WINNING IN TODAY’S FIERCELY COMPETITIVE GLOBAL ENVIRONMENT often means operating under strict cost controls. Superior products may succeed and gain market share initially, but those products will not dominate their category unless priced appropriately; this usually translates into minimizing manufacturing costs, including the use of consumables.

Examining the composition, use, and efficacy of wiping materials to gain a clear understanding of where they
are used and for which applications.

Nowhere is this more evident than in the semiconductor industry, which has engaged in continuous and pervasive cost-cutting measures. Wiping materials, comprising a major component of the cleanroom consumables budget, have been the focus of many cost-saving programs. The results have sometimes been mixed. End users have found that, indeed, there are ways in which wiping materials can be used more cost-effectively. But sometimes, when focusing strictly on price issues and ignoring performance and contamination control issues, some facilities have found, to their regret, that device yields can be compromised by wrong choices. This paper will address opportunities for smart consumable(wiper) usage without endangering end-product performance.

Wipers as Agents for Contamination Control
First, consider why wipers are used in semiconductor cleanrooms at all. Their primary purpose is for removal of surface contaminants — primarily particles and molecular condensables. Camenzind [1] points out that only 10-5 of a monolayer of surface molecular contaminants translates to one million 0.1 µm particles. Routine wiping prevents re-evaporation of sur face condensables, removes surface particles, and minimizes contact transfer of particles from one portion of the fab to another. Occasionally, wipers are used to mop up liquid spills or to provide clean surfaces on which to place sensitive articles. But if the main objective is contamination control, then obviously, the cleanliness of the wiper, rather than its absorbency, is thekey attribute for selection.

Paper Towels
If price were the only consideration, one could settle on something as simple and as inexpensive as paper towels for wiping activities in the cleanroom. A far-fetched idea? This possibility was explored previously [2], at least hypothetically, in an examination of the impact of wiper fabrics on semiconductor yield. The observation was made that since wiper expenditures are less than 0.1% of the cost of making a chip, then choosing a wiper material that is almost free (e.g. paper towel), and which might, for example, lessen yield by 0.1%, is not a cost-effective solution, as that 0.1% reduction in yield would result in significant monetary loss over time. Paper towels cannot be considered as appropriate contamination control tools in a semiconductor cleanroom because of the high particle, fiber, and ion residues that are left behind when they are used to wipe critical surfaces. A cost/benefit approach on evaluating the cost-effectiveness of cleanroom cleaning materialshas been proposed [3].

So if price is not the only consideration, how does one decide which wiper material to use where? Some guidance can be provided by Hattori [4] who used a bull’s-eye diagram to illustrate the need to increase contamination control efforts as one progresses from the least critical area — the building — through the cleanroom, the equipment, and the chamber, finally ending at the most critical area, the wafer (at the bulls-eye). In other words, the closer you are to the wafer, the cleaner the wiper needs to be. In the limit, only the very cleanest wipers should be used to clean the equipment that contacts or houses the wafer. In this regard, it becomes useful to think of the wipers used for equipment cleaning during preventative maintenance (PM) as part of the process. Entertaining use of a less expensive wiper for PM should be evaluated, tested and decided in the same way as using a less expensive process gas or a less expensive chamber component; i.e. what effect will it have on the process and the yields?