For half a century, silicon has been the workhorse of the semiconductor industry but it is now coming up against its physical limitations. Manufacturers are pairing silicon with other materials in order to enhance its properties and continue to drive the increased miniaturization, speed and functionality of microelectronics.
This “More than Moore” approach requires sophisticated characterization techniques that can prove complex nanoscale structures embedded in semiconductor devices.
Scanning X-ray diffraction microscopy (SXDM) is a powerful, non-destructive technique that was developed at ESRF beamline ID01, that enables industry researchers to detect the smallest imperfections in the crystalline structure of thin films and heterogenous structures, even deep within a stack of different layers.
This technique offers a spatial resolution of 100 nm and an unprecedented strain resolution of a few parts per million. IRT Nanotec partners are trialling it as a way of creating the next generation of micro- and nano-electronics.
The growth and performance of semiconductor devices can be severely affected by any imperfections in the microscopic structure of materials, such as variations or defects in the orientations of the crystal lattice.
This is especially important when matching silicon with another material, such as germanium, to create new chip architecture, as this process creates more complex structures and new interactions that need to be understood.
Wafer manufacturer Siltronic, in conjunction with ESRF staff, investigated a sample comprised of step-graded silicon-germanium layers on 300 mm silicon wafers. This is a promising substrate for advanced architectures including sub-20 nm CMOS transistors. The aim was to measure the composition, as well as the lateral distribution of strain and tilt across the system, and to compare these parameters when the wafer was treated both with and without chemical-mechanical polishing.
The small spot size and the high penetration of the ESRF X-ray enabled the team, in conjunction with laboratory-based Raman and AFM techniques, to establish a partial correlation between structural properties and real-space morphology of the sample at the micrometer scale.
The results demonstrate a strong local correlation between the composition distribution and the strain field. This indicates that the adatom surface diffusion during growth is driven by strain field fluctuations, which are induced by the underlying dislocation network.
The data also revealed that, even for very different surface morphologies, superficial chemical-mechanical polishing of the surface does not lead to any significant change of composition, strain or tilt variation compared to that of as-grown samples [see figure].
Maps showing the absolute lattice tilt in as-grown (left) and polished (right) Si0.3Ge0.7 layers, revealing the well-known crosshatch pattern caused by surface undulation of lattice-mismatched surfaces. The similarity between the two panels demonstrates that polishing does not affect lattice orientation, which is important during the growth of semiconductor structures. [Ref. ACS Appl. Mater. Interfaces 7 9031].
SXDM provides a non-destructive, quantitative and model free method for quickly extracting key parameters in silicon-germanium films, such as lattice tilt, fluctuations in composition and strain, without any morphological or surface limitations. Its exceptional resolution and strain sensitivity can be applied to any crystalline object, including polycrystalline thin films, and is relevant for MEMS, 3D integration of chips and ferroelectrics.
Therefore, this technique is of great value to device engineers when evaluating variations in advanced technologies, including state-of-the-art CMOS, that cannot be achieved with any other existing method.
- SXDM is a unique synchrotron X-ray technique that allows continuous and rapid mapping of lattice strain and tilt with sub-micrometer resolution
- Samples can be homogenous materials, semiconductor devices, thin films and beyond, and can be up to 400mm2 thick and 20mm2 in area
- X-rays are aligned around the nominal Bragg conditions of the sample and a 2D detector is used to monitor the diffraction signal, whilst the sample is moved using an x-y piezo stage
- Five-dimensional datasets built from millions of detector images are automatically processed to generate 2D maps of strain and tilt, using in-house software that allows preliminary results to be extracted during an experiment
- The application of such rapid scanning methods offers the possibility of performing in operando studies at high temperatures or in liquid or gas environments
This information has been sourced, reviewed and adapted from materials provided by The Platform for Advanced Characterisation Grenoble (PAC-G).
For more information on this source, please visit The Platform for Advanced Characterisation Grenoble (PAC-G).