JPH06160326A - Semiconductor thin film gas sensor - Google Patents

Abstract

PURPOSE:To prevent diffusion of a noble metal into a metal oxide semiconductor and to stabilize a resistance value against a change with time by forming a diffusion preventing layer between a lower electrode constituted of the noble metal and the metal oxide semiconductor and between the lower electrode and an upper electrode. CONSTITUTION:A lower electrode 3 formed of a platinum strip is attached on one side of an insulating base 1 being an alumina base, while a heater 2 is attached on the other, and a diffusion preventing layer 7 constituted of titanium nitride or the like is so formed as to cover the lower electrode 3. Moreover, an upper electrode 4 constituted of the same gold or the like as a lead 6 is formed as a lead takeout part, and an oxide semiconductor thin film layer 5 of which the main constituent is indium oxide or the like is formed between the electrodes 3 covered with the diffusion preventing layers 7. Thereby diffusion of the electrode 3 into the thin film layer 5 is prevented, a change with time of a resistance value in clean atmosphere is stabilized to be small and high sensitivity to a gas of low concentration can be obtained even in measurement for a long period. Besides, it is possible to prevent diffusion into the lower electrode 3 of the gold lead 6 welded by using a wire bonding technique and to eliminate disconnection of the lead and a break of a pattern.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a structure of a semiconductor thin film type gas sensor for detecting various gas concentrations.

[0002]

2. Description of the Related Art Conventionally, many examples of gas sensors using a metal oxide semiconductor as a gas sensitive material have been proposed. The principle is that when an n-type semiconductor such as zinc oxide, tin oxide, or indium oxide is used, its resistance value increases when it comes into contact with oxidizing gas, and it decreases when it comes into contact with reducing gas. Are using. On the other hand, when a P-type semiconductor is used, its resistance value decreases due to contact with an oxidizing gas, and its resistance value increases upon contact with a reducing gas. At present, semiconductor thin-film gas sensors have been provided for the purpose of downsizing gas sensors, integrating them for improved selectivity and multifunction, and reducing variations. A schematic structure of a conventional semiconductor thin film gas sensor is shown in FIG. A conventional gas sensor is a semiconductor thin film type gas sensor in which the above metal oxide semiconductor is formed as a thin film so as to cover a lower electrode made of a noble metal or the like. Further, in the semiconductor thin film gas sensor, it is necessary to reduce heat radiation in consideration of heating and heat retention, and a fixing method using a noble metal wire having a small wire diameter is generally used. As a fixing method, there is a method that effectively uses an assembly technique such as wire bonding established in the semiconductor field.

[0003]

However, the noble metal used as the lower electrode is a material that easily reacts with other substances as used as a catalyst or the like. Therefore, there is a drawback that direct contact with the metal oxide semiconductor causes diffusion of the noble metal into the metal oxide semiconductor and causes a change in the resistance value of the semiconductor thin film gas sensor with time. Furthermore, gold is used as a fine precious metal wire in wire bonding established in the semiconductor field. However, platinum is used for the heating element and electrodes of the semiconductor thin film gas sensor. Therefore, there is a drawback that different kinds of metals are bonded and diffusion occurs between gold and platinum, leading to cutting of leads and cutting of patterns.

[0004]

The present invention has been made by paying attention to the above situation. The means to do so is that in a semiconductor-type gas sensor in which a metal oxide semiconductor is formed on a film, the formula is diffused between a lower electrode made of a noble metal and a metal oxide semiconductor, and a formula between the lower electrode 3 and the upper electrode 4. It is characterized in that a preventive layer is formed.

[0005]

In the diffusion prevention layer established in the field of semiconductors, it is known that the diffusion prevention effect is produced by using a material having a higher melting point than the material of the upper and lower layers. In a semiconductor gas sensor, platinum is usually used for a heating element and electrodes. Therefore, M, which has a higher melting point than platinum, is used as a diffusion prevention layer.
By using o, Ta, Ti, W and further as a nitride that is stable even at a high temperature, the diffusion prevention layer becomes a layer that can obtain an excellent diffusion prevention effect and thermally stable contact.
By using such a diffusion prevention layer, it is possible to prevent the diffusion of the noble metal into the metal oxide semiconductor, stabilize the metal oxide semiconductor, and stabilize the change over time in the resistance value of the semiconductor thin film gas sensor. Becomes

[0006]

FIG. 2 is a schematic diagram for explaining an embodiment of the present invention. In this semiconductor gas sensor, as shown in FIG. 2, a lower electrode 3 made of platinum or the like is attached to one side of an insulating substrate 1 such as an alumina substrate, and a heater 2 is attached to the other side to cover the one lower electrode 3. Thus, the diffusion prevention layer 7 made of titanium nitride or the like is formed. Further, an upper electrode 4 made of gold or the like, which is the same as the lead 6, is formed as a lead extraction portion, and an oxide semiconductor thin film layer 5 containing indium oxide or the like as a main component is formed between the electrodes covered with the diffusion prevention layer. Composed. With the above structure, as shown in FIG.
In the case of the gas sensor with the conventional structure, the resistance value in the clean air has a large change with time and no stabilization tendency is observed, whereas the resistance value of the semiconductor gas sensor with the structure of the present invention has a small change with time. It was confirmed to be stable, and high sensitivity to low concentration gas was also confirmed in long-term measurement. In addition, it was confirmed that such a structure prevents diffusion to the lower electrode of the gold lead that is welded using wire bonding technology established in the semiconductor field, and eliminates lead breaks and pattern breaks. It was As shown in FIG. 4, electrodes and heaters are formed on the same surface of the insulating substrate. An insulating layer 8 of Al ₂ O ₃ or the like is provided between the electrode and the heater. It was confirmed that by adopting such a structure, a structure further utilizing the wire bonding technology can be obtained.

[0007]

EFFECTS OF THE INVENTION A diffusion prevention layer having a high melting point and being thermally stable is provided between the lower electrode made of a noble metal and the metal oxide which is a gas sensitive body, and between the lower electrode 3 and the upper electrode 4 of the lead-out portion. By forming it, there is no diffusion between the upper and lower layers, it is highly reliable for low-concentration gas, there is no change in output over time, and a structure that makes use of semiconductor technology such as wire bonding can be made compact and compact. A highly sensitive gas sensor can be provided. From the above, the present invention can provide an effective means for detecting a low-concentration gas for a long period of time, and can provide an effective means for automating and miniaturizing a gas sensor, which is industrially valuable.

[0008]

[Brief description of drawings]

FIG. 1 is a schematic sectional view of a conventional semiconductor thin film gas sensor.

FIG. 2 is a schematic sectional view of a semiconductor thin film gas sensor showing one embodiment of the present invention.

FIG. 3 is a diagram showing a change with time of a resistance value in a clean atmosphere serving as a baseline due to a difference in presence or absence of a diffusion prevention layer of a semiconductor thin film gas sensor.

FIG. 4 is a schematic sectional view of a semiconductor thin film gas sensor showing one embodiment of the present invention.

[Explanation of sign]

 1 Insulating Substrate 2 Heater 3 Lower Electrode 4 Upper Electrode 5 Oxide Semiconductor Thin Film Layer 6 Lead 7 Diffusion Preventing Layer 8 Insulating Layer

Claims (2)

[Claims] 1. A semiconductor gas sensor having a metal oxide semiconductor formed on a film on an insulating substrate having a heating element,
Between the lower electrode 3 made of a noble metal and the metal oxide semiconductor 5,
A semiconductor thin-film gas sensor, characterized in that a diffusion prevention layer (7) using at least one kind of nitride of Mo, Ta, Ti, W is formed. 2. Between the upper electrode 3 and the lower electrode 4, Mo, Ta,
The semiconductor thin film gas sensor according to claim 1, wherein the diffusion prevention layer 7 is formed by using at least one kind of nitride of Ti and W.