The invention relates to a method for manufacturing a humidity sensor and a humidity sensor.

Such relative humidity sensors are known in the art. US 4,651 ,121 discloses a moisture sensor comprising a substrate, a bottom electrode over the substrate, and an organic moisture sensitive film sandwiched between the bottom electrode and an upper electrode. The capacitance of the sensor changes when water vapor enters the moisture sensitive film. This effect is used to determine variations in the amount of water vapor in the atmosphere by detecting the corresponding changes of the capacitance. Organic material based relative humidity sensors are intrinsically submitted to degrading because of chemical ageing of the material and are heat sensitive, amongst others due to a low glass temperature transition.

US 8,783,101 B2 discloses another relative humidity sensor based on a nano structured aluminum oxide thin film. It comprises an anodic aluminum oxide thin film form from an Al substrate which also serves as one electrode. A porous metal layer is formed over the anodic aluminum oxide thin film as a second electrode. This sensor is obtained by stamping an aluminum sheet and anodizing it to form the porous aluminum oxide, then the porous metal layer is formed over the aluminum oxide by sputtering. Using solderable electrode pins connected to the electrodes using spring contacts or conductive glue, the obtained sensor can be plugged or soldered into a circuit. The fabrication method disclosed can, however, not be implemented into high yield microfabrication means which would allow to remove all mechanical assembly steps and to reduce the part to part difference. The parts assembly is tedious and can lead to a lack of alignment precision. Furthermore an unwanted

delamination of the parts can be observed.

It is therefore the object of the present invention to improve the manufacturing of relative humidity sensors using an inorganic humidity sensitive layer.

This object is achieved with a method of manufacturing a relative humidity sensor according to claim 1 comprising the steps of: a) providing a bottom substrate, b) providing a first electrode and an electrical connection means on or over the substrate, c) providing an insulating layer for electrically isolating the first electrode from the electrical connection means, d) providing an inorganic porous dielectric layer over the first electrode or the insulating layer in the area of the first electrode and e) providing a second electrode in particular a porous second electrode, in or over the porous dielectric layer by grazing incidence deposition and electrically connecting the second electrode to the electrical connection means. By providing an insulating layer it becomes possible to take advantage of microfabrication process steps leading to high yields and reliability. Using microfabrication the assembly of the various parts is simplified and alignment can be achieved within the tight limits of microfabrication in contrast to a mechanical assembly like in the prior art. At the same time the obtained sensor is less exposed to ageing than sensors with an organic dielectric and a wider temperature range, in particular for temperatures even exceeding 300 °C, can be used.

According to an embodiment, the inorganic porous dielectric layer can comprise Alumina which can be grown using high yield manufacturing methods.

According to an embodiment, step d) can comprise a step of depositing an aluminum layer and a step of growing a porous alumina layer by electrolytic growth. For example, a solution of oxalic acid can be used for the electrolytic growth, with the growth being driven by an electrolytic electrical current. Electrolytic growth can be realized in an environment of microfabrication and allows the growth of porous layers suitable for use in a relative humidity sensor.

According to an embodiment, an electric conductive drain can be provided in the insulating layer to electrically connect the aluminum layer with the first electrode. Thus during the electrolytic growth the drain allows draining the electrical current via the first electrode.

According to an embodiment, a plurality of relative humidity sensors can be formed on the same bottom substrate and the first electrodes are then provided such that they are electrically connected to each other. This feature simplifies the set-up to realize the electrolytic growth.

According to an embodiment, the porosity of the inorganic porous dielectric layer is in a range of 5% to 99%. In this range of porosity the moisture from the probed fluid or gas can enter the dielectric space and thus modify the capacitance by permeating through the porous network and can get absorbed on the structure. According to an embodiment, step e) of providing the second electrode can comprise depositing a conductive layer by grazing incidence deposition. Grazing incidence deposition has the advantage that the deposited particles or atoms do not penetrate deeply into the porous network so that the electrode layer essentially remains on the surface of the porous dielectric layer. The thickness of the penetration depth d scales as: d oc O tan cr EQ. 1

For each one of the pores, the length F being the pore diameter and a being the angle between the particle or atomic beam and the surface of the porous network.

According to an embodiment, the grazing incidence deposition can be a metal vapour grazing incidence deposition. With a vapour deposition technique, e.g. sputtering, ebeam evaporation, or any other suitable deposition method, a grazing incidence deposition can be realized by providing the target towards the side of the substrate instead of providing it opposite the surface of the substrate.

According to an embodiment, the angle of evaporated material flow and the surface of the inorganic porous dielectric layer can be less than 45°, in particular less than 30°. The lower the deposition angle, the less atoms will be deposited inside the porous layer and the lower the penetration depth into the porous material, thereby improving the electric properties of the sensor compared to a deposition under 90°.

According to an embodiment, the substrate is a passive substrate, in particular a Si substrate, or an application-specific integrated circuit (ASIC) type substrate.

According to an embodiment, the method can comprise a step of providing a conductive layer on the first electrode prior to step c). The conductive layer forms the counter electrode to the second electrode and its shape can be adapted to get an improved electric coupling and therefore improve the capacitive properties of the sensor.

According to an embodiment, the thickness of the inorganic porous dielectric layer can be in a range between 1 pm to 5pm and the lateral extension can be in a range between 10pm to 800pm at least in one dimension. Using the inventive method, it is possible to further reduce the size of the sensor while keeping the efficiency of detecting relative humidity. The object of the invention is also achieved, with a relative humidity sensor, in particular obtained by the method described above, and comprising: a substrate, a first and a electrical connection means on or over the substrate, an insulating layer over the first electrode and at least excluding the electrical connection means, an inorganic porous dielectric layer over the insulating layer and a second electrode, in particular a porous second electrode, electrically connected to the electrical connection means in or over the inorganic porous dielectric layer.

The inventive humidity sensor can be fabricated using microfabrication means with high yields of over 90% and reliability. Due the use of an inorganic dielectric, chemical ageing like for organic layers can be reduced or prevented. Further, a sensor working for temperatures of 300‘C or more can be achieved. Thus, the obtained inventive sensor functions in a range that is larger than the temperature range of sensors with an organic dielectric layer. The sensor can be used to detect humidity in gas and/or water in oil or other fluids.

According to an embodiment, the inorganic porous layer can comprise Alumina or other insulating porous inorganic material with a porosity of 5% to 99% and/or a thickness of 200nm to 10pm. According to an embodiment, the second electrode can be agold or any vapor phase deposited conductive porous layer.

The invention will now be described in greater detail using advantageous embodiments in an exemplary manner and with reference to the drawings. The described embodiments are merely possible configurations and it must be borne in mind that the individual features as described above can be provided independently of one another or can be omitted altogether while implementing this invention.

Figures 1 a-1 f schematically illustrate the method of manufacturing a relative humidity sensor according to the invention,

Figures 2a and 2b illustrate variants of the first embodiment illustrated in Figures 1 a to 1 f,

Figure 3 illustrates a top view of the porous dielectric layer,

Figure 4 shows details of the second electrode deposition by grazing incidence deposition. Figures 1 a to 1 f illustrate the method of manufacturing a relative humidity sensor according to the invention. It illustrates the formation of one relative humidity sensor, it is, however, to be understood that the described process is a micro fabrication process allowing the formation of a plurality of sensor structures at the same time.

Figure 1 a shows a substrate 1 . In the following process steps the various layers necessary to build the relative humidity sensor will be formed. The substrate 1 has already been processed using layer deposition and patterning steps, so that a first electrode 3 and an electrical connection means 5 are present in or on the surface of the substrate 1 . The substrate 1 can be a silicon substrate, a sapphire substrate or any other substrate used in microfabrication production lines. As an alternative an application-specific integrated circuit (ASIC) could also be used as a starting material of the process according to the invention. The first electrode and the electrical connection means are metallic, e.g. from Al, Cu, Au or from any other suitable material.

Figure 1 b illustrates the result of a further layer deposition and patterning step to obtain a conductive layer 7 on the first electrode 3. Preferably the conductive layer 7 is made of the same material as the first electrode. It has a thickness in a range of 10 to 500nm. The conductive layer 7 has a lateral extension in a range of 10pm to 800pm at least in one dimension.

Figure 1 c illustrates the result of an insulating layer deposition and patterning step. The obtained insulating layer 9 laterally extends over the conductive layer 7 and the first electrode 3 but does not cover the electrical connection means 5. The insulating layer 9 can be Si02 layer or made from any other suitable insulating material. It has a thickness in a range of 1 nm to 500nm.

Figure 1 d illustrates the result an aluminum deposition and patterning step. The aluminum layer 1 1 which will be transformed in a dielectric layer in the following process step, has a thickness of 1 nm to 1000nm and essentially has a lateral extension b corresponding to the lateral extension a of the conductive layer 7.

Figure 1 e illustrates the result of an electrolytic growth step transforming the aluminum layer 1 1 into a porous dielectric alumina layer. The electrolytic growth is realized in an acidic medium, e.g. oxalic acid and is electrically driven. It turns the Al layer 1 1 into the porous dielectric alumina layer 13. The dielectric alumina layer 13 has a thickness of 1 pm to 5pm and has essentially the same lateral extension as the conductive layer 7. The growth conditions are furthermore controlled such that the entire Aluminum layer 1 1 is transformed into alumina.

Figure 2a illustrates a first variant of the inventive process with additional process steps introduced between the process as illustrated by figures 1 d and 1 e. Prior to depositing the aluminum layer 1 1 , one or more via holes are etched into the insulating layer 9 in the area over the conductive layer 7 and filled with a conductive material, e.g. Al to provide an electrical drain 35 in the insulating layer 9. The drain 35 allows using the first electrode 3 as a current drain during the electrolytic growth, in particular when, on a wafer scale, the first electrodes 3 are all electrically interconnected.

Figure 2b illustrates a second variant of the inventive process. Instead of depositing the aluminum layer 1 1 onto the insulating layer 9 as illustrated in Figure 1 d, it is directly deposited onto conductive layer 7 on the bottom electrode 3. To do so, the patterning step associated with the step illustrated in Figure 1 c is realized such that the surface of the conductive layer 7 becomes at least partially free so that the aluminum, layer 1 1 can then be deposited onto layer 9. Also in this case the bottom electrodes are interconnected at the wafer scale and current drain during the electrolytic growth can be provided.

Figure 3 shows a top view onto the porous dielectric layer 13. It illustrates a channel matrix formed by Alumina walls 15 and voids 17. Depending on the growth conditions a porosity of 5% to 99% can be achieved.

Figure 1f illustrates the result of a deposition step to form a second electrode 19 over the porous dielectric alumina layer 13. The second electrode 19 is in electric contact with the electrical connection means 5. This can be achieved in the same process step or by an additional layer deposition and patterning step. In this embodiment the second electrode layer is a gold layer, but any other suitable conductor could be used in variants. The second electrode 19 has a typical thickness in a range of 0.2nm to 30nm.

Afterwards, the substrate is diced and individual sensors are packaged. When using interconnected first electrodes 3 as current drain, like explained above with respect to Figure 2a and 2b, the dicing will isolate the first electrodes 3 from each other. Figure 1f also illustrates the structure of the inventive relative humidity sensor 31. This sensor 31 functions as follows. Humidity in a gas or fluid can enter the porous channel matrix, thereby changing the dielectric properties of the porous alumina layer 13. This change in turn can be sensed by a change in the capacitance of the capacitor formed by the conductive layer 7, the porous Alumina layer 13 and the second electrode 19. Via the first electrode 3 and the electrical connection means 5 the corresponding signals can be output to the sensor’s control circuit. The absence of organic material provides long term stability and an extended temperature range in which the sensor functions. Indeed, the sensor can work for temperatures even exceeding 300 °C. Due to the use of micro fabrication process steps it is furthermore possible to mass produce with high yield and reliability.

Figure 4 illustrates schematically the deposition process of the step illustrated in Figure 1f. The deposition method used according to the invention is a grazing incidence deposition based on a vapour deposition method. In this method the flow of evaporated atoms 21 , e.g. Gold atoms, does not impinge perpendicular to the surface like in standard deposition methods but arrive on the surface of the porous dielectric alumina layer 13 under an angle a that is essentially less than 45°, in particular less than 30°, with respect to the surface 23 of the porous dielectric alumina layer 13 and taking into account the divergence of the beam. This can be achieved by providing the evaporation target 25 on the side of the substrate 1 instead of opposite to the substrate 1 like in conventional evaporation processes. In addition, the substrate 1 can be rotated around its normal axis during grazing incidence deposition to ensure the homogeneity of the deposited layer with respect to the substrate orientation.

The deposition under an angle smaller than 45°, in particular less than 30°, also has the advantage that the penetration depth d can be kept small (of. EQ. 1 ) due to the shading effect of the neighbouring alumina walls 15 leading to a shaded area 27 without deposited atoms. The penetration depth d scales like:

d oc F tan a

For each one of the pores, the length F being the pore diameter of the porous network, and a being the angle between the gold beam and the surface of the porous layer 13.

As can be seen on Figure 4, the second electrode 19 in this embodiment is porous like the underlying porous layer 13 with the alumina walls 15 and voids 17, which allows the moisture to enter the porous layer 13.

The areas 29 with deposited atoms actually form the second electrode 19. In the above description the terms deposition step and patterning relate to standard fabrication steps used in the semiconductor manufacturing. As an example deposition step can relate to chemical vapour deposition (CVD) or physical vapour deposition (PVD) and patterning step can relate to a lithography imaging and dry or wet etching step.

Modifications to embodiments of the invention described in the foregoing are possible without departing from the scope of the invention as defined by the accompanying claims.

Expressions such as“including”,“comprising”,’’incorporating”,� �consisting of”,“have”,“is” used to describe and claim the present invention are intended to be construed in a non exclusive manner, namely allowing for items, components or elements not explicitly described also to be present. Reference to the singular is also to be construed to relate to the plural.

Reference numerals:

I substrate

3 first electrode

5 electrical connection means

7 conductive layer

9 insulating layer

I I Aluminum layer

13 porous Alumina layer

15 Alumina walls

17 voids

19 second electrode

21 flow of evaporated atoms

23 surface of porous layer

25 evaporation target

27 shaded area

29 areas with deposited atoms

31 relative humidity sensor

35 electrical drain

a lateral extension of layer 7

b later extension of layer 1 1

d penetration depth

a angle between evaporated atoms impinging on surface 23