Research On Thermal Properties Of Thin Films

When the size of a device comes down into the range of micro/nano meter scale, thermal properties of the device are different from those of bulk materials, i.e. size effect appears. It has been reported that thermal conductivity and thermal diffusivity of thin films appear size effect. But further research on the theory of size effect is necessary. The size effect on specific heat of thin films has been only analyzed theoretically. Further study, especially experimental investigation is needed. To meet the requirement for developing the micro/nanometer devices, the thermal conductivity and thermal specific of thin films have been studied in this paper. The study mainly includes: developing a new structure to measure the thermal conductivity of silicon nitride thin films, fabricating the silicon nitride suspended membranes to measure their specific heat and putting forward a new method for measuring the heat capacity of microscale thin films.A new bridge membrane structure for measuring the thermal conductivity of thin film is presented which based on the static-state method. On the membrane, heat flux is very uniform and the temperature is evenly distributed, making the system error smaller than conventional structures. The silicon nitride bridge membranes with different thickness have been fabricated using micromachining process. Their thermal conductivity has been measured. The measured thermal conductivity shows clearly size effect, i.e. the thermal conductivity increases with film thickness. And the reason for the phenomenon has been analyzed in theory.A suspended silicon nitride membrane structure has been designed and fabricated by silicon micromachining for measuring the specific heat of the silicon nitride thin films. The specific heat of silicon nitride thin films with different thickness has been measured using the thermal time delay method. The results have shown clearly the specific heat of the silicon nitride thin film deviates from its bulk form.A new 3 method for measuring heat capacity of miroscale thin film has been put forward. The method have advantages of high measurement sensitivity and easyfabrication, for the heater and temperature sensor are integrated in the structure, which make it possible to miniaturize the area and the thickness of the membrane. The method has been used to measure the specific heat of aluminum thin film with different thickness. The results have shown size effect of the specific heat. And the size effect has been analyzed in theory.The above measurement structures and methods have higher precision than what have reported previously.