Gas flowmeter and manufacturing method thereof

Abstract

A gas flowmeter capable of reducing a secular change comprises a silicon semiconductor substrate formed with a cavity and a heat element formed above the cavity of the semiconductor substrate by way of an insulating film. The heat element is a silicon (Si) semiconductor thin film impurity-doped at high concentration. Stoichiometrically stable silicon nitride (Si 3 N 4 ) thin films as barrier layers which less permeate and less absorb hydrogen in the heat generating temperature range of the heat element are formed above and below the silicon (Si) semiconductor thin film.

What is claimed is:  
     
         1 . A gas flowmeter for measuring a flow rate of a gas to be measured 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;    wherein the heat element is a silicon (Si) semiconductor thin film applied with impurity doping at high concentration, and    the gas flow meter has barrier layers that disposed as an upper layer and a lower layer of the silicon (Si) semiconductor thin film and that are formed in a region to cover at least the cavity, the barrier layers less permeating and absorbing hydrogen in a heat generating temperature range of the heat element.    
     
     
         2 . A gas flowmeter as defined in  claim 1 , wherein the barrier layer comprises a stoichiometrically stable silicon nitride (Si 3 N 4 ) thin film.  
     
     
         3 . A gas flowmeter as defined in  claim 1 , wherein the silicon (Si) semiconductor thin film is a polycrystalline silicon (Si) semiconductor thin film applied with impurity-doping, and 
 the polycrystalline silicon (Si) semiconductor thin film is doped with phosphorus (P) or boron (B) as impurities at a high concentration.    
     
     
         4 . A gas flowmeter as defined in  claim 1 , wherein the doping is applied at such high concentration that resistivity of the silicon (Si) semiconductor thin film is 8×10 −4  cm or less.  
     
     
         5 . A gas flowmeter for measuring a flow rate of a gas to be measured 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;    wherein the silicon (Si) semiconductor thin film is a polycrystalline applied with impurity doping,    the polycrystalline silicon (Si) thin film is applied with doping of phosphorus (P) or boron (B) as impurities at high concentration, and    the gas flow meter has barrier layers that disposed as an upper layer and a lower layer of the silicon (Si) semiconductor thin film and that are formed in a region to cover at least the cavity, the barrier layers less permeating and absorbing hydrogen in a heat generating temperature range of the heat element.    
     
     
         6 . A gas flowmeter as defined in  claim 5 , wherein the doping is applied at such high concentration that resistivity of the silicon (Si) semiconductor thin film is 8×10 −4  Ωcm or less.  
     
     
         7 . 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: 
 forming a first insulating film containing a stoichiometrically stable silicon nitride (Si 3 N 4 ) thin film as at least a first barrier layer which less permeates and absorbs hydrogen, as an insulating film formed on the surface of the silicon semiconductor substrate;  
 forming a silicon semiconductor thin film, as an upper layer, on the first insulating film;  
 applying impurity doping by thermal diffusion to the silicon semiconductor thin film thereby doping impurities at high concentration;  
 patterning the silicon semiconductor thin film and, at least, forming a pattern of the heat element;  
 stacking a second insulating film containing a stoichiometrically stable silicon nitride (Si 3 N 4 ) thin film as at least a second barrier layer which less permeates and absorbs hydrogen on the heat element, as an upper layer of the heat element, and surrounding the heat element by the first and the second barrier layer;  
 applying a heat treatment annealing to the heat element in a gas atmosphere at least containing hydrogen or in an inert gas atmosphere;  
 forming an electrode film after forming through holes in the second insulating film so as to establish electrical connection between the heat element and an external circuit; and  
 forming a cavity at the rear face of the silicon substrate.  
   
     
     
         8 . A manufacturing method of a gas flowmeter as defined in  claim 7 , wherein the heat treatment annealing is a heat treatment at 550° C. or higher and 900° C. or lower.  
     
     
         9 . A manufacturing method of a gas flowmeter as defined in  claim 7 , wherein the heat treatment annealing is applied in an atmosphere at least containing a hydrogen gas in a case where hydrogen is contained in the gas to be measured.  
     
     
         10 . A manufacturing method of a gas flowmeter as defined in  claim 7 , wherein 
 the heat treatment annealing is applied in an atmosphere containing an inert gas in a case where hydrogen is not contained in the gas to be measured.    
     
     
         11 . 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 second 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.