Brian J.J. Zelinski

Critical cooling rate calculations for glass formation

Abstract An assessment is made of the various factors which are involved in the calculation of critical cooling rates for glass formation. In particular, attention is focused upon the sensitivity of computed critical cooling rates R c to various approximations and uncertainties in physical parameter data. SiO 2 and GeO 2 are chosen as model systems to illustrate the major features. It is shown that R c is rather insensitive to the mode of computation of the volume fraction crystallized, but is quite dddependent upon the material parameters which enter into the nucleation and crystal growth expressions.

Porosity and composition effects in sol-gel derived interference filters

The effects of porosity and composition on the optical thickness of sol—gel films is analyzed using the Clausius-Mossotti relationship. The optical thickness is predicted to decrease with shrinkage approximately linearly. For high density films the predictions agree with experimental data from SiO2 and SiO2-TiO2-Al2O3 coatings. At low densities, however, the optical thicknesses are much higher than predicted by the Clausius-Mossotti analysis. This difference is attributed to residual species in the unfired films. Using an empirical value for the ratio of the electronic polarizability αr to the molecular weight Wr of the residual species (αr/Wr= 1.65 × 10−25cm3 mol g−1) the difference between experiment and theory is accounted for quantitatively for both the single-component (SiO2) and multicomponent (SiO2-TiO2-Al2O3) coatings. Approaches to fabricating multicolor dichroic filters, which require large changes in optical thickness on heating, are presented.

Design and fabrication of a single leakage-channel grating coupler

We describe the modeling and fabrication of waveguide grating couplers with theoretical outcoupling efficiencies into a single diffracted order nearing 100%. Termed single leakage-channel grating couplers (or SLCGCs), these devices utilize a high-reflectivity dielectric stack to reflect the outcoupled beam diffracted toward the substrate and back up into the air region, where it constructively adds with the beam diffracted into the air region. Computer modeling shows that the branching ratio and the leakage rate can be independently controlled, and that the branching ratio is independent of grating depth and grating period. A SLCGC with a branching ratio of 97.1% is fabricated by the use of a combination of vacuum-evaporation and wet-chemical techniques.

Sol-Gel Derived Bismuth Titanate Thin Films with c-Axis Orientation

Bismuth titanate (Bi4Ti3O12), a member of the layered perovskite family, has a unique set of ferroelectric properties, which include a high remanent polarization, low coercive field, and high Curie temperature, that make it a possible candidate for data storage applications. For this investigation, bismuth titanate, or BiT, films were fabricated via sol-gel method to examine the effect of processing on phase development and orientation. Solutions were deposited onto platinized silicon, and then heat treated for one hour at temperatures ranging from 550°C to 700°C in 100% O2. It was found that c-axis orientated BiT films could be formed at temperatures as low as 550°C by using bismuth oxide template layers, while films without bismuth oxide templating possessed a random orientation over the same temperature range.

Sol-gel synthesis of optical thin films and coatings

Sol-gel methods offer a number of notable advantages for the synthesis of optical films and coatings. Areas of potential or actual application of this technology range from single layer and multilayer antireflection coatings to embossed planar waveguides and organic-modified oxide materials. The most notable advantages of these wet chemical nethods will be surveyed, as will progress achieved to date in a number of the most attractive representative areas. The technical bases for the success/failure in each case will be considered. Also to be discussed will be the prospects - in both the near-term and long-term - of future developments in the sol-gel synthesis of optical films, as well as the principal technical hurdles which must be overcome in order that such synthesis methods may achieve more widespread use in the future. Finally, a comparison will be made between the microstructures and characteristics of films and coatings deposited using sol-gel methods with those deposited from the vapor phase. In all cases, use will be made of recent advances in our laboratory in the subject area.

EMBOSSED GRATINGS IN SOL-GEL WAVEGUIDES : PRE-EMBOSS HEAT TREATMENT EFFECTS

Abstract An embossing technique is used to fabricate surface-relief gratings into SiOtn2TiO 2 dielectric, step-index, planar optical waveguides, fabricated using a sol-gel process. The influence that a pre-emboss bake cycle has upon the embossibility of the SiO 2 TiO 2 sol-gel films is described. Through SEM evaluation, the embossed grating ruling profile and general appearance are shown to be strongly dependent upon the duration of the bake at 70 ° C. In addition, two embossing surface mechanisms, molding and stress-induced fracturing, are identified and correlated with the duration of the pre-emboss bake. Embossed waveguide gratings with a 0.52 μm period and 100–200 nm peak-to-trough depth were fabricated.

Laser processing of channel waveguide structures in sol-gel coatings

Sol-gel derived silica, siica-titania, and tantala coatings were covered with a thin metal film and translated across a Nd:YAG laser beam (1.06 jim). The laser energy was absorbed by the metal film, which heated the underlying sol-gel coating. This heating densified the sol-gel coatings, thereby increasing the index ofrefraction of the laser heated region, and forming channel waveguide structures in all three systems. The channels formed by this technique were etched, to remove the undensified regions, which resulted in ridged waveguide structures. The structures were also produced by depositing a metal pattern using photolithographic techniques, and rastering the laser across the entire sample. The refractive indicies of laser densifled and furnace densified silica coatings were similar. Large differences were observed in the indicies oflaser and furnace densified coatings for the siica-titania and tantala systems.

Film/substrate/vacuum-chuck interactions during spin-coating

Thickness variations that are associated with the vacuum chuck were observed in wet-chemical-derived dielectric films applied by the spin-coating technique. These thickness variations are controlled by factors such as the thermal properties of the substrate material, the evaporation behavior of the coating solution, and the physical design of the vacuum chuck. Atechnique is described for evaluating the magnitude ofthis effect.

Infrared observation of evaporative cooling during spin-coating processes

To explore the origin of chuck-related surface nonuniformities in thin films deposited from solutions, real-time observations of evaporative cooling effects during spin-coating of solvents are made using an IR switched-field-effect-transistor (FET) array camera. The evaporative cooling depends strongly on the volatility of the solvent being tested. For acetone (the most volatile solvent tested) the cooling eftect can be as large as 10°C when a silicon wafer is used as the substrate. Comparisons are made between wafer type and position on the substrate and sources of the temperature differences are discussed within the framework of the basic spin-coating paradigm. It is likely that these evaporative cooling effects play an important role in the development of chuck-related surface nonuniformities during spin coating; solvent systems may be selected to help optimize film uniformity. These aspects of solution engineering are also discussed.

Attenuation of sol-gel waveguides measured as a function of wavelength and sample age

Solution chemistry techniques are of interest for the deposition of thin film planar waveguides due to the ease of fabrication, the compositional range available, and the routine synthesis of guides with losses < 0.5 dB ((lambda) equals 632.8 nm). In this work, the wavelength dependence and attenuation of sol-gel waveguides were measured as a function of composition and time in storage, or aging. The compositions investigated in this work were (in mole %) 65SiO2:35TiO2, 50SiO2:50TiO2 and 41.5SiO2:41.5TiO2:17Al2O3 waveguides. Initial losses of the waveguides were < 0.5 dB/cm ((lambda) equals 632.8 nm). The initial wavelength dependence of the guides was also measured. In waveguides where the losses are intrinsically or Rayleigh scatter limited, a dependence of (lambda) 4 is expected. However, the wavelength dependence measured for the sol-gel guides varied with composition. After periods in storage, waveguide attenuation was remeasured, and the guides were found to have deteriorated, with higher losses. The rate of deterioration varied with composition. Possible causes of the deterioration of the guides are presented and discussed.