Investigating ultrasonic direct chip attach of MEMS biosensors to flexible substrates

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Dr. Krammer Olivér
Department of Electronics Technology

To understand the behaviour of the brain, examination of its anatomical structure is not enough. Tests carried out during the active operation of the brain are needed, in order to obtain a more accurate picture of its behaviour. To achieve this there are several methods at our disposal, most of them are based on measuring the bioelectrical signals caused by the ionic currents induced by the nerve cells (neurons). One of these methods is the implantation of electrodes in the central nervous system. The subject of my thesis is the formation of thermosonic bonds between the implantable chip and a flexible substrate, as well as to optimise the process parameters. During my research, I studied several topics, including the materials and different types of rigid and flexible substrates, the deposit methods and materials of metallization used on semiconductor devices, methods of first level chip interconnections and direct chip attach, and the methods used for testing the electrical and mechanical properties of the created bonds. The samples were created by thermosonic flip chip bonding, and the effect of changing the main process parameters was examined on electrical and mechanical characteristics. To determine the electrical and mechanical properties, four wire resistance measurement and shear strength measurement were used. Based on the results of the available samples, the effect of altering the process parameters were less significant on the electrical resistance, than on the mechanical strength. On the resistance, the effect of the ultrasonic vibration power is the most prevailing, increasing this parameter the resistance decreases. The effect of the other parameters is minimal. On the shear strength, the bonding force has the greatest effect, increasing this parameter will result in a bond with considerably higher mechanical strength than the others. Apart from the bonding force, the effect of the duration of the ultrasonic vibration is considerable. In the case of the resistances, without exception, the samples made with the highest parameter values resulted in the best resistance values. However during the measurement of shear strength it became apparent, that excessive increase in the ultrasonic vibration power weakens the strength of the bonds. Based on the results of the examinations, to create the optimal bond, the greatest value of each parameter (in the examined parameter space) should be used with the exception of the ultrasonic vibration power.


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