Design And Fabrication Of Silicon-based Micromachined Infrared Emitter

With the rapid economic development, the global environmental condition sets to deteriorate because of the greenhouse and harmful gas produced by industry and transportation. The gas sensors become more and more needed for monitoring air quality. In many sorts of gas sensors, Infrared gas sensor has a wide range of applications in both qualitative and quantitative gas analysis because of its advantages of good selectivity and high sensitivity. As a key component in Infrared gas sensor, the performance of infrared light source determines the performance of infrared sensors. Comprared with the traditional infrared emitter, the micro infrared emitter fabricated by the micro-electro-mechanical systems technology has the advantages of small size, low power consumption, high frequency modulation and low fabrication cost. The MEMS infrared emitter has been paid great attention and was the focus of research for decades.In this paper, we designed and fabricated the silicon-based mircomachined infrared emitter, and the performance of the devices such as spectral characteristics, modulation characteristics and reliabilities were characterized. In fabrication process, polysilicon was used as the infrared emission material. By using the SOI wafer, the fabrication processes are simplified, so the production costs can be decreased. The temperature distribution of the light-emitting layer of the emitter was simulated by finite element analysis software—ANSYS. The temperature gradient distribution of the polysilicon membrane was obtained.The detail process flow has been designed and developed. The MEMS infrared emitter was successfully fabricated in SAH MEMS research center at Xiamen University. An appropriate boron (B) dope was used to realize the infrared absorption and transparence of device layer on SOI wafer for obtaining self-heating or body emitting effect. The resistance heating film used polysilicon deposited on the SOI wafer by LPCVD technology. The buried SiO₂ layer of the SOI wafer was used as etching stop layer to control the thickness of light-emitting layer. About four micrometer thickness of the light-emitting layer was fabricated by using deep reactive ion etching (DRIE) process from the backside of SOI wafer, thus this IR-emitter can be modulated at high frequency. The dimension of the MEMS infrared emitter wasabout 3 mm×3 mm.All parameter measurements of IR emitters had been performed on a TO packaging device without window. The surface temperature and emission spectrum of IR emitter were measured by thermal imaging system and spectroradiometer. The experimental results show that the IR emitter exhibits a strong emission in the wide spectrum of 1.5～20μm, which can meet the requirements of mostly gas absorption spectrums. The modulation characteristicsresearches show that the modulation frequency can reach to 40 Hz at 50 %modulation depth. The reliability of the emitter was tested by accelerated degradation test method. The lifetime was about 84 hours at the surface temperature of 1200K. The relative values of the resistance drift of the heater materials at each individual temperature T are plotted as a function of 1/T. From the Arrhenius plots, an activate energy Ea for the resistance degradation of the emitter was obtained. According to the activation energy Ea obtained by experiments, the lifetime of the device at different temperature could be obtained by extrapolation. It is expected that this IR-emitter can be used in low cost sensing system.In addition, the silicon-based mircomachined infrared emitter arrays for Combat Identification were also researched. The measured results show that the IR emitter arrays exhibit a strong, narrow-band emission in middle infrared range. The modulation frequency can be as high as 30 Hz at 50 %modulation depth.