Patent Publication Number: US-9414165-B2

Title: Acoustic sensor resonant peak reduction

Description:
BACKGROUND 
     Various embodiments of the invention relate generally to an acoustic sensor and particularly to the performance of the acoustic sensor. 
     Transducers of MEMS acoustic sensors have a frequency response with a gain peak that is quite steep relative to the remainder of the acoustic sensor&#39;s frequency response. Sounds or speech heard by a user of the MEMS acoustic sensor at frequencies of the gain peak or thereabout are unpleasant. An example of this unpleasantness is harshness of the voice. In some cases, the gain peak can degrade the intelligibility of speech that is recorded by the acoustic sensor, because it amplifies only the portions of the speech that are at frequencies substantially close to the gain peak. MEMS acoustic sensors employed in mobile devices, such as cell phones, exhibit additional unpleasant sounds because their gain peak shifts due to environmental changes. Another undesirable effect of high gain peak is noise amplification. 
     Therefore, the need arises for gain peak reduction in a higher performing MEMS acoustic sensor. 
     SUMMARY 
     Briefly, an embodiment of the invention includes a MEMS acoustic sensor having a transducer with a resonance frequency and a frequency response with a gain peak substantially at the resonance frequency, and a peak reduction circuit with a frequency response and coupled to the transducer. The frequency response of the peak reduction circuit causes attenuation of the gain peak. 
     A further understanding of the nature and the advantages of particular embodiments disclosed herein may be realized by reference of the remaining portions of the specification and the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a graph of the frequency response of a transducer of an acoustic sensor. 
         FIG. 2  shows an embodiment of peak reduction circuit employed by an acoustic sensor 
         FIG. 3  shows conceptually an embodiment of a peak reduction circuit employed with an acoustic sensor. 
         FIG. 4  shows a circuit, in accordance with another embodiment of the invention. 
         FIG. 5  shows a test system  500  of a peak reduction circuit, in an exemplary embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     In the described embodiments Micro-Electro-Mechanical Systems (MEMS) refers to a class of structures or devices fabricated using semiconductor-like processes and exhibiting mechanical characteristics such as the ability to move or deform. MEMS often, but not always, interact with electrical signals. A MEMS device may refer to a semiconductor device implemented as a micro-electro-mechanical system. A MEMS device includes mechanical elements and optionally includes electronics for sensing. MEMS devices include but not limited to gyroscopes, accelerometers, magnetometers, acoustic sensors and radio-frequency components. In an embodiment, acoustic sensors can include microphone. Silicon wafers containing MEMS structures are referred to as MEMS wafers. 
     In the described embodiments, MEMS structure may refer to any feature that may be part of a larger MEMS device. One or more MEMS features comprising moveable elements is a MEMS structure. A structural layer may refer to the silicon layer with moveable structures. MEMS substrate provides mechanical support for the MEMS structure. The MEMS structural layer is attached to the MEMS substrate. The MEMS substrate is also referred to as handle substrate or handle wafer. In some embodiments, the handle substrate serves as a cap to the MEMS structure. A cap or a cover provides mechanical protection to the structural layer and optionally forms a portion of the enclosure. Standoff defines the vertical clearance between the structural layer and the IC substrate. Standoff may also provide electrical contact between the structural layer and the IC substrate. Standoff may also provide a seal that defines an enclosure. Integrated Circuit (IC) substrate may refer to a silicon substrate with electrical circuits, typically CMOS circuits. A cavity may refer to a recess in a substrate. An enclosure may refer to a fully enclosed volume typically surrounding the MEMS structure and typically formed by the IC substrate, structural layer, MEMS substrate, and the standoff seal ring. A port may be an opening through a substrate to expose the MEMS structure to the surrounding environment. 
     In the described embodiments, an engineered silicon-on-insulator (ESOI) wafer may refer to a SOI wafer with cavities beneath the silicon device layer or substrate. Chip includes at least one substrate typically formed from a semiconductor material. A single chip may be formed from multiple substrates, where the substrates are mechanically bonded to preserve the functionality. Multiple chip includes at least 2 substrates, wherein the 2 substrates are electrically connected, but do not require mechanical bonding. A package provides electrical connection between the bond pads on the chip to a metal lead that can be soldered to a PCB. A package typically comprises a substrate and a cover. 
     In the described embodiments, a cavity may refer to an opening or recession in a substrate wafer and enclosure may refer to a fully enclosed space. Post may be a vertical structure in the cavity of the MEMS device for mechanical support. Standoff may be a vertical structure providing electrical contact. 
     In the described embodiments, back cavity may refer to a partial enclosed cavity equalized to ambient pressure via Pressure Equalization Channels (PEC). In some embodiments, back cavity is also referred to as back chamber. A back cavity formed with in the CMOS-MEMS device can be referred to as integrated back cavity. Pressure equalization channel also referred to as leakage channels/paths are acoustic channels for low frequency or static pressure equalization of back cavity to ambient pressure. 
     In the described embodiments, perforations refer to acoustic openings for reducing air damping in moving plates. Acoustic port may be an opening for sensing the acoustic pressure. Acoustic barrier may be a structure that prevents acoustic pressure from reaching certain portions of the device. Linkage is a structure that provides compliant attachment to substrate through anchor. Extended acoustic gap can be created by step etching of post and creating a partial post overlap over PEC. 
     Referring now to  FIG. 1 , a graph  100  of the frequency response of a MEMS device transducer is shown. The graph  100  shows an x-axis representing frequency in Hertz (Hz) and a y-axis representing magnitude in decibels (dB). The frequency range shown in the graph  100  is generally from 1 kHz to 30 kHz and the range of the magnitude is generally from −6 dB to 18 dB. It is noted that these numbers are merely used as examples and are not in any way intended to limit the various embodiments of the invention. 
     Also shown in  FIG. 1  is the curve  104  representing the frequency response of a MEMS device transducer when the gain peak  106  is attenuated. 
     In an embodiment of the invention, the frequency response of  FIG. 1  is for a MEMS acoustic sensor transducer. In such embodiments, the curve  102  is representative of the frequency response experienced by prior art devices. As shown at the gain peak  106  around frequencies higher than 10 kHz, an amplitude gain of more than 10 dB is shown over frequencies other than that of the resonance peak. Such increased magnitude causes unpleasant sounds and unintelligibility of speech. 
     The curve  104 , shown in  FIG. 1 , on the other hand, represents the desired response. It does not have a drastic gain peak, as does the curve  102 , and shows a frequency response generally similar to that of a low pass filter. The following figures and related text show various embodiments, although not inclusive, of apparatus and methods for achieving the response of curve  104  or thereabouts in a MEMS device that by itself would exhibit a frequency response resembling that of the curve  102 . 
       FIG. 2  shows an embodiment of a peak reduction circuit  200  employed by a MEMS acoustic sensor. The peak reduction circuit  200  is made of analog and non-tunable circuits and is generally an amplifier with a low-pass frequency response. The amplifier  200  is shown to include a transconductance element  201  with a gain of g M , shown coupled to a resistor  202  with resistance a′ and a capacitor  203  with capacitance ‘C.’ The peak reduction circuit  200  of  FIG. 2  is effectively an analog filter. 
     In operation, the stage  201  receives an input (“IN”), in the form of a voltage signal, and converts the same to a current signal, providing the current signal as input to the resistor  202  and capacitor  203 . The input to stage  201  is generated by a transducer of a MEMS device  204 . The transducer has a resonance frequency and a frequency response with a gain peak substantially at the resonance frequency. It is this gain peak, as shown by the gain peak  106 , in  FIG. 1 , that is undesirable and need be reduced to avoid noise amplification, harsh and unpleasant sounds or speech. 
     The circuit  200  has a frequency response that causes attenuation of the gain peak. The total bandwidth of the peak reduction circuit  200  is 1/(2πRC). Reducing the bandwidth of the peak reduction circuit  200  below the resonance frequency of the transducer of the MEMS device by increasing either ‘R’ and/or ‘C’ has the effect of reducing the height of the gain peak of the transducer. The peak reduction circuit  200  is effectively an analog low pass filter that reduces the gain peak of the frequency response of the MEMS device transducer. 
     In another embodiment of the invention, the peak reduction circuit  200  may be a digital filter. Other examples of filters that may be coupled to the transducer to reduce the gain peak are bandpass filter, stop-band filter, adaptive filter, high-pass or any suitable filter that reduces the amplitude of the gain peak. 
     In the case of an adaptive filter, parameters of the filter, such as capacitance in analog filters and coefficients in digital filters, are adjusted. The parameters may be adjusted once, when the MEMS device is powered on, and remain fixed thereafter, or they may be adjusted periodically while the MEMS device is powered on, or they may be continuously adjusted during operation. Obviously, in the last case, environmental changes resulting in shifts of the gain peak can be better compensated for. 
     In some embodiments of the invention, the peak reduction circuit and the transducer are in a single package. In some embodiments of the invention, the peak reduction circuit and the transducer are in multiple packages. In other embodiments of the invention, the peak reduction circuit and the transducer are in a single chip. In some embodiments, the peak reduction circuit and the transducer are in multiple chips. As shown and discussed herein, in some embodiments of the invention, the peak reduction circuit is an analog circuit and in other embodiments, it is a digital circuit. The analog and/or digital circuits may be adaptive or not adaptive. In cases where the analog and/or digital circuits are adaptive, either or both may have the transducer and the analog/digital circuit may be in multiple chips or multiple packages or a single chip or a single package. In cases where the analog and/or digital circuits are non-adaptive, the transducer and the analog/digital circuit may be in multiple chips or a single package or a single chip or a single package. 
       FIG. 3  shows conceptually an embodiment of a peak reduction circuit  300  employed with a MEMS device. In an embodiment of the invention, the peak reduction circuit  300  is an active damping circuit. In the peak reduction circuit  300 , the spring  302  with a spring constant ‘k’ and a moving electrode  304  with a mass ‘m’ together form a conceptual representation of a MEMS device. 
     The spring  302  is shown connected to a moving electrode  304  with a mass ‘m’, suspended on the spring  302  as to form a resonant mechanical system. Further shown in the active damping circuit  300  is a stationary electrode split into at least two parts, the sensing electrode  308 , and the driving electrode  306 . The sensing electrode  308  is shown coupled to a current-to-voltage (c2v) amplifier  310 , which converts a current signal from the sensing electrode  308  to a voltage signal. The capacitor  314  is shown coupled to the input and output of the amplifier  310  as well as to a feedback control network  312 . 
     The driving electrode  306  is responsive to feedback control network  312 . The capacitor  314 , feedback control network  312  and the amplifier  310  collectively form an active feedback loop. The feedback signal conditioning has a transfer function represented by ‘−G FB ’. The active feedback loop is used to apply a dampening force to the MEMS transducer around the resonant frequency of the transducer of the MEMS device to reduce the gain peak. The active feedback loop applies the damping force via the driving electrode  306 . 
     For further details of the operation of active damping circuits, such as the one shown in  FIG. 3 , the reader is directed to U.S. patent application Ser. No. 13/720,984, filed on Dec. 19, 2012, and entitled “Mode Tuning Sense Interface”, the disclosure of which is incorporated herein by reference as though set forth in full. 
     The feedback conditioning circuit  312  and the capacitor  314  in circuit  300  are tunable and, in this respect, peak reduction circuit  300  functions generally as an adaptive system, unlike the embodiment of  FIG. 2 , which is not tunable and therefore not adaptive. 
     In an exemplary embodiment of the invention, the MEMS device  302  is an acoustic sensor. In an embodiment where the MEMS device is an acoustic sensor, the adaptive characteristic of the circuit  300  compensates for the gain peak shift, such as air mass loading of the acoustic port in cell phone applications. Another way of estimating the shift in the gain peak is by use of a pilot test tone at a frequency near the gain peak with known relationship to the resonance frequency. The sensor&#39;s response to the pilot tone is tracked and where there is a shift in the gain peak, the sensor&#39;s response to the pilot tone should shift with it. 
       FIG. 4  shows a circuit  400 , in accordance with another embodiment of the invention. The circuit  400  is shown to include an amplifier  402 , an analog-to-digital converter (ADC)  404 , and a calibration circuit  406 . The amplifier  402  is shown to receive the transducer output  414  and includes a transconductance element  408 , a resistor  410 , and a variable capacitor  412 . The amplifier  402  is shown coupled to the ADC  404 , and the ADC  404  is further shown coupled to the calibration circuit  406 , which is shown coupled to the capacitor  412  of the amplifier  402 . The transconductance element  408  is shown coupled to the resistor  410  and the capacitor  412 . Opposite ends of the resistor  410  and capacitor  412  are shown coupled to ground. 
     The resistor  410  and capacitor  412  act as an adaptive filter with a parameter, such as the capacitance of the capacitor  412 , changed by the calibration circuit  406 . The transconductance element  408  converts the output  414  to current and provides the current to the filter made of the resistor-capacitor combination of the amplifier  402 . The output of the filter, which is in analog form, is converted to digital form by the ADC  404 . The ADC  404  provides a digital signal to the calibration circuit  406 , which uses the digital signal to adjust the resistor-capacitor filter. Varying the corner frequency response of the filter results in substantially better attenuation of the gain peak and because the filter is an adaptive filter, environmental effects on the acoustic sensor that cause a shift in the gain peak are compensated for. 
     In some embodiments of the invention, the calibration circuit  406  is located in the same chip as the amplifier  402 , or in the same package with the amplifier  402 . In other embodiments of the invention, as shown in  FIG. 4 , the calibration circuit  406  is located externally to the amplifier  402 . 
     It is understood that the embodiments of  FIGS. 2-4  are merely examples of filters and circuits for reducing the gain peak and that many other filters and circuits, too numerous to list, are anticipated. 
       FIG. 5  shows a test system  500  of a peak reduction circuit, in an exemplary embodiment of the invention. In  FIG. 5 , next to each block, a graph of the frequency response of the output of the block is shown. In  FIG. 5 , a pilot signal generator  502  is shown coupled to an acoustic sensor  504 , and the acoustic sensor is shown coupled to a calibration system  506  and to a peak reduction circuit  508 . The calibration system  506  is shown coupled to the peak reduction circuit  508 , as is the acoustic sensor  504 . 
     The pilot signal generator  502  generates pilot signals for the acoustic sensor  504 , which in an embodiment of the invention is a microphone. A graph of the pilot signal magnitude vs. frequency is depicted at  502   a . The output of the acoustic sensor  504  has a frequency response shown by graph  504   a . As shown in the graph  504   a , a peak is introduced into the frequency response of graph  502   a  due to the effects of the acoustic sensor. 
     The calibration system  506  uses the output of the acoustic sensor  504  to calibrate the peak reduction circuit  508  by adjusting the parameters thereof. The output of the peak reduction circuit  508  is a corrected output with no peaks in its frequency response, which is shown by the graph  508   a . Examples of the peak reduction circuit  508 , without limitation, are any of the peak reduction circuits shown and discussed herein. 
     Although the description has been written with respect to particular embodiments thereof, these particular embodiments are merely illustrative, and not restrictive. 
     As used in the description herein and throughout the claims that follow, “a”, “an”, and “the” includes plural references unless the context clearly dictates otherwise. Also, as used in the description herein and throughout the claims that follow, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise. 
     Thus, while particular embodiments have been described herein, latitudes of modification, various changes, and substitutions are intended in the foregoing disclosures, and it will be appreciated that in some instances some features of particular embodiments will be employed without a corresponding use of other features without departing from the scope and spirit as set forth. Therefore, many modifications may be made to adapt a particular situation or material to the essential scope and spirit.