PHOTOSENSITIVE PIXEL STRUCTURE WITH INCREASED LIGHT ABSORPTION AND PHOTOSENSITIVE IMPLANT

Abstract

The present invention refers to a photosensitive pixel structure comprising a substrate with a front surface and a back surface, wherein at least one photosensitive diode is provided on one of the surfaces of the substrate. A first material layer is provided at least partially on the back surface of the substrate, wherein the material layer comprises a reflective layer, in order to increase a reflectivity at the back surface of the substrate. Further, the present invention refers to an array and an implant comprising such a photosensitive pixel structure, as well as to a method to produce the pixel structure.

Claims

1. Photosensitive pixel structure comprising a substrate with a front surface and a back surface, wherein at least one photosensitive diode is provided on one of the surfaces of the substrate, characterized in that a first material layer is provided at least partially on the back surface of the substrate, wherein the first material layer comprises a reflective layer.

2. Photosensitive pixel structure according to claim 1, wherein the substrate comprises a material which is adapted to absorb light of a predetermined wavelength or wavelength range, in particular silicon.

3. Photosensitive pixel structure according to either one of claim 1 or 2, characterized in that the first material layer on the back surface of the substrate comprises a layer of buried oxide, preferably SiO2, or the material layer comprises a layer of metal, preferably a layer of aluminium or a layer of titanium.

4. Photosensitive pixel structure according to claim 3, characterized in that the first material layer is formed as an integral part of the substrate.

5. Photosensitive pixel structure according to one of claim 1 or 2, characterized in that the first material layer comprises a layer of buried oxide, preferably SiO2, and a layer of aluminium, wherein the layer of buried oxide is sandwiched between the substrate and the aluminium-layer.

6. Photosensitive pixel structure according to one of the preceding claims, characterized in that at least on a surface of the first material layer facing away from the substrate, a second material layer is provided, which hermetically covers at least the first material layer and/or the back surface of the substrate.

7. Photosensitive pixel structure according to claim 6, characterized in that the second material layer comprises or consists of titanium and/or a ceramic layer.

8. Photosensitive pixel structure according to either one of claims 3 to 7, characterized in that the first and/or the second material layer comprises titanium and the titanium layer has a thickness of not less than 100 nm, preferably more than 200 nm, most preferably a thickness of 500 nm or more.

9. Photosensitive pixel structure according to either one of the preceding claims, characterized in that the first material layer, preferably a buried oxide layer, has a thickness adapted to the material characteristics of the remaining materials, and wherein, in a case that a stack of titanium and buried oxide layer is used as a first material layer, the thickness of the buried oxide layer is in the range of about 65 nm and 210 nm, or, in a case that a stack of aluminium and buried oxide layer is used as a first material layer, the thickness of the buried oxide layer is in the range of about 90 nm and 170 nm, and preferably, the thickness of the buried oxide layer has a thickness of about 130 nm or 430 nm or 130 nm plus any multiple of 300 nm.

10. Photosensitive pixel array comprising a plurality of pixel structures according to one of the claims 1 to 9, wherein the plurality of pixel structures is arranged in an array.

11. Photosensitive pixel array comprising a plurality of pixel structures according to one of the claims 1 to 10, wherein second material layer is provided adjacent to the first material layer.

12. Photosensitive pixel array comprising a plurality of pixel structures according to claim 11 wherein between the second material layer and the first material layer there is arranged an adhesive layer having a thickness of preferably 5 nm to 50 nm, more preferably 10 nm to 30 nm, most preferred about 20 nm+/−5 nm and which is preferably formed of titanium.

13. Implant with a photosensitive pixel structure according to one of the claims 1 to 8 or with a photosensitive pixel array according to claim 10, wherein the implant further comprises at least one electrode, which is adapted to provide an electrical stimulation pulse generated by photoelectric generation in the pixel structure or pixel array.

14. Implant according to claim 13, wherein the implant is a retinal, preferably a subretinal, implant.

15. Method for providing a pixel structure according to one of claims 1-10, characterized in that the method comprises the steps of: providing a substrate adapted to absorb light of at least one predetermined wavelength, providing on the substrate, preferably on a front surface of the substrate, a photosensitive diode, providing on the substrate, preferably on a back surface of the substrate, a first material layer which comprises at least a reflective material layer, which is adapted to reflect light transmitted through the substrate to the first material layer back toward the substrate.

16. Method according to claim 15, characterized in that the first material layer is provided by ion-implantation and/or the first material layer is thermally grown from the substrate.

17. Method according to one of claim 15 or 16, characterized in that a second material layer is provided at least on a surface of the first material layer facing away from the substrate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Further details, preferred embodiments and advantages of the present invention will be found in the following description with reference to the drawings, in which:

[0046] FIG. 1 is an example of a photosensitive pixel with an electrode according to one embodiment of the present invention;

[0047] FIG. 2 is a schematic cross-sectional view of a semiconductor structure with two adjacent pixels according to an embodiment of the invention;

[0048] FIG. 3 displays an electrode array according to an embodiment of the present invention;

[0049] FIG. 4 shows a schematic cross section of (a) a substrate embedded in a retina; (b) a photosensitive pixel structure according to an embodiment of the present invention embedded in a retina; and (c) a photosensitive pixel structure according to another embodiment of the present invention embedded in a retina and (d) a photosensitive pixel structure according to another embodiment of the present invention embedded in a retina and

[0050] FIG. 5 shows a diagram representing the reflection coefficient in dependence from the thickness of a buried oxide layer on a back surface of a pixel structure according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0051] FIG. 1 shows an exemplified photosensitive pixel structure 10. The photosensitive pixel structure 10, in the following also referred to as a pixel, comprises two photosensitive diodes 12, 12′, a central electrode 14 and a resistor 16. At an outer periphery of the pixel structure 10, a counter electrode 18 is provided, which is also often referred to as return electrode. The counter electrode 18 can be placed on each individual pixel structure 10, for instance at the periphery of each pixel structure 10, as shown in FIG. 1. That means, the return electrode is local and in-between the different central electrodes of an array 1 of pixel structures. This is typically also referred to as a “bipolar” configuration.

[0052] For such a bipolar arrangement, two configurations are possible. The return electrodes may be disconnected from one another. That means, pixels in that case are completely independent from one another. Alternatively, all or groups of return electrodes of individual pixel structures or groups of pixel structures may be connected together, in order to effectively creating a sort of grid-like structure. Such a structure may, for instance, comprise a plurality of hexagonal pixels, which may extend over a whole pixel array 1. Examples for such pixel arrays are displayed in FIG. 3.

[0053] As a further alternative, a central return electrode (not shown) may be placed separate from the pixel structure 10, for instance at a position on a pixel array remote from the pixel structure. Such a central return electrode may in particular be provided at a remote location on the implant. Such a configuration may also be referred to as a monopolar configuration. It is to be noted that the return electrode does not necessarily have to be in a geometrical centre of the implant. Further, it is possible that a plurality of such central return electrodes are distributed over the implant or the pixel array. It will be understood that the present invention may be suitably used for either of these configurations.

[0054] The pixel structure 10 in the embodiment of FIG. 1 has a generally symmetric hexagonal shape. That hexagonal shape is defined by trenches 20 arranged around the pixel structure and electrically isolating the pixel structure from adjacent structures Adjacent to each of the sides of that hexagon of the embodiment shown, further pixels 10′ may be provided. An example for an embodiment of a pixel array 1 of pixels 10, also referred to as an electrode array in the context of the present invention, is shown in FIG. 3. In alternative embodiments, the shape of the individual pixels may also differ. For example, the pixels may have an octagonal or rectangular shape. The pixels may also have circular or diamond shape or any other, even arbitrary, shape, without departing from the scope of protection of the present invention.

[0055] Individual pixels are separated from each other by means of the trenches 20. A trench 20 comprises an electrically isolating material. Individual, adjacent pixels 10, 10′ preferably are electrically isolated from one another. The counter electrode 18 as shown in the embodiment of FIG. 1 is arranged along the extension of the trench 20 surrounding the periphery of active area of the pixel 10 thus with the same, here hexagonal, contour. A cross section through a pixel structure 10′ with an adjacent pixel structure 10′ is shown FIG. 2.

[0056] The two diodes 12, 12′ according to the embodiment of FIG. 1 are arranged inscribed within the area of the hexagonal pixel shape. Preferably, the diodes 12, 12′ are symmetrically arranged. Between the diodes 12, 12′, an isolating trench 20′ is provided. The isolating trench 20′ between the diodes 12, 12′ generally has the same properties as the isolating trench 20. The different diodes 12, 12′ of the pixel 10 are therefore basically electrically isolated from one another. It is to be understood that despite trenches 20′ arranged within the pixel, i.e. in a substrate 15 of the photosensitive element, electrical contact between objects separated and isolated by trenches 20, 20′ may still be established. In the embodiment according to FIG. 1, for instance, the diodes 12, 12′ are connected by an electrical contact 22. As will be further detailed with respect to FIG. 4, the diodes 12, 12′, that way, are serially connected with respect to one another in the embodiment according to FIG. 1.

[0057] The diodes 12, 12′ represent in the projection view of the embodiment according to FIG. 1 a photosensitive area of the pixel 10. In that embodiment, the surface area, i.e. the photosensitive area, of the diodes 12, 12′ is essentially symmetric around a symmetry axis of the pixel 10. In the embodiment of FIG. 1 such a symmetry axis may for instance coincide with the trench 20′ separating the diodes 12, 12′ of the pixel 10. In other embodiments, the number of diodes may be different. In particular, there may be only one diode 12 provided. That would allow to increase the photosensitive area of the pixel, as no trenches 20′ had to be provided to separate individual diodes within the pixel 10. In further embodiments, three diodes or more than three diodes may be provided in one pixel. If more than two diodes are provided in a pixel 10, the individual diodes may also be serially connected with one another, as already discussed for a two-diode pixel structure above.

[0058] As may be further seen in FIG. 1, in the centre of the pixel structure 10, an electrode 14 is provided. Due to its central position, that electrode 14 is also referred to as central electrode. Further, as that electrode typically is used for stimulation, that electrode is also referred to as stimulating electrode. The stimulating electrode 14 in the shown embodiment is provided having a circular shape. The electrode may also have different shapes, such as a shape similar to the shape of the return electrode 18 or the trench 20 reflecting the contour of the pixel 10. The circular shape of the presently shown embodiment was chosen such that the electrical field from the stimulating electrode 14 may be homogenous. Depending on the intended application, the shape may also include such shapes which allow less homogenous, locally enhanced field distributions.

[0059] According to some embodiments of the present invention, the electrode 14 of the pixel 10 shall be adapted for stimulation of surrounding tissue, preferably neural tissue, in particular neural tissue of a retina in vivo. Typically, the electrode comprises platinum, iridium oxide and/or titanium nitride. Alternatively, iridium, platinum iridium, doped diamond or diamond-like carbon or PEDOT:PSS, or other known materials may be used as electrode material. The preferred structure of the electrode material may in particular be a highly porous structure, such as a porous or fractal TiN, a platinum structure or SIROF. Such structures are known and found to be described to be, e.g., “black platinum” or “porous platinum”. The thickness of the electrodes may vary from about 100 nm to 3 μm. It is, however, also possible to have an electrode thickness up to or above 10 μm as well, or below 100 nm.

[0060] In the embodiment as shown in FIG. 1, the return electrode 18 is provided as an elongate electrode surrounding the pixel and following the contour of the pixels periphery, i.e., in the shown embodiment, the run of the trench 20. In alternative embodiments, the return electrode may also comprise a plurality of electrodes, which are distributed around the pixel structure 10 and around the stimulating electrode 14 in regular or arbitrary distribution. This may in particular be exerted at a peripheral portion of an electrode array 1.

[0061] Further, between the stimulating electrode 14 and the counter electrode 18, the resistor 16, also referred to as a shunt resistor, is arranged. That resistor 16 according to the embodiment shown in FIG. 1 of the present invention, is electrically connected to the stimulating electrode 14 and to the counter electrode 18.

[0062] As indicated above, a plurality of diodes, for instance two or three diodes, within one pixel 10, may be provided, if the voltage, as response to a light signal received, needs to be increased. The diodes may for such cases be serially connected, wherein the voltage of a number N of diodes is the factor N higher than the voltage created by one diode only. On the other hand, an increased number of diodes means that fewer light may be collected by each diode, per pixel. The electrical current created by each of those diodes connected in series may therefore be significantly lower when having a plurality of diodes compared to having only one or a few diodes. Typically, the current in a circuit with N diodes is N times less than the current in a circuit with one diode. It is therefore a matter of choice, which of the parameters, i.e., current or voltage, is more desirable for an individual application. In the specific case of neural stimulation, the required stimulation parameters may depend on the tissue and/or the individual cells, in particular neural cells, to be excited, the position of an implant and even individual specifics of a patient, possibly age, state of disease and general physiological condition.

[0063] In order to increase the current generated, thus, it is therefore desired to increase the light absorption in the substrate. FIG. 2 shows a sectional side view of a portion of an electrode array 1, showing two adjacent pixels 10, 10′. The pixels 10, 10′ correspond to the pixels of the pixel structure according to the embodiment as shown in FIG. 1, having two diodes 12, 12′. The same layer structure as shown in FIG. 1 for a two-diode pixel may essentially also be provided for a one-diode or three-diode pixel, analogously.

[0064] Further, in FIG. 2, a first material layer 30 is shown. That material layer 30 may be formed as an integral part of the substrate 15, as in the case for the embodiment shown in FIG. 2. Alternatively, the first material layer may at least partially be an integral part of the substrate 15, or may be a layer deposited on the substrate 15.

[0065] The first material layer 30 is provided adjacent and subsequent to a back surface of the substrate 15. The first material layer 30 may, for instance, comprise a buried oxide layer, in particular an SiO.sub.2 layer. The buried oxide layer may be thermally grown on the substrate 15. The substrate layer preferably comprises silicon. In addition, the first material layer 30 may be a stacked layer comprising, subsequent to the buried oxide layer, a metal layer, such as an aluminium or titanium layer.

[0066] In the embodiment according to FIG. 2, a second material layer 32 adjacent to the first material layer 30 is provided on a surface of the first material layer 30 which faces away from the substrate 15. The second material layer 32 may comprise a metal, such as aluminium or titanium, or a stack of metals. Preferably, the outermost layer of the pixel structure 10 at least on the back surface of the substrate 15, i.e., the outermost layer of the second material layer 32, comprises a material which allows a hermetic sealing of the back surface of the substrate 15, or of the back surface of the substrate and at least a part of the side portion of the pixel structure. That way, as may be seen in FIG. 2, the edge of the pixel structure may be hermetically sealed and the pixel structure may be protected from corrosion or decay due to environmental effects. In case that an entire pixel array 1, as shown in FIG. 3, or an implant shall be provided, the hermetic sealing may be provided on the outermost layer, edge and/or side portion of that pixel array 1.

[0067] It will be understood that the definition as a “layer”, in particular with respect to the first material layer 30, is used in order to better describe the characteristics of the pixel structure 10. However, as a consequence of the methods used to produce the pixel structure 10 according to the invention, the individual layers such as the substrate 15, the first material layer 30 or the second material layer 32 may be integrated into another. The Methods used to provide the layer structure according to the present invention may for instance include thermal growing, ion deposition, electrochemical deposition, physical vapour deposition, such as sputtering and electron beam evaporation, or other methods. Consequently, a pixel structure produced accordingly may actually not appear to have a layer appearance, or display separable layers, while, functionally, layers, e.g. according to embodiments of the present invention, are in fact provided therein. According to a special embodiment, at least two “layers” can be separated by one adhesive layer 33. Said adhesive layer 33 may have a thickness of preferably 5 nm to 50 nm, more preferably 10 nm to 30 nm, most preferred about 20 nm+/−5 nm. The adhesive layer 33 may be formed of titanium which has good adhesive properties. Preferably, there is no adhesive layer 33 between layer 15 and 30 when the first material layer 30 comprises buried oxide layer thermally grown on the substrate 15.

[0068] FIG. 3 shows an array of pixel structures 10, 10′, i.e., a pixel array 1. In the embodiment shown in FIG. 3, the pixel array 1 is an array of pixel structures 10, 10′ wherein each of the pixel structures 10. 10′ comprise a stimulating electrode 14 configured to stimulate cells or living tissue. Therefore, the pixel array 1 may also be referred to as an electrode array. The size of the individual pixel structures 10, 10′ in the array 1 may differ and can thus be tuned to different applications, without departing from the scope of the present invention. In the array 1 displayed in FIG. 3, the individual pixels 10, 10′ are hexagonally formed, which allows a space efficient distribution on the substrate 15. That way, the space available for light sensitive regions on the substrate 15 and within an array 1 may be increased and ideally maximized. A pixel array 1 as shown in FIG. 3 may for instance be used in an implant in order to stimulate cells or living tissue, in particular living tissue, such as neural tissue, or neural cells.

[0069] According to embodiments of the present invention, not shown in FIG. 3, first and/or the second material layer may be formed around the back surface of the array, i.e. the surface build by the plurality of back surfaces of the individual pixel structures. In addition, the first and/or second material layer may be formed around an edge of the array 1, in order to provide a sealing or protection to the array.

[0070] FIG. 4 (a) shows a schematic cross section of a pixel array 1, which is embedded in tissue, here for instance in a retina 3. The pixel array 1 is represented by the substrate 15, wherein any surface structures, such as diodes or electrodes, are not displayed in the figure.

[0071] Commonly, when implanting a pixel array 1, or an implant, into a retina 3, the substrate is arranged such that light, represented by the arrow 40 in FIG. 4 (a), which is incident on the eye may traverse the retina and be incident on a front surface of the substrate 15. From the front surface of the substrate 15, the light enters and traverses the substrate 15 where it is absorbed depending on the material of the substrate 15, the wavelength of the incident light, and other factors. The substrate 15 used typically comprises or consists of silicon.

[0072] For a typical thickness of 30 μm of silicon substrate, and at a wavelength of 830 nm of the incident light, about 85% of the incident light is absorbed. At a wavelength of 880 nm, 68% of the incident light are absorbed and at a wavelength of 915 nm, only about 53% of the incident light are absorbed. If the substrate is to be used in an implant in order to restore vision, the stimulation of a pixel structure 10 comprising the substrate 15 needs to be in the infrared or near-infrared region of the spectrum, such that residual vision of the retina is not disturbed. The light, which is not absorbed in the substrate 15 is incident on the back surface of the substrate 15. At the back surface of the substrate 15, due to the intrinsic material properties and the laws of reflection, about 21% of the light is reflected back into substrate (not shown in FIG. 4 (a)), while the bigger part of the light exits the substrate 15 as indicated with arrow 42 in FIG. 4 (a) and is lost.

[0073] As displayed in FIG. 4 (b), according to an embodiment of the present invention, on the back surface of the substrate 15 a first material layer 30 is provided adjacent to the substrate 15. That first material layer 30 may also be a stack of materials. The first material layer 30 comprises at least a reflective material layer, which increases the reflectivity at the back surface of the substrate. Thereby, an increased fraction of the light initially transmitted through the substrate 15 without being absorbed may be reflected back into the substrate 15, as indicated with the arrow 41 in FIG. 4 (b). Accordingly, less light will be lost for a photo-electrical reaction.

[0074] FIG. 4 (c) displays a further embodiment of the present invention, according to which a second material layer 32 is provided adjacent to the first material layer 30. Such a second material layer 32 may allow a further increase in reflectivity at the back surface of the substrate 15. That may further increase the absorption rate. The second material layer 32 may be a material which allows a hermetic sealing, such as titanium. Thereby, the reflectivity at the back surface may be increased, while, at the same time, hermeticity of the pixel structure 10 or the entire pixel array 1 or implant may be enabled. Further materials to provide a hermetic cover layer, coating or housing may be ceramic layers, such as aluminium oxide, silicon carbide or others.

[0075] FIG. 4 (d) displays a further embodiment of the present invention, according to which a second material layer 32 is provided adjacent to the first material layer 30 similar to the embodiment of FIG. 4 (c). Again, the second material layer 32 may allow a further increase in reflectivity at the back surface of the substrate 15. The first material layer 30 may be formed of silicon dioxide, while the second material layer 32 may be formed of Aluminium, having a thickness of 100 nm or more, or may be formed by a stack of Aluminium and Titanium, having a thickness of 100 nm or more each. Between the second material layer 32 of Aluminium, or Aluminium and Titanium, and the first material layer 30 of silicon dioxide there is arranged an adhesive layer 33 having a thickness of preferably 5 nm to 50 nm, more preferably 10 nm to 30 nm, most preferred about 20 nm+/−5 nm. The adhesive layer 33 may be formed of titanium which has good adhesive properties.

[0076] FIG. 5 shows a graph representing the reflection coefficient versus the thickness of a buried oxide layer of a stacked first material comprising the buried oxide layer and an aluminium layer (upper curve) and a buried oxide layer and a titanium layer (lower curve). It will be noted that the reflectivity of the BOX/Al-stack significantly surpasses that of the BOX/Ti-stack at and around a thickness of the buried oxide layer of 130 nm, 430 nm or 130 nm plus multiples of 300 nm. Further, the slope of the curve of the BOX/Al-stack is flatter in the range of that preferred thicknesses of 130 nm, 430 nm or 130 nm plus multiples of 300 nm of the buried oxide layer than the curve of the BOX/Ti-layer.

[0077] The graphs displayed in FIG. 5 are based on simulation results of a substrate 15 consisting of silicon, a first material layer 30 stacked of SiO.sub.2 and aluminium or titanium, and a subsequent retinal layer 3. Indices of refraction for that simulation were assumed to be 3.66 for Si, 1.4525 for SiO.sub.2, 2.58+8.21 I for Al, 3.06+3.305 I for Ti, and 1.36 for the retinal tissue at a wavelength of 880 nm. While these data represent specific embodiments of the present invention, similar or same conclusions, in particular to the BOX-thickness, may be drawn also for different wavelength or material properties. These examples shall not be construed to limit the scope of the present invention to the specific examples. Rather, various implementations with different materials, material thicknesses, layer numbers, reflective indices and so on may be applied within the scope of the present invention.

[0078] It is further to be understood that according to the present invention, the thickness of the BOX-layer may be varied to thicknesses higher or lower than the indicated preferred thickness of around 130 nm, 430 nm or 130 nm plus multiples of 300 nm.