Welding Technology Of Spindt Field Emission Array Cathode

Spindt type cathode is a very good source of electrons. A vacuum microelectronic device has both characteristics of solid state devices and electronic vacuum device, which works based on the cold cathode of field emission. It can start instantly, having the advantages of anti-radiation, high power, small volume and integration. However, the cathode can not be directly applied in practice. As the cathode is very brittle, it is easily damaged by the expansion of external friction, and it can not be energized. Therefore, it is needed to weld the molybdenum sheet on the silicon wafer. Molybdenum has the advantages of low coefficient of thermal expansion, high thermal conductivity, low resistivity, high temperature resistance, stable chemical properties and so on. On the one hand, it plays an important role in protecting and fixing silicon substrate; on the other hand, cathode needs external circuit power to achieve the electron emission.Because the solder is easy to tear the silicon wafer, a protective layer of molybdenum was deposited on the silicon wafer. Using the method of combining simulation analysis with experiment,brazing process parameters of the cooling rates, the brazing temperature, the pressure, the thickness of the solder layer and the thickness of the molybdenum on the influences of brazing were studied. The joints were examined by optical microscope, scanning electron microscope and energy spectrum analyzer. Finally, the thermal fatigue test was carried out on the part of the model. The main contents and conclusions of this paper are as follows:(1)Using ANSYS software to simulate the brazing, the effects of cooling rate, brazing temperature, pressure and thickness of solder layer on brazing were studied. The results showed that, the maximum equivalent stress increased with the increase of cooling rate. When the value reached a certain value, the maximum equivalent stress was reduced, while the maximum equivalent stress level was very low when the cooling rate was more than 25℃/min. With the increase of brazing temperature, the maximum equivalent stress was reduced. When the temperature reached near 850℃, the maximum equivalent stress increased with the increase of the temperature. When the pressure was applied to the welding part, the maximum equivalent stress value was the minimum, and with the increase of the pressure, the stress also became larger. With the increase of the thickness of the solder, the maximum equivalent stress also increased.(2)Microstructure analysis showed that, molybdenum coated on a silicon layer could effectively improve the quality of joints. A certain degree of diffusion occured in the base metal, the coating and the solder. When the solder spread to the coating, the combination was good. When the solder spread to the base metal, the combination was very poor. We can conclude that the increase of the thickness of the molybdenum coating is beneficial to obtain high quality of joints.(3)The thermal fatigue test of the solder joint showed that, applying a thermal cycle in a temperature range of 0~400℃ and cycles of 20 times, the coating and the substrate, the coating and the welding seam were well combined together.Through software simulation and experimental verification, the test results of the solder joint were in good agreement with the simulation results. The feasibility of the scheme was proved, promoting the application of the finite element method of brazing, it provides technical guidance for silicon brazing, and expands the application of silicon materials in engineering.