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Finding Defects In IC Packages

New inspection tools on the way, but there may still be holes in coverage.

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Several equipment makers are ramping up new inspection equipment to address the growing defect challenges in IC packaging.

At one time, finding defects in packaging was relatively straightforward. But as packaging becomes more complex, and as it is used in markets where reliability is critical, finding defects is both more difficult and more important. This has prompted the development of a new set of tools, but those tools are more expensive and there may still be some gaps.

The new inspection systems involve different techniques, such as optical, X-ray and others, to find defects and other problems in both legacy and advanced packages. Inspection systems are used to find defects in products. That’s different than metrology tools, which characterize structures.

In packaging, optical and X-ray inspection techniques are non-destructive, meaning they can inspect packages without damaging them. Both techniques are often complementary, but there are various tradeoffs. Optical tools are fast and used to find surface defects, but they are generally unable to see buried structures. In comparison, X-ray inspection can see buried structures with high resolutions. The problem with X-ray systems is speed, but some of the newer X-ray tools are faster.

Ultimately, packaging houses want non-destructive inspection tools that can help limit the use of mechanical destructive cross-sectioning methods. Often used in the flow, mechanical cross-sectioning is a process where the part is cut and examined with a tool to find buried defects and other issues in packages. Cross-sectioning works, but it’s a destructive method and not always practical for production.

All told, there is no one inspection system that can meet all requirements. Packaging houses require several inspection technologies, even mechanical cross-sectioning.

Nonetheless, IC package inspection is critical. Across the industry, packaging is playing a bigger role and becoming a more viable option to develop new system-level chip designs. “Advanced packaging schemes often incorporate many semiconductor devices in heterogeneous integration, resulting in increasingly high value packages,” said Stephen Hiebert, senior director of marketing at KLA. “Advanced inspection hardware and software solutions are necessary for accurate disposition of good versus bad die for multi-chip packages. All of these factors combined increase inspection loading, so advanced inspection technologies are necessary to provide the combination of throughput and defect detection.”

The challenges
There are many IC package types in the market. One way to segment the market is by interconnect type, which includes wirebond, flip-chip, wafer-level packaging (WLP) and through-silicon vias (TSVs).

Some 75% to 80% of packages are based on wire bonding, according to TechSearch. A wire bonder stitches one chip to another chip or substrate using tiny wires.

In flip-chip, copper bumps or pillars are formed on top of a chip. The device is flipped and mounted on a separate die or board. The bumps land on copper pads, forming electrical connections.

Fan-out, one WLP type, houses dies in a package. Meanwhile, TSVs are found in high-end packages like 2.5D technologies. In 2.5D, dies are stacked or placed side-by-side on top of an interposer, which incorporates TSVs. The interposer acts as the bridge between the chips and a board.

Each interconnect type enables a different package. Obviously, the goal is to develop packages with no defects. “You hope that your process has been set up with enough robustness. You don’t want to inspect quality in the design. You want to build it in,” said John Hunt, senior director of engineering at ASE.

Still, defects can crop up in packages during the production flow. Years ago, it was simple to find defects when packages were less complex with larger features. Now, packages are more complex with multiple dies. Plus, the features sizes are smaller.

“We are not down to the dimensions of the front end,” Hunt said. “But our optical inspections have been measuring about 5µm line and space. Now they are down to 2µm and below.”

At these dimensions, it’s difficult to find surface and buried defects. That’s where advanced packaging inspection fits in. “Advanced packaging continues to evolve to smaller packages with more functionality, more layers and finer features,” said Subodh Kulkarni, president and CEO of CyberOptics. “This increase in complexity is driving a growing need for specialized, high-precision measurement and inspection capabilities to detect defects and improve process control. Fast and accurate inspection and measurement are critical to improve yields and ensure long-term reliability.”

Going with optical
According to Zeiss, the packaging inspection/measurement process involves three different areas — failure analysis, technology development, and production. Failure analysis analyzes a package to determine the cause of a failure. Packaging houses use in-line and off-line inspection systems for production.

No one tool can do everything, so packaging houses require several inspection techniques, including optical, X-ray and others. It depends on the application. Typically, in production, packaging houses use optical inspection tools. Camtek, KLA and Onto Innovation sell optical systems for packaging. Recently, Nanometrics and Rudolph merged, creating a new company called Onto.

Optical inspection tools use brightfield and darkfield techniques. In brightfield imaging, light hits the sample and the system collects scattered light from the object. In darkfield imaging, the light hits the sample from an angle.

These systems are used in various places. In fan-out, for example, optical systems inspect each redistribution layer (RDL). RDLs are the metal traces that connect one part of the package to another. A fan-out package with three or four RDL layers may require between 10 to 15 inline inspection steps. Generally, the new optical tools can inspect RDLs at 5μm and below.


Fig. 1: Process flow for packaging Source: KLA

Optical systems are used for other package types as well. “We see an extensive in-line application of advanced optical inspections for complex processes such as fan-out, 2.5D and 3D-ICs,” KLA’s Hiebert said. “For advanced packaging schemes like 2.5D and fan-out, the need for advanced optical inspection is driven primarily by dimensional scaling, more complex processes, and higher requirements for quality. As feature dimensions for RDL lines, TSVs, or microbumps get smaller, the size of critical or killer defects also gets smaller. Optical inspection systems need to be more sophisticated in order to detect these smaller defects.”

Advanced packages may require other types of optical techniques. “With vertical integration, for example, there are bumps and pillars above the surface of the chip that have decreased in size and increased in number. Also, these bumps and pillars have shiny and specular, or mirror-like surfaces, as do the other substrates that create inspection and measurement challenges,” CyberOptics’ Kulkarni said.

For this, CyberOptics has developed an inspection/metrology unit based on phase shift profilometry. The technology, called Multi-Reflection Suppression (MRS), enables 2D and 3D inspections with data processing speeds in excess of 75 million 3D points per second.

“MRS sensor technology is designed to suppress errors caused by spurious multiple reflections from shiny and specular surfaces. The architecture with multiple cameras and projectors has an additional channel designed specifically to measure specular surfaces,” Kulkarni said.

Cross-sections vs. X-ray
Generally, though, many optical inspection tools are unable to detect defects in every part of a package. Some defects are buried or hidden and optical tools can’t see them.

In those cases, packaging houses may need to resort to other techniques, such as mechanical cross-sectioning, X-ray and others.

Mechanical cross-sectioning is a destructive technique that is widely used in the industry. For this, the package is encapsulated in epoxy, cut, and examined using a scanning electron microscope (SEM) or other tools.

Cross-sectioning is used to find buried defects and other issues in packages, which reside in the solder joints, interconnects and other regions. Cross-sectioning is destructive, as well.

“That’s a slow system. It’s limited in sample sizes. There are a lot of reasons that it’s just not acceptable for high-volume and flexible manufacturing,” said Thomas Gregorich, director of business development at Zeiss.

Fortunately, that’s not the only solution. Packaging houses also can use X-ray inspection to examine buried structures and other areas in packages. X-ray inspection is found in failure analysis, product development and production.

X-ray works, but it is also slow. This may limit its insertion points for production. Now, however, the industry is developing new systems that can help speed up the process.

Generally, X-ray has several advantages. “In principle, optical is usually faster, but it is limited to what you can see on the surface,” said Evstatin Krastev, director of applications at Nordson Dage, a supplier of X-ray inspection systems and other products. “With X-ray, you can go where optical cannot penetrate. X-ray goes through the body of the sample, so it can see what’s inside. Any optically-hidden joints, vias or interconnects is where X-ray comes into play. It’s anything optically hidden, which includes BGA joints, QFN packages, microbumps, TSVs, bond wires and MEMS.”

To be sure, X-ray inspection won’t replace optical. Each technology plays a key role in packaging. “Optical and X-ray should be considered complimentary. So we need them both,” he said.

X-ray inspection isn’t new. In the market for over 20 years, X-ray inspection for packaging incorporates 2D and/or 3D capabilities. Nordson, SVXR, Yxlon and Zeiss sell X-ray inspection systems, although not all tools are alike.

Basically, an X-ray inspection system for packaging is a 2D X-ray microscope. The latest systems have feature recognitions down to 0.1μm.

In operation, a sample is placed in the system. Then, a cone of X-rays about 180° wide is emitted by the X-ray source. “All materials absorb the X-ray radiation differently, depending on their density, atomic number and thickness,” Krastev said. “Thicker and/or denser material will absorb more of the X-rays.”

The inspection is conducted as the sample is being moved within the X-ray cone, including the top and angled views up to 70°. The latest X-ray systems create images with 6.7 mega-pixels, 30 frames per second and 65,000 grey levels.

In addition, Nordson offers a computerized tomography (CT) option for its 2D X-ray systems, which provides 3D capabilities. An X-ray imaging method, CT generates a 3D model of an object using multiple 2D images, enabling virtual micro-sectioning.

Originally, X-ray was used for PCB inspection and simple packages. Over time, the technology gained traction as packages became more complex.

“In many cases, 2D and 3D X-ray inspection work together hand-in-hand,” Krastev said. “Fine cracks and hands-on-pillow defects in BGA or bumped devices provide a good example for that. The 2D/2.5D inspection is efficient in finding the signature of the defect quickly. The next step is to use the slower, but more detailed 3D inspection to verify that these defects are what we think they are. Once this is done, the fast 2D inspection is used for further testing with high levels of accuracy, reliability and speed.”

Traditionally, X-ray inspection handles singulated parts. Recently, Nordson has expanded its efforts with wafer-level X-ray tools. These new X-ray inspection platforms provide an automated, high-throughput X-ray metrology and defect review capabilities for both optically hidden and visible features of TSVs, 2.5D and 3D IC packages, MEMS and wafer bumps.

Generally, X-ray inspection systems are based on a point projection source. Some use them for production, but these systems are slow.

In response, SVXR has developed a system based on High Resolution Automated X-ray Inspection (HR-AXI) technology. The system is targeted for fast in-line inspection/metrology for packaging. It also makes use of machine learning for defect detection.

“We have speeded up the process by about 100 times faster than what you can do with a point projection, failure-analysis type of X-ray machine,” said Scott Jewler, COO at SVXR, a startup supplier of X-ray inspection systems.

SVXR’s system is different than current technologies. In a traditional system, you take a sample and put it next to the source. The detector is situated away from the sample.

“In our case, the sample is against the detector directly. We use a flood source. It’s fast. You get a wide dynamic range. We can detect defects with a single image,” Jewler said. “We’re transmitting through the sample. What we are looking for is absorption, not refraction. We’re measuring how much the X-ray gets absorbed as it goes through the sample.”

The system incorporates a 30 mega-pixel sensor, which is used to image a 12mm x 18mm field-of-view. Each image can include tens of thousands of solder joints.

In just one example, the tool can inspect a 2.5D package with a logic die and high bandwidth memory (HBM). The package has 20,000 solder joints between the interposer and BGA laminate.

For this, the system takes 9 images in this part of the package. Each image takes about 3 or 4 seconds. So, it takes about 30 seconds to inspect 20,000 solder joints.

This is a critical application in other respects. HBM consists of a DRAM stack. Typically, X-ray exposures can damage DRAMs. But SVXR has demonstrated the ability to inspect DRAM (HBM) without causing a degradation in the refresh rates.

In addition, SVXR’s system incorporates machine learning algorithms, which automates and speeds up the defect detection process. In the system, the images are digitally aligned. Each solder joint is identified and a database is created for each structure.

“For inspection, individual solder joints are compared to a model of a normal solder joint and variation in the test solder joint from this normal structure is characterized,” Jewler explained in a recent paper. “Various techniques utilizing computer vision and machine learning are used to generate reference models and to determine how a test joint differs from the model. Solder joints that differ sufficiently from the normal model are further classified by comparing these to known defect types and their attributes. In this way, subtle differences such as those between a normal contact and a contact non-wet can be identified at high speed under continuous inspection operations.”

Meanwhile, Zeiss recently introduced an X-ray inspection system targeted for product development, design verification, process optimization and quality assurance/control.

Using 3D X-ray microscopy, Zeiss’ system provides volumetric and linear measurements for advanced packages. This enables the systems to see buried features that cannot be achieved with cross-sections, 2D X-ray and microCT.

It has a 500nm spatial resolution with minimum voxel size of less than 40nm. The automated system can be used for 2.5D interposers, high bandwidth memory stacks with TSVs and microbumps, wafer-level packages with package on package interconnects, and ultra-thin memory with multiple chips in a stack. “It is needed for these HBM 2.5D and advanced fan-out type packages,” Zeiss’ Gregorich said.

“Usually, customers look at metal structures. In particular, it’s the metal structures that are formed during the manufacturing process. For example, we can look at substrates. We can look at bumps. Those existed before the part went into final assembly,” Gregorich said. “But when it comes to the bump reflowing onto substrate or onto another chip, there was no way to inspect that before.”

X-ray can see the process in action. “Most of those tend to be solder-based. So we are looking at solder bumps, whether they’re C4, copper pillars, or micro-pillars,” he said.

More techniques
In packaging, there are other inspection/measurement techniques as well, such as infrared thermography (IRT), magnetic current imaging (MCI), scanning acoustic microscopy (SAM), and surface acoustic waves (SAW), according to a recent paper from KAIST.

IRT is used for material evaluation, while MCI measures magnetic fields. SAW uses ultrasonic waves for measurements. SAM generates an ultrasonic signal, which hits a sample. This creates images of variations in the mechanical properties of samples, according to KAIST.

Conclusion
Packaging vendors may require these techniques as well as optical and X-ray. Cross-sectioning is a necessary evil.

Over time, the industry may need to invent new inspection techniques. They may need to improve the existing ones. But this will take funding, and typically the packaging houses have tight budgets. That may need to change, especially as packaging is becoming a more important option for new designs.

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