Current Issue The International Reference for Chip-Scale Electronics, Flip-Chip Technology, Optoelectronic Interconnection and Wafer-Level Packaging July 2002 Email the editor

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By Dan Nelson and Greg Stark, August Technology, Bloomington, Minn.

Figure 1. Automated Visual Inspection (AVI), a critical component of advanced package characterization

The momentum to develop and implement advanced packaging technologies, such as wafer bumping and wafer-level packaging (WLP), like other aspects of chip making, has been affected by the industry downturn.

However, WLP is also one of the first areas witnessing signs of new activity. The major driver behind the advanced packaging initiative is its ability to improve device performance and manufacturability. It also promises, ultimately, to reduce the cost of making chips.

Characterization steps, such as Auto-mated Visual Inspection (AVI) and CD metrology, are playing significant roles in the evolution of advanced packaging.

Moreover, lessons from the front end are being applied at strategic points in the packaging process to catch defective product and to provide process-enhancing data. This article examines that trend and the characterization opportunities for an advanced packaging bumping line.

Learning from the Front End

The new manufacturing technologies in wafer bumping are reminiscent of the early years in front-end processing. Process flows are essentially dynamic and specific process technologies are not standardized.

Over the years, inspection and metrology applications at the front end have moved from a highly manual quality gate near the End of Line (EOL) to a fully automated quality gate, and finally to fully automated process control throughout manufacturing.

The result has been greatly improved yields and reduced manufacturing costs that could not otherwise have been achieved.

Figure 2. Data analysis is a key component of AVI.

The Evolution

Will final wafer manufacturing facilities and wafer-bumping lines follow the same evolutionary path toward fully integrated in-line process control inspection and metrology solutions as the front end?

In general, back end manufacturing has operated with smaller budgets and dealt with significantly less expensive tools. It has focused on final inspections rather than on in-line process control. Its mission has been simple: Don't destroy what the front end of the line produces.

However, with major cost reductions at the front-end, the cost and performance spotlight is now on final manufacturing, providing new incentives to implement yield-enhancing steps such as inspection and metrology.

It appears that final wafer manufacturers are beginning to recognize the need for process inspection and metrology for long-term cost reduction and yield improvements.

The trend toward fully integrated process control for final manufacturing-and particularly wafer bumping-is enabled by new technologies and products that provide the necessary characterization information. This information is presented in a nondestructive, high-throughput and low-cost format that allows these systems to be seamlessly and transparently implemented into the manufacturing line within tight budget constraints.

Figure 3. Projected growth of die inspection vs. front end equipment market (source: VLSI Research)

Bumping Line Characterization Applications

In general, characterization for final manufacturing falls into two categories: defect inspection and CD metrology.

Inspection is primarily the detection and classification of defects, while CD metrology is the measuring and monitoring of dimensions and thickness of features and films. If we look closely at a typical wafer bumping process, we can identify several opportunities for inspection and metrology to add value through integrated process control and characterization.

Each of the characterization opportunities identified in Figure 4 has a special purpose and provides a value that ultimately allows for process optimization, yield improvements and lower manufacturing costs.

The characterization steps in a bumping process, with the exception of measurements and inspection on the bumps themselves, are not necessarily new.

The challenge that exists for bumping and all advanced characterization opportunities is to adequately define the requirements so that cost-efficient technologies and equipment can be developed and implemented.

Figure 4. Critical bumping line process step characterization applications

Automated Visual Inspection (AVI) can be applied throughout the process to monitor and control particles, contamination and material handling issues. The defect trends captured from these in-line process monitoring steps can be used to ensure equipment and the facility are operating correctly, enabling cost-efficient preventive maintenance schedules.

A 100 percent defect inspection is still most often applied as a final inspection to verify the quality of the bumps and to ensure that die are defect, particle, and contamination free before they move on to their final assembly process.

There is a growing trend to apply statistical analysis and defect review at this final point in the process, to capture the wealth of available information about manufacturing issues. New data management and analysis software, including forms of automated defect classification, are streamlining the data analysis process and allowing for the quick identification of yield improvement opportunities.

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