Study Of Sacrificial Layer Etching Of Micro/Nano Structure And Its Model

In sacrificial layer etching technology, a sacrificial layer is grown on the substrate firstly, then a structure layer is grown on the sacrificial layer, finally a suspended element is obtained after the sacrificial layer is etched. It is one of the key technologies in MEMS and is widely used in MEMS devices. Though many researchers have studied etching mechanism, factors affecting the etching rate, etching flux about hydrofluoric acid etching SiO₂, there still exist some problems as follows, (1) The etching model can only predict the etching process in a short etching length. When the etching length and etching time become longer, errors increase. Finally, the model even fails to predict the etching process. (2) For a joint channel structure, besides the problem mentioned above, no one has studied the profile of etching front and changing of etching rate in detail. (3) It is not clear whether the etching model is still applicable when the thickness of sacrificial layer decreases to nanometer scale. And the factors, which may affect the etching rate in nanometer scale, are also unclear.Bearing these problems in mind, this dissertation studies the etching model of sacrificial layer in detail. It is found that there is a large error between experimental data and the etching model. The reason lies in that the diffusion coefficient and etching rate coefficient are considered as constant. A modified model is obtained by considering the diffusion coefficient as a function of temperature and etchant concentration, and the etching rate coefficient as a function of temperature.Based on the examples of port structure and bubble structure, experiments at different concentrations and different temperatures are done. For bubble structure, the experiments with different etching holes are also carried out. The results show that modified model can predict the etching process accurately at long etching length, which solves the problem that the error is large at long distance with the previous model.For the joint channel structure, a new mathematic model of the etching front profile is proposed. In this model, for a narrow-to-wide structure, the profile of etching front is an arc when the etching proceeds into the wide channel. For a wide-to-narrow structure, the profile of etching front still keeps a straight line when the etching front reaches the narrow channel. This mathematic model can predict the changing of profile at etching front, and it matches the experimental data well. Besides, the etching rates of narrow-to-wide structure and wide-to-narrow one are studied in detail. For a narrow-to-wide structure, when the etching front reaches the joint, the area of etching front increases abruptly. This results in the decreasing of etching rate suddenly. Then the etching front area increases gradually, which results in a further decrease of the etching rate. But the reduction is much less. When the etching front touches the sides of the wide channel, the area reaches a maximum, and the etching rate reaches a minimum. After that, the area begins to decrease till to the cross section area of the wide channel. So the etching rate increases gradually. On the other hand, because the diffusion restriction limited effect may reduce the etching rate, and then the etching rate may reach a minimum. Then the etching rate decreases slowly again. For wide-to-narrow structure, when the etching front reaches the joint, as the area of etching front decreases suddenly, the etching rate increases suddenly. After that, the etching rate goes down because of the diffusion restriction effect.The etching behavior of the sacrificial layer in tens of nanometers is also studied. The results show that the etching model deviates from the experimental data seriously when the thickness of sacrificial layer is in nanometer. Through the qualitative analysis of etching mechanism, it is proposed the reason for the deviation lies in that diffusion coefficient has changed in nanometer channel. Through simulation and curves fitting, the approximate reduction of the coefficient is obtained. The reasons for the reducing of diffusion coefficient may be EDL effect, surface tension, surface roughness, which may cause the change of viscoty and ion conductance and so on. In order to study these effects further, a new simple method for fabrication of tens of nanometers channel is proposed. And several nanometers channels are fabricated. By comparing the flow character of nanofluid with that of microfluid, several useful conclusions are drawn.In this dissertation, a modified sacrificial layer model is obtained, which enhances the correctness of the model. The elementary study for diffusion and flow behavior in nanofluid is firstly made. These results may promote the development of MEMS/NEMS technology.