Planning Model for OHB Storage Buffer of AMHS in Wafer Fabrication

Abstract

300mm wafer fab is an extremely automation intensive factory which both process restrictions
and the functions of OHB (Overhead Hoist Buffer) are increased significantly In the past, the
planning of OHB did not conform to the status quo due to its versatility. Without a complete
OHB control plan, factory will affect the flow of on-site production and impact the efficiency
of production badly.
In this work, a comprehensive planning model of OHB is proposed, which takes requirements,
functions and efficiencies into consideration. Because OHB is comparatively cheap,
convenient and not occupies the space of cleanroom, it will be constructed as many as the
factory can afford. Therefore, the focus of the plan is how many OHBs filled with nitrogen are
needed. In detail, there are two types of nitrogen filling: continuous filling and refilling. Under
the proviso, Queueing model is applied to evaluate the expected waiting time for the function
of continuous filling and the probability of over the certain waiting time. Little’s Law is used
to estimate the queue length. The quantity of refilling station can be calculated by the
probability and queue length. It is hoped that through this planning model for the OHB
temporary storage area, the 300mm fab can operate more efficiently on AMHS, thereby
improving the overall production efficiency.