M. Vallo

Stress investigation of the AlGaN/GaN micromachined circular diaphragms of a pressure sensor

In this paper, selected mechanical properties of a circular AlGaN/GaN diaphragm with an integrated circular high electron mobility transistor (HEMT) intended for pressure sensing are investigated. Two independent methods were used to determine the residual stress in the proposed diaphragms. The resonant frequency method using laser Doppler vibrometry (LDV) for vibration measurement was chosen to measure the natural frequencies while the diaphragms were excited by acoustic impulse. It is shown that resonant frequency is strongly dependent on the built-in residual stress. The finite element analysis (FEM) in Ansys software was performed to determine the stress value from frequency spectra measured. The transition behavior of proposed diaphragms between the ideal circular membrane and plate is observed and discussed. Secondly, the bulging method and white light interferometry (WLI) are used to determine the stress-dependent deflection response of the AlGaN/GaN diaphragm under static pressure loading. Regarding the results obtained, the optimal design of the sensing electrodes is outlined.

Modal Analysis of Gallium Nitride Membrane for Pressure Sensing Device

A circular high electron mobility transistor (C-HEMT) prepared on the AlGaN/GaN membrane surface has been investigated and its potential for pressure sensing has been already demonstrated. The key issue in the design process of such heterostructure based MEMS sensors is the stress engineering. This way we can scale the sensor performance, measured pressure range and sensitivity. Especially, the knowledge of the exact value of the residual stress in membranes (caused by deposition process) helps us to optimize the sensing devices. In this work, the residual stress determination method in gallium nitride circular shaped membrane is reported. It is shown that resonant frequency method using Laser Doppler Vibrometry (LDV) for membrane vibration measurement seems to be an appropriate technique to determine the residual stress in micro-scale membranes. Circularly shaped AlGaN/GaN micro-membranes are excited by acoustic short time pulse. The decay oscillating motion of the membrane is recorded by oscilloscope. By FFT spectral analysis of the signals the resonance frequencies are obtained. For the sample studied, the natural frequency mode resonance peak is used to define the residual stress level. To verify the observed stress in investigated membranes, prestressed modal analysis in finite element method (FEM) code ANSYS is performed. The stress extracted from the measured frequency is taken as an initial stress state of the modelled membrane. Experimentally obtained shock spectra are compared with that computed by FEM simulation.

Influence of temperature on the sensitivity of the AlGaN/GaN C-HEMT-based piezoelectric pressure sensor

We present a finite element method (FEM) analysis of the AlGaN/GaN diaphragm-based pressure sensor with integrated C-HEMT. Our concept uses the C-HEMT as a vertical ring gate capacitor to sense the changes in the piezoelectric charge generated while pressure loading. The lattice mismatch and different thermal expansion coefficients in manufacturing process put the diaphragm to the tension. The operating conditions, especially the elevated temperature, may cause the mechanical stress variations and therefore also the change in mechanical behavior of the pressure sensing diaphragm. Therefore we performed the FEM simulation to predict the influence of elevated temperature and to determine the operating temperature range of proposed circular diaphragm-based MEMS pressure sensor.

MEMS pressure sensor fabricated by advanced bulk micromachining techniques

We present the design and implementation of a MEMS pressure sensor with an operation potential under harsh conditions at high temperatures (T = 300 – 800°C). The sensor consists of a circular HEMT (C-HEMT) integrated on a circular AlGaN/GaN membrane. In order to realize MEMS for extreme conditions using AlGaN/GaN material system, two key issues should be solved: (a) realization of MEMS structures by etching of the substrate material and (b) formation of metallic contacts (both ohmic and Schottky) to be able to withstand high thermal loads. In this design concept the piezoresistive and piezoelectric effect of AlGaN/GaN heterostructure is used to sense the pressure under static and/or dynamic conditions. The backside bulk micromachining of our SiC wafer in the first experiment started with FS-laser ablation down to ~200 -270μm deep holes of 500μm in diameter. Because no additional intermediate layer can stop the ablation process, the number of laser pulses has to be optimized in order to reach the required ablation depth. 2D structural-mechanical and piezoelectric analyses were performed to verify the mechanical and piezoelectric response of the circular membrane pressure sensor to static pressure load (in the range between 20 and 100kPa). We suggested that suppressing the residual stress in the membrane can improve the sensor response. The parameters of the same devices previously fabricated on bulk substrates and/or membranes were compared. The maxima of drain currents of our C-HEMT devices on SiC exhibit more than four times higher values compared to those measured on silicon substrates.

Gates of AlGaN/GaN HEMT for high temperature gas sensing applications

The sensing capability of the Pt or Ir based Schottky gate electrodes on AlGaN/GaN heterostructures at elevated temperatures can be improved. It is carried by inserting a thin interlayer of conductive metal oxide between the sensing electrode and the semiconductor barrier layer. High temperature sensing ability of new composite gates is evaluated by detection of hydrogen. We investigate the different absorbing layers and the influence of composition and interfacial oxide layer thickness on sensitivity and response time of gas sensors.

Enhanced Sensitivity of Pt/NiO Gate Based AlGaN/GaN C-HEMT Hydrogen Sensor

In this article we demonstrate the high sensitivity AlGaN/GaN circular HEMT (C-HEMT) hydrogen gas sensor with new gate interfacial Pt/NiO layer. The wide band-gap III-nitride semiconductor heterostructure allows the sensor operation at elevated temperatures. Likewise, the C-HEMT sensing device is easy to prepare because the MESA insulation step can be omitted. Moreover, the I-V characteristics of ring gate diodes with a dominant thermionic emission of electrons can be easly achieved by elimination of tunneling currents induced on the MESA-etched edges. The Pt/NiO stacked gate absorption layer has nanocrystalline structure, what increases the surface-to-volume ratio. Consequently, the hydrogen gas is more efficiently dissociated at low temperature. Comparing to reference Pt/AlGaN/GaN diode sensor, the optimum operation temperature decreases from 250 oC towards 50oC and the hydrogen detection efficiency is enhanced about 10 times. This is desirable for battery-powered sensors with low current consumption. On the other hand, the fabricated sensor shows longer reaction and regeneration time constants. This is due to longer diffusion path that hydrogen atoms must overcome to reach the AlGaN semiconductor surface.

Conductive metal oxide based gates for self-aligned technology of AlGaN/GaN HEMTs

We present advanced gate interface of AlGaN/GaN HEMTs for self-aligned gate processing technology. The in-diffusion of the gate metal components to the AlGaN barrier layer can be eliminated. It can be done by inserting thin conductive metal oxide layer on the interface between the metal and the wide bandgap semiconductor layer. The proposed gate electrodes are prepared by thermal oxidation of high work function metals as Ni or Ir (RTA in pure O2 ambient at T=800 °C for 1 min). The highest value of oxidation temperature is limited by the optimal temperature of the ohmic contact formation. It is shown that high temperature formation of IrO2 and NiO gate contact layers has no significant impact on transport parameters of 2DEG channel of AlGaN/GaN HEMT structures thus it could be applied in self-aligned gate processing technology of the device.

Thermal stability of IrO 2 gate based AlGaN/GaN HEMT

We investigate a thermal stability of iridium oxide (IrO2) gate based AlGaN/GaN HEMT devices by using a long-term thermal stress at 300 °C and 450 °C for 48 hours in air atmosphere. AlGaN/GaN high electron mobility transistors with the gate contact based on IrO2 were designed for high temperature applications. IrO2 gate interfacial layer is formed by high temperature oxidation of 15nm thick Ir gate contact layer (T = 500 - 800 °C, for 1 min in O2 ambience) to provide a high temperature stable gate interface. IrO2 gate based AlGaN/GaN HEMT show excellent thermal stability after long-term storage tests at 450 °C for 48 hours in the air, and even leakage current of Schottky gate contacts is continual decreasing with the storage time. AlGaN/GaN HEMTs with Schottky gate contacts based on high temperature formed IrO2 could be predetermined for sensing applications at high temperatures.

HEMT-SAW structures for chemical gas sensors in harsh environment

A growing thirst for highly sensitive and sufficiently selective sensors for extremely harsh conditions can be seen. This fact excludes the use of conventional sensing devices and gives a space for Surface Acoustic Wave sensors with monolithically-integrated electronics. We have chosen the AlGaN/GaN material as a suitable material due to its excellent chemical inertness and stability of piezoelectric parameters. In this paper we test the possible HEMT transistor/SAW transducer monolithic integration and propose the design of an oscillator based on SAW.