| With the development of thick-film sensors towards miniaturization, integration and non-standardization, it is very important to develop a new processing technology, which can rapidly fabricate sensor with small-size, high precision, high reliability and no mask. With the support of National "863" Hi-Tech Research & Development Program of China and National Natural Science Foundation of China, a method of thick-film sensor fabrication by Micropen-Laser hybrid direct writing technique was presented in this paper firstly. With this novel method, thick-film temperature sensor, microheater and gas sensor were fabricated directly on ceramic substrates. At the same time, the concerned key scientific problems on the hybrid direct writing fabrication were studied systematically. Following are the main results on the project:This novel method combined Micropen direct writing technology and laser microcaldding electronic paste technology. Firstly, electronic paste was direct written or deposited on substrate by Micropen according to the designed pattern. Then it was dried by a heater. After that the laser beam was utilized to irradiate the coating according to Micropen movement track. Finally, the un-irradiated paste was removed by organic solvent and the pattern was left. Laser microcladding was selected as a post process, which can be used not only to functionalize the component but also to generate the fine pattern. It does not require the patterns and substrate to be post processed at high temperatures. At present, the minimum line width of thick-film PTC thermistor fabricated by Micropen-Laser hybrid direct writing was 40μm.Novel special equipment for Micropen-Laser hybrid direct writing technology was developed and manufactured on the basis of hybrid direct writing characteristics. This equipment was composed of Nd:YAG lasers, Micropen direct-write system and a 3-dimensional worktable. Laser beam delivery system and Micropen were installed on Z-axis of equipment. The scanning speed and directions were controlled by a numeric control system, by which Micropen direct writing and laser micro cladding process can be accomplished. The Open System Architecture of "NC embedded in PC" was used in the control system, which made the system more flexible and transplantable. "off-axis CCD + monitor" was used as workpiece position system. To measure the correlative coordinates between Micropen and laser beam, a simple method using direct-write marker was devised, which ensures that the movement tracks of Micropen and laser beam coincide with each other. Moreover, this equipment can be used in other fields such as laser precision machining, laser micro manufacturing and so on.The theoretical model of Micropen direct writing was established through rheology theoretical analysis. The sequence of the main process parameters which affect the line width is shown below in the order from primary to secondary: the viscosity of paste, the direct writing speed, the diameter of Micropen tip, gas pressure and the tip-to-substrate distance. It is obtained by orthogonal experiment. The main parameter which greatly affects the direct writing film thickness is the tip-to-substrate distance.Transformation character of functional electronic paste in the process of laser microcladding is similar to the process of furnace sintering accordint to the thermal analysis experiment. Laser power density has the similar effect on electrical properties as the temperature of furnace sintering. Laser microcladding electronic paste is essentially the thermal coupling process of laser beam withelectronic paste. There is liquid phase emerged during laser process. In essence, this process is an instantaneous liquid phase sintering during laser irradiation. The non-homogeneous distribution of the laser power intensity resulted in fluctuant phenomenon in the paste film during laser irradiation. The cracks can be found in the thick films when the film layer is thicker and the cooling process is non-uniform.The basic mechanisms of how to control the film quality of thermistor paste, galvanothermy paste and gas sensing paste by laser microcladding were studied systematically. The thermistors fabricated by laser microcladding exhibited excellent electrical performance and resistance tolerance equivalent to traditional oven-fired electrical paste. The TCR of thermistors reaches the maximum value of 2965×10-6/°C. Laser power density and the temperature of post-heat treatment had a great effect on sheet resistivities and TCR of thermistor. Optimal values of laser power density and temperature of post-heat treatment are 1.5×10-6W/cm2 and 400°C , respectively. The highest temperature of microheaters exceeded 500°C. The heating and cooling rates of the spiral-shaped microheater are about 7°C s-1 and 4°C s-1, respectively. The operating temperature has an obvious influence on the sensitivity of the sensor to ethanol vapor and the peak sensitivity occurs at operating temperature of 250°C. The sensitivity to 2000ppm ethanol vapor reaches a maximum value of 8.6, while that of screen-printed sample with 8.5μm thickness is 7.4. The measured response and recovery time of the sample was 7s and 20s, respectively, and 10s and 25s, respectively, for the sample fabricated by screen-printing. The gas sensor fabricated by laser microcaldding was packaged by TO-6 tube, which proved that the sensor fabricated by Micropen-Laser hybrid direct writing technology was compatible with traditional packaging technology.Summing up above mentioned, Micropen-Laser hybrid direct writing technology has created a new way for thick-film sensor fabrication with fast, maskless and high efficiency. Under the conditions of new product research and development, small-scale non-standardization production, this technique will be accepted by the industrial enterprises because of its low cost and high efficiency in comparison with traditional processes, which has a broad space for development and bright application prospects.
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