Issue



ISMI targets non-product wafer reduction


10/01/2008







EXECUTIVE OVERVIEW

The International SEMATECH Manufacturing Initiative (ISMI) has identified the use of non-product wafers (NPWs) as one of the significant costs to the fab. ISMI’s NPW working group is proactively identifying best practices to accelerate a reduction in NPW cost and use. To date, significant benefit has been realized by the working group participants, and further opportunities for NPW cost reduction have been identified. This paper describes one of those opportunities: tracking NPWs to further reduce manufacturing costs.

In addressing key factors of semiconductor manufacturing efficiency with member companies, ISMI identified the use of non-product wafers (NPWs) as one of the significant costs to the fab. NPWs are used for multiple purposes in semiconductor manufacturing, e.g., equipment qualification, process conditioning, and process qualification. Figure 1 shows typical NPW usage per tool type at multiple fabs. Since the quality of NPWs is almost the same as production wafers, the quantity of NPWs used in manufacturing impacts cycle time, manufacturing cost, and manufacturing capacity.

Each of ISMI’s member companies has been addressing the metrics of NPW usage, and now ISMI has a project to determine best-known methods and benchmark progress in reducing such usage. Tracking NPWs to further reduce manufacturing costs is one of the options.

Modern fabs use automated handling of product wafers with considerable wafer-level tracking. In contrast, NPWs are handled manually with only minimal tracking. To achieve a good balance between NPWs utilization and spending on silicon, the following capabilities should be provided for NPW tracking.

Visibility of NPWs. NPWs are currently handled separately from production wafers. They are frequently used for multiple purposes and reused until a criterion is reached. NPWs are internally recycled or externally reclaimed so they can be used multiple times. Because fabs use a large quantity of NPWs, it is important to collect NPW information for wafer location, specifications, and previous usage. It is also necessary to correlate these data with forecasted usage and improve visibility to help the fab operation reduce the number of NPWs used.

Maximized utilization of NPWs. The cost of a virgin wafer, a reclaimed wafer, a recycled wafer, and a reused wafer varies considerably. Using fewer virgin wafers is a cost-effective measure; hence an NPW tracking system is needed to maximize the usage of reclaimed, recycled, and reused wafers. Downgrading the NPWs to another usage is also cost-effective. To do this, the historical data for these various NPW flows must be adequately tracked.


Figure 1. Typical NPW usage per tool type at multiple fabs.
Click here to enlarge image

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Adjustability for multiple flows. There are various NPW flows in a fab, and the total number of NPW flows is quite large. These flows sometimes contain the same specifications during processes to create film, recycle, or reclaim. Because the NPW specification will easily affect the number of NPW flows, it is important to keep as common a specification as possible. Also, to recycle or downgrade NPWs effectively, they must be reallocated even if they have already started on another process flow. Because this operation is indispensable to the overall NPWs operation, it must be done correctly and be subsequently tracked.

Enhanced AMHS functions. The factory’s lot-handling system manages a variety of qualification wafers. If qualification wafers are handled manually, they can be easily damaged and data cannot be captured and stored correctly for subsequent decision making and analysis. Keeping coherent data and adequately handling qualification wafers are important activities.

Reporting and sampling frequency. NPW usage history must be reported not only to grasp current usage, but also to estimate future usage. To estimate the future required numbers of NPWs and their planned utilization date, a detailed usage report is required for each tool. Since tool qualification and process qualification sometimes use the same procedure, there is a potential for redundant qualification when using NPWs. Reporting detailed sampling data and analysis will help avoid redundant sampling. Continuously utilizing equipment data will allow equipment abnormalities to be detected earlier than with the normal NPW sampling period. By combining two or more equipment information streams, process performance can be predicted and substituted for measurements by NPWs.

Requirements for NPW tracking

Five key requirements for NPW tracking have been identified.

NPW visibility and wafer IDs. NPW visibility should be tracked under normal operation. Each NPW should be scribed just as production wafers are. The ID must comply with SEMI-M12-0706; it should also be both human- and machine-readable. The ID should be the same over the wafer’s lifetime to track its historical data, which need to identify production line, tool, and chamber for both the fab and the reclaim supplier.

It is advisable to use the same software for tracking both product and NPWs. The tracking capability should include the following features:

  • The software should provide basic operations such as dispatching, sorting, and verifications.
  • All dummy wafer information, which is located in the tool buffer station or port, must be kept.
  • The classification of a wafer, its history, the number of times it has been recycled or reclaimed, and its thickness are important for NPW operations and should be recorded in the same software used for product wafers and updated automatically.
  • If the MES does not adequately track NPW information, an electronic note associated with the carrier will help to understand the monitor type, thin-film type, thickness, and current number of times the wafer has been internally reclaimed. Because tracking is automatic, there are no sticky notes.
  • Reclaim suppliers must rescribe the same ID in the same position. If the ID cannot be read, the reclaim suppliers can rescribe it based on the IC maker’s requirements at that time.

Sorting, prioritizing, and managing NPWs. There are various activities and approaches to sorting and prioritizing NPWs. Figure 2 highlights the complexity of a typical NPW flow between process areas in a fab. Sorting is a basic function for NPW operation. Sorting should be supported by additional rules, such as the following:

  • Data mining approach for particle count wafers.
  • Downgrading rule
  • Reclaim wafer control rule
  • Cross-contamination rule


Figure 2. Typical NPW flow between process areas in a fab.
Click here to enlarge image

Each rule should work independently, without conflicting with other rules. Other features of these rules are as follows:

  • Managing the prioritization of each rule must consider actual fab inventory information.
  • A bank system helps to correct NPW information easily and execute downgrading effectively.
  • Tracking overall NPW history will minimize the sorting operation and maximize the NPW’s life.
  • Mutual data sharing between the fab and the reclaim supplier supports cohesive tracking.

Adjustability for multiple flows. Using NPWs, recycling internally, and downgrading wafers between process areas will complicate wafer-level tracking. Activities such as creating rules and establishing a banking system, registering total NPW flows, and operating with sufficient function will need to be supported. To use NPWs efficiently, a gradual introduction is suggested:

  • Consider internal reuse until a better wafer tracking system is installed.
  • Create a bank system and keep enough information so that a wafer-level tracking system for NPWs can execute downgrading and sorting.
  • Adopt a route-based cascade system that allows for alternative route move-outs. This system helps control where the wafers can go.

Enhanced AMHS function. The wafer handling system must be the same for NPWs as for product wafers. Just as for operational wafer-level tracking software, the AMHS operation should handle NPWs in the same way as product wafers. The goals are as follows:

  • Use the same handling system for product and NPWs
  • Use the same physical ID system for product and NPWs
  • Eliminate manual handling and manual data input
  • Maintain the same wafer handling system for NPWs as for product wafers

Any reporting done for product wafers must also be provided for NPWs. Additionally, NPW tracking requires other reports, e.g., recycling history, reclaim forecast, number of NPWs not utilized.

The periodic diagnosis and stability of a tool will be evaluated without NPWs by using tool fault detection and classification capability effectively. The practical use of equipment data and lot data can reduce the amount of NPW usage. This is a good opportunity to reduce the sampling frequency, which can be done by substituting tool monitoring data for NPW usage by using virtual metrology.

Conclusion

NPWs, which are used extensively by semiconductor fabs, have played an important role in diagnosing equipment condition and process performance. However, the total amount of NPWs used has a significant impact on production cost. A large number of process flows and specific rules govern NPW operation???some of them are very different from normal production flow. Recognizing these operations efficiently and conducting wafer-level tracking of NPWs are paramount to reducing their usage and improving fab productivity.

Akihiko Ikemura received his BS in mechanical engineering from Doshisya U. (Kyoto, Japan) and is a Panasonic assignee at International SEMATECH Manufacturing Initiative, 2706 Montopolis Dr, Austin, TX 78741 USA; ph.: 512-356-3915; email [email protected].

Harvey Wohlwend received his BS in mathematics from the U. of North Dakota and is manager of e-Manufacturing at International SEMATECH Manufacturing Initiative.

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