In situ characterisation of thin-film formation in molecular low-temperature plasmas

Thin films can be formed in low-temperature plasma in two ways: as a result of material modification usually in a discharge of non-organic gas and in the course of a deposition process in a discharge of organic vapour. Thin-film formation requires in situ control of plasma processing in order to get information on the growth mechanisms and control the deposition process. Here are shown some effective in situ techniques of thin-film characterisation: various kinds of infrared spectroscopy (infrared reflection absorption spectroscopy, attenuated total reflection spectroscopy, etc.), ellipsometry and microgravimetry. We discuss the application of these methods to polymer modification as well as plasma polymerisation. The applied techniques give very good spatial resolution, up to a few nm. The limitations of method or equipment on time resolution can be compensated by the appropriate experimental arrangement. Received: 4 October 2000 / Accepted: 12 December 2000 / Published online: 3 April 2001     

Spectroscopic in situ diagnostics of boron nitride film growth in plasma-enhanced deposition

The development of in situ diagnostics of the most important species and reactions in the plasma and/or on the surface during thin-film growth is one of the current topics in plasma-enhanced vapor deposition. In situ thin film diagnostic methods which could be used in plasma processing are restricted due to the presence of electrons and ions. The advantages and disadvantages of different applicable methods will be discussed. The spectroscopic in situ control of boron nitride film growth is presented as an example of surface modification in low-temperature, low-pressure plasma processing. The growth of cubic and hexagonal boron nitride is observed by polarized infrared reflection spectroscopy in absorption and ellipsometric configurations as well as by single-wavelength ellipsometry in the visible spectral range. Modeling of the experimental results gives detailed information on growth conditions and internal stress of the films. Received: 8 August 2000 / Accepted: 12 December 2000 / Published online: 3 April 2001     

Surface analytical studies of Ar-plasma etching of thin heptadecafluoro-1-decene plasma polymer films

An audio-frequency plasma polymerization set-up with a planar plasma source was used to deposit thin heptadecafluoro-1-decene (HDFD) plasma polymer films. The morphology and chemical structure of the films after deposition were compared with the state of the film after a subsequent Ar-plasma treatment by means of in situ Fourier transform infrared reflection absorbance spectroscopy (FT-IRRAS), X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS) and atomic force microscopy (AFM) as well as contact angle measurements. The results revealed the correlation of wettability of the model Teflon-like films with change of surface chemistry and surface topography as a result of Ar-plasma treatment.     

Effects of Plasma Etching on Surface Modification and Gas Permeability of Bisphenol‐A Polycarbonate Films

The surface fluorination of polycarbonate bisphenol‐A thin films was carried out by treatment with plasma of sulfur hexafluoride/argon (SF₆/Ar) using a radio frequency (RF) discharges. Effects of treatment time and SF₆ concentration have been studied. Surface characterization was performed using X‐ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy with attenuated total reflectance (FT‐IR/ATR), atomic force microscopy (AFM) and contact angle measurements. The effects of the surface modification on the carbon dioxide (CO₂) gas permeation of the fluorinated films were evaluated. The results showed effective fluorination of all samples, with C‐F, C‐F₂ and even C‐F₃ groups attached to the polycarbonate, depending on the treatment conditions. The presence of incorporated fluorine molecules absorbed into the polymer structure during treatment was also shown. The polymer films showed an increase of their surface roughness and a decrease in their surface tension due to the plasma treatment. Plasma fluorination also decreases polycarbonate CO₂ permeability considerably. The films treated with the lower concentration of SF₆ showed the highest barrier values; however, significant differences between the permeability values for the samples treated for different times were not observed.     

Plasma-induced graft-polymerization of polyethylene glycol acrylate on polypropylene substrates

A detailed study of argon plasma-induced graft-polymerization of polyethylene glycol acrylate (PEGA) on polypropylene (PP) substrates (membranes and films) is presented. The process consists of four steps: (a) plasma pre-activation of the PP substrates; (b) immersion in a PEGA solution; (c) argon plasma-induced graft-polymerization; (d) washing and drying of the samples. Influence of the solution and plasma parameters on the process efficiency evaluated in terms of amount of grafted polymer, coverage uniformity and substrates wettability, are investigated. The plasma-induced graft-polymerization of PEGA is then followed by sample weighting, water droplet adsorption time and contact angle measurements, attenuated total reflection infrared spectroscopy (ATR-IR), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) analyses. The stability of the obtained thin films was evaluated in water and in phosphate buffer saline (PBS) at 37 °C. Results clearly indicates that plasma-induced graft-polymerization of PEGA is a practical methodology for anti-fouling surface modification of materials.     

Electronic structure of GaN(0001)‐2 × 2 thin films grown by PAMBE

Gallium nitride thin films were grown on silicon carbide (0001) by plasma‐assisted molecular beam epitaxy (PAMBE). The samples were cooled down in nitrogen plasma and characterized in situ by reflection high energy electron diffraction (RHEED), photoelectron spectroscopy (XPS/UPS), and atomic force microscopy (AFM) revealing stoichiometric and smooth GaN films virtually free of contaminations. We present valence band data obtained by UPS with strong emission from surface states inside the fundamental band gap. These states and the observed 2 × 2 surface reconstruction are highly sensitive towards residual molecules. Once these surface states have disappeared the original state could not be recovered by surface preparation methods underlining the necessity of in situ investigations on as‐grown surfaces. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Mechanism of plasma polymerization: The effect of substrate and substrate topography

Surface properties of metals and plastic substrates and properties of plasma polymerized films on these substrates are reported. The surface and film properties were measured with ellipsometry, surface potential difference, photoelectron-emission and fluid contact angles. Light scattering, profilometry and scanning electron microscopy were used to characterize surface roughness and topography. The modification of the adhesive properties of low energy surfaces (such as Teflon) by plasma polymerization is shown. Adhesive properties of plasma films on metals is also shown. It is shown that photoelectron-emission may be a valuable tool for detecting the presence and decay of excited molecules and radicals formed during plasma polymerization.     

Effect of RF power on the optical and morphological properties of RF plasma polymerised linalyl acetate thin films

Thin films derived from linalyl acetate were fabricated using the Radio Frequency (RF) plasma polymerisation technique between RF power levels of 10 and 75 W. The optical properties of the films were investigated using spectroscopic ellipsometry and UV-vis spectroscopy between 200 and 1000 nm. An optical band gap of approximately 3 eV for all power levels was determined from Tauc plots. The surface morphology and hardness of the material were studied using AFM and nano-indentation respectively to determine the effect of RF power on the thin film properties. Smooth surfaces with an average roughness of approximately 0.2 nm with consistent morphology were obtained across all power levels, while hardness demonstrated a linearly increasing dependence on RF deposition power, with values ranging between 0.29 and 0.44 GPa. These studies indicate the ability to tailor film characteristics by varying the RF deposition power, and the potential for the films to be used within electronic devices as encapsulation layers, insulating layers, or as semiconducting layers with the introduction of charge carriers to the chemical structure of the material.     

Influence of Boron doping on microcrystalline silicon growth

Microcrystalline silicon (μc-Si:H) thin films with and without boron doping are deposited using the radio-frequency plasma-enhanced chemical vapour deposition method. The surface roughness evolutions of the silicon thin films are investigated using ex situ spectroscopic ellipsometry and an atomic force microscope. It is shown that the growth exponent β and the roughness exponent α are about 0.369 and 0.95 for the undoped thin film, respectively. Whereas, for the boron-doped μc-Si:H thin film, β increases to 0.534 and α decreases to 0.46 due to the shadowing effect.     

Immersion transmission ellipsometry (ITE): a new method for the precise determination of the 3D indicatrix of thin films

The new method of immersion transmission ellipsometry (ITE) [1] has been developed. It allows the highly accurate determination of the absolute three-dimensional (3D) refractive indices of anisotropic thin films. The method is combined with conventional ellipsometry in transmission and reflection, and the thickness determination of anisotropic films solely by optical methods also becomes more accurate. The method is applied to the determination of the 3D refractive indices of thin spin-coated films of an azobenzene-containing liquid-crystalline copolymer. The development of the anisotropy in these films by photo-orientation and subsequent annealing is demonstrated. Depending on the annealing temperature, oblate or prolate orders are generated. PACS 07.60.Fs; 42.70.Gi; 78.66.Qn     

Characterization of polymer thin films obtained by pulsed laser deposition

The development of laser techniques for the deposition of polymer and biomaterial thin films on solid surfaces in a controlled manner has attracted great attention during the last few years. Here we report the deposition of thin polymer films, namely Polyepichlorhydrin by pulsed laser deposition. Polyepichlorhydrin polymer was deposited on flat substrate (i.e. silicon) using an NdYAG laser (266 nm, 5 ns pulse duration and 10 Hz repetition rate).The obtained thin films have been characterized by atomic force microscopy, scanning electron microscopy, Fourier transform infrared spectroscopy and spectroscopic ellipsometry.It was found that for laser fluences up to 1.5 J/cm² the chemical structure of the deposited polyepichlorhydrin polymer thin layers resembles to the native polymer, whilst by increasing the laser fluence above 1.5 J/cm² the polyepichlorohydrin films present deviations from the bulk polymer.Morphological investigations (atomic force microscopy and scanning electron microscopy) reveal continuous polyepichlorhydrin thin films for a relatively narrow range of fluences (1-1.5 J/cm²).The wavelength dependence of the refractive index and extinction coefficient was determined by ellipsometry studies which lead to new insights about the material.The obtained results indicate that pulsed laser deposition method is potentially useful for the fabrication of polymer thin films to be used in applications including electronics, microsensor or bioengineering industries.     

The role of surface roughness in total internal reflection ellipsometry of hybrid systems

Total internal reflection ellipsometry (TIRE) technique was used to investigate the role of surface roughness in the hybrid system composed of octadecanethiole layer on Au thin film. The samples with Au films of different microstructure were explored. The experimental results were interpreted in the model, which took into account the surface roughness of Au film in the hybrid system. It was shown that optical parameters of octadecanethiole were in correspondence for samples of different microstructure in the case of adequate models used for interpretation of TIRE data.     

Surface modification of a polyamide 6 film by He/CF4 plasma using atmospheric pressure plasma jet

Polyamide 6 (PA 6) films are treated with helium(He)/CF₄ plasma at atmospheric pressure. The samples are treated at different treatment times. The surface modification of the PA 6 films is evaluated by water contact angle, atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The etching rate is used to study the etching effect of He/CF₄ plasma on the PA 6 films. The T-peel strengths of the control and plasma treated films are measured to show the surface adhesion properties of the films. As the treatment time increases, the etching rate decreases steadily, the contact angle decreases initially and then increases, while the T-peel strength increases first and then decreases. AFM analyses show that the surface roughness increases after the plasma treatment. XPS analyses reveal substantial incorporation of fluorine and/or oxygen atoms to the polymer chains on the film surfaces.     

Growth of thin silver films on silicon oxide pretreated by low temperature argon plasma

In the present study, we investigate the influence of low energy ion bombardment on nucleation and growth of thin silver films on silicon oxide by in situ photoelectron spectroscopy (PES) combined with specific resistivity measurements. Thermally grown thin silicon oxide films were exposed to a low temperature argon plasma for different time intervals resulting in changes in surface chemical composition as monitored by angle-resolved X-ray photoelectron spectroscopy (ARXPS). We demonstrate that irradiation of the oxide surface with low energy ions results in substantially changed nucleation of silver. Furthermore, silver films deposited on plasma treated oxide tend to have lower resistivity which is attributed to the effect of reduced grain boundary and surface roughness.     

椭偏光散射分析类金刚石薄膜的散射特性

 基于光学薄膜反射椭偏法的测量原理,对光学薄膜散射椭偏特性进行了研究。给出了光学薄膜散射逆问题的解决方法,并对不同脉冲频率下采用脉冲真空电弧离子镀技术沉积的类金刚石薄膜的散射特性进行了研究。分析了光学薄膜界面的相关特性以及膜层中局部缺陷对散射光椭偏特性的影响。结果表明:随着脉冲频率的增加,所沉积的类金刚石薄膜的相关性变差,且薄膜中的局部缺陷引起的体散射越明显。     

Ellipsometry as an aid in studying metallic corrosion problems

The application of ellipsometry to the study of corrosion is reviewed. It is concluded that the influences of surface roughness and film inhomogeneity are complicating factors in the interpretation of observations and that their effects are still insufficiently understood. Nevertheless, as a complementary tool to other methods of surface examination, ellipsometry is unique in its ability to detect and measure the formation and growth of surface films, continuously in situ; this can be done in either gaseous or liquid environments. Determination of the optical properties of thin films, and more especially spectral absorption characteristics, is potentially the basis for a valuable method of assessing averaged deviations from non stoichiometric composition of surface films.     

Planar quarter wave stacks prepared from chalcogenide Ge-Se and polymer polystyrene thin films

Planar quarter wave stacks based on amorphous chalcogenide Ge-Se alternating with polymer polystyrene (PS) thin films are reported as Bragg reflectors for near-infrared region. Chalcogenide films were prepared using a thermal evaporation (TE) while polymer films were deposited using a spin-coating technique. The film thicknesses, d∼165 nm for Ge₂₅Se₇₅ (n=2.35) and d∼250 nm for polymer film (n=1.53), were calculated to center the reflection band round 1550 nm, whose wavelengths are used in telecommunication. Optical properties of prepared multilayer stacks were determined in the range 400-2200 nm using spectral ellipsometry, optical transmission and reflection measurements. Total reflection for normal incidence of unpolarized light was observed from 1530 to 1740 nm for 8 Ge-Se+7 PS thin film stacks prepared on silicon wafer. In addition to total reflection of light with normal incidence, the omnidirectional total reflection of TE-polarized light from 8 Ge-Se+7 PS thin film stacks was observed. Reflection band maxima shifted with varying incident angles, i.e., 1420-1680 nm for 45° deflection from the normal and 1300-1630 nm for 70° deflection from the normal.     

Total internal reflection ellipsometry: ultrahigh sensitivity for protein adsorption on metal surfaces

Total internal reflection ellipsometry (TIRE) is used to study adsorption of human serum albumin and fibrinogen on thin gold films. TIRE shows very high sensitivity for protein monolayers adsorbed on metal surfaces when surface plasmon resonance effects are utilized. The measured data, expressed in ellipsometric angles psi and D are of several orders of magnitude larger in comparison with those from similar experiments performed with traditional ellipsometry. TIRE in spectral mode opens a new path for precise studies of organic layers adsorbed on metal surfaces, with a potential for resolving the adsorbed layer microstructure.     

Characteristics of nitrogen-incorporated silicon oxycarbide films and plasmas for plasma enhanced chemical vapor deposition with TMOS/N2/NH3

Plasma enhanced chemical vapor deposition of nitrogen-incorporated silicon oxycarbide thin films obtained from the gas mixture of TMOS (tetramethoxysilane), N₂, and NH₃ is studied. The effects of the TMOS to N₂ pressure ratio on the properties of the film and the plasma are investigated. The deposited films are analyzed by in situ ellipsometry, ex situ Fourier transform infrared spectroscopy (FTIR), and by X-ray photoelectron spectroscopy (XPS). The plasma is characterized by using optical emission spectroscopy (OES). The mass spectra of the constituents in the plasma are obtained by quadrupole mass spectroscopy. The correlation between the film properties and the plasma characteristics is explained wherever possible. As the partial pressure of N₂ is decreased, the refractive index begins to decrease, reaches a minimum, and then saturates. The FTIR absorption bands are observed from about 850 to 1000 cm⁻¹ and from 1000 to 1250 cm⁻¹, and can be attributed to the formation of a nitrogen-incorporated silicon oxycarbide thin film. The variation of the refractive index is discussed in relationship with the deposition rate, the OES spectra, the mass spectra of the plasma, the film composition obtained by XPS, and FTIR spectra.     

Tribological properties of chemically bonded polyimide films on silicon with polyglycidyl methacrylate brush as adhesive layer

Polyimide thin films, which possess good stability and film uniformity, are successfully fabricated on single crystal silicon wafers coated with a thin polymer brush by suface-initiated polymerization (SIP) as an adhesive layer. The growth kinetic of polyglycidyl methacrylate (PGMA) brush was studied by the means of ellipsometry. The nano-scale morphology and chemical composition of PGMA brush and polyimide film were studied with atomic force microscopy (AFM), Fourier transform infrared spectrum (FT-IR), and X-ray photoelectron spectroscopy (XPS). The tribological behaviors of the thin films sliding against AISI-52100 steel ball were examined on a static-dynamic friction precision measurement apparatus and UMT-2MT tribometer. The worn surface of the polyimide thin films was investigated with scanning electron microscopy (SEM). The results indicated that the chemically bonded polyimide films exhibited better friction reduction and antiwear behavior compared to the polymide films on bare silicon surface. At a load of 0.5 N and sliding speed of 20 mm s⁻¹, the durability life of the polyimide thin films is over 25,000 sliding cycles and the friction coefficient is about 0.08.