Humidity sensor and manufacturing method thereof

The invention provides a humidity sensor and a manufacturing method thereof. The humidity sensor comprises a substrate, an electrode structure, and a humidity sensing structure. The electrode structure is disposed on the substrate. The humidity sensing structure is disposed on the electrode structure. The humidity sensing structure includes a first humidity sensing layer and a second humidity sensing layer. The first humidity sensing layer is in direct contact with the electrode structure and has a first oxygen vacancy number. The second humidity sensing layer is disposed on the first humidity sensing layer and has a second oxygen vacancy number. The second oxygen vacancy number is greater than the first oxygen vacancy number.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to China Application Serial Number 201910807063.0, filed Aug. 29, 2019, which is herein incorporated by reference.

BACKGROUND

Field of Invention

The present disclosure relates to a humidity sensor and a manufacturing method thereof.

Description of Related Art

During a lamination or dispensing process in the manufacturing of mobile communication devices, the ambient humidity in the workshop may have a considerable influence on the product yield. Accordingly, if the ambient humidity of the workshop can be monitored at any time during the process, operators can readily adjust the ambient humidity to ensure a smooth process. However, a conventional humidity sensor can only measure an impedance value of 2 to 3 orders of magnitude, which is not sufficient to meet the aforementioned requirements. Therefore, a sensitivity sensor with a high sensitivity is needed.

SUMMARY

One aspect of the present disclosure provides a humidity sensor. The humidity sensor comprises a substrate, an electrode structure, and a humidity sensing structure. The electrode structure is disposed on the substrate. The humidity sensing structure is disposed on the electrode structure and comprises a first humidity sensing layer and a second humidity sensing layer. The first humidity sensing layer is in direct contact with the electrode structure and has a first oxygen vacancy number. The second humidity sensing layer is disposed on the first humidity sensing layer and has a second oxygen vacancy number greater than the first oxygen vacancy number.

In one or more embodiment, the humidity sensing structure further comprises a third humidity sensing layer disposed on the second humidity sensing layer and having a third oxygen vacancy number greater than the second oxygen vacancy number.

In one or more embodiment, the first humidity sensing layer and the second humidity sensing layer each comprises a perovskite-type oxide material with an ABO3structure.

In one or more embodiment, the electrode structure comprises an interdigital structure.

Another aspect of the present disclosure provides a method of manufacturing a humidity sensor. The method comprises steps of providing a substrate and an electrode structure, in which the electrode structure is located on the substrate; and forming a humidity sensing structure on the electrode structure. The formation of the humidity sensing structure on the electrode structure comprises forming a first humidity sensing layer at a first sintering temperature, in which the first humidity sensing layer is in direct contact with the electrode structure and has a first oxygen vacancy number; and forming a second humidity sensing layer on the first humidity sensing layer at a second sintering temperature, in which the second sintering temperature is lower than the first sintering temperature, such that a second oxygen vacancy number of the second humidity sensing layer is greater than the first oxygen vacancy number.

In one or more embodiment, the method further comprises steps of forming a third humidity sensing layer on the second humidity sensing layer at a third sintering temperature, in which the third sintering temperature is lower than the second sintering temperature, such that a third oxygen vacancy number of the third humidity sensing layer is greater than the second oxygen vacancy number.

In one or more embodiment, the first humidity sensing layer and the second humidity sensing layer each comprises a perovskite-type oxide material with an ABO3structure.

In one or more embodiment, the first sintering temperature ranges from 300° C. to 700° C.

In one or more embodiment, the second sintering temperature ranges from 200° C. to 600° C.

In one or more embodiment, the third sintering temperature ranges from 100° C. to 500° C.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, or examples, for implementing different features of the provided subject matter. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting.

One aspect of the present disclosure provides a method of manufacturing a humidity sensor.FIG. 1is a flow chart of method10of manufacturing a humidity sensor according to one embodiment of the present disclosure.FIG. 2toFIG. 5are schematic top views of various stages of the method10of manufacturing the humidity sensor according to one embodiment of the present disclosure. As shown inFIG. 1, the method10comprises step S01to step S04.

At step S01, a substrate110and an electrode structure200are provided, as shown inFIG. 2. Specifically, the electrode structure200is located on the substrate110. In one embodiment, the electrode structure200includes a first electrode layer210and a second electrode layer220. In one embodiment, the first electrode layer210has a first interdigital structure212, and the second electrode layer220has a second interdigital structure222. The substrate110includes an Al2O3ceramic substrate. The first electrode layer210and the second electrode layer220may include Ag—Pd electrode.

Next, step S02is performed to form a first humidity sensing layer on the electrode structure at a first sintering temperature. Referring toFIG. 3, a first humidity sensing layer310is formed on the electrode structure200at a first sintering temperature, in which the first humidity sensing layer310has a first oxygen vacancy number. In various embodiments, the first humidity sensing layer310is in direct contact with the electrode structure200. In one embodiment, the first interdigital structure212and the second interdigital structure222are entirely covered by the first humidity sensing layer310.

In various embodiments, the first sintering temperature ranges from 300° C. to 700° C., such as 400° C., 500° C., 600° C., 650° C., or 680° C. In various embodiments, the first humidity sensing layer310comprises a perovskite-type oxide material with an ABO3structure, such as bismuth ferrite (BiFeO3), lead zirconate titanate (Pb(ZrTi)O3, PZT), barium titanate (BaTiO3), lead titanate (PbTiO3), and the like, but is not limited thereto. In one embodiment where the first humidity sensing layer310is bismuth ferrite, the process of forming the first humidity sensing layer310includes but not limited to adding bismuth nitrate and iron nitrate to a mixed solution containing 2-ethoxyethanol and acetic acid and reacting bismuth nitrate and iron nitrate at 70° C. for 30 minutes, followed by drying overnight at 90° C. to form a paste. The paste is applied to the electrode structure200and the substrate110by spin-coating or other suitable processes. Subsequently, the paste is sintered at the first sintering temperature (i.e., 300° C. to 700° C.) to form the first humidity sensing layer310. In one example, since bismuth is volatile under high temperature, a molar ratio of bismuth nitrate to iron nitrate ranges from 1.01:1 to 1.09:1, such as 1.03:1, 1.05:1, or 1.07:1 to compensate the volatility of bismuth during the sintering process.

Next, step S03is performed to form a second humidity sensing layer on the first humidity sensing layer at a second sintering temperature. Referring toFIG. 4, a second humidity sensing layer320is formed on the first humidity sensing layer310at a second sintering temperature, in which the second humidity sensing layer320has a second oxygen vacancy number. It is noted that the second sintering temperature is lower than the first sintering temperature, such that the second oxygen vacancy number of the second humidity sensing layer320is greater than the first oxygen vacancy number of the first humidity sensing layer310. The oxygen vacancy number of bismuth ferrite varies under different sintering temperatures. The oxygen vacancy number of bismuth ferrite decreases along with an elevating sintering temperature. Conversely, The oxygen vacancy number of bismuth ferrite increases along with a decreasing sintering temperature.

In various embodiments, the second sintering temperature ranges from 200° C. to 600° C., such as 250° C., 300° C., 350° C., 400° C., or 500° C. It is understood that the second sintering temperature should be selected to be lower than the first sintering temperature. The arrangement of the difference in the first and second sintering temperatures may provide a specific technical effect, which will be described in detail below.

In some embodiments, the first humidity sensing layer310is entirely covered by the second humidity sensing layer320over the first interdigital structure212and the second interdigital structure222. The material and the forming process of the second humidity sensing layer320are similar to those of the first humidity sensing layer310and are not repeated herein.

It is understood that the method disclosed in the present disclosure includes forming at least two humidity sensing layers over the electrode structure. The at least two humidity sensing layers are respectively formed at different sintering temperatures, in which the humidity sensing layer closer to the electrode structure is formed at a higher sintering temperature, and the humidity sensing layer farther away from the electrode structure is formed at a lower sintering temperature.

It is noted that the method disclosed in the present disclosure may also include forming a plurality of humidity sensing layers over electrode structure, in which the plurality of humidity sensing layers are respectively formed at different sintering temperatures. The humidity sensing layer that is closest to the electrode structure is formed at the highest sintering temperature, and other humidity sensing layers sequentially disposed over the electrode structure are respectively formed at diminishing sintering temperatures. One of ordinary skill in the art can select the desired number of the humidity sensing layer based on actual needs. In one or more embodiment of the present disclosure, the sintering temperature ranges from 100° C. to 700° C., and one of ordinary skill in the art can select the suitable sintering temperature for each humidity sensing layer based on the desired number of the humidity sensing layer.

For example, method10may further include performing step S04, where a third humidity sensing layer is formed on the second humidity sensing layer at a third sintering temperature. Referring toFIG. 5, a third humidity sensing layer330having a third oxygen vacancy number is formed on the second humidity sensing layer320at a third sintering temperature. It is noted that the third sintering temperature is lower than the second sintering temperature, such that the third oxygen vacancy number of the third humidity sensing layer330is greater than the second oxygen vacancy number. In various embodiments, the third sintering temperature ranges from 100° C. to 500° C., such as 150° C., 180° C., 200° C., 300° C., or 400° C. It is understood that the third sintering temperature should be selected to be lower than the first sintering temperature and the second sintering temperature. The arrangement of the difference in sintering temperatures may provide a specific technical effect, which will be described in detail below.

In some embodiments, the second humidity sensing layer320is entirely covered by the third humidity sensing layer330over the first interdigital structure212and the second interdigital structure222. The material and the forming process of the third humidity sensing layer330are similar to those of the first humidity sensing layer310and are not repeated herein.

Reference is made toFIG. 5.FIG. 5is a schematic top view of a humidity sensor100according to one embodiment of the present disclosure. The humidity sensor100includes a substrate110, an electrode structure200, and a humidity sensing structure300. The electrode structure200is disposed on the substrate110. In one embodiment, the electrode structure200comprises a first electrode layer210and a second electrode layer220. The first electrode layer210has a first interdigital structure212, and the second electrode layer220has a second interdigital structure222. The humidity sensing structure300is disposed on the electrode structure200. In one embodiment, the first interdigital structure212and the second interdigital structure222are covered by the humidity sensing structure300.

Reference is made toFIG. 6andFIG. 7.FIG. 6is a schematic cross-sectional view taken along line A-A′ inFIG. 5.FIG. 7is a schematic cross-sectional view taken along line B-B′ inFIG. 5. In one embodiment, the humidity sensing structure300at least includes the first humidity sensing layer310and the second humidity sensing layer320. The first humidity sensing layer310is in direct contact with the first interdigital structure212of the electrode structure200and has a first oxygen vacancy number. The second humidity sensing layer320is disposed on the first humidity sensing layer310and has a second oxygen vacancy number. In one embodiment, the first humidity sensing layer310and the second humidity sensing layer320comprises bismuth ferrite (BiFeO3). It is noted that in the present disclosure, the second oxygen vacancy number of the second humidity sensing layer320is greater than the first oxygen vacancy number of the first humidity sensing layer310. This arrangement may provide a specific technical effect. Specifically, the oxygen vacancy number of bismuth ferrite may affect the conductivity of bismuth ferrite. Therefore, the more the oxygen vacancy number of bismuth ferrite, the lower the conductivity of bismuth ferrite. Conversely, the less the oxygen vacancy number of bismuth ferrite, the higher the conductivity of bismuth ferrite.

The inventor designed the humidity sensor of the present disclosure based on the principle above. In detail, the inventor uses bismuth ferrite to absorb the external moisture, in which the oxygen vacancy in bismuth ferrite is combined with oxygen in the external moisture. In other words, the oxygen in the external moisture can fill into the oxygen vacancy in bismuth ferrite, resulting in variation of conductivity and impedance of bismuth ferrite. By measuring the variation in the impedance value across two ends of the humidity sensor100of the present disclosure, the humidity value of the external environment can be detected. In one or more embodiment, the humidity sensor of the present disclosure is capable of measuring relative humidity of 0-100%, preferably 10-98%, and more preferably 35-95%.

Furthermore, in the humidity sensor of the present disclosure high sensitivity can be achieved as the oxygen vacancy in bismuth ferrite is combined with the oxygen in the external moisture, allowing the humidity sensor to measure an impedance value variation of more than 4 orders of magnitude. The humidity sensor of the present disclosure has a promising application prospect as compared to a conventional humidity sensor, which can measure an impedance value variation of only 2 to 3 orders of magnitude.

It is understood that the humidity sensing structure300includes at least the first humidity sensing layer310and the second humidity sensing layer320, but is not limited thereto. One of ordinary skill in the art can select the desired number of the humidity sensing layer in the humidity sensing structure300based on actual needs. Specifically, the humidity sensing structure300may include a plurality of humidity sensing layers sequentially disposed over the electrode structure200, in which the humidity sensing layer that is closest to the electrode structure200has a lowest oxygen vacancy number, and other humidity sensing layers sequentially disposed over the electrode structure200has an increasing oxygen vacancy number.

Still referring toFIG. 6andFIG. 7. In another embodiment, the humidity sensing structure300further includes a third humidity sensing layer330. The third humidity sensing layer330is disposed on the second humidity sensing layer320and has a third oxygen vacancy number. It is noted that in the present disclosure, the third oxygen vacancy number of the third humidity sensing layer330is greater than the second oxygen vacancy number of the second humidity sensing layer320.

In summary, the present disclosure provides a humidity sensor and a manufacturing method thereof. The method includes forming a humidity sensor by respectively forming at least two humidity sensing layers on an electrode structure at different sintering temperatures. The humidity sensor of the present disclosure is capable of measuring a wide range of relative humidity values and has a higher sensitivity, which is able to measure an impedance value variation of more than 4 orders of magnitude. Compared to the conventional humidity sensor, which can measure an impedance value variation of only 2 to 3 orders of magnitude, the humidity sensor of the present disclosure is eligible for accurate monitoring of the ambient humidity in a workshop during manufacturing processes. Moreover, since the humidity sensor of the present disclosure has a higher sensitivity, it can also play a role in the future application development of mobile communication devices and has a promising application prospect.