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Thick polysilicon layers (≥0.5 µm) become increasingly attractive for use as MEMS structure materials [1][2][3] . Increased film thickness improves mechanical rigidity in applications such as pressure sensor, provides increased surface area in comb-drive actuators, or increased mass in accelerometer applications. Thicker films however, can experience greater variability in morphology and in surface roughness due to longer deposition times.3.0 µm thick polysilicon films were deposited on silicon wafers by the lower pressure chemical vapor deposition (LPCVD) process. The films were grown by the thermal decomposition of silane in a horizontal hot walled LPCVD furnace at a temperature of 590°C. The problem addressed is the observed heterogeneity in the film quality that is associated with the position of the wafer in the furnace during film deposition. The microstructural characteristics of films produced in the same batch but differed in the location of the furnace were examined by means of high resolution scanning electron microscopy and electron diffraction analysis in a transmission electron microscope (TEM). Tripod polishing was used to prepare cross-sectional samples of the polysilicon films. Mechanical and electrical property measurements were also conducted to evaluate the effect of the heterogeneity in the films' performance.Asperities with diameters from 100 micron to 800 micron were observed on the surface of the polysilicon layers. TEM studies showed the co-existence of ellipsoidal fine grains and some discrete long columnar crystallites containing microtwins (fine laths) in the materials microstructure. The asperities on the surface are found to be the caps of the columnar grains that grow continuously out of the flat surface of the ellipsoidal grains. The density of these asperities is much more larger for the films deposited at the end of the furnace than those at the entrance.The structure evolution of the thick polysilicon layers was formed by the non-uniform conditions in the furnace that affected the nucleation and growth process, in particular the variation of silane gas pressure in the furnace [4] . At the entrance of the gas flow, the silicon crystal nucleation rate on the growing planes is high because of a higher silane pressure. Thus, a structure with more volume fraction of ellipsoidal fine grains is likely to be formed. While at the end of the furnace, the partial pressure of silane is low which results in a lower nucleation rate. So those favourably oriented nuclei whose fast growing direction is the same as the film growing direction are likely to form the observed long columnar crystallites, resulting in a microstructure with co-existence of both columnar and ellipsoidal grains.

We demonstrate a photonic integrated circuit using a novel monolithic integration platform combining InGaAsP gain elements and index matched amorphous silicon waveguide devices. The AWG based multi-frequency laser emits eight 100-GHz-spaced wavelengths near 1550 nm.OCIS codes: (250.5300) Photonic integrated circuits 1. Introduction Integration of photonic circuits is required to satisfy growing bandwidth demands in cost sensitive communication networks at affordable cost. Monolithic photonic circuits containing both active and passive components have been demonstrated using various integration techniques. Typically, the materials used for active as well as passive components in monolithic systems are InP and lattice matched epitaxially grown semiconductor compounds InGaAsP, GaInAlAs [1][2][3][4][5]. Most integrated devices require very low reflections and low loss at active-passive transitions. This makes materials that are popular for passive optical components such as SiO2/SiN/SiON and polymers less attractive for monolithic integration due to refractive indices much lower than those of the III/V compounds. Amorphous silicon alloys, transparent at telecommunication wavelengths (1310, 1550 nm), can be made with refractive indices matching those of III/V semiconductors but do not require lattice matched epitaxy. Optical waveguides made of amorphous silicon alloys deposited on silicon substrates have been explored in [6][7][8]. Low temperature PECVD deposition of amorphous silicon alloys makes this material system compatible with III/V components and substrates. In this paper we demonstrate the first monolithically integrated device combining amorphous silicon and III/V components. We have designed and fabricated a multi-frequency laser, which consists of InGaAsP/InP gain elements, and amorphous silicon waveguides, an amorphous silicon arrayed waveguide grating (AWG) and an amorphous silicon multimode interference (MMI) splitter monolithically integrated on an InP substrate.

Low loss, single mode rib waveguides, based on PECVD deposited multi-layer amorphous silicon are fabricated. These waveguide are refractive index and mode-matched to III/V laser waveguides. Methods for monolithic integration of these passive amorphous silicon waveguides with InGaAsP/InP gain sections are demonstrated. Results of a multiwavelength laser based on an amorphous silicon arrayed waveguide grating integrated on a single chip with InGaAsP gain sections are presented.