Characterization Another AVI application is driven by the growth of subcontract bumping services. The arrangement of multifacility manufacturing has created a growing demand for 100 percent defect inspection for both incoming and outgoing inspection to provide a means of accountability and liability protection from damaged wafers coming in and proof of quality going out. Technologies for AVI are based on machine vision, i.e., cameras, optics and software algorithms. Recent developments in high-speed cameras, data processing and application software allow AVI systems to offer extremely high throughput.
Photolithography Characterization Process control and defect capture in the photolithography bay are critical because wafers are at a point where they can be reworked. Edge bead removal, coat and develop defects, and pattern alignment errors in addition to thickness and uniformity can be quantified by data analysis techniques. Using this information to assess the potential yield, it can be determined whether a wafer should be passed or sent back for rework. The move to 300 mm wafers is driving these applications toward automation, and high-throughput AVI techniques are enabling the implementation of low-cost characterization solutions. Metallization/UBMCharacterization Under bump metallization (UBM) is most often characterized with resistivity measurements using four-point probe technology, by thickness measurements using profilometry and CD metrology using AVI. Resistivity is sampled across a wafer prior to etching, whereas thickness and CD metrology measurements are made on the patterned lines. Opportunities to consolidate characterization steps exist, as new high-speed scanning 3D inspection techniques based on confocal or interferometry technologies, primarily developed for bump height measurements, now have sufficient accuracy to measure UBM thicknesses for advanced packaging applications. We can expect to see increasing implementation of these consolidated 3D metrology tools in the future. Bump Characterization Bump material volume characterization is ideal prior to reflow, when there is an opportunity-with some processes-to rework faulty product. Sampling can provide useful process information at this stage, but 100 percent bump inspection is needed to ensure quality and ease rework of defective wafers. Bump characterization is critical after reflow to assess the quality of outgoing devices and to capture information that can be employed to improve and optimize the process. Although sampling is still used for some applications, 100 percent bump inspection is becoming standard. For example, 100 percent inspection is critical for the detection of non-co-planar bumps that may cause significant damage to vertical probe cards used during electrical test post wafer bumping. Metrology Bump metrology and inspection are applications that have no corollary in the front end. For the most part, the tools and technologies have been developed specifically for bumping as the process has evolved. The techniques used for this application include 2D and 3D metrology to understand solder volumes and bump co-planarity and AVI to capture defective bumps and defects caused by the bumping process. Additionally, bump shear testing characterizes bump adhesion, and online SEM analysis provides a detailed picture of the interconnects. As bumping processes have evolved, we have seen bump size and pitch decrease significantly over the past several years to the point where new technologies like Rapid Confocal Sensor needed to be developed to successfully characterize the bumps.
Looking Toward the Future In the future, we can expect to see the evolution of automated process control for advanced packaging applications become more and more integrated, again following the model of the front end, as technologies become available. Furthermore, "cluster" characterization tools that can effectively and completely characterize any portion of the process-in addition to in-situ AVI and metrology modules that will integrate into process or handling tools-will become available. Tool Requirements Process characterization tools must exhibit several characteristics for successful adoption by device makers. The following list summarizes the capabilities needed for a backend characterization tool: 2D Inspection and Metrology
3D Inspection and Metrology
- Accuracy better than 2 µm - Repeatability better than 1 µm
Automated Handling
Beyond the specific technical requirements of the characterization tool(s) the following general characteristics must also be met. The most important characteristic is throughput. Process characterization tools significantly reduce their value if they are a process bottleneck. High-throughput tools will find additional applications, as they can be transparently introduced at any point in the process flow. Next, low cost is critical. The budgetary paradigm of final manufacturing will not change overnight, and, therefore, the successful implementation of any product must include low cost (cost of ownership). Third, these tools need to be inline and nondestructive. The most effective process characterization will result from the information that only volume production analysis can provide. Fourth, final manufacturing requires flexibility, due to the wide variety of products and form factors involved and the automation for nonstandard (front end) applications like film frames with whole or diced wafers, hoops, strips or waffle packs. Finally, in bump characterization, close customer/supplier relationships are required. It is critical that suppliers are willing to meet the specific needs of each device manufacturer. Conclusion Final wafer manufacturing offers the opportunity to adopt fully integrated process characterization and control solutions to reduce the cost of manufacturing. Fortunately, there are a growing number of flexible, high-performance and cost-effective equipment solutions in the marketplace.
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