JP3388022B2 - Manufacturing method of flow sensor - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing a flow sensor used for measuring a gas flow rate.

[0002]

2. Description of the Related Art For example, in Japanese Patent Publication No. 3-52028, column 6, line 15 to column 8, line 22 is a thin film resistance element that generates heat by passing an electric current and its resistance value changes with temperature. There is disclosed a flow sensor having a structure in which the thin film resistance element is exposed to a gas flow path together with an insulating base for reinforcing the thin film resistance element. Specifically, there is disclosed a flow sensor having a structure in which a laminate of a thin film resistance element and an insulating film (silicon nitride film) as a base thereof is arranged in a bridge shape on a recess provided in a silicon substrate. There is. here,
The concave portion becomes a part of the gas flow path. In this flow sensor, the degree of cooling of the thin film resistance element changes according to the amount of gas passing therethrough, so the temperature of the thin film resistance element changes depending on the amount of gas passage. In addition, the resistance value of the thin film resistance element changes accordingly. Therefore, the gas flow rate can be measured by detecting the change in the resistance value.

Further, in Japanese Patent Publication No. 3-52028, for example, in column 24, lines 27 to 43, suitable material examples for forming a thin film resistance element are disclosed. Specific examples thereof include permalloy (iron-nickel alloy), a zinc oxide single crystal film, and a pyroelectric material. Furthermore, although no specific material name is described, a suitable temperature coefficient of resistance (TCR)
It is described that a metal film other than permalloy having a can be used as a constituent material of the resistance element.

[0004]

By the way, the thin film resistance element provided in the flow sensor is exposed to the gas atmosphere to be measured while being heated to a relatively high temperature in order to detect a temperature change. The gas to be measured may be corrosive or oxidative. Therefore, depending on the material forming the thin film resistance element, the resistance value of the thin film resistance element may change due to corrosion or oxidation, or in the worst case, the thin film resistance element may be disconnected. As one measure to prevent these, it is considered that the thin film resistance element is made of gold (Au). Gold is a chemically stable substance, and
The temperature coefficient (TCR) of resistance of gold to temperature is 402
Since it is as large as 0 ppm / ° C., in the thin film resistance element made of gold, the resistance value changes easily when the temperature of the thin film resistance element changes due to the gas flow rate, and therefore the change in the resistance value is easily detected. However, in the case of forming the resistance element with gold, in general, gold cannot be sufficiently adhered to the base when the thin film is directly formed on the base by vapor deposition or the like. An adhesive layer will be formed between the thin gold film. Examples of the adhesive layer include a laminated film of a titanium (Ti) film and a platinum (Pt) film or a thin film of chromium (Cr). And mainly next ~
For this reason, it is preferable to use a thin film of chromium as the adhesive layer.

When two layers of Ti film and Pt film are laminated, two types of vapor deposition are required, but when chromium is used, one type of vapor deposition is sufficient, so the film forming process is simple.

The material cost is low.

For example, when considering a flow sensor having a structure in which a thin film resistance element is provided on a concave portion provided in a silicon substrate, a silicon substrate portion below the thin film resistance element is selected in a state where the thin film resistance element is formed. The recess is formed by removing it by a wet etching method, but titanium is etched by the etchant at this time.

[0008] However, it has been clarified by an experiment conducted by the inventor of the present application that, when a chromium thin film is used as an adhesive layer, the following problems will occur unless some measures are taken.

That is, by using the laminated film of the chromium thin film and the gold thin film, the TCR is first reduced by about 500 to 1000 ppm / ° C. as compared with the case of using only gold (see the section of Examples described later). When this laminated film is annealed at a high temperature in a vacuum atmosphere or an inert gas atmosphere, the TCR becomes 1000 ppm / ° C. or less, and the TCR becomes 3000 ppm or more smaller than that of gold alone. In order to obtain a stable thin film resistance element, it was common sense to anneal a thin film for forming a thin film resistance element or a thin film resistance element obtained by patterning this at high temperature in an inert atmosphere or a vacuum atmosphere. In view of the above TCR lowering result, it is considered that such an annealing treatment is not always good when a thin film resistance element is formed using a laminated film of a chromium thin film and a gold thin film.

Further, there are the following problems as other problems of the prior art. In the actual configuration of the flow sensor, a bonding pad is often connected to the thin film resistance element via a lead electrode in order to connect the thin film resistance element to an external power source. In that case, the lead electrode and the bonding pad are made of a wire-bondable material such as a gold thin film or an aluminum thin film. Therefore, if the thin film resistance element is made of permalloy, it is necessary to form a thin film dedicated to forming the lead electrode and the bonding pad. And the step of patterning so as to be formed separately from the step of forming the thin film resistance element. Therefore, there is a problem that the number of steps for manufacturing the flow sensor increases.

[0011]

Therefore, according to the present invention, in manufacturing a flow sensor including a thin film resistance element which is a thin film resistance element which generates heat when energized on a base and whose resistance value changes with temperature, The formation of the thin film resistance element is
(A) a step of forming a chromium thin film and a gold thin film thicker than the chromium thin film on the base, and (b) diffusion of chromium from the chromium thin film to the gold thin film in an atmosphere containing oxygen. A heat treatment for accelerating and oxidizing chromium that has diffused to the surface of the gold thin film; and (c) before or after the heat treatment, patterning the underlying thin film into the shape of the thin film resistance element. Depending on the process.

In implementing the present invention, in the case where the flow sensor further comprises a lead electrode connected to the thin film resistance element and a bonding pad connected to the lead electrode, the thin film on the base is the thin film resistance element, The lead electrode and the bonding pad are patterned into a series of shapes.

[0013]

According to the structure of the present invention, a laminated film of a chromium thin film and a gold thin film (here, the laminated film is the laminated film before being patterned into the shape of the thin film resistance element). After patterning into the shape of the resistance element, whichever is acceptable), heat treatment is performed in an oxygen atmosphere. Since the chromium in the chromium film diffuses into the gold thin film by this heat treatment, it is considered that the laminated film becomes an Au-Cr diffusion layer. However, it is considered that chromium that has diffused in the gold thin film and reached the surface of the gold thin film is affected by oxygen in the heat treatment atmosphere to form an oxide (chromium oxide) and be trapped on this surface. Therefore, the amount of chromium in the gold thin film is smaller than that in the case where the laminated film of the chromium thin film and the gold thin film is annealed in a vacuum atmosphere or an inert gas atmosphere. It is considered that the decrease will be less. Regarding the point that a diffusion layer is formed in annealing in an oxygen atmosphere, see, for example, Document I.
((Thin Solid Films
s), 37 (1976) pp. 171-179), especially from the matters described on page 177. In addition, for example, the chromium diffused in the gold thin film and reaching the surface of the gold thin film is affected by oxygen to form an oxide (chromium oxide) and trapped on this surface, for example, the above-mentioned document I. For example, FIG. It can be seen from the items described in 1.

The thin film resistance element manufactured by the method of the present invention (thin film resistance element, lead electrode and bonding pad in the case of a structure further including a lead electrode and a bonding pad) contains chromium and gold and mainly contains gold. In the pattern of the thin film to be formed, the base side part is a layer mainly containing chromium, and the surface layer part is a layer mainly containing chromium oxide. It is considered that it is composed of a thin film pattern which is thicker than the above and is mainly composed of gold.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a method for manufacturing a flow sensor according to the present invention will be described below with reference to the drawings. However, the drawings used in the description schematically show the dimensions, shapes, and arrangement relationships of the respective constituents so that the present invention can be understood. In addition, in each drawing, the same components are denoted by the same reference numerals, and the duplicate description may be omitted. Further, in this embodiment, an example in which a flow sensor 10 having a structure described below with reference to FIGS. 1 and 2 (hereinafter also referred to as “flow sensor 10 to be manufactured”) is manufactured by the method of the present invention explain. Here, FIG. 1 is a plan view showing the overall shape of the flow sensor 10 to be manufactured,
2A is a cross-sectional view taken along line I-I of FIG.
(B) is a sectional view taken along line II-II.

First, the flow sensor 10 to be manufactured is
A silicon substrate 11 (shown only in FIG. 2), a stripe-shaped recess 13, a rectangular recess 15, a silicon nitride film 17 as a base, and four first to fourth thin film resistance elements 19a to 19d. , A lead electrode 21 and bonding pads 23a to 23d. Note that FIG.
Although it is a plan view, in order to clarify the silicon nitride film portion, the portion is hatched from the upper left to the lower right. Similarly, the portion formed of the thin film resistance elements 19a to 19d, the lead electrode 21, and the bonding pads 23a to 23d is hatched from the upper right to the lower left.

Here, the stripe-shaped recess 13 is provided in a predetermined portion of the silicon substrate 10. The stripe-shaped recess becomes a part of the gas flow path. Also, the rectangular recess 1
5 is provided in the vicinity of the stripe-shaped recess 13. The rectangular recess 15 is provided so that the thin-film resistance element 19a is also floated from the substrate 11 while the thin-film resistance element 19a is installed above the stripe-shaped recess 13. It is a thing. By doing so, the accuracy of the Wheatstone bridge formed of the thin film resistance elements 19a to 19b can be improved. The silicon nitride film 17 is provided on the entire surface of the silicon substrate 11. However, a silicon nitride film 17 is formed on each of the recesses 13 and 15.
Is provided so that a part of the recess extends across like a bridge. The first to fourth thin film resistance elements 19a to 19d are arranged on the silicon nitride film 17. However, here the first
The thin film resistance element 19a is provided on the silicon nitride film portion bridging over the stripe-shaped concave portion 13,
The second thin film resistance element 19b is provided on the silicon nitride film portion bridging over the rectangular recess 15.
The resistance values of the first thin film resistance element 19a and the second thin film resistance element 19b are matched or substantially matched. The resistance values of the third thin film resistance element 19c and the fourth thin film resistance element 19d are matched or substantially matched. Although not limited to this, in this embodiment, as shown in FIG. 3, the first to fourth thin film resistance elements 19a to 19d are thin films (which will be described later) having a meandering width W. It is configured by patterning into a shape. In addition, the lead electrode 21 has a line width such that wiring resistance does not matter, and the first to fourth thin film resistance elements 19a to 19d serve as respective resistances forming the Wheatstone bridge. These thin film resistance elements 19a
.About.19d are arranged to be connected. The bonding pads 21a to 21d are provided so as to be connected to the lead electrode portions corresponding to the respective sides of the Wheatstone bridge.

Next, the flow sensor 10 to be manufactured as described above.
An example will be described in which is manufactured by the method of the present invention. However, numerical conditions such as temperature, time, film thickness, and vapor deposition rate described in the following description are merely examples within the scope of the present invention. This description will be made with reference to FIGS. 1 to 3 and 4A to 4D. Here, FIGS. 4 (A) to 4 (D) are process drawings for explaining the main part of the manufacturing method of the present invention.

First, as shown in FIG. 4A, a silicon nitride film (SiN _x film) 17 in this case is formed on the (100) surface of the silicon substrate 11 as a base to a thickness of 2 μm. The SiN _x film 17 is formed here by the sputtering method. In this sputtering, a Si target was used as a target, Ar was supplied as gas at a flow rate of 20 sccm, and N ₂ was supplied at a flow rate of 10 sccm, and the pressure during sputtering was set to 0.
It was performed under the conditions of 6 Pa and RF power of 1 kW. On the back surface of the silicon substrate 11, the SiN _x film 17a is formed to a thickness of 3 μm under the same sputtering conditions as above (FIG. 4A). The silicon nitride films 17 and 17a are used later as etching masks when the stripe-shaped recess 13 and the rectangular recess 15 are formed in the silicon substrate 11. Also, the silicon nitride film 1 on the substrate surface
A part of 7 also functions as a reinforcing film when the thin film resistance element is floated on the recesses 13 and 15. The stress of the SiN _x films 17 and 17a formed on the front and back surfaces of the silicon substrate was 5 × 10 ⁹ dyn / cm ² when tensile.

Next, using an electron beam evaporator, a substrate temperature of 100 ° C., under a condition where the degree of vacuum and 10 ^-6 Torr order, a thin film 31 and the gold chromium (Cr) on the SiN _X film 17 ( A thin film 33 of Au) is deposited in this order (FIG. 4 (B)). At this time, various experimental samples were prepared by using the thickness of the Cr thin film, the thickness of the Au thin film, and the deposition rate of the Au thin film as parameters. The manufacturing conditions of each experimental sample are
The TCR values measured from these experimental samples are shown in Tables 1 and 2 below. However, in Table 2, the heat treatment conditions according to the present invention are also shown. In addition,
The deposition rate of the chromium thin film when making each experimental sample is
It is unified to almost 5Å / sec.

As is clear from the results shown in Table 1, A
It can be seen that the deposition rate of the u thin film affects the TCR of the laminated film of the chromium thin film and the gold thin film at the time of as depo. More specifically, under the film forming conditions of this example, the as de of the laminated film of the chromium thin film and the gold thin film formed in the range where the deposition rate of the Au thin film was up to 6Å
The TCR at po is 3400 ppm / ° C or higher.
On the other hand, the as d of the laminated film of the chromium thin film and the gold thin film formed in the range where the deposition rate of the Au thin film is 10 Å or more
The TCR at epo is 2800 ppm / ° C or less. Therefore, when the TCR is desired to be 3000 ppm / ° C. or higher, the deposition rate of the Au thin film is 1 at the maximum.
It can be seen that it is better to make it slower than 0Å, and more preferably 6Å or less.

Further, the sheet resistance of a laminated film of a chromium thin film and a gold thin film (also referred to as Cr / Au film) at asdepo is as follows: the Au film thickness is 2000Å and the Cr film thickness is 15Å. When measured on a certain sample, 130 mΩ
It was / □.

Next, Cr / A of each experimental sample shown in Table 2
Organic resist layer on the u film (here, Tokyo Ohka Kogyo Co., Ltd.)
Made of OFPR8300-30) with a thickness of 1.3 μm,
This resist layer is patterned into a predetermined shape by an ordinary photolithography technique (not shown). Here, the predetermined shape is a mask shape capable of patterning the Cr / Au film into a series of shapes of the first to fourth thin film resistance elements 19a to 19d, the lead electrode 21 and the bonding pads 23a to 23d shown in FIG. is there.

Next, using the resist pattern (not shown) as a mask, the Cr / Au film is selectively removed by ion milling with Ar here. With this,
A thin film pattern (however, before the heat treatment according to the present invention) forming the fourth thin film resistance elements 19a to 19d, the lead electrode 21 and the bonding pads 23a to 23d is obtained (see FIG. 1). Here, the first and second thin film resistance elements 19a and 19d are assumed to have a meander-shaped portion having a total length of 360 μm and a width W of 5 μm in FIG. In addition, the line width of the lead electrode 21 is set to a width that does not cause a problem of wiring resistance, here, a suitable width of 100 μm or more (a suitable size of 20 times or more of the line width 5 μm in the thin film resistance element portion).

Next, the resist pattern (not shown) is removed with a mixed solution of 5 parts of sulfuric acid and 1 part of hydrogen peroxide.

The heat treatment conditions shown in Table 2 for each of the experimental samples after the removal of the resist pattern were: no heat treatment, 350 ° C. for 2 hours, 450 ° C. for 2 hours, and 600 ° C. Each treatment is performed under various heat treatment conditions of temperature for 2 hours. In this embodiment, the atmosphere containing oxygen according to the present invention is an air atmosphere. However, the atmosphere containing oxygen is not limited to the atmospheric atmosphere. For example, it is considered that an atmosphere having a higher oxygen content than the air atmosphere (including almost an oxygen atmosphere) may be used.

In the heat treatment for each experimental sample, C
The chromium of the thin film 31 of r diffuses into the thin film 33 of Au,
Although a diffusion layer 35 of -Cr is produced, there is not only this but a diffusion layer 35 that diffuses to the surface of the Au thin film 33. Au
The chromium that has reached the surface of the thin film of is oxidized by the action of oxygen in the heat treatment atmosphere (in the air), and finally precipitates as Cr oxide 37 on the surface of the thin film of Au (some chromium is present on the surface of the thin film of Au). Will be trapped). Therefore, since only a small amount of Cr is present in the Au thin film, the decrease in Au purity of the Au film is suppressed. The occurrence of such a phenomenon is caused by Document I (Thin Solid Films (Thin Solid Films
Solid Films), 37 (1976) pp. 171-17
9), especially the section on discussion on page 177 and Fi
g. I think that it can be justified based on the facts described in 5.

When the TCRs of the respective experimental samples thus heat-treated were measured, the values shown in the right column of Table 2 were obtained. From the results shown in Table 2, when the present invention is carried out, the thickness of the Au thin film, the thickness of the Cr thin film, and the Cr
It is considered preferable to set the heat treatment conditions for the / Au film as follows. That is, first, the thickness of the Cr thin film is preferably thin as long as the adhesion strength between the underlayer and the Cr / Au film is secured. Further, it is preferable that the thickness of the Au thin film is thick as long as it does not impair characteristics other than TCR. of course,
If the thickness of the Au thin film is too large, the resistance value of the thin film resistance element becomes smaller than the desired value. Also,
The heat treatment temperature is preferably at least in a suitable range higher than the temperature at which the flow sensor is used. If it is too high, the TCR is unnecessarily deteriorated. On the other hand, if it is too low, the purpose of the aging treatment or the screening treatment for obtaining a stable thin film resistance element cannot be achieved. Good. When the TCR is at least about 3000 ppm / ° C., and considering the preferable range of the conditions such as the above film thickness in the case of this embodiment, the film thickness of the Cr thin film is preferably 50 Å at the maximum, more preferably 25 Å or less. Can be said to be good. Further, the heat treatment temperature of 350 to 600 ° C. and the working time of 2 hours tried in this example are considered to be a suitable range of the heat treatment conditions of the present invention. However, the conditions regarding the film thickness, the heat treatment temperature, and the heat treatment time are not limited to the above conditions because they are determined in consideration of the specifications of the flow sensor, particularly the TCR.

After the Cr / Au film was heat-treated in a vacuum atmosphere or an inert atmosphere, its TCR became 1000 ppm / ° C. or less, whereas the method of the present invention still has a high TCR of 3000 ppm / ° C. Therefore, the usefulness of the present invention can be understood.

Next, an organic resist layer (here, a layer of OFPR8300-30) is formed again on this sample (not shown). Then, this resist layer is patterned into a shape corresponding to the shape of an etching mask for forming the recesses 13 and 15 (see FIG. 1) in the silicon substrate 11. Next, using the resist pattern (not shown) as a mask, S
The iN _x film is etched. Here, the SiN _x film is etched with a mixed gas of CF ₄ and O ₂ (containing 5% of O ₂ ). Next, the resist pattern is changed to O ₂
Remove by ashing.

Next, this sample is immersed in a 500 g / l (liter) KOH aqueous solution for a predetermined time. The portion of the silicon substrate not covered with the SiN _x film is etched with KOH. The depth of 200μ in this etching
m, the width is 800 μm, and the stripe-shaped recess 13 is 3 mm in length and a predetermined size (second thin film resistance element 19b).
The rectangular recesses 15 each having a size capable of floating up are formed.

Next, a lid 41 (see FIG. 1) covering a predetermined region of the sample, in this case, the thin film resistance elements 19a to 19d and the stripe-shaped recess 13 is provided on the sample, for example, an epoxy adhesive. Adhesive fixing by. The lid 41 is configured so that the bonding pads 23a to 23d are located outside the lid 41. The sample with the lid 41 fixed is die-bonded on an alumina substrate (not shown) with Ag paste, for example. Then, the electric circuit pattern (for example, the pattern of the power supply line) on the alumina substrate and the bonding pad of the flow sensor are wire-bonded.

According to the method of the present invention, even when the thin film resistance element is formed by using chromium and gold, a of the Cr / Au film is formed.
It is possible to obtain a thin film low resistance element that substantially maintains the TCR at s depo. Further, as in the embodiment, the thin film resistance element, the bonding pad, and the lead electrode between them are made of the common Cr /
Since it can be formed of an Au film, patterning of the thin film resistance element, the bonding pad, and the lead electrode between them can be performed with one photomask. Other than that, Si
Since only the photomask is used to obtain the resist pattern for patterning the N _x film, the flow sensor can be conveniently manufactured with two photomasks.

In the flow sensor of this embodiment, the meander-shaped portion of the first thin film resistance element 19a in FIG. 1 has a total length of 360 μm, a width W of 5 μm, and a Au film thickness of 2 μm.
The resistance value of 000Å is almost 100Ω.
When a current of 6 mA is passed through the thin film resistance element 19a, the temperature of the thin film resistance element 19a rises to about 190 ° C. Further, when gas contacts the thin film resistance element 19a, the temperature of the thin film resistance element changes according to the flow rate of the gas, and the resistance value changes accordingly. This thin film resistance element 1
Since 9a constitutes a Wheatstone bridge with other thin film resistance elements 19a to 19c, the Wheatstone bridge which changes highly sensitively according to the change of the resistance value accompanying the temperature change of the thin film resistance element 19a due to the gas flow rate. The gas flow rate can be sensed with high sensitivity by measuring the central voltage output V _X. The voltage output V _{X at the} center of the Wheatstone bridge is the bonding pad 2 in the example of FIG.
It is a voltage generated between 3a and 23d. In addition, four thin film resistance elements 19 forming a Wheatstone bridge
Since a to 19d are heat-treated in an atmosphere containing oxygen during the manufacturing, the change with time when used as a sensor is
The amount of change in the value of V _X when no gas was flowing was as small as 5 μV / Hour or less. Since this value has no problem in measuring the gas flow rate, it can be seen that the method of the present invention can provide a flow sensor having excellent characteristics in this respect as well.

Although the embodiment of the method for manufacturing the flow sensor of the present invention has been described above, the present invention is not limited to the above embodiment. For example, the flow sensor to which the manufacturing method of the present invention can be applied is not limited to the one having the structure described with reference to FIGS. 1 and 2, but can be applied to all flow sensors intended to manufacture a thin film resistance element using Cr and Au. Is clear.

In the above-mentioned embodiment, the thin film of Cr and A
An example of forming the thin film of u by the electron beam evaporation method has been shown, but the film forming method of these thin films is not limited to this. For example, a sputtering method may be used. In this case, the formation of the silicon nitride film and the formation of the Cr thin film and the Au thin film can be performed continuously, which is preferable in terms of simplifying the process, reducing the number of film forming devices, and the like.

Further, in the above-mentioned embodiment, an example in which the heat treatment according to the present invention is performed after the Cr / Au film is patterned into the shapes of the thin film resistance element, the lead electrode and the bonding pad, is explained. On the other hand, the effect of the present invention can be obtained by first performing the heat treatment according to the present invention and then patterning the thin film into the shape of a thin film resistance element or the like.

[0038]

[Table 1]

[0039]

[Table 2]

[0040]

As is apparent from the above description, according to the method of manufacturing a flow sensor of the present invention, the Cr / Au film for manufacturing the thin film resistance element is subjected to the predetermined heat treatment in the oxygen atmosphere. , A thin film resistance element exhibiting a practical TCR can be obtained despite using chromium (Cr). Therefore, a flow sensor having a desired sensitivity and having various characteristics by using gold, for example, (1), a flow sensor having excellent corrosion resistance and oxidation resistance, (2). (3). A thin film resistance element, lead electrode, and bonding pad can be collectively formed, so that a flow sensor that is easy to manufacture can be obtained.

[Brief description of drawings]

FIG. 1 is a plan view for explaining the structure of a flow sensor (flow sensor to be manufactured) which is manufactured by the method of the present invention.

2 (A) and 2 (B) are cross-sectional views of a main part used for explaining the structure of a flow sensor (flow sensor to be manufactured) manufactured by the method of the present invention.

FIG. 3 is a diagram showing an example of the shape of a thin film resistance element.

FIG. 4A to FIG. 4D are process drawings for explaining the manufacturing method of the example.

[Explanation of symbols]

10: Flow sensor 11: Silicon substrate 13: Striped recess (a part of gas flow path) 15: Rectangular recess 17: Base (silicon nitride film) 19a to 19d: thin film resistance element 21: Lead electrode 23a-23d: Bonding pad 31: Chromium thin film 33: Gold thin film 35: Au-Cr diffusion layer 37: Cr oxide 41: Lid

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Toyohira Nakajima 4630 Shimotakanezawa, Haga-cho, Haga-gun, Tochigi Prefecture Inside the Honda R & D Co., Ltd. (56) Reference JP-A-2-121301 (JP, A) (58) ) Fields surveyed (Int.Cl. ⁷ , DB name) H01C ^7/ 02-7/22

Claims (2)

(57) [Claims] 1. When manufacturing a flow sensor comprising a thin film resistance element that heats up on the lower ground by energization and whose resistance value changes with temperature, the thin film resistance element is formed by forming chromium on the lower ground. Forming a thin film of gold and a thin film of gold thicker than the thin film of chromium, and promoting diffusion of chromium from the thin film of chromium to the thin film of gold in an atmosphere containing oxygen and diffusing to the surface of the thin film of gold. Flow sensor, which comprises a step of performing a heat treatment for oxidizing chromium, and a step of patterning the thin film on the underlayer into the shape of the thin film resistance element before or after the heat treatment. Manufacturing method. 2. The method of manufacturing a flow sensor according to claim 1, wherein the flow sensor further includes a lead electrode connected to the thin film resistance element and a bonding pad connected to the lead electrode. In the patterning step, the method of manufacturing a flow sensor, wherein the thin film on the base is patterned into a series of shapes of the thin film resistance element, the lead electrode and the bonding pad.