Session: 13-02-01: Design and Fabrication, Analysis, Processes, and Technology for Micro and Nano Devices and Systems

Paper Number: 143029

143029 - The Fabrication Process of Surface Acoustic Wave (Saw) Devices Using Maskless Photolithography With a Validation of Traditional Masked Photolithography for Microfluidic Applications 

Introduction: Piezoelectric Surface Acoustic Wave (SAW) devices, specifically Lithium Niobate (LiNbO₃) based ones, are extensively used in microfluidic applications for sensing, actuations, etc. This study describes the fabrication of a SAW device on LiNbO₃ substrate with an effective maskless photolithography process which removes the cost of mask making and provides flexibility to change layouts relatively quickly. The characterization results are validated through comparison with conventional masked-based photolithography. SAW-based devices fabricated as Interdigital Transducers (IDT) have been popularly used in microfluidic applications such as sensors, actuators, along with pumping and splitting droplets. The masked-based photolithography system is commonly used for fabricating IDTs on SAW devices both with the “lift-off” and “etching” technique. The process does not provide much freedom for changing the design once the mask is made. On a separate note, LiNbO₃ is very fragile and brittle in nature, which can lead to breakage and/or induce stress that ultimately impact piezoelectric properties when contact photolithography secures the mask on LiNbO₃ during UV exposure. To mitigate these limitations, a novel maskless photolithography process for such devices is introduced. In terms of cost, this maskless approach would be cheaper compared to creating a new mask each time a design change is needed. Finally, this process can be used for both the “lift-off” and “etching” techniques regardless of “Bright-field” or “Dark-field” masks once the design is created using CAD.

Methodology: The fabrication process starts with cleaning the LiNbO₃ wafer, and baking. Then, the LiNbO₃ undergoes the Hexamethyldisilazane (HMDS) process followed by spin-coating with photoresist (LOR 5A). Next, LiNbO₃ is baked on a hotplate, then spin coated with AZ1512 photoresist followed by a soft bake at 115ºC for 1 minute 30 seconds. After that, for maskless photolithography, the MicroLight3D Smart Print UV Maskless Lithography System is used. The system operates based on Digital Micromirror Device (DMD) with 1920 x 1080 mirrors. The machine is associated with an operating software called Phaos which converts the Drawing Exchange Format (dxf)/Graphic Data Stream (GDS) file into a stitch file to operate the exposure in a zigzag pattern instead of uniform exposure, like the masked photolithography. A visible image of Interdigital Transducers (IDTs) emerges through post-exposure baking aiding in their identification for dicing as the development of the whole wafer containing all devices does not occur simultaneously; therefore, before development, the device is diced and developed separately. Next, a Titanium adhesive (30-50 nm) and a gold layer (120-150 nm) are deposited using an e-beam evaporator. Lastly, a lift-off process involves immersion in AZ 400T stripper, followed by a rinse with DI water and successful creation of the device.

 

Results: For this experiment, four (4) devices were tested using a Vector Network Analyzer (VNA). These devices are: (a) Masked 60µm wavelength; (b) Maskless 60µm wavelength; (c) Masked 80µm wavelength; (d) Maskless 80µm wavelength. The result of device (a) at a resonant frequency of 64.5 MHz and S11 parameter of -15.6 dB was similar to device (b) which had a S11 parameter of -18.8 dB [higher than the device (a) at a resonant frequency of 63.6 MHz. Furthermore, the S11 parameter for the device (c) at a resonant frequency of 48.6 MHz is –17.6 dB while a higher S11 value of -32.6 dB was observed for device (d) at a resonant frequency of 47.3 MHz when compared to device (c).

 

Conclusion: Using a Maskless Photolithography technique to fabricate SAW devices on Lithium Niobate substrates has been introduced. This process can fabricate SAW devices which have negligible difference when compared to conventional masked lithography processing and can enable quicker design change flexibility and ease of fabrication

Presenting Author: Mohammad Salman Parvez The University of Texas at Dallas

Presenting Author Biography: Mohammad Salman Parvez is currently a PhD. candidate at the department of Electrical and Computer Engineering in the University of Texas at Dallas. He completed his MS in Electrical Engineering from the University of Texas at Rio Grande Valley on 2020. He completed his Bachelors in Mechanical Engineering from Bangladesh University of Engineering and Technology. He attended and published one Technical paper and on Poster Presentation at IMECE in the years 2019 and 2020 respectively.

Authors:

Mohammad Salman Parvez The University of Texas at Dallas
Shadhin Hussain The University of Texas at Dallas
Chadwin Young The University of Texas at Dallas
Jeong-Bong Lee Baylor University