The present invention relates generally to humidity sensors, more particularly to capacitive humidity sensor.

BACKGROUND

Conventional capacitive humidity sensor consists of substrate, insulator layer, interdigited fingers and contact pad. Polyimide is optionally used as moisture sensitive film. The contact pad should be in contact with humidity area to be measured.

The packaging for sensor area interdigited area must be exposed. However, wire bonds must be protected by resin. Resin can flow to the sensing part and influence the measurement. So the contact pads must be placed within a safe distance from fingers that are far from sensing membrane. The size of sensor is increased to maintain the distance between contact pad and sensing membrane.

US patent 6,580,600 is an example of humidity sensor, particularly a capacitive type humidity sensor. The capacitance type humidity detecting sensor has two electrodes opposing each other with a gap interposed between to form a capacitor on a silicon substrate with a silicon oxide film formed on a surface. A moisture sensitive film is formed so as to cover the two electrodes with a silicon nitride film interposed between. The silicon nitride film protects the two electrodes from moisture passing through the moisture sensitive film. The capacitance formed between the two electrodes changes in accordance with ambient humidity. A switched capacitor circuit formed in a circuit element portion processes a signal indicative of a change in the capacitance formed between the two electrodes.

There are many designs of humidity sensors, but most of the humidity sensors are complex and bulky. Prior art humidity sensor substrate is built on rigid and solid sensor platform, thus limiting the application in term of flexibility application on miniature and vital area measurement. Prior art humidity sensor design based on side by side or horizontal position of contact pad formation with the sensing area thus produce bulky sensor. Sensor size become saturated and cannot be miniaturize for further development. It is an object of the invention to provide a thin layer humidity sensor so that the size of the sensor can be further reduced.

SUMMARY OF INVENTION

The proposed structure provides a novel humidity sensor wherein the electrode contact pad is located at the bottom of sensing membrane. A thin layer of flexible humidity sensor can thus be produced.

This invention proposes all electrode contact pads to be formed at the midplane of the sensing area, with vertical contact pad formation, therefore reducing the whole size of the sensor. Moisture sensitive films are formed around the electrode contact pads. By forming a novel double sided and folded structure, the thin layer humidity sensor sensing area is maximized and sensor size is reduced. Consequently, the new design will expand the sensor limitation.

BRIEF DESCRIPTION OF DRAWINGS

The invention will now be described in greater detail, by way of an example, with reference to the accompanying drawings, in which: Fig. 1 shows a diagram of contact pads formed above silicon and oxide;

Fig. 2 shows a diagram of polyimide being patterned to cover contact pads;

Fig. 3 shows a diagram of metal being patterned above contact pads;

Fig. 4 shows a diagram of a second layer of contact pads patterned above metal; Fig. 5 shows a diagram of another layer of polyimide being patterned to cover contact pads;

Fig. 6 shows a diagram of a third layer of contact pads patterned above metal;

Fig. 7 shows a diagram of humidity sensor being connected; Fig. 8 shows an embodiment of humidity sensor; and Fig. 9 shows another embodiment of humidity sensor.

DESCRIPTION OF EMBODIMENTS

A method to fabricate humidity sensor will now be described. Silicon is prepared as substrate (20). A layer of oxide (22) is deposited on the silicon. Then, a layer of metal, selected from a material from a group consisting of platinum and aluminum, is selectively patterned and deposited on the oxide to form a pair of electrode contact pads (24) spaced apart, as shown in Fig. 1. A pair of contact pad is used as a basis as a capacitor.

Then, a first moisture sensitive film (26), particularly polyimide layer is coated and patterned around contact pads, as shown in Fig. 2. Polyimide layer acts as the sensing membrane. Then, a layer of metal (28) is deposited and patterned on top of contact pad, as shown in Fig. 3. Next, a second layer of contact pad (24) is deposited and patterned on top of the second layer of metal, as shown in Fig. 4. A layer of polyimide (26) is then coated and cured, as shown in Fig. 5. The first and second polyimide layer is coated to protect humidity sensor with CMOS device in between that two polyimide layer. Moving towards matured technology, fabricated humidity sensor using CMOS standard micro-fabrication surface micromachining offers tremendous and extreme miniaturization of the sensor devices as well as performance and sensitivity. The process was developed to be fully compatible with standard CMOS-IC processes.

The process can be optionally repeated to create another layer of metal (28) and contact pad (28), as shown in Fig. 6. The reason for creating another layer of contact pad for double sided contact pad area as design with double sided sensing area. Connection between sensor and other devices (30) can be formed, as shown in Fig. 7, to other module such as micro-sensor device or CMOS circuit, thus integration of CMOS and micro-sensor in the same platform can be produced.

Monolithic integration with integrated circuit or conditioning circuit can be formed with parallel fabrication process. The proposed on chip integration connection is formed during fabrication process for miniaturizations and integrated sensor system. The silicon and oxide layer is then removed with hydrofluoric acid. This etchant is CMOS compatible material as it used for thin layer oxide etch. It releases the sensor from the substrate. Now, a basic humidity sensor having a single contact pad area is formed, as shown in Fig. 8. Notice that a pair of l-shaped electrodes of a capacitor is formed, wherein the capacitance measured changes according to humidity. A double sided contact pad area sensor, after etched is shown in Fig. 9. The contact pads (24) are exposed at two sides of the polyimide film (26).

The contact pad, having metallic fingers, sandwiched with polyimide layers are capacitive to function as humidity sensors for measuring humidity as well as moisture. Polyimide layers act as dielectric material between capacitive contact pads. Polyimide also absorbs water molecule, which changes the permittivity between contact pads. The unique arrangements between combinations of polyimide layer within each porous insulating layer are claimed as the invention. Increased sensitivity of capacitance is achieved due to thicker permittivity of dielectric to absorption water molecule. Multi-layer electrodes are used for deeper and wider penetration area of water vapor absorption of the sensor, in order to increase the total capacitance and sensor sensitivity. For this novel design invention, the same sensor could now be used to measure water vapor and soil water.

The contact pads can be made to be symmetrical with the second polyimide layer and third contact pad layer, as shown in Fig. 9. Hence, it can be attached to package at either side. Folded structure allows flexible attachment of double sided sensing area application; front or back side have same properties and performances as humidity layer sensor. The selection of sensing area to face depends on user choice.

Accordingly, the invention disclosed a humidity sensor. It is the combination of the above features and its technical advantages give rise to the uniqueness of such invention. Although the descriptions above contain much specificity, these should not be construed as limiting the scope of the embodiment but as merely providing illustrations of some of the presently preferred embodiments.