Metrology can be considered a collection of subdomains, each subdomain representing a specific analysis technique. Think about secondary ion mass spectroscopy (SIMS), a technique for determining the composition of a material (surface) – with applications for organic and inorganic structures. Another subdomain is scanning probes, comprising several concepts for measuring, e.g., topography, adhesion, material hardness and chemical properties, and we have our own scanning spreading resistance microscopy (SSRM) technique, allowing to determine carrier profiles in a semiconductor. In recent years, many of these techniques have largely evolved and have become mature enough to be used in research or production of semiconductor applications. Lab-to-fab transition, and an increasing focus on volume, automation and throughput time are seen as important trends.
Orbi-trap SIMS: a revolutionary improvement in mass resolution
In recent years, progress in SIMS has been remarkable. SIMS allows studying the composition of a material surface by sputtering the surface with a beam of energetic ions. As a result, secondary ions are released and analyzed by means of a mass spectrometer. In 2017, the introduction of a new concept for mass spectrometry, the Orbi-trap, has enabled a significant improvement (10-50x) in mass resolution (>250,000), and in accuracy of mass determination (<2ppm). This way, complex molecules can be uniquely identified. Originally developed for applications in biological and medical research (single cell proteomics), these properties represent a quantum leap for the analysis accuracy and interpretation of SIMS data in semiconductor technology. With Orbi-trap SIMS, it is now possible to analyze, for example, photoresists and self-assembled monolayers, or to make a distinction between two elements with very similar mass (such as arsenic and germanium). The technique can also be used to support the self-focusing SIMS concept, which was developed at imec for analyzing extremely small structures.
Hybrid metrology: 1+ 1 =3
Over the last years, the concept of hybrid metrology has become increasingly important. Following this concept, different metrology techniques are used to measure one and the same structure.
This allows either correlating complementary information (such as structure and functional properties), or eliminating specific uncertainties of the individual techniques. An illustration is the combination of transmission electron microscopy (TEM, an imaging technique) with scanning probes (SPM, functional analysis). Combining TEM information (structure and composition) with the observation of functional properties at the nanometer scale via SPM (or, via SRRM for carriers, or via piezo-force for ferro-electrical properties,...) on the same structure provides a unique approach for generating insight in the functioning of new structures. Last year, imec played a pioneering role in the development of the SPM/TEM hybrid metrology – which was also presented at the IEDM conference.
In addition, our team made a major breakthrough towards more accurate 3D analysis, by combining the atom probe technique (APT, atom probe tomography) with atomic force microscopy (AFM). Prior to an atom probe measurement, a sample is prepared in the form of a sharp tip. From this tip, ions are evaporated, captured by a position sensitive detector and individually analyzed according to their mass. The tip acts as an ion-optical component, and creates an image magnification of > 106x. The result is a 3D analysis of the sample with a (theoretical) spatial resolution < 0.2nm. In practice, the exact value of the magnification (resolution) is unknown, because the detailed shape of the tip changes continuously and cannot be determined in-situ, until recently. Most of the labs and manufacturers explore the integration of TEM in an APT system as an expensive and complex solution to this problem. By showing that the APT tip can be imaged with AFM, imec demonstrated a promising, simpler, more quantitative and cost effective alternative. The further exploration of this concept will be complemented with new complex data algorithms that will be developed in collaboration with Vision Lab, an imec research group at the University of Antwerp.
As a final example of the increasing importance of hybrid metrology, I would like to mention a project in which imec combined a SIMS instrument with in-situ SPM: a world first. This will allow determining composition (SIMS) as well as functional properties (electrical SPM). In the future, the exploration and demonstration of this approach will be a main focus point of our research efforts.
Probing small confined volumes: a challenge for metrology
Another important milestone is the commercialization of the Fourier Transform-scanning spreading resistance microscopy (FFT-SSRM) technique. This novel technique is based on SSRM, an analysis technique that was invented at imec years ago. SSRM is among the few techniques that allows to determine carrier profiles in a semiconductor. But the standard SSRM technology (and its underlying approaches) cannot be applied to small volumes, such as FinFETs and nanowires, due to signal distortion by parasitic resistances. Imec’s FFT-SSRM concept overcomes this issue. In 2017, it was translated into a commercial product and installed at several partners of imec.