Premysl Marsik

Optical Property Changes in Low-k Films upon Ultraviolet-Assisted Curing

Ultraviolet-assisted curing (UV curing) has been applied recently to enhance the mechanical properties of low- k films. Knowledge about which UV energies are most effective is still limited and the consequences of applying the UV-curing process to integrated stacks in on-chip interconnects are unknown. To clarify these open questions, we investigated the optical properties of a SiCOH low- k layer by purged UV spectroscopic ellipsometry in the energy region 2-9 eV. The complex refractive index of the low- k film shows an absorption edge with a superimposed absorption band at 6.4 eV that vanishes upon UV-assisted curing. A comparison with Fourier transform infrared transmission demonstrates that the absorption at 6.4 eV must be attributed to the organic porogens, which also influence the absorption edge. Further analysis reveals the redshift of the absorption edge of silica-based low- k films with the presence of carbon species. The measured optical properties permit simulation of the standing wave pattern of light within the films in differently configured stacks.

Terahertz ellipsometry study of the soft mode behavior in ultrathin SrTiO3 films

We present a combined study with conventional far-infrared and time-domain terahertz ellipsometry of the temperature dependent optical response of SrTiO3 thin films (85 and 8.5 nm) that are grown by pulsed-laser deposition on LSAT substrates. We demonstrate that terahertz ellipsometry is very sensitive to the optical response of these thin films, in particular, to the soft mode of SrTiO3. We show that for the 85 nm film the eigenfrequency of the soft mode is strongly reduced by annealing at 1200 C, whereas for the 8.5 nm film it is hardy affected. For the latter, after annealing the mode remains at 125 cm-1 at 300 K and exhibits only a weak softening to about 90 cm-1 at 10 K. This suggests that this ultrathin film undergoes hardly any relaxation of the compressive strain due to the LSAT substrate.

Optical probe of ferroelectric order in bulk and thin-film perovskite titanates

We have measured the temperature dependence of the direct band gap Eg in SrTiO3 and BaTiO3 and related materials with quantum-paraelectric and ferroelectric properties using optical spectroscopy. We show that Eg exhibits an anomalous temperature dependence with pronounced changes in the vicinity of the ferroelectric transition that can be accounted for in terms of the Frohlich electron-phonon interaction with an optical phonon mode, the so-called soft mode. In addition, we demonstrate that these characteristic changes of Eg can be readily detected even in very thin films of SrTiO3 with a strain-induced ferroelectric order. Optical spectroscopy thus can be used as a rather sensitive probe of ferroelectric order in very thin films of these titanates and probably also in subsequent multilayers and devices.

Electric-field-induced polar order and localizition of the confined electrons in LaAlO3/SrTiO3 heterostructures

With ellipsometry, x-ray diffraction, and resistance measurements we investigated the electric-field effect on the confined electrons at the LaAlO3/SrTiO3 interface. We obtained evidence that the localization of the electrons at negative gate voltage is induced, or at least enhanced, by a polar phase transition in SrTiO3 which strongly reduces the lattice polarizability and the subsequent screening. In particular, we show that the charge localization and the polar order of SrTiO3 both develop below ∼50  K and exhibit similar, unipolar hysteresis loops as a function of the gate voltage.

Infrared Study of the Spin Reorientation Transition and Its Reversal in the Superconducting State in Underdoped Ba(1-x)K(x)Fe(2)As(2).

With infrared spectroscopy we investigated the spin-reorientation transition from an orthorhombic antiferromagnetic (o-AF) to a tetragonal AF (t-AF) phase and the reentrance of the o-AF phase in the superconducting state of underdoped Ba(1-x)K(x)Fe(2)As(2). In agreement with the predicted transition from a single-Q to a double-Q AF structure, we found that a distinct spin density wave develops in the t-AF phase. The pair breaking peak of this spin density wave acquires much more low-energy spectral weight than the one in the o-AF state which indicates that it competes more strongly with superconductivity. We also observed additional phonon modes in the t-AF phase which likely arise from a Brillouin-zone folding that is induced by the double-Q magnetic structure with two Fe sublattices exhibiting different magnitudes of the magnetic moment.

Low-energy interband transitions in the infrared response of Ba(Fe1-xCox)2As2

We studied the doping and temperature (T) dependence of the infrared (IR) response of Ba(Fe1_xCox)2As2 single crystals. We show that a weak band around 1000 cm_1, that was previously interpreted in terms of interaction of the charge carriers with magnetic excitations or of a pseudogap, is rather related to low energy interband transitions. Specifically, we show that this band exhibits a similar doping and T dependence as the hole pockets seen by angle resolved photoemission spectroscopy (ARPES). Notably, we find that it vanishes as a function of doping near the critical point where superconductivity is suppressed in the overdoped regime. Our IR data thus provide bulk specific information (complementary to the surface sensitive ARPES) for a Lifshitz transition. Our IR data also reveal a second low-energy band around 2300 cm_1 which further emphasizes the necessity to consider the multiband nature of these iron arsenides in the analysis of the optical response.

Local terahertz field enhancement for time-resolved x-ray diffraction

We report local field strength enhancement of single-cycle terahertz (THz) pulses in an ultrafast time-resolved x-ray diffraction experiment. We show that patterning the sample with gold microstruc ...

Optical constants, band gap, and infrared-active phonons of (LaAlO3)0.3(Sr2AlTaO6)0.35 (LSAT) from spectroscopic ellipsometry

Using spectroscopic ellipsometry, the authors determined the optical constants (complex dielectric function) for (LaAlO3)0.3(Sr2AlTaO6)0.35 (LSAT) from 0.01 to 6.5 eV. Above 0.5 eV, the data were described with a sum of two Tauc-Lorentz oscillators and two poles. A direct gap of 5.8 ± 0.1 eV was found. An Urbach tail extends to even lower photon energies and makes the crystal opaque above 4.8 eV. Using Fourier-transform infrared ellipsometry, the lattice dynamics was studied. Nine pairs of transverse/longitudinal phonons were found and attributed to disorder in the La/Sr sublattice, ordering in the Al/Ta sublattice, and two-phonon absorption.

Changes of UV Optical Properties of Plasma Damaged Low-k Dielectrics for Sidewall Damage Scatterometry

Porous low-k dielectrics were studied to determine the changes of optical properties after various plasma treatments for development of scatterometry technique for evaluation of the trench/via sidewall plasma damage. The SiCOH porogen based low-k films were prepared by PE-CVD. The deposited and UV-cured low-k films have been damaged by striping O 2 Cl 2 , O 2 , NH 3 and H 2 N 2 based plasmas and CF 4 /CH 2 F 2 /Ar etching plasma. Blanket wafers were studied in this work for the simplicity of thin film optical model. The optical properties of the damaged low-k dielectrics are evaluated the using various angle spectroscopic ellipsometry in range from 2 to 9 eV. Multilayer optical model is applied to fit the measured quantities and the validity is supported by other techniques. The atomic concentration profiles of Si, C, O and H were stated by TOF-SIMS and changes in overall chemical composition were derived from FTIR. Toluene and water based ellipsometric porosimetry is involved to examine the porosity, pore interconnectivity and internal hydrophilicity.

Granular superconductivity and magnetic-field-driven recovery of macroscopic coherence in a cuprate/manganite multilayer

We show that in Pr_(0.5)La_(0.2)Ca_(0.3)MnO_(3)/YBa_(2)Cu_(3)O_(7) (PLCMO/YBCO) multilayers the low temperature state of YBCO is very resistive and resembles that of a granular superconductor or a frustrated Josephson-junction network. Notably, a coherent superconducting response can be restored with a large magnetic field which also suppresses the charge-orbital order in PLCMO. This coincidence suggests that the granular superconducting state of YBCO is induced by the charge-orbital order of PLCMO. The coupling mechanism and the nature of the induced inhomogeneous state in YBCO remain to be understood.