Three major factors affect the cleanliness of process equipment. First, the amount of external contamination which the product or process is exposed to must be minimized. Second, the amount of internal contamination generated must be minimized. Third, the internally generated contamination must be kept away from the product and process. Each of these factors must be addressed in the design of the equipment if an optimally clean process is to be realized. Let us take a look at each and the mechanisms which allow us to produce the desired results.Three mechanisms or tools which allow us to minimize undesirable exposure of the product are prevention, isolation, and sweep.
Prevention
By designing (or redesigning and modifying) equipment, we can minimize internally generated contamination. For example, the use of sliding surfaces and belts should be minimized or isolated from the process. The use of air cylinders should be minimized and, when used, the exhaust should be contained and directed into the house exhaust. Materials should be analyzed for shedding and outgassing. For example, PFA is orders of magnitude cleaner than polypro while Teflon is an order of magnitude cleaner than PFA. Usually (but not always), aluminum is far less contaminating than steel. The selection of anodized aluminum over stainless steel may remove serious contamination from the process. And the process itself should not be overlooked.
In many cases we can redesign the process to be less contaminating or at least redesign the equipment to cope with the element of process contamination. For example, in evaporation or photolithographic equipment we can design the process chamber with economical removable shields in order to periodically remove process induced contamination. In wet stations we can design the process tanks with filtration or reprocessing systems to keep contamination levels low. And in dry processing systems we can utilize inert gas backflow, recirculation fluid filtration systems and self cleaning cycles.
The actual amount of the equipment and mechanisms exposed to the product should be minimized. If we could design the equipment such that all of the mechanisms and components except for a final, small process chamber were located behind a wall we would not only isolate the process from the majority of internal contamination, we would isolate it from external contamination such as maintenance activities. (Maintenance, likewise, would be simplified since the requirement for stringent clean operating conditions would not be necessary during much of the maintenance activity.) The required isolation takes two forms, external and internal isolation.
External isolation is isolation of the process from all external influence. External isolation can take the form of though the wall installations, flexible or hard shielding, or the remoting of contamination sources. Utilization of a variety of systems such as SMIF, vacuum processing, Nitrogen purged processing, isolation carriers, transport tunnels and hanging shields can dramatically reduce contamination of the product or process.
External isolation also includes isolation or consolidation of processes in the layout of equipment can have major benefits. Design of the equipment to accommodate this is desirable. For example, pre-diffusion clean needs to be as close as possible to the diffusion equipment and in an area of absolute minimum traffic or personnel exposure. Photolithographic processes and Ion Implant need to be isolated from all other processes.
Internal isolation is the isolation of the process from internal influence. Internal isolation usually takes the form of well designed process chambers. Examples are load locked vacuum processing chambers, photolithographic processing cups and miniature SMIF like enclosures. Essentially, the intent is to isolate the product or product from all internally induced contamination. A bonus that results is that this effort usually results in reduction of contamination from external sources as well.
In some cases we can redesign to move internal contamination to the external realm. For example, many wet stations encompass multiple processes. Contamination from the vapors (or even liquids) of one process may affect another process. Utilization of systems like the two tank auto etcher eliminates this problem, and in many cases provides an economical version of automation and process control, at very little increase in cost.
Sweep is the most overlooked function in a cleanroom or equipment. Great emphasis is placed on particle counts, with little attention to sweep. The fact is, 90% of the function of a cleanroom or clean device is to sweep internally generated contamination from the area of susceptibility. It is very important to maintain the proper velocity and direction of air flow around the product and process. If the velocity is too low, there will not be an effective sweep. If it is too high, turbulence may result.
The room or device must supply the proper velocities. However, unless the equipment is designed to utilize the mechanism of sweep, little is accomplished. The basic principle is to design the equipment to provide the proper amount of air at the right velocities and to aerodynamically direct the air from the points of contamination in a direction away from the product and process. Techniques like visible vapor demonstrations can readily pinpoint sweep problems.
Clean is a system. Each component of each piece of equipment must evaluated and optimized for minimum contamination effect. There are a multitude of techniques available, many of which are inexpensive or even cost reducing. The importance of the equipment as a function of an overall clean system should not be overlooked.
And that's the truth!
