Mahanth Prasad

Design and Fabrication of Si-Diaphragm, ZnO Piezoelectric Film-Based MEMS Acoustic Sensor Using SOI Wafers

This paper reports a simpler technique for fabricating an microelectromechanical system acoustic sensor based on a piezoelectric zinc oxide (ZnO) thin film, utilizing silicon-on-insulator wafers. A highly c-axis-oriented ZnO film of thickness 2.4 μm, which is covered with 0.2-μm-thick PECVD SiO2, is sandwiched between two aluminum electrodes on a 25- μm-thick silicon diaphragm. This diaphragm thickness has been optimized to withstand sound pressure level range of 120-160 dB. Stress distribution studies using ANSYS have been performed to determine the locations for placement of capacitor electrodes. This paper also reports a technique for the creation of a positive slope of the ZnO step to ensure proper coverage during Al metallization. In order to maximize yield, process steps have been developed to avoid the microtunnel blockage by silicon/glass particles. The packaged sensor is found to exhibit a sensitivity of 382 μV/Pa (RMS) in the frequency range from 30 to 8000 Hz, under varying acoustic pressure.

Fabrication and reliability study of a double spiral platinum-based MEMS hotplate

Abstract. A MEMS hotplate consisting of a double spiral platinum-based element was designed and simulated using MEMS-CAD tool COVENTORWARE. A platinum resistor of 115  Ω was fabricated on a 0.6  μm-thick SiO2 membrane of size 120  μm×120  μm. The hotplate consumes 54 mW when heated up to 756°C. The temperature coefficient of resistance of platinum was measured and found to be 2.19×10−3/°C. The fabrication and reliability testing of the hotplate are described. The test results show that the hotplate can continuously operate at 580°C for 5.5 h and it can sustain at least 60 cycles of pulse-mode operation at 530°C with very low temperature and resistance drifts. The maximum current capability of the hotplate was found to be 13.4 mA without any damage to the structure.

ZnO Etching and Microtunnel Fabrication for High-Reliability MEMS Acoustic Sensor

This paper describes a technique for uniform step coverage of aluminum metal (Al) on ZnO film in the fabrication of MEMS acoustic sensor. The MEMS acoustic sensors were fabricated by etching ZnO layer in three different etchants: HCl, NH4Cl with electrolytically added copper ions, and NH4OH with electrolytically added copper ions. For the first time, a technique is reported, which uses aqueous NH4OH solution with electrolytically added copper ions for etching of ZnO layer. For reliable operation of the device, the electrical testing of Al step coverage on ZnO layer was performed. The maximum currents that can be drawn across Al-deposited ZnO edge etched by HCl, Cu-added NH4Cl, and Cu-added NH4OH were 40 mA, 2.5 A, and 3.0 A respectively, without any damage to the structures. The investigations show that uniform Al step coverage on ZnO layer is obtained in case of NH4OH with electrolytically added copper ions. During fabrication of the device, a novel technique for building a microtunnel for pressure compensation was also developed. This microtunnel is used to compensate the pressure applied on the silicon diaphragm by connecting the cavity to the atmosphere. To realize the smooth inlet of microtunnel in the cavity, photoresist SU8 was used for patterning the cavity after microtunnel etching. The developed technique for microtunnel fabrication reduces the process complexity, providing improved yield of the device. The packaged device performed satisfactorily in the sound pressure level (SPL) of 120-160 dB over a wide frequency range of 30-8000 Hz. The maximum sensitivity of the sensor was measured as 380 μV/Pa.

Controlled Chemical Etching of ZnO Film for Step Coverage in MEMS Acoustic Sensor

In this letter, we report a novel wet etching technique of a c -axis-oriented ZnO film that solves the step coverage problem during formation of electrodes on this film. The negative profile or hanging structure of ZnO film deposited by RF magnetron sputtering was obtained during wet etching in HCl and NH4Cl solutions. The developed technique uses aqueous NH4Cl with electrolytically added copper ions. By suspending the wafer in the horizontal direction in a 20% NH4Cl solution, positive slope (more than 90 °) was obtained at the edge of the ZnO film. In this process, p-type 〈100〉 silicon wafers of 10-20-Ω·cm resistivity have been used. Al deposition was done to confirm the step coverage on ZnO film after getting the positive slope. The thickness of ZnO film was varied from 1.3 to 3.4 μm to observe the coverage of sidewall of ZnO film. The structure was also electrically tested and was found to function satisfactorily.

Long-Term Effects of Relative Humidity on the Performance of ZnO-Based MEMS Acoustic Sensors

This paper investigates the long-term repercussions of relative humidity on capacitance and dissipation factor tan δ of ZnO-based MEMS acoustic sensors. During the fabrication process, a ZnO layer covered with a 0.3-μm-thick PECVD layer was sandwiched between two aluminum (Al) electrodes on a 25-μm-thick silicon diaphragm made by a bulk micromachining technique. The fabrication of an acoustic sensor chip was then completed by etching a ZnO layer in the presence of strong acid (HCl) and weak acid (NH4Cl with electrolytically added Cu ions), separately. Post fabrication, under the humid environment conditions prevailing over a long period of time, viz., 150 days, with relative humidity between 60% and 80%, the capacitance values were found to be 1.5 times higher than the original values in the case of strong acid. The corresponding losses tan δ increased from 0.03 to 0.06. However, under the same conditions, the capacitance values did not change for the acoustic chips fabricated using weak acid. The deterioration in frequency and sensitivity responses of the packaged device has been also observed in the case of etching using strong acid. The investigations showed that a 0.3-μm-thick PECVD silicon dioxide as a passivating layer could protect the sensors from ambient humidity over a long period of time, because of a positive slope of a ZnO edge. However, the response of the devices for a negative slope of a ZnO edge was affected due to nonuniform step coverage of a ZnO layer.

Design and mathematical model of a ZnO-based MEMS acoustic sensor

The paper presents the design and mathematical model of a ZnO-based MEMS acoustic sensor. The structure consists with a 3.0 μm thick piezoelectric ZnO layer sandwiched between a pair of aluminum electrodes on a 30 μm thick silicon diaphragm. The size of silicon diaphragm is 3.1 x 3.1 mm2. Resonance frequency of the structure is determined using harmonic analysis by ANSYS and found to be 41.8 KHz. Sensitivity of acoustic sensor with and without the effect of residual stress are obtained as 334.7μV/Pa and 221.6μV/Pa respectively. The proposed model of the acoustic sensor operated over a wide frequency range from 30 Hz to 8 kHz.

Design and simulation of Pt-based microhotplate, and fabrication of suspended dielectric membrane by bulk micromachining

The paper presents the design and simulation of double spiral Pt-based microhotplate (MHP) for gas sensing application. A platinum resistor of 52 Ω has been designed and simulated on a 0.3 micron thick SiO2 suspended membrane of size 40 × 40 μm2 using ANSYS. The SiO2 membrane of size 40 × 40 μμm2 and thickness 0.3 micron has been fabricated successfully by bulk micromachining in <100< orientation P-type silicon using 25% tetra methyl ammonium hydroxide (TMAH) solution. The simulated temperature and transit time response of microhotplate were obtained as 600.5 °C and 0.2 ms respectively at 4.8 mW power consumption.

FEM simulation of platinum‐based microhotplate using different dielectric membranes for gas sensing applications

Purpose – The purpose of this paper is to help reduce power consumption by using platinum‐based microhotplate with different dielectric membranes SiO2 and Si3N4 for gas sensing applications, and to develop platinum lift‐off process using DC sputtering method for fabrication of platinum resistor.Design/methodology/approach – Semiconductor gas sensors normally require high power consumption because of their elevated operating temperature 300‐600°C. Considering the thermal resistant and sensitive characteristics of metal platinum as well as heat and electricity insulating characteristics of SiO2, Si3N4 and combination of both, a kind of the Si‐substrate microhotplate was designed and simulated using ANSYS 10.0 tool. Thermal oxidation of Si wafer was carried out to get a 1.0 μm thick SiO2 layer. Pt deposition on oxidized silicon substrate by lift‐off was carried out using DC sputtering technique.Findings – The platinum‐based microhotplate requires 31.3‐70.5 mW power to create the temperature 348‐752°C for gas...

Structural study of aluminium nitride thin film grown by radio frequency sputtering technique

Radio frequency sputtering technique was used to deposit high-quality Aluminium Nitride (AlN) on a silicon substrate. Structural parameters of deposited thin film are analyzed and reported. Structural parameters taken into account are dislocation density, crystalline size, lattice constants, and stress/strain. Deposited thin film has shown strong crystallographic orientation towards the c-axis (002) as revealed by X-ray diffraction analysis and elemental diffraction spectroscopy. Images obtained from Atomic Force Microscope (AFM) confirmed that deposited thin film was smooth and crack-free. Structural study of deposited films concludes that AlN thin films are a potential candidate for nano/micro-electro-mechanical systems (MEMS/NEMS), bulk acoustic and surface acoustic wave electronic device applications.

Design and modeling of a ZnO-based MEMS acoustic sensor for aeroacoustic and audio applications

In previous research work related to the acoustic sensors, the researchers had focused on the individuality of the sensors for aeroacoustic application and sensors for audio range application. This paper describes a simple and novel model of acoustic sensor for aeroacoustic and audio applications (microphone). The model of the device presented in this paper shows interoperability. The sensor reported has the bandwidth of~22 KHz, which covers the entire bandwidth of microphone and aeroacoustic sensors. A LEM (Lumped Element Model) is used to determine the characteristics of the device. The device has the square diaphragm of 1.5 × 1.5 mm2 and a nominal thickness of 15 μm to sustain the high SPL (Sound Pressure Level). A piezoelectric ZnO layer 2.4 μm-thick is sandwiched between two Al-top and bottom electrodes. The top electrode is segmented to enhance the sensitivity of the device. Furthermore, a microtunnel of 100 μm wide and 21μm deep is designed to achieve the lower cut-on frequency of ~5 Hz. The theoritical results show that the sensor has sensitivity (RMS) of 126.3μV/Pa and 96.6 μV/Pa in case of central and outer electrodes respectively. The resonant frequency of ~ 85 KHz is obtained from lumped model, simulated using MULTISIM 13.0. The result is verified with MEMS-CAD TOOL COVENTORWARE®.