INTEGRATED CIRCUIT COMPRISING A METAL-INSULATOR-METAL CAPACITOR AND FABRICATION METHOD THEREOF

Abstract

The disclosed technology relates to a metal-insulator-metal capacitor (MIMCAP) integrated as part of a back-end-of-line of an integrated circuit (IC). In one aspect, a MIMCAP comprises a first planar electrode having perforations formed therethrough, and a metal-insulator-metal (MIM) stack lining inner surfaces of cavities formed in the perforations and extending into the substrate. The MIMCAP additionally comprises a second electrode having a planar portion and metal extensions extending from the planar portion into the cavities. The first electrode and the planar portion of the second electrode are formed of or comprise planar metal areas of the respective metallization levels, which can be formed by a damascene process, which allows for a reduction of the series resistance. A low aspect ratio can be obtained using one electrode having a 3D-structure (the electrode having extensions extending into the cavities).

Claims

1. An integrated circuit (IC) comprising: a semiconductor substrate; and a plurality of metallization levels, each metallization level comprising a layer of intermetal dielectric having metal areas embedded therein, a metal-insulator-metal capacitor (MIMCAP) comprising a bottom electrode, a top electrode and a metal-insulator-metal (MIM) stack comprising a lower conductive layer, an upper conductive layer and an insulator layer sandwiched between the lower and upper conductive layers, wherein the bottom electrode comprises a planar metal area of a lower metallization level, the planar metal area having perforations formed therethrough, wherein the IC comprises cavities, each cavity extending through one of the perforations and into the semiconductor substrate, each cavity being separated from edges of a corresponding perforation by a first intermetal dielectric material (IMD1) of the lower metallization level, wherein the MIM stack comprises a planar portion on at least a part of an upper surface of the bottom electrode and a plurality of non-planar portions extending into the cavities, the MIM stack lining the sidewalls and bottoms of the cavities, and wherein the top electrode comprises: a planar portion formed by a planar metal area of an upper metallization level adjacent to the lower metallization level, the planar portion formed on at least a part of an upper surface of the planar portion of the MIM stack, and non-planar portions extending from the planar portion of the top electrode into the cavities.

2. The IC according to claim 1, wherein the planar portion of the top electrode comprising a plurality of perforations formed therethrough, each perforation having a central portion formed of a second intermetal dielectric material (IMD2) of the upper metallization level, wherein a side wall of the central portion is surrounded by the MIM stack.

3. The IC according to claim 1, wherein a portion of the substrate at which the MIMCAP is formed is laterally separated from the remainder of the substrate by a dielectric wall portion enclosing portions of the cavities extending into the substrate.

4. The IC according claim 1, wherein the bottom electrode further comprises an additional layer of a conductive material on the planar metal area of the lower metallization level, the additional layer being provided with a plurality of perforations corresponding to the perforations formed through the planar metal area.

5. The IC according claim 1, wherein a portion of the bottom electrode laterally extends beyond the top electrode, and wherein the extended portion of the bottom electrode is contacted by a via connection in the upper metallization level.

6. The IC according to claim 1, further comprising a power delivery network, and wherein the MIMCAP is a decoupling capacitor coupled between a supply terminal and a ground terminal of the network.

7. The IC according claim 1, wherein the MIMCAP is a part of a DC/DC converter.

8. The IC according claim 1, wherein the IC is an interposer chip.

9. The IC according claim 1, wherein the lower metallization level comprises a first metallization level (M1) and the upper metallization level comprises a second metallization level (M2).

10. The IC according to claim 1, wherein the planar portion of the top electrode has side walls lined by portions of the MIM stack.

11. A method for fabricating an integrated circuit (IC) comprising a metal-insulator-metal capacitor (MIMCAP), the method comprising: forming a first intermetal dielectric (IMD) layer over a semiconductor substrate, the IMD layer having a first region; patterning the first IMD layer within to remove portions of the first IMD layer from the first region while leaving the first IMD layer in a plurality of discrete areas; depositing a metal in the first region and planarizing the metal deposited in the first region, thereby forming a first metallization level comprising a first planar metal area, the first planar metal area having a plurality of perforations filled with the discrete areas of the first IMD layer; forming a second IMD layer over the first metallization level the second IMD layer having a second region; patterning the second IMD layer within a second region of the second IMD layer to at least partially remove the second IMD layer within a second region, thereby exposing an area that includes the perforations; etching cavities through the discrete areas of the first IMD layer, the cavities being spaced from the edges of the perforations in the first planar metal area, the cavities extending into the substrate; forming a metal-insulator-metal (MIM) stack, the MIM stack lining the sidewalls and bottoms of the cavities, the MIM stack covering the second IMD layer and the exposed area; depositing a metal on the MIM stack thereby filling the cavities with the metal, thereafter planarizing the metal, thereby forming a second metallization level, comprising a second planar metal area provided with non-planar extensions extending into the cavities, the second planar metal area and the extensions forming at least a part of a top electrode of the MIMCAP and the first planar metal area forming at least a part of a bottom electrode of the MIMCAP; and forming contacts that electrically contact the bottom and top electrodes.

12. The method according to claim 11, wherein the first planar metal area comprises a portion extending beyond the second planar metal area, and wherein forming the contacts comprises forming a via connection in the upper metallization level to the extended portion.

13. The method according to claim 11, further comprising: after forming the first planar metal area and prior to forming the second IMD layer, depositing and patterning to form a conductive layer on the first planar metal area, the conductive layer being patterned within the first region to have corresponding perforations to the perforations of the first planar metal area, the bottom electrode being formed by the first planar metal area and the conductive layer.

14. The method according to claim 11, wherein partially removing the second IMD layer within the second region, thereby forming a plurality of second discrete areas, such that the second planar metal area is formed with the plurality of discrete perforations comprising the discrete areas, the sidewalls of the discrete areas being surrounded by the MIM stack.

15. The method according to claim 11, further comprising: etching a trench in a backside of the substrate and around the MIMCAP, and filling the trench with an isolating material, thereby isolating a part of the substrate that comprises the MIMCAP from the remainder of the substrate by a dielectric wall portion laterally enclosing portions of the cavities extending into the substrate.

16. The method according to claim 15, wherein etching and filling the trench are performed as part of a via-last process.

17. The method according to claim 15, wherein the MIM stack is deposited in the exposed area such that the sidewalls of the second IMD layer is lined along the second region, and that after deposition and planarization of the metal on the MIM stack, the second planar metal area has side walls lined by portions of the MIM stack.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] FIGS. 1a to 1s illustrate intermediate structures at various stages of fabricating a metal-insulator-metal capacitor (MIMCAP), according to embodiments.

[0023] FIGS. 2a and 2b illustrate intermediate structures at various stages of electrically contacting a MIMCAP, according to embodiments.

[0024] FIGS. 3a to 3f illustrate a intermediate structures at various stages of electrically isolating a MIMCAP according embodiments.

DETAILED DESCRIPTION OF CERTAIN ILLUSTRATIVE EMBODIMENTS

[0025] FIGS. 1a to 1s illustrate a method of fabricating an integrated circuit (IC) comprising a MIMCAP, according to an embodiment of the disclosed technology. The method is performed on a semiconductor substrate 1. According to various embodiments disclosed herein, the MIMCAP is integrated with one or more metallization levels of the IC, e.g., in first (M1) and second (M2) metallization levels in a back end of line (BEOL) process performed on the semiconductor substrate 1. As described herein and throughout the specification, a metallization level refers to an interconnection level of an IC which includes conductive interconnection structures as well as intermetal dielectrics formed between the conductive interconnection structures. An nth metallization level refers to one of metallization levels of the IC having a plurality of metallization levels, where n indicates a relative proximity of the conductive interconnection structures relative to semiconductor substrate 1. For example, a first metallization level (M1) refers to an interconnection level that is closer to the substrate 1 than a second metallization level (M2). However, a first metallization level (M1) does not exclude the possibility that additional metallization level(s) may exist between the M1 and the substrate 1. For example, a zeroth metallization level (M0) may be formed between M1 and the substrate 1.

[0026] The drawings are for illustrative purposes only and are not drawn to the scale of the actual structures. The substrate 1 is preferably a silicon wafer, but it may be any type of semiconductor material. It may or may not comprise front-end-of-line processed semiconductor components. According to some embodiments, the substrate is a silicon substrate to be used as an interposer. As described herein, an interposer includes, a substrate onto which one or more integrated circuit chips can be mounted. The interposer is configured such that mounted IC chips can be connected to other components, e.g., a power supply, through one or more through silicon via (TSV) connections that are formed through the interposer. It can be advantageous to form a MIMCAP according to embodiments on such an interposer, as will be explained below. However, embodiments are not so limited and the embodiments describe with respect to FIGS. 1a to 1s can be applicable to any type of substrate 1 that is not an interposer.

[0027] In a first process, a pre-metal dielectric (PMD) layer 2 is deposited on the substrate 1 (FIG. 1a), the PMD layer being a layer suitable for producing thereon a metallization stack. The dielectric material and the application method used for this layer 2 may be according to known methods for depositing a PMD layer. As described herein, a metallization stack refers to a stack of metallization levels, where each metallization level, or each metallization layer, comprises a dielectric layer in which metal areas embedded therein. The metal areas may be planar areas, metal lines or metal vertical (so-called via) connections. The various levels of a metallization stack are indicated by their relative distance from the substrate 1, e.g., M1, M2, etc., where a lower metallization level is closer to the substrate 1 compared to a higher metallization level. Metallization levels of this type can be fabricated using single or double damascene processes.

[0028] As described herein, the dielectric layers into which the metal conductors are embedded are referred to as intermetal dielectric (IMD) layers. The production of a PMD and subsequent metallization levels M1, M2, etc., is commonly referred to as the back-end-of-line process (BEOL).

[0029] As described herein, a damascene process, or a single damascene process, is used to describe a process consistent with the usage of the term in the relevant industry. As consistent with the usage of the term in the relevant industry, a damascene process for forming a metallization level refers to an additive process in which an IMD layer is deposited and etched using a photolithographic process. Prior to deposition of a metal, a barrier layer and/or a seed layer may be deposited by, e.g., physical vapor deposition (PVD) on the patterned dielectric layer. Subsequently, a metal, e.g., copper, may be deposited by, e.g., electroplating, and planarized using, e.g., chemical mechanical polishing (CMP), thereby forming conductive structures such as metal lines that are electrically insulated by and embedded within the patterned IMD layer. These processes may be repeated n number of times for fabrication of n metallization levels.

[0030] As described herein, a dual damascene process is used to describe a process consistent with the usage of the term within the relevant industry. A dual damascene process is similar to a single damascene process, except that both lines and vias are formed by etching holes and trenches in the IMD layer, prior to depositing the metal. One photolithographic process may be used to form holes (vias) in the dielectric to make connection with underlying metal, and another photolithographic process may be used to form trenches for the metal lines. Subsequently, a metal, e.g., copper, may be deposited by, e.g., electroplating, and planarized using, e.g., chemical mechanical polishing (CMP), thereby forming conductive structures including metal lines and vias embedded within the patterned IMD layer. These processes may be repeated n number of times for fabrication of n metallization levels.

[0031] As illustrated in FIG. 1b, a first intermetal dielectric layer IMD1 is deposited onto the PMD layer 2. FIGS. 1c to 1f illustrate process of a damascene process: forming a lithographic mask 3, by depositing and photopatterning a patterning layer, e.g., a photoresist layer and/or a hardmask layer, etching the IMD1 layer according to the litho-mask 3, stripping the litho-mask 3, depositing a barrier and seed layer 4, electroplating a metal 5 (preferably Cu) onto the barrier/seed layer 4 thereby filling the areas removed by the etching process with metal, performing a planarizing process (e.g., by chemical mechanical polishingCMP), thereby planarizing the metal and removing the barrier/seed 4 from the upper surface of the IMD1 layer. As seen in the top view of FIG. 1f, the pattern defined by the litho-mask 4 comprises discrete areas 10 where the IMD1 layer remains, whereas all the IMD1 material around these discrete areas is removed within a well-defined boundary 11 (only the side edges 11 of the boundary are shown). The metal area created by the damascene process is therefore a planar metal area 5, hereafter referred to as a metal plate 5, with discrete perforations 10 filled with IMD1 material. The metal plate 5 will form at least part of the planar bottom electrode of the MIMCAP. As known in the art, the IMD1-filled perforations 10 have the function of maintaining a flat surface of the plate 5 during CMP planarization, i.e. avoiding the dishing effect occurring when planarizing a large area. The perforations 10 obtain an additional function in the construction of the MIMCAP according to embodiments, as described below.

[0032] In an optional number of processes shown in FIGS. 1g to 1i, a conductive layer 15 is deposited and patterned by a second litho-mask 16, in order to cover the perforated metal plate 5 with this conductive layer 15, leaving openings in the conductive layer 15 at the position of the perforations 10. When the metal of the metal plate 5 is Cu, the conductive layer 15 may be TaN or TiN. It is applied in order to avoid landing directly on Cu in subsequent etching process, this to avoid Cu contamination of the etch chamber. When layer 15 is used, the bottom electrode of the MIMCAP is formed by the layers 5 and 15 taken as a whole.

[0033] Then a second IMD layer IMD2 is deposited (FIG. 1j) and patterned by application of a third litho-mask 20, by etching the IMD2 layer in areas exposed by the mask and by stripping the mask (FIG. 1k and 1l). As seen in the top view of FIG. 1l, the litho mask 20 defines a pattern wherein all the IMD2 material is removed within a boundary 12 except for discrete islands 21 of IMD2 material located in between the perforations 10 in the bottom electrode 5/15, leaving the bottom electrode exposed within the boundary 12, except for the discrete islands 21.

[0034] Then a fourth litho-mask 22 is formed which covers the whole surface except for a number of openings 23, each opening being situated above a perforation 10 in the metal plate 5 (FIG. 1m). The openings 23 may have a circular cross-section and are preferably placed centrally with respect to the boundaries of each perforation 10. A deep semiconductor etch process is then performed, etching through the IMD1 layer, the PMD layer 2 and the semiconductor substrate 1, to create deep cavities 25, after which the fourth litho mask 22 is stripped (FIG. 1n). The cavities 25 are spaced from the edges of the perforations 10 in the metal plate 5. IMD1 material separates the cavities from these edges of the perforations.

[0035] Then a deposition is performed of three consecutive layers (FIG. 1o): a lower electrically conductive layer 26, a middle electrically insulating layer 27 and an upper electrically conductive layer 28, these three layers forming a metal-insulator-metal stack as known as such in the art. The conductive layers 26 and 28 may for example be Ta, TaN, Ti or TiN. The dielectric 27 of the MIM stack may be SiO.sub.2, silicon nitride (Si.sub.3N.sub.4), silicon carbide, Ta.sub.2O.sub.5, HfO.sub.2, TiO.sub.2, or another suitable material. The MIM stack is formed so as to cover the IMD2 layer and the exposed area of the bottom electrode 5/15 within the boundary 12, and to line the sidewalls and bottom of the cavities 25. In other words, the MIM stack comprises a planar portion on the upper surface of the bottom electrode and a plurality of non-planar portions extending in the cavities. The three layers 26/27/28 are preferably deposited by a conformal deposition technique, i.e. a technique wherein the deposited layers closely follow the topography of the surface on which they are deposited. In the embodiment shown in the drawings, this means that the MIM stack 26/27/28 follows the topography defined by the IMD2 islands 21 and the edges of the IMD2 layer along the boundary 12. The layers of the MIM stack may be deposited by Atomic Layer Deposition for example. The diameter of the cavities 25 and the thickness of the applied MIM stack layers 26/27/28 is chosen so that a central opening 29 remains in the cavities after application of the MIM stack layers. After deposition of a seed layer 30, a second metal layer 31 is deposited by electroplating, the metal filling the openings 29 and forming a layer on top of the MIM stack 26/27/28 which conformally covers the IMD2 islands 21 and the exposed area between said islands (FIG. 1p). This is followed by a CMP process (FIG. 1q) wherein the metal layer 31 is planarized and the MIM stack 26/27/28 is removed from the upper surface of the IMD2 layer. The islands 21 of IMD2 material ensure that the planarization does not cause a dishing effect of the metal layer 31. As seen in the top view illustrated in FIG. 1q, the planarization results in the formation of a planar metal area 32 (hereafter referred to as metal plate) 32 in level M2 of the BEOL stack, the plate 32 being located within the boundary 12, the plate being provided with perforations, each perforation comprising a central IMD2 island 21 the side walls of which are surrounded by the MIM stack layers 26/27/28, i.e. portions of the MIM stack are lining the side walls of the islands 21. The outer side walls of the plate 32 are equally lined with portions 35 the MIM stack 26/27/28. The metal plate 32 is furthermore provided with metal extensions 33 reaching down through the perforations 10 of the plate 5 in level M1. The metal plate 32 with extensions 33 will form at least a part of the upper electrode of the MIMCAP. In the embodiment shown in the drawings, the boundary 12 is sized with respect to the boundary 11 so that the bottom electrode 5/15 extends outward from the MIM stack 26/27/28 and the top electrode in an area 34. As explained hereafter, this extended portion 34 may be used for contacting the bottom electrode. The embodiments is however not limited to the shape, size and location of the second boundary 12 relative to the first boundary 11 as shown in the drawings. Generally, the boundary 12 is overlapping boundary 11 at least on an area comprising the perforations 10 in the bottom electrode. The boundary 12 may have the same shape and size as the lower boundary 11 and be aligned to said lower boundary, resulting in an equal-sized and aligned top and bottom electrodes. This would require another way of contacting the bottom electrode, instead of by an extended portion 34, e.g. by a conductor line in the M1 layer. Another alternative is an upper boundary 12 that has the same shape and similar size as the lower boundary 11 but that is shifted laterally with respect to the lower boundary 11. Yet another alternative is an upper boundary 12 that is larger than the lower boundary 11. The last two alternatives would result in the MIM stack 26/27/28 extending outward laterally with respect to the bottom electrode at least to one side of the bottom electrode.

[0036] At this point the MIMCAP is formed, having a bottom planar electrode 5/15, a 3D MIMCAP portion 26/27/28 and a top electrode, the top electrode having a planar portion 32 in the M2 layer and extensions 33 reaching down into the substrate 1. The 3D aspect of the MIMCAP allows for an increase in the capacitor area compared to a planar MIMCAP and compared to existing 3D MIMCAP designs. The series resistance of the MIMCAP is low given that the bottom and top electrodes consist of or comprise large metal areas incorporated in the M1 and M2 layers.

[0037] In the embodiment shown in the drawings, and as a consequence of the described process sequence, the planar portion 32 of the top electrode covers fully the planar portion of the MIM stack 26/27/28, and the MIM stack further comprises vertical wall portions 35 which line the side walls of the top electrode's plate portion 32. Alternative embodiments are possible wherein the planar portion 32 is smaller than the planar portion of the MIM stack, e.g. due to additional processes performed between the MIM formation and the second metal deposition. Also the MIMCAP according to embodiments could be formed without the MIM wall portions 35, e.g. by including a process wherein these portions are removed.

[0038] FIG. 1r shows a preferred way of producing a contact to the bottom electrode 5/15 of the MIMCAP, by a standard damascene process: applying a litho-mask (not shown), etching an opening through the IMD2 layer, depositing a barrier/seed layer 37 and electroplating+planarizing the electroplated metal, thereby creating a contact via 38, contacting the bottom electrode 5/15 on the extended portion 34. Optionally (FIG. 1s), a recess etch of the conductive layers 26/28 of the MIM stack may be performed, forming recesses 39 at the surface, in order to avoid shorting of the capacitor due to metal residues that are left on the surface.

[0039] As shown in FIG. 2a, a passivation layer 40 may be formed and patterned onto the structure shown in FIG. 1s, so as to form connections 41 to the MIMCAP's bottom and top electrodes, see FIG. 2a. Contact pads 42 may then be formed on top of the substrate, configured to receive further contact structures such as bumps or pillars. Possibly further BEOL-metallization levels M3, M4, . . . may be formed on top of the structure shown in FIG. 1s, before producing passivation layer 40 on top of the finished metallization stack. According to an embodiment illustrated in FIG. 2b, the production of the contact via 38 takes place together with the production of contact vias 43 in a further IMD layer 45.

[0040] As stated, the MIMCAP according to certain embodiments may be formed on any type of semiconductor substrate. One particular field of application is the building of decoupling capacitors in the power delivery network of an interposer substrate. The M1 and M2 levels described above are then coupled to a supply voltage and to ground respectively. The MIMCAP is designed to be able to compensate a voltage drop due to switching action of CMOS circuitry in the chips mounted on the interposer. The MIMCAP according to embodiments is also applicable for the building of DC/DC convertors on an interposer. An interposer according to embodiments is to be regarded as an integrated circuit. The MIMCAP of embodiments may be integrated in the production of any pair of adjacent metallization levels M.sub.n, M.sub.n+1 of a metallization stack. If formed between layers higher up than M1 and M2, the underlying layers must be designed suitably in order to allow the production of the cavities 25 through these underlying layers. No metal conductors can be present in these underlying layers at the position of the cavity areas.

[0041] When building multiple MIMCAPs according to embodiments on the same substrate, it is preferred to provide isolation between adjacent MIMCAPs. For example when adjacent MIMCAPs are coupled to different supply voltages, care must be taken to avoid shorting paths through the silicon substrate. One preferred way of isolating MIMCAPs according to embodiments is illustrated in FIG. 3. FIG. 3a shows the MIMCAP according to embodiments, formed on an interposer substrate 50. The substrate furthermore comprises metal areas 51/52 in the M1/M2 levels which are in electrical contact with each other. The substrate 50 is temporarily attached to a carrier substrate 53, by attaching it to the carrier through a releasable glue layer 54. The substrate is then thinned by a suitable grinding technique (FIG. 3b), e.g. from a thickness of about 775 m to about 50-100 m. Then a dielectric passivation layer 55 is applied, e.g. SiN (FIG. 3c). By a litho and etch process, a pattern of trenches is then etched in the substrate, stopping on the PMD layer 2 (FIG. 3c). These trenches comprise a trench 56 that encloses the MIMCAP, and a circular trench 57 aligned with the metal areas 51/52. The trenches are then filled with an electrically insulating material 58 (FIG. 3d), preferably a polymer material, deposited inside the trenches and on top of the substrate, thereby effectively isolating the part of the substrate where the MIMCAP is situated from the remainder of the substrate by a wall portion 70 surrounding the portions of the cavities 25 which extend into the substrate 50. After filling the trenches 56/57 with polymer, a litho+etch process is performed for removing a portion of the polymer, the substrate material enclosed by the trench 57, and a portion of the PMD layer 2, stopping on the upper metal contact area 51 (FIG. 3e). After stripping of the litho-mask, the thereby formed opening 59 is then plated with metal (after forming a seed layer, not shown), thereby forming a through-semiconductor-via 60 (FIG. 3f). The polymer forms the liner that isolates the TSV from the substrate 50. The polymer, simultaneously deposited in the TSV trench 57 and in the trench 56 surrounding the MIMCAP, thus performs the function of isolating the MIMCAP and of isolating the TSV. After releasing the interposer from the carrier 53, contact pads 61 are formed on backside for contacting the TSV. In terms of the production of the TSV 60, the above-described process is known as the via-last technique, wherein the liner is first formed and the opening for the TSV is etched after that. As shown, the isolation of the MIMCAP can be integrated in the known via-last process sequence. The isolating wall 70 may equally be formed independently from the formation of TSVs. In the embodiment shown, the layer 71 of the polymer formed on the passivation layer 55 is maintained in the final device. This layer 71 may alternatively be removed before producing the TSV 60.

An Example Implementation

[0042] Table 1 summarizes a set of preferred parameters in terms of dimensions and materials for a MIMCAP according to embodiments. With these parameters, it is possible to obtain a capacitance density of 100 nF/mm.sup.2. The voltage applied to the capacitor may be between 1 and 3 V, and the breakdown voltage is estimated at 10V. It will be understood that, due to inherent process variabilities, a structure having a dimension as described in Table 1 can vary by, without limitation, e.g., +/5%, +/10% or +/20%. For example, a thickness of about 600 nm may refer to a thickness range of 600+/30 nm, 600+/60 nm or 600+/120 nm, according to embodiments.

TABLE-US-00001 TABLE 1 Substrate 1 material Silicon wafer, e.g. about 770 m thick PMD Material Silicon oxide thickness 1 m M1/M2 IMD material A stack of SiO/SiN, 550 nm SiO with 50 nm SiN Thickness 600 nm MIMCAP Material conductive TiN layers 26/28 Material insulator 27 HfO.sub.2 Thickness layers 26/28 40 nm Thickness insulator 27 20 nm Diameter cavities 25 2 m Pitch cavities 25 3.5 m Depth cavities 25 23 m Aspect ratio cavities 25 11.5

[0043] While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure and the appended claims. In the claims, the word comprising does not exclude other elements or process, and the indefinite article a or an does not exclude a plurality. 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. Any reference signs in the claims should not be construed as limiting the scope.

[0044] Unless specifically specified, the description of a layer being present, deposited or formed on another layer or substrate, includes the options of said layer being present, formed or deposited directly on, i.e. in physical contact with, said other layer or substrate, and said layer being present, formed or deposited on one or a stack of intermediate layers between said layer and said other layer or substrate.