T. Lalinsky

Design and simulation of micromechanical thermal converter for RF power sensor microsystem

Abstract This paper discusses the thermo-mechanical simulations performed with the aim to optimize the temperature distribution of the microwave power sensor (MPS) microsystem keeping the thermal stress as low as possible. The concept of the absorbed power measurement is based on a thermal conversion, where the dissipated or absorbed RF power is converted into the thermal power, inside a thermally isolated system, so-called the micromechanical thermal converter (MTC) device. A new MTC approach uses a GaAs with an active high electron mobility transistor (HEMT) heater. New technology of low stress polyimide has been used for MTC thermal isolation. By means of thermo-mechanical simulations, we propose a GaAs micromechanical thermal converter design and a layout of the active sensor elements (HEMT heater and a temperature sensor TS) placed on the MTC structure. Spatial temperature distribution, thermal time constant, thermal stress and displacement and the power to temperature characteristics are calculated from the heat distribution. These findings are compared with results of thermo-mechanical measurement of real micromachined MTC devices. The 3-D thermal and thermo-mechanical simulations were performed, using the CoventorWare simulator.

Study of bimetallic effect in a GaAs cantilever beam of power sensor microsystem

A bimetallic effect in a 2-/spl mu/m-thick GaAs cantilever beam of power sensor microsystem (PSM) is studied using both the microscopic laser optical interferometry and laser optical reflectance measurement. The cantilever beam deflections induced by the different thermal expansion of the GaAs cantilever layer and the top device metallic leads are sensed at different power dissipations in the PSM heater. The key transfer characteristics of the PSM are found. The cantilever bimetallic effect is also considered to be used for the electrical power sensing.

High Frequency Characterization and Properties of AlGaN/GaN HEMT Structures

The paper reports microwave properties of AlGaN/GaN HEMT fabricated on sapphire substrate. The measured transition frequency as well as maximum frequency of oscillation was taken as a figure of merit for comparison of influence of different treatment. Using 2 mum length of gate electrode 7.425 GHz transition frequency as well as 23.437 GHz maximum frequency of oscillation was achieved. Significant influence of surface plasma treatment on HEMT microwave properties was found.

GaAs Cantilever and Bridge Membrane-Like Structures Fully Compatible with AlGaAs/InGaAs/GaAs and InGaP/InGaAs/GaAs Based HFETs

Silicon (Si) based MicroElectroMechanical Systems (MEMS) are now a well understood and widely used in various integrated micromachined microsensors and microactuators. In relation to this, Gallium Arsenide (GaAs) offers a number of material-related and technological advantages over Si. This paper is an attempt to demonstrate a high potential of GaAs based heterostructures for the development of a new generation of MEMS devices. There are 1 μtm-thick cantilever and bridge membrane-like structures fully compatible with both AlGaAs/InGaAs/GaAs and InGaP/InGaAs/GaAs based HFETs developed. The basic electro-thermo-mechanical properties of the micromechanical structures are investigated. A high electro-thermal conversion efficiency is studied in various ambient atmospheres. An internal mechanical stress induced in the micromechanical structures is evaluated to be admissible for their mechanical integrity and stability. This makes the structures very attractive for the design of new thermally based MEMS devices.

Thin film resistance temperature sensors on GaAs

Thin film resistance temperature sensors on GaAs has been developed as an alternative replacement to the temperature sensing Schottky diodes in the power sensor microsystem. Current-voltage and low frequency noise measurements of the temperature sensor have been carried out to analyze the sensor performances. Better noise performance of the resistance sensors compared to the Schottky one has been found.

AlGaN/GaN micromembranes with diamond coating for high electron mobility transistors operated at high temperatures

In this work, we present an application of NCD layers as backside cooling for AlGaN/GaN heterostructures grown on Si substrates. In this case, diamond nucleation is the most limiting technological step due to low mechanical stability of GaN membranes. We observed that standard nucleation techniques (ultrasonic seeding or bias enhanced nucleation) caused cracking of the membranes or not appropriate nucleation efficiency in the Z-depth of structures. Therefore we implemented PVA polymer consisting of diamond powder as seeding composite which resulted in a successful growth of diamond thin film.

Laser ablation: A supporting technique to micromachining of SiC

We present an effective fabrication method of AlGaN/GaN membrane on SiC for MEMS sensors applications. It employs laser ablation as a supporting technique to the plasma enhanced etching methods. Circular patterns transferred deeply into bulk SiC substrates fabricated by ablation using (1) excimer laser and (2) femtosecond (fs) laser tools were compared. We found that the fs laser tool is more suitable for bulk micromachining of SiC because of the clearness of the process. The additional higher thermal load can be also suppressed. A simple laser cleaning procedure was found allowing us to fabricate deep structures without the ablation process retardation by debris formation.

Preparation and properties of micro-hotplates for gas sensors based on GaAs

In this work, analysis of three different heating systems for two types of Pt micro-hotplate is reported: GaAs bulk structure (bulk GaAs), polyimide/GaAs bulk structure (PI-GaAs) and AlGaAs/GaAs suspended membrane structure (AlGaAs/GaAs). Complex electro-thermal characterization of prepared micro-hotplates was realized. Maximal reachable temperature of suspended membrane heating structure was 260degC with corresponding power 36 mW compared with the GaAs bulk structure with maximal temperature 220degC and corresponding power 1.5 W. At temperatures and powers above maximal limits, degradation and destruction of heating meanders occurred. Power consumption P200degC of sample on GaAs bulk substrate was 850 mW, and on PI/GaAs bulk substrate 380 mW, whereas power consumption of sample prepared on AlGaAs/GaAs suspended membrane was significantly lower about 26 mW.

Schottky contact metallization stability on AlGaN/GaN heterostructure during the diamond deposition process

The issue of gate metallization stability on AlGaN/GaN heterostructure during the diamond deposition process has been studied. Among tested Ni, Ir, NiO and IrO2 materials the iridium-based has the most promising characteristic to be used. The diamond growth in focused microwave plasma system on transistors with Ir and IrO2 Schottky contact metallization has been demonstrated and discussed.

Strain induced response of AlGaN/GaN high electron mobility transistor located on cantilever and membrane

The III-Nitrides, especially AlGaN/GaN devices can be widely used in micro-electro-mechanical sensors thanks to their excellent mechanical and electric properties. In this work, we investigate influence of the applied mechanical load on the source-drain current of AlGaN/GaN C-HEMTs located on the clamped edge of the cantilever beam and the membrane, respectively. A linear dependence of the output current on the calculated strain is observed.