B. Drévillon

Fast polarization modulated ellipsometer using a microprocessor system for digital Fourier analysis

A new type of spectroscopic automatic ellipsometer using a piezobirefringent element for polarization modulation at 50 kHz is described. Instead of lock‐in amplifiers the data‐acquisition system consists of a 12.8‐MHz digital sampling of the detected signal with a high word rate 8‐bit ADC, followed by on line Fourier transformation of the accumulated data with a short instruction cycle (∼200 ns) microprocessor, driven by a commercial microcomputer. The absolute minimum time required for measuring one set of Fourier coefficients is thus reduced to the modulation period of 20 μs. For digital error reduction purposes and signal‐to‐noise ratio improvement a basic 5 ms sequence of 256 accumulated periods per point is chosen. At this data‐acquisition rate a precision of 5×10−4 is obtained. Further accumulation over 10 s leads to 10−5 precision capability. A detailed analysis of various sources of inaccuracy leads to an estimate of 0.5° maximum systematic error on the ellipsometric angles and ψ and Δ. Applicatio...

Substrate selectivity in the formation of microcrystalline silicon: Mechanisms and technological consequences

We report the results of an in situ spectroscopic ellipsometry study concerning the substrate dependence of the evolution of microcrystalline silicon films deposited by alternating amorphous silicon deposition and hydrogen plasma treatment. The evolution of the composition of the films during growth, up to thicknesses of ∼100 nm, indicates that besides etching, the diffusion of atomic hydrogen efficiently promotes the growth (and/or nucleation) of buried crystallites. Moreover, the evolution of the films strongly depends on the nature of the substrate. This substrate selectivity is discussed in terms of initial growth processes. The effect of the hydrogen plasma well below the film surface, which produces the thickness‐dependent film composition, along with the substrate selectivity, may be of prime importance in technological applications of microcrystalline silicon.

Improvements of phase‐modulated ellipsometry

This article gives several new insights on ellipsometers using photoelastic modulators. The assumption that the modulation has the form δ=δ0+A sin ωt is ruled out. In contrast, it is shown that the presence of higher harmonics in the modulation affects the measured signal. A new formalism is proposed to take this effect into account, and experimental evidences of its consistency and relevance are exposed. Using this multiple‐harmonic model, ellipsometric measurements showed a dispersion four to five times less than using the conventional model. A new method is proposed to adjust the modulation amplitude during the measurements, by measuring the third harmonic of the signal. It is proven experimentally that this method actually improves the precision of phase‐modulated ellipsometry. Other sources of errors are reviewed, such as multiple reflections in coherent light. A practical procedure to test whether a modulator is well adapted to ellipsometry is given.

Stable microcrystalline silicon thin-film transistors produced by the layer-by-layer technique

Microcrystalline siliconthin films prepared by the layer-by-layer technique in a standard radio-frequency glow discharge reactor were used as the active layer of top-gate thin-film transistors(TFTs). Crystalline fractions above 90% were achieved for silicon films as thin as 40 nm and resulted in TFTs with smaller threshold voltages than amorphous siliconTFTs, but similar field effect mobilities of around 0.6 cm2/V s. The most striking property of these microcrystalline silicontransistors was their high electrical stability when submitted to bias-stress tests. We suggest that the excellent stability of these TFTs, prepared in a conventional plasma reactor, is due to the stability of the μc-Si:H films. These TFTs can be used in applications that require high stability for which a-Si:HTFTs cannot be used, such as multiplexed row and column drivers in flat-panel display applications, and active matrix addressing of polymer light-emitting diodes.

In situ investigation of the growth of rf glow‐discharge deposited amorphous germanium and silicon films

An in situ investigation of the growth of rf glow‐discharge amorphous germanium (a‐Ge:H) and silicon (a‐Si:H) films using fast real‐time spectroscopic phase‐modulated elipsometry is presented. The influence of the conditions of preparation is studied in both cases. The same behavior is obtained in a‐Ge:H and a‐Si:H films deposited in similar conditions. In particular, the initial stage of the growth can be described by a nucleation process in both cases, whatever the conditions of preparation. The incomplete coalescence of the nuclei leads to the formation of a surface roughness on a ∼40‐A scale which is observed during film growth. In comparing real‐time ellipsometry measurements performed at different wavelengths, a correlation between the internuclei distance and the thickness of the surface roughness is observed. An enhancement of the surface mobility of the reactive species due to an increase of substrate temperature and/or ion‐bombardment energy results in an increase in the density of the nucleatio...

Influence of the substrate on the early stage of the growth of hydrogenated amorphous silicon evidenced by kinetic ellipsometry

Fast real‐time ellipsometry is used to study in situ, as a function of the substrate, the growth of the first tens of monolayers of hydrogenated amorphous silicon (a‐Si:H) deposited by a rf glow discharge of SiH4. The high sensitivity of this technique is illustrated and the early stage of the growth is found to strongly depend upon the nature of the substrate. A nucleation mechanism followed by incomplete coalescence is observed on metal and hydrogenated amorphous germanium (a‐Ge:H) substrates. On the contrary, fused silica (SiO2) and tin dioxide (SnO2) are superficially reduced:  this reduction creates at the interface a mixed layer of a‐Si:H and silicon oxide on the silica substrate, and produces elemental tin at the surface of the SnO2 substrate. In this last case, tin is found to diffuse in the further a‐Si:H growing film. On crystalline silicon (c‐Si), the a‐Si:H growth shows incomplete coalescence followed by homogeneous growth, probably together with the reduction of the native c‐Si oxide layer.

A real time ellipsometry study of the growth of amorphous silicon on transparent conducting oxides

In situ fast kinetic ellipsometry is used to investigate the early stage of the growth of hydrogenated amorphous silicon (a‐Si:H) on tin‐doped indium oxide (ITO), tin oxide (TO), and zinc oxide substrates. Transparent conducting oxide (TCO) substrates obtained from various sources and deposited by different techniques are studied. A wide range of deposition parameters of a‐Si:H films is considered. For a comparison with growth of amorphous silicon from silane, the ellipsometry data on pure hydrogen plasma degradation of TCO surfaces are also presented. In case of ITO and TO films, a two‐step deposition is observed: the first step is dominated by the reduction of TCO and formation of metallic In or Sn (≊10 s time scale) and the second step is the subsequent growth of a‐Si:H on the reduced substrates. In contrast, ZnO does not show any reduction. The substrate temperature during a‐Si:H deposition is found to have the most important influence on the chemical reduction of the substrates. Variations of other d...

In situ investigation of the optoelectronic properties of transparent conducting oxide/amorphous silicon interfaces

Transparent conducting oxide (TCO)/hydrogenated amorphous silicon (a‐Si:H) interfaces are investigated combining kinetic ellipsometry and Kelvin probe measurements. It is shown that the correlation between both in situ techniques allows a detailed description of the optoelectronic behavior of these interfaces. The Schottky barrier at the TCO/a/Si:H interfaces, as revealed by Kelvin probe measurements, is correlated with the chemical reduction of the TCO surface during the early stage of a:Si:H growth, as evidenced by kinetic ellipsometry. In particular, indium tin oxide (ITO) and SnO2 are found to be reduced by the silane plasma at 250 °C. On the countrary, ZnO is found highly resistant upon plasma reduction. The influence of the substrate temperature during a‐Si:H deposition is analyzed. Finally, the technical consequences of this study are outlined.

In situ spectroellipsometric study of the nucleation and growth of amorphous silicon

A detailed in situ spectroellipsometric analysis of the nucleation and growth of hydrogenated amorphous silicon (a:Si:H) is presented. Photoelectronic quality a‐Si:H films are deposited by plasma‐enhanced chemical vapor deposition on smooth metal (NiCr alloy) and crystalline silicon (c‐Si) substrates. The deposition of a‐Si:H is analyzed from the first monolayer up to a final thickness of 1.2 μm. In order to perform an improved analysis, real time ellipsometric trajectories are recorded, using fixed preparation conditions, at various photon energies ranging from 2.2 to 3.6 eV. The advantage of using such a spectroscopic experimental procedure is underlined. New insights into the nucleation and growth mechanisms of a‐Si:H are obtained. The nucleation mechanism on metal and c‐Si substrates is very accurately described assuming a columnar microstructural development during the early stage of the growth. Then, as a consequence of the incomplete coalescence of the initial nuclei, a surface roughness at the 10–...

Role of Hydrogen Plasma during Growth of Hydrogenated Microcrystalline Silicon : In Situ UV-Visible and Infrared Ellipsometry Study

We have applied in situ UV-visible and infrared phase-modulated ellipsometry to investigate the role of hydrogen plasma during the growth of hydrogenated microcrystalline silicon (μc-Si:H) by plasma-enhanced chemical vapor deposition (PECVD). The results of the deposition of μc-Si:H from the SiH 2 highly diluted in H 2 , layer-by-layer (LbL) technique and post-hydrogenation experiments showed that the 3-dimensional cross-linking and relaxation of a Si network near the growing surface were essential for the formation of microcrystalline silicon. The major role of hydrogen plasma is the creation of the free volumes on the growing surface due to the inhomogeneous etching of the Si network and the promotion of the cross-linking reactions