Monolithic integration of MEMS thermal flow sensor and its driving circuit onto flexible Cu on polyimide film

A thermal sensor integrated with its driving circuit by using Cu on polyimide (COP) as a substrate was developed. A heater, working as the thermal flow sensor and constant temperature circuit (CTC), working as its driving feedback circuit, were successfully integrated on the COP substrate by photolithography, metal deposition, and sacrificial selective etching in the Cu layer. The Cu layer works to form a cavity that acts thermal isolation for the heater and electrical feedthroughs in the CTC. The electrical components, i.e., resistors, capacitors, and operational amplifiers, were mounted on the electrical pads using low-temperature solder paste of Sn–Bi at the CTC area. The areas of the thermal sensor and CTC were 11 × 23 mm and 16 × 24 mm, respectively. The area of the thermal sensor was inserted into a tube with a diameter of 5 mm acting as the flow sensor. The sensor outputs increased along with the airflow rate up to 10.0 L/min, and the calibration curve as the airflow-rate sensor was experimentally derived. The fabricated sensor outputs were not affected by variations in the airflow temperature and only depended on the airflow rate thanks to the added temperature compensation function, and a response time of 0.095 s was obtained.