Integrated circuit and manufacturing method

Abstract

Disclosed is an integrated circuit comprising a substrate (10); and an optical CO.sub.2 sensor comprising: first and second light sensors (12, 12′) on said substrate, said second light sensor being spatially separated from the first light sensor; and a layer portion (14) including an organic compound comprising at least one amine or amidine functional group over the first light sensor; wherein said integrated circuit further comprises a signal processor (16) coupled to the first and second light sensor for determining a difference in the respective outputs of the first and second light sensor. An electronic device comprising such a sensor and a method of manufacturing such an IC are also disclosed.

Claims

1. A method of manufacturing an integrated circuit comprising an optical CO.sub.2 sensor, comprising: providing a substrate including a first light sensor and a second light sensor spatially separated from each other and a signal processor coupled to the first and second light sensors for determining a difference in the respective outputs of the first and second light sensors; and forming a layer portion including an organic compound comprising at least one amine or amidine functional group for reacting with CO.sub.2 over the first light sensor.

2. The method of claim 1, further comprising: forming an interconnect structure over the substrate; forming a passivation layer over the interconnect structure, said passivation layer including a first area over the first light sensor; wherein the step of forming the layer portion includes forming the layer portion in the first area.

3. The method of claim 2, further comprising: forming a further sensor at least partially on the passivation layer by: forming a pair of electrodes on a further area of the passivation layer; depositing a gas or moisture sensitive layer over the passivation layer including the pair of electrodes; and patterning the gas or moisture sensitive layer such that the gas or moisture sensitive layer remains in the first and further areas; and wherein the step of forming the layer portion comprises forming the layer portion at least partially over the gas or moisture sensitive layer remaining in the first area.

4. The method of claim 3, further comprising including a dye in the gas or moisture sensitive layer prior to said patterning step.

Description

BRIEF DESCRIPTION OF THE EMBODIMENTS

(1) Embodiments of the invention are described in more detail and by way of non-limiting examples with reference to the accompanying drawings, wherein:

(2) FIG. 1 schematically depicts an IC according to an embodiment of the present invention;

(3) FIG. 2A schematically depicts the measurement principle of an IC according to an embodiment of the present invention;

(4) FIG. 2B is an image demonstrating the clouding of a diamine containing liquid upon exposure to CO.sub.2;

(5) FIG. 3 schematically depicts an IC manufacturing method according to an embodiment of the present invention;

(6) FIG. 4 schematically depicts a reaction scheme that may be used in an embodiment of the present invention;

(7) FIG. 5 depicts a measurement result of an IC according to an embodiment of the present invention;

(8) FIG. 6 depicts another measurement result of an IC according to an embodiment of the present invention; and

(9) FIG. 7 schematically depicts an IC manufacturing method according to another embodiment of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

(10) It should be understood that the Figures are merely schematic and are not drawn to scale. It should also be understood that the same reference numerals are used throughout the Figures to indicate the same or similar parts.

(11) FIG. 1 schematically depicts an IC comprising an optical CO.sub.2 sensor according to the general inventive concept of the present invention. A substrate 10, .g. a Si substrate, a SiGe substrate, a silicon on insulator (SOI) substrate and so on, which typically comprises a plurality of circuit elements such as transistors, diodes, and so on, is provided with a first photosensitive element 12 and a second photosensitive element 12′ spatially separated from the first photosensitive element 12. In FIG. 1, the first photosensitive element 12 and the second photosensitive element 12′ are laterally separated by way of non-limiting example only. It is equally feasible that the first photosensitive element 12 and the second photosensitive element 12′ are vertically separated, e.g. in the case of a vertically stacked diode comprising the photosensitive element 12 on top of the second photosensitive element 12′ or vice versa.

(12) The first photosensitive element 12 is covered by a layer portion 14 that includes an organic compound comprising at least one amine (e.g. a diamine) or amidine functional group. The layer portion 14 has a transmittance to a part of interest of the electromagnetic (EM) spectrum, e.g. visible light, which is a function of the amount of CO.sub.2 that is bound to the diamine compound in the layer portion 14.

(13) In an embodiment, the reaction of the organic compound with CO.sub.2 in the layer portion 14 causes a degree of clouding in the otherwise substantially transparent layer portion 14. This reduces the amount of light that is transmitted through the layer portion 14.

(14) In an alternative embodiment, the reaction of the organic compound with CO.sub.2 in the layer portion 14 causes a change in the absorbance spectrum of the organic compound, which may be detected by the photosensitive element 12 by a change in the intensity in light of a wavelength affected by such a change.

(15) Any suitable organic compound including at least one amine or amidine functional group that causes a transmissivity change at a defined wavelength upon reaction with CO.sub.2, e.g. through clouding or a change in its absorbance spectrum, may be used.

(16) The organic compound may be immobilized in the layer portion 14 in any suitable manner. For instance, the organic compound may be applied onto the substrate 10 in a solvent, which is subsequently evaporated to leave behind the organic compound in a neat form, e.g. a gel or solid. Alternatively, the organic compound, either in neat form or dissolved in a suitable solvent, e.g. an alcohol having negligible vapour pressure at room temperature (25° C.), such as oleyl alcohol, may be encapsulated in a polymer matrix, which may be formed by a curing reaction (chemically cross-linked polymers) or by self-assembly (physically cross-linked polymers).

(17) In yet another embodiment, the organic compound may be placed inside a limiting structure such as a polymer well structure formed on top of the passivation or metallization stack of the IC. Such a well structure may for instance comprise a polyimide well structure comprising an inner well in which the organic compound is placed and an outer well in which a gas permeable membrane covering the inner well is anchored. Such well structures and membranes are well known per se in the art, e.g. from the field of bodily fluid sensors. For instance, an example of a gas sensor utilizing such a well structure is disclosed in U.S. Pat. No. 5,376,255. An advantage of this embodiment is that the organic compound may be placed inside the inner well in liquid form, e.g. neat or dissolved in a suitable solvent, such that no separate immobilization steps are required.

(18) In an embodiment, the well structure is placed over the first photosensitive element 12, with the second photosensitive element 12′ not comprising such a well structure. In an alternative embodiment, the first photosensitive element 12 and the second photosensitive element 12′ each comprise separate well structures, in which the well structure over the second photosensitive element 12′ is filled with a reference liquid, such as a solvent used to dissolve the organic compound in the well structure over the first photosensitive element 12.

(19) In FIG. 1, the layer portion 14 is placed in direct contact with the first photosensitive element 12, e.g. a photodiode, by way of non-limiting example only. Alternatively, one or more layers (not shown) that are at least partly transparent to the part of the EM spectrum of interest may be present in between the photosensitive element 12 and the layer portion 14. For instance, the one or more layers may define a metallization stack of the IC for interconnecting the various circuit elements on the substrate 10 and for providing external connections to the circuit elements.

(20) The second photosensitive element 12′ acts as a reference photosensitive element, and is included in the design to filter out variations in the incident light levels to which the IC is exposed. A signal processing circuit 16 is also present in or on the substrate 10, which is connected to the respective outputs of the first photosensitive element 12 and the second photosensitive element 12′. Although in FIG. 1 these connections are shown as being embedded in the substrate 10, it should be understood that this is for the sake of clarity only. It is for instance equally feasible that such connections are routed over the substrate 10, e.g. form part of the aforementioned metallization stack (not shown).

(21) The signal processing circuit 16 is adapted to calculate a difference between the light intensities measured by the first photosensitive element 12 and the second photosensitive element 12′ respectively and to correlate this difference to a CO.sub.2 level. To this end, the signal processing circuit 16 may comprise or have access to a memory circuit (not shown), e.g. a look-up table, in which a measured difference in light intensity is correlated to a predetermined CO.sub.2 level, such that the signal processor can retrieve the appropriate CO.sub.2 level from the memory circuit.

(22) In an embodiment, the light intensities are measured at a selected wavelength, which is for instance advantageous in case the organic compound exhibits a change in its absorbance spectrum upon reaction with CO.sub.2, as for instance is the case for the spiropyran amidine compound as disclosed by Darwish et al.

(23) In an embodiment, the signal processing circuit 16 is adapted to determine a change in the ratio of the light intensities of the first photosensitive element 12 and the second photosensitive element 12′, e.g.:
R=[I(12)/I(12′)]
In which R is the dimensionless ratio and I(12) and I(12′) are the values of the voltage or current signals obtained from the first photosensitive element 12 and the second photosensitive element 12′ respectively. The value of the ratio can be correlated to a CO.sub.2 level as previously explained.

(24) In the absence of CO.sub.2 in the layer portion 14, the ratio R may be constant over a wide range of light intensities. Alternatively, the ratio R may be a function of the light intensity, in which case the above equation may be amended to:
R[(I(12)]=[I(12)/I(12′)]
In this embodiment, the memory circuit will store N look-up tables for the N different values of the ratio R, with N being a positive integer of at least 2. Other suitable algorithms to be implemented by the signal processing circuit 16 will be immediately apparent to the skilled person.

(25) The photosensitive elements 12 and 12′ may be realized in any suitable manner, e.g. photosensitive diodes, which may be single diodes, vertically stacked diodes and so on. In case of a vertically stacked diode, the substrate 10 is usually at least partially transparent such that the bottom diode can be illuminated through the substrate. This may for instance be achieved by thinning the substrate to a thickness such that it becomes as at least partially transparent.

(26) FIG. 2A schematically depicts the operating principle of the CO.sub.2 sensor of the present invention. The dashed line depicts the response or sensitivity of the CO.sub.2-sensitive photosensitive element 12 to a part of the EM spectrum, whereas the solid line depicts the response or sensitivity of the reference photosensitive element 12′ to the same part of the EM spectrum, e.g. a selected wavelength or wavelength range. The response curves have been horizontally displaced for the sake of clarity; it should be understood that normally the maximum response of the respective curves substantially coincide if the photosensitive elements 12 and 12′ are of the same design.

(27) The left hand pane depicts the response of the photosensitive elements 12 and 12′ in the absence of CO.sub.2 and the right hand pane depicts the response of the photosensitive elements 12 and 12′ after the diamine compound in the layer portion 14 has reacted with ambient CO.sub.2, thus causing clouding or a change in the absorbance spectrum of the layer portion 14, which alters, e.g. reduces, the transmittance of the layer portion 14 to the selected part of the EM spectrum. This increases the difference in the output signals produced by the photosensitive elements 12 and 12′, which can be converted by the signal processing circuit 16 into a detected level of CO.sub.2.

(28) As shown in FIG. 2B, a droplet of oleyl alcohol comprising 5% by weight of a meso-diamine ((1R, 2S)-1,2-diphenyl-1,2-ethanediamine) changes within 5 minutes from a clear to a milky, clouded appearance upon exposing the droplet to a CO.sub.2-flush, thus demonstrating the ability of diamine compounds to cloud a host material upon a binding reaction with CO.sub.2.

(29) FIG. 3 schematically depicts a method of manufacturing an IC in accordance with an embodiment of the present invention. The IC of the present invention may be provided using any suitable manufacturing technology, such as CMOS, silicon-on-insulator and SiGe technologies. As shown in FIG. 3(a), there is provided an IC that comprises a substrate 10, e.g. a Si substrate, a SiGe substrate, a silicon on insulator (SOI) substrate and so on, which typically comprises a plurality of circuit elements such as transistors, diodes, and so on, combinations of which from circuits including the signal processing circuit 16. These may be analog or digital circuits. The manner in which this substrate provided is not particularly limited.

(30) Any suitable manufacturing method may be employed to provide such a substrate. As such methods are numerous and commonplace, this will not be further explained for the sake of brevity only. It should further be understood that the present invention is not limited to specific types of ICs. The present invention may be included in any suitable IC, including digital ICs, analog ICs and mixed signal ICs.

(31) A first photosensitive element 12 and a second photosensitive element 12′ acting as a reference light sensor as previously explained are formed on the substrate 10. The photosensitive elements 12 and 12′ may take any suitable shape, e.g. photosensitive diodes, which may be single diodes, vertically stacked diodes and so on. In case of a vertically stacked diode, the substrate 10 is usually at least partially transparent such that the bottom diode can be illuminated through the substrate. This may for instance be achieved by thinning the substrate to a thickness such that it becomes as at least partially transparent. The photosensitive elements 12 and 12′ form part of the CO.sub.2 sensor of the IC and are typically connected to signal processing circuit 16.

(32) The interconnections between the circuit elements in the substrate 10 to define the circuits are typically provided by a metallization layer or layer stack 20, which by way of non-limiting example may comprise a plurality of patterned metal layers separated by dielectric layers. Any suitable number of metal layers and dielectric layers may be present. Metal portions in different metal layers may be conductively interconnected by one or more vias formed in a dielectric layer in between the respective portions of the metal layers. Any suitable material may be used to form the metallization stack 20, such as Ti, TiN, Al, Cu and combinations thereof to define the metal layers and silicon oxide, silicon nitride, low-k dielectrics and other dielectric materials as well as combinations thereof to form the dielectric layers.

(33) Each layer of the metallization stack 20 may in fact comprise a stack of layers, as is common design practice in contemporary semiconductor technologies such as sub-micron CMOS technologies. Any suitable manufacturing method may be employed to provide such an interconnect structure. As such methods are numerous and commonplace, this will not be further explained for the sake of brevity only.

(34) A passivation layer 30 is typically provided over the metallization stack 20 to protect the interconnect structure and the substrate 10 from damage, e.g. from exposure to excess moisture. Again, any suitable passivation layer 30 may be employed. Non-limiting examples of suitable materials for such a passivation structure include dielectric materials such as SiO.sub.2, Si.sub.3N.sub.4, low-k dielectrics and combinations thereof. In addition, the passivation structure may further comprise a moisture barrier material such Ta.sub.2O.sub.5. Preferably, the passivation layer 30 is a layer stack comprising one or more layers of a dielectric material, which may be formed in any suitable manner. As such methods are numerous and commonplace, this will not be further explained for the sake of brevity only.

(35) It will be clear that the metallization layer 20 and the passivation layer 30 are at least partially transparent to the part of the electromagnetic (EM) spectrum of interest, such that this light can reach the photosensitive element 12.

(36) A first electrode 42 and a second electrode 44 are formed on top of the passivation layer 30, which preferably is planarized prior to the formation of these electrodes. A suitable planarization method is chemical mechanical polishing. The electrodes 42 and 44 may be formed in any suitable manner, e.g. by depositing a metal layer on top of the passivation layer 30 and patterning this metal layer to obtain the first electrode 42 and the second electrode 44. In FIG. 3(a), the first electrode 42 and the second electrode 44 are shown as interdigitated electrodes by way of non-limiting example only. It will be appreciated that any suitable electrode design may be contemplated. Any suitable metal may be used for the electrodes. Preferably, the first electrode 42 and the second electrode 44 are formed of a metal that is also used to form the metal interconnects in the metallization layer 20 as this means that the electrodes may be formed by processes that are already available in the manufacturing process of the IC.

(37) The electrodes 42 and 44 form the electrodes of a capacitive or resistive gas or moisture sensor on top of the passivation layer of the IC. A non-limiting example of such a type of sensor is described in more detail in European patent application EP09166518.2. The electrodes 42 and 44 may be conductively connected for reading out purposes in any suitable manner. The electrodes 42 and 44 may be connected to circuitry on the substrate of the IC via the metallization stack 20, in which case respective electrically conductive portions extend from the electrodes 42 and 44 to different metal portions of the metallization stack 20 through the passivation layer 30. Alternatively, the passivation layer 30 may carry respective contact pads (not shown) to which the electrodes 42 and 44 are conductively connected such that the gas or moisture sensor may be read out externally by contacting these contact pads.

(38) According to an embodiment of the present invention, a gas or moisture sensitive layer 46 is formed over the passivation layer 20 including the first electrode 42 and the second electrode 44. In the context of the present invention, a gas or moisture sensitive material is a material that has electrical properties, e.g. conductive, resistive and/or capacitive properties that are a function of the gas or moisture content in the material. For instance, in case of a capacitive moisture sensor, the moisture sensitive material is a material that has a dielectric constant that depends on its moisture content, such that the moisture content can be determined by determining the capacitance of the sensor.

(39) Alternatively, an impedance measurement across the portion of the gas or moisture sensitive layer 46 in between the electrodes 42 and 44 can be performed to determine the gas levels in or relative humidity of the environment in which the IC is placed. Such a measurement could also be used to determine if the IC has been exposed to excessive humidity levels, e.g. has been immersed in water.

(40) It will be understood that in case of a moisture sensor on the passivation layer 30, this sensor may be used as a relative humidity sensor or as a liquid immersion sensor instead.

(41) Any suitable gas or moisture sensitive material may be used. For example, the moisture sensitive material may be a polymer selected from the group consisting of polyacrylates, polymethacrylates, polyimides, polyamides, polyamines, polypyridines, polycarbonates, polyacetates and polystyrenes and derivatives thereof. Polyimide is particularly preferred. In case of the layer 46 comprising such a polymer, the layer 46 may for instance be formed by spin-coating or any other suitable polymer deposition technique.

(42) Alternatively, polymers such as polyacetylenes, polyanilines, polypyrroles, polythiophenes, poly(phenyl vinylene) and derivatives thereof may be used, in particular if gases other than gaseous water (moisture) are to be detected. It is for instance known per se that several conductive polymers such as polypyrrole, polyaniline, polythiophene and their derivatives have successfully been used as gas sensitive layers in gas sensors. It is also known per se that for instance polythiophene and poly(dodecylthiophene) sensors can have sensitivities in the range of 0.2-1.8 DR/Rb for 300 ppm gas for 10 minutes for gases such as methane, chloromethane and ammonia, as for instance has been previously disclosed by Y. Sakurai et al. in Sensors and Actuators B: Chemical, Vol. 83, No. 1-3, pages 270-275.

(43) More generally, any suitable polymer may be used. An overview of some suitable polymers for use in gas sensors has been provided by Hua Bai et al. in Sensors 2007, Vol. 7, pages 267-307. Another overview of suitable polymers for gas sensor applications is provided by K. Arshak et al. in Sensor Review, 24(2), 2004, pages 181-198.

(44) To enhance the gas or moisture sensitivity of the polymers, at least the portion of the gas or moisture sensitive layer 46 forming part of the gas sensor may be coated with a noble metal such as Pt or Pd. Alternatively, such a noble metal may be dispersed in the polymer. For the interested reader, this is disclosed in more detail in the aforementioned article by Hua Bai et al.; see in particular Table 2 of this article.

(45) In a next step, as shown in FIG. 3(b), the layer 46 of the gas or moisture sensitive material is patterned to finalize the gas or moisture sensor 50 including a portion 46′ of the sensitive material and to provide a further portion 46″ that acts as a (wavelength) filter for the first photosensitive element 12. The portion 46′ and the further portion 46″ may still be interconnected or may be separated in the patterning step. The further portion 46″ is located on an area of the passivation layer 30 over the first photosensitive element 12 such that the majority of the incident light of the first photosensitive element 12 as indicated by the arrows in FIG. 3(b) passes through the further portion 46″. Preferably, substantially all incident light, e.g. at least 90% or even 99% of all incident light of the first photosensitive element 12 passes through the portion 46″ of the moisture sensitive material acting as the filter for this photosensitive element.

(46) Although not shown in FIG. 3, it should be understood that a second further portion of the gas or moisture sensitive layer 46 may extend over the reference photosensitive element 12′ such that both the CO.sub.2-sensitive photosensitive element 12 and the reference photosensitive element 12′ are exposed to the same selected, i.e. filtered, part of the electromagnetic spectrum.

(47) In an embodiment, the filter characteristics, i.e. the part of the incident electromagnetic (EM) spectrum absorbed by the portion 46″, are governed by the absorption characteristics, i.e. the absorption spectrum of the gas or moisture sensitive material. To this end, the gas or moisture sensitive material may be selected based on the nature of its absorption spectrum to ensure that it can effectively filter out the undesirable part of the EM spectrum. For instance, for a photopic light sensor 12, a material may be selected that strongly absorbs UV and IR irradiation.

(48) In a further embodiment, the absorption spectrum of at least the filter portion 46″ of the gas or moisture sensitive material may be tuned by adding a dye to the portion 46″. Due to the intense color of dyes, only a small amount of dye may need to be added to the gas sensitive material to alter its absorption spectrum. Any suitable dye may be used. The advantage of having to use only a small amount of dye is that the electrical properties of the gas or moisture sensitive material remain substantially unaffected. This means that the dye may also be present in the portion 46′ forming part of the gas or moisture sensor 50, which may simplify application of the dye to the gas or moisture sensitive material.

(49) For instance, the dye may be added after the layer 46 of the gas or moisture sensitive material has been deposited over the passivation layer 30, e.g. by absorption of the dye into the layer 46. Alternatively, in case of the gas or moisture sensitive material comprising a polymer, the dye may be incorporated into the polymer. This may for instance be achieved by using two different types of monomers, i.e. a monomer 100 without the dye chemically, e.g. covalently, bound thereto and a monomer 110 comprising the dye, for instance as a substituent, as schematically depicted in FIG. 4, to form a polymer 120 in which the dye is incorporated as a substituent to the polymer backbone. In FIG. 4, m and n are positive integers, with m typically being much larger than n as excess monomer 100 compared to monomer 110 is used in the polymerization reaction. In an embodiment, the ratio of monomer 100: monomer 110 may be at least 10:1, and may be at least 100:1 or even at least 1,000:1. As the chemistry of attaching a dye to a monomer is well-known per se and furthermore highly dependent on the nature of the dye as well as the monomer, this is not explained in further detail for the sake of brevity only.

(50) Upon returning to FIG. 3, the layer portion 14 comprising the organic compound comprising at least one amine or amidine functional group is subsequently formed over the further portion 46″, e.g. by depositing a droplet of a solution including the organic compound and a polymer precursor or a dissolved polymer and subsequently solidifying this layer portion, e.g. by curing or by evaporating the solvent to immobilize the organic compound on the further portion 46″, e.g. in a polymer matrix as previously explained.

(51) It should be understood that the several variations to the IC as shown in FIG. 3c may be contemplated. For instance, the IC may comprise at least four photodiodes 12, with a first photosensitive element not covered by any layer, a second first photosensitive element 12 covered by a moisture-sensitive layer such as a polyimide layer only to provide the relative humidity sensor, a third first photosensitive element 12 covered by the CO.sub.2-sensitive layer portion including the organic compound having at least one amine or amidine functional group and a fourth first photosensitive element 12 covered by both the moisture-sensitive layer portion and the CO.sub.2-sensitive layer portion, wherein the first photosensitive element may be used to generate a reference signal for the second photosensitive element, and wherein the third photosensitive element may be used to generate a reference signal for the fourth photosensitive element or vice versa. Many other variations will be apparent to the skilled person.

(52) To demonstrate the concept of the filter portion 46″ over the photosensitive element 12, FIG. 5 depicts the results of a measurement performed on a CMOS IC comprising a pair of vertically stacked photosensitive diodes labeled ALS1 and ALS2 in FIG. 3, in which only ALS2 is covered by a polyimide portion, i.e. the area of the passivation layer 20 over ALS2 comprises this portion. It is noted that during these measurements the CO.sub.2 sensitive material 14 was omitted for the sake of clarity.

(53) Three measurement curves are depicted in FIG. 5. The curve ALS1top−ALS2top depicts the differential spectrum obtained by subtracting the spectral response of the top diode of ALS2 from the spectral response of the top diode of ALS 1. The curve ALS2top depicts the spectral response of the top diode of ALS2 and the curve ALS1bottom−ALS2bottom depicts the differential spectrum obtained by subtracting the spectral response of the bottom diode of ALS2 from the spectral response of the bottom diode of ALS 1.

(54) As can be seen from curves ALS1top−ALS2top and ALS1bottom−ALS2bottom, a separation of the UV and IR parts of the EM spectrum can be achieved. This can be achieved by simple arithmetic manipulation of the ALS1 and ALS2 signals.

(55) FIG. 6 depicts the spectral response of the (unfiltered) ALS1 top and bottom diode before applying the data extraction shown in FIG. 5. Compared to the response of the ALS2 top diode in FIG. 5 it will be clear that the UV and IR wings of the spectral response of the ALS1 top diode have been effectively suppressed by the polyimide filter portion over the ALS2 top diode.

(56) FIG. 7 schematically depicts an alternative embodiment a method of manufacturing an IC in accordance with an embodiment of the present invention. Step (a) is identical to step (a) in FIG. 3 and will not be explained again for the sake of brevity. In step (b), the layer 46 of the gas or moisture sensitive material is patterned to finalize the gas or moisture sensor 50. The difference with the embodiment in FIG. 3 is that the further portion 46″ has been omitted. Instead, a well structure 60 is formed over the first photosensitive element 12, e.g. by spin-coating a polymer such as polyimide over the first photosensitive element 12 and subsequent selective patterning of the polymer into the well structure 60 shown in FIG. 7(b).

(57) Next, as shown in step (c), the organic compound comprising the diamine or amidine functional group is deposited in the well structure 60, e.g. as a neat or dissolved in a suitable solvent, thereby forming a liquid layer portion 14 within the well structure 60. Alternatively, the organic compound may be immobilized in a polymer deposited inside the well structure 60. This may for instance be achieved by depositing a curable mixture inside the well structure 60 and subsequently curing the curable mixture. The well structure 60 is subsequently sealed with a CO.sub.2-permeable but liquid-impermeable membrane 62 to retain the liquid layer 14 within the well structure 60.

(58) It is noted that in FIG. 7, a single well 60 is shown by way of non-limiting example only. In alternative embodiments, the well structure may comprise an inner well containing the organic compound 14 and an outer well acting as an anchor for the CO.sub.2-permeable membrane 62. In yet another alternative embodiment, an additional well structure may be provided over the second photosensitive element 12′, e.g. to place a reference liquid in the additional well structure, such as the solvent used to dissolve the organic compound 14 in the well structure 60. Further suitable variations will be apparent to the skilled person.

(59) It should furthermore be understood that other types of sensors may be added to the IC without departing from the teachings of the present invention. In particular, sensors that can be manufactured using process steps already in use for the manufacturing of the IC are preferred as they do not require a substantial cost increase of the manufacturing process to add such sensors to the IC. An example of a particularly preferable sensor is a temperature sensor such as a PTAT sensor, for which transistor-based implementations are readily available.

(60) It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word “comprising” does not exclude the presence of elements or steps other than those listed in a claim. The word “a” or “an” preceding an element does not exclude the presence of a plurality of such elements. The invention can be implemented by means of hardware comprising several distinct elements. In the device claim enumerating several means, several of these means can be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.