The collective term for the equipment that automatically moves wafer carriers such as FOUPs between tools and storage in a semiconductor fab, and the control system that runs it. It consists of OHTs, stockers and related equipment.
Japanese name: AMHS(自動搬送システム)
AMHS (automated material handling system) appears in 2 practice questions across 1 topic, including Cleanrooms and equipment.
Field: Cleanrooms and equipment
AMHS stands for automated material handling system and refers to the whole system that automatically moves and stores wafer containers in a fab. In a 300 mm fab it consists of OHT vehicles that run on ceiling rails and lower FOUPs onto tool load ports, stockers that are automated warehouses where robots store and retrieve FOUPs, buffers that hold FOUPs temporarily near the rails, and lifters that move them between floors. A transport control system runs all this, receiving transport requests from the manufacturing execution system (MES) and assigning vehicles and storage locations.
Many fabs have used a layout of bays, each grouping tools of the same kind. In a bay layout, interbay transport links distant bays via stockers and similar equipment, while intrabay transport links tools within a bay to each other and to stockers. In 200 mm fabs, often only interbay transport was automated and lots were carried by hand within a bay. Because each transfer adds waiting time, modern 300 mm fabs widely use OHTs that handle both in a single system and carry FOUPs directly from tool to tool.
A FOUP holding 25 wafers of 300 mm weighs about 8 kg, which is a heavy load to carry all day, and a drop can destroy expensive wafers. A wafer passes through hundreds of process steps before completion and moves between tools at each one, so the number of moves is very large. Automation transports FOUPs at a steady pace around the clock, lets the fab track where each FOUP is at any time and prevents mistakes such as delivering one to the wrong tool. Fewer people moving around also reduces particle contamination.
In large fabs, hundreds or more OHT vehicles run on a single rail network, so congestion and route selection strongly affect delivery time. If a tool stops while waiting for a FOUP, capacity drops and the time wafers spend in the fab (cycle time) grows. Fabs therefore anticipate transport requests to dispatch vehicles in advance and place storage near tools to cut waiting time. During fab design, transport volume is simulated to decide rail layout, vehicle count and storage capacity. Redundancy so that a local failure does not stop the whole system is also important.
Answer: A. Stocker
A stocker is an automated warehouse with shelves that a robot loads and unloads, temporarily storing FOUPs waiting for tools. Kept in clean air or nitrogen, it is linked to transport systems such as OHTs and releases FOUPs as directed by the manufacturing execution system. Recently, small storage racks beside the ceiling rails have also come into use.
Automated material handling system. In a semiconductor fab it refers collectively to the OHTs that carry FOUPs, the stockers that store them and the control system that runs them.
AMHS is the name for the whole transport and storage system, and OHT is the overhead vehicle within it that actually carries FOUPs. An AMHS also includes stockers, buffers and transport control software.
An automated warehouse with shelves where robots store and retrieve FOUPs, temporarily holding those waiting for tools to become free. It is filled with clean air or nitrogen and connected to transport systems such as OHTs.
It is the core of the automated material handling system (AMHS), and large fabs run hundreds of vehicles or more.
In 300 mm wafer fabs, which system automatically transports…As an Amazon Associate I earn from qualifying purchases.
FOUP OHT (overhead hoist transport) Airborne molecular contamination (AMC) Downflow EFEM FFU (fan filter unit) HEPA filter ISO 14644-1 Mini-environment ULPA filter