Piezoelectric air cooling device based on piezoelectric film PZT technology

Abstract

The invention discloses a piezoelectric air cooling device based on a piezoelectric film PZT technology, and relates to the technical field of MEMS. The device adopts a two-layer packaging structure of a piezoelectric cantilever beam MEMS chip and a PCB (Printed Circuit Board) with a vent hole. A cantilever beam of the MEMS chip is integrated with an SOI top silicon layer and an ultrathin PZT driving stack, low rigidity and high driving force are achieved, and the surface of the cantilever beam is covered with a PDMS flexible sealing layer to achieve dynamic sealing. When the device works, the cantilever beam is driven to resonate in an ultrasonic frequency band, and efficient synthetic jet is generated at the vent hole by periodically changing the volume of the pump cavity. The technical bottleneck that miniaturization and high performance are difficult to consider at the same time is broken through, and the device has the advantages of being compact in structure, mute and efficient and is suitable for precise heat dissipation of miniature electronic equipment.

Description

Technical Field

[0001] This invention relates to the fields of microelectromechanical systems (MEMS) and piezoelectric drive technology, specifically a piezoelectric air-cooled device based on piezoelectric thin film (PZT) technology. Background Technology

[0002] Piezoelectric fans, as brushless, solid-state fluid drive devices, utilize the inverse piezoelectric effect of piezoelectric materials to convert input alternating electrical signals into mechanical vibrations, driving a diaphragm to disturb the air and generate directional airflow. Compared with traditional electromagnetic fans, they have advantages such as simple structure, low noise, high reliability, and long lifespan.

[0003] However, current mainstream piezoelectric fans face serious challenges in miniaturization and performance: Bulk PZT ceramic materials: Although they have a high piezoelectric coefficient (d~33~), their preparation depends on ceramic sintering processes, making it difficult to achieve a thickness of less than 10 micrometers. Excessive material thickness leads to excessive stiffness in the drive structure, resulting in small vibration displacement, which limits heat dissipation airflow and miniaturization levels, making it difficult to adapt to millimeter-scale heat sources.

[0004] Aluminum nitride (AlN) thin film materials: Although they can be prepared using semiconductor processes (such as sputtering) and are easy to miniaturize, their piezoelectric coefficients (d33, d31) are much lower than those of PZT. Even with doping modification, their driving capability is still insufficient and cannot effectively drive the diaphragm to generate a sufficiently large displacement to form a cooling airflow with practical value.

[0005] Complex structure: Existing synthetic jet devices typically have complex airflow channels and outlets, often requiring separate nozzle components or multiple precision components for assembly. This increases manufacturing costs, assembly difficulty, and potential failure points, reducing the overall reliability of the device.

[0006] Therefore, the industry urgently needs a piezoelectric fan solution that can balance extreme miniaturization, large drive displacement, high operating frequency, and high reliability. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention provides a piezoelectric air-cooling device based on piezoelectric thin film (PZT) technology, solving the problem of the inability to simultaneously achieve miniaturization and driving performance in existing technologies. The invention also aims to simplify the device structure by directly integrating the airflow outlet onto the PCB board, eliminating complex independent nozzle components, thereby reducing manufacturing costs, improving packaging reliability, and increasing production yield. Ultimately, this results in a miniature cooling fan that can be used for extremely small heat sources, has a large airflow, low noise, and high reliability.

[0008] To achieve the above objectives, the present invention is implemented through the following technical solution: a piezoelectric air-cooled device based on piezoelectric thin film PZT technology, the core of which is a simple two-level structure consisting of a piezoelectric cantilever beam MEMS chip and a printed circuit board (PCB) with ventilation holes.

[0009] Piezoelectric cantilever beam MEMS chip: The MEMS chip is fabricated on a silicon-on-insulator (SOI) wafer using standard semiconductor MEMS technology. The SOI substrate consists of a substrate silicon, a buried oxide insulating layer, and a top silicon layer from bottom to top.

[0010] Fabrication of piezoelectric drive unit: On the top silicon surface of SOI wafer, a bottom electrode, a PZT thin film and a top electrode are sequentially deposited and patterned by photolithography to form a sandwich structure piezoelectric drive unit. The PZT thin film is grown by magnetron sputtering and its thickness is precisely controlled to 2 micrometers or thinner (e.g. 0.5μm~2μm). The ultrathin structure combines the piezoelectric properties of PZT material (providing large driving force / torque) with the low mechanical stiffness of the thin film morphology (allowing large displacement deformation).

[0011] Structure release: After the PZT layer and electrode layer are precisely patterned and defined by dry or wet etching processes, silicon is etched from the back and / or front using processes such as deep reactive ion etching (DRIE) to release the movable structure, ultimately forming a device consisting of a rigid substrate silicon frame and a micron-thick cantilever beam on it. This cantilever beam is a stacked composite structure of the top silicon layer and the piezoelectric drive stack above it (bottom electrode, PZT film, top electrode).

[0012] Dynamic sealing: A flexible superelastic sealing material is coated on the entire surface of the formed MEMS chip (including the cantilever beam vibration part and the fixed frame). The flexible superelastic sealing material is any one of polydimethylsiloxane (PDMS), platinum catalytic silicone rubber (Ecoflex), parylene-C, or other materials with superelastic properties, preferably polydimethylsiloxane (PDMS). The thickness of this coating is optimized to completely cover and seal the active gap between the root of the cantilever beam and the frame, but its superelastic properties ensure that it does not restrict the free vibration of the cantilever beam, effectively preventing airflow leakage from this dynamic gap during operation and improving the efficiency of the synthetic jet.

[0013] The flexible superelastic sealing material has a Young's modulus of less than 10 MPa and an elongation at break of more than 100%. The thickness of its sealing layer is 1 micrometer to 20 micrometers. While ensuring dynamic sealing, it adds almost no stiffness, thus avoiding significant attenuation of the resonant frequency and amplitude of the cantilever beam.

[0014] PCB board and integration: The PCB board serves as the mechanical support base and electrical connection platform for the device. At the same time, one or more vent holes are opened in the area corresponding to the inside of the MEMS chip frame. These vent holes serve as the only inlet and outlet for the synthetic jet.

[0015] Final assembly: The MEMS chip is directly aligned and fixed to the PCB board from the back of its substrate silicon bezel using an adhesive (such as conductive glue or non-conductive epoxy resin). The bezel of the MEMS chip, the PCB board, and the lower surface of the vibrating cantilever beam together form a sealed pump chamber. This back-to-back bonding method forms a minimalist two-layer packaging structure.

[0016] Working principle: Applying an alternating drive electrical signal of a specific frequency (preferably designed in the ultrasonic band, such as above 20kHz) to the upper and lower electrodes of the MEMS chip causes the ultrathin PZT film to reciprocate and expand under the action of the alternating electric field, driving the entire cantilever beam to produce large-amplitude up-and-down bending vibrations.

[0017] As the cantilever beam moves upward (away from the PCB), the pump chamber volume increases, the internal pressure decreases, creating a negative pressure, and external air is drawn into the chamber through the PCB vent.

[0018] When the cantilever beam moves downward (towards / impacts the PCB), the pump chamber volume decreases sharply, the internal pressure increases, creating positive pressure, and the air in the chamber is compressed and ejected at high speed through the same vent.

[0019] This periodic inhalation and jetting motion generates a pulsed, high-momentum synthetic jet at the vent, which directly impacts the target heat source, achieving efficient forced convection cooling.

[0020] This invention provides a piezoelectric air-cooled device based on piezoelectric thin film (PZT) technology. Compared with existing technologies, it has the following advantages: (1) The piezoelectric air-cooling device based on piezoelectric thin film PZT technology uses an ultra-thin (≤2μm) PZT thin film prepared by magnetron sputtering as the core driving material. It perfectly integrates the high voltage performance of PZT material itself (providing large driving force) and the low mechanical stiffness of the thin film morphology (allowing large displacement). It breaks through the technical barrier that miniaturization and high performance cannot coexist from the source, and realizes that effective heat dissipation air volume can still be generated at the millimeter or even sub-millimeter scale.

[0021] (2) The piezoelectric air-cooled device based on piezoelectric thin film PZT technology proposes a scheme to coat the surface of the MEMS movable structure with a flexible superelastic sealing material (such as PDMS), which realizes the effective sealing of the dynamic gap between the moving part and the fixed part, significantly reduces gas leakage during operation, greatly improves the jetting efficiency and heat dissipation performance of the synthetic jet, and due to the superelasticity of the material, it does not affect the vibration performance of the cantilever beam.

[0022] (3) The piezoelectric air-cooled device based on piezoelectric thin film PZT technology abandons the independent and complex airflow channels and nozzle components in the traditional design, integrates the function of the airflow channel on the PCB board, uses simple through holes as jet inlets, and achieves a minimal two-layer packaging structure by back-to-back bonding with MEMS chips. This greatly simplifies the assembly process, reduces manufacturing costs, and reduces the tolerance and failure risks caused by the assembly of multiple components, significantly improving the overall reliability, robustness and production yield of the device.

[0023] (4) The piezoelectric air-cooled device based on PZT thin film technology utilizes the characteristic that the thickness of the top silicon layer in the SOI substrate can be precisely controlled to design and adjust the stiffness and mass of the cantilever beam, thereby accurately setting its resonant frequency in the ultrasonic range (>20kHz). When operating in this frequency band, the human ear cannot perceive its operating noise, achieving true "silent" heat dissipation, which is particularly suitable for application scenarios such as wearable devices that are sensitive to noise. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the piezoelectric cantilever beam MEMS chip of the present invention; Figure 2 This is a schematic diagram of the structure of the present invention; Figure 3 This is a schematic diagram of the structure of the present invention.

[0025] Figure 4 This is a schematic diagram of the deformation of the MEMS device with the structure of the present invention under voltage drive; Figure 5 This is a schematic diagram of the second deformation of the MEMS device structure of the present invention under voltage drive; Figure 6 This is a schematic diagram illustrating the resonant frequency adjustment of the MEMS device parameters of the present invention. Figure 7 This is a schematic diagram of the stack of thin-film PZT piezoelectric materials (bottom electrode, PZT, top electrode) deposited on an SOI wafer according to the present invention. Figure 8 This is a schematic diagram of the photolithography and etching of the top electrode of the structure of the present invention; Figure 9 This is a schematic diagram of the photolithography and etching of the PZT thin film structure of the present invention; Figure 10 This is a schematic diagram of the photolithography and etching of the bottom electrode of the structure of the present invention; Figure 11 This is a schematic diagram of the photolithography and etching of the top silicon and oxide layer of the present invention. Figure 12 This is a schematic diagram of the electrode deposition structure of the present invention; Figure 13This is a schematic diagram of substrate thinning for the structure of this invention; Figure 14 This is a schematic diagram of silicon etching on the substrate of the structure of the present invention; Figure 15 This is a schematic diagram of the spin coating of the superelastic material structure of the present invention.

[0026] In the figure: 1. Piezoelectric cantilever beam MEMS chip; 2. Printed circuit board. Detailed Implementation

[0027] The technical solutions in the embodiments of the present invention have been clearly and completely described. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0028] Please see Figures 1 to 15 The present invention provides a technical solution: a piezoelectric air-cooled device based on piezoelectric thin film PZT technology, including a piezoelectric cantilever beam MEMS chip and a printed circuit board.

[0029] Example 1: A miniature piezoelectric fan for the optomechanical module of a smartwatch The fabrication of the piezoelectric cantilever beam MEMS chip 1 begins with the provision of a silicon-on-insulator (SOI) wafer as a substrate. The SOI wafer consists of a 300 μm thick substrate silicon layer, a 1 μm thick buried oxide layer, and a 5 μm thick top silicon layer from bottom to top.

[0030] On the top silicon layer, a piezoelectric drive stack is fabricated using semiconductor technology: A magnetron sputtering process in physical vapor deposition (PVD) was used to sequentially deposit a 150 nm thick platinum (Pt) film as the bottom electrode, a 1.5 μm thick lead zirconate titanate (PZT) film, and a 100 nm thick platinum (Pt) film as the top electrode. By controlling the sputtering power, atmosphere, and time, the thickness and composition of the PZT film were precisely controlled to ensure its piezoelectric properties.

[0031] Subsequently, the top electrode, PZT thin film and bottom electrode are patterned sequentially through photolithography and ion etching processes to form a piezoelectric drive unit with a predetermined pattern.

[0032] After completing the graphical representation of the drive stack, micromachining is performed to release the movable structure: Using photolithography and deep reactive ion etching (DRIE) technology, the top silicon layer and buried oxide layer of a specified area are etched away from the front side of the wafer to define the planar shape of the cantilever beam and the fixed boundary.

[0033] Then, the wafer is flipped, and DRIE is performed from the back side, etching the substrate silicon until the buried oxide layer stops, thereby releasing the composite cantilever beam structure composed of top silicon and piezoelectric stacks, and forming a rigid frame composed of the remaining substrate silicon. The final cantilever beam dimensions are: length 1.5 mm, width 1.0 mm.

[0034] To achieve dynamic sealing, a layer of polydimethylsiloxane (PDMS) prepolymer with appropriate viscosity was coated onto the entire surface of the MEMS chip (including the cantilever beam vibration section and the fixed frame) using a spin coating process. By controlling the spin coating speed, the thickness of the cured PDMS film was made to be approximately 10 μm, and then cured in an oven at 80°C for 2 hours. This PDMS layer can completely cover and seal the movable gap between the root of the cantilever beam and the frame, and due to its superelasticity, it does not significantly suppress the vibration of the cantilever beam.

[0035] Printed Circuit Board 2 Fabrication and Final Assembly A 0.6 mm thick FR-4 printed circuit board (PCB) was fabricated. A circular vent hole with a diameter of 0.8 mm was machined into the PCB in the area corresponding to the inside of the MEMS chip frame and directly below the cantilever beam.

[0036] During assembly, conductive silver paste with appropriate viscosity and bonding strength is used as the adhesive. The back of the substrate silicon frame of the MEMS chip is coated with silver paste, and then precisely aligned and placed on the PCB board, ensuring that the cantilever beam completely covers the vent holes on the PCB board and that the frame is in close contact with the PCB board. The silver paste is cured under specific pressure and temperature, completing the back-to-back bonding of the MEMS chip and the PCB board. This bonding process allows the frame of the MEMS chip, the PCB board, and the lower surface of the vibrating cantilever beam to collectively form a "pump chamber." Simultaneously, electrical connections to the MEMS chip electrodes are achieved through the silver paste and wires on the PCB.

[0037] Working principle and performance: The working principle of this invention is based on the synthetic jet effect: An alternating voltage signal (e.g., a sine wave with a peak voltage of 10V and a frequency of 25kHz) is applied to the upper and lower electrodes of the MEMS chip through an external driving circuit (not shown).

[0038] Under the influence of an alternating electric field, the ultrathin PZT film undergoes periodic expansion and contraction deformation due to its inverse piezoelectric effect. Since the PZT film is firmly bonded to the top silicon layer through the bottom electrode, its expansion and contraction deformation drives the entire cantilever beam structure to produce large-amplitude up-and-down bending vibrations.

[0039] Intake phase: When the cantilever beam bends and deforms upward (away from the PCB board) under the action of the drive signal, the volume of the lower pump chamber increases and the internal pressure decreases (forming a negative pressure). At this time, external air is drawn into the pump chamber through the vent on the PCB board under the action of pressure difference.

[0040] Injection Phase: When the cantilever beam bends and deforms downwards (moving towards the PCB board, and its free end may even approach or lightly touch the PCB surface under the PDMS coating), the pump chamber volume decreases sharply, and the internal pressure increases (creating positive pressure). At this time, the compressed air in the chamber is ejected at high speed through the same vent, forming a pulsed airflow.

[0041] The above-mentioned intake and ejection processes are repeated periodically with the drive signal, eventually synthesizing a continuous high-momentum, directional jet at the vent outlet. By directing this jet toward the heat source of the optomechanical module in the smartwatch, forced convection cooling of the heat source can be achieved.

[0042] By precisely designing the SOI top silicon thickness (5μm), cantilever beam geometry (1.5mm x 1.0mm), and piezoelectric stack structure, this embodiment sets the first-order resonant frequency of the cantilever beam at approximately 25kHz, which is within the ultrasonic frequency band insensitive to the human ear, thus achieving silent heat dissipation. The device can generate an average jet velocity of over 5m / s under 10Vpp drive, and when applied to a 1W heat source, it can reduce the temperature by more than 15°C.

[0043] According to the laser Doppler vibration meter, the free end of the cantilever beam in this embodiment can vibrate at a frequency of 25kHz and a driving force of 10Vpp, with a displacement amplitude of ±60μm, which fully demonstrates the advantages of ultrathin PZT film in large displacement driving.

[0044] Example 2: A miniature piezoelectric fan for AR glasses MEMS chip fabrication: SOI wafers with a top silicon thickness of 3 μm were used. The PZT film thickness was reduced to 1.0 μm, and a cantilever beam with dimensions of 0.8 mm x 0.6 mm was formed by photolithography and etching processes. The PDMS sealing layer coating process was the same as in Example 1.

[0045] PCB and Assembly: A 0.4mm thick PCB is used, and the diameter of the vent holes is reduced to 0.4mm. Low-stress epoxy resin is used for bonding and assembly.

[0046] The operating principle is the same as in Example 1. Due to the reduced structural size, the resonant frequency of the cantilever beam is increased to approximately 35kHz by adjusting the design parameters, further ensuring a quiet operation. Despite its smaller size, thanks to the efficient actuation of the ultra-thin PZT film, the device can still generate sufficient airflow for localized heat dissipation of the internal chip of the AR glasses.

[0047] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0048] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A piezoelectric air-cooled device based on piezoelectric thin film (PZT) technology, characterized in that: include: A piezoelectric cantilever beam MEMS chip, fabricated on an SOI substrate, comprises a rigid frame made of substrate silicon and at least one cantilever beam with a thickness of micrometers. The cantilever beam is a composite structure of a top silicon layer and a piezoelectric drive stack. The piezoelectric drive stack includes a bottom electrode, a PZT film and a top electrode arranged sequentially from bottom to top. The thickness of the PZT film is less than or equal to 2 micrometers. The entire surface of the piezoelectric cantilever beam MEMS chip is covered with a layer of flexible superelastic sealing material to seal the movable gap between the root of the cantilever beam and the frame. A printed circuit board having one or more vent holes of any kind; The piezoelectric cantilever MEMS chip is fixedly bonded to the printed circuit board via the back of its substrate silicon frame, so that the cantilever beam, the frame and the printed circuit board together form a sealed pump chamber, and the cantilever beam is directly opposite the vent hole on the printed circuit board.

2. The piezoelectric air-cooled device based on piezoelectric thin film PZT technology according to claim 1, characterized in that: The PZT thin film was prepared using a magnetron sputtering process.

3. The piezoelectric air-cooled device based on piezoelectric thin film PZT technology according to claim 1, characterized in that: The flexible, superelastic sealing material is any one of polydimethylsiloxane (PDMS), platinum-catalyzed silicone rubber (Ecoflex), parylene-C, or other materials with superelastic properties.

4. A piezoelectric air-cooled device based on piezoelectric thin film PZT technology according to claim 1, characterized in that: The resonant frequency of the cantilever beam is designed to be in the ultrasonic band, i.e., greater than 20kHz.

5. A piezoelectric air-cooled device based on piezoelectric thin film PZT technology according to claim 1, characterized in that: The resonant frequency of the cantilever beam is set by precisely controlling the thickness of the top silicon layer in the SOI substrate, the geometry of the cantilever beam, and the parameters of the piezoelectric drive stack.

6. The piezoelectric air-cooled device based on piezoelectric thin film PZT technology according to claim 1, characterized in that: The vent holes on the printed circuit board are located in the area corresponding to the inside of the MEMS chip bezel.

7. A piezoelectric air-cooled device based on piezoelectric thin film PZT technology according to claim 1, characterized in that: The piezoelectric cantilever beam MEMS chip and the printed circuit board are directly bonded back-to-back using an adhesive.

8. A piezoelectric air-cooled device based on piezoelectric thin film PZT technology according to claim 1, characterized in that: The bottom electrode and top electrode are made of platinum (Pt), iridium (Ir) or their oxides, and are composite metal layers consisting of an adhesion layer and a conductive layer. The PZT thin film has a perovskite structure with preferred crystal orientation.

9. An electronic device comprising a piezoelectric air-cooling device based on piezoelectric thin film (PZT) technology as described in any one of claims 1 to 8, for dissipating heat from heat-generating elements within the electronic device.

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