Self Focusing SIMS: Probing thin film composition in very confined volumes

Abstract

The continued downscaling of micro and nanoelectronics devices has increased the importance of novel materials and their interfaces very strongly thereby necessitating the availability of adequate metrology and very tight process control as well. For instance, the introduction of materials like SiGe or III-V compounds leads to the need for the determination of the exact composition and thickness of the resulting thin films. Concurrent with this trend, one is faced with layer growth concepts such as aspect ratio trapping, which exploit the reduced dimensionality of the devices. As this leads to films with very different characteristics as compared to their blanket counterparts, characterization now has to be performed on thin films grown in very confined volumes (with dimensions ranging down to less than 10–20nm) and standard analysis methods like X-Ray Photoelectron Spectroscopy, Secondary Ion Mass Spectrometry (SIMS) and Rutherford Backscattering Spectrometry, no longer seem applicable due to a lack of spatial resolution. On the other hand, techniques with appropriate spatial resolution like Atom Probe Tomography or Transmission Electron Microscopy are time consuming and suffer from a lack of sensitivity due to their highly localized analysis volume.In this paper, a novel concept termed Self Focusing SIMS, is presented which overcomes the spatial resolution limitations of SIMS without sacrificing the sensitivity. The concept is based on determining the composition of a specific compound using cluster ions which contain the constituents of the compound. Their formation mechanism implies that all cluster constituents originate from the same collision cascade and are emitted in close proximity (<0.5nm). As such, the composition information becomes confined (i.e. self focused) to the areas where all constituents are simultaneously present. The examples shown in this work are based on SiGe compounds and demonstrate that it becomes feasible to determine the composition of thin films in ultra narrow trenches (down to 20nm in width) with good accuracy and sensitivity. Whereas for the case where the probing beam is focused to a dimension smaller than the width of the structure, the analyzed volume/data point becomes very small (< a few tens of nm3), the simultaneous detection of many atoms from the multiple structures in the Self Focusing SIMS approach, represents effectively a much larger volume providing the abundant sensitivity as the analyzed area is now increased up to 100μm2. On the other hand, the minimum depth probed (in each trench) is still governed by the depth resolution of SIMS, hence it is similar to the one encountered in SIMS experiments performed on blanket films. This can be as small as 1–2nm/dec and is only limited by the energy of the sputter beam.