Term · General Industry
Wafer lot
A group of wafers, classically twenty-five, that travels through the fab together and shares a common process history. Lots are the unit of scheduling, costing and yield reporting, which is why fab data is almost always sliced lot by lot.
- Also known as
- Lot, Batch
- Type
- Term
- Domain
- General Industry, Silicon and form factors
Notes
Lot traceability is a formal requirement in automotive and aerospace supply, where a field failure must be tied back to its wafer.
Overview
A wafer lot is a group of wafers that a fab starts, moves, processes and records together, so that every wafer in it shares one route and one process history. Lots are the unit that dispatching, cost accounting, yield reporting and traceability are built around, and most fab data carries a lot identifier. How large a lot should be is a real operational choice: smaller lots move faster but create more work for transport, tools and the factory's software.
How it works
A lot is created at wafer start and given an identifier in the manufacturing execution system, which then records every move, measurement and hold against it. On 300 mm lines a lot usually travels in one carrier, typically of up to 25 wafers. Lots are sometimes split, for example to hold or rework a few wafers or to run an engineering experiment, and may later be merged again.
Tools treat lots differently. On per-wafer tools such as lithography steppers and implanters, wafers are processed one at a time, so each wafer waits for the rest of its lot; FabTime's worked example shows a wafer in a 50-wafer lot at a one-minute step waiting 49 minutes, against 24 minutes in a 25-wafer lot. Per-lot tools such as wet sinks take about the same time whatever the lot size, so halving lot size can double their load. Batch tools such as furnaces combine wafers from several lots into one run.
Smaller lots also give smoother flow, faster detection of yield problems and shorter waits for hot lots, the high-priority lots moved to the front of queues. Against that, they mean more transport moves and more transactions for the execution system, and more lots for dispatchers to choose between.
Examples
- A hot lot of first silicon for a new design pushed to the front of every queue so the design team gets parts sooner.
- A lot split at an inspection step, with flagged wafers held for review while the rest continue down the line.
- A fab weighing a move from 50-wafer to 25-wafer lots to cut cycle time, against the extra load on per-lot tools and transport.
- HBM stacks traced die by die back to their original wafer lots so that assembly defects can be tied to process history.
Why it matters
Lots are how fab industrial engineers, production planners and automation and MES engineers see the factory: cycle time, work in progress and on-time delivery are all counted in lots. Yield and quality engineers slice data lot by lot to spot problems, and the lot identifier is the thread that lets a failing packaged part be traced back to its wafers and process history.
Related terms
Part of the General Industry glossary, with every term in the topic and the lifecycles, tools and standards that use them.
Where it appears on the Hub
Used on 6 pages of the Hub. In each group, the pages that mention it most come first.
Lifecycle stages and gates
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Last reviewed . Written by The Semiconductor Hub editorial team. The full glossary