JP3383395B2 - Thin film gas sensor and driving method thereof - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thin film gas sensor for detecting the presence of gas in an atmosphere.

[0002]

2. Description of the Related Art A gas sensor is known in which a metal oxide semiconductor is used as a gas-sensitive substance and a change in resistance value is utilized by gas adsorption. The gas sensor is classified into a sintered type, a thick film type, a thin film type, etc. depending on the form. The thin film type has lower power consumption and better responsiveness than other types. Such a thin-film gas sensor detects a gas by heating a metal oxide semiconductor on an electrically insulating substrate with a heater.

[0003]

The thin film gas sensor is usually driven by being used at a high temperature of 300 to 450.degree. Since the driving temperature is an important factor that determines the sensitivity to various gases, it is necessary to accurately set the temperature of the heater. In the shape of “Substrate having overhanging portion extending in the air” (Japanese Patent Laid-Open No. 57-60887), which was filed earlier by the applicant, heat generated between the support, the heater and the electrode material by heat treatment during manufacturing. Due to the difference in expansion, the overhang portion warps.
The warp of these overhangs changes the heater resistance,
There was a problem that the control of temperature was different between individuals. According to the present invention, the structure of the overhanging portion is provided in which the stress generated by the difference in thermal expansion between the heater and the electrode and the support is mitigated to prevent the overhanging of the overhanging portion and stabilize the temperature control between the individuals. By pulsing the upper and lower heaters of the support of the thin film gas sensor of the present invention, displacement during driving is prevented,
Provided is a driving method that enables stable temperature control.

[0004]

According to a first aspect of the present invention, a substrate, a projecting portion provided on the substrate by projecting in the air, and a support functioning as an insulating layer provided on at least the projecting portion of the substrate and the projecting portion ( (This functions as an insulating layer), a metal oxide semiconductor layer for gas detection formed on the support, an electrode in contact with the metal oxide semiconductor layer, and a heater provided substantially in parallel with the electrode. In the gas sensor, a support for at least the protruding portion of the substrate and the protruding portion (this functions as an insulating layer)
The present invention relates to a thin film gas sensor characterized in that a stress relaxation layer is formed in a lower portion, and at least a part of the stress relaxation layer is formed in the same pattern as a heater lead and / or an electrode lead. By forming the stress relaxation layer under the support as described above, the stress due to the thermal expansion between the support, the heater and the electrode can be relaxed. Further, at least a part of the stress relaxation layer has the same pattern as that of the heater and the electrode, and further, the stress relaxation layer is made of the same material as that of the heater and the electrode, whereby the stress of the support can be more effectively relaxed. be able to. A second aspect of the present invention is that the stress relaxation layer formed under the support has the same layer structure as a plurality of layers formed on the support and is formed as a symmetrical layer centering on the support. Which is in the first thin film gas sensor. As described above, in the lower layer of the support, a plurality of layers having the same structure as the upper layers are symmetrically provided about the support to cancel the stress between the upper layer part including the heater and the electrode and the support. . A third aspect of the present invention is that the patterned stress relieving layer below the support of the first and second thin film gas sensors is electrically conductive. In particular, a means for pulse-driving the patterned stress-relieving layer under the support is provided with the same period as the heater on the support, and by pulse-driving by the means, the stress of the support and the upper and lower heaters is reduced. It is possible to mitigate and eliminate the change in the heater temperature during driving. A fourth aspect of the present invention relates to a method of driving the thin film gas sensor, characterized in that the heater lead portions above and below the support are pulse-driven at the same cycle.

[0005]

EXAMPLES The structure of the thin film gas sensor of the present invention will be described below based on examples, but the present invention is not limited to the following examples. Example 1 A thin film sensor having the structure shown in FIG. 2 was manufactured as follows. The shape of the sensor is the same as that of the applicant's earlier application, "Substrate having an overhang portion extending in the air" (Japanese Patent Laid-Open No. 57-608).
It has the same shape as that disclosed in 87). Substrate 1 is Si (100) that is easy to undercut and etch
To use. On the substrate 1, a layer made of a material having a thermal expansion coefficient substantially equal to that of a heater or an electrode material is laminated. As heater material, Pt, Ni, Cr, PtR
h, PtIr, SiC, TaN ₂ etc.
Examples thereof include t, Au, Rh, Ir, Ni, Cr, Mo, W, and the like. Here, the Pt layer 2 used for the heater and the electrode.
a'is formed on the substrate 1. However, since the stress varies depending on the process conditions and the like, a film thickness that does not cause warpage is set. On the Pt layer 2a ', the support 3a of the insulating layer of the overhang portion is formed in a thickness of 1.5 μm. A heater and an electrode Pt layer 2a are laminated on a support 3a, and an insulating layer 3b is further formed on the Pt layer 2a.
3 μm is formed. The heater and electrodes are patterned by photolithography etching. After patterning the heater and the electrode, an insulating layer 3c having a thickness of 0.5 μm is laminated, and the insulating layer on the electrode is removed by photolithography and etching. Next, Si of the substrate is etched to form a structure overhanging in the air.
A gas detection material is laminated on the uppermost layer of the overhang portion by 0.4 μm using a metal mask (gas detection layer 4). Finally, it is fired at 700 ° C. in an oxygen atmosphere. Layer 2a 'for relieving stress due to the coefficient of thermal expansion between the support, the heater and the electrodes
Is provided under the support to relieve stress and prevent warpage.

Example 2 A thin film gas sensor having the structure shown in FIG. 3 was manufactured as follows. In the thin film gas sensor of the example, the Pt layer 2
A'is formed by photolithography and etching similarly to the pattern 2a of the heater and the electrode. The film thickness that does not cause the warp varies depending on the process conditions and the like, so the film thickness that does not cause the warp is set. By forming a pattern similar to the heater and the electrode on the lower layer of the support, the stress caused by the thermal expansion coefficient between the support and the heater and the electrode can be more effectively offset and the warp can be prevented.

Example 3 A thin film gas sensor having the structure shown in FIG. 4 was produced as follows. The metal oxide semiconductor layer 4 ', the insulating layer 3c', the insulating layer 3b ', and the Pt layer 2a' are laminated on the Si substrate 1 symmetrically with the support as the center. The film thickness at which warpage does not occur varies depending on the process conditions and the like, so the film thickness at which warpage does not occur is set for each layer. By forming the layers symmetrically with respect to the support 3a, the stress in the support upper layer and the support lower layer is offset to prevent warpage.

Example 4 A thin film gas sensor having the structure shown in FIG. 5 was manufactured as follows. After the metal oxide semiconductor layer 4 ', the insulating layer 3c', the insulating layer 3b 'and the Pt layer 2a' are laminated symmetrically on the Si substrate 1 about the support, the Pt layer 2a 'is used as a heater and an electrode 2a. It is formed by photolithography and etching in the same manner as the pattern. The film thickness at which warpage does not occur varies depending on the process conditions and the like, so the film thickness at which warpage does not occur is set for each layer. By forming a layer symmetrically around the insulating layer 3a of the support and providing the same pattern as the heater and the electrode on the lower part of the support, the stress in the upper layer and the lower layer of the support can be more effectively offset and the warp can be prevented. prevent.

Example 5 When the support lower layer 2a 'having the same pattern as the heater and the electrode 2a is conductive, the same pulse as the heater is applied to the lower support layer. By heating in the upper and lower layers of the support,
The stress generated between the support and the upper and lower heaters during driving is offset.

[0010]

[Effect] According to the present invention, the stress due to the thermal expansion between the support and the heater and the electrode is relaxed, and the stress between the upper layer part including the heater and the electrode and the support is offset to prevent the warpage of the overhang portion. It is possible to realize a thin film gas sensor capable of preventing and controlling the heater temperature. Further, by driving the conductive layer on the lower portion of the support with a pulse having the same period as that of the heater on the upper portion, the stress on the support and the upper and lower heaters can be relaxed, and the heater temperature fluctuation during driving can be eliminated.

[Brief description of drawings]

FIG. 1 is a plan view of a sensor according to a first exemplary embodiment.

FIG. 2 is a sensor cross-sectional view (cross section A-B) of the first embodiment.

FIG. 3 is a sensor cross-sectional view (cross section A-B) of the second embodiment.

FIG. 4 is a sensor cross-sectional view (cross section A-B) of Example 3;

FIG. 5 is a sensor cross-sectional view (cross section A-B) of the fourth embodiment.

[Explanation of symbols]

1 substrate 2a Pt layer (heater lead or electrode lead) 3a Support (insulating layer) 3b insulating layer 3c insulating layer 4 Gas detector (metal oxide semiconductor) 5 Overhanging part 2a 'Pt layer 3b 'insulating layer 3c 'insulating layer 4'metal oxide semiconductor

─────────────────────────────────────────────────── ─── Continuation of the front page (56) References Japanese Patent Laid-Open No. 4-164243 (JP, A) Actual Development No. 1-91253 (JP, U) (58) Fields investigated (Int.Cl. ⁷ , DB name) G01N 27/12

Claims (6)

(57) [Claims] 1. A substrate, an overhanging portion provided in the air over the substrate, a support functioning as an insulating layer provided on at least the overhanging portion of the substrate and the overhanging portion, and on the support In a gas sensor having a formed metal oxide semiconductor layer for gas detection, an electrode in contact with the metal oxide semiconductor layer, and a heater provided substantially in parallel with the electrode, at least an overhang of the substrate and the overhang portion. A layer for stress relaxation is formed under the support of the portion, and at least a part of the layer for stress relaxation is formed.
A thin film gas sensor, which is formed in the same pattern as a heater lead and / or an electrode lead. 2. The stress relaxation layer is at least partially formed of the same material as the heater and / or the electrode.
The thin film gas sensor described. 3. The stress relaxation layer formed under the support is formed as a layer symmetrical to the support with the same layer structure as the plurality of layers formed on the support. The thin film gas sensor according to claim 1. 4. The thin-film gas sensor according to claim 2, wherein the patterned stress-releasing layer below the support is conductive. 5. The thin film gas sensor according to claim 4, wherein the pattern is provided on the lower portion of the support as a heater, and the thin film gas sensor is provided with a means for performing pulse driving with the same period as the heater on the upper portion of the support. 6. The method of driving a thin film gas sensor according to claim 5, wherein the heater lead portions above and below the support are pulse-driven at the same cycle.