Characterization of a MEMS acoustic/pressure sensor

~A new MEMS piezoresistive acoustic/pressure sensor has been developed for use to measure jet screech noise. A few samples of the new sensor with different sizes and 2. from different chips were calibrated in the sound field of an air siren. The results show that the new sensor has a flat response at least fi-om 1 kHz to 6 kHz and a 3. sensitivity that can be as high as four to five times that of the smallest-size silicon-based commercial sensors. However, these first generation devices lack the appropriate shielding and grounding and experience some variation in characteristics of different units with 4. the same nominal design parameters. Those drawbacks possible measurements where the spatial and temporal resolution requirements are highly demanding. The ability to manufacture MEMS sensors or acuators, in large arrays; thus providing the potential for distributed control and diagnostics. The compatibility with some of the manufacturing processes of Integrated Circuits. Therefore, complete (sesnor/actuator/controller) autonomous systems are realizable with MEMS. The potential for low power consumption for operation. can be remedied in future generations. Intmduct~ Background = Over the past decade there has, been a growing interest in exploring the ability of Micro Electra Mechanical Systems (MEMS) technology to provide sensors, actuators and ultimately, systems suitable for use in the control and diagnostics of flow phenomena. This interest has been motivated by a number of attractive charactristics of MEMS technology. These include: 1. The ability to manufacture extremely small sensors with very wide band widths, making Albeit the list of advantages of MEMS, the technology is currently at its frontiers and answers to various questions regarding the technolog and its utility are currently being researched. Some of the more important questions concern device characteristics, performance in the application environment and packaging of devices and systems. For an overview of MEMS and its aerospace and fluid mechanics applications, see Ho et al.’ Perhaps one of the most important applications of MEMS in fluid mechanics diagnostics is in conducting timeand space-resolved measurements of the surface l Member AIAA, Assistant Professor + Senior Member AIAA. Associate Professor Copyright 0 The American Institute of Aeronautics and Astronautics Inc. All right res#erved 1 American Institute of Aeronautics and Astronautics . (c)l999 American Institute of Aeronautics & Astronautics pressure. Such measurements would be useful for understanding flow-induced noise and vibrations. For example, the turbulent wall pressure fluctuations caused by the boundary layer flow structure over an airplane fuselage generate undesired cabin noise. Complete characterization of the wall-pressure fi-equency/wavenumber excitation of the fuselage at the high Reynolds numbers encountered during flight requires a large array of small (less than 100 pm) pressure sensors. Achievement of such an array may only be possible through MEMS. Other applications of surface pressure measurements include high-cycle fatigue in turbomachinery and characterization of the unsteady flow above a surface. Typically, pressure measurement is achieved through measurement of the deflect.ion of a thin elastic diaphragm due to the action of the unknown pressure. Although, there are several techniques for measurement of the diaphragm deflection (e.g., capacitive, inductive, optical, etc.), the one considered in this work is based on piezoresistive measurements of the diaphragm strain. MEMS-based piezoresistive pressure sensors have been fabricated and used by Liu et al.‘, Lofdahl et ah3 and Shcplak et aL4 Liu et al. used bulk and surface micromachining to construct a micro channel instrumente~d with an array of piezoresistive pressure senslors for measuring the pressure distribution along the channel length for investigating micro flows. The pressure sensors consisted of a 250 x 250 pm* silicon nitride diaphragm instrumented with p+ polysilicon resistors for strain measurements. The sensors had a static sensitivity of about 1-2 pVN Pa and were only used to conduct steady pressure measurements inside the micro channel. The piezoresistive sensor from Lofdahl et a1.3, on the other hand, was constructed using a 0.4 urn-thin polysilicon diaphragm. The deflection of the diaphragm was measured using a polysilicon piezoresistor deposited on top of the 100 x 100 pm* diaphragm. The sensor’s static and acoustic sensitivities were determined to be 0.12 pVN Pa and 0.09 uVN Pa, respectively. The acoustic response was uniform to within +/3dB from 10 Hz to 10 kHz. Lijfdahl et a1.3 demonstrated the utility of their sensor by conducting measurements beneath a turbulent boundary layer using an array of six pressure sensors. At MIT, Sheplak et al.’ constructed a silicon-based microphone with a piezoresistive sensing scheme for use in wind tunnel tests in bIASA’s High-Speed Civil Transport program. The priiary sensing element of the microphone was a 1.5 pm-thick, 210 pm-diameter silicon-nitride membrane. On top of the membrane, single-crystal silicon piezoresistors were used in half or full bridge configuration for detection of the diaphragm strain under the action of the measured sound-field pressure. A 10 pm x 10 pm x 2.25 mmlong channel provided static pressure equalization for the microphone. Sheplak et al.’ pointed out that the use of single-crystal silicon for construction of the piezoresistors resulted in about five times enhancement in the sensor sensitivity over that of commercial sensors with similar construction and sensing scheme. The sensitivity of the microphone was 2.2 pVN Pa and was flat, to within 3dB, from 200 Hz up to at least 6 kHz. Construction and fabrication of the current MEMS sensor The MEMS sensor used in the current investigation has been developed at the University of Michigan for use as part of an array to measure the sound field at the lip of an axi-symmetric jet during supersonic jet screech. Ultimately, the acoustic sensors, integrated with MEMS actuators are to be used to implement a feedback based control algorithm aimed at reduction/cancellation of screech noise. Development and testing of the actuators are not the subject of this paper. The MEMS acoustic sensor consists of a stresscompensated PECVD silicon nitride/oxide, 0.4 pm-thick diaphragm together with four mono-crystalline ionimplanted p* silicon piezoesistors. The coefficient, ~44, for this type of piezoresistors is about four times larger than that based on p-type polysilicon; thus, leading to a higher transducer sensitivity. The piezoresistors are arranged in a full Wheatstone-bridge configuration for detection of the diaphragm deflection. Two of the lead wires connected to the four comers of the bridge are used to provide 10 V excitation to the bridge. The remaining two wires carry the differential output signal of the bridge which is proportional to the measured pressure. Figure 1 (top) shows a SEM view of the pressure sensor, with a close-up view of one of the piezoresistors. Figure 1 (bottom) displays two of the sensors integrated with two actuators. The piezoresistive readout scheme for the sound detector is chosen in this research because of several reasons: 1. The sound level is high enough (> 100 dB SPL) that the slightly lower sensitivity of a piezoresistive readout does not limit the performance. 2. The fabrication and readout of a piezoresistive sound detector are much simpler than either a piezoelectric or capacitive microphone. American Institute of Aeronautics and Astronautics (c)l999 American Institute of Aeronautics & Astronautics 3. The bandwidth of a piezoresistive device is not affected by air damping typically encountered in a capacitive device with a small air gap. Figure 1. SEM Views of the Acoustic Sensor (top) and Integrated Sensor/Actuator System (bottom) A brief outline of the manufacturing process of the acoustic sensors is provided ji Figure 2. For a more detailed description of sensor manufacturing, see Huang et a1.5 Four different sensors from three different chips were tested in this study. Three of the sensors had a diaphragm size of 5 10 x 5 10 ~.un* while the fourth one was 710 x 710 pm*. One of the three chips contained the 710 pm sensor and one of the 5 10 pm sensors. The other two chips contained the remaining 510~pm sensors. The different chips will be referred to as MEMS chip #3, #4 and #5. Table 1 provides the resistance value for the pielzoresistors of the different sensors. Inspection of Table 1 shows that the nominal value of the piezoresistances for this first generation MEMS sensors are generally not precisely matched for a given sensor. This results in a fairly large DC output voltage for a zero-pressure measurement which makes difficult the amplification of the sensor output using large gain factors without saturating the output of the amplifier circuit. It is anticipated that future generation of the sensor will have a more precisely controlled value of the piezoresistors. a) Elch recess in silicon using RIE; selective deep boron diffusion b) Selective boron ion implantalion: RTA; LTO; PECVD c) Evapolale TiPt layer; pattern microstruclure using RIE: thinning d) Glass recess: evaporale melal; groove glass