Phase-modulated Ellipsometry Research Articles

Study of the effects of low-energy electron bombardment during the chemical vapor deposition of diamond

The effects of low-energy electron bombardment during the chemical vapor deposition of diamond were studied. The film growth was monitored in real time with in situ phase-modulated ellipsometry, in order to trigger the electron bombardment at different growth stages. Ex situ Raman spectroscopy and scanning electron microscopy were employed to evaluate the crystalline quality and the morphology of the grown films, respectively. The results indicated that triggering the electron bombardment after high-quality scattered diamond crystallites had formed results in finely grained smoother films of similar diamond yield and crystalline quality as those grown without bombardment. However, the electron bombardment deteriorates the film crystalline quality and the diamond yield when it was triggered from the start of deposition.

Structural evolution during chemical vapor deposition of diamond thin films

In situ phase-modulated ellipsometry was employed to monitor the nucleation and growth processes of diamond thin films fabricated by chemical vapor deposition. The effective extinction coefficient (k) at 1.96 eV was used as a basis for dividing the deposition process into intervals. The film growth was aborted at various k values yielding diamond film samples that represent snapshots of the growth process at different stages. Ex situ characterization of the films was performed using Raman spectroscopy, scanning electron microscopy, and x-ray diffraction. The results indicate that the diamond film deposition process consists of various stages in which the crystalline quality, the net compressive stress, and the relative amount of non-sp3 carbon follow different trends. A correlation between the effective k value measured in situ and the film microstructure characterized ex situ was established which enables the monitoring of the diamond film growth process in real time.

In situ phase-modulated ellipsometry study of the surface damaging process of silicon under atomic hydrogen

We employed in situ phase-modulated ellipsometry in the monitoring of surface damage to monocrystalline silicon (Si) under plasma conditions typical for the chemical vapor deposition of diamond. Single-wavelength kinetic and spectroscopic ellipsometry measurements were done and complemented with Raman spectroscopy, in order to characterize the surface conditions. It was found that heating the Si substrate to 700°C in the presence of molecular hydrogen produces etching of the native oxide layer. When the hot bare silicon surface is submitted to atomic hydrogen, it becomes rough in minutes. Modeling of the spectroscopic ellipsometry provided a quantitative physical picture of the surface damage, in terms of the roughness layer thickness and void fraction. The results indicate that by the time a thin film starts to grow on these silicon surfaces, like in the chemical vapor deposition of diamond, the roughness produced by the atomic hydrogen has already determined to a large extent the rough nature of the film to be grown.

Anisotropic incoherent reflection model for spectroscopic ellipsometry of a thick semitransparent anisotropic substrate

An anisotropic incoherent reflection model for the Mueller matrix elements of an optically thick uniaxial anisotropic semitransparent substrate with its anisotropy axis along its surface normal is developed. The Mueller matrix elements are measured by phase-modulated spectroscopic ellipsometry (SE) and compared with incoherent reflection model simulations. In the case of a sapphire substrate the oscillations observed are accurately modeled, and, in addition, the oscillating degree of polarization is correctly predicted. A straightforward generalization of the optical model, in the case of an arbitrary stack of layers containing a thick anisotropic semitransparent substrate, is also proposed and experimentally validated. The model is further applied to study the anisotropic dielectric function of a semi-insulating 4H-SiC wafer. An approximation based on a simple variation in the optical transition element is proposed to model the SiC birefringence. In conclusion, SE is shown to be a powerful alternative for investigating and predicting the behavior of optically thick birefringent materials.

Investigation of Si/SiGe heterostructure material using non-destructive optical techniques

Characterisation of UHV-CVD (ultra high vacuum chemical vapour deposition) grown Si/SiGe heterostructure field-effect transistor (HFET) material with a buried, strained silicon layer has been carried out using non-destructive optical techniques. The effect of thermal budget on the heterostructure was investigated by annealing samples at temperatures up to 900°C for 5 min and carrying out analysis using Raman back-scattering spectroscopy. An investigation of silicon cap loss due to native oxide removal etches was carried out using phase-modulated variable angle spectroscopic ellipsometry (VASE) and correlated with Rutherford back-scattering spectroscopy (RBS) measurements.

Phase-modulated Mueller ellipsometry characterization of scattering by latex sphere suspensions

Phase-modulated Mueller ellipsometry (PMME) is used to probe scattering by suspensions of polystyrene latex spheres, with particle diameters ranging from 400 nm to 3 microm. PMME allows simultaneous measurement of the 16 coefficients of the Mueller matrix. Furthermore PMME measurements can easily be carried out owing to a calibration procedure implemented in a scattering configuration. The measurements performed on low concentrations show good agreement with Mie theory. Moreover size distribution could be obtained with a least-squares method based on a genetic fit algorithm. Experimental evidence of multiple scattering on PMME measurements is also presented.

Process monitoring of semiconductor thin films and interfaces by spectroellipsometry

Real-time monitoring of the growth of plasma deposited microcrystalline silicon (μc-Si:H) by multi-wavelength phase-modulated ellipsometry is presented. Several growth models for process-monitoring are reviewed. In particular the inhomogeneity in μc-Si layers is treated by allowing graded-index profile in the bulk. Using the Brüggeman effective medium theory, we describe the optical properties of μc-Si and the monitoring of the crystallinity in the upper and lower parts of the layer, and the thickness. The inversion algorithm is very fast, with calculation times within 5 s using typical PC. This method opens up ways for precise control of surface roughness, bulk thickness, and crystallization of both the top and bottom interfaces of the layer during processing devices such as solar cells and thin film transistors.

Optimized calibration method for Fourier transform infrared phase-modulated ellipsometry

A new calibration method for FTIR phase-modulated ellipsometry is presented. This procedure is based on the multiple-harmonic modulation model and takes into account the effect of the multiple reflections of the beam between the modulator faces. With this procedure all the Bessel function background in the wavenumber range from 950 to 3500 cm −1 is removed. When this is achieved, the ellipsometric angles Ψ and Δ can be determined accurately using any of the standard measurement configurations. The measurement of the thickness of an amorphous silicon dioxide thin film, thermally grown on a crystalline silicon substrate, is used as a quality test of the new calibration procedure. FTIR phase-modulated ellipsometry and UV–Vis phase-modulated ellipsometry results agree within 0.1%, indicating that the FTIR measurements performed with the new optimized calibration method are accurate.

Spectroellipsometric Method for Process Monitoring of Semiconductor Thin Films and Interfaces

Real time monitoring by multiwavelength phase modulated ellipsometry (PME) of the growth of plasma deposited microcrystalline Silicon (μc-Si) is presented. The construction of a growth model for process-monitoring is discussed, and in particular the inhomogeneity in the μc-Si layer is modelled by using an approximation of the reflection coefficient known as the Wentzel Kramers Brillouin Jeffreys (WKBJ) approximation. By also using the Bruggeman effective medium theory to describe the optical properties of μc-Si, monitoring of the crystallinity in the upper and lower part of the layer, together with the thickness is demonstrated. The inversion algorithms remain thus extremely fast, with calculation times within 5 seconds on a standard Pentium computer. This opens up for precise control of thickness, and crystallisation of both the top and bottom interfaces of the layer during the production of devices such as solar cells and thin film transistors.

Spectroellipsometric method for process monitoring semiconductor thin films and interfaces

Real-time monitoring by multiwavelength phase-modulated ellipsometry of the growth of plasma-deposited microcrystalline silicon (muc-Si) is presented. We discuss the construction of a growth model for process monitoring, and, in particular, we treat the inhomogeneity in the muc-Si layer by using an approximation of the reflection coefficient known as the WKBJ method. By also using the Bruggeman effective medium theory to describe the optical properties of muc-Si, we demonstrate monitoring the crystallinity in the upper and the lower part of the layer together with the thickness. The inversion algorithms thus remain very fast, with calculation times within 5 s on a standard Pentium computer. This makes possible precise control of the thickness and the crystallization of both the top and the bottom interface of the layer during the elaboration of devices such as solar cells and thin-film transistors.

Real-time control of the deposition of optical coatings by multiwavelength ellipsometry

Real-time control, by multiwavelength phase modulated ellipsometry (PME), of the growth of optical coatings is reviewed. The structures consist of plasma deposited SiO 2 and SiN x stacks, and a SiO x N y gradient-index. A fast feedback method is used for the growth control of a standard quarterwave filter on c-Si, and on glass by incorporating incoherent modelling of the transparent substrate. This feedback control method is based on the comparison between the real-time PME measurements and pre-computed target trajectories. The resulting optical coatings characterized by spectroscopic PME and transmission measurements show a reproducible accuracy, with less than 1% deviation between target and measured spectral responses. A different feedback method is used to control of the deposition of gradient-index coatings. An efficient method for estimating the optical parameters based on a least square fitting of the most recent recorded measurements is used in the latter case. Furthermore, an integral expansion of the reflection coefficients, giving a continuous optical model describing inhomogeneous films is used in the real-time modelling of the deposited structure. As an example of application, a successful real-time control of the growth of a linear gradient index is demonstrated.

A direct robust feedback method for growth control of optical coatings by multiwavelength ellipsometry

Real time control by multiwavelength phase modulated ellipsometry (PME), as applied to the growth of optical coatings deposited on various substrates, is reviewed. The structures consist of plasma deposited SiO 2 and SiN x stacks and SiO x N y gradient-index coatings. A feedback method is used for growth control of a standard quarterwave filter on c-Si and on glass, the latter achieved by incorporating incoherent modelling of the transparent substrate. The feedback control method is based on a comparison between the real time PME measurements and pre-computed target trajectories. The resulting optical coatings characterised by spectroscopic PME and transmission measurements show reproducible spectral characteristics, with less than 1% deviation between the target and measured spectral responses. The feedback control method is further generalised to the control of gradient-index coatings.

Application of infrared Fourier transform phase-modulated ellipsometry to the characterization of silicon-based amorphous thin films

Multilayers of silicon-based amorphous thin films were characterized to test the performance of Fourier transform phase-modulated ellipsometry (FTPME) in absolute measurements (thickness and refractive index). The multilayers consist of a stack of silicon dioxide (SiO 2), hydrogenated amorphous silicon (a-Si:H) and NiCr thin films on c-Si substrates. Results provided by FTPME are compared with those from UV-visible phase-modulated ellipsometry and transmission electron microscopy. The thicknesses of the layers determined by the three techniques agrees within 3%. Similar volume void fractions of approximately 8% for the silicon dioxide layers are found by UV-visible and IR ellipsometry.

Infrared and UV-visible ellipsometric study of WO 3 electrochromic thin films

Infrared Fourier Transform phase-modulated and UV-visible ellipsometry techniques have been applied to study the colouring/bleaching process of WO 3 electrochromic thin films. The optical absorption of these films can be changed markedly in a reversible process induced by hydrogen incorporation. The colouring/bleaching process was carried out by using the sample as one electrode of an electrochemical cell filled with an electrolytic solution of H 2SO 4. The spectroscopic ellipsometric angles Ψ and Δ were obtained ex situ by an infrared Fourier Transform phase-modulated ellipsometer working in the wavenumber range from 800 to 4000 cm −1 and a rotating analyzer ellipsometer in the UV-visible range (1.7–4.8 eV). Significant optical absorption and vibrational changes associated with the colouring/bleaching process which follow the hydrogen incorporation in the film were observed in the UV-visible range from 1.7 to 3.8 eV and in the IR range at 1605 cm −1 and from 3000 to 3800 cm −1. The strong changes can be seen by changes in the spectroscopic dielectric function. For the coloured WO 3 layer, the main absorption contribution associated with the colour effect was found at 6032 cm −1, outside the analysed region.

In situ infrared ellipsometry study of the growth of hydrogenated amorphous carbon thin films

Infrared Fourier transform phase-modulated ellipsometry (FTPME) has been used as an in situ probe to study the structural and vibrational properties of thin films. To illustrate the performance of this technique, in situ (Ψ, Δ) measurements have been obtained during the growth of different types of hydrogenated amorphous carbon (a-C:H) films deposited from plasmas onto crystalline silicon (c-Si). The refractive index and thickness of the films have been determined through (Ψ, Δ) analysis over the CH n stretching region. Within this vibrational range, the chemical nature of the bonds formed during the growth can be identified. The role of ion bombardment and interlayer formation, which are both of crucial importance in a-C:H film properties, have been studied under soft plasma conditions. Through studies of the C–H vibrational intensity, a microstructure model can be given for a-C:H deposited at low ion bombardment.

High-performance beamline for vacuum-ultraviolet-excited material processing

A high-performance beamline dedicated to vacuum-ultraviolet (VUV)-excited semiconductor processing has been constructed on the compact electron storage ring “Super ALIS”. The first toroidal mirror accepts photons (emitted at a bending magnet) for 70 mrad in the horizontal direction, and the reflected parallel beam is focused onto the specimen surface by the second toroidal mirror. A brilliant VUV beam (with a photon density of 1.3×1016 s−1 mm−2 when the ring current is 300 mA) illuminates a circular field (10 mm in diameter) on the target surface. The eight 5-m-long differential pumping stages allow the reactant gas to be introduced into the reaction chamber at pressures up to 0.1 Torr. The processing is monitored in real time by using ultraviolet phase-modulated spectroscopic ellipsometry, and VUV-stimulated evaporation of SiO2 film has been demonstrated.

Real-Time Monitoring and Growth Control of Si-Gradient-Index Structures by Multiwavelength Ellipsometry

Real-time monitoring and control of the growth of plasma-deposited gradient-index structures by multiwavelength phase-modulated ellipsometry are presented. An efficient method for estimating the optical parameters based on a least-squares fitting on the most recent recorded measurements is described. This method is used for real-time monitoring of the outerlayer refractive index and rate of deposition during deposition of inhomogeneous transparent films. An accurate integral expansion of the reflection coefficient, giving a continuous optical model describing inhomogeneous films, is used in the real-time modeling of the deposited structure. A variety of increasing complexities of the optical model is studied within inversion algorithms. Furthermore several ways of defining the optical parameters to be estimated are discussed. Inversion of simulated growth trajectories by using the described algorithms show what kind of information is available from the various approximations and in what conditions they are useful. Since real-time measurements during growth involves statistical noise and systematic errors, it becomes necessary to stabilize the fit. Various stabilizing functionals are discussed and implemented to solve this problem. Several plasma-deposited silicon oxynitride gradient-index structures where both rate of deposition and the refractive index are varied continuously are inverted in real time. As an example of application, a successful real-time control of the growth of a linear gradient index is demonstrated by using inversion algorithms. Inversion algorithms are extremely fast, with calculation times from less than a second (for the lowest-order approximation) to approximately 3 s.

Real-time control by multiwavelength ellipsometry of plasma-deposited multilayers on glass by use of an incoherent-reflection model

Real-time control by multiwavelength phase-modulated ellipsometry (PME) of the growth of multilayer structures deposited on transparent glass is presented. The structures consist of plasma-deposited SiO(2) and SiN(x) stacks. A model that takes into account incoherent reflection in the substrate is described and tested. A generalized feedback control method that incorporates the incoherent modeling of the transparent substrate is further applied to the growth of a Fabry-Perot and a standard quarter-wave filter. The resulting optical coatings characterized by spectroscopic PME and transmission measurements show a reproducible precision, with less than 1% error between target and measured spectral responses.

Threshold behavior in synchrotron-radiation-stimulated recrystallization during Si homoepitaxy on Si(100)

A brilliant synchrotron-radiation beam with a large irradiation area has made it possible to use spectroscopic phase-modulated ellipsometry to monitor vacuum-ultraviolet-excited Si homoepitaxy. We find the crystallinity of Si films grown at 250 °C from Si2H6 depends strongly on the photon intensity. When the photon intensity is higher than a critical value, electronically excited crystallization yields epitaxial Si. At lower intensities, however, the initial epitaxy is terminated halfway by island-shaped regions of polycrystalline Si, which are eventually covered by a uniform nanocrystalline Si overlayer.

Silicon surfaces treated by CF4, CF4/H2, and CF4/O2 rf plasmas: Study by in situ Fourier transform infrared ellipsometry

The reaction layer formed on a silicon substrate in a CF4 rf plasma and in its admixtures with hydrogen and oxygen has been investigated by Fourier transform infrared phase-modulated spectroscopic ellipsometry (FTIR-PMSE). The results are compared with x-ray photoelectron spectroscopy spectra of the surface. It is shown that by using FTIR-PMSE, a significant gain in sensitivity is obtained as compared to ordinary reflection absorption measurements. In the case of a CF4 plasma, a large difference in the reaction layer at rf and the grounded electrode has been found. Samples treated on the rf electrode have a thin reaction layer consisting of SiFx (x=1, 2, 3, and 4). The reaction layer of samples treated on the grounded electrode also contain CFx species. In the case of a 50% CF4 50% H2 plasma, a thick carbon layer is formed on both the rf and the grounded electrode. While the layer formed on the grounded electrode contains a high amount of CFx (x=1, 2, and 3) bonds, that on the powered electrode does not contain as much. In the case of a 50% CF4 50% O2 plasma, a thin siliconoxide layer is formed. On the grounded electrode, it takes slightly longer to build up, but the final reaction layer on rf and grounded electrodes is the same. Small amounts of SiF4 are also present in the layer.