Engineering microscopes zoom in on defects

Ever since 17th century scientist Anton van Leeuwenhoek peered at microbes through a crude optical microscope, the quest for higher performance has been relentless. Just as Leeuwenhoek’s microscope impacted the science of his day, today’s microscopy systems affect the way engineers develop and test semiconductors and materials as well as technologies such as microelectromechanical systems (MEMS).

Both destructive and nondestructive microscopy tools now complement and surpass the scanning electron microscope (SEM), an iconic fixture of the well-equipped lab. Newer-generation instruments, some reasonably priced and remarkably user friendly, are moving imaging and analysis tasks that were once outsourced back into the manufacturing environment.

Makers of microelectromechanical systems, or MEMS, look to microscopes that can handle 3-D physical characterization and metrology, along with deep-submicron precision. Courtesy of Hyphenated Systems.

“3-D imaging is especially critical,” explained Terence Lundy, VP and managing director at Hyphenated Systems, a manufacturer of microscopes for wafer-fabrication applications. “Combining 3-D with metrology lets you look at and measure the dimensions of semiconductor features such as trenches and through-silicon vias [TSVs] while measuring attributes like surface edges and roughness.”

TSVs are created during wafer fabrication or during subsequent assembly and packaging. TSVs provide the interconnects for die-to-die and wafer-to-wafer stacks, eliminating wire bonds and resulting in highly dense, small form-factor devices.

To enable engineers to see nanotechnologies such as TSVs, Hyphenated’s hybrid HS Series NanoScale Optical Profilers combine conventional white-light optical techniques with a patent-pending approach dubbed Advanced Confocal Microscopy (ACM). ACM, as an open architecture, also supports enhancements.

ACM permits observation of sloped or rough surfaces, buried interfaces in transparent materials, and high-aspect ratio features found in MEMS, ink jets, digital light processors, and electrical test probe cards. Software provides a control interface for the imaging, metrology, and automation functions. In addition, measurement routines extract critical dimensions.

If need be, ACM can meld an atomic force microscope (AFM), spectroscopy, and interferometry, combining these to meet not only imaging and measurement needs but also analysis requirements. Hyphenated’s systems range in price from about $75,000 to about $250,000.

Lundy pointed to stacked-multichip technologies as an emerging application for ACM. “IC companies are taking 3-D packaging and interconnect seriously,” he said. “Some chip designers are even using hollow TSVs. Air or gas pumped through these flow holes cools the innards of a stack.”

The HS Series NanoScale Optical Profiler combines 3-D imaging and metrology. Software acquires and renders detailed 3-D models of samples in a matter of seconds, with deep-submicron lateral resolution and near-nanometer vertical resolution. Courtesy of Hyphenated Systems.

“In 3-D chip packaging, perfectly flat surfaces don’t exist,” said Lundy. “In fact, some surfaces are intentionally placed at angles. The limited ability to probe beyond certain depths, and the inability to peer through layers, is a roadblock to 3-D chip observation using interferometry.”

Likewise, AFMs, which actually touch DUT samples, can’t get inside 3-D structures. “You can see surfaces well enough,” said Lundy, “but not structures below the surface.”

In contrast, confocal microscopes require no sample preparation and no tipping or tilting of DUTs, and they can be used through transparent media such as glass. Hyphenated Systems’ instruments combine ACM profiling with off-the-shelf Nikon white-light inspection microscopes. The mix ensures z-axis resolution down to 5 nm. Moreover, unlike microscopes that generate ion or electron beams, the company’s instruments don’t require samples to be placed in tricky vacuum chambers.

“The 5-nm resolution is beyond the wildest dreams of most IC makers,” claimed Lundy. “Folks who used to use a SEM or AFM can now use ACM and avoid long preparation times and destructive testing.”

Hyphenated Systems’ microscopes employing ACM also make measurements from a single optical axis. When the need arises to make subsequent measurements, on-chip x-y coordinates can be accurately relocated.

In addition to providing 3-D images of ICs, the latest microscopy tools enable engineers to inspect for damage caused by wafer probes, including damage caused by the ultra-small probe tips and cards required for thinner semiconductor materials and ultra-small bonding pads. As chip density skyrockets, wafer probing requires more touch downs. If the pins that touch down on a pad are not coplanar, the probe tips can dig into surface material, damaging the pad or punching through to other layers.

Although a ball can be bonded to a damaged pad, downstream failures are likely to occur after a period of time. “Walking wounded chips can fail after some months,” said Lundy, “with balls breaking away from the pads. In an end product, this is disastrous.”

Probe marking is also an issue. Probe marks result from physical contact between probe tips and pads, and they’re fairly common. But excessive marking diminishes yield and device reliability. As with punch through, large or deep marks or misplaced probe marks can result in poor adhesion with wire bonds, especially in high-density multichip packages.

Some bonding pad layers and posts can also shed material during probing, resulting in conductive particulate contamination. IC makers want to be able to see these potential defects prior to sawing a wafer into dice.

SEMs and AFMs are usually used to inspect for punch through and probe marking, but observation can take hours or even days. That’s not optimum for wafer fabs cranking out ICs round-the-clock. “Chip makers want closed-loop metrology,” said Lundy. “People want to do trend analysis on-the-fly, with millions of touch downs in probing.” ACM supports such analysis.

In addition to performing failure analysis and device characterization, some microscopes offer features that help concurrent development cycles. Focused ion beam (FIB) products from FEI, for example, provide an interactive technique the company calls V600 Circuit Edit.

Circuit Edit lets you identify and correct problems at the nanoscale on a component by breaking and making electrical connections. It can actually produce functional prototypes to allow manufacturers to perform validation for product development in parallel with the production of corrected masks for 65-nm devices and smaller.

Depth of field is significant where there’s considerable variation in z-height. This image of pads and wire bonds shows how a large depth of field keeps the entire image in focus. Courtesy of FEI.

These multi-technology instruments use focused ions to drill holes in samples, with a typical hole measuring about 10x10 microns. The tool can delve inside the holes with an electron beam to inspect deep layers for shorts and opens. The locations and addresses of the failures are derived from automated test equipment (ATE) test vectors.

“You can cut a cross section with the ion beam and watch with the electron beam at the same time, to localize defects,” explained Carleson. “SEMs also use shorter wavelength beams than visible light systems, so resolution is higher.”

Another benefit is improved depth of field. “Optical microscopes have a depth of field on the order of a few microns, but SEMs and ion beams have 10-micron to 50-micron depths of field,” noted Carleson. Depth of field is significant when inspecting MEMS devices, where there’s a considerable variation in z-height. Multi-technology scopes let you see an entire sample with everything in focus at the same time.

If higher resolution is needed, ion-beam microscopes can also slice samples. Typically, a 50-nm-thick slice is placed inside a transmission electron microscope (TEM). Working much like a slide projector, the TEM then dishes up an image with atomic-level resolution.

Finally, an instrument like FEI’s Expida 1255S wafer DualBeam can also be used for electrical probing. “This is a fast-growing application area,” concurred Carleson.

Supporting development of next-generation products, FEI’s desktop-sized Phenom microscope spans millimeters to the nanoscale. Courtesy of FEI.

“It’s a back-end tool,” said Carleson, “letting you prepare samples for other microscopy techniques.” Operated by means of a touch screen, a Phenom provides magnification from 10X up to 20,000X (about 20 times higher than optical microscopes).

The product also incorporates an optical camera, which is used for so-called “never-lost” navigation, as well as an electron microscope. Samples are loaded using an automated vacuum technique that sidesteps cumbersome vacuum chambers. External vibration isolation isn’t needed, and samples can be loaded in less than 30 s.

Weighing in at just over 120 lbs, a Phenom also provides 12X digital zoom, delivering image resolution as high as 2k-by-2k pixels. Images are saved on USB memory sticks.

FEI expects the Phenom’s SEM-like performance will appeal to shops that can’t afford $250,000 or more for a SEM, to say nothing of the cost of specially trained SEM operating personnel and dedicated facilities. A $72,000 Phenom microscope can be used to inspect for packaging defects such as interconnect and whiskering failures.

“These types of faults are microscopic, but you don’t necessarily require SEM resolution and magnification to see them,” said Carleson. “In many applications, a Phenom can complement a SEM.”

Moving beyond SEM technology, at least one charged-particle manufacturer is moving into helium-ion beam microscopy. “There’s a paradigm shift in IC manufacturing,” explained Dr. Dirk Stenkamp, an executive member of the board at Carl Zeiss SMT. “Microscopes must keep pace.”

Material advancements and tighter process control is pushing the need for measurements with higher precision in manufacturing, said Stenkamp. “Nowadays, materials are critical factors in IC manufacturing.”

Complex materials such as SiGe, high- and low-K dielectrics, and new metals for interconnects demand new types of tools. That’s where classical techniques such as SEM run out of steam.

The Zeiss Orion microscope uses a stream of helium ions rather than electrons to generate signals. Helium-ion microscopy offers spatial resolution of 0.2 nm. Courtesy of Carl Zeiss SMT.

Orion microscopes use a stream of helium ions rather than electrons to generate signals. These instruments, able to observe features smaller than a micron, are useful for failure analysis, critical dimensional measurement, defect review, and material identification.

Helium-ion microscopy promises to improve spatial resolution, too. SEM resolution is between 1.5 nm and 2 nm, but helium-ion microscopy achieves 0.2-nm resolution, about an order of magnitude better. “The tool nicely bridges the gap between SEMs and TEMs,” said Stenkamp. “It also bridges the gap to the 0.2-nm resolution regime of the TEM.”

Stenkamp also pointed to helium-ion microscopy’s ability to use back-scattered ions to generate contrast features unobtainable with SEMs. “It lets you clearly see different types of materials in a sample. Helium-ion microscopy also provides a depth of field that’s also about an order of magnitude greater than a SEM.”

JEOL USA recently completed installation and acceptance of its first thermal field emission electron microprobe in the US. Installed at NIST, it is used for developing standards for a range of nanotechnologies. Courtesy of JEOL USA.

The EPMA’s wavelength-dispersive x-ray spectrometer (WDS) is capable of image magnification from 40X to 300,000X. What’s more, no ultra-high vacuum chamber is required.

“As dimensions shrink,” explained JEOL VP and product manager Charlie Nielsen, “electron-beam penetration must be reduced, and that’s where lower voltage systems come into play.” He noted that high-voltage predecessor microscopes typically required sample sizes on the order of a cubic micron, but EPMAs can work with samples only 100 nm in size.

JEOL’s nondestructive microscope can simultaneously deploy a spectrometer and up to five WDSs. “Vendors working with nanotechnologies, such as MEMS materials and thin films for hard disk pads, can use this technology to migrate microscopy out of the R&D lab, moving it into the realm of quality control,” said Nielsen.