Fabrication Of Microwave Devices Research Articles

New self-organised benzo[b]thiophene-based materials for GHz applications

ABSTRACT This research delves into the synthesis and characterisation of novel liquid crystal compounds derived from benzo[b]thiophene cores, focusing on their potential applications in microwave technology. Two synthetic strategies were developed to construct rigid cores, resulting in the successful synthesis of 10 compounds with varied terminal groups and lateral substituents. Correlations between molecular structure and mesomorphic properties were elucidated through extensive comparative analysis. Birefringence measurements and quantum chemical calculations further provided insights into the optical properties and polarisability anisotropy of the synthesised compounds. The results highlight the influence of structural diversity on the compounds’ suitability for microwave applications. Specifically, compounds featuring carbon–carbon triple bonds and polar terminal groups demonstrated enhanced birefringence and polarisability values, indicating their potential in microwave device fabrication. This study underscores the importance of molecular design in optimising liquid crystal materials for advanced technological applications.

Simple and Easily Connectable Transition from Empty Substrate-Integrated Waveguide to a 3D Printed Rectangular Waveguide

3D printing is one of the most promising manufacturing methods in the most developed technological fields, including microwave hardware fabrication. On the other hand, the well-known manufacturing methods of planar substrate integrated circuits allow high-quality prototypes to be made at low cost and with mass production capabilities. The combination of both manufacturing methods, 2D or 2.5D (substrate integrated circuits) and 3D (3D printed structures), will allow us to take advantage of the main strengths of each technology and minimise disadvantages. In this article, for the first time, a transition structure between the Empty Substrate-Integrated Waveguide (ESIW) technology—a planar waveguide integrated on a printed circuit board—and a standard rectangular waveguide manufactured by 3D printing is proposed. This transition will make it possible to combine planar circuits with 3D structures, thus taking advantage of the benefits of both types of technologies. The fabricated prototype presents low losses (0.6 dB for the transmission coefficient and 15 dB for reflection coefficient), good electrical response (very flat), and simultaneously good mechanical stability and robustness to manufacturing and assembly errors. The proposed design for this transition piece is easily realisable for a wide range of affordable 3D printers. Repeatability is guaranteed and the proposed transition allows us to combine different SIC structures to 3D printed circuits. Hence, this transition will enable advancements in the fabrication of microwave devices, particularly with regard to satellite communications.

Enhancement of microwave dielectric behavior by substitution of W6+ for Mo6+ in Li2Mg1.91Na0.18Mo3O12 ceramics

Microwave dielectric ceramics with low dielectric constants are widely used in the design and fabrication of microwave devices. Here, low-temperature sintered Li2Mg1.91Na0.18Mo3-x W x O12 (LMNMWO) (x takes values from 0 to 0.15) ceramics are prepared by the solid-phase reaction technology. XRD and BSE patterns indicated that W6+ is solidly dissolved into the lattice and exhibits an overall single phase. The dielectric properties of the ceramics are effectively enhanced by the substitution of Mo6+ for W6+. The trends of density and SEM microscopic morphology show that the trace W6+ substitution can effectively promote grain growth and enhance grain homogeneity. The results indicate that the Q×f values are related to the relative density and grain homogeneity. The EDS spectrum demonstrates that the ratio of elements is close to the chemical formula of Li2Mg1.91Na0.18Mo2.95W0.05O12. The trend of τf values is related to the lattice distortion generated when W6+ is irregularly solidified into the lattice. Specifically, dielectric properties of the substituted LMNMWO (x = 0.05) ceramics are determined as follows: εr = 8.3, Q×f = 48357 GHz, and τf = -59 ppm/°C.

Study on the transmission characteristics and band structure of 2D and 3D plasma photonic crystals.

In this paper, 2D and 3D plasma photonic crystals (PPCs) were constructed using argon-mercury glow discharge tubes. Firstly, the collisional radiative model (CRM) was established for the glow discharge process, the average electron density of plasma in the discharge tube was calculated, and the average electron density was experimentally diagnosed by microwave diagnosis. By comparing the calculated and experimental results, the variation range of the average electron density in the tube was given. Secondly, according to the analysis results of the average electron density, a layered model of the discharge tube was established, and the transmission characteristics of the 2D and 3D PPC were calculated and measured. Finally, to analyze the cause of the photonic bandgap (PBG) in the transmission spectra, the band structures of 2D and 3D PPC were calculated using the plane wave expansion (PWE) method and the symplectic finite difference time domain (SFDTD) method. The results show that: the layered model is a valid discharge tube model; for 2D PPC, the formation of Localized Surface Plasmons (LSPs) under TE polarization is the main cause of PBG generation, and the number of LSP bandgap increases as the electron density rises; The existence of cutoff bandgap under TM polarization is due to the cutoff effect of plasma on electromagnetic waves. For 3D PPC, both cutoff bandgap and LSP bandgap exist in the transmission spectra, and the cutoff bandgap is shifted to lower frequencies due to the presence of Fano mode. The results of the study can be used as a reference for communication, microwave device fabrication, and military target stealth.

A Model for an Anisotropic Flexoelectric Material with Cubic Symmetry

In this study, the responsive-behavior of an anisotropic, elastic, flexoelectric material with cubic internal structure to mechanical loads is formulated and examined. The elastic deformation and spontaneous polarization patterns of these materials, as well as the mechanical properties, are described. The first strain gradient theory is employed to describe flexoelectric phenomenon in dielectric materials. The equations governing the elastostatic equilibrium of flexoelectric materials under infinitesimal strain gradient regimes are derived depending on the elegant variational formulation introduced by Topuin [1956] and Mindlin [1972]. Explicit analytical expressions of the displacement and spontaneous polarization for the plane strain problem of the rectangular domain subjected to external forces are derived using the so-called semi-inverse method. The magnesium oxide MgO (magnesia) is used as an example of dielectric materials with cubic internal structure since it is widely used in thin-film substrate and fabrication of microwave devices. The results revealed that flexoelectric materials exhibit spontaneous polarization due to the elastic extensibility and elastic compressibility.

Crystal structure and microwave dielectric properties of [(Mg0.6Zn0.4)0.95Co0.05]2TiO4 modified Ca0.8Sm0.4/3TiO3 ceramics

High-quality x[(Mg0.6Zn0.4)0.95Co0.05]2TiO4-(1-x)Ca0.8Sm0.4/3TiO3 ceramics were prepared by solid-state reaction. [(Mg0.6Zn0.4)0.95Co0.05]2TiO4 has a dielectric constant (εr) of ~18, a high quality factor (Q×f) of ~206,000GHz, and a temperature coefficient of resonant frequency (τf) of ~−20.8ppm/°C. To produce a temperature-stable material, Ca0.8Sm0.4/3TiO3, which has a large positive τf value of +400ppm/°C, was added to [(Mg0.6Zn0.4)0.95Co0.05]2TiO4. 0.93[(Mg0.6Zn0.4)0.95Co0.05]2TiO4 −0.07 Ca0.8Sm0.4/3TiO3 has an excellent combination of microwave dielectric properties: εr~18.24, Q×f ~150,000 (at 9GHz), and τf ~1ppm/°C sinters at 1175°C, and can be utilized in the fabrication of microwave devices.

Synthesis of nickel–zinc–yttrium ferrites: Structural elucidation and dielectric behavior evaluation

Novel Ni0.6Zn0.4Y2xFe2−2xO4 soft spinel ferrites were prepared via double sintering ceramic route. The influence of Y3+ cations on structural, morphological, electrical and dielectric properties was investigated. X-ray diffraction (XRD) confirmed the formation of single phase spinel structure for 0≤x≤0.06 and thereafter a small peak of orthorhombic phase (FeYO3) appeared for x>0.06. The incorporation of yttrium altered the lattice constant. The variation in lattice constant with respect to the Y3+ contents was non-linear. The variation of lattice constant may be attributed to larger ionic radius of Y3+ as compare to Fe3+ cations and solubility limit of substituted cations. Fourier transform infrared (FTIR) spectra showed two strong absorption bands around 600cm−1 and 400cm−1 which confirmed the spinel structure. The dc electrical resistivity was found to increase from 5.67×105 to 8.48×108Ωcm with the increased Y3+ contents. The dielectric constant and dielectric loss tangent were significantly impeded by increasing the yttrium contents. The optimized electrical resistivity and reduced dielectric parameters suggest the possible use of these ferrites in the fabrication of microwave devices.

Microwave dielectric properties of x(Mg 0.7Zn 0.3) 0.95Co 0.05TiO 3–(1 − x)Ca 0.8Sm 0.4/3TiO 3 ceramics with a zero temperature coefficient of resonant frequency

High-quality x(Mg 0.7Zn 0.3) 0.95Co 0.05TiO 3–(1 − x)Ca 0.8Sm 0.4/3TiO 3 ceramics were prepared by solid-state reaction. The products were characterized by scanning electron microscopy, X-ray diffraction, and network analyzer. (Mg 0.7Zn 0.3) 0.95Co 0.05TiO 3 has a dielectric constant ( ɛ r) of ∼20, a high-quality factor ( Q × f) of ∼163,560 GHz, and a temperature coefficient of resonant frequency ( τ f) of ∼−65 ppm/°C. To produce a temperature-stable material, Ca 0.8Sm 0.4/3TiO 3, which has a large positive τ f value of 400 ppm/°C, was added to (Mg 0.7Zn 0.3) 0.95Co 0.05TiO 3. 0.85(Mg 0.7Zn 0.3) 0.95Co 0.05TiO 3–0.15Ca 0.8Sm 0.4/3TiO 3 has an excellent combination of microwave dielectric properties: ɛ r ∼ 29.5, Q × f ∼ 65,000 (at 9 GHz), and τ f ∼ 1 ppm/°C sintered at 1175 °C, and can be utilized in the fabrication of microwave devices.

Investigation of (1-x)(Mg0.6Zn0.4)0.95Co0.05TiO3–xSrTiO3 Microwave Dielectrics

The microwave dielectric properties and microstructures of (1-x)(Mg0.6Zn0.4)0.95Co0.05TiO3–xSrTiO3 ceramics were investigated using a network analyzer and scanning electron microscope (SEM). In order to achieve a temperature-stable material, we used a method of combining a positive-temperature-coefficient material with a negative one. SrTiO3 has a large positive temperature coefficient of resonant frequency (τf) of 1,700 ppm/°C. (Mg0.6Zn0.4)0.95Co0.05TiO3 possesses a negative temperature coefficient of resonant frequency of -65 ppm/°C. The dielectric constant and density increase with increasing SrTiO3 content. By appropriately adjusting x in the (1-x)(Mg0.6Zn0.4)0.95Co0.05TiO3–xSrTiO3 ceramic system, a near-zero τf value can be obtained. A new microwave material of 0.96(Mg0.6Zn0.4)0.95Co0.05TiO3–0.04SrTiO3 exhibits excellent dielectric properties including of a dielectric constant of 23.5, a Q ×f value of 92,000 GHz, and a temperature coefficient of resonant frequency of -2.5 ppm/°C, and can be utilized in the fabrication of microwave devices.

Infrared Reflectance of Extremely Thin AlN Epi-Films Deposited on SiC Substrates

Silicon Carbide (SiC) is an important wide band gap semiconductor with outstanding electronic properties. With figures of merit far better than silicon, SiC is believed to replace and outcompete silicon in many applications using high frequencies, high voltage and high temperatures. With the introduction of seeded sublimation technique, a realisation of substrates with large diameter and high quality became possible. Recent progress in the bulk growth using high temperature chemical vapour deposition (HTCVD) has shown excellent results with high purity substrates with semi insulating (SI) properties. The availability of high quality SI substrates allows the fabrication of microwave devices with low rf losses such as the Metal Schottky Field Effect Transistor (MESFET). With the introduction of the hot-wall CVD technique, thick low doped n-type epitaxial layers have been grown for high power devices (> 4 kV) such as the PiN diode. The main contribution of the present work relates to the investigation of growth of MESFET structures. The goal has been to demonstrate the ability to grow MESFET structures using the hot-wall CVD technique. The challenge with abrupt interfaces and controlled doping has been investigated. A comprehensive investigation has been made on how nitrogen and aluminum dopant atoms incorporate into the SiC lattice using the hot-wall CVD technique. Fundamental research of MESFET structures has been combined with growth of device structures for both Swedish and European groups as well as industries. The research has been focused towards the understanding of dopant incorporation, characterization of doped epitaxial layers, the growth of device structures, the modelling of temperature distribution in a hot-wall susceptor and the development of growth systems for future up scaling. In paper 1 we present how the nitrogen dopant is incorporated into the SiC lattice. The influence of several different growth parameters on the nitrogen incorporation is presented. Equilibrium thermodynamical calculations have been performed to give a further insight into the incorporation mechanism. The investigation shows that the N2 molecule itself does not contribute directly to the nitrogen incorporation, however, molecules like the HCN and HNC are more likely. In paper 2 the incorporation of the aluminum dopant into the SiC lattice is investigated in a similar way as the nitrogen incorporation in paper 1. The results show that the aluminum incorporation in SiC is mainly controlled by the carbon coverage on the SiC surface. The investigation shows that it is difficult to obtain high aluminum doping on carbon face whereas the silicon face is sensitive to changes of the growth parameters. High growth rate resulted in a diffusion controlled incorporation. In Paper 3 we present the results from the growth of MESFET structures as well as characterization of the structures and final device properties. Knowledge taken from paper 1 and 2 was used to improve the abruptness of the grown structures. Paper 4 presents the results obtained by low temperature photoluminescence (LTPL) on separately grown 4H-SiC epitaxial layers. Doping calibration curves for nitrogen in the doping range from 1⋅1014 to 2⋅1019 cm-3 are presented. A discussion concerning the Mott transition is also presented. Paper 5 presents the results of the use of simulation to investigate the heating of a hot-wall CVD reactor. New susceptor and coil design are tested. The simulation has been verified with experimental heating tests which show excellent agreement. The new design has a temperature variation of less than 0.5 % over more than 70% of the total susceptor length in addition to a decreased power input of 15 %. In the final two papers, paper 6 and 7, we present work of growth of AlN on SiC. Thin films were grown and characterized with different techniques concerning crystal quality and thickness. The use of infrared reflectance and the features of the AlN reststrahl reflectance band allowed us to determine the thickness of AlN films as thin as 250 A.

Processing and yield of Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-x/ thin films and devices produced with a BaF/sub 2/ process

High-quality films were produced by coevaporation and cosputtering of Y, Cu, and BaF/sub 2/ followed by an ex-situ anneal. The Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-x/ films produced had microwave surface resistances as low as 1/10 that of Cu at 77 K and 10 GHz. This process is especially useful for producing uniform films over areas larger than 1 in/sup 2/ and is compatible with lift-off or traditional etch patterning processes for fabrication of microwave devices. More than 100 films have been deposited on

The use of substrate annealing as a gettering technique prior to molecular beam epitaxial growth

Thermally stable GaAs substrates are needed for ion implantation and active layer growth for the fabrication of microwave devices and integrated circuits. The outdiffusion and redistribution of impurities and related deep levels from GaAs substrates during molecular beam epitaxial (MBE) growth is shown to affect interface quality and device performance. The present paper supports substrate annealing and repolishing as an impurity gettering technique prior to epitaxial growth to reduce interface and surface accumulation of unintentional substrate related impurities. The accumulation of impurities at the surface of GaAs substrates after prolonged annealing is identified by photoluminescence. Epitaxial layers grown on annealed substrates have reduced interface and surface accumulation of substrate related impurities as identified by secondary ion mass spectroscopy and capacitance–voltage profiling. The reduction of these substrate related impurities is observed to be important in the growth of homojunctions and heterojunctions by MBE.

Abstract: Ion implantation and proton−enhanced diffusion in semiconductors

In a period of less than ten years, ion implantation has become an accepted production process in the semiconductor industry, despite considerable initial skepticism about the merits of the process. More recently, proton−enhanced diffusion has been used to fabricate state−of−the−art microwave bipolar transistors and appears to have a number of other applications, including extensions of the frequency response of GaAs FETs. The purpose of this paper is to describe the basic physics involved in each of these processes and to show how the relative merits of each lead naturally to a set of applications in which ion implantation, enhanced diffusion, and conventional thermal processes are combined to produce devices with characteristics superior to those previously available. The principal topics to be presented are the following: (1) Basic operating features of implantation machines with special attention to those characteristics which are required for economic, efficient production; (2) Basic physics of the implantation process with special attention to range distribution theory and experiments, and damage production and annealing; (3) Applications of ion implantation to the production of semiconductor devices including MOSFETs, bipolar transistors, varactors, CCDs, and high−efficiency LEDs; and (4) Basic physics of the proton enhanced diffusion process and its emerging applications in the fabrication of microwave devices.