S. L. Lai

MELTING POINT DEPRESSION OF AL CLUSTERS GENERATED DURING THE EARLY STAGES OF FILM GROWTH : NANOCALORIMETRY MEASUREMENTS

This work investigates the thermodynamic properties of small structures of Al using an ultrasensitive thin-film differential scanning calorimeter. Al thin films were deposited onto a Si3N4 surface via thermal evaporation over a range of thicknesses from 6 to 50 A. The Al films were discontinuous and formed nanometer-sized clusters. Calorimetry measurements demonstrated that the melting point of the clusters is lower than the value for bulk Al. We show that the melting point of the clusters is size dependent, decreasing by as much as 140 °C for 2 nm clusters. The results have relevance in several key areas for Al metallization in micro-electronics including the early stages of film growth and texture formation, the Al reflow process, and the dimensional stability of high aspect ratio Al lines.

High‐speed (104 °C/s) scanning microcalorimetry with monolayer sensitivity (J/m2)

We introduce a high sensitivity (1J/m2) scanning microcalorimeter that can be used at high heating rates (104 °C/s). The system is designed using ultrathin SiN membranes that serve as a low thermal mass mechanical support structure for the calorimeter. Calorimetry measurements of the system are accomplished via resistive heating techniques applied to a thin film Ni heating element that also serves as a thermometer. A current pulse through the Ni heater generates heat in the sample via Joule heating. The voltage and current characteristics of the heater were measured to obtain real‐time values of the temperature and the heat delivered to the system. This technique shows potential for measuring irreversible heat of reactions for processes at interfaces and surfaces. The method is demonstrated by measuring the heat of fusion for various amounts of thermally evaporated Sn ranging from 50 to 1000 A.

1 000 000 °C/s thin film electrical heater: In situ resistivity measurements of Al and Ti/Si thin films during ultra rapid thermal annealing

We introduce a new technique for rapidly heating (106 °C/s) thin films using an electrical thermal annealing (ETA) pulse technique. By applying a high‐current dc electrical pulse to a conductive substrate‐heater material (Si), joule heating occurs thus heating the thin film. This method was demonstrated by heating thin films of aluminum at rates ranging from 103 to 106 °C/s. The temperature of the system is measured by using the substrate heater as a thermistor and is found to be within ≊±10 °C during anneals at ≊105 °C/s. Phase transformations in the Ti‐Si system were also observed using in situ resistivity measurements during ETA at ≊104 °C.

Heat capacity measurements of Sn nanostructures using a thin-film differential scanning calorimeter with 0.2 nJ sensitivity

We have developed a new thin-film differential scanning calorimetry technique that has extremely high sensitivity of 0.2 nJ. By combining two calorimeters in a differential measurement configuration, we have measured the heat capacity and melting process of Sn nanostructures formed via thermal evaporation with deposition thickness down to 1 A. The equivalent resolution of the calorimeter is 1 nanogram in mass or 0.4 A in thickness. We have observed a decrease of up to 120°C in the melting point of Sn nanostructures.

Scanning calorimeter for nanoliter-scale liquid samples

We introduce a scanning calorimeter for use with a single solid or liquid sample with a volume down to a few nanoliters. Its use is demonstrated with the melting of 52 nL of indium, using heating rates from 100 to 1000 K/s. The heat of fusion was measured to within 5% of the bulk value, and the sensitivity of the measurement was ±7 μW. The heat of vaporization of water was measured in the scanning mode to be within ±23% of the bulk value by actively vaporizing water droplets from 2 to 100 nL in volume. Results within 25% were obtained for the heat of vaporization by using the calorimeter in a heat-conductive mode and measuring the passive evaporation of water. Temperature measurements over a period of 10 h had a standard deviation of 3 mK.

Au-mediated low-temperature solid phase epitaxial growth of a SixGe1-x alloy on Si(001)

The evolution of microstructure during Au‐mediated solid phase epitaxial growth of a SixGe1−x alloy film on Si(001) was investigated by in situ sheet resistance measurements, x‐ray diffraction, conventional and high‐resolution transmission electron microscopy, energy dispersive x‐ray spectroscopy, and Rutherford backscattering spectrometry. Annealing amorphous‐Ge/Au bilayers on Si(001) to temperatures below 120 °C caused changes primarily in the microstructure of the Au film. Near ≊130 °C, Ge from the top layer diffused and crystallized along the grain boundaries of Au. The Ge that had reached the Au/Si (001) interface mixed with Si from the substrate, to form epitaxial SixGe1−x islands on Si (001). Si from the substrate had dissolved into Au before entering the growing epitaxial islands. Meanwhile, the Au that was displaced by Ge that filled the Au grain boundaries, diffused into the top layer along columnar voids in the amorphous Ge film. With increasing temperature, more Au was displaced to the top by ...

An ultrafast thin-film microcalorimeter with monolayer sensitivity (J/m{sup 2})

The authors introduce a high-sensitivity ({approximately}1 J/m{sup 2}) scanning microcalorimeter that can be used to perform direct calorimetric measurements on thin film samples at ultrafast heating rate ({approximately} 10{sup 4} C/s). This novel microcalorimeter is fabricated by utilizing SiN thin-film membrane technology, resulting in dramatically reduced thermal mass of the system. Calorimetric measurements are accomplished by applying a dc-current pulse to the thin-film metal (Ni) heater which also serves as a thermometer, and monitoring the real-time voltage and current of the heater. The temperature of the system and the energy delivered to the system are then determined. This calorimetric technique has been demonstrated by measuring the melting process of thin Sn films with thickness ranging from 13 to 1,000 {angstrom}, and shows potential for calorimetric probing of irreversible reactions at interfaces and surfaces, as well as transformations in nanostructured materials,

Evolution of Microstructure During Low-Temperature Solid Phase Epitaxial Growth of Si ξ Ge 1-ξ on Si(001)

The evolution of microstructure during Au-mediated solid phase epitaxial growth of a SiGe alloy film on Si(001) (c-Si) was investigated by in situ resistance measurements, X-ray diffraction, conventional and high-resolution transmission electron microscopy, and chemical microanalyis. Annealing a-Ge/Au bilayers on c-Si to temperatures below 120°C caused changes primarily in the microstructure of the Au film. Increases in temperature to ≃150°C resulted in the diffusion of Ge through the grain boundaries of Au. The Au, displaced by crystalline Ge at the grain boundaries, diffused along columnar voids of amorphous Ge (a-Ge) leading to the formation of Au-rich crystallites in the top layer. Results indicate that the Ge that had reached the Au/c-Si interface grew epitaxially on c-Si at temperatures below 150°C. As the temperature was further increased, some Si from the substrate dissolved into Au and got incorporated in the growing epilayer. At 310°C, the initial Au film was displaced completely by a laterally continuous Si ξ Gei. ξ (ξ ∼ 0.2) epilayer whose thickness was limited by that of the initial Au film. Twins, and residual amounts of Au near the SiGe/c-Si interface, were the predominant defects observed in the SiGe epilayer.