Modern Devices: Chapter 5 – Cleanrooms, Modern Devices: The Simple Physics of Sophisticated Technology Copyright © John Wiley and Sons, Inc. Chapter 5 – Cleanrooms, an enabling technology Modern Devices: The Simple Physics of Sophisticated Technology by Charles L. Joseph and Santiago Bernal

Modern Devices: The Simple Physics of Sophisticated Technology Copyright © John Wiley and Sons, Inc. Fig. 5.1 A typical floor plan for a cleanroom, showing the multiple decontamination stages used prior to entering the cleanroom. A floor plan reveals the multistage steps used to minimize contaminants being brought into the cleanroom. The first step is to remove shoe particulates, using treated fiber mats and an electric vacuuming shoe brush. Personnel enter the gowning room where they cover themselves with garments, boots, gloves, and hoods. Then, the individual walks across a sticky mat into an airlock room called the air shower, consisting of a number of jet nozzles. The main room is overpressurized so that any contaminants that have escaped these mitigation techniques will tend to flow away. There are three principle processes to maintain a pristine environment: 1. prevent external contaminants being introduced via personnel and equipment entering the room, 2. extensive air filtration plus a laminar flow of air, and 3. special cleaning procedures. Cleanroom Gowning Area Air Shower Small Equipment Air-Locked Pass-Through Box Main Treated Fiber Mats Electric Shoe Brush Sticky Mat Cleanroom Design

Protecting Equipment from Inherently Dirty Humans Modern Devices: The Simple Physics of Sophisticated Technology Copyright © John Wiley and Sons, Inc. Fig. 5.2 An individual completely garmented and an air shower that is used prior to entering a cleanroom. Cleanroom Design Protecting Equipment from Inherently Dirty Humans A complete covering is referred to colloquially as a bunny suit. These garments are tightly woven fabrics that retain the numerous particles per minute shed by humans. The fabrics also do not hold static charge that attracts particulates. Note: troublesome areas such as footwear and headwear may have two layers of covering and personnel are required to dress in a particular order. The air shower then becomes very effective at removing dust.

Turbulent vs. Laminar Flows Fig. 5.3 Two approaches to cleanroom design. Left: high turbulent flow that kicks up loose particles and forces these out the exit vents. Right: laminar flow, the more common type of cleanroom, moves and residual loose particles to the floor and traps these there until removed by routine cleaning procedures. Turbulent vs. Laminar Flows A pristine environment requires extensive air filtration, using one of two strategies. The vast majority of cleanrooms incorporate laminar airflow, designed to drive any residual particulates towards the floor and hold those there until the surface is cleaned. A key component is the use of high efficiency particulate air (HEPA) filters. Air turbulence tends to knock off particulates on surfaces. A single 0.5 m particle can destroy an integrated circuit. In recent years, a new ultra low particulate air (ULPA) filter has been introduced. ULPA were developed specifically for cleanrooms where the generation of large amounts contaminants were unavoidable. In these few cases, the airflow is deliberately made turbulent, stirring up surface particulates and seeping these out of the room. Laminar flows, however, generally provide superior air quality and ULPA filters have begun to be used in conjunction with HEPA filters to provide the best performance. Modern Devices: The Simple Physics of Sophisticated Technology Copyright © John Wiley and Sons, Inc.

Maximum Number of Particles / m3 Modern Devices: The Simple Physics of Sophisticated Technology Copyright © John Wiley and Sons, Inc. Historically, cleanrooms were classified according to the number of particulates per cubic foot, commonly Class 100, 1,000, and 10,000. This obsolete, but still used, designation has been replaced by an international standard, reflecting the modern requirements. ISO 14644-1 standards place limits on the numerical counts per cubic meter for each range in particle size. Cleanroom Standards Table 5.1 ISO 14644-1 Cleanroom Standards Class Maximum Number of Particles / m3 FED STD 209E Equivalent  0.1 m  0.2 m  0.3 m  0.5 m  1 m  5 m ISO 2 100 24 10 4   ISO 3 1,000 237 102 35 8 Class 1 ISO 4 10,000 2,370 1,020 352 83 Class 10 ISO 5 100,000 23,700 10,200 3,520 832 29 Class 100 ISO 6 1,000,000 237,000 102,000 35,200 8,320 293 Class 1,000 ISO 7 352,000 83,200 2,930 Class 10,000