Heiko Stegmann

Modeling of the F-actin structure

Actin has been a major target for structural studies in biology since F. B. Straub discovered it in 1942.1 This is probably because actin is one of the most abundant proteins in the eukaryotic cell as well as a key player in many physiological events, ranging from genetics to motility.

Preliminary results from the first monochromated and aberration corrected 200-kV field-emission scanning transmission electron microscope.

Experimental results from the first monochromated and aberration-corrected scanning transmission electron microscope operated at 200 kV are described. The formation of an electron probe with a diameter of less than 0.2 nm at an energy width significantly under 0.3 eV and its planned application to the chemical analysis of nanometer-scale structures in materials science are described. Both energy and spatial resolution will benefit from this: The monochromator improves the energy resolution for studies of energy loss near edge structures. The Cs corrector allows formation of either a smaller probe for a given beam current or yields, at fixed probe size, an enhanced beam current density using a larger condenser aperture. We also point out another advantage of the combination of both components: Increasing the convergence angle by using larger condenser apertures in an aberration-corrected instrument will enlarge the undesirable chromatic focus spread. This in turn influences spatial resolution. The effect of polychromatic probe tails is proportional to the product of convergence angle, chromatic aberration constant, and energy spread. It can thus be compensated for in our new instrument by decreasing the energy width by the same factor as the beam convergence is increased to form a more intense probe. An alternative in future developments might be hardware correction of the chromatic aberration, which could eliminate the chromatic probe spread completely.

Novel Carbon-Cage-Based Ultralow-

A new class of materials is presented that is supposed to be a potential candidate for isolating ultra low-k thin films between metal on-chip interconnects in future CMOS technology nodes. The ideal structure of the novel carbon-cage-based materials is described by a model that assumes an ordered network (mosaic structure) with fullerenes (C60) as the nodes and linker molecules along the edges of the mosaic cells. The interior of the network represents a nanopore of a 1-nm scale. According to the molecular design-based model, structures with simple cubic and diamond-like topology of the network are considered promising candidates. A dielectric constant (k value) of 1.7 and an elastic bulk modulus of about 20 GPa are predicted of ideal combinations of network topology and linker molecules. First experimental results, based on electron energy loss spectroscopy, X-ray absorption spectroscopy, nanoindentation, and atomic force microscopy are presented. A more controlled film fabrication process is needed to get more homogeneous thin films with characteristic material parameters as predicted by the model.

k

Analysis of samples from 3D integration, packaging and joining technologies more often than ever requires the removal of large amounts of material to access deeply buried target structures. Until now, such sample preparation has been achieved using demanding and slow techniques, such as metallographic cross-sectioning, or focused ion beam (FIB) milling. A new tool that combines pulsed laser ablation and FIB milling now allows precise target preparation of deeply buried features in a more efficient way.

Materials: Modeling and First Experiments

In chemical vapor deposited (CVD) organosilicate glasses (OSG), which are used as interlayer dielectric (ILD) materials, the substitution of oxygen in SiO2 by methyl groups (−CH3) reduces the permittivity significantly. However, plasma processing for resist stripping, trench etching and post‐etch cleaning removes C and H containing molecular groups from the near‐surface layer of OSG. Therefore, compositional analysis and chemical bonding characterization of structured ILD films with nanometer resolution is necessary for process optimization. OSG thin films as‐deposited and after plasma treatment are studied using X‐ray absorption spectroscopy (XAS) and electron energy loss spectroscopy (EELS). In both techniques, the fine structure near the C‐K absorption or energy loss edge, respectively, allows to differentiate between C–H, C–C, and C–O bonds, and consequently, between individual low‐k materials and their modifications. Examination of the C‐K near‐edge structures reveal a modified bonding of the remaini...

Efficient target preparation by combining laser ablation and FIB milling in a single tool

Today, focused ion beam (FIB) in combination with a scanning electron microscope is a widely used standard technique in research, analytics and for the modification and structuring of almost every material down to nanometer dimensions. Sample preparation for electron microscopy more often than ever requires the removal of large amounts of material e.g. to access deeply buried sample structures. Until now, such preparation has been achieved using demanding and slow techniques, such as metallographic cross sectioning and ion polishing, or FIB milling. To overcome this challenge Carl Zeiss combined a pulsed ns-laser with an Auriga CrossBeam® FIB/SEM system. The Laser system is attached to the load-lock chamber in order to avoid contamination of high voltage parts in the main chamber by laser sputtered material (Figure 1). The ablation of material volumes in the order of several 10 mm3 can be performed within minutes, followed by FIB preparation and SEM analysis in the same instrument [1,4,5]. This new tool now allows a time efficient target preparation with high positioning accuracy of deeply buried structures and of extremely large cross sections. One important dimension is the heat affected zone. It is necessary to know how much material have to be removed by subsequent FIB polishing until it is undisturbed. It has been shown, that the size of this zone is in the range of 5 μm [3,4]. Estimations about the whole preparation time taking into account the mentioned necessary subsequent FIB polishing will be shown in this presentation. That the laser FIB/SEM combination is capable to quickly expose deeply buried features in microelectronic devices in one workflow has been published [1,2,4]. Furthermore, to prepare large cross sections in brittle or soft materials can be very demanding and often conventional methods are not applicable. The feasibility of using laser cutting on these materials to get access to large cross sections for further FIB/SEM investigations has been presented also [5]. In addition, the ability to remove large arbitrary structures enables a wide field of novel sample preparation procedures. For example the preparation of structures for large volume FIB tomography. In order to avoid shadowing effects during the tomography run it is necessary to have as less as possible material surrounding the volume of interest. A FIB tomography of a through silicon via (TSV) will be shown. The first step is the annular removal of the surrounding material by means of laser ablation. This is followed by the FIB tomography under 0° stage tilt (Figure 2). These procedure enables a quick start of the overnight tomography run in just ca. 20 min.

Chemical Bonding in Low‐k Dielectric Materials for Interconnect Isolation: Characterization using XAS and EELS

Changing local electronic polarizability and chemical bonding in OSG in such a way that the effective permittivity - and consequently the electrical performance of the Cu/low-k structure - deteriorates only slightly and that adhesion and stiffness are improved significantly is an extremely challenging task [1], [2]. As the interconnect line spacings continue to shrink, optimization of the electrical and mechanical properties of the ILD material becomes increasingly important for Cu/low-k integration since the effect of thin regions that have been modified by special treatments on the effective material properties, e. g. keff, increases. Composition and chemical bonding, changed by plasma or beam treatments, effect the materials properties significantly. Plasma processes for resist stripping, trench etching and post-etch cleaning remove C and H containing molecular groups from the near-surface layer of OSG. Electron-beam interaction with OSG changes the chemical bonding in the low-k material. In this paper, the effect of chemical bonding on permittivity and elastic modulus is studied. Compositional analysis and chemical bonding characterization of structured ILD films with nanometer resolution is done with electron energy loss spectroscopy (EELS). The fine structure near the C-K electron energy loss edge, allows to differentiate between C-H, C-C, and C-O bonds, and consequently, between individual low-k materials and their modifications. Dielectric permittivity changes are studied based on VEELS (valence EELS) measurements and subsequent Kramers-Kronig analysis. The elastic modulus is determined with atomic force microscopy (AFM) in force modulation (FM) mode. Nanoindentation was applied as a complementary technique to obtain reference data.

Efficient and Precise Sample Preparation by Combination of Pulsed Laser Ablation and FIB Milling

The use of low dielectric constant materials in the on-chip interconnect process reduces interconnect delay, power dissipation and crosstalk noise. To achieve the requirements of the ITRS for 2007-2009 minimal sidewall damage from etch, ash or cleans is required. In chemical vapor deposited (CVD) organo-silicate glass (OSG) which are used as intermetal dielectric (IMD) materials the substitution of oxygen in SiO 2 by methyl groups (-CH 3 ) reduces the permit-tivity significantly (from 4.0 in SiO 2 to 2.6-3.3 in the OSG), since the electronic polarizability is lower for Si-C bonds than for Si-O bonds. However, plasma processing for resist stripping, trench etching and post-etch cleaning removes C and H containing molecular groups from the near-surface layer of OSG. Therefore, compositional analysis and chemical bonding characterization of structured IMD films with nanometer resolution is necessary for process optimization. OSG thin films as-deposited and after plasma treatment are studied using X-ray absorp-tion spectroscopy (XAS) and electron energy loss spectroscopy (EELS). In both techniques, the fine structure near the C1s absorption or energy loss edge, respectively, allows to identify C-H, C-C, and C-O bonds. This gives the opportunity to differentiate between individual low-k mate-rials and their modifications. The O1s signal is less selective to individual bonds. XAS spectra have been recorded for non-patterned films and EELS spectra for patterned structures. The chemical bonding is compared for as-deposited and plasma-treated low-k materials. The Fluo-rescence Yield (FY) and the Total Electron Yield (TEY) recorded while XAS measurement are compared. Examination of the C 1s near-edge structures reveal a modified bonding of the re-maining C atoms in the plasma-treated sample regions.

Chemical Bonding, Permittivity and Elastic Properties in Locally Modified Organosilicate Glass

Sample preparation is a critical step in TEM that significantly determines the quality of structural characterization and chemical analysis of the smallest and most critical structures in semiconductor manufacturing. In recent years, the accuracy requirements for the preparation of TEM cross-sections of nanoelectronic structures have drastically increased, and the rapid advance in TEM technology has demanded the highest standards of sample quality. Combination of a focused low-energy Ar ion beam column with a FIB and a SEM column in a triple beam instrument provides precise positioning as well as minimum thickness, surface damage and roughness of a lamella.

Characterization of Chemical Bonding in Low-K Dielectric Materials for Interconnect Isolation: A XAS and EELS Study

Sample preparation is a critical step in transmission electron microscopy (TEM) that significantly determines the quality of structural characterization and analysis of a specimen. In recent years, the accuracy and quality requirements for the preparation of TEM cross‐sections of nanoelectronic structures have drastically increased. Combining a focused low‐energy noble gas ion beam column with a FIB and a SEM column in a three beam system meets these requirements. It provides precise target preparation as well as minimum thickness and surface damage of the TEM sample.