MEMS BASED SENSOR FOR MEASURING ACOUSTIC PRESSURE

FIELD OF INVENTION

The invention is in the field of acoustic pressure measurement, and relates to the development of a Micro Electro Mechanical System (MEMS) based acoustic sensor for measuring strong sound pressure which can be used for the study of damaging effects of sound pressure on structures designed to perform in harsh environmental conditions. The sensor is very useful in high precision acoustic measurements of launch vehicles, aircrafts and structural test facilities. The invention is also useful for acoustic monitoring in automobile industries.

BACKGROUND OF INVENTION

Acoustic pressure measurement is essential in launch vehicles, aircrafts and automobile industries. The measurement is performed to study the effects of sound pressure on structures and sub assemblies. The instruments used for measurement should be designed to perform in harsh environmental conditions. They should be capable of measuring wide range of strong sound pressure.

An exhaustive prior art search has been conducted, which revealed the existence of following patents in the area of acoustic pressure measurement, which are on similar lines.

US 5,452,268 describes an acoustic transducer with improved low frequency response using capacitive sensing method for its operation. It is designed to have a perforated structure, which acts as the acoustic inlet, and a movable diaphragm spaced from the perforated structure, each structure acting as a plate of the capacitor. The limitation of this invention is that it requires external excitation voltage with charge-to-voltage converter and reference demodulator as the sensor uses capacitive sensing technology. Also the perforated structure makes it difficult to fabricate.

US 8,039,911 describes a MEMS sensor which measures the acoustic pressure by the capacitance variation of a fixed plate and a moveable one. The limitation of the invention is that the structure is a very complicated one with the diaphragm implemented in a floating state, while the supporting portions extend from the periphery of the main portion in the direction of the surface of the substrate. Also the vibration sensitivity is reduced by the modification of arrangement of multiple supporting structures.

US 2008/0019545 describes an acoustic sensor which uses a stacking method, wherein, multiple piezoelectric cells are arranged in different patterns to increase signal sensitivity.

The limitation of the sensor is that the arrangement of multiple piezoelectric cells in different patterns leads to complexity in assembly and integration. Also the sensitivity per cell is less.

It is noted that the prior arts provide either less sensitive or difficult to fabricate acoustic measurement sensors. Some of them require external excitation voltage and some having very complicated structure. All the above drawbacks of the prior art evolved the need for development of a new sensor devoid of these problems.

Objects of Invention:

In order to address the drawbacks of the prior art, the present disclosure teaches a sensor for measuring acoustic pressure with the following objectives:

The primary object of the invention is to develop a MEMS based sensor with high acoustic sensitivity.

Another object of the invention is to develop a MEMS based piezoelectric sensor which requires no external poling voltage or demodulator.

Another object of the invention is to develop a MEMS based sensor for measuring acoustic pressure with flat frequency response from 25 Hz to 4 KHz.

Still another object of the invention is to develop a compact, precision batch fabrication type and easy to manufacture sensor for measuring acoustic pressure.

Yet another object of the invention is to develop a MEMS based sensor for measuring acoustic pressure with low vibration sensitivity.

Still another object of the invention is to develop a MEMS based sensor for measuring acoustic pressure with low temperature sensitivity.

A further object of the invention is to develop a MEMS based sensor for measuring acoustic pressure with low noise level.

SUMMARY OF INVENTION

Disclosed herein is a MEMS based sensor device for high precision acoustic measurements. It consists of a silicon diaphragm created on a silicon wafer with a closed cavity under diaphragm. A Pyrex glass is bonded to a silicon chip (wafer) on the back side. A vent canal is provided to allow the movement of air in and out of cavity when any static, ambient pressure changes occur. Metal electrodes sandwiches a ZnO layer at the top of the assembly forms a central electrode and a rim electrode. The acoustic pressure is converted into stress or strain on the diaphragm which in turn is converted into piezoelectric charge variations over the central and rim regions.

The invention also relates to a MEMS based sensor system for acoustic pressure measurement including an electronics module integrated in a single casing with the sensor. The electronics module includes at least a charge to voltage converter to convert the charge variation output from the acoustic sensor to a measurable voltage.

The sensor according to this invention has many distinctive features including the following:

1. The sensor has high acoustic sensitivity through sensing of charge variations on both the rim and central capacitors and avoids the use of external signal conditioner.

2. The invention eliminates the need of any poling voltage or excitation voltage or demodulator by utilizing the self generating property of the piezoelectric material deposited.

3. The invention provides low frequency response up to 25Hz by achieving static pressure equalization up to 13 Hz through a vent canal.

4. The invention provides a compact and easy to manufacture sensor by depositing the sensing piezoelectric material and integrating associated electronics in the same case.

5. The invention develops an acoustic sensor with low vibration sensitivity by proper selection of mechanical dimensions and appropriate material for the diaphragm.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in greater detail with reference to the embodiments shown diagrammatically in the drawings wherein:

Figure 1A shows the MEMS acoustic sensor structure according to the present invention;

Figure 1B shows the backside of the silicon chip after anisotropic etching;

Figure 2 shows the cross sectional view of the etched vent canal;

Figure 3 shows the Acoustic Sensor layup with the different electrode layers according to
this invention;

Figure 4 shows the block diagram of MEMS acoustic sensor electronics according to the present invention; and

Figure 5 shows the chip located inside the header;

DETAILED DESCRIPTION OF THE DRAWINGS

The invention relates to a low cost MEMS based acoustic sensor which can be precision batch fabricated and which is very useful for high precision acoustic measurements, especially for the measurement of strong sound pressures. It is a MEMS sensor realized using bulk-micromachining technology. MEMS technology enables miniature, low weight, low volume and low power devices.

FIGS. 1A and IB shows the MEMS acoustic sensor structure according to this invention.

A Pyrex glass 1 is bonded to a silicon chip (wafer) 2 for providing strength to the chip. A silicon diaphragm 3 is obtained by time controlled anisotropic etching (KOH/TMAH) from back end of the silicon wafer 2 resulting in a closed cavity 4 under diaphragm 3.

Instead of time controlled etching SOI wafers with 25 micron oxide thickness can also be used. SOI wafers give better control of thickness of diaphragm. The diaphragm 3 is dimensioned for operation over 2Pa to 20kPa with a linearity of ±ldB and the range can be varied by modifying the dimensions of the diaphragm. In a preferred embodiment, the silicon diaphragm 3 has a size of 3mm x 3mm with a thickness of 25nm. A vent canal 5 is provided on the rear of the chip 2 which allows movement of air in and out of the closed cavity 4 when any static, ambient pressure changes occur. The width of the vent canal 5 can be about 50-150p.m, with a depth of 30-60nm and length of 10-12mm. An exemplary cross sectional view of the etched vent canal is illustrated in FIG. 2. The dimensions shown therein are only for illustrative purposes and should not be construed as the only possible values. The closed cavity 4 is charged by acoustic pressure (Pac) through the deflecting diaphragm 3 and through the vent canal 5. Electrical output of the sense layer depends on the stresses or the deflection of the diaphragm 3.

The volume of the closed cavity 4 should be very large as compared to the volume sweep of the diaphragm 3 in the entire range. The resistance offered by the vent canal 5 (viscous friction to air flow) should be such that 1/ (2it x effective resistance of the vent canal 5 x effective compliance of the closed cavity 4) be near the lower cut off frequency ensuring sensor blindness to low frequencies and full output to desired frequencies. The sensor operates over a frequency range of 25Hz to 4 kHz, with a response within ±2dB. The resonance frequency is designed to be 40 kHz. The factor of safety with respect to maximum stress in the diaphragm 3 is designed to be 100. The sensor system is capable of operation over a temperature range of -55 to +125°C, withstanding a vibration of 50grms and shock range of 50g half-sine over 10ms.

A central capacitor 9 and a rim capacitor 10 are formed on the top of the wafer 2. The signal transfer from the sensor is generated in the form of charge over the central and the rim regions. Metal electrodes sandwich a ZnO layer 7 of 3um thickness in both the central capacitor 9 and rim capacitor 10. ZnO is the piezoelectric material used in this sensor. The ZnO layer 7, insulator (Silicon dioxide/Silicon nitride) and metal electrode 8 are deposited by sputtering or CVD methods on the front (top) side of the wafer 2. The ZnO layer 7 is sandwiched between the bottom metal electrode 6 and top metal electrode 8. The metal electrodes 6, 8 are preferably aluminum electrodes. Aluminum metallization thicknesses for top and bottom electrodes 6, 8 are lum each. The acoustic sensor layup is shown in FIG. 3. Since ZnO 7 is used as the piezoelectric material, upon deposition of ZnO onto the wafer by sputtering process, the "C" axis (sensing axis) becomes normal to substrate 2 thereby making polling unnecessary. The sensing crystals of ZnO are oriented normal to the surface of the Silicon substrate. Zinc Oxide has the advantage that during deposition the sensing crystals get automatically polarized normal to the substrate. In other materials like PZT, the particles in the crystal are aligned in a random orientation. Hence, PZT requires a separate mechanism called polling (applying high voltage of the order of 200V) to re-orient the crystals so that their sense axis gets oriented normal to the surface. Zinc Oxide does not require this separate process. As mentioned before, appropriately patterned Aluminum electrodes 7,8 sandwich the ZnO layer 7.

Reverting to FIG 1 & 2, the silicon diaphragm 3 converts the acoustic pressure into lateral stresses in silicon and ZnO layers. Piezoelectric ZnO layer 7 converts the lateral stresses in it into transverse polarization to be picked up by aluminum electrodes 6,8.

There are two pairs of electrodes formed on the rim (near edge) region and on the central region. Diaphragm deflection causes opposite polarity stresses (compressive and tensile) as well as electric polarization in these two regions. Thus these two cross polarity signals, when added algebraically by means of the inbuilt signal conditioner, enhances acoustic sensitivity while canceling out any thermal (pyroelectric) polarisation. The piezoelectric material (ZnO Layer 7) does not respond to static changes in temperature but responds to dynamic changes in temperature. The vent canal 5 allows movement of air in and out of cavity 4 under the diaphragm 3 when any static, ambient pressure changes (or for low frequency acoustic pressure) while blocking out the measurement band.

The cavity 4 under the diaphragm 3 is a passive component, which provides large space for free oscillation of the diaphragm 3, along with the vent canal 5 which provides controlled resistance to air passage. It allows slow variations of the ambient pressure to the cavity thereby equaling ambient pressure on both sides of the diaphragm and also denying passage to acoustic pressure in the measurement band thereby allowing the diaphragm 3 to freely respond to measured acoustic pressure. The diaphragm 3 is the active component which converts the acoustic to stress in diaphragm and its piezoelectric layer 7 converts the stress to the charge output, which is now picked up by the metal electrode 6, 8. Unlike capacitive sensing, the sensor according to this invention needs no external excitation to sense the motion of the diaphragm 3. The central electrode assembly 9 alone can provide the required signal and the rim electrode 10 provides redundancy. The sensor can respond to sound pressure, fast variations of ambient pressure and dynamic acceleration like shock & vibration though the sensitivities need retuning as per sensing requirement. The sensor according to the present invention does not respond to magnetic field and gas properties.

FIG. 4 shows the block diagram of MEMS acoustic sensor electronics. The said electronics is integrated in the same casing of the sensor to form a MEMS based sensor system for measuring acoustic pressure. The MEMS sensor 12 converts the variation in acoustic signal 11 into charge 13. The sensor 12 can measure acoustic signals from l00dB to 180dB Sound Pressure Level (SPL) over a frequency range of 25 Hz to 8 KHz. The charge to voltage converter 14 converts the charge variation in the acoustic sensor to a voltage 15, which may be amplified using a voltage amplifier 16 to obtain the amplified measurable output voltage 17.

With the use of inbuilt signal charge amplifier 14 and voltage amplifier 16 signal conditioner, the output can be amplified to ± 2.5V so that it can be interfaced to next level digitization circuit without any intermediate amplifiers.

Further, the vent canal provided on the rear of the chip 2 provides static atmospheric pressure leveling. The vent should offer enough resistance to cut off leakage of pressures in measurement band frequencies. It is difficult to cut channel consistently to the given dimensions in glass. Instead, it is anisotropically etched on the backside of the silicon wafer 2 as shown in figure IB. The width of the vent canal can be 50-150um, depth can be 30-60nm and lengths looping around 3-15 mm.

Cc computed for the cavity = 2.84x1014.m4.sec2.kg1.

Reasonable values of lOOum width, 35nm depth and 12mm length for the canal give Rv = 4.4x1011kg.m-4.sec-1

The cut off frequency = = 13 Hz.

2n.Rv.Cc

Hence the usable frequency range starts from 25Hz.

Also, the invention provides a compact and easy to manufacture sensor by depositing the sensing piezoelectric material and integrating associated electronics in the same case.

FIG. 5 shows the chip 20 bonded on the header 21 by means of a non-conductive epoxy.

The header 21 is closed by a cap 22 with holes 23 so that acoustic pressure can fall on the diaphragm on the silicon chip. The electronics is integrated on the backside of the header 21, thereby eliminating external signal conditioners.

Estimation of vibration sensitivity

Usually the vibration sensitivity is expressed as equivalent SPL that produces the effect of 1 grms at 3 kHz vibration.

As noted earlier, Z-axis forces being negligible and at high resonance frequency of 39.5kHz, the diaphragm can be considered to be rigid through the thickness. The equivalent acoustic pressure as force per unit area produced by 1 grms gravity on the composite mass of the diaphragm is then computed.

Computing mass for l|i2 area through its thickness ( 3u ZnO,+ 25u Si) with margin for Aluminum and SiCh layers:

Mass = 8.10 14kg/u2

Pressure = 8.10 14.10 N n2 = 0.8 Pa = 92 dB SPL (Acceptable max = 105 dB)

Only as an example, the dimensional details and technical specifications of the MEMS Acoustic Sensor according to the invention is as follows:

Diaphragm size: 3mm x 3mm

ZnO thickness: 3um

Si thickness: 25um

Al metallization thickness: l|im

Electrical Specification:

Polarisation Data on the capacitor electrodes:

It is to be understood that both the foregoing general description and the following detailed description of the present embodiments of the invention are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments of the invention and together with the description serve to explain the principles and operation of the invention.

WE CLAIM

1. A MEMS based sensor for measuring acoustic pressure comprising:

a diaphragm formed on a silicon wafer with a closed cavity under the diaphragm;

a vent canal formed on the rear of the wafer allowing movement of air in and out of the
closed cavity, said closed cavity is charged by acoustic pressure through the diaphragm and through the vent canal;

a central electrode and a rim electrode formed on the wafer, each of said central and rim electrodes including a ZnO layer sandwiched between two metal electrodes, the diaphragm capable of converting the acoustic pressure into lateral stresses in the ZnO layer, which converts the lateral stresses into piezoelectric charge variation output picked up by the metal electrodes.

2. The sensor as claimed in claim 1, wherein the charge output is converted into measurable output voltage by a charge to voltage converter and amplifier integrated in a common casing with the sensor.

3. The sensor as claimed in claim 1, wherein deflection of the diaphragm provides the necessary electrical output.

4. The sensor as claimed in claim 1, wherein the volume of the cavity is larger than the volume sweep of the diaphragm.

5. The sensor as claimed in claim 1, wherein the diaphragm is dimensioned for operation over 2Pa to 20kPa with a linearity of ±ldB.

6. The sensor as claimed in claim 1, wherein the diaphragm is formed by time controlled anisotropic etching from a back end of the wafer.

7. The sensor as claimed in claim 1, wherein the ZnO layer is formed with 3nm thickness.

8. The sensor as claimed in claim 1, wherein the metal electrodes are preferably aluminum electrodes with thicknesses of lum each.

9. The sensor as claimed in claim 1, wherein the wafer is bonded on a header, and said header is closed by a cap with holes so that acoustic pressure can fall on the diaphragm.

10. The sensor as claimed in claim 1, wherein a resistance offered by the vent canal should be such that 1/ (effective resistance of the vent canal x effective compliance of the closed cavity) is near the lower cut off frequency.

11.The sensor as claimed in claim 10, wherein the sensor operates over a frequency range of 25Hz to 4 kHz, with a response within ±2dB.

12.The sensor as claimed in claim 1, wherein the ZnO layer is deposited onto the wafer by sputtering process.

13.The sensor as claimed in claim 1, wherein the vent canal is configured to offer resistance to cut off leakage of pressures in measurement band frequencies.

14.The sensor as claimed in claim 1, wherein the vent canal has a width of 50-150|im, a depth of 30-60um and a length of 10-12mm.

15.The sensor as claimed in claim 9, wherein the electronics is integrated on a backside of a header.

16.The sensor as claimed in claim 1, wherein the sensor is capable of operating over a temperature range of -55 to +125°C and withstanding a vibration level of 50grms and shock range of 50g half-sine over 10ms.

17. A MEMS based sensor system for acoustic pressure measurement comprising a MEMS based sensor as claimed in claims 1 to 16 and an electronics module integrated in a single casing with the sensor, said electronics module including a charge to voltage converter to convert the charge output from the sensor to a measurable voltage which may be amplified using a voltage amplifier to obtain the amplified measurable output voltage.