Minienvironments

Introduction

The potential for energy savings from minienvironments is substantial, if, upon implementing a mini-environment, the ventilation and filtration standards are correspondingly relaxed in the surrounding cleanroom space. Potential non-energy benefits are also significant, including higher obtainable cleanliness levels, improved worker safety, process control, fewer product defects, lower overall construction costs, etc. Cleanrooms have extraordinarily high rates of energy use in part because they are traditionally configured to maintain ultra clean conditions over very large areas. However, sensitive processes requiring high cleanliness levels are only carried out in relatively small areas within the larger cleanroom. "Minienvironment" technologies—essentially a cleanroom within a cleanroom—can be used to isolate these sensitive processes, thereby reducing the risk of contamination.

The concept of isolating critical spaces is not new, as evidenced by glove boxes used for many decades in hazardous research or manufacturing processes. Minienvironments go by various names, including "isolators", "separative enclosures or devices", or "barriers" in microelectronics, chemical, and pharmaceutical industry, and "safety cabinets" in the biomedical and healthcare industries. Isolation is desired for protecting contamination-sensitive products or processes and/or to protect people and the environment.

LBNL field tests have shown that—if cleanliness conditions are appropriately relaxed in the main cleanroom through the use of minienvironments—significant energy savings can also result. In addition, we found that the efficiency of minienvironments varies widely, depending largely on the quality of the fan filter unit used to provide filtered air to the minienvironment. Simply installing a minienvironment does not guaranty energy savings.

Case Study

LBNL conducted a case study, focusing on one of the promising applications: the semiconductor manufacturing sector, where particularly large amounts of energy are used in conventional cleanrooms. LBNL's initial research involves a collaboration with ASYST Technologies, Inc. to instrument and evaluate the energy performance of a minienvironment used to conduct research on the 300mm silicon wafer process. The current focus is on the operation of the air system associated with a minienvironment space, as distinct from tools or equipment within the minienvironment used to run the manufacturing process.

The dimensions of the first minienvironment studied are approximately 2'x4'x7.5', and it is located inside a traditional ballroom cleanroom. Once-through airflow is provided to the mini-environment by four 1'x2' fan filter units. Preliminary results are shown in the diagram. The positive pressure is maintained to control airflows and prevent contamination from the surrounding environment. In general, a lower air system EPI value indicates better minienvironment air system energy efficiency. Defining energy metrics such as this is not straight forward, e.g. W/ft2 versus, total facility energy, versus a production metric such as energy per cm2 of wafer produced. Processes vary widely, and thus so do the appropriate metrics.

The air-system's EPI (W/CFM) falls in the more efficient part of the range of those benchmarked in normal cleanrooms by LBNL (ISO Class 4 or Class 5), which suggests a significant overall energy savings potential due to the vastly smaller volumes of air that must be moved, conditioned, and filtered thanks to the smaller volume of minienvironments compared to that of full-scale cleanrooms.