Design and Fabrication of an Ultrasonic Waveguide for Microchip Cooling Applications

Abstract

1. Introduction Ultrasound is generated by a piezoelectric actuator and a waveguide, which can amplify the sub-micro displacement from electrical energy. It has been widely used in semiconductor cleaning, and one of challenging application area of the ultrasound is semiconductor chip including central processing unit (CPU) cooling, which can be performed by wave energy [1-3]. Previous researches were investigated, and main focuses of those studies were about exploring the methods of increasing passive cooling effect by enhancing the convective heat transfer coefficient. But in this research, an ultrasonic waveguide for microchip cooling that is operated by 40 kHz operating frequency, was designed and fabricated for active cooling. To develop the waveguide, finite element analysis using ANSYS software was processed. First, anti-resonance frequency for a piezoelectric actuator was predicted, the result was compared with the experimental result. Then the waveguide was fabricated, and impedance characteristics were measured and compared. Next, predicted anti-resonance frequency for the ultrasonic waveguide was compared with the experimental value. Finally, the application area of the developed ultrasonic waveguide to microchip cooling is discussed. 2. Design and fabrication of the ultrasonic waveguide For the generation of the ultrasound, the 40 kHz ultrasonic waveguide for microchip cooling is mainly composed of two parts, the ultrasonic unit and the electric generator. The fabricated ultrasonic waveguide system is shown in Fig. 1. The waveguide has a cylindrically shaped aluminium (Al) waveguide with a lead zirconate titanate (PZT) actuator attached on the top. In the waveguide, there are a resonator and stacks that can refrigerate. For the actuation, power is supplied to the PZT actuator, and the actuator vibrates. Subsequently, the displacement is transferred through the waveguide to the resonator part for cooling. To design the ultrasonic waveguide, FEM analysis using Ansys (Commercial FEM software) was performed. Firstly, the PZT actuator was modeled with the analysis tool. For the analysis, it was modelled axis-symmetrically and the nodes of the top and bottom electrodes were coupled to apply voltages. To obtain a solution, series of calculations were performed from 30.0 kHz through 40.0 kHz. As a result, the highest impedance value was 34.8 kHz, which was decided as a design frequency. Secondly, the waveguide with the PZT actuator was modelled. After analysis, the highest impedance value was 39.4 kHz as shown in Fig. 2. For the prediction of the displacement, analysis was performed using the same analysis model. The PZT actuator and the waveguide were modeled. And modal analysis was performed and structural motion could be predicted. As a result, displacements at the operating frequency of 39.4 kHz is shown in Fig. 3. 3. Experiments Based on the analysis results, ultrasonic waveguide was fabricated. And the peak frequency value of impedance was measured to be 34.7 kHz of the PZT alone and 39.8 kHz of the system, which agreed well with predicted value with 0.3% and 1.0% error, as shown in Fig 4 for the waveguide. 4. Conclusion In this paper, the design and fabrication processes of an ultrasonic waveguide for microchip cooling that is operated by 40 kHz operating frequency were explained. In the development stage, finite element analysis using ANSYS software was performed to design the waveguide. So the predicted anti-resonance frequency for a piezoelectric actuator was 34.8 kHz, which was in good agreement with the experimental result of 34.7 kHz with 0.3% error. The waveguide was manufactured, and impedance characteristics were measured and compared. The predicted anti-resonance frequency for the ultrasonic waveguide was 39.4 kHz, which coincided with the experimental value of 39.8 with 1.0% error. Based on these results, it is thought that the developed ultrasonic waveguide will be applicable in microchip cooling.