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A Near Field Scanning Optical Microscope (NSOM) with spectroscopic capability is applied to imaging semiconductor and microelectronic structures. NSOM combined with spectroscopic analysis provides physical and chemical information of thin films and defects with ultra high spatial resolution. We have studied epitaxial and bulk samples and partially fabricated SiO2/Si CMOS structures to investigate the spatial resolution and imaging modes of NSOM. Reflected intensity contrast in NSOM yields images of defect networks in InGaAs/InAlAs/GaAs epitaxial layers and shows thickness variations in SiO2 films on Si. Surface topological changes observed in NSOM demonstrate a spatial resolution of significantly better than 0.25 μm. Fluorescence imaging is examined for chemically identifying materials and defects.
Photoreflectance spectroscopy (PR) and spectral ellipsometry (SE) have been used to characterize the doping and structure of heterojunction bipolar transistors (HBT). This information provides a more complete description of the epitaxial HBT structure than is possible by relying solely on electrical characterization of specially processed test structures. Additional benefit is derived from the nondestructive nature of both SE and PR. The measurements are fast enough to be implemented on all production-bound HBT material. We describe our recent results comparing capacitance-voltage measurements with PRderived doping levels in the emitter layer of the HBT. We also describe some work comparing SE fit results with Auger electron spectroscopy depth profiles for InGaAs contact layer composition and thickness.
Material properties of GaAs films grown on Si(321) substrates using molecular beam epitaxy (MBE) were evaluated and compared to films grown on Si(100) and Si(211). Dislocation densities in the GaAs(321) films, determined using transmission electron microscopy (TEM), were lower than those observed in GaAs(100) and GaAs(211), and the density of stacking faults in GaAs(321) also was quantitatively lower than in GaAs(211). Low-temperature (4.2 K) photoluminescence spectroscopy (PL) indicated that the tensile stress on the GaAs(321) films was greater than that on GaAs(100). These differences are attributed to changes in the strain-relaxation process caused by variations in the number of geometric arrangement of active-dislocation glide systems with different orientations. In addition, Si uptake near the GaAs/Si interface was less than 1/100 of a monolayer in GaAs(321) and 1/4 of a monolayer in GaAs(100). This difference is attributed to the presence of a non-polar (neutral) interface in the (321), whereas the (100) has a polar (charged) interface. MODFET devices with a I-)m gate length exhibited a transconductance of 180–200 ms/mm, comparable to devices on homoepitaxial GaAs(100).
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