Infrared Laser Interferometry Research Articles

Experimental verification of theoretical calculations of the static electrical conductivity in moderately non-ideal helium plasmas produced in flashlamps

We present an experimental verification of the semiclassical theory [1] for static conductivity calculations in the case of moderately non-ideal plasmas which has been developed as a more simple approximation than the quantuum-mechanical theory based on the RPA method [2,3]. Such plasmas are produced in linear flashlamps (inner diameter 5mm, inter-electrode length 100mm) filled with pure helium and are characterized by on axis electron densities in the range 2.10 17 -1.7.10 18 cm -3 and temperatures around (2-3)×10 4 K, giving a plasma parameter r close to 0.1. Different methods of diagnostics based on measurements of continuum intensities, neutral line intensities and opacities (taking into account the effect of the statistical ionic microfields on the atomic levels), and infrared laser interferometry (3.39μm), are applied to determine the radial profiles of particles and temperature. Profiles so deducted are relatively flat and show a good filling of the plasma inside the flashlamp. Such experimental conditions coupled with precise measurements of the disharge electrical parameters (current intensity and electrical field) allow for a reliable evaluation of the validity of this semiclassical theory. Finally, for each case, the impedance of the plasma so calculated is in good agreement with the experimental value, proving the validity of the semiclassical theory, and thereby of the RPA theory in our plasmas.

Characterization of semiconductor devices by infrared laser interferometry

The principles of characterization techniques for semiconductor devices based on infrared laser interferometry are reviewed. Transient optical signals due to plasma-and thermo-optical effects are studied by experiment and numerical modeling, providing information on carrier concentration and lattice temperature in the device. The applicability of the techniques is demonstrated on smart power devices, VDMOSFETs, IGBTs and on sub-micron technology MOS- and bipolar transistors.

A laser beam method for evaluation of thermal time constant in smart power devices

We present a non-invasive optical method based on infrared laser interferometry to study the thermal time constant in dielectrically isolated smart power devices. By analyzing the time domain optical reflectivity signal, the thermal time constant is determined independently of the applied heating power. Experimental values of the time constant are confirmed by calculations using equivalent thermal RC-networks.

Temperature dependence of the near-infrared refractive index of silicon, gallium arsenide, and indium phosphide.

Infrared laser interferometry was used to measure the temperature dependence, \ensuremath{\beta}(T), of the refractive index of Si, GaAs, and InP at \ensuremath{\lambda}=1.15, 1.31, 1.53, and 2.39 \ensuremath{\mu}m. Semiconductor wafer samples that had been polished on both sides were either heated or cooled while measuring the sample temperature and the transmitted or reflected intensity of an infrared laser beam. The changing optical path length within the material causes alternating constructive and destructive interference between reflections off the front and back surfaces of the wafer. By subtracting the contribution of thermal expansion, \ensuremath{\alpha}(T), which is small and accurately known, \ensuremath{\beta}(T) was obtained. Representative values of \ensuremath{\beta} (293 K) at 1.53 \ensuremath{\mu}m are 5.15\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}5}$ ${\mathrm{K}}^{\mathrm{\ensuremath{-}}1}$, 6.65\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}5}$ ${\mathrm{K}}^{\mathrm{\ensuremath{-}}1}$, and 5.95\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}5}$ ${\mathrm{K}}^{\mathrm{\ensuremath{-}}1}$ for Si, GaAs, and InP. Polynomial expressions are presented for Si, GaAs, and InP, yielding values of \ensuremath{\beta}(T) that are accurate to within \ifmmode\pm\else\textpm\fi{}5%. $beta (T)--- increases with increasing temperature and decreases with increasing wavelength. There is a large resonance enhancement of \ensuremath{\beta}(T) in direct-gap semiconductors as the photon energy ${\mathit{E}}_{\mathit{h}\ensuremath{\nu}}$ approaches the band-gap energy ${\mathit{E}}_{\mathit{g}}$. Absolute values and temperature dependences of \ensuremath{\beta} calculated from published theory agree reasonably well with the measurements. The extreme accuracy in \ensuremath{\beta} needed for interferometric thermometry, however, cannot be met by these theoretical calculations, and so requires the experimental measurements.

Contactless local determination of recombination centre parameters in Cd0.3Hg0.7Te by infrared laser interferometry

We describe a method, and the results thereof, for contactless local determination of the excitation energy of recombination centres, their concentration, and the electron and hole capture coefficients in Cd0.3Hg0.7Te. The method suggested is based on analysis of the experimental temperature dependence of electron and hole lifetimes obtained by infrared laser interferometry.

The continuum absorption of argon, krypton and xenon dense plasmas produced in flashlamps

The optical thickness of dense plasma, created in linear flashtubes filled with noble gases, is measured over the wavelength range 466-780 nm using a CW laser beam. The electron density and temperature radial profiles of the plasma are determined by spectroscopic methods, infrared laser interferometry and local thermodynamic equilibrium equations. Knowing those profiles, the absorption coefficients and consequently the xi fb factors are deduced from optical thickness measurements. These factors are compared with theoretical values of Hofsaess (1978) and with previously published experimental results. The dependence of xi fb on the wavelength and the plasma parameters (electron density in the range 7*1017-1.7*1018 cm-3, temperature between 10900 and 17000 K) is discussed. The effect of the interparticle interaction on the absorption in the dense plasmas is only sensitive to the shift of the photoionization thresholds of atoms.

Intrinsic mechanism of smooth and rough morphology in etching of InP by Cl2 determined by infrared laser interferometry

The effect of Cl2 pressure and substrate temperature (Ts) on the absolute, steady-state etch rates of InP(100) has been measured with ∼1% accuracy from ∼0.02 to 1000 Å/sec by infrared laser interferometry. An unexpected finding was that the etch rate could exceed the calculated evaporation rate of InCl3 by a factor of 50, indicating a weaker binding energy of InCl3 to InP relative to itself. At sufficiently low Ts, the measured slow etch rate is only a factor of ∼3.5 above that predicted from the evaporation rate of InCl3. The transition between the fast and slow etch rates occurs as a first-order phase transition at a substrate temperature, T*s, which depends on the Cl2 pressure. If InP is etched at T*s, the surface roughens while at other Ts the surface remains smooth. The explanation for the surface roughening is that at T*s, InCl3 nucleates to form InCl3 islands. At T*s the etch rate is approximately fifteen times slower where the InP surface is covered by InCl3 compared to the bare InP surface. The large differential etch rates for bare and InCl3 covered InP produces a rough surface morphology. Below T*s, the surface is completely covered by an InCl3 multilayer and etches uniformly although slowly at the nominal InCl3 vaporization rate. This mechanism of surface roughening is intrinsic to the Cl2+InP reaction.

Maskless laser interferometric monitoring of InP/InGaAsP heterostructure reactive ion etching

Infrared laser interferometry is used to measure etch rate, measure wafer temperature, and identify heterostructure layers in situ during reactive ion etching, with or without masked regions. Interference between reflections from the etching wafer surface, buried heterointerfaces, and polished wafer back allows etch rate monitoring and endpoint determination. Changes in the optical path length that occur as a wafer heats and cools upon processing also produce reflected intensity oscillations that allow determination of the process-induced change in wafer temperature. We also show that λ=0.6238 μm light can be used to monitor optically thin heterostructure layers with enhanced depth resolution over infrared light.