Surface acoustic wave sensor working at constant temperature and preparation method thereof

Abstract

The invention belongs to the technical field of sensors, and particularly relates to a surface acoustic wave sensor working at constant temperature and a preparation method thereof. The sensor comprises a heat dissipation layer, a thermoelectric refrigeration layer and a sensing layer, the heat dissipation layer serves as a base of the whole surface acoustic wave sensor and has high heat conduction performance; the thermoelectric refrigeration layer adopts a semiconductor refrigerator, the upper surface of the thermoelectric refrigeration layer is connected with the heat dissipation layer through heat-conducting silicone grease or in a welding manner, and the lower surface of the thermoelectric refrigeration layer is connected with the sensing layer through heat-conducting silicone grease or in a welding manner; the sensing layer comprises a composite functional substrate, and an SAW sensing unit and a temperature sensing unit which are integrated on the composite functional substrate; the device is compact in structure, high in integration level, high in temperature control precision, fast in thermal response and uniform in temperature field, and has a good application prospect.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of sensors, and particularly relates to a constant-temperature working surface acoustic wave sensor and a preparation method thereof. BACKGROUND

[0002] The working principle of a surface acoustic wave (SAW) sensor is as follows: when the surface environment of a SAW device such as temperature, pressure, mass, etc. changes, the frequency of the SAW device drifts (for a resonant type sensor) or the transmission time changes (for a delay line type sensor), and the detection of a target object is realized by measuring the changes in frequency or time, thereby constituting various SAW sensors and sensor systems. The SAW sensor has the advantages of high sensitivity, small size and low price, but on the other hand, due to its high sensitivity, it has very poor anti-interference ability, especially poor temperature interference resistance. The temperature coefficient of a common lithium niobate or lithium tantalate substrate surface acoustic wave sensor is even-40~-80ppm / ℃. Therefore, temperature compensation is needed in use. There are mainly two kinds of existing temperature compensation schemes: 1. Algorithm compensation: the temperature drift is corrected through a software algorithm. This method cannot cope with rapid temperature fluctuations, and the compensation accuracy is limited, especially in extreme or transient temperature environments. 2. Constant temperature control: a heating and temperature control unit is added to the sensor, so that it always works at a set constant temperature point (usually higher than the maximum ambient temperature).

[0003] However, the common constant temperature SAW sensor structure has some defects:

[0004] Large thermal inertia: the integration of the traditional heater (such as thick film resistor) and the SAW chip is not high, the heat conduction path is long, the temperature rises slowly, and the temperature control response lags.

[0005] Large temperature gradient: the layout of the heater and the temperature sensor is unreasonable, resulting in uneven temperature in the sensing area, which affects the performance consistency of the SAW device.

[0006] Bulk structure: the separate installation of various functional units (SAW device, heater, temperature sensor) makes the whole sensor bulky, difficult to integrate and miniaturize.

[0007] In summary, there is an urgent need for a constant temperature SAW sensor with high integration, fast thermal response and uniform temperature field. SUMMARY

[0008] In view of the deficiencies of the prior art, the present application provides a constant-temperature working surface acoustic wave sensor and a preparation method thereof, the sensor comprising: a heat dissipation layer, a thermoelectric refrigeration layer and a sensing layer; the heat dissipation layer serves as the base of the entire surface acoustic wave sensor and has high heat conduction performance; the thermoelectric refrigeration layer adopts a semiconductor refrigerator, the upper surface of the semiconductor refrigerator is connected with the heat dissipation layer through heat-conducting silicone grease or welding, and the lower surface of the semiconductor refrigerator is connected with the sensing layer through heat-conducting silicone grease or welding; the sensing layer comprises a composite functional substrate and a SAW sensing unit and a temperature sensing unit integrated on the composite functional substrate.

[0009] Preferably, the bottom of the heat dissipation layer is provided with heat dissipation fins or heat dissipation channels, so as to enhance the heat dissipation effect.

[0010] Preferably, the SAW sensing unit is a delay line or a resonator structure composed of a double-end or multi-end interdigital transducer and a reflection grating.

[0011] Preferably, the temperature sensing unit is a SAW temperature sensor or a thin-film platinum resistance.

[0012] Preferably, the composite functional substrate comprises a quartz substrate and a uniform metal layer deposited on the lower surface of the quartz substrate.

[0013] Further, the temperature sensing unit and the SAW sensing unit are integrated on the upper surface of the same quartz substrate.

[0014] A preparation method of a constant-temperature working surface acoustic wave sensor, comprising:

[0015] S1: two SAW devices are made on the upper surface of a wafer, one serving as a SAW sensing unit and the other serving as a temperature sensing unit; a gold layer with a thickness of about 1-2 microns is deposited on the lower surface of the wafer by magnetron sputtering to form a metallized heat conduction layer, thereby obtaining a sensing layer chip;

[0016] S2: the heat dissipation layer with fins is fixed; high-performance heat-conducting silicone grease is coated on the heat dissipation layer, and the semiconductor refrigerator is attached to the heat dissipation layer and gently pressed to ensure good contact;

[0017] S3: heat-conducting silicone grease is coated on the upper surface of the semiconductor refrigerator, and the prepared sensing layer chip is attached to the center area of the semiconductor refrigerator with the metallized heat conduction layer facing downward;

[0018] S4: a gold wire bonder is used to connect the electrode pads of the SAW sensing unit and the temperature sensing unit with the pins of an external circuit board;

[0019] S5: the entire sensor structure is placed in a protective shell with a vent hole, and only the heat dissipation fins are exposed; the signal of the temperature sensing unit is connected to a PID controller, and the controller controls the current direction and size of the semiconductor refrigerator, so as to realize accurate closed-loop control of the temperature of the sensing layer.

[0020] The beneficial effects of this invention are as follows:

[0021] This invention integrates a SAW sensor and a temperature sensor onto the same quartz substrate and arranges them adjacent to each other. The temperature sensor measures the most accurate real-time temperature of the SAW device, avoiding measurement errors and control lag caused by the separation of the temperature measurement point and the sensing point, greatly improving the accuracy of temperature measurement and the response speed of the control system. This invention adopts a vertically stacked structure of "sensing layer-TEC-heat dissipation layer" to replace the traditional horizontal side-by-side layout, significantly reducing thermal inertia and enabling the sensor to heat up and cool down quickly. This invention designs a large-area metallized thermally conductive layer under the quartz substrate and ensures full and uniform contact between the upper surface of the TEC and the lower surface of the sensing layer, ensuring that heat can be evenly diffused throughout the quartz substrate, avoiding performance distortion of the SAW device caused by temperature gradients, and improving measurement consistency and accuracy. In addition, the structure of this invention is very compact, small in size, and has high mechanical strength, which facilitates subsequent packaging and integration into various detection systems. Attached Figure Description

[0022] Figure 1 This is a side view of the surface acoustic wave sensor in this invention;

[0023] Figure 2 This is a top view of the surface acoustic wave sensor in this invention;

[0024] Figure 3 This is a flowchart illustrating the operation of the surface acoustic wave sensor in this invention. Detailed Implementation

[0025] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. 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.

[0026] This invention proposes a surface acoustic wave sensor operating at a constant temperature and its fabrication method, such as... Figure 1 As shown, the sensor includes a heat dissipation layer, a thermoelectric cooling layer, and a sensing layer stacked from bottom to top;

[0027] The heat dissipation layer serves as the base of the entire surface acoustic wave sensor and possesses high thermal conductivity. In a preferred embodiment of the invention, the heat dissipation layer is made of a high thermal conductivity metal (such as aluminum or copper) or ceramic (such as aluminum nitride, AlN). The bottom of the heat dissipation layer is provided with heat dissipation fins or heat dissipation channels to enhance heat dissipation. Furthermore, the heat dissipation layer can dissipate the heat generated by the heat dissipation and cooling layer, maintaining the temperature difference between its hot and cold ends and ensuring cooling efficiency.

[0028] The thermoelectric cooling layer uses a thermoelectric cooler (TEC). Its upper surface is connected to the heat dissipation layer by thermal grease or soldering, and its lower surface is connected to the sensing layer by thermal grease or soldering. The TEC serves as the core actuator, which is used to heat or cool according to the instructions of the control system to achieve active temperature control of the sensing layer.

[0029] The sensing layer includes a composite functional substrate and SAW sensing units and temperature sensing units integrated thereon. For example... Figure 2 As shown, in a preferred embodiment of the present invention, the SAW sensing unit is a delay line or resonator structure consisting of a dual- or multi-terminal interdigital transducer and a reflective grating, fabricated on the surface of a quartz substrate using microelectronic processes. A preferred embodiment of the temperature sensing unit is an additional, simply designed SAW temperature sensor (such as a single-port resonator) whose frequency signal is temperature-sensitive and dedicated to real-time monitoring of the precise temperature of the SAW sensing unit region. Alternatively, a thin-film platinum resistance thermometer (Pt1000) can be used.

[0030] The composite functional substrate includes a quartz substrate and a uniform metal layer deposited on the lower surface of the quartz substrate as a metallized thermally conductive layer. By designing a large-area metallized thermally conductive layer under the quartz substrate and ensuring sufficient and uniform contact between the upper surface of the TEC and the lower surface of the sensing layer, heat can be uniformly diffused throughout the entire quartz substrate, forming a highly uniform temperature field in the SAW sensing area. This avoids performance distortion of the SAW device caused by temperature gradients and improves the consistency and accuracy of measurements.

[0031] The temperature sensing unit and the SAW sensing unit are integrated on the same quartz substrate surface; preferably, the quartz substrate can be an ST-cut quartz crystal (with a near-zero temperature coefficient).

[0032] This invention integrates a SAW sensor and a temperature sensor onto the same quartz substrate and arranges them adjacent to each other. The temperature sensor measures the most accurate real-time temperature of the SAW device, avoiding measurement errors and control lags caused by the separation of the temperature measurement point and the sensing point. This greatly improves the accuracy of temperature measurement and the response speed of the control system, laying a structural foundation for achieving high-precision constant temperature control (±0.1°C or even higher).

[0033] The three-layer structure designed in this invention is tightly bonded by a highly thermally conductive material to form an efficient thermal management path in the vertical direction. Specifically, by depositing a uniform metal layer (such as gold or silver) on the lower surface of the quartz substrate of the sensing layer (the surface in contact with the TEC), it can be ensured that the cold / heat generated by the TEC can be quickly and uniformly conducted to the entire quartz substrate, eliminating local hot spots or cold spots.

[0034] This invention employs a vertically stacked structure of "sensing layer-TEC-heat dissipation layer," replacing the traditional horizontal side-by-side layout. This design significantly shortens the heat conduction distance between the heat source (TEC) and the controlled object (SAW chip). It significantly reduces thermal inertia, enabling the sensor to heat up and cool down rapidly, greatly reducing the time required to reach the set constant temperature point (down to the second level), and effectively suppressing rapid fluctuations in ambient temperature.

[0035] like Figure 3 As shown, the working process of the isothermal surface acoustic wave sensor designed in this invention includes:

[0036] The temperature of the temperature sensing unit (SAW temperature sensor or thin-film platinum resistance thermometer) is read, and the temperature is observed to see if it changes within a certain period of time. If the temperature rises, the TEC is controlled to cool; if the temperature drops, the TEC is controlled to heat. The sensor temperature is read again after the temperature stabilizes. If the temperature does not change within a certain period of time, the SAW sensing unit outputs the measured value of the target object (it can detect various objects such as gas concentration, pressure, humidity, and torque, and outputs the corresponding measured value according to the specific sensor).

[0037] The present invention also proposes a preparation method for preparing the above-mentioned temperature-controlled surface acoustic wave sensor, comprising:

[0038] S1: Two SAW devices are fabricated on the upper surface of the wafer, one as a SAW sensing unit and the other as a temperature sensing unit; a gold layer of about 1-2 μm thickness is deposited on the lower surface of the wafer by magnetron sputtering to form a metallized thermal conductive layer, thus obtaining the sensing layer chip.

[0039] like Figure 2 As shown, an ST-cut quartz wafer was selected as the quartz substrate 4. Two SAW devices were fabricated on the upper surface of the wafer using microfabrication processes such as photolithography, deposition, and etching: one is a SAW sensing unit 3 for detecting target parameters (such as gas), and the other is a dedicated SAW temperature sensing unit 2 for temperature measurement. After completion, a gold layer approximately 1-2 μm thick was deposited on the lower surface of the wafer by magnetron sputtering to form a metallized thermally conductive layer.

[0040] S2: Fix the finned heat dissipation layer 7; apply high-performance thermal grease to the heat dissipation layer, attach the semiconductor cooler 5 to it, and press gently to ensure good contact;

[0041] The heat dissipation layer can be made of aluminum, and multiple heat dissipation channels can be set at the bottom of the heat dissipation layer.

[0042] S3: Coat the upper surface of the semiconductor cooler with thermal grease, and attach the prepared sensing layer chip with the metallized thermally conductive layer facing down to the central area of ​​the semiconductor cooler.

[0043] S4: Use a gold wire bonding machine to connect the electrode pads of the SAW sensing unit and the temperature sensing unit to the pins of the external circuit board.

[0044] External circuits include signal readout circuits, processing circuits, and signal transmission circuits. Figure 2 The spot welding line 1 in the circuit leads the signal from the sensing unit to the external circuit board.

[0045] S5: The entire sensor structure is placed inside a protective housing with ventilation holes, with only the heat dissipation fins exposed; the signal from the temperature sensing unit is connected to a PID controller, and the controller's output drives an H-bridge circuit to control the direction and magnitude of the TEC current, thereby achieving precise closed-loop control of the sensing layer temperature.

[0046] This invention integrates all functional units (SAW sensor, heater / cooler, temperature sensor) into a compact three-dimensional module using microfabrication and integrated packaging technologies. The structure is extremely compact, small in size, and has high mechanical strength, facilitating subsequent packaging and integration into various detection systems, and thus showing promising application prospects.

[0047] The above-described embodiments further illustrate the purpose, technical solution, and advantages of the present invention. It should be understood that the above-described embodiments are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made to the present invention within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A surface acoustic wave sensor operating at a constant temperature, characterized in that, include: Heat dissipation layer, thermoelectric cooling layer, and sensing layer; The heat dissipation layer serves as the base of the entire surface acoustic wave sensor and has high thermal conductivity. The thermoelectric cooling layer uses a semiconductor cooler, and its upper surface is connected to the heat dissipation layer by thermal grease or soldering, while its lower surface is connected to the sensing layer by thermal grease or soldering. The sensing layer includes a composite functional substrate and the SAW sensing unit and temperature sensing unit integrated on it.

2. The surface acoustic wave sensor operating at a constant temperature according to claim 1, characterized in that, The bottom of the heat dissipation layer is provided with heat dissipation fins or heat dissipation channels, thereby enhancing the heat dissipation effect.

3. A surface acoustic wave sensor operating at a constant temperature according to claim 1, characterized in that, The SAW sensing unit is a delay line or resonator structure consisting of a dual- or multi-terminal interdigital transducer and a reflective grating.

4. A surface acoustic wave sensor operating at a constant temperature according to claim 1, characterized in that, The temperature sensing unit is a SAW temperature sensor or a thin-film platinum resistance thermometer.

5. A surface acoustic wave sensor operating at a constant temperature according to claim 1, characterized in that, The composite functional substrate includes a quartz substrate and a uniform metal layer deposited on the lower surface of the quartz substrate.

6. A surface acoustic wave sensor operating at a constant temperature according to claim 5, characterized in that, The temperature sensing unit and the SAW sensing unit are integrated on the same quartz substrate surface.

7. A method for preparing a surface acoustic wave sensor as described in any one of claims 1 to 6, characterized in that, include: S1: Fabricate two SAW devices on the upper surface of the wafer, one as a SAW sensing unit and the other as a temperature sensing unit. A gold layer of about 1-2 μm thickness is deposited on the lower surface of the wafer by magnetron sputtering to form a metallized thermal conductive layer, thus obtaining the sensing layer chip. S2: Secure the finned heat dissipation layer; apply high-performance thermal grease to the heat dissipation layer, attach the thermoelectric cooler to it, and press gently to ensure good contact; S3: Coat the upper surface of the semiconductor cooler with thermal grease, and attach the prepared sensing layer chip with the metallized thermally conductive layer facing down to the central area of ​​the semiconductor cooler. S4: Use a gold wire bonding machine to connect the electrode pads of the SAW sensing unit and the temperature sensing unit to the pins of the external circuit board. S5: The entire sensor structure is placed inside a protective housing with ventilation holes, with only the heat dissipation fins exposed; the signal from the temperature sensing unit is connected to a PID controller, which controls the direction and magnitude of the current in the semiconductor cooler, thereby achieving precise closed-loop control of the sensing layer temperature.

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