## 1 Introduction  
In recent decades, the semiconductor and optoelectronics industries have become important contributors to Taiwan's economic development. For example, the semiconductor industry, especially the wafer foundry, is the most competitive Taiwanese industry in the international market. Therefore, developing ways to reduce the manufacturing costs and improve the wafer yield are an important concern for the semiconductor manufacturers during the production of highly complex devices, using advanced manufacturing processes. The cleanliness of the manufacturing environment influences the product yield and the device performance [1; 2; 3]. For example, moisture and oxygen will induce oxidation or a loss of Cu on a wafer during the waiting period [4; 5]. In addition, there are three major factors (materials, contamination prevention or removal, and cleaning methods) that influence the cleanliness level of a storage box (such as a front opening unified pod [FOUP]) [6]. [PERSON] et al. [7] proposed a novel cleaning system of the FOUP for the next generation of manufacturing. [PERSON] et al. [8] explored the cross-contamination between FOUPs and wafers, and their resultsdemonstrated that the contamination concentration was affected by the solubility or diffusivity of the FOUP. They further suggested that the choice of FOUP materials should be based on the fabrication processes of the products. However, the traditional large-space clean room does not meet the rigorous cleanliness requirements (e.g., a lower particle concentration and lower degree of relative humidity [RH]) that have resulted from the rapid development of semiconductor technology. Therefore, an ultra-clean environment, which has been called a mini-environment, is generally installed inside a clean room, in order to reduce the possibility of cross-contamination (such as contamination from workers) and to maintain a high-cleanliness area for the process tools. The size of this mini-isolation region is much smaller than that of the external clean room, which can significantly reduce the maintenance (or operating) cost of this high-cleanliness region. In general, wafers are stored in a FOUP while waiting in the line of the manufacturing processes, and this FOUP will be mounted on the load port unit (LPU) in the mini-environment. Many studies have focused on the prevention (or removal) of the contamination that occurs while the FOUP door is closed. Purging a FOUP with a gas (such as nitrogen or clean dry air [CDA]) is an efficient method to prevent (or remove) contamination from the FOUP. The vacuum method is another efficient way to maintain the cleanliness of a FOUP [9; 10]. For example, [PERSON] et al. [11] employed a purge or vacuum approach to remove the moisture from a FOUP. Their results demonstrated that a higher purging flow rate using CDA provided better moisture removal. In addition, the moisture removal performance was improved by introducing flow distributors into the FOUP. They also reported that the RH distribution was more uniform at different locations in the FOUP when using the vacuum method, compared with the CDA purge process; and the RH evolutions during the purging and vacuuming processes showed exponential and linear decrease trends, respectively. Their results indicated that the vacuum method could successfully meet the rigorous RH limitation and rapidly reach the ultra-low RH level. For practical fabrication processes, the FOUP door is frequently opened, and during this time, a vertical air flow occurs in the downward direction and over the open cavity. Here, the FOUP is considered to be a cavity, and the air flow is provided by fan filter units (FFUs) on the clean room ceiling or on the top of the mini-environment. [PERSON] and [PERSON] [12] reported that the length-to-depth ratio of the cavity (L/D) affected the number of vortices generated in the cavity.