AMHS (Automated Material Handling System)

Notes

A 300 mm FOUP with wafers weighs around 9 kg, which is why automated handling became mandatory rather than optional.

Overview

An AMHS, or automated material handling system, is the network of overhead vehicles, storage stockers and other transport equipment that moves wafer carriers between process tools and storage in a fab without people carrying them. In a 300 mm fab it handles sealed FOUPs, each holding up to 25 wafers, under the direction of a material control system that takes its orders from factory software. Its speed and reliability affect how long wafers wait between steps and how fully tools are used.

How it works

The main vehicle in most 300 mm fabs is overhead hoist transport (OHT): small vehicles run on a track hung from the ceiling and lower a belt-driven hoist to pick up or set down a FOUP directly on a tool's load port or a stocker port. The same vehicles can serve moves within a bay of tools and across the whole factory. Stockers hold carriers in automated storage served by cranes, buffers in front of tools hold FOUPs waiting for their next step, and some systems add floor or rail-guided vehicles. Stored FOUPs can also be purged with nitrogen.

Software coordinates the movement. Factory dispatching decides what each lot should do next; a material control system turns that into transport jobs, commands vehicles and stockers, and tracks where every carrier is. When a vehicle reaches a tool, a hand-off sequence over a parallel input and output interface, defined in SEMI E84, makes sure the vehicle and the load port agree before a carrier is placed or removed, with defined steps for errors and recovery. Other SEMI standards define how transport and storage equipment report their state to the factory host.

A loaded 25-wafer FOUP weighs several kilograms, which is one reason automated handling is essential in all but the smallest fabs. Traffic congestion, vehicle faults and poorly placed storage all add waiting time, so the layout and dispatch rules of an AMHS have a direct effect on cycle time.

Examples

• Overhead hoist vehicles on a ceiling track lowering FOUPs straight onto a process tool's load port.
• Tool-front FOUP buffers, offered by Murata Machinery among its AMHS products, holding carriers close to busy tools.
• FOUPs purged with nitrogen while they wait in a stocker.
• An E84 hand-off sequence between an overhead vehicle and a load port, with defined error detection and recovery.
• A material control system commanding every vehicle and stocker in a fab and tracking where each carrier is.

Related terms

• FOUP
• MES
• Cycle time

Where it appears on the Hub

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