Showing papers by "Xiang Zhang published in 1996"

Transient Temperature During the Vaporization of Liquid on a Pulsed Laser-Heated Solid Surface

Abstract: The thermodynamics of the rapid vaporization of a liquid on a solid surface heated by an excimer laser pulse is studied experimentally. The transient temperature field is measured by monitoring the photothermal reflectance of an embedded thin film in nanosecond time resolution. The transient reflectivity is calibrated by considering a temperature gradient across the sample based on the static measurements of the thin film optical properties at elevated temperatures. The dynamics of bubble nucleation, growth, and collapse is detected by probing the optical specular reflectance. The metastability behavior of the liquid and the criterion for the onset of liquid-vapor phase transition in nanosecond time scale are obtained quantitatively for the first time.

Secretory expression of a single-chain insulin precursor in yeast and its conversion into human insulin

Abstract: A synthetic single-chain porcine insulin precursor (PIP) gene and an alpha-mating factor leader sequence (alpha MFL) gene obtained by the PCR method are inserted between the promoter and 3'-terminating sequence of the alcohol dehydrogenase gene ADH1 in plasmid pVT102-U to form plasmid pVT102-U/alpha MFL-PIP. The single-chain insulin precursor is expressed and secreted to the culture medium by Saccharomyces cerevisiae transformed by pVT102-U/alpha MFL-PIP. The precursor is purified and converted into human insulin by tryptic transpeptidation. The purified human insulin is fully active and can be crystallized. The overall yield of human insulin is 25 mg per liter of culture medium.

Lattice locations of silicon atoms in δ-doped layers in GaAs at high doping concentrations

Abstract: Low-noise infrared (IR) absorption measurements of localized vibrational modes (LVM's) showed that ${\mathrm{Si}}_{\mathrm{As}}$ acceptors, ${\mathrm{Si}}_{\mathrm{Ga}}$-${\mathrm{Si}}_{\mathrm{As}}$ pairs, and a deep trap Si-X (${\mathit{V}}_{\mathrm{Ga}}$-${\mathrm{Si}}_{\mathrm{As}}$-${\mathrm{As}}_{\mathrm{Ga}}$), as well as isolated ${\mathrm{Si}}_{\mathrm{Ga}}$ donors, were present in silicon \ensuremath{\delta}-doping superlattices in (001) GaAs grown under an As flux by molecular-beam epitaxy (MBE) at 400 \ifmmode^\circ\else\textdegree\fi{}C for areal concentrations (per layer) 0.05 ML\ensuremath{\leqslant} [Si${]}_{\mathit{A}}$\ensuremath{\leqslant}0.5 ML. These observations supersede previous data, and agree with recent Raman-scattering measurements. For [Si${]}_{\mathit{A}}$\ensuremath{\geqslant}0.5 ML, the LVM's were not detected by either technique, but Raman measurements revealed a broad line that has been attributed to small two-dimensional Si clusters. For [Si${]}_{\mathit{A}}$\ensuremath{\geqslant}0.5 ML, electrical conductivity was lost. These observations led to a reappraisal of simulations of high-resolution x-ray 002 and 004 diffraction profiles. IR and Raman measurements for \ensuremath{\delta}-doping superlattices that all have [Si${]}_{\mathit{A}}$=0.01 ML (per layer) showed only the ${\mathrm{Si}}_{\mathrm{Ga}}$ LVM as the interlayer spacing was reduced to 5 ML when the volume carrier concentration n approached \ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}${10}^{19}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$. For interlayer spacings of 2 and 1 ML, compensating complexes ${\mathrm{Si}}_{\mathrm{As}}$, ${\mathrm{Si}}_{\mathrm{Ga}}$-${\mathrm{Si}}_{\mathrm{As}}$, and Si-X were present, and n tended to zero. Compensating complexes were also present in homogeneously doped MBE GaAs grown at 350 \ifmmode^\circ\else\textdegree\fi{}C, but n remained at a value of 2\ifmmode\times\else\texttimes\fi{}${10}^{19}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ as [Si] was increased to 1.3\ifmmode\times\else\texttimes\fi{}${10}^{20}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$. N never exceeded 2\ifmmode\times\else\texttimes\fi{}${10}^{19}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ in any sample. The formation of ${\mathit{V}}_{\mathrm{Ga}}$, ${\mathrm{As}}_{\mathrm{Ga}}$, etc. is attributed to diffusion jumps of Si atoms originally located on Ga lattice sites. The formation of the ``Si-like'' structure in \ensuremath{\delta} layers must result from the aggregation of such displaced atoms. We speculate that these processes are facilitated by the initial displacements of ${\mathrm{Si}}_{\mathrm{Ga}}$ donors to DX locations. \textcopyright{} 1996 The American Physical Society.

Heat and mass transfer in pulsed-laser-induced phase transformations

Abstract: Publisher Summary This chapter reviews a recent research on pulsed-laser-induced phase-change transformations at the nanosecond time scale. The melting of semiconductor materials was probed by optical reflectance, transmittance, electrical conductance, and infrared pyrometry. The experimental results were in general agreement with the thermal model, except for the high-irradiance regime. Direct measurements of the solid-liquid interface in melting of polysilicon films on glass substrates showed superheat by over 100° C. The rapid mass transfer in the liquid silicon phase allowed successful fabrication of well-controlled and sharply defined box-shaped ultrashallow dopant junction profiles. The chapter discusses that utilizing heat transfer and stability analysis, the generation of surface morphology was attributed to Rayleigh-Taylor instability triggered by the reduction of density on melting. Time-of-flight measurements of the kinetic-energy distribution in near-threshold sputtering of gold produced values exceeding the thermal model expectations. A computational heat-transfer analysis of the heat transfer and the vapor gas dynamics adopting the transparent vapor assumption are also discussed.

Excimer laser projection micromachining of polyimide thin films annealed at different temperatures

Abstract: A KrF excimer laser projection micromachining tool has been designed and implemented aiming to accomplish one-step etching with micron resolution for applications such as chip module packaging and polyimide technology in semiconductor manufacturing. Two polyimide (Probimide/sup (R/) 7020) thin films spun on the silicon wafers are annealed at 100/spl deg/C and 400/spl deg/C, respectively, in order to investigate the effect of the annealing temperature on the laser micromachining process. The micro-machined polyimide surface morphology is studied by scanning electron microscopy (SEM), atomic force microscopy (AFM), and a surface profilometer for determination of the etch rate at different laser fluences. Micromachining with micron resolution is achieved by this excimer laser projection tool. Measured on a 25 /spl mu/m line, the etch rate dependence on laser fluence both below and above the ablation threshold is investigated over the fluence range from 10-1000 mJ/cm/sup 2/. The polyimide thin film optical properties, i.e., the components of the complex refractive index, n and k at /spl lambda/=248 nm are measured by a spectroscopic ellipsometer for both films. The etch rate above the ablation threshold for the film annealed at 100/spl deg/C is found 60% higher than that for the film annealed at 400/spl deg/C. This behavior originates from the correspondingly higher by 82% absorption coefficient, /spl alpha/ in the 100/spl deg/C film. The ablation thresholds for both films are found the same at about 100 mJ/cm/sup 2/. Calculated etch rates agree with experimental values within 15%. Etch rates of the order of nanometer can be achieved near the ablation threshold of 100 mJ/cm/sup 2/ for both films. The linearity between the etched depth and the number of the laser pulses prevails for the micromachining process at and above the threshold.

Excimer laser-induced heating, melting, and mass diffusion in crystal silicon in nanosecond and nanometer scale

Abstract: Heat and mass transfer at the nanosecond time scale and the nanometer length scale in pulsed laser fabrication of ultra-shallow p+-junction is studied in this work. A technique is developed to fabricate the ultra-shallow p+-junctions with pulsed laser doping of crystalline silicon with a solid spin-on-glass (SOG) dopant, through the nanosecond pulsed laser heating, melting, and boron mass diffusion in the 100 nm thin silicon layer close to the surface. High boron concentration of 1020 atoms/cc and the `box-like' junction profile are achieved. The key mechanism determining the `box-like' junction shape is found to be the melt-solid interface limited diffusion. The ultra-shallow p+-junctions with the depth from 30 nm to 400 nm are successfully made by the excimer laser. The optimal laser fluence condition for SOG doping is found about 0.6 - 0.8 J/cm2 by studying the ultra-shallow p+-junction boron profiles measured by the secondary ion mass spectroscopy versus the laser fluence and the pulse number. The 1D numerical analysis agrees reasonably with the experiment, within the available physical picture. Possible mechanisms such as boron diffusivity dependence on the dopant concentration in the molten silicon are proposed.