HX Film Research Articles

The Chemistry and Physics of Bayfol® HX Film Holographic Photopolymer.

Holographic photopolymers are a new technology to create passive diffractive optical elements by a pure laser interference recording. In this review, we explain the chemistry concepts of light harvesting in an interference pattern and the subsequent grating formation as chemical response. Using the example of the newly developed Bayfol® HX film we discuss the reaction-diffusion driven photo-polymerization process for an index modulation formation to create volume phase gratings. Further we elucidate the selection of monomer chemistry and discuss details of the recording conditions based on the concept of exposure dosage and exposure time. Influences ranging from high dosage recording to low power recording are explained and how to affect the desired diffraction efficiency. Finally, we outline and demonstrate the process to mass manufacturing of volume phase gratings.

Reliability in Modeling of Spectroscopic Ellipsometry

Spectroscopic ellipsometry (SE) [1, 2] is a powerful non-destructive technique to determine the structure of silicon-based materials such as amorphous silicon (a-Si) and hydrogenated amorphous silicon (a-Si :H). In analyzing the SE spectra, the Bruggeman effective medium approximation (EMA) [3, 4] is usually used to model a thin film as a mixture of amorphous silicon and voids. In these models, however, precise knowledge of the dielectric functions of the various components is required in order to obtain reliable estimation of the structural parameters. This requirement is particular critical to the amorphous component because the structure of amorphous silicon is known to depend considerably on the preparation procedures [5]. So far, the dielectric functions used for modeling a-Si1 x :Hx have been based on certain dielectric functions without consideration of the preparation method. It may not be clear, particularly for thin films of amorphous silicon prepared by novel method, whether it is relaxed or unrelaxed dielectric functions which are appropriate for the modeling. In the present work the scope of the problem is assessed by constructing dielectric functions for relaxed and unrelaxed a-Si1 x :Hx thin films using measured a-Si dielectric functions, and fitting, respectively, using simulated dielectric functions for bulk unrelaxed and relaxed a-Si1 x :Hx on the surface roughness and void concentration parameters. The tetrahedron model [3, 4] is applied to construct the complex dielectric functions for relaxed (e 1⁄4 e1r þ ie2r) and unrelaxed (e 1⁄4 e1ur þ ie2ur) a-Si1 x :Hx, by using the dielectric functions for relaxed and unrelaxed a-Si [5], respectively. For details we refer to the work of Mui and Smith [3], [4]. The complex dielectric functions constructed for relaxed and unrelaxed a-Si1 x :Hx components with various hydrogen concentrations are shown in Fig. 1. It is seen that the relaxed and unrelaxed spectra exhibit different peak position and amplitude. Moreover, as the hydrogen content increases, the complex dielectric functions shift to higher photon energy with lower peak values. The complex dielectric functions for the relaxed (unrelaxed) a-Si1–x :Hx film are constructed using a three-layer structure: the first layer is a surface roughness layer, modeled using the EMA, consisting of 50% voids, 50% layer 2. Layer 2 is modeled using the EMA, consisting of a mixture of relaxed (unrelaxed) a-Si1 x :Hx and voids. The substrate is smooth silicon dioxide (SiO2). Fractional void concentrations in layer 2 are varied from 0.01 to 0.26 in the simulations. The thickness of layer 1 is set to be 0.001 mm, while the film thickness (layer 2) is fixed at 1 mm. The incidence angle is set to be 75 . The constructed complex dielectric functions for relaxed a-Si1 x :Hx films are then fitted using unrelaxed a-Si :H spectra, applying a nonlinear regression based on the Levenberg-Marquardt method [6], and vice-versa. In order to avoid interference caused by the interface, the fits are done above 1.75 eV. We denote the void concentration and the thickness of the surface roughness by Cvoid and TR respectively. The fitting error of the void concentration and the thickness of surface roughness are denoted by DCvoid and DTR, respectively, i.e. Cvoid 1⁄4 Cconstructed void Cfit void , similarly for DTR. In order to reveal the quality of the modeling, we define

Chemical influence of inert gas on the thin film stress in plasma-enhanced chemical vapor deposited a-SiN: H films

The growth of amorphous hydrogenated silicon nitride (a-SiN:H) films by plasma enhanced chemical vapor deposition (PECVD) of SiH4−NH3−N2 reactive gas mixtures has been studied. Films were deposited at low temperature (T < 250 °C) in a commercial PECVD system commonly used to grow a-SiN: H for semiconductor integrated circuit passivation. It has been observed that the stress of the a-SiN: H film can be controlled through dilution of the film precursors with an inert gas. Experiments indicate that the influence of the inert gas on the process extends from growth kinetics and plasma chemistry to hydrogen bonding, elemental composition, and biaxial elastic modulus. The stress in films deposited without dilution is tensile. When argon is added to the plasma, Si–Hx plasma chemistry and film hydrogen bond density change producing a reduction in the amount of tensile stress. Dilution with helium can be used to shift the film stress from tensile to compressive with minimum change in growth rate. The observed helium/film stress relationship is associated with helium-based Penning ionization processes, which create metastable reactive gas species. In turn, the metastables influence nitrogen and hydrogen incorporation into the film. Nitrogen incorporation produces volume expansion of the film, increasing the compressive character of the film stress. This effect is similar to that observed when the RF power is varied or when low or multifrequency plasma excitation is used during PECVD growth of a-SiN: H.