JP2001165737A - Microheater and its production method and flow sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a flow sensor with high reliability by suppressing generation of bubbles above a heater region when covered with water. SOLUTION: A heater region 50 is flattened, and at the same time an uneven region 60 is provided around the heater region 50, for promoting bubble generation by vaporization. By this, bubble generation by vaporization above the heater region 50 is suppressed and so the break of diaphragm 10 due to the temperature difference caused by insulation effect of bubbles can be prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a micro heater having a thin film structure provided above a cavity formed in a substrate and having a heating element region in which a heating element is formed, a method of manufacturing the same, and a flow sensor. .

[0002]

2. Description of the Related Art Conventionally, in a flow sensor, a humidity sensor and the like, a micro heater capable of heating to several hundred degrees has been required due to its detection principle. This micro heater is
In order to suppress the escape of heat to the substrate at the time of temperature rise, a thin film structure of several μm is formed, and the thin film structure employs a heat insulating structure such as forming a through hole. Furthermore, by adopting these structures and reducing the size of the sensor, thermal responsiveness and power consumption are reduced.

Generally, a thin film structure forming a micro-heater is a structure in which a thin film of a heating element material is sandwiched between upper and lower protective films. Here, as the heating element material, a conductive material such as Pt (platinum), polysilicon, NiCr (nickel chrome), TaN (tantalum nitride), SiC (silicon carbide), W (tungsten) is used. As MgO (magnesium oxide), S
A thin film of an insulating material such as iO ₂ (silicon dioxide), Si ₃ N ₄ (silicon nitride), Ta ₂ O ₅ (tantalum oxide), or Al ₂ O ₃ is used.

Further, a thin film structure constituting a microheater is provided as a diaphragm or a bridge above a cavity formed in a substrate. For this reason, the strength is extremely disadvantageous. Therefore, as disclosed in Japanese Patent Application Laid-Open No. H11-271123, the film stress is adjusted to improve the thermal stress resistance.

[0005]

As a sensor having the above-described micro heater, in a flow sensor or the like used for controlling the amount of intake air in an engine, it is conceivable that water may enter the intake pipe due to rain or the like. In order to guarantee the operation of the flow sensor in this state, it must be durable without breaking even underwater.

The present inventors have studied the durability of a micro heater in a flow sensor in water, and found that water evaporates on the micro heater due to heat to form bubbles, and the bubbles are thermally insulated. And the temperature of the microheater rises, but the temperature of the microheater does not rise above 100 ° C. where it is in contact with water. For this reason, it has been found that there is a possibility that a temperature difference is partially generated, a distortion is generated due to a difference in thermal expansion caused by the temperature difference, and the micro heater is broken.

An object of the present invention is to solve the above problem by controlling the generation of bubbles.

[0008]

In order to achieve the above object, according to the first aspect of the present invention, a heating element (4) is provided on a cavity (1a) formed in a substrate (1).
In the micro heater having the thin film structure having the heating element region (50) on which the (0) is formed, nucleation of evaporation occurs in the heating element region (50) when liquid is applied to the surface of the heating element region (50). It is characterized by having a flat surface.

As a result, nucleation of evaporation can be reduced in the heater region 50 when liquid is applied.

In this case, if a nucleation promoting region (60) for promoting nucleation is formed in a portion other than the heating element region (50), the nucleation of boiling can be prevented. Since it is possible to start from the nucleation promoting region (60), nucleation of evaporation in the heater region (50) can be further reduced.

Further, in the invention according to claim 3,
A micro heater having a thin film structure provided on a cavity (1a) formed in a substrate (1) and having a heating element region (50) in which a heating element (40) is formed,
A nucleation accelerating region (60) for accelerating nucleation of evaporation when the liquid is attached is formed in a portion other than the heating element region (50).

Thus, nucleation of boiling can be started from the nucleation accelerating region (60), so that nucleation of evaporation in the heater region (50) can be reduced.

The nucleation promoting region (60)
Is a heating element region (5).
Preferably, it is formed around 0).

The nucleation promoting region (60) may be a region provided with a projection (61) on the surface as in the invention of the fifth aspect, or may be as in the invention of the sixth aspect. In addition, the surface roughness of the surface may be higher than the surface roughness of the heating element region (50). In the latter case,
Further, as in the invention according to claim 7, a region where the surface roughness of the surface of the uppermost film of the thin film structure is higher than the film formation state, or as in the invention according to claim 8, the surface film has a polycrystalline structure. The region may be a film (62).

According to the ninth aspect of the present invention, the heating element region (5) provided above the cavity (1a) formed in the substrate (1) and having the heating element (40) formed therein.
In the micro heater having a thin film structure having 0), the surface film of the heating element region (50) is a hydrophilic film (70).

By providing the hydrophilic film (70) in this manner, the bubbles can be promoted to separate from the film (70), and as a result, the generation of bubbles in the heater region (50) is suppressed. be able to.

[0017] The hydrophilic film (70) can
As in the invention described in No. 10, TiO _TwoForm using
be able to.

According to the eleventh aspect of the present invention, the substrate (1)
A micro heater having a heating element region (50) provided above a cavity (1a) formed therein and having a heating element (40) formed therein when a liquid is applied thereto. (50) is characterized in that nucleation of evaporation is prevented from occurring.

This also enables the heater region (50)
Nucleation of the evaporation in the reactor can be reduced.

According to the twelfth to nineteenth aspects, it is possible to provide a method for manufacturing the above-described micro heater.

According to the twentieth aspect of the present invention,
A flow sensor having the microheater according to any one of claims 1 to 11 and configured to detect a flow rate of a fluid can be provided.

Note that the reference numerals in parentheses of the above means indicate the correspondence with specific means described in the embodiments described later.

[0023]

(First Embodiment) FIG. 1 is a perspective view of a flow sensor according to a first embodiment of the present invention, and FIG.
1 shows a cross-sectional view taken along the line AA in FIG.

In this flow sensor, a silicon nitride film 11 and a silicon oxide film 12 serving as lower insulating films are formed on a semiconductor substrate 1 made of single crystal silicon or the like.
A fluid temperature detector 20 and a flow rate detector (temperature measuring element) 30 forming a thermometer are formed thereon, and a heater (heating element) 40 is formed thereon. Further, a silicon oxide film serving as an upper insulating film is formed thereon. It has a structure in which a film 13 and a silicon nitride film 14 are formed.

As shown in FIG. 2, a cavity 1a is formed in the semiconductor substrate 1, a diaphragm 10 having a thin film structure is formed on the cavity 1a, and a flow rate detector 30 and a heater 40 are provided in the diaphragm 10. And are arranged.

The fluid temperature detector 20, the flow detector 30 and the heater 40 are arranged in that order from the upstream side in the direction of the flow of the fluid (indicated by the white arrow in FIG. 1). The pattern is formed by a resistor film made of a wiring material such as Pt.

The fluid temperature detector 20 detects the temperature of the fluid, and is disposed at a position sufficiently separated from the heater 40 so that the heat of the heater 40 does not affect the temperature detection. The heater 40 is controlled by a control circuit (not shown) so that the reference temperature is higher than the temperature detected by the fluid temperature detector 20 by a certain temperature.

In the flow sensor constructed as described above, when a fluid flows, the temperature of the fluid is changed to the fluid temperature detector 2.
The heater 40 is energized so that the reference temperature is measured at 0 and the reference temperature is higher by a certain temperature than the measured temperature. Then, the heat distribution of the heater 40 changes according to the magnitude of the flow of the fluid, and the change in the heat distribution causes the flow rate detector 3 to change.
The flow rate is detected by changing the resistance value of 0.

Here, in this embodiment, as shown in FIG.
Has a flat surface. Thereby, it is possible to prevent nucleation of evaporation from occurring in the heater region 50 when the liquid is applied.

Further, around the vicinity of the heater region 50,
A protrusion 61 is formed on the surface by the convex portion 15 formed on the silicon oxide film 13 so as to surround the heater region 50. The uneven area where the projections 61 are formed serves as a nucleation promoting area 60 for promoting nucleation of bubbles. The nucleation promoting region 60 may not be formed so as to surround the heater region 50, but may be formed only in a region sufficient to promote nucleation of bubbles.

In such a structure, when water is attached while the heater 40 is generating heat, the heater region 50 generates heat at 100 ° C. or higher, but the attached water is very clean. Boiling does not start on a smooth surface even if the temperature exceeds the boiling point. Incidentally, as shown in FIG. 3, a silicon nitride film 11 and a silicon oxide film 12 are laminated on the semiconductor substrate 1, and a flow rate detector 30, a heater 4
0, and a silicon oxide film 1 is further formed thereon.
In the case of a structure in which the silicon nitride film 3 and the silicon nitride film 14 are stacked, irregularities are present on the surface of the heater region 50 due to the pattern of the heater 40, and boiling starts from the nucleus, and bubbles are generated on the heater region 50.

In this embodiment, the heater region 50
Is flattened and the nucleation promoting region 60 is provided around the heater region 50, so that the nucleation of boiling starts from the nucleation promoting region 60, whereby the boiling occurs around the nucleation promoting region 60. Get up. For this reason, the water on the heater region 50 is stirred by the bubbles to improve the heat transfer, and no boiling occurs on the heater region 50.

Therefore, according to this embodiment, since air bubbles on the heater area 50 are suppressed from adhering, no temperature difference occurs due to the heat insulation effect of the air bubbles in the heater area 50, and the coefficient of thermal expansion is reduced. The difference can prevent the diaphragm 10 from being broken.

Next, a method for manufacturing the above-described flow sensor will be described in order with reference to the process charts shown in FIGS. 4 and 5 (corresponding to the AA cross section in FIG. 1). [FIG. 4 (a)
Step] A single-crystal silicon substrate 1 is used as a semiconductor substrate, and a silicon nitride film 11 is
VD method or the like, and a silicon oxide film 12
It is formed by a VD method or the like.

By laminating the silicon oxide film 12 on the silicon nitride film 11 in this manner, the adhesion with the wiring material formed on the silicon oxide film 11 is improved,
Further, when the thin film structure is formed, the moisture resistance of the thin film structure can be improved because the water-resistant silicon nitride film 12 is disposed outside. [Step of FIG. 4 (b)] A Pt film is formed as a resistor material by 200
The Pt film is deposited on the silicon oxide film 12 by a vacuum evaporation machine at
The wiring pattern of the flow rate detector 30 and the heater 40 is patterned.

Here, the resistor material may be polysilicon, NiCr, TaN, SiC, W or the like. In this case, a single film is more preferable than a multilayer film. Further, a Ti layer or a Cr layer may be inserted as an adhesive layer between the silicon oxide film 12 and the Pt film for the purpose of increasing the adhesion, and the adhesive layer may also be inserted between the resistor material and the upper film. May be inserted. [Step of FIG. 4C] A silicon oxide film 13 is deposited for insulation between the fluid temperature detector 20, the flow detector 30, and the heater 40. Here, the surface of the silicon oxide film 13 has irregularities due to the pattern on the fluid temperature detector 20, the flow detector 30, and the heater 40. [Step of FIG. 4D] In order to remove irregularities formed on the surface of the silicon oxide film 13, a CMP (Chemical
The surface of the silicon oxide film 13 is flattened by performing Mechanical Polishing or the like. The flattening may be performed by using an etch back. [Step of FIG. 5 (a)] A silicon nitride film is formed on the silicon oxide film 13 and is patterned by etching to form the projections 15. In this case, for example, the silicon nitride film can be selectively removed by etching using phosphoric acid or the like, and the silicon oxide film 13 can be hardly etched. The protrusions 15 can be formed by patterning the silicon oxide film 13 after the CMP step shown in FIG. 4D. However, it is considered that unevenness due to etching unevenness may occur. It is desirable to form the convex portion 15 by the following method. [Step of FIG. 5B] A silicon nitride film 14 as a surface protection film is formed. In this state, the silicon nitride film 1
The projection 61 is formed on the surface of the substrate 4.

Thereafter, although not shown, openings are formed in the silicon nitride film 14 for forming electrode pads of the fluid temperature detector 20, the flow detector 30, and the heater 40. [Step of FIG. 5C] A mask material (for example, a silicon oxide film or a silicon nitride film) 16 is formed on the back surface of the silicon substrate 1.
Is formed and etched to form an opening 17. [Step of FIG. 5 (d)] The back side of the silicon substrate 1 is anisotropically etched until the silicon nitride film 11 is exposed to form a cavity 1a. At this time, the end point is detected by, for example, using TMAH (tetramethylammonium hydroxide) as an etching solution, so that the silicon nitride film 1
1 can be easily stopped because the etching rate is very low.

Thus, the flow sensor shown in FIGS. 1 and 2 can be manufactured. (Second Embodiment) In the first embodiment, the protrusion 61
Shows the formation of the nucleation promoting region 60 by
The surface roughness of the nucleation promoting region 60 may be higher than that of the heater region 50. For example, as shown in FIG. 6, the surface of the silicon nitride film 14 may be roughened by etching to form the nucleation promoting region 60. This makes it easier to generate bubbles in the nucleation promoting region 60 having a higher surface roughness than on the smooth heater region 50.

In this embodiment, after the step of FIG. 4D in the first embodiment, the silicon nitride film 14 is formed in the step of FIG. Then, as shown in FIG.
7 and the heater region 50 are protected by the resist 18, and as shown in FIG. 7B, portions other than the portion protected by the resist 18 are processed by, for example, dry etching to increase the surface roughness of the surface. I do. After this, FIG.
By performing the step shown in (d), the flow sensor shown in FIG. 6 is manufactured.

Instead of roughening the surface of the silicon nitride film 14, a film 62 having a high surface roughness such as polycrystalline silicon is formed as shown in FIG.
It may be set to 0. Also in this case, bubbles can be easily generated in the region where the film 62 having a higher surface roughness than the smooth heater region 50 is formed.

It should be noted that, in addition to the configuration shown in FIG.
Even if only the concavo-convex region 60 and the regions 61 and 62 having higher surface roughness than the heater region 50 as shown in the second embodiment are formed, boiling occurs around such a region. Boiling can be suppressed. Third Embodiment In the first and second embodiments described above, the surface of the heater region 50 is flattened to suppress the generation of bubbles in the heater region 50. However, as shown in FIG. Alternatively, a film 70 having good wettability (hydrophilicity) may be formed on the heater region 50. The hydrophilic film 70 is a bubble film 7
Since the adsorption force to zero can be reduced, it is possible to promote the separation of bubbles from the film 70. As a result, generation of bubbles in the heater region 50 can be suppressed.

The mechanism in this embodiment will be described.

As shown in FIG. 10A, if a drop of liquid is dropped on the substrate, the interface tension σ (liquid-gas) between the liquid and gas in the tangential direction of the liquid on the hemisphere becomes Is the interfacial tension σ (solid-liquid), and between the substrate and the bubbles is σ (solid-gas)
The interfacial tension between the liquid and the gas is generated at an angle of θ with respect to the substrate. In this case, σ (solid-gas), σ (solid-liquid), σ
(Liquid) has the relationship of Equation 1.

[0044]

Σ (solid-gas) −σ (solid-liquid) = σ (liquid-gas) cos θ Further, assuming that the substrate is immersed in a liquid and the substrate is heated to generate bubbles, similarly, FIG. The result is as shown in FIG. At this time, since the buoyancy and the adhesion to the substrate of the gas bubbles are balanced, the relationship of Expression 2 is established (Fritz W., Phys. Zeitsch., 3).
6 (1935), p. 379).

[0045]

(Equation 2)

Here, D is the bubble diameter, σ (liquid-gas) is the interfacial tension, θ is the contact angle at the time of separation, g is the gravitational acceleration, ρ _v is the density of gas, and ρ _l is the density of liquid.

From equation (2), it can be seen from the equation that the interfacial tension σ (liquid-gas) is small (when the substrate is replaced, σ (solid-gas) is large or σ
(Small (solid-liquid)), that is, if the film has good wettability (is hydrophilic), the bubble diameter D becomes small, and the bubbles are released quickly.

Accordingly, as shown in FIG. 9, by forming the film 70 having good wettability (hydrophilicity) on the heater region 50, bubbles on the heater region 50 can be easily released. it can. Here, as a material having good wettability and exhibiting hydrophilicity, for example, a material obtained by irradiating TiO ₂ (titanium oxide) with ultraviolet rays can be used. When TiO ₂ is irradiated with ultraviolet rays, the TiO ₂ becomes in a super-hydrophilic state. By forming such a film, the release of bubbles can be promoted.

In the various embodiments described above, the thin film structure of the flow rate detector 30 and the heater 40 has a diaphragm structure, but may have a bridge structure. In addition, the flow rate detector 30 includes the heater 4
0 may be provided not only on one side but also on both sides. In this case, the flow rate is measured based on the detected temperature difference between the flow rate detectors 30 provided on both sides.

[Brief description of the drawings]

FIG. 1 is a perspective view of a flow sensor according to a first embodiment of the present invention.

FIG. 2 is a sectional view taken along the line AA in FIG.

FIG. 3 is a diagram showing a cross-sectional configuration of a comparative example with respect to the embodiment shown in FIGS. 1 and 2;

FIG. 4 is a process chart showing a method of manufacturing the flow sensor according to the first embodiment of the present invention.

FIG. 5 is a process chart showing a step shown in FIG. 4;

FIG. 6 is a cross-sectional view showing a first example of a flow sensor according to a second embodiment of the present invention.

FIG. 7 is a process chart showing a partial manufacturing method of the flow sensor shown in FIG. 6;

FIG. 8 is a cross-sectional view illustrating a second example of the flow sensor according to the second embodiment of the present invention.

FIG. 9 is a sectional view showing a flow sensor according to a third embodiment of the present invention.

FIG. 10 is an explanatory diagram for explaining a mechanism of a flow sensor according to a third embodiment of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Silicon substrate, 1a ... Hollow part, 10 ... Diaphragm, 11 ... Silicon nitride film, 12 ... Silicon oxide film, 1
3 ... silicon oxide film, 14 ... silicon nitride film, 15 ... convex part, 20 ... fluid temperature detector, 30 ... flow rate detector, 40 ...
Heater, 50: heater area, 60: uneven area, 61: area with high surface roughness, 62: film with high surface roughness such as polycrystalline silicon.

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Toshimasa Yamamoto 1-1-1, Showa-cho, Kariya-shi, Aichi F-term in DENSO Corporation (reference) 2F035 AA02 EA08

Claims (20)

[Claims] 1. A cavity (1a) formed in a substrate (1).
A micro-heater having a heating element region (50) provided with a heating element (40) formed therein, wherein when the surface of the heating element region (50) is covered with a liquid, A micro heater characterized in that the heating element region (50) has a flat surface so that nucleation of evaporation does not occur. 2. The micro-heater according to claim 1, wherein a nucleation promoting region (60) for promoting the nucleation is formed in a portion other than the heating element region (50). 3. A cavity (1a) formed in a substrate (1).
A micro-heater having a heating element region (50) in which a heating element (40) is formed inside, and a nucleation promoting region (50) for promoting nucleation of evaporation when a liquid is attached. 60) is formed at a location other than the heating element region (50). 4. The micro heater according to claim 2, wherein the nucleation promoting region (60) is formed around the heating element region (50). 5. The micro-heater according to claim 2, wherein the nucleation promoting region (60) is a region provided with a projection (61) on the surface. 6. The nucleation promoting region (60) is a region where the surface roughness of the surface is higher than the surface roughness of the heating element region (50). The micro heater according to one. 7. The microstructure according to claim 6, wherein the region having a high surface roughness of the surface is a region in which the surface roughness of the surface of the uppermost film of the thin film structure is higher than that in a film formation state. heater. 8. The micro-heater according to claim 6, wherein the region having a high surface roughness is a region in which the surface film is a polycrystalline film. 9. A cavity (1a) formed in a substrate (1).
A micro-heater having a heating element region (50) having a heating element (40) formed therein, wherein the surface film of the heating element region (50) is a hydrophilic film (7).
0) A micro heater characterized in that: 10. The hydrophilic film (70) is made of TiO ₂
10. The micro heater according to claim 9, wherein the micro heater is formed by using. 11. A cavity (1) formed in a substrate (1).
a) a thin film-structured micro heater having a heating element region (50) having a heating element (40) formed therein, the evaporation of the liquid in the heating element region (50). A microheater configured to prevent nucleation from occurring. 12. A plurality of films (11 to 1) on a substrate (1).
4, 20, 30, 40) in a method of manufacturing a micro heater having a thin film structure having a heating element region (50) in which a heating element (40) is formed. The film formed after the formation (1
3) A method for manufacturing a micro heater, comprising the step of flattening a film surface by eliminating irregularities caused by the formation of the heating element (40). 13. The step of flattening the unevenness is performed by CMP.
(Chemical Mechanical Polis
13. The method for manufacturing a micro heater according to claim 12, wherein the step of flattening the film surface by ching). 14. The method according to claim 12, wherein the step of flattening the irregularities is a step of flattening the film surface by etch-back. 15. A plurality of films (11 to 1) on a substrate (1).
4, 20, 30, 40), wherein the heating element region (50) has a heating element region (50) in which a heating element (40) is formed. A nucleation promoting region (60) that promotes nucleation of evaporation when liquid is attached to places other than
Forming a micro-heater. 16. The method according to claim 15, wherein the step of forming the nucleation promoting region (60) is a step of forming projections (61) on the surface. 17. The method according to claim 15, wherein the step of forming the nucleation promoting region (60) is a step of roughening the surface by etching. 18. The method according to claim 15, wherein the step of forming the nucleation promoting region is a step of forming a polycrystalline film as a surface film. Method. 19. A plurality of films (11 to 1) on a substrate (1).
4, 20, 30, 40), wherein the heating element region (50) has a heating element region (50) in which a heating element (40) is formed. Forming a hydrophilic film (70) as a surface film of (1). 20. A flow sensor comprising the micro heater according to claim 1 and configured to detect a flow rate of a fluid.