D. Wolfframm

Influence of local heating on micro-Raman spectroscopy of silicon

In micro-Raman spectroscopy strong local heating by the tightly focused laser beam results in an unexpectedly small temperature shift of the Raman peak. The experimental results of shift, broadening, and asymmetry are discussed in view of restricted thermal expansion and a temperature dependent Raman cross section. The Raman signal originates mostly in the wings of the beam profile, where the temperature is substantially lower than at the center. Consequently, the Raman peak shift cannot be used to estimate the maximal surface temperature: despite local melting in the center of the probed spot, the maximal detected shift is only about 2 cm−1, 10 times less than expected from the center temperature.

Laser-induced ion emission from dielectrics

Photo-ablation of sapphire by ultrashort laser pulses was investigated by time-of-flight mass spectrometry. Experiments were performed on a laser fluence range below the single shot damage threshold. The dependence of emitted positive ion intensity on both the laser fluence and the number of laser pulses, hitting the same target site, was studied. In addition, the ion kinetic energy distribution was analysed. We find that the ablation is caused by surface explosion. The origin of this explosion, however, is still unknown.

Ultrafast laser-induced index grating in transparent insulators

Abstract We report on new results of third harmonic generation and self-diffraction. These effects are related to the occurrence of a refractive index grating produced in transparent dielectrics, in particular in barium fluoride, under irradiation by femtosecond laser pulses. Because only nonresonant electronic nonlinearities are exploited, all described effects are quasi instantaneous. As a result of the high intensities at weak energy obtainable from femtosecond laser systems these effects are very efficient.

Ultrafast laser desorption from transparent insulators

Abstract Positive ion emission upon irradiation with trains of ultra short laser pulses from an amplified titanium:sapphire laser (pulse duration 120 fs, photon energy 1.55 eV) shows significant differences between Al2O3, and NaCl targets. For both types of crystals, an incubation phase of several hundred laser pulses is necessary to prepare the target for the emission of detectable amounts of positive ions. Thereafter, in the case of Al2O3, ion bursts are observed depending stochastically on the number of additional laser pulses, while such an explosive ablation is not observed for NaCl. Evidence for different incubation and ablation mechanisms for both materials is presented, pointing to an important role of recondensation during multi-shot laser ablation.

Explosive Femtosecond Ablation from Ionic Crystals

Femtosecond laser ablation of positive ions from transparent ionic crystals is studied by time-of-flight mass spectrometry. We find an explosive emission of positive ions. The ion yield dependence eon the laser fluence is highly nonlinear. The material is emitted in characteristic bursts, depending chaotically on the number of laser pulses hitting the sample. The mean kinetic energy of the positive ions is on the order of 100 eV while their temperature is only around 1 eV, very similar to supersonic expansion of a molecular jet. The last observation is independent of the ion species, indicating that all ions were born at the same instant and kicked out of the material simultaneously with identical kinetic energy. Negative ions, on the contrary, appear considerably later and are much slower. Al ablation is preceded by effective electron emission. We suggest that the laser generates a high-density plasma. The resulting electrons may possibly escape, due to the short pulse duration, without being disturbed by the build-up of a space charge zone. Subsequently, positive ions are expelled by Coulomb explosion of the unstable surface .Negative ions may be produced much later form the hot sample or by secondary processes.

PLD of high-k dielectric films on silicon

Thin films of high-k materials praseodymium oxide (PrxOy) and hafnium oxide (HfOx) were deposited on silicon (100) surfaces by pulsed laser deposition (PLD), using the third harmonic of a Nd:YAG laser. The two materials are compared with respect to their morphology, in dependence on the substrate temperature during deposition, and their chemical composition and crystalline structure, in particular at the interface. The films of both oxides exhibit a grainy structure when deposited at substrate temperatures below 750°C, with the grain size increasing from ≈ 40 nm at room temperature to ≈ 100 nm at 750°C. However, the PrxOy films are much more uniform than hafnia, the latter exhibiting increasingly larger holes, reaching several nm into the silicon substrate. For a substrate temperature of 900°C, the film morphology for PrxOy completely changes to much larger crystalline areas, while for HfOx the role of holes in the film becomes substantial. Also, the interface chemistry is significantly different for both materials: a silicate formation for PrxOy and a rich abundance of SiO2 and a silicide for hafnia. Finally, our PrxOy-films exert compressive stress on the substrate, for HfOx-films tensile stress is observed which, however, may also result from interfacial SiO2.

Nonlinear optical characterization of the surface of silicon wafers: In-situ detection of external stress

Abstract The potential of nonlinear optical techniques for a rapid on-line and non-destructive inspection and characterization of silicon wafers is discussed. As an example, the in-situ detection of external stress on the wafer is reported, resulting from specific mounting conditions. As an outlook, the problem of radially non-uniform growth of the silicon crystal when utilizing the Czochralski-growth method is addressed. A simple technique is proposed to discriminate those sections of the wafer which are ready for use in further applications from those which are not useable for proper device fabrication, thus enabling the selection of appropriate material from as-grown crystals.

Femtosecond index grating in barium flouride: efficient self-diffraction and enhancement of surface SHG

A transient refractive index grating is formed in barium fluoride crystals under irradiation with femtosecond laser pulses from two non-collinear beams. At low intensities energy coupling takes place. At high intensities, a typical self-diffraction pattern is obtained with a diffraction efficiency better than 10%. Simultaneously, an enhancement of the SHG signal from the surface, as well as the generation and diffraction of the third harmonic is observable. For all effects the nonlinear Kerr-effect is responsible, with the response time being limited only by the temporal pulse shape.

Hafnium Oxide on Silicon: A Non-Destructive Characterization of the Interfacial Layer

Quality and stability of the substrate/dielectric interface r challenges for the implementation of new highk gate dielectrics into silicon technology. For our studies, we inve stigated nanometer thin hafnium oxide layers grown on Si substrates by Hf-Si -O-N MOCVD. Subsequent N2 annealing provides a high concentration of nitrogen close to the interf ace, as confirmed by AES, XRR and XPS measurements, indicating a silicon-nitrogen rich interm ediate layer. Samples annealed at low N 2 pressure (with an amorphous hafnium oxide layer, according to XRD mea surements) are compared to samples annealed at high N 2 pressure (with a crystalline HfSiO 4 layer). Micro-Raman measurements reveal that the silicon substrate is under compres sive stress, with similar intensity for both samples. In contrast, band-edge photoluminescence and defect luminescence exhibit pronounced differences: The sample with the crystalline HfSiO 4 phase shows a significantly higher band-edge luminescence than the sample with an amorphous layer. A higher defect luminescence occurred for the sample with the crystalline layer. We c onclude, that the photoluminescence results are strongly related to non-radiative interfacial rec ombinations. Interface-sensitive SHG investigations clearly show the four-fold symmetry of the substrate, indicatin g n intact interface.