N. David Theodore

Epitaxial CoSi2 films on Si(100) by solid‐phase reaction

The inversion of a bilayer of Co on top of Ti and a Si(100) substrate upon steady‐state annealing, and the resultant formation of an epitaxial CoSi2 layer have been studied using both reactive (N2, N2+5% H2, He+14% H2) and nonreactive (vacuum) annealing ambients. In nitrogen, a high‐quality, single‐crystalline CoSi2 layer forms above 600 °C for 30 min, with an abrupt interface to the substrate. As the fraction of hydrogen in the ambient increases, the abruptness of the interface deteriorates slightly. On top of this silicide, the Ti is chemically bound with oxygen present as a contaminant. In the case of a nonreactive annealing ambient, the Co/Ti inversion still takes place, although it is only partial. Moreover, the interface is very rough. The structure is unstable above 800 °C for 30 min annealing and transforms into a mixed layer of Co0.25Ti0.75Si2 and epitaxial CoSi2 grains. Using isothermal vacuum annealings with varying durations, a square‐root time dependence is observed for the growth of the epit...

Dopant activation in ion implanted silicon by microwave annealing

Microwaves are used as a processing alternative for the electrical activation of ion implanted dopants and the repair of ion implant damage within silicon. Rutherford backscattering spectra demonstrate that microwave heating reduces the damage resulting from ion implantation of boron or arsenic into silicon. Cross-section transmission electron microscopy and selective area electron diffraction patterns demonstrate that the silicon lattice regains nearly all of its crystallinity after microwave processing of arsenic implanted silicon. Sheet resistance readings indicate the time required for boron or arsenic electrical activation within implanted silicon. Hall measurements demonstrate the extent of dopant activation after microwave heating of implanted silicon. Physical and electrical characterization determined that the mechanism of recrystallization in arsenic implanted silicon is solid phase epitaxial regrowth. The boron implanted silicon samples did not result in enough lattice damage to amorphize the s...

Plasma hydrogenation of strained Si∕SiGe∕Si heterostructure for layer transfer without ion implantation

We have developed an innovative approach without the use of ion implantation to transfer a high-quality thin Si layer for the fabrication of silicon-on-insulator wafers. The technique uses a buried strained SiGe layer, a few nanometers in thickness, to provide H trapping centers. In conjunction with H plasma hydrogenation, lift-off of the top Si layer can be realized with cleavage occurring at the depth of the strained SiGe layer. This technique avoids irradiation damage within the top Si layer that typically results from ion implantation used to create H trapping regions in the conventional ion-cut method. We explain the strain-facilitated layer transfer as being due to preferential vacancy aggregation within the strained layer and subsequent trapping of hydrogen, which lead to cracking in a well controlled manner.

Effects of hydrogen implantation temperature on ion-cut of silicon

We have studied the effect of ion implantation temperature on the nature of cleavage and layer transfer, and the electrical properties in hydrogen implanted p-Si. The lattice damage and the hydrogen concentration in the as-implanted Si and transferred Si films were analyzed with elastic recoil detection, respectively. Implantations performed at −140 °C [low temperature (LT)] and room temperature (RT) resulted in a variation in the thickness and surface morphology of the transferred layers. The transferred layer from room temperature hydrogen ion implantation was both thicker and atomically smoother than the transferred layer produced by −140 °C hydrogen implantation. The as-transferred layer obtained from RT-implanted p-Si wafer was n-type, but converted to p-type after annealing at 650 °C or higher. The transferred layer obtained from LT-implanted Si wafer was highly resistive even after high temperature annealing. These variations were observed to be correlated with the damage profiles measured by ion c...

Strain relaxation in single crystal SrTiO3 grown on Si (001) by molecular beam epitaxy

An epitaxial layer of SrTiO3 grown directly on Si may be used as a pseudo-substrate for the integration of perovskite oxides onto silicon. When SrTiO3 is initially grown on Si (001), it is nominally compressively strained. However, by subsequent annealing in oxygen at elevated temperature, an SiOx interlayer can be formed which alters the strain state of SrTiO3. We report a study of strain relaxation in SrTiO3 films grown on Si by molecular beam epitaxy as a function of annealing time and oxygen partial pressure. Using a combination of x-ray diffraction, reflection high energy electron diffraction, and transmission electron microscopy, we describe the process of interfacial oxidation and strain relaxation of SrTiO3 on Si (001). Understanding the process of strain relaxation of SrTiO3 on silicon will be useful for controlling the SrTiO3 lattice constant for lattice matching with functional oxide overlayers.

Investigation of plasma hydrogenation and trapping mechanism for layer transfer

Hydrogen ion implantation is conventionally used to initiate the transfer of Si thin layers onto Si wafers coated with thermal oxide. In this work, we studied the feasibility of using plasma hydrogenation to replace high dose H implantation for layer transfer. Boron ion implantation was used to introduce H-trapping centers into Si wafers to illustrate the idea. Instead of the widely recognized interactions between boron and hydrogen atoms, this study showed that lattice damage, i.e., dangling bonds, traps H atoms and can lead to surface blistering during hydrogenation or upon postannealing at higher temperature. The B implantation and subsequent processes control the uniformity of H trapping and the trap depths. While the trap centers were introduced by B implantation in this study, there are many other means to do the same without implantation. Our results suggest an innovative way to achieve high quality transfer of Si layers without H implantation at high energies and high doses.

Effectiveness of reactive sputter-deposited Ta–N films as diffusion barriers for Ag metallization

Tantalum nitride films on silicon were prepared by reactive sputtering of Ta under nitrogen partial flow rates varying from 15% to 40% N2. Rutherford backscattering spectroscopy (RBS) and x-ray diffraction (XRD) analysis revealed that the composition and phases of the Ta–N films were influenced by the N2 flow rate. Increasing the nitrogen partial flow rate from 25% to 40% N2, results in the films changing from metal-rich to stoichiometric Ta–nitride. High N2 flow rates (30%–40% N2) resulted in a disordered tantalum–nitride. The tantalum nitride films were evaluated as potential diffusion barriers for Ag metallization. Sheet resistance measurements, XRD and RBS analysis confirmed that Ta–N films, used as diffusion barriers in the Ag∕Ta–N∕Si system, were thermally stable up to 650°C when annealed for 30min in vacuum. The thermal stability was independent of N2 flow rate within this temperature range. However, at 700°C, the barrier failed as a result of Ta–silicide formation by reaction with the underlying S...

Influence of interfacial copper on the room temperature oxidation of silicon

Thick (∼1.3 μm) oxide films were grown by room‐temperature oxidation of silicon after low‐energy copper‐ion implantation. The structural properties of the silicon dioxide layer and the implanted silicon were characterized by Rutherford backscattering spectrometry and transmission‐electron microscopy. During the room temperature oxidation a portion of the implanted copper resided on the surface and a portion moved with the advancing Si/SiO2 interface. This study revealed that the oxide growth rate was dependent on the amount of Cu present at the moving interface. The surface copper is essential for the dissociation of oxygen at the surface, and it is this oxygen that participates in the oxidation process. The resulting oxide formed was approximately stoichiometric silicon dioxide.

Effects of hydrogen implantation temperature on InP surface blistering

We have investigated the effects of hydrogen implantation temperature on the ion-cut process of InP by examining the correlation between surface blistering and the ion induced damage, hydrogen distribution, and strain. Using Rutherford backscattering spectrometry, elastic recoil detection, and x-ray diffraction, it was found that both the point defects induced by the hydrogen implantation and the in-plane compressive stress were necessary for hydrogen trapping and H-platelet nucleation and growth. The control of implantation temperature is crucial for creating sufficient defects and strain to induce surface blistering or layer exfoliation.

H-induced platelet and crack formation in hydrogenated epitaxial Si∕Si0.98B0.02∕Si structures

An approach to transfer a high-quality Si layer for the fabrication of silicon-on-insulator wafers has been proposed based on the investigation of platelet and crack formation in hydrogenated epitaxialSi/Si0.98B0.02/Si structures grown by molecular-beam epitaxy. H-related defect formation during hydrogenation was found to be very sensitive to the thickness of the buried Si0.98B0.02 layer. For hydrogenated Si containing a 130nm thick Si0.98B0.02 layer, no platelets or cracking were observed in the B-doped region. Upon reducing the thickness of the buried Si0.98B0.02 layer to 3nm, localized continuous cracking was observed along the interface between the Si and the B-doped layers. In the latter case, the strains at the interface are believed to facilitate the (100)-oriented platelet formation and (100)-oriented crack propagation.