HK1170305B - Rapid response relative humidity sensor using anodic aluminum oxide film - Google Patents

Description

Rapid-response relative humidity sensor applying anodic aluminum oxide film

Technical Field

The present invention relates to anodic alumina thin film sensors, and more particularly, to a rapid response capacitive relative humidity sensor based on nanostructured alumina thin film materials.

Background

Humidity sensors are used in a wide range of applications, for example in weather stations, air conditioners, household and office appliances, and industrial instruments, among others. For electronic type relative humidity sensors, polymer based humidity sensors have monopolized the market due to their low cost and mature manufacturing processes. For high performance humidity sensors, semiconductor material based sensors and ceramic material based sensors are more commonly used.

Anodic porous alumina (AAO) membranes (aluminum foils anodized in an acidic solution) are well known ceramic materials for the manufacture of humidity sensors. The humidity sensitive anodized aluminum oxide layer on the first aluminum substrate was reported in 1953, which found that the capacitance of the structure increased with relative humidity. Since the development of alumina materials of regular honeycomb structure in 1978, moisture-sensitive materials based on alumina have also attracted increasing attention. Research on high performance and reliable sensors based on the novel nanostructured materials is very popular.

The creation of a protective AAO layer by anodic oxidation is a well established technique, for example, the application of protective AAO layers on aluminum structural materials like window frames. However, the anodization techniques used to create the protective AAO application are not suitable for direct use to create an AAO-based humidity sensor. The humidity response of the AAO film is largely dependent on the anodization parameters. For example, existing humidity sensors often have a capacitance versus humidity response that is non-linear, with a "flat" response in low or high humidity; the sensor cannot operate in the full humidity range. Long-term stability of humidity sensors is another major concern; existing AAO sensors can age when exposed to high humidity environments. Previous solutions to the technical problems of existing humidity sensors have been based on complex structural designs and/or manufacturing processes, which have resulted in unacceptable manufacturing costs in commercial production. Therefore, limited progress has been made only in the field of forming absolute humidity sensors based on anodized aluminum.

In view of the foregoing, there is a need in the art for an anodized aluminum based, low cost, fast response humidity sensor whose capacitance response is substantially linear with humidity and which can measure a full range of humidity conditions.

Disclosure of Invention

The present invention provides a simple, low cost, and reliable process for manufacturing stable, fast response, and full range humidity sensors based on anodized aluminum film. Anodized aluminum films contain nanoscale channels with well-controlled pore diameter (from a few nanometers to hundreds of nanometers), pore depth, and pore density by adjusting the anodization conditions. In the extended surface area of the porous structure, high humidity sensitivity and fast response can be achieved. Also, the anodized aluminum film is thermally and chemically stable, and can form a stable humidity sensor that can be used even in harsh environments.

Drawings

FIG. 1: schematic side view of an anodic aluminum oxide film based capacitive humidity sensor.

FIG. 2: a flow chart of a process for manufacturing an anodized aluminum oxide film based capacitive humidity sensor.

FIG. 3: a humidity sensor according to one embodiment of the present invention.

FIG. 4: a humidity sensor according to yet another embodiment of the present invention.

FIG. 5: for the AAO-based humidity sensor of the present invention, the capacitance versus humidity curves are at 5 ℃, 20 ℃, 35 ℃, 50 ℃.

Fig. 6A and 6B: AAO-based humidity sensor capacitance versus humidity curves measured at 20 ℃ before and after heat treatment.

Detailed Description

Turning in detail to the drawings, embodiments of the present invention illustrate a capacitive humidity sensor employing an anodized aluminum oxide film. FIG. 1 is a schematic view of a sensor. The various elements are not shown to scale relative to each other.

The body of the sensor is an Anodized Aluminum (AAO) film 120 having a thickness of about 2-20 μm. The AAO film 120 is sandwiched between a top metal layer electrode 130 having a thickness of about 20-200nm and an aluminum substrate 110 as another electrode. The AAO thin film includes nanoscale channels having diameters of about 10-100 nm. In operation, water vapor is drawn onto the channel surfaces of the AAO membrane, changing the capacitance of the AAO membrane. By careful control of the nanostructure film thickness and the size of the channels therein, a large surface area can be provided by the nanoscale channels, resulting in a fast response humidity sensor.

Referring to the flow chart of FIG. 2, the process of manufacturing the humidity sensor of the present invention is next outlined. The conditions of the following process are exemplary conditions for forming the particular dimensions of fig. 2.

In step 201, the starting material is an aluminum sheet with a thickness of 0.6 mm. To produce the proper shape of the capacitive wetness sensor, the aluminum sheet is pressed into the shape shown in FIG. 3.

Alternatively, in step 201, the starting material is an aluminum sheet with a thickness of 0.8mm and is pressed into the shape shown in fig. 4.

In step 202, the aluminum template constructed in step 201 is anodized. In one exemplary embodiment, a film having a channel and a thickness of 15 μm is formed, wherein the channelThe diameter of the track is 40 nm. For membranes of this size, anodization was performed in 0.3M oxalic acid at 20 ℃. The current density is kept at 4A/dm²The voltage is kept at about 30V. For a 15 μm film, a 4 hour anodization time was used. After anodization, the sensor plate is rinsed in water to remove acid residues and then dried in an environment of elevated temperature.

For the manufacture of the humidity sensor, the channel diameter is preferably in the range of 10 to 100nm and the thickness is 2-20 μm. This can be adjusted by varying the composition and concentration of the acid, as well as adjusting the current density and operating temperature and time. Typically, higher voltages result in larger channel diameters, and the thickness of the anodized alumina is generally determined by the time of anodization. The technique of anodizing is generally classified into a potentiostatic anodizing technique and a galvanostatic anodizing technique. In potentiostatic anodization, a more uniform channel size is achieved through the entire anodized layer. However, the current density and the growth rate of alumina decrease with time. In galvanostatic anodization, the alumina increases at a more uniform rate, although the voltage and pore diameter increase with time of anodization. Combinations of these techniques can be used sequentially to achieve the desired film layer structure and commercially acceptable processing times.

FIG. 3 schematically depicts a side view of a humidity sensor structure of an embodiment, wherein 310 is an unanodized aluminum substrate, 320 is anodized aluminum oxide on top of the aluminum substrate 310, and 330 is a permeable metal layer formed over the AAO and aluminum substrates. The permeable metal layer may be a porous metal layer, such as a porous aluminum metal layer that covers the top of the channel edges and allows water vapor to permeate to the AAO layer.

FIG. 4 schematically depicts a side view of another embodiment of a humidity sensor structure. 410 is an unanodized aluminum substrate, 420 is an AAO formed on top of the aluminum substrate 410, 430 is an AAO layer 420 and a permeable metal layer on top of the aluminum substrate 410, and 440 is a solderable electrode pin attached to the sensor by a spring contact or conductive paste.

To fabricate the humidity sensor of fig. 3 and 4, only regions 320 and 330 and regions 420 and 430 of fig. 3 require an anodized layer. Regions 310 and 410 must be conductive to function as electrodes. This can be achieved by different methods:

i) masking regions 310 and 410 (e.g., protective covering) during anodization, or avoiding regions 310 and 410 from contacting acidic solution during anodization;

ii) anodizing the entire template structure (i.e., the entire shape of fig. 3 and 4 without electrode 440), followed by removing the anodized layer from regions 310 and 410; or

iii) anodizing the entire template and depositing a metal layer on regions 310 and 410 and creating contact between the deposited metal layer and the aluminum substrate.

The permeable metal layers 330 and 430 are preferably formed by deposits on the surface of the AAO layer in these regions. The thickness of the conductive layer is preferably 20-200 nm. The conductive layers are preferably of the same material, such as gold or aluminum. The deposit can be made, for example, by sputter coating. As mentioned above, since the AAO layer is itself porous, the deposited metal layer will cover the edges of the porous channels, resulting in the metal layer permeating moisture.

Since only regions 330 and 430 are covered by the conductive layer in fig. 3 and 4, regions 310 and 320 and regions 410 and 420 may be masked or masked during deposition of the conductive layer.

To achieve a more linear response of the humidity sensor of the present invention, the structure of fig. 3 and 4 is heat treated after the metal deposition process described above. Exemplary heat treatment conditions are listed below:

i) annealing the structure at a temperature of 85 ℃ under 90% relative humidity for 5-50 hours; or

ii) annealing the structure in dry air at a temperature of 90 ℃ for 5 to 50 hours; or

iii) soaking the structure in boiling water at a temperature of 100 ℃ for 10-100 minutes.

For the embodiment of fig. 3, the fabricated sensor utilizes regions 310 (bare aluminum pins) and 330 (metal pins on AAO on aluminum base) as two electrodes. The structure of fig. 3 can be inserted directly into the circuit as a humidity sensitive capacitor.

For the embodiment of FIG. 4, two external pins 440 are assembled in the configuration as shown, as electrodes for the humidity sensor. Two pins are attached to the structure by spring contacts or conductive glue, one of which is in contact with region 410 (bare aluminum region) and the other pin is in contact with region 430 (metal region on AAO on aluminum base) and serves as two electrodes. The two pins are optionally made of a solderable metal or metal alloy, such as brass and tin. The device fabricated as described above can be inserted or soldered into a circuit as a humidity sensitive capacitor.

In an exemplary process of manufacturing an AAO-based relative humidity sensor (fig. 2), the main processes include: the method comprises the steps of constructing an aluminum template (201), anodizing (202), electrode deposition (203), heat treatment (204), and pin assembly (205, optional).

The simple template design of fig. 3 and 4 allows for the production of a final humidity sensor with high productivity and low manufacturing costs. In the batch production, the template design can be customized according to the specific conditions of the equipment of anodic oxidation, electrode deposition and heat treatment, thereby realizing the high-efficiency and simple production flow.

The present invention utilizes an anodized aluminum film as a sensing element. The thin anodic alumina membrane contains nanoscale channels that have fully controllable pore diameters, depths, and densities by adjusting the anodization conditions. For example, FIG. 5 is obtained from a sensor having nanoscale channels with pore diameters of about 40nm and depths of about 15 μm. The capacitance and humidity curves of FIG. 5 were measured at 5 deg.C, 20 deg.C, 35 deg.C and 50 deg.C, respectively. As a result of the process of the present invention, the curve exhibits an approximately linear response over the full humidity range of 0-100% relative humidity. At 20 ℃ and 60% relative humidity, the typical capacitance is 165 pF.

The sensitivity of the sensor of the present invention can be improved by increasing the effective sensing area of the AAO film or decreasing the thickness of the AAO film. The response time is mainly determined by the pore size of the nanochannel. For example, for a sensor having nanoscale channels with pore diameters of about 40nm, the response time is less than 1 second at a humidity range of 30-90% relative humidity. For another sensor having nanoscale channels with pore diameters less than 10nm, the response time is about 6 seconds at a humidity range of 30-90% relative humidity.

The heat treatment improves the performance of the sensor. As shown in fig. 6A, the sensor fabricated before heat treatment exhibited a non-linear response. In a flat relative humidity region below 30% relative humidity, the capacitance read is almost the same. Thus, the humidity sensing range of the device is limited to about 30-100% relative humidity. For the same sensor after the heat treatment shown in fig. 6B, the capacitance response becomes lower. However, the capacitance versus humidity curve exhibits an approximately linear response over the full humidity range of 0-100% relative humidity. Furthermore, the hysteresis of the sensor manufactured according to the invention is negligible.

The heat treatment also improves the stability of the sensor. The heat treated sensor maintained the same capacitance and humidity response for about one year when it was operating normally at temperatures below 50 c and humidity below 90% relative humidity. While sensors that have not been heat treated are less stable and age rapidly under high humidity conditions.

Industrial applicability

Compared to previous AAO humidity sensors, the sensor made according to the present invention combines the following features: (1) competitive sensitivity and response time, (2) operation in the full humidity range of 0-100% relative humidity, (3) relatively long-term stability, and (4) a simple manufacturing process at low cost. It is suitable for mass production and has wide applications, for example, in air conditioning systems, humidifiers, dehumidifiers, industrial processes such as annealing processes requiring precise humidity control, and measurement of ambient humidity for weather forecasting, etc.

Claims (8)

1. A capacitive humidity sensor having a substantially linear response and capable of detecting relative humidity over the full range of 0-100% relative humidity, the humidity sensor comprising:

an aluminum substrate forming one electrode of said capacitive humidity sensor;

an anodized aluminum thin film formed by anodizing at least a part of the aluminum substrate, the anodized aluminum thin film being formed on a part of the aluminum substrate to have a thickness of 2 to 20 μm, the anodized aluminum thin film having a multi-channel structure for absorbing water vapor from the ambient environment, the channel diameter being 10 to 100 nm; and

a porous metal layer deposited on a portion of said anodized aluminum film formed on an aluminum substrate, said porous metal layer forming a second electrode of said capacitive humidity sensor, said porous metal layer having a thickness of 20-200nm, said porous metal layer overlying the tops of the channel edges and allowing water vapor to permeate to the anodized aluminum film, said aluminum substrate, anodized aluminum film and porous metal layer being heat treated to produce a substantially linear humidity and capacitance sensor response.

2. The capacitive humidity sensor according to claim 1, having a response time of less than 1 second.

3. The capacitive humidity sensor according to claim 1, further comprising metal electrode pins extending from the aluminum substrate and the porous metal layer.

4. The capacitive humidity sensor according to claim 1, wherein said aluminum substrate is flat and has two integrally formed electrode legs extending from said aluminum substrate, one electrode leg comprising unanodized aluminum and the other electrode leg comprising anodized aluminum, wherein said anodized aluminum has a porous metal layer formed thereon.

5. The capacitive humidity sensor according to claim 1, wherein the aluminum substrate is flat, said aluminum substrate having an additional electrode leg extending from a non-anodized portion of the aluminum substrate and another additional electrode leg extending from an anodized portion of the aluminum substrate, wherein the anodized portion of the aluminum substrate has a porous metal layer formed thereon.

6. A method for manufacturing the capacitive humidity sensor of claim 1:

forming an aluminum template from an aluminum substrate;

anodizing at least a portion of the aluminum template to form an anodized aluminum film, wherein a combination of potentiostatic anodization techniques, in which a more uniform channel size is achieved through the entire anodized aluminum film, and galvanostatic anodization techniques, in which alumina is increased at a more uniform rate, are used in sequence; wherein the anodic aluminum oxide film comprises nanoscale channels, and the diameter, the depth and the density of the pores of the nanoscale channels can be completely controlled by adjusting the anodic oxidation conditions; wherein, during anodization, the entire aluminum template is anodized to form an anodized aluminum film, and then the anodized aluminum film is removed from areas that must be electrically conductive to serve as electrodes;

depositing a porous metal layer on at least a portion of the anodized aluminum template; and

heat treating the resulting multilayer structure to produce a humidity sensor having a substantially linear humidity to capacitance response, wherein the heat treating is one of:

annealing at 85 deg.C for 5-50 hr under 90% relative humidity; annealing at 90 deg.C in dry air for 5-50 hr.

7. The method of claim 6, wherein the anodic oxidation is performed in oxalic acid, or sulfuric acid, or phosphoric acid, or nitric acid, or a mixture thereof.

8. The method of claim 6, wherein the porous metal layer is aluminum, or copper, or gold, or platinum, or palladium, or nichrome.