Sunday, November 8, 2009

Nanotechnology Cleanroom - Design on A Dime

By: Raymond K. Schneider P.E.
October 2009

The tight budget of today’s nanotechnology facility start-ups makes the need for small scale cleanroom solutions as important as ever.

Over the past several years the term “nanotechnology” has been growing in the public consciousness. The science of small things holds the same allure that “microelectronics,” the buzzword of the 80’s, held at one time. Entrepreneurs see new generations of microelectronic devices, optics, pharmaceutical delivery systems, medical diagnostic devices, and an array of molecular level products about to breakthrough to the marketplace as “nanotechnology.”

Attend a nanotechnology forum or trade show and the exhibit floor is crammed with great ideas available for license or investment or partnering opportunities. What is rarely found is the much needed attention to detail regarding how the device is to be produced in a cost effective manner, to insure that initial investments are protected by profitable production. The requirement for a high production yield generally follows on the heels of the “gee-whiz” stage, characterized by “Wow, we did it....who woulda believed it possible?” First we invent/design, then we prototype, then we produce. Failure to produce economically carries the risk that others, who can produce at a profit, will be able to exploit the market window of opportunity.

As start-ups blossomed in “high-tech” corridors around the country a bit more than two decades ago, the search for product yield was a driver of cleanroom design and construction. Today the excitement of that era is returning through nano technology research and development. Not surprisingly, the manufacturing facility budget of today’s high tech nanotechnology start-up is as tight as the microelectronic counterpart of years ago. The requirement for cleanroom solutions “on a dime” is as important as ever.

Contaminants that negatively affect product yield can be particles or gases. The purpose of the cleanroom is to isolate the product from contaminants that cause product rejection by your quality control. This is done by keeping contaminants out of the facility or, should they enter, by removing them before they do damage. A variety of strategies are employed in large, well-financed facilities, but how does the start-up nanotechnology company, with relatively few dollars and many priorities, begin to address the need for an appropriate facility that will support small production runs of high quality, contaminant-free product?

Assess the process to be contained within the space. While it might be convenient to house all phases of your process within a cleanroom, the size, hence cost, of the facility will immediately begin to grow. Identify those processes that must be conducted within a clean environment and limit this first clean area to only those process machines, WIP storage, materials, etc. necessary to support those processes.

Lay out the clean facility incorporating people and material flow and integrate it into the other, non-clean areas of the manufacturing facility that relate to the clean area. After careful consideration it may turn out that only a small percentage of the floor space needs to be cleanroom rated with the majority of the square footage being a “controlled environment,” that is, conditioned and maintained but not HEPA/ULPA filtered, and having only a modest air exchange rate.

Developing a “clean workspace” mindset is a challenge for those accustomed to working in an uncontrolled lab environment, however if cleanliness is critical to the end product the discipline associated with working clean is vital. A gowning protocol should be established, as should a janitorial protocol. By using cleanroom garments, you are protecting the product from the people. By regularly cleaning the cleanroom, in a meticulous manner, the debris that is inevitable in a workspace, and that in turn may become a product contaminant, is removed.

The cleanspace itself, whatever the size, should have walls, floors, and a ceiling (Figure 1) that do not contribute to the to particle count within the space, and are easily cleaned. A hard, cleanable floor, such as epoxy clad concrete or a seamless (if possible) sheet vinyl would be an appropriate floor surface. As part of the gowning protocol dedicated slip-on lab shoes would be helpful (shoe covers are good, too), as the floor will inevitably be the dirtiest area, and a major source of contaminating particles. Walls can be as simple as drywall or concrete block with an epoxy or polyurethane finish. The ceiling can also by drywall, though, depending on the application, an inverted “T” grid ceiling with non-shedding panels (such as those clad with high pressure laminate, aluminum, or vinyl sheeting) offers considerable flexibility. There are many materials that will suffice, but when cost is a key, simplicity in installation and maintenance is best.

After the appropriate sized space has been identified and a plan has been developed for the clean, easily maintained envelope, other key control parameters should be identified. It is advisable that these parameters be related to product yield requirements rather than some preconceived notion of how a “cleanroom” should perform, or designed around a specification that is not relevant to your product or your industry. There is no standard “nanotechnology” cleanroom particle count or critical particle size. There is a particle count and size appropriate for your product. Frequently, however, you will not know what it is.

Before you invest in a cleanroom, you should have a fairly good idea of your product’s “killer particle size” and how many of these particles you can tolerate within your cleanroom space. Whether you start with the gold-plated cleanroom or work your way up to it over time is a business decision you face. As a rule, the smaller the particle (0.1 micron vs 0.5 micron) and the fewer of them to be tolerated (10 vs 100 vs 1000), the more airflow, the larger the air conditioning load, the higher the energy requirement and the higher the first cost and operating cost of the facility.

Similarly your products’ sensitivity to gases and vapors that may be present in the ventilation air stream should be assessed. Treatment of make-up air with activated carbon filters modified to remove specific gaseous contaminants may be required.

Temperature and humidity are most economically supplied if worker comfort is the driver (68-75ºF and 30-70% RH). Unusually high or low temperature, or humidity specification with tight tolerances (eg. +/-.2ºF), can require high end equipment and controls. This is particularly true if there is a large exhaust air volume, and therefore outside make-up air requirement, that has to be conditioned (and cleaned).

Cleanrooms are typically kept at an air pressure slightly higher than surrounding space to provide a limited amount of exfiltration and thereby prevent leakage of contaminated air into the cleanroom. A pressure on the order of 0.05 inches of water column is appropriate for your “starter” cleanroom. Higher pressures are more expensive, produce higher air noise, and are rarely required. A negative pressure cleanroom, one intended to prevent cleanroom air from leaking outward is possible, but unusual, and the need for such a design should be carefully studied before making an investment.

Other parameters, such as lighting level and quality, sound level, vibration, and air velocity, should be addressed as appropriate for specific nanotech applications.

When carving out a cleanroom within an existing plant space, it is tempting to maximize the use of existing plant services. Certainly if these services can be adapted to use within the cleanroom, economies can be realized. However some cleanroom specific caveats ought to be observed.

An air conditioning system should be dedicated to the cleanspace. Mixing air from cleanroom and “nonrated” adjacent spaces is not generally recommended. If an existing air conditioning system is available and can be dedicated to the cleanspace, the interior of the supply ducts should be cleaned. A HEPA filter should be installed on the supply side of the air handler. The extra pressure drop associated with this higher efficiency filter may require adjustment of the air handler fan speed or replacement of the fan motor.

Note also that standard comfort air conditioning, when called upon to provide humidity control, particularly dehumidification, may have to be reconfigured (or replaced) to provide such control. Humidification, typically required during cold times of the year, can usually be readily added on to an existing system. Depending on the size and complexity of the cleanroom, and its environmental parameters, it may be most economical to install a new system selected based on process requirements than attempt to modify an existing system.

The air handler of a comfort conditioning system generally provides approximately six air changes per hour (ACH) to the conditioned space. Utilizing this type of system may meet the cooling and heating requirements of the space but once the cleanliness requirement of the space exceeds “controlled environment,” additional HEPA/ULPA filtered airflow will be required. It is common for filtered airflow to increase from dozens of ACH at lower cleanliness classifications to hundreds of ACH in the most stringent cleanrooms. Therefore, the design of the conditioning system and of the filtered air recirculation system are usually best addressed separately. Application of filter fan units offers a neat solution to the requirement for high airflow rates of filtered air (Figure 2).

(Click Image For A Larger Version)

Nanotechnology product research and development eventually calls for a pilot line that permits the production process to be examined, fine tuned, and scaled up as volume increases. Often the facility serves to generate cash as small lots of product are manufactured for introduc tion to the marketplace. The small scale cleanroom facility offers the environment of a full scale production facility, provides the stringent environment commonly required for nanotechnology product manufacture, enables the process to be evaluated in a “real” environment, and offers the opportunity to establish real yields upon which financial projections can be based, all this at a reasonable cost per square foot so important to a start-up operation.

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