Overhead OHT AMHS in Semiconductor Fabs: Boosting Wafer Transport

Upon entering a semiconductor fabrication cleanroom, one is immediately struck by the sheer size and complexity of the equipment. However, the second most noticeable feature is the multitude of small, automated vehicles traversing the ceiling, stopping, starting, switching tracks, and handling boxes of wafers. These overhead trolleys are a critical component of modern wafer fabrication, moving wafers efficiently across the facility.

Bays and Stockers in a Fab

Modern semiconductor manufacturing involves hundreds, if not thousands, of process steps over several months. A typical fab might house over a thousand pieces of equipment. To manage this complexity, fabs are organized into processing zones (e.g., Lithography, Etch, Ion Implantation), each containing rows or clusters of equipment known as bays. Some larger fabs may further group bays into cells, though this is less common.

Wafers are moved in two primary ways: - Intrabay: Movement between tools within a single bay. - Interbay: Movement between different bays.

Each bay includes automated storage areas called stockers, which function like automated parking garages for wafers. A fully stocked stocker can hold 150-250 boxes of wafers. These stockers replaced open racks, saving valuable cleanroom floor space, improving work-in-progress (WIP) tracking, and serving as buffer overflow and entry/exit points for a zone.

Wafers undergo extensive travel within a fab. In the late 1990s, a 200mm wafer in a 400-step process node could travel 8-10 miles and interact with up to 250 tools. With leading-edge process nodes now exceeding 1,000 steps, this travel has significantly increased. The extensive travel is due to: 1. Process Layout: Zone and bay layouts are dictated by process requirements, including chemical contamination concerns and the need to group tools for easier maintenance. 2. Repetitive Processes: Wafers often undergo the same process multiple times. For instance, in a 45-nanometer process node, a wafer might undergo lithography 12 times, requiring multiple visits to the same bay.

Moving Wafers in the Fab

To prevent contamination from dirt, dust, or human skin particles, wafers are never exposed outside of processing tools. They are stored in bulky containers called Front Opening Unified Pods (FOUPs).

Historically, FOUPs were transported manually on carts. However, this method was inefficient and risky, especially given the high value of WIP wafers (around $100,000 per FOUP in the past). This led to the introduction of Automated Material Handling Systems (AMHS) in the 1980s.

The first AMHS were Automatic Guided Vehicles (AGVs), which resembled serving carts with robotic arms. These AGVs moved on the floor, guided by overhead cameras or floor tapes, primarily for interbay transport. They would pick up 4-5 FOUPs from a bay's stockers and deliver them to another bay. Stockers also served as buffers, simplifying the process for human operators who loaded wafers into tools.

AGVs offered flexible routes but were limited by safety concerns when sharing floor space with humans, typically moving at about 1 foot per second. Obstructions caused delays. Some fabs adopted Rail Guided Vehicles (RGVs) on fixed rails for faster movement and higher throughput, but these still required dedicated floor space and safety partitions, which was an inefficient use of expensive cleanroom real estate. Cleanroom floor space costs have escalated significantly, from $350-$700 per square foot in the past to $89,000-$265,000 per "ping" (a common real estate metric) for ISO 4-3 cleanroom space today.

To address the floor space issue, PRI Automated introduced the Aerotrak in the 1980s, a monorail system mounted on the ceiling. This freed up floor space, with stockers raising wafers for the Aerotrak to transport between bays. By 1988, such interbay AMHS were common.

From Interbay to Intrabay Automation

By the early 1990s, the bottleneck shifted to human operators moving wafers between stockers and tools within bays. While an experienced operator could change out a wafer lot in 30-90 seconds, the sheer number of tools in a bay made this a significant constraint. The constant beeping of tools signaling for wafer changes highlighted the need for intrabay automation.

Intrabay AMHS proved more challenging to implement due to the need for systems to: - Monitor tool processing status. - Automatically interface with tools for loading and unloading wafers. - Handle potential misalignment and timing errors. - Overcome the lack of industry standards for tool interfaces, leading fabs to develop proprietary solutions.

Early intrabay systems used floor-running AGVs and rail-mounted trolleys, but these faced issues with limited floor space within bays. Automation remained restrained during the 200mm wafer era, often supplemented by human labor for "hot lots" (high-priority wafers) due to the complexity of programming exceptional behaviors.

The 300mm Problem

The transition to 300mm silicon wafers in the late 1990s fundamentally changed fab operations. This shift, driven by economics of scale, made manual transport unsustainable. - Tool Economics: Tools became significantly more expensive, necessitating continuous operation. Studies showed 15-20% of tool time was wasted waiting for operators or FOUPs in older fabs. - Physical Constraints: 300mm wafers are 2.25 times larger and proportionally heavier than 200mm wafers. A fully loaded 300mm FOUP weighs about 19.8 pounds (9 kg), which is beyond the "maximal acceptable weight of lift" for repeated manual handling over an 8-hour workday without injury. Surveys in 200mm fabs already showed high rates of shoulder and back discomfort among workers. - Increased Traffic: The higher cost of 300mm wafers meant more frequent movements (300+ per hour in high-traffic bays, compared to 125-175 for 200mm wafers).

These factors compelled fabs to adopt comprehensive automation.

Going Overhead

To address the challenges of 300mm wafers—larger tools, less floor space, and the need for intrabay automation—fabs adopted a second generation of AMHS that operated entirely from the ceiling. The key technology was the Overhead Hoist Transport (OHT).

OHT vehicles align above a tool and lower a hoist to pick up or drop off FOUPs directly from the tool's frontend interface. This process is quick and efficient. - Space Efficiency: Ceiling-mounted tracks free up cleanroom floor space. - Speed: OHTs can travel up to 60 meters per minute. - Challenges: Installation costs are higher, flexibility is reduced, and there's a particle risk as vehicles operate directly above tools.

Successful OHT implementation required close coordination with tool vendors. The SEMI industry group developed standards for tool loadports and optical signals to ensure seamless handoffs between OHT vehicles and tools, reducing costs.

While OHT systems can be augmented with RGVs or AGVs, a fully unified system capable of interbay and intrabay movement, stocker interaction, and tool loading/unloading typically relies solely on OHTs.

TSMC was an early adopter, opening Fab 12 in Hsinchu with a full ceiling-mounted AMHS for 300mm wafers, featuring 2,000 vehicles and 600,000 daily transport trips. Despite the benefits, the sheer size, complexity, and cost ($50-$100 million and 2 years for installation) of these systems initially made them intimidating, leading many fabs to continue relying on human wafer transport for some time.

Dispatching Vehicles

Designing an AMHS is akin to designing a complex transportation network. A central control center manages all interbay and intrabay vehicles and stockers, dispatching vehicles for jobs, optimizing routes, and mitigating issues like congestion, deadlocks, or downed tools. Fabs typically don't have surplus vehicles, so efficient dispatching is crucial. Special conditions, such as "hot lots" or rework, must also be programmed.

There is no single optimal algorithm for AMHS dispatching. Fab operators use simulations to optimize for goals like average delivery times, due date satisfaction, cycle time, or throughput.

Daifuku: A Leader in AMHS

The largest provider of OHT systems today is the Japanese company Daifuku. Founded in 1937 as a manufacturer of forging machines, Daifuku expanded into cranes and hoists. After World War II, they adopted the name Daifuku (combining characters from Osaka and Fukuchiyama) to distance themselves from their former conglomerate.

A pivotal moment for Daifuku was a licensing partnership with Jervis B. Webb, an American company known for rivetless chain conveyors used in automotive assembly lines. This partnership provided Daifuku with the expertise to supply high-quality chain conveyor systems to Toyota in 1959, benefiting from the rise of Japanese automakers. Daifuku later acquired Webb in 2007.

Daifuku entered the semiconductor industry in the mid-1980s, producing cleanroom items like stockers and the "Cleanway" shuttle. During the 200mm wafer era, they competed with domestic rival Murata Machinery and American company PRI Automated, which dominated with its Aerotrak monorail.

However, the transition to 300mm wafers reset the market. PRI struggled with the R&D costs for its Aeroloader hoist system and eventually sold to Brooks Automation, which later exited the industry. This allowed Daifuku and Muratec to consolidate the AMHS market, dominating it to this day.

Conclusion

AMHS significantly impacts a fab's productivity, particularly cycle time—the time required to process a wafer. Shorter cycle times enable faster iteration and turnaround. Fabs continuously experiment with new routing rules, algorithms, and dispatch strategies to optimize these systems. The intricate network of these automated vehicles highlights the advanced nature of modern manufacturing, making fabs feel like organic, highly sophisticated entities.