I. Core Research Summary

• Precise aerosol fabrication of 37 μm high-precision SAW interdigital transducers

By simulating the Reynolds number, the optimal printing working distance was determined to be 4 mm, and the sheath‑gas/carrier‑gas ratio was optimized. The resulting electrode exhibits an average line width of 32–37 μm, a metallization ratio of 0.465, and a thickness of approximately 5.3 μm, with smooth edges free of overspray or wire‑break defects. At a low sintering temperature of 200 °C, the silver film undergoes grain growth, leading to a 22.2% improvement in film densification and a 34% reduction in resistivity compared to bulk silver.

• Dual-mode high-precision temperature sensing characteristics

The device has a reference resonant frequency of 20.75 MHz, with a sensitivity of −96.9 × 10⁻⁶/°C in the pulse‑echo (S11) mode and −92.0 × 10⁻⁶/°C in the pulse‑receive (S21) mode. Over the full temperature range, the linear fit achieves an R² as high as 0.999, and the temperature measurement resolution is as low as 1.66 × 10⁻⁷ °C, with stable performance and no resonant drift between 25 and 200 °C.

• 6-hour long-term printing process is stable and controllable.

In the initial stage, the equipment exhibits a small amount of overspray; after 2.5 hours of continuous operation, the uniformity of the mist beam improves significantly, and over the full 6-hour run, the line width shows minimal variation. Ultrasonic treatment of the ink continuously breaks up agglomerated particles, leading to a gradual reduction in particle size during printing, thereby validating long-term production‑level consistency.

• Maskless Digital Rapid Iterative Design

CAD allows direct modification of interdigital spacing and electrode count without the need to re‑fabricate the photolithography mask, enabling SAW device fabrication in a single batch within just a few hours and facilitating rapid optimization of resonant frequency and temperature‑sensing sensitivity parameters.

• Damage-free, gentle processing of piezoelectric substrates

The entire process employs low-temperature forming at 70°C on the substrate, followed by only a brief post‑processing sintering at 200°C, thereby preserving the piezoelectric effect of the lithium niobate crystal and preventing substrate cracking or polarization degradation.

II. Complete SAW Fabrication Process and Core Advantages for Aerosol Jet Printing

(1) Complete SAW Thermometer Fabrication Process

• Y‑cut LiNbO₃ piezoelectric wafer cleaning, without the need for substrate surface modification;
• Commercial silver ink is diluted and formulated with deionized water, then ultrasonically dispersed to obtain a low-viscosity atomization ink suitable for use.
• Optomec AJ ultrasonic atomization module, with a carrier gas flow of 35 sccm and a sheath gas flow of 55 sccm, and a constant nozzle-to-substrate distance of 4 mm.
• At low speed, 0.25–0.4 mm/s, print the interdigitated active region; at high speed, 0.8 mm/s, deposit the solder pads.
• Sintering in a 200°C oven for 1 hour promotes silver particle grain growth and yields a dense, conductive film.
• The probe station, paired with a vector network analyzer, is used to acquire S11 and S21 RF curves for characterizing the temperature-sensing performance.

(II) Five Core Processes (Comparison of Lithography and Inkjet)

• The subtractive process is perfectly replaced, with a green, non-corrosive waste liquid.

By eliminating the conventional microfabrication workflow involving sputtering, photolithography, and chemical etching, and avoiding the use of strong acids and bases, environmental‑friendly processing costs are significantly reduced, enabling small and medium‑sized laboratories to develop SAW devices without requiring a cleanroom production line.

• Micron-level high-precision molding, with edge morphology far superior to inkjet.

Aerosol-focused microbeams suppress spattering, while the electrode edges are smooth and exhibit uniform line width; in contrast, inkjet-printed lines suffer from spreading and burrs, severely degrading the SAW resonator’s quality factor.

• Wide safety spray distance compatible with irregular-shaped piezoelectric substrates.

A stable working distance of 4 mm ensures that even slight surface undulations on the wafer do not compromise print quality, with the capability to extend to conformal printing of curved‑surface piezoelectric components in the future.

• Redesign and zero‑tooling costs, with R&D cycles significantly shortened.

Changing the interdigitated electrode layout requires only revising the design files; lithography for each mask set takes 7–15 days, and aerosol‑based prototyping of the new structure is completed on the same day.

• Low-temperature flexibility and compatibility, universally applicable to all types of piezoelectric materials.

During the printing process, the substrate temperature is only 70°C, making it compatible with the entire range of piezoelectric substrates—including lithium niobate, lithium tantalate, and aluminum nitride—without compromising the crystal’s polarization properties.

III. Five Core Technological Innovations of This Study

Innovation 1: A SAW-based aerosol standardization process window optimized through Reynolds-number simulations.

Innovative Scheme for Long-Term Stable Atomization of Ink

Innovative SAW device with three dual‑RF‑channel cooperative temperature‑sensing capabilities

Innovative Low-Temperature Sintering Scheme for Densifying Aerosol-Printed Silver Films

Innovative New Paradigm for Low-Cost, Passive MEMS Sensor Additive Manufacturing

IV. Existing Process Challenges and Corresponding Solutions for Overcoming Them

Challenge 1: Traditional lithography processes are complex, highly polluting, and involve long prototyping cycles.

Challenge 2: The edges of inkjet-printed electrodes are rough, resulting in significant SAW resonator loss and poor linearity.

Challenge 3: Aerosol-based inks tend to agglomerate during prolonged printing, causing the printed lines to gradually thicken.

Challenge 4: The nozzle-to-substrate gap is highly sensitive, and uneven wafers are prone to wire breakage.

Challenge 5: High-temperature treatment can degrade the polarization properties of piezoelectric crystals.

V. Future Diversified Industrial Application Scenarios

1 High-Temperature Industrial Passive Temperature-Sensing Node

2 Onboard Wireless Temperature Measurement for Aircraft Engines

3. Internal Condition Monitoring of Nuclear Power Equipment

4 Acoustic Devices for Microfluidic Chips

5. University-level MEMS Teaching and Research Platform

Figure 1 shows a schematic of aerosol jet printing for SAW devices. In this process, ultrasonically atomized nanoparticle ink droplets are delivered from the ink reservoir to the print head, where they are focused into an ordered stream and deposited onto the surface of a piezoelectric substrate. The operating principle of a SAW device is to convert an applied voltage into an acoustic wave, which propagates along the substrate surface at a material‑specific speed, VSAW; upon reaching the receiver, the acoustic wave is converted back into a readable electrical signal.

Figure 2. Rheological properties of silver nanoparticle ink for AJP: a) Dynamic light scattering spectrum of silver nanoparticles suspended in water (used for hydrodynamic particle size determination); b) Drop‑test images (used to measure the ink’s surface tension); c) Contact angle receding curve of the silver nanoparticle ink on a lithium niobate surface, with the inset showing the Young’s contact angle measured 10 seconds after deposition.

Table 1: Summary of AJP Parameters

Figure 3 shows the instrument drift and print‑quality assessment during a 6‑hour AJP experiment: (a) the time‑dependent curves of carrier‑gas and sheath‑gas feedback pressures; (b) a 50× micrograph of a printed line, with a measured line width of approximately 35 μm; and (c–e), 5× micrographs illustrating, at (c) 1 hour, (d) 3 hours, and (e) 5 hours, the improved print‑line quality and reduced spattering.

Figure 4. Structure and electrical properties of printed silver devices. a X-ray diffraction patterns of silver nanoparticle ink deposited on a glass substrate, showing the spectrum before and after sintering at 200°C; crystallographic planes of the silver thin film are labeled above the corresponding diffraction peaks. b Stitched micrographs with dimension annotations of a printed four‑point device used for electrical characterization. c Tip‑profile measurement of the printed four‑point structure. d Average line profiles of the effective conductive paths in the four‑point device. e Resistance–temperature dependence of the printed silver four‑point structure.

Figure 5. Schematic diagram of the design and structure of a printed surface acoustic wave (SAW) device. a shows a stitched micrograph of the printed SAW device, with dimensions labeled; the inset clearly illustrates the effective electrode width and pitch. b and c are tip‑profile measurements of the printed SAW device. d presents the average height and width data of the finger‑like electrode structures obtained via three‑dimensional scanning.

Figure 6. Characteristics and temperature response of AJ‑printed surface acoustic wave devices: (a) S11 and (d) S21 measured before and after time‑gating at room temperature; (b) S11 and (e) S21 measured at room temperature and 200 °C after time‑gating; and the thermal drift of the fundamental frequency f(T), obtained from (c) S11 and (f) S21 after gating, as the ambient temperature is ramped up from 25 °C in 5 °C increments to 200 °C in 5 °C increments.

Table 2 Performance of the AJ-Printed SAW Thermometer