Nasrat A. Raouf

Optical and electrical properties of reactively sputtered TiN, ZrN, and HfN thin films

Thin films of titanium, zirconium, and hafnium nitride are prepared by DC magnetron reactive sputtering at room temperature on fused silica, optical glass and silicon substrates. Deposition parameters are investigated in order to obtain stoichiometric films. The optical and electrical properties of the films as a function of nitrogen partial pressure are determined. The results show that an inverse correlation exists between the optical reflectance and the electrical resistivity of the films. The optical constants of the films are determined by Variable Angle Spectroscopic Ellipsometry (VASE) measurements from 240- 1700 nm at 10 nm steps. Deposited film composition is obtained by the Rutherford Ion Back Scattering (RBS) method. The rms roughness of the films is measured by using an optical scatterometer. Ellipsometer data for all three films show that their refractive index (n) in the visible spectrum is decreased by increasing the film thickness while the extinction coefficient (k) is unchanged. Thin films of TiN have the lowest room temperature resistivity (approximately equals 75 (mu) (Omega) - cm) relative to ZrN and HfN thin films.

Aluminum Mirror Coatings for UVOIR Telescope Optics Including the Far UV

NASA Cosmic Origins (COR) Program identified the development of high reflectivity mirror coatings for large astronomical telescopes particularly for the far ultra violet (FUV) part of the spectrum as a key technology requiring significant materials research and process development. In this paper we describe the challenges and accomplishments in producing stable high reflectance aluminum mirror coatings with conventional evaporation and advanced Atomic Layer Deposition (ALD) techniques. We present the current status of process development with reflectance of ~ 55 to 80% in the FUV achieved with little or no degradation over a year.

Minimizing instrumental polarization in the Multiangle SpectroPolarimetric Imager (MSPI) using diattenuation balancing between the three mirror coatings

Special enhanced silver mirror coatings were designed and fabricated to minimize the polarization introduced by a three-mirror off-axis high-accuracy telescope. A system diattenuation of approximately 1% in the VIS-NIR was achieved by both reducing the diattenuation from each mirror individually and by balancing the diattenuations introduced by the three mirrors over the spectral range. This process of low-polarization engineering involves minimizing system polarization introduced by surface geometry, thin film coatings and birefringent elements, and measuring the system. In this report we will outline a methodology to minimize instrumental polarization aberrations, with an emphasis on achieving low diattenuation in the MSPI camera, given its off-axis geometry and coating design constraints imposed by the space-based application. This polarization balancing technique for mirror coatings can be applied to astrophysics applications.

Characterization of UV-visible filters for the Wide Field Camera 3 of the Hubble Space Telescope

A set of 48 ultraviolet-visible filters in the Wide Field Camera 3 will be deployed on the Hubble Space Telescope in 2004. We summarize the specifications for the filters, derived through interaction with the Science Oversight Committee. A detailed characterization of the 48 filters is presented.

Certification of the full size double corner cube fiducials for the Space Interferometer Mission-PlanetQuest test bed

A full size Double Corner Cube (DCC) assembly was delivered recently to NASAs Space Interferometer Mission (SIM) PlanetQuest testbed at JPL. The DCC was developed at CSIROs Australian Centre for Precision Optics (ACPO) to demonstrate the fabrication of the flight size DCC fiducials. The DCC was assembled from three 30°, high precision ULE glass wedges and a 132 mm diameter base plate. After alignment to sub arc-second angular tolerances, the three wedges were chemically bonded to the base-plate. Comprehensive testing was performed on the assembly to certify the compliance of several parameters including the dihedral angle errors, figure of all reflecting surfaces and the Non Common Vertex Error (NCVE) of the DCC. This paper elaborates on some of the metrology and the certification results of the delivered DCC assembly as well as the chemical bond strength tests.

Mirror coatings for large aperture UV optical infrared telescope optics

Large space telescope concepts such as LUVOIR and HabEx aiming for observations from far UV to near IR require advanced coating technologies to enable efficient gathering of light with important spectral signatures including those in far UV region down to 90nm. Typical Aluminum mirrors protected with MgF2 fall short of the requirements below 120nm. New and improved coatings are sought to protect aluminum from oxidizing readily in normal environment causing severe absorption and reduction of reflectance in the deep UV. Choice of materials and the process of applying coatings present challenges. Here we present the progress achieved to date with experimental investigations of coatings at JPL and at GSFC and discuss the path forward to achieve high reflectance in the spectral region from 90 to 300nm without degrading performance in the visible and NIR regions taking into account durability concerns when the mirrors are exposed to normal laboratory environment as well as high humidity conditions. Reflectivity uniformity required on these mirrors is also discussed.

Microstructure of metal coatings deposited using ion-beam processes

Ion beam deposition processes were used to influence the microstructure and scattering characteristics of gold and platinum coatings. These metallic coatings were deposited onto super-polished fused silica and Zerodur substrates. They were deposited using thermal evaporation, ion assisted deposition, sputtering, and ion assisted sputtering. The microstructure of these films was characterized using an optical scatterometer, an optical profilometer, a stylus profilometer, scanning tunneling microscopy, and atomic force microscopy. We found that these metallic coatings deposited using ion beam deposition produced lower optical scattering and related surface microroughness.

Phasing the segments of the Keck and Thirty Meter Telescopes via the narrowband phasing algorithm: chromatic effects

The narrowband phasing algorithm that was originally developed at Keck has largely been replaced by a broad- band algorithm that, although it is slower and less accurate than the former, has proved to be much more robust. A systematic investigation into the lack of robustness of the narrowband algorithm has shown that it results from systematic errors (of order 20 nm) that are wavelength-dependent. These errors are not well-understood at present, but they do not appear to arise from instrumental effects in the Keck phasing cameras, or from the segment coatings. This leaves high spatial frequency aberrations or scattering within 60 mm of the segment edges as the most likely origin of the effect.

Toric offset three-reflector antenna for an advanced microwave limb sounder

An advanced Microwave Limb Sounder (MLS), now in concept development for a potential future mission, is a space-borne heterodyne instrument to measure pressure, temperature, and atmospheric constituents from thermal emission between 120 and 2400 GHz. Previous MLS instruments used pencil-beam antennas sized to resolve ~1 vertical scale height. Current atmospheric models need better horizontal resolution than orbit spacing provides. To meet these needs, a new antenna concept combines the wide scan range of the parabolic torus with unblocked offset Cassegrain optics. The resulting system is diffraction-limited in the vertical plane but extremely astigmatic, with beamwidths 0.13×2.5°. Nadir axis symmetry ensures that this Beam Aspect Ratio (BAR) is invariant over ±33 degrees of azimuth. The antenna can feed either an array of receivers or multiplexed low-noise receivers whose FOVs are swept by a small scanning mirror. We describe 3 stages of antenna design: First, using a paraxial-optics method, we choose conic profiles given vertical resolution orbit geometry, then develop the surfaces by nadir axis rotation, matching axisymmetric feeds to the BAR. A ray-trace program validates the design and generates alignment and deformation tolerances. Finally, a physical optics analysis verifies reflector surface currents and radiation patterns.

Optical scattering and microstructure of ion beam deposited metal coatings

Thin films of gold and platinum have been deposited onto super-polished fused silica substrates using thermal evaporation, ion assisted deposition (IAD), sputtering and ion assisted sputtering. The influence of ion beam flux, coating material and deposition rate on the films microroughness have been investigated. Coatings of gold and platinum have been bombarded with low energy (10-20 eV) Ar ions from an electron cyclotron resonance (ECR) ion source during deposition. Short range surface microroughness of coated surfaces has been examined using scanning tunneling microscopy (STM) and atomic force microscopy (AFM), while long range surface microroughness has been characterized using an angle resolved optical scatterometer. Results indicate that bombardment with low energy ions cause significant reduction in microroughness of metal coatings.