Characterization of microstructure and dopant distribution of laser diffused resistors

In order to completely understand the laser-processing mechanisms and accurately control the electrical and material properties of the LDRs, the research during this thesis was focused on microstructural characterization and dopant distribution measurements of LDRs fabricated under various laser parameters and different initial device structures The aim of the project was to use theses results to improve the accuracy, thermal coefficient of resistance (TCR) and long-term stability of those microdevices.;Three-dimensional (3D) and two-dimensional (2D) periodic silicon nanostructures formed by polarized focused Nd:YAG laser irradiation (532 nm) with spot size less than 3 μm on Si covered by SiO2 are presented in this thesis. We observed different structures, including periodic coexisting of liquid and solid, and ripples, were obtained under different laser intensities. However, when the light intensity is out of those ranges, either no melting was created or the periodicity was destroyed. The periodicity of these periodic structures is 360 nm related to the wavelength of frequency doubled Nd:YAG laser and the index of refraction of SiO2. We propose a model based on the fact that as the oxygen is diffusing locally from Sin, into the melted Si thus forming SiO with a lower melting point.;The resistivity and dopant distribution in these arrays of periodic submicron resistive links between the heavily doped regions were characterized. Then, the widths were measured by scanning capacitance microscopy (SCM), the depths were obtained by TEM and DSE, and then resistance values were measured using a four-point probe technique. Those arrays with different resistance values in LDRs can be accurately achieved by finely controlling the laser power and pulse widths. The SCM technique not only detects material properties at surface but also detects those at pn junctions near surface regions as a consequence of the built-in depletion based on different polarity of the applied DC bias voltage (VDC). A mechanism related to oxygen activation as donors is also proposed to explain the measurements of SCM signal vs. VDC. Moreover, the dopant uniformity in LDRs, fabricated with laser power of 3.10 W and 3.75 W, is also deduced. It was found that between 2 to 7 submicron conductive links are formed in the focused spot size of 3 μm and their number depends only on laser intensity ranging from 3.10 W to 3.75 W, while their average width (15–300 nm) and depth (108–147 nm) strongly depends on both laser intensity and number of laser pulses. The resistances of these links are between 363–493 Ω and the effective average doping levels are from 1.5×1018 to 2.4×1019 cm -3.;The physical structures, induced by laser irradiation with various process parameters, were obtained by microstructural analytical methods, such as TEM and AFM (atomic force microscopy), in combination with chemically selective etching technique. Point defects, dislocations, interdiffusion of both oxygen and silicon between the dielectrics and the melted silicon as well as mass transportation due to liquid convection are determined. Furthermore, the formation mechanisms of these point defects and dislocations are also proposed. The presence of point defects and dislocations implies that long laser pulses should be used in the LDRs' fabrication. The regrowth of Si crystal simultaneously starting from both the dielectrics/Si interface and the Si-liquid/solid interface is for the first time suggested. The origin of this phenomenon is based on the increase of heat conductivity of the dielectrics due to diffusion of Si from the melting, pool into the hot dielectrics. The liquid convection induces non-ideal dopant transportation. Therefore, the oxygen and silicon inter-diffusion and liquid convection suggests that short laser pulses should be used in fabrication process. All these structural information suggests that fabrication of high quality LDRs requires considering and controlling all processing conditions.;Obtaining accurate quantified electrically activated dopant distribution in the actual microelectronic devices was a real challenge, especially when one considers microdevices (LDRs) having a very small area of few μm 2. By considering a vector as etching rate instead of a scalar (as considered by previous researchers) and by using a novel calibration method, the reliability, reproducibility and accuracy of quantification of dopant concentrations, ranging from 9×1016 to 6×10 19 atoms/cm3, of the dopant evaluation technique have been significantly improved. Therefore, the dopant profiles in the non-irradiated heavily doped regions in our devices were obtained using dopant selective etching (DSE) in combination with cross-sectional transmission electron microscopy (TEM) and focused ion beam (FIB) techniques. Furthermore, the developed method has been applied to our LDRs and the two-dimensional (2D) dopant distributions with wide range of boron concentrations of the resistive links were quantified with spatial resolution of 1 to 5 nm. This results shows that the LDRs with boron concentrations up to ∼0.24×1018 atoms/cm 3 and 8.9×1018 atoms/cm3 can be produced depending on different structures of initial microdevices and laser parameters. Those profiles were accurately compared with numerical simulation results based on heat transfer and diffusion equations.