Bulk- micromachined dielectric tunable optical filter realized with inductively coupled plasma chemical vapour deposition

We present the characterization of silicon oxide (SiOx) and silicon nitride (SiNx) films deposited by inductively coupled plasma chemical vapour deposition (ICP-CVD) at low temperature (< 100 ◦ C). A tunable optical Fabry- Perot (FP) -filter operating at a wavelength around 1.5µm is realized. It is hybridly assembled with two dielectric distributed Bragg reflectors (DBR). One of the DBR- mirrors is intentionally curved using the intrinsic stress inside the films. Our aim is the development of a tunable surface micromachined VCSEL with a curved dielectric mirror. Therefore ICP-CVD with a low deposition temperature is used for SiOx and SiNx films. As a first step the realization of a tunable bulk- mircomachined optical FP- filter is presented. The refractive index, deposition rate, stress and etching rate in buffered hydrofluoric acid (BHF) of thin dielectric films (<500 nm) in dependence on deposition temperature and on gas flow ratio are investigated. The knowledge of the deposition characteristics of the dielectric films is used to realize DBRs with a given curvature that are applied to electrothermally actuated, optical tunable FP- filters. The presented filter has a free spectral range of 29 nm, an insertion loss of 10 dB and a full width half maximum of 0.16 nm. The thickness of the deposited dielectric films are below 500 nm. The investigation of these films is concentrated on the refractive index, deposition rate, stress and etching rate and their dependence on the process parameters from the ICP-CVD. The obtained knowledge of characterization of dielectric films is necessary for the realization of high reflectivity mirrors or so called as distributed Bragg reflector (DBR). These mirrors can be used for optical filters and lasers. The development of a tunable surface micromachined VCSEL (vertical cavity surface emitting laser) operating around 1.5µm is our final aim. The realization of a tunable surface micromachined VCSEL is presented by Tayebati et.al. 1 . The VCSEL consists of a tunable SiO2/TiO2 mirror membrane and emits around 850 nm with a tuning range around 30 nm. In ref.( 2) a hybridly assembled bulk- micromachined VCSEL is presented with a tuning range of more than 50 nm. The tunable mirror membrane consists of the semiconductor Al0.86Ga0.14As/GaAs. For the realization of tunable VCSELs it is necessary to acquire basic knowledge of the manufacturing of the tunable optical filter. The reason is the cheaper and easier manufacturing process and the lower time requirement for the realization of the filter compared with the production time for the VCSEL. The presented optical filter is a Fabry- Perot (FP) interferometer whose dielectric mirrors are produced separately and afterward hybridly assembled. Whereas one of the mirror is flat, the other mirror is realized as a curved mirror- membrane that is intentionally caused by the stress inside the dielectric films. The reason for this curvature is discussed in the following. The cavity length of the FP- filter is in the µm- domain and the curved mirror- membrane can be actuated mechanically by an electrical current that has an influence on the filter