manufactured within cleanrooms and with the help of contamination control procedures already totals many hundred billion U.S. dollars a year and this value continues to grow at an impressive pace. Developments in advanced technologies based on miniaturization and high precision manufacturing impose ever higher requirements on the degree of cleanliness of production environments. Computer and optical industries as well as semiconductor and pharmaceutical industries around the world, needing ultra-pure working space, are purchasing new cleanrooms at the rate of 6,000 per year, according to the World Cleanrooms Projects report by the McIlvaine Company [1]. The USA has the most semiconductor cleanrooms, South East Asia is the leading operator of disc drive cleanrooms and Europe has the highest number of pharmaceutical cleanrooms. But a high degree of cleanliness means high cost. Prices in the year 2000 can go as high as $US 60 million for a 60 000 ft2 (⬃6 000 m2) Class 1 (ISO Class 3) cleanroom for * Corresponding author. Tel.: ⫹44-24-7652-3121; fax: ⫹44-24-76418922. E-mail address: [email protected] (D.G. Chetwynd).
chip manufacture [1]. These prices are prohibitive for small manufacturers using super-precise surfaces in small quantities, such as those working in replication processes. However, because only small quantities are needed, the economics are different and more manual operations can be accepted. Further, simpler processes may allow different approaches, making it easier to remove the human operator from the controlled environment. In the most sophisticated systems this is desired to avoid contamination; in simple ones it permits the use of conditions unpleasant for humans. Portable cleanrooms exist on the market for a lower price but with most models the size is too small to accommodate the necessary equipment or instrumentation and they usually consist of a plastic curtain and a HEPA filter only. It should be born in mind that when a portable room is setup it is absolutely necessary to provide a clean environment where the portable room should stand, otherwise the filter will clog up quickly and lose its efficiency.
2. Cleanrooms in general A suitable way to define a cleanroom could be that of an enclosure in which the air purity is controlled and maintained to a pre-determined level. Frequently this is accomplished by continuously “flushing” the cleanroom with highly filtered air that is forced in through High Efficiency Particulate Air (HEPA) filters. HEPA filters can prevent over 99.997% of particles measuring greater than 0.3 m in size from entering the cleanroom. This is remarkable considering that the outside air we normally breathe may con-
0141-6359/02/$ – see front matter © 2001 Elsevier Science Inc. All rights reserved. PII: S 0 1 4 1 - 6 3 5 9 ( 0 1 ) 0 0 0 9 3 - 9
V. Anghel, D.G. Chetwynd / Precision Engineering 26 (2002) 122–127
123
Fig. 1. Typical particulate contaminants—size and visibility (after Barrett, [2]).
tain up to 5 million suspended particles of dust, smog, and pollen in one cubic foot of air (⬃0.03 m3). The dimensions of particles that are of concern to cleanroom operators are of the order of 10 m or less. Inevitably, those that are most numerous are also the smallest and mostly invisible to the naked eye under normal conditions of illumination (see Fig. 1) [2]. People are considered to be the greatest source of contamination in a cleanroom. By simply walking around, the human body can generate 5 to 10 million skin, hair, dirt and clothing particles every minute. Thus the reduction of particulate contamination from people is a paramount condition for the successful operation of cleanrooms; hence the need to wear a cleanroom suit, overshoes, mask, gloves, and cap. Cleanrooms are classified according to the quality of the cleanroom environment with several standards covering this issue [3], for example: Y ISO 14664 [4], regarding cleanrooms and associated controlled environments, Y US FED STD-209E, 1992 [5], which replaces US FED STD-209D, 1988. However, the requirements set in different standards are not fully comparable. When comparing the UD FED STD-209 and ISO 14664 –1, the cleanroom classes are approximately in agreement, but in general, the requirements set in the ISO standard are more stringent than those set in the US standard. The “traditional” approach and still used by, e.g., US standards define class number as the permissible number of particles greater than 0.5 m diameter per cubic foot of air. ISO classes are ranking numbers based on similar notions. Common cleanroom status levels are Class 10 000, Class 1,000 and Class 100 according to Fed. Standard 209D corresponding to classes 7, 6, and 5 according to ISO EN 14644 –1. Cleanrooms of Class 10 and Class 1 (ISO Classes 4 and 3, respectively) are used in the silicon wafer/microchip industry. With contamination control requirements becoming more stringent, there is increased demand for a consistent standard for cleanrooms and particle tests. This will probably be achieved when all parts from ISO 14664 are finally
published (parts 2 and 4 are at present in the Draft International Standards, DIS, stage). To maintain the air purity, continuous recycling with some “fresh” air is done through appropriate laminar air flow filters. However, none of the above Standards sets the requirements for the number of air changes in the cleanrooms and 20 changes per hour is arbitrarily considered as a minimum requirement [3] for Class 10 000 (ISO Class 7) cleanliness, for example. Many more air changes are required to maintain a room at Class 100. To prevent ingress of unwanted air and air-borne particulates cleanrooms are normally slightly pressurized (some mm H2O) above atmospheric pressure. Another aspect is the environment within the cleanroom regarding temperature and humidity control. Since people dressed in suitable attire (non-shedding suits, caps, face masks, footwear, gloves) have to work in such rooms, the temperature and humidity need to be controlled; usually between 15–18°C and 25– 40% RH, respectively. Precise temperature and humidity control is frequently necessary to maintain thermal and chemical stability of components.
3. Starting point The work described in this paper was part of a European research project [6] concerned mainly with the development of calibration artifacts for surface topographic instruments, which also included the specification of safe cleaning procedures for master and working standards. The target is to produce conditions approaching those obtained from expensive automated cleaning plants within a budget acceptable to users such as metrology laboratories. This task required the design and construction at accessibly low cost of a pure environment for testing the validity of the cleaning procedures. A clean manufacturing facility is a complex system incorporating the building, supporting facilities and process tools. Adapting an existing facility to a cleanroom status is very different from starting everything from the scratch. One has to make the most of what is available and also find
124
V. Anghel, D.G. Chetwynd / Precision Engineering 26 (2002) 122–127
Fig. 2. View of the 2 Hepaire cabinets and the air conditioning unit.
solutions to problems that would not arise if money were available for purchasing a completely new commercial installation. The conditions at Warwick were those offered by a 1960s style building of reinforced concrete and glass. Rooms and laboratories provided with suspended lightweight ceilings were divided by semi-permanent coated mild steel panel partitions. The laboratory prior to conversion to a surface cleaning unit was part of a three-room suite which previously had been used for sample preparation, lapping and polishing. The suite was provided with two wall-mounted Hepaire filtering units but they were out of use because both the input and output filters were blocked by abrasive and polishing compounds. Thus up-grading a small room (3.5 ⫻ 3 ⫻ 2.5 m3) to a Class 10 000 (ISO Class 7) facility was a real challenge bearing in mind the type of construction, the cleanliness status and the small budget available. One of the main problems was the air circulation in the space between the lightweight ceiling panels and the concrete beams, which was carrying particulates and could contaminate the room air. On the other hand there was space between the steel panels and the edge of the ceiling, which again would allow air-borne particulates to ingress the room.
4. The up-grade debate Considerations of budget and room geometry indicated that the best that could be achieved was to add two more filter units, making four in total. The two additional filters were contained within basic laminar flow cabinets (approximately 500 ⫻ 700 mm2 table area). In our case, the choice was governed by the availability of second-hand units but the approach is quite cost-effective even if new ones must be acquired. Initially the intention was to bring the whole room to Class 100, but careful analysis of the number of air changes obtainable led to a different strategy. Calculations for the whole room showed the need for 90 and 62.5 air
changes/hr (at 50% flow capacity) for using four and three filter units, respectively. If one unit was used to feed a tunnel of 3.5 ⫻ 0.9 ⫻ 1 m3 with clean air from the other units, the number of air changes in the tunnel would be 213 per hour at 50% flow capacity (the actual tunnel sizes were dictated by the room and convenient bench widths). Better overall quality could be obtained by using three units to clean the room air and the fourth one to feed an enclosed tunnel (housing all the cleaning, drying and inspection equipment) with air filtered to Class 100 (ISO Class 5).
5. Room modifications Three filtering units were used for cleaning the air inside the room. Two Hepaire laminar air flow cabinets were mounted on the wall. In our case there was only one possible place to put the filters and that was the solid wall where the air conditioning unit was also mounted (as seen in Fig. 2). Because of this limitation, the air flow inside the room was not laminar but this was not so critical since all the important work was done inside the purpose built tunnel which is providing a laminar air flow. New filters were installed in the Hepaire units, both for the air input and output. The pre-filters are cardboard cased V pleat of 380 ⫻ 380 ⫻ 50 mm3 nominal dimensions. The output filters are Class 100 quality (99.997% efficiency) and their dimensions are 610 ⫻ 610 ⫻ 66 mm3. The third filter unit, a WHS cabinet, was adapted to bring fresh air from outside the room. The air is drawn from the corridor area through a plastic pipe which is connected to the pre-filter via a box partly covering it. Thus about 50% of the air intake of this filter unit comes from outside the room. It is worth mentioning that the corridor area is always kept in a spotless condition and has little traffic. Keeping the intact cabinet provided a secondary work area that was cleaner than the room average condition. The filters in this unit were also replaced with new ones of the same quality
V. Anghel, D.G. Chetwynd / Precision Engineering 26 (2002) 122–127
125
Fig. 3. Cleaning sequence inside the enclosed tunnel.
and efficiency as those installed in the Hepaire units for both input and output. An air conditioning system had to be installed because our room suffered a large and varying thermal input from adjacent parts of the building. It may not be needed elsewhere. To avoid air contamination from unfiltered air, the room was sealed all the way around the ceiling and all the steel panel edges were sealed to the aluminum support frame by the use of silicone sealant. This was done to slightly pressurize the room in keeping with best cleanroom practice, and to maintain the best air quality. Other tasks to ensure the best environmental conditions included repainting of the walls, trunking of the electrical wires and fitting a circuit breaker within the tunnel and thoroughly wiping all the surfaces including the ceiling and the walls. A small changing room was created by partitioning walls. It provides the only entrance to the cleanroom as well as storing the overall suits and slippers, etc. “Tack mats” on the changing room floor and inside the entrance area to the cleanroom remove particulates from the soles of slippers while entering the cleanroom. With 62 air changes/hour at 50% operating capacity, the room is well above the requirements (20 air changes/hour) for a cleanroom Class 10 000 (ISO Class 7). Bearing in mind that this air forms the input for the enclosed tunnel and that inside the tunnel the air changes are of the order of a couple of hundreds, we believe that the air quality we have achieved is more than suitable.
6. The flow tunnel system Working surfaces for the clean tunnel (with cupboards and drawers below) were built from good quality standard
plastic-coated kitchen furniture units. Care was taken to seal the top bench edges to the room walls with silicone sealant. A mask was also built to cover the area under the sink. Since it was made out of wood the whole surface needed to be painted and then thoroughly sealed all around the edges. The idea was to build an enclosed tunnel containing high purity air around an ultrasonic cleaning unit. The pre-cleaning, drying and inspection stages were to be performed in the confined space as well (see Fig. 3) where there is a continuous laminar flow of air of at least Class 100 (ISO Class 5) purity. Packaging and sealing required in a later stage of the project would also take place in the same enclosure. The tunnel setup is shown in Fig. 4. Commercial double glazed windows in PVC-U frames built to order were used for the top and front of the tunnel mainly because of the good quality obtained for a reasonably low price. The front and the top were silicone sealed to the benches and to the existing plastic windows separating the laboratory from the adjacent room. The filter unit used to improve the air quality inside the tunnel was built inhouse by using the component parts from a Slee filter cabinet. The air fan was vertically mounted in the left hand side of the tunnel. The air is drawn from the room through a cardboard V pleat pre-filter and blown into the tunnel enclosure through a Class 100 filter of the size 914 ⫻ 457 ⫻ 50 mm3. The cleaning sequence starts from the far end of the enclosure (i.e., the right hand side) in the opposite direction of the laminar flow to ensure that the last stages (packaging and sealing) will be performed in the area closest to the filter where the air is the most pure. The sample is transported inside the enclosure from one stage to the next by suspending and manually sliding it along a rail. Access is provided through several small doors
126
V. Anghel, D.G. Chetwynd / Precision Engineering 26 (2002) 122–127
Fig. 4. Double glazed class 10 laminar air flow tunnel.
that are opened and closed in sequence to minimize contamination. The last door is kept slightly open to exhaust the air and maintain the flow directionality, but is closed when the tunnel is not in use. The flow rate, 213 changes per hour at 50% flow, is much higher than could be used in an occupied room and results in Class 10 (ISO Class 4) conditions in the tunnel. Contamination could be reduced further by using gloves fixed in the window panels instead of opening doors and by motorizing the rail transport. However, this seems uneconomic since no back-flow of air towards the cleanest end has been detected when using the doors. Table 1 Cleanroom evaluation results Cleanroom air data Instrument used – Airflow Air Velocity Meter model LCA6000 UK Location Air velocity (m s⫺1) ROOM Target ⫽0.45 WHS unit ⫽0.44 Hepaire unit 1 ⫽0.45 Hepaire unit 2 ⫽0.47 TUNNEL Pre US station ⫽0.47 US unit ⫽0.53 Drying/inspection station ⫽0.53 Differential pressure measurement Instrument used – Druck Differential Pressure Indicator, model DPI 705, UK Differential pressure between room and main building ⫽ 0.1 mm H2O Differential pressure between tunnel and pre-US door open ⫽2.2 mm H2O – door open 50 mm ⫽1.0 mm H2O – door open 100 mm ⫽0.3 mm H2O – door open fully Particles per cubic foot of air greater than 0.5 m Instrument used – LASAIR model 310 from Particle Measuring Systems Inc. USA - Under air conditioning unit ⫽2400 - Above US unit inside tunnel ⫽4 - WHS unit (200 mm from filter, ⫽6 200 mm above bench) - WHS unit (400 mm from filter, ⫽1 200 mm above bench) - 500 mm below Hepaire 2 ⫽2200 - 500 mm below Hepaire 1 ⫽2300
7. Measurement of cleanroom and tunnel status It was deemed imperative that the evaluation of the cleanroom and tunnel status should be carried out by an independent evaluator. This was done with the room in normal operating conditions with the evaluator inside the room. The results of the cleanroom status evaluation are given in Table 1.
8. Conclusions Very expensive and very effective cleanrooms are available on the market but they are only affordable to largescale industries. This paper has shown that cheaper options can be achieved, with quite impressive outputs. For general guidance the capital outlay on filters, double glazed windows, benches and cabinets and a second hand laminar air flow cabinet was a little over £3 000 ($4500). Based on a modest capital outlay, a small cleaning facility was built which can provide Class 10,000 (ISO Class 7) air environment. It also has a self contained tunnel enclosure in which all the operations are conducted i.e., cleaning, drying, inspection and packaging which is in better than Class 10 (ISO Class 4) condition.
Acknowledgments The authors wish to thank Mr. Rob Smith, Managing Director of Speedfam UK Ltd. for generously donating the Slee Class 100 (ISO Class 5) laminar air flow unit to the project. The work was supported by EU grant SMT4-CT972 176.
References [1] Anon. News report by McIlvane Company. Cleanroom Technology June 2000; p. 5. [2] Barret GFC. Stamping out particulates. Cleanroom Technology October 1997; p. 11.
V. Anghel, D.G. Chetwynd / Precision Engineering 26 (2002) 122–127 [3] Winter BR, Holmgren H. A filter guide for cleanrooms in the pharmaceutical industry. Cleanroom Technology August 2000; p. 24. [4] Draft International Standard ISO/DIS 14644-1, Cleanrooms, and associated controlled environments—Part 1: classification of air cleanliness. Geneva: International Organisation for Standardisation, December, 1996.
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[5] U.S. Federal Standard 209E, Airborne particulate cleanliness classes for cleanrooms and clean zones. Washington, September 1992. [6] “Calibration Standards for Surface Topography Measuring Systems down to Nanometric Range”, European Union grant SMT4-CT972176, 1997–2000.