Phase-modulated Ellipsometry Research Articles

Deposition of SiO 2 in integrated distributed electron cyclotron resonance microwave reactor

The deposition of dielectrics onto large areas is mostly carried out in traditional capacitively coupled 13.56 MHz RF plasma enhanced chemical vapour deposition reactors. Integrated distributed microwave 2.45 GHz electron cyclotron resonance (IDECR) configuration may be a possible alternative, if its potential advantages are realised. In this work we report on the results of SiO 2 deposition obtained with the IDECR system. Thin films of silica were deposited from the mixtures of SiH 4 with O 2 at different process conditions. Growth rates of silica up to 10 Å s −1 were obtained. Optical properties in UV-visible range were studied with spectroscopic phase-modulated ellipsometry. The first results suggest that films have high material quality and the deposition technique provides a broad process window. We study the influence of working pressure, power and gas composition on optical properties of SiO 2.

The effect of strain on the dielectric constants of strained In0.7Ga0.3AsyP1−y films

We measured the dielectric constants of strained In0.7Ga0.3AsyP1−y (y=0.2, 0.4, 0.8, 1.0) and lattice-matched 1.32 μm In1−xGaxAsyP1−y thin films grown on InP substrates by metalorganic chemical vapor deposition. Measurements were performed by phase-modulated spectroscopic ellipsometry in the range of 0.76–4.9 eV. Our data bridge the gap between literature data in the near-infrared region and those in the visible-ultraviolet region. The critical point energies of strained In0.7Ga0.3AsyP1−y were compared with unstrained counterparts and were found to be shifted in accordance with the theory, which predicts that the compositional shift is compensated. Thus, the critical point energies of strained In1−xGaxAsyP1−y thin films of arbitrary composition can be estimated accurately and, conversely, the composition of strained In1−xGaxAsyP1−y thin films can be estimated by measuring their critical point energies, as for unstrained materials.

Real time control of the growth of silicon alloy multilayers by multiwavelength ellipsometry

Real time control, by multiwavelength phase modulated ellipsometry (PME), of the growth of plasma deposited optical structures is presented here. The transparent multilayers consist of SiO 2 and SiN 1 alloys. Various on-line methods are compared. The best results are obtained with a feedback method based on comparison between the real time PME measurements and pre-computed target trajectories. It can provide the high precision required to deposit high performance optical coatings. In particular, an overall accuracy better than 1% is obtained on a 15-layer quaterwave filter, designed at 670 nm. This real time procedure, which is not limited to transparent materials or plasma processes, appears a useful tool for process control.

Calibration improvement of Fourier transform infrared phase-modulated ellipsometry

An improved self-consistent calibration procedure has been developed for FTIR phase-modulated ellipsometers. The new calibration method basically considers the effect of multiple reflections between the modulator faces in the intensity detected. The alignment, orientation, and calibration procedures are described in detail. The use of extra polarizers or analyzers is also discussed. The new calibration method removes the Bessel function background with an accuracy better than 0.5% in the wave-number range from 1000 to 4000 cm−1. The examples provided illustrate the accuracy in the measurement of Ψ and Δ angles, which turns out to be less than 1° on average.

Phase modulated ellipsometry used for composition control during MBE growth of CdHgTe: An analysis of instrumental factors and an assessment of the material produced

We have found phase modulated ellipsometry (PME) to be a sensitive analytical technique capable of providing real time information on composition, epilayer thickness, growth rate, interdiffusion and surface roughness. To fully exploit the benefits of PME, the instrument must be carefully calibrated and the many factors affecting its performance need to be understood and allowed for. In this paper we examine how the accuracy of the determination of composition of molecular beam epitaxially grown CdHgTe alloy films is affected by misalignment of the optical components, the presence of vacuum windows, signal conditioning prior to analog to digital conversion, temperature changes, and modulator settings. We conclude by presenting results which demonstrate the quality of CdHgTe layers grown on (211)B CdZnTe and (211)B GaAs substrates using our techniques.

Real time control of plasma deposited multilayers by multiwavelength ellipsometry

A robust direct method for real time control, by multiwavelength phase modulated ellipsometry (PME), of the growth of plasma deposited structures is presented here. Transparent multilayers consisting of SiO2 and SiNx alloys are investigated. This feedback control method is based on the comparison between the real time PME measurements and precomputed target trajectories. It can provide the high precision required to deposit high performance optical coatings. In particular an overall accuracy better than 1% is obtained on a fifteen layer quarter-wave filter, designed at 670 nm. This real time procedure, which is not limited to transparent materials nor plasma processes, appears to be a useful tool for process control.

Thin BN films obtained by dual-ion-beam sputtering: an FT-IR and spectroscopic ellipsometry characterization

Thin BN films (100–250 nm) have been deposited on Si(100) substrates in a double-ion-beam sputtering system from a h-BN target. The films have been characterized by FT-IR spectroscopy and spectroscopic phase-modulated ellipsometry as a function of the deposition and irradiation parameters. Assistance with Ar + ions result in boron rich films with refraction indexes above 2 or even higher as the deposited energy of the assisting ions increases. N 2 +-bombardment causes stoichiometric and even nitrogen-rich BN films characterized by refraction indexes around 1.85. The stability of the films is enhanced when the deposited energy of the N 2 + ions is above 30 eV/atom and/or the deposition is performed on substrates at temperatures above 100°C.

Modulated photoellipsometry: Application to the measurement of GaAs internal field

A new optical system allowing photoellipsometric measurements is presented. Photoellipsometry (PE) is a modulation spectroscopy technique based on ellipsometry in presence of a chopped external light excitation. PE measurements are obtained using a double modulation system, combining spectroscopic phase-modulated ellipsometry (SPME) with a laser pump beam. The experimental system described here takes advantage of the high frequency polarisation of SPME ( 50 kHz). As a consequence the frequency of the pump beam can be varied up to 5 kHz. The field-induced changes in the real and imaginary parts of the bulk dielectric function can be directly measured and analysed in terms of the pump beam power and/or the probe beam photon energy. Demonstration of this method is made with measurements, recorded in the band-gap E 0 region ( 1.4 eV), on n-type GaAs sample. In particular, Franz-Keldysh oscillations are observed with good sensitivity. PE measurements are successfully compared with a theoretical model. From this preliminary study, it can be inferred that PE appears as a promising technique for semiconductor characterisation.

Observation by infrared transmission spectroscopy and infrared ellipsometry of a new hydrogen bond during light-soaking of a-Si:H

Abstract Evidence for participation of atomic hydrogen in the light-induced metastability mechanism of hydrogenated amorphous silicon (a-Si:H) has been observed by low-temperature infrared transmission spectroscopy and room-temperature infrared phase-modulated ellipsometry. A band at 1730cm−1 accompanied by an increase of the amplitude of the bending mode or a new broad band at ∼ 870 cm−1 was observed after 1h of intense illumination of an annealed a-Si:H sample. A shift to lower frequencies of this band is observed with prolonged light-soaking time and the band disappears after ∼ 4h of illumination. At the same time the density of atomic hydrogen bonded to silicon decreases, as inferred from the area under the broad band at ∼ 640 cm−1. Our results can be interpreted by a hydrogen diffusion model in which the hydrogen atom of a Si-H bond next to a weak Si-Si bond diffuses to form a three-centre bond at intermediate illumination times, then diffuses further to form a stable Si-H bond at longer times.

Improvements of Fourier transform phase-modulated ellipsometry

Improvements of the Fourier transform phase-modulated ellipsometry (FTPME) technique are described. Measurements performed on the silicon oxide-silicon wafer system are used to illustrate FTPME performances. In particular, the chemistry of Si(100) and Si(111) surfaces after hydrofluoric acid (HF) treatment is investigated. Precisions on the ellipsometric angles Ψ and Δ of ±0.003° and ±0.008°, respectively, are obtained in the SiHn stretching mode region. SiH and SiH2 vibrations are identified at the Si surface revealing that submonolayer sensitivity can be achieved with FTPME. As a consequence, FTPME appears as a promising technique to perform detailed studies of interface formation and thin-film growth.

Surface Morphology and Crystallite Size during Growth of Hydrogenated Microcrystalline Silicon by Plasma-Enhanced Chemical Vapor Deposition

The correlation between the surface morphology and the crystallite size during growth of hydrogenated microcrystalline silicon (µ c-Si:H) has been discussed using spectroscopic UV-visible phase-modulated ellipsometry, Raman spectroscopy and atomic force microscope (AFM). Through the deposition study of µ c-Si:H on Corning 7059 glass and Cr substrates from SiH4 highly diluted in H2 and layer-by-layer (LbL) technique by plasma-enhanced chemical-vapor deposition (PECVD), we found that the surface roughness during µ c-Si:H growth strongly influenced the creation of the crystallite phase and the relaxation of Si network. The ion bombardment induces surface roughness and decreases the crystallinity in µ c-Si:H growth. The LbL technique using H2 plasma promotes the nucleation and coalescence processes in the early stage of µ c-Si:H growth.

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 SiH4 highly diluted in H2, 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.

In situ investigation of amorphous silicon-silicon dioxide interfaces by infrared ellipsometry

An in situ investigation of the a-Si:H-a-SiO2 interfaces by infrared phase modulated ellipsometry is presented. 20–30-Å-thick intermixing layers are clearly revealed at the interface whatever the deposition sequence. In particular, the presence of SiO and (O)SiHn bonds in the first monolayers of the growth of the ‘‘top’’ a-Si:H is identified. Likewise a hydrogen accumulation, revealed by SiH vibrations, is observed during the early stage of the growth of a-SiO2 on top of a-Si:H. The nature of the a-Si:H-a-SiO2 and a-Si:H-a-SiNx interfaces is compared.

In situ spectroscopic ellipsometry studies of interfaces of thin films deposited by PECVD

The formation of a-Si:H a-SiN x and p-doped a-Si:H ( p) interfaces has been investigated in situ by spectroscopic phase modulated ellipsometry (PME) from UV to IR. The thin films are deposited by plasma enhanced chemical vapor deposition (PECVD). In both cases, the nature of the interface is found to be influenced by the deposition sequence affecting the corresponding devices' behaviour. The a-SiN x / a-Si:H and p/i interfaces are found to be atomically sharp. A different behaviour is observed when p-doped a-Si:H or a-SiN x is deposited first. In the latter case, the ellipsometric measurements clearly reveal a nitrogen incorporation in the first monolayers of a-Si:H together with an increase of intensity of the SiH bonds at the interface. Likewise, a hydrogen accumulation, at a level of a few monolayers, is observed at the i/ p interface. More generally, the monolayer sensitivity of IRPME to vibrational absorption is emphasized.

Adhesion of a thin silicon oxide film on a polycarbonate substrate

The present paper analyzes the adhesion mechanisms of a silicon oxide film on a polycarbonate substrate, combining XPS, UV-visible ellipsometry and Fourier transform infrared phase-modulated ellipsometry (FTPME) measurements. In particular, we present the new FTPME technique: Due to its submonolayer sensitivity to vibrational properties, it suits well to physico-chemical studies, such as plasma-surface interactions. A preliminary plasma treatment of the polymer substrate before film growth is found to increase the sticking of the first monolayers of the film, and therefore its adhesion, as shown by conventional thermal adhesion tests. The treatment consists of three successive steps: argon plasma, (NH 3, Ar) plasma and (SiH 4, He, Ar) plasma. The first two steps induce a densification of the bulk and an activation of the surface (formation of CN bonds), while the silane plasma leads to the growth of a very thin silicon oxide layer. In particular, the increase in the film adhesion appears to be closely related to the presence of a SiO vibrational mode located at 1030 cm −1.

Spectroscopic variations of optical constant and emissivity of Pt-Al2O3 cermet thin films

Thin films of Pt-Al2O3 cermets were prepared by r.f. cosputtering on glass and metallic substrates. Wide ranges of composition and thickness were investigated. Spectroellipsometric determination of the complex optical constant n∗ = n−ik of semitransparent cermet thin coatings, from UV to IR, are presented. n∗ was measured by spectroscopic phase-modulated ellipsometry in the UV - visible and IR ranges. These results are used for the theoretical analysis of normal emissivity measurements. Examples of normal emission direct measurements are illustrated and compared with calculations. The variations of emissivity with film thickness can then be explained in terms of interference effects.

Ellipsometry study of the adhesion of dielectric thin films on polymer substrates

The adhesion mechanisms of plasma-deposited silicon alloys on polymer substrates are studied by IR ellipsometry. The IR ellipsometer combines the high sensitivity of Fourier transform spectroscopy with the fast modulation (37 kHz) of the phase-modulated ellipsometry technique. The film adhesion is characterized from the evolution of the vibrational properties of the polymer surface during the early stage of the film preparation. The influence of preliminary plasma treatments of the polymer surface is emphasized. Successive exposure of the polymer (polycarbonate) to argon and (NH 3, Ar) plasmas induces a substrate activation revealed by the appearance of CN bonds at the substrate surface. Then it is shown that the SiH 4 plasma treatment leads to the formation of a very thin SiO x layer at the substrate surface. In particular, the enhancement of the film adhesion appears to be correlated with the presence of a vibration located at 1030 cm −1. The influence of the plasma conditions is also studied. In particular, the ion bombardment can enhance the film adhesion.

In situ investigation of amorphous silicon/silicon nitride interfaces by infrared ellipsometry

A detailed in situ study by infrared phase-modulated ellipsometry of interfaces between plasma-deposited amorphous silicon (a-Si:H) and silicon nitride (a-SiNx) is presented. The structure of the interface is affected by the deposition sequence. A behavior compatible with a sharp interface is observed when a-SiNx is deposited on top of a-Si:H, the underlayer material being very weakly influenced by the deposition of the overlayer. In contrast, a graded transition is observed when a-SiNx is deposited first. In the latter case, the infrared measurements directly reveal a nitrogen tail incorporated in the first monolayers of a-Si:H (10–20 Å thick). The formation mechanisms of the interfaces are discussed.

Effect of hydrogen dilution on the growth of hydrogenated amorphous silicon studied by in-situ phase-modulated ellipsometry

We have conducted a study of the growth of hydrogenated amorphous silicon (a-Si:H) thin films, deposited in SiH 4H 2 r.f. discharges, through spectroscopic and real-time phase-modulated ellipsometry. The study focused on the microstructure of the a-Si:H-crystalline Si interface and the a-Si:H bulk layer. Real-time data were analysed using a theoretical growth model, previously proposed, which assumes cylindrical nucleation, coalesence and long-term growth phases. For high silane fractions (above 60%) the results are compatible with the existence of an abrupt interface, whereas for low silane fractions (20%) the real-time ellipsometric data can be modelled assuming the existence of an interface, 1.2 nm thick, composed of SiO 2 (95%) and a-Si:H (5%). The ellipsometric results indicate an optimal degree of silane fraction for which the bulk void fraction is minimized. The results are discussed in terms of growth kinetics.

Optical properties of Zr films grown under ion bombardment

Zr films (300–600 nm) have been deposited on Si〈100〉 in a double-ion-beam system (residual pressure, less than 10 −7 Torr) using Ar + for both sputtering and irradiation of the growing film. The films have been optically characterized by spectroscopic phase-modulated ellipsometry in the range 1.5–4.5 eV as a function of the deposition variables. The results show significant changes in the real part n and in the imaginary part k of the complex refractive index ( N ̃ = n + ik) . These changes mainly depend on the flux and energy of the Ar + ions impinging the growing film. Furthermore, the stability of the films against the exposure to atmosphere and annealing in vacuum (10 −7 Torr) was also investigated. The results are discussed in terms of packing densities, damage and surface oxidation of the films.