Dual Beam Targets New Materials, 45 nm Node

As once-unthought-of materials are plucked from the periodic table and are either used in or being considered for IC structures at the 45 nm node and beyond, the engineer faces an increased need for a dual-beam (FIB/SEM) instrument with the capability to produce a sufficiently clear image that allows the study of increasingly smaller feature sizes and enables the preparation of ever-thinner samples in the constant quest for yield enhancement. Although there are a number of available tools potentially capable of meeting these needs, they can sometimes be difficult and time-consuming to operate and maintain at the performance level needed.

FEI Co. (Hillsboro, Ore.) has introduced a family of dual-beam instruments that has at its core an electron column optimized for imaging in that configuration. According to the company, its Helios NanoLab family provides imaging performance with the highest information content of any comparable platform: 0.9 nm at 15 kV. These systems can operate at low accelerating voltages, which eliminate charging artifacts and avoid damage to sensitive electronic materials. It is possible to get a 1.5 nm resolution at 1 kV.

The imaging is done through a Schottky field emission gun that enables the necessary stability and brightness for the required imaging and analysis performance. Through-the-lens detectors and a low-noise imaging chain further augment imaging, while a 4 mm SEM working distance at the coincident point improves resolution. Added scanning precision enables the gathering of data for precise measurements. Previous tools have had a longer working distance and an image resolution of 2.5 nm at 1 kV. The new family provides a 40% resolution improvement — tested <1.5 nm at 1 kV, using a gold-on-carbon resolution standard — that surpasses that of other similar currently available systems.

The platforms are designed for 2-D and 3-D analysis. The ion column can produce accurate site-specific cross-sections in a few minutes, while monitoring with the SEM provides real-time imaging of the ion-milled surface for end-pointing purposes. The dual-beam platforms’ software automates the collection of data from sequential slices, which can then enable reconstruction into 3-D models. Automated routines simplify and provide repeatable STEM and TEM sample preparation, resulting in 100 nm thick samples in <20 min. The ion beam is further capable of providing low-voltage sample cleaning down to <1 kV, reducing surface damage and preserving detail in samples thinned to <100 nm.

A cross-section of a logic device imaged at 6 kV (image width 650 nm, left). The sample has been FIB-sectioned and delineated with in situ gas chemistry to enhance the edge contrast between materials. The cross-section of logic device imaged at 2 kV (image width 1000 nm). Although no chemical etching has been applied, the image displays good materials contrast, even showing grains within the tungsten contacts. (Source: FEI Co.)

The SEM column has been designed to operate using constant power lens technology. This provides the capability to alter the strength of the system lenses without changing the overall input power, avoiding thermal fluctuations that sometimes result when column conditions are altered. This simplifies column alignment and operation while reducing drift, providing more stability and faster focusing. Redesign of the through-the-lens detector, collection efficiency, and the signal-to-noise ratio of the overall imaging chain from the detector to the screen has made it possible to use a lower beam current, resulting in less sample damage while still providing an image with the needed information.

Platform design is aimed at coping with the novel materials being used in structures or considered for other nodes. While the capability to do effective preparation by milling and cross-sectioning is important, the other side of the equation is effectively imaging those different materials. Some of these, particularly low-k dielectrics, have posed a considerable problem because of the fact that they are composed of sensitive, delicate films. The platforms are also equipped with a module that allows a “hot swap” to exchange the source tip and minimize down time, with improved column shielding against electromagnetic interference and vibration.

Increasingly, dual-beam platforms combining FIBs and SEMs are becoming crucial in the development and/or analysis phase to learn what takes place with the new materials being integrated. The capability to use an electron beam to view the sample being prepared by the ion beam to see the region within the sample, either to take a high-quality picture of it or prepare it as a TEM sample that will be passed on for a more detailed analysis, is becoming of prime importance.

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