Florian Hüe

Nanoscale holographic interferometry for strain measurements in electronic devices

Strained silicon is now an integral feature of the latest generation of transistors and electronic devices because of the associated enhancement in carrier mobility. Strain is also expected to have an important role in future devices based on nanowires and in optoelectronic components. Different strategies have been used to engineer strain in devices, leading to complex strain distributions in two and three dimensions. Developing methods of strain measurement at the nanoscale has therefore been an important objective in recent years but has proved elusive in practice: none of the existing techniques combines the necessary spatial resolution, precision and field of view. For example, Raman spectroscopy or X-ray diffraction techniques can map strain at the micrometre scale, whereas transmission electron microscopy allows strain measurement at the nanometre scale but only over small sample areas. Here we present a technique capable of bridging this gap and measuring strain to high precision, with nanometre spatial resolution and for micrometre fields of view. Our method combines the advantages of moiré techniques with the flexibility of off-axis electron holography and is also applicable to relatively thick samples, thus reducing the influence of thin-film relaxation effects.

On the influence of elastic strain on the accommodation of carbon atoms into substitutional sites in strained Si:C layers grown on Si substrates

Measurements of strain and composition are reported in tensile strained 10- and 30-nm-thick Si:C layers grown by chemical vapor deposition on a Si (001) substrate. Total carbon concentration varies from 0.62% to 1.97%. Strain measurements were realized by high-resolution x-ray diffraction, convergent-beam electron diffraction, and geometric phase analysis of high-resolution transmission electron microscopy cross-sectional images. Raman spectroscopy was used for the deduction of the substitutional concentration. We demonstrate that in addition to the growth conditions, strain accumulating during deposition, thus depending on a layer thickness, has an influence on the final substitutional carbon composition within a strained Si:C layer.

Invited) Strain Mapping of Layers and Devices Using Electron Holography

We present the HoloDark technique which has recently been invented and allows one to map strain in two dimensions in layers and devices with nanometer resolution, high precision and large field of view. The technique is based on electron holography and is applicable to all standard focused-ion beam FIB prepared crystalline samples. We show a panorama of typical results obtained in SiGe stacks, ion implanted silicon, strained silicon channel nMOS and pMOS type transistors and in the challenging case of strained silicon FinFETs, In such materials and structures, the HoloDark technique, although still perfectible, appears as the only technique able to provide reliable and extended data against which simulations can be calibrated.

Strain metrology of devices by dark-field electron holography: A new technique for mapping 2D strain distributions

The authors present the latest results from the new technique of dark-field electron holography (HoloDark) which combines the advantages of the conventional transmission electron microscopy (TEM) with the precision of electron holography and is applicable to standard focused-ion beam (FIB) prepared samples. The authors will present measurements of strain in the active regions of a strained-silicon n-MOSFET device and a test structure for CESL induced strain.

CHAPTER 6 - Aberration Correction With the SACTEM-Toulouse: From Imaging to Diffraction

This chapter focuses on the aberration correction with the spherical aberration corrected transmission electron microscope (SACTEM)-toulouse. The benefits of aberration correction have been underestimated and largely unexpected. Aberration correction allows strain mapping to be performed on thick and damaged focused ion beam (FIB)-prepared specimens. The least expected benefits have been for electron diffraction experiments. Aberration correction allows large-angle convergent-beam electron diffraction (LACDIF) and convergent-beam electron holography (CHEF) configurations to be used effectively and efficiently. The extra lenses provided by the corrector allow original optical configurations to be explored. Indeed, the stability of the microscope and the computer-assisted alignment has allowed the SACTEM to be used continuously and routinely in a wide range of modes ever since its installation.

Strain mapping in MOSFETs by transmission electron microscopy

In this paper, we present two methods to map strain in MOSFETs at the nanometer scale. Aberration corrected high resolution transmission electron microscopy (HRTEM) coupled with Geometric Phase Analysis (GPA) provides sufficient signal/noise to measure the displacement fields accurately. Finite Element Method simulations confirm our measurements. The field of view is however limited to an area of 200 nm times 200 nm. To overcome this, we have developed a new technique called dark-field holography based on off-axis electron holography and dark-field imaging. This new technique provides us a better strain resolution than HRTEM (reaching 0.05%), a spatial resolution of 4 nm and a field of view of 1 mum.

Dark-field electron holography for the measurement of strain in nanostructures and devices

We present a new method for measuring strain in nanostructures and electronic devices [1]. It is based on a combination of the moire technique and off-axis electron holography. A hologram is created from the interference between the diffracted beam emanating from an unstrained region of crystal, which serves as the reference, and a beam from the region of interest containing strained crystal. A typical example for these two regions would be the substrate and an active region of a device. The aim is to measure geometric phase differences, from which the deformation can be calculated [2]. Naturally, any other phase contributions should be minimised, notably, dynamic phases due to thickness variations. For this reason, specimens should be prepared with suitably uniform thickness and regions exhibiting bend contours avoided.

Strain measurements in electronic devices by aberration-corrected HRTEM and dark-field holography

The recent introduction of strained silicon in electronic devices has led to tremendous performance increases [1]. Strain can be engineered in active areas by different processes such as biaxial and uniaxial technologies. Measuring strain at the nanometre scale is therefore essential and has lead to the development of techniques like convergent-beam electron diffraction (CBED) and nanobeam diffraction [2]. Now, high-resolution transmission electron microscopy (HRTEM) can map strains in devices [3] and the new technique of dark-field holography appears extremely promising [4].

Strain determination by dark-field electron holography

Accurate determination of strain in electronic devices has been the subject of intense work during the last decades. Few techniques are able to provide highly localized and accurate information at the nanoscale. Among these, convergent-beam electron diffraction (CBED) combines the advantages of very small probes and remarkable sensitivity to small variations in the lattice parameter [1]. However, elastic relaxation effects make the analysis extremely difficult, necessitating time-consuming dynamical simulations combined with finite element modeling [2].

Critical Analysis of Different Techniques for Measuring Strain in Si1-yCy Layers Grown by CVD on a Si Substrate

In this work, we performed quantitative measurements of strain in structures consisting of a 30 nm-thick Si1-yCy layer grown by chemical vapour deposition (CVD) on a Si (001) substrate. Total C concentrations vary from 0.67 to 2%. Geometric phase analysis (GPA) of high-resolution transmission electron microscopy (HR TEM) cross-section images and convergent-beam electron diffraction (CBED) with a nanometer resolution as well as Raman spectroscopy and high-resolution X-ray diffraction with a micro- and millimeter resolutions, respectively, were used to deduce the strain within these Si1-yCy layers. These results were compared with the predictions of elasticity theory. Particular interest is paid to the formation of the structural defects within Si1-yCy layers as a function of C concentration, growth temperature and incorporated strain.