Cleanroom Demands for the Next Generation

		At a Glance
	The semiconductor industry's demands in terms of more advanced cleanroom and contamination control capabilities include the elimination or reduction of airborne gases (not just particles); greater work comfort and efficiency; better control over static electricity; improved detection techniques; and, of course, the new demands created by the trend to 300 mm wafers and minienvironments.

Nearly 40 years ago the cleanroom industry flexed its tiny muscles in the aerospace industry. Today it is a multibillion-dollar, worldwide industry with its base in semiconductor manufacturing. As shown in Figure 1, the industry has grown at double-digit rates for the last decade and will continue to do so through the year 2000. The semiconductor industry portion of the market has grown rapidly, but other cleanroom applications have kept pace so that the semiconductor industry has captured slightly less than 50% of the total market on a historical basis.



1. The cleanroom industry has grown at double-digit rates for the last decade and will continue to do so through the year 2000.

The industry has undergone a great deal of change and adjustment over the years. Where once the suppliers were mostly small local companies, now large international corporations are focusing on this industry. The largest corporation volumes $2 million per day in the worldwide cleanroom industry. The industry has had to keep pace with rapid technological change in the semiconductor industry. If anything, the technology changes over the next decade will be even more demanding on the cleanroom industry than in previous decades. Here are some of the challenges ahead.

Global supply

Semiconductor manufacturers are locating their plants where the markets are. Since their markets are global, their plants are spread worldwide. Cleanroom hardware and consumables are a significant cost to a semiconductor manufacturer. More importantly, they have a substantial impact on yield. Furthermore, there are thousands of different cleanroom items that  must be evaluated and purchased. All these factors lead the purchaser to demand global supply and to attempt to standardize on product, irrespective of plant location. This demand has created the need for larger global suppliers. As a result, there have been a number of mergers and acquisitions in the industry that could be characterized as consolidation. Equally important, cleanroom companies have attained that critical mass to make public offerings and attract investment capital so they in turn can make their own acquisitions.

One-stop shopping

Along with global supply, semiconductor manufacturers are increasingly interested in   purchasing from fewer numbers of vendors. Several distributors are offering worldwide services that allow the manufacturer to write one yearly contract instead of thousands of purchase orders for individual consumables and repair part items.

Because the semiconductor industry typically uses outside garment processing facilities for cleaning and renting garments, it has also become convenient to deal with suppliers who will both furnish consumables and process and clean garments. This synergy has led to joint ventures in the United States and also to expansion of local European distributors to supply both distribution and processing services in multiple countries throughout Europe; originally the industry was comprised of individual distributors and laundries each operating in one country.

In the Asian cleanroom market there have been two distinct trends. One has been the expansion of international companies from the United States, Europe and Japan, and the other has been the growth and expansion of local companies. Korean cleanroom suppliers have been active throughout Southeast Asia. Malaysian and Australian companies have also experienced substantial growth. So by and large, cleanroom suppliers are meeting the needs of their clients. However, there is the need to keep up with the fastest growing markets and to make sure that there is an adequate supply of cleanroom products in these areas. As shown in Figure 2, China, Taiwan and the UK have all shown rapid growth, and each has a significant share of both the new equipment and the cleanroom consumables markets.

2. Suppliers of cleanroom products must address growing markets in China, Taiwan and the UK.

New cleanliness demands

In the past, the assignment for cleanroom manufacturers was to keep the air free of particles. That role is now expanded to clean the air of objectionable gases as well. The effects of airborne molecular contamination on defect density and product yield have become an important concern in semiconductor plants. The measurement of airborne molecular contamination at the parts-per-billion (ppb) level is now becoming a routine part of some fabrication facility certification protocols. However, more work is needed to understand contaminant concentration flux and its effect on semiconductor manufacturing processes. The challenge of the cleanroom industry is not only to remove these contaminants once they are in the airstream, but to avoid generation in the first place. This means selection of materials for cleanroom consumables and hardware that minimize the outgassing of objectionable organic vapors.

HEPA and ULPA filters used in cleanroom air filtration are made of fiberglass. There has been some evidence in recent years to indicate that boron contamination may be caused from boron in the fiberglass. One option for eliminating this source of contamination is boron-free glass. The other is a new design of ULPA filters made by W.L. Gore (Newark, Del.) from expanded PTFE membrane. Some side-by-side tests have shown substantially less boron contamination from these new membrane filters than from those exposed to fiberglass. As the line sizes are reduced, the effect of particulate contamination increases.

For highly mobile ions (such as sodium), a very small amount of contamination, present in trace quantities, can go a long way. A single crystal of table salt (NaCl) contains enough sodium to deposit one quadrillion atoms per square centimeter on 5000 150 mm wafers and thereby electrically destroy all of their circuits.

The need for greater particulate control is placing new demands on cleanroom manufacturers. The synthetic membrane filters mentioned above are one option for achieving greater particle reduction. However, there is a need for suppliers of all the products and consumables to design them for minimum particle generation.

The design of chairs is one example. BioFit (Waterville, Ohio) believes it has an advantage with its chrome-plated finish over companies that use painted or powder-coated finishes that may generate particles as they are exposed to shoe or boot abrasion. In addition to the particles, the case for electrostatic discharge (ESD) is also important. Painted or powder-coated surfaces will be insulative and not dissipate the electrostatic charges as a chrome-plated finish would.

Users of a chair equipped with a chrome-plated metal base or polished aluminum will have a conductive surface to rest their feet, which provides a natural path for the electrostatic charges to drain away when used on a conductive floor or mat. The base portion is a very critical part in the process of eliminating ESD from chairs and its user and should always be chrome-plated or polished aluminum.

Greater work comfort and efficiency

Cleanroom clothing has undergone a great change and continues to be improved to meet the increased cleanliness requirements. Whereas inner layers of clothing were uncommon a few years ago, they now are required in many fabs. An inner layer of full polyester underwear is utilized in addition to the outer bunny suits. Operators are also typically required to take a shower and use lotion to prevent skin flaking prior to final dressing and entering fabs. When they do put on the clothing they are likely to also don a bubble helmet with a breathing recirculator and exhaust blower that pumps the user's breath through a HEPA filter pack strapped to the waist. This prevents exhaled particles from contaminating the fab work space. Despite all these elaborate procedures and products, the wafer fabs of the future will require even greater cleanliness.

The semiconductor industry is necessarily concerned about the shortage of highly skilled workers. Future rapid growth is going to make it even more difficult to recruit and maintain the needed workforce. It is therefore essential that cleanroom product designers make the environment more pleasant and comfortable. This extends from the original design up through the clothing itself.

One area for improvement is gloves. Skin problems caused by latex gloves have been widely reported in the industry. At one semiconductor facility, nitrile is used for the inner glove, and vinyl is used for an outer glove to protect against the elevated sulfur in the nitrile gloves. Glove makers have been improving designs, such as increasing gripping potential under wet or dry and hot or cold conditions. A slippery glove leads to wasted time recovering dropped parts and increased employee frustration, as well as damage to parts. Baggy gloves can cause wearers to execute procedures awkwardly. One approach used by Berkshire (Great Barrington, Mass.) is to offer knitted polyester glove liners that increase comfort by wicking away perspiration from the skin's surface.

Reduction of static electricity

Besides attracting and holding dirt, dust, bacteria and other airborne contaminants, static electricity can damage integrated circuits (ICs) in process via a discharge mechanism. The sensitivity to ESD damage depends on the specific devices being fabricated. Typically, charges as low as 1 V or less will cause failure in modern ICs. Products such as floor material, gowns, carts, packaging, etc., must be considered for their static dissipating capability.

There are three methods of static control:

• Grounding to prevent charge storage. This means using measures such as grounding of work surfaces and product, static floor mats and the use of personal wrist straps.
• Antistatic coating, additives and sprays.
• Air ionization to increase air conductivity.

Gearing up for 300 mm wafers

The development of the 300 mm wafer is significant for cleanroom suppliers. There are a number of orders presently being processed by the cleanroom industry that are directly attributable to the conversion from 200 mm to 300 mm wafers. SEMI (Mountain View, Calif.) estimates that the transition to 300 mm wafers will initially cost $14 billion, but could eventually cut semiconductor manufacturing costs by as much as 30%, while increasing chip production quantities by a factor of 2.5 over the current 200 mm wafer. At least nine pilot production lines with 500-1000 wafer starts per month are projected to be in operation between the second half of 1998 and the first half of 1999.

The potential cost of this conversion has been greatly reduced by an agreement between the United States and Japanese consortia. This agreement has been documented in a report titled 'Global Joint Guidance for 300 mm Semiconductor Factories.' It identifies the need for automation, simplified automated material-handling interfaces and the use of front-opening unified pod and open cassette carriers.

Perfecting the minienvironment

The minienvironment is a localized environment created by an enclosure to isolate the product from contamination and people. The goal of the minienvironment is to attain the highest possible level of cleanliness around a product, usually with a minimum of space required. Minienvironments are adaptable to many situations.

Controlling molecular contamination on a global basis in a fab is impractical, whereas it is possible and being done with minienvironments. The most common airborne molecular contaminant is water vapor. Water vapor can be damaging to metalized wafer surfaces and vacuum processing tool performance. It remains as a dominant gas load for hours during pumpdowns of vacuum processing chambers and can influence the nucleation of particles and the regrowth of native oxide on silicon in an uncontrolled manner. These problems can be controlled with a minienvironment by introducing a relatively inert gas, such as dry nitrogen, argon or even dry air, in selected locations.

Any environment requires a fan and a filter. One of the biggest new trends in the cleanroom industry has been the use of fan/filter units in which each filter is integral with a fan dedicated to that filter unit. One of the problems in using these fan/filter units in many environments is that the heat generation within the process tool required an air conditioning system as well. Stand-alone air conditioning systems are awkward and expensive. Therefore, there is a real need for a combined filter/fan and air conditioning system. Hepair is now supplying such a system.

Improved contamination detection

Fabrication facilities of the future are going to need improved contamination detection in order to provide improved contamination control. The introduction of the minienvironment complicates the requirement since there can be as many as 400 individual environments to monitor. Of equal importance now is the ability to measure molecular contaminants.

At present, aerosol particle detection is the most advanced area in particle detection. Condensation nucleus counters (CNC) can successfully detect particles down to 3 nm.

Improvements still needed in CNC counters include larger sample volumes with shorter sample times and less expensive point-of-use of detectors capable of 0.1 µm detection. It is anticipated that in situ aerosol particle detection will become more widely used for characterizing process vacuum equipment and preventing cross-contamination as more different steps in processing are linked together. To be effective tools for this use, CNCs will need to function effectively at 0.1 µm and have capture efficiencies of 90%.

There is a need for refinement in the ability to detect particles on the wafer surface. The current state-of-the-art technology permits detection of particles in the 0.1 µm range using light scattering. Smaller geometries will continue to drive this size downward.

Topcon Technologies Inc. (Paramus, N.J.) has introduced the compact Wafer Surface Particle Measuring System (WM-3). The WM-3 can reliably detect particles on a wafer surface and transfer the data to a host computer for expanded analysis. Mean time between failures for the WM-3 exceeds 10,000 hours.

The problems associated with contaminants such as ammonia and n-methyl pyrrolidone and their impact on the DUV lithography process are well known. Molecular Analytics (Sparks, Md.) has developed two new products that allow monitoring at the low parts-per-billion to parts-per-trillion levels required. The ProSentry-IMS for ammonia and the AirSentry-IMS for n-methyl pyrrolidone are designed to monitor each specific gas without interference from each other or any other compound used in the semiconductor industry. Ion mobility spectrometry is possibly the only method of detection that can monitor as low as 500 parts-per-trillion and is unaffected by temperature and humidity.

TeloSense (Fremont, Calif.) offers a centralized, 20-point toxic gas monitor for semiconductor fab EHS program support. The system features a centralized multipoint approach to toxic gas monitoring. The system uses molecular emission spectrometry for parts-per-billion detection sensitivity.

Expanded supply capability

The growth of the industry combined with the switch to 300 mm wafers will require a substantial expansion by the world's cleanroom industry. The new fabs scheduled to go on-line are typically very large in size so that the supply requirements will be uneven, surging with the start-up of each new fab. The semiconductor industry has had mostly ups but has had its downs as well. Consequently, it has been difficult for the cleanroom suppliers to meet the delivery dates during the up periods and to stay profitable in the down periods. The trend toward larger facilities will therefore present still another challenge to the industry. However, the cleanroom industry has proved fully capable of meeting the challenges put to it by the semiconductor industry, and there is every expectation that it will continue to perform as it has in the past.

Robert McIlvaine is president of the McIlvaine Company, which he founded in 1974. He supervises the activities of 40 individuals including engineers, scientists and journalists involved in the air, water and energy fields. He has a Bachelor of Arts degree from Princeton University 1956. He was president of Environeering from 1968 to 1974 and an employee from 1962 to 1974. Phone at 847-272-0010 FAX at 847-272-9673.