As semiconductor process geometries shrink to the 32-nm node and below, obtaining clear images and data of critical IC structures is becoming a lot tougher using traditional SEMs (scanning electron microscopes) for surface inspection. Scientists in process-development labs and engineers at logic and memory manufacturers are struggling to quickly see complex 3-D surfaces, as well as materials interfaces and profiles. They also need to see all of this from multiple angles and at resolutions below 1 nm without increasing the electron-beam voltage.

This XHR SEM image shows a tilted view of a deprocessed logic device sample, imaged at 1-kV beam voltage and 600,000 times magnification. Courtesy of FEI and STMicroelectronics, Malta/Grenobl.

To address these problems, FEI has begun leveraging techniques and components used in dual-beam systems, which integrate SEM and FIB (focused ion beam) methods. The result is a new class of instruments that the company calls XHR (extreme high-resolution) SEMs. FEI introduced the first such machine, the Magellan XHR SEM, at Semicon West (July 15–17, San Francisco, CA). The new system achieves surface sensitivity and subnanometer resolution at voltage levels from 1 kV to 30 kV.

Traditionally, SEMs have been favored in both semiconductor labs and production facilities for their nanometer-level imaging and ability to handle bulk samples, as well as for their wide imaging range, simple sample-preparation requirements, and analytical capabilities. But just as capturing clear images of smaller features has become more challenging, so has achieving good contrast on the surfaces of sensitive dielectrics and resists without increasing voltage. Lower voltages are needed to minimize the damage to sample materials. They also avoid the distortions caused when a higher voltage beam passes below the sample’s top surface and into the material below, creating artifacts in images.

Solving these challenges with UHR (ultra-high-resolution) SEMs has been difficult, said FEI’s Richard Young, technologist for scanning electron microscope and small dual-beam systems. Some of these machines have stringent sample-preparation requirements.

“Many UHR SEMs are limited to samples less than 10 mm per side and do not allow the rotation and tilt flexibility needed for good examination of edges and interfaces,” said Young. “They often require extra preparation steps just to get the sample into the system.” Other UHR SEMs use beam voltages of 5 to 10 kV in order to get higher-resolution images.

This XHR SEM image shows a top-down optimal working-distance view of a deprocessed logic device sample, imaged at 1-kV beam voltage and 600,000 times magnification. Courtesy of FEI and STMicroelectronics, Malta/Grenoble.

Due to these constraints, most UHR SEMs are not designed for high sample throughput. Yet, demand for higher throughput is increasing at process-development labs, where scientists work to reduce development time, and at manufacturing support labs, where engineers must speed up the ramp-to-volume-yield rates in new devices.

Other technologies, such as FIB or TEM (transmission electron microscope) systems, are not designed to achieve the XHR SEM’s combination of features and abilities. The XHR SEM addresses multiple limitations of UHR SEMs, including high beam voltages, sample handling constraints, and low throughput. It combines the column, stage, and platform from FEI’s dual-beam FIB/SEM products with new technology that permits the system to offer improved low-voltage performance by reducing the electron beam’s energy spread. This results in decreased probe size and improved sample contrast, producing rapid surface imaging at tens of images per hour. The machine’s 100-mm, high-precision, high-stability, five-axis tilting/rotating stage accommodates large samples or multiple smaller samples, while an optional automated loadlock further improves sample throughput.

Although FEI has pushed its XHR SEM’s low-voltage resolution below 1 nm, that doesn’t mean that the need for FIBs or TEMs is going away, said Todd Templeton, the company’s product marketing manager for scanning electron microscope and small dual-beam systems. FIBs and TEMs may not offer the XHR SEM’s combination of features and abilities, but both can reveal defects below the surface in advanced materials and devices. For example, even though TEM samples take longer to prepare and image, TEM systems provide chemical analysis and resolutions as high as 10 times those of the XHR SEM. Both of these capabilities are sometimes needed in process development and failure analysis where engineers need to get more accurate profile and metrology information.