Vacuum cavity encapsulation for response time shortening in flexible thermal flow sensor

A vacuum cavity encapsulation fabrication process for a thermal sensor based on a Cu on polyimide (COP) substrate was studied to shorten the response time, and it was applied to a flow sensor application. First, the sacrificial etching properties of a Cu layer for designing and forming a cavity under a heater for thermal isolation and an electrical feedthrough into the Cu layer were evaluated. The etching rates depended on the hole size of an etching mask pattern and the distance between two holes. Then, the relationship between the amount of parylene dimer and the sealing thickness in a lateral direction in the etching hole by its deposition was investigated to encapsulate the cavity under a vacuum condition. The thickness increased linearly with the amount of dimer, and the hole was successfully sealed by the parylene film under a vacuum condition. The vacuum cavity encapsulation was applied to the flexible thermal flow sensor to shorten the response time. The response time value was successfully shortened to three-fifths by applying the vacuum cavity encapsulation, and it did not change after 2 months.