Plasma enhanced chemical vapor deposition(PECVD) is being increasingly used for the fabrication of transparent dielectric optical films and coatings. This involves single-layer, multilayer, graded index, and nanocomposite optical thin filmsystems for applications such as optical filters, antireflective coatings, optical waveguides, and others. Beside their basic optical properties (refractive index, extinction coefficient, optical loss), these systems very frequently offer other desirable “functional” characteristics. These include hardness, scratch, abrasion, and erosion resistance, improved adhesion to various technologically important substrate materials such as polymers, hydrophobicity or hydrophilicity, long-term chemical, thermal, and environmental stability, gas and vapor impermeability, and others. In the present article, we critically review the advances in the development of plasma processes and plasmasystems for the synthesis of thin film high and low index optical materials, and in the control of plasma–surface interactions leading to desired film microstructures. We particularly underline those specificities of PECVD, which distinguish it from other conventional techniques for producing optical films (mainly physical vapor deposition), such as fabrication of graded index (inhomogeneous) layers, control of interfaces, high deposition rate at low temperature, enhanced mechanical and other functional characteristics, and industrial scaleup. Advances in this field are illustrated by selected examples of PECVD of antireflective coatings, rugate filters, integrated optical devices, and others.

Plasma deposition of optical films and coatings: A review

The scaling of complementary metal oxide semiconductor (CMOS) transistors has led to the silicon dioxide layer used as a gate dielectric becoming so thin that the gate leakage current becomes too large. This led to the replacement of SiO2 by a physically thicker layer of a higher dielectric constant or ‘high-K’ oxide such as hafnium oxide. Intensive research was carried out to develop these oxides into high quality electronic materials. In addition, the incorporation of Ge in the CMOS transistor structure has been employed to enable higher carrier mobility and performance. This review covers both scientific and technological issues related to the high-K gate stack – the choice of oxides, their deposition, their structural and metallurgical behaviour, atomic diffusion, interface structure, their electronic structure, band offsets, electronic defects, charge trapping and conduction mechanisms, reliability, mobility degradation and oxygen scavenging to achieve the thinnest oxide thicknesses. The high K oxides were implemented in conjunction with a replacement of polycrystalline Si gate electrodes with metal gates. The strong metallurgical interactions between the gate electrodes and the HfO2 which resulted an unstable gate threshold voltage resulted in the use of the lower temperature ‘gate last’ process flow, in addition to the standard ‘gate first’ approach. Work function control by metal gate electrodes and by oxide dipole layers is discussed. The problems associated with high K oxides on Ge channels are also discussed.

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High-K materials and metal gates for CMOS applications

Presents an efficient method for the design of complex fiber Bragg gratings. The method relies on the synthesis of the impulse response of the grating by means of a differential layer-peeling algorithm. The algorithm developed takes into account all the multiple reflections inside the grating, giving an exact solution to the inverse scattering problem. Its low algorithmic complexity enables the synthesis of long fiber gratings. The method is illustrated by designing several filters with interest for optical fiber communication systems: dispersionless bandpass filters and second- and third order dispersion compensators.

An efficient inverse scattering algorithm for the design of nonuniform fiber Bragg gratings

The authors' experience with the application of the needle optimization technique to the design of optical coating is summarized. A physical interpretation of the technique is provided, and its main features are identified. Guidelines on the application of the needle optimization technique to various types of design problems are given.

Application of the needle optimization technique to the design of optical coatings.

Tin oxide films doped with oxygen vacancies, F, Sb, or Mo were made by reactive rf magnetron sputtering of Sn, Sn‐Sb, or Sn‐Mo in Ar+O2(+CF4) onto glass heated to a temperature up to 530 °C. Electrical dc resistivity, mobility, free‐electron density, spectral optical properties, and microstructure were investigated as a function of sputtering parameters. Optimized deposition parameters gave SnOx:(Sb,F) films with high luminous transmittance, low luminous absorptance, high infrared reflectance, and dc resistivity down to 9.1×10−4 Ω cm. Refractive index n and extinction coefficient k were evaluated from spectrophotometric transmittance. In the luminous range, the films had 1.90<n<2.0 and k of the order of 10−2. Hall‐effect measurements showed n‐type conduction with electron densities in the 1020–1021 cm−3 range. Band‐gap broadening from 4.06 to 4.45 eV was observed with increasing electron density. X‐ray diffractometry and transmission electron microscopy showed that the structure factor of the films depend...

Optical and electrical properties of radio frequency sputtered tin oxide films doped with oxygen vacancies, F, Sb, or Mo

The principal aspects of rugate filter theory are reviewed and expanded to show how the Fouriertransform technique can be used to design rugate filters that fulfill many optical coating functions.

Rugate filter theory: an overview

Optical and microstructural properties of hafnium dioxide thin films

The Fourier transform technique developed for the design of variable refractive index coatings such as rugate filters is improved to achieve an accurate correspondence between optical properties and the refractive index profile. An application to the design of narrowband reflectors is presented.

Rugate filter design: the modified Fourier transform technique

In optical coating calculations, a 2 × 2 matrix formalism is generally used to describe the effect of each layer on the electromagnetic field. In current applications, the layers are considered homogeneous, and, therefore, the matrix used is restricted to this particular case. In this paper, a 2 × 2 matrix describing an inhomogeneous dielectric thin film is derived from Maxwell’s equations. The inhomogeneity is assumed to vary along the direction perpendicular to the film plane. No restriction is made on the amplitude of its variation. The matrix is illustrated in the case of a rugate filter designed according to Sossi’s Fourier transform technique. An improved approximation for the Fourier transform technique is then introduced.

Derivation of a matrix describing a rugate dielectric thin film

Diamond films, deposited on silicon substrates through microwave-assisted chemical vapor deposition, have been polished with a 500-eV beam of oxygen ions through the use of a planarizing layer. The films' surface roughness, in excess of 1 microm (rms) and 6 microm (peak to valley), was reduced to 35 nm (rms) and 217 nm (peak to valley). The polished films retained their diamond character and surface contamination was negligible. With this polishing process, optical applications of diamond films in the near infrared region are realizable.

Bertrand G. Bovard

Papers

Rugate filter theory: an overview

Optical and microstructural properties of hafnium dioxide thin films

Rugate filter design: the modified Fourier transform technique

Derivation of a matrix describing a rugate dielectric thin film

Planarizing technique for ion-beam polishing of diamond films.