Yield management is a key performance metric in the semiconductor industry. It is used to gauge the efficiency of a semiconductor manufacturing process by determining the percentage of wafers that are considered functional and are up to the mark based on predetermined standards. The higher the yield, the more efficient the manufacturing process is considered. Consequently, as the industry evolves and the demand for semiconductors increases, the focus on enhancing yield is becoming progressively more pronounced. The overarching goal of a
yield management system is to improve production yield outcomes while reducing cost inefficiencies associated with waste.
Understanding Yield in Semiconductor Manufacturing
In the context of semiconductor manufacturing, yield is the proportion of usable wafers generated against the total number introduced at the beginning of the process. Yield can be broadly classified into wafer fabrication yield, wafer sort yield, and packaging yield.
Wafer Fabrication Yield
This refers to the percentage of wafers that successfully undergo the fabrication process, i.e., where the semiconductor devices are formed on the silicon wafer. This stage is most prone to defects and, consequently, yield loss.
Wafer Sort Yield
Also known as probe yield, this is the percentage of dies on a wafer that passes the wafer testing, often referred to as the ‘wafer sort’ or ‘probe test’.
Packaging Yield
This measures the percentage of dies that successfully go through the assembly, packaging, and final test semiconductor processes to emerge as a complete, working chip.
Yield loss can occur at any of these stages, and identifying, rectifying, or preventing the root causes is crucial for overall yield improvement.
Yield Ramp: Maximizing Efficiency in New Technology Integration
Yield ramp refers to the period when process adjustments are made to a new technology or design to enhance its yield. It is a critical phase in the life cycle of a semiconductor product, given the significant financial implications associated with the speed of yield improvement. The semiconductor yield ramp includes the characterization of new process technologies, equipment, and structures, the identification and rectification of process flaws, and the optimization of equipment performance.
Equipment Performance: The Impact on Semiconductor Yield Rates
The performance of the equipment used in the manufacturing process significantly influences the yield in semiconductor manufacturing. Older or outdated equipment often becomes a potential point of failure, triggering sequences of failures that could detrimentally impact the entire manufacturing process. They may cause defects during wafer fabrication or even during the wafer sort and packaging stages, leading to yield loss.The primary challenge in maintaining older equipment lies in the costly upkeep and the potential shortages of replacement parts for systems that are over two decades old. Resorting to second-hand parts from unverified sources or transitioning to newer models introduces an additional layer of risk and cost. This underscores the need for a cost-effective and reliable solution that maintains high yields while ensuring the integrity of the manufacturing process.
Leveraging Equipment Upgrade Kits for Yield Improvement
Upgrade kits provide an efficient, drop-in, plug-and-play solution that can enhance the performance of older equipment without necessitating an extensive overhaul of the manufacturing process. These kits allow operators to address and rectify individual issues in a targeted manner without causing extensive disruptions.Moreover, upgrade kits maintain the form and function of the original equipment, eliminating the need for further fabrication. By doing so, they support continuity in the manufacturing process and minimize downtime, both of which contribute to maintaining and potentially improving the yield.
Utilizing Wafer Map Generators for Yield Enhancement
A
wafer map generator is a tool that helps visualize the location of defects on a silicon wafer. It creates a ‘map’ of the wafer, highlighting the areas where defects have been detected during testing. By identifying the type and location of defects, a wafer map can provide valuable insights into the possible causes of yield loss.The data from a wafer map generator can be used in a yield management system to drive continuous improvement efforts, identify patterns of failure, and make informed decisions on potential modifications in the process or equipment.
Interpreting the Semiconductor Wafer Yield Formula
The wafer yield formula is a mathematical representation used to calculate the yield in semiconductor manufacturing. There are several variations of the yield formula, but the most commonly used one is the Seeds formula:
Yield = (1 – (Defect Density x Die Area)) ^Die CountThis formula represents the probability that a die will not have any defects, considering the defect density and the total number of dies on the wafer. A lower defect density or a smaller die area would generally result in a higher yield.
Conclusion
Improving yield in
semiconductor manufacturing involves a multi-pronged approach encompassing the careful monitoring of the manufacturing process, maintaining the equipment’s optimal performance, and making informed, data-driven decisions for continual improvement. Tools such as upgrade kits and wafer map generators, coupled with a thorough understanding of the yield formula, can contribute significantly towards this goal.
References
- Bhardwaj, N., et al. (2018). Semiconductor Manufacturing Technology. Springer International Publishing.
- Campbell, Stephen A. (2001). The Science and Engineering of Microelectronic Fabrication. Oxford University Press.
- Ayers, John E. (2007). Digital Integrated Circuits: Analysis and Design. CRC Press.
- May, Gary S., and Costello, C. J. (1997). Fundamentals of Semiconductor Manufacturing and Process Control. Wiley-IEEE Press.
- SICARA. (2018). Semiconductor Yield Management: the “Big Data” Problem. SICARA Blog.