Heterogeneous integration of microwave and millimeter wave diodes on silicon and flex substrates
HETEROGENEOUS INTEGRATION OF MICROWAVE AND MILLIMETER WAVE DIODES ON SILICON AND FLEX SUBSTRATES By Amanpreet Kaur Millimeter waves (MMW) are electromagnetic (EM) signal between microwave and far infrared, i.e. frequencies between 30GHz (10mm wavelength) and 300 GHz (1mm wavelength) It has applications in future high speed communications, automotive collision avoidance and navigation, homeland security, and high speed chip interconnects. To make commercial MMW integrated circuits a reality, low cost approaches for wafer level integration of all components (actives, passives, back end CMOS) is critically needed. The current state of art MMW circuits utilizes expensive compound semiconductors (CS) for active devices. To meet the future need, there is growing interest in heterogeneous integration of CS and other non Si materials (novel materials) on large area, low cost substrates such as Si, glass and flexible substrates. Integration compatibility with flexible substrate is of special interest for applications such as wearable medical and communication devices. Diodes are of particular interest for high frequency applications such as rectification, frequency mixing and multiplication. So, there is a need for heterogeneous integration compatible diodes for MMW circuits. Key focus of this thesis is to demonstrate carefully engineered high frequency diodes that allow higher levels of integration on existing silicon ICs and on other lower cost materials such as flex polymer substrates. This work focuses on three types of diodes: Graphene based diodes and Metal Insulator Metal (MIM) tunneling diode for low power applications and excimer laser synthesized Silicon Carbide/ Silicon (SiC/Si) heterojunction diodes for high power devices. Graphene is a good candidate for flexible GHz circuits as it possesses excellent electronic and mechanical properties. In its natural state, graphene does not have a band gap which limits its use in the design of diodes. In this thesis, graphene based diodes have been demonstrated by first opening its band gap through chemical modification. The modified graphene or reduced graphene oxide (r GO) based diodes are fabricated on flexible substrates. The r GO diodes show strong non linearity with current in the micro Amp range. This diodes also work well as microwave rectifiers up to 22 GHz as well as frequency multiplier and mixer over a wide frequency range. In parallel, a non semiconductor alternative technique, i.e. MIM diodes, is also demonstrated for low power GHz circuits on flex substrates. Their high speed and frequency response in comparison to III V Schottky diodes and compatibility with variety of substrates (flexible substrate, SiO2, on top of existing CMOS circuitry) makes them a good choice for MMW integrated circuits. Two different types of insulators (TiO2 and NiO) with different dielectric constants are used here. The diodes were also characterized for rectification, multiplication and mixing circuit applications. Diodes for high power MMW applications require wide band gap materials such as Silicon Carbide (SiC) or Gallium Nitride (GaN) Integration of these materials with Si using conventional techniques is very challenging. This thesis presents a new technique for local growth of SiC on Si using high power KrF excimer laser under ambient air conditions. The fabricated diodes show high breakdown voltage >200 V) high rectification ratio and low leakage current densities. The diode also works efficiently as high power microwave rectifier and as frequency doubler.