The objective of the project was to develop and manufacture µSJA based on silicon technologies. To reach the required output velocities of the µSJA, the chamber and the exit channel or nozzle were optimised using different analytic and numeric methods. Based on the results of the optimisation, a design was investigated and transferred to silicon based structures. The production of the µSJA was performed in the clean rooms of the ZfM and realized on wafer-level. The two different parts of the µSJA, the cavity and the diaphragm were micro-machined and wafer bonded in order to form the actuator. The diaphragm-wafer’s backside was wet etched in order to produce a membrane in two different thicknesses, 40 µm and 70 µm. The cavity-wafer in contrast, was prepared by a sequential etching process with the same etchant. In the first step the orifice was preliminary etched with a depth of 400 µm. The second step equated the continuing etching of the orifice and the production of the cavity geometry. The achievements of these two processes were 3D nozzle structures of the orifice in a required aspect ratio and a very small cavity, resulting in a higher pressure gradient which leads to a much higher jet velocity compared to a dry anisotropic etched orifice combined with a huge cavity. This created the main advantage compared to known layouts.
Then, the two parts were bonded to form a small cavity with an orifice on the one side and a diaphragm on the opposite side. Finally, the piezo disc was mounted on the back side of the diaphragm into the wet etched cavity. To integrate the electro mechanical transducer, in this case the PZT element, into the system, reactive bonding technologies for the use of PZT silicon bonding were investigated. Thus, the strength of the low temperature bond of the ceramics, concerning low inducted mechanical stress, can be optimised.
In this report the design, manufacturing and characterisation of the micro-technology based synthetic jet actuator is described. In the design and optimisation work package a Combined Simulation μSJA-model was created. Therefore, the functionality was virtually split into two sub-systems. The Finite Element Method (FEM) was used to determine relevant lumped parameters of a piezoelectric transducer - electromechanical sub-system, which can be used with the lumped element method (LEM). Furthermore, lumped element models for the acoustic sub-system already exist. These were adapted for the μSAM geometry and a complete electro-acoustic simulation was done by bringing together and solving the LE models. Relevant constraints and parameters for the simulation had been defined and determined. Further acoustic investigations identified possible failure sources due to additional eigenmodes. Other failures sources were discussed. The simulation results were compared with optical deflection measurement results of the piezoelectric transducer and with flow measurements of the μSJA. Out of the preliminary simulation results an optimal design had been chosen.
The wafer preparation and wafer bonding work packages manufactured the whole μSJA chip. This includes the silicon manufacturing of the nozzle- and the membrane wafers, the bonding of the membrane- and the nozzle wafers to the µSJA system, the dicing of these bonded μSJA wafers to μSJA chips as well as the bonding of the piezoelectric transducer, (PZT ceramics) to the diced μSJA chips. The most important challenge of the μSJA manufacturing is the integration of the PZT ceramic into the μSJA system because this faces some limitations concerning the whole manufacturing- as well as the integration concept. Therefore two wafer bonding concepts as well as different PZT ceramic to silicon bonding concepts were investigated regarding their feasibility to fulfill the aimed requirements. The wafer bonding concept has been split into the 'wafer bonding first, followed by the PZT ceramic bonding' and the 'PZT ceramic bonding first, followed by the wafer bonding' approaches.
The PZT ceramic bonding concept concerning several bonding techniques such as bonding with reactive layers, bonding with the negative photoresist SU8, bonding with epoxies and bonding with different types of adhesive tapes have been investigated. Finally, corresponding the aimed requirements such as a low bonding temperature and a low bonding pressure as well as a low stress input into the whole system especially during the PZT ceramic bonding step one approach could be proven with excellent results. This approach is characterized by the first wafer bonding concept that is a direct bonding step with a special pre-treatment of the bonding areas and a followed temperature step at 200 °C. Afterwards the bonded μSJA system is diced into chips and the PZT ceramics are bonded on the thin silicon membrane of the μSJAs. This is realized with an electrically conductive adhesive foil with a bonding temperature of 130 °C and a bonding pressure of 0,5 bar. The connection to the overall system is realized by the integration of the μSJAs into the system and by wire bonding.
After bonding of the transducers and the wafer, the actuators are tested to evaluate the functionality and to verify the single actuator characteristics. Beside the development and manufacturing of the silicon based actuators the design and the development of an electronics system for the controlling of the actuator are done. The system is divided into signal generator and transducer electronics. The signal generator provides the actuation signal depending on the simulation results and the geometrical data of the actuator. The transducer electronics amplifies this signal to meet the requirements of the actuators power consumption depending on the used actuation principle. To test the whole system including a number of actuators and the driving electronics, the single components are integrated into a mock-up. This mock-up will be system designed in that way that it can be integrated into a wind tunnel model. After finalizing the manufacturing and assembly of all the components of the mock up, the system performance is verified by measuring the output velocities of the actuator. Besides that, test and characterisation of the test board are performed. All the results and manufacturing details are summarised in this final report.