DESC:TECHNICAL FIELD
[0001] The present disclosure relates generally to the field of sensors. In particular, the present disclosure relates to a microfluidic device for real-time sensing and monitoring of the density of industrial fluids as well as biofluids.

BACKGROUND
[0002] Background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
[0003] Fluids have certain physical properties which, if tracked, can provide information about their quality and condition. Often this information is related to the mechanical properties of the fluid such as density, dynamic viscosity, volume viscosity, bulk modulus, and surface tension. Monitoring these properties is highly desirable in both industrial and healthcare applications. These properties can be monitored in real-time using fluid transducers, which can be fabricated in various sizes with diverse specifications.
[0004] Transducers for sensing and monitoring the density of fluid are of real importance, as they allow determining properties of industrial fluids, as well as that of the biofluids such as blood, cerebrospinal fluid, sweat, urine, tear, and the likes. These transducers are already realized at the microscale, and a class of piezoelectric-MEMS-based fluid transducers using 2D resonators in the form of piezoelectric micromachined ultrasound transducers (PMUTs) have already been fabricated for density sensing. Such a transducer that used through the transmission of ultrasound in an arrangement of PMUT-Fluid-PMUT (PFP) in order to monitor the fluid density in real-time is already available.
[0005] Although there are several advantages of this transducer, however, it has a serious drawback in terms of its size and complex packaging. Also, it requires a pair of PMUTs, one acting as a transmitter and the other as a receiver in order to determine the density of a fluid, which makes it complex and difficult to fabricate.
[0006] There is, therefore, a requirement in the art to overcome the limitations, drawbacks, and complications associated with the PMUTs. Further, there is a requirement for a reliable and highly sensitive microfluidic device for real-time sensing of the density of fluids, and which is highly compact and consumes extremely low volume of fluids in various industrial and healthcare applications.

OBJECTS OF THE PRESENT DISCLOSURE
[0007] An object of the present disclosure is to provide a microfluidic device and a method that eliminates the effect of cross-coupling between electrodes.
[0008] Another object of the present disclosure is to provide a microfluidic device and a method that functions efficiently utilizing a single PMUT.
[0009] Another object of the present disclosure is to provide a microfluidic device that is portable and of minimal size.
[0010] Another object of the present disclosure is to provide an easy to fabricate microfluidic device for sensing fluid density.
[0011] Another object of the present disclosure is to provide a reliable and highly sensitive microfluidic device for real-time sensing of the density of fluids.
[0012] Another object of the present disclosure is to provide a microfluidic device, which is highly compact and consumes extremely low volume of fluids in various industrial and healthcare applications.

SUMMARY
[0013] Aspects of the present disclosure relate generally to the field of sensors. In particular, the present disclosure relates to a microfluidic device for real-time sensing and monitoring of the density of industrial fluids as well as biofluids.
[0014] An aspect of the present disclosure pertains to a microfluidic device for sensing density of a fluid. The device includes: an active layer made of lead zirconate titanate (PZT) and having a thickness of first predefined value, configured over a substrate made of platinized silicon-on-insulator having a device thickness of second predefined value, and a driving electrode and a sensing electrode configured at least partially over the active layer and separated from each other by an electrical ground line.
[0015] In an aspect, the active layer may have the thickness of 500 nm, and the substrate may have the device thickness of 10 µm.
[0016] In an aspect, the device may be circular in shape; the driving electrode may be circular in shape and positioned centrally covering 70% of the upper area of the active layer; and the sensing electrode may be annular in shape placed 20 µm away from the central driving electrode towards fixed edge of the active layer, wherein the central driving electrode and the annular sensing electrode may be separated by the electrical ground line, to eliminate the effect of cross-coupling.
[0017] In an aspect, the device may be adapted to be operated at one or more frequencies by varying any or a combination of diameter of the device, thickness of diaphragm stack, and interlayer stresses.
[0018] In an aspect, the thickness of the diaphragm stack is 10 µm and the interlayer residual stresses is 700 MPa.
[0019] In an aspect, an AC voltage of predefined magnitude may be applied to the driving electrode while sweeping the signal frequency over a desired range, which correspondingly may vibrate diaphragm of the device, wherein a peak displacement of the diaphragm may be recorded at its resonant frequency, which is indicative of the density of the fluid being tested.
[0020] Another aspect of the present disclosure pertains to a method for fabricating a microfluidic device for fluid density sensing. The method includes the steps of: spin coating lead zirconate titanate (PZT) over a substrate made of platinized silicon-on-insulator wafer having a predefined device layer thickness; annealing, the spin coated PZT at a predefined temperature, which results in formation of a PZT film as active layer over the substrate; patterning top electrode in a predefined pattern using lithography followed by wet etching of the PZT layer; patterning backside to etch backside oxide; and removing a handle layer, followed by etching buried oxide to form a stress-free microfluidic device for sensing density of a fluid.
[0021] In an aspect, the method includes spin coating the PZT over the substrate using sol gel technique, and the step of annealing takes place at 650°C.
[0022] In one aspect, the predefined device layer thickness is 10 µm, and the top electrode is made of a material selected from Cr and Au having thickness in the range of 30-120 mm.
[0023] In another aspect, the backside oxide may be etched using reactive ion etching, and the handle layer may be removed using deep reactive ion etching.

BRIEF DESCRIPTION OF THE DRAWINGS
[0024] The accompanying drawings are included to provide a further understanding of the present disclosure, and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the present disclosure and, together with the description, serve to explain the principles of the present disclosure. The diagrams are for illustration only, which thus is not a limitation of the present disclosure.
[0025] In the figures, similar components and/or features may have the same reference label. Further, various components of the same type may be distinguished by following the reference label with a second label that distinguishes among the similar components. If only the first reference label is used in the specification, the description is applicable to any one of the similar components having the same first reference label irrespective of the second reference label.
[0026] FIG. 1 illustrates an exemplary 3-D schematic of the proposed single-cell dual-electrode PMUT, in accordance with an embodiment of the present invention.
[0027] FIG. 2A illustrates a method for fabricating a microfluidic device for fluid density sensing, in accordance with an embodiment of the present invention.
[0028] FIG. 2B illustrates an exemplary process flow diagram describing the steps involved in the fabrication of the proposed PMUT, in accordance with an embodiment of the present invention.
[0029] FIGs. 3A and 3B illustrate an SEM of the ross-section of the layered stack in a PMUT, and ?/2? scan of the thin film stack used to make the PMUT, respectively.
[0030] FIGs. 4A and 4B illustrate a Polarization vs. applied voltage graph showing the ferroelectric hysteresis loop, and a Capacitance vs. applied voltage graph showing the hysteresis behavior, respectively.
[0031] FIG. 5 illustrates the frequency response of the proposed PMUT in air and water.
[0032] FIG. 6 illustrates an exemplary 3-D schematic of the PMUT-Micorfludic-Integration, in accordance with an embodiment of the present invention.
[0033] FIG. 7 illustrates an exemplary 3-D schematic of the fabrication process flow for fabricating PMI of FIG. 6.
[0034] FIG. 8A illustrates the PMI fabricated using a linear array of three 500 µm diameter PMUTs. FIG. 8B illustrates the PMI wire bonded to a custom-made PCB.
[0035] FIG. 9 illustrates an exemplary experimental setup designed for microfluidic density sensing using the PMI of FIG. 6, in accordance with an embodiment of the present invention.
[0036] FIG. 10 represents a maximum deflection obtained from the laser Doppler vibrometer (LDV) compared with the maximum voltage received from the sensing electrode using the lock-in amplifier.
[0037] FIG. 11 represents a frequency vs density graph obtained from the PMI.
[0038] FIG. 12 represents a frequency vs density graph obtained from the PMI in the blood hemoglobin range.
[0039] FIG. 13 illustrates a plot representing real-time density using the proposed PMI.

DETAILED DESCRIPTION
[0040] The following is a detailed description of embodiments of the disclosure depicted in the accompanying drawings. The embodiments are in such detail as to clearly communicate the disclosure. However, the amount of detail offered is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure as defined by the appended claims.
[0041] Various terms as used herein are shown below. To the extent a term used in a claim is not defined below, it should be given the broadest definition persons in the pertinent art have given that term as reflected in printed publications and issued patents at the time of filing.
[0042] In some embodiments, the numerical parameters set forth in the written description and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by a particular embodiment. In some embodiments, the numerical parameters should be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of some embodiments of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as practicable. The numerical values presented in some embodiments of the invention may contain certain errors necessarily resulting from the standard deviation found in their respective testing measurements.
[0043] As used in the description herein and throughout the claims that follow, the meaning of “a,” “an,” and “the” includes plural reference unless the context clearly dictates otherwise. Also, as used in the description herein, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise.
[0044] The recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each separate value falling within the range. Unless otherwise indicated herein, each individual value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g. “such as”) provided with respect to certain embodiments herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the invention.
[0045] The present disclosure relates to a reliable and highly sensitive microfluidic device for real-time sensing of the density of fluids, and which is highly compact and consumes extremely low volume of fluids in various industrial and healthcare applications
[0046] Referring to FIG. 1, in an aspect, the present disclosure elaborates upon a single cell of the proposed dual-electrode PMUT based fluid density sensing device (also referred to as device 100 or microfluidic device 100, herein). The device 100 uses a lead zirconate titanate (PZT) as the active layer 102-1 and 102-2 (collectively designated as 102, herein). A PZT thin film 102 of thickness~500 nm can be deposited on a platinized silicon-on-insulator (SOI) substrate 104-1 to 104-2 (collectively designated as substrate 104, herein) having a constant device layer thickness of 10 µm. The fabricated PMUTs 100 can include two active electrodes, namely a driving electrode (D.E.) 106, and a sensing electrode (S.E.) 108.
[0047] In an embodiment, the fabricated PMUTs 100 can be circular in shape and can be designed to operate at desired frequencies. This can be done by either varying the diameter, the thickness of the diaphragm stack, or the interlayer stresses. The diameter of the PMUTs 100 can be varied in order to obtain desired frequencies while keeping all other design variables fixed (thickness of the diaphragm was fixed at ~ 10.5 µm and the interlayer residual stresses at ~ 700 MPa, tensile, but not limited to the likes). It is to be appreciated by a person skilled in the art that the topology and design of the fabricated PMTUs 100 in the proposed invention is not just limited to the ones as disclosed in various figures and embodiments of the present disclosure, however, there can be various topologies and approaches to design and fabricate PMUTs that one can follow, and for a desired resonant frequency, there can plenty of geometric design choices, and all such embodiments are well within the scope of the present disclosure.
[0048] The resonant sensing for fluid density in the proposed device 100 works on a principle called the virtual added mass effect, which refers to the fluid density dependent extra mass that acts on a vibrating object when it vibrates in the fluid of interest. This virtual mass can shift the resonant frequency of the structure in direct relation to the medium density, and thus this shift can track even a minute dynamic change in the density of the medium if the change in the resonant frequency can be tracked at a much faster time scale than the change in density.
[0049] A denser medium can cause a decrease in the resonant frequency of the structure, and a rarer medium can increase the frequency. For instance, the frequency obtained from a 750 µm PMUT in carbon tetrachloride, having 1496 kg/m3 is ~ 86 kHz, whereas the frequency obtained from ethanol, having density 774 kg/m3 is ~ 65 kHz, thereby making the resonators a preferred candidate for density sensing. At microscales, one of the most suitable candidates for resonant density sensing is a PMUT, which is in essence a 2D microplate resonator.
[0050] In an embodiment, the single-cell PMUTs 100 being fabricated can have two electrodes, one circular central electrode (driving electrode 106) covering 70% of the PMUT area, and the other annular electrode (sensing electrode 108) placed 20 µm away from the central electrode towards the fixed edge. The central 106 and the annular electrodes 108 can be separated by an electrical ground line, GND 110, to eliminate the effect of cross-coupling. The central electrode 106 can be used as the driving electrode, and the annular electrode 108 can be used as the sensing electrode. To actuate the PMUT, an AC voltage of constant magnitude can be applied to the driving electrode 106 while sweeping the signal frequency over the desired range, which, in turn, vibrates the PMUT 100 diaphragm. The peak displacement of the diaphragm is recorded at its resonant frequency, which depends on the medium density. The corresponding strain developed in the diaphragm gives rise to a proportionate voltage output from the sensing electrode 108 through the direct piezoelectric effect. This output voltage is plotted against the swept frequency to generate a frequency response function that contains the resonant frequency corresponding to the density of the medium.
[0051] Referring to FIG. 2A, the proposed method 200 (also, referred to as method 200, herein) can facilitate fabricating a microfluidic device for fluid density sensing. In an embodiment, the method 200 can include the steps of: spin coating, at step 202, lead zirconate titanate (PZT) over a substrate made of platinized silicon-on-insulator wafer having a predefined device layer thickness, and then annealing, at step 204, the PZT, being spin coated at the step 202, at a predefined temperature, which results in formation of a PZT film as active layer over the substrate.
[0052] In one embodiment, the method 200 can include patterning, at step 206, top electrode in a predefined pattern using lithography followed by wet etching of the PZT layer. In other embodiment, the method 200 can include patterning, at step 208, backside to etch backside oxide.
[0053] In an embodiment, the method 200 can include removing, at step 210, a handle layer, followed by etching buried oxide to form a stress-free microfluidic device for sensing density of a fluid.
[0054] In an embodiment, the method 200 can include spin coating the PZT over the substrate using sol gel technique, and the step of annealing can take place at 650°C.
[0055] In an exemplary embodiment, the predefined device layer thickness is 10 µm, and the top electrode can be made of a material selected from Chromium (Cr) and Gold (Au) having thickness in the range of 30-120 millimetre (mm).
[0056] In another exemplary embodiment, the backside oxide can be etched using reactive ion etching, and the handle layer can be removed using deep reactive ion etching.
[0057] Referring to FIG. 2B, in an implementation, the process flow describing the steps involved in the fabrication of the proposed PMUT is disclosed. The process starts with a platinized silicon-on-insulator wafer having a 10 µm device layer (at step a). Lead zirconate titanate (PZT) is spin-coated repeatedly using the sol-gel technique to achieve the desired thickness (~ 0.5 µm) and then annealed at 650°C, leading to the formation of a uniform thin film (at step b). The top electrode (Cr/Au – 30/120 nm) is patterned using lithography (at step c) followed by wet etching of the PZT layer (at step d). The stack is then patterned from the backside to etch the backside oxide using reactive ion etching (RIE) (at step d). Finally, the handle layer is removed using deep reactive ion etching (DRIE) in order to release the devices, followed by the etching of the buried oxide in order to make the released stack stress-free (at step f).
[0058] In order to assess the quality of the PZT thin film, it was characterized using a scanning electron microscope (SEM). The film was found to be free of defects and uniformly spread over the Ti/Pt thin film. The thickness of the PZT film was found to be 466.67 nm as shown in FIG. 3A. To assess the orientation of the deposited PZT film, an X-ray diffraction scan was carried out. The ?/2? plot of the thin film stack used to make the PMUT is shown in FIG. 3B. The peaks are indexed as per the JCPDS card no: 33-0784 which confirms the tetragonal perovskite structure of the PZT film. The deposited film showed the most prominent peak along (111) orientation. This indicates that the highly oriented bottom Pt (111) electrode acts as a nucleation center for a favored orientation of the PZT thin film. This preferred orientation influences its electrical and mechanical performance.
[0059] To ascertain the quality of the thin-film PZT electrically, polarization was done by sweeping the applied voltage using the Precision Materials Analyzer from Radiant Technologies Inc., and the hysteresis loop so obtained is shown in FIG. 4A. It shows a typical ferroelectric switching behavior of the thin film PZT with the applied electric field. The obtained remanent polarization (Pr), saturated polarization (Ps), and the coercive field (Ec) from the plot are found to be 26 µC/cm2, 44 µC/cm2, and 3.02 V, respectively. Capacitance is also plotted with respect to the applied voltage, and a hysteresis plot is observed as shown in FIG. 4B, which suggests the ferroelectric switching phenomenon. The peak capacitances were observed to be 140.4 nF and 105.3 nF, respectively. The asymmetry in the peak capacitances can be due to the lead vacancies in the deposited PZT film. To understand the AC characteristics of the film, electrical characterization was conducted using 4294A Precision Impedance Analyzer, Agilent Inc. The frequency was swept from 230 kHz to 245 kHz. The peak impedance was found to be 101.8 O and the peak capacitance was calculated to be 6.7 nF.
[0060] PMUTs of two different sizes were fabricated and characterized for vibration response using a laser Doppler vibrometer (Micro System Analyzer, MSA 500 from Polytec Inc.) The resonant frequencies of 750 µm and 500 µm diameter PMUTs were found to be 235.1 kHz and 496.8 kHz in air, and 79.2 kHz and 196 kHz in water, respectively, as shown in FIG. 5. In order to find the actual deflection of the PMUTs and their working as an actuator at their resonance, a frequency sweep was carried out over an appropriate frequency band using the peak hold settings and by actuating the driving electrode with 0.5V AC. Similarly, experiments were carried out using a lock-in amplifier (MFLI) from Zurich Instruments to verify the performance of the PMUT as a sensor. The experiments were carried out in air. The response obtained from the LDV and the lock-in amplifier are tabulated in Table I.
TABLE-1- ELECTROMECHANICAL RESPONSE OF DUAL ELECTRODE PMUTS
Diameter (µm) Resonant Frequency in air (kHz) Peak deflection measured with LDV. (µm) Peak voltage sensed using lock-in amplifier (mV)
750 235.1 1.5 8.6
500 496.8 0.85 3.3

[0061] In order to fabricate the microfluidic density sensor, it was necessary to create a microfluidic channel and integrate it with PMUTs. This can be done by fabricating a single PDMS channel having an inlet 604 and an outlet 606 over a silicon wafer 602, and aligning and bonding it to a specially designed PMUT array containing one or more PMUTs 100-1 to 10-3 (collectively designated as PMUTs 100) as shown in FIG. 6. For convenience, we name this device PMUT-Microfluidic-Integration (PMI) 600. It can form a self-sensing platform to monitor fluid density in microfluidic regimes.
[0062] Referring to FIG. 7, a 3-D schematic of the fabrication process of the PMI 600 is disclosed. In order to make the PMI, a microfluidic mold was fabricated using a silicon wafer 602 at step a. The wafer 602 was patterned and then time-etched using deep reactive ion etching to achieve the desired channel geometry at step b. PDMS was mixed with a curing compound (10:1 w/w), poured over the mold, and desiccated to release any trapped air bubbles at step c. Subsequently, it was cured at 120°C at step d. After hardening, the PDMS was stripped from the mold at step e and punched with two 1 mm holes for the inlet 604 and outlet 606 at step f. The bonding surfaces were cleaned using plasma cleaner PDC-32G (Harrick Plasma Inc.) for 3 minutes followed by the alignment of the channel with the linear array of PMUT cells 100-1 to 1003 and bonded to realize the PMI 600. Post bonding, the PMI 600 was cured at 110°C for 1 hour to ensure the formation of strong bonds.
[0063] The fabricated channel of the PMI was characterized using an optical profilometer (Talysurf CCI, Taylor Hobson Precision Inc.). The channel depth and width were measured to be 250 µm and ~ 1 mm, respectively, which agreed with the design dimensions. In order to understand the effect of the microfluidic channel integration on the PMUT’s properties, vibration responses were obtained before and after the channel bonding. It was found that the channel dampens the vibration from the PMUTs, thereby significantly reducing their quality factor.
[0064] Referring to FIG. 8A and 8B, the PMI can be firstly die bonded to a custom-made printed circuit board using the H70E epoxy from Epotek Inc. and then wire bonded using HB16 wire bonder from TPT Inc.. Fluid I/O connections can be made by inserting tubings of 1 mm outer diameter into the punched holes. Teflon dissolved in FC-40 (3:7 v/v) can be pushed through the microchannel and the PMI can be cured at 60°C for 2 hours to establish an insulated Teflon coating on the PMUT-fluid interface, thereby electrically insulating the connections from any fluid interference. A lock-in amplifier can be connected to the PMI and the input signal frequency can be swept in the desired range to observe the shift of resonant frequency with the change in density at a driving voltage amplitude of 1V. The tubing at the inlet can be connected to a syringe pump for fluid injection. The PMI can be isolated from background vibrations using a pneumatic vibration isolation table, and shielded cables can be used to minimize electromagnetic interference. An exemplary experimental setup for designing the proposed microfluidic density sensing device using the PMI is shown in FIG. 9.
[0065] To verify the relation between the deflection caused by the driving signal and the electrical voltage received from the sensing electrodes, a PMUT having 750 µm diameter was driven at 0.1 V to 1 V by sweeping the frequency over the desired range to record the maximum deflection using the laser Doppler vibrometer (LDV). Simultaneously, the maximum voltage received from the sensing electrode at the resonance was recorded using the lock-in amplifier. The results so obtained are plotted in FIG. 10. It is observed that the maximum deflection obtained from the LDV shows a linear relationship with the voltage received from the lock-in amplifier. The slope obtained from the graph indicates a sensitivity of 0.56 mV/µm.
[0066] Several samples of a test fluid were prepared using ethanol and carbon tetrachloride in ten different solutions in the range of 774 kg/m3 to 1496 kg/m3 and each of them were individually pushed in the PMI that contained a linear array of three 750 µm diameter PMUTs. The PMUTs were excited and their resonant frequency under the test fluid was found by sweeping the input signal and looking for the peak response obtained from the sensing electrode. The mean resonance frequency is plotted against the test fluid density FIG. 11. It is observed that the variation of the resonance frequency follows a linear relationship with the fluid density. From the experimental results in FIG. 11, the sensitivity of the sensor is found to be 25.9 Hz/(kg/m3). The measurements were repeated thrice, and the obtained data were found to be repeatable. The density vs. frequency relationship obtained with a linear fit shows an R-squared value of 0.96, thereby suggesting that PMI can successfully work as a sensor to sense fluid density. Also, the effect of channel depth on the resonant frequency of the PMUTs was simulated. It was found that the resonant frequency shifts by 37%, on varying the channel depth from 10 µm to 1000 µm. After 1000 µm, no significant variation was observed. Thus, to select an optimum channel depth, 250 µm was chosen for, in which the resonant frequency differs from the saturated frequency by 6%. Also, the repeatability of the data suggested that a channel of perhaps any reasonable dimension could be used for resonant density sensing.
[0067] In order to evaluate the potential of the PMI as a microfluidic hemoglobin sensor, test fluids were created in the range of 1020 – 1090 kg/m3. This is the range in which the density of human blood varies solely due to the variation in the hemoglobin content. The shifted resonant frequency in each case was recorded and plotted as shown in FIG. 12. The plot shows a linear variation of the resonant frequency with the density of the test fluids. The sensitivity of the PMUT as a density sensor is obtained from the slope of the graph which is found to be 26.3 Hz/kg/m3. From the literature, it is found that 1% change in hemoglobin content results in a 0.8 kg/m3 change in blood density. Hemoglobin content in human blood generally varies up to 24.6% for males and 24.8% for females. Thus, the sensitivity obtained here corresponds to a density sensitivity of 21.4 Hz/1% change in blood hemoglobin concentration and is considered to be sensitive enough to sense hemoglobin variation in the human body.
[0068] The performance of the PMI was finally tested for a real-time density sweep in the range of 777.3 to 1281 kg/m3. Carbon tetrachloride was added to ethanol in fixed steps of 100 s and frequency shifts observed after every 10 sec using the lock-in amplifier and keeping the fluid discharge constant at 1 mL/min. The data obtained are plotted in FIG. 13. The plot shows the variation of the resonant frequency of a typical PMUT in the PMI with the time and density of the fluid mixture. It is observed that the frequency switch resulting due to the change in fluid density is almost instantaneous thereby depicting the responsiveness of the PMI towards real-time density sensing.
[0069] The proposed PMUT-Microfluidic-Integrated (PMI) device can be used for real-time monitoring of the density of any fluid. In particular, due to the small volume requirement (in microliters) of the fluid sample, this device has immense potential for characterizing biofluids such as blood, cerebrospinal fluid, sweat, urine, tear, and the likes. in terms of their density, thereby providing an alternative path for disease detection and human health monitoring based on pathology related variation in the density of these biofluids.
[0070] While the foregoing describes various embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. The scope of the invention is determined by the claims that follow. The invention is not limited to the described embodiments, versions or examples, which are included to enable a person having ordinary skill in the art to make and use the invention when combined with information and knowledge available to the person having ordinary skill in the art.

ADVANTAGES OF THE PRESENT DISCLOSURE
[0071] The present disclosure provides a microfluidic device and a method that eliminates the effect of cross-coupling between electrodes.
[0072] The present disclosure provides a microfluidic device and a method that functions efficiently utilizing a single PMUT.
[0073] The present disclosure provides a microfluidic device that is portable and of minimal size.
[0074] The present disclosure provides an easy to fabricate microfluidic device for sensing fluid density.
[0075] The present disclosure provides a reliable and highly sensitive microfluidic device for real-time sensing of the density of fluids.
[0076] The present disclosure provides a microfluidic device, which is highly compact and consumes extremely low volume of fluids in various industrial and healthcare applications.

,CLAIMS:1. A microfluidic device (100) for sensing density of a fluid, the device (100) comprising:
an active layer (102) made of lead zirconate titanate (PZT) and having a thickness of first predefined value, configured over a substrate (104) made of platinized silicon-on-insulator having a device thickness of second predefined value; and
a driving electrode (106) and a sensing electrode (108) configured at least partially over the active layer (102) and separated from each other by an electrical ground line (110).
2. The device (100) as claimed in claim 1, wherein the active layer (102) has the thickness of 500 nm, and the substrate (104) has the device thickness of 10 µm.
3. The device (100) as claimed in claim 1, wherein:
the device (100) is circular in shape;
the driving electrode (106) is circular in shape and positioned centrally covering 70% of the upper area of the active layer (102); and
the sensing electrode (108) is annular in shape placed 20 µm away from the central driving electrode (106) towards fixed edge of the active layer (102), wherein the central driving electrode (106) and the annular sensing electrode (108) are separated by the electrical ground line (110), to eliminate the effect of cross-coupling.
4. The device (100) as claimed in claim 3, wherein the device (100) is adapted to be operated at one or more frequencies by varying any or a combination of diameter of the device, thickness of diaphragm stack, and interlayer stresses.
5. The device (100) as claimed in claim 4, wherein the thickness of the diaphragm stack is 10 µm and the interlayer residual stresses is 700 MPa.
6. The device (100) as claimed in claim 1, wherein an AC voltage of predefined magnitude is applied to the driving electrode (106) while sweeping the signal frequency over a desired range, which correspondingly vibrates diaphragm of the device (100), wherein a peak displacement of the diaphragm is recorded at its resonant frequency, which is indicative of the density of the fluid being tested.
7. A method (200) for fabricating a microfluidic device for fluid density sensing, the method (200) comprising the steps of:
spin coating (202) lead zirconate titanate (PZT) over a substrate made of platinized silicon-on-insulator wafer having a predefined device layer thickness;
annealing (204), the spin coated PZT at a predefined temperature, which results in formation of a PZT film as active layer over the substrate;
patterning (206) top electrode in a predefined pattern using lithography followed by wet etching of the PZT layer;
patterning (208) backside to etch backside oxide; and
removing (210) a handle layer, followed by etching buried oxide to form a stress-free microfluidic device for sensing density of a fluid.
8. The method (200) as claimed in claim 7, wherein the method (200) comprises spin coating the PZT over the substrate using sol gel technique, and the step of annealing takes place at 650°C.
9. The method (200) as claimed in claim 7, wherein the predefined device layer thickness is 10 µm, and the top electrode is made of a material selected from Cr and Au having thickness in the range of 30-120 mm.
10. The method (200) as claimed in claim 7, wherein the backside oxide is etched using reactive ion etching, and the handle layer is removed using deep reactive ion etching.