Low-loss Alumina Research Articles

Low-loss and low-temperature Al2O3 thin films for integrated photonics and optical coatings

Amorphous aluminum oxide (Al2O3) is a key material in optical coatings due to its notable properties, including a broad transparency window (ultraviolet to mid-infrared) and excellent durability. Moreover, its higher refractive index contrast relative to silica cladding layers and high solubility of rare-earth ions make it well suited for optical waveguides and the development of various functionalities in integrated photonics. In many coatings and integrated photonics applications, the substrates are temperature and stress sensitive, while relatively thick (∼1 μm) alumina layers are required; thus, it is crucial to fabricate low optical loss alumina thin films at low deposition temperatures, while maintaining high deposition rates. In this study, plasma-assisted reactive magnetron sputtering, operated in an alternating current mode, is investigated as a reliable, straightforward, and wafer-scale compatible technique for the deposition of high optical quality and uniform Al2O3 thin films at low temperature. One-micrometer-thick amorphous Al2O3 planar waveguides, deposited at 150 °C and a rate of 23.3 nm/min, exhibit optical losses below 1 dB/cm at 638 nm and as low as 0.1 dB/cm in the conventional optical communication band.

Evaluation of low-loss alumina material for high-power RF windows

Conventional RF vacuum windows are made of metalized ceramics, hermetically brazed to a pillbox cavity. High-power windows, operating in ultra-high frequency (UHF) band, require the fabrication of ceramic disks with diameters on the order of 9”. Furthermore, a Titanium Nitride (TiN) multipactor suppression coating must be applied to the ceramic surfaces. The large size and complex internal geometry of these windows create challenges in validating the coating in the fully fabricated assembly. This study evaluates a novel low-loss alumina AO479U, provided by Kyocera, and a reactive sputtering process suitable to deposit a 10-20 nm thick TiN coating on a large diameter window. The paper will report the changes in the TiN coating through chemical cleaning and vacuum braze processes using stylus profilometry, optical microscopy, Scanning Electron Microscopy (SEM), and Rutherford Backscattering Spectroscopy (RBS).

Mitigating Quantization Lobes in mmWave Low-Bit Reconfigurable Reflective Surfaces

We present a method for the mitigation of quantization lobes in single-bit reconfigurable reflective surfaces (RRSs). Typically, RRSs are planar beamforming structures consisting of hundreds or thousands of antennas with integrated tunable switches. Under plane-wave illumination, single-bit RRSs suffer from undesired side lobes or quantization lobes, which are caused by the periodicity of the errors due to the limited number of bits used in phase quantization. In this article, we present a topology that suppresses the quantization lobes using single-layer, 1-bit RRSs, by implementing a fixed but random phase delay in every unit-cell. The introduction of phase randomization breaks the periodicity of the quantization errors, thus reducing the quantization lobe level (QLL). We carry out a theoretical analysis to demonstrate the effect of phase randomization in RRSs, and for the first time, provide the condition for choosing the range of randomization required to achieve the lowest sidelobe level (SLL). Leveraging this condition, we design a single-layer, 1-bit 30 × 30 randomized RRS at 222.5 GHz. The reflective surface is fabricated on a thin, low-loss alumina ribbon ceramic wafer from Corning Inc. using a simplified fabrication technique suitable for large-scale production of mmWave/THz RRSs. Finally, we present the radar cross-section (RCS) characterization results obtained from a quasi-optical measurement setup validating the mitigation of quantization lobes using the proposed randomization technique.

High-Q Tuneable 10-GHz Bragg Resonator for Oscillator Applications.

This paper describes the design, simulation, and measurement of a tuneable 9.365-GHz aperiodic Bragg resonator. The resonator utilizes an aperiodic arrangement of non ( /4) low-loss alumina plates ( , loss tangent of to ) mounted in a cylindrical metal waveguide. Tuning is achieved by varying the length of the center section of the cavity. A multi-element bellows/probe assembly is presented. A tuning range of 130 MHz (1.39%) is demonstrated. The insertion loss varies from -2.84 to -12.03 dB while the unloaded Q varies from 43 788 to 122 550 over this tuning range. At 10 of the 13 measurement points, the unloaded Q exceeds 1 00 000, and the insertion loss is above -7 dB. Two modeling techniques are discussed; these include a simple ABCD circuit model for rapid simulation and optimization and a 2.5-D field solver, which is used to plot the field distribution inside the cavity.

Broadband 1-to-N Circuit Combiner for mm-wave Applications

Packaging mm-wave and THz circuits, poses many challenges due to the sensitivity of the circuits to any mismatch at the input or output. Traditional interfaces for combining circuits produce significant losses, and standing waves. This paper presents a unique approach to combing circuits at high frequency that results in reflection cancellation, and significant isolation between the circuits. The novel ‘iπ-wave’ approach relies on introducing a set of resistors and 180/N delay lines; where N is the number of circuits to be combined. The approach enables combing 1-to-N circuits with low loss due to its use of low impedance transmission lines, and provides a relative bandwidth of 50 – 200%. The concept is studied theoretically using simulations, and demonstrated experimentally with a 1-to-4 divider. The concept is demonstrated with a 5 – 45 GHz combiner circuit on low loss Alumina substrate featuring high isolation, low loss, and excellent reflection cancellation. The concept can be implemented in hybrid circuits, or MMICs.

Effective dielectric response of polymer composites with ceramic coated silver flakes

High permittivity polymer composites were prepared using Bisphenol A epoxy resin and low loss alumina (Al2O3) coated silver (Ag) flakes for embedded passives. The uniformity and thickness of the Al2O3 layer were studied using both scanning and transmission electron microscopy (SEM and TEM). SEM was also used to investigate the morphology of the composites and the particle distribution. The dielectric properties of the composites were measured as a function of frequency and temperature. The composites showed a high relative permittivity of 370 with a low loss tangent of 0.07 at 1 MHz and a temperature coefficient of permittivity ~160 ppm/K.

Low loss alumina dielectrics by aqueous tape casting: The influence of composition on the loss tangent

Polycrystalline aluminas with various concentrations of oxide dopants CaO, MgO, and TiO2, ranging from 0.05wt.% to 5wt.% (3wt.% in case of MgO), as well as pure alumina references were prepared by tape casting of aqueous suspensions and sintered in air at 1600°C for 4h for applications as low dielectric loss electroceramics. Loss tangents were measured at room temperature in the frequency range between 1 and 100kHz as the key parameter for the intended application. The values of loss tangents of doped materials were influenced by the concentration of dopants. The addition of 0.5 and 5wt.% of TiO2 and 3wt.% of MgO decreased the value of loss tangent in the whole frequency range. The addition of these dopants eliminated abnormal grain growth and decreased the amount of residual porosity. By doing so, the dopants compensated the negative influence of process impurities and decreased the loss tangent values. The cations (Ti4+) with high solubility in the Al2O3 crystal lattice were preferably built into the grain boundary glass, thus efficiently reducing the concentration of polarizable defects in corundum matrix; the formation of vitreous phase had a positive effect on the value of loss tangent in TiO2 doped samples. The increased values of loss tangent were related to lower density of prepared materials, and the presence of residual porosity. Other contributing factors were especially the formation of calcium-containing secondary crystalline phases, and the increased concentration of lattice defects due to incorporation of atoms with different valencies to alumina crystal lattice.

Electromagnetic absorption in anisotropic photonic crystal of alumina cylinders

We present a theoretical and experimental study of the effective permittivity of a photonic crystal (PC) made of parallel dielectric rods in the long-wavelength limit. Using the plane-wave expansion method we derive an analytical expression for the imaginary part of the dielectric tensor. Angular dependence of the dissipative part of permittivity turns out to be more complicated than that for the real part. In particular, it exhibits stronger anisotropy for a PC with rectangular unit cell. Theoretical and numerical results are compared to the experimental measurements of microwave reflection and transmission through a PC fabricated from low-loss alumina rods. A good agreement between the theory and experiment was obtained in the limit where the effective medium approximation is valid.

High-Q X-band distributed Bragg resonator utilizing an aperiodic alumina plate arrangement

This paper describes a high-Q X-band distributed Bragg resonator that uses an aperiodic arrangement of non-lambda/4 low loss alumina plates mounted in a cylindrical waveguide. An ABCD parameter waveguide model was developed to simulate and optimize the cavity. The dielectric plates and air waveguide dimensions were optimized to achieve maximum quality factor by redistributing the energy loss within the cavity. An unloaded quality factor (Q(0)) of 196,000 was demonstrated at 9.93 GHz.

Integrated self-biased hexaferrite microstrip circulators for millimeter-wavelength applications

Planar microstrip Y-junction circulators have been fabricated from metallized 130-/spl mu/m-thick self-biased strontium hexaferrite ceramic die, and then bonded onto silicon die to yield integrated circulator circuits. The impedance matching networks needed to transform the low-impedance circulator outputs were deployed on low-loss alumina or glass dielectrics to minimize circuit losses. These magnetically self-biased circulators show a normalized isolation and insertion loss of 33 and 2.8 dB, respectively, and a 1% bandwidth for an isolation of 20 dB. Application of small (H<1.5 kOe) magnetic bias fields improved the isolation and insertion loss values to 50 and 1.6 dB, respectively. This design may form the basis for future monolithic millimeter-wave integrated circulator circuits that do not require magnets.

Coupling with an Adjustable Compound Iris in a Single Mode Applicator

ABSTRACTCoupling of the input power to a dielectric in a rectangular microwave cavity by using a compound iris having both inductive and capacitive slits, has been investigated. The compound iris is used in combination with a variable tuning plunger to maintain a resonant frequency of 2.45 GHz and minimize the reflected power. Under certain conditions, the compound iris has been observed to achieve an improved coupling of the input power, resulting in higher temperature of cylindrical 0.95 cm rods of silicon carbide (SiC) and alumina (A12O3). By placing the SiC perpendicular to the applied E field (with SiC enclosed in a porous insulation partially filling the applicator), coupling with the compound iris by adjusting both inductive and capacitive slits was achieved for a wide range of values compared to the use of a single pair of inductive slits. The coupling was more sensitive to a precise positioning of the single inductive slit. Such a difference was not observed for the case where the sample was placed parallel to the E field. In temperatures over 1600°C have been reached with a 600 W of power injected into the cavity. In tests with low loss alumina samples (99.8% pure) placed parallel to the E field, the compound iris permitted a greater coupling and higher temperature. The presence of alumina insulation had a significant effect in achieving higher temperatures, and occassionally preventing the initiation of arcing.

A Compact Hydrogen Maser with Exceptional Long-Term Stability

A compact passive hydrogen maser with many unique features, including a significant reduction in size over previous hydrogen masers, is described. It uses the passive mode of operation, thereby permitting use of a small microwave TE011 cavity which is dielectrically loaded by a low-loss alumina (Al2O3). The cavity is 14.6 cm outside diameter and 13.7 cm high, and weighs only 4.4 kg. The unloaded-cavity Q factor is about 6000. With a conventional source, hexapole state selector, and four magnetic shields, the volume of the entire hydrogen maser resonator package is only about 20 l. The FEP-coated quartz bulb which is common in other masers has been replaced by a similar coating on the inside wall of the cavity. This has yielded a simpler design and more rugged hydrogen maser package. The technique for the application of liquid emulsion FEP is discussed. The cavity and attached endcaps compose the vacuum envelope, thus allowing use of a single vacuum system. The dimensional stability of the ceramic cavity under barometric changes is sufficiently within the range of the electronic cavity servo that a second vacuum system is not needed. For temperature control, a single oven is located in the magnetic shield nest. The most recent stability data compiled from four compact hydrogen masers built at NBS is presented. Comparisons are made against the cesium standard NBS-4 and a time scale comprised of nine commercial cesium standards (denoted as 8s).