Legacy Tools, New Tricks: Optical 3D Inspection

White light interferometry and other optical approaches can help detect defects in advanced packages


Stacking chips is making it far more difficult to find existing and latent defects, and to check for things like die shift, leftover particles from other processes, co-planarity of bumps, and adhesion of different materials such as dielectrics.

There are several main problems:

  • Not everything is visible from a single angle, particularly when vertical structures are used;
  • Various structures, such as pillars or 3D structures on interposers (3D-IC, advanced fan-outs, 5.5D) can cast shadows or otherwise obscure details, making it imperative to supplement traditional inspection tools with new inspection methods;
  • Different materials have different reflectivity, and in a heterogeneous, multi-chip implementation, failure to fully inspect a device can allow defects to slip through.

One solution that appears to be gaining traction is a legacy technique called interferometry, in which light is split and recombined to form interference patterns. Because white light interferometry (WLI) is non-destructive and can penetrate great depths, it is starting to play a more prominent role as the industry moves into advanced packaging.

WLI, a subset of optical profilers, is one of several methodologies, including X-ray and ellipsometry, that were largely sidelined in the past because they were considered too slow. But as the value of chips developed at the most advanced nodes rises, and as some of these chips are used in safety- and mission-critical applications, the economics of chipmaking are changing.

In addition, the tools themselves are becoming faster and better. All of these approaches are non-destructive, and they can provide relatively quick results during production versus other methods, like atomic force microscopes (AFMs) and scanning electron microscopes (SEMs). Still, each tool has a specific, and sometimes overlapping niche, and every fab and OSAT utilizes one or more, depending on their particular needs.

“Optical profilometers have been widely used in wafer level packaging applications (such as copper pillar Cu profile and more) due to the advantages of non-destructive measurement and fast throughput (compared to the stylus type),” said Xiaosheng Wang, senior manager for metrology at Lam Research. “Among the techniques — white light confocal, laser confocal, white light triangulation, just to name a few — white light Interferometry has excellent z resolution and a reasonable throughput. It might not be fast enough for full wafer inspection in HVM (high-volume manufacturing) applications, but it is very suitable for R&D environments to cross-check on full wafer inspection results with other techniques.”

Several major vendors provide inspection tools, including Bruker, Camtek, KLA, Nova, Park and Zygo.  Some, like KLA, are optical and e-beam specialists, while others, like Bruker, provide complementary instruments, such as AFMs and x-ray tools.

“In the ’90s, due to shrinking transistor dimensions, FEOL already was dominated by atomic force microscopy (AFM),” said Samuel Lesko, director of technology and applications development at Bruker. “However, with the rise of advanced packaging, it became extremely important to effectively measure laterally. White light interferometry provides enough lateral and vertical metrology to successfully inspect advanced packages. So, for example, a WLI can examine die-to-die stacking, die-to-wafer, heterogenous integration for chiplets, and all the fan-out processes.”

In advanced packages, WLI can inspect fine pitch bumps laterally and vertically. (See figure 1).

However, it cannot inspect voids because the visible light does not go through. For that purpose, a different technology is needed, such as X-ray inspection.

Fig 1: WLI inspection in packaging. Source: Bruker

Fig 1: WLI inspection in packaging. Source: Bruker

How interferometry works
The basic principle behind interferometry hasn’t changed since 1881, when physicist Albert Michelson developed a beam splitter to divide one beam of light into two perpendicular beams. Similarly, in today’s commercial interferometers, now made with precision electronics, a beam of light from an LED is split into a reference beam, which goes to a mirror, and a measurement beam, which focuses light on the sample. The reference beam always travels the same distance, but the measurement beam’s path length changes as it scans the sample’s contours. The two beams are combined into an overlap called interference fringes, which appear as alternating bands of brightness and darkness. The moiré pattern of the interference fringes can be analyzed by computer or even the naked eye to find anomalies.

“This is an optical microscope basically,” Lesko said. “You can see gray level intensity images. And we use that image, for instance, to automatically extract lateral dimensions (CD), like inner and outer diameter together with the relative positions between the two, which is a key overlay parameter for packaging. The smallest details we can see in terms of lateral size is in the range of 0.5µm. And we can see that with a precision down to 0.02µm. On the height, we can go to 0.1nm, which is just a single layer of atoms.”

Fig 2: On left, WLI schematic. On right, moiré patterns. Source: Bruker

Fig 2: On left, WLI schematic. On right, moiré patterns. Source: Bruker

Modern interferometers (considered a subset of optical profilers) also can switch between broad spectrum white light and monochromatic light, such as green light, allowing inspection to balance between speed and precision. Interferometry based on white light uses a broader spectrum, which is useful for measuring the height profile, but it requires additional effort.

“Monochromatic light is for flat, smooth surfaces where you want a high-throughput measurement,” said Lesko. “White light is used to narrow the moiré into a specific height, allowing you to focus in on the defect. Our objective moves in a continuous motion at a constant speed and the camera takes about a hundred frames per second. So we are able to derive the z position for every pixel captured by the camera on a wider vertical range, from micron to millimeter.”

But all of this generates a huge amount of data, which in turn creates another problem. “For every device, whether it’s a sensor or something else, the gating factor often is the data transmission,” said Subodh Kulkarni, CEO of CyberOptics. “How much data you can extract per second often becomes the limiting factor on resolution and accuracy. In the backend, we’re using the most advanced GPUs, but we are cranking out 100 megapixels of data, 100 frames a second, and doing this with four or five cameras. It’s literally tens of gigabits per second. We have to be able to take the information from the data and throw away the rest.”

Use cases at the front end of line
Because of its ability to relatively quickly and non-destructively trace topography, the canonical use case for WLI is ensuring die flatness. At FEOL, it is used for failure analysis and optimization of the CMP processes. “WLI played a critical role in enabling monitoring and improvement of Chemical Mechanical Planarization (CMP) for 300mm wafers. This CMP control is vital for integration processes such as flip-chip and Wafer Level Packaging,” said a CEA Leti researcher. The inspection takes a few minutes for a 300mm wafer.

Another use case involves hybrid bonding. For example, during the CMP process, copper will erode more than the dielectric, causing a dishing effect. Currently, most of the inspection for this profile is done with AFMs, but the process can be slow. “People have something like 5 to 10µm diameter of copper and 100,000 paths they want to inspect to make sure that none of them get a critical recess that would prevent connection,” said Lesko. “If you miss one connection, the full die assembly would not work. There is no other way than just to measure the pad shape down to sub-nanometer. Regular inspection tools are not capable here. With a WLI and a software package, you could automatically measure full die in about two hours.”

Once the defects are found, an AFM or SEM EDX, which offer high lateral resolution, can be used for a more detailed inspection to understand the root cause of the defect.

Fig 3: Processing of lateral and vertical data reveals CMP recess (blue) and protrusion (red). Source: Bruker

Fig 3: Processing of lateral and vertical data reveals CMP recess (blue) and protrusion (red). Source: Bruker

Use cases in the back end of line
While WLI isn’t used inline in FEOL, for back-end processing it can be fully automated and utilized 24/7, allowing metrology to drive production for the next process layer. Consider TSV processes, for example, where the depth and accuracy of each etch needs to be assessed. Sometimes this measurement is performed by a cross-section with a TEM, but that technique is destructive, and thus, not scalable. By contrast, WLI can view the necessary unfilled depth directly on the wafer, as part of an inline process that doesn’t sacrifice good dies or production time. If there is a problem, a wafer can be sent back to the process equipment to undergo additional etching, and then returned to the WLI system to confirm the correction.

For fan-out packaging, the redistribution layer consists of multiple layers of interconnects that need to be aligned for proper electrical connection. WLI profiler provides the via depth, thickness of polyimide layers, and the overlay between the top layer versus previously processed bottom layer in a single measurement. It conveniently combines topography and intensity image measurements, with automatic extraction of CD and overlay, which in turn are used to correct the subsequent lithography step.

Different techniques
Other techniques, which are use-case dependent, don’t so much compete with WLI as play alongside it. For example, at vertical resolution, WLI can discover if a material is one atom off. Nevertheless, it’s rarely used for measuring roughness down to the sub-nm level. A faster choice would be a tool that uses scatterometry.

As Nova, a supplier of scatterometry tools explains, “Optical scatterometry is a method of characterizing unknown properties of a sample by measuring the reflection of broadband light from an object. The reflection varies by wavelength (color), polarization, and angle-of-incidence.” Like most optical techniques, scatterometry is non-destructive. Its particular advantage in the fab is that it is especially fast.

WLI isn’t the only legacy technique that’s now being used for inspection and metrology. Another is ellipsometry. “Ellipsometer is pretty simple,” says Nick Keller, senior technologist at Onto Innovation. “It’s been around for over 100 years. Basically, what you’re doing is you are throwing incident light, for instance, near infrared, and you are probing the polarization change of a sample.

As Keller explained in more technical detail in a recent article, “Using a mid-IR ellipsometer with a high brightness light source that operates from roughly 5µm to 11µm, the mid-IR wavelength range exploits inherent absorption bands in the common dielectric materials used in the manufacturing of 3D NAND to attain ultra-high z dimension fidelity (angstrom-level uncertainty) of etched holes.”

There are a range of tools to fill a variety of needs, but one tool doesn’t do everything. For a full inspection, each method has strengths and weaknesses. Unlike stylus profilers, such as AFMs, WLIs can inspect a surface without touching it, thus eliminating risk of damage. They also can produce images more quickly. However, unlike AFMs, they can only inspect up to 0.5µm.

“AFM will be used in the inline front end to measure the quality of the chips and NAND/DRAM memory with unprecedented capability to measure narrow opening with challenging high aspect ratios. X-rays measure through structures,” said Lesko. “So they examine stacks: Is there any void in-between the stack? Are there any voids in the bump? WLI can cover a wider area than an AFM and also be faster, as well as non-destructive. AFM is limited to approximately a 100 x 100µm scanning, and 10µm micron height. WLI overlaps with AFM because the smallest range it examines is closer to 50 by 50µm laterally, and it can be expanded to tens of square millimeters laterally and hundreds of microns vertically.”

Certain AFMs also can be used to push atoms and fix nanoscale lithography errors on photomasks. Optical tools, meanwhile, reach much higher throughputs and have greater depth range than AFMs.

“Admittedly, AFM metrology is not the fastest,” said Stefan Kaemmer, president of Park Systems America. “On the other hand, we had one instance where optical metrology wasn’t producing results for months, and our AFM got results in three hours. So you have to take it on a case-by-case basis.”

The same advice applies when deciding about which inspection tools to use. It remains important to ask about, and test, the depth and resolutions that vendors claim. But remember, there is no all-in-one answer. That can lead to a footprint problem in a fab, due to the necessity of installing several large machines. Recognizing this, some vendors offer combined instruments to better accommodate spatial requirements. Other vendors claim that a combined instrument may not be as good at each individual function as separate instruments. For now, there are no definitive answers, just competitive ones. But the good news is that the interiors of multi-layered structures are no longer impervious to inspection.

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