Patent ID: 7228614

Claim:
A method of manufacturing a gas flowmeter comprising a semiconductor substrate formed with a cavity and at least a heat element formed by way of an insulating film above the cavity of the semiconductor substrate to measure a flow rate of a gas to be measured, wherein the semiconductor substrate is a silicon semiconductor substrate, and the method comprises the steps of: stacking a first silicon dioxide film, a stoichiometrically stable silicon nitride (Si 3 N 4 ) thin film as a first barrier layer which less permeates and absorbs hydrogen, and a second silicon dioxide film on the surface of the silicon semiconductor substrate; forming a silicon semiconductor thin film on the second silicon dioxide film; applying an impurity doping by thermal diffusion to the silicon semiconductor thin film to apply doping of phosphorus (P) at high concentration such that the resistivity is 8×10 −4 Ωcm or less; patterning the silicon semiconductor thin film to form at least a pattern for the heat element; stacking a third silicon dioxide film, a stoichiometrically stable silicon nitride (Si 3 N 4 ) thin film as a second barrier layer which less permeates and absorbs hydrogen and a fourth silicon dioxide film further on the heat element, and surrounding the heat element with the first and the second barrier layer which less permeate hydrogen; applying a heat treatment annealing to the heat element in a gas atmosphere at least containing hydrogen or in an inert gas atmosphere at a temperature of 550° C. or higher and 900° C. or lower; forming an electrode film after forming through holes in the third silicon dioxide film, the second barrier layer and the fourth silicon dioxide film so as to establish electrical connection between the heat element and an external circuit; and forming a cavity in the rear face of the silicon substrate.