A significant portion of the total wafer inventory in a fab consists of test wafers (ranging between 25 - 50%). Managing such amounts of test wafers in semiconductor fabs presents a unique set of challenges essential to maintaining productivity and efficiency. Test wafers, which include all non-product wafers, are fundamental to fab operations. Their importance often becomes visible only when they are not available, impacting operations significantly. Unlike product wafers, test wafers serve multiple specialized functions, such as process equipment qualification, conditioning for production readiness, and calibration of measurement tools. Consequently, they come in many varieties, including thickness or particle measurement wafers, conditioning wafers, filler wafers, monitor wafers, mechanical handling wafers, and calibration wafers, each requiring specific handling and preparation.
Ensuring Availability and Minimizing Downtime
One of the main challenges in test wafer management is ensuring the continuous availability of the "right" test wafer type when needed. This is critical because a lack of suitable test wafers at key moments, such as after maintenance or equipment re-qualification, can extend downtime. This in turn increases cycle time, thus reducing fab capacity and impacting productivity. To mitigate this, fabs maintain substantial inventories of various types of test wafers. For some fabs, these inventories are strikingly high, —sometimes reaching a 1:1 ratio between product and test wafers. This sheer volume emphasizes how critical and complex test wafer logistics is within a semiconductor fab.
Carrier Management: An Overlooked Complexity
Another challenge lies in carrier management, as test wafer carriers typically hold fewer wafers than product carriers. While product carriers can contain up to 25 wafers, test wafer carriers frequently carry just one to three wafers. This difference requires fabs to carefully manage both the carriers and storage space, ensuring enough carriers are always available for both product and test wafers.
Prioritization and Automation: Key Solutions
To address these complexities, test wafers need to be prioritized alongside product wafers, with well-defined setup protocols in the MES (Manufacturing Execution System) and clear ownership among engineering teams. Automating processes like kitting, de-kitting, and the transport of test wafer carriers plays a vital role in streamlining test wafer management and maintaining the overall efficiency of fab operations.
More storage capacity needed?
A combination of the TWC and our floor-based, fully automated storage system CubeStocker, gives you all the flexibility you need!
CubeStocker is a space-saving, fully automated storage system e.g. for open 200mm wafer carriers in cleanrooms. It creates an ISO 3 cleanroom environment for particle-free intermediate storage, helping buffer production without impacting yield quality.
The system can be placed in greyrooms (ISO 6) to save cleanroom space. It holds up to 200 wafer cassettes, uses RFID for material identification, and features a robot portal for precise, vibration-free handling.
