The outlook for new metrology approaches

BY PAUL VAN DER HEIDE, director of materials and components analysis, imec, Leuven, Belgium

To keep up with Moore’s Law, the semiconductor industry continues to push the envelope in developing new device architectures containing novel materials. This in turn pushes the need for new solid-state analytical capabilities, whether for materials characterization or inline metrology. Aside from basic R&D, these capabilities are established at critical points of the semiconductor device manufacturing line, to measure, for example, the thickness and composition of a thin film, dopant profiles of transistor’s source/drain regions, the nature of defects on a wafer’s surface, etc. This approach is used to reduce “time to data”. We cannot wait until the end of the manufacturing line to know if a device will be functional or not. Every process step costs money and a fully functional device can take months to fabricate. Recent advances in instrumentation and computational power have opened the door to many new, exciting analytical possibilities.

One example that comes to mind concerns the development of coherent sources. So far, coherent photon sources have been used for probing the atomic and electronic structure of materials, but only within large, dedicated synchrotron radiation facilities. Through recent developments, table top coherent photon sources have been introduced that could soon see demand in the semiconductor lab/fab environment.

The increased computational power now at our finger tips is also allowing us to make the most of these and other sources through imaging techniques such as ptychography. Ptychog- raphy allows for the complex patterns resulting from coherent electron or photon interaction with a sample to be processed into recognizable images to a resolution close to the sources wavelength without the requirement of lenses (lenses tend to introduce aberrations). Potential application areas extend from non-destructive imaging of surface and subsurface structures, to probing chemical reactions at sub femto-second timescales.

Detector developments are also benefiting many analytical techniques presently used. As an example, transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) can now image, with atomic resolution, heavy as well as light elements. Combining this with increased computational power, allows for further devel- opment of imaging approaches such as tomography, holography, ptychography, differential phase contrast imaging, etc. All of which allow TEM/STEM to not only look at atoms in e.g. 2D materials such as MoS2 in far greater detail, but also opens the possibility to map electric fields and magnetic domains to unprecedented resolution.

The semiconductor industry is evolving at a very rapid pace. Since the beginning of the 21st century, we have seen numerous disruptive technologies emerge; technologies that need to serve is an increasingly fragmented applications space. It’s no longer solely about ‘the central processing unit (CPU)’. Other applications ranging from the internet of things, autonomous vehicles, wearable human-electronics interface, etc., are being pursued, each coming with unique requirements and analytical needs.

Looking ten to fifteen years ahead, we will witness a different landscape. Although I’m sure that existing techniques such as TEM/STEM will still be heavily used – probably more so than we realize now (we are already seeing TEM/STEM being extended into the fab). We will also see developments that will push the boundaries of what is possible. This would range from the increased use of hybrid metrology (combining results from multiple different analytical techniques and process steps) to the development of new innovative approaches.

To illustrate the latter, I take the example of secondary ion mass spectrometry (SIMS). With SIMS, an energetic ion beam is directed at the solid sample of interest, causing atoms in the near surface region to leave this surface. A small percentage of them are ionized, and pass through a mass spectrometer which separates the ions from one another according to their mass to charge ratio. When this is done in the dynamic-SIMS mode, a depth profile of the sample’s composition can be derived. Today, with this technique, we can’t focus the incoming energetic ion beam into a confined volume, i.e. onto a spot that approaches the size of a transistor. But at imec, novel concepts were intro- duced, resulting in what are called 1.5D SIMS and self-focusing SIMS (SF-SIMS). These approaches are based on the detection of constituents within repeatable array structures, giving averaged and statistically significant information. This way, the spatial resolution limit of SIMS was overcome.

And there are exciting developments occurring here at imec in other analytical fields such as atom probe tomography (APT), photoelectron spectroscopy (PES), Raman spectroscopy, Rutherford back scattering (RBS), scanning probe microscopy (SPM), etc. One important milestone has been the development of Fast Fourier Transform-SSRM (FFT-SSRM) at imec. This allows one to measure carrier distributions in FinFETs to unparalleled sensitivity.

Yet, probably the biggest challenge materials characterization and inline metrology face over the next ten to fifteen years will be how to keep costs down. Today, we make use of highly specialized techniques developed on mutually exclusive and costly platforms. But why not make use of micro-electro-mechanical systems (MEMS) that could simultaneously perform analysis in a highly parallel fashion, and perhaps even in situ? One can imagine scenarios in which an army of such units could scan an entire wafer in the fraction of the time it takes now, or alternatively, the incorporation of such units into wafer test structure regions.

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