Research On The Key Technologies Of Micro-Gas Flow Sensors

Gas flow rate is an important measurement parameter in some fields like industrial production and scientific research.Gas flow meter is widely used in automotive,power generation,metallurgy,aerospace,microelectronics manufacturing,medical and other fields.Micro-gas flow sensors are small in size and low in power consumption,which can accurately measure the flow rate,and attracted more and more attention.And thermal mass gas flow sensors are used more widely.In this dissertation,the working principle and fabrication process of three kinds of temperature measuring elements of thermal temperature-difference gas flow sensor were studied.The working principle of thermal resistors connected to a Wheatstone bridge for temperature detection was described,for the problem that platinum resistors is difficult to fabricate by wet etching,we achieved the stripping of platinum resistors with good quality.The fabrication process of the heavily doped polysilicon resistors was studied.Fabrication process of thermopile is also studied in this paper,several key problems that should be paid attention to in the fabrication process were put forward,and we successfully fabricated thermopiles using aluminum and polysilicon.The temperature detecting principle of micro bridge resonant beam is introduced,using ICP etching,the micro bridge resonant beams were released finally through vertical and horizontal etching of silicon.Choosing NiCrSi as the heating element material of these flow sensors,this dissertation explains the changing mechanism of the TCR of NiCrSi.Using magnetron sputtering,lithography and wet etching technology,we fabricated NiCrSi resistors with different thickness,and got their TCR,then those NiCrSi resistors were annealed in different temperatures in the heating furnace,we tested their TCR again and finally got the appropriate annealing parameters to reduce the TCR of NiCrSi.Considering the corrosion of different element material in different etchants,we adjusted the sequences of some wet etching steps,and finished the fabrication process of the chips of hot film gas flow sensor and resonant gas flow sensor.Several key technologies during the fabrication process were studied.(1)The stress matching of silicon dioxide and silicon nitride with compressive press and tensile stress respectively was achieved through thickness calculation.(2)The ICP etching technology of LPCVD and PECVD silicon nitride was studied.(3)For the protection of resistors and aluminum wires in the front side of the silicon wafer,we used a kind of fixture and corrosion resisting glue for KOH solution and successfully achieved the protection of those elements and wires on the front side.(4)Back etching process was studied,and the silicon on the back surface of the substrate was corroded by KOH solution,and the silicon cavity at the backside of the flow sensor chip was fabricated.Through the whole experiment,we verified the technical feasibility,and laid the foundation for the subsequent package and test of these four kinds of micro gas flow sensor.