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MONOLITHIC PIEZOELECTRIC ALUMINUM NITRIDE MEMS-CMOS MICROPHONE J. Segovia-Fernandez1*, S. Sonmezoglu1, S. T. Block1, Y. Kusano1, J. M. Tsai2, R. Amirtharajah1, and D. A. Horsley1 1 University of California, Davis, USA 2 InvenSense, Inc., USA has grown in popularity as a piezoelectric material after its use in commercial film bulk acoustic resonators (FBARs) [5]. A photograph of the 5.2×4.5 mm2 MEMSCMOS chip, composed of 4 different AlN microphone topologies, is shown in Figure 1.a. The characterized microphone consists of a multilayer AlN-SiO2 circularshaped membrane, whose top and intermediate surfaces are partially covered by 0.2 μm-thick Al and Mo electrodes, respectively (Figure 1.b).
ABSTRACT A monolithic piezoelectric Aluminum Nitride (AlN) MEMS-CMOS microphone for high-sensitivity, lowpower applications is presented. The MEMS microphone that is directly bonded to a CMOS buffer for current-tovoltage conversion consists of a circular-shaped AlN-SiO2 unimorph membrane. The radius, package dimensions, and electrode layout were optimized to maximize the MEMS sensitivity. The integrated device was fabricated by wafer-scale eutectic bonding of a 0.18 μm CMOS buffer to the MEMS microphone. In this work, the piezoelectric microphone is formed by 1 μm-thick AlN and 1 μm-thick SiO2 layers and provides an average offresonance sensitivity of 0.68 mV/Pa, a resonance frequency of 11.2 kHz, and a floor noise of 0.03 μV/Hz.
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Piezomicrophone
Bond pads
MEMS die
microphone,
Aluminum
Membrane
Nitride, CMOS die
MEMS port
Figure 1: a) Optical image of a 5.2×4.5 mm2 bonded die assembly containing 4 different AlN microphone topologies and b) illustration of the circular-shaped piezoelectric microphone with the outer ring electrode.
INTRODUCTION In a continuously growing market, MEMS microphones have become pivotal elements in mobile phones and wearable electronics, such as smart watches and hearing aids, and have potential applications in emerging internet of things and smart home devices. To be successful in such ubiquitous technologies, the next generation of MEMS microphones requires having improved reliability, high sensitivity and low power consumption. However, most of today’s MEMS microphones are based on capacitive transduction, which shows a high risk for stiction between the transducer’s diaphragm and back-plate [1], and include off-chip CMOS electronics, which degrades the microphone sensitivity due to the high parasitic capacitance of the wirebonds. In comparison to capacitive transducers, piezoelectric microphones have a more robust mechanical structure with no air gap, do not require a DC voltage to operate, which reduces the overall power consumption, and display a more linear response [2]. Moreover, previous works have demonstrated the possibility of fabricating the piezoelectric MEMS microphone and CMOS amplifier on the same chip, which lowers the interconnect parasitics. The aforementioned piezoelectric MEMS-CMOS microphones consist of a square-shaped ZnO-SiN membrane and report off-resonance sensitivities per volume equal to 1.83 mV/mm3/Pa in [3] and 42.06 mV/mm3/Pa in [4] before amplification. In this work, we present the design, fabrication and operation of a monolithic piezoelectric MEMS-CMOS microphone based on AlN, whose unamplified offresonance sensitivity per volume is 169 mV/mm3/Pa. AlN
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Top electrode
AlN
KEYWORDS Piezoelectric MEMS-CMOS.
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MICROPHONE DESIGN Typical MEMS microphones contain three parts that are critical to the overall performance: MEMS transducer, CMOS buffer, and acoustic package. To describe the microphone response in the frequency domain we can use an equivalent circuit model in which the electrical components represent the main parameters of a low-pass filter (Figure 2). Here the motional resistance, inductance, and capacitance are expressed as a function of the microphone stiffness (Km), angular resonance frequency (ωr), quality factor (Q) and transduction coefficient (η), and the purely-electrical capacitance (C0) takes into account the dielectric polarization of AlN. Moreover, the input capacitance of the CMOS buffer and interconnect parasitics lumped into a parallel capacitance (Cp), and stiffness of the acoustic package (Kp) are included in the circuit.
Figure 2: Equivalent circuit representation of the physical sensor combining acoustic, mechanical, and purely-electrical components.
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Eq. (1) computes the microphone sensitivity that is defined as the ratio of output voltage (Vout) and input pressure (Pin). 1 (1) 1
The packaging requirements for piezoelectric microphones are equivalent to those based on capacitive transduction. Essentially, one side of the piezoelectric membrane needs to be encapsulated so that it remains at atmospheric pressure while the open side is exposed to the acoustic pressure signal. However, the air enclosed in the acoustic cavity reacts to compression by stiffening the mechanical structure and reducing the acoustic sensitivity. Therefore, it is important to consider the package acoustics when designing the microphone. To model the microphone, we use a lumped element model in which the equivalent force related to the distributed acoustic pressure Pin is concentrated on a single point and the displacement (u) is averaged over the membrane’s surface area (Am). Therefore, we can approximate the membrane stiffness (Km) by the following equation [2]:
where the term η/(Km+Kp)(C0+Cp) determines the acoustic sensitivity at frequencies below resonance. Note that (1) neglects any shunt resistance to ground existing at the buffer input or due to parasitics in the AlN film that, otherwise, could induce a low-frequency cut-off that we do not observe experimentally. ⁄2 ) is a key The resonance frequency ( parameter for the design of MEMS microphones since it determines the operational bandwidth of the acoustic sensor. The fr of a circular membrane structure can be written as follows [6]: 3.19 2
∬
(2)
In the case of a circular membrane clamped at the edges and carrying a load uniformly distributed over the entire surface, the mode shape can be expressed as follows [6]:
where R, D, and µ represent the radius, flexural rigidity, and mass per unit area of the membrane, respectively. In the case of a multilayer membrane, the varying material constants need to be introduced, and D and µ become the following expressions: 1
;
(4)
(5)
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(3)
where r represents the radial coordinate. The package stiffness is computed based on the compliance of the enclosed air volume. The equivalent stiffness of the package (Kp) is related to the cavity volume (V0) [2]:
where E, ν, and ρ stand for the Young’s modulus, Poisson ratio, and mass density of the layers forming the membrane. Assuming very thin Mo and Al layers (the electrodes are 5 times thinner than the piezoelectric and passive layers) we can model the membrane as a unimorph structure formed by SiO2 and AlN. From (2) and (3), we notice that the fr is a function of R and mechanical properties of the combined SiO2 and AlN stack. Since the piezoelectric microphone is formed by 1 μm-thick AlN and 1 μm-thick SiO2 layers, we can optimize R to increase fr without excessively stiffening the device structure. Figure 3 shows the resonance frequency as a function of the membrane radius. For practical applications we choose R=800 µm that results in fr=11 kHz (this frequency is above 10 kHz, which is usually considered as the minimum bandwidth for low-noise high-sensitivity microphones).
(6) where ρ0 and c represent the density and acoustic velocity of air. By substituting (5) into (4), the ratio Kp/Km is found from (4) and (6) to be: 64
(7)
As a rule of thumb, we set the ratio Kp/Km2.5x2.5x2.5 mm3 at room temperature. While commercial MEMS microphones typically have a maximum height of 1 mm, reducing the package height to this level would result in only a 2.5 % reduction in sensitivity. For ease of assembly, we used a larger 3D printed lid with a 20x20x10 mm3 internal volume for testing. In a piezoelectric sensor, the charge produced by mechanical deformation of the piezoelectric layer is collected by means of metal electrodes forming an output capacitor (C0). For the membrane design used here, in which the electrodes are placed at the edge of the circular membrane that coincides with the region of largest radial stress, C0 can be obtained from:
Figure 3: Resonance frequency vs. membrane radius of a unimorph 1µm-thick AlN-1µm-thick SiO2 circular-shaped membrane.
(8)
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piezo and SiO2 standoff layers. The standoff layer is formed on the MEMS wafer to provide separation between the MEMS structure and the CMOS wafer. Then, the AlN piezo layer is patterned to form Mo bottom electrode contact. Al, Ti and Ge are then deposited in sequence from bottom to top and patterned in order to define the Ge bonding pads and the Al top electrode. A bottom cavity is etched in the CMOS wafer (Figure 5.b) to allow clearance for the out-of-plane moving MEMS microphone. The MEMS and CMOS wafers are then bonded using an Al-Ge eutectic bond to create a hermetic seal around the MEMS structures as well as electrical interconnects between the MEMS structure and CMOS circuit. Finally, after thinning the MEMS wafer to 200 µm thickness by grinding, the MEMS port is etched through the MEMS wafer to expose the membrane to the surrounding environment and the CMOS port is etched through the CMOS wafer. The CMOS port is necessary to open the back of the membrane to increase the volume of the packaged cavity.
where εAlN, tAlN stand for the dielectric permittivity and thickness of the AlN layer, and (R-Re)/R represents the outer electrode coverage. On the other hand, the parasitic capacitance of the bonding pads and CMOS buffer input capacitance form an additional parallel capacitance (Cp) that causes a reduction in the overall microphone sensitivity (see Eq. (1)). For this work, both MEMS sensor and CMOS electronics are monolithically integrated through eutectic wafer bonding, which minimizes the parasitic capacitance to an estimated value of 50 fF. Moreover, the circuit input capacitance is optimized to be about 100 fF. As a result, Cp=150 fF. To have C0>Cp we selected the electrode coverage ratio to be (R-Re)/R=0.15. Under the conditions of large package volume (10x10x5mm3) and small parasitic and buffer input capacitances (Cp=150 fF), Kp and Cp are considered to be negligible and removed from (1) that remains as follows: 1 (9) 1
a)
The off-resonance MEMS sensitivity (η/KmC0) is estimated via COMSOL simulations for the specific piezoelectric microphone under study (Figure 4). The circular membrane including the outer ring electrodes is represented as a 2D axisymmetric geometry in COMSOL and modeled through the acoustic-piezoelectric interaction physics interface. According to simulations, the fr=11.644 kHz, which is very close to our analytical predictions (11 kHz), and the off-resonance MEMS sensitivity is 1.1 mV/Pa.
SiO2 DEPOSITION AND DEFINE Mo
b) CMOS wafer
CMOS CAVITY ETCH CMOS wafer
DEFINE STANDOFF
DEFINE BOTTOM ELECTRODE CONTACT
MEMS wafer
AlGe BONDING CMOS wafer
Al DEPOSITION AND DEFINE Ge PADS
MEMS wafer
DEFINE TOP ELECTRODES
CMOS AND PORT ETCH
Figure 5: Piezoelectric microphone fabrication steps including a) MEMS wafer and b) MEMS-CMOS integration process flows.
MICROPHONE CHARACTERIZATION To characterize the frequency response, linearity and floor noise of the piezo-microphone, an acoustic pressure (Pin) is applied by means of a mid-range speaker (Yamaha NS-6490) and calibrated by using a reference microphone (ICS-40300). To generate a variable Pin the speaker is connected to a power amplifier and controlled via a USB data acquisition system (NI USB-6343), which simultaneously measures the piezo-microphone and reference microphone outputs. The frequency response of the microphone is recorded by sweeping an acoustic signal from low (20 Hz) to high (12 kHz) frequencies (Figure 6). By using (9) and setting the off-resonance MEMS sensitivity, fr and Q equal to 0.68 mV/Pa, 11.2 kHz and 3.7, respectively, we can fit the experimental data. Note that the extracted sensitivity from the actual transducer differs by 38 % from COMSOL predictions (1.1 mV/Pa). This deviation is most likely due to the presence of a film stress gradient between piezo and passive layers and the lack of system
Figure 4: COMSOL simulations of the AlN/SiO2 circular membrane a) first mode eigenfrequency and b) offresonance sensitivity.
MEMS-CMOS PROCESS The AlN MEMS-CMOS fabrication process used to build the microphone under study consists of two starting wafers (MEMS and CMOS) [7]. The MEMS wafer (Figure 5.a) is first patterned with back-side alignment marks used for front-to-back alignment after fusion bonding. The MEMS wafer is then deposited with the SiO2 passive layer, AlN seed, Mo bottom electrode, AlN
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level optimization. In fact, previous studies have reported sensitivity degradation due to residual film stress [8].
CONCLUSION A monolithic piezoelectric MEMS-CMOS microphone based on AlN has been presented in this paper. The MEMS microphone consists of a circularshaped membrane formed by 1 μm-thick AlN and 1 μmthick SiO2 layers. To describe the frequency response of the microphone an equivalent electrical circuit that combines acoustic and purely-electrical components has been used. To optimize the microphone bandwidth and reduce the impact of acoustic package and parasitics and input buffer capacitance on sensitivity, the membrane radius, package dimensions and electrode coverage have been set to 800 µm, 10x5x5 mm3 and 15 %, respectively. The integrated device, which was fabricated by eutectic bonding of MEMS and CMOS wafers, provides with an average off-resonance sensitivity of 0.68 mV/Pa, a resonance frequency of 11.2 kHz and a floor noise of 0.03 μV/Hz.
Figure 6: Piezoelectric microphone frequency response. The linearity of the microphone is verified at 1 kHz by varying the Pin amplitude from 0.1 to 10 Pa and fitting the outcome to a linear regression with excellent agreement (R2=0.9998) (Figure 7). In contrast to previous fitting we observe a higher sensitivity, 0.74 mV/Pa.
ACKNOWLEDGEMENTS This work was sponsored by DARPA, Microsystems Technology Office under the N-ZERO Program, Contract HR0011-15-C-0145.
REFERENCES [1] T. Goida, “Reduced Footprint Microphone System with Spacer Member Having Through-Hole,” WO/2012/015584, 03-Feb-2012. [2] R. J. Littrell, “High Performance Piezoelectric MEMS Microphones,” PhD thesis, The University of Michigan, 2010. [3] E. S. Kim, “Integrated Microphone with CMOS Circuits on a Single Chip,” University of California. Berkeley, 1990. [4] R. P. Ried, E. S. Kim, D. M. Hong, and R. S. Muller, “Piezoelectric microphone with on-chip CMOS circuits,” J. Microelectromechanical Syst., vol. 2, no. 3, pp. 111–120, Sep. 1993. [5] R. Ruby, “A decade of FBAR success and what is needed for another successful decade,” in 2011 Symposium on Piezoelectricity, Acoustic Waves and Device Applications (SPAWDA), 2011, pp. 365–369. [6] S. Timoshenko and S. Woinowsky-Krieger, Theory of Plates and Shells. New York: McGraw-Hill, 1959. [7] J. M. Tsai et al., “Versatile CMOS-MEMS integrated piezoelectric platform,” in 2015 Transducers - 2015 18th International Conference on Solid-State Sensors, Actuators and Microsystems (TRANSDUCERS), 2015, pp. 2248–2251. [8] A. Dehe, M. Wurzer, M. Fuldner, and U. Krumbein, “The Infineon Silicon MEMS Microphone,” presented at the AMA conferences, 2013, pp. 95–99.
Figure 7: Linearity of piezoelectric microphone response measured at 1 kHz. The microphone noise spectrum at ambient conditions can be seen in Figure 8. We characterize the piezo-microphone floor noise by computing the FFT of the measured output when no acoustic signal is applied and comparing the result with a dummy buffer output located on the same chip. In this configuration, we observe that the minimum detectable signal is limited by the buffer floor noise (0.03μV/Hz).
Output floor noise [V/√Hz]
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Dummy CMOS buffer AlN MEMS-CMOS microphone
1.E-5 1.E-6 1.E-7
white noise
1.E-8 fc=13 Hz 1.E-9 1.E+0
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1.E+2 Frequency [Hz]
CONTACT
1/f noise
*J. Segovia-Fernandez, telephone: +1-267-3044725, e-mail:
[email protected]
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Figure 8: Output noise characteristic of the AlN MEMSCMOS microphone and dummy CMOS buffer located on the same chip. The corner frequency (fc) in which both white and 1/f noise plots cross is equal to 13 Hz.
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