Pavel V. Bulkin

Fourier-transform design and electron cyclotron resonance plasma-enhanced deposition of lossy graded-index optical coatings

A Fourier-transform synthesis technique is applied to the design of inhomogeneous refractive-index optical thin films with dispersion of refractive index and absorption taken into account. Using measured properties of SiO(x) N(y), we have designed a broadband reflector and two three-line filters with high reflectance. One of the three-line filters was manufactured by electron cyclotron resonance plasma-enhanced chemical vapor deposition from a mixture of SiH(4), N(2), O(2), and Ar. Good agreement between design and measured performance proves the feasibility of applying the Fourier-transform technique to the design of reflection filters even in the case of highly dispersive, absorbing material, if its optical properties are well characterized.

Properties and applications of electron cyclotron plasma deposited SiOxNy films with graded refractive index profiles

Abstract In the experiments in the present study, electron cyclotron resonance plasma enhanced chemical vapour deposition was used for the deposition of SiOxNy films from O 2 Ar , N2 and SiH 4 Ar mixtures. Optical properties of the films were studied by means of transmission spectroscopy in the wavelength range 200–2600 nm. Refractive indices ranged between 1.5 and 3.5 (measured at 632.8 nm) for different film compositions. The extinction coefficient showed a strong blue shift with decrease of silane content in the gas mixture. Optical interference filters with continuously varying refractive index profiles were designed, and manufactured by our computer controlled electron cyclotron resonance system. Designs which take into account dispersion of refractive index, and absorption in the constituent SiOxNy material, differ only significantly from designs in which it is neglected at wavelengths shorter than 700 nm.

Electron cyclotron resonance plasma enhanced chemical vapour deposition and optical properties of SiOx thin films

Abstract Silicon–oxygen alloys ranging from amorphous silicon to silicon dioxide have been deposited from electron cyclotron resonance SiH4/O2/Ar plasmas onto radio frequency biased substrates at room temperature. Hundred Watt microwave power, a working pressure of 2 mTorr, a total gas flow rate of 20 sccm, and 13.5 Watt radio frequency bias power were used. Refractive indices of 3.8 to 1.48 were measured by ellipsometry at a wavelength of 632.8 nm. Deposition rates for different SiH4/(SiH4+O2) gas flow ratios were between 4.3 and 7.5 nm min−1. Fourier transform infrared spectroscopy revealed a gradual evolution of the chemical composition of the films from amorphous silicon to silicon dioxide with change in gas phase composition. For silicon dioxide films, the centre frequency of the main Si–O stretching band is 1060 cm−1, whereas full width at half maximum is approximately 94 cm−1. Presence of Si–OH bonds is not detectable for any of the films. The amount of hydrogen in the films is considerably less than that in similar layers deposited by radio frequency plasma enhanced chemical vapour deposition, and for amorphous silicon it does not exceed 1.5×1022 cm−3. The presence of a single band at 486 cm−1 in the Raman spectrum of the a-Si film confirmed its amorphous structure.

Rugate filter manufacturing by electron cyclotron resonance plasma-enhanced chemical vapor deposition of SiNx

Electron cyclotron resonance plasma-enhanced chemical va- por deposition (ECR-PECVD) is used to deposit thin films of SiN x of different composition from mixtures of N2 and 30% SiH4 in Ar onto dif- ferent substrates. Measured values of the complex refractive index over the wavelength range of interest are used in the design and computer- controlled fabrication of one- and two-band rugate optical interference filters. The excursion of the continuously varying refractive index is cho- sen to be between 0.8 and 1.2, and window functions and matching layers at both sides are employed for sidelobe suppression. Measured reflection patterns of filters agreed well with simulated results. Filters designed for a center wavelength of 1000 nm and a total thickness of 2.1 mm, have a reflectance of 97% in the stopband and a bandwidth of typically 240 nm. The two-band filter is designed for stopbands at 1000 and 770 nm with reflectance of 98 and 96% and bandwidths of 150 and 120 nm, respectively. The total thickness of this filter is 3.33mm. Filters are successfully deposited on optical fiber ends using one of the fibers for in situ monitoring of the deposition process.

Electron cyclotron resonance plasma deposition of SiNx for optical applications

Abstract Thin films of SiN x were prepared from mixtures of SiH 4 /Ar and N 2 employing a compact 2.45 GHz electron cyclotron resonance plasma source. The composition of these layers as a function of gas flows, extraction magnetic field and substrate bias was studied. Growth rates of 3–4 nm min −1 are typical. The refractive index can be varied from 1.92 to 3.6, depending on experimental conditions. Computer control was implemented to achieve controlled growth of graded refractive index profiles suitable for application in rugate optical interference filters. Rugate filters were grown by the electron cyclotron plasma deposition technique for the first time. Optical reflectance measurements in the 3800–13 000 cm −1 range were used to evaluate characteristics of the rugate filters.

Effect of process parameters on the properties of electron cyclotron resonance plasma deposited silicon-oxynitride

Abstract Silicon-oxynitride layers were deposited by electron cyclotron resonance plasma enhanced chemical vapour deposition onto silicon and glass substrates. Working pressure ranged from 0.8 to 4 mTorr. Thin films of SiOxNy with different composition were obtained by a change of gas flow ratio under computer control. The optical properties of thin SiOxNy films grown from O 2 Ar , N2 and SiH 4 Ar gas mixtures were determined by optical transmission spectroscopy and Fourier transform infrared spectroscopy. Growth rate was in the range 2–10 nm/min, while refractive index changed from 2.6 to 1.48 with a corresponding change in O 2 ( N 2 + O 2 ) from 0.01 to 0.318 at constant total flow and constant silane flow. The influence of gas flow ratio and substrate bias on refractive index, growth rate and on chemical composition of grown layers was also studied.

Optical properties of SiNx deposited by electron cyclotron resonance plasma-enhanced deposition

The properties of thin SiNx films deposited by computer-controlled electron cyclotron resonance plasma-enhanced chemical vapor deposition (ECR-PECVD) from a mixture of SiH4 and N2 onto sapphire substrates were investigated by optical transmission spectroscopy in the wavelength range 200 to 2600 nm. The wavelength dependencies of the refractive index n, extinction coefficient k, absorption coefficient α, and the optical bandgap Eg of the films were studied. The optical properties of SiNx, were found to be strongly dependent on the gas flow ratio during deposition. The optical band gap displays a blue shift with increase of nitrogen content in the layer with values ranging from 1.7 to 4.2 eV.

Understanding the amorphous-to-microcrystalline silicon transition in SiF4/H2/Ar gas mixtures

We report on the growth of microcrystalline silicon films from the dissociation of SiF4/H2/Ar gas mixtures. For this growth chemistry, the formation of HF molecules provides a clear signature of the amorphous to microcrystalline growth transition. Depositing films from silicon tetrafluoride requires the removal of F produced by SiF4 dissociation, and this removal is promoted by the addition of H2 which strongly reacts with F to form HF molecules. At low H2 flow rates, the films grow amorphous as all the available hydrogen is consumed to form HF. Above a critical flow rate, corresponding to the full removal of F, microcrystalline films are produced as there is an excess of atomic hydrogen in the plasma. A simple yet accurate phenomenological model is proposed to explain the SiF4/H2 plasma chemistry in accordance with experimental data. This model provides some rules of thumb to achieve high deposition rates for microcrystalline silicon, namely, that increased RF power must be balanced by an increased H2 flow rate.

Mueller polarimetry as a tool for detecting asymmetry in diffraction grating profiles

Reflected Mueller matrix spectra were measured and simulated for asymmetrical photoresist master diffraction gratings in conical mounting (i.e., the direction of grating grooves was not perpendicular to the plane of light incidence). From the electromagnetic reciprocity theorem, Mueller matrix of symmetric grating (composed of only reciprocal materials, and operating in zeroth-order diffraction) is invariant under transposition ( M=Mt). For zeroth-order diffraction of asymmetric gratings, the lack of profile rotational symmetry violates this reciprocity and, consequently, breaks the symmetry of the above-mentioned matrix. This property of the Mueller matrix of asymmetric gratings was experimentally observed and numerically modeled at all experimental illumination conditions with the exception of planar mounting (the direction of grating grooves was perpendicular to the plane of light incidence), where there is no cross-polarization effect for the gratings composed of isotropic materials. It was demonstrat...

Automated electron cyclotron resonance plasma enhanced chemical vapor deposition system for the growth of rugate filters

We developed a control system for an electron cyclotron resonance plasma enhanced chemical vapor deposition (ECR-PECVD) reactor for the growth of inhomogeneous dielectric thin films for optical applications. The system supervises the growth of films of a required refractive index profile on different substrates including optical fiber ends. It controls the gas flow rates, microwave power, radio frequency substrate bias, and the ECR and extraction magnet power supplies. The system also adjusts the microwave generator to minimize reflected power during deposition. The characteristics of the ECR-PECVD system and the automation of the deposition process are discussed using the growth of a complex rugate optical filter as an example.