Time synchronization a challenge for fab managers - By Gino Crispieri, Sematech

 Gino Crispieri
  7th-Dec-2006
Time Synchronization
The exponential increase in data collection rates required to meet future technology goals has been a cross cutting issue in the International Technology Roadmap for Semiconductors (ITRS). In particular, today’s factories have separate applications that must work together gathering context information to determine the next steps to be taken. In the Factory Integration Chapter is stated that new yield enhancement systems need to be developed to access and correlate information form multiple data sources. The greatest challenge to a comprehensive data management system required for yield learning is the ability to deal with and integrate data streams that are continuous, periodic, sporadic, and interval-based such that they can all be linked through some common coupling system or user interface and be resolved accurately.
Data collection in the semiconductor industry has been plagued with unreliable and inaccurate timestamp information. The critical cause can be attributed to the inability to distribute accurate time to the clocks within the tool, where timestamp latencies of up to two minutes have been observed. As the semiconductor industry adopt more advanced techniques for controlling processes and tools such as Advanced Process Control (APC) applications, time stamped and synchronized sensor data needs to be available. Usually, APC applications require between 10 ms to 1ms time stamping precision. Sampling data rates are also needed up to 10,000 Hz. As data becomes shared with other applications accurate synchronization of data from multiple sources is needed as it is the case of clustered tools, facilities and automation applications.
Accurate clock synchronization and timestamps are fundamental to achieving e-Manufacturing paradigms such as Advanced Process Control (APC), e-Diagnostics, real-time control and development of an intelligent automated knowledge base for rapid yield learning. Because of the multiple uses of time, and the need for consistency in manufacturing processes, time synchronization is essential in distributed factory environments to ensure quality and rapid diagnosis of process faults. In order to ensure the factory can readily synchronize a new equipment clock and determine the quality of the time and timestamps generated by that clock, equipment and 3rd party application suppliers should adhere to standards and techniques for time synchronization.
Synchronizing the systems in a distributed environment is not a trivial issue. Many elements in the end systems, networking devices, and network contribute to synchronization performance degradation. Because of these issues careful attention must be given to the architectural design of the time synchronization system as shown in Figure 2. Time synchronization issues in manufacturing are inherently similar, and therefore the solutions developed have been applicable to a wide range of industries spanning from telecommunications to automotive. Cross-industry solutions are ever-evolving, as the timing synchronization requirements become increasingly stringent. While each industry has its own specific timing requirements, there are a few common components among the solutions. Maintaining precise, accurately synchronized time in a distributed environment requires a solution comprised of a quality reference time source, a synchronization protocol for distributing time information to the network nodes, software to process the timing information, and a precision clock at each node requiring highly precise time accuracy and stability.
The international standard time, UTC, provides the most accurate and stable time reference and can be best acquired via a GPS receiver. Establishing a sound time synchronization and time stamping system requires an understanding of methods to obtain and distribute accurate time within the equipment and throughout the factory network. Evolving over 20 years, Network Time Protocol (NTP) provides a viable means to synchronize clocks down to millisecond accuracies using a pure software implementation over existing standard network components. Optimizing time synchronization performance requires the equipment, network, and communication paradigm to be amenable to leveraging the synchronization protocols. NTP is a time-tested, evolving synchronization protocol that has been used pervasively in Internet and enterprise LAN environments for accuracies on the order of 1 millisecond in a LAN environment synchronized to a GPS time source. Although under review at this time, an internationally recognized solution adopted by ANSI and IEC (IEC 61588), IEEE 1588 supports sub-millisecond synchronization accuracy requirements for next-generation industrial automation.
Time Synchronization Guidelines
ISMI has developed in conjunction with the Time Synchronization Working Group a set of guidelines to facilitate the establishment of effective factory time synchronization architecture. These guidelines provide direct recommendations to ensure synchronization quality. It provides methods for meeting the upcoming SEMI Time Synchronization standard’s requirements. Pointing out specific guidance on data time-stamping for ensuring data quality and listing in a comprehensive table current and upcoming time synchronization accuracy requirements for equipment and factory applications.
Figure 2 Accuracy/Precision Guidelines for e-Manufacturing Applications
Equipment Clocks Related Guidelines
Equipment clock related guidelines include recommendations for using NTP to synchronize to factory servers that provide stable and accurate time. Defines the base format for exchanging synchronized date/time based on ISO 8601. The guidelines include requirements for quality computer clocks and the implementation of a Clock object to provide synchronization quality parameters and error messages as defined in the SEMI ballot 4291B.
Factory Time Synchronization Recommendations:
The factory is also required to provide a fault-tolerant synchronization architecture that is capable of provide a quality time synchronization source. By providing a high quality clock and stable time source, the computers in the factory will eventually minimize the synchronization cycles. Network security is paradigm to ensure time synchronization is not affected by external attacks. The factory must adopt network security measures to avoid this type of problems. Minimizing jitter in network and applications during synchronization is also critical such that the network does not come overloaded. Sufficient clock resolution has to be ensured for the most stringent requirements and synchronization quality must be monitored for quality time synchronization and control.
Data time stamping
Data timestamps must adhere to an standardized format like ISO 8601 or a format adopted by SEMI standards. Timestamps must reflect the resolution of the clock. There is no reason to require high accuracy when the internal clocks do not support such resolution. Timestamps must reflect the time when the data was measured such that data correlation and proper analysis is possible.
Specifications in new standard for time synchronization
The Time Synchronization working group defined immediate needs and specific tasks for updating and developing SEMI standards to facilitate rapid deployment of factory and equipment clock synchronization. The new standard for time synchronization establishes factory clock synchronization to facilitate data merging by using a common time-stamp format, synchronizing to a consistent point of time-stamping, creates a timestamp traceability/quality and maintains compatibility with legacy systems. The new standard defines a Clock object. This Clock object is not used to synchronize the clocks internally but to allow a host system to query about the equipment or application time synchronization quality and status. The attribute values in the Clock object are based on current attributes defined in NTP.
Conclusion: Final thoughts Data from sensors and tools cannot be readily merged today to meet factory application needs because they lack accurate timestamps or because their values reflect an internal clock setting that is not synchronized to a factory date/time. Unreliable timestamps prevent advanced capability analysis of applications in the factory. It is expected that in the near future more tools will have their clock and internal ones readily synchronized with the factory. This implies that when possible all tool and tool components’ clocks should support synchronization. As described in this paper, synchronization solutions exist. NTP being one that is readily available in multiple platforms and operating systems. The key of successful time synchronization architecture if for the factory to provide accurate timing. Factory time servers and time sources should propagate timing information to all equipment and equipment modules within the manufacturing environment using a mainstream synchronization protocol like NTP. Although additional standards/guidelines are being developed to improve synchronization communication of equipment clocks, provide consistent time-stamping, and verify clock synchronization/time-stamping quality. Time synchronization is an initial step to addressing the overall data quality issue encountered today in the semiconductor manufacturing that limits the industry from achieving e-Manufacturing paradigms such as Advanced Process Control (APC), e-Diagnostics, real-time control and development of an intelligent automated knowledge base for rapid yield learning. With NTP as the current prevailing solution for disseminating time information, the industry has a time synchronization protocol sufficient enough to meet near-term accuracy demands. Meeting more intelligent and complex automation needs will require sub-millisecond accuracies. The industry should continue to follow and even actively participate in the developments of IEEE 1588 in order to incorporate the capabilities into next-generation equipment, network and other system design for meeting industry requirements. High-end factory networks with message latencies of less than one millisecond are sufficient to support available time synchronization protocols to their maximum capability. Therefore initial industry efforts should be focused on the equipment. The industry should amass experts to draft guidelines and standards to facilitate the communication of timing information between equipment and within a single tool. Equipment suppliers should also be aware of best practices relative to time synchronization and time stamping when designing future processing and metrology equipment tools.
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Domain: Electronics
Category: Semiconductors
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